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 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. 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- MnOϩ10/7 Cϭ1/7 Mn C ϩCO ...............(3) lurgical coke. A rising cost of electrical energy and envi- 7 3 ronmental concerns associated with production of metallur- DG°ϭ257 753.71Ϫ159.82T 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: ϩ ϭ ϩ ϩ MnO 10/7 CH4 1/7 Mn7C3 CO 20/7 H2 ......(1) DG°ϭ377 682Ϫ314.44T 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 atmo- ϩ ϩ sphere. 2MnO2 H2 Æ Mn2O3 H2O ....................(5) ϩ ϩ 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 ϩ ϩ ϩ 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 (99ϩ%, maximum size 0.17 mm) ϩ 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. 1481 © 2004 ISIJ ISIJ International, Vol. 44 (2004), No. 9 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).

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