Dow Corning has been working towards the advancement of silicon smelting in a closed furnace over the past four years. A 200 kVA closed furnace pilot plant unit was built to investigate the operating parameters for smelting silicon. The single electrode furnace is operated under totally sealed conditions. The feed from the feed hoppers is fed through air locks to the furnace. The off-gas from the furnace, consisting of by-product CO as well as volatiles from the feeds, pass through a venturi scrubber, where water is introduced to scrub out the fume from the furnace and cool the gas. The mixed scrubber water and off-gas pass into a centrifugal mist eliminator where the water and fume disengage from the gas. The fume slurry is passed through bag filters where the fume is separated from the water. The clean off-gas from the furnace was evaluated for its calorific value and evaluated for conversion to useful products. A number of silicon smelting tests were conducted during this program. Various levels of charcoal and coal mixtures were evaluated to determine the optimum mix. A low volatile coal was preferred over typical Blue Gem coal. The coal amount in the mix was maximized without compromising the smelting performance. A raw material mix consisting of 30% charcoal and 70% low volatile coal was determined to be an optimum mix for closed furnace operation. Silicon recoveries in the low nineties were demonstrated using this mix. Four quartz sources were also evaluated in the closed furnace. The closed furnace operation for silicon smelting was identified to offer significant advantages over an open furnace from the standpoint of reduced carbon oxidation losses, electrode consumption, electrical energy consumption and silicon yield improvement. Other advantages in addition to process off-gas recovery included improved safety from reduced heat and fume exposure, and improved pollution control to the environment. 1 ref
