Hemimorphite Determination Of Thermodynamic Data

More and more attentions were given to the development and utilization of low-grade zinc oxidized ores with the exhaustion of zinc sulfide ores. The main researches of leaching for low-grade zinc oxidized ores were focused on the optimization of process conditions, but few on the thermodynamics of leaching process. Hemimorphite was a refractory oxidized mineral. These basic thermodynamic data played an important role in the study of leaching process. It was necessary to study the basic thermodynamic properties of hemimorphite.A thermogravimetry and a differential scanning calorimeter were used to measure the thermodynamic data of hemimorphite Zn4Si2O7(OH)2·H2O and dehydrated hemimorphite Zn4Si2O7(OH)2 respectively. The crystal structure was characterized by IR and XRD determination in order to prove the extent of the reaction.The thermodynamic datas of Zn4Si2O7(OH)2·H2O and Zn4Si2O7(OH)2 were calculated based on a designed thermodynamic cycle under the condition of standard pressure and 298.15 K.△rGmΘof Zn4Si2O7(OH)2·H2O and leaching in various solutions were calculated based on the calculated△fGmΘ, and the effects of leaching conditions on the leaching ratio were also investigated. The main conclusions were summarized as follows:1.The thermodynamic data of CuSO4·5H2O and specific heat capacity of sapphire were measured, which verified the accuracy of the thermodynamic data determined by the differential scanning calorimeter.2.The DSC curves for the heating and dehydrating process of Zn4Si2O7(OH)2·H2O were obtained in the temperature range of 573-998K. And the specific heat capacities Cp of Zn4Si207(OH)2 and Zn4Si2O7(OH)2·H2O were measured in the temperature range of 363-563K. Then the enthalpy change and the reaction temperature of each step in dehydration process, and the functions between the molar heat capacities and temperature were obtained. The△fHmΘ,△fSmΘand△fGmΘfor Zn4Si2O7(OH)2·H2O were-4007.156 kJ·mol^-1 -1140.290J·mol^-1·K^-1,-3667.179 kJ·mol^-1 respectively, and those for Zn4Si2O7(OH)2 are-3527.307 kJ-mol^-1,-717.780 J-mol^-1·K^-1, 3313.301 kJ·mol^-1 respectively.3. It was shown that Zn4Si2O7(OH)2·H2O could not be leached in alkaline leaching system, ammonium leaching system and ammonia-ammonium system, but was refractory in Acid leaching systemunder the conditions of standard state and 298.15K. While Zn4Si2O7(OH)2 could be leached in Acid leaching system and ammonia-ammonium system, but was refractory in alkaline leaching system and ammonium leaching system.