Flue gas desulfurization by-product weathering by acidic mine drainage

The authors examined the suitability using a flue gas desulfurization grout (FGDG) for the attenuation and abatement of acidic mine drainage (AMD). The FGDG used was a mixture of fly ash (FA) and filter cake (FC) with a FA/FC ratio of 1:1 to improve handling. Five percent of lime (CaO) was added to improve strength development and allow the use of this FGDG as a hydrologic seal for underground mines. Acidic mine drainage solutions collect from wells located within mine voids were reacted with samples of FGDG for up to 168 d, to evaluate the potential for grout dissolution subsequent to subterranean implacement. Shortly upon reaction with AMD, FGDG released a number of ions into solution (As, B, Ca, K, Na, Se, SO4), a concomitant with a rapid increase in solution pH (8.5), causing decreases in the solubility of most cations (Al, Fe, Mn, Zn). Significant increases in dissolved As and B concentrations were noted. Both elements were present in solution at levels below respective regulatory limits for drinking water. Of the original quantities of As and B present in FGDG, 1.3 and 45.6%, respectively, were released to solution over a 168-d reaction period. Concomitant with changes in solution composition, reaction of FGDG with AMD resulted in a loss of ettringite and hannebachgite and a growth of gypsum. Additional changes in mineralogy were observed as FGDG equilibrated with AMD solutions. From these reactions, the long-term stability of FGDG in underground acidic mine environments is questionable and warrants study in situ