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Gold production

ANGLOGOLD ASHANTI’S CORE BUSINESS IS THE PRODUCTION OF GOLD

Gold production can be divided into six main activities supported by mine planning, engineering services, ventilation, rock engineering, procurement, finance, social and environmental services and human resources, among others. The six core production processes are:

1. Exploration – Finding the orebody

AngloGold Ashanti’s exploration work is split into two functions. The company’s greenfield exploration team identifies and evaluates targets on its own or in conjunction with joint venture partners. The brownfield exploration team is responsible for identifying the limits of known deposits or finding additional deposits close to existing operations to facilitate organic growth. All discoveries undergo a well structured and intensive evaluation process aimed at improving confidence in the Mineral Resource and Ore Reserve estimates before developing or expanding the mine.

2. Development – Creating access to the orebody

Two types of mining are used to access orebodies:

  • Underground mining: a vertical or decline (inclined) shaft is sunk deep into the ground to transport people and mining materials to underground levels from which the orebody is accessed through horizontal tunnels known as haulages and cross-cuts. Further development is then undertaken to open the orebody so mining can take place; and
  • Open-pit mining: this method is employed when ore lies close to surface and can be exposed for mining by stripping overlying, barren material.

3. Mining – Removing the ore

In underground mining, holes are first drilled into the orebody, filled with explosives and blasted. The blasted ‘stopes’ or ‘faces’ are then cleaned and the ore released by blasting is then ready to be transported to surface.

In open-pit mining, the material may be ‘free digging,’ although drilling and blasting is usually necessary to break the ore and waste prior to transportation. Excavators then load the material onto haul trucks which transport the material to the plant, ore stockpiles or waste dump facility.

4. Transportation – Moving broken material from mining face to the plant

Underground material is brought to surface by a combination of horizontal and vertical transport systems. Once on surface, ore is transported to the processing facilities by surface rail or overland conveyors and waste material is deposited on low grade dumps.

In open-pit operations, haul trucks deliver ore directly to the processing facilities.

5. Processing – Treating the ore to recover the gold

Liberation is the first step in processing and involves breaking up ore, which is delivered as large rocks, into small particles so contained gold minerals are exposed and available for recovery. This is usually undertaken by a combination of multistage crushing and milling circuits with associated screening and classification processes to ensure that material of the correct size is removed promptly from the milling circuit. Coarse, liberated gold particles, which may not dissolve fully during the cyanide leach process, are removed by gravity concentration during milling with the resultant concentrate undergoing separate processing.

Recovery of gold can then commence, depending on the nature of the gold contained in the ore.

There are two basic classes of ore:

  • Free-milling: where gold is readily available for recovery by the cyanide leaching process; and
  • Refractory ores: where gold is not readily available for leaching because it is locked within a sulphide mineral matrix (e.g. pyrite), extremely finely dispersed within the host rock (not yet exposed), or alloyed with other elements which retard or prevent leaching (e.g. tellurides).

Free milling and oxidised refractory ores are processed for gold recovery by leaching ore in agitated tanks in an alkaline cyanide leach solution which dissolves the gold. This is generally followed by adsorption of the dissolved gold cyanide complex onto activated carbon at a significantly higher concentration. In some operations, the gold bearing solution is filtered from the pulp and gold is then precipitated by the addition of zinc dust.

Refractory ores undergo pre-treatment to make them more amenable to cyanide leaching. This commonly takes the form of separating the gold-bearing sulphide materials from the barren gangue material by using flotation to produce a highgrade sulphide concentrate. The sulphide concentrate is then oxidised by either roasting – as at AngloGold Ashanti Córrego Do Sítio Mineração; bacterial oxidation (BIOX) – as at Obuasi; or in pressure oxidation units. This oxidation destroys the sulphide matrix and exposes the gold particles thereby making them amenable to recovery by the cyanidation process.

An alternative to the milling and leaching process is the heap leach process, generally applicable to high-tonnage, low-grade ore deposits. It can, however, also be successfully applied to medium-grade deposits where smaller ore deposit tonnages cannot economically justify a capital-intensive milling and leaching plant. In this process, ore is simply crushed to a coarse size and heaped on a lined leach pad. Low-strength alkaline cyanide solution is dripped onto the heap for periods of up to three months. The gold dissolves and the 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 barren solution is refreshed and recycled to the top of the heap.

Gold which has loaded (adsorbed) onto activated carbon is recovered by a process of re-dissolving it from the activated carbon (elution), followed by precipitation in electro-winning cells and subsequent smelting of the precipitate into doré bars, which have a gold content of between 85% and 95%. These bars are shipped to gold refineries for further processing. Valuable by-products are generated during the gold recovery process at certain AngloGold Ashanti operations. These by products are:

  • Silver, which is associated with the gold at some of our operations;
  • Sulphuric acid, which is produced from the gases generated during sulphide roasting; and
  • Uranium, which is recovered in a process which involves sulphuric acid leaching, followed by recovery of the leached uranium onto resin and subsequent stripping of the resin by sulphuric acid and precipitation of ammonium diurinate (yellow cake) using ammonia. Uranium oxide is then produced by calcination (heating) of the yellow cake.

Residue from processing is pumped to well-designed tailingsstorage facilities, where the solids settle to form a beach, while the water is reused.

6. Refining – Preparing the gold for market

The doré bars are transported to a precious metal refinery, where the gold is upgraded to a purity of 99.5% or greater, for sale to a range of final users. High-purity gold is referred to as ‘good delivery’, which means it meets the quality standards set by the London Bullion Market Association and gives the buyer assurance of its gold content and purity.