Laboratory simulation of flocculation processes in a flooded tailings impoundment at the Kristineberg Zn–Cu mine, northern Sweden

A laboratory mixing experiment was performed to simulate the flocculation processes that can be expected when natural stream water mixes with the saline water in flooded tailings impoundments. Mixing plots where dissolved (<0.22 μm) Ca, Mg, Na, K, S and Si were plotted vs. a conservative mixing index revealed a conservative mixing behaviour for these elements. Similar plots for dissolved Fe, Mn, Al and total organic C (TOC) showed that dissolved Fe and Al flocculated within 24 h after the mixing of the waters. Dissolved Mn was removed from solution 13–98 days after mixing, presumably due to the oxidation of Mn2+. Removal of TOC could not be detected in the mixing bottles. However, the flocculants that settled in the mixing bottles (1.7–3.4 mg flocs L−1 of stream water) contained 18 wt% C and 14–19 wt% acid-leacheable Fe. Organic C and Fe oxyhydroxides thus appear to form substantial fractions of the flocculants. If stream water is diverted through tailings ponds as part of a remediation programme, flocculation processes are likely to contribute to natural sedimentation in the pond. However, the relative contribution from flocculated particulate matter may be significant only when the stream-water transport of suspended matter into the impoundment is low (<10–15 mg L−1). Trace metal uptake in the flocculants that settled in the mixing bottles resulted in removal of Cd (0.024–0.028 μg L−1), Co (0.15–0.17 μg L−1), Cu (1.8–3.5 μg L−1) and Zn (15–29 μg L−1) from the dissolved phase. Relative to the dissolved trace metal concentrations in the tailings pond water used in the experiment (Cd=0.435–0.438 μg L−1; Co=0.738–0.763 μg L−1; Cu=3.16–5.05 μg L−1; Zn=26.6–32.6 μg L−1), the trace metal uptake exceeded 50% of these concentrations only for Cu and Zn.