Investigation On Sintering And Leaching Process Of Zircon Sand Decomposed With CaCO₃-CaCl₂

Zirconium oxychloride is the main raw material for the preparation of zirconium series chemicals,which plays an extremely important role in nuclear industry,aerospace,machinery and electronics fields.China is the largest producer of zirconium oxychloride,with the highest output and production capacity in the world.The research on production process of zirconium oxychloride is significant to zirconium metallurgical industry in China.The process of producing zirconium oxychloride by lime sintering has the advantages of low production cost,low corrosion to equipment and easy to realize Automated manufacturing,which is concerned by zirconium metallurgical enterprises.However,the lime sintering process has the technical defects of high sintering temperature and low production efficiency,and has not been widely used in the zirconium chemical industry.It is very essential to reduce the sintering temperature of lime sintering process and improve the production efficiency for industrial application of zirconium oxychloride production by lime sintering process.In the present work,zircon sands were decomposed by CaCO₃-CaCl₂ using CaCl₂as additive.The effect of CaCl₂ additive on the process of producing zirconium oxychloride by lime sintering method was investigated.First,on the basis of Thermodynamic analysis of CaCO₃-CaCl₂-ZrSiO₄ ternary system,sintering process of lime sintering is optimized by designing orthogonal test and single factor experiment.Second,the leaching process of lime sintering method was studied by calculate the removal efficiency of the thermodynamic analysis of CaCO₃-CaCl₂-ZrSiO₄ ternary system was carried out.The sintering process conditions were optimized by detecting the removal efficiency of Ca²⁺and Si⁴⁺.The decomposition rate of zircon sand was determined by EDTA complexometric titration.The phase composition and morphology of decomposition products of zircon sand and leaching products were analyzed by XRD and SEM.Combined with TG-DSC analysis of CaCO₃-CaCl₂-ZrSiO₄ system,the effect of CaCl₂on the decomposition process of zircon sand by lime sintering was discussed.The main results are as follows:The thermodynamic analysis of CaCO₃-CaCl₂-ZrSiO₄ reaction system shows that CaCO₃ decomposes to form CaO and CO₂ during the reaction,and CaO further reacts with zircon sand to form CaZrO₃ and Ca₂SiO₄.The Gibbs free energy of the zircon sand decomposition reaction is more negative with the increase in sintering temperature.Additionally,increasing the sintering temperature is beneficial to the decomposition of zircon sand.Based on the sintering process of orthogonal test,the significant order of affecting the decomposition rate of zircon sand is that the molar ratio of CaCO₃/ZrSiO₄>sintering temperature>reaction time>CaCl₂ addition.When sintering process is further optimized by single factor experiment,the decomposition rate of zircon sand increases with the increase of CaCO₃/ZrSiO₄ molar ratio,sintering temperature and reaction time.And as the amount of CaCl₂ added increases,the decomposition rate of zircon sand shows a trend of increasing first and then decreasing.The suitable parameters for the sintering process are as follows:the CaCO₃/ZrSiO₄ molar ratio is3.5,sintering temperature is 1200℃,sintering time is 300 min,CaCl₂/ZrSiO₄ molar ratio is 0.24.The decomposition rate of zircon sand reaches to 93.99%under this process condition.The study on the reaction process of CaCO₃-CaCl₂ decomposition of zircon sand indicates that a eutectic molten salt could be synthetized with CaCO₃ and CaCl₂ during the heating process,and decomposition temperature of zircon sand is reduced to 850°C.In the sintering process,the insufficient addition of CaCO₃ leads to incomplete decomposition of zircon sand,and Ca₃ZrSi₂O₉ appears in the product.When the zircon sand is fully decomposed by CaCO₃,the main phase composition of the product is CaZrO₃,Ca₂SiO₄,CaO and ZrSiO₄,in which the particle size of CaZrO₃ is 1-3μm with approximately spherical particles,and Ca₂SiO₄ is present in a glassy agglomerate.The leaching process of zirconium oxychloride prepared by lime sintering method reveals that the removal efficiencies of Ca²⁺and Si⁴⁺increase upon hydrochloric acid concentration,liquid-solid ratio and leaching time growing.The suitable leaching process parameters are that:hydrochloric acid concentration,liquid-solid ratio and leaching time are set 2mol/L,6ml/g,and 30min.The removal efficiencies of Ca²⁺and Si⁴⁺under this process condition are 61.22%and 71.96%respectively.The zircon sand decomposition product is leached by hydrochloric acid,Ca₂SiO₄ and CaO are decomposed by hydrochloric acid and dissolved in the filtrate in the form of silicic acid and calcium chloride.The solid phase is mainly CaZrO₃,and CaZrO₃ is 1-3μm,with the spherical particles and of well dispersion.The decomposition of zircon sand by CaCO₃-CaCl₂ can decrease the decomposition temperature of zircon sand,so that energy and the production cost are reduced,and establish the foundation for industrial application of lime sintering method.