METHOD AND APPARATUS FOR PURIFYING METALLURGICAL GRADE SILICON BY DIRECTIONAL SOLIDIFICATION AND FOR OBTAINING SILICON INGOTS FOR PHOTOVOLTAIC USE

摘要

A method and an apparatus for purification of metallurgical grade silicon by directional solidification and for obtaining silicon ingots for photovoltaic use. The method comprises a preheating step, up to a temperature that is higher than the melting point of silicon, of a quartz crucible (18) that is accommodated in a containment enclosure (19) arranged inside a chamber (4) of a furnace. The chamber (4) of the furnace is delimited by a covering structure (3) and by a footing (2), which can move with respect to each other, or vice versa, toward or away from each other along a vertical direction respectively for opening and closing the chamber (4). Heating occurs by way of heating means (10) of the electric type, which are associated with the walls of the covering structure (3). The metallurgical grade silicon obtained at the end of a carbon reduction cycle in a carbon reduction furnace, from which it exits in the molten state, is transferred in the molten state directly into the quartz crucible (18) thus preheated inside the furnace chamber, which is closed and inside which an atmosphere of inert gas at a pressure that is higher than the atmospheric pressure is generated. Transfer of the silicon in the molten state occurs through a barrier of at least one inert gas that is generated proximate to at least one opening (13) formed in the top (7b) of the covering structure (3). The method then comprises a step for directional solidification of the silicon in the molten state, by removing heat from the bottom of the quartz crucible and by means of the selective control of the heating means of the electric type and the modulation of the power delivered by them, until the silicon solidifies completely in an ingot. During the solidification step, the furnace chamber is closed and an atmosphere of an inert gas at a pressure that is higher than atmospheric pressure is maintained inside it. At the end of solidification, the quartz crucible (18) accommodated in the containment enclosure (19) and containing the ingot thus obtained is extracted from the furnace chamber, which is opened by removing the covering structure (3) from the footing (2).