Purification of metallurgical grade silicon by a solar process

Abstract

The purification of upgraded metallurgical silicon by extraction of boron and phosphorus was experimentally demonstrated using concentrated solar radiation in the temperature range 1550–1700 °C. The process operated with a flow of Ar at reduced pressure (0.05 atm) for elimination of P, and with a flow of H₂O for elimination of B. Impurity content decreased by a factor of 3 after a 50-min solar treatment, yielding Si samples with final average content of 2.1 ppm_w B and 3.2 ppm_w P.

Introduction

The worldwide solar cell production reached 1256 MW_p in 2004, a 67% increase over the 2003 output [1]. Crystalline silicon solar cells, particularly multi-crystalline silicon, made up of more than 90% of the world PV production [2]. Most of the solar grade silicon (SGS) used for PV applications stems from rejects of electronic grade silicon (EGS) used in the electronic industry. However, its annual supply is surpassed by its demand, urging the need for novel technological solutions for the processing of SGS at a target price of 12–$15€/\mathrm{kg}$ and target demand of 10 g/W_p [3], [4]. Similarly to EGS production, the route for SGS processing uses metallurgical grade silicon (MGS) as starting material. Annual world production of MGS amounts to 1 million tons, obtained by quartz reduction in arc furnaces. The process reaction is

$${\mathrm{SiO}}_2+2\mathrm{C}\to \mathrm{Si}+\mathrm{CO},\mathrm{\Delta }{\mathrm{H}}_{298\mathrm{K}}=695\mathrm{kJ}\text{.}$$

The production of 1 ton of MGS requires 2.9–3.1 ton quartz, 1.2–1.4 ton coke, 1.7–2.5 ton charcoal, 0.12–0.14 ton graphite (electrodes), and 12.5–14 MWh electricity [5]. The purity varies from 98.5% up to 99.5%, and the average price is 1.5–$2.5€/\mathrm{kg}$, depending on quality [3]. Some of the typical impurities are Fe, Al, Ca, Ti, Cr, B, P, O, and C. The production of EGS from MGS requires a reduction of these impurities by at least five orders of magnitude. Consequently, EGS processing occurs via distillation of silicon gaseous compounds at 35–$50€/\mathrm{kg}$. Purification of MGS is an optional chemical path among the routes for processing SGS. Other routes include reaction of silicon dioxide and carbon with high purity and silicon halogenide reduction with high-purity reduction materials. Purification can be accomplished using plasma techniques [6], [7]. In particular, an inductive plasma torch combined with an inductive cold crucible in a chamber was used to treat 30 kg of upgraded metallurgical silicon (UMGS) per batch. PV cells processed with the resulting material exhibited a conversion efficiency of 12.4% [7], compared with 10.12% measured on the starting material [8]. Boron removal is considered to be critical due to its rather high segregation coefficient that prevents its removal by controlled solidification. Plasma treatment with steam and directional solidification steps allowed the removal of B and other metallic impurities [9]. The main drawback of the plasma purification process is its high electricity consumption that governs the process cost ($21€/\mathrm{kg}$ in 2002, without manpower [7]). Alternatively, concentrated solar energy can replace the plasma torch and be used as the source of high-temperature process heat. In contrast to a plasma-driven process, no excited species are formed at the reaction surface in the solar-driven process. This paper examines the solar purification of MGS, aimed at the removal of boron and phosphorus by vaporisation. Thermodynamic equilibrium computations are carried out for determining temperature and pressure requirements and product gas compositions. The experimental set-up and main experimental results are presented and discussed.