The State of Canada's Environment — 1996 Chapter 11 — Human Activities Minerals, Metals, and Mining Environmental impacts of mining Mining and milling Acid mine drainage/acid rock drainage

Acid mine drainage/acid rock drainage

Most metal mines in Canada contain sulphide minerals, particularly iron pyrite (fool's gold), in natural association with the desired minerals. When these sulphide minerals are exposed to air and moisture, oxidation occurs, resulting in the generation of sulphuric acid. Because mining leaves waste that consists largely of small pieces of broken waste rock and finely ground tailings, it dramatically increases the surface area that can be subjected to this process. If the rock has a natural buffering capacity (i.e., carbonate minerals), the acid generated will be neutralized until either the buffering capacity or the acid-generating potential is consumed. The most significant problem with acid generation is that it may accelerate over time and, once begun, is almost impossible to stop. Certain microorganisms that feed on sulphur, particularly Thiobacillus ferrooxidans, accelerate the production of acid (Ripley et al. 1996). Increased acidity promotes the mobilization of contaminants such as heavy metals. This set of processes and the acidic effluents that result are grouped into what is known as "acid mine drainage" or "acid rock drainage." The latter term originates from the fact that this phenomenon is not necessarily confined to mining activities but can occur wherever sulphide-bearing rock is exposed to air and water.

Acidic drainage is the most serious environmental problem facing the global metal mining industry. Although steps can be taken to slow the process or treat the acidic effluent, it is difficult to prevent completely.

The Canadian mining industry generates approximately 650 million tonnes of tailings and waste rock annually, half of which are from sulphide ore operations. Table 11.16 provides estimates of all mine wastes as well as acid-producing and potentially acid-producing mine wastes in Canada. Technologies to reduce and prevent acidic drainage are being researched and developed in Canada under the Mine Environment Neutral Drainage (MEND) program, a cooperative program between industry and both federal and provincial governments (MEND is described in more detail in this component, under the section "Cooperative efforts to address environmental concerns"). Promising techniques developed under MEND for preventing or controlling acidic drainage are summarized in Box 11.13.

Table 11.16 Estimates of all mine wastes as well as acid-producing and potentially acid-producing mine wastes in Canada as of 1994

Keywords for Table 11.16
Province; surface; coal; mining; oil; sands; sites; rehabilitated; water; cover; territory; Newfoundland; New Brunswick; Nova Scotia; Quebec; Ontario; Manitoba; Saskatchewan; British Columbia; Yukon; Northwest Territories; Canada; Waste; rock; tailings.

Box 11.13 Promising techniques for preventing and controlling acidic drainage Several promising techniques for acidic drainage are being developed under the Mine Environment Neutral Drainage (MEND) program. Acid generation may be prevented or reduced by submerging sulphide materials in shallow or deep water. This creates an anaerobic environment that discourages oxidation and bacterial action. Tailings disposal in the deep water of northern lakes, such as at the Polaris Mine, has produced excellent results: sulphides appear to remain stable, and dissolved metal concentrations are low (Ripley et al. 1996). Unfortunately, subaqueous disposal is not an option for more than half of the existing tailings sites in Canada, because of the following factors: the lack of suitable water bodies to flood the wastes, thermal turnovers in lakes, the permeability of the underlying ground, the presence of oxidation products in the tailings , or the location of the waste material at the surface — waste rock is impractical to flood unless the rock is returned as backfill in underground mines or to mined-out open pits (Feasby and Tremblay 1995; Mine Environment Neutral Drainage Program 1995). Layered earth covers are being investigated. They are effective but costly to instal in many areas of the country (Feasby and Tremblay 1995; Mine Environment Neutral Drainage Program 1995). Passive treatment methods, such as engineered wetlands, have been investigated for application to metal mine acidic drainage in Canada. In this process, metals are absorbed by plants, such as cattails. This technique has been used successfully in Elliot Lake. Adding lime to neutralize the acid is another technique that is being used for treatment of acidic drainage (Ripley et al. 1996). Long-term methods for the management of tailings and waste rock are being researched. Flooding impoundments for uranium tailings look promising. The East Sullivan Mine in Quebec is testing an organic cover made of 2 m of softwood and hardwood bark overplanted with grasses. Along the same lines, Falconbridge is experimenting with compost, which creates anaerobic conditions and effectively halts oxidation, "thus reversing acid drainage processes and possibly even reducing trace metals" (Ripley et al. 1996). Municipal sewage is one form of compost being tested at Falconbridge. Another engineered cover being tested for durability for large-scale applications on acid-generating waste rock piles is the use of a cementitious cover. The prohibitive cost of aggregate for the cementitious covers has steered the MEND program away from further research into these areas (Ripley et al. 1996). What has been confirmed in the MEND program is that prevention is the best solution for acidic drainage (Feasby and Tremblay 1995). The best preventive measure is to design an impoundment for subaqueous disposal of tailings, taking into consideration water sources, climate, and geology of each site and factoring in the chemical and physical properties of the waste material (Ripley et al. 1996).