A Review of Slag Refining of Crude Silicon
- PDF / 1,181,322 Bytes
- 22 Pages / 593.972 x 792 pts Page_size
- 0 Downloads / 214 Views
https://doi.org/10.1007/s11837-020-04470-4 Ó 2020 The Minerals, Metals & Materials Society
SILICON PRODUCTION, REFINING, PROPERTIES, AND PHOTOVOLTAICS
A Review of Slag Refining of Crude Silicon SRIDEVI THOMAS
,1,3 LIUQING HUANG,1,2 and MANSOOR BARATI1
1.—Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, ON M5S 3E4, Canada. 2.—Fujian Key Laboratory of Advanced Materials, College of Materials, Xiamen University, Xiamen 361005, People’s Republic of China. 3.—e-mail: [email protected]
Metallurgical refining of crude silicon for low-cost, mass production of solar silicon has been studied extensively over the past two decades. One of the methods attempted to target the deleterious impurities of Si is slag refining, where liquid Si or its alloys are treated with a flux. This article presents an overview of the impurity response to various slag treatment conditions for crude Si metal from both fundamental and practical perspectives, aiming to provide a basis for the design and application of more effective fluxes and alloys. It was found that removal of B and P through slag refining alone is not practical on an industrial scale as it would require a large volume of slag, however the removal percentage can be significantly increased if the silicon has been alloyed with a suitable metal.
INTRODUCTION
THERMODYNAMIC ASPECTS
Silicon has been the predominant photovoltaic material used in solar cells. Efficient conversion of solar power to electric power requires silicon of high purity, typically 6–7N (N = nines, e.g., 3N = 99.9). Two of the major impurities that have proven to be most difficult to remove are boron (B) and phosphorus (P). Being dopant elements used in photovoltaic (PV) silicon, these elements warrant the strictest control in the silicon. Consequently, many methods have been studied to reduce the B and P content of silicon, including directional solidification, acid leaching, evaporative refining with vacuum or inert gases, reactive gas injection, electrorefining, solvent refining, and slag treatment, to name a few. Slag treatment is an attractive method for the refining of metals due to its low cost, fast kinetics, and effective removal of impurities. Use of slag for the removal of B and P from silicon has been widely examined over the past two decades. This article presents a critical review of the studies in this area, aiming to evaluate the prospects of slag treatment as part of an integrated metallurgical process for silicon refining.
Slag refining involves exposing liquid silicon to molten slag to thermodynamically promote transfer of impurities to the slag. Liquid states of both phases are preferable as mass transfer rates are faster, and ideally the phases should remain immiscible and in distinct layers to allow for easy separation. The widely used quantifier for how effectively a slag removes impurity i from a metal is the distribution coefficient Li, defined in Eq. 1. ði Þ ð1Þ Li ¼ ½i
(Received June 24, 2020; accepted October 27, 20
Data Loading...
