Ore from the open pit is delivered to the process plant by haul truck. Argyle's process plant is one of the most efficient in the world. Over time it has been upgraded and is now able to process up to 11 million tonnes of ore per annum.
Diamonds are liberated and recovered from the host lamproite ore in six stages:
- The mined material goes through a primary crusher that reduces it to a maximum size of 150mm. Approximately 2,500 tonnes of ore are dumped into the primary crusher each hour. The secondary crusher's product is conveyed to the primary stockpile, which has a capacity of 10,000 tonnes.
- The ore is extracted from the primary stockpile at a rate of 1,500 tonnes per hour. It is conveyed to high pressure rolls-crushers, where its maximum size is reduced to 30mm.
- Ore is then scrubbed and screened, where it is separated into three sizes. Oversized material (larger than 15mm) is further reduced, and undersized material (smaller than 1.5mm) is rejected to the tailings dump, as diamonds in this undersized ore cannot be extracted profitably. Ore between 1.5mm and 15mm is conveyed to a Heavy Media Separation (HMS) Plant Feed Stockpile.
- The majority of the material in the HMS stockpile is lamproite ore. It contains diamonds and some other high density minerals. The ore is then processed in a cyclonic separation plant. Heavy media consisting of ferrosilicon powder mixed with water is used to separate the lamproite ore from the diamonds and heavy minerals which sink to provide a diamond rich concentrate.
- In the recovery plant, the diamond concentrate is fed through a series of custom-designed x-ray sorters. Diamonds fluoresce when exposed to x-rays. Sensors detect the flashes of light emitted by the diamonds. These send signals to the microprocessor that fires an air blaster valve at the appropriate moment, blowing the diamonds into a collection box.
- The diamonds are acid-cleaned, washed, weighed and transferred to Rio Tinto Diamonds’ central sales and marketing organisation in Antwerp, Belgium where they are prepared for sale.
The future of mining at Argyle, and plans for the development of an underground mine beyond 2010
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