Assmang Manganese Mines, South Africa

The Kalahari Manganese Field, located in Northern Cape Province, about 700km southwest of Johannesburg, contains perhaps 80% of the world's known high-grade manganese ore reserves. The district yields about 4Mt/y, mined mainly by Samancor and Assmang.

Originally established in 1935 and now jointly owned and managed by African Rainbow Minerals and Assore, Assmang wholly owns two mines near the community of Black Rock. The company also produces manganese alloys at its Cato Ridge works in KwaZulu Natal.

Assmang commissioned Nchwaning in 1972 while the Gloria mine, to the south, started production in 1978. Each had a plant nominally rated to treat 1Mt/y ore.

Early in 2000, Assmang announced an expansion involving the development of a new shaft complex at Nchwaning to add about 2Mt/y of run-of-mine ore capacity, to make Nchwaning the world’s lowest-cost underground manganese mine, and to extend its lifetime by about 30 years.

The expansion, including additional treatment capacity, was commissioned in May 2004 and was completed in May 2005 at a capitalised cost of R780m. In the future, the company would like to build a new manganese-alloy plant near the mine. Assmang works three eight-hour shifts per day from Monday to Friday, as well as a single shift on Saturday, employing a total of 2,510 people at its various operations.

GEOLOGY AND RESERVES

The Kalahari Manganese Field lies within a large structural basin that extends approximately 40km south to north and 5km to 15km east to west, dipping gently northwest. At Black Rock, near the northern end of the basin, the Transvaal System rocks lie about 300m from surface, beneath Kalahari Formation sands and calcretes, Karroo System tillites and Waterberg System shales and quartzites.

The sub-horizontal stratabound manganese ore horizons occur in banded ironstone of the Voëlwater Formation at the top of a sequence of Transvaal System rocks. As well as being faulted, the horizons are folded.

The ore is massive and averages 48% manganese; at Nchwaning it is particularly low in phosphorus while Gloria has a higher manganese-to-iron ratio. At the end of 2006 Assmang's reserves totalled 192Mt with mineral resources of 428.4Mt.

MINING

Both mines are underground operations, using the room-and-pillar method. Nchwaning started with one vertical hoisting shaft, with the 450m-deep No.2 vertical skip shaft added and the plant upgraded in 1981. Gloria combined a vertical shaft for personnel and materials hoisting with a long incline shaft for vehicle access and conveyor hoisting of the ore to a surface crushing, screening and washing plant.

The new expansion follows this design, having a 2,200m-long incline shaft and the 500m-deep Nchwaning No.3 personnel shaft. There is also a new ventilation shaft, and a workshop located on the 400 level.

Bolted development entries access rooms that are 7m to 8m wide and 3.2m to 3.5m high, while the pillars are normally 8m x 8m. Low-grade ore is left to form both the floor and roof of the rooms.

The established areas use Boart Longyear hydraulic drilling rigs and Wagner Load-Haul-Dump (LHD) machines and trucks to transport the ore to storage silos, primary crushers and screens that feed the hoisting systems. Roofs are scaled using a modified three-wheeled loader.

For the new area, Assmang requested a three-boom rig fitted with two rock drills for face work plus one for roof-bolting, so the mine can drill holes for roof bolts and face-blast holes from the same set-up. Atlas Copco has supplied four purpose-designed Rocket Boomer M3D rigs.

Assmang also added to its fleet of Atlas Copco Wagner LHDs but opted for an alternative truck brand, ordering three Caterpillar AD 30 machines.

PROCESSING AND PRODUCTION

The run-of-mine ore is crushed, washed and screened, with no other processing needed. After the ore has passed through the plants, it is stacked according to size and grade. The capacities of the stacks vary between 280t and 320t each, and are numbered and sampled.

In total, Assmang produced about 2.5Mt of manganese ore in the 2006 financial year, not including sales to Cato Ridge. Exports travel by the main South African Railways route to Port Elizabeth on the Indian Ocean. Lesser tonnages are directed to the domestic steel industry.

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