Up to now, electroplating industry is an indispensable link in industrial development. Now, in addition to developing technology that can replace its function, the focus is on the prevention and control of electroplating pollution. The sludge produced in electroplating production process contains many existing treatment technologies, so the pure and harmless treatment of electroplating sludge is still the mainstream of the existing treatment technologies. In a word, the recycling technology of electroplating sludge will be the key research direction of heavy metal pollution control.
1 harmless treatment of electroplating sludge
1. 1 curing/stabilization technology
Solidification/stabilization technology is an important technology for harmless treatment of electroplating sludge. It mainly includes cement curing, lime curing and thermoplastic curing. Commonly used curing agents are cement, lime, asphalt, glass, soil curing agent and so on. Mixed with sludge for solidification, so that harmful metals in sludge are sealed in the solidified body, and the purpose of eliminating pollution is achieved. Among them, cement solidification is the most commonly used technology, and it should also be a relatively mature treatment technology. Wang Jiyuan and others obtained through experiments that the compressive strength can reach above 30MPa by adding appropriate additives and adjusting cement: electroplating heavy metal sludge: river sand: activated alumina: sodium silicate = 1:0.8:0.2:0.08:0.06. ARoy et al. further studied the method of curing heavy metals (including chromium, nickel, tin, etc.). ) with cement and fly ash, so as to achieve the purpose of treating waste with waste and saving cost. Tu Jie et al. used HAS soil curing agent to cure electroplating sludge at room temperature instead of traditional curing substrate, and obtained slope protection bricks with good anti-leaching, anti-corrosion, anti-permeability and sufficient mechanical strength. This solidification process opens up a new way for the resource utilization of electroplating sludge. Zhong Yufeng and others used cement and fine sand as solidification matrix to treat electroplating sludge containing heavy metals Ni, Cr and Cu. Through the leaching experiment of solidified blocks, it is found that cement can effectively solidify electroplating sludge, and adding a proper amount of chelating agent KS-3 in the solidification process can improve the solidification effect.
1.2 thermochemical treatment technology
Thermochemical treatment technologies (such as incineration, roasting, smelting, ion arc and microwave, etc.). Decomposition of waste at high temperature reduces the toxicity of some highly toxic components, realizes rapid and significant volume reduction, and utilizes the useful components of waste. At present, the research on thermochemical treatment technology of electroplating sludge, especially the migration characteristics of heavy metals during incineration treatment of electroplating sludge, has the advantage of greatly reducing the volume of electroplating sludge and its harm to the environment. However, due to high energy consumption and certain requirements for incineration equipment and conditions, it is difficult for ordinary small electroplating manufacturers to bear huge treatment costs, and it is easy to cause secondary pollution to the environment during incineration, so this treatment method is relatively difficult to popularize.
2 electroplating sludge resource treatment
Electroplating sludge itself is also a kind of resource, which contains a variety of metals necessary for industry, such as chromium, nickel and zinc. Therefore, the heavy metals can be recycled or directly used as production resources, which not only solves the heavy metal pollution, but also realizes the recycling of non-renewable resources, and truly forms a sustainable development model.
2. 1 Chemical recovery of valuable metals
The chemical recovery of valuable metals only refers to the separation and recovery of heavy metals from electroplating sludge by chemical separation and extraction. These methods include acid leaching, ammonia leaching, chemical precipitation, ion exchange membrane method and so on.
2. 1. 1 acid leaching method and ammonia leaching method
Acid leaching is one of the most widely used leaching methods in hydrometallurgy. Commonly used leaching agents are hydrochloric acid, sulfuric acid, nitric acid and aqua regia. Most metals in electroplating sludge exist in the form of hydroxide or oxide. Heavy metals in sludge can exist in the form of ions or complexes by acid leaching, and then the heavy metals in leachate can be separated and selectively recovered by mixed ammonia solution or organic solution. The recovered heavy metals include high-grade metal simple substance or metal salt.
The ammonia leaching method usually uses ammonia water solution as leaching agent, and adopts the process of ammonia complexation leaching-ammonia evaporation-sulfuric acid leaching of hydrolysis residue-solvent extraction-metal salt crystallization recovery, in which the recovery rates of copper, zinc, nickel, chromium and iron are greater than 93%, 9 1%, 88%, 98% and 98% respectively.
When electroplating sludge is treated by acid leaching or ammonia leaching, the total recovery rate of valuable metals and the difficulty of separating them from other impurities are mainly controlled by the leaching rate of valuable metals and the selectivity of leaching solution to valuable metals and impurities. The main feature of acid leaching method is that the leaching effect of valuable metals such as copper, zinc and nickel is good, but the selectivity to impurities is low, especially to impurities such as chromium and iron. Ammonia leaching method has high selectivity for impurities such as chromium and iron, but low leaching rate for copper, zinc and nickel.
2. 1.2 ion exchange membrane method
Due to the selective permeability of ion exchange membrane to ions, many separation methods of ion exchange membrane in industry have important application value in the process of metallurgical solution separation. The ion exchange membrane method is to put the liquid membrane in the sludge leaching solution, and the mobile carrier selectively complexs the metal ions outside the membrane, then diffuses into the membrane and contacts and complexs on the membrane, and finally makes the metal ions enter the membrane. This method is repeated repeatedly, and finally the metal ions in the membrane are enriched, the wastewater is purified, and the metal ions are reused.
2.2 Biological treatment technology
Biological treatment technology mainly reduces and metabolizes some heavy metals in sludge through microorganisms. However, this technology is still in the exploratory stage and has not yet formed a systematic treatment scheme. Only through some individual experiments, it has been proved that microorganisms can reduce and metabolize some heavy metals, but the mechanism of microbial reduction and metabolism of heavy metals has not been fully understood. For example, SSilverMarques et al. use Pseudomonas to reduce Cr3+. Bewtra's experiment shows that bacteria can effectively transform metal ions in electroplating sludge into water-insoluble sulfides. Wu Ganjing and others have studied a new process for treating electroplating wastewater and sludge by microorganisms. The purification rate of Cr(VI), Cr ~ (3+), Ni ~ (2+) and Cu ~ (2+) is over 99.9%, and the metal recovery rate is 85%.
2.3 making all kinds of industrial materials
After treatment, electroplating wastewater can be made into different industrial materials because of its different composition and content. For example, chromium-containing electroplating sludge can be made into magnetic materials by ferrite method. In China, chromium-containing sludge has been used to make MX-400 medium-wave antenna magnetic rod, a kind of manganese-zinc ferrite. In addition, the process has the advantages of simplicity, high yield, no secondary pollution and low treatment cost. Chromium-containing sludge and iron filings produced by electrolysis can be made into industrial catalysts, and some scientific research units have used this sludge to make intermediate shift catalysts for synthetic ammonia, such as C4-2, C6 and B 104.
2.4 made into fertilizer
Fertilizer production from electroplating sludge is a process of artificially controlling microbial fermentation under certain moisture and ventilation conditions, and then making compound fertilizer from fermentation products and fertilizers. The research shows that after treatment, the physical and chemical properties of chromium-containing sludge in electroplating waste chromium solution are obviously changed, the content is obviously reduced, and the harm to plants is also obviously reduced. Then the treated chromium-containing sludge and chemical fertilizer are made into compound fertilizer, which has obvious effect on the good growth of plants. Therefore, the electroplating sludge is made into fertilizer, which not only solves the sludge pollution, but also improves the agricultural output and realizes double benefits.
3 Material treatment of electroplating sludge
Material treatment of electroplating sludge is a process of producing building materials or other materials with sludge as raw material or auxiliary material. The material technology of electroplating sludge mainly includes: firing bricks and tiles, producing modified plastic products, etc.
3. 1 firing bricks and tiles
A method for treating and utilizing electroplating sludge can absorb and maintain a large amount of electroplating sludge by firing bricks and tiles. The experiment shows that the leaching concentration of metals in red bricks and blue bricks made of electroplating sludge and clay in a certain proportion can meet the water quality standard of domestic drinking water source and sanitary standard of domestic drinking water, so the method of burning bricks is also reasonable.
3.2 Production of Modified Plastic Products
The production of modified plastic products is a new technology, which is jointly developed by many scientific research units in Shanghai. Basically, the modified plastic products are made of electroplating sludge as filler, mixed with waste plastics at a proper temperature, and pressed and molded. In addition, the leaching test of the product also meets the national standard. Using electroplating sludge and waste plastics to jointly produce modified plastic products not only solves the safe disposal of waste plastics, but also makes full use of waste resources and realizes waste resource treatment, which has good social and environmental benefits.
4 Prospect analysis and prospect
The composition and properties of electroplating sludge are very complex, and its effective treatment has always been the focus and difficulty of research. However, among all the treatment and utilization methods of electroplating sludge at home and abroad, solidification/stabilization technology and material chemistry technology are relatively mature, but the attitude towards heavy metal recovery is basically not recycling, so the economic benefit is extremely low and the comprehensive benefit is average, which is only suitable for local use. Although thermochemical technology can greatly reduce the volume of electroplating sludge and reduce its harm to the environment, it also has its own shortcomings, such as easy to cause secondary pollution to the environment when burning, the need to add auxiliary fuel when burning, high investment and operating costs, difficult to popularize in a large scale, and need to be further improved. The resource treatment of electroplating sludge, especially the recovery technology of valuable metals, has been studied for a long time, which is relatively mature, with high recovery rate of heavy metals and good economic and environmental benefits. It is the best treatment and utilization technology at present. Microbial treatment technology has the advantages of low cost, high efficiency, no secondary pollution and wide sources of adsorption materials, and has the most development potential. However, how to reduce the toxic effect of high-content heavy metals in electroplating sludge on microorganisms, how to cultivate strains with strong adaptability and high waste treatment efficiency, and how to understand the mechanism of heavy metal treatment by microorganisms is still a challenge. The resource utilization of electroplating sludge meets the requirements of sustainable development in today's society, which can not only effectively eliminate the harm of electroplating sludge, but also bring considerable economic and environmental benefits, and become the focus of electroplating sludge treatment technology development. Among them, using chemical methods to treat and recover useful metal elements is the main content of future research, and applying biotechnology to electroplating sludge treatment is a brand-new development direction.
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