A fly ash/biosludge dry cover for the mitigation of AMD at the falun mine

Abstract

The sulphide mine tailings in Falun, Sweden is covered with a mixture of fly ash and biosludge as a measure against acid mine drainage. This is a study of the tailings geochemistry by means of pH, Eh, iron, sulphate, calcium and organic carbon concentrations from a field control site and field studies of the covered deposit. The sulphate/iron mole ratios of the interstitial solutions can be used as a proxy to indicate dominating chemical processes in the tailings. In the Falun tailings no ferric iron oxidation of the mining waste seems to take place. Thermodynamic calculation of the redox potential demonstrates which half-cell reaction is governing the redox potential variations. The organic carbon concentrations are very high in the covered Galgberget tailings deposit compared to the control site. This carbon originates from the cover material and is used as a measure against oxygen penetration to the mine tailings. The drawback of the dissolved organic material is that it can act as complexing agents for heavy metals. Also, the dissolved organic material is according to an empiric model decreasing rapidly and of negligible importance after a short time, approximately 30 years. The Galgberget tailings deposit is greatly oversaturated with respect to gypsum making the formation of hardpans possible. Compared to the uncovered control site the field studies of the covered Galgberget tailings deposit show a decrease in the oxidation of iron sulphide minerals and an increase in pH of the leachate with time.