Factors affecting the resistivity and reactivity of carb()n,aceous reducing agents for the electric-smelting

Transcription

1 Fators affeting the resistivity and reativity of arb()n,aeous reduing agents for the eletri-smelting industry by H. M. DJS*, B.S. (Chem. ng.) and D. J. SMTHt, B.S. (Chem. ng.), M.. Chem.., S.A.. Chem.. SYNOPSS Resistivity tests indiated that, at temperatures lower than 145 C, reduing agents alone, as well as burdens onsisting of high-arbon ferrohromium and reduing agents, exhibit wide differenes in resistivity. These differenes are less pronouned at elevated temperatures. The resistivity of a reduing agent and the volume fration it oupies are important fators ontrolling the resistivity of burdens of high-arbon ferrohromium. A mathematial model is formulated to desribe the dependene of the resistivity of mixtures of onduting and insulating partiles on the partile sizes and the volume fration oupied by the onduting phase. The resistivity of burdens of high-arbon ferrohromium is high if the partile size of the reduing agent is larger than that of the other omponents of the burden. A number of reduing agents showed small variations in their reativity towards hromite ore in the solid state. However, with hromite ore in the liquid state, redution proeeded more rapidly with reduing agents oupying larger volume frations. During the smelting of manganese ore, high initial rates of redution were observed for reduing agents that had a high reativity towards arbon dioxide and a high ontent of volatile matter, and that oupied a large volume fration. High liq'uid-state reativities having a ow degree of graphitization. were observed for reduing agents oupying a large volume fration and SAMVATlNG Resistiwiteitstoetse het getoon dat reduseermiddels alleen, asook ladings bestaande uit hoekoolstofferrohroom en reduseermiddels by laertemperature as 145 C groot verskille in resistiwiteit toon. Hierdie verskille is nie by hoe temperature so duidelik nie. Die resistiwiteit van 'n reduseermiddel en die volumefraksie rike faktore wat die resistiwiteit van ladings van hoekoolstofferrohroom bepaal. wat dit beslaan is belang- Daar word 'n wiskundige model geformuleer om die afhanklikheid van die resistiwiteit van mengsels van geleidende enisolerende partikels van die partikelgroottes en die volumefraksie wat die geleidende fase in beslag neem, te beskryf. Die resistiwiteit van ladings van hoekoolstofferrohroom is hoog as die partikelgrootte van die reduseermiddel groter as die van die ander bestanddele van die lading is. 'n Aantal reduseermiddels het klein variasies in hul reaktiwiteit teenoor hromieterts in die vaste toestand getoon. Met hromieterts in die vloeibare toestand het die reduksie egter vinniger plaasgevind met reduseermiddels wat groter volumefraksies beslaan. Gedurende die uitsmelt van mangaan erts is daar hoe aanvanklike reduksietempo's waargeneem vir reduseermiddels wat 'n hoe reaktiwiteit teenor koolstofdioksied en 'n hoe inhoud aan vlugtige stowwe het en wat 'n groet volumefraksie beslaan. Daar is hoe reaktiwiteit vir die vloeibare toestand waargeneem vir reduseermiddels wat 'n groat volumefraksie beslaan en 'n lae graad van grafitisering het. NTRODUCTON The ferro-alloy industry in South Afria has expanded very rapidly over the past thirty years, and this ountry is now a major supplier of ferro-alloys on the world market. n 1976, a ommittee, known as the Char Committee, was onvened to examine the supply of arbonaeous reduing agents to the eletri-smelting industry. The Committee onluded that there was likely to be a signifiant shortage in the supply of some reduing agents in the future, and that the bulk of this shortage would have to be made up by har. As part of the Committee's suggested programme to identify suitable hars for the ferro-alloy industry, the National nstitute for Metallurgy (NM) undertook to determine the eletrial resistivity and metallurgial reativity of various types of reduing agents and to assess their suitability for the eletri-smelting industry2. The reduing agents onsidered inluded standard reduing agents (suh as okes and har) and non- *Formerly of the National nstitute for Metallurgy; now at Onderwei 43, 1273 TB, Huizen, Holland. tnational nstitute for Metallurgy, Private Bag X315, Randburg 2125, South Afria. g 18. ommerial hars produed from a number of oals by the Fuel Researh nstitute (F.R.). These oals were harred by the F.R. under oxidizing and reduing onditions at temperatures ranging from 6 C to 12 C. Table summarizes the properties of these reduing agents. MASURMNT OF LCTRCAL RSSTVTY The resistane measurements omprised the following: (1) determination of the resistivity of the reduing agents at room remperature and during heating to 155 C, (2) determination of the resistivity during heating to 15 C of typial burdens used in the prodution of standard ferrohromium ontaining various types of reduing agents, and (3) a study of the effet of the partile size of the reduing agent and other omponents of the burden on the resistivity of burdens of high-arbon ferrohromium. Resistivity at Room Temperature The resistane of a bed of dried reduing agent was measured at room temperature with the equipment Hhown in Fig.!. 286 AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY

2 The ell onstant of the system was measured by the use of sodium hloride solutions of known resistivity so that the resistane values for the reduing agents ould be expressed as resistivity values. The resistivities at room temperature of two of the hars produed by the F.R. are shown in Table l. The other F.R. hars exhibited the same trends. The following onlusions an be drawn. (1) The resistivity of a har produed under reduing onditions is usually higher than that of a har produed from the same oal and at the same temperature under oxidizing onditions. (2 Variable load t Brass PVC tube Charge eletrode Brass plate Fig. -Apparatus for resistane measurements at room temperature (2) An inrease in the temperature of harring results in a signifiant derease in resistivity and an inrease in the fixed arbon. Resistivity at levated Temperatures Fig. 2 shows the apparatus used to obtain the resistane measurements at elevated temperatures. The partile size of the reduing agent (a mean partile size TABL ANALYSS OF RDUCNG AGNTS (N MASS PRCNTAG ON DRY BASS) Reduing agent Fixed arbon Volatile Ash material Rand Carbide har 79,8 4,5 15,7 sor oke 83,6 1,1 15,3 Vryheid oke 79,8 1,3 18,9 Anthraite 8,5 9,9 9,6 letrode graphite 99,1,9, Springbok no. 5 har 79, 4,3 16,8 Delmas no. 2 har 76,1 5, 19, Delmas no. 4 har 73,1 5,3 21,6 Matla no. 5 har 8,9 4, 15,5 Utreht har 82,4 3,9 13,7 U mgala har 82,1 4,4 13,6 Blesbok no. 5 har 81,2 4,1 14,8 TABL RSSTVTY OF RDUCNG AGNTS AT ROOM TMPRATUR Reduing Charring AtmOSPhere Resistivity agent temperature QC Q.m Umgala 6 Reduing 2,8 x 13 Umgala 6 Oxidizing,8 x 13 Umgala 8 Reduing 27,2 Umgala 8 Oxidizing 8,8 Umgala 1 Reduing 2,3 Umgala 1 Oxidizing 2,3 Delmas no. 4 6 Reduing 2,8 x 13 Delmas no. 4 6 Oxidizing 2,39 x 13 Delmas no. 4 8 Reduing 22,1 Delmas no. 4 8 Oxidizing 14,7 Delmas no. 4 1 Reduing 3,6 Delmas no. 4 1 Oxidizing 3,3, Converter amplifier reorder nsulating tube.' Argon nlet Graphite ruible Thermoouples 22V D 5 Hz '-U ndution Voli Charge e --- letrode Fig. 2-Apparatus for resistane measurements at elevated temperatures JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY AUGUST

3 of 3,6 mm) orresponds to one-quarter of that used in industry for the prodution of ferrohromium. The ell onstant of the system was measured with sodium hloride solution. The results of the resistane measurements on standard reduing agents during heating to a temperature of 155 C are shown in Fig. 3. At temperatures lower than approximately lloo C, the ranking of the resistivities followed the ranking of the ash ontents of the reduing agents. At higher temperatures, the differenes in resistivity beame muh less pronouned, and it was observed that the ranking of the reduing agents in terms of resistivity was not the same as that at lower temperatures. For the oals harred under different oxidizing and reduing onditions, it was observed that the resistivity beame independent of the harring onditions at temperatures higher than lloo C. Resistivity of Burdens Containing Reduing Agents Resistivity measurements were onduted with the apparatus shown in Fig. 2 on harges onsisting of a reduing agent, Steelpoort hromite ore, dolomite, and quartzite. The amount of arbon added was 115 per ent of the stoihiometri amount, and the ompositions were alulated to give the same theoretial slag omposition for eah test. The results of the resistane measurements during heating to 15 C are given in Fig. 4. At temperatures lower than 14 C, fairly wide differenes an exist in the resistivities of burdens of high-arbon ferrohromium. However, the shapes of the urves for resistivity versus temperature were similar for most of the reduing agents onsidered, inluding the various oals that were harred. The resistivity peaks observed at temperatures between 9 C and lloo C ould possibly have been aused by thermal ontration of the partiles of the reduing agent3. The resistivities of harges ontaining standard reduing agents other than graphite inreased in the order fi!or oke < Rand Carbide har < anthraite, whih is the same order as that established for these reduing Anth,";'. Rand Ca,b;de ha, 6 so< ok. 2 ', "--" "'""'" '" "" ":',::::::-:::::. G,aph;t T.mp.,otu<e,C Fig. 3-Resistivity of standard reduing agents during heating to SO C ; ;: i- '; ' &! 15 Anth,"lt Tempe,etu,e, 'C Fig. 4-Resistivity, during heating to SOO C, of burdens of high-arbon ferrohromium ontaining standard reduing agents agents in the absene of ore and fluxes. The fat that graphite oupied the lowest volume fration of all the standard reduing agents ould aount for the higher resistivity of high-arbon ferrohromium ontaining this reduing agent as ompared with the resistivity of graphite alone. The resistane measurements on harges ontaining hars from the F.R. showed the same trends as those observed in the absene of ore fluxes: that is, hars produed in a reduing atmosphere and at low temperatures gave higher resistivities than those produed from the same oal in an oxidizing atmosphere and at higher temperatures. Burdens ontaining hars that oupied a low volume fration had resistivities higher than those of burdens ontaining hars that oupied a higher volume fration. ffet of Partile Size The effet of the partile size of the reduing agent and of other omponents of the burden on the resistivity of typial burdens used in the eletri-smelting industry has been the subjet of a number of investigations3-7. However, some of the reported results appear to be ontraditory. So that this matter ould be resolved, a detailed examination was made of the effet of partile size on the resistivity of burdens of high-arbon ferrohromium. Theoretial Considerations A burden of high-arbon ferrohromium an be rgarded as a two-phase system of onduting partiles (reduing agent) in an insulating matrix (ore and fluxes). The eletrial ondution in suh mixtures 288 AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY

4 ours by means of ontinuous arrays of onduting partiles randomly distributed throughout the insulating matrix. The resistivity of the mixture (p ) an be expressed m as follows2; R L 1 p = for N < m ' a- ' A {N (a-)}m- p and 1 Pm = 1,, for Ne (1) a-' where R is the resistane of a ontinuous array of onduting partiles per input length of the array, L is the length of a ontinuous array of one duting partiles per unit length of the mixture, A is the number of onduting partiles per Pnit area of the mixture, N is the number fration of onduting partiles in the mixture, a is the o-ordination number of a onduting partile, and m is the number of onduting partiles in a ontinuous array. xperimental Proedure The experiments were aimed at a determination of the quantities ontained in equation (1) as a funtion of the partile sizes of the reduing agent and other omponents of the burden, and as a funtion of the volume fration oupied by the reduing agent. The resistane measurements were made at room temperature, Rand Carbide har being used as the reduing agent. Results The resistane of a ontinuous array of onduting partiles per unit length of the array was determined for various partile sizes of Rand Carbide har in the absene of silia. Least squares fitting of the results gave R = (,89+,7 ) Q.m, d2 C TABL COMPARSON BTWN TH MASURD AND FTTD RSSTVTY VALUS OF MXTURS OF CONDUCTNG AND NSULATNG PARTCLS d Conduting phase: Rand Carbide har nsulating phase: Silia dj,45,81,25,81 V Fitted Measured ----,1 >2 X 1' >2 x 1',3 5,46 x ,4,5 22,5 2,,7 8,9 8,3,9 5,8 5,5 1, 4,7 4,6,1 1,24 x 1' > 2 X 1', ,5 42,4 59,8,7 13,6 15,,9 9,5 1,6 1, 8,1 7,8 (2) where d is the mean partile size of the onduting phase (har) in entimetres. The number of onduting partiles per unit of the mixture A pe was determined from a ount of the numbel' of har partiles on any ross-setion of the har-silia mixtures after the mixtures had been set in epoxy resin. The dependene of A pe on the partile size and volume fration of the onduting phases is given by A p,695 V d2 C where V e is the volume fration oupied by the onduting phase. The remaining parameters were derived on the assumption that the partiles in the mixture were spherial: and N = m=, 1 d 1- V +()3() d. V 1 C LL s where dj is the mean partile size of the insulating phase in entimetres, and Ls is the sample length in entimetres. Least squares fitting of the values obtained for Le ga ve the following expression: L =exp{,8(-v)}, where,8 is given by,8 = 4,4-,8d -4,8d., 1 A omparison of the measured resistivity values with the values obtained theoretially indiates that the model represents the data for measured resistivity for V e values of,4 or greater (see Table ll). Disussion letrial ondution in arbonaeous materials has been the subjet of several investigations8-14, whih showed that the eletrial resistivity of suh materials is influened by the following fators. (i) The resistivity of a reduing agent dereases with an inrease in its arbon ontent. (ii) The volatile matter, onsisting of ompounds of low moleular mass and high resistivity, an be regarded as being in a mixture of maromoleular material with low resistivity. An inrease of volatile matter in the reduing agent therefore inreases the resistivity of the reduing agent. (iii) The resistivity of oal inreases with an inrease in its ash ontent. This effet is not pronouned unless the ash ontent is more than 2 per ent. (iv) Other fators affeting the eletrial resistivity of arbonaeous materials are moisture ontent, partile size, mirostruture, presene of miro- (3) (4) (5) (6) (7) JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY AUGUST

5 sopi raks, ash distribution, and geometrial arrangement and spaing of the graphiti rystallites in the material. The effets of harring onditions on resistivity an be explained in terms of the effets of these onditions on the fixed arbon and volatile matter ontained in the har. An oxidizing atmosphere and high harring temperatures result in the presene of less volatile matter (and therefore more fixed arbon), whih explains the lower resistivities of the hars produed under these onditions. n eletri-smelting operations, it is onsidered desirable for the solid burden to have high resistivity. Suh burdens require a reduing agent that has high resistivity and oupies a small volume fration in the burden. Reduing agents with high resistivity are those that have been arbonized at ow temperatures under reduing onditions. For the volume fration oupied by the reduing agent to be small, the reduing agent should be dense and should have a high ontent of fixed arbon. Figs. 5 to 7 show the effet of the partile sizes of the omponents of the burden, and of the volume fration oupied by the reduing agent, on the resistivity of burdens of high-arbon ferrohromium as predited by the mathematial model presented earlier. Fig. 5 shows that, for a volume fration of,5, the effet of the partile size of the reduing agent was most pronouned when the size of the insulating phase was small, and that, for any given partile size of the insulating phase, the resistivity of the mixture passed through a minimum as the partile size of the reduing agent inreased. These minima arose beause an inrease in partile size of the reduing agent tends to derease,j :': " ;e de -.,8 de -,6 de =,4 de= 1,,2,4,6,8 Ve =,5 Fig. 6-Predited effet of the partile size of ore and fluxes on the resistivity of burdens of high-arbon ferrohromium 2 d" m de = dj =,4 m 1, 2 18 dj di =.4 =,3 V =, dj =', Q. 1 C: 1 " ':e di,2,4,6,8 1, d, m =,8 do = 1,,5 V 1, Fig. S-Predited effet of the partile size of the reduing agent on the resistivity of burdens of high-arbon ferrohromium Fig. 7-Predited effet of the volume fration oupied by the reduing agent on the resistivity of burdens of higharbon ferrohromium 29 AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNiNG AND MTALLURGY

6 Thermoouple- Argon purge to furnae wlndlngs.. Thermometer Gas hromatograph and mass spetramater Temp. ontroller and temp. and mass...reorders Cherge Roheter --- Manometer --- Furnae windlngs Furnae tuba Needle valve Argon supply Fig. 8--Apparatus for reativity measurements 2., Ė 1 Rend Carbide hbr Vrylleld oke Anthraite Graphite.J 5 3 SOD Temperature, 'C Time, min Fig. 9-Redution of hromite ore in the solid state with standard reduing agents,9,8,7,6.n :,5.,4....,3..J,2, Temperature, 'C Time, mln Fig. O--Rate of loss in mass of a mixture of reduing agent and hromite ore (reduing agent: Dalmas no. 4 seam oal harred at OOO Cin a reduing atmosphere) JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY AUGUST

7 the resistivity of the mixture by dereasing the resistane of a ontinuous array of onduting partiles, but also tends to inrease it by dereasing the number of suh arrays. For a given partile size of reduing agent, the model predits that the resistivity of burdens of higharbon ferrohromium wih derease with an inrease in the size of the insulating omponents. The resistivity of these burdens an be expeted to be high as long as the partile size of the reduing agent is signifiantly larger than that of the other omponents of the burden. Conlusions 1. The resistivity of the reduing agent, the volume fration it oupies, and its partile size and that of the other burden omponents are important parameters influening the resistivity of burdens of high-arbon ferrohromium. 2. The resistane measurements on various types of har show that the resistivity of a har depends on the type of oal from whih it is produed and on the onditions under whih it is produed. Chars produed in a reduing atmosphere had a higher resistivity than those produed in an oxidizing atmosphere at the same temperature and from the same type of oa1. However, the effet of the harring atmosphere was sma11 ompared with that of the harring temperature. 3. A mathematial model was formulated to desribe the dependene of the resistivity of burdens of higharbon ferrohromium on the partile sizes of the reduing agent and other omponents of the burden, and on the volume fration oupied by the reduing agent. High resistivity of the burden an be expeted if the partile size of the reduing agent is large in relation to that of the other omponents of the burden. RACTVTY MASURMNTS The reativity measurements involved determination of the reativity of various reduing agents towards hromite and manganese ores in the solid and liquid states. Reativity towards Chromite Ore in the Solid State The reativity of reduing agents towards hromite ore in the solid state was determined with the apparatus TABL RSULTS OF RACTVTY TSTS ON CHROMT OR N TH SOLD STAT (maximum temperature 15 C) Heduing agent Charring temperature Rand Carbide har Vryheid oke Anthraite Graphite Delmas no. 4 Delma. no. 4 Springbok no. 5 Springbok no. 5 Delmas no. 2 Delmas no. 2 Matla no. 5 Mata no. 5 C V Atmosphere Oxidizing Reduing Oxidizing Reduing Oxidizing Reduing Oxidizing Reduing Redution % 55 54,52 47, ", S " 4 (,) Temperature. C Time. min Temperature. C Time, min Fig. -Analyses of gases emitted during the redution of hromite ore in the solid state (reduing agent: Springbok no. 5 seam oal harred at OOO C in a reduing atmosphere) ihustrated in Fig. 8. The losses in mass for the mixture of hromite ore and reduing agent in the alumina ruible were measured on an eletroni balane. The gases emitted from the furnae were analysed with a mass spetrometer and a gas hromatograph for arbon monoxide, arbon dioxide, methane, hydrogen, and argon. The sample was heated from room temperature to 15 C at the rate of 4 C per hour, and was then held at 15 C for 2 hours. For the tests with standard reduing agents, the amount of arbon added was the stoihiometri amount required to redue the iron and hromium oxides to their respetive metals, but, for the tests with hars produed by the F.R., the amount of arbon added was 12 per ent of the stoihiometria11y required amount. The results of the experiments are summarized in Table V, whih shows the final perentage redution. This is the amount of oxygen removed expresi3ed as a perentage of the total oxygen ontained by the iron and hromium oxides in the ore and of the perentage metahization of iron and hromium as determined from hemial analyses of the reation produts. The reativities of the standard reduing agents towards hromite ore in the solid state were not signifiantly different, exept for graphite, whih showed signifiantly better reativity than the rest. This an be explained by the fat that graphite has the highest 292 AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY

8 ontent of fixed arbon and the lowest ontent of ash and volatile matter, oupies the smallest volume fration (i.e., it has the highest value for fixed arbon ontent x apparent density), and has a low reativity towards arbon dioxide. This agrees with the findings of Dewar et al.15 and Fremont-Lamouranne et al.16. The atmosphere under whih a har from the F.R. was produed had little effet on the perentage redution. Fig. 9 shows plots of the perentage loss in mass versus temperature and time for the standard reduing agents. Curves of similar shape were obtained for the other reduing agents tested. Plots of the rate of loss in mass versus temperature and time showed three distint maxima at the following temperatures: (i) 2 C, owing to the evolution of water, (ii) 75 C, owing to the evolution of volatile matter (hydroarbons), and (iii) 15 C, owing to the redution of hromite ore with the evolution of arbon monoxide. These maxima are shown in Fig. 1, whih is typial for the reduing agents used in the tests. Fig. 11 shows a typial analysis of the gases emitted from the furnae. The evolution of hydrogen usually started at about 5 C, reahing a maximum between 85 C and 12 C, after whih its onentration dereased ontinuously, although its presene ould be deteted during the entire reation period. Carbon monoxide evolved from the volatile matter and as a result of the redution of the hromite ore. Variations in the partile size of the reduing agent (see Fig. 12) resulted in an insignifiant differene in the reativity of the reduing agent. However, the rate of redution of hromite ore inreased with a derease in the partile size of the reduing agent, as ould be expeted on the basis of the larger surfae area available for reation and the improved inter-partile ontat, as shown by Barza et al,17. As ould be expeted, the rate of redution inreased with an inrease in the perentage of stoihiometri arbon added, the effet being most marked for the lower perentages. The results also indiated that the differenes in the reativity of these reduing agents tend to derease with an inrease in the perentage of stoihiometri arbon added. Silia flux added at 15 C had no effet on the metallization of iron, but slowed down the metallization of hromium. At temperatures higher than the fusion point of the ore, silia enhaned the redution, whih is in agreement with the findings of Urquhartl8. Reativity towards Chromite Ore in the Liquid State A variation of the SCC (stationary harge in ontrolled environment) tehniques 19 was used in an assessment of the reativity of reduing agents towards hromite ore in the liquid state. Charges onsisting of reduing agent, hromite ore, and fluxes were heated from room temperature to 15 C at a rate of 35 C per hour, and were then maintained at 15 C for a period of 2 hours. A final temperature of 15 C was seleted beause the preliminary experiments had shown that this temperature was suffiiently high to ause fusion of the ore and fluxes in the harge, whereas, at higher temperatures, the thermoouples failed rapidly. The reations taking plae during heating were studied from a ontinuous analysis (by use of a mass spetrometer) of the gases emitted from the furnae. Fig. 13 shows the results as a plot of perentage redution of the ore versus temperature and time of holding at 15 C. The perentage redution was alulated from the gas analyses, an example of whih is shown in Fig. 14. Carbon dioxide was first deteted at about 4 C, and probably originated mainly from deomposition of the dolomite added to the harge as a fluxing agent. The onentration of arbon dioxide reahed a maximum at about 8 C, whih was the {!. 7 : 6 5: 5 11) 4.J 3 2 _._-.'-"" 4,76 to 6,4 mm 2,38to4,OOmm,84 to 2, mm '", " ",,, """'- ''. ', ' ",- ",- 1 3:iJ, 'SOl Temperature, C Time. mln Fig. 12-ffet of partile size of reduing agent (Rand Carbide har) on the redution of hromite ore J,OURNALOF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY AUGUST

9 same temperature as that at whih arbon monoxide first appeared in the effluent gas. As the redution of hromite ore by solid arbon does not take plae at temperatures as low as 8, it is believed that arbon monoxide was formed by the reation of arbon dioxide with the arbon of the reduing agent aording to the Boudouard reation 2+=2. The onentration of arbon monoxide reahed a peak at about 1, probably beause the deomposition of the dolomite was nearing ompletion, and started to inrease again at about 1l to 12, indiating the onset of the redution of the hromite ore. A seond maximum was reahed at about 15, after whih, during holding at 15, the onentration of arbon monoxide dereased ontinuously. Table V lists the reduing agents tested in order of inreasing reativity. Reativity towards Manganese Ore Charges onsisting of an intimate mixture of reduing agent and manganese ore were heated in the apparatus (see Fig. 8) in an argon atmosphere at a rate of 4h until a temperature of 145 was reahed, after whih this temperature was maintained for 2 hours. A final temperature of 145 was seleted beause preliminary experiments had shown that this temperature was suffiiently high for liquid metal and slag to be formed. n all the experiments, the arbon added was 1 per ent of the stoihiometri requirement. Fig. 15 shows the urves for loss in mass when stan- 7 6 so Rand Carbide har -_ soroke -'- Anthraite """""""" Graphite it 4 ii ::J! Temperature, C Time, mln Fig. ]-Redution of hromite ore, in the liquid state with standard reduing agents 6 i 5 > 4 'ii ! \ CO2! \ Temperature, C Time, mln Fig. 4-Analyses of gases emitted during the redution of hromite ore in the liquid state (reduing agent: Rand Carbide har) 294 AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY

10 dard reduing agents were reated with natural Mama. twan ore. These urves were orreted for the evolution of water and volatile matter from the reduing agents. The urves obtained for the other reduing agents were very similar in shape. Redution was rapid at temperatures between 5 C and 1l C, muh slower at temperatures between 1l C and 14 C, and again rapid during holding at 145 C. n aordane with the work reported by Grimsley2, it an be expeted that the first stage of redution orresponds to the redution of iron oxide to metalli iron and the redution of the higher manganese oxides to MnO and CaMn24' in addition to thermal deomposition of the ore. The seond stage of redution orresponds to the redution, in the solid state, of MnO and CaMn2O4 by arbon dissolved in the ferro- TABL V RDUCNG AGNTS N ORDR OF NCRASNG RACTVTY TOWARDS CHROMT OR N TH LQUD STAT (temperature 15 C) Reduing agent Charring Atmosphere Redution from temperature metal analyses C % Utreht 1 Reduing 41,9 Utreht 1 Oxidizing 42, Graphite ,6 Delmas no. 2 1 Reduing 43,5 Springbok no. 5 6 Oxidizing 44, Umgala 1 Reduing 44,2 Utreht 6 Reduing 44,8 Utreht 6 Oxidizing 46,7 Springbok no. 5 6 Reduing 47,3 Anthraite - - 5,3 Utreht 8 Reduing 51,8 Utreht 8 Oxidizing 53,6 Springbok no. 5 8 Reduing 54,9 Springbok no. 5 8 Oxidizing 56,4 Delmas no. 4 1 Reduing 58,7 Matla no. 5 1 Reduing 58,7 Rand Carbide har ,9 sor oke ,7 manganese phase. The third stage of redution therefore probably orresponds largely to the redution of MnO dissolved in the liquid slag. Any effet of the type of reduing agent on the redution of manganese ore was usually apparent only during the first and final stages of redution, was usually small, and was very similar for the natural and the preheated ore. However, the redution behaviour of mixtures of manganese ore and graphite differed from that of mixtures ontaining the other reduing agents in that fusion of the ore did not take plae upon heating to 145 C. ven when the ash of Rand Carbide har was added to the mixture of graphite and ore, fusion did not our, indiating that the distribution of ash in a reduing agent determines its fluxing behaviour. Table V summarizes the results obtained with various reduing agents and preheated Mamatwan ore, the values for reativity being saled with respet to the values obtained for Rand Carbide har. The results showed that the atmosphere under whih a har is produed has little effet on its reativity. An inrease in the temperature of harring aused a derease in the hars produed from Springbok no. 5 seam oal. However, this effet was not observed with hars produed from Utreht oal. TABL RSULTS OF RACTVTY TSTS ON PRHATD MAMATWAN OR Reduing agent Solid-state Liquid-state reativity reativity Rand Carbide har 1, 1, sor oke,7,9 Anthraite 1, 1, Graphite,8 Springbok 1,25 1,1 Utreht 1,,75 Delmas no. 2 1,1 1, Delmas no. 4 1,2,7 Matla no. 5 1,2,8 Blesbok no. 5 1,2 1,15 V 6 5 Rand Carbide har sor oke -.-'- Anthraite Graphite 8 4 x 111'G.. = :! 3 :11! 1 :11! Mamatwan only 145'C Temperature, C C Time, Fig. 15- Redution of natural Mamatwan ore with standard reduing agents mln.journal OF TH SOUTH AFRCAN NSTTUT OF MNNG AND MTALLURGY AUGUST

11 n general, it was found that reduing agents that have a high ontent of volatile matter, that oupy a large volume fration, and that have a high reativity towards arbon dioxide tend to have high solid-state reativities. Reduing agents that oupy a large volume fration and have a low degree of graphitization tend to have high liquid-state reativities. As ould be expeted, the solid-state and liquid-state reativity dereased with an inrease in the partile size of the reduing agent, although the effet on the liquid-state reativity was more pronouned than that on the solid-state reativity. Silia added as a flux to a mixture of hromite ore and reduing agent had little effet on the solid-state reativity, exept where the amount added was large (2 per ent). However, the liquid-state reativity tended to derease with an inrease in the amount of silia added. The formation of siliates in the slag is known to hamper the redution proesses 18. Conlusions 1. Different types of reduing agents have very similar reativities towards hromite ore in the solid state. 2. Reduing agents oupying a large volume fration in the burden tend to have high reativities towards hromite ore in the liquid state. 3. Reduing agents that have a high reativity towards arbon dioxide and a high ontent of volatile matter, and that oupy a large volume fration, tend to have a high reativity towards manganese ore in the solid state. 4. Reduing agents that oupy a large volume fration and have a low degree of graphitization tend to have a high reativity towards manganese ore in the liquid state. ACKNOWLDGMNT This paper is published by permission of the National nstitute for Metallurgy. RFRNCS 1. JOCHNS, P. R. (National nstitute for Metallurgy). Private ommuniation, DJs, H. M., TAYLOR, J. F., SCHOUKNS,A. F. S., FARRL, L. J., FAGAN, R. G., NWY, V., and FLTCHR, G. W. A laboratory investigation of reduing agents for use in the eletri-smelting industry. Randburg, National nstitute for Metallurgy, Report no Mar W1LLAND, K., and JOCHNS, P. R. Measurement of the eletrial resistane of ferrohromium furnae harges. Johannesburg, National nstitute for Metallurgy, Report no Apr RNN1, M. S., JOCHNS, P. R., and SOMMR, G. The eletrial-resistane harateristis of the harge in the eletri redution furnae. Johannesburg, National nstitute for Metallurgy, Report no Feb LORNZ, M., and MARNK, B. The eletrial resistane of harge materials in eletri smelting furnaes. Shweizer Arhiv, vol. 35. Mar. 19f\9. pp DA SLVRA, R. C., and LMA D MLo, M. A. The influene of sizing of haroal on the produtivity of eletri redution furnaes. Metalurgia A.B.M., vol. 29, no Aug pp DA SLVRA, R. C. letrial harateristis of some raw materials and loads for the prodution of ast iron in eletrial reduing furnaes. Metalurgia A.B.M., vol. 23, no pp POP, M. 1., and GRGG, S. J. The speifi eletrial ondutivity of oals. Fuel, vol pp WALKR, P. L., and RUSNKO, F. Determination of the eletrial resistivity of partiulate arbons. Fuel, vol pp OCH, K. letrial properties of oal and arbonated materials. Fuel, vol pp MAZUMDAR, S. K., and BBLAB MTRA. A study of the eletrial resistivity of hemial oke and other heat-treated arbonaeous materials. ndian J. Tehn., vol. 1. Nov pp UKANAKOV, P. M., MzN, V. G., and SROV, G. V. letrial resistivity of different forms of arbonaeous reduing agents. Coke and Chemistry USSR, no pp GLANACHV, V. S., et al. Temperature dependene of the eletrial ondutivity of ertain arbographite materials. norg, Mater., vol. 12, no pp SCHUYR, J., and VAN KRvLN, D. W. Chemial struture and properties of oal. X. Semi-ondutivity of high rank oals. Fuel, vol pp DWAR, K., S, J. B., KNG, R. P., and FLTCHR, G. W. The influene of arbonaeous reduing agents on the rate of redution of representative manganese and hromium ores. Randburg, National nstitute for Metallurgy, Report no May FReMoNT-LAMOURANN, R., GRLLT, Y., and GueRN, H. The reativity of arbons with respet to solid oxides. Bulletin de la Soiete Chimique de Frane, no ' pp BARzA, N. A., HOWAT, D. D., JOCHNS, P. R., and PAYNTR, J. C. Studies of inipient fusion in the system hromite MgO - - A.OJ - SiO. - C. Johannesburg, nstitute for Metallurgy, Report no National 18. URQUHART, R. C. A study of the prodution of high arbon ferrohromium in the submerged ar furnae. Ph.D. thesis, University of the Witwatersrand, JOHNSON, A. Blast-furnae proess researh by the SCC tehnique. J. ronsteelnst., vol. 26. Jul pp GRMSLY, W. D. A study of the redution harateristis and mehanism of redution of manganese ore by solid arbon. M.S. thesis, University of the Witw atersrar.d, 1977 Tunnelling The Fourth Australian Tunnelling Conferene, organized by the Australasian nstitute of Mining and Metallurgy, is to be held in Melbourne from 16th to 18th Marh, t is hoped that part of the Conferene will be devoted to engineering aspets of the Melbourne Underground Rail Loop, inluding items suh as arhitetural, eletrial, and mehanial works. The remainder of the Conferene will over the following themes: (i) Future projets in mining or ivil engineering involving underground exavation. (ii) nvestigation, design, and monitoring of underground exavations. (iii) Case histories, new equipment, reent developments and modern management of underground onstrution. (iv) Possible innovations in underground onstrution by the year 2. All orrespondene relating to the Conferene should be addressed to The Chief xeutive Offier, The Australasian nstitute of Mining and Metallurgy, P.O. Box 31, Carlton South, Vitoria, Australia AUGUST 198 JOURNAL OF TH SOUTH AFRCAN NSTTUTO MNNG AND MTALLURGY