The Company’s two Montana mines produce PGMs and minor amounts of other metals (by-products). Facilities are located at both mines to upgrade mined ore to a concentrate. The Company operates a smelter and base metals refinery in Columbus, Montana, to remove by-product metals and further upgrade the concentrate to a PGM-rich filter cake. The filter cake is shipped to a third-party custom refiner for final refining before the PGMs contained in this material are sold.
Mine Site Concentrators
Mine site concentrators are located at both the Stillwater and East Boulder mines. After removal from the mines, ore is fed into a concentrator to begin the refining process. This activity produces a concentrate that is transported to the metallurgical complex for further refining. The daily capacities of the concentrators are 3,000 short tons (2,700 metric tons) and 2,000 short tons (1,800 metric tons) for the plants located at the Stillwater and East Boulder mines, respectively.
The concentrator process begins when ore is fed into the concentrator, mixed with water and ground to a slurry. PGM-bearing sulfide minerals are liberated from the rock matrix. Reagents are added to the slurry to separate the valuable sulfides from the waste rock. Sulfide minerals are floated, recycled, reground and refloated to produce a concentrate suitable for further processing. The flotation concentrate, weighing about 2 percent of the weight of the original ore, is filtered and transported in bins to the Company’s metallurgical complex in Columbus, Montana. Approximately half of the tailings material from this process is returned to the mines and used for backfill to provide a foundation upon which additional mining activities can occur. The balance is placed in tailings confinement areas.
Concentrates from the Stillwater and East Boulder mine sites and crushed recycled catalyst material 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 are fed into the smelter furnace. Furnace matte is tapped from the furnace and granulated. The granulated furnace matte is re-melted in a top-blown rotary converter (TBRC), which separates most of the 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, nickel and iron sulfides, along with PGMs.
The gases 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. The gypsum solids are sold for use as soil amendments and water treatment additives.
The base metals refinery (BMR) is adjacent to the smelter at the Company’s metallurgical complex in Columbus, Montana. Converter matte from the smelter that contains copper, nickel, iron and PGMs is transported from the smelter to the BMR. The matte is then sampled, weighed and assayed before being fed to the refinery.
The role of the BMR is to extract the base metals (copper, nickel and iron) and upgrade the PGM content of the matte. This process utilizes a sulfuric acid media that effectively leaches only the copper, nickel and iron while the PGMs remain inert to the leach process. This occurs in a three-step leach procedure, which is initiated by conducting a nickel atmospheric leach (NAL) utilizing sulfuric acid, oxygen and elevated temperatures to preferentially leach the nickel and iron. The nickel-laden solution is then directed to a crystallization circuit where a crude nickel sulfate crystal is produced and sold as a by-product.
The remaining copper/PGM matte proceeds to the second step or the copper dissolve autoclave, which also utilizes sulfuric acid, oxygen and elevated temperatures along with elevated pressure to leach the copper. Once complete, the copper-laden solution is directed to an electrowinning circuit that removes the copper from solution as an impure copper cathode that is then sold as a by-product.
Once the bulk of the copper, nickel and iron have been removed, the matte proceeds to the third and final step, or the polish leach, that again utilizes sulfuric acid, oxygen and elevated temperatures and pressure to remove any remaining copper, nickel and iron. After completion of the polish leach, the PGMs in the matte are ready to be shipped for final metal refinement as PGM-rich filter cake.
The state-of-the-art process utilized at the BMR results in a zero-discharge operation.
The Company operates an analytical laboratory at its metallurgical complex in Columbus, Montana. The lab receives and analyzes samples from the Stillwater Mine/concentrator, East Boulder Mine/concentrator, recycling facility, smelter and refinery. After initial preparation, samples are directed to areas of the lab suitable for the analytical work to be conducted. The Company’s state-of-the-art assay lab utilizes an automated X-ray facility that provides accurate results with faster processing times than conventional fire assay methods. Other laboratory analytical methods include fire assay, acid dissolution and inductively coupled plasma (ICP) spectroscopy.