Distribution of boron between silicon and CaO-SiO2, MgO-SiO2,CaO-MgO-SiO2 and CaO-Al2O3-SiO2 slags at 1600°C

Abstract

New energy sources are needed for a sustainable future. Solar cells have a huge potential as a sustainable energy source but further development of this technology is needed for solar cells to become cost-competitive with other energy sources. Silicon produced by metallurgical refining methods has the potential of reducing the cost of crystalline silicon solar cells significantly but boron has proven to be hard to remove from silicon by these methods. Slag refining is however a promising refining method for removal of boron from silicon. The aim of this thesis was to determine accurate data for the distribution of boron between silicon and selected slags.

All published values of the distribution coefficient of boron have been critically reviewed in this work. The thermodynamic properties and distribution of other major components of the system have also been reviewed. Several experiments with silicon and CaO-SiO2, MgO-SiO2, CaO-MgO-SiO2 and CaO-Al2O3-SiO2 slags have been conducted at 1600 °C under argon atmosphere.

A series of experiments with ferrosilicon and CaO-SiO2 slags have also been carried out.

Accurate data for the distribution of boron between silicon and CaO-SiO2, MgO-SiO2, CaO-MgO-SiO2 and CaO-Al2O3-SiO2 slags at 1600 °C has been determined. The distribution of other major slag components between slag and silicon has also been found in these slag systems. The distribution of calcium and boron between ferrosilicon and CaO-SiO2 slags at 1600 °C has been determined. Activities of slag forming components have been determined in the CaO-SiO2, MgO-SiO2 and CaO-MgO-SiO2 systems. The activity coefficient of BO1.5 at infinite dilution in these slags has also been determined. Activity coefficients of calcium, magnesium and aluminium at infinite dilution in silicon and the activity coefficient of calcium and boron at infinite dilution in ferrosilicon have been determined. An alternative equation for mass transfer has been derived and the mass transfer coefficient of boron in a 37.9%CaO-62.1%SiO2 slag has been estimated to be ks = 5.2 · 10-7m/s.

The refining efficiency of CaO-MgO-SiO2 slags has been found to be approximately the same independently of slag composition with a distribution coefficient of boron between 2 and 2.5. This also includes the binary CaO-SiO2 and MgO-SiO2 systems. The activity coefficient of BO1.5 at infinite dilution in CaO-MgO-SiO2 slags, including the binary CaO-SiO2 and MgO-SiO2 systems, has been found to follow the activity coefficient of SiO2 where  . A linear decrease of the distribution coefficient was found with increasing Al2O3 content in a ternary CaO-Al2O3-SiO2 slag. This has been found to be caused by an increasing activity coefficient of BO1.5 at infinite dilution in slag relative to the activity coefficient of SiO2 where. The distribution coefficient of boron between ferrosilicon and CaO-SiO2 slags was found to be unchanged with iron content in silicon