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As we all know, alluvial gold usually refers to the gold in sand. But how to extract gold from sand? Compared with the rock gold, the method of extracting gold from sand is simpler. The mining and extraction of gold from sand are usually carried out at the same time. The process of extracting gold from sand mainly consists of three steps: crushing and screening, de-sludge stage, and processing stage.
Most of the alluvial gold contain cemented mud, which is attached to the gravel or pebbles. If it is not broken in advance, it will cause gold loss during the screening process. Moreover, because of their different particle sizes, if the particle size is larger than 100 mm, they will be discharged along with the waste rock during the screening process, which also causes the loss of gold. So the crushing stage is very important.
On gold mining vessels, crushing and screening operations are usually carried out simultaneously in the trommel. The trommel is internally provided with intermittent spiral angle steel. When working, the washing water pressure inside the trommel should be no less than 35 kPa.
Screening process can remove 20%-40% of waste rock (gravel, pebbles), which is an essential part of gold ore dressing. Screen parameters required for screening operations often require reference to the particle size of the gold. At present, the screen holes selected by many gold mines are generally 10-20mm. If the chute is used for coarse sorting equipment, the screen hole can be larger, but generally does not exceed 60 mm. The equipment used in different types of gold extraction plants is different, the screening equipment used in fixed gold extraction plants is mostly vibrating screen, while the trommel is used in gold mining ships. Generally, water should be added in the screening operation, and its flushing capacity should be determined according to the requirements of ore washing, and should be meet the requirements of concentration in the next stage of separation operation as far as possible. For example, when using a chute for rough processing, the amount of flushing should be 8-14 times that of sand. Flushing on the sieve can not only improves the screening efficiency, but also further break down the cement.
Materials with particle size less than 0.1 mm in alluvial gold generally contain little or no gold with particle size less than 0.1 mm is commonly known as floating gold. This kind of gold is difficult to recover in the separation process, and will play a certain interference role in the extraction process. Therefore, in an alluvial gold extraction plant, it is necessary to try to remove the slime less than 0.1 mm in size. The de-sludge equipment commonly used in actual production is the mud hopper of various specifications. However, when gold is processed by chute, the particle size of materials allowed is larger and the processing capacity is larger, so the de-sludge stage may not be performed before the chute.
Practice has proved that the most effective and economical method to extract gold from sand is gravity separation method. Due to the different particle size composition of gold in the alluvial gold mine, the effective grain size limits of the various gravity separation equipment are different, so a reasonable alluvial gold processing line should be a joint operation of several gravity separation equipment. There are currently three methods for the separation of gold-bearing concentrates:
(1)Discard the heavy sand after manually washing out the gold particles with a gold pan;
(2)Carry out internal amalgamation with the amalgamation cylinder, and discard heavy sand after obtaining the amalgam;
(3)After manual elutriation or gold extraction by amalgenation of mercury, heavy sand shall be concentrated and sent to a concentrate for treatment, and various heavy sand minerals shall be recovered by magnetic separation, electrostatic separation and other methods.
The above are the main methods to extract gold from sand. At present, the process of extracting gold from sand has been relatively mature, and the recovery rate can reach more than 90%.