Example of treatment of cyanide-containing industrial wastewater by acid chloride

China Gold Corporation Jinchangyu Gold is a gold mine owned mining Xuanye comprehensive daily processing 1000t ore, gold extraction process ten gold flotation concentrate cyanidation process, the daily processing of gold concentrates 30t, cyanide The process is two dip and two washes. The bottom washing slurry of the secondary washing thickener is the final cyanide tail. After the cyanide tailing gold selection operation, it is discharged into the cyanide tail clarifier. The cyanide-containing sewage is mainly derived from the cyanide tailing liquid and the partial replacement lean liquid. Cyanide clarification liquid has about 120m ³ per day, cyanide concentration is 200mg/L, replacement poor liquid discharges about 50m ³ per day, cyanide concentration is about 550mg / L, so about 170m ³ per day, cyanide concentration is about 300mg / The cyanide-containing wastewater of L needs to be purified.

1. The process of sewage treatment in Jinchangyu Gold Mine

In 1970, the cyanide workshop was put into operation. The original design wastewater treatment process was bleaching powder oxidation. Due to the large consumption of bleaching powder (22.5kg/m ³ ), the sewage treatment cost was high and the cyanide removal effect was poor. Therefore, in 1972, it was changed to an open-circuit alkaline liquid chlorine process for the treatment of cyanide-containing sewage. It has been proved that the process is open, the reaction is incomplete, the oxidation efficiency is low, and the sewage can not reach the standard. In 1979, the open circuit treatment process was changed. Process for large cycles. In 1986, cyanide tailings were added to the cyanidation process. The activator was used in the flotation operation, so that the amount of sodium cyanide in the cyanidation operation increased correspondingly. The cyanide concentration in the sewage increased, the nature of the sewage deteriorated, and the chlorine consumption in the sewage treatment increased to 10.62 kg. /m ³ , residual chlorine 1280.3mg / L, the treatment cost increased. In view of the problems of the lye chlorine method, after analysis, the small test proposed the acid liquid chlorine method to remove cyanide.

Second, acidic liquid chlorine treatment of cyanide-containing sewage can reduce chlorine consumption

(1) Removal of cyanide by liquid chlorine method is mainly based on oxidation of chlorine gas after hydrolysis

The chlorine gas has a solubility in water of 0.091 mol/L at room temperature, wherein it is 0.061 mol/L in the form of a solvent and 0.03 mol/L in the form of hypochlorous acid.

Hypochlorous acid is a weak acid that balances in aqueous solution:

HClO=ClO ^- +H ⁺ K=3.4×10 ^-8

HClO solution and ClO ^- proportional to the pH dependent: When at pH <. 5, virtually not dissociate HClO, HClO mainly in the molecular form; when the pH = 7.5, about 50% dissociation of HClO ClO ^-; When pH > 10, HClO is almost completely dissociated into ClO ^- .

It can be seen that in the treatment of cyanide-containing wastewater, alkaline chlorination is used, and the solution is mainly ClO ^- oxidation. If acidic conditions are used, mainly HClO is oxidized. HClO is a neutral molecule, a negatively charged than ClO ^- can be more easily diffused to the negatively charged cyanide (the CN ^-) group to accelerate the oxidation of cyanide. Therefore, acidic conditions can reduce chlorine consumption.

(two) from the electrode potential

OH ^-

Under alkaline conditions: ClO ^- +H ₂ O+2e→Cl ^- +2OH ^- E ⁰ = 0.89V

H ⁺

Under alkaline conditions: HClO+H ⁺ +2e→Cl ^- +H ₂ O E ⁰ =1.49V

Strong oxidizing ability ^- so, HClO under acidic conditions than under basic conditions of ClO.

(3) From the mechanism of removing cyanide

Cyanide containing waste water, there is a large amount of thiocyanate (1986 CN2166.7 Analysis Total CN ^- from as ^{575.7, SCN - 1228mg / L)} . Chlorine is the order of addition of cyanide: first reacted with a free cyanide, and then with SCN ^- At the end, the reaction with the cyanide complex.

Thiocyanate is a relatively stable group and is not easily decomposed. The cyanide-containing wastewater is treated under alkaline conditions. In order to remove the cyanide, it is necessary to first remove the thiocyanate, which increases the difficulty of removing cyanide.

Under acidic conditions, the complexed cyanide compound dissociates into a simple cyanide, exemplified by Zn(CN) ₄ ^{2 -} .

_{^{Zn (CN) 4 2 - +}} 4H + → Zn 2 + + 4HCN K = 10 -16.9

After the cyanide dissociation, the wastewater is mainly in the form of free cyanide (in this case, SCN ^- and some metal cyanide precipitate). This simplifies the cyanide composition in the wastewater. Therefore, the effective time of HClO formed by hydrolysis of chlorine gas can be fully utilized, the oxidation of cyanide can be accelerated, the action rate and oxidation efficiency of effective chlorine can be improved, and the chlorine consumption can be reduced.

Third, the industrial practice of acid liquid chlorine cyanide removal process

The acid liquid chlorine cyanide removal process is mainly composed of two parts, the acidic phase and the alkaline phase, and the chlorine is added in parallel by 5 chlorinators. The acid phase is in a fully enclosed form and is divided into three operations of mixing-oxidation-mixing. The oxidative decomposition process of cyanide under acidic conditions is completed. It has been proved that the cyanide efficiency is above 98%, and the acidic condition is controlled to control pH<5.

The alkaline phase consists of two operations of absorption-complete oxidation, mainly controlling the pH above 10, in order to absorb the cyanogen chloride gas generated under acidic conditions.

The cyanide removal process is carried out by the acidic liquid chlorine method, the chlorine consumption is 3.36kg/m ³ , the lime consumption is 15kg/m ³ , the chlorine consumption is about 68.3% compared with the alkaline liquid chlorine treatment process, and the lime is saved by about 57.1%. The residual chlorine in the waste liquid is generally below 300mg/L, and the treated sewage completely meets the national emission standards.

Fourth, the economic effect comparison

The economical effects of the treatment of cyanide-containing wastewater by the alkaline liquid chlorine method and the acidic liquid chlorine method in Jinchangyu Gold Mine are shown in Table 1.

Table 1 Comparison of the economic effects of the two processes

The nature of cyanide-containing wastewater in Jinchangyu Gold Mine is relatively complicated. The original process has large chlorine consumption and high cost, which is not suitable for the change of sewage properties. The acid liquid chlorine method has been successfully applied in the mine to achieve continuous production and one qualified discharge; reduce chlorine consumption by more than 50% and lime by 50%; the residual chlorine in the treated waste liquid is 300mg/L lower, and the external drainage is prevented. Secondary pollution reduces unit processing costs, avoids chlorine and cyanide, and prevents air pollution in the workshop.