The vastness and geophysical complexity of the Great Lakes Basin become apparent in this satellite photograph.
Acid mine drainage (AMD) has plagued the sulphide mining industry since Roman times. The German scientist, Georg Agricola, mentioned it as far back as the late medieval era. Concerns about AMD pertain to the generation of acid by sulphide oxidation and the mobilization of iron and base metals by acid reactions.
Nowadays, mine discharge, process effluents, seepage from tailings and contaminated site run-off are channelled through one or two treatment plants. There, the acid is neutralized and soluble iron and base metals are precipitated with lime. Excess alkalinity is then neutralized with carbon dioxide before discharge to a water body. Traditionally, this was considered sufficient protection from adverse ecological impact. Now, however, the significance of sulphate ions in downstream systems is gradually receiving more attention as the supplies of fresh water become constrained.
In regions with net annual precipitation, stream dilution usually keeps sulphate concentrations well below the level where solids precipitation occurs. But where mine discharge is a significant proportion of stream flow, concerns about fish spawning and water potability may arise.
In arid areas, the evaporation of effluents serves to concentrate the salts and precludes recycling to reduce fresh water intake from finite aquifer supplies. Water recycling in such locations may necessitate water treatment to remove sulphate ions. Techniques such as ion exchange, reverse osmosis, barium precipitation and microbial reduction to hydrogen sulphide can remove sulphate, but involve high capital and operating costs.
The alternative, in principle, is to avoid the formation of sulphate by inhibiting sulphide oxidation. Mine decommissioning plans often attempt such action retroactively. Increasingly, operations also attempt to reduce sulphates made during active operation through various actions under a progressive reclamation plan. However, designing operations to minimize the rate of sulphate formation from the outset is still rare.
This perspective is gradually changing and there are a number of approaches being investigated, including:
- placing sulphide underground as backfill in a cement-bonded paste fill;
- co-disposal of sulphide-bearing waste rock with desulphurized tailings slimes;
- covering tailings with composted materials as a combined oxygen-water barrier and growing medium; and
- storing high-sulphur (pyrrhotite and pyrite) concentrates under water and promoting the formation of biofilm oxygen barriers over sulphide surfaces in rock by supplying trace nutrients in the form of natural rock phosphate.