Study On Removal Of Boron Impurities From Industrial Silicon By Composite Melting

Nowadays, solar energy has been widely attentioned as a kind of renewable and clean energy and the demand of Solar-grade silicon (SOG-Si) as the main material increases sharply.There are three major methods to produce polysilicon which are the Siemens process, the fluidized bed reactor process and metallurgical technologies for SOG-Si refining. Although the Siemens process and the fluidized bed rector process have some sort of technical progress in the region of the control of production cost and the enhancement of purification efficiency, lower production cost has been limited by the two steps of phase change (solid-gas, gas-solid) which are the most important links for the two processes. It can not meet the requirements of the photovoltaic industry. Metallurgical technologie for SOG-Si, as a purification method which does not change the chemical property of Si, has been well researched by experts extensively.In the process of refining, it is difficult to remove B from Si during solidification refining and vacuum treatment due to its large segregation coefficients. Composite solvent refining was a purification process which combined solvent refining with slag oxidization treatment. In this paper, it is effective to remove boron by composite solvent refining and could get the shorter flow and the lower energy consumption. This paper has discussed the effect on the thermodynamics, dynamics and property from the slag and alloy. And we make a combination between raw material alloying and aggrandizement of electric field. A slag was used to treat Sn-Si alloy when voltage was joined between alloy phase and slag phase.(1) A novel approach by combined solvent refining to enhance the purification efficiency of boron.The results showed that tin proportion had significant influence on the distribution of B. The partition coefficient of boron was increased from 3.16 to 13.8 with the highest removal rate of 93.3% and the lowest boron content of 0.89ppmw.The viscosity of Sn-Si alloy in the composite solvent refining system had directly affected the interfacial mass transfer and the purification efficiency. It was found that the viscosity was substantial reduced with the increase of Sn content of alloy when the Sn content was larger than 30%. Both the LB and viscosity had the similarly changing trend. When the viscosity was 0.61 mPa·s, it was found that the highest values of LB of the two slags were 10.1 and 12.3, respectively. Because of the difference of density and toughness between the slag and Sn-Si alloy, it has been well solved the problem of the separation of slag and silicon.(2) A CaO-SiO2-Al2O3 slag was used to remove boron at 1773 K (1500℃), and the effect and refining mechanism with the changing of optical basicity and slag composition were investigated. The results showed that enhance of optical basicity and tin proportion had significant influence on the distribution of B. The partition coefficient of boron was increased from 3.2 to 10.08 with the lowest boron content of 1.2 ppmw. It can be increased the optical basicity with the addition of Na2O content to enchence oxidizing ability. The Sn-Si alloy was subjected to refining with the purpose of removing boron by CaO-SiO2-Al2O3-Na2O slag and the highest LB was 11.(3)A new formula has been put forward to estimate the relationship between LB and the factors like activity coefficient of B and electric field intensity and others. The LB was increased when the yB and AE was enhanced through the formula.It was found that the partition ratio of B was remarkably increased from 0.43 to 32.8 with the increase in the Sn content from 0 pct to 50 pct and the voltage was enhanced from Ov to 6v at the CaO-SiO2-20%Al2O3 slag composition. There were three major maters as follows:the increase of activity coefficient of B, the high potential and the enhancement of mass transfer. And electrode reaction process was also discussed.