Cyanide (often referred to as sodium cyanide and potassium cyanide) is a chemical product, which is mainly used in gold production, electroplating and other industries, resulting in a large number of toxic cyanide containing wastewater in these industries. At the same time, most gold ores are associated with a variety of valuable elements such as silver, copper, lead, zinc and sulfur. However, many gold mining enterprises only focus on the recovery of heavy gold, and the recovery rate of associated elements is generally low, resulting in waste of resources. In addition, cyanide is widely used in metallurgy, electroplating, printing and dyeing, daily chemical industry and agriculture. Fodamon engineers summarized that cyanide widely exists in various organisms, including photosynthetic bacteria, algae and animals. More cyanide comes from human production activities, such as electroplating, smelting, printing and dyeing wastewater, landfill leachate, cyanide containing chemicals, etc. From the perspective of environmental engineering and biosafety, great attention should be paid to the detoxification of cyanide containing wastewater.
Cyanide consumption in production is mainly in the following aspects:
(1) Mechanical consumption, such as running, running, etc.
(2) When the pH value in the pulp is too low, HCN will escape.
(3) The formation of thiosulfate is easy to form CNS when there is a large amount of S2 – in the pulp.
(4) Generate metal complexes, such as those with Cu2S, ZnS, FeS, etc. in minerals. 2.7, 3.0 and 5.26 g sodium cyanide are consumed per gram of copper, zinc and iron respectively.
(5) Silicate colloid or silicon oxide in the pulp adsorbs cyanide.
- Hazards of cyanide
Cyanide refers to the substance containing cyano group (CN -) in the compound molecule. Cyanide can be divided into two categories according to whether the elements or groups connected with cyano are organic or inorganic, namely, organic cyanide and inorganic cyanide. Inorganic cyanide is widely used and comes from many varieties. According to its nature and composition, it can be divided into two types, namely simple cyanide and complex cyanide.
As we all know, most inorganic cyanide is highly toxic and highly toxic. A very small amount of cyanide will kill people and livestock in a very short time, and also cause crop yield reduction. Cases of acute poisoning of fish, livestock and people caused by cyanide pollution of water have been reported at home and abroad.
1.1 Impact of cyanide on human
Cyanide does great harm to warm blooded animals and people, characterized by high toxicity and quick action. CN – generates hydrogen cyanide after entering the human body, which has a very rapid effect. In the air containing hydrogen cyanide at a very low concentration (0.005 mg/L), it will cause headache, discomfort, palpitations and other symptoms in a very short time; In the air with high concentration (>0.1mg/L) of hydrogen cyanide, people can die in a very short time. The United States Environmental Protection Agency (USEPA) proposes that the maximum cyanide concentration in drinking water and ecological water is 0.05mg/L and 0.20mg/L respectively. The maximum mass concentration of cyanide in domestic drinking water and industrial wastewater is 0.05mg/L and 0.50mg/L respectively.
1.2 Toxicity of cyanide to aquatic organisms
Cyanide is very toxic to aquatic organisms. When the concentration of cyanide ion is 0.02~1.0 mg/L (within 24 hours), the fish will die. When the concentration was 3.4 mg/L, Daphnia died in 48 hours; The maximum allowable concentration of cyanide in water by plankton and crustaceans is 0.01mg/L. Microbes in water can destroy cyanide with low concentration (<2 mg/L), making it a simple non-toxic substance, but it will consume dissolved oxygen in water, reduce biochemical oxygen demand, reduce digestion, and cause a series of water quality problems.
1.3 Effect of cyanide on plants
When the concentration of cyanide in irrigation water is below 1 mg/L, the growth and development of wheat and rice are normal; When the concentration was 10 mg/L, rice began to suffer, the yield was 78% of that of the control group, and wheat did not suffer significantly; When the concentration was 50 mg/L, both rice and wheat were obviously injured, but rice was more seriously injured, the yield was only 34.7% of the control group, and wheat was 63% of the control group. When the cyanide content was 1 mg/L in water culture, the growth and development of rice began to be affected; When the concentration was 10 mg/L, the growth of rice was obviously inhibited, and the yield was 50% lower than that of the control group; At 50 mg/L, most of the plants were killed, and a few of the remaining plants could no longer bear fruit. In the land seriously polluted by cyanide containing wastewater, the yield of fruit trees decreases and the fruit becomes smaller. In addition, when cyanide containing wastewater is used to irrigate rice, wheat and fruit trees, the fruits will contain a certain amount of cyanide.
2 Study on process mineralogy of cyanide tailings
Cyanide tailing is the tailing obtained from flotation gold concentrate after cyanidation operation and pressure filtration. Due to different ore properties and gold extraction processes, the properties, types and contents of valuable metals and minerals in the tailing are different. The main metal minerals in the cyanide tailings are sulfides: pyrite, chalcopyrite, galena, sphalerite, etc; Gangue minerals are mainly quartz, a small amount of chlorite, mica, feldspar, kaolinite, etc.
The main copper mineral in cyanide tailings is chalcopyrite, so copper can be recovered by conventional flotation method. As sphalerite and galena have less content and pyrite has the largest content, copper and sulfur can be recovered.
Cyanide tailings have some common features: fine mineral particle size and serious argillization; The mineral composition is relatively complex; It contains a certain amount of CN – and some residual reagents. Due to the influence of the above factors, the floatability of some minerals in the cyanide tailings is greatly reduced, and it is difficult to recover valuable elements.
The recovery of metal from cyanide tailings is different from that from raw ore, mainly because:
1) After regrinding and long-time aerated stirring of gold concentrate, the mineral particle size is very fine (- 0.045mm particle size content reaches 95%, or even finer, the specific surface is increased, and the colloidal dispersion system appears, which makes flotation separation difficult.
2) The metal minerals are seriously overworn, and some of them are strongly inhibited due to excessive oxidation in the cyanidation process, so they are difficult to activate.
3) A large amount of argillaceous silicate minerals and cyanide in the pulp will deteriorate the flotation process and affect the grade and recovery of the concentrate.
3 Cyanide treatment
There are many methods to treat cyanide containing wastewater, including alkali chlorination method, acidification recovery method, sulfur dioxide air oxidation method, electrochemical method, activated carbon adsorption catalytic oxidation method, peroxide oxidation method, ferrous sulfide method, biochemical method, ion exchange method, natural purification method, ozone oxidation method, emulsion liquid membrane method and pressurized hydrolysis method. The main treatment methods of cyanide and cyanide tailings are summarized as follows.
3.1 Chemical treatment alkali chlorination
Alkali chlorination method is very effective in treating high concentration cyanide, and the treatment equipment is simple and easy to operate. Both small batch processing and large-scale processing can achieve good results. This method has been applied in the treatment of waste cyanide for enterprises and institutions, and achieved very good results.
The reaction principle is as follows: under alkaline conditions, CN – in the solution is oxidized to extremely weak toxic CNO – with bleaching powder [the main component is calcium hypochlorite Ca (ClO) 2], and CNO – is further oxidized to non-toxic carbon dioxide and nitrogen. The reaction formula is:
ClO- + CN- + H2O → ClCN + 2OH-
ClCN + 2OH → NCO-+ Cl- + H2O
2NCO- + 3ClO- + H2O → 3Cl- + N2↑+ 2CO2↑+ 2OH-
First, add sodium hydroxide into the container containing water, adjust the pH of the aqueous solution to 11~12, and then add the waste cyanide into the aqueous solution with the pH adjusted, gently stir to make it completely dissolved. Then add industrial bleaching powder and continue to stir gently to facilitate the reaction. During the reaction, cyanide is first oxidized to cyanate, and the toxicity of cyanate is only 1 ‰ of that of cyanide. Cyanate continues to be oxidized and finally completely oxidized to carbon dioxide and nitrogen.
3.2 Biological treatment
There are four main biochemical processes of cyanide degradation by microorganisms: hydrolysis, oxidation, reduction and substitution. Among them, hydrolysis and oxidation are dominant.