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In production, the common process mainly includes six stages: ore preparation, pad construction, heap building, leaching solution preparation, liquid distribution and collection, and precious liquid processing.
The ore preparation stage mainly includes crushing, granulation and pretreatment operations, the purpose of which is to improve the ore leaching ability and heap permeability, avoid uneven liquid flow, blockage, ditch flow and other phenomena, improve the gold recovery rate and reduce the consumption of leaching solution.
According to the properties of gold ore and process requirements, different crushing process flows are implemented. Waste ore heap leaching is generally not crushed (accounting for 30%), the ore particle size after the second stage crushing is 30-50mm (accounting for 53%), and the ore particle size after the third stage crushing is 9-19mm (accounting for 17%). Practice has proved that crushing is one of the key technical links to ensure that the ore has good permeability and improve the leaching rate of gold ore.
For powder ore and ore with high clay content, granulation operation must be carried out first. Add a certain amount (3-6kg/t) of cement and lime adhesive to the ore, and add an appropriate amount of sodium cyanide solution to keep the ore at 8-12% humidity, solidify for 8-12 hours, and form a mass ore. Through granulation, the permeability of the ore pile is improved, the oxygen supply is sufficient, and the gold is dissolved by cyanide in advance, which greatly improves the leaching rate of gold.
In order to collect rich liquid and prevent solution leakage, the bottom of the yard needs to be trimmed and paved (or called bottom paving) before building the pile. The materials for building the immersion pad are mainly clay, sand, gravel, asphalt, reinforced concrete, plastic film or rubber plastic board. Depending on the material, the thickness of the cushion layer is generally 300-600mm. The longitudinal and transverse drainage ditches are paved on the cushion layer with block ore, and finally a layer of large ore is laid on the entire bottom plate to protect the bottom pad.
The purpose of pile building is to make the ore pile have good and uniform permeability and ensure that the slope will not collapse. The height of the ore pile mainly depends on the permeability of the ore pile, the content of the leaching agent in the leaching solution, and the equipment and methods for building the pile. Generally, the pile height is 3-9m, and the original ore heap leaching pile can be as high as 46m. The heap building methods include multi-pile method, multi-layer method, slope method, etc.
Water and leaching agent are formulated according to a certain formula to prepare a solution or leaching solution for leaching useful components from gold ore. Common leaching agents include sulfuric acid, nitric acid, hydrochloric acid, sodium carbonate, sodium bicarbonate, ammonium carbonate, ammonium bicarbonate, iron sulfate, chloride, sodium iodide, potassium iodide, ammonium sulfate, air, oxygen, permanganate, nitrogen oxides, hydrogen peroxide and chlorate. The formula and dosage of the leaching solution need to be determined according to the type of ore, mineral composition and chemical composition. Alkaline sodium cyanide solution is often used as the leaching solution for gold ore heap leaching.
Liquid distribution means spraying the leaching liquid evenly on the top surface of the ore pile. The liquid distribution system consists of a liquid distribution tank, a pump, a liquid infusion pipe, and a spray pipe and a sprinkler laid on the ore pile.
Taking spraying as an example, the spray pipe network uses high-strength polyethylene plastic pipes, and the nozzle uses a rotating swing nozzle with a large spray radius, large and uniform spray droplets, not easy to atomize, and easy to install. During the spraying process, one is to make the leaching liquid spray evenly on the ore pile, and the other is to achieve the required spraying intensity. After spraying, the rich liquid from the bottom of the pile flows into the liquid collection tank through the liquid ditch, and then is pumped to the workshop for processing.
Use carbon to adsorb and extract gold ore. Activated carbon becomes gold-loaded carbon due to the adsorption of gold in the ore pulp. After the adsorption operation is completed, the ore pulp containing gold-loaded carbon is sent to the carbon screen through an air lift to separate the ore pulp from the activated carbon. After screening and washing, the ore pulp is sent to the desorption electrolysis operation.
The above are the six common stages of heap leaching of gold mines. Sometimes, heap leaching can be used in conjunction with cyanidation or zinc replacement, where high-grade gold ore is sent to a cyanidation plant for treatment, while low-grade ore, off-balance ore or waste rock is treated by heap leaching.
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