Reduction of Manganese Oxides by Methane-Containing Gas

ISIJ International, Vol. 44 (2004), No. 9, pp. 1480–1487

Reduction of Manganese Oxides by Methane-containing Gas

Nathaniel ANACLETO, Oleg OSTROVSKI1) and S. GANGULY2)

Formerly PhD Student, School of Materials Science and Engineering, University of New South Wales, Sydney, now at MSU- IIT, Tibanga, Iligan City, 9200 Philippines. 1) School of Materials Science and Engineering, University of New South Wales, Sydney, 2052 Australia. E-mail: [email protected] 2) Tasmanian Electrometallurgical Company, Bell Bay, Tasmania 7253, Australia. E-mail: [email protected] (Received on February 12, 2004; accepted in ﬁnal form on June 8, 2004)

The paper presents results of reduction of pure manganese oxides by methane containing gas in non- isothermal and isothermal experiments and reduction mechanisms. The extent and rate of manganese oxide reduction were determined by on-line off-gas analysis using a mass-spectrometer in a ﬁxed bed labo- ratory reactor in the temperature range 1 000–1 200°C at different gas compositions.

Manganese oxides were reduced to carbide Mn7C3. High extent and rate of reduction by methane-containing gas in comparison with carbothermal reduction were attributed to high carbon activity in the reducing gas, which was in the range 15–50 (relative to graphite). The rate of reduction of manganese oxide in-

creased with increasing temperature. Increasing methane content in the reducing gas to 10–20 vol% CH4 favoured the reduction process. Increase in hydrogen partial pressure had a positive effect on the reduction

rate. Addition of carbon monoxide to the reducing gas retarded the reduction process. The addition of Fe3O4 to manganese oxide increased the rate of reduction. Reduction by methane-containing gas occurs through adsorption and cracking of methane with formation of active adsorbed carbon. Deposition of solid carbon retarded the reduction. KEY WORDS: manganese; oxide; methane; carbothermic reduction; mechanisms; activity; partial pressure.

At standard conditions, reaction (1) proceeds spontaneously 1. Introduction at temperatures above 928°C, while carbothermal reduction Ferromanganese is produced in blast and electric ferroal- of MnO by the reaction loy furnaces using lump manganese ore, sinter, and metal- MnO10/7 C1/7 Mn C CO ...... (3) lurgical coke. A rising cost of electrical energy and envi- 7 3 ronmental concerns associated with production of metallur- DG°257 753.71159.82T J/mol2) ...... (4) gical coke and sintered ores are behind a search for alterna- starts at 1 340°C (standard conditions). tive technologies. Pre-reduction of manganese ores could The equilibrium constant for reaction (1) is equal to 8.5 be an attractive route to increase efﬁciency of ferroman- at 1 000°C, 114 at 1 100°C and 1 075 at 1 200°C. This indi- ganese production.1) cates that MnO reduction to manganese carbide may have a Manganese oxides in manganese ore in the solid state by high extent at 1 000–1 200°C using appropriate gas compo- coke, hydrogen or carbon monoxide are reduced only to MnO. This is seen from the Mn–O–C stability diagram in Fig. 1. At temperatures, at which manganese ore is solid (below 1 200°C), low oxygen partial pressure needed for reduction of MnO to metallic manganese or manganese carbides cannot be achieved (in practical sense) using solid carbon, hydrogen or carbon monoxide. However, it can be achieved using methane-hydrogen gas mixture with appropriate CH4/H2 partial pressure ratio and temperature, when thermodynamic activity of carbon is high, above unity (relative to graphite). Reduction of manganese oxide by methane to manganese carbide occurs in accordance with the following reaction: MnO 10/7 CH4 1/7 Mn7C3 CO 20/7 H2 ...... (1) DG°377 682314.44T J/mol2) ...... (2) Fig. 1. Stability diagram for the Mn–O–C system at 1 000°C.

© 2004 ISIJ 1480 ISIJ International, Vol. 44 (2004), No. 9 sition. Only one publication on reduction of manganese ore by methane-containing gas was found in literature. A Japanese patent4) described the production of manganese carbide by reducing pyrolusite (MnO2) by methane–hydrogen gas in a fluidised-bed furnace at 250–520°C. At these temperatures, only manganese dioxide can be reduced by methane to manganese carbide. Thermo- dynamic calculations show that reduction of Mn2O3, Mn3O4 and MnO to manganese carbide requires higher temperatures. The rate of manganese dioxide reduction in the temperature range of 250–520°C is very slow. In the example given in the patent, reduction of pyrolusite to manganese carbide took 48 h (!). Meantime, pyrolusite is easily reduced by hydrogen to Mn2O3 and Mn3O4 by reactions: Fig. 2. Non-isothermal reduction of MnO2 under argon atmosphere. 2MnO2 H2 Æ Mn2O3 H2O ...... (5) 3Mn2O3 H2 Æ 2Mn3O4 H2O ...... (6) the exit gas. The extent of reduction was determined as a These reactions are expected to be much faster at 250– ratio of oxygen loss to initial oxygen in manganese oxides 520°C than the carbide formation by reaction: MnO2 or MnO. The extent of reduction was also calculated on the basis of oxygen content in the reduced sample, 7MnO2 3CH4 8H2 Æ Mn7C3 14H2O ...... (7) which was measured using a LECO oxygen analyser. In the Therefore, not MnO2 but Mn2O3 and/or Mn3O4 will react non-isothermal reduction experiments, a sample was heated with methane-hydrogen gas. However, at these tempera- with a ramping rate of 2°C/min. Non-isothermal experi- tures, the reduction of Mn2O3 or Mn3O4 to manganese car- ments were performed only with manganese oxide MnO2. bide is infeasible. Samples containing manganese carbide quickly swelled The aim of this paper is to examine the extent and rate and decrepitated in air; this made them difficult to analyse. of reduction of pure manganese oxides MnO2 and MnO by Kuo and Persson,5) Kor6) and Tanabe et al.7) also observed CH4–H2–Ar gas mixture at different temperatures and gas the rapid decomposition of manganese carbides in air and compositions, and to establish the reduction mechanism. found it very difficult to prepare a sample for metallograph- Results on the study of manganese ores reduction will be ic examination. They reported that manganese carbide can presented in another paper. be stabilised by adding at least 5% Fe. Because of this, in some experiments magnetite (50 mm powder) was added to manganese oxide in the amount of 10 wt% to have about 2. Experimental 7wt% Fe in the carbide phase. Experiments were conducted in a fixed bed reactor heat- The data obtained from the mass spectrometer and dew ed in a vertical tube furnace with molybdenum disilicide point meter in volume percentage were first converted to heating elements. The experimental set up and procedure the molar flow rate, using argon gas as reference. The ex- were described elsewhere.4) A 2 g sample was held at the tent of reduction was determined by integrating the oxygen bottom of the inner alumina tube of 11 mm inside diameter. removal rate. The gas flow rate was 1.0 L/min. At this flow The bed height was about 10 mm. The raw materials were rate, the external mass-transfer resistance was negligible. manganese oxides MnO (99%, maximum size 0.17 mm) and MnO2 (99 %, maximum size 0.23 mm). 3. Results The reducing gas mixture was made from high purity argon, ultra high purity hydrogen, chemically pure carbon 3.1. Non-isothermal Reduction of MnO2 monoxide and chemically pure methane. Before being in- The manganese oxide MnO2 was subjected to non- troduced to the reactor, all gases were cleaned using a isothermal reduction under pure argon atmosphere from Hydro Purge purifier to remove moisture and carbon diox- 200 to 1 000°C, under hydrogen–argon gas from 200 to ide. The hydrogen gas line had an additional activated char- 900°C and under CH4–H2–Ar gas from 200 to 1 225°C. coal purifier to remove hydrocarbons. Brooks mass flow Results on the non-isothermal reduction of MnO2 (pyro- meters regulated gas flow rates. The exit gas was analysed lusite) under argon atmosphere are shown in Fig. 2. using a mass spectrometer PRIMA 600 supplied by Fisons Pyrolusite started to decompose to Mn2O3 (bixbyite) at ap- Instruments, UK. Water vapour in the exit gas was also proximately 480°C. At about 675°C, Mn2O3 decomposed to analysed using a dew point sensor. Mn3O4 (hausmannite). At 915°C, the oxide was pure XRD analysis was carried out using SIEMENS D5000 Mn3O4. X-Ray Diffractometer with monochromator and a copper Reduction of MnO2 by H2–Ar gas (20 vol% H2 and Ka X-ray source. Scanning range was from 20° to 80° at a 80 vol% Ar) started at 305–320°C as shown in Fig. 3. Pure speed of 0.6°/min, with a step of 0.01°. MnO2 was completely reduced to MnO at 610–620°C with Oxygen removed from the sample in the reduction exper- no sign of further reduction with further increase in temper- iment was calculated on the basis of CO and H2O content in ature.

Fig. 3. Non-isothermal reduction of MnO by hydrogen–argon 2 Fig. 5. Reduction of pure MnO and MnO with addition of mixture (20 vol% H2 and 80 vol% Ar). 10 wt% Fe3O4 by 15vol%CH4–20vol%H2–65vol%Ar mixture at different temperatures.

Fig. 4. Non-isothermal reduction of MnO2 by methane– hydrogen mixture (10vol%CH4–20vol%H2–70vol%Ar).

In the non-isothermal reduction by CH4–H2–Ar gas Fig. 6. Reduction of MnO by methane-containing gas with different methane content at 1 200°C (hydrogen content was (10 vol% CH4, 20 vol% H2 and Ar the balance) presented in constant at 20 vol% H2). Fig. 4, MnO2 was reduced to Mn3O4 and further to MnO by hydrogen at temperatures very close to the reduction by

H2–Ar gas. The reduction of MnO to manganese carbide pure MnO reduction produced only CO. The ﬁrst stage of started at 760°C and was completed at about 1 200°C. In reduction comprised the reduction of Fe3O4 by hydrogen to the process of MnO reduction to manganese carbide, only metallic iron, which proceeded quickly. CO was detected in the gas phase. The reduction reaction (reaction (1)) is strongly en- dothermic, and the decrease in the bed temperature during 3.2. Isothermal Reduction—Effect of Temperature the MnO reduction was observed. The temperature drop and Iron Addition on MnO Reduction was particularly strong (50–60°C) in the beginning of the Results of the isothermal MnO reduction by CH4–H2–Ar reduction process. gas (15 vol% CH4, 20 vol% H2 and 65 vol% Ar) in the temperatures range 1 000–1 200°C showed that the rate of man- 3.3. Effect of Methane Content on MnO Reduction ganese oxide reduction increased with temperature (Fig. 5). The effect of methane content in the gas mixture on the Oxygen analysis by LECO detected no oxides in a sample rate of MnO reduction was examined at 1 200°C at constant after reduction. hydrogen content of 20 vol%. The methane content was

The addition of Fe3O4 increased the rate of reduction at varied from 2.5 to 20 vol%. The extent of reduction versus all temperatures in the range of 1 000 to 1 200°C. Its effect time is shown in Fig. 6. The rate of reduction increased on the rate of MnO reduction was particularly strong at with increasing methane content to 10–15 vol%. Increase in 1 000 and 1 050°C. However, at these temperatures, com- the methane content above 15 vol% had only a slight effect plete reduction of the MnO–Fe3O4 samples was not on MnO reduction and was accompanied by strong carbon achieved due to blockage of a porous plug by solid carbon deposition. Deposited carbon blocked an access of the re- deposits. The exit gas contained a small amount of H2O, ducing gas to the particle interior, affected the gas ﬂow which was attributed to the iron oxide reduction whereas through the reactor and hindered the reduction process.

Fig. 7. Reduction of MnO by methane-containing gas with dif-

ferent hydrogen content at 1 150°C (CH4 content was constant at 15 vol%). Fig. 9. X-ray diffraction patterns at various stages of reduction of MnO by methane-containing gas mixtures (15 vol%

CH4, 20 vol% H2, Ar the balance) at 1 100°C.

Fig. 8. Effect of CO contents on the reduction of MnO by

CH4–H2–Ar mixture (10 vol% CH4, 20 vol% H2, Ar the balance) at 1150°C. Fig. 10. X-ray diffraction patterns at various stages of reduction of MnO by methane-containing gas mixtures (15 vol% 3.4. Effect of Hydrogen Content on MnO Reduction CH4, 20 vol% H2, Ar the balance) at 1 200°C. The effect of hydrogen content in the gas mixture on MnO reduction was investigated at a constant methane content of 15 vol% at 1 150°C. The hydrogen content in the gas CO for the calculation of the extent of reduction. Reduction mixture was varied from 10 to 85%. The extent of reduc- curves for different carbon monoxide concentrations are tion at different hydrogen contents is shown in Fig. 7. The shown in Fig. 8. rate of MnO reduction increased slightly with the increase The addition of CO to the gas mixtures had a strong re- in hydrogen content in the gas mixtures. tarding effect on the extent of MnO reduction, particularly when CO content in the inlet gas was above 1.5 vol%. The 3.5. Effect of Carbon Monoxide on MnO Reduction degree of reduction experienced after 3 h by gas containing The effect of carbon monoxide in the gas mixture on 3vol% CO was less than 40%. MnO reduction was examined at 1 150°C at constant methane and hydrogen content of 10 and 20 vol%, respec- 3.6. XRD Patterns of Oxides at Various Stages of Re- tively. The carbon monoxide content of the gas mixtures duction was varied from 0 to 5%. It should be noted that CO mea- Reduced samples were subjected to XRD analysis. sured by the mass spectrometer in reduction experiments Figures 9 and 10 display X-ray diffraction patterns at vari- was a sum of CO formed by the reduction reaction and the ous stages of reduction of pure MnO and MnO–Fe3O4 mix- initial CO in the inlet gas mixture. Because of this, the gas ture at 1 100 and 1 200°C. Iron stabilises manganese car- composition was examined at the identical conditions but bide; its addition in the amount of 7 wt% had insigniﬁcant without MnO sample to determine the background level of effect on the X-ray diffraction pattern of a reduced sample.

Fig. 12. Non-isothermal reduction of pelletised MnOC mix-

Fig. 11. X-ray diffraction patterns of reduced pure MnO, MnO– ture under 80vol%H2–20vol%Ar gas.

Fe3O4 and synthetic carbide.

XRD spectra of manganese–iron carbides with high carbon concentrations are not reported in literature. There are only three iron–manganese carbides with lower carbon concentration listed in the XRD database Traces version

4 (Powder Diffraction File, 1997), namely: Fe0.6Mn5.4C2 (PDF No. 20-522), Fe0.4Mn3.6C (PDF No. 20-521) and Fe0.25Mn1.4C0.6 (PDF No. 41-1220). To identify the manganese-iron carbide formed in the reduction process, synthetic carbide was smelted using 7.5 wt% Fe, 84.0 wt% Mn and 8.5 wt% C (graphite) in graphite crucible at 1 350°C under argon atmosphere. The synthetic carbide had a composition close to (Mn, Fe)7C3. X-ray diffraction patterns of reduced pure MnO, reduced

MnO–Fe3O4 and the smelted synthetic carbides are very close to one another (Fig. 11). This observation provides a Fig. 13. X-ray diffraction patterns of MnO reduced under hydro- basis to conclude that pure MnO is reduced to Mn7C3 and gen carbothermally with different MnO/C ratio at MnO–Fe3O4 sample is reduced to (Mn, Fe)7C3. According to XRD data, formation of manganese carbide 1 200°C. was completed in 60 and 30 min at 1 100 and 1 200°C, re- spectively. 4. Discussion 3.7. Carbothermal Reduction of MnO under Hydro- 4.1. Progress of Reduction gen As it follows from results presented in Figs. 2–4, the re-

Carbothermal reduction of a MnO intimately mixed with duction of MnO2 to Mn7C3 follows the sequence graphite was also studied in non-isothermal and isothermal MnO2 Æ Mn2O3 Æ Mn3O4 Æ MnO Æ Mn7C3 .....(8) experiments in H2 atmosphere. The aim of these experiments was to make a comparison of carbothermal reduction When heated under argon, pyrolusite (MnO2) decomposed with the reduction by methane containing gas. with bixbyite (Mn2O3) formation at temperature above In the non-isothermal experiment presented in Fig. 12, 480°C, and bixbyite transformed to hausmannite (Mn3O4) MnO reduction started at 920°C with formation of CO. at temperature above 675°C. These decomposition tempera- Methane started to form at around 900°C. In the isothermal tures are relatively close to the decomposition temperature 8) reduction, the rate of reaction was strongly affected by the under N2 atmosphere reported by Krogerus et al. : 510°C MnO/graphite ratios where MnO reduction was notably for MnO2 Æ Mn2O3 transformation and 725°C for faster at a higher proportion of graphite. Mn2O3 Æ Mn3O4 transformation. Under oxidising gas at The XRD patterns of samples reduced at 1 200°C is 1 atm, MnO2 is stable up to 500°C, Mn2O3 up to 900°C and 9) shown in Fig. 13. After reduction, the main phases present Mn3O4 up to 1 600°C. were Mn, MnO and small amount of Mn23C6 when MnO/C During non-isothermal reduction of manganese oxide, molar ratio was 0.9/1. When the MnO/C molar ratio was MnO2 is reduced to MnO by hydrogen only, methane does 0.7/1, the Mn23C6 peaks became stronger and the Mn peaks not participate in the reduction process. Reduction of MnO weaker while the MnO almost disappeared. When the to manganese carbide proceeds with formation of CO. MnO/C molar ratio was 0.5/1, the main phase present was However, it does not exclude reduction of MnO by hydro-

Mn7C3 (close to 100%). gen (reaction (9)) followed by reaction (10) of water vapour with methane:

7MnO 3CH4 H2 Mn7C3 7H2O ...... (9) CH4 H2O CO 3H2 ...... (10) 4.2. Reaction Products Five stoichiometric carbides are known in the Mn–C system, namely; Mn7C3, Mn5C2, Mn3C, Mn15C4 and Mn23C6. Different carbides are stable under different conditions. Rankin and Van Deventer,10) Eric and Burucu11) and Akdogan and Eric12) reported that in the carbothermal reduction of manganese oxides, Mn5C2 was formed. Terayama and Ikeda13) and Ostrovski and Webb14) conclud- ed that the reaction product was Mn7C3. In this investiga- tion, the manganese carbide produced in the reduction by

CH4–H2–Ar gas was identified as Mn7C3 (XRD, PDF No. 36-1269). The formation of different phases in carbothermal reduction of MnO under hydrogen was dependent on the amount of graphite added. However, in the case of MnO reduction Fig. 14. Calculated equilibrium partial pressure of CO in the by methane, only Mn C or (Mn, Fe) C was formed even in MnO reduction by methane and graphite as a function 7 3 7 3 of temperature. the very early stages of reduction. Metallic manganese and other ferromanganese carbides were not observed in the reduction process. Rate of reduction of pure MnO was slightly affected by hydrogen content when the methane concentration was 4.3. Effect of Iron Addition fixed. The slight increase in reduction rate with increasing The addition of Fe3O4 to manganese oxide increases the hydrogen content might be due to the suppression of solid rate of reduction in the whole temperature range of 1 000 to carbon deposition. Hydrogen does not directly reduce MnO 1 200°C. This can be attributed to two factors; first, Fe3O4 is to Mn and therefore its role is to control the carbon activity easily reduced to metallic iron, and second, metallic iron in the gas phase. acts as nuclei in formation of carbide, which accelerates The addition of carbon monoxide to the reducing gas re- MnO reduction.12) tarded the reduction of pure MnO, which can be explained However, addition of iron to manganese oxide had also a either by the re-oxidation of manganese carbide or CO ad- negative effect: iron catalyses methane cracking and carbon sorption onto active sites of the manganese oxide surface 15) deposition. Complete reduction of the MnO–Fe3O4 sam- thus hindering the adsorption of methane. ples was not achieved due to blockage of a porous plug by Manganese carbide can be re-oxidised by the reaction solid carbon. This occurred at low temperatures of 1 000 Mn C (s)7CO (g)7MnO (s)10C (s) ...... (11) and 1 050°C, at which the reduction rate was slow (Fig. 5). 7 3 Accumulation of deposited carbon in the sample and The calculated equilibrium partial pressure of CO for MnO porous plug could be observed by the gradual increase of reduction to Mn7C3 by methane and graphite at different the pressure of the inlet methane-containing gas. This was temperatures is shown in Fig. 14. It shows that, the CO also confirmed by the total carbon analysis using LECO equilibrium partial pressure in reaction (11) is much lower and by the XRD analysis. The graphite peak in the XRD than in reduction by methane-containing gas by reaction pattern of reduced MnO–Fe3O4 sample is higher than of re- (1). The reduction reaction by gas containing 10 vol% CH4 duced pure MnO. and 20 vol% H2 is practically irreversible at high tempera- The stabilising effect of iron on the manganese carbide ture. The equilibrium CO partial pressure is very high. can be explained by formation of iron-manganese substitu- However, a partial pressure of CO of 0.01–0.1 atm can be tion solution, which was confirmed by the XRD analysis. above equilibrium for carbothermal reduction and can re- oxidise manganese carbide to MnO especially at lower tem- 4.4. Effect of Gas Composition on Reduction of MnO peratures according to reaction (11), which explains the The MnO reduction rate increased with increasing frac- strong retarding effect of CO addition to the reducing gas tion of methane in the reducing gas from 2.5 to 10 vol%, on the reduction rate of MnO. and practically was not affected by further increase in the methane content. Deposition of carbon was caused either 4.5. Comparison of MnO Reduction by Methane-containing Gas with Carbothermal Reduction by high methane partial pressure (high CH4/H2 ratio which controls carbon activity at constant temperature) or addition Figure 15 presents reduction curves for pure MnO re- of iron, or high temperature. Deposited carbon had a retard- duced at 1 200°C by graphite in argon atmosphere (Rankin ing effect on MnO reduction. This was convincingly and Van Deventer10)), at 1 320°C by graphite in CO 16) demonstrated by experiments with MnO–Fe3O4 samples. In (Yastreboff et al. ), at 1 350°C by graphite in CO the absence of iron, MnO reduction and carburisation by (Yastreboff et al.17)), at 1 350°C by graphite in He methane-containing gas under conditions employed in this (Yastreboff et al.16)), at 1 200°C by graphite in He work was faster than carbon deposition. This is a necessary (Terayama and Ikeda13)), at 1 100°C by graphite under hy- condition for complete reduction process. drogen and at 1 100°C and 1 200°C by CH4–H2–Ar gas

4.6. Mechanism of Reduction by Methane-containing Gas This is the first work, in which manganese oxide was reduced to metallic state (carbide) by gas; and only two publi- cations are known on chromium oxide reduction to chromium carbide by methane-containing gas, which findings and conclusions are in contrast with this work. Read et al.18) in a study of chromium oxide reduction suggested that the oxide is reduced by solid carbon deposited as a result of methane cracking. Findings of this work show that solid carbon deposition has a detrimental effect on the rate and extent of MnO reduction. The mechanism of reduction by methane-containing gas is different from the mechanism of carbothermal reduction. Reduction and carburisation of iron oxides by methane- containing gas has been examined in a number of works, and even, at one stage, was commercialised for iron carbide Fig. 15. Reduction curves for MnO reduced by graphite in CO, production. A significant difference in the reduction/car- 17) Ar and He (Yastreboff et al. ), Ar (Rankin and Van burisation of iron oxides with manganese oxides by Deventer10)), He (Terayama and Ikeda13)), H atmo- 2 CH –H gas is that iron oxide is firstly reduced by hydrogen sphere and by CH –H –Ar gas at different temperatures. 4 2 4 2 to metallic iron and then metallic iron is carburised by methane with formation of cementite. Metallic manganese

was not observed in the reduction by CH4–H2–Ar gas in this work; the product was manganese carbide. Neverthe- less, formation of manganese in the course of reduction cannot be excluded. Manganese has much higher afﬁnity for carbon in comparison with iron; formation of manganese carbides could be much faster than reduction of oxides, which makes metals undetectable in the phase analysis, following the reduction process. Fundamentals of iron oxide reduction by methane-containing gas to iron carbide (cementite) are, to some extent, applicable to manganese oxide reduction. Cementite is unstable and decomposes to iron and solid carbon; in other words, cementite is not formed by reaction of solid carbon (graphite) with iron. Formation of cementite Fig. 16. Reduction curves obtained in non-isothermal reduction in the process of iron oxide reduction by methane-contain-

of MnO by 10vol%CH4–20vol%H2–70vol%Ar and car- ing gas is attributed to high carbon activity in the reducing bothermal reduction in the 80vol%H2–20vol%Ar gas at- gas, which is above unity relative to graphite. This is also a mosphere. key factor in the reduction of manganese oxides. Reduction process starts with adsorption of methane on the active sites of the oxide surface and its decomposition mixture. to carbon and hydrogen, which includes the following reac- The reduction of MnO with solid carbon under argon is tions19): very slow and is not complete even at 1 200°C after 2 h. The rate of carbothermal reduction depends strongly on the CH4 (gas) CH4 (ad) ...... (12) gas atmosphere. At 1 200°C, the carbothermal reduction of CH (ad)CH (ad)H (ad) ...... (13) pure MnO under He, investigated by Terayama and Ikeda13) 4 3 is about 7 times faster than under Ar atmosphere measured CH3 (ad) CH2 (ad) H (ad) ...... (14) 10) by Rankin and Van Deventer. However, it should be em- CH (ad)CH (ad)H (ad) ...... (15) phasised that the reduction rate of MnO by methane-con- 2 taining gas is faster than the carbothermal reduction of CH (ad)C (ad)H (ad) ...... (16) MnO under Ar, He or H2. 2H (ad)H (g) ...... (17) In non-isothermal experiments, reduction of MnO by 2 methane gas starts at 760°C, while carbothermal reduction Overall reaction of methane adsorption and cracking may under hydrogen only at 920°C. This is clearly seen from re- be presented as: duction curves in non-isothermal reduction of pure MnO in CH Æ ···Æ C 2H ...... (18) Fig. 16. The rate of MnO reduction by methane containing 4 ad 2 gas is faster than that by carbothermal reduction under hy- On the basis of this reaction, thermodynamic activity of ad- drogen. sorbed carbon may be deﬁned, with graphite as the standard state, by

Ê ˆ 5. Conclusion PCH aK Á 4 ˜ ...... (19) C Á P2 ˜ Manganese oxides are reduced to the manganese carbide Ë H2 ¯ Mn7C3 by the methane-containing gas through the se- where K is the equilibrium constant of the reaction (20) of quence: MnO2 Æ Mn2O3 Æ Mn3O4 Æ MnO Æ Mn7C3. The methane cracking to graphite and hydrogen, reduction rates increase with increasing temperature in the temperature range of 1 000–1 200°C. At 1 200°C, reduction CH C 2H ...... (20) 4 gr 2 of MnO is close to completion in less than 30 min. The (P /P 2 ) ratio in the gas phase in the equilibrium CH4 H2 The reduction rate increases with increasing methane with graphite is ﬁxed at constant temperature and will be content in the gas mixtures up to 10–15 vol% CH4. referred to as (P /P 2 ) . The Eq. (19) may be re-written CH4 H2 gr Increasing hydrogen content above 20 vol% favours the re- in the form: duction process. Addition of CO to the reducing gas strongly retards the Ê ˆ reduction process. The addition of Fe O to manganese PCH 3 4 Á 4 ˜ oxide increases the rate of reduction. Á P2 ˜ Ë H2 ¯ MnO is reduced to Mn C by reaction (1). Reduction a ...... (21) 7 3 C Ê ˆ process starts with adsorption of methane on the active sites PCH Á 4 ˜ of the oxide surface and its decomposition in the following Á 2 ˜ PH Ë 2 ¯ gr sequence: CH Æ CH H Æ CH 2H Æ CH3H Æ C 2H When (P /P 2 )(P /P 2 ) , activity of adsorbed carbon 4 3 2 ad 2 CH4 H2 CH4 H2 gr will be 1. This active carbon provides higher extent of the The key factor in the reduction process is high carbon ac- reduction reaction in comparison with the carbothermal re- tivity in the reducing gas. Deposition of solid carbon in the duction. course of oxide reduction has a strong retarding effect on Adsorbed carbon is consumed by reduction/carburisation the reduction process. reaction. The key factor is a high rate of this reaction in comparison with the rate of carbon deposition (reaction REFERENCES (20)). If solid carbon is formed on the oxide surface, the 2 1) V. Misra: Proc. 14th CMMI Cong., Institution of Mining and (PCH /P H ) ratio at the gas/solid interface will be maintained 4 2 2 2 Metallurgy, (1990), 39. equal to (PCH /P H )gr regardless of the high (PCH /P H )gr 4 2 4 2 2) Thermochemical Properties of Inorganic Substances, Second Ed., ratio in the inlet gas. Then the reductant will be solid car- ed. by O. Knacke, O. Kubaschewski and K. Hesselmann, Springer bon (aC 1) deposited by the reaction of methane cracking; Verlag, Berlin, (1991). under given experimental conditions, the extent and rate of 3) Nippon Denko KK: Japanese Patent No. 08-253308, (1996). carbothermal reduction of MnO will be low. 4) G. Zhang and O. Ostrovski: Metall. Trans. B, 31B (2000), No. 2, The rate of reaction (18), R, is proportional to the frac- 129. 5) K. Kuo and L. Persson: J. Iron Steel Inst., 9 (1954), 39. tion of the oxide surface area available for adsorption, 6) G. Kor: Metall. Trans. B, 10B (1979), 397. (1q), and in the general case is a function of partial pres- 7) I. Tanabe, T. Toyota and H. Komo: J. Jpn. Inst. Met., 24 (1960), 272. sures of methane P and hydrogen P : 8) D. Krogerus, J. Vehvilainer and M. Honkaniem: INFACON 8, China CH4 H2 Science and Technology Press, Beijing, (1998), 271. RkAf (P , P )(1q) ...... (22) CH4 H2 9) K. Berg and S. Olsen: Proc. 54th Elect. Fur. Conf., ISS Warrendale, PA, (1997), 217. A strong effect of the surface area on the rate of manganese 10) W. Rankin and J. Van Denventer: J. S. Afr. Inst. Min. Metall., 80 oxides reduction observed in this work is evidence that re- (1980), No. 7, 239. action (18) can be a rate controlling stage or a contributing 11) R. Eric and E. Burucu: Miner. Eng., 5 (1992), No. 7, 795. stage in the case of mixed control. Sintering or formation of 12) G. Akdogan and R. Eric: Metall. Trans. B, 26B (1995), No. 1, 13. molten phases decreases the surface area available for 13) K. Terayama and M. Ikeda: Trans. JIM, 26 (1985), 108. 14) O. Ostrovski and T. Webb: ISIJ Int., 35 (1995), No. 11, 1331. methane adsorption, having a strong retarding effect on the 15) K. Hutchings, R. Hawkins and J. Smith: Ironmaking Steelmaking, 15 reduction. (1988), No. 3, 121. Reduction/carburisation reaction serves as a sink for ad- 16) M. Yastreboff, O. Ostrovski and S. Ganguly: INFACON 8, China sorbed carbon. After completion of this reaction, adsorbed Science and Technology Press, Beijing, (1998), 263. carbon is not consumed and forms solid carbon. Deposition 17) M. Yastreboff, O. Ostrovski and S. Ganguly: INFACON 9, The Ferroalloys association, Washington, DC, (2001), 286. of solid carbon in the reduction of manganese ores is much 18) P. Read, D. Reeve, J. Walsh and J. Rehder: Can. Metall. Q., 13 less in comparison with iron ore, where reduced iron cataly- (1974), 587. ses the methane cracking. 19. H. Grabke: Metall. Trans. B, 1B (1970), 2972.