Treatment of cyanide in waste heap after heap leaching

Our Liaoning gold mine located near a water source, the use of cyanide heap leaching method. In order to prevent wastewater from polluting water sources, the required discharge of sewage must meet the environmental standards of the national first-level source area. After the heap is immersed, the waste heap is repeatedly washed three times. Each time the rinse water is added with an appropriate amount of bleaching powder and calcium oxide, and the activated carbon is adsorbed to form a closed loop.
After the heap is immersed, the cyanide in the waste heap can undergo natural degradation and chemical destruction. It has been reported in the United States and has experience to follow.
The waste heap is exposed to air, especially when the pH of the solution is below 10.5, and the HCN cyanide may evaporate from the cyanide-containing solution. When the pH is 7-8, about 95% or more of the free cyanide is present in the form of HCN molecules and has a relatively high vapor pressure. Materials containing large amounts of sulfur may become acidic over time. The rate and amount of production of hydrocyanic acid gas is low. The dissipation of the wind makes this gas so thin that it cannot be measured and reaches a safe concentration. Volatilization is the reason why some cyanide naturally disappears.
First, natural degradation
Natural degradation can occur in a variety of ways, such as photolysis, oxidation, volatilization, adsorption, and biodegradation. The natural degradation of cyanide in heap leaching occurs continuously, and this destruction can make the waste heap environmentally friendly. There are various mechanisms for the breakdown or loss of cyanide in the heap. These mechanisms include: (1) bacteria, ie microbial action; (2) the action of air; (3) the action of sunlight; and (4) the action of the pile. .
(1) Microbial action
Cyanide contains two basic life elements, carbon and nitrogen. These elements in cyanide combine with a high-energy-carbon-nitrogen bond, so cyanide is an ideal food for microorganisms. When cyanide is combined with oxygen, soda ash and trace amounts of phosphorus , the microorganisms can grow vigorously, and the cyanide destruction by the microorganisms is complete. Cyanide is oxidized to form CO 2 or hydrocarbons, while nitrogen is converted to nitrogen or protein.
The biological treatment process also removes thiocyanate, free and metal complexed cyanides, metals and ammonia. Biodegradable products include sulfur, nitrogen and carbon.
(2) Air action
Oxygen in the air is an effective oxidant for cyanide. This oxidation process must have a catalyst. Many compounds are effective catalysts, and sulfur oxide or sulfite ions have recently been used as catalysts. Materials have some activity, such as activated carbon, iron oxide, manganese oxide, clay (clay), zeolites, and active silica, it can also act as a catalyst. Some of the above compounds, cyanide and air may be present in the heap leach or heap material.
4NaCN + 5O 2 + 2H 2 O → 2N 2 + 4CO 2 + 4NaOH
(3) Sunlight effect
Sunlight also acts to catalyze the reaction of oxygen and cyanide, which is especially effective. When cyanide on the surface of the heap or cyanide is sprayed into the air, it can be oxidized with cyanide under exposure to ultraviolet radiation.
Second, chemical damage
The fastest way to destroy cyanide is chemical oxidation. Many chemical agents oxidize cyanide. The most commonly used chemical reagents contain oxygen and chlorine. The oxidation reaction can be returned to the reactor after the reaction outside the reactor, or the oxidant can be added to the reactor. Chemical reagents are a major part of the cost of the destruction process.
(a) chlorination
Chlorine is a very effective oxidant for cyanide. Commonly used chlorine reagents are chlorine gas, sodium hypochlorite and calcium hypochlorite. Alkaline liquid chlorination is the most common method of destroying cyanide in solution. The liquid discharged from the heap can be chlorinated and returned to the heap for destruction of residual cyanide in the heap. Other leachates can be treated similarly. The reaction of free cyanide and chlorine is fast and efficient. Only iron complexed cyanide is resistant to chlorination.
At the end of the leaching, the leaching residue is treated with an industrial grade calcium hypochlorite solution containing 0.5 g/L, and 272.4 g of calcium hypochlorite is consumed per ton of residual slag. This method is fast and effective.
(2) Air - sulfur dioxide
Recently, INCO used air-SO 2 or air-sulfite to destroy cyanide to a relatively low level. Under the condition of pH control and a small amount of copper as a catalyst, the cyanide can be rapidly decomposed by mixing with wastewater and SO 2 air. The cyanide and metal cyanide complexes can be selectively oxidized, and the ferricyanide complex which cannot be removed by the alkaline chlorination method can also be removed. This treatment can be carried out at room temperature using only lime water, SO 2 and air, and the residual total cyanide content is between 0.05 and 1 mg/L (between pH 9 and 10). It is currently considered to be the most economical and effective method.
(3) Hydrogen peroxide
Cyanide can also be destroyed by H 2 O 2 , but the cost is too high.

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