| As a big country of alumina production,China's alumina production and production capacity are ranked first in the world.However,as a major raw material in the production of alumina,bauxite resources are relatively shortage in China.At present,it needs to import a large number of bauxite from abroad to meet the domestic alumina production,which restricts the development of China's alumina industry.Therefore,the literatures on the extraction of alumina from other aluminum resources are increasing year by year.As a large thermal power generation country,a large amount of coal fly ash(CFA)is produced by burning coal in China.Of all the CFA,the high alumina CFA with alumina content above 30%is about 30%.Considering its high alumina content,high alumina CFA is equivalent to mid-grade bauxite ores,considered as a potential substitute producing alumina.Therefore,it is of great practical and long-term strategic significance to develop and reutilize high alumina CFA to produce alumina and realize green resource production from solid waste,and it has become one of the research hotspots in recent years.However,there are many different shortcomings in the traditional extraction process,which makes it difficult to be applied in actual production.Based on the sintering of CFA with ammonium sulfate,a new technology for extracting alumina from CFA by direct leaching with ammonium hydrogen sulfate solution has been already developed in our laboratory.In this paper,the leaching thermodynamics,the transformation law of the material properties in the leaching process,and the crystallization characteristics of ammonium aluminum sulfate solution were studied,so as to provide theoretical support for the popularization of this technology.The thermodynamic data for different temperatures were obtained by thermodynamic calculations,and the Eh-pH diagrams for the systems Al-,Fe-,Al-Fe-H2O at different temperatures were constructed.The results showed that the stability regions of each substance in the leaching system moved toward the direction of relatively low pH and high potential with the increase of temperature.The stable region of Al3+became narrower at high temperature,which indicated that the extraction efficiency of alumina will reduce under high temperature conditions.Based on the Eh-pH diagram for the system Al-Fe-H2O,the separation of Al and Fe in the leaching solution was realized by increasing the value of potential and pH of the solution.The properties and composition of the solution in leaching system were studied.The conductivity of ammonium hydrogen sulfate solution increased firstly and then decreased with the increase of solution concentration.The structure of the solution changed with the change of concentration.There was a solute-solute interaction in ammonium hydrogen sulfate solution,which disappeard gradually with the increase of temperature.Experimental findings revealed that the prolonged reaction time and the increased reaction temperature are all beneficial to the formation of ammonioalunite.However,the increase of ammonium aluminum sulfate solution concentration and the ammonium hydrogen sulfate added were all harmful to the formation of ammonioalunite.The as-obtained ammonioalunite possessed a cube-like structure and became decomposed with the increasing temperature.?-Al2O3 was generated at 1200°C,and ammonioalunite dissolved in alkaline conditions with the generation of ammonia.The solubility of NH4Al(SO4)2·12H2O was measured by the polythermal method,and the relationship between the solubility and temperature was obtained.Furthermore,the supersolubility and the metastable zone width(MSZW)of NH4Al(SO4)2·12H2O were determined.The results showed that the solubility of NH4Al(SO4)2·12H2O increased with the increase of temperature,and it was fitted as a function of temperature with a correlation coefficient of 0.9906.The growth of NH4Al(SO4)2·12H2O crystals in water followed a continuous growth mode.In phase diagrams of(NH4)2SO4-Al2(SO4)3-H2O ternary system,the crystallization field of NH4Al(SO4)2·12H2O reduced with the increase of temperature.The as-obtained crystals possessed a rhombic shape with a smooth surface,and the particle size increased with the increasing crystallization time.The solubility of NH4Al(SO4)2·12H2O decreased to a certain extent with the addition of NH4HSO4 and(NH4)2SO4,in other words,the supersaturation of NH4Al(SO4)2·12H2O was increased.Therefore,the crystallization induction period of NH4Al(SO4)2·12H2O was shortened and the nucleation rate was accelerated,which leaded the decrease of crystal size.The purity of alumina products was improved by stirring crystallization.It was found that the MSZW of NH4Al(SO4)2·12H2O decreased slightly with the increasing stirring speed and the decreasing cooling rate. |
PDF Full Text Request