A Review of Slag Refining of Silicon Alloys
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https://doi.org/10.1007/s11837-020-04474-0 Ó 2020 The Minerals, Metals & Materials Society
SILICON PRODUCTION, REFINING, PROPERTIES, AND PHOTOVOLTAICS
A Review of Slag Refining of Silicon Alloys SRIDEVI THOMAS
,1,5 MANSOOR BARATI,1 and KAZUKI MORITA1,2
1.—Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, ON M5S 3E4, Canada. 2.—Institute of Industrial Science, The University of Tokyo, 4-61 Komaba, Meguro-ku, Tokyo 153-8505, Japan. 5.—e-mail: [email protected]
Slag refining of silicon has been modified in recent years by combining the process with solvent refining, in which an alloy of silicon is first treated by slag and then solidified under controlled conditions to yield high-purity silicon crystals. This paper discusses the effect of alloying elements on the efficiency of slag treatment. A set of criteria are established and quantified for potential alloying elements to guide their proper selection.
(i) [i] aO2 pO2 cimpurityoxide ci aSi
List of Symbols Concentration of impurity i (B/P) in slag Concentration of impurity i (B/P) in metal Activity of oxygen ions in slag Oxygen potential Activity coefficient of impurity oxide in slag Activity coefficient of impurity i in metal Activity of silicon
To this end, significant effort has been expended to study slag and solvent refining, methods that are considered straightforward and thermodynamically feasible. A number of review papers on both methods have been published,1–3 but recently a third approach has emerged: a combination of solvent and slag refining. The objective is to use a slag to treat an alloy of silicon at first to remove B and P, followed by controlled cooling of the alloy to achieve high-purity silicon phases, leaving the remainder of the B and P in the alloy phase. A review article published along with this paper discusses slag refining of silicon metal with the following key takeaways: 1. 2.
INTRODUCTION Solar-grade silicon of 6–8 N purity is mainly produced by blending ultrapure, electronic-grade silicon (8–9 N), which is generated by the expensive and energy-intensive Siemen’s method, with upgraded metallurgical grade silicon (3–5 N). Research in this field has been largely directed towards developing high-throughput methods that can achieve solar-grade purity in fewer steps or by a simpler process. The focus of purification is on removal of boron (B) and phosphorus (P), as these two elements are the most challenging to eliminate. (Received June 24, 2020; accepted October 27, 2020)
3. 4.
The basicity (aO2 ) and oxygen potential (pO2 ) of the slag work in tandem to maximize B and P removal from silicon. To quantify the efficacy of slag refining, the distribution coefficient, Li, was defined as the ratio of the concentration of the impurity in the slag to that in the silicon, Li ¼ xi;slag =xi;metal . Li is usually too low to allow effective removal of B/P to a desired concentration with a reasonable slag mass. Alloying the silicon with another metal can lessen or enhance the rem
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