CHARACTERIZATION OF WEATHERING AND CONTAMINATION POTENTIAL OF NICKEL SULFIDE MINE TAILINGS, HITURA, WESTERN FINLAND

The Hitura nickel mine and processing plant, located in Western Finland, have produced ~ 9,500,000 tons of tailings during the 32 years of operation. The tailings have been disposed as slurry into a tailings pond being nowadays 110 ha in size and 27 m in depth. Water in the tailings area is mainly circulated back to the enrichment process. Yet, ~ 10 % of the water leaches through the tailings dam to the seepage ditch and to the groundwater around the tailings pond causing deterioration of the water quality with increased concentrations of Ni, SO4, Cl, Mg and Fe. However, pH in the polluted groundwater is near neutral, suggesting that the tailings are not acid producing.

To understand the chemical processes causing the contamination of the groundwater, a mineralogical examination of the unremediated tailings and analysis of the tailings pore water chemistry were made. Primary mineralogy was studied using XRD and optical microscopy, and weathering of sulfide minerals and mineralogy of the secondary precipitates were examined with SEM-EDS. Sequential extractions were used to study the sorption of heavy metals and finally, a chemical model was created to combine these data to quantify the chemical processes.

Results show that weathering is restricted to the topmost 60 cm with Fe oxyhydroxide alteration rims around pyrrhotite grains. The most active oxidation zone, with thickest alteration rims and absence of carbonates, reaches the depth of 25 cm. Pore water concentrations and sequential extraction analyses indicate that Ni has been released from the sulfide fraction and partly coprecipitated with Fe oxyhydroxides in the oxidation zone. Neutral pH in pore water and the absence of carbonates demonstrates that the tailings have the ability to neutralize the acidity produced by the sulfide weathering. Furthermore, high Mg concentrations in pore water (ca. 2,000 mg/l) suggest that Mg-serpentinite is also involved in the neutralization process apparently through cation exchange reactions. Below the oxidation zone, only slight alteration of sulfides can be detected and most of the Ni is fixed in the sulfide fraction.