Study of Pellets and Lumps as Raw Materials in Silicon Production from Quartz and Silicon Carbide
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TALLURGICAL silicon is produced by the reduction of silica in an electric arc furnace by means of carbonaceous materials. As alternatives to the dominating Siemens chemical route, two main direct metallurgical routes have been established for the production of low-cost solar grade silicon in Norway: the Elkem route (developed by Elkem Solar[1]) and the Solsilc route (developed by Fesil Sunergy[2]). The Solsilc route uses high-purity raw materials in form of pellets. It aims at ‘‘direct’’ high-purity silicon production, avoiding several downstream refining steps. Elkem produces a high-grade MG-Si, which is subsequently refined using pyrometallurgical and hydrometallurgical processes. For both routes, the raw materials play an important role for the optimization of the process. The main quartz specifications are the purity chemistry, lump size, lump mechanical and thermal strength, and softening properties. The absence of fines (particles less than 2 mm in size) and a softening temperature close to the quartz melting point are desirable to maintain a high gas E. DAL MARTELLO, Ph.D. Student, G. TRANELL, Professor, and L. ARNBERG, Research Scientist, are with the NTNU Department of Materials Science and Engineering, 7491 Trondheim, Norway. Contact e-mail: [email protected] S. GAAL, O.S. RAANESS, and K.TANG, Research Scientists, are with the SINTEF Materials and Chemistry, 7465 Trondheim, Norway. Manuscript submitted February 26, 2011. Article published online May 20, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
permeability in the furnace burden.[3] Common impurities in quartz, like Na, K, Al, and Fe, come from minerals like mica and feldspar. Hydrothermal quartz and pegmatite core are the purest among the silica sources. These can be purified even more by means of mineral liberation, separations, and acid treatments.[4,5] The purification processes must be carried out with fine particles (in the range of micrometers). To charge these materials in an electric arc furnace, agglomeration (pellets or briquettes) is necessary. SiO reactivity[6] and chemistry are the most important distinguishing properties of carbon. High-reactivity carbon materials are desirable as reductants, because they preserve matter and energy in the process by rapidly reacting with energy-rich SiO(g). The SiO reactivity of coal based reductants depends mainly on the ranks of the coals, on its petrographic properties,[7] on the carbon particle size and on the SiO diffusion in pores.[8] Coal and charcoal contain both B, P, and ash minerals mainly consisting of SiO2, Fe2O3, Al2O3, TiO2, CaO, and MgO. B and P originate from the plants, whereas ash containing oxides derive from clay minerals deposited onto the plant material precursors.[9] Highpurity carbon black is a synthetic carbonaceous powder material produced from pure liquefied natural gas or methane. Low B and P content in the metal produced is necessary because these elements are difficult to remove by directional solidification in the subsequent ingot production of silicon for photovoltaic purp
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