THERMODYNAMIC STUDY OF CHROMITE CAUSTICFUSION PROCESS

ACTA METALLURGICA SINICA (ENGLISH LETTERS)Vol. 14 No.1 pp 47-55 February 2001THERMODYNAMIC STUDY OF CHROMITE CAUSTICFUSION PROCESSS.L. Zheng and Y. ZhangTFO AInstitute of Chemical Metallurgy, the Chinese Academy of Sciences, Beiing 100080, ChinaManuscript received 8 June 2000; in revised form 21 October 2000A new method for chromate cleaning production named chromite caustic fusion pro-ceas, is aduanced by Institute of Chemical Metallurgy, the Chinese Academy of Sci-ences. With sodium hydroxide as reaction medium, the new process is composed ofthree procedures: liguid phase oridation of chromite - metastable phase separation -carbonalion ammonium transition. Generally ilustrating the new process and its fea-tures, this paper mainly studies the thermodynamica of chromite oridation. The neuprocess has much better practical results than the conventional chromate productionprocess in which sodium carbonate is used a8 reaction medium. The superiority is alsoshown through thermodynamic studies.KEY WORDSchromite, oxidation, flowsheet, recycling, thernodynamic1. General IntroductionChromate production from chromite is an important basic industry in the field of chem-ical engineering and metallurgy. The conventional chromate production process consists ofthree procedures: roasting of chromite with oxygen at high temperature (1150C) - waterleaching- multistage evaporation crystallization.. It works at a quite low utilization ef-ficiency of resource and energy. A large amount of chromium-containing residue, chromitedusts and waste gases are discharged from chromate plants. Millions of tons of wastechromium-containing residues present a potential threat to underground water. Many newmethods2-8] such as roasting with less calcium or even without calcium have been de-veloped, but there still exist many dificulties in the industrialization and the pollutionproblem has not been thoroughly resolved yet.In China, chromate production has long been ranked top on the list of severe pollutionindustries. In order to solve the problem of environmental pollution, Institute of ChemicalMetallurgy, the Chinese Academy of Sciences first put forward a new process for chromatecleaning production9. The new process is made up of three procedures: liquid phaseoxidation of chromite - - metastable phase separation - - carbonation ammionium transition.The first procedure focuses on oxidizing chromium with air from trivalent to hexavalentat 530C in the medium of molten sodium hydroxide. As we know, Cr, Fe, Al, Mg, Si are themain elements of chromite. When the oxidation is finished, Cr and Al will enter liquid phasein the form of sodium chromate and sodium aluminate together with the excessive sodiumhydroxide, while Fe, Mg, Si will become ferric oxide, magnesia and sodium aluminumsilicate respectively, and they are the main components of residue. The fllowing is themain reactions in this procedure:FeO . Cr2O3(s) + 4NaOH()+ -O2(g) = 2Na2Cr04()+ FFe2Os(s) + 2H2O(B) (1)中国煤化工MYHCNMHG●48FeO . Al2Os(s) + 2NaOH()+ =02(8) = 2Nal02()+ ,Fe2O3(s) + H20(g) (2)MgO . Cr2O3(s) + 4NaOH(I) + 2O2(g) = 2Na2CrOx(s) + MgO(s) + 2H2O(g) (3)The fusion is directly diluted and its heat energy is fully used, then the liquid can beseparated from the residue by gravity separation. The liquid will enter the next procedurefor NaOH-Na2CrO4-NaAlO2 separation, and the residue will take countercurrent washing.The washing water obtained from residue washing makes sodium hydroxide recycle as thediluent of the fusion. The washed residue is suitable for iron making after extracting Mgby carbonate leaching method.The second procedure is the separation of components (NaOH-Na2CrO4-NaAlO2) inthe liquid phase obtained from the frst procedure. By comparing many separating ways,the best one is vacuum cooling crysallization, and sodium aluminate will take crystalliza-tion with sodium chromate. The liquid after crystallization that contains excessive sodiumhydroxide, saturated sodium chromate and saturated sodium aluminate returns to the firstprocedure to recycle alkali, chromium and aluminum. The mixed crystal takes counter-current washing to separate sodium aluminate from sodium chromate by taking advantageof the supersaturation property of sodium aluminate. So, the eligible sodium chromateand sodium aluminate crystals will be obtained respectively. Sodium aluminate crystalsare used to produce alumina by the traditional Bayer method, and the water for washingcrystal returns to the first procedure to recycle its valuable components. In this procedure,the sodium carbonate produced in the first procedure can be separated by adjusting theconcentration of sodium hydroxide in washing water. Sodium in sodium carbonate takesrecycle by the reaction of lime with sodium carbonate.The third procedure studies the production of sodium dichromate from sodium chro-mate crystal obtained in the second procedure by taking NH3 and CO2 as reaction medium.The main equations are listed as follows:Na2CrO4 + 2NH3 + 2CO2 + 2H2O = 2NaHCO3↓+(NH4)2CrO4(4)Na2CrO4 + (NH4)2CrO4 = Na2Cr2O7 + 2NH3 + H2O(5)NaHCO3 produced in Eq.(4) is used as raw material to produce sodium bhydroxide bycalcination and reaction with lime. In this procedure, ammonia and carbon dioxide arethe inner recycle reactants.To sum up, the new process goes like this: firstly, chromite is oxidized continuously withair in the medium of molten sodium hydroxide at 530C and the fusion is directly dilutedby the washing water to 50% NaOH; secondly the eligible products of sodium chromateand alumina can be obtained respectively after separation of NaOH, Na2CrO4 and NaAlO2in the high concentration medium of NaOH; lastly, sodium bichromate is produced fromsodium chromate crystal by carbonation ammonium transition method. All the washingwater containing sodium hydroxide, sodiurn aluminate and sodium chromate is back to thefirst procedure to recycle its valuable components. Its schematic fow sheet is shown inFig.1.The new process has many advantages. In the conventional process which is widely usedin China at present, the inert flling, twice the quantity of the chromite ore, is needed toadd into the feed to avoid agglomeration and overcome the difficulties in mass transferring中国煤化工MYHCNMHG， 48through the thick liquid membrane embracing the chromite particle. It is the inert flingthat causes a large amount of chromium-containing residue. But in the new process,the inert flling is unnecessary because the reactants are highly dispersed in the floatingfusion. As a result, the quantity of residue reduces to one-fourth as much as that in thetraditional process. Furthermore, instead of gas-solid reactions in the traditional process,the gas-liquid-solid heterogeneous reactions in fusion in the new process can intensify thetransferring of mass and heat greatly, so the reaction temperature is greatly lowered. andno dust is produced. Meanwhile, Cr recovery yield reaches above 95%，After magnesiumextraction by carbonate leaching method, the quantity of chromium-containing residue is .reduced more, and the residue is made suitable for refning iron.Thus, chromite ores and air are only used theoretically to produce chromium productsand ferrate products. The general reaction equation is: .FeCr2O4 + O2recycle reactantsFe2O3 + Cr series products(6)The experiments both in laboratory and in small-scale pilot plant have been carriedout. The main results in technical and economic index are given in Table 1.Table 1 Comparison of the new process with old one in technical and economic indexItemsNew processOld process*TechnicalCr recovery yield95%76%indexChromium residue0.6t/ton of Na2Cr2O>.2H2O 2.5t/ton of Na2Cr2O>.2H2OCr content in residue0.50%Cr+ in residue0.15%0.7%Cr-containing waste gasNoSeriousEconomicOre consumption1.07t/ton of Na2Crz2O7 2H2O1.35t/ton of Na2Cr2O+2H2OAlkali consumption0.35 ton of NaOH1.0 ton of Na2CO3Assisting roasting limestone2.5tEnergy cost(natural gas)Decreased by 20%* Data obtained In China.The features of the new process are discussed below. The reaction temperature in the :ew process is 5309C, about 600C lower than that in the conventional process. No inertflling added brings forth the great decrease of the quantity of residue, and the reactionsin fusion make no dust, which make the new process simpler, safer and more effective. Inaddition, Cr extraction yield increases to nearly 100%, and the other valuable elementsFe, Al, Mg are all fully utilized as final eligible products of iron ore, Al2O3 and MgOrespectively. So, the new process really realizes the deep comprehensive utilization ofresources and the zero emission of chromium-containing residue.Here we mainly discuss the thermodynamics of the new process. As discussed above,only the first procedure requires high temperature, and the other two procedures are justgoing at the normal conditions. So only the procedure of chromite oxidization need to bestudied its thermodynamic behaviour.The detailed experimental studies on the new process please see the future publications.中国煤化工MYHCNMHG502. Thermodynamic StudyChuomue AIThe criterion for the new process is orig-ordson ofChromuteinally based on its thermodynamic analysis.Thermodynamic analysis on the new processis systematically carried out in this paper soSodiumoycdeNOHCO,YAO: 1as to cooperate with the design of the newLashngSseaionprocess.The general formula of spinel-structuredvypout loaoreNa.CO，AILO,chromite is (Fe,Mg)O-(Cr,Al,Fe)2O3. To ex.Co.rasyletract chromium, trivalent chromium must beaO. cio， N.C.O.,ene poustransformed into hexavalent chromium in theform of soluble salt. The solid- liquid -gas het~Fig.l Schematic flow sheet of the new process.erogeneous reactions in molten NaOH occurin the procedure of liquid phase oxidization of chromite, the most important procedurein the new process. So, NaOH-(Fe,Mg)O.Cr2O3-O2 is the principal reaction system. Bythe Thermodynamic Calculation Software (TCS) designed by the authorl10-12), Gibbs freeenergy and reaction heat are calculated. Here are the equations for calculation:巴s,G°=-2nGA,H°=--二v;H°where Gq, H? are standard enthalpy and heat of component i at temperature T, and v isthe coefficient matrix of all components.The relationship diagrams between standard free energy change O,G° and reactiontemperature T, standard reaction heat change△; H° and reaction temperature T, vaporpartial pressure PH2O and oxygen partial pressure PO2 are plotted on the basis of calculationresults. The diagrams are very useful in evaluating the reaction, comparing thermodynamicadvantages over the conventional process and guiding the new process study.The reaction temperature of chromite oxidization is about 530C, so the temperaturefor calculation ranges from 400 to 1000K. As the reaction temperature of the conventionalroasting process is about 900- 1200C, the temperature range for calculation is accordinglybroadened from 400 to 1600K.2.1 Thermodynamics of the new process2.1.1 Decomposition of chromiteThe spinel-structured chromite contains the four following possible compounds:FeO-Cr2O3, Mg0-Cr2O3, FeO. Al2O3 and MgO Al2O3. The chromite may decompose ther-mally with the following reaction equations.FeO. Cr2O3 = FeO + Cr2O3(7)MgO . Cr2O3 = MgO + Cr2O3(8)FeO.Al2Oz=FeO+Al2O3.(9)MgO . Al2O3 = MgO + Al2O3(10)中国煤化工MYHCNMHG，5As for the above four thermal decomposition reactions, its relationship between stan-dard free energy change and temperature is shown in Fig.2. As shown, different tempera-ture has a different effect on the standard free energy change of every reaction, and theirfree energy changes are all positive between 400 and 1000K. As far as thermodynamics isconcerned, it is impossible for chromite to decompose thermally.2.1.2 Oxidation of chromiteChromite will take the fllowing principle reactions with air in molten NaOH to breakits spinel structure.FeO . Cr2O3() + 4NaOH() + -O2(g) = 2Na2Cr04() + Fe2O3(s) + 2H20(g) (1)Cr2O3(s) + 4NaOH() + 202(g) = 2Na2CrO4() + 2H20(g)(12)MgO. Cr2O3(s) + 4NaOH() + ;02(g) = 2Na2CrO4(1) + Mg0(s) + 2H2O(g) (13)Based on the thermodynamic calculation of the above three reactions, the relationshipbetween the standard free energy change and the reaction temperature is plotted in Fig,3.As shown, the free energy changes of main reactions in the procedure of liquid phaseoxidation of chromite are all very negative. That is to say, trivalent chromium is oxidizedto hexavalent chromium easily in molten NaOH by air at the investigated temperature.The reaction tendency is FeO.Cr2O3>Cr2Oz>MgO.Cr2O3. Destroying the spinel structureof chromite, chemical reactions will generate the liquid sodium chromate, solid magnesiumoxide and ferrate oxide. As shown in Fig.3, it is clear that every main reaction has aminimum peak of free energy near 570K. Below the temiperature, the thermodynamictendency of chromite oxidation is stronger with reaction temperature increasing. Above it,the higher reaction temperature is, the weaker the thermodynamic tendency of chromiteoxidation becomes.28E9.(13). MgO.CrO5MgoOCrOsmgO.Cle322340Eq.(12)-C.0, .MgO.ALO, sMgO +A923602 -380FeO.ALO,=FeO+ALO,Fo.CrO.=FeO+CrO120050000700800 900 100001400 500 600700800900~ 100Temperature, KFig.2 Standard free energy versus temperature inFig.3 Standard free energy versus temperaturethe reactions of chromite decomposition.in the procedure of liquid phase oxidationof chromite.In comparison with the analysis ilustrated in the above section, we know that it is veryeasy to break the spinel structure of chromite by chemical reaction. It can be concludedthat the spinel structure of chromite is destroyed by chemical reactions, not by thernaldecomposition.中国煤化工MYHCNMHG.5It is observed from experiment that chromite can not be oxidized to sodium chromatebelow 450C in spite of the strong thermodynamic tendency of chromite oxidization at suchtemperature. The optimization of the procedure of liquid phase oxidization depends onkinetic factors. It includes seeking the optional reaction conditions and intensifying thetransfer of mass and energy so as to make the procedure more conveniently. The kineticexperiments showed the oxidation of chromite were controlled by surface chemical reaction.So, an efficient additive may be found to accelerate reaction rate, shorten reaction time andlower reaction temperature in order to improve the maneuverability of the new process.On basis of the calculation of the other systems in the procedure of liquid phase oxida-tion, the following products may be formed possibly: sodium ferrate, sodium aluminate.sodium silicate, magnesium silicate, and sodium aluminum silicate.Fig.4 is the relationship diagram of theabove three reactions between the standard.240reaction heat and the temperature. Asshown, they are all exothermic reactions,20E9.(13)- MgO. CrO,in which reaction (11) has the largest heatEq.(12)-Cr0,quantity. These exothermic reactions bring360forth lowering energy consumption greatly,which is very benefcial to the new pro-cess. Temperature has similar effect on theE0.11)- FeO.Cr0.480three reactions. While the temperature in-creases, the exothermic quantity of reaction400500600 700800 90010000increases. Interestingly, all the three reac-Temperalure,Ktions have a minimum heat peak during 560-700K. If the reaction temperature can be de- Fig4 Standard reaction heat quantity versuscreased to less than 700K by some eficienttemperature in the procedure of liquidphase oxidation of chromite.way, the reaction heat can be utilized to itsutmost to lower energy consumption.The relation between vapor partial pressure and oxygen partial pressure at a definitetemperature can be deduced on the basis of the relation between the reaction equilibriumconstant and the activities of all its components. Limited by the size of the article, reaction(12) and (13) will not be discussed here. For the main reaction (11) which represents theoxidation of FeO-Cr2O3, there exists:lgPH20- lBPO2 = EIgK -二lgaFerOs - lgaNazCrO. + 2IgoNaOH + -lgaFeOCr2O3 (14)We assume that the activity of solid is 1, the activity of liquid is its weight percent, andthe standard state of gas is 1.013x10Pa. At the practical concentration of liquid phaseoxidation, the activity of NaOH is 0.8 and that of sodium chromate is 0.2. Based on the :above assumption and Eq.(12), Figs.5 and 6 can be obtained.As shown in Fig.5, the practical vapor partial pressure and oxygen partial pressure arein the stable zones of sodium chronate and ferric oxide at the temperature range from400 to 1000K. It indicates that the products of oxidation of chromite are sodium chromateand ferric oxide in the practical reaction atmosphere. The stable zones of Na2CrO andFe2O3 enlarge with the temperature decreasing. High temperature is not beneficial to the中国煤化工MHCNMHG53reaction (11). Air, as oxidant, is not beneficial to the reaction (11) as a result of the smallerrange of vapor partial pressure at the same temperature.However, the vapor partial pressure for producing sodium chromate is relatively wider(~107 atm). The procedure of liquid phase oxidation is operated at the atmospheric pres-sure, so there is a lttle difference between air as oxidant and oxygen in the procedure. Oraccount of the same reason, temperature has little efect on the oxidation of chromite.Fig.6 shows the relationship between reaction atmosphere and temperature more di-rectly, which is consistent with the results as shown in Fig.5.3030 FeO.Cr.O, + NaOH 4002:500K2520600FoO.Cr20, + NaOH7O0Ks 201000K”15Ne.,CrO. +Fe2O,Ng.CrO, +Fe,03.54↓21 0- ↑5 2'900 5000600700800900 1000Temperature, KPo2Fig.5 Vapor partial pressure versus oxygenFig.6 Relation of reaction atnosphere and tem-partial pressure in the oxidation 0perature in the oxidation of FeO-Cr2O3.FeO-Cr2O3.2.2 Thermodynamic of the conventional processThe thermodynamic analysis on the roasting process of conventional chromate pro-duction is carried out in order to conveniently compare with the new process. In the oldprocess, chromite roasts with air at high temperature using sodium carbonate as reactionmedium. Its main reactions are listed as follows:FeO . Cr2O3 + 2Na2CO3 + O2 = 2Na2CrO4 + Fe2O3 + 2CO2(15)Cr2O3 + 2Na2CO3 + 302 = 2Na2CrO4 + 2CO2(16)MgO . Cr2O3 + 2Na2CO3 + "O2 = 2Na2CrO4 + MgO + 2CO2(17)The relationship of the above three reactions between the standard free energy changeand reaction temperature is shown in Fig.7. As shown, the standard free energy changesare all negative at the temperature range from 400 to 1600K. The results indicate it isfeasible in thermodynamic. In addition, temperature has the similar effect on the threereactions. With the temperature increasing, the standard free energy changes towardsnegative, which is favorable to prompt reactions.Fig.8 shows the relationship between standard reaction heat of reaction (15- 17) and .reaction temperature. Similarly to the effect of temperature on free energy change, tem-perature has the same infuence on heat quantities of all the three reactions. There existsa positive peak at the temperature range from 700 to 1120K. At the range from 1060 to中国煤化工MYHCNMHG●541120K, the exothermic quantity decreases significantly. We can see, at such temperaturerange, the reaction (17) becomes endothermic reaction instead of exothermic reaction. Theeffect of temperature on the conventional process is not beneficial in practice because itwill increase the energy consumption.4Eq.(17)- MgO Cry0..120592."E9.(16). Cro.180-120-.160-Eq.(15).Fe0.CrQ.280200 t3200600600010001200 14001600400 600 BOO 1000 1200 1400 160Tomperaturo. KTompuraturo, KFig.7 Standard free energy change versus tem-Fig.8 Standard reaction heat quantity versusperature in the conventional process.temperature in the conventional process2.3 Comparison of the new process with the conventional oneAs calculated above, the thermodynamic analyses of the procedure of liquid phaseoxidation and conventional roasting process are respectively shown in Fig.2 and Fig.6.The standard free energy change of the new process is -426.56kJ/mol, about twice thevalue of the roasting method. Furthermore, the technological condition of the new processis much wider than that of the traditional process. The new process can be operatedmuch more easily. The new process does not emnit carbon dioxide to environment whilethe old process does. As we all know, the emitted carbon dioxide may be another sourceof environmental pollution if not treated properly.The new process is superior to the old one not only in thermodynamic but also inexothermic quantity. What's more, the exothermic negative peak in the new process asshown in Fig.3 can decrease energy consumption greatly. On the contrary, the effect oftemperature on beat quantity as shown in Fig.8 is not favorable to the conventional processat all.3. ConclusionsThis paper generally ilustrates the flow sheet, practical results and features of the newprocess for chromate cleaning production. As discussed above, it can be seen that the newprocess has an attractive prospect for chromate cleaning production. On the basis of thetbermodynamic analysis on chromite oxidation of the new process, the fllwing resultscan be obtained:(1) There are strong reaction tendency and large exothermic quantity in the new pro-cess.(2) Thermodynamically, air as an oxidant has the same efect on oxidation of chromiteas oxygen.(3) The spinel structure of chromite is destroyed by chemical reactions, not by thermal中国煤化工MYHCNMHG55decomposition.(4) The products of sodium ferrate, sodium aluminate, sodium silicate, magnesiumsilicate and sodium aluminum silicate may be formed possibly.(5) In comparison with the conventional process, the new process has much widerreaction conditions, and no carbon dioxide emitted.REFERENCES1 s.W. Cheng, Y. Ding and C.R. Yang, Chromate Production Proceses (Beijing; Chemical IndustryPress, 1987).2 s. Aquayo and J.C. Aghumade, Rev, Metal, 27(6) (1992) 403.3 D. Chandra, C.B. Magee and L. Lfter, PB83- 106781. Denver Research Inst., 1982.4 G.L. Hundley, PB88 -240529. (US)Bureau of Mines, Albany, OR., 1988.5 G.L. Hundley, D.N. Nilsen and R.E. Sienens, PB86- 132719. (US)Bureau of Mines, Albany, OR., 1985.6 J.W. Holtz and N.Y. Solvay, USA Pat. 4173618, 1979.7 M.A. Kapland, M.D. Annapolid and M.W. Robinson, USA Pat. 4504321, 1985.8 R. Weber, B. Rosenow and H.D. Block, USA Pat. 5273735, 1993.9 Y. Zhang, Z.H. Li, ZK. Wang and J.Y. Chen, Progress n Chenistry 10(2) (1998) 172.10 S.L. Zheng, Y. Zhang and JL. Cui, Computer and Applied Chemistry 15(6) (1998) 373.11 Y.J. Liang and Y.C. Che, Thermodynamic Dalabase Handbook of Inorganic Substances (Shenyang:Northeast University Press, 1996).12 0. Knacke, Thermochemical Properties of Inorganic Subotances (Duseldorf: Springer Verlag, 1991).中国煤化工MYHCNMHG