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Dwarsrivier Part of the ‘New Rustenburg’

The Eastern Limb of the Bushveld Complex, centred around the towns of Steelpoort and Burgersfort in Mpumalanga in South Africa is undergoing a boom, with some 18 mines in existence and more in the offing. Located in this area is the Dwarsrivier Mine, in operation since 2000, and which three and a half years ago went underground after exhausting most of its surface mining options.

Dwarsrivier, owned by Assmang Limited, has opted to outsource many of its mine functions. This includes management of its lamp room, and of course its main outsourced contract, the mining of this 120,000t a month operation.

Murray & Roberts Cementation, which sunk Dwarsrivier’s decline shafts, part of the R247 million capital cost of the mine, is the mining contractor and mine overseer Pieter Rooy says the project is looking to ramp up to 140,000t a month by about July 2008. That is the full capacity of the Dwarsrivier plant; any further expansions will require an expansion of the plant.

Dwarsrivier, which mines the LG6 chromite seam, does have the capacity to expand, to the north on the other side of a national road that bisects the property. It could conceivably add a second set of declines, effectively a doubling capacity, Dwarsrivier Mine’s production manager, Meiring Burger says. The spot price for chrome ore has been in the region of R700 to R800 per tonne.

Rooy told Mining Review Africa during a site visit that development at Dwarsrivier that work is progressing at an average of 25m a month per decline which is in line with the mining rate of 25m a month per decline and is being done to ensure sufficient face availability. The mine has two sections, South and North, each similar in size and providing 60 000t a month of ore. “Our goal is to reach the point where development is one year ahead of mining and if we maintain our current rate of development we will achieve that,” Rooy says. With the odd geological fault in the orebody, which for certain sections may require a month or so to bypass, to ensure there is no fallback in production that flexibility will be important.

The average mining depth is now 110m below the surface with Dwarsrivier’s decline shafts having extended some 800m. Its reserves of 36.4 million tonnes and resources of 102.5 million tonnes are all delineated at above a depth of 350m, and taking into account an annual mining rate of 1.4 million tonnes, this gives Dwarsrivier a life of mine of 26 years on its reserve base alone. Typically for planning purposes the mine looks ten years ahead in terms of reserves and the orebody is defined and consistent enough that it requires no geological face sampling for grade control.

Two declines extend from surface, a conveyor decline and a decline for personnel and material, and after passing an upthrow fault zone these branch into three declines. In addition a 5.3m diameter downcast ventilation shaft was sunk as well as two 3.1m diameter upcast ventilation shafts.

Burger describes the Dwarsrivier orebody as a relatively simple one to deal with, the reef width being 1.86m, dipping to the west at 10° with the strike going north to south. The ore body in situ contains 39% chrome oxide (Cr2O3) which stays relatively stable over the whole property with increases to 414% in specific areas. Beneficiation takes the Cr2O3 up to 46%.

The mine is one of those that produces a fair amount of lumpy ore (about 42% of its output) and Assmang Chrome’s Dwarsrivier production manager, Meiring Burger, says that it produces two lumpy products, one of +25mm to -100mm and one of +6mm to -25mm. This represents the addition of a new lumpy product compared with a couple of years ago and coincides with the separation of the cyclone and drum products in the DMS section of the plant, which lead to an increase in the lumpy portion of the ore, and less fines creation. Dwarsrivier produces three fines products, one being a metallurgical fines product of 44% Cr2O3 with <3% silica, a chemical grade product of 46% Cr2O3 with <0.8% silica and 42% Cr2O3 product. This latter is also an addition to the mine’s product range compared with a couple of years ago. Recently a wet screen was installed in the screening plant to remove excessive fines from the lumpy feed to reduce the recirculating fines in the DMS section. The stockpile capacities are; 30,000t for the chemical grade fines, 30,000t for the metallurgical grade fines and 16,000t for the 42% chrome oxide product.

One would expect a premium for lumpy ore, because that eliminates the need for a pelletising plant by the ferrochrome smelters, and that premium does still exist,Burger says however that this is to a degree cancelled out by the fact that pelletising of the fines from the mine gives a higher grade product. Dwarsrivier supplies 60% of its output to Assmang’s Machadadorp ferrochrome smelter, with the remainder sold to the spot and long term markets.

The operation is predominantly mechanised, though a few hand-held rock drills are used to help out in sections where the angle of the reef becomes steep. The mechanised mining fleet consists of 18 LHDs(mostly Tamrock EJC 115s), five underground utility vehicles for explosives and materials, six drill rigs, and two MahindraLDVs. These are leased to the mine within the contract with Murray & Roberts Cementation.

Rooy is very aware that companies such as Grinaker LTA, Shaft Sinkers and Anglo are also on the lookout for skilled miners and artisans in the region. “Having compared the salaries of drill rig operators, LHD drivers, miners and artisans, we are fairly sure we are paying in the range to retain our people.” One of the other ways Murray & Roberts Cementation retains its workforce is by providing a bus service to its employees whereas Assmang provides some of the Murray & Roberts Cementation and their own employees with housing in Lydenburg which is some 60km distance away. Housing is, however, provided at market related rental. Rooy says these methods are working. Murray & Roberts Cementation is always on the lookout for skilled employees to ensure they can deliver to required commitments, says Rooy.

Murray & Roberts Cementation has, in addition, employed an above average number of female miners with more than 10% of its workforce being women, including a section leader.

However, even with the management effort put into ensuring a stable workforce, new projects and expansions lead to a poaching of experienced and skilled mining people. This coupled with the fact that Dwarsrivier, like many mines in the area, has a new workforce, means there has been a lot of turnover. But Rooy says things are stabilising as Dwarsrivier completes its move to a more steady state operation. The remaining equipping is now down to some final conveyor sections, as well as new installations such as dams, electrical cables and permanent piping and pumps. Its workforce including contractors is about 780 people, of which 500 are the Murray & Roberts Cementation mining team.

“Shareholders require a year by year decline in costs and we are most probably somewhere in the middle of the cost curve for chrome operations. Our aim is to be in the lower quadrant of the cost curve, but further decreases in our costs require a stable workforce,” Burger says.

The fact that the underground workshop has recently been completed will help reduce costs by saving time on refuelling and increase the availability and utilisation of the mining fleet. It will enable the mechanics to have a properly lit place where they can fix the machines. The target is an availability of 85% and a utilisation of 80% of that availability, whereas at the moment the fleet is running at a 68% availability and a 65% utilisation. This is also due to the fact that the loading of vehicles with waste from development is having a material impact on tramming.

The mine is innovating with new methods in other ways. It is testing blasting using emulsion, as opposed to Anfo, and this entails more skilful preparation and less people at the face. “It also increases the lumpy portion of the feed, ensures better control of the hanging wall, and better advance rates,” Burger says. The mine will also introduce a vehicle detection system, something that is now becoming a requirement for all mines as specified by the Department of Minerals and Energy (DME).

Burger says that Dwarsrivier has also made a few adjustments to its bord and pillar mine layout which will improve cost efficiency. “In the beginning we used nine 12m wide roadways per section for the first few sections, but now are using seven ten metre wide roadways per section.”

Dwarsrivier is undertaking rehabilitation of its old open pit sites, with backfill and revegetation rehabilitation completed on most of the old pits. The plant is ISO 9001:2000 accredited while the total mine is ISO 14001:2004 and OHSAS 18001 accredited. The mine has a good recent safety record with the last lost time injury being in March. Underground one can observe the attention to the details that mark a good mining operation such as ensuring the tipping area is kept at a suitable distance from the face (no more than 75m in this case) to reduce tramming times, and details such as the white washing of the worked out areas 20m behind the working face. In summation, mining and planning is not any less polished due to a contractor doing the job, and is arguably much more so than would be the case of smaller mining groups doing the job in-house.

Tonnage did fall short of the targets during 2006 during the build up phase, though since April 2007 the operation has moved closer to achieving the target tons and twice gone above its target. Rooy points out that the contract has been sufficiently beneficial to the extent that after completion of the first three year term, an additional three year contract has been signed with Dwarsrivier based on mostly similar terms as the original.

Rooy has purchased a house in Burgersfort and moved there, an indication of his confidence in Murray & Roberts Cementation’s longer term future as a contract miner on the Eastern Limb.

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