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Abstract

Methods and apparatus for the commercial-scale production of purified polycrystalline silicon granules with one or more tailored levels of n- and p-type impurities from an impure silicon source such as, for example, metallurgical-grade silicon. Purification systems and methods involve: (1) one or more series of temperature controlled reactors or vessels provided with dual fluidized beds wherein solids and gases are transported so that varying degrees of purification and deposition of solid silicon is accomplished by strict control of temperature and residence time; (2) separation and recovery of the compounds of high-melting-point impurities such as, for example, FeSi and FeI₂; (3) purification, separation, and recycling of silicon tetraiodide; (4) separation and recovery of iodide compounds of lower-boiling-point liquid impurities such as for example, AlI₃, in a continuous fractional distillation column, facilitated by an iodine reflux; (5) separation and recovery of very fine solid particles including impurity iodides and elemental silicon in a liquid mixture downstream of a fractional distillation column; (6) recovery of input iodine from the oxidation of both solid and liquid iodide impurity waste streams from the process.

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