Although the Merensky Reef is generally regarded as a uniform reef type, large variations occur in reef thickness, reef composition, as well as the position of the mineralisation.

The rock-forming minerals of the Merensky Reef comprise approximately equal amounts of dark iron-magnesium silicate minerals and lighter calcium-aluminium-sodium silicate minerals (called a feldspathic pyroxenite) under- and overlain by thin (5 to 15 mm) often discontinuous layers of chrornite concentrations. The total thickness of this package is generally less than 30 cm. This zone, commonly known as the Merensky pegmatoid, contains the base metal sulfide grains and associated platinum group minerals. The Merensky Reef has been traced for 300 km around the entire outcrop of the eastern and western limbs of the Bushveld Complex, and to depths of 5 km.

The rock-forming silicate minerals of the Merensky Reef consist predominantly of orthopyroxene (- 60 per cent), plagioclase feldspar (- 20 per cent), pyroxene (- 15 per cent), phlogopite (5 per cent), and occasional olivine which lists the composition of some of the common minerals found in the Bushveld-type rocks. Secondary minerals such as talc, serpentine, chlorite and magnetite have widespread occurrence.

The base metal sulfides consist of pyrrhotite (-40 per cent), pentlandite (- 30 per cent), chalcopyrite (- 15 per cent), and trace amounts of millerite (NiS), troilite (FeS), pyrite (FeSJ, and cubanite (Cu5FeS4). The major platinum group minerals are cooperite (PtS), braggite [(Pt,Pd)NiS], sperrylite (DtAsz) and PGE alloys, although in some areas minerals such as laurite) can be abundant.

The UG-2 Reef is a platiniferous chromititc layer which, depending on the geographic ocation within the Complex, is developed some 20 to 400 metres below the better known Merensky Reef. The chromitite itself is usually 1 m thick but can vary from - 0.4 to up to 2.5 m. Thin chromitite seams (generally less than 20 cm in thickness) may be present in both the footwall and, more commonly, in the hanging wall rocks. The UG-2 consists predominantly of chromite (60 to 90 per cent by volume) with lesser silicate minerals (5 to 30 per cent pyroxene, and 1 to 10 per cent plagioclase). Other minerals, present in minor concentrations, can include the silicates: phlogopite and biotite, the oxides: ilmenite, rutile and magnetite, and base metal sulfides. Secondary minerals include quartz, serpentine and talc.

The base metal distribution follows a similar trend to that of the PGE, with mat of the values occurring in the bottom and top part of the reef. The base metal content of a typical UG-2 Reef is approximately 200 to 300 ppm nickel occurring as nickel sulfide and less than 200 ppm copper occurring as copper iron sulfide.

The platinum group minerals present in the UG-2 Reef are highly variable, but generally the UG 2 is characterised by the presence of abundant PGE sulfides, comprising predominantly laurite (RuOsIr sulfide), cooperite (PtS), braggite (Pt, Pd, NiS), and an unnamed PtRhCuS. The platinum group minerals only reach an average size of approximately 12 pm, with particles larger than 30 pm being extremely rare. Most of the platinum group minerals occur in association with the base metal sulfides and silicates. It is only the mineral laurite which exhibits a preferred association with the chromite grains. Both the grain size and associations are extremely important as these affect the metallurgical behaviour during subsequent processing.

Union’s infrastructure consists of two vertical shafts, namely Spud Shaft and Richard Shaft, and a declines section consisting of four decline complexes. The 1S section was closed early in 2014 as part of the decisions reached through Amplats’ Platinum Review of 2013.

The other three decline sections were closed by the end of 2014 owing to their unprofitable nature. The operating depth of the current workings is between 100 metres and 1,500 metres below surface.

The mine extracts mostly UG2 Reef ore, but also produces limited Merensky Reef ore and treats low-grade surface ore and tailings. Three-quarters of Union Mine’s underground production is done conventionally (using breast stoping with strike pillars), while hybrid mining occurs at the declines.

The ore is smelted at Anglo Platinum's Mortimer Smelter located near the mine, while converting is done at the Waterval Smelter located in Rustenburg.

The smelter objective is to process wet concentrate to produce crushed, slowcooled, sulphur-deficient nickel-copper matte rich in platinum group metals, gold, and base metals for despatch to the Magnetic Concentration Plant at the Base Metals Refinery.

Wet concentrate is dried, then melted in electric furnaces, and excess iron sulphides are removed in the ACP converter. A slag cleaning furnace treats gangue from the converter, while a slag milling and flotation circuit treats the slag from the electric furnaces.

Wet concentrate with an average moisture content of 16-17% by mass is delivered by road in flexi-side tippers or containers, and offloaded in the concentrate-receiving shed. Slurries with a solids content of 50-60% are pumped or delivered by tanker to the Waterval Complex Concentrate Handling plant (WCCH) for dewatering.

In the flash drying process, moisture is removed from the wet concentrate, producing a dry furnace feedstock (<0.5% H2O). The energy for the process is provided by burning coal in a fluidised bed (silica sand) combustor known as a hot gas generator (HGG).

The two electric furnaces are of Hatch design, with a rated transformer capacity of 39 MVA (34 MW maximum or 30 MW nominal design), and a design operating factor of 90%. The rectangular furnaces have internal dimensions of 8.0 x 25.8 m, and the calculated power flux at nominal design is 146 kW/m². The shell is constructed from refractory bricks held together by a precisiondesigned tension system.

Waterval furnace matte (WFM) is tapped into matte ladles and transported by crane to one of two granulation stations. The ladle is placed on a hydraulically driven tilter that pours the matte into a granulation launder at a predetermined rate. The launder discharges above a stream of granulating water that shearquenches the matte, forming fine particles suitable for dry feeding into the ACP converter.

The slag-cleaning furnace (SCF) is a 12 m diameter round furnace, designed by Pyromet, rated at 30 MVA (23 MW). Three Söderberg electrodes, each 1 400 mm in diameter, deliver power into the bath at currents up to 80 kA. The high currents are required due to the high conductivity of the slag. The resulting high slag temperatures led to the design choice of water-cooled copper sidewalls in the form of Pyromet Maxicoolers. This maximises energy removal, and eliminates the need for annual refractory repairs.

The flotation circuit consists of two 30 m³ cascading Outokumpu tank cells, followed by 14 flotation cells, currently functioning as a single rougher bank. Concentrate is pulled from each of the two tank cells and every flotation cell, then sent to the flue dust thickener. Tailings from the flotation cells are pumped to a tailings dam.