Evaluation of membrane filtration for Sulphate and Uranium removal from mine water: Assessing the impact of Manganese scaling

Sammanfattning

Stricter environmental legislation has set new standards for mine water discharge and created the need for more advanced treatment. This study investigates a pilot-scale filtration system designed for removal of sulphate (SO₄²⁻) and uranium (U) in Aitik copper mine, Northern Sweden. The filtration system is divided into three treatment steps connected in series, consisting of greensand filtration, ultrafiltration and nanofiltration. The system demonstrated stable removal rates, over 99%, successfully achieving concentrations below the set threshold value for SO42- and the environmental quality standards (EQS) for U. Based on theoretical projections used in the design of the pilot plant, together with literature values, manganese (Mn) concentration above 50 μg/L was expected to cause scaling within the nanofiltration unit. However, scanning electron microscopy (SEM) analyses revealed no Mn scaling on the membranes. Instead, scaling was found to be caused by barite (BaSO4) while surface abrasion of the filters appeared to be caused by silicates. To ensure sufficient Mn removal, chemical oxidation with potassium permanganate (KMnO4) was used before further treatment. The formation of Mn-oxides was found to adsorb Si and Ba, creating a possible solution for an increased membrane lifespan. The findings from this study challenge the initially set threshold limit for Mn, due to the fact that no indication of Mn scaling was observed in the nanofilter (NF) membranes and instead highlights the need for further investigation of several factors affecting membrane fouling.