The acid generation potential of iron precipitates and their sludge in decommissioning with ecological engineering

Abstract

The Ecological Engineering approach to decommissioning mine sites, which has been applied at the South Bay Waste Management Area (SBWMA), takes advantage of a number of naturallyoccurring processes to correct problems associated with acid mine drainage. One of these natural processes is to allow the formation of iron precipitates to occur without adding neutralisation chemicals such as lime as this requires the maintenance of a chemical treatment plant, often in perpetuity. Such plants produce, as a secondary waste, a sludge containing high metal concentrations, which needs disposal. For South Bay, a former Cu/Zn Mine with tailings and underground workings producing AMD, natural sludge production is estimated to be about 30 m 3 annually. The estimated sludge production for a high-density treatment plant would generate about 1,200m3 of sludge per year, including spent lime, while a general treatment plant would generate 3000 m3 per year (Kalin, 2001). The tailings consist of 41 % pyrite and 4 % pyrrhotite and are expected to generate AMD for anywhere between 1000 to 35,000 years. From an economic and environmental viewpoint the Ecological Engineering technology developed at this site in the last 15 years should not be dismissed without careful consideration. Ecological Engineering integrates natural iron - precipitation, alkalinity generation by microbes and biological polishing along with measures to reduce the rates of acid generation. The objective of this report is to summarize the acidification associated with iron precipitation which occurs in the discharge of contaminated ground water seepages. It will address precipitate formation in mine working discharges to the Backfill Raise Ditch (BRD), in the relevant water bodies of the SBWMA, which include Boomerang and Mud Lakes which have been utilized as polishing ponds and Armanda Lake, inadvertently acidified by effluent from Mud Lake.