Basic Research On Ozone Oxidation Removal Of Impurity Iron In Zinc Hydrometallurgy Solution

The main raw materials of zinc hydrometallurgy are zinc sulfide concentrates,which are usually associated with high contents of iron.In the leaching process,iron is dissolved into the leaching solution together with the main metal zinc.In the pre-neutralization process,Fe³⁺is easy to be removed by hydrolytic precipitation.However,the pH value of the complete precipitation of Fe²⁺at the beginning of precipitation(6.45)is higher than that of the main metal Zn²⁺(5.54),and it is not possible to remove Fe²⁺by hydrolytic precipitation through further increase of the solution pH value so as to avoid the loss of Zn.Usually,a large amount of Fe²⁺will be enriched in ZnSO₄ solution.If it is not removed deeply,it will have serious adverse effects on the subsequent zinc electrowinning process,such as reducing current efficiency and cathode zinc quality,increasing power consumption and production cost,etc.Therefore,it is necessary to deeply remove Fe(mainly Fe²⁺)from ZnSO₄ solution to meet the requirements of normal zinc electrowinning and improving current efficiency.Aiming to solve above problems,this paper carried out the thermodynamics,experiment and kinetics of removal of impurity iron in ZnSO₄ solution by ozonation,and the conclusions were as followings:(1)The thermodynamic analysis of iron removal by ozonation was carried out.It is shown that the Gibbs free energy of ozonation is negative when the temperature is 298?373 K.Ozone can oxidize Fe²⁺to Fe³⁺and remove it from the solution by hydrolysis precipitation,while the Gibbs free energy of hydrolysis reaction of Zn²⁺is positive,indicating that the reaction cannot be spontaneous.Therefore,Fe in the solution can be removed effectively by ozone oxidation without Zn loss.(2)And parallel experiments were carried out under conditions of ozone and air as oxidant,respectively.The results showed that the percentage removal of iron was about 3.0?3.5 times higher using O₃ compared with air under parallel conditions,which proves that the effective removal of impurity Fe from ZnSO₄ solution can be achieved by ozone.Meanwhile the ozonation experiments were carried out to obtain the optimized process parameters to remove Fe from ZnSO₄ solution.The optimum conditions were as followings:M_ZnO/Fe(molar ratio of ZnO to total iron)1.45,reaction temperature 90?,ozone flow rate 3.12 g/h,reaction time 30 min,and stirring speed 300 rpm.Under these conditions,removal rate of Fe was 99.98%,removal rate of coexisting impurity Mn was 23.08%,loss rate of Zn was 0.14%,and amount of precipitate was 22.56 g(amount of precipitate produced by 1 L ZnSO₄ solution).The precipitated slag was characterized by XRD,SEM and EDS.It was found that the precipitated slag was mainly composed of FeOOH,MnO₂ and ZnSO₄·H₂O,which were irregular massive aggregates.The elements of Fe,Zn,S,O and Mn were evenly distributed in the precipitated slag.(3)The reaction kinetics of Fe removal by ozone in simulated ZnSO₄ solution was studied.The results showed that increasing the initial Fe²⁺concentration in the solution would reduce the oxidation reaction rate of O₃ and Fe²⁺,while increasing the pH value of the solution,reaction temperature,and O₃ flow rate increased the reaction rates of O₃ and Fe²⁺,which promotes the removal of Fe.At the same time,the increase of the above factors will make the potential value of ZnSO₄ solution increase significantly at first and then tend to be flat.And XRD,SEM and EDS analysis showed that the precipitated slags were the mixtures of FeOOH and ZnSO₄·H₂O at different reaction conditions,which were irregular massive aggregates.(4)The effects of initial Fe²⁺concentration,solution pH,reaction temperature and ozone flow rate on the kinetics of iron removal were studied.The reaction kinetics of removal impurity Fe from zinc hydrometallurgy solution by ozone was obtained.