c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a01 557

Aluminum Oxide 1

Aluminum Oxide

L.Keith Hudson, Aluminum Company of America, Alcoa Center, Pennsylvania 15069, USA

Chanakya Misra, Aluminum Company of America, Alcoa Center, Pennsylvania 15069, United States

Anthony J. Perrotta, Aluminum Company of America, Alcoa Center, Pennsylvania 15069, USA

Karl Wefers, Aluminum Company of America, Alcoa Center, Pennsylvania 15069, USA

F. S. Williams, Aluminum Company of America, Alcoa Center, Pennsylvania 15069, USA

1. General Aspects . . . . . . . . . . . . 11.1. Aluminum Hydroxides . . . . . . . 21.2. Aluminum Oxide Hydroxides . . . 31.3. Aluminum Oxide, Corundum . . . 41.4. The Al2O3 –H2O System . . . . . . 41.5. Thermal Decomposition of Alu-

minum Hydroxides . . . . . . . . . . 51.6. Aluminates and Related Com-

pounds . . . . . . . . . . . . . . . . . . 72. Bauxite, the Principal Raw Mate-

rial . . . . . . . . . . . . . . . . . . . . . 72.1. Definition and Geology . . . . . . . 72.2. Composition and Properties . . . . 72.3. Genesis of Bauxites . . . . . . . . . . 92.4. Major Bauxite Deposits . . . . . . . 102.5. Economic Aspects . . . . . . . . . . . 113. Bayer Process . . . . . . . . . . . . . . 113.1. History and Procedure . . . . . . . . 113.1.1. Bauxite Preparation . . . . . . . . . . 123.1.2. Digestion . . . . . . . . . . . . . . . . . 133.1.3. Equipment . . . . . . . . . . . . . . . . 153.1.4. Residue Separation . . . . . . . . . . . 173.1.5. Precipitation . . . . . . . . . . . . . . . 203.1.6. Impurities . . . . . . . . . . . . . . . . . 233.1.7. Calcination . . . . . . . . . . . . . . . . 25

3.1.8. Evaporation . . . . . . . . . . . . . . . 263.1.9. Residue Disposal . . . . . . . . . . . . 273.1.10. Energy in the Process . . . . . . . . . 283.2. Economic Aspects . . . . . . . . . . . 294. Other Processes for Alumina Pro-

duction . . . . . . . . . . . . . . . . . . 294.1. Raw Materials . . . . . . . . . . . . . 294.2. Alkaline Processes . . . . . . . . . . . 304.3. Acid Processes . . . . . . . . . . . . . 315. Metallurgical Alumina . . . . . . . . 325.1. Alumina Properties Required for

Smelting . . . . . . . . . . . . . . . . . 325.2. Typical Specifications forMetallur-

gical Alumina . . . . . . . . . . . . . . 336. Industrial Alumina Chemicals . . 336.1. Aluminum Hydroxides . . . . . . . 336.2. Adsorbent and Catalytic Aluminas 346.2.1. Preparation of Activated Aluminas . 346.2.2. Adsorbent Applications . . . . . . . . 366.2.3. Catalytic Applications . . . . . . . . . 377. Ceramic Uses of Alumina . . . . . . 387.1. Calcined Alumina . . . . . . . . . . . 397.2. Fused Alumina . . . . . . . . . . . . . 398. Toxicology and Industrial Hygiene 399. References . . . . . . . . . . . . . . . . 39

1. General Aspects

Almost 4000 years ago Egyptians and Baby-lonians used aluminum compounds in variouschemicals and medicines. Herodotus men-tioned alum in the fifth century b.c. and Plinyreferred to “alumen,” now known as alum, as amordant to fix dyes to textiles around 80 a.d.In 1754Marggraf showed that a distinct com-pound existed in both alum and clays. In 1761the French chemist Guyton de Morveau pro-posed the name “alumine” for the base in alum,identified in 1787 by Antoine Lavosier as the

oxide of a then-undiscovered element. By the1700s the earthy base alumina was recognizedas the potential source of a metallic element.Greville (1798) described a mineral from

India that had the composition Al2O3 andnamed it corundum [1302-74-5], derived fromthe native name of this stone. Hauy (1801)called a mineral diaspore [14457-84-2] (fromthe Greek “diaspora” meaning dispersion) be-cause it decrepitated on heating. Its composi-tion, Al2O3 · H2O, was determined by Vaque-lin in 1802. Gibbsite [14762-49-3], named afterthe American mineralogist G. Gibbs, was found

2 Aluminum Oxide

by Dewey in 1820; Torrey (1822) showed thismineral to have the composition Al2O3 · 3H2O.The name hydrargillite [14762-49-3] was givento a similarmineral found later in theUralMoun-tains. Using the newly developed technique ofX-ray diffraction, Bohm and Niclassen [13]identified a crystalline aluminum oxide hy-droxide, Al2O3 · H2O, later named bohmite(boehmite). Bohm discovered a second type oftrihydroxide,Al2O3 ·3H2O, a year later.Fricke[14] suggested the name bayerite [20257-20-9]for this compound, believing it to be the prod-uct of the Bayer process, which he later identi-fied as gibbsite. Only few occurrences of naturalbayerite have been reported [15]. van Nord-strand et al. [16] reported a third form of tri-hydroxide, which was later named nordstrandite[13840-05-6] in his honor.

1.1. Aluminum Hydroxides

A general classification of the various modifica-tions of aluminum hydroxides is shown in Fig-ure 1. The best defined crystalline forms are thethree trihydroxides, Al(OH)3: gibbsite, bayerite,and nordstrandite, and two modifications of alu-minum oxide hydroxide, AlO(OH): boehmiteand diaspore. Besides these well-defined crys-talline phases, several other forms have been de-scribed in the literature [2], [3]. However, thereis controversy as to whether they are truly newphases or simply forms with distorted latticescontaining adsorbed or interlamellar water andimpurities.

Identification of the different hydroxides andoxides is best carried out by X-ray diffractionmethods [3]. Mineralogical and structural dataare listed in Tables 1 and 2.

Gelatinous hydroxides may consist of pre-dominantlyX-ray indifferent aluminumhydrox-ide or pseudoboehmite. The X-ray diffractionpattern of the latter shows broad bands thatcoincide with strong reflections of the well-crystallized oxide hydroxide boehmite.

The aluminum hydroxides found abundantlyin nature are gibbsite, boehmite, and diaspore.Gibbsite and bayerite have similar structures.Their lattices are built of layers of anion octahe-dra in which aluminum occupies two thirds ofthe octahedral interstices. In the gibbsite struc-ture, the layers are somewhat displaced relative

to one another in the direction of the a axis. Thehexagonal symmetry of this lattice type (brucitetype) is lowered tomonoclinic. Triclinic symme-try was found in larger gibbsite single crystalsfrom the Ural Mountains [17].

In bayerite the layers are arranged in approx-imately hexagonal close packing. Because ofshorter distances between the layers, the den-sity is higher than in the case of gibbsite. Thecrystal class and space group of bayerite havenot yet been established clearly.

The individual layers of hydroxyl ion octahe-dra in both the gibbsite and the bayerite struc-tures are linked to one another only throughweak hydrogen bonds. Bayerite does not formlarge single crystals. The most commonly ob-served growth forms are spindle- or hourglass-shaped somatoids. The long axis of these soma-toids stands normal to the basal plane; i.e., thesomatoids consist of stacks of Al(OH)3 layers(Fig. 2). The effect of alkali ions on the structuresof Al(OH)3 types was investigated by severalworkers [3], [18], [19]. Intercalation of Li+ ionstransforms gibbsite to the hydrotalcite structure[20].

Figure 2. Somatoids of bayerite

Gibbsite crystals of appreciable size are notuncommon. Clear pseudohexagonal plateletsabout 1mm in diameter are known from Aro inNorway. Prismatic crystals 0.5 – 1mm in lengthare occasionally produced in the Bayer process.Nordstrandite, the third form of Al(OH)3,

was described by Van Nordstrand [16] andothers [21]. The structures of nordstrandite andbayerite were investigated [22] and comparedwith those of monoclinic and triclinic gibbsite,which had been determined previously [17].

Aluminum Oxide 3

Figure 1. Classification of aluminum hydroxides(All figures courtesy of Aluminum Company of America)

Table 1. Mineralogic properties of oxides and hydroxides [3]

Phase Refractive index n20D Cleavage Brittleness Mohs Luster

hardnessα β γ Average

Gibbsite 1.568 1.568 1.587 – (001) perfect tough 2.5 to 3.5 pearly vitreous

Bayerite – – – 1.583 – – – –

Boehmite 1.649 1.659 1.665 – (010) – 3.5 to 4 –

Diaspore 1.702 1.722 1.750 – (010) perfect brittle 6.5 to 7 brilliant pearly

ε ω Average

Corundum 1.7604 1.7686 – none tough whencompact

9 pearlyadamantine

The lattice of nordstrandite is built of thesame, electrically neutral Al(OH)3 octahedrallayers that form the structural elements of gibb-site and bayerite [14]. The lattice period amountsto 1.911 nm in the direction normal to the layer.This corresponds to the sum of identical layerdistances of bayerite plus gibbsite. The identi-cal nordstrandite structure consists of alternat-ing double layers, in which the OH octahedraare arranged once in the packing sequence ofbayerite, and then in that of gibbsite. Materialcontaining continuous transitions from bayeritethrough nordstrandite to gibbsite has been pre-pared through the proper selection of precipita-tion conditions [18].

1.2. Aluminum Oxide Hydroxides

Pseudoboehmite is formed during aging of X-ray indifferent hydroxide gels as a precursor oftrihydroxide. The reflections of pseudoboehmiteare broadened not only because of the very smallparticle size, but also because of variable dis-tances of the AlO(OH) double chains, whichform the structural element of pseudoboehmiteas well as of well-crystallized boehmite. Thelattice spacing in the direction of the c axis in-creases by 0.117 nm for eachmole of excess wa-ter [22].Boehmite consists of O, OH double lay-

ers in which the anions are in cubic packing.

4 Aluminum Oxide

Table 2. Structural properties of oxides and hydroxides [3]

Phase Formula Crystal system Spacegroup

Moleculesper unit cell

Unit axis length, ×10−1 nm Angle Density,g/cm3

a b c

Gibbsite Al(OH)3 monoclinic C52h 4 8.68 5.07 9.72 94 ◦34 ′ 2.42

Gibbsite Al(OH)3 triclinic – 16 17.33 10.08 9.73 94 ◦10 ′

92 ◦08 ′

90 ◦0′

–

Bayerite Al(OH)3 monoclinic C52h 2 5.06 8.67 4.71 90 ◦16 ′ 2.53

NordstranditeAl(OH)3 triclinic C1i 4 8.75 5.07 10.24 109 ◦20 ′

97 ◦40 ′

88 ◦20 ′

–

Boehmite AlO(OH) orthorhombic D172h 2 2.868 12.227 3.700 – 3.01

Diaspore AlO(OH) orthorhombic D162h 2 4.396 9.426 2.844 – 3.44

Corundum Al2O3 hexagonal (rhomb.) D63d 2 4.758 – 12.991 – 3.98

The aluminum ions are octahedrically coordi-nated. These layers are composed of chains of[AlO(OH)]2 extending in the direction of the aaxis [23]. The double layers are linked by hy-drogen bonds between hydroxyl ions in adjacentplanes. Boehmite crystals exhibit perfect cleav-age parallel to the (010) plane.

In the diaspore structure the oxygen ions arenearly equivalent, each being joined to anotheroxygen center through a hydrogen ion. The an-ions are hexagonally close packed [24]. The po-sition of the hydrogen ion has been establishedby neutron diffraction [25]. The O – H – O dis-tance is 0.265 nm. By infrared studies, the bondenergy for the hydrogen bridges in diaspore wasdetermined to be 28.7 kJ/mol, compared with20.1 kJ/mol for boehmite [26].

1.3. Aluminum Oxide, Corundum

The hexagonally closest packed α-Al2O3 mod-ification is the only stable oxide in theAl2O3 –H2O system. Corundum is a commonmineral in igneous and metamorphic rocks. Redand blue varieties of gem quality are called rubyand sapphire, respectively. The lattice of corun-dum is composed of hexagonally closest packedoxygen ions forming layers parallel to the (0001)plane. Only two-thirds of the octahedral inter-stices are occupied by aluminum ions. The struc-ture may be described roughly as consisting of

alternating layers of Al and O ions. The corun-dum structure was determined in the early 1920s[27]; numerous workers later confirmed and re-fined these data [3]. Properties of corundum arelisted in Tables 1 and 2.

1.4. The Al2O3 –H2O System

Under the equilibrium vapor pressure of wa-ter, crystalline Al(OH)3 converts to AlO(OH)at about 375K. The conversion temperature ap-pears to be the same for all three forms ofAl(OH)3. At temperatures lower than 575K,boehmite is the prevailing AlO(OH) modifica-tion, unless diaspore seed is present. Sponta-neous nucleation of diaspore requires temper-atures in excess of 575K and pressures higherthan 20MPa. In the older literature, therefore,diaspore was considered the high-temperatureform of AlO(OH). The first reaction diagram ofthe phase transitions in the Al2O3 –H2O sys-tem was published in 1943 [28]. These workersdetermined the gibbsite→ boehmite conversiontemperature to be 428K. Boehmite transformedto diaspore above 550K; diaspore converted tocorundum, α-Al2O3, at 725K. Similar resultswere reported in 1951 [29].

The system was reinvestigated in 1959 [30]and in 1965 [31]. A phase diagram based onthese data is shown in Figure 3. Diaspore is

Aluminum Oxide 5

the stable modification of AlO(OH); boehmiteis considered metastable, although it is ki-netically favored at lower temperatures andpressures. This is because the nucleation en-ergy is lower for boehmite than for the con-siderably more dense diaspore. Nucleation isadditionally facilitated by the possibility thatboehmite can grow epitaxially on Al(OH)3. Inthe Al2O3 – Fe2O3 –H2O system, the presenceof the isostructural goethite, α-FeO(OH), low-ers the nucleation energy for diaspore so that thisAlO(OH) modification crystallizes at tempera-tures near 373K [32]. This observation explainsthe occurrence of diaspore in clays and bauxitedeposits that have never been subjected to hightemperatures or pressures.

Figure 3. The Al2O3 –H2O systemDashed lines [21], solid lines [22], [28]

Nomenclature. Although there is fairly goodagreement in the more recent literature on phasefields and structures of the crystalline phases inthe Al2O3 –H2O system, the nomenclature isstill rather unsystematic.

Bayerite, gibbsite (hydrargillite), and nord-strandite are trihydroxides of aluminum, andnot oxide hydrates. The designation “aluminumoxide monohydrate” for boehmite and diasporeis also incorrect. Both are true oxide hydrox-ides. Molecular water has been determined onlyin poorly crystallized, nonstoichiometric pseu-doboehmite.

The designation of the modifications of alu-minum hydroxides and oxides lacks unifor-mity just as much as does the nomenclature ofthe compounds. According to the general us-age in crystallography, the most densely packedstructures are designated asα-modifications [3].Bayerite, diaspore, and corundumfallwithin thisclass. The compounds with cubic packing se-quence, gibbsite and boehmite, have been des-ignated by the symbol γ. Nordstrandite can beclassified as β-Al(OH)3 when regarding thiscompound not as an intergrowth of bayerite andgibbsite, but as an independent modification.

1.5. Thermal Decomposition ofAluminum Hydroxides

When aluminum hydroxides or oxide hydrox-ides are heated in air at atmospheric pressure,they undergo a series of compositional and struc-tural changes before ultimately being convertedto α-Al2O3. These thermal transformations aretopotactic. Despite a loss of 34 or 15 % of massfor the trihydroxides or oxide hydroxides, re-spectively, the habit of the primary crystals andcrystal aggregates changes very little. This leadsto considerable internal porosity, which may in-crease the specific surface area of the material toseveral hundred m2/g. Structural forms developthat, although not thermodynamically stable, arewell reproducible and characteristic for a giventemperature range and starting material. Thesetransition aluminas have been the subject of nu-merous investigations because of their surfaceactivity, sorptive capacity, and usefulness in het-erogeneous catalysis. The literature in this fieldof physical chemistry has been reviewed up to1987 [3].

The simplest transformation is that of dias-pore to corundum. As the structures of thesetwo compounds are very similar, the nucle-ation of α-Al2O3 requires only minor rear-rangement of the oxygen lattice after the hy-drogen bonds are broken. A temperature below860 – 870K is sufficient for complete conver-sion. The newly formed corundum grows epi-taxially on the decomposing diaspore, with the(0001) plane ofAl2O3 parallel to the (010) planeof AlO(OH) [33]. Transformation to corundum(α-Al2O3) proceeds through an intermediateα’-Al2O3 phase [34].

6 Aluminum Oxide

Figure 4. Gibbsite heated to 573K

Figure 5. Specific surface area, loss on ignition (LOI), anddensity of heated Al(OH)3

The thermal transformation, at ambient pres-sure, of boehmite and the trihydroxides toα-Al2O3 requires considerably more struc-tural rearrangements and is generally not com-pleted until the temperature reaches at least1375 – 1400K. The first step in the reaction se-quence is the diffusion of protons to adjacentOHgroups and the subsequent formation of water[35], [36]. This process begins at a temperaturenear 475K. If this water cannot diffuse rapidlyout of larger trihydroxide particles, hydrother-mal conditions may develop locally, resulting

in the formation of γ-AlO(OH). With increas-ing loss of water, a large internal porosity devel-ops. The lattice voids left by the escaping waterare not readily healed because of the slow diffu-sion in this low temperature range. The voids areoriented parallel and perpendicular to the basalplane of the trihydroxide crystals (Fig. 4).

The highest surface area and lowest crys-talline order of the solid (not counting newlyformed boehmite) is obtained at a temperaturearound 675K. With increasing temperature thesurface area decreases, while the density of thesolid shows progressively higher values (Fig. 5).This trend is the result of progressive reorderingand consolidation of the solid.

During the thermally driven consolidationand reordering, the solid goes through structuralstages that are influenced by the nature of thestarting material as well as by heating rates, fur-nace atmosphere, and impurities. The generalreaction paths are illustrated in Figure 6, whichshows the various intermediate transition formsthat have been identified during the reorderingprocess.

Figure 6.Decomposition sequence of aluminum hydroxide

Transition oxides formed at lower tem-peratures are mostly two-dimensional, short-range ordered domains within the texture ofthe decomposed hydroxides. Extensive three-dimensional ordering begins at about 1050K.Until completely converted to corundum, thesolid retains considerable amounts of OH− ions.Most likely protons are retained to maintainelectroneutrality in areas deficient of cations.Therefore, the presence of protons may retardthe reordering of the cation sublattice. The highsurface area (> 75m2/g) of γ-Al2O3 has beenshown to provide thermodynamic stability [37].Addition of fluorine to the furnace atmosphereremoves protons. As a result, rapid transitionto α-Al2O3 occurs at temperatures as low as

Aluminum Oxide 7

1150K. Markedly tabular corundum crystalsform, possibly because the preceding transitionalumina is mostly two-dimensionally ordered[38].

Transition forms other than those shown inFigure 6 can be obtained by hydrothermal treat-ment [3]. The structures of various transitionforms have been investigated [17], [22], [39].

1.6. Aluminates and RelatedCompounds

Sodium oxide forms several compounds withaluminum oxide. These so-called β-aluminasrepresent a group of aluminates having the same,or very similar, structures but variable chemicalcomposition. Their molar ratios (Na2O : Al2O3)can vary between 1 : 1 and 1 : 11. The1 : 1 sodium aluminate, NaAlO2 [1302-42-7],exists in at least two allotropic modifica-tions. Orthorhombic β-NaAlO2 is stable be-low 750K; the higher – temperature γ modifica-tion is tetragonal. For preparation and propertiesof technical sodium aluminate, →AluminumCompounds, Inorganic, Chap. 2.1.

The 1 : 11 β-Al2O3 crystallizes from meltscontaining aluminumoxide and sodium oxide orother sodium compounds. A 1 : 5 β-alumina hasbeen prepared by heatingα-Al2O3 withNaAlO2or Na2CO3 at about 1325K.

Several other β-aluminas containing CaO,BaO, or SrO in 1 : 6 ratio also have been re-ported [3].

β-Alumina is of interest to its use as a solidelectrolyte in sodium – sulfur secondary batter-ies (→Batteries).

2. Bauxite, the Principal RawMaterial

2.1. Definition and Geology

The term bauxite [1318-16-7] is used for sed-imentary rocks that contain economically re-coverable quantities of the aluminum mineralsgibbsite, boehmite, and diaspore. The name de-rives from the description byBerthier, in 1821,of a sediment that occurred near the village ofLes Baux in the Provence, France. Originally

assumed to be a dihydrate of aluminum oxide,bauxite was later recognized as being composedof aluminum hydroxide, iron oxide and hydrox-ide, titanium dioxide, and aluminosilicate min-erals [2].

Early in this century, major bauxite depositswere found in various parts of the Tertiary andCretaceous limestone formations of the Euro-pean Alps; also in several locations on the NorthAmerican continent, e.g., inArkansas,Alabama,andGeorgia. Since the 1920s, extensive depositshave been discovered in the tropical and subtrop-ical climate belts.

The oldest known bauxites developed in thePrecambrian; the youngest are of recent ori-gin. Deposits may occur as extensive, flat bod-ies blanketing areas of many square miles; theymay form irregularly shaped fillings of dolinasin old karst surfaces, or lenses several hundredmeters in diameter and tens of meters thick.Allochtonous, i.e., displaced bauxites, are alsocommon; erosion of primary deposits and rede-position of the detritus in valleys or alongmoun-tain slopes has frequently led to mineable accu-mulations of ore.

Many of the geologically younger bauxitesare covered only by thin layers of soil; others areburied by coastal or alluvial sediments. Olderdeposits, especially in the Balkans or the UralMountains, often are overlain by carbonate rockshundreds ofmeters thick. Regardless of their ge-ologic age, all bauxites were formed during con-tinental periods [40].

2.2. Composition and Properties

The chemical composition of bauxites of variousorigin is given in Table 3 [2]. Aluminum oxide,iron oxide, and titanium and silicon dioxides arethe major chemical components of all bauxites.Alkali and alkaline earth compounds are rarelyfound.

Gibbsite, γ-Al(OH)3, is the predominantaluminum mineral in the geologically youngbauxites of the tropical climate belt. Mesozoicand older bauxite contain mostly boehmite (γ-AlO(OH)) or diaspore (α-AlO(OH)). Since theirformation, many of the older bauxites have beenburied under considerable layers of youngersediments and often were subjected to tectonicstress. Boehmite and diaspore formation appears

8 Aluminum Oxide

Table 3. Principal chemical constituents of various bauxites, wt %

Country and location Al2O3 SiO2 Fe2O3 TiO2 Loss on ignition

AustraliaDarling Range 37 26.5 16.4 1.1 19.3Weipa 58 4.5 6.9 2.5 26.8

BrazilTrombetas 52 5.1 13.9 1.2 28.1

FranceSouthern districts 57 4.6 22.6 2.9 15.1

GuyanaMackenzie 59 4.9 2.9 2.4 30.4

GuineaFriguia 49 6.1 14.2 1.6 28.1Boke 56 1.5 7.9 3.7 30.1

HungaryHalimba 52 6.6 23.5 2.9 18.1

IndiaOrissa 46 2.7 22.4 1.1 24.2

IndonesiaBintan 53.5 3.9 12.1 1.6 29.2

JamaicaClarendon 47.8 2.6 17.6 2.3 27.3

SurinamOnverdacht 59 4.3 3.1 2.5 30.9Moengo 54 4.2 10.4 2.8 28.9

United StatesArkansas 51 11.2 6.6 2.2 28.4

Former Soviet UnionSeverouralsk 54 6.2 14.8 2.4 15.7

Former YugoslaviaMostar 52 3.9 21.2 2.7 16.2

to be related to an increasing degree of meta-morphism. However, both minerals also occurin young deposits, although in minor quantities.The chemical environment obviously plays asimportant a role in the formation of boehmiteand diaspore as do pressure and stress [40].

Dissolution of gibbsite requires the mildestconditions in the Bayer process (see Sec-tion 3.1.2). Higher temperatures and alkali con-centrations are necessary for the digestion ofboehmite and diaspore. Technically, both oxidehydroxides can be processed without difficul-ties. The abundance of high-grade gibbsiticbauxites, however, has made boehmite- anddiaspore-rich ores economically less attractive.

Goethite, α-FeO(OH) [1310-14-1], andhematite, α-Fe2O3 [1309-37-1], are the mostprevalent iron minerals in bauxites. They arepractically inert under the conditions of theBayer process. In both materials, some of theiron may be replaced isomorphically by alu-minum ions. This amount of aluminum is in-cluded in the chemical analysis of the bauxitebut is normally not extracted in the digest. Mag-netite (Fe3O4) is found in some European baux-ites; pyrite (FeS2) and siderite (FeCO3) alsomayoccur. Decomposition of pyrite may lead to highsulfur levels in the process solutions.

Anatase, TiO2 [1317-70-0], is the titaniummineral found most frequently in bauxites. Ru-tile, TiO2 [1317-80-2], occurs in someEuropean

Aluminum Oxide 9

deposits; FeTiO3 is also present in minor quan-tities, especially in titanium-rich bauxites. Tita-nium dioxide minerals are attacked under onlythe most severe conditions of the Bayer process.

Silicon dioxide may occur as quartz[14808-60-7], as in the bauxites of the DarlingRange in Western Australia. Most commonly,however, SiO2 is associated with the clay min-erals kaolinite, halloysite, or montmorillonite.These aluminosilicates react with sodium alu-minate solutions to form insoluble sodium alu-minum silicates during digest, causing loss ofsodium hydroxide and extractable alumina. Theamount of so-called reactive silica is one of themajor factors determining quality and price ofthe ore.

Minor constituents, such as chromium, vana-dium, zinc, and gallium, have little effect on theBayer process or on the quality of the final prod-uct. Some tend to accumulate in the recirculatedprocess solutions (e.g., gallium and vanadium)and must be removed periodically by appropri-ate treatments.

The physical properties of bauxites, i.e., tex-ture, hardness, and density, can vary widely.Geologically old diaspore bauxites, especiallythose high in iron oxide, are very hard and canreach densities of 3.6 g/cm3. Young tropical de-posits, in contrast, may have an earthy, soft tex-ture and a density around 2.0 – 2.5 g/cm3. Al-lochtonous bauxite often consists of hard nod-ules embedded in a soft, usually clayey matrix.Porous textures also occur. The color of baux-ites is largely determined by the type and par-ticle size of the prevalent iron mineral. Highlydispersed goethite tends to be yellow to orange,whereas dark brown tones usually are associatedwith coarser hematite. Colors can vary greatlywithin a single ore body.

Hardness, texture, and the amount of overbur-den determine the methods applied for bauxitemining. Deposits in Greece, former Yugoslavia,Hungary, and the former Soviet Union requiredeepmining to depths of several hundredmeters,often complicated by the difficulties of control-ling water levels in the porous limestone forma-tions. Tropical bauxites frequently are located soclose to the surface that they can be recoveredwith normal earth-moving equipment.

2.3. Genesis of Bauxites

Manyof the geologically young bauxite depositsare located in the savannah region, which ex-tends north and south of the tropical rain forestbelt. The climate of this region is characterizedby a high mean annual temperature and abun-dant precipitation during the rainy season. De-posits occur on gently sloping hills or on pene-plains. The stratigraphic evidence shows thatthese bauxites have formed in situ. Parent rocksmay be coarse-grained, igneous rocks such assyenite, phonolite, basalt, or gabbro. However,large deposits also developed on kaolinitic sand-stones, on phyllites, and on schists. A layer ofkaolinitic clay is frequently found between theore body and the parent rock.

Bauxite probably forms during long peri-ods of low geologic activity when the combi-nation of high temperature, abundant precipita-tion, and good vertical drainage favors intensivechemical weathering. The sequence of leach-ing begins with removal of alkali followed bythe removal of alkaline earths. Oxides of iron,aluminum, titanium, and silicon are mobilizedand reprecipitated as hydroxides and oxides.Aluminum hydroxide and silica form kaolinite,Al4(OH)8Si4O10. This sequence first leads tothe formation of tropical soils (laterites). Baux-itization follows when the climatic, chemical,and topographic conditions prevail long enoughto allow the removal of silicon dioxide as well.

The bauxite deposits of the Mediterraneanregion, the Caribbean Islands, and many otherlocations that are associated with tertiary andolder limestone formations (karst bauxites)wereformed by a similar weathering process. Par-ent materials were lateritic soils and clays trans-ported into the karst region and deposited in de-pressions. During extended terrestrial periods,high mean temperatures, copious precipitation,and good vertical draining through the porouslimestone bedrock facilitated a thoroughdesilifi-cation. In the older literature, the parent materialfor karst bauxitization was reported to be clayeyresidue left after weathering of substantial lay-ers of carbonate rocks [41]. Researchers haveshown conclusively that igneous rocks were thesource of this material [42–44].

Comprehensive reviews on the geology, min-eralogy, and genesis of bauxites have been pub-lished [40], [45].

10 Aluminum Oxide

Table 4. World bauxite reserves and production, 103 t [46]

Country Mine production Reserves

1996 1997

United States n.a. n.a. 40 000Australia 43 100 43 500 7 900 000Brazil 9700 9700 2 900 000China 6 200 7000 2 000 000Guinea 14 000 14 000 900 000Guyana 2000 2000 900 000India 5100 5500 200 000Jamaica 11 829 12 000 2 000 000Russia 3300 3300 200 000Surinam 4000 4000 600 000Venezuela 5600 5600 350 000Other countries 8928 8900 4 400 000World total (rounded) 114 000 115 000 28 000 000

2.4. Major Bauxite Deposits

Until the 1950s, the European aluminum indus-try was supplied from the karst bauxite depositsof France, Hungary, former Yugoslavia, andGreece. United States sources (Arkansas) andore from Surinam provided the raw material forNorth American production. Since then, the pic-ture has changed dramatically. The four largestbauxite producers of 1990, namely, Australia,Guinea, Jamaica, and Brazil (Table 4), hardlywould have been mentioned in 1950. Today,they provide more than half of the world’s to-tal bauxite output.Africa. The Savannah region covers an area

of theAfrican continent that has experienced lowgeologic activity for a long time. In this belt,which stretches from the Ivory Coast to Mada-gascar, very large bauxite deposits were found.The major production is currently concentratedin Guinea and Ghana. Cameroon, Sierra Leone,Mali, and the Congo region, among other areas,have substantial reserves.Australia. Major deposits are located in the

DarlingRange inWesternAustralia, on theGovePeninsula in the Northern Territory, and on theCape York Peninsula in Queensland. Bauxitealso occurs in New South Wales, in Victoria,and on the island of Tasmania. The Australiandeposits developed between the Eocene andPliocene epochs on substrates ranging from Pre-cambrian sandstones to Tertiary basalts. Gibb-site is the predominant aluminum mineral, al-though someboehmite occurs in all but theWest-ern Australian deposits.

South America. On the outer slopes of theold Guyana Shield, many economically impor-tant deposits have been found. They are lo-cated in the Amazon Basin of Brazil, in Colom-bia, Venezuela, Surinam, Guyana, and FrenchGuyana. Surinam and Guyana have been pro-ducing for more than 60 years, whereas theBrazilian Amazon deposits have been minedonly recently. Bauxite also is produced in thePocos de Caldas area in Southern Brazil, stateof Minas Gerais. The South American bauxitesgenerally are geologically young, gibbsitic ores.Caribbean. Jamaica and the Dominican Re-

public have major reserves of karst bauxites thatoccur on Tertiary limestones under generallyvery thin overburden. Although gibbsite is themain aluminum mineral, some boehmite also ispresent. Jamaica has been one of the world’sleading bauxite producers for the past 20 years.North America. The only economically im-

portant deposit is located in Arkansas, wheregibbsitic bauxite developed on nepheline syeniteduring the Eocene. Less than 5× 107 t of bauxiteremain, and the grade is declining.Europe. Except for a few commercially in-

significant occurrences, all European bauxitesare of the karst type. The oldest deposits (De-vonian/Mississippian) are those of the Tikhvinarea in the former Soviet Union; the youngestare the Eocene bauxites of former Yugoslavia.Most European deposits developed during theLower and Upper Cretaceous, e.g., the diasporeand boehmite bauxites of France, Greece, andRomania, and the gibbsite and boehmite baux-ites found in Hungary, former Yugoslavia, and

Aluminum Oxide 11

Italy. Today, all European mines combined con-tribute only about 15 % of world production.Asia. Major deposits of gibbsite bauxites oc-

cur in India; the island of Kalimantan (In-donesia) has large potential reserves. Numerousbauxite deposits, most of them containing geo-logically old, diaspore-rich ore, were found inChina. Substantial deposits also are located inWestern Siberia.

2.5. Economic Aspects

The proved reserves of bauxite shown in Table 4are sufficient to supply the world aluminum in-dustry for a few centuries. Total resources areestimated by the U.S. Geological Survey at 55to 75× 109 t. Because of the worldwide dis-tribution of significant ore deposits, a disrup-tion of bauxite supply for political reasons ap-pears highly unlikely. In 1974 several majorbauxite-producing nations formed the Interna-tional Bauxite Association (IBA) with the in-tent of increasing control over the exploitationof their bauxite deposits. Although levies wereincreased substantially, competition from coun-tries not associatedwith the IBAhelpedmaintaina reasonable price structure.

Dissolution of gibbsite requires the mildestconditions in the Bayer process (see Sec-tion 3.1.2). Higher temperatures and alkali con-centrations are necessary for the digestion ofboehmite and diaspore. Technically, both oxidehydroxides can be processed without difficulty.The abundance of high-grade gibbsitic bauxites,however, has made boehmite- and diaspore-richores economically less attractive.

Economic and political considerations favorrefining of bauxite near the deposit and shipmentof either alumina or aluminum ingot. Brazil,Surinam, and Australia have smelters, althoughtheir refining capacity by far exceeds the de-mands of domestic metal production. Jamaicaand Guinea refine a substantial portion of theirown bauxite production.

About 25 % of all bauxite mined is used forproducing refractories, abrasives, catalysts, ad-sorbents, and other industrial chemicals.

3. Bayer Process

3.1. History and Procedure

In 1855 the French mining engineer Louis leChatelier obtained alumina from bauxite bysintering with sodium carbonate at 1200 ◦C andleaching the sodium aluminate with water. Alu-minumhydroxidewas then precipitated from thesodium aluminate solution by carbon dioxide.Early use of aluminum hydroxide was chiefly asa mordant in the textile dyeing industry. The de-mand for pure alumina increased rapidly whenit became the raw material for aluminum pro-duction upon development of the Hall –Heroultcell. The Austrian chemist Karl Josef Bayerreceived German patent 43977 in August, 1888for a new, improved process for production ofaluminum hydroxide from bauxite, and the pro-cess became known as the Bayer process in hishonor. Bayer initially worked on his process atthe Tentelev chemical plant near St. PetersburginRussia. Between 1888 and 1900 he supervisedthe construction of Bayer process plants in Ger-many, England, France, Italy, and the UnitedStates. The Merrimac Chemical Company in

Table 5. World alumina production, 103 t Source: Roskill’s MetalsDatabook 1997

Country Production

1996 1997

Australia 13 008 13 349Brazil 2 147 2 800Canada 1 064 1 100China 2 080 n.a.France 425 430Germany 994 1 000Greece 630 630Guinea 630 622Hungary 353 n.a.India 1 650 1 660Ireland 1 186 1 300Italy 857 880Jamaica 3 030 3 200Japan 743 750Spain 1 095 1 100Surinam 1 579 1 600Turkey 172 170United Kingdom 110 110United States 4 530 4 780Venezuela 1 641 1 700Yugoslavia 150 300World total 43 600 45 000

12 Aluminum Oxide

Massachusetts first used the Bayer process in theUnited States, and the Alcoa plant in East St.Louis, Illinois, was constructed in 1901. Sincethen, plants have been built in more than 25countries, and the present world alumina capac-ity is over 40× 106 t/a (Table 5).

The important features exploited in the Bayerprocess are that boehmite, gibbsite, and diasporecan be dissolved in NaOH solutions under mod-erate hydrothermal conditions; the solubility ofAl2O3 in NaOH is temperature dependent; mostother components of the bauxite are quite inert inthe process; and the silica that does dissolve sub-sequently forms a nearly insoluble compound.These features permit formation of a sodiumalu-minate solution, physical separation of the im-purities, and precipitation of pure Al(OH)3 fromthe cooled solution.

The main features of the Bayer process haveremained unchanged for the last 100 years, al-though the scale of operations has been enlargedconsiderably due to chemical engineering devel-opments. Figure 7 shows the flow sheet of theprocess as it is now practiced.

Each operation in the process is carried outin a variety of ways. The process begins withpreparation of the bauxite by blending for uni-form composition followed by grinding. In mostplants the bauxite is ground while suspended ina portion of the process solution. This slurry ismixed with the balance of the heated NaOH so-lution, then treated in a digester vessel at wellabove atmospheric pressure. The digest reactionis:

Al(OH)3 +Na+ +OH− −→ Na+ +Al(OH)−4

Additional reactions convert impurities suchas SiO2, P2O5, and CO2 to relatively insolublecompounds. The slurry leaves the digester at atemperature above its atmospheric boiling pointand is cooled by flashing off steam as the pres-sure is reduced in several stages. The flashedsteam is used to heat the slurry and the solu-tion going to the digester. The bauxite residuesolids are separated from the sodium alumi-nate solution in two steps so that the coarsefraction is processed separately from the fine.Both residue fractions arewashed anddiscarded.The solution, being free of solids, is cooled andseededwith fine crystals of Al(OH)3; this causesthe Al(OH)−4 ions to decompose to Al(OH)3,

thereby reversing the reaction that previouslyhad taken place in the digester. Again, the heatremoved in cooling the solution is used to heata colder stream in the process. After the pre-cipitation reaction has proceeded to the pointthat about half of the Al2O3 in the solution hasbeen removed, the mixture of solids and solu-tion is sent to classifiers. The fine Al(OH)3 par-ticles are returned to the process to serve as seed.The coarse particles are washed and calcined toAl2O3. Excess solution introduced in washingthe product and the residue must be removed byevaporation. In some cases the solution is treatedto remove both organic and inorganic impuritiesbefore the solution is recycled through the plant.

3.1.1. Bauxite Preparation

The bauxite entering the refinery must be uni-form and sufficiently fine that extraction of theAl2O3 and the other operations are successful.The chemical composition of bauxite varies.Uniformity is improved by blending materialmined from several pits and, if necessary, byadding bauxite from storage piles. Ground baux-ite cam be stpred as a slurry in surge tanks be-fore it is pumped to digestion. These agitatedtanks are operated so that plant feed is uniformlyblended for several hours. Sometimes bauxitesare dried to improve handling or washed to re-move clay.

Hard bauxite is reduced to particles finer than2 cm in roll or cone crushers and hammermills.Before it enters the process it is ground furtherto less than 0.15 cm.

Previously, fine grinding was done in drymills operating in closed circuit with vibratingscreens. Such operation required very dry baux-ite to avoid blinding the screens. This resulted ina dusty working environment. In most modernplants, the bauxite is mixed with a portion of theprocess solution and is ground as a slurry. Rodmills and ball mills are used most frequently.The ground slurry may be passed over screensor through cyclones, with the fine particles pro-gressing and the coarse ones being returned tothe mills.

In all-wet grinding the bauxite feed and theflow of solution are controlled to keep the solidscontent of the slurry between 45 and 55 %. Thepower consumed by themill drive is an indicator

Aluminum Oxide 13

Figure 7. Bayer process flow sheet

of the amount of grinding being done andmay beused to control the feed to the mill. On a longertime scale the particle size of the product can beused to make changes in the grinding operation.

3.1.2. Digestion

In digestion, all of the Al2O3 in the bauxite mustbe extracted. A solution is produced that con-tains the maximum Al2O3 concentration thatcan remain stable through the rest of the pro-cess. This must be accomplished while using aminimum amount of energy.

The conditions for digestion can and do varywidely. The first consideration is whether the

availableAl2O3 is present as gibbsite, boehmite,diaspore, or a mixture of these minerals. Thedissolution rates of the three are quite differ-ent. Generally, if the bauxite contains mixedphases, the digestion conditions will be chosenon the basis of the least soluble compound. AnyAl(OH)3 or AlOOH left undissolved can act asseed in the clarification step, causing precipi-tation of Al(OH)3 while the residue is still incontact with the solution. In the sweetening pro-cess, boehmitic bauxite is digested under rela-tively mild conditions, producing an interme-diate Al2O3 concentration. In a separate ves-sel, gibbsitic bauxite is added to the flow fromthe first digest to raise the Al2O3 concentra-tion to the desired level. Another approach is

14 Aluminum Oxide

to digest only the gibbsite in a mixed bauxite.The residue is separated from the solution andredigested under more severe conditions to re-cover the boehmite. This method reduces theflow to the high-temperature digest and so re-quires lower capital and operating costs.

Figure 8. Phase diagram Na2O–Al2O3–H2O

The solubility data (Fig. 8) show that theAl2O3 concentration in the process solution canbe increased by increasing the temperature, the

NaOH concentration, or both [1–4]. As a re-sult, operating conditions in plants vary widely.Higher digest temperatures result in higher pres-sures, making the equipment more expensive.There also is the need for more heat-exchangeequipment,which further increases capital costs.High concentrations, on the other hand, permitincreased production from a given flow rate and,hence, from a given plant installation. Precipita-tion is thought to occur better at lower concentra-tions, but use of low precipitation concentrationswhile digesting at high concentrations requiresdilution and additional operating costs for subse-quent evaporation. Choosing digester conditionsinvolves balancing these physical factors withlocal economics, togetherwith the designer’s ex-perience. This has resulted in the spectrum ofoperating conditions given in Table 6.

The conditions listed in the first line of Ta-ble 6 are those for digesting bauxite at the at-mospheric boiling point. Quite high alkali con-centrations are required and the evaporation re-quirement is an extraordinary 5.3 t of water for atonne of Al2O3. Most plants digesting gibbsiticbauxite use the conditions on the second line ofTable 6. Boehmitic bauxite is digested using oneof two general sets of conditions. The first is Eu-ropean practice, in which higher concentrationsand dilution prior to precipitation are preferredto higher digest temperatures. When Americancompanies began processing boehmitic bauxitefrom the Caribbean area, most chose the sameconcentrations used for gibbsitic bauxite; there-fore, a higher temperature was required.

The second important reaction in digestion isdesilication. In the equation below, kaolinite isused to illustrate the reaction of siliceous min-erals with the process solution:

Al2O3 · 2 SiO2 · 2H2O+6NaOH

→ 2NaAlO2 + 2Na2SiO3 + 5H2O

The soluble products react to form aseries of precipitates with zeolite struc-ture, having a composition of approximatelyNa8Al6Si6O24(OH)2.

Depending on temperature and concentra-tion, the ratio of Al2O3 :×SiO2 in the zeolite(tectosilicate) structure can vary. For each Al3+

replacing Si4+ in the lattice, one Na+ is takenup tomaintain charge neutrality. Anions, such asSO2−

4 or CO2−3 may substitute for OH− in the

structure. The formation of the zeolitic desilica-

Aluminum Oxide 15

Table 6. Commercial digestion conditions

Bauxite type Temperature, K cNaOH, g/L Final cAl2O3 , g/L

gibbsitic 380 260 165415 105 – 145 90 – 130

boehmitic 470 150 – 250 120 – 160510 105 – 145 90 – 130

diaspore∗ 535 150 – 250 100 – 150

∗ CaO is added to digests to accelerate dissolution of diaspore.

tion product (DSP) therefore leads to costly lossof sodium hydroxide. This reaction, however,is necessary for lowering the level of dissolvedSiO2 to less than 0.6 g/L, the maximum accept-able concentration.

The rate-determining step in the desilicationreaction is the nucleation and crystallization ofthe desilication product. Therefore, the presenceof seed particles is important; without seeds, so-lutions containing 0.75 g/L SiO2 will not reactin 40min at 415K. This has led to the paradoxi-cal situation that some bauxites contain too littlereactive SiO2 for good desilication. The slurryblending and storage operation discussed earliercan be an important part of the desilication pro-cess. If the storage temperature is above 355K,about 80 % of the reaction takes place in 8 h.More important, seed is formed so the desilica-tion reaction in the digest is not delayed. Verylow SiO2 concentrations can be achieved if anexcess of CaO is charged. At high temperatures,and with the lime additions, a less soluble desil-ication compound (cancrinite) is formed. Euro-pean plants use longer holding times during orafter digestion to facilitate desilication.

Some CaO is added to the digest even whenextremedesilication is not required. The calciumreacts with the carbonate and phosphate com-pounds as follows:

CaO+H2O→Ca(OH)2

Ca(OH)2 +Na2CO3 →CaCO3 + 2NaOH (causticization)

5Ca(OH)2 + 3Na3PO4 →Ca5(PO4)3OH+9NaOH

The last reaction controls the level of phosphatesin the process solution; this impurity can affectclarification adversely. Causticization in the di-gest was more important when prices were such

that the sodium required by the process was sup-plied more cheaply as Na2CO3. Its current im-portance is in control of theCO2−

3 concentration,which, at high levels, can affect precipitation.In modern practice, the causticization reactionis carried out on dilute process solutions outsidethe digester.Maintaining lowconcentrations andhavingmuch of the NaOH combined as NaAlO2are favorable tomore complete reactionofCO2−

3with Ca2+.

3.1.3. Equipment

The equipment for digestion includes the reactorvessel, heat-exchange equipment, and pumps.The first reactors were horizontal vessels withcrude agitators mounted on axial shafts. Thesewere filled with a slurry of bauxite, lime, andprocess solution. They were closed and heatedindividually by injecting high-pressure steam.At the end of the designated holding time, adischarge valve was opened and a slurry wasforced into another vessel at atmospheric pres-sure. Only a small portion of the steam flash-ing from the slurry as the pressure was reducedcould be recovered. Continuous operation wasintroduced in the 1930s in which heat could beexchanged between the hot stream leaving thedigester and the cooler, incoming stream. Thisgreatly reduces the energy requirements.

Reactors. Digester vessels, whether batch orcontinuous, provide intimate contact betweenthe bauxite and the solution for a period longenough to complete the extraction and desili-cation processes. The equipment designed forthis purpose is quite diverse. Agitated horizon-tal vessels have been replaced by vertical unitsto avoid the mechanical problems with sealing

16 Aluminum Oxide

Figure 9. High-temperature digestion and heat recovery

the agitator shaft. Both designs have used a se-ries of vessels to minimize short circuiting ofthe flow. European practice includes use of a se-ries of vertical agitated vessels, each operatingat a higher temperature than the preceding one.These units are heated by flashed steam in inter-nal steam coils. A German digester design usesconcentric pipes; the liquid being heated flowsthrough the inner pipe while the hot slurry fromthe digester is returned through the outer pipe.This tube digester has been modified by placingseveral tubes for the cold slurry in parallel in-side a larger outer tube. This reduces the lengthof the unit. The variety of digester design is illus-trated in Figures 9, 10. The flow through single-digester units may be as high as 12m3/min.

Figure 10. Tube digestera) Piston pump; b)Heat; c) Condensate; d) Flow controlvalve; e) Steam

The usual construction material for digestersis mild (low-carbon) steel, despite data show-ing that, at the temperatures and NaOH con-

centrations involved, there may be stress cor-rosion cracking resulting from alkali embrittle-ment. The presence of the [Al(OH)4]− ions re-duces the activity of OH− in the solution. Finelydivided Fe2O3 in the bauxite quickly saturatesthe solution with Fe3+, suppressing corrosion ofthe metal. Some digestion equipment has beennickel plated for safety.Flashing. In most plants the hot slurry from

the digester is flashed in a series of pressurevessels until it reaches the atmospheric boilingpoint. The steam generated in flashing is used inheat exchangers to heat the flow of liquor andbauxite coming to the digester. Flashing also re-moves water from the process stream, therebyaccomplishing a significant portion of the evap-oration needed. Normally, heaters are tubular,but coils of tubing also are used. As few as threestages of flashing may be needed when the di-gestion temperature is 418K, and this may beincreased to ten stages for a unit operated at515K. Figure 9 is a schematic diagramof a high-temperature digestion unit.

Heat Exchangers. Operating practice hasbeen divided as to whether the bauxite slurryshould be heated separately or whether it shouldbe mixed with the rest of the solution and thenheated. These twomodes of operation have beendesignated the single- and the double-streamprocesses. Those favoring separate heating ofthe bauxite slurry, the double-stream option,feared rapid deposition of desilication producton the heat-transfer surfaces. Experience hasshown that, particularly if the bauxite slurry is

Aluminum Oxide 17

stored for blending, those fears are unfounded.The rate of fouling is no higher than for heat-ing clean liquor. In both options there is slowdeposition of desilication product on the heat-exchanger surface because the desilication reac-tion does not reach equilibrium in the digesterand continues at a slow rate wherever the solu-tion temperature is raised. The encrustationmustbe removed periodically by washing with diluteHCl orH2SO4, or bymechanicalmeans tomain-tain a high heat-transfer rate.

In some plants using the double-stream sys-tem, the bauxite slurry is heated in contactheaters. These heaters are designed much likebarometric condensers. The liquid is forced tofall through the steam in the heater in dropletsor in thin films. The large surface area exposedresults in high heat-transfer rates, and becausethere is no metallic surface, there is no fouling.The penalty is higher dilution.

The heat needed to raise the temperatureabove that achieved in the flashing system canbe introduced by direct injection of steam.Moreoften the final heating to the digest temperatureis done in a separate heat exchanger, with theenergy coming from steam, hot oil, or moltensalts.

Pumps. Centrifugal pumps are used in mostdigester systems. Those used for pumping slur-ries are built of wear-resistant alloys. A greatdeal of maintenance is required to keep the shaftseals operating well. In some plants, concernabout pumpwear has led to the use of diaphragmpumps. In such pumps a check valve admits theslurry to a chamber on one side of a flexible di-aphragm. When that chamber is full, oil at highpressure is forced into a chamber on the otherside of the diaphragm. This moves the slurrythrough another check valve into the digester.Therefore, no rapidly moving parts are in con-tact with the slurry. In a Japanese modification,called theHydrohoist, the diaphragm is replacedby a sphere that is free to move the length of acylinder. Slurry is kept in one end of the cylinderand the drive fluid, which is clean process solu-tion, is in the other end. Because of this choice ofdrive fluid, leakage past the sphere is of no con-sequence and tight clearances are not required.

Measurement and Control. Operatingvariables that must be controlled are temper-

ature, solution concentration, and degree of de-silication. The holding time is fixedby the designof the digester vessels. As noncondensable gasesare formed by the reaction of the solution withorganic materials in the digest, it is necessaryto vent these gases so that the digester remainsfull. Venting can be controlled by sensing theliquid level with a float or a radioactive sensor.Temperature measurement and control are rela-tively simple because only the temperature fromthe final heater is critical. Concentration controlcomes from blending the bauxite and carefulproportioning of the slurry and clean solutionflows. Originally, feedforward control was usedin which the composition of the incoming ma-terials was used to set the rates at which theywere added. In modern practice feedback con-trol is based on the composition of the exitingsolution. Chemical analyses of the solution canbe used, but the time lag involved makes thiscontrol imprecise. Nearly real-time control hasbeen achieved by using the fact that the con-ductivity of the solution at a given temperatureis a linear function of the mass ratio of Al2O3and NaOH. This is the variable of interest. Inmodern plants the output of the sensor goes to acomputer, which in turn controls the amount ofbauxite slurry pumped. Such control has reducedthe variance in concentrations to about a thirdof the values experienced earlier. This permitsoperation closer to the maximum safe concen-tration with less risk of premature precipitationof Al2O3 in clarification.

3.1.4. Residue Separation

The next step in the process is the separation ofbauxite residue solids from the solution. A widevariety of equipment and procedures is used inthis operation. The methods chosen depend onthe quantity and properties of the residue.

The particle size distribution of the residuesolids is usually bimodal. The coarse fractionover 100µmindiameter is termed sand,whereasthe rest of the solids are finer than 10µm. Thesand fraction may range from 5 to nearly 50 %of the total. The lowest amounts are in theCaribbean bauxites and the highest are in theresidue from Western Australian bauxite. In theplant the sand fraction is separated from the pro-cess stream in liquid – solids cyclones or inmore

18 Aluminum Oxide

Figure 11. Residue thickenera) Feedpipe; b) Feedwell; c) Motorized lifting device; d) Liquid level; e) Overflow launder; f) Weir; g) Off-tank support;h) Tank; i) Cone scraper; j) Discharge cone

primitive settling devices. A variety of equip-ment, including Dorr, Hardinge, and Aikensclassifiers, have been used to wash the sand freeof process solution before it is discarded. In allof these units the sand moves countercurrentlyto the wash water so that a maximum of wash-ing is done with a minimum of water. The washsolution is added to the balance of the processflow and proceeds to further clarification stepsfor removal of the fine solids.

In most plants the fine residue fraction is set-tled in raking thickeners of the type illustrated inFigure 11. These vessels may exceed 49m in di-ameter. Some older plants used multideck thick-eners. Difficulties in keeping themultideck unitsin balanced operation have led to almost exclu-sive use of single-deck units in newconstruction.

The desanded slurry is fed at the center of thethickener and clarified solution overflows at theperimeter. As the solution flows radially acrossthe thickener, both the horizontal and verticalvelocities become very low. The solids, havinga higher specific gravity than the solution, sinkto the bottom of the thickener. A settling rate of1.5m/h is sufficient for commercial operation.The solution overflowing the thickener usuallycontains less than 0.3 g/L solids, whereas the un-derflow ranges from 15 to 35wt % solids. Therotating mechanism has plows mounted at anangle to the arm. These slowly move the set-tled pulp across the bottom of the thickener to adischarge port. A few units have been designedwith the discharge at the perimeter rather than atthe center. The objective was to avoid the need

to elevate the units to provide access to a centerdischarge.

The fine solids behave as a relatively stablecolloidal suspension, so they settle slowly if nottreated further. The addition of flocculants im-proves the clarity of the thickener overflow, thesettling rate of the solids, and the solids contentof the underflowpulp. Flocculants act bybindingthe very fine particles into flocs that may be sev-eral millimeters in diameter. The ratio of mass todrag forces is increased so the flocs settle morerapidly.

Starch from grains or roots was the firstflocculant; the dosage varied from 0.5 to 3 kgper tonne of bauxite. The amount required in-creases as the surface of the residue increasesand is affected by the mineral composition ofthe residue. Starch has the advantages of be-ing inexpensive and ubiquitous. It does reactwith the NaOH in the solution to add organiccompounds to the solution. In the 1950s water-dispersible polymers were proposed as floccu-lants. The first compounds were effective whenadded at 10 % of the starch charge, but theywere expensive so they offered no economicadvantage. More recently, flocculants based onacrylate – acrylamide copolymers have begun toreplace starch. The functional groups of theseanionic flocculants can be modified to suit spe-cific operating conditions. Different materialsmay be used in the thickeners and in the wash-ing operation. In some cases a small amount ofstarch is added to the synthetic flocculant to im-prove overflow clarity. These syntheticmaterials

Aluminum Oxide 19

now offer an economic advantage because theyhave been made less expensive and more effec-tive.

With the residue concentrated in the thickenerunderflow, the next task is to wash it free of pro-cess solution so that it can be discarded. This isusually done in countercurrent decantation sys-tems using washing thickeners that are similarin design to those used for settling. The washingoperation is done in asmany as seven stageswiththe solids moving countercurrent to the washstream. This is illustrated in Figure 12. An equa-tion can be written for each stage expressing amaterial balance between soluble materials anddilution. Computer models of the washing sys-tem allow accurate prediction of performance.

Figure 12. Flows at stage n in countercurrent decantation(O= overflow; U= underflow)

In residue washing, the objective is to mini-mize both the value of the soluble salts lost andthe cost of evaporating the added wash water.The solubles in the discarded pulp can be re-duced by increasing the solids content of theunderflow. This reduces the amount of solublescarried from one stage to the next so that dilutionismore effective and can be accomplished by us-ing more flocculant or by increasing the holdingtime in the thickener. There is some risk in thelatter change. If some undissolved gibbsite orboehmite remains in the residue, it may induceprecipitation with a resulting loss of Al2O3.

In some plants vacuum drum-type washingfilters are used to replace some or all of thewash-ing thickener stages. The solids content of theresidue may be increased to 50 – 60wt % by fil-tration. Therefore, only about 33 % as much so-lution is needed compared to plants using thick-eners and this results in a great increase in theeffectiveness of washing. Calculations for coun-tercurrent washing show that two stages dis-

charging at 55 % solids are more effective thansix stages operating at 20 % solids.

Another approach only recently introducedinto plant practice is called the deep thickener.This equipment was developed in the Britishcoal industry to concentrate the slimes from coalwashing. In the deep thickener, increased quan-tities of flocculant are used, and a column ofresidue pulp up to 10m deep is maintained. Thehydraulic pressure of the deep column appearsto densify the pulp so that underflow solids ap-proaching the concentration of filter cake are ob-tained. The overflow from these units containsmore solids than the flow from standard thicken-ers, but the units are finding use in concentratingresidue for washing and for disposal. The sim-plicity of the equipment adds to its appeal.

The final stage of clarification is polish fil-tration of the solution, sometimes called clearpressing. Most of the residue solids have beenremoved from the process solution in the previ-ous steps, but product purity must be protectedby removing the few particles of solids remain-ing. Filter presses are used most widely.

Another approach to polish filtration is useof a stationary filter in which the medium is uni-form, fine sand a meter deep. Gravity providesthe force to move the solution through the sandbed. During operation, the fine solids are mixedwith the sand, from which they are elutriated atthe end of the cycle. Such filters can be auto-mated and are especially effective with residuesthat are difficult to process.

Close attention is required to keep the solidscontent of the filtrate down to 0.5mg/L to protectproduct purity. Light transmission of the filtrateis used as an index of clarity. Purity originallywas determined visually, but nephelometers areused now. If evidence of solids is found, the fil-trate from each filter is scanned to locate thesource. Some solids may pass through the filtercloth before a layer of solids forms to serve as abarrier, so it is common practice to recycle thefirst portion of filtrate.Control in clarification has two major objec-

tives; the solidsmust be removed and thewashedresidue must be prepared for discard, using aminimum of wash water. The first objective isachieved by measuring the cloudiness of the fil-trate. High solids in the underflow are dependentin part on the retention time in the thickener, someasurements must be made of the pulp level in

20 Aluminum Oxide

the unit. Automatic determination of the inter-face between clarified liquid and pulp has beendone by ultrasonic and by optical scanning. Thedensity of the underflow pulp is monitored by adevice that measures absorption of radiation orby gravimetric measurement. If the pulp is toodilute the rate of withdrawal is decreased. Thiscontrol can be overridden if the pulp level in theunit rises too high so that there is concern thatthe solids in the filter feed may become exces-sive. The flows of solids and wash water alsomust be measured and proportioned for efficientoperation.

3.1.5. Precipitation

The filtered solution has a temperature of 375Kand it must be cooled to 335 – 345K before pre-cipitation. This cooling is usually done in a flash-ing system analogous to that used in recoveringheat in digestion. Because the temperatures arebelow the atmospheric boiling point, the flashvessels and the heat exchangersmust operate un-der vacuum. Most of the heat removed by flash-ing is transferred to the colder solution return-ing from precipitation. Plate-type liquid – liquidheat exchangers have also been used. The solu-tion can be cooled further midway through theprecipitation operation. This allows higher re-covery of Al2O3 without using conditions thatlead to excessive nucleation of product.Recovery of the Al(OH)3 from the process

solution is known as precipitation, decomposi-tion, or Ausruhrung. The reaction is the reverseof the digester reaction given earlier. The cool-ing done after digestion and filtration has movedthe solution into an area of the solubility diagramknown as the metastable region. The concentra-tion and temperature are such that the solution issupersaturatedwithAl(OH)3 but is not saturatedenough for spontaneous crystallization. This isillustrated by Figure 13. At the digest tempera-ture, T1, the Al2O3 concentration is increased topoint P on the diagram. Cooling to temperatureT2 results in crossing the solubility curve into themetastable region. Addition of Al(OH)3 seed atthis point causes precipitation and the concen-tration of Al2O3 approaches the solubility curveat T2. The additional cooling recently adoptedmoves the solution conditions further to the left

in the diagram above a lower point on the solu-bility curve.

Figure 13. Generalized solubility curve

The kinetics of the precipitation reaction arerepresented by the equation:

−dc

dt= kexp (−E/RT ) A (c−c∞)2

where:

c =Al2O3 concentration, g/Lk = constantE = activation energy, about 59000 J/molR = 8.31441 J mol−1 K−1

T = temperature, KA = seed area, m2/Lc∞ = concentration at equilibrium

The reaction is second order, i.e., the rate isaffected by the square of the difference betweenthe actual Al2O3 concentration and the equilib-rium concentration. The operating temperatureaffects the equilibrium concentration and the re-action rate. Because seed area is important, theseed chargemust be as high as can bemaintainedwhile meeting other operating objectives.

The first objective in precipitation is to pro-duce Al(OH)3 that, when calcined, meets thespecifications for metallurgical alumina. Thesespecifications are discussed in Section 5.2. Thesecond objective is to obtain a high yield fromeach volume of solution. This increases the plantcapacity and reduces the amount of energy spentin heating and pumping the solution. At thesame time, the processes serving to create newAl(OH)3 particles must be controlled so that the

Aluminum Oxide 21

number of seed particles created equals the num-ber of particles leaving the system as product.This requires balancing nucleation, agglomera-tion, growth, and particle breakage, so a combi-nation of science and art has developed.

About 2× 1010 nuclei must be formed ineach liter of solution to balance the process. Therate of nucleation is a strong function of boththedegreeof supersaturation andof temperature.Very few nuclei are formed if the temperature isabove 350K. A change in the operating temper-ature of only 5Kwill change a system from a netproducer to a net consumer of nuclei. Tempera-ture changes of 2 – 3K are the most frequentlyused control measures.

The nuclei grow by accumulation ofAl(OH)3on their surface if they become large enough tobe viable seeds. The rate of growth can be as highas 9µm/h, but it is generally much lower. Thegrowth rate is independent of the particle diame-ter. Particles also increase in size by agglomera-tion.By a process that is notwell understood, thenuclei form clusters in the first few hours of thecycle in batch precipitation. When the supersat-uration is low, this agglomeration does not occur.Some speculate that when the rate of precipita-tion is high, the Al(OH)3 is not well crystallizedwhen it is first deposited on the seed. This latermay serve as the bond for agglomeration. At lowsupersaturation the deposited material is betterordered. The action of mechanical equipment inprecipitation can break fragments off crystals orbreak up agglomerates, creating secondary nu-clei, but this is not a major factor in most plants.

Figure 14. Technical aluminum hydroxide (gibbsite)

Figure 14 is an electron micrograph of someAl(OH)3 produced in a plant operation. The par-

ticles are obviously the result of agglomeration.Those agglomerates having fewer particles canbe shown by thin-section micrographs to havegrown radially from an agglomerate of relativelyfew nuclei.

In Figure 15, the Al2O3 concentrations in ex-perimental precipitations is shown as a functionof time and temperature. The slopes of thesecurves indicate the rates at which precipitationis progressing. All curves show a high rate atthe start and appear to be approaching a finalvalue asymptotically. The final concentrationswere well above the saturation level, but in theseas in plant operation the rate became unusuallylow, so the process was terminated.Precipitators are vertical cylinders; the

height, which may be 30m, is usually 2.5 – 3.0times the diameter. The seeded slurry is circu-lated tomaintain intimate contact between solidsand solution. Early precipitators circulated thematerial by means of a central air lift pipe. Airintroduced at the bottom of the pipe reduced theapparent density of the slurry within the pipe.This caused slurry to flow into the bottom of thepipe to establish circulation. Modern precipi-tators are circulated mechanically by impellersup to 3m in diameter operating in a draft tube.In these units the flow is downward through thetube, so the tank bottoms are nearly flat to re-verse the flow and cause upward motion of theslurry. Tanks with air lifts have conical bottomsto direct the flow to the central air lift.

Originally, precipitators were filled individu-ally in batch operation. This method had severaldisadvantages, chief of which was the need formany operators because the operating cycle of asingle precipitator required at least 15 separateoperations. Control was difficult and batch op-eration left equipment out of service part of thetime.

Nearly all plants built in the last 30 years haveused the continuous system. Up to 13 tanks areplaced in series so that the flow of seed and solu-tionmoves by gravity through channels connect-ing the tank tops. Continuous flows are mucheasier to measure and to automate. The numberof flows is reduced, so control is better and thelabor requirement is reduced. There are someoperating problems. One is to avoid passage ofthe slurry across the tank tops, thereby reduc-ing the retention time. Movement of solids fromtank to tank is also a problem. An ideal system

22 Aluminum Oxide

Figure 15. Effect of temperature on precipitation

Figure 16. Al2O3 concentration profiles in batch and con-tinuous precipitation

would retain fine particles in the tanks and moveonly the coarse material to the discharge. This isnot possible, so a compromise method of oper-ation must be found. Precipitation takes place atconstant conditions in each tank instead of fol-lowing a continuous curve, as in Figure 16. Thistends to decrease production per tank, but theloss is more than recovered because the tanksare always in use.

Several patterns have been used to supply theseed for precipitation. The earliest was to neu-tralize a portion of the solution. An Al(OH)3 gelwas precipitated that changed into fine crystal-lites on aging. This very finematerial was grownthrough at least one cycle to provide an activeseed for product precipitation. The more com-mon approach is to use fine particles, producedin the operation, as seed. The slurry leaving pre-cipitation is classified into a coarse product frac-tion and one or more fine fractions. The finesare returned to the process to grow to productsize. Classification is done by controlling theflow rate through tanks so that the fines are elu-triated. Liquid – solid cyclones are coming intouse because they provide more accurate sepa-rations. The seeding and classification systemshave become increasingly complex. In modernplants at least three fractions are separated. Insome plants the agglomerates formed from fineseed must pass through the system once more

before becoming product. This reflects the be-lief that the agglomerates must be made tougherby deposition of additional Al(OH)3 so that theydo not disintegrate in calcination and subsequenthandling.

In a modification [47], a relatively smallcharge of fine seed is added to the first tank in aseries. This promotes agglomeration. Then, ad-ditional coarser seed is added to the second orthird tank to provide the large seed area neededfor high production. As indicated previously,cooling may be done between units in a seriesto increase the supersaturation in the final tanks,thereby increasing the production rate. A fur-ther change in modern plants is to filter the seedslurry being recycled. This decreases the amountof spent solution returning to the system and in-creases both the Al2O3 concentration in the firsttank of the series and the overall production.

The control variables available to the pre-cipitation operator are the temperatures in thesystem; the flow per unit, which translates intoholding time; the amount of seed; and the parti-cle size of the seed. Some of these variables aredifficult to control, so the system usually cyclesslowly from coarser than desired to finer. Thereis enough inventory in the process so that minorcycles can be tolerated without affecting plantoutput.

Aluminum Oxide 23

3.1.6. Impurities

The impurities in the product Al2O3 are affectedby all of the foregoing operations, including se-lection of the bauxite. There are at least threeclasses of impurities. The first is residue solidsthat are not removed in clarification. The largesurface area of Al(OH)3 in precipitation effec-tively sweeps up these solids into the product.The preventive measure is increased vigilancein filter maintenance and operating. The secondclass is materials that are in true solution, suchas sodium and gallium. The third group includesmaterials that are in solute form at the tempera-ture of digestion but precipitate later in the pro-cess. Their solubilities may be only milligramsper liter and return of the solution to equilibriumat lower temperature may be slow. If precipita-tion is not completed before filtration, the re-maining impurities appear in the product. Thereare also other impurities that do not fall neatlyinto these categories.Sodium is themain component of the process

solution and is also the largest contaminant of theproduct. TheNa2Ocontent of alumina canbede-creased by increasing the seed charge and by in-creasing the precipitation temperatures. The realvariable that these operating changes help con-trol is the rate of crystal growth. Higher temper-atures, greater seed area, and lower Al2O3 con-centrations all slow growth and give impuritiesmore time to diffuse from the surface. By propercontrol of these variables an order of magnitudedecrease in the Na2O content of a commercialproduct has been achieved. Some sodium is lostin theHall –Heroult cells during smelting by ab-sorption into the cell lining and by conversion tosodium metal, so complete absence of sodiumin metallurgical alumina is not necessary. Thetarget is to balance the input to the cells withthe losses. This requires Al2O2 containing about0.4 – 0.5wt % Na2O.Gallium is a ubiquitous component of alu-

minous ores. Its chemistry is similar to that ofaluminum, so it accumulates in Bayer processsolutions until an equilibrium is reached at about0.2 g/L. The gallium content of Al2O3 is a lin-ear function of the gallium concentration in thesolution. Therefore, the amount leaving in theproduct equals the input. Because gallium is intrue solution, the same changes that lower theNa2O content of the product lower the gallium

content. Only in the production of high-puritymetal has there been concern about the galliumcontent of alumina. About 10 t/a of gallium isrecovered from Bayer process solutions. Thisis the best source for the production of gallium(→Gallium and Gallium Compounds).Silicon is a component ofmany aluminum al-

loys, yet the specification for Al2O3 is less than0.02 % SiO2. This value is only 25 % of thatspecified 50 years ago. The SiO2 is in solution,but it appears to be added to the product througha slow continuation of the desilication reaction(Section 3.1.2). There is also evidence that if therate of Al2O3 deposition on seed is not high,the seed surface becomes at least partially cov-ered with desilication product, thereby becom-ing less active as a seed for Al(OH)3 precipita-tion and more active toward SiO2. This impurityis controlled by driving the desilication reactionnearer to completion. Lowering the SiO2 con-centration of the solution below 0.1 g/L gives aproduct containing less than 0.003 % SiO2.Potassium is undesirable in the Al2O3

because it may destroy the graphite inHall –Heroult cells by intercalation, i.e., it dif-fuses between the layers of the graphite struc-ture, thus expanding its volume. Although it issoluble in Bayer solutions, there has not beena recorded instance of K2O concentrations be-coming high enough to affect the Al2O3 quality.Iron(III)oxide (Fe2O3) as an impurity in

Al2O3 has been the subject of much investiga-tion. It is soluble up to ≈ 0.1 g/L in the solu-tions at digest temperatures (400 – 500K), butonly to ≈ 0.001 g/L at 333K [48]. This recon-ciles the observations that Fe2O3 behaved attimes as if it were in solution and as a colloid atothers. There are several iron minerals that canserve as sources for the impurity. Hematite (α-Fe2O3) and goethite (α-FeO(OH)) are presentin most bauxites. Goethite is slowly convertedto hematite in high-temperature digests, particu-larly if CaO is present. This conversion improvesclarification because hematite settles better thangoethite. The mineral siderite (FeCO3) presentin some bauxites not only is a source of iron butalso reacts with NaOH to increase the amountof causticization needed. Pyrite (FeS2) is almostinsoluble in Bayer solutions; its adverse effectsoccur when the sulfur is oxidized to sulfate inthe presence of air and water. The sulfate will

24 Aluminum Oxide

react with NaOH to form Na2SO4 which has tobe removed periodically.

Paradoxically, there is less difficulty withFe2O3 as an impurity in processing bauxitescontaining a large amount of iron oxides. This isprobably because the iron oxides serve as seedand hasten hydrolysis of the NaFeO2 formed inthe digest. Running the solution over iron oxideparticles before filtration has been effective incontrolling this impurity. Another process forlowering the iron content of Al2O3 is termedstep precipitation. The filtered Bayer solution islightly seeded with Al(OH)3 for a short time toprecipitate up to 3 g/L Al2O3. The seed and thesmall amount of precipitation effectively sweepthe colloidal iron hydroxide particles from thesolution, leaving an Fe2O3 concentration of lessthan 0.005 g/L.Calcium also is a common impurity. The

presence of increasing amounts of Na2CO3 inthe Bayer solution seems to increase the CaOin the product. The calcium content of alumina,as with sodium, needs to be controlled such thatthe Hall –Heroult cell bath can come to an equi-librium level acceptable for cell performance.Calcium can be controlled by step precipitationor by lowering the carbonate content of the so-lution.Lithium is also acceptable in Hall –Heroult

cells. It can be removed by step precipitation.Other metallic impurities that appear in

many bauxites in small amounts are chromium,copper, manganese, titanium, vanadium, andzinc. Generally, these impurities are not so con-centrated that removal processes are required.Sulfide salts have been added to precipitate cop-per and zinc. The chromium is a problem only ifit is oxidized; the organic salts in the solution areusually a sufficient reductant. The addition ofmanganese compounds as oxidants for organicmaterial has been reported.Anions, such as carbonate, chloride, fluoride,

and sulfate, are known to interfere withAl(OH)3precipitation. The carbonate ion is controlledby addition of lime, although processes havebeen reported in which Na2CO3 is removed byevaporative crystallization or by cooling the so-lution to 260K. The sulfate and chloride ionsusually are controlled naturally, as their saltsare incorporated in the desilication product insufficient quantities to maintain an equilibrium.There are commercial operations in which the

sulfate salt is removed by evaporation and crys-tallization. The least soluble salt is schairerite(Na2SO4 ·NaF), andwhen the fluoride ion is de-pleted, burkeite (2Na2SO4 ·Na2CO3) appears.The fluoride ion is almost never present in quan-tities large enough to be harmful.

Many bauxites contain phosphate miner-als. Apatite (3Ca2P2O8 ·CaF2) does not re-act in the process; indeed, this compound orits hydroxy counterpart is formed when solu-ble phosphates react with calcium in the di-gest. Aluminum phosphates, such as wavellite(4AlPO4 ·Al(OH)3 · 9H2O) do react, formingNa3PO4 as the soluble phosphate. The phos-phate ion interferes with flocculation of bauxiteresidues containing goethite, probably by com-peting for attachment sites [49]. The concentra-tion of phosphates never is high enough to inter-fere with Al2O3 purity. Addition of CaO to thedigest is the control measure.Organic matter in bauxite, whether it be

roots and twigs from plants or humic acids fromdecayed matter, reacts in the digest to form awide variety of organic compounds. Some op-eration process streams have exceeded 15 g/Lorganic carbon in solution. There is evidencethat larger molecules are oxidized in the pro-cess all the way to Na2CO3 and sodium oxalate(Na2C2O4). In addition some, but not all, of theorganic compounds may interfere with precipi-tation [4].

The effect of sodium oxalate on process-ing has not been quantitatively determined, yetmany plants incorporate equipment for crystal-lization of the oxalate salt from the solution.Oth-ers wash this salt from the Al(OH)3 seed andrecover it from the wash water. The evidence isoverwhelming that operations are improved bythe oxalate removal techniques. Early removalprocesses included evaporation to high concen-trations followed by centrifugal separation of thegelatinous salt mass. In other plants, a portion ofthe solution was mixed with bauxite or Al(OH)3and heated to 1280K. This destroyed the organiccompounds and formed NaAlO2, which couldbe recovered in the process. A German processaddsmagnesium salts to the digest to remove thedeleterious organic compounds. Still others areinvestigating methods of oxidizing the organicmaterial in the solution [50].

Aluminum Oxide 25

3.1.7. Calcination

The final operation in production of alumina iscalcination. The temperature of the Al(OH)3 israised above 1380K resulting in the reaction:

2Al(OH)3 →Al2O3 + 3H2O

As discussed in Section 2.2, this reaction cantake several pathways and several transitionforms may appear. The end of all pathways isα-alumina. In older European practice, a ma-jor portion of the alumina was converted to theα-phase, sometimes by the addition of a fluo-ride salt to lower the temperature of transforma-tion. In American practice, calcination alwayshas been less severe; so, normally, � 20 % ofthe alumina is in the α-phase.

Before calcination, it is necessary towash theprocess solution from the coarse Al(OH)3. Thisis done countercurrently, using storage tanks andfilters. As in residue washing, increasing thenumber of washing stages and the concentrationof solids leaving each stage improves the effec-tiveness of washing. Vacuum filters of severaldesigns have been used for the final separation.The early Oliver andDorr drum filters have beenreplaced by horizontal rotary filters because bet-ter washing can be achieved. The quality of thewash water is of some concern because such im-purities as calcium and magnesium can be ad-sorbed on the surface of the Al(OH)3.

Earlier, calcinationwas done in reverberatoryfurnaces. These had neither the capacity nor thethermal efficiency required, so they were soonreplaced by rotary kilns. These kilns are cylin-ders thatmay be 3.5m in diameter and over 80mlong. They are mounted on bearings and rotateabout an axis inclined at a small angle to thehorizontal. Damp Al(OH)3 from the filters en-ters the upper end and slowly tumbles towardthe lower end, traveling against a stream of hotgas formed by combustion of natural gas or oilat the discharge end. Early kilns were less than40m long, but longer units have better thermalefficiency and higher capacity.Product coolers were added to kilns so that

part of the heat in the incandescent product couldbe transferred to the combustion air. Early cool-ers followed designs developed in the cementindustry and were themselves rotary units muchlike, although smaller than, the kilns. Improved

coolers fluidize the product Al2O3 around heat-exchange surfaces, often in several stages. Insuch coolers, the heat can be recovered in com-bustion air or in wash water and the product canbe cooled readily to 425K.

The progressive adoption of large-capacitystationary calciners has reduced fuel consump-tion for alumina calcination to ca. 3.1GJ pertonne of alumina, which compares to ca. 4.5GJper tonne for earlier rotary kilns. Several sta-tionary calciner designs with capacities of> 2500 t/d alumina are now commercially avail-able. In stationary calciners, a combination offluid beds and stages in which the solids aretransported in suspension is used. Figure 17 isa schematic diagram of a commercial unit capa-ble of calcining 1500 t/d.

TheAl(OH)3 is washed and dewatered on thefilter (a) and is conveyed into the flash dryer (b),where it encounters hot gas from cyclone (g).The gas evaporates the free water from the par-ticle surfaces. Because the temperature exceeds480K, a portion of the water of hydration also isdriven from the crystals. Cyclone (c) separatesthe solids from the gas. The gas, cooled almostto its dew point, leaves the unit through the elec-trostatic precipitator (j), where dust is recovered.Vessel (d) serves as a surge buffer to smooth theflow of solids through the unit. The solids areseparated in cyclone (g) to enter the vessel (f)where combustion of fuel is taking place. Vessel(e) allows a holding time at the calcination tem-perature so that the water content of the productis low. A small amount of fuel can be burned inthis vessel if a higher temperature is desired. Inthis design the flow of solids is counter to theflow of the gas.

The solids are cooled, also in counterflow, bythe combustion air in unit (h). Final cooling isin a fluidized bed unit (i). In the fluidized unita small amount of air is introduced through amembrane at the bottom. This lifts but does notentrain the solids, so they behave much like afluid. Heat is transferred at a high rate to air orwater flowing through tubes submerged in thebed. The product Al2O3 can be cooled below400K with all of the recovered energy beingused in the process.

The great advantage of the stationary unitsis that the heat required for calcination is3100MJ/t. The capital costs are lower than forrotary kilns of equal capacity, and the mainte-

26 Aluminum Oxide

Figure 17. Alcoa flash calcinera) Filter; b) Flash dryer; c) Cyclone; d) Fluidized-bed dryer; e) Holding vessel; f) Furnace; g) Cyclone; h)Multistage cyclonecooler; i) Fluidized-bed cooler; j) Electrostatic precipitator

nance costs are lower primarily because the re-fractories are not subjected to rotational stresses.Because the capacity per unit is high and au-tomation is quite easy, the labor requirementsare low. The stationary units are being installedin retrofitting older plants as well as in new con-struction.

Alumina calcined in stationary kilns hasa lower α-alumina content and a higher sur-face area than alumina calcined at the sametemperature in a rotary kiln. This reflects theshorter residence time and the absence of a high-temperature flame in the static units. Photomi-crographs show that the rapid temperature risecreates vapor pressure within the particles thattends to fracture large crystals.

The characteristics of the metallurgical alu-mina are controlled mostly by the time and tem-perature of calcination. The important parame-ter is the reaction between the Al(OH)3 feed andthe flow of fuel. The fuel must be free of impuri-ties that can contaminate the alumina. Sulfur andvanadium in fuel oil are limited by concern forproduct purity. The amount of combustion airmust be sufficient to burn the fuel completelywithout the use of a great excess that increasesthe gas flow and thereby reduces the thermal ef-ficiency. Factors affecting fuel economy and alu-

mina quality in modern stationary calciners arereviewed in [51].

3.1.8. Evaporation

As is shown on the flow sheet for the Bayer pro-cess (Fig. 7), the solution continuously cyclesthrough the plant. Consequently, any wash wa-ter used must be evaporated so that the solutionvolume can be controlled. About 10 % of theflow is evaporated in the two cooling areas bybeing flashed into steam. In high-temperaturedigesters, the amount of flash evaporation iseven larger. At least one plant processing high-grade bauxite was designed without any addi-tional evaporation capacity. In most cases, theeconomics require installation and operation ofevaporators. The objective is to minimize thesummed costs of evaporation and the value ofthe soluble salts lost by incompletewashing. Theminimum is usually attained with a net dilutionof the residue of 1.5 – 2.0 kg/kg.

Table 7 lists the input and exit streams thatcompromise the dilution balance in an operat-ing plant. Efficiency demands that all of theseflows be monitored.

Aluminum Oxide 27

Table 7. Dilution balance

Inputs Losses

residue wash evaporationsand wash –Al(OH)3 wash heat interchange flashfree moisture in bauxite Al(OH)3 to calcinationwater in gibbsite and boehmite free water with Al(OH)3injected steam water with residuepurge water vapor from solution surfacessodium hydroxidecleanup water (maintenance)uncontrolled dilutions

Several evaporator designs are used in Bayerplants, but nearly all designs are used inmultiple-effect configurations. In such units each stageoperates at a lower pressure than the preced-ing one. Therefore, the vapor evaporated fromthe solution in the first stage is at a temperaturehigh enough to heat the solution in the secondstage, causing it to boil. This continues to thefinal stage, where the vapor is condensed. Thecondenser operates under vacuum so a jet pumpis necessary to remove noncondensable gasesfrom it. The more times the latent heat is used,i.e., the more stages that are present, the morewater can be evaporated by the fuel steam usedin the first effect.With Bayer solutions themaxi-mumnumber of effects is six. This is because theboiling point of the solution is elevated 5 – 8Kby the dissolved solids. This reduces the steamtemperature in each effect; with many effectsthere is little temperature difference available tocause evaporation.

An unconventional design called continuousregenerative evaporators has been used in someplants. These units are similar to the heating andflashing equipment used in digestion in that thefeed is heated through up to ten stages with-out evaporation; the hot solution is then flashedthrough an equal number of stages and the steamis used for heating the feed. These units are notas efficient as most multiple-effect evaporators.The design choice is based on economics. Spe-cial evaporator designs may be used when itis necessary to crystallize organic or inorganicsalts from the solution. In such designs pumpsare used to increase the thermally induced flowthrough the tubes in the heaters. The heat-trans-fer surfaces in evaporators must be cleaned peri-odically to remove encrustations of desilicationproduct and soluble salts.

Evaporators are controlled primarily bychanging the amount of steam used. The flowof feed solution is regulated so that crystalliza-tion of soluble salts is either induced or avoided,depending on the operating mode. Temperaturereadings at each effect are compared to designvalues to indicate operating difficulties.

3.1.9. Residue Disposal

The most important environmental problem inthe Bayer process is disposal of the bauxiteresidue. The solution left with the residue af-ter an economical amount of washing is stillvery alkaline and cannot be allowed to contam-inate ground water. Furthermore, the desilica-tion product in the residue which is in contactwith water has the capacity to exchange sodiumfor hydrogen. An aqueous slurry of the residuethat had been washed with 1000 times its massof distilled water still reached a pH value of10.5 on standing. The undrained fine residue,even after years of consolidation, does not haveenough strength to support buildings or equip-ment. These properties make disposal a difficultproblem.

Previously, disposal was to a marine environ-ment where the alkalinity was diluted by largequantities of water. This method has been usedin the seas off Europe and Japan and in a river inthe United States. Studies by environmentalistshave indicated little damage to flora or fauna bythe residue in a disposal area [52]. Today, how-ever, environmental concern is so great that anynew refinery is unlikely to be permitted to usemarine disposal.

Early inland refineries simply dammed a con-venient valley or built retaining dikes on flat landto form residue disposal areas. In some cases,the sandy portion of the residue was used tobuild the dikes. Thismethod can be effective andcheap if care is taken to protect the surroundingsby proper sealing techniques. The compactedresidue has a lower permeability than manyclays; yet, there have been isolated leaks intoaquifers from such impoundments. Improveddesigns have been developed.

In Germany, retaining dikes are built with aportion of the structure designed to be porous.The dilute solution draining from the residue ischanneled by the porous sections into water-

28 Aluminum Oxide

treatment facilities before being discarded. Insome cases, the dike material exchanged ionswith the solution so that little treatment wasneeded. The draining allowed consolidation ofthe residue so that it could support equipment.An American firm used a drained lake in whichthe bottom of the disposal area was covered withsand in which draining pipes were laid. Theresidue was pumped to these areas in the tra-ditional manner as a dilute slurry. Most of theconveying water was decanted, while some per-colated through the deposit to the drains. Thisdesign has two advantages: the residue consoli-dates better, so the storage capacity of an area isincreased, and the hydraulic head on the bottomis reduced to zero, so that leakage is unlikely.Lakes have been built using the sand drains ontop of seals of clay and plastic film for additionalenvironmental safety.

Another modern disposal method, called drystacking, takes advantage of the thixotropic na-ture of the residue. By some method, usuallyvacuum filtration, the residue is concentrated to35 – 50 wt % solids. The slurry is agitated toreduce its viscosity by as much as two ordersof magnitude and pumped to a disposal area.There it flows in lava-like fashion over the sur-face, establishing a slope away from the dis-charge point. In the absence of shear, the vis-cosity of the slurry increases and flow stops.Water does not separate from the slurry and theslope causes rain to run off rapidly, so the sur-face usually is losing water to the atmosphere.The surface becomes deeply fissured, further as-sisting drying. In about 90 days the residue maydry to 75wt % solids, far drier than in any ofthe other disposal methods. In this state, it cansupport heavy earth-moving equipment and canbe recovered or used for increasing the heightof retaining dikes. This method maximizes thestorage capacity of a given area and seems topose the minimum threat to the environment. Itdoes require construction of a permanent lakearea for water storage and for cooling water. Ifany use is to be made of the residue, recovery isrelatively easy.

Many investigations have been directed to-ward finding a commercial use for bauxiteresidue. The high iron content of some residuessuggested production of pig iron. The qualityof the iron was poor and the amount of slagformed exceeded the original amount of residue.

Similarly, chemical processes have been devel-oped for recovery of Al2O3, Na2O, and TiO2from residue. Although all are technically possi-ble, none has been feasible economically. Smallquantities of residue have been used in mak-ing Portland cement, and smaller quantities havebeen used as a mold wash, as an insulating ma-terial, and, after reaction with H2SO4, as a watertreatment. No chemical use is likely to consumea significant part of the residue [1], [5].

The residue is claylike and can be used in ce-ramicmaterials. The sodiumcauses formation ofglasses at 1450K, giving a vitreous bond. Brickshave been made commercially, but economicfactors and other shortcomings of the brick haveeliminated this use. Sintering the residue into ag-gregate for concrete may be economical wherenatural aggregate is not available.

In Texas, agronomists have shown that theresidue can be used to neutralize acidic soil,replacing limestone. The cost of preparing thematerial for application and the logistics argueagainst extensive use. The conclusion is thatlarge-scale use of bauxite residue is unlikely, soefforts should be directed toward reclamation ofdisposal areas.

3.1.10. Energy in the Process

Approximately 16MJ are required to producea kilogram of Al2O3. The worldwide range is7.4 – 32.6MJ/kg [48]. Variations in the qualityof bauxite, plant design, and the size of the plantsare the reasons for the large differences. Evenwith existing plants, large improvements in en-ergy usage can be made. The Aluminum Asso-ciation (USA) reports that the energy used perunit of alumina production in 1980 was 68 % ofthe energy used in 1972 [54]. The energy used inmining is less than 5%of the total and is difficultto change. The energy for transporting bauxiteis double that for mining it, because the aver-age tonne of bauxite used in the United States istransported 4700 km. The situationworldwide isdifferent because much of the bauxite is refinedclose to the mines.

Within the refinery, more than half of the en-ergy is used for pumping and heating the so-lution and for evaporation. The previous dis-cussion (Section 3.1.3) has shown that plant de-sign can affect heat recovery and minimize en-

Aluminum Oxide 29

ergy use. Emphasis is being placed on increasingthe amount of Al2O3 recovered from less than50 g/L to over 65 g/L; some claim yields as highas 100 g/L. Because energy is more closely re-lated to the flow of solution than to yield, theenergy savings are large. The static calciners arenearly as efficient as they can be made becausethe temperatures of the gas and of the productleaving the units are very low. The change fromrotary kilns has made up a large portion of thesavings. There are still opportunities to improveoperating practice and design to reduce energyconsumption [55].

3.2. Economic Aspects

The largest cost elements in alumina productionare raw materials, energy, and capital-relatedcosts for the production equipment. Labor, oper-ating supplies, andmiscellaneous costs aremuchsmaller than these three.

Bauxite is the most important raw material.Its cost includes those for mining, transport,levies, and taxes. In addition, the relative qual-ity of the bauxite ore influences the expendituresfor necessary reagents.Mining is relatively inex-pensive because most deposits are covered withonly a shallow overburden and can be minedwith efficient equipment. Since the formationof the International Bauxite Association (IBA),levies and taxes have become a large part of thecost of bauxite. These costs have become rela-tively stable because most are related to the sell-ing price of aluminum. The availability of baux-ite from outside the IBA countries has allowedthe effect of supply and demand to influence thiscost. Where levies are not imposed, some sys-tem of taxation provides income to the produc-ing nation. Transportation costs vary from verysmall to as much as half the delivered cost ofthe bauxite. Some bauxite travels less than 5 kmby truck or conveyor belt to the refinery. Otherbauxite may travel more than halfway aroundthe world. The characteristics of bauxite mostimportant in influencing its value are the avail-able alumina and the reactive silica contents. Thelatter quantity has a great effect on the amountof sodium hydroxide and lime required in pro-cessing. Bonuses are given for high availablealumina values, and penalties are charged forexcessive silica.

In page 28, values were given showing thefourfold range in energy used to produce alu-mina. Themost important variables are the plantdesign and the quality of the bauxite being pro-cessed. The most energy-efficient bauxites aregibbsitic, with very low residue content.

The capital costs also show wide variation.In 1990 U.S. dollars new installations may costfrom $ 700 to $ 900 per annual tonne of capac-ity. The variables are design factors, location,capacity, and the properties of the bauxite tobe processed. Much of the world capacity wasconstructed before construction costs were in-flated and so has lower capital costs. The effectof capacity is also large, and plants producing2× 106 t/a have costs well below those of plantsproducing less than 105 t/a.

Alumina is traded on the world market as acommodity. Not all smelters have alumina ca-pacity dedicated to them; they buy alumina oncontract. Long-term contracts are common. Inother instances, alumina is bought when neededat spot market prices. Since 1980, alumina hassold on the spot market for less than $ 150 toover $280 per tonne, reflecting a change in therelationship between supply and demand. Eco-nomics do not always control production be-cause many corporations own refineries to sup-ply their smelters. In other instances, nationalgovernments own a significant portion of a re-finery and for political reasons may choose tooperate in a noncompetitive situation. Costs pertonne of alumina can, under unfavorable con-ditions, exceed $ 200, although efficient plantsmay produce at roughly half this figure.

4. Other Processes for AluminaProduction

4.1. Raw Materials

Many investigations have sought processes to re-place the Bayer process by using raw materialsother than bauxite [2]. Clay, primarily kaolinite,has been most considered because it can containup to 39 % Al2O3 (→Clays). Other materials,including anorthosite, nepheline, coal wastes,and fly ash, have been candidate raw materials.Yet, less than 2%of theworld supply of aluminais not made by the Bayer process. This happens

30 Aluminum Oxide

only where use of a domestic raw material andproduction of a desirable byproduct change theeconomic picture. Alumina from ores other thanbauxite normally costs 1.5 – 2.5 times that fromthe Bayer process.

Because aluminum is amphoteric, both acidand alkaline processes have been developed.Most of the processes in each class follow thesame general flow sheets.

4.2. Alkaline Processes

Sodium compounds, such as Na2CO3, reactat 1280K with the Al2O3 in aluminous oresto produce water-soluble NaAlO2. This com-pound can be leached from the sinter with waterand the solution treated to remove impurities;the purified solution is neutralized with CO2 torecover Al(OH)3. The last step regenerates theNa2CO3 for recycle. Difficulty arises becausethe aluminous ores contain SiO2, which also re-acts in the sinter to form soluble Na2SiO3. Inprocessing, the desilication reaction discussed inSection 3.1.2 takes place, so the net recovery ofAl2O3 is small or zero. If 2mol of CaO, usuallyas limestone, is charged for every mole of SiO2in the raw material, insoluble calcium silicatecompounds are formed. Under proper leachingconditions the NaAlO2 can be recovered fromsuch sinters with only slight loss.

The process has been used commercially intwo American plants to recover most of theNa2O and Al2O3 lost in the desilication productformed while treating high-silica native baux-ite. A schematic flow sheet is given in Fig-ure 18. The sameprocesswas investigated by theU.S. Bureau ofMines in a large pilot plant usinganorthosite as the raw feed [51]. [Anorthosite is amixture of the minerals anorthite (CaAl2SiO8)and albite (NaAlSi3O8).] Unless the composi-tion, sintering, and leaching were closely con-trolled, the residue formed a gel in leaching andbecame very difficult to filter.

In a variation of the sintering approach, cal-cium replaces the sodium so that calcium alu-minate (3CaO ·Al2O3) is formed as well as cal-cium silicate (CaSiO3). The calcium aluminatereacts with Na2CO3 in an aqueous leach to formNaAlO2.

3 CaO ·Al2O3 + 3Na2CO3 + 2H2O→2NaAlO2 + 4NaOH+3CaCO3

Figure 18. Lime/soda sinter process flow sheet

In carbonation, Al(OH)3 is formed and theNa2CO3 solution is regenerated. Nepheline,(Na,K)AlSiO4, is treated by this process inthe former Soviet Union. The calcium silicateresidue is processed to make about 10 t of ce-ment per tonne of alumina. The two productsmake the process viable here.

In the Pedersen process, sintering was re-placed with a reducing fusion so that ferric ionsin the orewere reduced tometal. Iron and the cal-cium slagwere separated by decantation.Duringcooling, the CaSiO3 passes through a crystallinephase change and the resulting stresses reducethe slag to powder [1], [2].

The sinter processes suffer economically forseveral reasons. The energy for sintering and forevaporation of the leach solution must be addedto the energy required from operations analo-gous to those in Bayer processing. The capitalinvestment is increased by the need for sinteringequipment, and although limestone is inexpen-sive, the quantity required is so large that theexpense is considerable.

Aluminum Oxide 31

4.3. Acid Processes

All of the acid processes follow the generalflow sheet given in Figure 19. Usually the clayis prepared by grinding and roasting to about1000K. The roasting changes the kaolinite tometa-kaolin, from which the aluminum can bedissolved as the acid salt. The roasted clay isleached in an acid solution, usually at the atmo-spheric boiling point. Some investigators havechosen to leach at elevated temperature for pro-cessing advantages even though corrosion prob-lems become more severe [1]. The siliceousresidue is separated using sedimentation and fil-tration as in theBayer process. Small amounts ofiron salts remain in the clarified solution. Thesesalts are removed by extraction with an organiccompound that forms a complex with the ironbut not with the aluminum salts. The organic so-lution is decanted from the aqueous phase and istreated to separate the iron salt and to regeneratethe organic extractant.

Figure 19. Generalized acid process flow sheet

A hydrated aluminum salt is recovered fromthe aqueous solution, usually by evaporation andcooling to cause crystallization. In the hydrogenchloride process, advantage is taken of the lowsolubility of AlCl3 in HCl. The HCl gas is ab-sorbed in the solution both to regenerate the acidfor leaching and to precipitate AlCl3 · 6H2O. Inall acid processes the hydrated salt is decom-posed to Al2O3 by heating to 1300K. Both wa-ter and the acid radical are driven off, and theacid is absorbed for recycling.

The U.S. Bureau of Mines sponsored a coop-erative effort with several aluminum companiesto investigate acid processes for recovery of alu-mina from clay [56]. For several reasons theHClprocess was preferred: the reagent is inexpen-sive, processing conditions are not severe, andthe salt can be recovered without evaporation.The HCl is not decomposed in calcination, andthe water content of the acid salt is half that ofother acid salts. Despite these features the pro-cess is not competitive economically with theBayer process. A major fault is the amount ofenergy used to roast the clay, to evaporate thesolution, and to decompose the acid salt. Theinvestment is increased because of the corro-sive solutions. A process disadvantage is that theAl2O3 is physically different from Bayer alu-mina, so smelting practice has to be changed forthis aluminum source.

Pechiney and Alcan jointly operated a pilotplant using the H+ process, a modification ofthe HCl process in which H2SO4 is added tothe digest [52]. The combination of acids elimi-nated the need to roast the clay. Other processesuse H2SO4, H2SO3, HNO3, or NH4HSO4 asreagents to attack the clay, but no commercialuse has been made of any of them.

Clay is a major impurity of coal, so the ashfrom coal may be considered an overroastedclay. The acid processes, with minor modifi-cations, will extract alumina from ash [57].Anorthosite that is high in anorthite reacts withboiling HCl; this approach has been investigatedin Norway and Canada [58]. So far, all of thesemethods have served only to enrich the technicalliterature.

Two aluminous minerals that do not containsilica have attracted attention and, because oftheir composition, require different technology.Alunite, K2SO4 ·Al2(SO4)3 · 2Al2O3 · 6H2O[12588-67-9], has been used commercially in

32 Aluminum Oxide

Russia. Both H2SO4 and K2SO4 are useful by-products of alumina production by this method.The alunite is heated to drive off the hydratedwater; additional heating under reducing condi-tions decomposes the aluminum sulfate withoutadversely affecting alumina recovery. The solidresidue from the roast is a mixture of K2SO4,Al2O3, and gangue. The K2SO4 is dissolvedin water and recovered. Alumina is extractedfrom the gangue in a modification of the Bayerprocess. In other approaches using alunite, thesolutions are kept acidic so the alumina is re-covered as a salt [59].

Dawsonite, Na2O ·Al2O3 · 2CO2 · 2H2O[12011-76-6], is found in some oil shales. Thetemperatures used to recover the oil from theshale decompose the dawsonite to NaAlO2,which is soluble in dilute NaOH. The Al(OH)3and Na2CO3 can be recovered from the solu-tion by carbonation. The economics are suchthat recovery of the oil must be competitivebefore mining and byproduct recovery can beconsidered.

5. Metallurgical Alumina

Aluminum production is the principal applica-tion for alumina; more than 92 % of world alu-mina production is used for this purpose. Theproperty requirements for commercial metal-lurgical alumina are therefore of considerableimportance to the alumina industry. Specifica-tions have responded to changes in energy costs,aluminum cell design, cell gas scrubbing tech-niques, environmental regulations,working con-ditions in smelters, and the technology for alu-mina calcination. In general, there has been ashift away from relatively small-particle-size,highly calcined, “floury” alumina to a coarse,free flowing, dust-free, less calcined, “sandy”alumina of narrower particle sizing and higherchemical purity (→Aluminum).

5.1. Alumina Properties Required forSmelting

Five developments have had a significant impacton alumina properties.

Cell Design. New, high current efficiency,prebaked anode cells with automatic center-feedsystems require that the alumina be consistent,with trouble-free handling properties to insureproper conveying and volumetric metering fromthe feeder. These criteria are best met by a free-flowing,moderate- to low-calcined aluminawithrelatively coarse and narrow particle size distri-bution.

Cell Gas Dry Scrubbing. The use of dryscrubber systems employing cell-feed aluminaas adsorbent for fluorides in effluent gas fromthe cells dictates other requirements for metal-lurgical alumina:

1) High adsorption capacity for hydrogen flu-oride. This property is closely related to thespecific surface area of the alumina, whichis higher for the lower calcined aluminas.

2) Attrition resistance.3) Free flowability.4) Higher chemical purity to compensate for

capture in the dry scrubber of impuritieswhich are recycled to the cell.

Pot RoomWorking Conditions. Use oflow-calcined, high surface area alumina as acover for the cell bath reduces fluoride evolu-tion within the pot room. Working conditions inthe smelter are degraded by dust caused by fineparticles of alumina. Reduction of the fines frac-tion (< 44µm) in the alumina and high attritionresistance are important for reducing dust.

Use of Stationary Calciners and Pneu-matic Handling Systems for Alumina. The re-placement of rotary alumina kilns by the energy-efficient stationary calciners has resulted in pro-ducing a different type of calcined alumina forsmelter feed. This difference is reflected in theinterrelationship between degree of calcination(measured by themass loss on ignition), specificsurface area, and the α-Al2O3 content [60].

The strength of the alumina particles has be-come of concern not only because of relativelyhigher breakdown in the stationary calciners,but also because of attrition occurring in pneu-matic unloading and conveying equipment andin fluid-bed dry-scrubbing systems. The gener-ation of fine particles in such equipment, apartfrom causing unacceptable dusting conditions in

Aluminum Oxide 33

the smelters, often results in troublesome seg-regation problems in alumina storage bins andbunkers.

Electrolyte Composition and Tempera-ture. The trend to operate cells at lower temper-atures with electrolyte having a lower bath ratio(NaF : AlF3) has decreased the solubility andrate of dissolution of alumina in the electrolyte.The rate of dissolution is greater for aluminashaving higher surface areas and low content ofα-Al2O3. Both properties can be achieved by alow degree of calcination.

5.2. Typical Specifications forMetallurgical Alumina

The considerations discussed inSection 5.1 havecontributed to the evolution of the general spec-ifications used in production and internationaltrading (Table 8). These values are only rep-resentative and considerable variation exists inactual practice depending on price, availability,smelting practices, andmany other factors. Con-sistency of quality is a major consideration.

Table 8. Typical properties of metallurgical alumina

Physical property

Particle size distribution, wt %+ 100 mesh (Tyler) < 5+ 325 (44µm) > 92− 325 < 8

Bulk density, kg/Lloose 0.95 – 1.00packed 1.05 – 1.10

Specific surface area, m2/g 50 – 80

Moisture (to 573K), wt % < 1.0

Loss on ignition (573 – 1473K),wt %

< 1.0

Attrition index (modifiedForsythe –Hertwig method)

increase in < 44µm particles4 – 15wt %

α-Al2O3 content (by optical orX-ray method), %

< 20

Chemical analysis wt %Fe2O3 < 0.015SiO2 < 0.015TiO2 < 0.004CaO < 0.040Na2O < 0.400

6. Industrial Alumina Chemicals

Alumina, in various forms, is one of the inor-ganic chemicals produced in greatest volumetoday. Although production of aluminum metalconsumes ca. 90 % of all alumina, an increasingamount is being applied in the chemical industryfor fillers, adsorbents, catalysts, ceramics, abra-sives, and refractories. With the developmentand growth of applications and markets for alu-mina chemicals, all themajor alumina producershave, over the years, converted a part of their ca-pacity to produce various alumina chemicals. Infact, some of the older, smaller alumina refiningplants have been totally converted to aluminachemicals production in order to be economi-cally viable. Chemical uses account for nearly8 % of the world production.

6.1. Aluminum Hydroxides

Aluminum hydroxides constitute a versatilegroup of industrial chemicals. Important usesrequiring large quantities are as fillers in plas-tic and polymer systems and for the productionof aluminum chemicals. A moderate amount isused for the production of alumina-based adsor-bents and catalysts.

Aluminum hydroxide meets most of the re-quirements for an effective filler: white or near-white color; large volume production base, re-sulting in price and supply stability; consistencyof physical and chemical properties; a widerange of particle size distributions; chemical in-ertness; and nontoxicity. However, its increas-ing popularity as a plastics filler is strongly re-lated to its fire-retardant and smoke-suppressantproperties, which justify the somewhat higherprice compared with calcium carbonate andother mineral fillers. Aluminum hydroxide actsas a fire retardant by adsorbing heat through en-dothermic dehydration and dilution of pyrolyti-cally produced combustion gas by the releasedsteam. Dehydration and water release processesbecome significant at temperatures above 500K.The smoke-inhibiting activity of aluminum hy-droxide filler has been attributed to promotionof solid-phase charring in place of soot forma-tion. Although offering these desirable features,

34 Aluminum Oxide

aluminum hydroxide has certain disadvantagesthat impose some limitations on its uses as afiller. Like other nonreinforcingmineral fillers, itgenerally lowers strength. Because it undergoesthermal decomposition, it is not suitable for pro-cessing above 500K. These factors are respon-sible for the larger use of aluminum hydroxidein latex carpet backings and in glass-reinforcedpolyesters, where processing temperatures be-low about 480K generally are used. These arealso chemically cross-linked or fiber-reinforcedsystems, in which loss of strength caused by anonreinforcing filler may not be very important.Aluminum hydroxide filler has been used lessextensively in thermoplastics, e.g., poly(vinylchloride) and polyethylene, and elastomericma-terials. For use as a filler, the crystalline alu-minum hydroxide from the Bayer process isdried and ground to particles� 10µm size. Spe-cial grades with increased whiteness and a va-riety of both particle size ranges and chemicalpurity also are available commercially.

Fine, precipitated aluminum hydroxide hav-ing a uniform particle size (≈ 1µm) is used inpapermaking as a filler pigment and as a coating.As a filler, it disperses rapidlywith low sedimen-tation. Improved printing properties are reportedfor the hydroxide-filled paper. The application offine, platy aluminum hydroxide as a paper coat-ing is well established in the paper industry. Itgives a coating of high brightness, opacity, andgloss (→Paper and Pulp).

Technical aluminum hydroxide obtainedfrom the Bayer process is 99.5 % pure. It dis-solves readily in strong acids and bases. Forthese reasons, aluminum hydroxide is the pre-ferred raw material for the production of a largenumber of aluminum compounds. These includepure, iron-free aluminum sulfate (used in thepaper industry and for water purification), alu-minum fluoride, synthetic zeolites, and sodiumaluminate. Aluminum hydroxide also is used inthe glass industry and in cosmetic and phar-maceutical preparations. An important cosmeticuse of aluminum hydroxide is in toothpaste. Themildly abrasive hydroxide cleans and polishesteeth.

The price of aluminum hydroxides rangedfrom $0.20 to $0.60 per kg in 1990, the price

range reflecting the cost of additional processingof the usual Bayer process product to suit ap-plication requirements. These include grinding,higher purity, classification, surface treatment,etc. Pharmaceutical-grade gel hydroxides wereat the top of the price range.

6.2. Adsorbent and Catalytic Aluminas

Activated aluminas represent another group oftechnically important alumina chemicals. Prin-cipal uses are as drying agents, adsorbents, cat-alysts, and catalyst carriers. These products areobtained by thermal dehydration of different alu-minum hydroxides in the 520 – 1070K temper-ature range (see Section 1.5).

6.2.1. Preparation of Activated Aluminas

Bayer aluminum hydroxide is the chief sourceof commercial activated alumina products. Pow-der forms of activated alumina are produced byheating the hydroxide directly at 575 – 1825K inovens or in rotary or fluidized-bed calciners. Theproducts have surface areas of 200 – 350m2/gand losses on ignition of 3 – 12wt % (at575 – 1473K). Such products are used as de-colorizing agents for organic chemicals and asstartingmaterial for the production of aluminumfluoride (→Fluorine Compounds, Inorganic).Other uses include chromatographic and cat-alytic applications in organic chemistry.

Granular activated alumina produced fromBayer plant crust is one of the oldest commercialforms of this product and still is used widely. Aflow sheet of the production process is shown inFigure 20. The activated material is a hard, non-dusting product. Table 9 lists some properties ofa commercial product of this type (Alcoa F-1).A similar granular product has been producedby compacting Bayer hydroxide by mechanicalpressure (Martinswerk GmbH, Federal Repub-lic of Germany). The process utilizes a roll-typecompactor. The product from the compactor isbroken up and sieved to the required size frac-tions and activated at 675 – 875K in a rotary cal-ciner.

Aluminum Oxide 35

Table 9. Typical properties of Alcoa activated aluminas

F-1 H-151 S-100

Al2O3, % 92 90 95Na2O, % 0.58 1.6 0.35SiO2, % 0.12 2.0 0.03Fe2O3, % 0.06 0.03 0.05Loss on ignition, % 7 6 5Loose bulk density:g/cm3 0.83 0.82 0.80lb/ft3 52 51 50

Packed bulk density:g/cm3 0.85 0.85 0.75lb/ft3 55 53 47

Pore characteristics and surface area of typical Alcoa activated aluminas:Helium (true) density, g/cm3 3.25 3.40 3.15Mercury (particle) density,

g/cm31.42 1.38 1.24

Micro pore volume (pores< 3.5 nm), cm3/g

0.017 0.023 0.012

Macro pore volume (pores> 3.5 nm), cm3/g

0.023 0.020 0.037

Total pore volume, cm3/g 0.40 0.43 0.49Total porosity, % 56.3 59.4 60.6Pore diameter at 50 % total

pore volume, nm17.7 3.5 4.7

Primary pore size range, nm 0 – 10000 0 – 40 0 – 500BET surface area, m2/g 250 360 260Pore diameter, nm106 – 107 0.0031 0.0003 0.0000

Pore volume, cm3/g105 – 106 0.0045 0.0001 0.0001104 – 105 0.0059 0.0001 0.0004103 – 104 0.0029 0.0004 0.0033102 – 103 0.0045 0.0093 0.004735 – 102 0.0021 0.0099 0.02892 – 35 0.0165 0.0232 0.0115

0.0395 0.0433 0.0489

Figure 20. Flow sheet for production of activated aluminafrom Bayer plant crust

Fast dehydration of Bayer aluminum hydrox-ide, either by vacuum or by exposure to high-temperature gas (1050 – 1300K) for a few sec-

onds, has been used for the production of ball-shaped, activated alumina having properties su-perior to the granular product. This process re-sults in the formation of nearly amorphous �-Al2O3. The product is finely ground and usingwater as binder, formed into spherical agglomer-ates in a rotating pan agglomerator. Rehydrationof �-Al2O3 with water leads to crystallizationof bayerite, causing the agglomerates to harden.Reactivation of the hard balls at 675 – 775Kpro-duces the activated product (5 – 20mm in diam-eter) having a surface area of 320 – 380m2/g.

Alumina gels also have been used for themanufacture of activated aluminas. These gelsare produced by neutralization of aluminum sul-fate or ammonium alum by NH4OH, or fromsodium aluminate by neutralization with acids,CO2, NaHCO3, and Al2(SO4)3. The gelatinousaluminum hydroxide precipitate is filtered andthoroughly washed and dried. The dried prod-

36 Aluminum Oxide

uct is activated, milled, and agglomerated toa spherical product. Other forming processes,such as extrusion, pelletizing, and tableting, alsocan be used. The product is finally activatedat 675 – 875K to a loss on ignition value ofabout 6 %. Although gels of various texturescan be prepared, the usual industrial adsorbentproducts have very small pores (less than 4 nmin diameter) and surface areas in the range of300 – 400m2/g. A flow sheet of a manufactur-ing process is shown in Figure 21. Table 9 re-ports data on a commercial, gel-based product(Alcoa H-151).

Figure 21. Production of gel-based activated alumina

Aluminumoxide hydroxide (boehmite), a by-product from the Ziegler process for linear alco-hol production, is another source of activatedalumina. The high purity of this material favorsits use in catalytic applications. The fine-particleboehmite is normally extruded to various shapes.Thematerial is claimed to serve as its ownbinderwhen peptized with glacial acetic acid. The ex-trudate is cut to the required size, dried, and acti-vated at 775 – 875K. The surface area of the ac-tivated product is 185 – 250m2/g. Commercialbayerite also has been used to produce activatedalumina; the product is preferred in some cat-alytic applications.

6.2.2. Adsorbent Applications

Amajor application of activated alumina is in thefield of adsorption, where its high surface area,pore structure, strength, and chemical inertnessfavor its use. The alumina performs important

technical functions, such as gas and liquid dry-ing, water purification, and selective adsorptionin the petroleum industry.

Gases that have been dried successfully byalumina desiccants are:

acetylene cracked gas hydrogen oxygenair ethane hydrogen

chloridepropane

ammonia ethylene hydrogensulfide

propene

argon freon methane sulfur dioxidecarbon dioxide furnace gas natural gaschlorine helium nitrogen

In many applications an alumina desiccant candry gas to a lower dew point than any other com-mercially available desiccant. The static wateradsorption capacities of two typical activatedaluminas (Alcoa F-1 and H-151) in contact withair at different relative humidity conditions areshown in Figure 22.

Figure 22. Static water adsorption capacity of typical com-mercial activated aluminas at 25 ◦C (298K)

Moist gas usually is dried by passing itthrough a column (or tower) packed with theadsorbent. Adsorption of water on activated alu-mina is strongly exothermic, releasing between45 and 55 kJ of heat per mole of water adsorbed.This factor must be considered in the design ofthe drying tower. Both granular and sphericalforms of activated alumina can be used in desic-cant beds. Sizes from 2 to 20mm are availablecommercially. The granular products have lower

Aluminum Oxide 37

surface areas and their application is based onlow cost per kilogram, low water load, and lowstream pressure. Spherical products, producedby either the gel or the fast dehydration pro-cesses, have larger surface areas, a narrower porestructure, and a high adsorption capacity, andthey are relativelymore expensive than the gran-ular variety. The spherical kind usually is spec-ified for high-pressure, high-moisture removalduties. The alumina desiccant is regenerated bypassing a current of hot, dry gas (475K) throughthe bed, usually countercurrent to the main gasflow.

Liquids that can be dried with activated alu-mina include aromatic hydrocarbons, highermolecular mass alkanes, gasoline, kerosene, cy-clohexane, power system coolants, lubricants,and many halogenated hydrocarbons. Liquidsthat are highly adsorbed on alumina (e.g.,ethanol and methanol), react or polymerize incontact with activated alumina. Those contain-ing components that tend to deposit on the alu-mina surface cannot be dried by activated alu-mina. Regeneration schemes for liquid dehydra-tion units are varied and depend on the liquid be-ing dried. In some applications the liquid beingdried is vaporized, heated, and passed throughthe desiccant bed to adsorb water. Hot, dry gasesalso are used for regeneration.

An evolving application of activated aluminais in water purification. Several important con-taminants have been removed from water suc-cessfully and economically in pilot plants aswellas in large-scale treatment plants. These includereduction of fluoride concentration in drinkingwater and in some industrial effluents, and re-moval of color and odor fromeffluentwater fromdye works and paper mills. Removal of phos-phate and arsenic also has been investigated.

Activated alumina can be used to separate oneor more components from a gas or liquid streamby taking advantage of differences in adsorp-tion or desorption kinetics. For example, a shortcycle process has been used to recover heavyhydrocarbons from a stream of lighter hydro-carbons. Often a regeneration scheme can bedevised that permits cyclic use of the alumina.In other instances, such as removal of catalystin polyethylene and hydrogen peroxide produc-tion, it is more economical not to recycle thealumina.

6.2.3. Catalytic Applications

Alumina is used inmany industrial catalytic pro-cesses, both as the catalyst and as a supportfor catalytically active components. In many in-stances, the alumina support contributes to cat-alytic activity and so assumes an essential rolein the catalyst system. Other catalytic uses ofalumina take advantage of its strength, heat re-sistance, and inertness.

Although the common adsorbent aluminasderived from Bayer hydroxide have many cat-alytic applications, high-purity materials (e.g.,boehmite from the Ziegler process for linearalcohol production) are sometimes preferred.Because of the lower cost of Bayer process-based products, such techniques as water andacid washing of the activated product have beenemployed to reduce the alkali (Na2O) contentof the Bayer material. The sodium oxide isknown to have a negative influence in manycatalytic processes. Catalyst-forming methodsinclude tableting, pelletizing, compacting, ballforming (agglomeration), and extrusion. Suchfactors as purity, surface area, pore volume, andsize distribution, and rate of deactivation influ-ence catalyst performance and selectivity. In ad-dition, crushing strength and resistance to attri-tion of the catalyst pellets are important con-siderations in practical operation of catalytic re-actors. Although these physical aspects of alu-mina catalysts are well characterized, the sur-face structure and chemistry responsible for cat-alytic activity still remains unclear. Many inves-tigators have attributed catalytic activity to theintrinsic acidity of the surface of activated alu-mina. First, the combination of two neighbor-ing OH− groups to form water during the dehy-dration process leaves behind an exposed Al3+

ion,which, because of its electron deficiency, be-haves as a Lewis acid site. In addition, hydroxylgroups are retained during thermal decomposi-tion of aluminum hydroxide (see Section 1.5).These OH− ions on the surface may act as pro-ton donors (Brønsted acids). These Lewis andBrønsted acid sites have been looked upon as theactive catalytic centers. Further “defect” struc-tures are formed with increasing degree of de-hydration [56]. Of the types of defects createdduring dehydration, those assumed to have thegreatest catalytic importance are the triplet va-

38 Aluminum Oxide

cancies. They provide unusual exposure of thealuminum ions in the underlying layer and con-stitute strong acid sites.

Catalytic applications of alumina are exten-sive. Important industrial processes employingalumina as a catalyst by itself include alco-hol dehydration and the Claus process for sul-fur recovery (→Sulfur). Dehydration of alco-hols over activated alumina is one of the oldestcatalytic processes. Typical reaction conditionsfor olefin production are 575 – 675K and atmo-spheric pressure. Lower temperatures favor theformation of ethers. The most suitable alumi-nas for alcohol dehydration catalysis are thosethat have large surface areas (150 – 200m2/g)and possess good thermal and hydrothermal sta-bility. Coke formation occurs over a period ofseveral hundred operating hours and the catalystmust be regenerated by burning off the carbonwith hot air at 773 – 873K.

The largest present-day catalytic applicationof activated alumina itself is in theClaus process,which is used to recover sulfur from hydrogensulfide (H2S).

2H2S+SO2 → 2H2O+3S ∆H =− 147 kJ

The reaction is carried out catalytically in twoor more conversion stages using alumina cat-alysts. Reaction temperature in the first stageis around 625K. At this temperature, the con-version of H2S is only about 65 %. Subsequentlower temperature catalytic stages are used tofurther reduce the H2S concentration. Spheri-cal, high-strength activated alumina catalysts areused in the Claus converters. Service life as highas 5 years has been reported. Deactivation of thealumina catalyst occurs by sulfation, thermal ag-ing, and carbon and/or sulfur deposition. Regen-eration of Claus catalyst involves removal of sul-fur and burning off of carbon deposits.

Alumina-supported catalysts are used exten-sively in the petroleum and chemical industries.In general, the petroleum industry catalysts havehigh surface area and high porosity; the sup-port is mostly activated alumina. On the otherhand, many typical chemical process catalysts(e.g., ammonia synthesis, steam reforming) arecharacterized by lower surface area (< 20m2/g)and are nonporous or have very large diameterpores. The carrier in this case is inert and consistsmostly of calcined or sintered alumina products.

Two different methods have been used com-monly for preparation of alumina-supported cat-alysts: impregnation and coprecipitation. Thealumina support used in the impregnation pro-cess generally has been formed into its finalshape (extrudates, tablets) prior to the impreg-nation step. Impregnation with a salt solution ofthe active species is then carried out, followedby drying and thermal decomposition of the salt.In the coprecipitation procedure, hydroxides ofaluminum together with the active componentare precipitated from a salt solution by neutral-ization with ammonia or alkali. The washed pre-cipitate is dried, powdered, and processed (e.g.,by extrusion) to the desired shape and finallyactivated by thermal dehydration. The coprecip-itation method is used when the active speciesmust be present in high concentrations or whenmore uniform distribution of the active compo-nent is desired.

The technical and patent literature containsinnumerable examples of the use of alumina asa catalyst support. Some important examples arethe catalytic dehydration of n-butane to butadi-ene (used in synthetic rubber), using a chromia-impregnated alumina catalyst; molybdenum –alumina catalysts, used in hydrorefining opera-tions (e.g., desulfurization) in petroleum refin-ing; and pelleted catalysts containing platinum,palladium, and rhodium on an alumina base,used in automobile exhaust catalytic converters.

United States production of adsorbent-gradeactivated aluminas, both granular and spherical,amounted to nearly 250 000 t in 1990. Price ofthe cheaper, granular productwas quoted around$ 0.40 – 0.50 per kg. Price of the spherical prod-uct ranged from $ 0.55 to $0.65 per kg. Totalcatalytic applications of alumina in the UnitedStates were estimated to be around 400 000 t in1990. Price of preformed alumina for catalyticapplications ranges from $ 0.60 to $ 4.00 per kg,depending on source and purity. Alumina-basedClaus catalyst was priced between $ 3.00 and$ 3.50 per kg.

7. Ceramic Uses of Alumina

Alumina is used extensively as a ceramic ma-terial (→Ceramics, Advanced Structural Prod-ucts). Products range from relatively low-calcined grades of polishing aluminas to the

Aluminum Oxide 39

extremely hard, fused alumina and syntheti-cally produced sapphire. The characteristics thatmake alumina valuable in ceramic applicationsare high melting point (2325K), hardness (9 onthe Mohs scale), strength, dimensional stabil-ity, chemical inertness, and electrical insulatingability. These, together with availability in largequantities at moderate prices, have led to exten-sive and varied uses of alumina as a ceramicmaterial.

7.1. Calcined Alumina

Ceramic aluminas are generally produced bycalcining Bayer aluminum hydroxide at tem-peratures high enough for the formation of α-Al2O3. By control of calcination time and tem-perature and by the addition of mineralizers,such as fluorine and boron, the crystallite sizein the calcined product can be varied from 0.2 to100µm. These calcined aluminas can be catego-rized broadly according to their sodium content.There are two general types: those having about0.5 % Na2O and low-soda grades with a content< 0.1 %.

Reactive alumina is a material manufacturedby dry grinding calcined alumina to particlesizes smaller than 1µm. The large surface areaassociated with very fine particles and the highpacking densities obtainable considerably lowerthe temperatures required for sintering.

Tabular aluminas are manufactured by grind-ing, shaping, and sintering calcined alumina.The thermal treatment at 1900 – 2150K causesthe oxide to recrystallize into large, tabular crys-tals of 0.2 – 0.3mm.

7.2. Fused Alumina

For ceramic applications and for the productionof abrasives, fused aluminas are manufacturedby melting a suitable raw material in an elec-tric arc furnace (→Abrasives). Calcined alu-mina from the Bayer process is used as a startingmaterial for the highest quality fused alumina.Bauxites with varying levels of iron oxide, sili-cates, and titanium minerals are melted to pro-duce the brown or less pure black qualities.

8. Toxicology and Industrial Hygiene

For a discussion concerning the toxicologyand industrial hygiene of aluminum oxides,→Aluminum, Chap. 9.

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40 Aluminum Oxide

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Amalgams → Mercury, Mercury Alloys, and Mercury CompoundsAmericium → Transuranium ElementsAmidosulfuric Acid → Sulfamic AcidAmine Oxides → Surfactants