Neutralization of acid by steel-making slags

Steel-making slag residues are potentially useful in the neutralization of acidic liquors such as acid-mine drainage (AMD) because of their high conte nt of basic minerals. This work examined the neutralization capacity of sla gs by batch titration with sulfuric add. Measurements of pH and elemental s olution composition over time provided input to a dissolution model based o n saturation driven surface kinetics, a process dependent on the relative d egree of saturation of the dissolving solid, and solution equilibrium. The heterogeneous composition of slag grains was simplified to three compositio nal groups based on their solubility. Group 1 included free lime, magnesium and silicon oxides, and dicalcium ferrite. Group 2 included weakly bound l ime and magnesium oxides. Group 3 contained tightly bound lime and magnesiu m oxides, and wustite. Neutralization profiles consistently revealed a two- phased kinetics. Group 1 oxides dissolved almost entirely in the first days according to saturation driven kinetics. Group 2 dissolved over the next 1 0 to 80 days, driven by a pH gradient in a quasi-first order reaction rate with respect to H+. Group 3 did not dissolve. Indirect evidence suggests th at the transfer from saturation to pH driven dissolution would be caused by armoring of the grain surface by precipitating iron oxides. Although only 60% in volume of slag dissolves, it still possess the same neutralizing cap acity as calcite and can neutralize acidic liquors like AMD. However, due t o the slag having slow dissolution kinetics and the necessity to use fine p owders, rapid neutralization is hindered because of the high cost of crushi ng.