Ore Melting and Reduction in Silicomanganese Production
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GROWING steel production dictates a strong demand for manganese alloys, which production reached 13.1 Mt in 2007 (an 11 pct growth relative to 2006), including 4.5 Mt of high carbon ferromanganese (HC FeMn) and 7.5 Mt of silicomanganese.[1] The design of new technologies and an optimal choice of the raw material mix requires an improved understanding of the materials’ melting behavior, reaction mechanisms, and kinetics. A description of the melting and the reduction reactions, as presented for selected manganese bearing raw materials in this article, is one of the steps required to acquire this understanding. A standard silicomanganese alloy contains 67 to 70 wt pct manganese, 17 to 20 wt pct silicon, and 1.5 to 2 wt pct carbon. It can be produced from different charge mixes of manganese ore, ferromanganese slag, quartz, and fluxes (dolomite or calcite). Metal also may be present in the feed materials either as entrapped droplets in ferromanganese slag or as scrap such as silicon alloys. Typically 500 to 1000 kg of ferromanganese slag, containing 35 to 45 wt pct manganese oxide and 15 to ELI RINGDALEN and SEAN GAAL, Research Scientists, are with Materials and Chemistry, SINTEF, 7465 Trondheim, Norway, MERETE TANGSTAD, Professor, is with the Department of Materials Science and Engineering, NTNU, 7491 Trondheim, Norway, and OLEG OSTROVSKI, Professor, is with the School of Materials Science and Engineering, UNSW, 2052 Sydney, Australia. Contact e-mail: [email protected]. Manuscript submitted July 15, 2009. Article published online September 3, 2010. 1220—VOLUME 41B, DECEMBER 2010
30 wt pct silica,[2] is formed during the production of 1000 kg of HC FeMn alloy. This slag often is used as a source of manganese in the production of silicomanganese, although some plants use only manganese ore as a source of manganese. Utilization of the ferromanganese slag in silicomanganese production increases the energy consumption by approximately 500 kWh/t slag.[3] During the smelting of silicomanganese, additions of dolomite or calcite to the charge increase the slag basicity, which improves the slag fluidity[2] and facilitates the manganese oxide reduction from the slag. The typical silica content in silicomanganese slag is in the range 35 to 45 wt pct. This slag has a liquidus temperature between 1573 K and 1653 K (1300 °C and 1380 °C), depending on the slag composition. Increasing the temperature or the silica content in the slag and decreasing the (CaO + MgO)/Al2O3 ratio all increase the silicon metal–slag partitioning coefficient. Excavations of industrial furnaces provide information about the distribution of phases in the reaction zones in the furnace. The interior of the silicomanganese furnace, described on the basis of the excavation of a 16-MW furnace,[3] includes a preheating and a prereduction zone as well as a coke bed zone. In this furnace, it was observed that manganese oxides were reduced almost totally from the ferromanganese slag and the manganese ore at the top of the coke bed. Different charge materials exhibit
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