Reduction of Manganese Oxides by Methane-containing Gas - J-Stage

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 MSUIIT, 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 nonisothermal 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 laboratory 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 increased 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.

1.

At standard conditions, reaction (1) proceeds spontaneously at temperatures above 928°C, while carbothermal reduction of MnO by the reaction

Introduction

Ferromanganese is produced in blast and electric ferroalloy furnaces using lump manganese ore, sinter, and metallurgical coke. A rising cost of electrical energy and environmental concerns associated with production of metallurgical coke and sintered ores are behind a search for alternative technologies. Pre-reduction of manganese ores could be an attractive route to increase efficiency of ferromanganese production.1) Manganese oxides in manganese ore in the solid state by 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:

MnO10/7 C1/7 Mn7C3CO ...............(3) DG°257 753.71159.82T J/mol2) ............(4) starts at 1 340°C (standard conditions). The equilibrium constant for reaction (1) is equal to 8.5 at 1 000°C, 114 at 1 100°C and 1 075 at 1 200°C. This indicates that MnO reduction to manganese carbide may have a high extent at 1 000–1 200°C using appropriate gas compo-

MnO10/7 CH41/7 Mn7C3CO20/7 H2 ......(1) DG°377 682314.44T J/mol2) ........................(2) Fig. 1.

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Stability diagram for the Mn–O–C system at 1 000°C.

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. Thermodynamic 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.

2MnO2H2 Æ Mn2O3H2O ....................(5) 3Mn2O3H2 Æ 2Mn3O4H2O .................(6)

the exit gas. The extent of reduction was determined as a ratio of oxygen loss to initial oxygen in manganese oxides MnO2 or MnO. The extent of reduction was also calculated on the basis of oxygen content in the reduced sample, which was measured using a LECO oxygen analyser. In the non-isothermal reduction experiments, a sample was heated with a ramping rate of 2°C/min. Non-isothermal experiments were performed only with manganese oxide MnO2. Samples containing manganese carbide quickly swelled and decrepitated in air; this made them difficult to analyse. Kuo and Persson,5) Kor6) and Tanabe et al.7) also observed the rapid decomposition of manganese carbides in air and found it very difficult to prepare a sample for metallographic examination. They reported that manganese carbide can be stabilised by adding at least 5% Fe. Because of this, in some experiments magnetite (50 m m powder) was added to manganese oxide in the amount of 10 wt% to have about 7 wt% Fe in the carbide phase. The data obtained from the mass spectrometer and dew point meter in volume percentage were first converted to the molar flow rate, using argon gas as reference. The extent of reduction was determined by integrating the oxygen removal rate. The gas flow rate was 1.0 L/min. At this flow rate, the external mass-transfer resistance was negligible.

These reactions are expected to be much faster at 250– 520°C than the carbide formation by reaction: 7MnO23CH48H2 Æ Mn7C314H2O ..........(7) Therefore, not MnO2 but Mn2O3 and/or Mn3O4 will react with methane-hydrogen gas. However, at these temperatures, the reduction of Mn2O3 or Mn3O4 to manganese carbide is infeasible. The aim of this paper is to examine the extent and rate of reduction of pure manganese oxides MnO2 and MnO by CH4–H2–Ar gas mixture at different temperatures and gas compositions, and to establish the reduction mechanism. Results on the study of manganese ores reduction will be presented in another paper.

2.

Non-isothermal reduction of MnO2 under argon atmosphere.

Experimental

Experiments were conducted in a fixed bed reactor heated in a vertical tube furnace with molybdenum disilicide heating elements. The experimental set up and procedure were described elsewhere.4) A 2 g sample was held at the bottom of the inner alumina tube of 11 mm inside diameter. The bed height was about 10 mm. The raw materials were manganese oxides MnO (99%, maximum size 0.17 mm) and MnO2 (99%, maximum size 0.23 mm). The reducing gas mixture was made from high purity argon, ultra high purity hydrogen, chemically pure carbon monoxide and chemically pure methane. Before being introduced to the reactor, all gases were cleaned using a Hydro Purge purifier to remove moisture and carbon dioxide. The hydrogen gas line had an additional activated charcoal purifier to remove hydrocarbons. Brooks mass flow meters regulated gas flow rates. The exit gas was analysed using a mass spectrometer PRIMA 600 supplied by Fisons Instruments, UK. Water vapour in the exit gas was also analysed using a dew point sensor. XRD analysis was carried out using SIEMENS D5000 X-Ray Diffractometer with monochromator and a copper Ka X-ray source. Scanning range was from 20° to 80° at a speed of 0.6°/min, with a step of 0.01°. Oxygen removed from the sample in the reduction experiment was calculated on the basis of CO and H2O content in

3.

Results

3.1. Non-isothermal Reduction of MnO2 The manganese oxide MnO2 was subjected to nonisothermal reduction under pure argon atmosphere from 200 to 1 000°C, under hydrogen–argon gas from 200 to 900°C and under CH4–H2–Ar gas from 200 to 1 225°C. Results on the non-isothermal reduction of MnO2 (pyrolusite) under argon atmosphere are shown in Fig. 2. Pyrolusite started to decompose to Mn2O3 (bixbyite) at approximately 480°C. At about 675°C, Mn2O3 decomposed to Mn3O4 (hausmannite). At 915°C, the oxide was pure Mn3O4. Reduction of MnO2 by H2–Ar gas (20 vol% H2 and 80 vol% Ar) started at 305–320°C as shown in Fig. 3. Pure MnO2 was completely reduced to MnO at 610–620°C with no sign of further reduction with further increase in temperature. 1481

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Fig. 3.

Non-isothermal reduction of MnO2 by hydrogen–argon mixture (20 vol% H2 and 80 vol% Ar).

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 (10 vol% CH4, 20 vol% H2 and Ar the balance) presented in 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 started at 760°C and was completed at about 1 200°C. In the process of MnO reduction to manganese carbide, only CO was detected in the gas phase.

Reduction of pure MnO and MnO with addition of 10 wt% Fe3O4 by 15vol%CH4–20vol%H2–65vol%Ar mixture at different temperatures.

Fig. 6.

Reduction of MnO by methane-containing gas with different methane content at 1 200°C (hydrogen content was constant at 20 vol% H2).

pure MnO reduction produced only CO. The first stage of reduction comprised the reduction of Fe3O4 by hydrogen to metallic iron, which proceeded quickly. The reduction reaction (reaction (1)) is strongly endothermic, and the decrease in the bed temperature during the MnO reduction was observed. The temperature drop was particularly strong (50–60°C) in the beginning of the reduction process.

3.2.

Isothermal Reduction—Effect of Temperature and Iron Addition on MnO Reduction Results of the isothermal MnO reduction by CH4–H2–Ar 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 manganese oxide reduction increased with temperature (Fig. 5). Oxygen analysis by LECO detected no oxides in a sample after reduction. The addition of Fe3O4 increased the rate of reduction at all temperatures in the range of 1 000 to 1 200°C. Its effect on the rate of MnO reduction was particularly strong at 1 000 and 1 050°C. However, at these temperatures, complete reduction of the MnO–Fe3O4 samples was not achieved due to blockage of a porous plug by solid carbon deposits. The exit gas contained a small amount of H2O, which was attributed to the iron oxide reduction whereas © 2004 ISIJ

Fig. 5.

3.3. Effect of Methane Content on MnO Reduction The effect of methane content in the gas mixture on the rate of MnO reduction was examined at 1 200°C at constant hydrogen content of 20 vol%. The methane content was varied from 2.5 to 20 vol%. The extent of reduction versus time is shown in Fig. 6. The rate of reduction increased with increasing methane content to 10–15 vol%. Increase in the methane content above 15 vol% had only a slight effect on MnO reduction and was accompanied by strong carbon deposition. Deposited carbon blocked an access of the reducing gas to the particle interior, affected the gas flow through the reactor and hindered the reduction process. 1482


Fig. 7.

Fig. 8.

Reduction of MnO by methane-containing gas with different 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.

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% CH4, 20 vol% H2, Ar the balance) at 1 200°C.

3.4. Effect of Hydrogen Content on MnO Reduction 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 mixture was varied from 10 to 85%. The extent of reduction at different hydrogen contents is shown in Fig. 7. The rate of MnO reduction increased slightly with the increase in hydrogen content in the gas mixtures.

CO for the calculation of the extent of reduction. Reduction curves for different carbon monoxide concentrations are shown in Fig. 8. The addition of CO to the gas mixtures had a strong retarding effect on the extent of MnO reduction, particularly when CO content in the inlet gas was above 1.5 vol%. The degree of reduction experienced after 3 h by gas containing 3 vol% CO was less than 40%.

3.5. Effect of Carbon Monoxide on MnO Reduction The effect of carbon monoxide in the gas mixture on MnO reduction was examined at 1 150°C at constant methane and hydrogen content of 10 and 20 vol%, respectively. The carbon monoxide content of the gas mixtures was varied from 0 to 5%. It should be noted that CO measured by the mass spectrometer in reduction experiments was a sum of CO formed by the reduction reaction and the initial CO in the inlet gas mixture. Because of this, the gas composition was examined at the identical conditions but without MnO sample to determine the background level of

3.6.

XRD Patterns of Oxides at Various Stages of Reduction Reduced samples were subjected to XRD analysis. Figures 9 and 10 display X-ray diffraction patterns at various stages of reduction of pure MnO and MnO–Fe3O4 mixture at 1 100 and 1 200°C. Iron stabilises manganese carbide; its addition in the amount of 7 wt% had insignificant effect on the X-ray diffraction pattern of a reduced sample. 1483

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Fig. 12. Non-isothermal reduction of pelletised MnOC mixture under 80vol%H2–20vol%Ar gas.

Fig. 11. X-ray diffraction patterns of reduced pure MnO, MnO– 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 basis to conclude that pure MnO is reduced to Mn7C3 and MnO–Fe3O4 sample is reduced to (Mn, Fe)7C3. According to XRD data, formation of manganese carbide was completed in 60 and 30 min at 1 100 and 1 200°C, respectively.

Fig. 13. X-ray diffraction patterns of MnO reduced under hydrogen carbothermally with different MnO/C ratio at 1 200°C.

4.

3.7.

Carbothermal Reduction of MnO under Hydrogen Carbothermal reduction of a MnO intimately mixed with graphite was also studied in non-isothermal and isothermal experiments in H2 atmosphere. The aim of these experiments was to make a comparison of carbothermal reduction with the reduction by methane containing gas. In the non-isothermal experiment presented in Fig. 12, MnO reduction started at 920°C with formation of CO. Methane started to form at around 900°C. In the isothermal reduction, the rate of reaction was strongly affected by the MnO/graphite ratios where MnO reduction was notably faster at a higher proportion of graphite. The XRD patterns of samples reduced at 1 200°C is shown in Fig. 13. After reduction, the main phases present were Mn, MnO and small amount of Mn23C6 when MnO/C molar ratio was 0.9/1. When the MnO/C molar ratio was 0.7/1, the Mn23C6 peaks became stronger and the Mn peaks weaker while the MnO almost disappeared. When the MnO/C molar ratio was 0.5/1, the main phase present was Mn7C3 (close to 100%).

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Discussion

4.1. Progress of Reduction As it follows from results presented in Figs. 2–4, the reduction of MnO2 to Mn7C3 follows the sequence MnO2 Æ Mn2O3 Æ Mn3O4 Æ MnO Æ Mn7C3 .....(8) When heated under argon, pyrolusite (MnO2) decomposed with bixbyite (Mn2O3) formation at temperature above 480°C, and bixbyite transformed to hausmannite (Mn3O4) at temperature above 675°C. These decomposition temperatures are relatively close to the decomposition temperature under N2 atmosphere reported by Krogerus et al.8): 510°C for MnO2 Æ Mn2O3 transformation and 725°C for Mn2O3 Æ Mn3O4 transformation. Under oxidising gas at 1 atm, MnO2 is stable up to 500°C, Mn2O3 up to 900°C and Mn3O4 up to 1 600°C.9) During non-isothermal reduction of manganese oxide, MnO2 is reduced to MnO by hydrogen only, methane does not participate in the reduction process. Reduction of MnO to manganese carbide proceeds with formation of CO. However, it does not exclude reduction of MnO by hydrogen (reaction (9)) followed by reaction (10) of water vapour with methane: 1484


7MnO3CH4H2Mn7C37H2O ............(9) CH4H2OCO3H2 .....................(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) concluded that the reaction product was Mn7C3. In this investigation, 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 by methane, only Mn7C3 or (Mn, Fe)7C3 was formed even in the very early stages of reduction. Metallic manganese and other ferromanganese carbides were not observed in the reduction process.

Fig. 14. Calculated equilibrium partial pressure of CO in the MnO reduction by methane and graphite as a function of temperature.

Rate of reduction of pure MnO was slightly affected by hydrogen content when the methane concentration was fixed. The slight increase in reduction rate with increasing hydrogen content might be due to the suppression of solid carbon deposition. Hydrogen does not directly reduce MnO to Mn and therefore its role is to control the carbon activity in the gas phase. The addition of carbon monoxide to the reducing gas retarded the reduction of pure MnO, which can be explained either by the re-oxidation of manganese carbide or CO adsorption onto active sites of the manganese oxide surface thus hindering the adsorption of methane. Manganese carbide can be re-oxidised by the reaction

4.3. Effect of Iron Addition The addition of Fe3O4 to manganese oxide increases the rate of reduction in the whole temperature range of 1 000 to 1 200°C. This can be attributed to two factors; first, Fe3O4 is easily reduced to metallic iron, and second, metallic iron acts as nuclei in formation of carbide, which accelerates MnO reduction.12) However, addition of iron to manganese oxide had also a negative effect: iron catalyses methane cracking and carbon deposition.15) Complete reduction of the MnO–Fe3O4 samples was not achieved due to blockage of a porous plug by solid carbon. This occurred at low temperatures of 1 000 and 1 050°C, at which the reduction rate was slow (Fig. 5). Accumulation of deposited carbon in the sample and porous plug could be observed by the gradual increase of the pressure of the inlet methane-containing gas. This was also confirmed by the total carbon analysis using LECO and by the XRD analysis. The graphite peak in the XRD pattern of reduced MnO–Fe3O4 sample is higher than of reduced pure MnO. The stabilising effect of iron on the manganese carbide can be explained by formation of iron-manganese substitution solution, which was confirmed by the XRD analysis.

Mn7C3 (s)7CO (g)7MnO (s)10C (s) ......(11) The calculated equilibrium partial pressure of CO for MnO reduction to Mn7C3 by methane and graphite at different temperatures is shown in Fig. 14. It shows that, the CO equilibrium partial pressure in reaction (11) is much lower than in reduction by methane-containing gas by reaction (1). The reduction reaction by gas containing 10 vol% CH4 and 20 vol% H2 is practically irreversible at high temperature. The equilibrium CO partial pressure is very high. However, a partial pressure of CO of 0.01–0.1 atm can be above equilibrium for carbothermal reduction and can reoxidise manganese carbide to MnO especially at lower temperatures according to reaction (11), which explains the strong retarding effect of CO addition to the reducing gas on the reduction rate of MnO.

4.4. Effect of Gas Composition on Reduction of MnO The MnO reduction rate increased with increasing fraction of methane in the reducing gas from 2.5 to 10 vol%, and practically was not affected by further increase in the methane content. Deposition of carbon was caused either by high methane partial pressure (high CH4/H2 ratio which controls carbon activity at constant temperature) or addition of iron, or high temperature. Deposited carbon had a retarding effect on MnO reduction. This was convincingly demonstrated by experiments with MnO–Fe3O4 samples. In the absence of iron, MnO reduction and carburisation by methane-containing gas under conditions employed in this work was faster than carbon deposition. This is a necessary condition for complete reduction process.

4.5.

Comparison of MnO Reduction by Methane-containing Gas with Carbothermal Reduction Figure 15 presents reduction curves for pure MnO reduced at 1 200°C by graphite in argon atmosphere (Rankin and Van Deventer10)), at 1 320°C by graphite in CO (Yastreboff et al.16)), at 1 350°C by graphite in CO (Yastreboff et al.17)), at 1 350°C by graphite in He (Yastreboff et al.16)), at 1 200°C by graphite in He (Terayama and Ikeda13)), at 1 100°C by graphite under hydrogen and at 1 100°C and 1 200°C by CH4–H2–Ar gas 1485

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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 publications 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 methanecontaining gas has been examined in a number of works, and even, at one stage, was commercialised for iron carbide production. A significant difference in the reduction/carburisation of iron oxides with manganese oxides by CH4–H2 gas is that iron oxide is firstly reduced by hydrogen 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. Nevertheless, 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 in the process of iron oxide reduction by methane-containing gas is attributed to high carbon activity in the reducing gas, which is above unity relative to graphite. This is also a 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 to carbon and hydrogen, which includes the following reactions19):

Fig. 15. Reduction curves for MnO reduced by graphite in CO, Ar and He (Yastreboff et al.17)), Ar (Rankin and Van Deventer10)), He (Terayama and Ikeda13)), H2 atmosphere and by CH4–H2–Ar gas at different temperatures.

Fig. 16. Reduction curves obtained in non-isothermal reduction of MnO by 10vol%CH4–20vol%H2–70vol%Ar and carbothermal reduction in the 80vol%H2–20vol%Ar gas atmosphere.

mixture. The reduction of MnO with solid carbon under argon is very slow and is not complete even at 1 200°C after 2 h. The rate of carbothermal reduction depends strongly on the gas atmosphere. At 1 200°C, the carbothermal reduction of pure MnO under He, investigated by Terayama and Ikeda13) is about 7 times faster than under Ar atmosphere measured by Rankin and Van Deventer.10) However, it should be emphasised that the reduction rate of MnO by methane-containing gas is faster than the carbothermal reduction of MnO under Ar, He or H2. In non-isothermal experiments, reduction of MnO by methane gas starts at 760°C, while carbothermal reduction under hydrogen only at 920°C. This is clearly seen from reduction curves in non-isothermal reduction of pure MnO in Fig. 16. The rate of MnO reduction by methane containing gas is faster than that by carbothermal reduction under hydrogen.

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CH4 (gas)CH4 (ad) .............................(12) CH4 (ad)CH3 (ad)H (ad) .................(13) CH3 (ad)CH2 (ad) H (ad) ................(14) CH2 (ad)CH (ad)H (ad) ..................(15) CH (ad)C (ad)H (ad) .......................(16) 2H (ad)H2 (g) .....................................(17) Overall reaction of methane adsorption and cracking may be presented as: CH4 Æ ··· Æ Cad2H2 .....................(18) On the basis of this reaction, thermodynamic activity of adsorbed carbon may be defined, with graphite as the standard state, by 1486


ÊP CH aC K Á 2 4 Á P Ë H2

ˆ ˜ ...........................(19) ˜ ¯

5.

Conclusion

Manganese oxides are reduced to the manganese carbide Mn7C3 by the methane-containing gas through the sequence: MnO2 Æ Mn2O3 Æ Mn3O4 Æ MnO Æ Mn7C3. The reduction rates increase with increasing temperature in the temperature range of 1 000–1 200°C. At 1 200°C, reduction of MnO is close to completion in less than 30 min. The reduction rate increases with increasing methane content in the gas mixtures up to 10–15 vol% CH4. Increasing hydrogen content above 20 vol% favours the reduction process. Addition of CO to the reducing gas strongly retards the reduction process. The addition of Fe3O4 to manganese oxide increases the rate of reduction. MnO is reduced to Mn7C3 by reaction (1). Reduction process starts with adsorption of methane on the active sites of the oxide surface and its decomposition in the following sequence:

where K is the equilibrium constant of the reaction (20) of methane cracking to graphite and hydrogen, CH4Cgr2H2 ..........................(20) The (PCH4/P 2H2) ratio in the gas phase in the equilibrium with graphite is fixed at constant temperature and will be referred to as (PCH4/P 2H2)gr . The Eq. (19) may be re-written in the form: ÊP ˆ CH Á 24 ˜ Á P ˜ Ë H2 ¯ aC ...........................(21) ÊP ˆ CH Á 24 ˜ Á P ˜ Ë H2 ¯ gr

CH4 Æ CH3H Æ CH22H Æ CH3H Æ Cad2H2

When (PCH4/P 2H2)(PCH4/P 2H2)gr , activity of adsorbed carbon will be 1. This active carbon provides higher extent of the reduction reaction in comparison with the carbothermal reduction. Adsorbed carbon is consumed by reduction/carburisation reaction. The key factor is a high rate of this reaction in comparison with the rate of carbon deposition (reaction (20)). If solid carbon is formed on the oxide surface, the (PCH4/P 2H2) ratio at the gas/solid interface will be maintained equal to (PCH4/P 2H2)gr regardless of the high (PCH4/P 2H2)gr ratio in the inlet gas. Then the reductant will be solid carbon (aC1) deposited by the reaction of methane cracking; under given experimental conditions, the extent and rate of carbothermal reduction of MnO will be low. The rate of reaction (18), R, is proportional to the fraction of the oxide surface area available for adsorption, (1q ), and in the general case is a function of partial pressures of methane PCH4 and hydrogen PH2:

The key factor in the reduction process is high carbon activity in the reducing gas. Deposition of solid carbon in the course of oxide reduction has a strong retarding effect on the reduction process. REFERENCES 1)

V. Misra: Proc. 14th CMMI Cong., Institution of Mining and Metallurgy, (1990), 39. 2) Thermochemical Properties of Inorganic Substances, Second Ed., ed. by O. Knacke, O. Kubaschewski and K. Hesselmann, Springer Verlag, Berlin, (1991). 3) Nippon Denko KK: Japanese Patent No. 08-253308, (1996). 4) G. Zhang and O. Ostrovski: Metall. Trans. B, 31B (2000), No. 2, 129. 5) K. Kuo and L. Persson: J. Iron Steel Inst., 9 (1954), 39. 6) G. Kor: Metall. Trans. B, 10B (1979), 397. 7) I. Tanabe, T. Toyota and H. Komo: J. Jpn. Inst. Met., 24 (1960), 272. 8) D. Krogerus, J. Vehvilainer and M. Honkaniem: INFACON 8, China Science and Technology Press, Beijing, (1998), 271. 9) K. Berg and S. Olsen: Proc. 54th Elect. Fur. Conf., ISS Warrendale, PA, (1997), 217. 10) W. Rankin and J. Van Denventer: J. S. Afr. Inst. Min. Metall., 80 (1980), No. 7, 239. 11) R. Eric and E. Burucu: Miner. Eng., 5 (1992), No. 7, 795. 12) G. Akdogan and R. Eric: Metall. Trans. B, 26B (1995), No. 1, 13. 13) K. Terayama and M. Ikeda: Trans. JIM, 26 (1985), 108. 14) O. Ostrovski and T. Webb: ISIJ Int., 35 (1995), No. 11, 1331. 15) K. Hutchings, R. Hawkins and J. Smith: Ironmaking Steelmaking, 15 (1988), No. 3, 121. 16) M. Yastreboff, O. Ostrovski and S. Ganguly: INFACON 8, China Science and Technology Press, Beijing, (1998), 263. 17) M. Yastreboff, O. Ostrovski and S. Ganguly: INFACON 9, The Ferroalloys association, Washington, DC, (2001), 286. 18) P. Read, D. Reeve, J. Walsh and J. Rehder: Can. Metall. Q., 13 (1974), 587. 19. H. Grabke: Metall. Trans. B, 1B (1970), 2972.

RkAf (PCH4, PH2)(1q ) ....................(22) A strong effect of the surface area on the rate of manganese oxides reduction observed in this work is evidence that reaction (18) can be a rate controlling stage or a contributing stage in the case of mixed control. Sintering or formation of molten phases decreases the surface area available for methane adsorption, having a strong retarding effect on the reduction. Reduction/carburisation reaction serves as a sink for adsorbed carbon. After completion of this reaction, adsorbed carbon is not consumed and forms solid carbon. Deposition of solid carbon in the reduction of manganese ores is much less in comparison with iron ore, where reduced iron catalyses the methane cracking.

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