Vacuum evaporation is usually utilized to remove volatile impurities in metallurgical grade silicon to prepare solar grade silicon by metallurgical routes. Especially phosphorus needs to be removed by vacuum evaporation. And the removal efficiency of impurities strongly influenced the quality and performance of products. In this paper, the removal efficiency of impurities is studied by using different raw material. The results indicated that the content of impurities in raw material had deep effect on the removal efficiency of vacuum evaporation. The high quality product can be obtained by vacuum evaporation only once from raw materials with the lower content of impurities whereas it is not for high content of impurities in raw material. This is due to the vapor-liquid equilibrium in the vacuum furnace. The impurities can be removed effectively by vacuum evaporation many times.

Calcium is one of the main impurity elements in silicon. The removal of calcium strongly affects the quality of the polycrystalline silicon ingot produced by a vacuum directional solidification method. Based on the considerations of the theory of segregation, mass transfer and evaporation during vacuum directional solidification process, a mathematical model for calcium distribution in silicon was proposed and it can be used to explain the removal mechanism. In order to confirm the mathematical model, an industrial scale experiment on upgraded metallurgical grade silicon (UMG-Si) with an initial purity of 99.98 wt. % was performed. Since the reaction temperature strongly influences both the evaporation and segregation of calcium, the dependences of effective segregation coefficient (k eff) and the evaporation coefficient (k E) on temperature were carefully investigated. The results showed that the proposed mathematical model was highly consistent with the experimental data and the calcium removal efficiency mainly relied on the evaporation step.

We used high-purity multicrystalline silicon prepared by metallurgical method for the study of directional solidification. The optical microscope was used to observe the etch pits on the surface of silicon wafers, and we calculated their dislocation density. The result showed the space distribution of dislocation density presented “V” shape for each ingot produced at different drop-down rates. The dislocation density of slicon ingots followed the order 10<20<30<40μm/s. The high-resolution glow discharge mass spectroscopy was used to analyze the concentration of transition metal impurities. The macro-morphology of vertical-section of silicon ingots growth at different drop-down rates was observed. The x-ray diffraction measurement was performed to analyze the crystallographic orientations of the silicon ingot growth at 20μm/s, which was a better drop-down rate for producing high-quality multicrystalline silicon.

There are many different types of valence of sufur in aluminate sodium solution during the process of the bauxite dissolution, which mainly including S2-, S22-, S, S2O32-, SO32-, SO42- and it may have an effect on the mineral dissolution process. Therefore, it is necessary that the thermodynamic properties of S-H2O system were investigated in sodium aluminate solution. In this paper, the reactions between different valence states of sulfur and lye (with or without oxygen participation) were studied in sodium aluminate solution. The thermodynamic software Factsage 7.0 was carried out to calculate the standard reaction Gibbs free energy (ΔGTθ). The results showed that S2- and SO42- are the most stability in terms of the autoreactive reaction of different valence states at 298-573 K, and that the stability of different valence states of sufur order is “S22- >(S2O32-, SO32-) >S”. At 523 K, SO32- is more stable than S2O32-. On the other hand, the low-valent sulfur was more likely to be oxidized into SO42- under oxidizing conditions at 298-573 K. Besides, it was also found that S22- is most susceptible to oxidation because of the lowest stability. Finally, the stability of the other valence states of sulfur under oxidizing conditions order is “SO32- >S2O32- >S2- >S”.

In present, refining of metallurgical grade silicon is one of the promising routes to low-cost solar grade silicon for solar cells. Alloying with Ca has shown a great potential as efficient refining method of MG-Si in combination with acid leaching. Compared with Ca metal, Ca-Si alloy is cheaper and more secure. Great removal of impurity depends on microstructure of MG-Si after alloyed with Ca-Si alloy. In the work, the change of impurity phase which was performed by the change of the microstructure of MG-Si before and after alloyed with Ca-Si alloy has been analyzed. It was determined that CaSi2phase contained significant phosphorus content after alloying with Ca-Si alloy, It also investigated the optimal acid leaching condition after leaching, which confirmed that metallurgical grade silicon with Ca-Si alloy addition followed by acid leaching could be a potential route to remove phosphorus from MG-Si.