Finsch prepares for middle age — Increased automation important to underground success

Lime Acres, South Africa — The miner’s instinct to get an obstacle out of the way with a drill and a few sticks of powder is usually a reliable one.

But what do you do with an obstacle that is made of air, rather than rock?

That’s a flippant way to introduce the problem faced at the Finsch diamond mine here in Northern Cape province. An open pit can only go down so far, but once it’s there it profoundly affects the kind of underground mine that can be made to work on the rest of the deposit.

At Finsch, operated by De Beers Consolidated Mines (DBRS-Q), the diamond-bearing kimberlite pipe complex — which intrudes a sequence of dolomites — is being mined by driving workings to the face of the open pit, then retreat-mining. This solution is a lesson in solving some of the unique problems of underground diamond mining.

Finsch was discovered by Thorny Fincham, Brahm Papendorf, William Schwabel and Schwabel’s son, Ernst. The group had been prospecting in the Lime Acres area since the mid-1950s.

The Precious Stones Act of the time prohibited diamond exploration on public lands, so Fincham registered several base-metal claims. He and the elder Schwabel then set up shop under the name Finsch Base Minerals, ostensibly looking for asbestos.

Amendments to the law made in 1960 permitted the company to look for diamonds, and Finsch started small-scale production from an open pit in 1960. In 1963, De Beers bought the company for R4.5 million.

The first underground workings date to 1978, when De Beers drove a decline below the pit and began sinking a production shaft, which went into operation in 1982. The open pit was mined until late 1990, reaching a depth of 423 metres and producing 79 million carats from 98 million tonnes of ore.

The 764-metre shaft now hoists about 3.7 million tonnes of ore annually, while a second shaft hoists roughly twice as much waste rock. The underground levels currently in operation are mining a collar of kimberlite that surrounds the lower levels of the open pit. Each production level consists of two sublevels — a drilling level and a loading level. Drifts along the sublevels are driven more or less radially toward the centre of the pipe, breaking out into the open pit. Mining then retreats from the pit face, using blasthole stoping. The lowest blasting pattern in the stope has a triangular cross-section, with the apex at the bottom, to force broken rock downward to the draw-points on the loading level. Loader operators at the pit face do their work by remote control, minimizing miners’ exposure to possible rock falls.

A primary concern in mining at Finsch is dust control. Workings in kimberlite are stable until they are wet, when they become friable and prone to swelling. This means water cannot be used for dust control underground.

Instead, large vacuum and filter units are stationed near every working face, with suction heads mounted on the machines. That makes Finsch an unusually dry mine, but no dustier than average. A single ventilation raise supplies fresh air that discharges through the shafts and the open drifts.

Because the mill is sensitive to steel among the feed — a piece of steel in the crusher can shut the machinery down for up to two weeks — ground control staff at Finsch also try to keep bolting to a minimum by maintaining a dry mine and coating drift walls with concrete.

Ore is hauled by trackless loaders to a pass system, where it falls to an underground crusher that reduces the muck to a maximum 30 cm in size.

Finsch’s automation program has centred on the ore handling system, and has included developing a remote-controlled breaking system at the tipping points and automatic monitoring of load levels in the passes and loaders.

Monitoring is done from a central control room underground, normally manned only by the breaker operator. “All the conveyor operation, the vital signs monitoring, we do from here,” says Junior Keyser, Finsch’s production overseer.

A modification now being considered is to move the mill’s gyratory crusher underground so that material is ready for the mill when it is hoisted.

The future at Finsch is block caving: the levels currently being mined are near the bottom of the existing pit, and the next block of ore, Block 4, lies beneath the pit floor. “It’s a challenge,” says mine planner Ross Norris, “it’s almost like a mine within a mine, changing from open stoping to block caving.”

Finsch’s plant is scheduled to treat 3.9 million tonnes of ore in 1998, 3.7 million from underground and 200,000 tonnes of re-processible tailings.

Production is expected to be just over 2 million carats.

The plant’s primary crusher reduces the ore to 150 mm, then the ore is scrubbed to wash away fines and clay. Secondary and tertiary crushing circuits bring the ore down to a process size range of 1.5 to 18 mm.

The plant has a 2-stage dense-medium-separation (DMS) plant, with an intermediate grinding stage in a rod mill bringing the feed to the secondary DMS down to a maximum 3 mm. Concentrate from the DMS goes to a standard x-ray separation unit, with the separated material sent to the sorthouse.

Finsch also had a pan plant, but it was closed when the price of small stones fell in the 1980s.