With the continuous improvement of people's living standards and the development of industry, people pay more and more attention to the living environment. As we all know, almost all industrial sectors have electroplating processing, and it is inevitable to discharge waste water. However, chromium in wastewater is very toxic. The chromium in the electroplating of chromium-containing wastewater exists in the form of Cr 3+ and Cr 6+ , among which Cr 6 + is the most toxic, about 100 times that of Cr 3+ . Cr 6+ can cause lung cancer, intestinal diseases and anemia. Although China has dealt with electroplating wastewater very early, the treatment of electroplating wastewater has not been satisfactory to date. Therefore, electroplating wastewater must be strictly controlled and properly disposed of.
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I. Research progress of chromium-containing wastewater treatment methods at home and abroad Domestic and international chromium-containing wastewater treatment methods are widely used in industry, such as chemical reduction method, ion exchange method, adsorption method, electrolytic treatment method, countercurrent rinsing-evaporation concentration method, etc. . The author mainly introduces the methods of biological method, membrane separation method, xanthate method, photocatalysis method, and groove edge chemical rinsing which have appeared in recent years. It has developed rapidly and has become the focus of everyone's attention.
(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 (sulphate-reducing bacteria), SR-series complex-function bacteria, SR-complex bacteria, desulfurization bacteria, Bac. Dechromaticans, Zoolocaramigera, yeast, ambiguous pseudomons Bacteria, Pseudomonas fluorescens, Streptococcus faecalis, Enterobacter cloacae, and chromate reducing bacteria have been studied, from the single species of the past to the combination of the current multi-species, so that the treatment of wastewater is clean and pollution-free. The way to deal with. 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 Cr 6+ and Cr 3+ and oxidize NO 3 to NO 3- . It has been used in the treatment of chromium-containing wastewater from Egyptian light vehicle companies.
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, safe and reliable device for the discharge of water and other bacteria culture used; and less sludge, metal sludge recycling; to achieve the cleaner production, no sewage and waste discharge. Low investment, low energy consumption and low operating costs.
(2) Membrane separation method Membrane separation method uses a selective permeation 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. Membrane for the purpose of separating and 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. Treatment of wastewater containing chromium film, ionophores as TBP (tributyl phosphate), Span80 membrane stabilizers, easy to process operation, the device is simple, cheap and easily available raw materials. There is also the choice of non-ionic carriers, such as neutral amine used Alanmine336 (trioctylamine), with 2% Span80 as a surfactant, the choice of hexachlorobenzene 1,3 - butadiene (19%) and polybutadiene (74 The mixture of %) is used as a solvent, and the separation process is divided into steps of extraction, stripping, and the like. 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. [next]
(III) Xanthogenate Method In the 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 not only removes a variety of heavy metal ions, but also reduces Cr 6+ to Cr 3+ 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.
(4) Photocatalysis
   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 the semiconductor oxide (Zno/TiO 2 ) as a catalyst, the electroplating chromium-containing wastewater was treated by a solar light source, and the hexavalent chromium was reduced to trivalent chromium and then hydrogen by 90 min sunlight (1182.5 W/m 2 ). The chromium oxide form removes trivalent chromium and the removal rate of chromium is over 99%.
  (5) Slot-edge chemical rinsing This technology was developed by ERG/Lancy of the United States and Effluent Treatment Lancy of the United Kingdom, so it is also called 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.
(6) Cement-based curing method for neutralizing waste slag For toxic wastes that cannot be disposed of temporarily, solidification technology can be used to convert harmful dangerous materials into 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.
Second, the comprehensive utilization of electroplating chromium- containing waste liquid and sludge Because of the high content of harmful substances in electroplating chromium-containing aging waste liquid, the composition is complex, and various waste liquids should be treated separately and classified before comprehensive utilization. Passivation of zinc plating solution, copper and aluminum passivation solution containing electrolytic polishing with phosphoric acid are the pH adjusted; the anion exchange resin, it becomes simply to Na 2 CrO 4.
(1) Production of red strontium sodium by using chromium sludge In the high temperature alkaline condition medium Na 2 CrO 4 , trivalent chromium can be oxidized by air to Na 2 Cr 2 O 7, and the iron , zinc and the like contained in the sludge are converted into corresponding Soluble salt NaFeO 2 , Na 2 ZnO 2 . When the alkali melt is leached with water, most of the iron is decomposed into Fe(OH) 3 precipitate and removed. The filtrate is acidified to pH<4, and Na 2 CrO 4 is converted to Na 2 . Cr 2 O 7 was crystallized by the difference in solubility between Na 2 SO 4 and Na 2 Cr 2 O 7 . The conditions for preparing red strontium sodium using high-temperature alkaline chromium oxide sludge are n(Na 2 CO 3 ):n(Cr 2 O 3 )=3.0:1.0, temperature 780 ° C, time 2.5 h, and the conversion of chromium is above 85%.
(2) Production of chrome yellow The use of soda ash as a precipitant to remove 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 Na 2 CO 3 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 Cr 3+ in the filter residue was oxidized to Cr 6+ with H 2 O 2 under basic conditions and filtered, and the filtrate was 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.
(3) Production of liquid chrome tanning agent and leather tanning agent Basic chromium sulphate chromium- containing waste liquid First remove the metal ion impurities with sodium hydroxide, control pH=5.5-6.0, then filter, the filtrate is ready for use, sludge ferrite Harmless treatment. Then, the reducing agent glucose is added to the filtrate to reduce Na 2 Cr 2 O 7 to Cr(OH)SO 4 , and further polymerization is carried out at 100 ° C. When the basicity is 40%, the molecular formula is 4Cr (OH). 3 • 3Cr 2 (SO 4 ) 3 , which is a 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.
III. Concluding remarks The treatment methods and resource utilization of chromium-containing wastewater introduced 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.