Study On A New Process For Separation Of Chromium From Iron By Selective Complexation-Preferential Hydrolysis Precipitation

On the basis of the review of the development of Cr/Fe separation techniques,the Cr/Fe separation and the further recovery of Cr in sulfuric acid leaching solutions of mixed electroplating sludge are focused and a new process of selective complexation – preferential hydrolysis precipitation is proposed to separate Fe³⁺ from Cr³⁺ successfully.This research has supplied a new feasible economic process for the recovery of Cr from electroplating sludge,which is also eccential to the harmless disposal and resourcization of Cr from other Cr-containing wastes.The main research contents are as follows:Based on the thermodynamics,the dissolution behavior of Fe³⁺ and Cr³⁺ as well as the effects of different factors on the dissolution behavior of Fe³⁺ and Cr³⁺ has been studied.Considering the iron precipitation rate and the filtration property of the hydrolysis residue,some common precipitants have been studied and further compared and MgO is finaly is selected as the precipitant.On the basis of simulated solutions experiments,the optimal precipitation conditions for Fe precipitation are determined as follows: reaction temperature of 90 ?,initial p H value via MgO of 2.5,reaction time of 30 min,agitation speed of 350r/min.Under above conditions,the precipitation rate of Fe can arrive at 98.90% and the deep precipitation of Fe has been achieved.The experimental study on Cr/Fe separation was carried out with sodium formate as the complexant and magnesium oxide as the precipitant.The optimal conditions are determined as follows: the dosage of sodium formate as 5.5 times of the HCOONa/Fe³⁺mole ratio with a fixed p H value of 1.7,reaction temperature of 90 ?,reaction time of 6 h.Afterwards,the p H value is raised as 2.5 with MgO at the same temperature,the mixture time of 30 min under stirring condition.Under the conditions stated above,the precipitation rate of Fe can be reached as 93.65% while that of Cr is only 12.37%.A desired separation of Cr/Fe has been achieved.The complexation kinetics of the process of HCOONa complex Cr³⁺ and Fe³⁺ under90 ? has been studied based on the Ultraviolet spectrum analysis respectively.The results show that the complexing action between formic acid and Fe³⁺ is stronger than that of Cr³⁺and the existence of Cr³⁺ will reduce the complexing reaction rate of Fe³⁺.Furthermore,the complexation thermodynamics shows the complexing reactions are endothermal process,the reaction enthalpy of the reaction between formic acid and Cr³⁺ is 10.68 k J/mol,whilethat for Fe³⁺ complexing is 37.37 k J/mol.The reaction enthalpy between formic acid and Fe³⁺ is reduced to 22.00 k J/mol in to the mixed Cr³⁺/Fe³⁺ solutions.Based on L9?34?orthogonal experiments,the effects of initial p H,reaction temperature and agitation speed on the precipitation rate of Fe and Cr were discussed by range analysis and variance analysis respectively.The results show that the most significance factor on the precipitation rate of Fe and Cr is the initial p H of the system.