Advancing Sustainability in Solar-Grade Silicon Production: Enhanced Boron and Phosphorus Removal via Silicon Refining from Al–Si Melt

Abstract Solar radiation is a renewable and practically infinite source of energy that creates no greenhouse gas emissions such as $${\text{CO}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>CO</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> . Photovoltaic devices that turn solar energy directly into electricity are commonly made of high-purity solar-grade silicon, (SoG-Si). The SoG-Si is conventionally produced by a carbothermic reduction of quartz ( $${\text{SiO}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>SiO</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> ), resulting in roughly 98 wt.% metallurgical grade silicon (MG-Si), which is then further purified into SoG-Si using the Siemens process. The carbothermic step releases a large quantity of $${\text{CO}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>CO</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , and conventional purification methods require technically complicated equipment, consume intensive energy, and involve the use and production of highly toxic silane gases. This work has investigated a metallurgical purification method of MG-Si by solidification from aluminum-Si melt. Four purification rounds have been applied to ensure that boron and phosphorus are reduced to 314 $${\text{ppb}}_{{\text{a}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>ppb</mml:mtext> <mml:mtext>a</mml:mtext> </mml:msub> </mml:math> and 96 $${\text{ppb}}_{{\text{a}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>ppb</mml:mtext> <mml:mtext>a</mml:mtext> </mml:msub> </mml:math> respectively. A &gt; 99.99 wt.% Si has been obtained, which can be further purified by the directional solidification method, and the purity of Si can be improved to 6N by effectively removing the other elements. The raw material of this process, the MG-Si, was produced by an aluminothermic process without direct $${\text{CO}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>CO</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> emissions. Such a process, purification of MG-Si obtained from the aluminothermic reduction of a ferrosilicon slag, by a metallurgical route plus an additional step of directional solidification eliminates the use of highly hazardous silane gases associated with the conventional processes and requires much simpler equipment and lower operational energy, cost, and $${\text{CO}}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>CO</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:math> emissions.