The kinetics of the reduction of manganous oxide by graphite

Transcription

1 The kinetis of the redution of manganous oxide by graphite by W. J. RANKIN*, Ph.D., M.S.A.I.M.M. and J. S. J. VAN DEVENTERt, Hons. B. Ing. (Chem. Ing.) M.S.A.I.M.M. SYNOPSIS Mixtures of MnO and graphite were reated under argon at temperatures ranging from 12 to 1425 C, and the loss in mass of the mixture was reorded ontinuously. The results showed that the rate of redution of MnO inreased as the temperature inreased and the ratio of MnO to C dereased. The effet of the partile size of MnO and of graphite on the rate of redution at I35 C was very small in the range 63 to 25 flm for MnO and 45 to 355 flm for graphite. The rate of redution at I35 C was inreased markedly by the addition of Na.CO. and K.CO., was slightly inreased by the addition of Fe.O., was unaffeted by the addition of CaD, MgO, and AI.O., and was dereased by the addition of SiO Some CO. was always present in the effluent gas, and this was taken as evidene that the redution of MnO ourred by way of CO. as an intermediate produt. The ativation energy of the rateontrolling step was 57,5 kal, and this was similar to the measured ativation energy for the gasifiation of graphite by CO., namely, 61,S kal per mole of graphite. A simple kineti model was derived on the assumption that the gasifiation of graphite was the rate-ontrolling step, and the results over the temperature range 12 to I425 C, when plotted in the appropriate form, yielded straight lines as predited by the model. SAMEVATTING Mengsels van MnO en grafiet is onder argon by temperature van 12 tot 1425 C laat reageer en die massaverlies van die mengsel is voortdurend geregistreer. Die resultate het getoon dat die reduksietempo van die MnO toegeneem het namate die temperatuur en die verhouding van MnO tot C gestyg het. Die uitwerking van die partikelgrootte van die MnO en die grafiet op die reduksietempo by 135 C was baie gering. Die partikelgrootte was in die bestek van 63 tot 25 flm vir MnO en 45 tot 355 flm vir grafiet. Die reduksietempo by 135 C is merkbaar verhoog deur die byvoeging van Na.CO. en K.CO., net effens verhoog deur die byvoeging van Fe.O., glad nie deur die byvoeging van CaD, MgO en AI.O. be"invloed nie en verlaag deur die byvoeging van SiO Daar was altyd CO2 in die afvalgas aanwesig en dit is beskou as 'n bewys dat die reduksie van MnO plaasvind met CO. as 'n tussenproduk. Die aktiveringsenergie van die tempobeherende stap was 57,S kal per mol grafiet en dit stem ooreen met die gemete aktiverengsenergie vir die vergassing van grafiet met CO2, naamlik 61,5 kal. Daar is 'n eenvoudige kinetiese model, afgelei met die veronderstelling dat grafietvergassing die tempobeherende stap was en toe die resultate oor die temperatuurbestek van 12 tot 1425 C in die gepaste vorm gestip is, het hulle reguit Iyne gegee soos die model voorspel het. This work was undertaken to provide fundamental information on the kinetis and mehanism of the redution of manganous oxide by graphite, and on the effet of the gangue omponent of ores on the rate of redution. This information should further the knowledge and understanding of the omplex reations and transformations that our in the redution of manganese ores during the prodution of ferromanganese alloys. Manganese is present in manganese ores as higher oxides and siliate, and during the early stages of redution these are redued to MnO by the reduing atmosphere in the furnae!. The redution of MnO to arbide and metal are the most important reations in the prodution of ferromanganese alloys, and an our by two mehanisms: (i) redution in the solid state by arbon, and (ii) dissolution of MnO in the slag, followed by its redution from the slag by a solid arbonaeous reduing agent or arbon-saturated alloy. The latter mehanism has been observed in laboratorysale experiments! and is most probably the major mehanism in the submerged-ar furnae, but whether the former mehanism also ours to a signifiant extent is not known. The former mehanism is the subjet of the study reported in this paper. It is generally aepted2, a that, when a metal oxide is redued by arbon, the reation ours by way of *Comalo Researh, Thomastown, Australia; formerly University of Stellenbosh. ton national servie. arbon dioxide as a gaseous intermediate produt: MO+CO=M+CO2 (1) """""'" CO2+C=2CO. (2) """"""'" The overall reation is the sum of equations (1) and (2), and is the same as if redution had ourred by diret ontat of the oxide and arbon: MO+C=M+CO (3) Thermodynamially, there is no differene between equations (1) and (2) on the one hand and equation (3) on the other. If reations (1) and (2) our under a ertain set of onditions, so an reation (3). However, equation (3) involves reation between solids, whereas equations (1) and (2) involve reations between solids and gases and, from kineti onsiderations, reations (1) and (2) should our muh faster than reation (3). This is the main argument in favour of the 'gaseous intermediate' mehanism. The rate of redution of many oxides by graphite is ontrolled by the rate of gasifiation of arbon by equation (2), the so-alled Boudouard reation. This reation is atalysed by small amounts of impurities, partiularly alkali-metal oxides and to a lesser extent by metals and other metal oxides4. When gangue materials are present, the rate of redution of MnO by arbon may be inreased beause of the atalyti effet of the gangue on the Boudouard reation. The typial omposition of a South Afrian Mamatwan manganese ore is given in Table I, from whih it is apparent that the major gangue omponents are CaO, SiO2, MgO, and iron oxides. The effet of these, as well as of A12a, Na2COa, and K2COa, JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JULY

2 on the rate of redution of MnO was exaed in this study. Thermodynamis of Redution of MnO by Carbon For the redution of MnO by arbon, a system of at least three omponents, namely, manganese, oxygen, and arbon is neessary. For a two-dimensional representation of the stability fields of the system, two degrees of freedom are possible, and these are most onveniently taken as oxygen potential and temperature. Therefore, from the phase rule, there are three o-existing phases. Sine one of these is always a gaseous phase, two ondensed phases must be present in eah stability field. Eah pair of ondensed phases possesses a region of stability in the spae between oxygen potential and temperature, and this is detered by the reations between oxygen and three ondensed phases. A urve an be drawn for eah ombination; on one side of the urve two of the three phases an o-exist, and on the other side another two of the phases an o-exist. In this way, the entire spae is divided into regions in eah of whih only one pair of ondensed phases o-exists. A graphial representation of this type is known as a Pourbaix diagram. The omposition of the gas phase in equilibrium an be detered from the reation between oxygen, the ondensed phases, and the gaseous speies. When the pressure of the gas speies is fixed, a urve an be drawn in the region of the two ondensed phases, and by variation of the pressure a surfae an likewise be drawn. Eah gaseous speies has a partial-pressure surfae over eah pair of ondensed phases, the total pressure being the sum of these surfaes. The Pourbaix diagram onstruted by Downing5 for the Mn-O-C system is shown in Fig. 1. The gaseous phase is omposed of CO, CO2, and manganese vapour, and the total pressure at any point is the sum of the partial pressures of CO, CO2, and Mn. Lines of onstant total pressure are shown in Fig. 1 for values of 1, 1-2, 1-4 and 1-6 atm (1 atm=1o1,325 kpa). At a total pressure of 1 atm, Mn2a and Mna4 are unstable in the presene of arbon at all temperatures and reat to form MnO. The sequene of redution of MnO by arbon at atmospheri pressure is shown by the urve for 1 atm of total pressure from O C (line AB in Fig. 1). Initially, the gaseous phase is pure CO2, but the amount of CO inreases with inreasing temperature, and TABLE I TYPICAL ANALYSIS OF MAMATWAN ORE Constituent Mass % MgO AI.O. SiO- CaO Mn_O. Fe_O. Cr_O. p.os K.O CO- H_O 3,1 <,1 4,27 12,96 55,9* 5,99,36,17,21 14,91,54 This is the total manganese ontent of the ore expressed as * Mn_O. ii u '" er Go III.S! 5 ~ 1 I 15 A ',Mnlll, " \ ", M7Ca ", 1 2B Temperature. K Fig. I-Pourbaix diagram for the Mn-O-C systems log PM. log Peo log Peo. above about 11 K the gas i3 virtually pure CO. The redution of MnO starts at about 154 K to form Mn7Ca, and this then reats with exess MnO to form liquid manganese metal at about 2 K. Pure manganese metal an be prepared by the mixing of MnO and C in equimolar amounts and heating to at least 2 K (1727 C): (A) (B) lomno+lc Mn7Ca+3MnO lomno+loc - Mn7Ca+7CO+3MnO (154 K) = lomn+3co = 1Mn+lOCO In step A, the amounts of CO2 and Mn vapour in the gaseous phase are negligible. In step B, the amount of CO2 is again negligible but, beause of the higher temperature, the vapour pressure of manganese is appreiable and, although low-arbon manganese ould be produed by reation B, the yield would be relatively low. At lower total pressures, the redution sequene is the same but the temperatures for the reations are lower. Experimental Method The redution step to form manganese arbide was investigated in the present study. At a total CO pressure of 1 atm, the imum temperature required to form arbide is about 154 K (1267 C). So that reasonable reation rates would be obtained, a temperature of 135 C was used for most of the experimental work, although temperatures in the range 12 to 1425 C were inluded to show the effet of temperature. Equimolar amounts of MnO (2 g) and graphite (,34 g) were mixed intimately, with or without the addition of one of the following: CaO, MgO, Si2, K2COa, Na2COa, Fe2a, and A12a. The mixture was plaed in a small rerystallized-alua ruible. A ontinuous weighing and reording eletroni balane was mounted above a tube furnae of 5 mm internal diameter, whih was heated by six silion arbide elements and maintained at onstant temperature by a Eurotherm on-off ontroller. The furnae assembly is shown in Fig. 2. The ruible, 24 JULY 198 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

3 with a Pt-13%Rh support wire, was introdued into the bottom of the tube, whih was then sealed and flushed with argon. Over a period of 3 seonds, the ruible was raised to a predetered position and was supported from the balane, whih had automati taring. During a run, the tube was flushed with argon at the rate of 63 ml and the loss in mass of the ruible was reorded ontinuously. The temperature of the ruible was also reorded ontinuously as a hek that it remained onstant during the run. After the reation, the ruible was lowered into the water-ooled quenhing hamber at the bottom and was allowed to ool. The MnO was prepared from manganese dioxide of analytial-reagent grade, whih was heated for 5 hours in a large magnesia ruible at 1l C under a hydrogen atmosphere. The X-ray-diffration pattern of the produt revealed only MnO. The size distribution of the MnO is given in Table 11. For most runs, the MnO was unsized and the distribution of partile sizes in a sample then orresponded to that given in Table 11. However, so that the effet of partile size on the rate of redution ould be investigated, several different size frations were prepared. The graphite was Ellor 7, a high-purity form with a maximum impurity level of,6 per ent, and it was milled and separated into several size frations. The most frequently used size fration was 12 to 18 Lm and, unless otherwise stated, this size fration was used in all the runs. In some instanes, the phases present in the produts of reation were exaed by X-ray diffration. A Pillips diffratometer with a graphite monohromator was used, the radiation being Cu Ka. ~ G' D G- J Results and Disussion The data on loss in mass were onverted to frational reation on the assumption that the loss in mass of the sample was due entirely to the evolution of CO. This permitted the alulation of the mass of oxygen removed from the MnO and hene the frational reation (x): mass of oxygen removed x= total mass of removable oxygen' For the ase of 2 g of MnO and,34 g of graphite, x - 7,94 m 2 X 28,1-1,26m, (4) where m is the measured loss in mass. When Fe23 was added, the oxygen ontent of the Fe23 was inluded as removable oxygen. The rate of redution of MnO is defined as dxdt and is equal to the slope of the urve for frational reation versus time.. G' H (5) L,I 11 I1 G : TABLE SIZE DISTRIBUTION OF THE MnO USED IN THE REDUCTION TESTS Size fration, Lill Mass, % <45 45 to to to 9 9 to to to to 355 > 355 II 1,9,5,8 1,7 3,7 27,9 58,1 2, 3,2 Fig. 2-Furnae assembly for the MnO-graphite redution tests A Insulating briks B Pythagoras work tube C Alua reation ruible D Pt-13%Rh support wire E Silion arbide element F Pushrod G Water for ooling H Argon inlet (during run) I PtfPt-13%Rh indiating thermoouple J PtfPt-13%Rh ontrol thermoouple K Argon inlet (for flushing) L Removable plate for insertion and removal of ruible JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JULY

4 The results for runs in whih no gangue omponents were added are shown in Figs. 3 to 6. These show the effets of temperature, ratio of MnO to C, and partile sizes of graphite and MnO. In all the tests, the rate of redution was greatest at the start of the redution, remained fairly onstant until most of the reation was omplete, and then dereased rapidly to zero. Wherever the frational reation was less than 1, X-ray diffration revealed unreated MnO but no graphite. This shows that the slowing down and eventual essation of the reation was due to depletion of graphite. The produt of reation was identified by X-ray diffration as MnsC2 (and not Mn7Ca, as expeted from the Pourbaix diagram), and it appears that MnsC2 is,6 :!2 Ụ. "~ :,4.2 Ụ. u: Theoretial limit 1 redution Fig. 3- The effet of temperature on the redution of MnO by graphite the stable arbide of manganese under the experimental onditions. The effet of inreased temperature is an inrease in the rate of redution of MnO as shown by the sjopes of the first portion of the frational reation urves in Fig. 3. Aording to stoihiometri alulations, the maximum frational reation if all the oxygen is re. moved as CO is,72 for equimolar mixtures of MnO and graphite. The alulated frational reations in Fig. 3 are from,74 to,77, whih are slightly greater than the theoretial value. The reason for the slightly higher values of frational reation is most probably that a small amount of CO2 esaped from the reation mixture. Sine CO2 arries twie as muh oxygen per mole of arbon as does CO, the presene of CO2 in the gas raises the apparent frational reation. Calulations show, for example, that, if all the oxygen were removed as CO2, there would be more than enough arbon in an equimolar mixture of MnO and graphite to entirely redue the manganese to MnsC2. Thermodynamially, the existene of an appreiable amount of CO2 in the presene of graphite is not possible at temperatures above about 1O C. However, during redution, non-equilibrium onditions exist in the mixture and onsiderable amounts of CO2 are formed: 5MnO+9CO=MnsC2+7C2. (6) If only CO were evolved from the mixture, all the CO2 formed by reation (6) would reat with graphite to reform CO. It is most likely that the greatest proportion of CO2 will be evolved at the stage when the urves for frational reation begin to flatten out, beause this is the stage at whih arbon is beog defiient and the diffusion of CO and CO2 between the MnO grains and 1, R=,23.!:!,8 ' III ~ III.2 ";;,6 III It o~,2 Theoretial of redution limit,8,6 MnO 18 to 25 IIm ' C 125 to 18 flm III Temp. 135 C ~iij mol MnO.2 R=,4 mol C ' ~u R = 3,39 Theoretial limit of redution,2 3 4 Theoretial limit of redution MnO 18 to 25 IIm. C 1 45 to 63 IIm 2 75 to 9 J.lm to 18 IIm 4 25 to 355 flm Temp.135oC 2 4 mfn Fig. 4- The effet of MnO-to-C ratio on the redution of Fig. 5- The effet of the partile size of graphite on the redution of JULY 198 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

5 Theoretial limit of redution ~,6 Cl) a,8 :;:;,4 () a u.,2 ' C 125 to 18}Jm MnO 1. < 63 }Jm 2. 9 to 125}Jm to 25}Jm to 355}Jm Temp Fig. 6- The effet of the partile size of HnO on the redution of HnO by graphite 6 U,6 a Cl) tij :.~ 4 U ' a Lt.,8 2,2 1. No Fe23 :.- Theoretial limit of redution 2. 2,5, 5, 1% Fe23 Temp. 135oC 2 4 Fig. 8- The effet of additions of Fe.Oa on the redution 1 HnO by graphite 6 U,6 a Cl),8 a :;::,4 () a u.,2 1. No SiO2 2. 2,5% SiO % SiO2 4. 1% SiO2 Temp. 135 C Time. Theoretial limit of redution Fig. 7- The effet of additions of SiC. on the redution by graphite U,6 a Cl) a.~ 4, () a u.,8,2 1. No CaO 2. 2,5%CaO 3. 5 %CaO 4. 1 %CaO 2 Temp Theoreti:11 limit of redution Fig. 9- The effet of additions of CaO on the redution of HnO by graphite 6 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JULY

6 ,8 ~,6 as Q) as :;:,4 CJ as u,2 1. No MgO 2. 2,5% MgO 3. 5 % MgO 4. 1 % MgO Temp. 135oC Theoretial limit of redution Fig. 1- The effet of additions of MgO on the redution of graphite grains is the rate-ontrolling step. At this stage it is possible for part of the CO2 formed by reation (4) to esape from the mixture before it enounters suffiient graphite to redue it to CO. The evolution of CO2 from the reation mixture is evidene that redution oured by way of CO2 as an intermediate produt. The alternative mehanism, redution of MnO by solid graphite at points of ontat, would produe no detetable amount of CO2 at temperatures above about 1O C. The effet of the MnO-to-C ratio on the rate of redution is shown in Fig. 4. The rate of redution is inreased as the MnO-to-C ratio dereases. The final values for frational reation are slightly higher than the theoretial values at MnO-to-C molar ratios of 3,39 and 1,, indiating the evolution of a small amount of CO2, At ratios of,38 and,23, the final frational reation is 1, whih is the theoretial value sine there is an exess of graphite, as thus indiating that only CO was evolved in these ~ instanes; there was suffiient graphite throughout the iii reation for the redution of CO2 to CO derease in the rate of redution. The most likely explanation of this is that MnO and Si2 reat to form a slag. There is insuffiient Si2 to flux anything but a fration of the MnO, and the redued rate of redution is probably due to a oating of slag on the MnO partiles that redues the area of ontat between the MnO grains and the reduing atmosphere. The presene of,b-mnsioa was deteted in the reation produts by X-ray diffration, and this supports the above argument. The effet of Fe2a is shown in Fig. 8. Sine Fe2a, as well as MnO, is redued, the oxygen ontent of the Fe2a was inluded in the alulation of the frational reation. The addition of Fe2a inreases the rate of redution slightly, and this may be due to two fators. (i) Fe2a is muh easier to redue than MnO, and hene the rate of redution of a mixture of the two is higher than that of MnO. However, the amount of Fe2a present is too small for this effet to be appreiable. (ii) Metalli iron and iron arbide, when present even in very small amounts, atalyse the Boudouard reation6-8. If the rate of gasifiation of graphite is the rate-ontrolling step, the redution of Fe2a very early in the reation leads to an inrease in the overall rate of redution. The effet of CaO, MgO, and Al2Oa is shown in Figs.9 to H, and it is evident that the effet on the rate of redution is very small, if not negligible, indiating that CaO, MgO, and Al2Oa do not atalyse the Boudouard reation. However, for eah mixture, the final frational redution was raised, showing that a greater amount of CO2 evolved than when there were no additions. The reason for this is not ertain, but it may be due to the diluting effet of the additions on the graphite in the mixture, whih ~,6 The partile size of the graphite (Fig. 5) has no effet ~,4 CJ on the reation rate. As shown in Fig.6, the rate of redu- ~ tion is also almost independent of MnO partile size in u the size range 63 to 25 p.m. However, there is a large derease in reation rate when the MnO size is inreased to between 25 and 355 p.m. The reason for this is not ertain, but it is probably due to a hange in the rateontrolling step from the rate of gasifiation of graphite to the rate of reation of the MnO. The Effet of Gangue Components The effet of Si2 on the rate of redution of MnO is shown in Fig. 7. Although there is some rossing of the urves, it is evident that Si2 auses an appreiable 244 JULY 198 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY,8,2 1. No Alz3 Theoretial limit of red ution 2. 2,5% AlzO % AlzO % Alz3 Temp. 135 C ,2 Fig. 11- The effet of additions of AI.O3 on the redution of

7 ,8 uo,6 IV QI IV C :;; (.),4 IV LL,2 4 3 Theoretial limit of redution 1. No K2CO3 2. 2,5 % K2CO % K2CO % K2CO3 Temp. 135 C Boudouard reation has been the subjet of several laboratory investigations, and the results have been reviewed by Rao4. The values obtained vary from 59 to 97 kal per mole of graphite, and appear to depend on the form of the arbon and probably also on the impurities in the arbon. For this reason, the ativation energy of the Boudouard reation was detered again, this time from the graphite with whih the MnO redution study was onduted. Samples of powdered graphite of 2 g eah were reated with arbon dioxide at a flowrate of 15mlj in the same apparatus as that used for the tests on the redution of MnO. The urves for frational reation were linear at temperatures below lloo C but urved slightly at higher, Fig. 12-The effet of additions of K.CO. on the redution of redued the hane of CO2 enountering graphite partiles for reation. No reation between MnO and the additives was indiated by the X-ray-diffration patterns of the reated mixtures. The effet of K2C3 and Na2C3 is shown in Figs. 12 and 13, from whih it an be seen that there is a marked inrease in the rate of redution of MnO when K2C3 or Na2C3 is present. Also, the final value of frational redution is greatly inreased owing to the evolution of CO2 but, as before, the reason for this is not lear. The explanation for the marked inrease in the rate of redution is almost ertainly that K2C3 and Na2C3 atalyse the gasifiation of graphite. Ativation Energy for the Redution of M no by Graphite Aording to the Arrhenius equation, Rate=Ae-EjRT,, (7) where A is a onstant, R is the universal gas onstant, T the absolute temperature, and E the ativation energy, and the relation between the logarithm of the rate of reation and the reiproal of the absolute temperature is linear with a slope equal to -EjR. The rate of redution of MnO at the start of the redution was detered from the urves in Fig. 3. As shown in Fig. 14, the relation is linear, as expeted, and is desribed by the following equation obtained by linear regression: In: 11= 6 =-28 95jT+14, (8) Hene, the ativation energy is 57,5 kal. The value orresponds to the ativation energy of the rate-ontrolling reation, whih was suggested earlier to be the Boudouard reation. The ativation energy of the uo,6 IV QI IV.24, (.) IV LL,2 Theoretial limit of redution 1. No Na2CO3 2. 2,5% Na2CO % Na2CO3 4. 1% Na2CO3 Temp Fig. 13- The effet of additions of Ha.CO. on the redution of,, 11-3, -4 "" "I" ' '---J.E -5, 6, 6,2 6,4 6,6 6,8 11r Fig. I4-Arrhenius plot for the redution of based on the data of Fig JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JULY

8 temperatures. The slopes at the origin are shown as a funtion of temperature in Fig. 15. The relationship is linear, as expeted, up to lloo C, but then deviates from linearity. The reason for this is probably that, at the flowrate of CO2 used, the rate of reation is ontrolled hemially below lloo C but diffusion-ontrolled above that temperature. Nevertheless, there are suffiient points to give a fairly aurate value of the ativation energy. The linear portion of the urve is desribed by the equation (obtained by linear regression): In:lt=o=379jT+17,134. (9) Hene, the ativation energy of the Boudouard reation is 61,5 kal per mole of graphite. This value agrees fairly well with the value for the redution of, and this is onsidered to be very strong evidene that the rate-ontrolling step is the gasifiation of graphite. Model for the Redution of MnO by Graphite As a further test of the above onlusion, the experimental data were treated by a modifiation of Rao's model4 for the redution of hematite by graphite, The following assumptions were made. (i) The gasifiation of graphite by CO2 is rate-ontrolling. (ii) The temperature of the reation mixture is onstant. (iii) The penetration of argon is limited, and for pratial r , -7, >< "C"C " 1 J ' C -8, 6, 7, 4 1 i T 8, Fig. IS-Arrhenius plot for the gasifiation of graphite by arbon dioxide 9, 5, 4, 3, >< I - I 2, 1, DC 14DC I ;' I", " DC, 135D I I 2 4 ' 6,.,. Fig. I6-Data of Fig. 3 plotted aording to equation (2) purpoiles the gas phase in the mixture is omposed prinipally of CO and CO2, (iv) The mixture behaves as if it were an assemblage of numerous individual partiles eah entirely submerged in an atmosphere of CO and CO2. (v) For the greater part of the redution, the omposition of the gas phase hanges only slowly. (vi) Mass-transferresistans ar negligible ompared with the hemial resistane of the Boudouard reation. (vii) The gasifiation of graphite in an atmosphere of CO and CO2 obeys first-order kinetis. Therefore, the rate of reatioa of graphite is given by _dw<+rw Cl (1) dt where R is a rate onstant and W is the mass of arbon in the mixture. Integration gives.6.w = W~[I-e-Rt]., (ll) where.6.w is the mass of arbon gasified in time t and W~ is the mass of arbon initially in the mixture. If f3is the fration of CO utilized in the redution, 8.6.Woxlde=3"f3.6.W, (12) where.6.w oxide is the mass loss of the oxide in time t..6.wtotal=.6.w +.6.Woxlde =.6.W + 8 3" f3.6.w 8 =.6.W [I + 3f3J (13) 246 JULY198 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

9 For the simplest ase where no CO2 is evolved from the reation mixture, p=,5, as an be seen by referene to equations (1) and (2), and 7 6Wtotal=3W~ [1-e-Rt] (14) The frational reation, x, is given by x=6wtotaljqwo,..... (15) where Q is the maximum frational reation and Wo is the initial mass of the oxide-graphite mixture. For a mixture of 2 g of MnO and,34 g of graphite, and Wo=6,88W(; (16) x-,339[1- e-rt] Q (17) Theoretially, Q is equal to,72, but in all ases the atual values were somewhat higher beause of the evolution of some CO2, Sine the ase (:3=,5 was taken, equation (17) applies only to ases in whih the evolution of CO2 was small. This is best approahed by the data in Fig. 3 for the effet of temperature on the rat of redution Equation (17) an be simplified further if xl=qxjo,339, (18) i.e., """"" xl=l-e-rt (19) and therefore -In (I-Xl) = Rt. (2) If the assumptions made above apply to the redution of, then the relation between In (I-Xl) and t is linear. The data of Fig. 3 were expressed in the form In (I-Xl), and these values are presented as a funtion of time in Fig. 16. The relationships are approximately linear over a onsiderable period for eah run, and the deviation generally ours late in the run at a time when the graphite is nearing depletion. At that time, the reation beomes diffusion-ontrolled and the model no longer applies. Conlusions The following onlusions were drawn from the study. (i) At temperatures between 12 and 1425 C, the redution produt of mixtures of MnO and graphite is Mn5C2, (ii) The rate of redution of is inreased by an inrease in the temperature and a derease in the ratio of MnO to C. The effet of the partile size of MnO and of graphite on the rate of redution at 135 C is very slight in the range 63 to 25 Lm for MnO and 45 to 355 Lm for graphite. (iii) The rate of redution of at 135 C is inreased markedly by the addition of Na2C3 and K2C3, slightly inreased by the addition of Fe23' unaffeted by the addition of CaO, MgO, and A12O3' and dereased by the addition of Si2. (iv) The gas evolved from the mixture is predoantly CO, but in all ases some CO2 was evolved, this being greatest when one of the additives (CaO, MgO, A12O3' Na2C3, or K2C3) was present. (v) The presene of CO2 in the effluent gas is evidene that redution ourred by way of CO2 as an intermediate produt. The alternative mehanism, the redution of at points of ontat, produes only CO at the temperatures used in the work. (vi) The ativation energy of the rate-ontrolling reation in the redution of is 57,5 kal. This is similar to 61,5 kal, the experimentally detered ativation energy of the Boudouard reation, and suggests that the rate of gasifiation of graphite is the rate-detering step in the redution of MnO. (vii) The relation between In (I-Xl), where Xl is defined by equation (18), and time is linear for the redution of MnO by the graphite over the range 12 to 1425 C. This is to be expeted if the Boudouard reation is rate-ontrolling. Aknowle~ements This work was arried out in the Department of Chemial and Metallurgial Engineering, University of Stellenbosh, and was finaned by the National Institute for Metallurgy. The work was onduted by J. S. J. van Deventer as a final-year projet for the degree of Hons. RIng (Chemiese Ingenieurswese). The tehnial assistane of Mr W. L. Hall and Mr C. Atkins is gratefully aknowledged, as is the work of Dr D. I. Ossin, who provided the neessary liason with the National Institute for Metallurgy, and Dr D. Cornell of the Department of Geology, University of Stellenbosh, who arried out the X-ray-diffration work. This paper is published by permission of the National Institute for Metallurgy. Referenes 1. KOURSARIS, A., and SEE, J. B. Reations in the prodution of high-arbon ferromanganese from Mamatwan manganese ore. Johannesburg, National Institute for Metallurgy, Report no th Aug., pp. 2. TrEN, R. H., and TURKDOGAN,E, T. Mathematial analysis of reations in metal oxidearbon mixtures. Metall. Trans., vol. 8B pp BORN, H. Y., and SZEKELY, J. Reations through gaseous intermediates. 1. Reations between solids ontrolled by hemial kinetis. Chem. E~. Si., vol pp RAo, Y. K. The kinetis of redution ofhematite by arbon. Metall. Trans., vol pp DOWNING, J. H. Fundamental reations in submerged ar furnaes. Eletri Furnae Conferene Proeedings 1963, pp KING, J. G., and JONES, J. H. The reativity of oke. J. Inst. Fuel., vol pp GULBRANSEN, E. A., and ANDREW, K. F. Reation of arbon dioxide with pure artifiial graphite at temperatures of 5 to 9 C. Ind. E~ng Chem., vol pp RAKSZAWSKI,J. F., RUSINKO, F., and WALKER, P. L. Catalysis of the arbon-arbon dioxide reation by iron. Pro. Conf. Carbon, 5th, vol. 2., pp JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JULY