Phase Characterization of High Basicity Manganese Slags
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INTRODUCTION
THE most prevalent industrial high carbon ferromanganese production technology in use is the submerged arc furnace (SAF) process. This process requires lump ore and reductant feed materials of +6 to 75 mm to ensure sufficient gas permeability through the material bed.[1] The AlloyStream process was developed to produce high carbon ferromanganese, as well as other ferroalloys, from ore fines and thermal coal as described in U.S. patent 6146437.[2] The furnace cross-section diagram in Figure 1 illustrates the main process features: 10 mm raw material mixture is fed through the furnace combustion zone (freeboard) 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 raw material feed consists of a mixture of coal, ore, and flux material. 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 alloy 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 likely mechanisms of manganese ore reduction, based on mineralogical investigation of heap samples periodically extracted THERESA COETSEE is with the University of Pretoria, Pretoria, 0002, South Africa. Contact e-mails: [email protected], [email protected] JOHANNES NELL is with Hatch, Woodmead, Johannesburg, 1609, South Africa. PETRUS CHRISTIAAN PISTORIUS is with the Carnegie Mellon University, Pittsburgh, PA, 15213, USA. Manuscript submitted August 23, 2016. Article published online February 14, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
from the pilot furnace, have been discussed in detail.[3] The objectives of this study are the mineralogical characterization of tapped slag from the same pilot plant campaign, and interpretation of the results in terms of the slag chemistry operational options in the AlloyStream process. This interpretation shows that equilibrium slag chemistry calculations and mass balance calculation methods were successfully applied to calculate the feed material mixture required to attain magnesia saturation to limit refractory wear, whilst maintaining successful process operation. Trials were performed in a furnace with internal diameter of approximately 2.5 m, with a typical power input (to the induction heater) of 500 kW, and a typical manganese ore feed rate of 450 kg/h. The furnace was tapped when the slag layer thickness approached 100 mm, typically every 2 hours. The average furnace operational parameters, 4-hour average
