Alluvial mining

The process involves removing the overlying barren ground, digging up the bearing ground, and then extracting the diamonds.

Overburden removal
The first step in the mining process is to remove the overburden – the surface material covering the diamonds. This can be anything up to 40m thick and can vary from loose sand to cemented conglomerate.

Various methods and equipment are used for this process:

• Conventional stripping using bowl scrapers and bulldozers, although flexible, it is a costly system
• ADT stripping uses excavators and articulated dump trucks (ADTs) and is flexible and cost-effective
• Stripping by bucket-wheel excavator
• Stripping by dragline, although the lowest-cost stripping method, reduces flexibility as the dragline is cumbersome

Ore excavation and bedrock cleaning
Most of the ore is excavated by trackdozers and excavators. They create windows from which front-end loaders remove ground and place into ADTs and haultrucks.

The bedrock is then cleaned by employees using pneumatic breakers and a containerised vacuum cleaning system. In areas where cemented conglomerates occur, it is necessary to drill and blast.

For lightly cemented areas, we use hydraulic impact hammers to separate the diamond-bearing material from the bedrock, or we use the trackdozers’ ripping tool to break it up.

Once excavated, the ore is transported to ore processing plants where it is crushed to pieces smaller than 150mm. A process of dense medium separation, using ferrosilicon as the medium, produces a diamond-rich concentrate from the crushed gravel.

The final concentrate represents around 1% of the material originally fed to the plant. The concentrate is sent to a central recovery plant, which produces the final 100% diamond product.

Dewatering
As alluvial mining takes place close to rivers and/or the ocean, water has to be drained from the working area. At Namdeb, for instance, beach walls help us to mine the area between the original sea low-water mark and the original high-water mark. 

These walls prevent wave erosion and are created and maintained by dumping overburden sand onto a wall and then pushing it out to sea with trackdozers. Mining now takes places some 300m beyond the original high-water mark and 19m below mean sea-level.

In areas where the water table is above the diamondiferous horizon, we install a system of well points along the landward side of the beach wall.  This stabilises the wall and reduces seepage into the working area.

Further dewatering is carried out by conventional pumps and this allows the mining operations to proceed below sea-level.

Surf Zone Mining
When we mine the surf zone (the area 30m from the high-water mark) we scour out gullies in the zone. The process is weather-dependent and forms a small part of the total mining volume.  Generally, contractors are used to mine the surf zone.

Dredging
Dredging is used to remove overburden in areas with very wet ground conditions. The dredge is floated in an initial pond and then moved into the mining block where its cutter suction head excavates down to the diamondiferous material and the bedrock.

This generally takes place down dip, which allows us to control the water level and use conventional bedrock cleaning once the water level has dropped sufficiently. The dredge discharges on the seaward side of the seawall, effectively “pushing” back the sea and resulting in more ore reserves becoming accessible.

We pump water from the sea to supplement water for the dredge pond. The dredge is also equipped with a floating treatment plant to treat low-grade diamond-bearing overburden.

De Beers alluvial mining operations include:

• Namaqualand (South Africa) - opencast and surf zone mining of large-scale alluvial deposits  
• Elizabeth Bay (Namibia)
• Mining Area 1 (Namibia)
• Oranjemund (Namibia) – Orange River mines

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