The Capacity of Natural Peat to Attenuate Metals from Simulated Waste Rock Drainage : A Laboratory Study

Northland Resources’s Tapuli iron ore mine site is located in North East of Sweden. The waste facilities for tailings and waste rock are surrounded by peatland and the leachate from the waste will in the future infiltrate the peat area over a long period. Northland therefore wanted to investigate the natural capacity of peat to attenuate metals from a simulated waste rock leachate, produced to correspond to waste rock leachate that will be generated in field. Peat was sampled as undisturbed as possible in December 2012. Waste rock leachate was produced from a waste rock mixture provided by SRK by using the method for Net Acid Generation test, which include dissolution with hydrogen peroxide. Since the NAG test procedure is a method to dissolved sulphide-rich material, it was not an appropriate method of choice to dissolve carbonate-rich waste rock, such as the waste rock used in this study, to simulate waste rock drainage.The peat and waste rock was analyzed for its chemical composition and waste rock for its acid generation potential. The waste rock had very low heavy metal content as well as rather low concentrations of S, but high content of carbonates as marble. The simulated waste rock drainage showed low metal concentrations (much lower than anticipated), and only Cr, Mo and S showed higher elemental concentrations in the simulated waste rock leachate than in the peat itself. The waste rock was considered as non-acid producing and the high marble content (enrichment of calcite and dolomite) promoted formation of neutral rock drainage. The feed solution was flowing up through the peat in a column for three weeks at an average temperature of 40C. The L/S ratio was 100 the first two weeks and 9 the third week. Inflow and outflow leachate was analyzed for anions and elemental concentration, as well as pH, redox potential and electrical conductivity. In the outlet water pH decreased from 6.6 to 3.4 during the first week and the release of dissolved organic material was high, probably due to that the peat was oxidized by the feed solution, which might have contained oxidation species from the hydrogen peroxide. Due to the very low sulfur content as well as the high buffering capacity in the Tapuli waste rocks, the NAG test was not a suitable way to prepare a simulated leachate from waste rock. This was confirmed by the high oxygen concentration determined in the feed solution compared to the outlet water. The peat showed variations in attenuation during the three weeks with a rather constant attenuation of the metals such as Cr and Mo, and an initial release of primarily Al and Fe. If the waste rock mixture provided by SRK Consulting was representative, the waste rock leachate should not be an environmental problem in itself. Nor should it, in a long term perspective, affect the peat in a greater extent than the release of metals that were already found in the area.Validerat; 20131204 (global_studentproject_submitter