Acidification-volatilization-re-neutralization treatment of cyanide-containing wastewater

HCN is highly volatile (boiling point 26 ° C) and is therefore easily separated from the solution by air separation, especially at low pH. In order to promote metal - cyanide complex dissociates into the applicable speed HCN, it is necessary to lower the pH.

The method consists of three steps: acid method, volatilization, and neutralization (A·V·R=AVR): (1) acidification, dissociation of CN in metal cyanide complex, and conversion to HCN; (2) strong The air bubbling volatilizes HCN, and the recovered HCN is repeatedly recycled by the lime liquid; (3) the acid-poor liquid after the aeration is neutralized to remove the metal ions (then, the metal precipitate is recovered, and the solution is discharged). The chemical process of this method is as follows:

Acidification Ca(ON) ₂ +H ₂ SO ₄ →CaSO ₄ +2HCN

Absorption 2HCN+Ca(OH) ₂ →Ca(CN) ₂ +2H ₂ O

In fact this method is carried out in a series of bubblers and absorption towers. The pH of the spent lean liquid is first lowered to 2.5 to 3 with H ₂ SO ₄ , and the acidification solution is passed from top to bottom through a stacking bubble column at any time, and the gas flow rises countercurrently from below. The HCN formed from the cyanide complex of cyanide, zinc , copper and nickel is carried into the absorption tower by the air stream. The HCN is in contact with the misty, dispersed lime slurry. The absorption tower lime liquor is repeatedly circulated until it accumulates into a cyanide concentration and returns to the cyanidation process.

In this process, ferricyanide and thiocyanate salts are not decomposed. However, the latter can be removed by precipitation with other metals released from the metal cyanide complex. A large amount of solids are formed in the spent cyanide solution, which are calcium sulphate and some calcium carbonate. Whether cyanide and thiocyanate of copper and cyanide of zinc, copper, nickel and iron are present as solids depends on the composition of the lean liquid, the final pH of the treated liquid and the contact time. Some zinc, copper, nickel, and iron may also remain in solution. The treated solution is also acidic, so to achieve metal recovery, the spent lean liquid must be neutralized and filtered prior to disposal.

The AVR method was studied by the Canadian Centre for Minerals and Energy Research and tested with lean liquids from six different gold mines with satisfactory results. At pH 2.5-3.0, after 2 to 4 hours of treatment, the total cyanide concentration was reduced to 0.1 mg/L. China's Shandong Jiaojia, Xincheng and Linglong gold mines have been used for many years. For example, in the Zhaoyuan Gold Mine Cyanide Plant, the tailings containing 1236 mg/L of cyanide can be reduced to 58.1 mg/L after acidification, and the cyanide recovery rate is 95.3%.

The advantages of this method are: (1) discharge of cyanide up to 0.1mg / L; (2) cyanide can be recycled and returned to use; (3) metal can be removed and recovered; (4) the required reagents are cheap; (5) There is no special problem with the absorbed HCN returned. Disadvantages are: (1) HCN vapor is dangerous and must be sealed; (2) Energy consumption is greater than other methods.