Literature review of electroplating wastewater
Design requirements: (1) water quality: copper ion 30mg/L, hexavalent chromium 25mg/L, zinc ion 12mg/L, nickel ion 16mg/L, cyanide 8mg/L, other trace amounts, lead, etc. ，Ph4.5。
(2) Treatment requirements: The first-class standard of Integrated Wastewater Discharge Standard (GB8978- 1996) shall be implemented.
Although the amount of wastewater from electroplating industry accounts for a small proportion in the wastewater from the whole industrial system, electroplating wastewater contains metal pollutants such as cyanide, acid, alkali, hexavalent chromium, copper, nickel, zinc and cadmium, which is seriously harmful to the environment. Therefore, the treatment methods of this kind of wastewater have been actively studied and applied at home and abroad. Based on the advantages of absorbing micro-electrolysis and biosorption to treat heavy metal ion wastewater and the existing experiments to treat single heavy metal ion wastewater, this paper determines to adopt micro-electrolysis and biofilm composite process to treat actual electroplating wastewater.
Keywords: chromium-containing wastewater treatment and emission reduction
Abstract: Electroplating wastewater contains cyanide, acid, alkali and heavy metals such as chromium, copper, nickel, zinc and cadmium. Although it accounts for a small proportion of all industrial wastewater, it seems to have caused serious damage to the environment. At present, the research and application of wastewater treatment at home and abroad have begun to advance. In this paper, on the basis of micro-electrolysis and biosorption methods for treating heavy metal ion wastewater and single heavy metal ion wastewater with experimental advantages, the micro-electrolysis and biofilm composite electroplating process is determined to treat the actual wastewater.
Keywords: electroplating wastewater, treatment, recovery
The main forms of chromium in water environment are trivalent chromium (Cr (Ⅲ) and hexavalent chromium (Cr (ⅵ)), and their migration and transformation in water has certain regularity. Cr (Ⅲ) is mainly adsorbed on solid substances and exists in sediments. Cr (Ⅵ) is soluble in water and stable. Only under anaerobic conditions can it be reduced to Cr (Ⅲ). The toxicity of chromium is related to its existing state. It is generally believed that the toxicity of Cr (ⅵ) is much greater than that of Cr (Ⅲ) [1]. Cr (Ⅵ) is the main characteristic pollutant in chromium-containing electroplating wastewater.
Sources of1Cr (Ⅵ) pollution
Hexavalent chromium compounds are important raw materials in metallurgical industry, metal processing and electroplating, leather making, pigment, textile production, printing and dyeing, chemical industry and other industries. These industries are distributed in many areas, and a large amount of chromium-containing wastewater is discharged every day. The discharge of these wastewater will cause water and soil pollution, and directly affect the sanitary condition of human drinking water. The standard of Cr (ⅵ Ⅵ) content in drinking water stipulated by WHO is 1 ~ 2 μ mol/L [2]. Due to the pollution of industrial wastewater or geological background, the content of Cr (ⅵ) in drinking water in many places in China is seriously exceeding the standard.
2 Treatment technology of wastewater containing chromium (ⅵ)
According to the data, the most commonly used methods for treating chromium-containing wastewater in electroplating industry are reduction method and electrolysis method, which are mature and have good operation effect. However, many other methods have been developed recently, and a comprehensive comparison will show that these methods also have their own advantages and disadvantages. As a new method, they have their own experience.
2. 1 reduction precipitation method
Electroplating wastewater containing chromium (ⅵ) was treated by chemical precipitation method. One method is to reduce Cr (ⅵ) to Cr (Ⅲ) by reduction, and then precipitate. The other is to use barium salt to make chromate generate barium chromate precipitation. Yuan Zhibin [3] has a regulating tank, and the chromium-containing wastewater enters the reduction tank after passing through the regulating tank. In the reduction tank, H2SO4 is added to control the pH value to 2.5 ~ 3, and NaHSO3 is added to reduce Cr (Ⅵ) to Cr (Ⅲ). In the reaction tank, NaOH was added to form Cr(OH)3 precipitate. Dou Xiudong et al. [4] found that the removal rate of Cr by reduction precipitation method was above 99%, and the settling performance of MgO chromium mud was excellent. Adding part of MgO into NaOH and CaO can greatly improve the performance of chromium sludge, and the optimum dosage is pH≈8.3. Zheng Xinqing [5] studied and analyzed the process steps of treating chromium-containing wastewater by reduction precipitation, the content of Cr (ⅵ) in supernatant and settled sludge after solid-liquid separation, and the correlation between the forms transformation of Cr (Ⅲ) and Cr (ⅵ), and put forward that special attention should be paid to controlling the rebound of chromium in chromium-containing wastewater and the integrity of the whole process.
2.2 electrolytic precipitation filtration
1. Overview of process flow
Chromium-containing electroplating wastewater first flows into the regulating tank through the grid to remove the suspended solids of larger particles, so as to balance the water quantity and water quality, and then is pumped to the electrolytic cell for electrolysis. In the process of electrolysis, the anode iron plate is dissolved into ferrous ions, which reduce hexavalent chromium ions to trivalent chromium ions under acidic conditions. At the same time, due to the precipitation of hydrogen on the cathode plate, the pH value of wastewater gradually increased and finally became neutral. At this time, both Cr3+ and Fe3+ are precipitated by hydroxide, and the electrolytic effluent first passes through the primary sedimentation tank and then continuously passes through the secondary sedimentation and filtration tank (wastewater from top to bottom). The primary filter tank is filled with fillers: charcoal, coke and slag; The second filter tank is filled with anthracite and quartz sand. The sediment in the sewage is filtered and adsorbed by the filter tank filler, and the effluent flows into the drainage inspection well. And then enters a circulating water pool as cooling water through a pump. Charcoal, coke, anthracite and slag for filtration are collected regularly and mixed in the boiler room.
2. Main equipment
1 regulating tank; 1 primary sedimentation tank and 2 sedimentation and filtration tanks; 1 circulating pool; Power control cabinet, electrolytic cell, electrolytic power supply and electrolytic voltage 1 set; Five pumps.
3. Results and analysis
Under normal working conditions, the electroplating wastewater treatment equipment of an electroplating factory samples several times at different time intervals.
Electroplating wastewater containing chromium was treated by electrolytic precipitation filtration process, and all of it was reused. The filler in the filter pool is regularly concentrated in the boiler room for mixed combustion, so as to achieve the purpose of comprehensive treatment of chromium-containing electroplating wastewater.
Although the treatment technology is reliable and the operation is simple, several aspects should be paid attention to:
A) The polar plate needs to be replaced regularly;
B) In an acidic medium, chromium hydroxide can be redissolved;
C) The filler in the sedimentation and filtration tank must be treated regularly and burned completely, otherwise it will cause secondary pollution. It can be seen that it is very important to strengthen the management of treatment facilities.
4. Conclusion
1) treatment process thoroughly treats chromium-containing electroplating wastewater, and the filler in the filter tank is treated regularly and evenly, which will not cause secondary pollution; All the treated clean water can be reused, which can save water resources and has obvious economic benefits.
2) The process has the advantages of low investment, mature technology, stable and reliable operation, convenient operation and easy management, and is suitable for electroplating production enterprises of different scales.
2.3 adsorption method
Adsorption method is a common method to treat wastewater by using porous solid substances to adsorb pollutants in water. The key technology of adsorption method is the selection of adsorbent. At present, the most commonly used adsorbent in industrial application is activated carbon, which has large adsorption capacity and strong reduction effect on Cr (ⅵ) cation [6]. After soaking in 20% sulfuric acid solution, the removal rate of Cr (ⅵ) reached 9 1.6%, which was easy to regenerate [7]. Valix et al. [8] studied the influence of heterocyclic atoms (such as S, N, O, H, etc.). ) and the structural characteristics of activated carbon on the adsorption of Cr (Ⅵ), and it is considered that heterocyclic atoms assist activated carbon to play the role of reducing agent, which improves the adsorption of chromate ions on activated carbon, and increasing the total surface area of activated carbon is helpful to improve the adsorption capacity and bring out Cr (Ⅵ).
Although the performance of activated carbon is excellent, the output of activated carbon in China is low and the price is relatively expensive, which limits its use in some economically underdeveloped areas and some industries. Therefore, many types of adsorbents have been developed. One is to use industrial and agricultural waste as adsorbent, and treat waste with waste, which not only has good adsorption effect, but also has low price and wide sources. Li Xinjin et al. [9] used activated red mud to treat chromium-containing wastewater, and the concentration of Cr(III) in the treated wastewater was below 300 mg/L, and the removal rate was above 99%. When treating wastewater containing Cr (Ⅵ), ferrous sulfate is added for reduction first, which can also make the wastewater with Cr (Ⅵ) concentration below 300 mg/L reach the national standard after treatment. Ma Shaojian et al. [10] used steel slag to adsorb Cr(III), the removal rate was over 99%, and at the same time, more than 94% of Pb2+ in wastewater could be removed. Jiang et al. [1 1] studied the adsorption characteristics of chromium ions by blast furnace slag. In the range of pH 4 ~ 12, the removal rate of Cr(III) by blast furnace slag can reach above 97%, and Cr (VI) needs to be reduced and treated with ferrous sulfate. Hu et al. [12] studied the adsorption of Cr (ⅵ Ⅵ) by maghemite nanoparticles. Compared with activated carbon, its adsorption capacity is not affected by other ions, and it is easy to regenerate, so it can be used to recover Cr (ⅵ) from wastewater. Cheng Yonghua et al. [13] studied the efficient adsorption of chromium-containing wastewater by chitosan. In strong acid, the adsorption speed of chitosan for Cr (ⅵ) is faster, while in weak acid, it is beneficial for chitosan to adsorb Cr (Ⅲ). By controlling the pH value, the chromium content in wastewater can be effectively removed.
The other is to use modified materials as adsorbents. Because the adsorption effect of some natural materials (or wastes) is not ideal, many scholars have modified them, and there are many reports in this regard at present. Han Yi et al. [14] modified red mud with ferric chloride, Ren Nailin et al. [15] acidified and soaked the modified sawdust with 8- hydroxyquinoline metal complexing agent, etc. [16] activated calcium-based bentonite with inorganic acid, Li et al. [17] Sui Guoshun et al. [/kloc]
2.4 Research progress of other chromium-containing wastewater treatment methods at home and abroad
1. 1 biological method
In recent years, biological treatment of chromium-containing wastewater has been started at home and abroad. Biological method is a high-tech biotechnology to treat electroplating wastewater, which is suitable for wastewater treatment in large, medium and small electroplating plants, and has great practical value and is easy to popularize. SRB bacteria (sulfate reducing bacteria), SR series compound functional bacteria, SR compound functional bacteria, desulfurization solitary bacteria and decolorizing bacteria (bac) at home and abroad. Decolorizing bacteria), fauna, yeast, Pseudomonas, Pseudomonas fluorescens, Streptococcus lactis, Enterobacter cloacae, chromate reducing bacteria, etc. I've studied it. From the single strain in the past to the combined use of multiple strains now, electroplating wastewater is mixed with other industrial wastes and human feces, and lime is used as coagulant, and then chemical-coagulation-precipitation treatment is carried out. The research shows that the biological treatment method of mixed activated sludge can remove Cr6+ and Cr3+, and NO3- is oxidized to NO3-. It has been used in the treatment of chromium-containing wastewater from Egyptian light vehicle company.
The biological treatment of electroplating wastewater depends on artificially cultured functional bacteria, which has the functions of electrostatic adsorption, enzyme catalytic conversion, complexing, flocculation, inclusion precipitation and buffering pH value. The method is simple to operate, the equipment is safe and reliable, and the discharged water is used for the purposes of cultivating bacteria, etc. The amount of sludge is small, and the metals in the sludge are recycled; Cleaner production is realized, and no sewage and waste residue are discharged. Low investment, low energy consumption and low operation cost.
1.2 membrane separation method
The membrane separation method takes the selectively permeable membrane as the separation medium. When there is some driving force (such as pressure difference, concentration difference, potential difference, etc.). ) On both sides of the membrane, the components on the raw material side selectively permeate through the membrane to achieve the purpose of separating and removing harmful components. At present, the mature technologies used in industry include electrodialysis, reverse osmosis, ultrafiltration and liquid membrane. Other methods, such as membrane bioreactor and microfiltration, are still in the basic theoretical research stage and have not been applied in industry. Electrodialysis is a method to purify wastewater by using the selective permeation of ion exchange membrane under the action of DC electric field and the potential difference as the driving force. The reverse osmosis method realizes the separation through the diffusion of solvent under a certain external pressure. Ultrafiltration is also a solute separation membrane process driven by static pressure difference. Liquid films include carrier-free liquid films, carrier-based liquid films and immersion liquid films. When the liquid film is dispersed in electroplating wastewater, the moving carrier selectively complexs heavy metal ions at the outer interface of the film, then diffuses in the liquid film, separates at the inner interface of the film, and the heavy metal ions enter the inner phase of the film for enrichment, and the moving carrier returns to the outer interface of the film. This process continues and the wastewater is purified. Advantages of membrane separation method: high energy conversion rate, simple equipment, easy operation, easy control and high separation efficiency. But the investment is large, the operating cost is high, and the film life is short. Mainly used for recycling high value-added substances, such as gold.
The recovery of rinsing water in electroplating industry is the main application of electrodialysis in waste liquid treatment. Water and metal ions can be completely recovered, the whole process can be operated at high temperature and wide pH value, and the concentration of the recovered solution can be greatly improved. The disadvantage is that it can only be used to recover ionic components. The chromium-containing wastewater was treated by liquid membrane method. The ionic carrier is TBP (tributyl phosphate) and the membrane stabilizer is Span80. The process operation is convenient, the equipment is simple, and the raw materials are cheap and easy to obtain. Non-ionic carriers, such as neutral amine, commonly used alanine 336 (trioctylamine), 2%Span80 as surfactant, and the mixture of carbon hexachloride 1, 3- butadiene (19%) and polybutadiene (74%) as solvent were also selected. The separation process is divided into extraction, back extraction and other steps. Recently, microfiltration has also been used to treat wastewater containing heavy metals, which can remove toxic heavy metals such as cadmium and chromium from industrial wastewater such as metal electroplating.
1.3 xanthate method
In 1970s, the United States developed a new type of insoluble heavy metal ion remover ISX, which is convenient to use and low in water treatment cost. ISX can not only remove a variety of heavy metal ions, but also reduce Cr6+ to Cr3+ under acidic conditions, but its stability is poor. Insoluble starch xanthate has good chromium removal effect, the removal rate is >: 99%, the residue is stable and will not cause secondary pollution. Zhong Changgeng and others used rice straw instead of starch to make rice straw xanthate to treat chromium-containing wastewater. The removal rate of chromium is high, and it is easy to meet the discharge standard. Researchers believe that the removal of chromium from rice straw xanthate is the simultaneous action of xanthate and chromium hydroxide through precipitation and adsorption, but xanthate plays the main role. The method has the advantages of low cost, fast reaction, simple operation and no secondary pollution.
1.4 photocatalytic method
Photocatalytic method is a new method developed rapidly in recent years to treat pollutants in water, especially to treat organic pollutants in water with semiconductor as catalyst. Using semiconductor oxide (ZnO/TiO2) as catalyst, chromium-containing electroplating wastewater was treated by sunlight. After 90 minutes of sunlight irradiation (1 182.5W/m2), hexavalent chromium was reduced to trivalent chromium, and then trivalent chromium was removed in the form of chromium hydroxide, and the removal rate of chromium reached over 99%.
1.5 circulating chemical cleaning of tank edge
This technology was jointly developed by ERG/ Langzi Company in the United States and Ef fluentTreatmentLancy Company in the United Kingdom, so it is also called Langzi Method. A recovery tank, a chemical circulation rinsing tank and a water circulation rinsing tank are arranged behind the electroplating production line, and the treatment tank is arranged outside the workshop. The plated parts are rinsed with low-concentration reducing agent (sodium bisulfite or hydrazine hydrate) in the chemical circulation rinsing tank, so that the liquid brought out is reduced by 90%, and then the plated parts enter the water rinsing tank, and the solution after chemical rinsing continuously flows back to the treatment tank and circulates continuously. Alkali precipitation is carried out in the treatment tank, and its sludge discharge cycle is very long. Guangzhou Electric Apparatus Research Institute has developed three chemical rinsing processes for electroplating wastewater. The recycling rate of water is as high as 95%, which has the advantages of less consumption, less sludge and high purity. Sometimes, tank side circulation is combined with workshop circulation.
1.6 cement-based solidification method for treating neutralized waste residue
For toxic wastes that can't be treated temporarily, curing technology can be used to convert harmful and dangerous substances into harmless substances for final disposal. This can prevent the toxic ions of waste residue from entering water or soil again under natural conditions, causing secondary pollution. Of course, the leaching rate of hexavalent chromium in cement solidified blocks treated in this way is very low.
2. Comprehensive utilization of chromium-containing electroplating waste liquid and sludge
Because of the high content of harmful substances and complex components in aging wastewater containing chromium, it is necessary to treat and classify all kinds of wastewater separately before comprehensive utilization. For zinc plating passivation solution, copper passivation solution and aluminum electrolysis polishing solution containing phosphoric acid, the pH value is adjusted with acid and alkali. Anion exchange resin, just replace it with Na2CrO4.
2. 1 production of sodium dichromate from chromium sludge
Trivalent chromium can be oxidized by air into Na2CrO4 in high-temperature alkaline medium, and iron and zinc contained in sludge can be converted into corresponding soluble salts NaFeO2 and Na2ZnO2. When the alkaline melt is leached with water, most of the iron is decomposed into Fe(OH)3 and removed. Acidify the filtrate to ph value.
2.2 Production of Chrome Yellow
Sodium carbonate is used as precipitant to remove impurity metal ions from electroplating waste liquid, and then lead chrome yellow is produced by replacing part of sodium chromate with purified electroplating waste liquid. After adding saturated Na2CO3 solution into the electroplating solution, adjust the pH to 8.5 ~ 9.5, and filter the filtrate for later use. Under alkaline conditions, Cr3+ in the filter residue is oxidized to Cr6+ by H2O2, and then filtered, and the filtrate is mixed with the above filtrate. The filtrate reacts with lead nitrate solution and additives at 50 ~ 60℃ for 65438+/-0h, then it is filtered, washed with water, washed away soluble impurities such as chlorine and sulfate, dried and crushed to obtain the finished product lead chrome yellow. Using electroplating waste liquid to produce lead-chromium yellow not only solves the pollution problem, but also recovers chromium from electroplating waste liquid. It is estimated that treating 200 tons of electroplating waste liquid every year and recovering sodium chromate 18t annually can achieve an annual income of more than 40,000 yuan. The benefits are considerable.
2.3 Production of liquid chrome tanning agent and leather tanning agent basic chrome sulfate
The chromium-containing waste liquid is first treated with sodium hydroxide to remove metal ion impurities, and the pH is controlled to be 5.5 ~ 6.0, and then filtered, and the filtrate is reserved, and the sludge is harmlessly treated with ferrite. Then reducing agent glucose was added to the filtrate to reduce Na2Cr2O7 to Cr(OH)SO4, and further polymerization was carried out at 100℃. When the alkalinity is 40%, the molecular formula is 4Cr(OH)3.3Cr2(SO4)3, which is a chrome tanning agent. A leather factory in Wuji County, Hebei Province is using chromium-containing electroplating wastewater to produce liquid chrome tanning agent. According to the daily production of 5t liquid chrome tanning agent, the daily profit can reach more than 6000 yuan. It can be seen that the economic benefit of producing chrome tanning agent from chrome-containing waste liquid is very significant. In addition, chromium-containing sludge can be mixed with carbon powder and calcined at high temperature to prepare metallic chromium. Because chromium-containing sludge is the main variety of sludge in electroplating workshop, the recycling of sludge is different according to different electroplating treatment methods.
Electrolytic sludge:
(1) as the raw material of the medium temperature shift catalyst;
(2) Used as raw material of iron chromium red pigment.
Chemical sludge:
(1) Recovery of chromium hydroxide;
(2) Recovering chromium trioxide and polishing paste. Ferrite sludge is used as raw material for magnetic materials.
3. Concluding remarks
Some of the treatment methods and resource utilization of chromium-containing wastewater introduced above have been industrialized, and some are still in the basic research stage in the laboratory. In the actual use process, it is not necessarily limited to the above treatment methods, and the above treatment methods can also be used together. From the point of view of environmental protection, people will abandon the traditional chemical methods and choose microbial method and membrane separation method. Microbial method will represent the development trend of electroplating wastewater treatment in 2 1 century, and it can be predicted that microbial method will be more widely used in the near future.
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