Research On Purification And Mechanism Of Phosphoric Acid Leaching–Sodium Carbonate Roasting Of Vein Quartz

With the depletion of crystal resources in China,the use of vein quartz for the purification of fluorine-free phosphoric acid leaching is a technological exploration of a new approach to green purification.The so-called fluorine-free phosphoric acid purification refers to the complete absence of fluoride in the purification process of vein quartz,using phosphoric acid molecules to ionize H⁺and PO₄^3-and react with the main impurity elements Fe and Al in quartz,forming a stable compound to achieve separation between impurity metal and quartz.Using carbonate roasting technology for trace amounts of impurities in chemical leached quartz,reducing the phase transition temperature of quartz and promoting the cracking and activation of metal atoms due to volume shrinkage during the heating process of quartz,which is beneficial to the effective separation of trace meta elements impurities.Phosphoric acid leaching process parameters under atmospheric pressure and hot-press were studied separately,and the mechanism and reaction mechanism of the reaction of phosphoric acid and metal oxides to form complexes were revealed.The reaction mechanism of reducing the quartz phase transition temperature and activating the impurity metal elements by carbonate roasting was discussed and analyzed.Provides new ways and theoretical basis for green manufacturing of high-purity quartz.The main results of the study are as follows:（1）The analysis of the characteristics of vein quartz shows that the SiO₂ content is 99.95%,the total impurity elements are 619.0μg/g,and the content of main impurity element Al is as high as 352.7μg/g;the impurity elements mainly occur in muscovite,apatite,hematite and aluminosilicate mineral inclusions.（2）Phosphoric acid leaching was performed on vein quartz by atmospheric pressure and hot-press leaching respectively.Atmospheric pressure leaching process conditions were:reaction at 80℃ for 12 h,phosphoric acid concentration 1.2 mol/L,and leachate liquid to solid ratio 10:1.After leaching reaction,the total amount of impurity elements was 148.59μg/g,and the total removal rate was 76.00%.The content of element Al was 77.87μg/g and the removal rate was 77.92%.The hot-press leaching process conditions were:reaction at 260℃ for 4h,phosphoric acid concentration of 0.6 mol/L,and leaching liquid to solid ratio of 5:1.The total impurity elements after leaching reaction were 91.22μg/g,and the total removal rate was85.26%.The element Al content was 45.33μg/g,the removal rate was 87.15%,and the SiO₂ mass fraction was 99.99%.Hot-press leaching is significantly more efficient than atmospheric pressure leaching purification.The reduction rate of the total amount of impurity elements was 9.26%higher than that of the former,and the reduction rate of element Al was increased by 9.23%.（3）The hot-press leached quartz was subjected to oxidation roasting and 5‰sodium carbonate roasting comparative test.The leaching results after roasting showed that the addition of sodium carbonate roasting was superior to oxidation roasting in the removal of element Al.The optimum conditions of oxidation roasting were roasting temperature 1200℃,roasting time 20 h,total content of impurity elements 49.98μg/g,total removal rate 91.93%,element Al content 30.02μg/g,removal rate 91.49%.The optimum conditions of sodium carbonate roasting were roasting temperature 1000℃,roasting time 15h,total impurity content 46.75μg/g,total removal rate 92.45%,element Al content 25.22μg/g,removal rate 92.85%.The SiO₂ content after calcination purification is≥99.995%.（4）Thermodynamic analysis of vein quartz phosphoric acid leaching indicated that the Gibbs free energy of phosphoric acid reacted with muscovite,hematite and other impurity minerals was less than 0 at both 80℃ atmospheric pressure and 260℃ hot-press leaching conditions.Both leaching reactions can proceed spontaneously.The kinetics analysis of element Al leaching temperature was carried out.The results show that both the atmospheric and hot-press leaching process conform to the Avrami model.The atmospheric pressure leaching process is controlled by internal diffusion and chemical reaction.The apparent activation energy of the reaction is 38.04 kJ/mol.For hot-press leaching,when the temperature is lower than 140℃,the leaching process is controlled by internal diffusion and chemical reaction.When the temperature is higher than 140℃,the leaching process is mainly controlled by internal diffusion,and the apparent activation energy of the reaction is 9.13.kJ/mol.It shows that the hot-press leaching process is more conducive to the leaching of impurity elements.（5）The advantage of incorporation of sodium carbonate roasting is that it lowers the crystal phase transition temperature of quartz,which is accompanied by changes in the volume of quartz,cracks appear on the quartz surface,crystal defects of quartz are created,and the impurity metal is activated,which create favorable conditions for subsequent leaching.Comparing the two roasting methods,sodium carbonate roasting not only improves the purification efficiency,but also significantly reduces the thermal energy consumption.The roasting temperature is reduced by 200℃,and the roasting time is reduced by 5 h.It is a new energy-saving technology.（6）Leaching process completely without hydrofluoric acid,hydrochloric acid,sulfuric acid and nitric acid is an innovation to the traditional leaching technology.It has positive significance and important role in reducing equipment corrosion,reducing the degree of environmental pollution,improving the operating environment,and reducing manufacturing costs.The industrial wastewater of phosphoric acid leaching is easy to handle up to standard emissions.