The Smokey Hills project is located in the central portion of the eastern limb of the Bushveld complex and is underlain by rocks of the upper critical zone, which hosts two of the more important PGE-bearing units in the Rustenburg Layered Suite: the Merensky Reef and the UG2. In the Smokey Hills Project Area, the Merensky Reef is expected to occur approximately 300 m above the uG2. However, a minimum of approximately 140 m of the Merensky Reef footwall units have been removed by erosion and therefore no Merensky Reef is preserved at the Smokey Hills project. In the central portion of the eastern limb of the Bushveld complex, five chromitite layers have been identified in the Upper Critical Zone, each of which forms the base of a cyclic unit. The cyclic units are termed, from oldest to youngest the UG1, UG2, UG3 UG3A and UG3B. Each of these chromitites carries varying concentrations of PGEs. However, concentrations in the UG1 chromitite layer in all areas of the RLS have been found to be subeconomic. The UG3, UG3A and UG3B chromitite layers are generally too thin and contain insufficient concentrations of PGE mineralisation to have any reasonable expectation to support economic extraction. The UG1 chromitite layer occurs over 100 m below the UG2. The UG3 chromitite layer is approximately 0.25 m thick and occurs approximately 15 m above the UG2. The UG3A and UG3B chromitite layers are approximately 0.1 m thick, occur approximately 5 m above the UG3 and are separated by approximately 0.3 m of pyroxenite. Individual layers in the Upper Critical Zone can generally be correlated over several kilometers and as such, detailed local stratigraphic nomenclature can be established.

Dykes in this area are primarily composed of dolerite, which is generally fine-grained and of good competence, with associated areas of dense jointing and alteration. Underground dyke intersections are generally less than 10 m in width. An airborne magnetic survey has identified several swarms of northeast-striking dolerite dykes. No serious problems were encountered during mining through these features, and no significant displacements have been associated with them.

Potholes are defined as areas where normal reef characteristics are destroyed and hence, pothole areas are believed to be un-mineable and considered a geological loss. The position and size of potholes are typically unpredictable. In most instances, the potholes affect only one of the economic units. However, in minor instances, the underlying or overlying economic unit may be affected by the development of the pothole.

The Bushveld complex stratigraphy is occasionally replaced by late-stage pegmatite bodies. These pegmatite bodies have a range of compositions, from highly ultramafic to felsic.

Rolling reef and small scale undulations of the UG2 do not create geological losses but can contribute towards mining difficulties. Undulations and highly dipping (up to 50°) have been observed at Smokey Hills in the outcrop and during mining. Miners have tried to minimize difficulties associated with the mining of these areas by using up-dip methods. An area of rolling reef has been encountered during underground mining centred on the 4.4 Raise. The UG2 elevation, in relation to adit 4, was reduced by up to 40 m. This has made it difficult to mine because of the proximity of the mining lease boundary.

When in production, the Smokey Hills mine utilises a combination of conventional narrow reef stoping methods with trackless equipment to remove blasted ore from underground workings. The mine plan has changed from an up-dip stoping method to a (mainly) breast stoping configuration. However, some up-dip mining is still planned for future operations. Stope panels and reef raises are drilled with hand-held jack hammers, and the blasted ore is removed from the working faces by means of scraper winches. The centre gully (reef raise) winch scrapes all blasted ore into a large muck bay. The ore is loaded in the muck bay with trackless equipment and transported in dump trucks to the concentrator plant. Access to the underground workings is via adits, which are developed on strike and reef.

The Smokey Hills Plant consists of a typical UG2 processing plant with a mill-float-mill-float (or “MF2”) configuration with a capacity of 60 ktpm.

The plant consists of the following circuits:
• jaw crushing;
• cone crushing in closed circuit with a screen;
• primary ball mill in closed circuit with a screen;
• primary flotation;
• chrome spirals recovery circuit;
• secondary ball mill in closed circuit with a cyclone;
• secondary flotation;
• PGM concentrate and tails thickening;
• PGM concentrate filtering and storage;
• reagent make-up and dosing circuits; and
• water recycling and storage.

Primary and Secondary Crushing.
The crushing circuit is of a compact design and consists of a primary crushing with secondary crushing in closed circuit with a screen.

Underground RoM material is transported via trucks and tipped onto a static grizzly. Static grizzly undersize gravitates into an ore receiving bin and onto a vibrating grizzly feeder (“VGF”). Oversize from the VGF feeds a 55 kW Sandvik jaw crusher with its product combined with the VGF undersize on a conveyor.

The jaw product and VGF undersize feeds a vibrating screen. Screen oversize (+25 mm) is transferred into a feed bin and vibrating feeder ahead of the secondary crusher (132 kW Sandvik cone crusher). The cone crusher product gravitates onto a conveyor and is recycled back to the vibrating screen.

Screen undersize is transferred into a concrete silo prior to milling. A belt magnet located at the head of the sizing screen feed conveyor removes tramp metal.

Primary Milling and Classification.
Ore is transferred from the mill feed silo via a variable speed belt feeder. A weightometer is installed on the mill feed conveyor to measure the feed tonnes. The primary milling circuit comprises of a 1,400 kW Outotech ball mill in closed circuit with a screen. Ball mill discharge passes over a scalping screen, the oversize (scats) from which will be returned to the ball mill feed conveyor via a load-in chute.

Scalping screen undersize gravitates into the sump and is diluted with process water prior to being pumped to the classification screen. Screen undersize is pumped to the primary rougher circuit through an automated cross stream sampler. Screen oversize gravitates back to the primary mill feed chute.

Steel balls are added to the mill using an automated ball charger. Steel balls are automatically added. The addition rate is based on the mill feed tonnage.

Primary Flotation.
The primary roughers (“PR”) flotation circuit comprises of a bank of three forced air tank cells in series. As per the original designs seen in Figure 26, tails from the PR was pumped to the chrome removal circuit. Currently the plant is setup to pump PR tails directly to secondary milling with the secondary rougher tails treated through the chrome spirals circuit.

A fast floating concentrate is removed from the first PR and pumped to the fast cleaners (“FC”). A medium floating concentrate is removed from the last two PRs and combines with the tails from the FC before being pumped to the medium cleaners (“MC”). The FC concentrate is pumped to the PGM concentrate thickener.

The MC concentrates are pumped to the medium recleaners (“MRCs”). The MRC concentrate is pumped to the PGM concentrate thickener while the MRC tails is recycled back to the MC cells.

The MC tails is pumped to the slow cleaner (“SC”) circuit. Concentrate from the SC cells are pumped to the slow recleaners (“SRC”) while the SC tails are pumped to the slow cleaner scavengers (“SCS”). SCS concentrate is recycled back to the SC while the SCS tails is pumped to the SCS tails cyclone. The cyclone overflow is sent to the process thickener prior to being pumped to the secondary mill. The SCS cyclone underflow is pumped directly to the secondary mill.

The SRC consists of two sets of cells, namely the SRC1 and SRC2. The concentrate from SRC1 is pumped to the SRC2 cells with its concentrate subsequently pumped to the PGM concentrate thickener. The SRC2 tails is recycled to the front of the SRC1 cells. The SRC1 tails is pumped to the SC circuit.

Chrome Removal Circuit.
The chrome removal circuit is made up of a series of spirals which produces a chrome concentrate and tails. The spirals circuit is made up of roughers, cleaners, middlings and middling cleaner spirals.

In the original plant design shown in Figure 26, the spirals treated PR tails. With this scenario, the spiral tails streams are classified with cyclones with the cyclone overflows thickened in the process thickener prior to secondary milling. The cyclone underflow is pumped directly to the secondary mill. Currently, the spirals circuit has been modified to treat secondary flotation tails. As a result, the spiral tails will be thickened and then pumped to the TSF.

Secondary Milling and Flotation.
The secondary milling circuit comprises of a 1,400 kW Outotech ball mill in closed circuit with a cyclone. The mill discharge gravitates into a sump where water is added prior to being pumped to the secondary mill cyclone. Cyclone overflow is pumped to the secondary roughers (“SR”) through an automated crosscut sampler. The cyclone underflow gravitates back to the secondary mill feed chute. Steel balls are added manually on a daily basis to the mill using a dedicated ball kibble and hoist arrangement.

Secondary Flotation.
The SR circuit consist of two rougher cells. The SR concentrate is combined with the MRC tails and pumped to the MC cells. The SR tails is treated in three secondary scavengers (“SS”). The SS concentrate is combined with the SRC tails and pumped to the SC circuit. Currently, the SS tails is pumped to the spirals recovery plant to produce a chrome concentrate. The spirals circuit tails is thickened prior to being pumped to the tailings storage facility (“TSF”).

Concentrate Handling and Filtration.
Final concentrate from the flotation plant is pumped to a single rake driven concentrate thickener via a trash screen. The purpose of the trash screen is to prevent ingress of foreign trash into the concentrate thickening and filtration circuits. Diluted flocculant is added in the feed launder and/or feed well to assist in the flocculation and settlement of the solids. Thickener underflow is pumped to an agitated filter feed tank and from there pumped to the Larox filtration circuit.

Thickener overflow gravitates to a collection tank and is pumped to the flotation circuit for use as launder wash down water.

Underflow from the concentrate thickener is stored in the filter feed tank before batch pumping to the Larox filter. An automatic cross-cut sampler is installed above the filter feed tank on the concentrate thickener underflow line. This sample represents the product from the flotation circuit and is used for metal accounting purposes.

The filter cake from the Larox filter discharges onto a conveyor and is transferred onto stockpiles for loading and despatch. The filtrate from the concentrate filter is recycled within the filtration circuit. Excess filtrate is pumped back to the concentrate thickener.