Study on the Treatment of Cyanide-ContainingWastewater from Gold Mine by Sodium Metabisulphite /AirMethod and Na2S Precipitation Method

The cyanidation method is the most widely used gold extraction method at home and abroad. In this process, cyanide is used in large quantities, so a large amount of cyanide-containing wastewater and cyanide-containing tailings will be produced in the gold production process.

Cyanide is a highly toxic substance, and a very small amount can kill organisms. When the concentration of cyanide in water (calculated as CN-) is 0.04～0.1 mg/L, it can kill aquatic organisms, such as fish, plankton and crustaceans.

Therefore, the cyanide-containing wastewater must be treated to reach the standard before it can be discharged into the sewer or stream. The Chinese national standard GB 8978-1996 stipulates that the total cyanide and copper ion discharge standards of wastewater are both below 0.5mg/L.

The principle of the sodium metabisulphite/air method is: under the condition that the wastewater contains copper ions and the pH is 7-10, the SO2 -3 produced by the hydrolysis of sodium metabisulphite and the oxygen in the air work together to oxidize cyanide, so that CN- is oxidized to CNO- and CNO- are then hydrolyzed to generate NH3 and HCO-3 to achieve the purpose of removing cyanide.

The reaction equation is as follows:

The method for treating cyanide-containing wastewater can not only remove free cyanide and most metal cyanide complexes, but also treat ferricyanide complexes and ferrocyanide complexes that are difficult to treat by other methods. In addition, the method has a rapid reaction process, a wide range of pharmaceutical sources required, and low treatment costs, and the treated wastewater can meet the discharge standard.

Experiments show that:

1. The influence of PH value

As the pH rises, the concentration of Cu(CN)3 -4 and CN- in the solution gradually increases and accounts for a larger proportion. They are the main cyanide-containing substances that are oxidized. Therefore, as the pH rises, the total cyanide is removed The effect gradually increases, but when the pH is too high, the concentration of Cu(CN)2 -3 is too low, which weakens the catalytic effect of Cu(CN)2 -3.

2. The influence of sodium metabisulphite dosage

With the addition of sodium metabisulphite, the removal rate first increases and then decreases. When the dosage of sodium metabisulphite is less than 6g/L, as the dosage of sodium metabisulphite increases, the remaining total cyanide concentration in the solution after treatment gradually Decrease, the removal rate of total cyanide gradually increases; when the dosage of sodium metabisulphite is greater than 6g/L, as the dosage of sodium metabisulphite increases, the concentration of total cyanide remaining in the solution after treatment gradually increases, and the total cyanide The removal rate of cyanide is gradually reduced; when the dosage of sodium metabisulphite is 6g/L, the treatment effect of cyanide-containing wastewater is the best. After the dosage of sodium metabisulphite is greater than 6g/L, the removal rate will decrease with the increase of dosage. This is because excessive sodium metabisulphite will consume the [O] reaction produced during the reaction, thereby reducing [O] and CN— The reaction reduces the utilization rate of sodium metabisulphite, which reduces the treatment effect of cyanide .

3. The influence of stirring speed

As the stirring speed increases, the removal effect of cyanide-containing wastewater first increases and then decreases. The best stirring speed is 1600r/min (aeration column is 100r/min); with the increase of pH, the removal effect of cyanide-containing wastewater is first enhanced The optimal pH is 9.0; with the increase of sodium metabisulphite dosage, the removal effect of total cyanide first strengthens and then weakens, and the optimal dosage is 6g/L; with the increase of the aeration, the total cyanide removal effect is 6g/L; The removal effect of cyanide is first enhanced and then weakened. The best aeration amount is 130L/min; with the increase of the residence time, the removal effect of total cyanide has been enhanced, but the removal effect will not change after the residence reaction time exceeds 1h; as the water temperature rises , The removal effect of total cyanide is gradually enhanced, and the best water temperature is 30℃.

4. Impact at best

As the amount of aeration increases, the removal rate of total cyanide first increases and then decreases. When the amount of aeration is less than 130L/min, the concentration of remaining total cyanide in the solution after treatment gradually decreases, and the removal rate of total cyanide gradually Increase; when the aeration volume is greater than 130L/min, the concentration of the remaining total cyanide in the solution after treatment gradually increases, and the removal rate of total cyanide gradually decreases; when the aeration volume is 130L/min, the treatment effect is the best

5. Response time

As the residence time increases, the removal effect of cyanide-containing wastewater first increases and then remains unchanged. When the residence time is less than 60 minutes, the concentration of total cyanide remaining in the solution after the dissolution reaction decreases, and the removal rate of total cyanide increases; After the residence time exceeds 60 minutes, the concentration of total cyanide remaining in the solution after treatment remains unchanged, and the removal rate of total cyanide remains unchanged

Conclusion: When the sodium metabisulphite/air method is used to treat cyanide-containing wastewater, the pH is 9.0, the sodium metabisulphite dosage is 6g/L, the aeration volume is 130L/min, the reaction time is 1h, and the water temperature is 30℃. The flotation machine with a speed of 1600r/min is the best treatment effect as a reaction device.