The treatment methods of cyanide-containing wastewater that have not been industrialized include ozone cyanidation, emulsion liquid membrane, electrodialysis and electrowinning. At present, these methods still have some problems in technology, equipment, reagents and theory, or they have not received due attention and in-depth research. Looking at the problem from the perspective of development, with the rapid development of science and technology, the changes in the prices of various raw materials in gold cyanide plants, and the in-depth implementation of national environmental protection policies, some of them may be rapidly extended to industrial production in the near future.
1 1. 1 ozone oxidation method
Ozone, molecular formula O3, English name ozone. The molecular weight is 48, and the specific gravity is 2. 154g/L(0℃, 0. 1MPa), which is widely known because of the existence of atmospheric ozone layer, but few people know about the application and production technology of ozone in the last hundred years.
Ozone has a special fishy smell. In nature, ozone exists in the ozone layer at an altitude of 15 ~ 35km. It absorbs most ultraviolet rays from the sun and is an umbrella to protect human life.
Ozone is an allotrope of oxygen, and its oxidation ability is second only to fluorine. Unstable, easy to decompose, and can only be used while producing. The device that produces ozone is called an ozone generator. Since the first large-scale ozone treatment of tap water in Germany in 1902, thousands of waterworks around the world have adopted corona discharge to introduce dry oxygen or air into the discharge chamber, and through alternating high-voltage corona discharge, some oxygen is decomposed and polymerized into ozone, which is an allotropic transformation process of oxygen. Since the 1857 glass tube generator came out, the technical specifications and types of the generator have developed greatly, and one has been formed abroad.
Since 1980s, China has been studying the treatment of cyanide-containing wastewater by ozone oxidation, and has made some progress. However, due to the small capacity and large investment of domestic ozone generators, no one has started industrial experiments yet.
11.1.1the reaction mechanism of ozone oxidation.
There are different views on the reaction mechanism of cyanide oxidation by ozone. It is generally believed that the first step of the reaction is the local oxidation of cyanide, which is similar to chlorine oxidation.
CN-+O3→CNO-+O2
Simple cyanide is easy to cyanide, while complex cyanide is more difficult. This reaction is completed in a few minutes. When the ozone concentration increased from 1% to 2%, the reaction completion time decreased from 12 minutes to 4 minutes, and when the CN- concentration decreased to 3 ~ 4 mg/L, the reaction speed slowed down.
With the continuous oxidation of cyanide, the cyanide ions in the solution gradually increased, and about 30% of CNO- was further oxidized and decomposed:
2CNO-+3O3+H2O→N2↑+2HCO3-+3O2
The oxidation rate of CNO- is related to the pH value of the reaction. When the pH value is greater than 10, CNO- weighing 100% is oxidized and decomposed according to the above formula. If the pH value of the reaction is lower than 10, the remaining CNO- is decomposed:
CNO-+2H2O→ bicarbonate -+NH3
The optimum pH value of hydrolysis is 6 ~ 8, and the ammonia produced by hydrolysis will quickly react with ozone to produce nitrate.
NH3+4O3+3OH-→NO2-+3H2O+4O2
The reaction process between cyanide and ozone is about 7 times slower than that between cyanide and ozone, and its reaction speed is equivalent to the decomposition speed of ozone in alkaline solution. Therefore, in the limited time of actual wastewater treatment, ozone oxidation process is similar to chlorine oxidation process, which can only oxidize cyanide into cyanate, and cyanate can only be partially decomposed.
Cyanide factory wastewater usually contains reducing substances, such as thiocyanate, which reacts with ozone as follows:
SCN-+2o 3+2OH-→CNO-+SO32-+2o 2+2H2O
CN-+SO32-+2O3→CNO-+SO42-+2O2
Because the generated cyanate consumes ozone, the ozone consumption is much larger than the theoretical value (O3/SCN-=3.3).
Ozone has no oxidation ability to cyanide in iron and ferrocyanide, but it can oxidize Fe(CN)64- to Fe(CN)63-. Therefore, when the wastewater contains Fe(CN)64-, cyanide is not easily reduced unless it is irradiated with ultraviolet rays. Some experiments at home and abroad show that the actual consumption of ozone is generally O3/CN-= 3 ~ 10. If the pulp is treated, the ozone consumption will be greater, and copper salt is the catalyst for ozone oxidation.
Research on 1 1. 1.2
1979 A gold mine in Canada has carried out an expanded experiment on the treatment of cyanide-containing wastewater by ozone oxidation. In less than 1 hour, cyanide decreased from 450 mg/l to 2 mg/l, and thiocyanate decreased to the analytical limit. The treatment capacity is 40m3/d, but cyanide can't be reduced because of the existence of ferricyanide. The scale of another gold mine was 20m3/d, and the cyanide and thiocyanate decreased from 2.5mg/L and 13.5mg/L respectively to below 0.2 mg/L. The remaining cyanide was caused by the presence of ferricyanide.
Domestic research has drawn a * * * same law. When the concentration of cyanide in wastewater is lower than 30mg/L, the treated CN-
During the period of 1988, Changchun Gold Research Institute of Ministry of Metallurgy conducted an experiment on the wastewater from a gold mine in China. Conclusion As above, it is found that the cyanide removal rate decreases when the wastewater contains iron, which is similar to chlorine oxidation. Because the gold industry generally needs a lot of ozone (a 50t/d all-mud cyanidation plant consumes at least 5kg/h), the ozone generator is expensive and consumes a lot of electricity, so far it has not been tested in industry.
1 1. 1.3 Problems in Ozone Cyanidation
The advantage of ozone cyanidation is that raw materials (air) are easily available, which is especially beneficial to cyanidation plants in remote mountainous areas. In addition, no other ions are introduced into the wastewater, which is beneficial to wastewater circulation. But there are still some problems to be solved.
1. Poor adaptability. Ozone method is only suitable for treating clarified liquor with CN- content less than 30mg/L, while wastewater from gold cyanide plant is mostly pulp with CN- content above 80mg/L, so this method can only be used as a secondary treatment method.
2. Large investment. Except for the high price of equipment, taking the ozone oxidation unit supporting a 50t/d all-mud cyanidation plant as an example, the total investment of equipment is about1000000 yuan.
3. High power consumption. Every time 1kg ozone is produced, it consumes 15 ~ 20 kWh of electricity, while the 50t/d all-mud cyanidation plant needs more than 100 kg of ozone every day, which is equivalent to increasing the production capacity of 100kw. Compared with the current wastewater treatment capacity of 15kw, the power consumption is too large, the power input is large, the electricity price is high, and the treatment cost is inevitably high, so it is difficult to compete with the sulfur dioxide-air oxidation method and chlorine oxidation method. Japan has recently developed a new type of ozone generator, but it is not clear whether it can produce a large amount of ozone.
4. The oxidation process is not selective. When reducing substances such as SCN- and SO32- in wastewater are high, ozone consumption is greater.
5. Secondary pollution. The treated wastewater contains a certain concentration of ozone. If discharged directly, it will have an impact on the water environment and must be treated. In addition, in the process of water treatment, ozone will also escape from wastewater into the air, polluting the operating environment.
6. The equipment of the ozone generator is complicated, the maintenance is difficult, and the requirements for operators are high.
1 1.2 emulsion liquid membrane method
Emulsion liquid membrane method, referred to as liquid membrane method, was invented by American Dr. Nianzhi Li. It is a new science that rose in the early 1970s. Compared with the solid membrane separation method, it has stronger directionality and selectivity, minimum permeability and high enrichment ratio. This is very noticeable when selecting and extracting some substances from wastewater. In recent years, the research on it is very active at home and abroad. Many magazines introduced the research reports of extracting gold, copper, zinc and cyanide by this method.
1 1.2. 1 latex film
There are two kinds of emulsion films, one is oil-in-water type and the other is water-in-oil type. The latter is used to treat cyanide-containing wastewater, in which the same phase is usually diesel or kerosene, the water phase is NaOH solution, and the oil-water interface is composed of surfactant. In this way, the oil-water two phases form a relatively solid and stable system.
The particles in the liquid film are actually emulsified, that is, the emulsified droplets contain countless tiny dispersed droplets, the organic solvent is the continuous phase, and the NaOH aqueous solution is the dispersed phase. The process of cyanide removal by liquid membrane is as follows.
After the cyanide-containing wastewater is acidified, cyanide exists in the form of HCN, which is added to the liquid film and fully stirred to form an emulsion. HCN first dissolves in the membrane phase with concentration gradient (kerosene or diesel oil) and diffuses into the membrane. When the internal phase NaOH solution is encountered, NaCN is generated by the reaction. NaCN is an ionic compound and cannot return to the aqueous solution through the liquid membrane. When the sodium hydroxide in the internal phase is almost exhausted, the stirring is stopped and the liquid film is still for a period of time.
Research progress of 1.2.2 liquid membrane method
The liquid membrane separation system for treating cyanide-containing wastewater is introduced. The membrane phase consists of surfactant polyamine EN3064 and kerosene as the flow carrier of TBP. The internal water phase is NaOH solution, and the oil-water ratio is 1: 1.3. The external water phase is a clear solution containing HCN, which has been acidified and precipitated, and the pH value is 2 ~ 3. The extraction rate in the rotary table tower can reach more than 95%, and the residual cyanide is said to be less than 0.5mg/L (the analysis method is unknown). After demulsification, the concentration of sodium cyanide in the internal water phase is 100 times higher than that in the original wastewater. After 2000 ~ 3000 V electrostatic demulsification, the separated oil phase can be reused for many times.
Another document introduces the results of treating cyanide-containing wastewater by liquid membrane method in a university. When the oil-water ratio is 2∶ 1 and the emulsion ratio is 20 ~ 40, after continuous extraction 1 hour, the cyanide content in the wastewater is directly determined by ion-selective electrode, and the concentration of sodium cyanide in the internal water phase is 30 times that in the wastewater. After two-stage extraction,
During the period of 1987, Dalian Institute of Physics and Chemistry of Chinese Academy of Sciences completed a small-scale experiment in a gold mine in Shandong, and some results were basically consistent with the above documents. But it has not yet reached the stage of industrial application.
1 1.2.3 Problems in liquid membrane method
First, the applicability of liquid film
The treatment of cyanide-containing wastewater by liquid membrane method introduced in various literatures is to prepare water or simply leach poor solution at one time, instead of cyanide-containing wastewater really produced in industry. Even so, the report talked about the damage of liquid film. If the real wastewater from cyanide plant is used, the damage rate of liquid film is high because of its complex composition and high content of other components. Therefore, it is difficult to industrialize this method before developing a liquid film with good strength.
Two. Treatment effect
It is a matter of principle that the analysis method is not explicitly mentioned in the treatment effect reports of various literatures. According to the regulations of the State Environmental Protection Bureau, the analysis of cyanide in industrial drainage should be pre-distilled by tartaric acid-zinc nitrate method, and then silver nitrate titration or colorimetry should be adopted according to the cyanide content. It is mentioned in some literatures that the results of direct determination by ion-selective electrode method or determination by adding ion intensity regulator are different from those of the method stipulated by the state, so it is incorrect to say that cyanide meets the standard. At present, it can only be said that the cyanide removal effect of liquid membrane is good, but it cannot be said that the cyanide in treated wastewater reaches the standard.
Three. processing costs
The amount of acid (sulfuric acid) required by liquid membrane method is similar to that of acidification recovery method, and the amount of NaOH is higher than that of acidification recovery method. Take the wastewater with CN- concentration of 1000mg/L as an example. If the acidification recovery method is adopted, only 1.6kg/m3 NaOH is needed, while the liquid membrane method needs 2.5 kg/m3 NaOH. The lime used to neutralize acidic wastewater is the same, but it is one more than the acidification recovery method.
Four. investment
Before cyanide removal by liquid membrane method, wastewater must undergo acidification and precipitation, which is similar to acidification and recovery method. Therefore, the equipment is almost the same as that of the acidification recovery method, and the investment of liquid membrane extraction device, milk making and demulsification device is similar to that of the acidification recovery method. Therefore, the total investment is equivalent to the acidification recovery method.
Verb (abbreviation for verb) Feasibility of equipment
Up to now, there is no successful industrial test report on the treatment of cyanide-containing wastewater by liquid membrane. Whether the extractant is suitable for practical wastewater and whether the power consumption is acceptable needs further study.
1 1.3 electrodialysis method
Basic principle of electrodialysis: 1 1.3. 1.
Electrodialysis can not be used alone, but must be used in combination with acidification and recovery. It can be called electrodialysis-acidification recovery process.
The electrodialysis method is to make the ions in the solution flowing through the desalination compartment (dilution compartment) enter the adjacent compartment (concentration compartment) through the membrane under the action of the electric field, and the solutions in the concentration compartment and dilution compartment flow out of the dialyzer through their respective water channels, thus obtaining two solutions with different concentrations.
Research status of 1.3.2 electrodialysis
As early as 1970s, the Soviet Union began to study the electrodialysis method to treat cyanide-containing wastewater. The calcium and magnesium ions in the wastewater were removed by cation exchange resin, and then passed through an electrodialyzer consisting of 49 compartments, with a treatment capacity of 1.5l/m3. h and a power consumption of 1.2kwh/m3. Before electrodialysis, calcium and magnesium must be removed to avoid the formation of sediment on the electrodialysis ion exchange membrane, thus blocking the membrane. The content of cyanide and heavy metals in dilute phase is low, which can be recycled in cyanide section.
The research results of Kunming Metallurgical Research Institute in China show that when the wastewater contains 540 mg/L NaCN, the removal rate of NaCN in the dilute phase is more than 90%, that is, the NaCN in the dilute phase is less than 50mg/L, the original wastewater in the concentrated phase is 2.9 times higher, and all kinds of heavy metals also enter the concentrated phase. The power consumption is about 3kwh/m3, and the dilute phase accounts for 70% of the treated wastewater, and the concentrated phase accounts for 30%. The concentrated phase is treated by acidification recovery method, and the dilute phase can be recycled in the cyanidation section.
The electrodialysis-acidification recovery process creates conditions for recovering low-concentration cyanide and heavy metals in wastewater and reduces the scale of acidification recovery device, which is its advantage, but there are still the following problems in this method.
1. The process is complex, the process is long and the investment is large.
When treating cyanide-containing wastewater in gold industry by electrodialysis, it can not be used alone, and may include four sections, which are called electrodialysis-acidification recovery process, as shown in Figure 1 1-2. The first step is to remove calcium and magnesium by ion exchange, which requires regular regeneration. If this section is omitted, the electrodialyzer should be cleaned regularly, the second section is to treat wastewater by electrodialyzer, which also needs regeneration, the third section is to treat concentrated phase by acidification recovery device, and the fourth section is to treat substandard cyanide wastewater produced by acidification recovery method. It can be seen that the process flow is too long, which is equivalent to the cooperation of four treatment devices, and the investment is bound to be high, and the operation and management are not easy, and the investment is not less than that of the acidification recovery method.
Two. processing costs
The concentrated phase produced by electrodialysis-acidification recovery method is 1/3 of the original wastewater, so the acid consumption of concentrated phase treated by acidification recovery method is less than that of wastewater directly treated by acidification recovery method, and the power consumption of electrodialysis-acidification recovery method is similar to that of wastewater directly treated by acidification recovery method. If the wastewater is directly treated by acidification recovery method, the amount of acidic cyanide-containing wastewater (which can only be discharged after secondary treatment) is large, and the concentrated phase of electrodialysis is only the raw wastewater. In a word, electrodialysis-acidification recovery process is to concentrate low-concentration wastewater and then treat it with acidification recovery method. Its cost is lower than that of acidification recovery method. Considering the value of recovering cyanide and heavy metals, it may be profitable, but the technological process is long and it is difficult to estimate the management and maintenance. Moreover, the concentration of cyanide in wastewater should not be too low, otherwise, due to the small concentration ratio, (the concentration of concentrated cyanide is only three times that of the original wastewater) is still uneconomical.
Three. Equipment maintenance is difficult
The ion exchange membranes in electrodialyzer are installed accurately, up to hundreds, and it is difficult to maintain.
1 1.4 electrolytic deposition method
Electrowinning is an electrochemical treatment of cyanide-containing wastewater, but its purpose is to greatly reduce the content of heavy metals in wastewater, so as to ensure that the recovery rate of gold will not be affected after the wastewater returns to the cyanidation section, rather than destroying cyanide in wastewater. Therefore, this method may be more suitable for cyanide plants that buy gold concentrate to extract gold.
Electrodeposition method does not want to remove cyanide from wastewater, but hopes that cyanide in wastewater can be converted into free cyanide as much as possible because central ions are reduced by electrodeposition, so as to save cyanide when wastewater is recycled, which is completely different from previous electrolytic wastewater treatment, which means that cyanide in wastewater treated by electrodeposition method cannot meet the standard and cannot be used to treat cyanide-containing wastewater that needs to be discharged.
1 1.4. 1 Basic principle of electrowinning method
The basic principle of electrolytic deposition is similar to that of direct electrolytic deposition or electroplating in expensive solution:
Copper (CN)32-+e→ copper +3CN-
Zinc (CN)42-+2e→ Zinc +4CN-
Lead (CN)42-+2e→ Lead +4CN-
Fe(CN)62-+Pb2+→Pb2Fe(CN)6↓
Fe(CN)62-+Zn2+→Zn2Fe(CN)6↓
Other impurities will also be deposited in the negative deposition region. See table 1 1- 1 for the composition analysis of typical cathode deposits under gold electrodeposition conditions.
Table 1 1- 1 Composition of electrodeposited sediments
Elements gold, silver, copper, lead, zinc, iron and calcium
Content (%) 2.41.36 25.96 8.28 27.438+0.1.48 4.9
The electrodeposition equipment for removing impurities in the lean solution can be the same as the gold electrodeposition equipment, except that the electrodeposition voltage is higher than the voltage when gold is electrodeposited. The anode can be graphite or stainless steel, and the cathode can be stainless steel plate or steel wool. In order to minimize the harm of cyanide during electrolysis, diaphragm electrolyzer should be used.
Research status of 1.4.2 electrowinning method
At present, there is little research in this field, and most of them are compared and discussed when extracting gold from your solution by direct electrodeposition.
Changchun Gold Research Institute, Ministry of Metallurgy, 65438-0983 found that the deposition rate of heavy metals and other impurities in your solution was relatively high during gold electrodeposition. However, the loss of cyanide during electrolysis was not carefully studied in the experiment, so it is impossible to judge whether electrolytic deposition method can be used in lean liquid circulation method.
The industrial test of cyanidation of concentrates from Hangzhou University and Zhuji Gold Mine in Zhejiang Province-direct electrowinning of expensive solution proves that the effect of electrowinning is good, but unfortunately there is no data of cyanide damage. If the cyanide loss rate is high, the economic benefit is not obvious or even no economic benefit.
The possible way to extend electrowinning to industry is to adopt diaphragm electrolyzer without cyanide loss.