Mining tails yield diamonds.
By Ettagale Blauer
Is it possible to have a million carats of diamonds lying around in the open, just waiting to be claimed? It is possible but, of course, there’s a catch: Those diamonds must first be separated from vast piles totaling 100 million tons of pulverized waste rock. That’s the task facing De Beers Consolidated Mines (DBCM) and a variety of contractors and partner firms as they seek the very valuable needles in these enormous haystacks located at De Beers mines in South Africa. When DBCM sold Kimberley Underground to Petra Diamonds in 2010, it did not include the mountains of tailings that had piled up over 130 years of mining. Like a thrifty housewife, De Beers kept the tailings that were created from the original open-pit mine, the Big Hole, as well as the three main pipes that comprised the Kimberley Underground mine: Bultfontein, Dutoitspan and Wesselton. These mines were worked as open pits until the 1950s, when they were converted to underground operations. It is estimated that there are some 360 million tons of old tailings around the Kimberley mines. Various companies are involved in reprocessing the tailings, with the goal of unearthing diamonds that were missed the first time around. How could that happen?
|Processed diamond-bearing ore is carried by overhead conveyor belt and added to tailings pile. Photograph by Jason Lauré.|
Tailings are the residue of the diamond-bearing ore that was processed during the original mining operation. At that time, decisions were made as to the most likely size diamonds that could be extracted from the surrounding ore. Every mine has a known profile, a geologist’s estimation of the number of carats expected to be found per hundred tons of treated ore, as well as the size and type of those diamonds. Within that profile, there are exceptions, but miners process the ore according to that profile. The key to success is to crush the ore to the correct size. It’s a Goldilocks kind of process: Crushing the ore too finely means possibly destroying valuable larger stones. Conversely, setting the crusher to leave larger pieces can mean missing smaller stones. The decision is made based on the mine’s expected production but is also guided by the market value placed on different types and sizes of stones at the time. A just-right crushing strength enables the mining company to recover the greatest number of potentially profitable stones for the least cost. But times change — and technology changes, too. Stones that were considered too small to be viable in years past are now an important part of the diamantaire’s mix. A main reason: The cutting revolution in India and China that yields fully faceted tiny melee has sent mining companies back to their stockpiles of processed ore. One of the prime stockpiles now undergoing reprocessing is the mountain range of tailings generated at Kimberley, the mine that is synonymous with diamonds and mining. A De Beers spokesperson says, “Mining of the kimberlite deposits at the various pipes resulted in the creation of a large number of surface tailings resources that are scattered over a large area in and around Kimberley.” Another reason for the new interest in the tailings, according to De Beers, is that “New refinements in diamond recovery made retreatment of the Kimberley ore dumps economically viable.” Developments in such technical areas as dense-media separation and high-pressure roll crushers along with advances in sorting equipment all make it possible to economically sort for very small diamonds.
In 2004, De Beers opened a combined treatment plant (CTP) that extended the life of its surface operation at Kimberley. How worthwhile was the investment? According to De Beers, in 2013, the company recovered 815,036 carats of diamonds from 6,133,799 tons of tailings. The new plant, commissioned in 2001, is expected to continue operations beyond the year 2030. It has a capacity of processing 7 million tons of ore a year. What was deemed uneconomic for De Beers expansive corporate structure was considered very viable for a smaller company and in 2007, DBCM committed to sell Kimberley Underground to Petra Diamonds. The complexity of the sale kept the two firms from fully implementing the transition immediately. From 2007 until 2010, when the deal was finalized and Petra proceeded with full-scale mining, it entered into an agreement with De Beers for what is known as “care and maintenance.” During those three years, Petra built up its own ore stockpile of some 500,000 tons, estimated to contain 90,000 carats of diamonds. All these operations lie cheek by jowl in the Kimberley area. There may be some lovely surprises in that stockpile. Kimberley is known for producing both high-value gem-quality diamonds and fancy yellow diamonds.
At the same time as the Petra-De Beers deal was being negotiated, the South African government instituted a program called Black Economic Empowerment (BEE). The goal was to broaden the economic base of the country, while trying to redress the economic disparities and economic deprivation of the majority of South Africa’s citizens that had occurred during apartheid, which was dismantled beginning in 1991 when Nelson Mandela was released from prison. A few years later, in 2007, the government modified the program and introduced the Broad Based Black Economic Empowerment Act (BBBEE) to extend economic incentives to other disadvantaged groups. It focused not only on black South Africans but also on women, young people, the disabled and those in rural communities where there were scant opportunities for employment. De Beers complied with this new law in part by working with local groups to bring black South Africans into the mining economy, which has been the engine of growth for South Africa for more than a century. According to a De Beers spokesman, “There are several smaller-scale mining companies processing tailings from the mines in Kimberley.” Four of these companies — Super Stone, Superkolong, Sedibeng Mining and Samba Trading — have joined together to form the Small Miners Forum, also known as the Kimberley Miners Forum. This marginal material could be worked more profitably by a small group with low overhead. These small-scale miners made strategic alliances with other companies to gain access to the equipment to work the tailings and recover the diamonds. Superkolong, for example, entered into a venture with Batla Minerals, a specialist contract miner working in the field of processing tailings dumps. The firm had a history of contract mining with De Beers. In 2013, Batla announced plans to upgrade its processing plant capacity from 150,000 tons to 230,000 tons of tailings per month. The recovered diamonds are then sold via tenders. Before the upgrade, the firm was recovering up to 15,000 carats of diamonds per month. Like the diamonds found during the original recovery process, diamonds from the tailings include both industrial and gem-quality stones. At the end of the day, perhaps by 2030, all the mountains of tailings will have been reworked and presumably, most of the diamonds in them will have been recovered. The only question that remains is: What happens to the dumps? The crushed ore will still be there, just a little more depleted. One of the more exotic proposals is to put it back where it came from — into the Big Hole, the site of the original mining operation in the late 1860s, and now a tourist attraction. It would be a bit like filling in the Grand Canyon.
|De Beers combined treatment plant in the Kimberley mining area processes diamond-bearing ore. Vast dumps, the residue of decades of mining, are seen in the background. Photo courtesy De Beers.|
Article from the Rapaport Magazine - September 2014. To subscribe click here.