Thermodynamics of Gas-Solid Reactions for Purification of Bauxite at Moderate Temperatures Davld J. Mllne” and Robin D. Holliday University of Newcastle, New South Wales 2308, Australia
Removal of iron to low residual levels is essential in preparation of AIC13 feed for chloride electrolysis cells for AI production. The number and complexity of systems affording possibilities for beneficiation of bauxite by gassolid reaction is such that computer techniques are needed to analyze them in an ordered way. Predominance area diagrams have been used in conjunction with free energy minimization calculations to define routes for selective removal of iron. The systems AVFe-CI-0, AI/Fe-S-0, AI/Fe-S-CI and AI/Fe-S-CI-0 have been analyzed in this way. Several indirect routes in which Fez03 is first converted to an easily chlorinated compound have been identified; in particular, sulfidization by SO2-CO mixtures of high sulfur potential appears attractive. Other routes involve direct chlorination of Fez03 to FeC12 or FeCI3 under conditions where AI203 is nonreactive. Volatilization of AIC13 with retention of FeS is possible in principle with gas reactants based on H2S-CI2 mixtures.
Empirical studies of the beneficiation of bauxite by anhydrous methods for use in the electrolytic production of aluminum or in special grades of high-alumina refractories have an extensive history. In aluminum reduction technology, the principal objective has been to reduce the capital costs by utilizing compact gas-solid reactors to replace the costly hydrometallurgical systems of the Bayer process. The recent development of attractive new electrolytic processes based on A1C13 (Haupin, 1973; McIntosh and Holliday, 1974) has focussed renewed interest in the problem, since chlorination by gas-solid reaction must be used to manufacture feed material suitable for electrolysis. The reduction process requires a feed material which can be introduced into the molten salt electrolyte in an anhydrous condition as nearly as possible free from contaminants which will be preferentially reduced under cell operating conditions. These impurities include FeCl3, TiC14, and SiC14, all of which will be formed during chlorination of the aluminous component of the bauxites under nonselective conditions. Separation of AlC13 from Tic14 or SiC14 presents no special problems, because of the satisfactory difference in relative volatility and the absence of complications such as the formation of vapor-phase complexes. Removal of iron from the mixed chloride product is considerably more difficult because FeAlC16 forms in the vapor and makes a clean separation impossible despite the high volatility of pure AlC13 relative to a pure FeC13 phase (Semenenko and Naumova, 1964). The present work, using modern computer calculation techniques, focusses on the determination of conditions for selective removal of iron from bauxite using moderate temperature gas-solid reactions that can be integrated into a process for production of anhydrous AlC13. The alternative of selectively removing AlC13 without reaction of iron also appears to offer some prospects for success, as revealed by the present analysis, although previous workers have discounted the possibility because of the generally more favorable free energy changes for the corresponding reactions involving Fe2O3. Recent work (Russell et al., 1973) has shown that the aluminous component of bauxite may be chlorinated relatively rapidly with CO-Cl2 or other chlorinating mixtures Address correspondence to this author at Comalco Ltd., 95 Collins St., Melbourne, Australia. 442
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
a t temperatures in the region of 500-750°C, provided the oxide is calcined at low temperatures to produce alumina in the reactive y-form. These results have been confirmed and extended in our own laboratory. It is highly important, therefore, that the beneficiation reactions should proceed below the temperature where the high surface area of the reactive alumina is destroyed by sintering. Although the standard free energy change for the reaction Fen03
+ 2AlCl3 = 2FeCl3 + A1203
is negative over the range of temperature of interest, in practice the A1203 component of bauxite is chlorinated concurrently with the Fen03 unless the alumina is preheated to llOO°C or above to produce the non-reactive aform. Several investigators (Buntin et al., 1962; Khundkar and Ahmad, 1955; Spitzvin, 1930) have reported that HC1 gas may be used to beneficiate bauxite under reducing conditions but the reactions are very slow. In spite of the low reactivity of A1203 with HC1, the long reaction times lead to appreciable losses of y-alumina unless an undesirably high temperature calcination is carried out. More promising results have been attained by converting the iron to an intermediate compound or reducing it to elemental form before removing it by chlorination or leaching. Thus under strongly reducing conditions, Fe may be formed and partially removed. If carbon is the reducing agent, temperatures above 1000°C are necessary to attain satisfactory reaction rates, and the final iron remains at 0.5 to 1.096 (Foley and Tittle, 1971). Better results have been reported with H2 as reducing agent, followed by chlorination of the FeO intermediate with HCl (Khundkar, 1955). Rather precisely controlled reaction conditions were needed to secure good iron removal, and under these conditions reaction rates were too low to find useful application. Sulfidization of the iron prior to its removal as chloride has been investigated (Finke and de Marchi, 1938). Severe reaction conditions were employed, and considerable losses of A1203 were encountered. Under extreme conditions (Perieres, 1967), sulfidization and reduction can lead to nearly complete conversion of the iron, which may then be removed by chlorination. The temperature of 134OOC and the use of elemental sulfur make this route unpromising for the large scale. Treatment of aluminous materials with SO3 gas affords
0
50
100
200
400
MI
1000
Figure 2. Vapor pressures of chlorides.
F2 6
-100
F 500
TEMP,
O K
Figure 1. Change in standard free energy per mole of A1203 or Fez03 for the following reactions. (Al) A1203 + 3so2 + 3c12 = 2AlCl3 3so3; (A2) A1203 3soc12 = 2AlCl3 3so2; (A3) A1203 + 3so2 3c12 + 3Hz0 = 2AlC13 3HzS04; (A4) A1203 + 3szc12 = 2AlCl3 %SO2 + 'hS; (A5) A1203 + 6HC1 + 3CO = 2A1C13 + 3co2 + 3H2; (A6) A1203 6HC1 + 3C = 2AlC13 3CO 3H2; (Fl) Fez03 + 3so2 + 3c12 = 2FeC13 3so3; (F2) Fez03 + 3soc12 = 2FeC13 3so2; (F3) Fez03 + 3so2 + 3C12 + 3H20 = 2FeC13 + 3HzS04; (F4) Fez03 + 3szc12 = 2FeC13 + %SO2 + 9hS; (F4A) Fez03 + 2S2C12 = 2FeC12 + %SO2 %S; (F5A) Fez03 + 4HC1 + 3CO = 2FeC12 + 3co2 + 2H2; (F6) Fez03 6HC1+ 3C = 2FeC13 + 3CO 3Hz; (F6A) Fez03 + 4HC1 3C = 2FeC12 + 3CO + 2H2; (F7A) Fez03 4HC1+ H2 = 2FeC12 + 3H20.
+ + +
+
+
+
+
+
+
+
+
+
+
+ + +
one of the few feasible routes for preferential reaction of A1203. Removal of the sulfate requires a leaching step that would be ill-adapted to an integrated process for anhydrous AlC13. In addition, under the conditions investigated, beneficiation was poor and the reaction was slow (Chao, 1966).
Possible Routes for Beneficiation of Bauxite The thermodynamic approach to the problem of selecting reactants and conditions to separate compounds such as Fez03 and A1203is conventionally based on the calculation of free energy changes for proposed reactions. This method was used in the first stages of the present work. In the preliminary study, over 450 possible gas-solid reactions were formulated in terms of simple metered-in mixtures of economically feasible reactants. The reactions were selected on the basis that they could lead to selective chlorination of either A1203or FezO3, by direct reaction or via formation of a reactive intermediate. In the gas-solid systems presently considered, the following problems are encountered. 1. The simple metered-in binary or ternary gas mixtures usually react to form complex new systems at high temper-
atures: thus CO-Cl2 mixtures may contain significant proportions of components such as COC12, C C 4 , CO2, and C10 (Table 11). 2. The nature of the solid product depends on the reaction conditions: thus Fez03 may react with HCl-CO mixtures to form either FeC12, FeC13, Fe304 or even carbon, depending on the gas ratio and temperature of reaction. 3. Under flow conditions commonly encountered in industrial reactors, the solid tends to attain equilibrium with the incoming gas mixture, rather than with the depleted composition produced in the reaction. Thus, the end point predicted by the conventional closed-box equilibrium calculations is not necessarily the same as that produced under flow conditions. The complexity of the systems requires the study of a very large number of reaction paths, and this may make results obtained by the conventional methods invalid if important reactions are neglected. Computer techniques for finding the condition of minimum free energy of the complex systems can be employed very usefully to check and extend the predictions from the initial simplified calculations.
Results of Preliminary Simplified Calculations A computer program was written to calculate changes in standard free energy, enthalpy, and entropy of reactions a t several temperatures for a very large number of arbitrarily selected routes. The findings from this study confirmed that no routes existed for direct chlorination of A1203 without reaction of Fe2O3, but showed several possibilities for selective removal of Fez03 either directly as FeCl2 or FeC13, or via an easily chlorinated compound such as FeS or Fe2N. Thus the reactions summarized in Figure 1 may lead to separation of iron and aluminum, in the absence of the complications previously listed. The reactions chosen yield a negative change in free energy for Fez03 and a positive change for the corresponding reaction with A1203. Reactants which gave negative changes for both oxides are not considered, since experience has shown that in such circumstances the active y-alumina will react concurrently with FezO3. Reactants which produce volatile FeC13 with no reaction of A1203 are SO2-Cl2, S2C12, SOC12, and S02-ClpH20. Gaseous HC1 may possibly come into this category also. Formation of FeC12 is favored by the combinations SzClC CO, HC1-CO, HCl-Hz and HC1-C. FeCl3 vaporizes a t 312°C and is therefore easily removed (Figure 2), while FeCl2 may be volatilized above 650°C, by chlorination to FeC13 under neutral conditions, or may be removed by leaching. Alternately, purified AlC13 may be vaporized while FeCl2 Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
443
Table I. Reactions Producing Iron Intermediates Removable by Chlorination Reaction F e 2 0 3 + 3.5s = 2FeS + 1.5Soz F e 2 0 3 + 5.5s = 2FeS2 + 1 . 5 S o z FezO, + 2SO2 + 7CO = 2FeS + 7c02 Fe,O, + 4SO2 + l l C 0 = 2FeS2 + l l C O z FezO, + 2H$ + H2 = 2FeS + 3 H 2 0 Fe203+ 2NH3 = Fe2N + 3HzO 2 F e 2 0 3 + NH, + 4.5H2 = Fe4N + 6 H 2 0 2 FezO, + 1/2N2+ 6Hz = Fe4N + 6 H z 0 Fe20, + C 0 2 + CO = 2FeC0, Fe203+ 3H2 = 2 Fe + 3 H z 0 F e z 0 3 + 3CO = 2 F e + 3 c 0 2 Fe203+ CO = 2 F e 0 + COz Fe20s+ 2S02 + 72 02 = 2FeS04 3 FezO, + 2C + 9H2= 2Fe3C + 9Hz0 FezOB+ 3S02 = F e 2 ( S 0 4 ) 3
-~ Temp range, "K
9 00 -1 3 00 300-900 800-1300 300400 800-1300 800-1 3 0 0 800 -1 3 0 0 1000-1 3 00 800 -1 000 800-1300 3 00 -1 3 00 300-1300 300-1000 3 0 0-1 3 00 300-900
is retained because of its much lower volatility (Figure 2). This route is feasible only if p(Cl2) is maintained at a very low level, dependent upon the degree of contamination of AlC13 that may be tolerated. Thus a t 700°K, p(Cl2) must atm if p(FeCl3) is not to be maintained below 7.6 X exceed 0.01 atm. This level of p(Cl2) is not exceeded for reactants which produce FeC12, but it is exceeded in most reactions for chlorination of A1203.However, the initial calculations showed that S2C12-CO2 mixtures may provide suitable conditions in the reaction A1203 4- 3s2c12 4- 3CO = 2A1C13 4- 3 c o 2 4- 3s2 The simplified analysis indicated that FeCl2 should not react to FeCl3. Most significantly, the initial analysis revealed a number of routes whereby a reactive iron intermediate compound could be formed under conditions such that A1203 was unreactive. These are listed in Table I. Production of iron nitride from ammonia under streaming conditions may be favored if thermal dissociation of NH3 can be suppressed. Removal of iron intermediates by high-temperature chlorination is appropriate in an integrated process for anhydrous AlC13 manufacture; the alternative is to remove compounds such as FeS, FeS2, Fe2N, or FeC03 by acid leaching. Refinement by AG-Minimization i n Conjunction with Predominance Area Diagrams. The complications arising from parallel competing reactions and the multiplicity of possible solid phases can be handled by the technique of free energy minimization. This is an optimization method for determining the combination and proportion of components in a reaction system of given elements which yields the minimum value for the total free energy, and thereby defines the point of equilibrium (van Zeggeren and Storey, 1970; Oliver and Baier, 1962; White et al., 1958). A novel and important feature of the calculation techniques in the present study for AG-minimization was that they permitted components in complex gas mixtures to be determined to levels below atm. This allowed the gas mixtures to be characterized by two, or a t most three, significant components in accordance with the phase rule. The accuracy of the technique used in this way is demonstrated in Appendix I. The solid phases in equilibrium with the gas could be 444
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
Figure 3. The Fe/Al-Cl-0 system at 900OK. Equilibrium p(Cl2) and ~ ( 0 2 values ) are superimposed for the gaseous mixtures indicated. Line A-B is the A120eAlC13 (0.001 atm) phase boundary.
characterized in terms of the same components on a conventional predominance area diagram, in which fields of existence of solid phases are represented as a function of the activities of the gaseous species. Application of Refined Techniques to Bauxite Systems T h e System M-C1-0 ( M is A1 or Fe). This system describes the behavior of bauxite when the carbon potential of the reactant gas is low, as in the commonly proposed chlorination mixtures of CO and Cl2. Up to 1200°C, no solid carbon may exist at equilibrium, so the oxygen potential is always higher than the equilibrium value with solid carbon. The proportion of new species increases markedly a t low temperatures, but the equilibrium is not readily established in the absence of a catalyst. The predominance area diagram (Figure 3) shows that FeC13 and AlC13 coexist over the whole range of CO-Cl2 compositions. The broken line indicates the equilibrium between A1203and p(AlC13) at atm. The shaded area thus delineates conditions for producing an iron chloride with essentially no reaction of Al2O3. Suitable conditions are not attainable with CO-Cl2 mixtures, but a separation is thermodynamically possible at lower temperatures when HCl-CO mixtures may be used to produce FeC12. Some deposition of carbon occurs also under these conditions. SO2-Cl2 mixtures may be capable of selectively producing FeCl3 at moderate temperatures; the required ~ ( 0 2in ) the vicinity of 10-lO atm is not readily attainable in other systems. The SO&& system is of further interest because, with small injections of 02, FeS04 may be produced below 1000OK. This may be represented on a predominance area volume basis with axes for p(O2),p(Clp), and p(S2). T h e System M-S-0 (M is A1 o r Fe). The predominance area diagram, Figure 4, defines the conditions for formation of iron sulfides as a first stage in the beneficiation of bauxite. The AG- minimization technique permits the calculation of p(S2) and ~ ( 0 2 )values for S O r C O mixtures over a range of compositions (Figure 5). The combined data plotted as in Figure 4 may then be used to select the gas composition and temperature which yields the desired FeS and avoids wasteful consumption of sulfur in forming FeS2. The occurrence of a maximum sulfur-potential at about the 3:7 ratio of SO2/CO is suggestive that this composition may be associated with particularly favorable reaction kinetics. This result was demonstrated experimentally to afford pleasing confirmation of the predictive value of the technique employed. The AG minimization technique predicts that FeS may be produced also by the reactant gas systems HzS-H~,H2S-CO, and SO2 after equilibration with solid carbon. Formation of Al& will not occur above 60OOC with SOz/
Table 11. Equilibrium Compositions of Equimolar Mixtures of CO and Clz at 1atm ~~~~~~
Partial pressures, atm Temp, "K
co
ClZ
700 900 1100
0.051 0.372 0.491
0.051 0.372 0.491
COClZ
0.028 0.023 0.003
cc1,
COZ
c10
0.434 0.116 0.007
0.434 0.116 0.007
6.8 x 1.6 x lo-'* 1.0 x 1 o - t j
0 2
2x 4 x 10-22
Figure 4. The Fe-S-0 system a t 90O0K.Equilibrium p ( S 2 ) and ~(02) values are superimposed for SO2/CO mixtures of the following composition ratios: a, 1:lOO;b, 1:lO;c, 1:l;d, 1O:l;e, 1OO:l.
CO mixtures and the A1203-AlzS3 phase boundary is not included in Figure 4. However, after equilibration with solid carbon, SO2 yields a reactant gas with high p(S2) and low ~ ( 0 2 which ) is possibly capable of converting A1203 to Al2S3. Data on Al2S3 are very limited so that experimental confirmation of these predictions is required. T h e System Fe/Al-S-Cl. The predominance area diagram, Figure 6, defines conditions for selective removal of AlC13, with retention nonvolatile FeS (or FeS2 at lower temperatures). AG minimization applied to H2S-Cl2 mixtures over a range of compositions produced the p(S2)-p(C12) curve superposed on the diagram. With this reactant system, no hydrides are formed, so only a twocomponent diagram is needed. Equilibrium lines are drawn for Al2S3 with p(AlC13) at 0.1 and 0.001 atm. It is evident that a 7:3 ratio H&C12 mixture a t 1000°K lies within the region for separation of aluminum and iron in a one-stage process, and a fairly wide range of reactant gas composition may be tolerated about the initial level. Further, the oxygen potential is low enough so that reduction could proceed readily. T h e System M-S-(21-0 (M is A1 or Fe). This system is of particular interest since it affords possibilities for separation of AlC13 by volatilization under conditions where iron is retained as FeC12. The reaction A1203
+ 3s2c12 + 3CO = 2A1C13 + 3C02 + 3sz
appears to provide the desired selectivity, on the basis of the initial simplified thermodynamic analysis. Table XI1 gives results of the AG-minimization calculatioh for S2C12-CO mixtures over a range of temperatures. Although p(C12) generally is low, it is still above the value required to chlorinate FeC12 to a significant p (FeCld. Hence the desired selectivity is attainable only at very low temperatures. The preliminary analysis further suggested that selective production of FeCl3 could occur, via Fez03
+ 3soc12 = 2FeC13 + 3so2
under conditions where A1203 would not react (Figure 1).
x
GO
Figure 5. Equilibrium partial pressures of the principal components of variable SO&O mixtures a t 980OK.
FeS
5
-
:I I
Fe
A
B
'
FeC$
Figure 6. The Fe/A1-S-C1 system at 1000OK.Equilibrium values for p ( S 2 ) and p(Cl2) are superimposed for H2S/C12 mixtures of the following composition ratios: a, 991;b, 9:l;c, 7:3;d, 1:1;e, 3:7;f, 1:9;g, 1:99. Line A is the Al&-AlC13 (0.1atm) phase boundary; line B is for AlzS3-AlC13 (.001 atm).
The AG-minimization calculations summarized in Table XI indicate that relatively high p (C12) results from dissociation in the reactant gas, so that AlC13 formation is to be expected. Certain mixtures of SO2 and Cl2 (Table 111) do have ap) p(Cl2) to preferentially react propriate values of ~ ( 0 2 and with Fez03 and leave A1203unreacted. Flow-through versus Closed-Pore Reaction Conditions Under flow conditions where the solid is approaching Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
445
Table 111. Equilibrium Compositions in the S-Cl-0 System a t 1 atma ~~
Partial p r e s s u r e s , a t m Component s2c12
co sc12
SR
co2 cos cs2 cs C12 coc12
cc14
SO2 02 SOCl2
so3
A 0.063 0.034 0.427 2 x 10-9 0.116 0.237 0.111 1 . 5 x lo-' 4.4 x 10-3 1.6 x 1 0 - ~ 5.4 x 2 x 10-1'
...
5.1 x
...
B
Moles C
7 x 10-6
...
5.4 x
10-l~
*..
3 x 10-2
9.7 x 10-8
... ... ... ... ...
... ... ... ...
...
3 x 10-2
0.428
... ...
... ...
3 x 10-2 6.5 x 0.911 2 x 10-1'
0.571 4.4 x 10-11 1.3 x 1.3 x lo-'
D 2.2 x 10'10 2.2 x 10-7 1 . 3 x 10'" 7.4 x 10-2 0.999 4 . 3 x 10-4 4.3 x 10-10 1 8 x 10''' 6.2 x 10-15 2.5 x lo-"
... 0.250 1.8 x loqz6 2 x 10-12
...
FeC12(s )
1.00 SO) 1.16 Fe203( s ) 4.50 s2 1 . 9 x 10-4 2 10-l~ 1.4 x 9.9 x 10-4 A: equimolar SzCIz-CO at 700°K. B: SOClz at 500°K. C: S02-Clz in ratio 4:3 at 800°K. D: SzC12,l mol; CO, 1 mol; Fez03,5 mol at 600°K.
equilibrium with the feed gas, the gaseous reactant changes composition only to a small extent. This condition has been assumed in the preceding equilibrium calculations. Within the inner regions of a system of Angstrom-sized pores such as is found in the structure of typical calcined bauxites (Glass and Ross, 1972), the real situation may more resemble a closed-box equilibrium, even during the initial stages of the reaction. The resulting changes in gas composition can markedly affect the nature of the product in systems where ~ ( 0 2exerts ) strong influence on the reaction. This can occur, for example, in the equilibration of S2C12-CO mixtures in the presence of excess FezO3. At 600°K in this system, heavy deposition of liquid sulfur is to be expected, from AG-minimization calculations (Table 111).This deposition can cause pore-blockage and stifle further reaction. Such a situation contrasts markedly with the equilibrium in the final stages of reaction, where complete conversion of Fez03 to FeC12 takes place. Conclusions The number and complexity of systems affording possibilities for beneficiation of bauxite by gas-solid reaction is great. By employing computer calculation techniques, it has proved possible to analyze the system in an ordered way. Promising beneficiation schemes which may be integrated into a process for manufacture of anhydrous AlC13 have been identified and described, and were selected for experimental study. Most of the suitable systems involve direct chlorination of the iron content to FeC12 or FeCl3 under conditions where A1203 is nonreactive. Several indirect routes where iron is first converted to an easily chlorinated compound such as FeS or Fe2N showed particular promise for removal of iron at high rates a t moderate temperatures, using inexpensive reagents. In particular, sulfidization by S02-CO mixtures of high sulfur potential seemed to afford a likely basis for an efficient, relatively low-temperature process. A few processes were identified permitting the preferential volatilization of AlC13 from bauxite, leaving iron either unreacted or as a nonvolatile compound. In these processes, the reagents required are not common, but there are 448
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975
no fundamental obstacles to their application. A study of the reaction rates and reaction paths is reported separately. Acknowledgment The authors wish to thank Comalco Ltd., Melbourne, Australia, for financial assistance toward this work. Appendix Free Energy Minimization. The technique used was originally proposed by Oliver et al. (1962) for determining the equilibrium distribution of a large number of species within a system of known elemental composition a t a specified temperature and pressure. The distribution which gives the minimum free energy of the system is determined by a computer iteration process (Ketteridge, 1969). The primary steps in the procedure are as follows. (1)An expression for the free energy of each component in the system is developed as a function of temperature and pressure. (2) An expression is written for the free energy of a mixture of assumed composition. (3) The free energy of the equilibrium mixture (of unknown composition) is expressed, using a Taylor series, in terms of the known mixture and of unknown increments representing changes necessary to bring the assumed composition to equilibrium. (4) The expanded function is minimized, subject to mass balance constraints, using the technique of Lagrangian multipliers. ( 5 ) Thus a series of linear simultaneous equations is set up and is solved using matrix algebra (method of Gaussian elimination with a row pivoting and back substitution). When solved, the system yields a new composition that represents a first approximation to the composition of minimum free energy. (6) The iteration is continued until the new composition is the same as the previous one to the accuracy desired. At this point the free energy is a t a minimum. Details of the mathematical procedure are outlined in supplementary material available in the microfilm edition of this journal. (See the paragraph a t the end of the paper regarding this material.) The computer program has been described by Ketteridge (1969). In the present investigation, the program was run in two parts: the initial run was
carried out using a relatively high discard level of the order of atm for all species, and this served to establish an accurate estimate of the input distribution for a second run. The second calculation was then able to establish equilibrium levels of gaseous species to below atm. This was the fundamental feature that enabled a predominance area diagram to be used for a wide range of gas compositions. The method of checking the accuracy of calculation of these low component levels may be demonstrated by referring to Figure 4. The triple point for Fe-FeS-FeO was calculated by conventional means, using data from Kubaschewski et al. (1967). The unique values for sulfur and oxygen pressures at equilibrium a t lOOOOK were 0.96 X 10-lo atm, respectively. The triple point could and 2.33 X be reproduced by the minimization program by equilibrating a closed system containing 3.6 mol of SO2, 6.4 mol of CO and excess iron. The equilibrium distribution was found to contain the solid components a t sulfur and oxygen and 2.4 X atm, respectively. pressures of 1.2 X This degree of precision was attained in other similar systems, confirming that the use of the low calculated partial pressures was justified. Literature Cited Buntin, A,, et al., Tr. Tomsk. Gos. Univ.: Ser. Khim., 154, 52 (1962). Chao. T.. Ph.D. Thesis, Pennsylvania State University, 1966. Finke, C., de Marchi, V., Trans. Electrochem. Soc.,74, 469 (1938). Foley, E., Tittle, K., Proc. Aust. lnst. Min. Metall., 239, 59 (1971).
Glass, A.. Ross, 0.. Can. J. Chem., 50, 2537 (1972). HaUpin, W. E. (to Aluminum Co. of America), U S . Patent 3,755,099 (Aug 28, 1973). Ketteridge, i., Ph.D. Thesis, University of Adelaide, Adelaide, Australia, 1969. Khundkar, M.. Ahmad, N., J. lndlan. Chem. SOC.,18,109 (1955). Kubaschewski, O., et ai., “Metallurgical Thermochemistry,” Pergamon. London, 1967. McIntosh, P., Holliday, R. D. (to Conzinc Riotinto of Australla), Australian Patent 445,623 (Mar 4, 1874). Oliver, S.,et el., Chem. Eng., 80, 121 (1962). Perieres. R. (to Cie. Pechiney), French Patent 1,495,002 (Sept 15, 1967). Russell, A. S., et ai. (to Aluminum Co. of America), German Patent 2,244,041 (Mar 22, 1973). Semenenko, T., Naumova, T., Do&/.Akad. Nauk SSSR, 154, 169 (1964). Spitzvin, V., 2.Anorg. Allg. Chem., 180, 337 (1930). van Zeggeren, F., Storey, S., “The Computation of Chemical Equilibria,” Cambridge University Press, Cambridge, 1970. White, W., et al., J. Chem. Phys., 28, 751 (1958).
Receiued for reuiew December 2,1974 Accepted May 14,1975
Supplementary Material Available. Details of the mathematical procedure will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Business Office, Books and Journals Division, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order for $4.00 for photocopy or $2.50 for microfiche, referring to code number PROC-75-442.
Experimental Evaluation of Routes for Purification of Bauxite by Gas-Solid Reactions Robin D. Holliday and Davld J. Mllne’’ University of Newcastle, New South Wales 2308, Australia
Conditions for preparation of AIC13 of low iron content were studied in a thermogravimetric apparatus designed so that sensitive components were shielded from the gaseous reagents. Reaction of natural bauxite pisolites of diameters between 1.5 mm and 12 mm with gaseous reagents based on S02-C0, HCI-CO, HCI, HPS-CI~, S2C12-CO, S02-C12 and CO-CI2 was analyzed. Predictions that Sop-CO mixtures of high sulfur potential would be particularly suitable for preparation of FeS were confirmed. Maximum sulfidization rates occurred at the SO21 CO ratio of 3565. Rates were proportional to (p(S2)2 p(COS).p(Sd p(COS)2). FeS could be rapidly removed to residual Fe levels of 0.1 to 0.3% by direct chlorination below 750OC. Direct chlorination of FepO3 using CO-CI2, HCI, or HCI-CO gave much less favorable results. The possibility of removing AIC13 while retaining FeS is unique to the H2S-C12 reagent system, but the reaction kinetics were unfavorable. The sulfidizationchlorination route appears to afford the best combination of characteristics sought in a beneficiation process suitable for integration with AIC13 production.
+
A detailed thermodynamic analysis of routes for removal of iron from bauxite under anhydrous conditions defined a number of reactions that merited experimental study (Milne and Holliday, 1975). The experimental program aimed to establish which of the prospective routes could be realized under the least severe reaction conditions. An attractive process was defined as one which afforded the best Address correspondence to this author at Comalco Ltd., 95 Collins St., Melbourne, Australia.
+
combination of the desired characteristics: low temperatures to minimize corrosion, high reaction rates to minimize reactor size, together with low reactant costs and ease of recovering or recycling intermediates. The essential criterion was that iron removal be highly efficient, so in practice only processes found capable of producing iron levels below 0.5% were selected for detailed study. Further, the temperature of reaction must be kept below 75OOC to avoid serious impairment of reaction rates in the subsequent chlorination of yA1203. Id.Eng. Chem., Process Des. Dev., Vol. 14, No. 4. 1975
447
