Separation Of Chromium From Electroplating Sludge Sulfuric Acid Leaching Solution

On the basis of the review of the development of the treatment of electroplating sludge as well as the Cr/Fe separation processes, the Cr/Fe separation in sulfuric acid leaching solutions of electroplating sludge is focused and the selective phosphate precipitation is further proposed to achieve the deep separation of Cr from Fe on the basis of simulated solutions and real solutions experiments. This research has supplied a new feasible economic process for the Cr/Fe separation in acidic solutions, which is eccential to the harmless disposal and resourcization of Cr in electroplating sludge. The main subjects in this dissertation are as follows:The feasibility of the phosphate separation of Cr is discussed by thermodynamics and the solution pH condition is further discussed for the separation process.Orthogonal experimental designing method is employed to evaluate the effects of temperature, pH of the solution and sodium phosphate as the precipitant on the precipitation of Cr. The results show that the phosphate precipitation reaction can be carried out quickly and the dosage of sodium phosphate has a great effect on the precipitation of Cr and its contribution rate is also the highest. The variance analysis also shows that the reaction temperature, the dosage of sodium phosphate and the interaction of temperature and pH of the solution has significant effects on the precipitation of Cr. The precipitates are further analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy spectrum (EDX) and optical microscopy.The simulated solutions experiments prove the feasibility of selective phosphate precipitation process in sulfuric acid solutions to achieve deep separation of Cr/Fe. It’s further indicated that the selective phosphate precipitation process is applicable for a wider range of iron concentration and the impurities such as Cu2+, Zn2+, Ni2+, Cl" effect the Cr/Fe separation rarely.The optimized phosphate precipitation technique is studied for the Cr/Fe separation in sulfuric acid leaching solutions of mixed electroplating sludge. Prior to the phosphate precipitation process, the real leaching solutions are treated by solvent extraction of copper and active carbon adsorption for the removal of organics from the solutions firstly. The optimal technique for the Cr/Fe separation can be determined as follows:the dosage of sodium sulphite as1.46～1.73times of the theoretical amount for the reduction reaction of Cr6+and Fe3+, the mixture time of10min under the ambient temperature, the further mixture time of10min under90℃, the constant pH of2.0, the secondary addition of sodium sulphite as (1.0-2.0) g/250mL, the dosage of sodium phosphate as the molar ration of PO43-/Cr+of1.1～1.2, the agitation speed of400r/min, the precipitation time of30-60min. The experimental results show that the selective phosphate precipitation process can be carried out stably and the concentration of Cr in the solutions can be lowered down to about0.04g/L. The phosphate precipitates contain about13%of Cr and the impurities such as Ni, Fe and Zn can be decreased to～0.4%,～0.1% and～0.2%respectively. Chromium can be further recovered from the phosphate precipitates as valuable products, which will be studied subsequently.