Flotation of Metallurgical Grade Silicon and Silicon Metal from Slag by Selective Hydrogen Fluoride-Assisted Flotation
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RODUCTION
SILICON is an important industrial metalloid used in the production of ferrosilicon and aluminum alloys, fumed silica, silanes, silicones, and high purity silicon products used in poly- and monocrystalline solar cells and microelectronics. Accompanying the span in applications, there is a corresponding span in the chemical purity of the respective silicon products. High quality demands are put on silicon used in the electronics and photovoltaic industries, and the subsequent down-stream processing is costly and energy intensive. Hence, silicon for such uses is a highly priced commodity. Flotation could be an indispensable and cost-effective tool in the upgrading of metallurgical grade silicon concentrates for higher purity uses, as impurities can be retained in the tailings. This paper describes results that could lead to improvements in the production of higher purity silicon, increased recovery of silicon from slags, and upgrading of products that contain silicon impurities. As shown by this study, silicon can be floated in aqueous solutions of hydrogen fluoride (HF) without the use of a flotation collector. This is similar to the findings of Larsen and Kleiv,[1,2] who showed that quartz would float in a diluted solution of HF and a frother. The process described in this study uses HF to produce a directly floatable silicon surface. E. LARSEN and R.A. KLEIV are with the Department of Geoscience and Petroleum, Norwegian University of Science and Technology, Sem Sælandsvei 1, 7491 Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted June 4, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Several researchers have investigated the reactions between HF/silica and HF/silicon. Ernsberger[3] proposed that under ‘‘less extreme conditions of acidity,’’ F ions replace a significant proportion of silica tetrahedron surface OH ions, and that the resulting silicon atom coordination deficiency could lead to a continuous adsorption of further fluoride ions, thus suggesting that quartz surfaces can be fluorine-terminated (F-terminated). Although there were several early reports of hydrogen-termination (H-termination) and not F-termination subsequent to HF etching of Si surfaces, it was not until 1984 that Ubara et al.[4] clearly showed that HF etching of oxygenated silicon resulted in the formation of Si-H bonds on the etched Si surface. Trucks et al.[5] presented a mechanism that explains H-termination of silicon surfaces by HF etching, in which he explained that the surface oxide removal by HF initially results in F-termination, but that the subsequent HF attack leads to a H-terminated surface due to the instability of the F-terminated surfaces. Although the F-terminated surface is more thermodynamically stable, the reactions leading to an H-terminated surface are more exothermic and more energetically accessible. Their work indicates that kinetic rather than thermodynamic considerations are responsible for H-termination. Several workers have reported that HF-etched and H-terminated silicon surfaces ar
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