Selective Reduction of Laterite Nickel Ore

Sungging Pintowantoro+ and Fakhreza Abdul

Department of Materials Engineering, Faculty of Industrial Technology, Institut Teknologi Sepuluh Nopember, Arief Rahman Hakim Street,Surabaya, East Java, 60111, Indonesia

Nickel is an important metal in the industry. To obtain nickel metal, the extraction process from nickel ore should be conducted. Recently,nickel ore resources from sulphide ore becomes rare which makes nickel laterite ore as the future of nickel extraction. Unlike sulphide nickel oreprocessing, laterite nickel ore processing requires higher processing energy through smelting. Therefore, a novel method to process laterite nickelore using lower energy is needed. The novel method is done via direct reduction and magnetic separation. In laterite nickel processing by directreduction, the challenges are to conduct selective reduction of nickel and to let iron unreduced. The addition of additive in the direct reductionprocess is needed to achieve this selective reduction. Additives used for the selective reduction includes Na2SO4, MgCl2, CaSO4, NaCl andCaCl2·H2O. This article will further review the role of additive used in selective reduction of laterite nickel ore and the current trends on selectivereduction research. [doi:10.2320/matertrans.MT-M2019101]

(Received April 1, 2019; Accepted August 22, 2019; Published October 25, 2019)

Keywords: laterite nickel ore, selective reduction, direct reduction, additives

1. Nickel ­ General

Nickel is an important alloying metal with widerangeapplication in the industry.1) Nickel is one of the mostimportant strategic metals which is widely applied tostainless steel, electroplating, catalyst, and petrochemicalindustry.2,3) Nickel, found in products produced fromsmelters and refineries, is classified into three categories,i.e. Refined nickel (Class I), Charge nickel (Class II) andChemicals. Class I has nickel content of 99% or higher. Theproduct examples of class I are electrolytic nickel, pellets,briquettes, granules, rondelles and powder/flakes. Class IIhas nickel content less than 99%. The product examplesof class II are Ferro-nickel, nickel oxide sinter, utility, andNickel Pig Iron. Chemicals category examples are chemicalnickel oxide, nickel sulphate, nickel chloride, nickel carbon-ate, nickel acetate, nickel hydroxide, etc.4)

2. Laterite Nickel Ore and Its Processing

About 70% of nickel reserve is nickel laterite, but only40% nickel laterite is processed.5) The small number of nickellaterite ore processed is due to the difficulty of nickel lateriteore processing when compared to sulphide nickel oreprocessing. Nickel laterite ore needs complex treatment toextract the Ni metals causing the nickel laterite ore processingmore expensive than nickel sulphide ore processing. This isbecause Ni content of nickel laterite ore is more difficult toupgrade. Unlike nickel sulphide ore, the distribution of nickelmetal in nickel laterite is distributed uniformly and act asinterstitial condition in molecular lattice of particles. It makesthe concentration process of nickel through flotation andgravity separation can not be applied. Based on this reason,almost all nickel laterite ore from mine have to be fed intothe whole process. This results in the high operational costrequired for the processing.6) In recent years, the increasein nickel laterite production still happens because of highdemand of stainless steel and because of decrease of sulphide

nickel ore reserve.7) This fact causes nickel laterite orebecoming the major source for production of nickel metal.8,9)

The lateritic nickel ore is an ore resulting from longweathering process, derived from ultra-basic rocks containingsilicate and magnesium minerals. During this process, Mgand Fe in the silicate lattice are partially replaced by Ni inorder to form mineral deposits with different nickel andimpurity contents.10) Nickel laterite ore was classified intotwo types of ore, i.e. limonitic and saprolitic nickel lateriteore. Limonitic nickel ore has low content of Ni. The nickelcontent of limonitic nickel ore is 1.1 until 1.8wt%. On theother hand, the saprolitic nickel ore has higher nickel contentthan limonitic nickel ore. The nickel content in saproliticnickel ore is about 2% or higher.11) In detail, laterite depositsare classified into four categories, including: i) Limonitezone, (ii) Non-nontronite zone, (iii) Serpentine zone and (iv)Garnierite (Saprolite) zone.5,12) Based on Fe and Mg contentin laterite nickel ore, laterite nickel ore can be classified intothree classes, including (a) Class A - Garnieritic laterite type(Fe <12% and MgO >25%); (b) Class B - Laterite limonitictype (Fe: 15­32wt% and MgO <10%); and (c) Class C -Intermediate type (Fe: 12­15wt% and MgO: 25­35wt% or10­25wt%).12,13)

Nickel laterite processing through pyrometallurgy andhydrometallurgy have been commercially applied to extractnickel metal. The pyrometallurgical routes method wassuitable for saprolitic nickel ore processing. The examplesof pyrometallurgy processing which have been commerciallyapplied are Blast Furnace and Rotary Kiln ­ Electric Furnace(RKEF).14) Conversely, limonitic nickel laterite ore wasprocessed through hydrometallurgical route. The examplesof hydrometallurgical route for limonitic nickel laterite oreprocessing is High Pressure Acid Leaching (HPAL).15­17) Anumber of hydrometallurgy processes have been applied inindustrial scale, but this application cannot meet the need ofproduction level and its operational cost was too high.16­19)

In addition to the hydrometallurgical process, a combinedprocess between pyro and hydro is also carried out to processlimonitic nickel ore.20,21) Using this method, the nickelrecovery obtained is about 80%, so this process is generally+Corresponding author, E-mail: sungging@mat-eng.its.ac.id

Materials Transactions, Vol. 60, No. 11 (2019) pp. 2245 to 2254©2019 The Japan Institute of Metals and Materials REVIEW

not economical.17) Figure 1 below shows the processingroute of nickel laterite ore based on nickel ore types.

2.1 Limonitic nickel oreLimonitic nickel ore contains iron oxide in the form of

goethite (¡-FeO·OH). The limonitic nickel ore is alsorelatively rich in cobalt and chromium. Iron oxide fromlimonitic nickel ore has an amorphous crystal structure witha crystal size in a nanometer scale and a large surface area.These characteristics can cause the absorption of largeamounts of Al3+ ions from the soil. Iron oxide is rarelypresent in laterites in pure form because of the substitution ofiron ions.22) In nature, the majority of limonite has dominantcompound of 2Fe2O3·3H2O. The compound has amorphousstructure, jelly as iron oxide and amorphous hydroxide.11)

The process of dehydroxylation of goethite into hematite(in oxidation state) occurs due to heating and it is the basis ofsome manufacturing processes. The process of dehydrox-ylation of goethite is a complex process as shown below.23)

¡-FeOOH ðgoethiteÞ ! Fe5=3ðOHÞO2 ðprotohematiteÞ! Fe11=6ðOHÞ1=2O5=2 ðhydrohematiteÞ ! ¡-Fe2O3

The first transformation of goethite is the release of OHgroup-which causes a significant change in crystal structure.The newly formed hematite has an imperfect crystal lattice.At 800°C, recrystallization and grain growth occur. However,there is a reduction in the effect of size caused by highconcentration of impurities. This is partly prevented by thediffusion of atoms into the crystal lattice of hematitesubstitutionally resulting in significant increase in structuraldisorders.24)

The water contained in limonitic nickel ore is classifiedinto free water, crystalline water and hydroxyl group.According to the types, while heating, there are severalphenomena happening. At temperature of 25­140°C, the firstphenomenon is the removal of free water. Then, at atemperature of 200­480°C, the second phenomenon is theremoval of crystal water. Subsequently, at a temperature of500­800°C, the third phenomenon is the removal of hydroxylgroup. The goethite dehydroxylation process for limoniticnickel ore is the highest dehydroxylation process comparedto other lateritic nickel ore types. It occurs at temperatures of261­270°C, while the removal of the hydroxyl group in thelimonite nickel ore occurs at a temperature of 400­600°C.12)

The removal of hydroxyl groups in chlorite (Fe,Mg,Al)3-(Si, Al)2O5(OH)4 starts at 602°C and ends at 760°C. Thechlorite group, then decomposes into MgO and SiO2. Allcrystal waters of serpentine Mg21Si2O28(OH)34H2O disappearabove 480°C, which will form Mg3Si2O5(OH)4, and itshydroxyl group is lost at 580°C and 700°C. With the onsetof this reaction, Mg3Si2O7 is formed, then the compound will

decompose at 795°C.25) So, above 795°C, the dominantphases are Fe2O3, MgSiO3 and Mg2SiO4.

2.2 Saprolitic nickel oreNickel in saprolitic nickel ore occurs in the form of

garnierite (Ni3Mg3Si4O10(OH)8). The garnierite minerals willbe decomposed while heating at 700°C. The decompositionof garnierite is shown in eq. (1).26)

Ni3Mg3Si4O10ðOHÞ8ðsÞ ! 3NiOðsÞ þ 3MgOðsÞþ 4SiO2ðsÞ þ 4H2OðgÞ ð1Þ

In addition to garnierite, the minerals contained in saproliticlaterite ore are goethite, serpentine, and quartz. When thetemperature is raised to 400°C, the phases of ore change. Allof phases will transform to serpentine. There are two typesof dehydroxilation process occurred when the saproliticlaterite ore was heated. The first dehydroxilation ofserpentine is completed when the temperature of the processis above 650°C. The second dehydroxilation process iscompleted at temperature of 750°C.27) A detailed study ofthermal treatment of garnieritic nickel ore was conductedby Yang, J.28) The initial phase of ore are chlorite((Mg, Fe, Ni)6(Si,Al)4O10(OH)8), talc ((Mg, Fe, Ni)3(Si,Al)4-O10(OH)2), quartz (SiO2), and hematite (Fe2O3). The resultshowed that roasting at lower temperature of 400°C and500°C made no change in minerals phase. However, whenthe temperature reaches 600°C, the minerals are dominatedby chlorite and partially dehydrated chlorite. When thetemperature is raised to 700°C, a number of serpentine wasdecomposed into forsterite (Mg2SiO4). Above 800°C, thechlorite transformed into forsterite and enstatite (MgSiO3). At1000°C, talc minerals transformed into forsterite and enstatiteas well. So, after 1000°C, the phases of ore are forsterite,enstatite, hematite and quartz. When the temperature is raisedto 1300°C, the phase of saprolitic nickel ore changes intoa complex mineral. The mineral is dominated by olivine(Mg0.5Fe0.5)2SiO4.29)

2.3 Direct reduction of laterite nickel ore in industrialscale

Most laterite nickel ore processing is carried out usingElkem process. The Elkem process uses Rotary Kiln andElectric Furnace (RKEF) to reduce and smelt laterite nickelore. Commercialization of Elkem process itself began withthe development of a pilot plant in 1953­1954. At that time,the Elkem process was used to process garnieritic nickel orein New Caledonia. Furthermore, the Elkem process under-went significant development and increased productioncapacity. Until now, the Elkem process has been widelyapplied in various countries in the world, including Brazil,Japan, New Caledonia, Indonesia, Yugoslavia, Colombia,

Fig. 1 Nickel laterite ore classification and its processing methods.

S. Pintowantoro and F. Abdul2246

etc.30) In Rotary Kiln, nickel ore was calcined at 1000°C andthen smelting is carried out in Electric Furnace. Molten Fe­Ni and slag were tapped out from Electric Furnace at 1600°Cfor molten Fe­Ni and 1400°C for molten slag.31)

However, the Elkem process requires an enormouselectrical energy. Therefore, Nippon Yakin modifies theKrupp-Renn process. The Krupp-Renn process itself is aprocess of direct reduction and smelting of iron ore using arotary kiln with a length of 70 meters and a diameter of 4.2meters. Iron ore was smelted in semi-fused condition withoutfully melted. The advantages of the Krupp-Renn processinclude: 1) Investment costs are relatively low, 2) Productshave relatively few impurities due to low smelting temper-atures, 3) Does not require metallurgical coke and 4) Able todirectly produce crude steel.32) Based on the Krupp-Rennprocess, Nippon Yakin modifies it so that it can be used toprocess laterite nickel ore. Furthermore, the process isreferred to as “Nippon Yakin Oheyama Process”. TheNippon Yakin Oheyama Process is one of the nickel oreprocessing that utilizes the principle of direct reduction andhas been proven commercially. At the beginning of itsconstruction, the Nippon Yakin Oheyama Process had a lowrecovery of Ni (80%) because it only uses rotary kilnswithout any pretreatment process. However, with modifica-tions of the preheater, dust scrubber, burner and thebriquetting process, the Nippon Yakin Oheyama Processcan achieve a recovery Ni of 95% and a production capacityof 9585 Ton Ni/year. The total energy requirement of theNippon Yakin Oheyama Process is about 5,000 kWh/Ton Ni.

The Fe­Ni particle products were referred to as “Luppe”which still contains 2% of slag.33) Luppe has Fe content ofbetween 75­80% and Ni between 25­20%, particle size ²0.1and an average diameter of 1mm. The Ni losses due to thepresence of fine Fe­Ni particles (in the size of µm) whichis joined in the slag. Therefore, in the Nippon YakinOheyama Process, the growth of Fe­Ni particles has animportant role in determining Ni recovery. One of the factorsthat affect the growth of Fe­Ni particles, namely softeningslag34) and the formation of secondary melt (Low MgO·FeO,high SiO2·CaO·Al2O3 silicate) at 1250­1300°C.35) In theoperation, there are major problems that disrupt Rotary Kilnproductivity, namely the formation of Slag Ring around theinner walls of the Rotary Kiln.36) The Nippon Sure OheyamaProcess flow sheet process is shown in Fig. 2, while thetypical Luppe composition is shown in Table 1.

3. The Selective Reduction of Laterite Nickel Ore

Selective reduction of nickel aims to greatly reduce nickeloxide into metallic nickel and to maintain the iron oxideunreduced or to minimize the reduction of iron oxide intometallic iron. With this selective reduction process, thereduction product will have high Ni content and moderate Fecontent. Selective reduction of nickel is strongly influencedby the thermodynamics of the reduction reaction of nickeland iron compounds in laterite nickel ore. Many researchershave conducted research and modelling on thermodynamicaspects in the selective reduction of laterite nickel ore.2,17,37)

Fig. 2 Flowsheet of Nippon Yakin Oheyama Process for production of Luppe from Laterite Ni-ore, Adopted from Refs. 33, 34).

Table 1 Typical composition of Luppe.33)

Selective Reduction of Laterite Nickel Ore 2247

From research and modelling that have been done, thereare several thermodynamic aspects that are important ininfluencing selective reduction, including: 1) Potentialreduction (CO/CO2 ratio), 2) Reduction temperature, and3) Phase/compound transformation. Carbothermic reductionreactions that occur are shown by eq. (2)­(7):37)

NiOðsÞ þ COðgÞ ¼ NiðsÞ þ CO2ðgÞ ð2ÞNiFe2O4ðsÞ þ COðgÞ ¼ NiðsÞ þ Fe2O3ðsÞ þ CO2ðgÞ ð3Þ3Fe2O3 þ COðgÞ ¼ 2Fe3O4ðsÞ þ CO2ðgÞ ð4ÞFe3O4 þ COðgÞ ¼ 3FeOðgÞ þ CO2ðgÞ ð5ÞFeOþ COðgÞ ¼ FeðsÞ þ CO2ðgÞ ð6ÞFeSiO4ðsÞ þ 2COðgÞ ¼ 2Feþ SiO2ðsÞ þ 2CO2ðgÞ ð7Þ

From eq. (2)­(7), it can be seen that thermodynamically, thecontrol of CO and CO2 can affect the occurrence of Ni andFe reduction reactions. In selective reduction Ni, as far aspossible the reduction of iron oxide should be prevented sothat the reduction reaction of iron oxide only occurs untileq. (5). So, there is still a lot of iron metal that has not beenformed. In other words, in the process of selective reductionof laterite nickel ore, Fe is a kind of opponent of selectivereduction of Ni. In order to get Fe­Ni alloy products (whethercontaining ferronickel, Fe­Ni crude oil or NPI) with higherNi grade, it must inhibit the process of iron oxide reduction tometallic iron or carrying out a compounding process betweeniron and the other elements (such as with oxygen, sulfur orchloride).

Generally, the higher the potential reduction and temper-ature reduction, the higher Ni recovery and the lower the Nigrade. In addition, the presence of non-oxide iron compoundswill make the reduction of Fe into metals will be inhibited.Thermodynamic relationships between reduction potential,temperature and compounds formed to the degree of nickelmetallization are shown by stability diagram (Fig. 3) whichhas been studied by Hallet, 199738) and Elliot R., 2016.37)

Based on Fig. 3, it appears that the final product of thecarbothermic reduction process will be affected by two mainthings, namely temperature and reduction potential (ln CO/CO2). The combination of these two process variables willaffect the final product of reduction. In general, to produce

the final product in the form of metallic Fe­Ni (100%metallization), the higher the reduction temperature, thehigher the reduction potential needed. For example, if thereduction process occurs at a low temperature (300­600°C),then a moderate reduction potential is needed. On the otherhand, if the reduction process occurs at high temperatures(900­1100°C), a high reduction potential is needed. For moredetails, analysis will be carried out for each zone.Zone 1

Zone 1 is a zone where the reduction product is an Fe­Nialloy. So all Fe and Ni are ideally reduced. In this zone,theoretically, recovery of Fe and Ni can reach 100%. Whilethe grade of Fe and Ni depends on the content of Fe and Ni ofnickel ore being processed. For example, if a limonite ore tobe processed has a Ni content of 1.2% and Fe of 37%, thenthe Ni grade in this zone is: (1.2/(1.2 + 37)) © 100% =3.14%. The Ni grade is in accordance with the assumptionthat there are no other impuritic elements present in Fe­Nialloys. In this zone, the Ni grade is still low, especially if thenickel ore used has a high Fe content. In general, zone 1 canbe easily achieved by reducing the reduction temperature andincreasing the reduction potential by increasing the CO/CO2

ratio. By increasing the ratio of CO/CO2 it will causeatmospheric conditions to be very reductive, causing manyoxides of reduced Fe to become metals. As a result, Ni gradewill decrease while grade Fe will increase. Because thereduction temperature tends to be low, the reduction reactionrate will run slowly in this zone. Thus, the level ofproductivity in this zone is relatively lower.Zone 2

Zone 2 is a zone where the reduction product is(Fe,Ni)1¹yO + (Fe,Ni). In this zone, there are still exist ironand nickel oxide compounds. The iron and nickel oxides inthis zone have an oxidation number of +2. The reductionproduct in Zone 2 has a higher Fe and Ni content than thereduction product in Zone 3. In addition, the recovery ofFe and Ni is also higher compared to Zone 3. Zone 3 can beachieved by using a moderate CO/CO2 ratio and hightemperature. Zone 2 is the most suitable zone to be used as areference for selective reduction process because the producthas a moderate Ni grade and high Ni recovery. In addition,with a higher temperature applied, then qualitatively, thereduction reaction rate will be faster than the other zones.Zone 3

Zone 3 is a zone where the reduction product is(Fe,Ni)O·Fe2O3 + (Fe,Ni). In this zone, there are still existnickel and iron oxide. Ni in the form of NiO and Fe in theform of FeO and Fe2O3. From the reduced compounds, itappears that in zone 3, there is still a lot of Fe that has notbeen reduced to metal, because FeO and Fe2O3 are stillfound. Therefore, zone 3 has a reduction product that has thehighest Ni content. To reach Zone 3, a low reductiontemperature and a moderate CO/CO2 ratio are needed. Witha low temperature and a low ratio of CO/CO2, the reductionof Fe into metals will be inhibited, so Fe is still present in theform of FeO or Fe2O3. As a result, the reduction product willhas high Ni content.Zone 4

Zone 4 is a zone where the reduction product is(Fe,Ni)O·Fe2O3. In this zone, Ni and Fe metallization has

Fig. 3 The stability diagram of Fe­Ni­O system for limonitic nickel orecontaining 1.2wt% Ni 1997.37,38)

S. Pintowantoro and F. Abdul2248

not occurred. Thus, Zone 4 is not desirable in reductionprocess, especially in selective reduction process. Zone 4 willbe occurred when the CO/CO2 ratio is too low and thetemperature is too high. The low CO/CO2 ratio makes COinsufficient for the reduction process and makes the reductionatmosphere not reductive. Thus, Fe and Ni oxides have notbeen reduced to metal.

On the other hand, the addition of additives can also beused to increase the selective reduction of nickel. Theseadditives are classified into two groups, namely additivesin the form of sulfocompound (Na2SO4

39,40) and CaSO441))

and additives in the form of chlorine compounds (MgCl2,42)

NaCl43) and CaCl2·H2O44)). In additive additions in the formof sulfocompounds, selective reduction occurs when Fe inFe­Ni or iron oxide reacts with sulfur compounds to produceTroilite (FeS) compounds. On the other hand, in addition toadditives in the form of chlorine compounds, selectivereduction occurs when Ni oxide and Fe react with Cl to formNiCl2, FeCl2, and FeCl3. While some other iron oxides willreact with CaO or MgO to form CaFe2O4 or MgFe2O4. TheNiCl2 and FeCl2 then react with H2 to form Fe­Ni alloys.With controlling of additives dosage, the chloridization ofFe can be controlled. So, there is still Fe in the form of oxide.As a result, iron oxide will not be completely reduced by H2

and the Ni grade will be higher.43)

Despite increase the tendency for selective reduction of Ni,the additive addition will increase Ni recovery and enlargeFe­Ni particle products because it can trigger the formationof compounds that have low melting points on the surface ofFe­Ni particles and promote the formation of compounds that

have a low melting point at impurity around Fe­Ni particles,such as NaO, CaO or MgO. The difference mechanism ofsulfo and clorin compounds on selective reduction of Ni isshown in Fig. 4. To study the mechanism for selectivereduction for each additive, a selective reduction mechanismwill be described in the next section for each type of additiveand its reaction.

Temperature which can be used for selective reduction is inthe range of 1100­1450°C. Too low temperature will causeslow reaction rate.37) On the other hand, too high temperaturewill cause the formation of a liquid phase.45) In addition,the high temperature reduction causes the possibility of theformation of olivines getting higher, thereby lowering thereduction of Ni. In addition, too high reduction temperaturecauses reduction of chromium and silicon oxide compounds,so Cr and Si will also dissolve into the Fe­Ni crystal lattice.As a result, both Ni and Fe contents will decrease.46) Thus,the optimal temperature for selective reduction is at1200°C.39) In his research, the selectivity factor achievedwas 21.8. Even so, when considering the size of Fe­Niparticles formed, the optimal temperature is at 1420°C. Thisis because the size of Fe­Ni particles obtained is incentimeter size.47) So from the industry perspective, it ismore efficient because Fe­Ni recovery can be maintainedduring magnetic separation process. The role of each additivein selective reduction of Ni will be described below.

3.1 Na2SO4 additiveNa2SO4 can increase the selective reduction of Ni. This

happens because Na2SO4 will decompose to Na2S, Na2O, and

(a)

(b)

Fig. 4 Mechanism of additive on promoting selective reduction of Nickel for: a) Sulfo compound additives and b) Chloride compoundadditives. (Adapted from Ref. 43)).

Selective Reduction of Laterite Nickel Ore 2249

S under reducing atmospheric condition. The decompositionreaction is shown in eq. (8)­(10).39,40,47­50)

Na2SO4 þ 4CO ! Na2Sþ 4CO2 ð8ÞNa2SO4 þ 3CO ! Na2Oþ Sþ 3CO2 ð9ÞNa2SO4 þ 4C ! Na2Sþ 4CO ð10Þ

The reduction of Na2SO4 occurs at the temperature range of850­900°C.47) The products of Na2SO4 reduction are Na2O,Na2S, CO and CO2. Na2O can improve Ni recovery becauseNa2O will react with iron silicate and form sodium silicateas can be seen in eq. (11).39) The sodium silicate compoundshave low melting temperatures, thus accelerate the rate ofaggregation of metal particles.

Na2Oþ 2Fe2SiO4 ! 4FeOþ Na2Si2O5 ð11ÞOn the other hand, S can significantly increase the grade ofnickel. Further, iron recovery is decreased due to theformation of Troilite (FeS) as can be seen in eq. (12).39,51,52)

Feþ S ! FeS ð12ÞMeanwhile, Na2S can also reduce Fe recovery because Na2Swill react with FeO to form FeS as can be seen ineq. (13).39,40) Therefore, FeO can no longer be reduced byreducing agents.

Na2Sþ FeOþ 2SiO2 ! FeSþ Na2Si2O5 ð13ÞTroilite acts as an activating agent to facilitate the formationof the melting phase and then increase the transport rate toaccelerate the aggregation of Fe­Ni particles.40) This FeScompound cannot be drawn by the magnet in the followingprocess after reduction, thus increasing the Ni grade. Sulfurcan cause an increase in the surface of metal particles and candecrease the surface tension of metal particles, therebyincreasing the growth of metal particle size. However, Na2Owill react with silicates and form minerals with low meltingtemperatures.39) The mechanism of selective reduction of Niby using Na2SO4 additive is shown in Fig. 5.

3.2 MgCl2 additiveThe addition of MgCl2 may improve Ni grade and Ni

recovery. The process is called as process of segmentation ofchloridation. In this process, chlorination of Ni, Co and Feoxide from laterite nickel ore happen. First of all, there is aninteraction of MgCl2 with water content in the moisture andSiO2 from the laterite nickel ore impurities. The reactionproduces magnesium silicate and HCl compounds as can beseen in eq. (14).42)

MgCl2 þ SiO2 þ H2O ¼ MgO�SiO2 þ 2HCl ð14ÞThen, HCl will react with NiO and Co2SiO4 to form nickelchloride and cobalt chloride compounds as can be seen ineq. (15) and (16).42)

NiOþ 2HCl ¼ NiCl2 þ H2O ðT � 600KÞ ð15ÞCo2SiO4 þ 4HCl ¼ 2CoCl2 þ SiO2 þ 2H2O

ðT � 600K{900KÞ ð16ÞSubsequently, NiCl2 and CoCl2 will react with carbon in thereductant and with water content in the moisture resulting inNi and Co metal as can be seen in eq. (17) and (18).42)

NiCl2 þ Cþ H2O ¼ Niþ 2HClþ CO ðT � 900KÞ ð17ÞCoCl2 þ Cþ H2O ¼ Coþ 2HClþ CO ðT � 900KÞ ð18ÞThe higher MgCl2 added, the higher Ni grade and Nirecovery will be. However, when the addition of MgCl2 ishigher than 6%, the grade and recovery of Ni no longerchange significantly. Hence, an important point in theselective reduction of Ni using the addition of MgCl2additive is the amount of HCl formed from the MgCl2reaction with H2O from the moisture feed.42)

3.3 CaSO4 additiveCaSO4 may increase the selective reduction of Ni from the

reduction process of limonite laterite nickel ore. This isbecause CaSO4 will produce reaction of fayalite (FeSiO3)formation. Thermodynamically, CaSO4 will decompose intoS2 (g), O2 (g), and CaO under reducing atmosphericconditions. CaO will then react with SiO2 and FeO to formkirschsteinite (CaFeSiO4) which will accelerate the formationof fayalite. In addition, S2 (g) will react with the iron oxide ofFe and Ni. Thus, the reaction result of S2 (g) and Fe will forman iron sulphide having a low melting point. As a result, itcauses an increase in particle size of Fe­Ni metal. Thereactions occurred during the process of reduction of limonitelaterite nickel ore with the addition of CaSO4 additives are asfollows (eq. (19)­(30)).41)

2Fe3O4 þ 2COðgÞ ¼ 6FeOþ 2CO2ðgÞ ð19Þ2=5Fe3O4 þ S2ðgÞ ¼ 6=5FeSþ 4=5SO2ðgÞ ð20Þ4=3FeOþ S2ðgÞ ¼ 4=3FeSþ 2=3SO2ðgÞ ð21Þ

Fig. 5 Mechanism of Na2SO4 in selective reduction of laterite nickel ore.

S. Pintowantoro and F. Abdul2250

CaOþ FeOþ SiO2 ¼ CaFeSiO4 ð22Þ2FeOþ 2COðgÞ ¼ 2Feþ 2CO2ðgÞ ð23Þ2NiOþ 2COðgÞ ¼ 2Niþ 2CO2ðgÞ ð24Þ2CoOþ 2COðgÞ ¼ 2Coþ 2CO2ðgÞ ð25Þ2Feþ S2ðgÞ ¼ 2FeS ð26Þ2Niþ S2ðgÞ ¼ 2NiS ð27Þ2Coþ S2ðgÞ ¼ 2CoS ð28Þ2Feþ 2NiS ¼ 2FeSþ 2Ni ð29Þ2Feþ CoS ¼ 2FeSþ 2Co ð30Þ

From the above reaction, it appears that the increase in Nicontent depends on the inhibition of FeO reduction to Fe dueto the formation of fayalite or kirschsteinite. Thus, a sufficientamount of SiO2 is required so that FeO is not reduced into Fe.Limonite ores have high Fe content and low SiO2, so mixinglimonite and saprolite nickel ore is required.41)

3.4 NaCl additiveSimilar to MgCl2, NaCl is added as an additive to become

the chlorination agent Ni and Fe ore laterite nickel. As for theMgCl2 reaction, in the presence of moisture, Ni and Fe oxideswill be chlorinated by HCl, which is formed from NaClpyrohydrolysis process with a SiO2 catalyst. HCl then reactswith Ni and Fe oxides to form chlorine compounds, NiCl2,and FeCl2 (eq. (31)­(33)).43,44)

2NaClþ SiO2 þ H2O ¼ Na2O�SiO2 þ 2HCl

ðT � 2403KÞ ð31ÞFeOþ 2HCl ¼ FeCl2 þ H2O ðT � 647KÞ ð32ÞNiOþ 2HCl ¼ NiCl2 þ H2O ðT � 1021KÞ ð33Þ

In addition, iron and nickel chlorine will react with moistureas can be seen in eq. (34)­(36).

NiCl2 þ 6H2O ¼ NiCl2�6H2O ðT � 179KÞ ð34ÞFeCl2 þ 2H2O ¼ FeCl2�2H2O ðT � 214KÞ ð35ÞFeCl2 þ 4H2O ¼ FeCl2�4H2O ðT � 192KÞ ð36Þ

Then, the NiCl2, FeCl2 and CoCl2 will react (water gasreaction) with C and H2O to form Ni, Fe and Co as can beseen in eq. (37)­(39).

NiCl2 þ Cþ H2O ¼ Niþ 2HClþ CO ðT � 900KÞ ð37ÞFeCl2 þ Cþ H2O ¼ Coþ 2HClþ CO ðT � 900KÞ ð38ÞCoCl2 þ Cþ H2O ¼ Niþ 2HClþ CO ðT � 900KÞ ð39ÞOn the other hand, oxide compounds in nickel ore will reactin accordance with eq. (40)­(43).

2Mg2SiO4 þ SiO2 ¼ Mg2SiO4 þ 2MgSiO3

ðT � 24{1098KÞ ð40ÞFe2O3 þMgO ¼ MgFe2O4 ðT � 1473KÞ ð41ÞFe2O3 þ NiO ¼ NiFe2O4 ðT � 1473KÞ ð42ÞMgNiSi2O6 ¼ MgSi2O5 þ NiO ðT � 1473KÞ ð43Þ

At the end of the process, the compounds formed includeMg2SiO4 (Most dominant), MgFe2O4, NiFe2O4, SiO2,MgNiSi2O6 and Fe2O3.43)

3.5 CaCl2·2H2O additiveSimilar to NaCl, the mechanism of CaCl2·2H2O promotes

selective reduction of Nickel in lateritic nickel ore throughchloridation. First, CaCl2·2H2O was decomposed into CaCl2and 2H2O. The reaction is shown in eq. (44) below.53)

CaCl2�2H2OðsÞ ¼ CaCl2ðsÞ þ 2H2OðgÞ ðT � 20KÞ ð44ÞThen, the CaCl2 will react with SiO2 (in ore) and H2O (formmoisture of ore and decomposition product of CaCl2·2H2O)to form CaSiO3 and HCl as shown in eq. (45).44)

CaCl2ðsÞ þ SiO2ðsÞ þ H2OðgÞ ¼ CaSiO3ðsÞ þ 2HClðgÞðT � 1168KÞ ð45Þ

The HCl gas product has a role in the chloridation process forselective reduction. HCl(g) will react with NiO from lateriticnickel ore to form NiCl2. The reaction is shown in eq. (46).44)

NiOðsÞ þ 2HClðgÞ ¼ NiCl2ðsÞ þ H2OðgÞ ðT � 1021KÞ ð46ÞOn the other hand, carbon from reductor will react with H2Oto form CO gas and H2 gas. The reaction is shown ineq. (47).44)

CðsÞ þ H2OðgÞ ¼ COðgÞ þ H2ðgÞ ðT � 947KÞ ð47ÞFinally, the NiCl2 will react with H2 gas or directly react withcarbon and H2O to form metallic Nickel. Both reactions areshown in eq. (48) and (49).44)

NiCl2ðsÞ þ H2ðgÞ ¼ NiðsÞ þ 2HClðgÞ ðT � 720KÞ ð48ÞNiCl2ðsÞ þ CðsÞ þ H2OðgÞ ¼ COðgÞ þ NiðsÞ þ 2HClðgÞ

ðT � 831KÞ ð49Þ

4. Novel Research of Selective Reduction of LateriteNickel Ore

The summary of novel research of selective reductionof lateritic nickel ore is shown in Table 2. There are twoparameters to compare each method of experiment, i.e.selectivity factor and separation efficiency. The selectivityfactor shows the level of selective reduction of nickel thatoccurs. On the other hand, the separation efficiency showsefficiency of the process that occurs and this can be relatedto economical consideration.

The selectivity factor was calculated using eq. (50).Conversely, the separation efficiency was calculated usingeq. (51).

Selectivity Factor ¼ XNiYFeXFeYNi

ð50Þ

Where: X is the grade of Ni or Fe in lateritic nickel ore and Yis the grade of Ni or Fe in reduction product.54)

Separation Efficiency ¼ 100RNimðYNi �XNiÞðm�XNiÞXNi

ð51Þ

Where: RNi is the Recovery of Nickel, m is percentage ofnickel in valuable mineral (assumed in the form of NiO, som is 78.58%), YNi is the Nickel grade in reduction product,XNi is the nickel grade in lateritic nickel ore.55,56)

Table 2 shows the summary of novel research aboutselective reduction of nickel obtained from limonitic,saprolitic, and other types of laterite nickel ore. The bestseparation efficiency of limonitic type nickel ore processedis conducted by Elliot, R.37) using carbon as reductant with

Selective Reduction of Laterite Nickel Ore 2251

7.07 kg of carbon of 100 kg ore. Nevertheless, there is no datashowing the selectivity factor for its process. Finally, the bestmethod is conducted by Lu, C.57) The study used 8wt% ofcoal and 20wt% of Na2SO4 as additive. The selectivity factorof this method is 6.01 and the separation efficiency of thismethod is 69.93%. On the other hand, when saprolitic nickelore is processed, the best method was conducted by Ref. 58).The study used coal and composite additive of 14wt%. Usingthis method, separation efficiency of 70.36% was obtained.However, the selectivity factor was still lower than othermethods. And for the best selectivity factor for saproliteprocessing, it was conducted by Ref. 39). Coal was used asreductor and 10wt% of Na2SO4 as additive. As a result,selectivity factor of 21.8 (The highest selectivity factoramong other methods) was obtained.

5. The Feasibility and Potential Problems of AdditivesAddition in the Process

The selective reduction process using a number ofadditives is proven to be able to increase the selectivityfactor, thus producing products with high Ni grades.However, to date, the process of selective reduction usingadditives is still at the research stage and has not been appliedon a pilot or industrial scale. That is caused by severalfactors, including:1. Production cost

In the selective reduction process using additives, theaverage additive is needed as much as between 8­10wt% of the entire raw material used. Moreover, todate the selective reduction studies have used analytic

Table 2 Summary of novel research of selective reduction of lateritic nickel ore.

S. Pintowantoro and F. Abdul2252

grade additives. Thus, the cost of procuring rawmaterials will increase significantly. On the other hand,product sales will be based on the tonnage of the nickelmetal produced. Thus, increasing Ni levels does notsignificantly influence product sales revenue. On theother hand, there is an increase in production costs dueto the addition of additives for selective reduction ofNi. Therefore, there is a need for a complete economicfeasibility analysis to determine whether this selectivereduction process will provide financial benefitscompared to the reduction process currently applied.

2. The formation of slag which contains compounds thatare harmful to industrial operationsAddition of additives which have the element of Na,will produce slag with high NaO content. Althoughfrom the viewpoint of Ni recovery, it will help increaseNi recovery because NaO will significantly reduce theviscosity, softening temperature, and melting temper-ature of the slag. However, an increase in NaO contentwill cause slags to become very corrosive to reactorrefractories. If it left for a long time, the thickness ofthe refractory will decrease due to high temperaturechemical reaction between the slag (with a high NaOcontent) and the refractory constituent compounds. If itis related to production costs, the reactor maintenancetime will be faster and cause an increase in maintenancecosts.

3. Byproducts in the form of gases that harm theenvironmentIn the selective reduction process using additives, theadditives used are sulfate compounds or chloridecompounds. To have a selective effect, S or Cl onadditives will react with Fe to reduce the total mass ofFe in the product and to reduce the interface energy ofthe particles. In the perspective of selectivity reduction,it has been proven to improve Ni grades. However,there are potential problems, namely environmentalissues as an impact on the addition of this additive. Inthe pyrometallurgy process, all processes utilize hightemperature and there are many transport phenomenathat occur. Thus, there is also the possibility of theformation of toxic gases such as H2S and HCl as abyproduct of selective reduction reactions usingadditives. The gas will be dangerous both to theoperator and the environment.

6. Conclusion

Nickel selective reduction is a challenge in ferronickelproduction research through carbothermic reduction. Thereare several criteria in nickel selective reduction process.The criteria needed for increasing Ni grade during thecarbothermic reduction process are:(1) Optimum Reduction Potential (CO/CO2 gas ratio)

The reduction potential will be affected by the CO/CO2

gas ratio. In general, for laterite nickel ore, a decrease inreduction potential (by reducing the CO/CO2 gas ratio)will increase the nickel grade. The increase in Ni gradeis caused by the low reduction potential which willprevent reduction of Fe oxide to Fe metal. In the case of

carbothermic reduction, the use of C (from the reducingagent)/O (from iron oxide and nickel oxide) ratio is0.75­0.80.

(2) Optimum Control of Reduction TemperatureThe optimal reduction temperature for selectivereduction of nickel is at 1100°C­1400°C. When thereduction temperature is higher, it will increase thepossibility of formation of olivine ((Mg, Fe, Ni)2SiO4).As a result, the reduction of nickel oxide will be moredifficult.

(3) Promotion of Iron Compound Formation ReactionThe formation of iron compounds has an important rolein the selective reduction process of laterite nickel ore.The formation of iron compounds is facilitated by theaddition of additives during the reduction process.Additives proven to be able to encourage selectivereduction are sulfate or chloride compounds. For sulfocompounds, the sulfur content in additives will facilitatethe formation of FeS (Troilite) reactions. The FeScompounds will then separate from Fe­Ni metal andmake a mixture with slag. While for chloridecompounds, the Cl content in additives will facilitatethe formation of NiCl2 and leave Fe2O3 or FeO stillunchloridized (with controlling of chloride additivedosage addition). To obtain a high nickel selectivityfactor, 20wt% Na2SO4 additives can be used forlimonitic nickel ore and 10wt% Na2SO4 additives canbe used for saprolitic nickel ore.

Until now, the application of selective reduction of Ni inindustrial scale still requires a more detailed study, such asstudies of economic feasibility, impacts on the environmentand impacts on unit operations and unit processes. Therefore,in the future, these studies are important to be carried out, soit can be known whether direct reduction with the additionof additives is feasible to be applied on a pilot plant scale oreven on an industrial scale.

Acknowledgements

The authors express gratitude to Institut Teknologi SepuluhNopember for the financial support provided for thisresearch.

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