Assessing the feasibility of silica-based media for coal preparation operations: A novel method of carbon feedstock production for the silicon market

Production of silicon for solar cells and semiconductors requires the use of ultra-clean coal as the main reducing agent in the silicon production process. Most coal preparation plants employ magnetite suspensions as the dense medium in coal cleaning processes. Although downstream operations are designed to remove and recover magnetite, the unavoidable presence of residual magnetite in the final coal product dramatically decreases the purity and suitability of the coal for the silicon market. Given that silicates are the main raw materials in the silicon production, a systematic study through laboratory dense-medium cyclone tests was used to evaluate fine silica-based alternative materials as a substitute for magnetite in coal cleaning operations. These alternative silica materials include byproducts generated during the silicon production process. The results indicate that laboratory tests with silica-based materials were able to achieve similar separation performance when compared to plant operations using magnetite. Following the outcomes of the experimental study, a novel flowsheet arrangement was designed using silica-based media rather than magnetite. If properly implemented, this new separation system can eliminate iron contamination and provide coal that can be marketed as a high-quality silicon feedstock.