Separation And Recovery Of Zn/Al/Ni From Electroplating Sludge

The electroplating industry is an important part of the manufacturing industry,and China is a large manufacturing country,the area of processed products in the electroplating industry continues to grow,and at the same time,the production of electroplating sludge also increases rapidly.Plating sludge contains a large amount of heavy metals,which is extremely harmful to the environment.The traditional curing and stabilization process is accompanied by the risk of heavy metal leaching and also causes a serious waste of heavy metal resources.The development of efficient electroplating sludge resource recovery technology to minimize pollution in the electroplating industry has become an urgent industry bottleneck to break through.To address this technical challenge,this study proposes the R&D idea of"selective leaching of valuable metals from electroplating sludge-high-value recovery of target metals-the secondary dissolution of leaching residue-separation and removal of impurity metals",and conducts a series of experiments to obtain the following conclusions:（1）Zinc powder was recovered from zinc-containing electroplating sludge by alkaline solution leaching-electrodeposition process,and zinc was recovered from the residue by the secondary dissolution of alkaline leaching sludge-iron removal by hydrothermal method-chemical precipitation method.Optimization of the alkaline leaching process parameters resulted in the leaching of 58.6%of zinc,96.7%of aluminum and 54.1%of silicon from the alkaline solution.The recovery of zinc powder from the alkaline leaching solution was achieved by electrodeposition.Over 95.0%of the zinc element was recovered by a single electrodeposition process,while the aluminum and silicon elements were retained in the solution and subsequently removed from the solution by the addition of limestone,a process in which no zinc element was lost,resulting in the regeneration of the alkaline solution.The role of additives in the leaching-electrodeposition process of alkaline solutions was also investigated.The results show that additives such as EDTA-2Na are less helpful for the leaching of alkaline solutions.While in the electrodeposition stage,additives such as EDTA-2Na can slightly improve the current efficiency and the additive gelatin can change the surface morphology of zinc powder.The lye-leaching residue was secondarily dissolved by a mixed acid solution of hydrochloric acid and nitric acid,and 98.2%of zinc,98.4%of iron and97.9%of calcium were leached into the solution.The removal of iron from the solution using the hydrothermal method was investigated.After 10 h of hydrothermal reaction at 160°C,more than 99.9%of the iron in the solution was removed in the form of hydroxy iron oxide（β-FeOOH）with almost no loss of elemental zinc.Subsequently,the solution after iron removal was added to a sulfuric acid solution to precipitate the calcium element and then the pH was adjusted to about 8 to precipitate the zinc element in the solution.The recovery of nearly 99.3%of zinc from the electroplating sludge was achieved by a two-stage recovery process.（2）Zinc and aluminum elements were recovered into zinc oxide and sodium square product by a two-step hydrothermal process using zinc-containing electroplating sludge as raw material.The electroplating sludge was leached using a high solid-to-liquid mass-to-volume ratio,and the concentration of elemental zinc in the solution could reach more than 50g/L.The two-step hydrothermal method was used to recover zinc oxide and sodalite products from alkaline leaching solutions,and a single hydrothermal process was able to recover more than 63.0%of zinc elements.The elemental aluminum was retained in the solution after the hydrothermal reaction and was subsequently removed from the solution in the form of siderite due to the addition of sodium silicate and sodium chloride,without loss of zinc in this process.Factors such as pre-hydrothermal pH,hydrothermal reaction temperature and hydrothermal reaction time were investigated for the reaction of zinc oxide generation in alkaline leach solution.The results showed that no ZnO was produced after direct reaction of the alkaline leachate without pH adjustment,while hydrothermal reaction after adjusting the initial pH to13.8 was the best reaction condition,hydrothermal reaction at 160°C was the best hydrothermal reaction temperature,and hydrothermal reaction time of 8 h was the best hydrothermal reaction time.The leaching residue was also secondarily dissolved by a mixed acid solution of hydrochloric acid and nitric acid,and the iron in the solution was removed in the form of hydroxy iron oxide（β-FeOOH）by the aforementioned hydrothermal iron removal method.The solution after iron removal was added to a sulfuric acid solution to precipitate the calcium element and the pH was adjusted to about 8 to precipitate the zinc element in the solution.The recovery of 99.5%of zinc from electroplating sludge was achieved by a two-step zinc recovery process.（3）Nickel sulfate products were recovered from nickel-containing electroplating sludge by a sulfuric acid priority leaching and two-step evaporation crystallization process,and nickel was recovered from the residue of the priority leaching to nickel hydroxide by a secondary dissolution of the sulfuric acid priority leaching residue,hydrothermal removal of iron and aluminum,and chemical precipitation.By controlling the pH of the acid leach solution,most of the nickel in the electroplating sludge is dissolved into the solution,while ensuring the lowest possible leaching of iron and aluminum from the electroplating sludge at this time,and then the refined nickel sulfate product is recovered from the leach solution by a two-step evaporation and crystallization process,and the product purity reaches the Class I grade in the relevant standards.After the secondary dissolution of the preferred leach residue by sulfuric acid,the iron and aluminum impurities in the solution were removed by hydrothermal method.After 10 h of hydrothermal reaction at 160°C,the iron and aluminum in the solution were removed in the form of sodium yellow iron alum and sodium alumina,while there was almost no loss of nickel elements at this time.The solution after iron and aluminum removal was subsequently adjusted to pH 9.5,and the nickel element in the solution was precipitated in the form of Ni（OH）₂ to obtain a nickel hydroxide product.In this study,a new method of resource utilization of electroplating sludge was proposed to resourcefully recover zinc,aluminum and nickel elements from electroplating sludge to prepare zinc powder,zinc oxide,nickel sulfate and nickel hydroxide products,demonstrating good application prospects and environmental benefits.