The Stillwater Complex is a layered ultramafic to mafic igneous complex formed by the gradual cooling of an enormous subterranean intrusion. In such intrusions, the gradual cooling leads to the formation of layers of crystals with similar geochemical properties. Some of these layers are essentially monomineralic (anorthosites composed of high-calcium plagioclase), while others have additional minerals such as troctolites, which contain both high-calcium plagioclase and olivine. The layering is very distinct and resembles the layering of sedimentary rocks even though the rocks of the complex are igneous. These layers form over extremely large areas. The fact of this layering is critically important to understanding the deposition of the palladium- and platinum- bearing minerals in the J-M Reef.

The J-M Reef is recognized by the presence of chalcopyrite, pyrrhotite, pentlandite, and, rarely, pyrite within the reef package. Other base-metal sulfides have been identified, but their presence is subordinate. The platinum group minerals can rarely be recognized with a hand lens. SMC (2001) reported that platinum occurs primarily (67%) as sulfide, 25% as the metal alloy, isoferroplatinum, and 8% as the telluride, moncheite. Palladium occurs primarily (80%) as a solid-solution in pentlandite, 15% as sulfides, and 5% in the telluride, moncheite. The platinum:palladium ratio averages 1:3.5 in-situ at the Stillwater Mine, except for the Dow (Upper West) area of the mine, where the platinum:palladium ratio increases to 1:3.9 in- situ, and averages 1:3.6 to 3.7 in-situ at the East Boulder Mine. The currently delineated proven reserves at the East Boulder Mine have a lower PGM grade and higher copper plus nickel content than the Stillwater Mine. At the west end of the Stillwater Mine in the Dow sector, an area of higher copper plus nickel content has been identified that is similar to the East Boulder Mine mineralization.

The three principle mining methods are conventional overhand cut and fill (Conv. C&F), as captive stopes; ramp and fill (R&F) as both overhand and underhand cut and fill; and sublevel extraction (SLE) by longhole open stoping with subsequent backfill. Since considerable tonnage is produced from the development of the sublevels in the Reef, this is carried separately tonnage as sublevel development (SLD) tonnage.

The other variation of captive overhand cut and fill involves using a raise-bored access hole, rather than an Alimak raise climber, which remains captive to the stope. Uncemented sand fill (cycloned mill tailings) is used for the backfill material.

Sublevel Extraction and Sublevel Development: Where the reef structure and the hanging wall are much more competent, sublevel open stoping, using relatively short “long holes” (compared to other mining districts) can be applied. The sublevels are driven on the reef structure at 25- to 50-foot intervals. Since considerable amount of tonnage is generated by driving sublevels in the Reef, this tonnage is accounted for as a separate mining method of SLD, as separate from SLE tonnage.

The East Boulder concentrator was constructed to allow expansion to 4,000 tons per day from the originally planned 2,000 tons per day operation. The mill is capable of processing in excess of 2,000 tons per day of feed, but concentrator throughput has been significantly limited by mining rates. The concentrator operated on a 24 hours per day, 7 days per week schedule, but it shuts down 3 days per week, depending upon ore supply and maintenance requirements. In 2010, it averaged 1,932 tons per day throughput calculated on a 208 operating day basis. PGM recoveries in 2010 to mid-year were similar to previous year recoveries, averaging 89.5%.

The ore feed to the concentrator is generally uniform in grade, with a lower hardness (Bond Work Index 18), lower talc content (5% to 10%), lower platinum plus palladium grade, and higher base metal content (copper and nickel) than composite feed to the Stillwater concentrator. The processing circuit at East Boulder was modeled directly after the Stillwater concentrator experience, although there are a few differences.

A surface jaw crusher reduces ore to minus 6 inches, and the ore is conveyed to a stockpile. An apron feeder recovers ore from the stockpile and conveys it to a SAG mill.

The grinding circuit consists of a 2,000-HP SAG variable speed mill and a 2,000-HP ball mill in closed circuit with a cluster of 15-inch cyclones. The SAG trommel oversize is recycled back to the SAG mill, and undersize is fed either directly to the flotation circuit or to the ball mills for further grinding. Downstream, a tertiary grinding 125-HP Verti mill is installed in closed circuit with a cluster of 10-inch cyclones to regrind rougher cleaner tails, middling cleaner tails, and scavenger concentrate.

A test of copper sulfate additions to the East Boulder concentrator resulted in an increase in PGM recovery. Copper sulfate is a traditional activator for sulfide minerals that are slow to float and acts by coating mineral surfaces with a layer of copper ions, which readily attach to collectors, thereby promoting flotation. The addition of copper sulfate was initiated on a regular basis during 2006. The Stillwater concentrator has a copper sulfate system installed but has discontinued its use as it was no longer effective relative to its cost. The recycle water at Stillwater has a greater content of dissolved elements compared to East Boulder, as it has been in pr duction for a much longer time; so the surface chemistry issues are somewhat different.

A flash flotation cell installed to process cyclone underflow ahead of the ball mill recovers readily floatable sulfides, reducing the possibility of over grinding liberated material. The flash flotation cell recovers up to 50% of the total recovered PGMs, which report directly to a cleaner circuit and then to concentrate containers following thickening and pressure filtration.

Tails from the flash flotation circuit report to the ball mill and then to a Knelson gravity concentrator that recovers 2.9% of total platinum and 2.3% of the total recovered gold. The gravity concentrate is shipped separately from the flotation concentrates to the smelter for refining. In-house flotation tests on the gravity concentrate revealed less than optimum platinum and gold recoveries, justifying the inclusion of the Knelson concentrator as a means to improving overall precious metal recoveries. The presence of a ferro-platinum mineral has been confirmed.

SAG and ball mill discharge is fed to the 15 inch cyclones. The process cyclone overflow product, at P80 of 97 to 100 microns, reports to the rougher conditioner for subsequent flotation. Rougher and middling concentrate undergoes cleaning and joins the final concentrate stream. Scavenger concentrate joins the rougher and middling cleaner circuit tails and undergoes regrinding in closed circuit with cleaner and column cell flotation prior to discharge into the final concentrate stream. As in the Stillwater concentrator, the use of carboxymethyl cellulose to suppress talc is effective.