Concentrates from the Stillwater and East Boulder mine sites and crushed catalyst material for recycling are processed through the Company’s smelter in Columbus, Montana. This process smelts and separates the concentrates into a silica oxide-rich slag, and PGM-rich converter matte. The converter matte is transferred to the Company’s refinery located adjacent to the smelter for further processing.
Mine concentrate and spent catalyst material is fed into the smelter furnace. Furnace matte is tapped from the furnace and granulated. The granulated furnace matte is remelted in a top-blown rotary converter (TBRC), which separates iron from the converter matte. The converter matte is poured from the TBRC, granulated and transferred to the refinery for further processing. The granulated converter matte consists of copper and nickel sulfides along with PGMs.
The gasses released from the smelting operations are routed through a gas/liquid scrubbing system, which removes approximately 99.8 percent of the sulfur dioxide. Spent scrubbing solution is treated in a process that converts the sulfur dioxide to gypsum, or calcium sulfate, and regenerates clean scrubbing solution. Local farmers use the gypsum as a soil amendment.