There are two main types of pipe mining: open-pit and underground mining.
Open-pit mining involves removing surface layers to expose the kimberlite, which is then blasted—each blast breaking around 3,000 tonnes of ore. Excavators load the ore into trucks that transport it to a crusher for processing. The Kimberley Big Hole is a well-known example.
Underground mining requires tunneling to reach the kimberlite pipe. Two tunnel levels are built, with funnels connecting them. Ore is blasted on the upper level, falls through the funnels, and is collected on the lower level, then brought to the surface for extraction.
Over thousands of years, the kimberlite pipe that reaches the Earth’s surface is eroded and weathered by wind, rain, rivers and streams.The eroded kimberlite bears rough diamonds, which are carried downstream. The first diamonds discovered in South Africa were from alluvial deposits.
Today, industrial alluvial mining involves building a large wall to collect the water in one area. Diamonds are often found in the gravel layer, which collects under layers of other material, such as mud, clay and underwater plant-life. Once the gravel is collected, it is hauled to the surface and prepared for processing.
Marine mining involves extracting diamonds from the seabed, hundreds of meters under water. The earliest form of marine mining entailed shore diving, where a swimmer would collect diamond bearing gravel from the shallow seabed.
Today technology has evolved to specialised ships that mine for diamonds deep out at sea. These specialised ships use a powerful crawler that sucks gravel on the seabed up through flexible hoses/pipes. Alternatively, they use a large scale drill mounted to the ship to excavate diamonds.
The coast of Namibia is the richest known source of marine diamond deposits which account for approximately 64% of Namibia’s total diamond production.
Once the diamond bearing ore and gravel is collected, it is transported to a primary crusher. The primary crusher is responsible for reducing the size of the ore into smaller, more manageable pieces or chucks measuring no larger than 150mm. A secondary crusher, known as a roll-crusher, may also be used to reduce the size of the ore even further.
In this stage, the ore are scrubbed to remove loose excess material and is screened. Material smaller than 1.5mm is discarded because it is too costly to extract diamonds from such a small piece of ore.
During this stage the diamond bearing ore is mixed with a solution of ferrosilicon powder and water, which is measured to a specific relative density. This solution is fed into a cyclone, which tumbles the material and forces a separation. Materials with a high density sink to the bottom, which results in a layer of diamond rich concentrate.
Here, the diamond rich concentrate is put through a series or processes involving magnetic susceptibility, X-ray luminescence and crystallographic laser fluorescence that are calculated based on the specific properties of diamonds. These processes are designed to separate the rough diamond from any other heavy density materials collected by the cyclonic separation plant. For example, diamond’s fluorescence when exposed to an X-ray. Sensors detect flashes of light emitted by the diamond and sends a signal to a microprocessor that fires a blast of air at the diamond, which spits it into a collection box.
The diamonds collected in the recovery process are cleaned in an acid solution, washed, weighed and packaged in sealed containers for transport. In accordance with the Kimberley process, these containers are sealed with a tamper resistant seal, numbered on site, and a certificate of origin is issued.
