A Laboratory Study of ARD Neutralization and Prevention by Alkaline Rest Products : Waste rock from Maurliden

Boliden’s Maurliden mine, located in Northern Sweden, is a polymetallic VMS deposits with pyrite as the dominant sulfide mineral. At closure, waste rock will be backfilled into the open pit, covered, and the pit will then be allowed to flood to reduce oxidation. Boliden is considering the addition of alkaline material as a safeguard to buffer potential acid rock drainage (ARD). Therefore, characterization and leaching tests of waste rock were completed as part of this study. Characterization of alkaline rest products and leach tests with mixtures of waste rock and alkaline rest products were performed. Acid generation potential of the waste rock was determined together with neutralization experiments with alkaline rest products. Leach tests of the waste rock were conducted in 1m3 tanks under saturated and dry conditions, dewatered once a week. The pH in the water saturated tank was observed to increase from 2.4 to 5.6 as maximum over a 4 months period. pH was observed to decrease in the water saturated tank after 2 months of sampling to pH 3.5, most likely due to a higher evaporation and a lowered water table. A decrease of the pH in the dry tank from 6.7 to 1.4 over an 8 months period, with a minimum pH of 0.9 was observed. ICP-analysis of the leachate from the dry tank showed a significant increase in concentrations of elements such as S and Fe from µg/L to g/L, while the concentrations were constant at µg/L in the saturated tank. This shows the importance of saturated conditions to reduce the oxidation of sulfides. Alkaline rest products mesa lime, fly ash (FA) and green liquor dregs (GLD) from paper and pulp mills together with slag from steel industry were evaluated for their ARD neutralization potential. Waste rock crushed down to <6mm were mixed with different fractions of alkaline waste, 0.5%-5%, and leached in small scale for 1, 3, 8, 21 and 65 days. The 5% mixtures were leached for 115 days. After 65 days the analyzed pH was circum-neutral for most materials expect AOD slag with pH >10, mesa lime with one sample with a pH at 4 and Iggesund FA with 2 samples with a pH at 3. Samples with pH <7 showed increased concentrations of elements such as As, Cr, Fe and S, and low element concentrations were reported in all samples with pH >7. This shows how important it is to keep pH in the solution at 7 to 9, to minimize the leaching of elements. After 65 days of leaching, almost all water had been replaced with fresh water. It takes many years before all water in an open pit is replaced with new water. The material in the pit is significantly coarser grained than the size fraction 6mm used in the experiment, which also influence the oxidation of the waste rock. A material with fine particle size has a larger surface area, which increases the oxidation. The results of this master thesis are a preliminary step in evaluating the neutralization capacity of alkaline rest products and their effect on leachate quality when mixed with waste rock. The investigation could be used in further studies of neutralization. Rest products other than mesa lime can be used with the purpose to neutralize ARD when mixing the alkaline material with waste rock and backfill it to an open pit. Further investigations will be undertaken to determine the long-term ARD neutralization capacity of the alkaline wastes for use in ARD mitigation.Validerat; 20121112 (global_studentproject_submitter