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Effect of zinc oxide addition in slag system and heating manner on boron removal from metallurgical grade silicon

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Abstract

Different amount of ZnO was added to calcium silicate reagent to improve boron removal from metallurgical grade silicon (MG-Si). The thermodynamic analysis showed that the reagent CaO-SiO2-ZnO had a stronger ability to boron removal than the reagent CaO-SiO2. Two kinds of heating manners of electromagnetic induction and resistance were used to refine MG-Si. Compared with the resistance heating, the induction heating showed a much higher efficiency of boron removal. The mechanism of boron removal from MG-Si by ternary CaO-SiO2-ZnO slag reagent was put forward and the role of ZnO in slag to boron removal was described. The by-product Zn in refined silicon can be thoroughly removed by a vacuum distillation treatment, which showed an absolute advantage to the residual zinc removal in silicon by comparison with the acid leaching treatment. The residual zinc in silicon was reduced to lower than 0.05 ppmw after a vacuum treatment at 1723 K.

Introduction

The demand for solar grade silicon (SoG-Si) has been rapidly growing annually. The metallurgical route for the yield of low-cost SoG-Si materials is a promising method. Boron removal from metallurgical grade silicon (MG-Si) to SoG-Si by this route is still the technical difficulty [1]. Slag refining, traditionally used as the purification of steel making [2], is also very efficient technique for boron removal from MG-Si. The refining flux for boron removal is based on the calcium silicate slag system. Some attempts had also been made to improve the boron removal efficiency and some additives were used to calcium silicate slag. They include binary, ternary and polynary slag reagents such as CaO-SiO2 [3], CaO-SiO2-CaF2 [4], Al2O3-CaO-MgO-SiO2 [5], CaO-CaCl2-SiO2 [6], Na2O-CaO-SiO2 [7], CaO-SiO2-K2O [8] and so on. Morita [6] et al. found that the boron removal efficiency reached to 86% while adding CaCl2 to CaO-SiO2 binary slag. As described by Tan [9] et al., boron can be reduced from 25 ppmw to 4.4 ppmw with Na2O addition. In the experimental results by Wu et al. [10], the boron in MG-Si can be reduced from 22 ppmw to 1.8 ppmw at 1823 K with a composition of 40% CaO-40% SiO2−20% K2CO3 and the removal efficiency of boron reached to 91.8%.

In our recent study [11], a ZnO addition to 50% CaO-50% SiO2 slag showed a 20% increase in boron removal efficiency and a 40% increase in distribution coefficient due to a stronger oxidizing ability to boron for ZnO than SiO2. The saturated vapor pressure of Zinc is very large at high temperature, and it can easily volatilize during the high temperature experiment. In this study, the effects of an electromagnetic heating manner and a resistance heating manner on boron removal using CaO-SiO2-ZnO slag reagent were investigated. Subsequently, a vacuum distillation treatment to remove residual zinc in refined silicon was studied compared with the results of acid leaching treatment.

Section snippets

Experimental

The MG-Si powder of 30 g with a boron concentration of 12.94 ppmw and the ternary CaO-SiO2-ZnO slag reagent of 30 g were mixed and used as raw materials. The refining experiments were carried out in a medium frequency induction furnace (20 kHz) and in a resistance furnace (~1650 °C), respectively. The schematics of apparatuses are shown in Fig. 1. Consequently, a high-purity graphite crucible and a corundum crucible were accordingly used as the reactors of raw materials in the apparatuses. The

Results and discussion

The photographs of refined silicon samples using CaO-SiO2-ZnO slag reagent are shown in Fig. 2. The samples both in the induction furnace and the resistance furnace show that the refined silicon and the refined slag are thoroughly separated after experiments. The refined silicon is surrounded by the slag, which aggregate on the inner wall of graphite or corundum crucible on account of the larger viscosity and surface tension than those of molten silicon [12], [13], [14].

The chemical reaction

Summary

The boron removal by slag refining with different amount of ZnO addition to silicate slag reagent was employed in the electromagnetic induction and resistance heating, respectively. After the addition of ZnO to CaO-SiO2 slag reagent, the ternary system showed a stronger ability to boron removal than the binary one. The removal efficiency of boron in the electromagnetic induction furnace was much higher than that in the resistance furnace. The residual zinc in refined silicon can be removed to a

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (No. 51574133 and 21563017) and the Natural Science Foundation of Yunnan Province in China (2014FB124 and 2016FA022).

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