Our open-pit mines – in Botswana, Canada and South Africa – are impressive feats of engineering.
This type of mining is the most common method of recovering our diamonds. It’s used when diamond ore appears near the surface or is covered by a relatively thin layer of sand, cinder or gravel. Once the ore has been exposed, we break it up before taking it to be processed.
The layout of each mine depends on the size and shape of the diamond deposit as well as the characteristics of the rock. When an open-pit mine reaches a certain depth, we may continue mining by constructing an underground mine, as we are doing at our Venetia operation in South Africa.
Underground mining is a technically complex way of extracting ore and recovering diamonds. It’s used when open-pit mining becomes uneconomic. As the open-pit excavation around a kimberlite pipe goes deeper, we balance the cost of removing waste materials against the rate of diamond recovery. If the kimberlite pipe continues to produce a high quality of diamonds, and is of a suitable structure, we may decide to mine it underground.
Occasionally, horizontal and sloping deposits are discovered, where weaknesses in the structure of the rock have allowed the kimberlite to form branches and dykes. If these occur at sufficient depth, we mine them underground. Our Snap Lake Mine in Canada's Northwest Territories, which was placed into care and maintenance in December 2015, has this kind of deposit.
We mine diamonds from the seabed at depths of 90 to 140 metres in the Atlantic Ocean off the coast of Namibia. The Orange River that carried diamonds from the centre of South Africa millions of years ago also deposited its precious cargo across the ocean floor.
Debmarine Namibia’s fleet of five specialised marine mining vessels screen material recovered from the ocean floor. These deposits are then airlifted by helicopter for further processing on shore. We use two methods for our marine operations – horizontal marine mining and vertical marine mining. Horizontal marine mining involves a seabed crawler that uses flexible hoses along the ocean floor to bring diamond-bearing gravels to the surface. Vertical marine mining employs a large diameter drill for the same purpose.
Diamonds have been carried by wind and water down rivers over millions of years. In Namibia, deposits of the resulting alluvial diamonds stretch north from the Orange River delta along the west coast.
Our alluvial mining operations, in partnership with the Namibian Government, involve four stages: exposing diamonds, holding back the sea, excavation and processing.
Diamond ore is buried under as much as 40 metres of sand, gravel and cement-like materials.
We use a variety of large-scale industrial methods to remove this material, known as ‘overburden’.
Alluvial mining takes place near rivers and the ocean, so holding back the sea is essential before we can excavate the diamond ore.
In Namibia, we mine 19 metres below sea level. We build seawalls from the excavated overburden. At the end of the mine's economic life, the ocean will quickly reclaim this land.
Once the bedrock is exposed and cleaned, we excavate diamond-bearing ore for processing.
We crush the ore to free the diamond concentrate at its core.
The final concentrate, less than one per cent of the original material, goes on to a central plant where diamonds can be seen for the first time.
Our aim is to maximise the value of our diamonds by the way we process them. The method we choose to release diamonds from their surrounding ore – usually kimberlite – depends on the properties of the diamonds and the rock. We carry out our processing operations carefully, recognising that the world’s supply is finite and shrinks with every diamond recovered.
We break and fracture the ore (the technical term is comminution) using crushing and milling processes to reduce the size of particles fed into the automated facilities that free our diamonds.
Diamonds are heavier than the material around them. Once the ore has been crushed to a manageable size, we mix it with a slurry made of finely ground ferrosilicon prepared to a specified density.
In a method known as Dense Medium Separation, we feed the ore into a cone containing fluid. This causes a vortex, and centrifugal action makes the diamondiferous material sink to the bottom, while the lighter, waste material rises to the top where it is drawn off. Further along the line, we recover the ferrosilicon for re-use. At this point, we normally repeat the crushing and separation stages to free more diamonds, usually smaller ones.
Diamonds have several properties that help us to recover them. They emit light under x-rays, enabling us to detect and separate them. They also repel water and are attracted to grease. When we mix the diamond-rich material with water and pass it over a grease belt, the diamonds stick to the grease. And they fluoresce under the laser sorters we use to concentrate the processing stream further.