Study On The Dissolution Behavior Of Oxidized Zinc Ores In Ammoniacal Solution

Since the leaching of metals in ammoniacal solutions is applied increasingly to the extraction of nonferrous metals, the application of the leaching technology in zinc hydrometallurgy has been given more attention. In view of the defects in treating high-alkalinity low grade oxidized zinc ore using traditional acid leaching process and the advantages of ammoniacal leaching and chloride hydrometallurgy, the ammonia-ammonium chloride solution was introduced to dissolve the oxidized zinc ore. Therefore, the study of the dissolution behavior of oxidized zinc ore in ammonia-ammonium chloride solution will provide theoretical foundation and basic data for the zinc hydrometallurgy process.The thermodynamics and kinetic properties of oxidized zinc ore dissolved in ammoniacal solution were investigated. A series of measurements, TG/DTA, XRD and IR were used to compare the thermal property and dissolution behavior of natural and synthetic hemimorphite. The dissolution equilibrium and kinetics of hemimorphite and willemite in ammoniacal solution were also studied. Their dissolution mechanism in ammoniacal solution was discussed. The behaviors of different components in Zn(?)-NH3-H2O, Zn(?)-NH3-Cl--H2O, Zn(?)-NH3-CO32--H2O and Zn(?)-NH3-Cl--CO32--H2O systems were investigated using the chemical equilibrium software GEM-Selektor. The equilibrium thermodynamic properties and codeposition behavior of the stable and metastable components were discussed in the multi-component system of zinc complexes. The predominance area diagrams of these systems were constructed using "projective technique". The dissolution behaviors of ZnO and Zn5(CO3)2(OH)6 in ammonia, NH4Cl solution and NH3-NH4Cl solution were studied by experiments and calculations, and the solubility of the two minerals in NH3-NH4Cl solution was predicted. The main conclusions were drawn as follows:1. Comparison of the thermalmodynamic properties and dissolution behaviors of the synthetic and natural hemimorphite was made. It was found that the dissolution rate of synthetic hemimorphite is different from natural hemimorphite in NH3-NH4Cl solution. But the final states are the same when they are completely dissolved. It is reliable to study the dissolution equilibrium by using the synthetic hemimorphit.2. The dissolution behaviors of hemimorphite in ammonia or ammonium chloride solution were investigated separately. The equilibrium concentration of Zn(?) is very low. There is a relatively high equilibrium concentration of Zn(?) in the solution of [NH3]/[NH4C1] 1:2. The dissolution kinetics of hemimorphite in NH3-NH4Cl solution can be described by Elovich equation with an activation energy of 57.64 kJ/mol, which is of the characteristics for a chemically controlled process. The dissolution mechanism can be inferred that the complex reactions of hemimorphite on the surface with the cation or? in solution lead to the collapse of the structure. Then the dissolved SiO2·H2O coagulates and adsorbs back onto the hemimorphite surface, which may prevent hemimorphite from further dissolution.3. When willemite is dissolved in ammonia or ammonium chloride solution, the equilibrium concentration of Zn(?) is very low. The equilibrium concentration of Zn(?) in the solution of [NH3]/[NH4C1] 1:2 is relatively high. The concentration of Zn(II) by the dissolution of willemite is much smaller than that by the dissolution of hemimorphite in NH3, NH4Cl and NH3-NH4Cl solution. The dissolution of willemite is relatively slow in NH3-NH4Cl solution, and the dissolution kinetics can be described by the grain pore model with an activation energy of 54.47 kJ/mol and a reaction order of about 3 with respect to NH3, which is controlled by pore diffusion.4. The thermodynamical behaviors of different components in Zn(?)-NH3-H2O, Zn(?)-NH3-Cl--H2O, Zn(?)-NH3-CO32--H2O and Zn(?)-NH3-Cl--CO3-2-H2O systems were studied. In acidic region, the free Zn2+ and zinc chloride complexes are coexisted in Zn(?)-NH3-Cl--H2O and Zn(?)-NH3-Cl--CO32--H2O system. In neutral region, there exist small amount of Zn(?) in solution and precipitates in all systems. In Zn(?)-NH3-H2O and Zn(?)-NH3-CO32--H2O system, the zinc ammonia complexes are codependent in the solution and Zn(OH)2(S) and hydrozincite are precipitated respectively. Zn(NH3)2Cl2(S), Zn(OH)1.6Cl0.4(S) and Zn(NH3)2Cl2(S), Zn5(CO3)2(OH)6(s) are precipitated in Zn(II)-NH3-Cl-H2O and Zn(II)-NH3-Cl--CO32--H2O system, respectively, and zinc chloride complexes, zinc ammonia complexes Zn(NH3)Cl3- and Zn(NH3)3Cl+are coexisted in solution. In weak alkaline region, Zn(NH3)42+is the main component in all the systems studied. When pH >10, there are hydroxyl zinc ammonia complexes in solution. When pH >12, Zn(OH)2(S) is precipitated. The components and precipitates are related to [NH3]T, [C1-]T and [CO32-]T. The pH ranges for stable zinc ammonia complexes and hydroxyl zinc ammonia complexes are broadened with the increase of the total ammonia concentration. The precipitates of Zn(NH3)2Cl2(s) and Zn(OH)i.6Cl0.4(s) appear at high chloride concentration in Zn(II)-NH3-Cl--H2O system. The precipitate of Zn(NH3)2Cl2(s) appears at high chloride concentration in Zn(II)-NH3-Cl--CO32--H2O system. In Zn(II)-NH3-CO32--H2O and Zn(II)-NH3-Cl--CO32--H2O systems, [CO32-]T has a little influence on the components in solution. The pH range for the predominant area of Zn5(CO3)2(OH)6(S) increases with the increase of [CO32-]T·5. The dissolution equilibrium of ZnO in ammonia, NH4Cl solution and NH3-NH4Cl solution was investigated by experiments and calculations. The solubility of ZnO is small in ammonia solution and is determined by the solubility of Zn(OH)2(s) only. Zn(NH3)2Cl2(s) and Zn(OH)1.6Cl0.4(s) are precipitated in NH4Cl solution, and the concentration of Zn(?) is determined by the solubility of Zn(NH3)2Cl2(s) at high concentration of NH4Cl. When NH3/NH4C1=1, there is a maximum concentration of Zn(?) in the solution. Zn(NH3)2Cl2(s) is formed at high concentration of NH4Cl and Zn(OH)1.6Cl0.4(S) is formed at low concentration of NH3 and high concentration of NH4Cl in NH3-NH4Cl solution.6. The dissolution equilibrium of hydrozincite in ammonia, NH4Cl solution and NH3-NH4C1 solution was studied. In ammonia solution, Zn(OH)2(S) is precipitated and the dissolved CO32- can form a buffer solution. The concentration of Zn(II) in NH4Cl solution is relatively low and hydrozincite or Zn(NH3)2Cl2(S) is precipitated. When NH3/NH4C1=2, the solubility of Zn(?) is the largest. Zn(OH)2(S) is precipitated from the ammoniacal solution at high NH3/NH4Cl, while Zn(NH3)2Cl2(s) is precipitated from the ammoniacal solution at low NH3/NH4Cl.