Progress on the Dow Corning Process for Solar-Grade Silicon

Microstructure of metallurgical grade silicon and its acid leaching behaviour by alloying with calcium

“…The accuracy of the measurements includes the unevenness of impurity distribution, and is shown for the source MG-Si and is also applicable to the treated MG-Si samples. It is evident that fine grinding and thorough leaching with hydrochloric and hydrofluoric acids are successful in the removal of a significant part of the metallic impurities and the leaching results agree well with other data (Santos et al, 1990;Dietl, 1983;Hunt et al, 1976a;Juneja and Mukherjee, 1986). Inductively coupled plasma optical emission spectroscopy data on impurities in MG-Si remaining after leaching with two different acids (Table 1) show that hydrofluoric acid solution is more efficient: removal of y67% of aluminium and y88% of iron compared to 5% and y25%, respectively, with hydrochloric acid solution.…”

“…Amongst other detected phases were Al-Fe-Ti-Si, Al-Ca-Fe-Si, Al-Ca-Cu-Fe-Si, Ca-Cu-Si, Fe-Si-Ti, Ca-Fe-Si; some of them are most likely non-equilibrium phases (Meteleva-Fischer et al, 2012a). Leaching using water only (Hunt et al, 1976a(Hunt et al, , 1976b would not be efficient and much more time consuming, taking into account the low solubility in water of the abovementioned impurity phases. Thus, acid leaching is addressed to impurities precipitated in inclusions, and for MG-Si with iron as a major impurity hydrofluoric acid solution is more effective.…”

“…Quantitative results, selection of leaching agents and optimum conditions of the leaching process, obtained by various authors, differed remarkably from each other. Numerous acids (nitric (Santos et al, 1990), sulphuric (Voos, 1961), hydrochloric (Santos et al, 1990;Zhang et al, 2009) and hydrofluoric (Voos, 1961;Santos et al, 1990;Ma et al, 2009)), their mixtures (Dietl, 1983;Norman et al, 1985), and even water (Santos et al, 1990;Hunt et al, 1976aHunt et al, , 1976bChu and Chu, 1983) were found in different studies as efficient leaching agents. Double-step leaching of MG-Si at elevated temperature using hydrochloric acid (16%, 5 h, 80uC) and a relatively coarse particle fraction (116 mm), followed by hydrofluoric acid (2?5%, 2 h, 80uC), removed y85% of the impurities and resulted in 99?9% pure upgraded metallurgical grade silicon (UMG-Si) (Santos et al, 1990).…”

“…Shimpo et al (2004) reduced the phosphorus content in silicon to 16?7 ppmw by alloying with calcium followed by acid leaching. Particle size was found to be important and grinding to below the grain size was preferred (Santos et al, 1990;Zhang et al, 2009;Ma et al, 2009;Dietl, 1983;Hunt et al, 1976aHunt et al, , 1976bChu and Chu, 1983). At the same time large 3-12 cm pieces of MG-Si with calcium as a dominant impurity were purified up to 99?95% by a combination of two leaching steps: treatment with a water solution of iron (III) chloride or iron (III) chloride and hydrochloric acid, followed by 5% hydrofluoric acid or mixed hydrofluoric/ nitric acid solution (Ceccaroli and Friestad, 2005).…”

An Overview of Photovoltaic Power Systems

“…Since solar cells do not require the high purity material usually required in the semiconductor industry, it is feasible that alternative processes could be used to produce lower cost polycrystaline material. Recent studies by several large chemical companies have indicated that a variety of processing techniques could be used to produce polycrystaline silicon suitable for solar cells at less than $10/kg (20,21,22). Solar cells produced from this silicon material have shown conversion efficiencies of 11-12%.…”