Recovery of Multi-Metallic Components from Bottom Ash by Smelting Reduction Under Plasma Environment
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considerable attention has been devoted for the recycling of industrial wastes because of increasing its generation and also their adverse impact on environment day by day.[1] Bulk utilization toward making constructional bricks does not give much value addition as well as not possible to recover value-added metals.[2] Some countries like China have a shortage of bauxite ore for the production of aluminum. Like those countries, efforts are made toward recovery of aluminum from coal combustion residues, especially from fly ashes for replacement of bauxite ore.[3] The commercial technology for aluminum production includes production of alumina from bauxite and smelting reduction of
ARUP KUMAR MANDAL, Research Scholar, and OM PRAKASH SINHA, Associate Professor, are with the Department of Metallurgical Engineering, IIT(BHU), Varanasi, India. Contact e-mail: [email protected] Manuscript submitted September 4, 2015. Article published online October 27, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
alumina to aluminum. Carbothermic reduction of alumina is a promising alternative technology for aluminum and aluminoalloy production. Compared with the ‘Hall-Heroult’ process, carbothermic reduction of alumina offers advantages of a simpler process, lower cost, and a lower requirement for raw materials. Previous assessments showed that carbothermic reduction has the potential to reduce energy consumption by up to 38 pct, capital costs by more than 60 pct and decrease CO2 emissions by up to 30 pct. It has no fluoride emission, and may decrease overall operating costs by 25–30 pct.[4–6] Wang et al. patented a technology to reduce bauxite flotation tailings in an electric arc furnace in order to produce a high aluminum alloy at 2573 K to 2773 K (2300 C to 2500 C). They also reported that the formation of an Al-Si alloy at above 2073 K (1800 C) temperature. They further claimed that the optimum conditions for Al-Si alloy production are as follows: pressure at 0.1 MPa, temperature of 2173 K (1900 C), carbon content of 95 pct of the theoretical amount, and one-hour heating time.[7] The major crystalline phases characterized are mullite and quartz in most of the ash and major components are metallic oxides with varying contents of unburnt carbon. The contents of principal oxides are in descending order: SiO2>Al2O3>Fe2O3>CaO>MgO>K2O. Coal fly ash is rich in aluminum, making it a potential source of alumina. With the diminishing of bauxite resources as well as the increase in alumina demand, the profitable industrial utilization of coal fly ash in alumina recovery has attracted extensive attentions. But the alumina is present in bottom ash mainly as a combined form of mullite, which requires more energy to break the bonds.[8] Pickles et al., showed that the recovery of metal values from the fly ash by argon plasma in an extended arc flash reactor (EAFR).[9] Silicon and iron were successfully recovered by that process. At the high temperature, produced aluminum became vaporized. In the present work, an effort was made to recover aluminum
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