Stillwater Mine

 mine, platinum mine/processing

 Upload a photo

This mine is a producer of Platinum and palladium. The Stillwater platinum / palladium mine is in the Beartooth Mountains, 130km southwest of Billings, Montana, USA. It is wholly owned by Stillwater Mining Co. The East Boulder project, some 20km west of Stillwater, began commercial production in 2002 following an investment of $370 million. In mid-2003, the Russian PGM producer, Norilsk Nickel, took a 50.8% holding in Stillwater Mining, in an agreement that included Stillwater marketing 877,000oz of palladium from Norilsk's holdings.

In 2005, Stillwater Mining produced a total of 554,000oz of palladium and platinum, which included 428,000oz of palladium and 126,000oz of platinum. The average cash cost of production was US$324/oz, with a total production cost of US$472/oz. The average price received for sales was US$467/oz. During the year, Stillwater Mining delivered a further 439,000oz of palladium on behalf of Norilsk, in addition to the 375,000oz sold in 2004.

GEOLOGY AND RESERVES

The deposit is in the J-M reef of the Stillwater complex. This is a large syngenetic, mafic/ultramafic layered complex in which the layering consists bands of norite, gabbro and anorthosite. Crystal settling allowed the heavy minerals to settle to the base with the lighter siliceous minerals remaining at the top. Platinum-group metals (pgms) occur in a layer averaging 2.4m thick, and are associated with pyrrhotite, chalcopyrite and pentlandite.

As of end-2005, the company's total proven and probable ore reserves were 38.1Mt at a grade of 19.5g/t, containing 24.1Moz of palladium and platinum at an in-situ ratio of 3.5 to 1. At Stillwater, proven and probable reserves totalled 16.4Mt at a grade of 21.6g/t, containing 11.5Moz, while at East Boulder, proven and probable reserves were 21.7Mt at a grade of 18.2g/t, containing 12.6Moz.

MINING
The mine has adits and laterals driven in from the Stillwater Valley side. There is also an internal decline and shaft. Three stoping methods are used: mechanised captive cut-and-fill; ramp-and-fill using hydraulic drill jumbos and LHDs; and sub-level stoping. Declines parallel to the orebody allow entries to be made into the stoping area by overhand methods. Sub-levels are driven on lifts and drilled downwards. Blasted ore is recovered from below using LHDs and stopes are paste backfilled.

The 5,443t/d underground crushing station receives ore from upper levels via ore passes and crushes it to –150 mm. The ore is then conveyed to holding bins before loading in the 9t-capacity skips. Approximately 60% of the ore is transported to the mill via a 600m shaft, while the remainder is transported by rail or truck.

ORE PROCESSING
Ore is stored in a 3175t silo before being fed to a SAG mill. Oversize is passed to a ball mill while undersize is cycled to flash flotation circuits. The float product is thickened, the concentrate containing 1,700g/t PGM. Tailings from the flash flotation are fed to hydrocyclones and the overflow is fed via conditioner tanks to the rougher/scavenger/cleaner flotation circuits. Scavenger float is recycled via a regrind mill while the tailings are drawn off. Hydrocyclone underflow is fed to a flash flotation cell from which the underflow is fed to the thickener along with the cleaner overflow. Thickened concentrate is transported to the smelter. The float tailings are sent to the sand plant for incorporation in backfill.

The company's smelter and base metals refinery are located in Columbus, Montana. At the smelter, where a new 90t/d furnace was commissioned in late 1999, concentrate is upgraded to produce matte containing approximately 16,000g/t pgms, plus small amounts of rhodium, gold, nickel, copper and silver. Matte is refined to produce filter cake containing 60% pgms, which is shipped to metal refiners in the US and Germany. The smelter also recycles spent auto-catalysts to recover their pgm content.

THE FUTURE

Stillwater Mining is now targeting an increase in the use of selective mining at its operations, in an effort to improve resource recovery and cut production costs. It is also aiming to ramp up outut at both mines to their nominal combined capacity of 4,300t/d of ore. Actual ore production in 2005 averaged 3,000t/d. It is also aiming to increase its platinum-group metals recycling business, and to diversify away from sole dependence on platinum-group metals.

Reference for information:
www.mining-technology.com/projects/stillwater/