Through the investigation of data, the most common methods for the treatment of chromium-containing wastewater in the electroplating industry are reduction method and electrolysis method. The process is mature and the operation effect is good. However, many other methods have been studied recently, and comprehensive comparisons will find that these methods also have advantages and disadvantages. As a new method, they have their own lessons.
The data on the treatment of chromium-containing wastewater from electroplating production lines are summarized as follows:
A reduction precipitation method
The chemical reduction method uses a reducing agent such as ferrous sulfate, sulfite or sulfur dioxide to reduce hexavalent chromium in the wastewater to trivalent chromium ions, and adjusts the pH value by adding alkali to form a chromium hydroxide precipitate by removing the trivalent chromium. This method has low equipment investment and operating costs and is mainly used for batch processing.
The common treatment process is to adjust the pH of the wastewater to 2 to 3 with sulfuric acid in the first reaction tank, then add the reducing agent, and adjust the pH value to 7-8 with NaOH or Ca(OH)2 in the next reaction tank. Cr(OH)3 was precipitated, and a coagulant was added to precipitate Cr(OH)3. The improved process is to directly add ferrous sulfate in the first reaction tank, adjust the pH value to 7-8 with NaOH or Ca(OH)2, form Cr(OH)3 precipitate, and add coagulant to make Cr ( OH) 3 precipitate removed. After using this technology, the daily treatment volume of chromium-containing wastewater is 1000 M3, and the chromium content in wastewater is 10 mg/l. This technology is suitable for the treatment of chromium-containing industrial wastewater.
In some reports, it has also been mentioned that the treatment of electroplating chromium-containing wastewater by using polyaluminum ferric chloride is mentioned. Polyaluminum ferric chloride has the advantages of traditional flocculants PAC and PFC. The formed flocs are large and heavy, and the sedimentation speed is fast. Its effluent color is better than that of polyferric chloride, and the turbidity removal effect and floc sedimentation performance are better than polyaluminium chloride. The specific report content is attached to the text.
2. Electrolytic precipitation filtration
1. Process overview
The electroplating chromium-containing wastewater is first passed through the grid to remove the suspended solids of the larger particles, then flows to the regulating tank, equalizes the water quality, and then is pumped to the electrolysis cell for electrolysis. During the electrolysis process, the anode iron plate dissolves into ferrous ions, in acidic conditions. The lower ferrous ion reduces the hexavalent chromium ion to the trivalent chromium ion, and at the same time, the pH of the wastewater is gradually increased due to the precipitation of hydrogen on the cathode plate, and finally it is neutral. At this time, Cr3+ and Fe3+ are precipitated by hydroxide precipitation, and the effluent after electrolysis first passes through the primary settling tank, and then continuously passes through (two-stage sedimentation filter tank from top to bottom). The primary filtration tank has packing: charcoal, coke, slag; secondary filtration tank contains filler: anthracite, quartz sand. The sediment in the sewage is filtered and adsorbed by the filter tank packing, and the effluent flows into the drainage inspection well. It is then pumped into the circulating pool as cooling water. The charcoal, coke, anthracite and slag used for filtration are collected regularly in the boiler room and mixed.
2. Main equipment
1 adjustment tank; 1 primary sedimentation tank, 2 sedimentation filter tanks; 1 circulating water tank; 1 power supply control cabinet, electrolyzer, electrolysis power supply, 1 electrolysis voltage; 5 pumps.
3. Results and analysis
The electroplating wastewater treatment equipment of an electroplating plant is sampled multiple times at different intervals under normal working conditions.
The electroplating chromium-containing wastewater is completely reused by the electrolytic precipitation filtration process, and the filler in the filter tank is regularly concentrated in the boiler room to be mixed, thereby achieving the purpose of comprehensively treating the chromium-containing wastewater.
Although the treatment technology is reliable and easy to operate, it should pay attention to several aspects: a) the need to periodically replace the plates; b) the possibility of redissolving of chromium hydroxide in certain acidic media; c) in the sedimentation filter tank The packing must be treated regularly and burned thoroughly, otherwise it will cause secondary pollution. This shows that it is very important to strengthen the management of the processing facilities.
4 Conclusion
1) The treatment process is thorough for the treatment of chromium-containing wastewater. The filler in the filter tank is treated regularly and will not cause secondary pollution. After treatment, the clean water can be reused, which can save water resources and has obvious economic benefits.
2) The process investment is small, the technology is mature, the operation is stable and reliable, the operation is convenient, the management is easy, and it is suitable for electroplating production enterprises of different scales.
Research progress in other domestic and international chromium-containing wastewater treatment methods
1.1 Biological law
The biological treatment of chromium-containing wastewater has started in recent years at home and abroad. The biological method is a high-tech biotechnology for treating electroplating wastewater. It is suitable for wastewater treatment in large, medium and small electroplating plants. It has great practical value and is easy to promote. SRB bacteria (sulfate-reducing bacteria), SR-series complex-function bacteria, SR-complex bacteria, desulfurization bacteria, Bac. Deomaticans, Zoolocaramiger a, yeast, ambiguous Cytobacteria, Pseudomonas fluorescens, Streptococcus faecalis, Enterobacter cloacae, Chromate-reducing bacteria, etc., from the past single species to the combination of the current multi-species, so that the treatment of wastewater is clean, no The road to pollution treatment. The electroplating wastewater is mixed with other industrial wastes and human waste, using lime as a coagulant, and then subjected to a chemical-condensation-deposition treatment. Studies have shown that biological treatments mixed with active 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 companies [9].
The biological treatment of electroplating wastewater technology relies on artificially cultured functional bacteria, which have electrostatic adsorption, catalytic conversion of enzymes, complexation, flocculation, occlusion coprecipitation and buffering of pH. The method is simple in operation, safe and reliable in equipment, and the discharged water is used for cultivating bacteria and other uses; and the amount of sludge is small, and the metal in the sludge is recycled; the clean production, no sewage and waste residue are realized. Low investment, low energy consumption and low operating costs.
1.2 Membrane separation method
Membrane separation method uses a selective permeable membrane as a separation medium. When there is some driving force (such as pressure difference, concentration difference, potential difference, etc.) on both sides of the membrane, the raw material side component selectively permeates the membrane to achieve separation. The purpose of removing harmful components. At present, the more mature processes applied in the industry are electrodialysis, reverse osmosis, ultrafiltration, and liquid membrane. Other methods such as membrane bioreactors and microfiltration are still in the basic theoretical research stage and have not yet been applied in industry. The electrodialysis method uses a potential difference as a driving force under the action of a direct current electric field, and utilizes the selective permeability of the ion exchange membrane to purify the wastewater. The reverse osmosis method achieves separation by diffusion of a solvent under a certain applied pressure. Ultrafiltration is also a membrane process that performs solute separation under the push of static pressure. The liquid film includes an unsupported liquid film, a carrier liquid film, an impregnated liquid film, and the like. When the liquid film is dispersed in the electroplating wastewater, the flow carrier selectively complexes the heavy metal ions at the interface of the outer membrane, then diffuses in the liquid membrane, decomposes at the interface inside the membrane, and the heavy metal ions enter the inner phase of the membrane to be enriched, and the mobile carrier returns. The interface of the outer membrane is so continuous that the wastewater is purified. The advantages of membrane separation method are high energy conversion rate, simple device, easy operation, easy control and high separation efficiency. However, the investment is large, the running cost is high, and the life of the film is short. It is mainly used to recover substances with high added value, 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 fully recycled. The whole process can be operated under high temperature and wider pH conditions, and the concentration of recovered liquid can be greatly improved. The disadvantage is that it can only be used to recover ionic components. Liquid membrane treatment of chromium-containing wastewater, ionophore is TBP (tributyl phosphate), and Span80 is a membrane stabilizer. The process is convenient, the equipment is simple, and the raw materials are cheap and easy to obtain. Nonionic carriers are also used, such as neutral amines, Alanmine 336 (trioctylamine), 2% Span 80 as surfactant, and hexachloro1,3-butadiene (19%) and polybutadiene (74). The mixture of %) is used as a solvent, and the separation process is divided into steps of extraction, stripping, etc. [10,. Recently, microfiltration has also been used to treat heavy metal-containing wastewater, and it can remove toxic heavy metals such as cadmium and chromium in industrial wastewater such as metal plating.
1.3 Xanthate method
In the 1970s, the United States developed a new type of insoluble heavy metal ion remover ISX, which is easy to use and low in water treatment costs. ISX not only removes a variety of heavy metal ions, but also reduces Cr6+ to Cr3+ under acidic conditions, but with poor stability. The insoluble starch xanthate has a good effect of removing chromium, the removal rate is >99%, and the residue is stable, and does not cause secondary pollution. Zhong Changgeng et al. used rice straw instead of starch to make straw xanthate, and treated chromium-containing wastewater. The removal rate of chromium is high, and it is easy to reach the discharge standard. The researchers believe that the removal of chromium from straw xanthate is a combination of the chromium salt of xanthogen and chromium hydroxide through several processes of precipitation and adsorption, but the chromium salt of xanthogen plays a major role. The method has low cost, quick response, simple operation and no secondary pollution.
1.4 Photocatalysis [20, 21]
Photocatalysis is a new method that has been rapidly developed in the treatment of pollutants in water in recent years. In particular, there have been many reports on the use of semiconductors as catalysts for the treatment of organic pollutants in water. Using a semiconductor oxide (ZnO/TiO2) as a catalyst, the chromium-containing wastewater was treated by a solar light source. After 90 minutes of sunlight (1182.5 W/m2), the hexavalent chromium was reduced to trivalent chromium and then to chromium hydroxide. The form removes trivalent chromium and the removal rate of chromium is over 99%.
1.5 groove side cycle chemical rinsing
This technology was developed by ERG/Lancy of the United States and Ef Fluent Treatment Lancy of the United Kingdom, hence the name Lancy. It is provided with a recovery tank, a chemical circulation rinsing tank and a water circulation rinsing tank after the electroplating production line, and the treatment tank is arranged outside the workshop. The plated part is rinsed in a chemical circulating rinse tank with a low concentration of reducing agent (sodium bisulfite or hydrazine hydrate), so that 90% of the stripped liquid is reduced, and then the plated part enters the water rinse tank, and the chemically rinsed solution is Continuously flow back to the processing tank and continuously circulate. The alkali precipitation is carried out in a treatment tank, which has a long sludge discharge period. Guangzhou Electrical Apparatus Research Institute has developed a tank-side chemical rinsing treatment process for three types of electroplating wastewaters. The water reuse rate is as high as 95%, with less drug administration, less sludge and high purity. Sometimes, a combination of a grooved cycle and a shop cycle is used.
1.6 Cement-based curing method for neutralizing waste residue
For toxic wastes that cannot be disposed of temporarily, curing techniques can be used to convert harmful hazardous materials into final disposal methods for non-hazardous materials. In this way, the toxic ions of the waste residue can be prevented from entering the water body or the soil again under natural conditions, causing secondary pollution. Of course, the leaching rate of hexavalent chromium in the cement cured block thus treated is very low.
2 Comprehensive utilization of electroplating chromium-containing waste liquid and sludge
Due to the high content of harmful substances in the electroplating chrome-containing waste liquid and the complex composition, various waste liquids should be treated separately and classified before comprehensive utilization. For galvanized passivation solution, copper passivation solution and aluminum electrolysis polishing solution containing phosphoric acid, the pH is adjusted with acid and alkali; for anion exchange resin, it is only necessary to change it to Na2CrO4.
2.1 Production of sodium sulphate using chromium sludge
In the high temperature alkaline condition medium Na2CrO4, the trivalent chromium can be oxidized by air to Na2Cr2O7, and the iron, zinc and the like contained in the sludge are converted into the corresponding soluble salts NaFeO2 and Na2ZnO2. When the alkali melt is leached with water, most of the iron is decomposed into Fe(OH)3 precipitate and removed. The filtrate was acidified to pH<4, and Na2CrO4 was converted to Na2Cr2O7, and the difference in solubility between Na2SO4 and Na2Cr2O7 was crystallized separately. The conditions for preparing red strontium sodium using high-temperature alkaline chromium oxide sludge are n(Na2CO3):n(Cr2O3)=3.0:1.0, temperature 780 ° C, time 2.5 h, and the conversion of chromium is above 85%.
2.2 Production of chrome yellow The use of soda ash as a precipitant to remove the impurity metal ions in the electroplating waste liquid, and then use the purified electroplating waste liquid to replace part of the red strontium sodium to produce lead chrome yellow. After adding the Na2CO3 saturated solution to the plating solution, the pH was adjusted to 8.5 to 9.5. Filtration was carried out and the filtrate was used. The Cr3+ in the filter residue is oxidized to Cr6+ with H2O2 under alkaline conditions, and after filtration, the filtrate is mixed with the above filtrate. The filtrate is reacted with lead nitrate solution and auxiliary agent at 50-60 ° C for 1 h, then filtered, washed with water, washed away with chlorine, sulfate and other soluble impurities, and then dried and pulverized to obtain the finished lead chrome yellow. The use of electroplating waste liquid to produce lead chrome yellow not only solves the pollution problem, but also recycles the chromium in the electroplating waste liquid. It is estimated that the annual treatment of electroplating waste liquid 200t, the annual average recovery of 18t red strontium sodium, can achieve 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 chromium chromate chromium-containing waste liquid First remove the metal ion impurities with sodium hydroxide, control the pH = 5.5 ~ 6.0, then filter, the filtrate is ready for use, the sludge is harmless with ferrite Processing. Then, the reducing agent glucose is added to the filtrate to reduce Na2Cr2O7 to Cr(OH)SO4, and further polymerization is carried out at 100 ° C. When the basicity is 40%, the molecular formula is 4Cr(OH)3 ̇3Cr2(SO4). 3, that is, chrome tanning agent. A leather factory in Wuji County, Hebei Province, uses liquid chromium-containing wastewater to produce liquid chrome tanning agent. According to the daily production of 5t liquid chrome tanning agent, the profit per day is more than 6,000 yuan. It can be seen that the economic benefit of using chromium-containing waste liquid to produce chrome tanning agent is very significant. Alternatively, the chromium-containing sludge may be mixed with carbon powder and calcined at a high temperature to obtain metallic chromium. Because chromium-containing sludge is the main type of sludge in electroplating workshops, depending on the plating treatment method, the recycling of sludge is also different. Electrolytic sludge: (1) as a raw material for the medium temperature shift catalyst; (2) as a raw material for the iron chrome red pigment. Chemical sludge: (1) recovery of chromium hydroxide; (2) recovery of chromium oxide polishing paste. Ferrite sludge is used as a raw material for magnetic materials.
3 Conclusion
The treatment methods and resource utilization of the chromium-containing wastewater described above have already achieved industrialization, and some are still in the basic research stage of the laboratory. It is not necessarily limited to the above-described processing method in actual use, and the above-described several processing methods may be used together. From the perspective of environmental protection, people will abandon the traditional chemical method and choose the microbial method and membrane separation method. The microbiological method will represent the development trend of the treatment of chromium-containing wastewater in the 21st century. It is expected that the microbial method will be more widely used in the near future.
The data on the treatment of chromium-containing wastewater from electroplating production lines are summarized as follows:
A reduction precipitation method
The chemical reduction method uses a reducing agent such as ferrous sulfate, sulfite or sulfur dioxide to reduce hexavalent chromium in the wastewater to trivalent chromium ions, and adjusts the pH value by adding alkali to form a chromium hydroxide precipitate by removing the trivalent chromium. This method has low equipment investment and operating costs and is mainly used for batch processing.
The common treatment process is to adjust the pH of the wastewater to 2 to 3 with sulfuric acid in the first reaction tank, then add the reducing agent, and adjust the pH value to 7-8 with NaOH or Ca(OH)2 in the next reaction tank. Cr(OH)3 was precipitated, and a coagulant was added to precipitate Cr(OH)3. The improved process is to directly add ferrous sulfate in the first reaction tank, adjust the pH value to 7-8 with NaOH or Ca(OH)2, form Cr(OH)3 precipitate, and add coagulant to make Cr ( OH) 3 precipitate removed. After using this technology, the daily treatment volume of chromium-containing wastewater is 1000 M3, and the chromium content in wastewater is 10 mg/l. This technology is suitable for the treatment of chromium-containing industrial wastewater.
In some reports, it has also been mentioned that the treatment of electroplating chromium-containing wastewater by using polyaluminum ferric chloride is mentioned. Polyaluminum ferric chloride has the advantages of traditional flocculants PAC and PFC. The formed flocs are large and heavy, and the sedimentation speed is fast. Its effluent color is better than that of polyferric chloride, and the turbidity removal effect and floc sedimentation performance are better than polyaluminium chloride. The specific report content is attached to the text.
2. Electrolytic precipitation filtration
1. Process overview
The electroplating chromium-containing wastewater is first passed through the grid to remove the suspended solids of the larger particles, then flows to the regulating tank, equalizes the water quality, and then is pumped to the electrolysis cell for electrolysis. During the electrolysis process, the anode iron plate dissolves into ferrous ions, in acidic conditions. The lower ferrous ion reduces the hexavalent chromium ion to the trivalent chromium ion, and at the same time, the pH of the wastewater is gradually increased due to the precipitation of hydrogen on the cathode plate, and finally it is neutral. At this time, Cr3+ and Fe3+ are precipitated by hydroxide precipitation, and the effluent after electrolysis first passes through the primary settling tank, and then continuously passes through (two-stage sedimentation filter tank from top to bottom). The primary filtration tank has packing: charcoal, coke, slag; secondary filtration tank contains filler: anthracite, quartz sand. The sediment in the sewage is filtered and adsorbed by the filter tank packing, and the effluent flows into the drainage inspection well. It is then pumped into the circulating pool as cooling water. The charcoal, coke, anthracite and slag used for filtration are collected regularly in the boiler room and mixed.
2. Main equipment
1 adjustment tank; 1 primary sedimentation tank, 2 sedimentation filter tanks; 1 circulating water tank; 1 power supply control cabinet, electrolyzer, electrolysis power supply, 1 electrolysis voltage; 5 pumps.
3. Results and analysis
The electroplating wastewater treatment equipment of an electroplating plant is sampled multiple times at different intervals under normal working conditions.
The electroplating chromium-containing wastewater is completely reused by the electrolytic precipitation filtration process, and the filler in the filter tank is regularly concentrated in the boiler room to be mixed, thereby achieving the purpose of comprehensively treating the chromium-containing wastewater.
Although the treatment technology is reliable and easy to operate, it should pay attention to several aspects: a) the need to periodically replace the plates; b) the possibility of redissolving of chromium hydroxide in certain acidic media; c) in the sedimentation filter tank The packing must be treated regularly and burned thoroughly, otherwise it will cause secondary pollution. This shows that it is very important to strengthen the management of the processing facilities.
4 Conclusion
1) The treatment process is thorough for the treatment of chromium-containing wastewater. The filler in the filter tank is treated regularly and will not cause secondary pollution. After treatment, the clean water can be reused, which can save water resources and has obvious economic benefits.
2) The process investment is small, the technology is mature, the operation is stable and reliable, the operation is convenient, the management is easy, and it is suitable for electroplating production enterprises of different scales.
Research progress in other domestic and international chromium-containing wastewater treatment methods
1.1 Biological law
The biological treatment of chromium-containing wastewater has started in recent years at home and abroad. The biological method is a high-tech biotechnology for treating electroplating wastewater. It is suitable for wastewater treatment in large, medium and small electroplating plants. It has great practical value and is easy to promote. SRB bacteria (sulfate-reducing bacteria), SR-series complex-function bacteria, SR-complex bacteria, desulfurization bacteria, Bac. Deomaticans, Zoolocaramiger a, yeast, ambiguous Cytobacteria, Pseudomonas fluorescens, Streptococcus faecalis, Enterobacter cloacae, Chromate-reducing bacteria, etc., from the past single species to the combination of the current multi-species, so that the treatment of wastewater is clean, no The road to pollution treatment. The electroplating wastewater is mixed with other industrial wastes and human waste, using lime as a coagulant, and then subjected to a chemical-condensation-deposition treatment. Studies have shown that biological treatments mixed with active 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 companies [9].
The biological treatment of electroplating wastewater technology relies on artificially cultured functional bacteria, which have electrostatic adsorption, catalytic conversion of enzymes, complexation, flocculation, occlusion coprecipitation and buffering of pH. The method is simple in operation, safe and reliable in equipment, and the discharged water is used for cultivating bacteria and other uses; and the amount of sludge is small, and the metal in the sludge is recycled; the clean production, no sewage and waste residue are realized. Low investment, low energy consumption and low operating costs.
1.2 Membrane separation method
Membrane separation method uses a selective permeable membrane as a separation medium. When there is some driving force (such as pressure difference, concentration difference, potential difference, etc.) on both sides of the membrane, the raw material side component selectively permeates the membrane to achieve separation. The purpose of removing harmful components. At present, the more mature processes applied in the industry are electrodialysis, reverse osmosis, ultrafiltration, and liquid membrane. Other methods such as membrane bioreactors and microfiltration are still in the basic theoretical research stage and have not yet been applied in industry. The electrodialysis method uses a potential difference as a driving force under the action of a direct current electric field, and utilizes the selective permeability of the ion exchange membrane to purify the wastewater. The reverse osmosis method achieves separation by diffusion of a solvent under a certain applied pressure. Ultrafiltration is also a membrane process that performs solute separation under the push of static pressure. The liquid film includes an unsupported liquid film, a carrier liquid film, an impregnated liquid film, and the like. When the liquid film is dispersed in the electroplating wastewater, the flow carrier selectively complexes the heavy metal ions at the interface of the outer membrane, then diffuses in the liquid membrane, decomposes at the interface inside the membrane, and the heavy metal ions enter the inner phase of the membrane to be enriched, and the mobile carrier returns. The interface of the outer membrane is so continuous that the wastewater is purified. The advantages of membrane separation method are high energy conversion rate, simple device, easy operation, easy control and high separation efficiency. However, the investment is large, the running cost is high, and the life of the film is short. It is mainly used to recover substances with high added value, 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 fully recycled. The whole process can be operated under high temperature and wider pH conditions, and the concentration of recovered liquid can be greatly improved. The disadvantage is that it can only be used to recover ionic components. Liquid membrane treatment of chromium-containing wastewater, ionophore is TBP (tributyl phosphate), and Span80 is a membrane stabilizer. The process is convenient, the equipment is simple, and the raw materials are cheap and easy to obtain. Nonionic carriers are also used, such as neutral amines, Alanmine 336 (trioctylamine), 2% Span 80 as surfactant, and hexachloro1,3-butadiene (19%) and polybutadiene (74). The mixture of %) is used as a solvent, and the separation process is divided into steps of extraction, stripping, etc. [10,. Recently, microfiltration has also been used to treat heavy metal-containing wastewater, and it can remove toxic heavy metals such as cadmium and chromium in industrial wastewater such as metal plating.
1.3 Xanthate method
In the 1970s, the United States developed a new type of insoluble heavy metal ion remover ISX, which is easy to use and low in water treatment costs. ISX not only removes a variety of heavy metal ions, but also reduces Cr6+ to Cr3+ under acidic conditions, but with poor stability. The insoluble starch xanthate has a good effect of removing chromium, the removal rate is >99%, and the residue is stable, and does not cause secondary pollution. Zhong Changgeng et al. used rice straw instead of starch to make straw xanthate, and treated chromium-containing wastewater. The removal rate of chromium is high, and it is easy to reach the discharge standard. The researchers believe that the removal of chromium from straw xanthate is a combination of the chromium salt of xanthogen and chromium hydroxide through several processes of precipitation and adsorption, but the chromium salt of xanthogen plays a major role. The method has low cost, quick response, simple operation and no secondary pollution.
1.4 Photocatalysis [20, 21]
Photocatalysis is a new method that has been rapidly developed in the treatment of pollutants in water in recent years. In particular, there have been many reports on the use of semiconductors as catalysts for the treatment of organic pollutants in water. Using a semiconductor oxide (ZnO/TiO2) as a catalyst, the chromium-containing wastewater was treated by a solar light source. After 90 minutes of sunlight (1182.5 W/m2), the hexavalent chromium was reduced to trivalent chromium and then to chromium hydroxide. The form removes trivalent chromium and the removal rate of chromium is over 99%.
1.5 groove side cycle chemical rinsing
This technology was developed by ERG/Lancy of the United States and Ef Fluent Treatment Lancy of the United Kingdom, hence the name Lancy. It is provided with a recovery tank, a chemical circulation rinsing tank and a water circulation rinsing tank after the electroplating production line, and the treatment tank is arranged outside the workshop. The plated part is rinsed in a chemical circulating rinse tank with a low concentration of reducing agent (sodium bisulfite or hydrazine hydrate), so that 90% of the stripped liquid is reduced, and then the plated part enters the water rinse tank, and the chemically rinsed solution is Continuously flow back to the processing tank and continuously circulate. The alkali precipitation is carried out in a treatment tank, which has a long sludge discharge period. Guangzhou Electrical Apparatus Research Institute has developed a tank-side chemical rinsing treatment process for three types of electroplating wastewaters. The water reuse rate is as high as 95%, with less drug administration, less sludge and high purity. Sometimes, a combination of a grooved cycle and a shop cycle is used.
1.6 Cement-based curing method for neutralizing waste residue
For toxic wastes that cannot be disposed of temporarily, curing techniques can be used to convert harmful hazardous materials into final disposal methods for non-hazardous materials. In this way, the toxic ions of the waste residue can be prevented from entering the water body or the soil again under natural conditions, causing secondary pollution. Of course, the leaching rate of hexavalent chromium in the cement cured block thus treated is very low.
2 Comprehensive utilization of electroplating chromium-containing waste liquid and sludge
Due to the high content of harmful substances in the electroplating chrome-containing waste liquid and the complex composition, various waste liquids should be treated separately and classified before comprehensive utilization. For galvanized passivation solution, copper passivation solution and aluminum electrolysis polishing solution containing phosphoric acid, the pH is adjusted with acid and alkali; for anion exchange resin, it is only necessary to change it to Na2CrO4.
2.1 Production of sodium sulphate using chromium sludge
In the high temperature alkaline condition medium Na2CrO4, the trivalent chromium can be oxidized by air to Na2Cr2O7, and the iron, zinc and the like contained in the sludge are converted into the corresponding soluble salts NaFeO2 and Na2ZnO2. When the alkali melt is leached with water, most of the iron is decomposed into Fe(OH)3 precipitate and removed. The filtrate was acidified to pH<4, and Na2CrO4 was converted to Na2Cr2O7, and the difference in solubility between Na2SO4 and Na2Cr2O7 was crystallized separately. The conditions for preparing red strontium sodium using high-temperature alkaline chromium oxide sludge are n(Na2CO3):n(Cr2O3)=3.0:1.0, temperature 780 ° C, time 2.5 h, and the conversion of chromium is above 85%.
2.2 Production of chrome yellow The use of soda ash as a precipitant to remove the impurity metal ions in the electroplating waste liquid, and then use the purified electroplating waste liquid to replace part of the red strontium sodium to produce lead chrome yellow. After adding the Na2CO3 saturated solution to the plating solution, the pH was adjusted to 8.5 to 9.5. Filtration was carried out and the filtrate was used. The Cr3+ in the filter residue is oxidized to Cr6+ with H2O2 under alkaline conditions, and after filtration, the filtrate is mixed with the above filtrate. The filtrate is reacted with lead nitrate solution and auxiliary agent at 50-60 ° C for 1 h, then filtered, washed with water, washed away with chlorine, sulfate and other soluble impurities, and then dried and pulverized to obtain the finished lead chrome yellow. The use of electroplating waste liquid to produce lead chrome yellow not only solves the pollution problem, but also recycles the chromium in the electroplating waste liquid. It is estimated that the annual treatment of electroplating waste liquid 200t, the annual average recovery of 18t red strontium sodium, can achieve 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 chromium chromate chromium-containing waste liquid First remove the metal ion impurities with sodium hydroxide, control the pH = 5.5 ~ 6.0, then filter, the filtrate is ready for use, the sludge is harmless with ferrite Processing. Then, the reducing agent glucose is added to the filtrate to reduce Na2Cr2O7 to Cr(OH)SO4, and further polymerization is carried out at 100 ° C. When the basicity is 40%, the molecular formula is 4Cr(OH)3 ̇3Cr2(SO4). 3, that is, chrome tanning agent. A leather factory in Wuji County, Hebei Province, uses liquid chromium-containing wastewater to produce liquid chrome tanning agent. According to the daily production of 5t liquid chrome tanning agent, the profit per day is more than 6,000 yuan. It can be seen that the economic benefit of using chromium-containing waste liquid to produce chrome tanning agent is very significant. Alternatively, the chromium-containing sludge may be mixed with carbon powder and calcined at a high temperature to obtain metallic chromium. Because chromium-containing sludge is the main type of sludge in electroplating workshops, depending on the plating treatment method, the recycling of sludge is also different. Electrolytic sludge: (1) as a raw material for the medium temperature shift catalyst; (2) as a raw material for the iron chrome red pigment. Chemical sludge: (1) recovery of chromium hydroxide; (2) recovery of chromium oxide polishing paste. Ferrite sludge is used as a raw material for magnetic materials.
3 Conclusion
The treatment methods and resource utilization of the chromium-containing wastewater described above have already achieved industrialization, and some are still in the basic research stage of the laboratory. It is not necessarily limited to the above-described processing method in actual use, and the above-described several processing methods may be used together. From the perspective of environmental protection, people will abandon the traditional chemical method and choose the microbial method and membrane separation method. The microbiological method will represent the development trend of the treatment of chromium-containing wastewater in the 21st century. It is expected that the microbial method will be more widely used in the near future.
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