Selective Extraction Of Valuable Metals From Electrplating Sludge In Chloride Medium

With the blooming development of electroplating industry, a lot of electroplating sludge was produced, which must be recycled because they contained a lot of valuable metals. The typical treatment for electroplating sludge was leached by strong acid, it not only need a large number of chemical reagents, the leaching was lack of selectivity, the impurity metals content were high somewhat, and strong acid could cause severe corrosion for the equipment. To avoid more consumption of acid and reduce the content of imputy metals in leaching solution, chlorination roasting as pretrement was adopted in this paper. And the calcined materials were leached by weak acid. Finally, the gradual separation process was adopted to purify the leachate which could recover valuable metals effectively.On the basis of chloridizing roasting thermodynamic analysis, ammonium chloride was selected as a chlorinating agent for electroplating sludge pretreatment. Experimental results showed that the mass ratio of the chlorinating agent vs. the material was 0.8, the roasting temperature was 673 K, the roasting time was 60 min and the particle size of the material was 200 mesh, the effect of chlorination roasting was the best. Then the material roasted by chlorination reagent was leached by weak acid. The leacing conditions were that acidity was 1mol/L, leaching time was 30 min, leaching temperature was 318 K and liquid-solid ratio was 4:1, the results showed that nickel, copper leaching rate could reach 97.48% and 87.65%, while the leaching rate of chromium and iron were 45.07% and 26.83%, respectively, which accorded with the thermodynamic analysis of electroplating sludge chlorination roasting. To investigate the mechanism of chloride electroplating sludge, the XRD analysis for raw materials, leaching slag and chlorination roasting material were finished.. The results showed that chlornation roasting of electroplating sludge were in favor of selectively leaching of valuable metals by weak acid.After chloridizing roasting-weak acid leaching for electroplating sludge, the leaching solution contained nickel, copper, iron, chromium, calciumand magnesium. In order to obtain pure nickel chloride solution, the removal of impurity metals step by step should researched. Firstly, the copper could be reduced by iron powder. The best conditions were that the amount of iron powder was 1.2 times than that of copper on the basis of the theoretical amount, the reaction temperature was 60℃, the reaction time was 15 min, the results showed the removal rate of copper was 99.17%, the loss rate of nickel was 1.58%. And the XRD of copper slag phase showed that only copper phase existed without other elements phase. Secondly, the high temperature hydrolysis was adopted to remove iron. The best experimental conditions were that p H value was 3.5, the reaction time was 6h and the reaction temperature was 95℃, the experimental results showed the removal of ironwas 98.59%, the loss rate of was 2.84%. Then phosphate salt was used to remove chromium from the previous leaching solution. The best experimental conditions were that the initial p H value was 2, the amount of phosphate sodium was 0.5 times than that of the theoretical amount, the reaction time was 50 min and the reaction temperature was 80℃, the experimental results showed the removal of chromium was 97.08%, the loss rate of nickel was 3.81%. Finally, the removal of calcium and magnesium by fluoride salt was studied. The best experimental conditions were that the reaction p H value was 5.5, the amount of ammonium fluoride was 1.2 times than that of the theoretical amount, the reaction temperature was 95℃, the reaction time was 2.5h, the experimental results showed that the removal of calcium and magnesium were 92.39% and 93.12%, the loss rate of nickel was 2.39%. The XRD analysis of precipitation of calcium and magnesium slag showed that there was only calcium fluoride phase and no other elements phase.The chlorination roasting-weak acid leaching process of electroplating sludge showed that it could achieve selective extraction of valuable metals such as nickel and copper, and could reduce the extraction of iron, which had very importent significance for the treatment of electroplating sludge, which also could be used in treatment of other secondary resources.