Enhanced separation of phosphorus from metallurgical grade silicon by CaAl2Si2 phase reconstruction

To find a high-efficiency approach for removing phosphorus from metallurgical grade silicon, a novel method of silicon-aluminum-calcium alloy refining was investigated through phase reconstruction to form CaAl2Si2 phase. The distribution of phosphorus between purified solid silicon and liquid silicon-aluminum-calcium alloy was determined to investigate the possibility of removing phosphorus from the silicon by alloy refining with silicon-aluminum-calcium melt. It was found that the CaAl2Si2 phase had a high affinity for phosphorus resulting in dissolving a large quantity of phosphorus. Decreasing the amount of getter materials was not good for the phosphorus removal from the silicon, while adopting twice alloy refining could strengthen the removal with obtaining as high as 99.99 wt% purity silicon. In addition, the purity of refined silicon depends on the quenching temperature and the cooling rate. Moreover, to evaluate efficiency of phosphorus removal from silicon by silicon-aluminum-calcium alloy refining, the thermodynamic properties of phosphorus in solid silicon and liquid alloy phase at 1273-1473 K were further evaluated by the chemical equilibrium method.