Lignite fly ash utilization for acid mine drainage neutralization and clean-up

Two fly ashes (FAs) originating from Kardia and Megalopolis power plants are used for the first time to amend the acid mining drainage (AMD) from the abandoned Ermioni and the active Kassandra Greek mining sites. The fly ashes employed are of type C, with Kardia fly ash (KFA) being mostly calcareous and Megalopolis’ (MFA) mostly siliceous. Both FAs were tested for their pH-regulating and metal-ions-removal efficiency. Regarding the acidity reduction both FAs proved equally efficient; by regulating the FA to AMD solution ratio, final pH values between 6 and 12.5 were attained using KFA while weakly acidic (pH = 6) to neutral solutions were produced in the case of MFA. Except for chromium ions, both FAs, also, proved equally effective in (heavy-)metal-removal for loadings higher than 5 g/50 mL acid waters. KFA, particularly, cleaned up the acid mine drainage solution from all metals even at a 0.5 g/50 mL loading. For KFA, metal removal was attributed to precipitation while in the case of MFA retention was found to occur via adsorption and/or complexation as well. Both FAs and AMD are largely produced as side effects of the two major Greek economic activities, i.e., electricity production and mining; using FAs to assess the AMD environmental impact can contribute to circular economy and, therefore, is of significant financial interest for Greece.