Electroplating sludge treatment and recycling technology
Electroplating sludge is a discharge produced during the electroplating wastewater treatment process, which contains large amounts of toxic heavy metals such as chromium, cadmium, nickel, and zinc. The ingredients are very complex. Among the 47 categories of hazardous waste listed in my country's "National List of Hazardous Wastes" (Huanfa [1998] No. 89), electroplating sludge accounts for 7 major categories and is a typical hazardous waste. At present, due to many problems such as many factories, small scale, low equipment level and low pollution control level in my country's electroplating industry, most of the electroplating sludge is still simply landfilled or even piled randomly, causing serious pollution to the environment. . Therefore, how to adopt effective technical treatment and disposal of electroplating sludge and achieve its stabilization, harmlessness and resource utilization have always been the focus of research at home and abroad.
1. Solidification/stabilization technology of electroplating sludge
At present, research on the solidification/stabilization of electroplating sludge mainly focuses on the mechanism and microscopic mechanism of the solidification block stabilization process. etc. Using ordinary Portland cement as the curing agent, Roy et al. systematically studied the effects of the addition of copper-containing electroplating sludge and the interfering substance copper nitrate on the long-term change behavior of cement hydration products. They found that copper nitrate and copper-containing electroplating sludge have an impact on the cement hydration products. The crystallinity, porosity, morphology of heavy metals, and pH of the hydration product have an important impact on trace chemical and microstructural characteristics. For example, the pH of the solidified body shows an obvious downward trend with the increase in the amount of copper nitrate added, and the porosity decreases. It increases with the increasing amount of copper nitrate added. Asavapisit et al. [3] studied the solidification effect of cement, cement and fly ash solidification systems on electroplating sludge, analyzed the compressive strength, leaching characteristics and microstructure changes of the solidified body, and found that electroplating sludge can significantly The compressive strength of the final solidified block of the two systems is reduced because the electroplating sludge covering the surface of the cementitious material inhibits the hydration of the solidified system, but the addition of fly ash can not only minimize this inhibitory effect, but also It can also reduce the leaching rate of chromium in the solidified body. The reason may be that fly ash partially replaces the high-alkalinity cement, which reduces the alkalinity of the mixed system to a level that is conducive to the stabilization of heavy metal hydroxides.
Sophia et al. believe that the compressive strength of electroplating sludge treated with single cement is better than that of a mixed system of cement and fly ash, but as long as the ratio of cement and fly ash is appropriate, the solidification of chromium can also be satisfied. need. The use of fly ash during the curing process does not benefit the long-term stability of copper.
The use of additives can improve the solidification effect of electroplating sludge. In the solidification treatment of electroplating sludge, adding appropriate additives according to the nature of harmful substances can improve the solidification effect, reduce the dissolution rate of harmful substances, save the amount of cement, and increase the strength of the solidified block. There are many types of additives used in the curing method using cement as the curing agent, and their functions are also different. Common ones include activated alumina, sodium silicate, calcium sulfate, sodium carbonate, activated chaff ash, etc.
2. Thermochemical treatment technology of electroplating sludge
Thermochemical treatment technology (such as incineration, ion arc and microwave, etc.) is to decompose waste under high temperature conditions to make it Reduce the toxicity of some highly toxic components, achieve rapid and significant volume reduction, and utilize the useful components of waste. In recent years, the use of thermochemical treatment technology to achieve pretreatment or safe disposal of hazardous waste electroplating sludge is attracting people's attention.
At present, research on thermochemical treatment technology of electroplating sludge focuses on the migration characteristics of heavy metals during the incineration treatment of electroplating sludge. Espinosa et al. studied the thermal characteristics of the incineration process of electroplating sludge in the furnace and the migration patterns of heavy metals in it, and found that incineration can effectively enrich chromium in the electroplating sludge. The residual rate of chromium in the ash is as high as more than 99%. During the incineration process, most of the sludge components are lost in the form of CO2, H2O, SO2, etc. Therefore, the volume and weight reduction effect is very obvious, and the weight reduction can reach 34%.
Barros et al. used a cement rotary kiln to study the process of mixed incineration of electroplating sludge. They analyzed the impact of adding chlorides (KCl, NaCl, etc.) on the migration patterns of Cr2O3 and NiO in electroplating sludge. They believed that chloride affects Cr2O3 and NiO during incineration. The residue in the ash has almost no impact, and both Cr2O3 and NiO can be effectively solidified in the incineration residue during the incineration process. When Liu Gang et al. used a tube furnace to simulate the incinerator to study the heat treatment characteristics of electroplating sludge, they analyzed the migration characteristics of various heavy metals such as chromium, zinc, lead, copper, etc., and concluded that when the incineration temperature is below 700°C, the moisture in the sludge will , organic matter and volatile matter can be well removed, and high temperature can effectively inhibit the leaching of heavy metals in sludge, but this inhibition has different effects on various heavy metals. For example, nickel is a non-volatile heavy metal, and in incineration ash The residual rate in the slag is 100%, and the residual rate of chromium in the ash is also as high as more than 97%. The precipitation rates of zinc, lead, and copper increase to varying degrees as the incineration temperature increases.
In terms of research on other thermochemical treatments such as ion arc and microwave, Ramachandran et al. used DC plasma arc to treat electroplating sludge under different atmospheres, and analyzed the treated residue and powder generated during the treatment. Research has been conducted and it is believed that this method can realize the recovery of valuable metals such as copper and chromium while converting the residue into stable inert slag. Gan et al. conducted detoxification and heavy metal solidification experiments on electroplating sludge through microwave radiation, and found that microwave radiation treatment has a significant effect on the solidification of heavy metal ions in electroplating sludge. The reason may be the combination of high-temperature drying and electromagnetic waves. Under the action of **, it is conducive to the strong interaction between heavy metal ions and bipolar polymer molecules to combine together. The electroplating sludge treated by microwave has the characteristics of fine particle size, high specific surface area, and easy agglomeration.
In addition, thermochemical treatment is beneficial to reducing the toxicity of chromium in electroplating sludge. Ku et al. studied the changes in the toxic valence state of chromium during the high-temperature heat treatment of electroplating sludge, and believed that high-temperature heat treatment can convert chromium (VI) into chromium (III), and the higher the temperature, the more obvious the conversion effect; in high-temperature treated electroplating sludge, Ku et al. In mud, it is mainly chromium (III). Cheng et al. [16] analyzed the valence state of chromium in a mixture of electroplating sludge and clay after heat curing in an electric furnace at 900°C and 1100°C for 4 hours, and found that in the mixture treated with heat curing at 900°C, Chromium (VI) has an absolute advantage, and in the mixture treated by heat curing at 1100°C, chromium mainly exists as chromium (III).
3. Recovery technology of valuable metals in electroplating sludge
3.1 Acid leaching method and ammonia leaching method
Acid leaching method is one of the solid waste leaching methods. The most widely used method, the specific acid used for leaching depends on the nature of the solid waste. Sulfuric acid is the most effective leaching reagent for the treatment of industrial wastes such as electroplating, casting, and smelting. It is widely used because of its cheap price, low volatility, and resistance to decomposition. Silva et al. used diisooctyl phosphate as the extraction agent to conduct research experiments on the recovery of nickel and zinc by sulfuric acid leaching from electroplating sludge. Research by Vegli et al. shows that the leaching rate of copper and nickel by sulfuric acid can reach 95% to 100%, and in the electrolytic recovery process, the recovery rate of both copper and nickel is also as high as 94% to 99%.
Other acidic extractants (such as acidic thiourea) can also be used to leach heavy metals in electroplating sludge. Paula et al. used cheap industrial hydrochloric acid to leach chromium from electroplating sludge. During leaching, 5mL of industrial hydrochloric acid (purity: 25.8%, mass concentration: 1.13g/mL) was added to about 1g of pre-prepared sample, and then leached at 150r /min shaker for 30 minutes, the chromium leaching rate was as high as 97.6%.
Although the technology of extracting metals by ammonia leaching has a certain history, compared with acid leaching, there are relatively few research reports on using ammonia leaching to treat electroplating sludge, and most of them are domestic research reports. The ammonia leaching method generally uses ammonia solution as the leaching agent because ammonia has the advantages of moderate alkalinity, easy use, and recyclability.
Using ammonia complex group leaching-steamed ammonia-hydrolysis residue sulfuric acid leaching-solvent extraction-metal salt crystallization recovery process, most valuable metals can be recovered from electroplating sludge, including total recovery of copper, zinc, nickel, chromium, and iron. The rates are greater than 93%, 91%, 88%, 98%, and 99% respectively. In response to the problem of difficulty in selecting an extractant suitable for separating copper from ammonia leaching liquid systems, Zhu Wanpeng et al. developed an extractant called N510, which can effectively recover electroplating sludge ammonia leaching in a kerosene-H2SO4 system. Cu2+ in the liquid, the recovery rate is as high as 99%. Wang Haodong et al. [26] studied the ammonia leaching method to recover nickel from electroplating sludge. It was shown that the nickel-containing sludge was roasted after oxidation and roasting. The roasted sand was roasted with ammonia water with a mass fraction of NH3 of 7% and CO2 of 5% to 7%. The sand is stirred and leached with oxygen to obtain a solution containing Ni(NH3)4CO3. The solution is then evaporated to convert Ni(NH3)4CO3 into NiCO3·3Ni(OH)2, and then calcined at 800°C to obtain Commercial nickel oxide powder.
When acid leaching or ammonia leaching is used to treat electroplating sludge, the total recovery rate of valuable metals and the ease of separation from other impurities are mainly affected by the leaching rate and leaching rate of valuable metals during the leaching process. Selective control of valuable metals and impurities in the liquid. The main feature of the acid leaching method is that it has a good leaching effect on valuable metals such as copper, zinc, and nickel, but its selectivity for impurities is low, especially for impurities such as chromium and iron; while ammonia leaching has poor selectivity for impurities such as chromium and iron. The method has high selectivity for impurities such as chromium and iron, but has a low leaching rate for copper, zinc, nickel, etc.
3.2 Bioleaching method
The main principle of bioleaching method is to use the biological acid production of chemoautotrophic Thiobacillus acidophilus to remove poorly soluble heavy metals from the solid phase. It dissolves and enters the liquid phase to become soluble metal ions, which are then recovered from the leach solution using appropriate methods. The mechanism of action is relatively complex, including microbial growth metabolism, adsorption, and transformation. Judging from the literature that can be collected so far, there are relatively few research reports on the use of bioleaching methods to treat electroplating sludge. The reason is that the high content of heavy metals in electroplating sludge has a toxic effect on microorganisms, which greatly limits the application of this technology in this field. application. Therefore, how to reduce the toxic effects of high levels of heavy metals in electroplating sludge on microorganisms and how to cultivate strains with strong adaptability and high waste treatment efficiency are still a major problem faced by the bioleaching method [30], but they are also The key to solving the application of this technology in this field.
3.3 Smelting method and roasting and leaching method
The main purpose of smelting electroplating sludge is to recover the copper and nickel contained in it. The smelting method uses coal and coke as fuel and reducing substances, and the auxiliary materials include iron ore, copper ore, limestone, etc. When smelting sludge mainly composed of copper, the furnace temperature is above 1300°C, and the melted copper is called matte; when smelting sludge mainly composed of nickel, the furnace temperature is above 1455°C, and the melted nickel is called crude matte. nickel. Matte and crude nickel can be directly separated and recycled by electrolysis. Slag is generally used as raw material for building materials.
The principle of the roasting and leaching method is to first use high-temperature roasting to pretreat impurities in the sludge, and then use acid, water and other media to extract valuable metals in the roasted product. Pyrite waste is used as acidification raw material, mixed with electroplating sludge and roasted, and then the roasted product is leached and separated with deionized water at room temperature. The recovery rates of zinc, nickel and copper are 60% and 43 respectively. %,50%.
4. Materialization technology of electroplating sludge
Materialization technology of electroplating sludge refers to the process of using electroplating sludge as raw materials or auxiliary materials to produce building materials or other materials. Ract conducted an experiment to partially replace cement raw materials with electroplating sludge to produce cement. It is believed that even when the chromium-containing electroplating sludge is added to the raw material in an amount as high as 2% (dry basis mass fraction), the cement sintering process can proceed normally. , and the residual rate of chromium in the sintering product is as high as 99.9%.
Magalh es et al. analyzed the factors that affect the firing of ceramics from a mixture of electroplating sludge and clay, and believed that the physical and chemical properties of the electroplating sludge and the stirring time when prefabricating the mixture of electroplating sludge and clay are the dominant factors that determine the quality of ceramics, such as the original The type and content of heavy metals in electroplating sludge (such as aluminum, zinc, nickel, etc.) obviously determine the leaching characteristics of electroplating sludge and its mixture with clay. When prefabricating the mixture of electroplating sludge and clay, severe or long-term leaching Stirring is beneficial to the homogenization of the mixture and the progress of the sintering reaction. In addition, mixing electroplating sludge with beach mud can produce up to standard ceramics.
5. Conclusion
The treatment of electroplating sludge has always been the focus of research at home and abroad. Although relevant personnel have carried out a lot of research in this field and achieved certain results, there are still There are many problems that urgently need to be solved, such as the risk of secondary pollution to the environment caused by traditional cement-based solidification technology and leaching methods for the purpose of recovering valuable metals. New research approaches must be adopted to solve these problems. In recent years, the use of thermochemical treatment technology to achieve pretreatment or safe disposal of electroplating sludge provides broader development space and prospects for future electroplating sludge treatment. Recent research shows that thermochemical treatment technology has obvious advantages in the reduction, resource utilization and harmlessness of electroplating sludge. Therefore, it will become an important research direction in the field of electroplating sludge treatment in the future.
However, due to the relatively small application and research of thermochemical treatment technology in electroplating sludge treatment, many issues need further exploration, such as the migration characteristics of heavy metals during thermochemical treatment of electroplating sludge, The residual characteristics of heavy metals in ash and slag, the precipitation characteristics and evaporation characteristics of heavy metals during thermochemical treatment need to be studied in depth.