Study On The Sulfur Behavior In The Process Of Extracting Alumina From Fly Ash By Sub-molten Salt Method

Fly ash the largest single emission of waste residue has brought about serious problems in China. At the same time, there are large reserves of high-alumina fly ash in the northwest of China. It is a good non-traditional resource for extracting aluminum. Accordingly, a new clean process of extracting alumina from the fly ash effecently and cleanly namely sub-molten salt method has been created by the Institute of Process Engineering, Chinese Academy of Sciences (IPE, CAS). However, the sulfur impurity’s influence on the new process is not yet clear and the control of it is also undefined. Taking sulfur impurity in the process as research object, this paper focused on experimental investigation of sulfur occurrence state, material flow of sodium sulfate, equilibrium of sodium sulfate in NaOH and aluminate solutions and crystallization kinetics of sodium sulfate in the aluminate solution.Innovative achievements in this paper including:(1) The sulfur occurrence state in the fly ash was obtained. The results show that after burning in the circulating fluidized bed boiler, sulfur minerals in fly ash converted into anhydrite and ettringite. This is remarkably different from bauxite mineral resources. Ettringite is deduced to be a kind of product formed by the reaction of anhydrite and active alumina in fly ash during stocking. In the end, sulfur in the fly ash converted into sodium sulfate.(2) The material flow of sodium sulfate in the process was determined. Based on the data obtained the place where to remove the sodium sulfate was confirmed. Sulfur in the process mainly removed out of the system along with desiliconed residue and washing liquor of leached residue.(3) Phase diagram of Na2SO4-NaOH-H2O and Na2SO4solubility in the sodium aluminate solution with caustic ratios of12and15were drawn. The phase diagram shows that the Na2SO4solubility declines as the concentration of NaOH in the ternary system increases. At a NaOH concentration exceeding33%(mass percent), the Na2SO4solubility is aboutlower than1%. The Na2SO4solubility in the quaternary system approximates the values obtained in the Na2SO4-NaOH-H2O system. At NaOH concentrations lower than72.97%the equilibrium solid is anhydrous sodium sulfate. Compare the solubility of ternary system with the quartery system, the addition and the amount of A1(OH)3have insignificant effect on Na2SO4solubility.(4) Sodium sulfate crystallization kinetics study shows:sodium sulfate supersaturation is not obvious and the sulfur removal method is eastablished. Sodium sulfate can be removed out using evaporative crystallization. Meanwhile, the influences of impurities such as SiO2and Na2CO3were studied in this thesis.