“There’s gold in them thar hills”1
The retreatment business is high-volume and low-risk. Vast quantities of material are processed monthly through the plant in order to recover gold from old mine dumps at a recovery rate that varies depending on the material being treated. As each old dump or dam is depleted, others are brought on stream. Teething problems are inevitable during the commissioning of new dumps and decommissioning of exhausted reclamation sites, and these can negatively affect recovery rates in the short term. Excessive rain can also affect recovery rates although DRDGOLD has taken the appropriate measures to better manage weather-related issues in future.
Ergo Gold Mining Proprietary Limited (Ergo), one of the world’s largest gold surface tailings retreatment facilities, owns the rights to treat around 100 sand dumps and slimes dams on the Witwatersrand. Some of these are currently being treated.
Sand and slime
Sand dumps are the result of the less efficient ‘stamp-milling’ process employed in the early gold mining years. They consist of coarse-grained particles which generally contain higher quantities of gold. Sand dumps are reclaimed mechanically using front end loaders that load sand onto conveyor belts. The sand is fed onto a screen where water is added to wash the sand into a sump, from where it is pumped to the plant. Most sand dumps have already been retreated using more efficient milling methods.
Slimes dams are also the result of older treatment methods, although they are more recent than sand dumps, and contain lower grades of gold. However, this material has become economically more viable to process owing to improved treatment methods and a higher gold price. The material from the slimes dams is broken down using monitor guns that spray jets of high pressure water at the target area. The resulting slurry is then pumped to a treatment plant for processing.
1 Attributed to the assayor of the Dahlonega Mint in 1849; Dahlonega was the site of the first major gold rush in the United States. Source: Wikipedia