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Diamond Processing Remains the Premier.

Byline: Louis W. Cope, Contributing Writer

The last few years have seen the greatest shift in the diamond market in nearly two hundred years. South Africa, the home of the world's largest diamond producer, De Beers, adopted a new mining charter to encourage black ownership in the industry; mining companies are behind a groundbreaking push to combat the raging epidemic of HIV/AIDS and eliminate "conflict diamonds," which are used to fund wars in across the continent. The most profound change, however, is coming from the outside. The diamond industry is increasingly moving away from its home in southern Africa. The newest mines are in Canada, and exploration is continuing around the world. Profound as these changes are, processing the ores remains an integral step in supplying the global jewelry market, and understanding the way diamonds are processed is essential to understanding the changes occurring around it.

In South Africa, one of the largest and most important operations is the Premier mine, a name it didn't get by accident. In 1905, the 3,100-carat Cullinan diamond was found at this site. This stone, the size of a baseball, is the largest stone ever found. The Premier mine has a habit of coming up with unusually large diamonds, so proper handling of the kimberlite material is particularly important.

Processing diamonds is an art that has been devised by De Beers through a century of experimentation, research, and practical experience. Although the company allowed E&MJ the opportunity to visit the Premier operation, taking photos of the inner workings was not allowed.

De Beers started mining Premier, 40 kilometers northeast of Pretoria, as an open pit. After the pit reached 500 meters deep, the mine went underground in 1945, using block caving from the 763-meter level. Since then, De Beers has been pulling out 4 million metric tons (mt)/year of kimberlite ore from the Cullinan pipe, the largest and richest of 12 pipes in the region. The kimberlite produces a handful of diamonds each day, 10% of which are gem quality.

Before being hoisted to the surface, the dark colored kimberlite ore is transported by 13-ton trucks to 6-ton hopper railcars to the underground crushing station. A gyratory crusher reduces the size of the ore to less than 250 millimeters, which is then sent by conveyor to the 500-meter level for hoisting.

The Premier mill is a busy place, involving more than 250 seemingly criss-crossing conveyor belts that carry material from ore crushing through diamond recovery. As complex as it is, the system recovers 97% of large and small diamonds. The ore first goes to the washing plant for trommel scrubbing and screening at a rate of 800 mt/hour.

Then the ore is crushed again, but to avoid breaking any large diamonds, the crushing is done in incremental stages, each of which uses a small ratio of reduction. The ore passes through a Symons cone crusher and McCully crusher and is screened into four size fractions. The largest fraction, more than 60 millimeters, is returned to the cycle for further crushing; the 32- to 60-millimeters goes to the X-ray machines, which look for large-sized diamonds. Some of the material goes to an intermediate interparticle pressure roll crusher, made by Krupp-Polysius. The material below 32 millimeters is sent to the dense media separation plant for recovery of smaller stones; and the slimes are sent to the tailings dam.

Using of X-rays for diamond recovery has been a standard for decades. The technique works because a diamond will floresce for a few seconds when exposed to an X-ray beam. An optical scanner detects the stone, which is then removed from the conveyor belt and sent to the sorthouse for diamond removal.

The smaller-sized ore from the crusher is sent directly to the dense media separation plant for further scrubbing and screening, where it is further reduced in size. The heavier material is concentrated in a slurry of magnetic ferrosilicate particles, which can be removed and reused.

The tailings are also recrushed through a shorthead cone crusher and pumped to a wet cyclone, where the heavy material is separated for diamond recovery and the overflow is screened again. The coarse fractions, with the diamonds removed, goes to tailings and the fine fractions are sent to the slimes.

At the recovery plant, as much as 10 mt/hour of concentrates from various parts of the mill pass through X-ray machines and over grease tables, where diamonds are separated. The grease is then carefully burned away from the diamonds, which are sorted out by hand behind closed doors, under utmost security.

C-Cut Delayed

The health of the U.S. economy is very important to the global diamond jewelry trade, as Americans account for half of all purchases. Changes in economic growth rates and consumer confidence can have a big impact on the industry.

In addition to the many changes in the diamond industry, De Beers has witnessed a lot of changes of its own. Anglo American, the Debswana Group, and Central Holdings Group bought up all the shares of De Beers in 2001, making it a private company. The privatization included a reshaping of the Central Selling Organization, the branch of the company that sold all its diamonds, into the Diamond Trading Company. These events took place at the same time that stock markets were falling around the world. While this has not immediately affected the bottom line, it has forced De Beers to scale back many of its operations. Sales of rough stones topped $2.8 billion in the first six months of the year, and carat production is climbing, replenishing the supply that had fallen due to a brisk Christmas season last year. Nevertheless, De Beers has decreased throughput levels at its operations by 5%. It put the Snap Lake project in Canada on hold, as well as the C-Cut project at the Premier mine.

The C-Cut project is the next step in the development of the Premier. De Beers completed a detailed feasibility study for deeper block caving of the Cullinan pipe, bottoming at 1,100 meters below the surface. While the $800 million project has been delayed, it is expected to mine 9,000 mt/day and recover 126 million carats over 13 years. Production would begin five years after the company gives the go-ahead. Under the proposed plan, De Beers would sink two shafts from the surface, and a winze from the 763 Level to the 1,100 Level, which later would be a ventilation shaft. The company would also construct a newer, more modern processing facility to treat the larger tonnages. In addition, the coarser tailings will be reprocessed to recover much of the diamonds lost during a century of operations.

Keeping Up with the Changes

In the last century, the Premier mine has seen considerable change within the operation and from the outside world. The most recent changes, however, promise to herald a new direction in the industry. For decades, black participation has been limited to the lowly, hazardous jobs down in the mine or in the plant. The recent mining charter, published by the South African government, plans to give blacks a greater stake in the industry. The charter seeks to put blacks in the boardroom, through encouraging investment in the industry by black-owned businesses. It hopes to relieve the burden of poverty from the population by encouraging an entrepreneurial spirit.

De Beers and Anglo American have expressed support of the charter, which calls for black participation in the industry to reach 15% in five years. In ten years, black-owned businesses should control 26%. De Beers and other companies in the Anglo American family also have taken a leading role in providing anti-retroviral drugs for its workers infected with HIV/AIDS. In January, the companies expect to begin a two-year pilot program to administer the medications to their workers who are infected with the virus free of charge. By the end of this year, the diamond industry will implement a program, called the Kimberley Process, to certify diamonds, stamping out the trade in "conflict diamonds," which have been tarnishing the industry's image. The illegal trade in diamonds, representing only 4% of the global market, has been cited as providing millions of dollars to buy weapons for wars across the African continent, which have had a devastating effect on the human and animal populations.

These changes will have a permanent affect on the diamond market, though the transfer of knowledge becomes critical as the business becomes more and more global. Understanding the basics behind diamond processing will allow the industry to flourish around the world.
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Title Annotation:Premier mine, near Pretoria, South Africa
Author:Cope, Louis W.
Publication:E&MJ - Engineering & Mining Journal
Geographic Code:6SOUT
Date:Nov 1, 2002
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