Phase diagrams of the MgSO4-Al2(SO4)3-(NH4)2SO4-H2O system at 25 and 55 °C and their application in mineral carbonation

A route for the indirect mineral carbonation of blast furnace slag using (NH4)2SO4 as a recyclable reagent has recently been proposed. The main challenges associated with this route are reducing its energy consumption and improving its cost-efficiency. In this process, the recycling of (NH4)2SO4 by evaporation of the carbonation liquor make up a large proportion of the total energy consumption. Thus, controlling the water dosage during leaching of the products of roasting the blast furnace slag with (NH4)2SO4, which are mainly various sulfates of Ca, Mg, Al, and NH4+, is of great importance. In this study, the solubilities of the quaternary MgSO4-Al2(SO4)3-(NH4)2SO4-H2O system were measured at 25 and 55 °C using the isothermal method. The resulting phase diagrams are instructive for determining the lowest liquid-to-solid ratio required for leaching and for recovery of the value-added byproduct, ammonium alum, from the leachate. The results show that the crystalline region of NH4Al(SO4)2·12H2O dominates in the phase diagrams, indicating that it is easily crystallized out and that the use of this route for the separation of aluminum as ammonium alum is feasible. The liquid-to-solid ratios for the leaching unit were calculated to be 4.55 and 1.97 mL/g at 25 and 55 °C, respectively. The liquid-to-solid ratio at 55 °C was reduced by 56.7% compared to the ratio at 25 °C.