Reduction Mechanisms in Manganese Ore Reduction
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INTRODUCTION
THE submerged arc furnace (SAF) process is most widely used in the industrial production of high-carbon ferromanganese. This process requires lump ore and reductant feed materials of +6 mm 75 mm to ensure sufficient gas permeability through the material bed.[1] The AlloyStream process was developed to produce highcarbon ferromanganese, as well as other ferroalloys, from ore fines and thermal coal as described in U.S. patent 6146437[2]; see the schematic drawing of the furnace in Figure 1. The furnace raw material feed mixture consists of mixed ore, coal, and fluxes of 10 mm. The raw material mix is fed onto a liquid alloy bath, forming heaps of reacted material. The heap material is heated at the heap top surface by heat generated from burning combustibles with oxygen-enriched air in the freeboard. The combustibles consist of fuel gas, coal volatiles, coal carbon, and reduction reaction product gas emanating from the heaps. Final reaction and smelting of the heap material is supported by energy transferred from the induction-heated metal bath. The material mixture is reacted at temperatures of 1673 K to 1823 K (1400 °C to 1550 °C). These temperatures are in a much narrower band compared to that expected in the SAF, ranging from room temperature to very high temperatures in excess of 1873 K (1600 °C) below the electrode tips. In development of the AlloyStream process, several campaigns were completed at pilot plant and demonstration plant level. The objective of this study is to THERESA COETSEE, Principal Specialist, is with Exxaro Resources, Pretoria, South Africa. Contact e-mail: theresa.coetsee@ exxaro.com CHRISTIAN REINKE, formerly Senior Instrument Scientist, Mintek, is with the University of Johannesburg, Johannesburg, South Africa. JOHANNES NELL, formerly Senior Consultant, Mintek, is with the Hatch, Johannesburg, South Africa. PETRUS CHRISTIAAN PISTORIUS, POSCO Professor, is with the Carnegie Mellon University, Pittsburgh, PA. Manuscript submitted March 26, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
show the likely mechanisms of manganese ore reduction, based on mineralogical investigation of heap samples periodically extracted from the pilot furnace.
II.
PREVIOUS WORK
The reduction behavior of different manganese ores has been studied in several experimental programmes. Because manganese ores are highly heterogeneous and contain various minerals with different levels of contained manganese and iron, ore reduction behavior is not uniform. For example, in Wessels ore reacted with graphite under hydrogen at 1473 K (1200 °C) (isothermally), a small proportion of ore particles was reduced to approximately 30 pct reduction level while the overall degree of reduction was 80 pct.[3] This makes the experimental study of reduction mechanisms on the micron scale complex. Also, the use of synthetically prepared oxide mixtures to simulate the ore in reduction experiments cannot easily replicate the complex reduced ore phase morphology which develops as reduction progresses, and such results should be in
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