Effect of Ettringite and Monosulfate Formation on Elution of Lead from CaO-Al₂O₃-CaSO₄ Solid Hydrate

Abstract

Ash from waste incineration and coal combustion processes contains various kinds of toxic heavy metals. It has been proposed that the elution of these metals is possibly restricted by solidification of the ash with hydrate formation. Typical hydrates of ettringite and monosulfate are formed from lime, alumina and gypsum, which are often contained by the various kinds of wastes. Thus, it is expected that mixing various wastes in adequate ratis restricts the elution of these metals. In the present study, first of all, the mixtures of the pure reagents with various ratios were cured and X-ray diffraction analysis was conducted for the formed solids. The intensity of the diffraction peak of ettringite increases with increasing ratio of gypsum to lime, while that of monosulfate has a maximum at a mixture ratio of 3 : 4 : 3 for lime, alumina, and gypsum. Secondly, the eluted amount of lead from the solid hydrates with addition of lead was evaluated by the standard method for soil environment. It is found that the elution of lead is suppressed under conditions of ettringite formation and lower pH value of the effluent than around 12.