Cyanide Wastewater Treatment in Gold Cyanidation Plant

In gold cyanidation plant, although part of the cyanide wastewater has been returned for reuse, there are still some cyanide wastewater or cyanide residue discharged. Cyanide is highly toxic, so these cyanide wastewater must be treated professionally before discharging to avoid the environment pollution and damage to human and animal health. At present, the following are the commonly used cyanide wastewater treatment methods in gold cyanidation plants.

1. Medium and Low Concentration Cyanide Wastewater Treatment Methods

(1) Alkaline Chlorination Method

Alkaline chlorination is a method commonly used in various countries. This method requires adding chlorine oxidant highly-charged oxidation states in alkaline cyanide wastewater. The commonly used oxidants are ClO2, Cl2 (gas and liquid), bleach powder, sodium hypochlorite, calcium hypochlorite, chlorite and so on. In alkaline solutions, OCl- or chloride highly-charged oxidation are generally generated. The Cyanide is first oxidized to cyanate and further oxidized to carbon dioxide and nitrogen.

Merits

• The reagent sources are wide and price is low.
• The processing effect is good, the equipment used is simple, and it is easy to realize automation.

Demerits

• Since the cyanogen chloride produced is toxic, it is very harmful to operators.
• Cyanogen chloride will produce corrosive fumes when it comes in contact with water, and the equipment will be severely.

(2) Inco Method

Inco method was developed by Inco Ltd. in 1982. It is mainly to add the mixture of SO2 and air into the cyanide wastewater, and control the pH value at 8～l0. And the cyanide in the wastewater was oxidized by the catalysis of bivalent copper ion.

Merits

• Inco method is simple, the equipment used is not complicated.
• The treatment effect is generally better than the chlorine oxidation process (without considering the toxicity of thiocyanide).
• The source of reagent is relatively wide, and the investment is lower than alkaline chlorination process.

Demerits

• Inco method is difficult to oxidize SCN-, and SCN- can dissociate CN- later, so it is not suitable for treating cyanide wastewater with high concentration of SCN-.
• SO2 is also the air pollutant, which may escape and leak during the reaction process, polluting the environment.

(3) H2O2 Oxidation Method

H2O2 oxidizes cyanide to generate CNO- under the condition of pH 9.5 ~ 11, normal temperature and copper (Cu2+) ion as the catalyst. CNO－ will be further hydrolyzed to produce NH4+ and CO32－. And the hydrolysis rate depends on the pH.

Merits

• The treatment effect of cyanide wastewater is good and the process is simple.
• H2O2 oxidation method is suitable for treating low-concentration cyanide wastewater, cyanide concentration after treatment is less than 0.5mg/L.

Demerits

• H2O2 is toxic and corrosive，so its transportation and use are dangerous.
• H2O2 oxidation method is difficult to oxidize the SCN in the wastewater, and the wastewater processed are still toxicity.

(4) Ozone Oxidation Method

The oxidation ability of ozone is extremely strong, the electrode potential is 2.07mV, only second to fluorine. It can easily decompose the components that other oxidants cannot. The chemical reaction mechanism of ozone oxidation process is: ozone reacts with cyanide to produce cyanate, which hydrolyzes to produce nitrogen and carbonate.

Merits

• It only needs ozone generating equipment, no need to purchase and transport chemicals.
• The process is simple and convenient, no secondary pollution.

Demerits

• Due to the high cost of the ozone generator and the difficulty of equipment maintenance, there are some restrictions in industrial applications.
• Ozone oxidation method consumes a lot of electric energy, so it is difficult to apply in places where electric power is lacking.

2. High Concentration Cyanide Wastewater Treatment Methods

(1) Acidization Method

Acidification method can process most of the solution with high cyanide concentration (60*10-6 + NaCN) discharged from factories. The concentration of free cyanide ion in solution processed can be reduced to 1*10-6.

Merits

• It can recover cyanide to the maximum extent, the resources can be recycled, and the economic benefits are significant.

Demerits

• One-time investment is too large, and some small and medium enterprises cannot afford it.
• The operation is complicated, and the treated pulp is still difficult to meet the discharge standard.

(2) Natural Degradation Method

Natural degradation method is a purification method that uses natural factors such as illumination to decompose cyanide. It includes the effects of cyanide volatilization, self-decomposition, oxidation, photochemical degradation, biodegradation, precipitation and adsorption, which is the result of a complex comprehensive action of physical chemistry, photochemistry and biochemistry.

Merits

• Natural purification method can achieve the purpose of removing cyanide without equipment or any chemical. So the cost is low.

Demerits

• The process is very slow and the treated wastewater can’t meet the discharge standard.

(3) Two-stage Precipitation Method

The two-step precipitation method is a high-efficient closed-circuit full-circulation method developed for small and medium gold cyanidation plant with high concentration of SCN- wastewater, which realizes the “zero release” of wastewater. The two-step precipitation method is mainly to add catalyst and enough oxygen to the wastewater cyanide, and remove cyanide by the following reaction on the gold loaded carbon.

2Cu+＋2SCN－→Cu2(SCN)2↓（White）
Ca2+＋SO42－→CaSO4↓（White)
Pb2+＋SO42－→PbSO4↓（White）
H+＋CN－→HCN

Merits

• It can remove heavy metal ions in the solution and realize the recycling of wastewater.

Demerits

• The first step must be completely precipitated. Otherwise, when adding alkali, cuprous rhodanide will re-dissolve, which will affect the process effect.
• Unprecipitated CaSO4 will cause valve blockage.

To Wrap Up

The above are the common cyanide wastewater treatment methods in gold cyanidation plant. In addition to the cyanide removal method described above, the discharge of cyanide wastewater can also be reduced by reducing the use of cyanide in gold cyanidation process. YANTAI CNLITE MINERAL PROCESSING REAGENTS CO., LTD. has developed a high-tech, low-toxic gold dressing agent, which can 100% replace sodium cyanide for gold extraction and is suitable for all gold cyanidation processes.

If you want to know more about cyanide wastewater treatment, gold cyanidation process, or want to consult cyanide alternatives (low-toxic gold dressing agent), you can consult the online customer service or submit a message on the website, we will contact you as soon as possible!