Aspects of alumina dissolution during the smelting reduction of chromite in MgO-Al 2. - CaO-SiO 2

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1 Aspects of lumin dissolution during the smelting reduction of chromite in MgO CO slg system t 1650 o C MASIYA, TRUST T 1, TSAPAYI ALLAN, TSOMONDO BODMAN MC, 1 KUIPA PARDON K 3 AND SIMBI DAVID J 3* 1 Institute of Mining Reserch, Fculty of Science, University of Zimbbwe, MP 167, Mount Plesnt, Hrre, Zimbbwe Deprtment of Metllurgicl Engineering, Fculty of Engineering, University of Zimbbwe, P.O. Box MP167, Mount Plesnt, Hrre 3 School of Engineering Sciences nd Technology, Chinhoyi University of Technology, P. Bg 774, Chinhoyi, Zimbbwe 3* Corresponding uthor emil ddress: dsimbi@cut.c.zw or dsimbi@gmil.com Abstrct The effect of MgO/ rtio on the reduction kinetics of chromite in MgO CO slg system by crbon dissolved in high crbon ferrochromium lloy melts ws investigted. Experiments were performed in n induction furnce t 1650 o C with chromite, Cr concentrtion of 4 wt%. The results show tht the dissolution of chromite s CrO is sensitive to chnges in MgO/ rtio nd not the combined MgO + totl. Chnges in the MgO/ rtio ffect the slg composition, therefore, influencing the trnsport properties of rectnts through chnges in the viscosity of the slg. The conditions which minimize CrO frction in the slg such s the replcement of CO by MgO nd high MgO/ rtio were identified s enhncing the chromite reduction rection nd extent. The sme conditions lso influence the kinetics of dissolution, hence the extent of lumin crucible dmge. Since the period of fst chromite reduction coincides with minimum dissolution of the lumin crucible, the overll reduction process could be terminted fter 30 minutes to minimize the erosive wer on the refrctory. There is no benefit to be derived in running the smelting process for more thn 30 minutes serve to llow sufficient time for lloy nd slg to completely seprte. Key words: chromite, spinel, smelting reduction, dissolution kinetics, slg bsicity, lumin crucibles. Introduction Chromite occurs in nture s solid solution of spinel compounds hving the generl formul RO.R in which the divlent ions (Fe +, Mg + ) occupy tetrhedrl sites while trivlent ions (Al 3+, Fe 3+, Cr 3+ ) occupy octhedrl sites. The spinel compounds hve complex minerlogicl properties nd these hve diverse influences on the smelting reduction chrcteristics of chromite. Mny studies hve been conducted to provide thermodynmic nd kinetic dt on chromite smelting reduction (Curr et l., 1988; Morit et l., 1988, Morit et l., 1988b; Morit nd Sno, 1988; Hino et l., 1994; Tsomondo et l., 1997; Hino et l., 1998; Divkr et l., 00; Vldez et l., 00). Brcz et l. (1971) proposed generl eqution for chromite dissolution in smelting slg: (Fe, Mg)O.(Cr, Al) O 6 + C (FeO) + (CrO) + (MgO + ) + CO (1) As reported by Murkw et l. (1985), the dissolution of chromite is to lrge extent influenced by spinel (MgO. ) formtion nd this occurs when the combined MgO nd content is in the rnge 945 wt% depending on the temperture. It is for this reson tht the spinel formtion phenomenon nd the ttributes regrding the retrdtion of kinetics of chromite reduction hve received much ttention in the vilble literture (Med nd Sno, 1986; Xio nd Hollp, 1993; Tsomondo, 1998). Minerlogicl studies by Oosthüyzen nd Viljoen (198) show tht slgs ssying >11 wt% chromium contin undissolved nd prtly ltered chromite prticles nd these hve been demonstrted to exhibit vrying degrees of metlliztion. The solubility of Cr in the COCr slg system t 1500 o C, for exmple, increses with decresing slg bsicity nd oxygen prtil pressure (Mun, 1985). It would pper tht silic, SiO dissolves the MgO. spinel coting round the unreduced chromite grins, thereby fcilitting their dissolution (Morit et l., 1988; Morit et l., 1988b). There is lso the ttendnt lowering of the liquidus temperture of the slg s the formtion of the higher melting point MgO. spinel is circumvented. This would explin the role of silic s flux in fcilitting s well s improving the reduction of chromite (Neuschutz, 199) s indicted by eqution (): (MgO.Cr + nsio + C (CrO.nSiO ) liq + CO + MgO () The liberted MgO rects with the in the slg to form the MgO. spinel. Tking the specific cse (eqution ) when n=1, it would seem tht Cr goes into solution s the chromous orthosilicte, nd this is consistent with the results observed by Hely nd Schottmiller (1964). As noted Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 15

2 16 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 previously, the ssocition of the chromous ions with the silic implies generl segregtion of the slg components with silicrich phse tht is lso being enriched with divlent chromium, CrO. The enhnced solubility of the MgO.Cr in MgO CO slgs by lumin under reducing conditions is ttributed to the libertion of Cr with the MgO proceeding to prticipte in forming the MgO. spinel. The improved solubility my lso rise from decrese in slg bsicity, whereby cts s n cidic oxide. The formtion of the bsic divlent chromium oxide is then more fvourble. This is observed on mcroscle s n improvement in chromium oxide solubility in which the reduction of CrO from the liquid CrO slg is considered to proceed firly rpidly. There is serious flw, however, in overlooking the thermodynmic connottions of the predominnce of CrO in the slg, nd in mking the ssumption tht reduction would proceed t fster rte simply becuse the chromium oxide is in solution (Morit nd Sno, 1988; Tsomondo et l., 1997; Simbi nd Tsomondo, 00). Work by Med nd Sno (1986) nd lter Xio nd Hollp (1993) shows tht incresing in CO CrO x system t constnt slg bsicity generlly increses the ctivity of Cr whilst depressing tht of CrO, implying tht the solubility of Cr is hindered by. This is contrry to the finding of Morit et l. (1988), who investigted Cr solubility in MgO CO slgs to which incresing mounts of lime, CO, were dded t tmospheric pressure. As suggested by Tsomondo (1998), it could be tht prtil oxygen pressure exerts greter influence thn bsicity. At high oxygen prtil pressure, the iron oxide of the chromite is preferentilly reduced leving behind unltered nd virtully undissolved spinel phses rich in Cr. Under these conditions, the formtion of the divlent chromium oxide is unfvourble, nd this my well explin the prevlence of the unltered spinel phse. Simplifying rection (1) for the chromium oxide nd expressing it in its ionic form, we obtin eqution (3): Cr 3+ + O Cr +O (3) in which oxygen toms re removed by rection with crbon to form crbon monoxide. C + O CO (4) The equilibrium constnt for rection (3) is given by the expression: K = Cr Cr (5) where Cr + nd Cr 3+ re the ctivities of the divlent nd trivlent chromium, respectively nd O nd. 0 the oxygen (prtil pressure) nd oxy nion ctivities, respectively. By rerrnging eqution (5), the divlent chromium oxide s function of oxygen prtil pressure nd trivlent chromium oxide concentrtion cn be written s: Cr Cr + 3+ O = O (6) It becomes pprent, therefore, tht the lower the oxygen ctivity (or prtil pressure with reference to the CO equilibrium), the higher is the divlent chromium oxide frction in the slg. With the bove nlysis in mind, it follows tht the dissolution of chromium spinel is chemiclly driven process s opposed to simple phse trnsformtion (Tsomondo, 1998). The solubility of chromite in MgO SiO slgs is enhnced by the ddition of CO to the system, under ordinry tmospheric conditions (Morit et l., 1985). However, t low oxygen prtil pressure, the ddition of CO results in lowering of the Cr + concentrtion in the slg, hence the solubility of chromite. This observtion is consistent with the dicttes of rection (3). Hyhurst (1974) identified the mjor slg components ssocited with the production of ferrochrome s comprising mgnesium spinel (MgO. ), forsterite (MgO.SiO ), northite (CO..SiO ) nd cordierite (MgO..5SiO ). The reltive mounts of these phses in the slg will vry considerbly depending on composition nd temperture. Anorthite nd cordierite generlly hve low liquidus tempertures, wheres the mgnesium spinel nd forsterite hve higher melting points. Depending on slg composition, both the spinel nd forsterite cn coexist in the liquid phse with northite nd cordierite t tempertures <1500 o C. However, when the proportion of spinel nd forsterite re rised, ssuming the temperture is mintined constnt, precipittion of solid spinel nd forsterite phses will occur nd this will be ccompnied by simultneous increse in slg viscosity. Clcultions by Tsomondo (1998) of slg viscosity from isoviscosity contours for the MgO CO system contining 50% SiO t 1500 o C seem to indicte tht it is not the MgO+ combintion tht is primrily responsible for viscosity chnges, but rther the proportions of these compounds tht will be present in the slg. From the foregoing discussion, there is need to define n operting window with respect to slg composition nd temperture if we re to successfully smelt nd reduce

3 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, chromite to chromium. This would men selection of n pproprite slg composition tht would minimize spinel formtion (nd/or other phses) nd would concurrently stbilize Cr rther thn the divlent, CrO species. The mount of silic to be dded should be just sufficient to flux the MgO nd since excess SiO would led to the formtion nd stbiliztion of CrO. Experimentl Method Apprtus The pprtus used in this investigtion hve been previously described (Simbi nd Tsomondo, 00). Rectnts contined in density lumin crucible of inner dimeter 50 mm nd height of 70 mm were plced in grphite crucible, which provided resistive heting. The induction furnce ws equipped with Eurotherm PID regultor for temperture control. The grphite crucible ws shielded from the tmosphere by the cst lumin crucible. To minimize temperture losses, the spce between the crucible rrngements ws filled with high temperture fibre insultion mteril. An inert tmosphere ws mintined bove the slg bth by rgon gs flowing t rte of Lmin 1. Mterils The mterils used in this study hd the following composition by weight: 50.08Cr 14.8Fe 4.71SiO MgO0.30CO nd 64.78Cr6.3Fe 6.90C1.4Si0.04S0.07Ti0.014V. Regent grde oxides of mgnesium, clcium, silicon nd luminium were used to prepre the mster slgs for the MgO CO system. The formultions, nticipted liquidus temperture, T liq, nd bsicity B of the three slg types re given in Tble 1. Tble 1: Composition of slg types in wt% Bse Slg MgO CO SiO T liq ( o C) B Type ~ Type ~ Type ~ Experimentl Procedure For n experimentl run, bout 10 g of high crbon ferrochromium lloy (HCFeCr) ws melted down in high frequency induction furnce. The temperture ws llowed to stbilize for 10 minutes t the preset vlue, fter which smple of the lloy ws tken. The weight of metl remining in the bth ws clculted s the difference between the initil lloy weight nd tht of the smple. This ws followed by the ddition to the molten lloy of 65 g of the MgO CO slg to which 10 wt% of pulverized Cr hd been dded. The moment or time of ddition ws tken s zero time for the commencement of the Cr reduction rection. In ech cse, drop in temperture of bout 50 K ws observed, but the temperture reverted to the preset vlue within less thn minutes. A liquid ws formed during the sme time intervl. Molten slg smples were collected periodiclly using molybdenum wire. Helium gs quenching ws pplied to ll the smples, followed by weighing to enble the determintion of the mount of metl nd slg remining in the crucible. The smples were lter nlyzed for constituent phses by Xry fluorescence (XRF) spectroscopy nd by wet chemicl nlysis in the cse of CrO. The rection rte ws obtined from the grdient of concentrtion versus time curves. Results nd Discussion Effect of MgO/ rtio Typicl curves for the reduction of chromite in MgO CO slgs with vrying MgO/ rtios re shown in Figure 1. Incresing the MgO/ from 0.33 to unity cn be seen to improve the kinetics with reduction rtes of 1.13 x 10 4 nd.7 x 10 4 mol cm 3 s 1, respectively being obtined for the first 10 minutes of the experimentl run. The profiles of the two curves re similr indicting tht the mechnism for the reduction of chromite in MgO CO FeO Cr slgs is not ffected by chnges in the MgO/ rtio. It would pper, therefore, tht t constnt temperture trnsport properties of the slg re determined by composition rther thn by the MgO/ rtio. Consequently, chnges in the composition re expected to lter the viscosity of the slg tht in turn hs direct effect on the dynmic properties of rectnts present. In the first 10 minutes of the rection, the reduction of chromite displys mrked deprture from the theoreticl reduction pthwy with pseudoequilibrium condition being estblished within 30 minutes of commencing of the rection. In contrst, the reduction process in the slg with low MgO/ rtio ws still in progress t the end of 50 minutes when the rection ws terminted. Previous investigtors (Simbi nd Tsomondo, 00) hve shown tht Cr reduction increses s the bsicity is rised. In the current work, bsicity djustments were chieved t constnt MgO by vrying the CO nd SiO contents of the slg, nd t unity bsicity the MgO/ rtio ws fixed t 1.6. On the bsis of these results, it would seem resonble tht MgO increments during bsicity djustments improve Cr reduction kinetics better thn when CO is used for the sme purpose. Jen nd Dressler (1994) showed tht for specific MgO/ rtio, recovery

4 18 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 Figure 1: Kinetics of Cr reduction from slgs of different MgO/ rtios with the sme totl MgO nd contents t 1650 o C flls with incresing CO content, but the uthors were unble to explin this observtion. Unlike MgO, lime stbilizes CrO in the slg, nd this my well be the reson for the discrepncy observed in chromite reduction kinetics when the two bsic oxides re exmined seprtely. Eqully, MgO stbilizes Cr in the slg through the formtion of MgO.Cr, but this only occurs under reducing conditions. However, in the presence of, the formtion of MgO. is thermodynmiclly fvourble, nd Murkw et l. (1985) ttribute the limit in the extent of smelting reduction process encountered to the spinel formtion s thin shell round the chromite grins. A constnt mount of MgO+ ws used in the current work, thereby circumventing the possibility of spinel formtion, hence its retrding effect on chromite reduction kinetics. In order to illustrte the effects of MgO/ rtio on slg viscosity (Slg Atls, 1995), use ws mde of the MgO CO system t 40% SiO nd temperture of 1500 o C (Figure ). Figure : Isoviscosity MgO CO system t 40% SiO nd temperture of 1500 o C (Slg Atls, 1995)

5 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, From the isocontours displyed, incresing the MgO/ rtio from 0.33 to unity lters the viscosity by pproximtely poise. If such viscosity vritions were ssumed to hold in chromium oxide contining slgs, then for constnt Cr concentrtion, the slg with high MgO/ rtio should be substntilly less viscous, nd is expected to exhibit fster reduction kinetics. Compring the chromium oxide mss trnsfer coefficients further highlights the influence of the MgO/ on the smelting reduction kinetics. In slgs with 10MgO30, the chromium oxide mss trnsfer coefficient ws found to be 3.6 x 10 3 cms 1, while in the slg contining 0MgO0, it ws n order of mgnitude higher t 1.35 x 10 cms 1. The mss trnsfer coefficient results drw direct correltion with slg viscosity mesurements previously noted. In the foregoing discussion, the effect of lumin on chromium oxide reduction hs been wholly ttributed to its effect on the liquidus temperture, hence slg viscosity. There re no specific ssumptions regrding the bove. Kinetics of lumin dissolution during chromite smelting reduction Severe loclized erosive wer of lumin crucibles ws observed t the slgmetl (lloy) interfce. Similr observtions were mde by Iguchi et l. (1978) nd were ttributed to the effect of metl phse cting s chrge crrier for the rection: O+ e ( O ) (7) The overll effect of the dditionl supply of oxygen nions to the slgmetl lloyrefrctory contct re is intense ttck of the refrctory. In the present work, lumin pickup from crucible erosive wer during slg preprtion ws estimted t <.0 wt% fter 60 minutes of homogeniztion. Physicl exmintion of the crucible reveled little if ny evidence of loclized erosive wer. However, exmintion of the lumin crucibles fter chromite smelting reduction experiments showed extensive erosive wer of the slg/ crucible contct re. Chemicl nlysis of slgs smples collected during experimentl runs showed progressive increses in lumin content (Figure 3). Figure 4 shows the vrition of concentrtion in the slg t 1650 o C s the bsicity is rised from 1 to 1.6 with the chromium oxide concentrtion t 1.5 x 10 mol (~4 wt% Cr ). There is grdul increse in content in the slg s the bsicity is rised. Iguchi et l. (1978) used electrochemicl cells to study the nture of the preferentil ttck of refrctory mterils by slgs t the slgmetl lloy phse boundry. Their mesurements indicted tht ttck ws either cidic or bsic depending on the composition of the slg. The effect of slg bsicity ws ttributed to bsic ttck of the lumin: Al O + ( O ) ( AlO ) 3 (8) This reltionship explins the role of slg bsicity in promoting the dissolution of of the crucible. In this instnce, the mphoteric behves s wek cid, nd the rection will proceed subject to the provision of oxygen nions by the bsic slg. However, the requirement for oxygen nions to sustin rection (8) merits further considertion. In this regrd, comprison cn be drwn Concentrtion (wt%) Figure 3: Vrition of the slg composition with rection time

6 0 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 Figure 4: Vrition of content with slg bsicity during chromite reduction t 1650 o C between the reltively smll mount of tht dissolves during the premelting stge of slg preprtion, nd the high vlues reported following the smelting reduction rection. dissolution is enhnced considerbly by the presence of the divlent chromium oxide, which is wekly bsic. In highly bsic slgs, the CrO frction is low, nd the mjor influence on the dissolution of is due minly to the MgO nd CO. At low bsicity, the CrO frction is considerbly high s to contribute significntly to the dissolution of. While the reduction of Cr from MgO CO slgs reches pseudoequilibrium conditions in bout 0 minutes (Tsomondo nd Simbi, 00), the dissolution of lumin nd its subsequent ingress into the slg will continue throughout the experimentl run period. It is lso possible tht the observed erosive wer of the lumin crucibles is due to mechnicl effects rising from convective mss flow conditions in the metl nd slg phses (Szekely nd Stnek, 1969). Nito et l. (199) used the rotting cylinder technique to study the dissolution chrcteristics of in CO nd N O slgs, nd concluded tht incresing slg bsicity increses both the rte nd extent of dissolution. However, Endell et l. (1939) were ble to demonstrte mechnicl contribution to the overll destruction of refrctory mterils during the smelting reduction of chromite which ws ttributed to the combined effect of fluid flow, hydrodynmics nd solvent ction of the slg. The effect of Cr concentrtion on the dissolution of during chromite reduction is shown in Figure 5. Our results suggest tht the dissolution of lumin decreses with incresing Cr concentrtion in the slg. This behvior my be explined on the bsis of the vrition in the physicochemicl properties of the slg tht ccompnies the chnges in Cr concentrtion. Firstly, there is generl increse in slg viscosity, which retrds the diffusion of products ( ) from the erosive wer relted rections into the slg wy from the slgcrucible interfce. Secondly, the mount of CrO decreses s the totl chromium oxide concentrtion in the slg is rised with slg becoming more cidic on the bsis of the predominnce of the cidic trivlent chromium oxide. Xio s work confirms the decrese in the CrO frction s totl chromium oxide concentrtion in the slg is rised (Xio, 1993). The effect of temperture on dissolution is shown in Figure 6. Temperture exerts mrked influence on the kinetics of dissolution during the smelting reduction of chromite. At 1650 o C nd bove, most of the chromite will hve undergone reduction within the first 10 minutes of rection time (Simbi nd Tsomondo, 00). Figure 6 shows tht

7 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 1 Figure 5: The effect of initil Cr concentrtion in the slg on the dissolution kinetics of t 1650 o C t slg bsicity ~1.3 Figure 6: The vrition of dissolution during chromite reduction with temperture during the sme time intervl, only smll mount of underwent dissolution while Figure 1 shows tht Cr reduction improved s the rtio of MgO/ ws rised. The vrition in concentrtion with time for two MgO/ rtios is presented in Figure 7. A low MgO/ rtio implies miniml dissolution of crucible into the slg. Thus for the slg with the low MgO/ rtio of 0.33, the reduction kinetics of the chromite within the first 10 minutes will be significntly retrded by n increse in the concentrtion. By combining the dt

8 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 Figure 7: The effect of initil MgO/ rtio in the slg on the dissolution of during chromite reduction t 1650 o C in Figures 1 nd 7, nd ssuming tht the chromite smelting reduction rection ttins pseudoequilibrium conditions fter 0 minutes, it is possible to terminte the process thereby sving the lumin crucible from unnecessry chemicl erosive wer. There is therefore nothing to be gined by operting the smelting reduction process for period longer thn 0 minutes. Conclusions 1. Chromite reduction experiments in the slg system MgO CO verified tht the spinel, MgO. dissolution is not of prmount importnce in determining the extent of the reduction process. Rther, spinel precipittion s distinct solid phse hs the tendency to rise the viscosity of the slg thereby dversely ffecting the mobility of rectnt species in the slg phse. While spinel formtion hs retrding effect on the reduction rte, it is probbly not responsible for the reported rection ceiling.. There is no evidence to suggest tht spinel formtion plys ny role during the smelting reduction of Cr in MgO CO slgs. Increments in chromic oxide insted influence the reduction process indirectly by its effect on liquidus temperture of the slg, nd consequently slg viscosity. 3. By mking pproprite composition djustments, the chromium concentrtion in the slg my decrese to ~40 wt% in less thn 60 minutes of process time. 4. Slgs sturted with lumin ( ) minimize refrctory lining wer, nd the use of short process time is n dded dvntge. Acknowledgements This pper is dedicted to the work of the lte Dr B.M.C. Tsomondo of the Institute of Mining Reserch who s postgrdute reserch student in the Deprtment of Metllurgicl Engineering, University of Zimbbwe under the supervision of Professors D.J. Simbi nd E. Nvr crried out the experimentl work reported. The uthors cknowledge mteril nd finncil support from the Zimbbwe Mining nd Smelting Compny, the Swedish Agency for Reserch nd Economic Development (SAREC) nd the Germny Technicl Coopertion (GTZ). References Brcz, N.A., Jochens, P.R. nd Howtt, D. D. (1971). The mechnism nd kinetics of reduction of Trnsvl chromite ores. Electric Furnce Proceedings, Toronto, Cnd, Iron Steel Society of Americn Institute of Mechnicl Engineers, 9: Curr, T.R., Wedepohl, A. nd Eric, R.H. (1988).Molten slgs nd fluxes. Proceedings of the 3 rd Interntionl Conference on Molten Slgs nd Fluxes, Glsgow, Scotlnd. The Institute of Metls, pp Divkr, M., Gönerup, M. nd Lhiri, A.K. (00). Simultneous reduction of oxides nd dissolution of lumin in stinless steelmking slgs. Ironmking Steelmking, 9: Endell, K., Fehling, R. nd Kley,R. (1939). Influence of fluidity, hydrodynmic chrcteristics solvent ction of

9 Journl of Zimbbwe Studies: Science, Technology & Helth, Vol. 1, 01 3 slg on the destruction of refrctories t high temperture. Journl of the Americn Cermic. Society, : Hyhurst, A. (1974). Crystlliztion processes in highcrbon ferrochromium slg. Proceedings Interntionl FerroAlloys Conference Johnnesburg. South Africn Institute of Mining nd Metllurgy, 74: Hely, G.W. nd Schottmiller, J.C. (1964). The chromium oxide silicte system t low oxygen pressure. Trnsctions of Metllurgicl Society, Americn Institute of Mechnicl Engineers, 30: Hino, M., Higuchi, K., Ngsk, T. nd Bny, S. (1998). Thermodynmic estimtion on the reduction behviour of ironchromium ore with crbon. Metllurgicl nd Mteril Trnsctions, 9B: Hino, M., Higuchi, K., Ngsk, T. nd Bny, S. (1994). Phse equilibri nd ctivities of the constituents in FeO.Cr MgO.Cr spinel solid solution sturted with Cr. Iron nd Steel Institute of Jpn Interntionl, 34: Iguchi, Y., Yurek, G.T. nd Elliot, J. F. (1978). Electrochemicl spects refrctory ttck. Proceedings of the 3 rd Iron nd Steel Congress, Americn Society of Mining nd Iron Steel SocietyAmericn Institute of Mechnicl Engineers. pp Jen, B.C. nd Dressler, W. (1994). Optimistion of the slg composition nd operting temperture for smelting of Cndin chromites. Proceedings of Extrction nd Process Division (EPD) Congress. G.W. Wrren, (editor). The Minerls, Metls nd Mterils Society. pp Med, M. nd Sno, N. (1986). Thermodynmics nd kinetics of reduction of chromium oxide in silicte slgs. Proceedings of the 6 th Technicl Conference Iron Steel SocietyAmericn Institute of Mechnicl Engineers, 6: Mun, A. (1985). Slgmetl equilibri involving chromium s component. Proceedings of the Interntionl Conference Mining, Science. nd Technology. : , Sndton, South Afric. Morit, K. nd Sno, N. (1988). Solubility of MgO.Cr in MgO CO. Proceedings of the 3 rd Interntionl Conference on Molten Slgs nd Fluxes. Glsgow, Scotlnd. The Institute of Metls, pp Morit, K., Sno, N. nd Tkym, N. (1985). Effect of slg composition nd temperture on the reduction rte of chrome oxide. TetsutoHgnè (Journl of Iron Steel, Institute of Jpn), 71: S Morit, K., Inoue, A., Tkym, N., nd Sno, N. (1988). The solubility of chromite in MgO CO melts t 1600 o C under reducing conditions. Tetsu to Hgné. (Journl of Iron Steel Institute of Jpn), 74: Morit, K., Shibuy, T. nd Sno, N. (1988b). The solubility of chromite in MgO CO melts t 1600 o C in ir. TetsutoHgné. (Journl of Iron Steel Institute of Jpn), 74: Murkw, K., Anezki, S., Hirt, T. nd Ishikw, M. (1985). Report No , Steelmking Committee of Jpn Society for Promotion of Science, S98. Nito, H. Isomoto, M., Kishimoto, M., Nkshim, K. nd Mori, K. (199). Rte of dissolution of solid lumin into molten slgs. Proceedings of the 4 th Interntionl Conference on Molten Slgs nd Fluxes, Sendi. Iron Steel Institute of Jpn, pp Neuschutz, D. (199). Kinetic spects of chromite ore reduction with col t 100 to 1500 o C. Proceedings of Interntionl FerroAlloys Conference 6, Cpe Town, South Africn Institute of Mining nd Metllurgy, 1: Oosthüyzen, E.J. nd Viljoen, E.A. (198). Mechnism of reduction of chromium oxide dissolved in the CO MgO slg by solid crbon. Proceedings of the XIV Interntionl Mining Congress, Toronto, Cnd. Cndin Institute of Mining, 6.1. Simbi, D.J. nd Tsomondo, M.B.C. (00). Aspects of smelting reduction of chromite ore fines in COFeO Cr slg system by crbon dissolved in high crbon ferrochromium lloy bth. Ironmking nd Steelmking, 9: Slg Atls, (1995). nd Edition, Verlg, Sthlheisen, M.ß.H. Düsseldorf, pp Szekely, J. nd Stnek, V. (1969). The effect of surfce tension on the dissolution of solids in lminr flow nlysis. Chemicl Engineering Science, 4: 1117 Tsomondo, M.B.C. (1998). Smelting Reduction of Chromite Ore by Crbon Dissolved in Highcrbon Ferrochromium Alloy Bth. PhD Thesis. Deprtment of Metllurgicl Engineering, University of Zimbbwe, Hrre, Zimbbwe. Tsomondo, B.M.C. nd Simbi D.J. (00). Kinetics of chromite ore reduction from MgOCO slg system by crbon dissolved in high crbon ferrochromium lloy bth. Ironmking Steelmking, 9: 31. Tsomondo, B.M.C., Simbi, D.J. nd Nvr, E. (1997).Thermodynmics chromite reduction in crbon sturted highcrbon ferrochromium lloy bth. Ironmking nd Steelmking, 4: Vldez, M., Prpkorn, K., Crmb, A.W. nd Sridhr, S. (00). Dissolution of lumin prticles in CO SiO MgO slgs. Ironmking nd Steelmking, 9: Xio, Y. (1993). Thermodynmic Study of CrOx Contining Slgs. Doctor of Technology Thesis, Helsinki University of Technology, Helsinki, Norwy. Xio, Y. nd Hollp, L. (1993). Determintion of ctivities in slgs contining chromium oxides. Iron Steel Institute of Jpn Interntionl, 33: 6674.