ABOUT THIS REPORT
About this report
The process of producing gold
|There are six main activities in which the company engages in the process of producing gold:|
1 Finding the orebody
AngloGold Ashanti's global exploration programme generates targets and undertakes exploration, on its own or in conjunction with joint venture partners.
2 Creating access to the orebody
There are two types of mining which take place to access the orebody:
- underground - a vertical or decline shaft (designed to transport people and/or materials) is first sunk deep into the ground, after which horizontal development takes place at various levels of the main shaft or decline. This allows for further on-reef development of specific mining areas where the orebody has been identified; and
- open-pit - where the top layers of topsoil or rock are removed in a process called 'stripping' to uncover the reef.
3 Removing the ore by mining or breaking the orebody
- In underground mining, holes are drilled into the orebody, filled with explosives and then blasted. The blasted 'stopes' or 'faces' are then cleaned and the released ore is now ready to be transported out of the mine.
- In open-pit mining, drilling and blasting may also be necessary to release the gold-bearing rock; excavators then load the material onto the ore transport system.
4 Transporting the broken material from the mining face to the plants for treatment
- Underground ore is transported by means of vertical and/or horizontal transport systems. Once on surface, conveyor belts usually transport the ore to the treatment plants.
- Open-pit mines transport ore to the treatment plants in vehicles capable of hauling huge, heavy loads.
- Comminution, which is the breaking up of ore to make gold available for treatment. Conventionally, this process occurs in multi-stage crushing and milling circuits. Modern technology is based on large mills fed directly with run-of-mine material.
- Gold ores can typically be classified into:
- refractory ores, where the gold is locked within a sulphide mineral and not readily available for recovery by the cyanidation process; or
- free milling, where the gold is readily available for recovery by the cyanidation process.
- Refractory ore treatment - after fine grinding the sulphide materials are floated away from the barren gangue material to produce a high grade sulphide concentrate. The sulphide concentrate is oxidised by either roasting as at AngloGold Ashanti Mineração or bacterial oxidation (BIOX) as at Obuasi. The oxidation process oxidises the sulphide minerals liberating the gold particles making them amenable to recovery by the cyanidation process.
- Free milling and oxidised refractory ores are processed for gold recovery by agitator leaching the ore in an alkaline cyanide leach solution followed generally by adsorption of the gold cyanide complex onto activated carbon-in-pulp (CIP).
- The alternative process is the heap leach process. Generally considered applicable to only high tonnage, low grade ore deposits, AngloGold Ashanti has successfully applied this to medium grade deposits where the ore deposit tonnage cannot economically justify constructing a process plant. Here, the run-of-mine ore is crushed and placed on the leach. 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 the carbon-in-solution (CIS) columns where the gold cyanide complex is adsorbed onto activated carbon. The stripped solution is recycled back 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 bars that are shipped to the gold refineries.
- The retreatment of tailing stockpile from previous decades' operations is also practised 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 major by-products produced are:
- silver, which is associated with gold in ratios ranging from 0.1 to 1 to 200:1 silver to gold
- sulphuric acid which is produced by scrubbing the off gases from the roasting plants; and
- uranium which is recovered in a process which involves initial acid leaching followed by recovery of the leached uranium onto resin and subsequent stripping with ammonium hydroxide and precipitation of crude yellow cake.
- The tailings from the process operations are stored in designated Tailings Storage Facilities designed to enhance water recovery and prevent contaminant seepage into the environment.
The gold dust is then smelted into gold bars, which are transported to a refinery for further refining, to as close to pure gold as possible - good delivery status. This gives the assurance that the bar contains the quantity and purity of gold as stamped on the bar.