This section illustrates our key sustainability issues during the life cycle of mining and processing gold.
Identifying targets and undertaking exploration.
Two types of mining take place: underground mining – a vertical or decline shaft is sunk deep into the ground; and open-pit mining – where the top layers of topsoil or rock are removed to uncover the reef.
In underground mining, holes are drilled, filled with explosives and then blasted; stopes are cleaned and ore is then transported. In open-pit mining, drilling and blasting may be necessary to release the gold-bearing rock while excavators load the material onto the ore transport system.
Underground ore is transported by means of vertical and/or horizontal transport systems, while open-pit mines transport ore in vehicles capable of hauling large, heavy loads.
Mining activities require extensive services, both on the surface and underground, including:
Comminution (the process of breaking up ore) occurs in multi-stage crushing and milling circuits, or large mills fed directly with run-of-mine material. For refractory ores (gold locked within sulphide mineral and not readily available for recovery by the cyanidation process) sulphide material is separated using flotation. Sulphide concentrate is oxidised by roasting or bacterial oxidation, to liberate gold. Ore is agitated in an alkaline cyanide leach solution, followed by adsorption of the gold cyanide complex onto activated carbon-in-pulp (CIP). In the heap-leach process, run-of-mine ore is crushed and heaped on a leach pad; low strength alkaline cyanide solution is applied to the top for up to three months, and the dissolved gold-bearing solution is collected from the base of the heap and transferred to carbon-in-solution (CIS) columns where the gold cyanide complex is adsorbed onto activated carbon. Gold is recovered by redissolving gold, followed by precipitation in electro-winning cells and smelting of precipitate into dor? bars.
Dor? bars are transported to a refinery for further refining, to as close to pure gold as possible.
Once mining has been completed, operations are ‘closed’ and rehabilitation activities begin to return the land to a productive state. (Rehabilitation is the process of reclaiming mined land to the condition that existed prior to mining or to a pre-determined post-mining use.) Planning for this process is undertaken during the life of mine.
AngloGold Ashanti Annual Report 2006 - Report to Society