Abstract
In this work, Si-Ca-Mg alloys were made with different compositions and solidification conditions to investigate the impurity segregation and separation performance from Si, especially for the crucial P impurity at several ppmw levels. Varying acid leaching parameters were also employed to investigate the optimized process window. Results indicate that the novel Si-Ca-Mg alloying-leaching system is valid for high P extraction. The ternary intermetallic phase Ca7Mg7.5±δSi14 appears as the main precipitate in all alloys to gather other minor impurities. Rapid cooling significantly reduced the size of precipitates and Si grain, the impurity segregation was also limited. In the acid leaching experiments, HCl is found as the most economical leaching agents among the studied combinations. Smaller particle size promotes the leaching efficiency, but the increment narrows with increasing Ca/Mg ratio. Leaching kinetics of the studied alloys was found following the modified Kröger-Ziegler model based on a cracking-shrinking mechanism. The impurity purification efficiency increases with increasing Ca/Mg mixing ratio, but significantly reduced by rapid cooling. An analytical model was developed for ternary alloy system to predict the P segregation and its removal with varying alloy concentration through the thermodynamic approach, which shows good agreements of the experimental results.
Highlights
There is growing interest in developing a more environmentally friendly approach to produce solar-grade silicon (SoG-Si, purity 99.9999%) for the photovoltaic (PV) industry
It is seen that the alloy microstructure is significantly affected by the alloy composition as the large phases are marked in the backscattered electrons (BSE) image
Unlike the microstructure observed in binary Si-Ca and Si-Mg alloys, in the Si-Ca-Mg alloying system, Al is found most likely to be dissolved in the ternary Ca7Mg7.5±δSi14 phase as the form of the solid solution rather than to form specific compounds
Summary
There is growing interest in developing a more environmentally friendly approach to produce solar-grade silicon (SoG-Si, purity 99.9999%) for the photovoltaic (PV) industry. The principle of acid leaching purification is to digest the impurity phase precipitated out of Si phase; in practice, the final purification efficiency is always affected by several processing factors. One successful approach is known as the solvent refining, which aims at low-temperature operation by adding large amounts of alloying elements such as the well-studied Si-Al [36–41], Si-Cu [12,42], and Si-Fe [43–45] systems. Another alloying approach is adding only a limited amount of additives but with strong impurity affinity as an impurity getter. A purification model for P removal was established for the ternary alloy system through a thermodynamic approach and Gulliver-Scheil solidification principles
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