Effect of ettringite and monosulfate formation on elution of lead from CaO-Al2O3-CaSO4 solid hydrate

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 fo rmation. Typical hydrates of ettringite and monosulfate are formed from lim e, alumina and gypsum, which are often contained by the various kinds of wa stes. Thus, it is expected that mixing various wastes in adequate ratis res tricts 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 dif fraction analysis was conducted for the formed solids. The intensity of the diffraction peak of ettringite increases with increasing ratio of gypsum t o 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 fro m the solid hydrates with addition of lead was evaluated by the standard me thod for soil environment. It is found that the elution of lead is suppress ed under conditions of ettringite formation and lower pH value of the efflu ent than around 12.