I. PAULIN ET AL.: MECHANISMS OF HF BONDING IN DRY SCRUBBER IN ALUMINIUM ELECTROLYSIS

MECHANISMS OF HF BONDING IN DRY SCRUBBER INALUMINIUM ELECTROLYSIS

MEHANIZMI VEZAVE HF V ^ISTILNEM SISTEMU PRIELEKTROLIZI ALUMINIJA

Irena Paulin1,2, ^rtomir Donik2, Monika Jenko2

1 TALUM, d. d., Kidri~evo, Tovarni{ka cesta 10, 2325 Kidri~evo, Slovenia2 Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia

irena.paulin�imt.si

Prejem rokopisa – received: 2009-04-21; sprejem za objavo – accepted for publication: 2009-05-26

Modern dry scrubbers in electrolytic winning of aluminium operate with very high efficiency, typically above 99 %, and all butthe fugitive fluoride losses are returned into electrolytic cell. The principle of purification of exhaust gases from electrolyticcells is adsorption of HF onto primary alumina in dry scrubber reactors, where gases from cells and flow of active (primary)alumina run into each other and react. It is very important how the reaction between gases and primary alumina takes place andwhich chemical mechanisms are taking part. Nevertheless, also industrial conditions are playing an important role in efficiencyof the scrubbing reaction.In references on dealing with related systems there can be found that most research was made to improve dry scrubbing systemsand their efficiency, but not much was done to explain chemical mechanisms of fluoride bonding on active alumina that isessential for the cleaning process of exhaust gases. Chemical and micro-chemical analyses enabled to determine the fraction ofbonded fluoride on the surface and in the interior of the fluorinated alumina grains. Also the correlation between the mostefficient fluoride bonding on active alumina and real industrial conditions was determined.Key words: chemical adsorption, HF-alumina interaction, fluoride bonding mechanism, EDS, dry scrubber

Ve~ina ~istilnih naprav pri proizvodnji primarnega aluminija ima u~inkovitost ve~jo od 99 % in ves ujeti fluor se vra~a v proceselektrolize. ^i{~enje plinov HF iz elektroliznih celic deluje po principu vezave fluora na primarno glinico, ki se dozira v reaktor~istilne naprave, skozi katero te~ejo plini. Zato je zelo pomembno, kako poteka vezava fluora na aktivno glinico in kak{ni sonajugodnej{i pogoji za to.V prej{njih raziskavah je bilo veliko narejeno na podro~ju u~inkovitosti ~istilnih sistemov, mehanizmi vezave fluora na aktivnoglinico pa {e niso bili podrobno raziskani. S kemijsko in EDS-analizo elementov smo ugotovili dele` vezanega fluora napovr{ini in v notranjosti zrn glinice. Poiskali smo povezavo med u~inkovitostjo mehanizma kemijske vezave fluora na aktivnoglinico in industrijskimi pogoji, pri katerih poteka ~i{~enje plinov, ki nastajajo med procesom elektrolize.Klju~ne besede: kemijska adsorpcija, interakcija glinica-HF, mehanizmi vezave fluora, EDS, ~istilni sistem

1 INTRODUCTION

Aluminium is one of the most important and widelyused metals in industry and in everyday life in modernworld, also. Pure aluminium is extracted by electrolysisof alumina (Al2O3) prepared from bauxite. Primaryaluminium is extracted from alumina with electrolyticprocess according to the 2 Al2O3 + 3 C (graphite) � 4 Al + 3CO2 (g) reaction. The process, with additives of cryolite,AlF3, CaF2 etc. into electrolyte, proceeds at temperaturesbelow 1000 °C and at constant DC, most frequently 180kA to 190 kA. Electrolytic cells are connected in seriesin order to keep constant electrical current in all the cellsaccording to the Ohm’s law.

Like in all the other industries, the major drivingforce in the primary aluminium industry over the last 20years has been to produce more metal of better quality atlower costs. But production of greater quantities of metalresults in significant increase of emissions. Hence,aluminium industry has developed unique industrialemission control and recovery systems. Fluorideemissions of the highest concern during the electrolyticprocess are those related to cryolite, (Na3AlF6), chiolite,

(Na5Al3F14), and atmolite, (NaAlF4) particulates, and toHF, CF4, and C2F6 gases1. The focus in this paper wasdirected to fluorides that were caught and bonded ontothe alumina surface during the cleaning process.

Minimizing the fluoride emission in aluminiumelectrolytic cells has many positive effects; reduction oflocal and global environmental impacts of the process,improved working environment as well as economiceffects and advantages in cell operation.

Efficiency of removal of fluorides from flue gases inmodern dry scrubbing systems is now higher than 99 %,and fluoride emissions from smelters are typically closeto 0.5 kg F/ton Al 2,3. Nevertheless, further reduction offluoride losses through emissions can lead to chemicallymore stable electrolyte and the level of fugitive fluorideemissions (emissions through the pot-room roof) is thusreduced, too. Fugitive emissions are estimated to be 4–5times higher than the stack ones3 of which up to 80 %may be represented by standard background emissions4–6.

In the past there have been some significant butlimited studies on exact contributions of various sources

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UDK 669.71:546.16:620.18 ISSN 1580-2949Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 43(4)189(2009)

to the total fluoride emissions from cells. 7,8 In 1963Henry 7 measured the influence of a number of variablesin 10 kA experimental aluminium reduction cells onparticulate and gaseous emissions. In 1972 Grjotheim etal. 8 prepared a review of available data for carefulassessment of contributions of important parameters.Mathematical models to predict total fluoride losseswere based on these data. 9,10 Over 30 years of researchbrought to improved efficiency of existent cleaningsystems and to an examination of fluoride bondingmechanism on active alumina in dry scrubbing reactors.

Principle of the dry scrubbing system is simple.Scrubbers operate by direct extraction of flue gases fromelectrolytic cells into reactors where gases react withactive alumina. At first, poisonous fluoride and HF areadsorbed from the gas mixture onto active aluminasurface. Then gases with active alumina are transportedthrough bag filters where solid sandy alumina withadsorbed gases is removed. The fluoride-enrichedalumina is after filtration returned back into electrolyticprocess. Effectiveness of the cleaning process dependson details of the system and the process. Continuoussustaining "equal parts of alumina" in the process gasstream represents the opportunity of modern dryscrubbers to reduce fluoride emissions11.

Most of dry methods essentially consist of adsorptionand chemisorption of HF from flue gases coming fromaluminium cells by active alumina. Research projectsmainly deal with various technologies and theirimprovements, rarely with exact mechanisms how HF isactually bonded onto raw alumina. Dando13 made reviewof experiments that were performed from 1970 till now.However, it is well known that more than one mecha-nism of HF bonding onto primary alumina was proposedbut none of existent hypotheses was undoubtedlyconfirmed with analyzing techniques, such as XRD,XPS, TGA, NMR etc., were used in various experimentsand analyses.

Primary focus in previous examinations of alumina –HF systems was directed to various types of alumina andtheir capacity for HF adsorption. The results of thesestudies have been used to propose mechanisms of HFadsorption on alumina. Two basic models emerged:1) physical adsorption of HF on the alumina surface

(hydroxyl groups bonded to surface and/or physi-sorbed by water) to form alternating layers of HF andH2O 14, or

2) direct chemisorption of HF at reactive sites on thealumina surface with formation of Na-F and/or Al-Fspecies and with additional monolayer beingphysically adsorbed on top of chemisorbed layer 15.However, Wagener et. al. observed that alumina

readily adsorbs HF under either hydrous or anhydrousconditions16. Some authors17,18 believe that certain num-ber of H2O molecules is needed to bind HF molecules tothe surface of alumina. Others19 believe that anAl2O3·nHF type of compound is formed which is trans-

formed into AlF3 when heated above 300 °C. Severalresearch works have shown that both mechanisms tookplace depending on process conditions. However,Wagener et. al.16 concluded that H2O did not play anessential role in the alumina-HF interaction. HF was notonly adsorbed physically, but it reacted with the exposedalumina surface to form an AlOF or Al(F,OH)3 type ofcompound. If a large excess of HF has passed overalumina also AlF3 was formed (and alumina particlesbecame soft and friable).

Our research represented a more theoretical analysisof chemical bonding reactions. There exist twopossibilities for HF bonding on primary alumina. Thefirst one is the Lewis acid-base complex and the otherone is the theory of hydrogen bonding. Lewis theoryclaims that acid can take electron pair and base candonate it. Al3+ is by definition Lewis hard base and F- isLewis hard acid. Due to their chemical properties theyform together most stable complex.

The other theory is the theory of hydrogen bonding.This theory was already well described in somepapers14,20,21. Basically, it is a surface process wherehumidity is present. The reaction mechanism involvesfew steps: H2O adsorption forming an aqueous layer onthe alumina surface; HF adsorption to acidify the surfacewater layer; dissolution of the alumina surface to formAlO2

– and AlOH2–, and precipitation of AlFx(OH)3–x ·nH2O. The overall adsorption rate is controlled by therate of the surface chemical reactions rather than bytransport of fluid phase, mass transfer, and intraparticlediffusion.

2 EXPERIMENTAL WORK

Smelting grade alumina samples, both primary andreacted, were analyzed as-received from TALUMcompany, Kidri~evo. The reacted alumina was preparedin industrial conditions. Primary alumina passed throughthe dry scrubber system where HF from electrolytic cellswas bonded in controlled reactor conditions to vapour-phase HF. The temperature in the dry scrubber systemwas about 90 °C (363 K). The flow rate of air suckedfrom electrolytic cells into dry scrubber system was of500 000 m3/h and all the air passed through six reactorsinto which new (primary) alumina was dosed.

Electron microscope was used to analyze bonding offluoride onto primary alumina. Sandy alumina beforeand after the dry scrubbing was examined in scanningelectron microscope (SEM – JEOL-JSM 6500F). Therewere made also micro-chemical analyses with energydispersive X-ray spectroscopy (EDS Oxford InstrumentsINCA x-sight ENERGY 400). This technique is ananalytical technique used for the analysis or of asample, limited to detection of about 0.1 % of chemicalelements and to 1–3 µm of analyzed volume, dependingon analyzed materials.

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Cross-section micro-chemical EDS analyses offluorinated alumina grains were made with larger grainsto find differences in fluoride concentrations on thesurface and in the interior of grains. Alumina sample wasprepared as metallographic sample with grinding andpolishing.

Chemical analyses of fluoride in the fluorinatedalumina were made by standard Pechiney method withion-selected electrode (Thermo ORION EA940, elec-trode: Thermo 9409 BM).

3 RESULTS AND DISCUSSION

Alumina grains had large surface area because oftheir morphology (Figure 1). Larger was the surfacemore of vapour-phase HF could be bonded onto aluminain reactors, and less HF escaped into environmentthrough the chimney after dry scrubbers.

Some authors22 used TGA, XRD and NMR andproposed that fluorides were adsorbed onto alumina in atleast 2 ways, i.e. as mobile and as rigid species. Thoughno real structures were identified, it was hypotheticallyproposed that adsorbed fluoride existed as surface-bonded species or as fluoride incorporated into thealumina structure network (Al-F-Al bonds), respectively.We have used EDS technique to analyze fluorinatedalumina grain to find differences in fluoride concen-trations on the surface of alumina grain and in itsinterior. Figure 2 shows fluorinated alumina grain with aline of discrete spots analyzed with the EDS.

Results of EDS analyses are presented in Figure 3.More fluoride was detected on the grain surface than inthe interior. This indicated that adsorption as well asdiffusion progressed from surface into interior. However,there was difference in intensity of fluoride bonding;greater amount of fluoride was found on the surface.Reduced oxygen content and increased fluoride content

were determined on the grain surface. The ratio ofelements detected with the EDS enabled to propose thatAlF3 and Al2O3 were the most probable compounds onthe grain surface. Alumina represented substratum on topof which aluminium fluoride was bonded.

Results confirmed the theory of Lewis acid-basecomplex since by definition Lewis hard acid Al3+ andLewis hard base F- most likely formed strong complex.Reaction: Al2O3 + 6 HF � 2 AlF3 + 3 H2O took place onthe surface of alumina grains. Less oxygen and morefluoride was present there (Figure 3). On the other hand,alumina content was constant through the whole graincross section.

However, the hydrogen bonding reaction was stilltaking place in the process. It depended on temperaturein reactors that varied with the surrounding temperatureand with the humidity level in reactors. Results ofchemical analyses of fluoride in fluorinated aluminafrom dry scrubbers enabled to determine fluoridecontents in different periods of year. In summer whenthe average day temperature of surrounding was between

I. PAULIN ET AL.: MECHANISMS OF HF BONDING IN DRY SCRUBBER IN ALUMINIUM ELECTROLYSIS

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Figure 3: EDS discrete-point line analysis of fluorinated aluminaSlika 3: Linijska analiza fluoriranega zrna glinice z EDS-tehniko

Figure 1: SE image of grain of aluminaSlika 1: Slika sekundarnih elektronov z vrsti~nim elektronskim mi-kroskopom zrna glinice

Figure 2: SE image of EDS discrete-point line analysis of fluorinatedaluminaSlika 2: Slika sekundarnih elektronov z vrsti~nim elektronskim mi-kroskopom zrna glinice z ozna~enimi mesti linijske analize EDSfluoriranega zrna glinice

20 °C and 25 °C (month average; every-day temperaturewas measured at 2 p. m.) also temperature in reactorswas higher, above 105 °C. In winter, temperature inreactors was only about 80 °C since the surroundingtemperature was about 0 °C. When temperature inreactors was above the temperature of water evaporationmore fluoride was bonded onto alumina. Thus in warmermonths, from June to September, more fluoride wasdetected in the fluorinated alumina (Figure 4). Inconditions when humidity was low more fluoride wasbonded as AlF3 compound by the principle of Lewisacid-base complex. In colder months when the tempe-rature in reactors did not exceed 85 °C, more H2O waspresent and more fluoride was bonded by hydrogenbonding mechanism. However, relation between thehumidity level and the fluoride bonding was obvious;higher temperature, less humidity, and more fluoride wasloaded onto alumina. Molecules of water occupied thesurface of alumina and thus less fluoride could beadsorbed. Mechanisms of possible bonding 13 ofvapour-phase HF on new (primary) alumina are shownin Figure 5. One possibility was that fluoride wasbonded directly on alumina (aluminium atoms) insteadof on terminal hydroxyls (OH groups) (Figure 5 A). Onthe other hand, fluoride bridging to alumina by hydrogenbonding could exist too. The most efficient bonding offluoride was achieved when fluoride was bonded as

bridging fluoride and also as terminal fluoride (Figure 5B).

Small amounts of fluoride were detected with theEDS analytical technique also in the interior of grains.Continuous adsorption of fluoride, in excess of surfacemonolayers, took place with diffusion of surface bondedfluor into the alumina lattice to displace oxygen inbridging sites. This could also occur by direct continuousreaction of alumina with vapour-phase fluoride todisplace the bridging oxygen, followed by continuousdiffusion of fluoride into alumina surface. Growth ofaluminium fluoride on surface impeded but did not stopthe continuous diffusion of fluor into the alumina latticethat could cause alumina decay in reactors due to limitedcontact time inside the reactor.

4 CONCLUSIONS

Comparison of data obtained in this study withobservations collected from previously publishedexaminations of related systems strongly supportsfindings on the dry scrubber reactor chemistry andretention of fluoride. Significant conclusions can bemade from chemical microanalyses of alumina grainsand relations between the fluoride bonding and industrialconditions depending on external conditions.

Micro-chemical analyses confirmed the theory offluoride bonding on the surface of alumina. A very smallamount of fluoride was also found in the interior ofgrains, but diffusion gradient was not large since thelayers of bonded fluoride on the surface have hinderedpenetration of fluor into the interior of grains.

Important finding of this paper was the correlationbetween chemical reaction of vapour-phase HF andprimary alumina in dry scrubber reactors and theindustrial temperature conditions. In the warmer seasonwhen average daily temperature was 20 °C to 25 °C,temperature in reactors increased to over 100 °C andchemical analyses confirmed higher contents of fluorideon reacted alumina. Less fluoride was detected inperiods when the average daily temperature was below 5°C and temperatures in reactors were around 80 °C. Thistemperature difference influenced the fluoride adsorp-tion, because it was related to chemical reaction that tookplace on the surface of alumina grains. Less fluoride wasbonded with hydrogen bridges and more of it waschemisorbed directly onto the surface as AlF3.

5 REFERENCES

1 M. M. Hyland, B. J. Welch, J. B. Metson, in Book Changing Know-ledge and Practices Towards Minimising Fluoride and SulphurEmissions from Aluminium Reduction Cells, Editor, TMS, Norway,2000, 119

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Figure 4: Chemical analyses of fluoride in fluorinated aluminaSlika 4: Kemijska analiza fluora v fluorirani glinici

Figure 5: Scheme of mechanisms of vapour-phase HF bonding ontoprimary alumina13

Slika 5: Shema vezave HF na primarno glinico13

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