Venetia a mine of its time

The processing plant at the Venetia mine, west of Messina, South Africa

Every mine is a creature of its age, a reflection of the kind of thinking that marks the time during which it was planned. In South Africa, times have changed a great deal, and mines have changed with them.

At Venetia, 70 km west of the northern border city of Messina, De Beers Consolidated Mines (DBRS-Q) has deliberately gone about building a mine that fits its physical environment, while subtly but surely changing its social environment.

Venetia, De Beers’ newest South African mine, was discovered in September 1980 — the fruit of an indicator mineral survey that started in 1969. It was evaluated over a 4-year period beginning in 1985, and opened in 1992.

The mine exploits two kimberlite pipes, K1 and K2, part of a swarm of 12 pipes aged 510 million years. The present K1 and K2 pits are being progressively joined as mining proceeds, and will ultimately form a single pit. The pipes intrude early Archean granitic gneiss and quartzite.

The pit, as designed, has a 20-year life. Last year’s production was 4.2 million carats from 3.3 million tonnes of ore grading 128 carats per 100 tonnes, suggesting a minimum reserve of 60 million tonnes. The pit has a stripping ratio of 4-to-1.

Independent estimates have put the Venetia ore reserve near 31 million tonnes grading 120 carats per 100 tonnes, with an average price near US$120 per carat.

The main equipment in the pit is a team of 11- and 20-cubic-metre diesel shovels, feeding 85- and 136-tonne trucks. There is a primary crusher in the pit, which feeds the processing plant via a conveyor. All the conveyors have steep slopes to minimize the size of their footprint.

At the processing plant, the ore is first reduced to a size range of 8 to 25 mm, then sent through dense-medium separation (DMS) units, X-ray sorters and grease tables to separate the diamonds. Process tailings go to a tertiary crushing unit, which produces a size range of 1 to 8 mm. Tertiary crusher run goes back to the DMS, X-ray sorters and grease tables for a final recovery.

The tailings, entirely made up of minus-1-mm material, go to the slimes dam by conveyor, where a crawling unit positions the conveyor belt to distribute the slimes evenly. There is no release of decant water at Venetia: the concern instead is that too much water will evaporate.

In this environment, where annual evaporation rates are around 2.6 metres and annual rainfall is 0.35 metre, water supply is a vital concern. Venetia’s solution was a field of 27 drilled wells in two fields near the Limpopo River, connected to the site by a 32-km pipeline.

The wells exploit the Greefswald formation, a gravel aquifer, and were deliberately drilled in two fields to spread around the water demand and so minimize depression of the water table.

The mine is allowed to draw down the water table 2 metres in surface pools and 4 metres in forested areas. Both limits corresponding to the dry-season water table in 1989.

When moving to a 7-day work week increased the mine’s water requirements, De Beers constructed an off-stream storage dam near the wellfield, which holds a 1-year water supply for the mine. Three to four days’ supply is held in reservoirs at the mine site.

Further work on the processing plant has reduced the water requirement again, so that Venetia’s current water demand is met by its original allocation.

On top of being designed with the environment in mind, Venetia also marks a departure in the social design of a South African mine: there is no on-site hostel. The mine has a policy of hiring locally whenever it can get a qualified worker, and it maintains buses to bring in its workforce daily from Messina, 80 km to the east, and Alldays, 37 km to the southwest.

The mine, which has a total of 730 employees, still finds most of its technical talent elsewhere. “For the higher skills, we have to import,” says Russell Rood, mine manager. Over the long term, the operation will be encouraging local education. Says Daan Smith, personnel manager, “We are concentrating on getting the education levels in place so we can continue recruiting from this area.”

Venetia is the safest of De Beers’ South African mines, with a lost-time injury frequency rate of 0.08 shift per 200,000 hours worked. In recognition, the National Occupational Safety Association rated Venetia a “5-star” mine.