Phase Diagrams of (NH4)(2)SO4-Al-2(SO4)(3)-H2O Ternary System: Effect of Sulfuric Acid and Its Application in Recovery of Aluminum from Coal Fly Ash

摘要

Coal fly ash (CFA) is the powdery waste generated in coal-fired power plants. The content of Al2O3 in CFA can be up to 40 wt % and can serve as an alternative to bauxite for aluminum production. Recently, the ammonium sulfate (AS) roasting process has attracted wide interest for extracting aluminum from CFA. During the process, aluminum enters the leaching solution and forms the (NH4)(2)SO4-Al-2(SO4)(3)-H2O ternary system or (NH4)(2)SO4-Al-2(SO4)(3)-H2SO4-H2O quaternary system. In this study, the aforementioned phase equilibrium data were measured using the isothermal method to minimize the water used in the leaching unit. The results showed that the crystallization fields of NH4Al(SO4)(2)center dot 12H(2)O were larger than those of other species, which indicates that it will easily crystallize. The presence of H2SO4 at a concentration of 2.5 wt % had nearly no effect on the solubilities of (NH4)(2)SO4 and Al-2(SO4)(3) but 10 wt % H2SO4 decreases the solubilities at high temperature. According to the composition of the roasted slag obtained at 500 degrees C, the minimal liquid-to-solid ratio during the leaching unit was calculated as 3.32 and 1.4 mL/g at SS and 80 degrees C, respectively. Following crystallization at 25 degrees C, NH4Al(SO4)(2)center dot 12H(2)O can be recovered at crystallization ratios of 44.58% and 60.89% with leaching at 55 and 80 degrees C, respectively. A modified process, high-temperature leaching with cooling crystallization, is proposed. Compared to the original process, the liquid-to-solid ratio can be decreased by approximately 66.8% and 86% at 55 and 80 degrees C, respectively. As a result, the energy consumption during the (NH4)(2)SO4 recycling is significantly reduced.