Both the Merensky and UG2 Reefs are exploited. The Merensky Reef is generally composed of an upper feldspathic pyroxenite, overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall. Locally this is termed a “pyroxenite reef”. Occasionally a pegmatoidal pyroxenite and a second chromitite stringer may be developed between the feldspathic pyroxenite and the footwall units. This is termed a “pegmatoid reef”. As an aid to mining operations the Merensky Reef is further defined as being “A”, “B” or “C” Reef where it rests on specific footwall units – locally called Footwall 1, 2 and 3 respectively.

The UG2 Reef is defined as a main chromitite layer, with most of the PGM and base metal mineralisation confined to this unit, followed by a poorly mineralised pegmatoidal pyroxenite footwall. The hangingwall to the main chromitite layer is a feldspathic pyroxenite containing up to four thin, poorlymineralised chromitite layers.

Both mineralised horizons dip gently away from the sub-outcrop in a north-easterly direction at 10° to 12°. The vertical separation between the Merensky and UG2 Reefs varies from about 125m in the south to 45m in the north of the mining area. The reefs may be disrupted by minor and major faults, lamprophyre, syenite and dolerite dykes, late stage ultramafic replacement pegmatoid bodies and potholes. The latter features are generally circular in shape and represent “erosion” or “slumping” into the footwall units. They vary in size from a few metres to tens of metres across and up to tens of metres in depth.

The shafts across the property can be divided into three groupings – the old shafts (the ‘Old Men’ comprising 4, 6, 7, 7A, 9 and E/F), the mature shafts (the ‘Big 5’ include 1, 10, 11, 12 and 14) and the ‘Build-up shafts’ (16 and 20).

The Merensky and UG2 Reefs are mined concurrently. The mining method is predominantly conventional breast mining. Stoping at the operations is carried out through conventional double-sided breast mining in accordance with Impala’s best practice principles. The haulages are developed in opposite directions from cross cuts connected to a central shaft position, following the two reef horizons on strike in the footwall and are defined as half levels. Footwall drives are developed at approximately 18m to 30m below the reef horizon with on-reef raise/winze connections being between 180m and 250m apart. Panel face lengths vary from 15m to 30m for both Merensky and UG2 Reefs, with panels being typically separated by 6m x 3m grid pillars with 2m ventilation holes. Stoping widths are approximately 1.3m and 1.1m for conventional Merensky and UG2 Reefs respectively, depending on the width of the economical reef horizon. Mechanised (trackless) bord and pillar mining occurs in selected Merensky Reef areas at 14 Shaft. The average stoping width of the mechanised panels is about 1.9m.

Mining currently extends to a depth of around 1 000 meters, with most operations occurring at an average depth of 800 meters. The mining width, including dilution, for Merensky Reef is typically about 1 meter, whilst that for UG2 is about 0.9 meters. Each shaft develops and mines about 8 square kilometres.

Mineral Processes houses the concentrator and smelter operations and is located on the mine property in Rustenburg. Current smelting capacity is 2.6 million ounces of platinum. Refineries, located in Springs, comprises a base metal refinery (BMR) and a precious metal refinery (PMR). Current refining capacity is 2.3 million ounces of platinum at the PMR.

Ore is delivered from underground via a rail network of 92km to two concentrator plants: the central concentrator, comprising the Merensky and MF2 sections, and the UG2 concentrator.

Merensky ore is crushed to a top size of 150 mm by jaw crushers, prior to delivery to the silos. The 15 Merensky mills run in parallel as 15 single-stage run-of-mine ball milling circuits with single-stage hydrocyclone classification. The milling circuit is followed by a single-stage bulk sulphide flotation circuit. Two flotation banks are run in parallel, receiving feed from 8 and 7 mills, respectively.

UG2 ore is largely processed at the UG2 plant, with the excess being taken up at the MF2 plant, along with some Merensky and all the opencast material. The UG2 plant consists of two primary autogenous mills, which both receive run-of-mine ore. The mill discharge is screened, and this effects the ore separation for which the circuit is known: the screens separate the chromite-rich fraction from the silicaterich fraction. The fine, chromite-rich material reports to the undersize, along with about 90% of the PGMs, and is therefore known as the high-grade fraction. The remaining 10% of the PGMs are associated with silicates, and report to the screen oversize with the coarse silicate, or low-grade, fraction. The high-grade fraction is treated through a classic MF2 circuit, with the low grade treated in a MF1 circuit, similar to the Merensky ore.

The conversion of the UG2 plant to an open-circuit primary milling operation has led to more efficient milling of the high-grade fraction, and has reduced over-grinding of chromite. The effect of this has been seen as a reduction in chrome in concentrate, and an associated increase in PGM recovery. However, increased throughput at the UG2 plant has actually resulted in a larger quantity of chromite being processed at the smelter.

Flotation concentrate is treated together with IRS toll concentrates, through a thickening circuit to maximize on water recovery, and to facilitate blending. The thickened product is fed to four operational coal-fired Niro-technology spray-drying units, where an almost bone-dry product is pneumatically discharged to silo storage units. Two rectangular six-in-line furnaces, with a combined power of 73 MW, are used for sulphide concentration. The sulphide matte (which is the PGM carrier) is further concentrated through iron removal in Peirce-Smith converters.

The Impala Platinum Limited base metals refinery in Springs processes ore containing base metals and platinum group metals (PGM’s) sourced from mines in the Rustenburg area and supplied to the refinery in the form of convertor matte. At the BMR, a hydrometallurgical process is used to refine the matte.