AngloGold Ashanti is in the business of exploring for, mining and processing gold. These activities alter the original nature of environment, and of land in particular. AngloGold Ashanti has in place closure plans at all its operations to return the land to a productive state once mining has ceased.
The process of producing gold can be divided into six main phases:
This basic process applies to both underground and surface operations.
AngloGold Ashanti’s global exploration group identifies targets and undertakes exploration, on its own or in conjunction with joint venture partners.
There are two types of mining which take place to access the orebody:
Mining activities require extensive services, both on the surface and underground, including:
Comminution is the process of breaking up ore to make gold available for treatment. Conventionally, this process occurs in multi-stage crushing and milling circuits. Modern technology is to use large mills fed directly with run-of-mine material.
Gold ores can typically be classified into:
Refractory ore treatment: after fine grinding, the sulphide materials are separated from the barren gangue material using flotation to produce a high-grade sulphide concentrate. The sulphide concentrate is oxidised by either roasting as at AngloGold Ashanti Minera??o or bacterial oxidation (BIOX) as at Obuasi. The oxidation process oxidises the sulphide minerals, liberating the gold particles and making them amenable to recovery by the cyanidation process.
Free milling and oxidised refractory ores are processed for gold recovery by leaching the ore in agitated tanks in an alkaline cyanide leach solution. This is generally followed by adsorption of the gold cyanide complex onto activated carbon-in-pulp (CIP).
An alternative process is the heap-leach process. This process is generally considered applicable to high-tonnage, low-grade ore deposits, but it can be successfully applied to medium-grade deposits where the ore deposit tonnage cannot economically justify constructing a process plant. Run-of- mine ore is crushed and heaped on a leach pad. Low strength alkaline cyanide solution is applied, generally as a drip, to the top of the heap for periods of up to three months. 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. The stripped solution is recycled to the top of the heaps.
Gold adsorbed onto activated carbon is recovered by a process of re-dissolving the gold from the activated carbon (elution), followed by precipitation in electro-winning cells and subsequent smelting of that precipitate into dor? bars that are shipped to the gold refineries.
Retreatment of tailing stockpile from previous decades’ operations is also practiced by AngloGold Ashanti. The old tailings are mined by water sluicing followed by agitator leaching in alkaline cyanide solution and recovery of dissolved gold onto activated carbon.
At AngloGold Ashanti operations, the main by-products produced are:
The tailings from the process operations are stored in designated tailings storage facilities designed to enhance water recovery.
The dor? bars are transported to a refinery for further refining, to as close to pure gold as possible. This is known as good delivery status. This gives the assurance that the bar contains the quantity and purity of gold as stamped on the bar.
In all the jurisdictions in which the company operates, it is required to conduct closure and rehabilitation activities to return the land to a productive state once mining has been completed. Additionally, the company is required to provide financial assurance, in a form prescribed by law, to cover some or all of the costs of the anticipated closure and rehabilitation costs for the operation. Rehabilitation refers to the process of reclaiming mined land to the condition that existed prior to mining or to a pre-determined post-mining use.
Closure plans are devised prior to the start of operations and are regularly reviewed to take into account life-of-mine projections. Although the final cost of closure cannot be fully determined ahead of closure, appropriate provision is made during the mine’s economic operation.
AngloGold Ashanti Annual Report 2007 – Annual Financial Statements