Impurity removal from Si by Si-Ca-Mg ternary alloying-leaching system

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.

Keywords

• Silicon
• Phosphorous
• Impurity
• Microstructure
• Segregation
• Thermodynamics