Preparation of ores for extraction of precious metals

Now gold and silver are extracted from primary ores: 1) by hydrometallurgical processes; 2) by combined circuits in which the various methods of concentration play the big role. As the extracted ore usually represents the large pieces up to 500 mm, they first of all are crushed and grinded.

Crushing and grinding of gold-containing ores. The aim of these processes - the full or partial release of grains of gold-containing minerals (basically particles of native gold) and reduction of ore to condition providing successful course of subsequent concentrating and hydrometallurgical processes.

Processes of crushing and grinding are the power-intensive and expensive processes (40-60 % of all charges), therefore grinding always needs to finish at that stage when precious metals are enough opened for extraction. The necessary degree of grinding should provide the good contact of solutions to the opened grains of golden and silver minerals.

The choice of the circuit of crushing and grinding depends on structure and physical properties of ore. First of all the ore is subjected to primary and secondary (average) crushing in jaw and cone-type crushers. Received material has the size 20 mm. Sometimes the third stage of fine crushing is applied, size of received material - 6 mm. The crushed material is given on wet grinding which is carried out in ball and bar mills.

Than a classifying (sizing) of crushed material is carried out. The rough classifying is carried out in drum-type sizing screen, for thinner classifying the classifiers and hydrocyclones are use.

Gold ores before hydrometallurgical processing or concentration (dressing) by flotation are subjected sludge removal if sludge has the low content of gold and negatively influences on technological operations. Hydrocyclones or thickeners are applied for sludge removal. Up to 30-40 % of poor material is removed in dump by such ways that improves the technological parameters and reduces the volume of equipment for carrying out of the subsequent operations.

Grading (sorting) and primary dressing of lump ore. Grading is used for removal from rock mass the gold poor rock and for allocation of ore fractions enriched by gold or other minerals. Removal of the dead or poor rock allows to direct on processing the richer material with the smaller content of sludge. The general rule in this case consists in that the gold content in allocated rock was less, than the gold content in tails of factory.

Hand-sorting is applied seldom and only for the lump material. The sorting conveyor tapes vibrate for improvement of inspection of pieces.

Process of float-and-sink separation is more often applied. This process is the most effective for sulphidic ores. At float-and-sink separation gold with sulfides is concentrated in heavy fractions, and enclosing rocks - in easy fractions.

Other ways of mechanization of sorting:

1. Nuclear methods (separation of low-sulphide gold ore and shale rock on the basis of difference in radioactivity of quartz, containing gold and sulfides, and radioactivity of shale);

2. Radiometric method of sorting of uranium-containing gold ores (it is based on definition of radioactivity of each piece);

3. Automatic sorting on color of pieces (when barren rock is submitted as shale with grey and black color).