Since the mid-1990s, the majority of new mine development and mine closure in Canada has focussed on the northern regions. In response to this activity, the Mine Environment Neutral Drainage (MEND) Program sponsored a series of projects involving review of literature and some data collection to increase the understanding of mine waste behaviour under cold conditions and to evaluate opportunities to exploit low temperatures for control of acid rock drainage (ARD). Over the past ten years, data on low temperature effects have been collected at several sites. The current review, to compile this data, was commissioned by MEND.
The scope of this review covered eight mechanisms that were either expected to occur differently at low temperatures, or were unique to low temperature conditions: oxidation rates of iron sulphide minerals; oxidation rates of other sulphide minerals; activity of different types of bacteria; solubility and reactivity of acid buffering minerals including carbonates and silicates; formation and solubility of secondary minerals (weathering products); freeze concentration effects; physical exposure of minerals due to freeze-thaw processes; and solubility of oxygen in waters used to flood reactive wastes. The primary focus of the report was on understanding these mechanisms at small scales although implications to full-scale facilities were considered.
Materials for the review were obtained from the authors’ files, contact with mining companies and consultants, and through a general literature search. The majority of information obtained related to specific sites. Fourteen case histories were compiled and form the basis for the review findings. Overall, the authors found that specific information on the reactivity of iron sulphides at low temperatures has been collected from eleven sites and seven of these had comparative temperature data from laboratory test work. These data indicated rate reductions at low temperatures were generally consistent with the Arrhenius Equation, though some exceptions were apparent, which could be related to compounding effects caused by the temperature differences. Data on the reactivity of other sulphides was very limited.
Considerable progress has been made on understanding the activity of bacteria at low temperatures. It is clear that bacteria can be active at sub-zero temperatures and that bacteria adapt to low temperature conditions.
Specific information on the relative solubility and reactivity of acid neutralizing minerals was not found. Theoretical calculations demonstrate that carbonate minerals are more soluble under cold conditions. This represents a potential opportunity for blending of waste rock since it means that alkalinity may be more available. No relevant data on the weathering of silicates at low temperatures was found. A specific issue caused by the solubility of carbonates at lower temperatures is the greater solubility of heavy metals (e.g. zinc and cadmium) which can be expected. This may explain the observation from test work that zinc leaching increases as the temperature falls. Specific data on the solubility of other secondary minerals at lower temperatures were not obtained, but in general, lower temperatures should result in lower solubility.
Freeze concentration can be an important effect since it results in increasing dissolved concentrations of contaminants and freezing point depression. The former can result in expulsion of waters containing unacceptable concentrations during freezing. The latter is important because it may prevent waste from freezing.
The effect of physical exposure due to freeze thaw has potential implications for the exposure of reactive minerals to oxidizing conditions. No specific references to the geochemical implications of this process were found.
Finally, the solubility of oxygen in water is greater at lower temperatures than higher temperatures but the decrease in oxygen diffusivity is expected to offset this effect. Specific case studies to demonstrate this effect were not found.
The authors concluded that the main technology gaps exist in; the areas of specific methods for low temperature test work; understanding of the reactivity of sulphide minerals other than the main iron sulphide minerals at low temperatures; the effect of temperatures on the dissolution and weathering of carbonates and silicates; and understanding of the low temperature solubility of secondary minerals (particularly carbonates). The opportunities to harness low temperature conditions to limit ARD and metal leaching remain the same as were identified in previous MEND studies, with the possible addition of blending as an opportunity because carbonates are more soluble at lower temperatures. Limited research has been completed on the effect of low temperatures on the geochemical performance of mine waste management facilities. Research to address some of these gaps is recommended.
Practitioners who wish to invoke the benefit of low temperatures should complete suitable test work for minesite characterization projects because the data demonstrating the effects of low temperatures are sparse and indicate site-specific effects. Larger scale experiments conducted on site are highly recommended.
