Neutralization of Acid by Steel-Making Slags

Abstract

Steel-making slag residues are potentially useful in the neutralization of acidic liquors such as acid-mine drainage (AMD) because of their high content of basic minerals. This work examined the neutralization capacity of slags by batch titration with sulfuric acid. Measurements of pH and elemental solution composition over time provided input to a dissolution model based on saturation driven surface kinetics, a process dependent on the relative degree of saturation of the dissolving solid, and solution equilibrium. The heterogeneous composition of slag grains was simplified to three compositional groups based on their solubility. Group 1 included free lime, magnesium and silicon oxides, and dicalcium ferrite. Group 2 included weakly bound lime and magnesium oxides. Group 3 contained tightly bound lime and magnesium oxides, and wüstite. 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 10 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 that 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 capacity as calcite and can neutralize acidic liquors like AMD. However, due to the slag having slow dissolution kinetics and the necessity to use fine powders, rapid neutralization is hindered because of the high cost of crushing.