New production process for polycrystalline silicon is necessary in order to increase the actual feed stock of this material for the solar cell market. Metallurgical grade silicon can be used for the production of solar cells if the required degree of purity is achieved. The slag refining of silicon for boron removal using silicate slags is one of the available techniques. In this work, the thermodynamics of CaO-SiO_2 and CaO-SiO_2 - CaF_2 slags were evaluated at 1823 K using metal-slag equilibration method. The activity coefficient of boron oxide and the borate capacity of the slag were calculated for low boron containing slags (boron concentration lower than 0.25 %). For basicities lower than 0.8 in the CaO-SiO_2 binary system, as the content of silica increases, an unexpected decrease in the activity coefficient of boron oxide was observed. To explain this decrease, a model where the boron in slag phase is incorporated in the silicate network was proposed. 11B solid state NMR analyses and quantum chemical calculations have shown that this boron exists in tetrahedral form connected to four silica groups and having a Ca~(2+) coordinated close to it.
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