MEND Project 2.25.1a
July 1992

EXECUTIVE SUMMARY

Falconbridge Limited requested that the Centre in Mining and Mineral Exploration Research at Laurentian University conduct a literature review and make recommendations on the use of municipal compost in the reclamation of sulphide tailings. The extent and continuing burden of acid mine drainage (AMD) from sulphide tailings has prompted the federal and provincial governments, and the mining industry to look for permanent solutions. This report describes the basis of the AMD problem and the major solutions that have been proposed for tailings abandonment. One promising approach is the creation of an oxygen barrier on tailings that would prevent penetration of atmospheric oxygen, which is the main agent of tailings oxidation and AMD production.

The establishment of artificial wetlands on tailings is being researched and applied at some sites to form an oxygen barrier and create a chemical reducing regime in the tailings. However, many tailings sites are not amenable to flooding or the maintenance of wetlands. New forms of oxygen barriers are under development that will maintain a layer of water saturated material on top of the tailings and greatly suppress oxygen diffusion. The cover layer could consist of fine, silt-like material, but this material is very expensive or not locally obtainable in the tailings areas of the north. Municipal solid waste (MSW) compost is proposed as a material which could be used as an oxygen barrier cover for tailings. The compost layer would function as both a physical barrier and as an oxygen-consuming layer that would permanently prevent sulphide oxidation and the resultant AMD.

The vast areas of sulphide tailings in Canada would require a great amount of MSW compost for such reclamation. In 1991, Ontario municipalities were told by the provincial government to divert 50% of the solid waste stream from landfills and incineration by the year 2000. An important component of meeting this goal is the recycling of organic waste by making compost. The tailings cover approach could utilize all the MSW compost that could be produced in Ontario for many decades. This application appears to be a Awin-win@ situation for both the mining industry and the municipalities, providing it is technically feasible, environmentally safe and socially acceptable.

Study Objectives

This literature review addresses the following objectives:

1. Review what is already understood about remediation of acidic mine tailings to avoid the problem of acid mine drainage,
2. Determine what is known about the characteristics of MSW compost and other types of organic matter, and how these wastes could be used in tailings reclamation,
3. Examine environmental regulations and socioeconomic concerns about the use of MSW compost, sewage sludge and other large volume sources of organic matter,
4. Examine the availability and costs of using MSW compost,
5. Recommend experimental studies to address unresolved technical questions about MSW compost and other organic materials as oxygen barriers on tailings.

Conclusions

A literature review of what is known about the physical and chemical characteristics of MSW compost, and other organic materials, revealed that a compost layer on tailings could be beneficial in five ways:

1. Physical oxygen barrier - the compost would be saturated with water over at least part of its depth so that the limiting factor in oxygen diffusion would be the diffusivity of oxygen in water;
2. Oxygen-consuming barrier- the continued decomposition of compost creates a large biological oxygen demand that acts as a sink for diffusion of atmospheric oxygen or dissolved oxygen;
3. Chemical inhibition - compounds and decomposition products in the MSW compost that leach into the tailings inhibit the growth and metabolism of sulphate-producing bacteria;
4. Chemical amelioration - organic constituents in the MSW compost can cause the reductive dissolution of ferric oxides and prevent indirect ferric sulphide oxidation and acid generation;
5. Reduced water infiltration - reduced hydraulic conductivity of compacted compost may prevent infiltration of precipitation, thus decreasing tailings groundwater flow.

Three compost cover layer models are proposed to produce and maintain these functions. Two models consist of a layer of compost (of undetermined depth), which is compacted by an overburden layer of sand and gravel. In one model the bottom of the compost layer is separated from the tailings by another coarse layer to hydraulically isolate it from the tailings. In one model, compost is ploughed into the upper layer of tailings before the compacted compost is placed, so that there is chemical contact between the compost and the oxidized portion of the tailings. The main purpose of the overburden layer is to keep the compost layer permanently compacted so that air-filled pore space is minimized and incoming precipitation can produce a high degree of saturation. The overburden will also be a protective layer against erosion, evaporation and runoff which could destroy the compost layer as an oxygen barrier. The third and most inexpensive compost model consists of a deep layer of compost placed directly on the tailings.

Experimental investigations in the laboratory are required to assess whether the compost cover layer models will function as expected, in the field. Other waste materials can also be incorporated into the compost cover layer to investigate how they alter its physical and chemical properties. Analyses are also required of the leachates that come out of the compost cover layers and tailings, since MSW compost may release heavy metals, organic chemicals and pathogens into the environment. Ontario has released strict guidelines for compost quality which necessitate excellent source separation and production control in order to meet the specifications for Aunrestricted use@ compost.

The risks of contaminants are largely a function of the quality of the feedstocks and the optimization of the composting process. For MSW compost, separation of organic waste from non-biodegradable garbage and hazardous chemicals is essential in producing uncontaminated, high-quality compost. Sewage sludge compost may also become a high-quality material if pathogens are killed and if the wastewater stream is uncontaminated by industrial effluents. However, immature, uncured composts offer advantages for use in tailings reclamation because of their high oxygen-consuming demand and the presence of a wide variety of organic compounds that could function in chemical amelioration of AMD. Immature compost could also be diverted much earlier in the composting process and be much cheaper for a municipality to produce.

The quality of leachates from mature and immature MSW compost and other organic waste components used on tailings will require careful experimental examination to ensure public health and environmental safety. Early and genuine involvement by the community is essential to the acceptance of any tailings reclamation plan involving MSW compost or other organic wastes.

Other waste materials offer some or all the benefits of MSW compost as an oxygen barrier. Forest industry and paper mill wastes are plentiful and a nuisance in many of the areas of Canada where tailings are located. Peat is a common and abundant natural material in bogs throughout the north and may be useful for tailings covers, if it can be managed as a renewable resource. Tailings leachates will have to be tested for harmful compounds from these wastes, as well as for any beneficial effects on AMD.

MSW compost or other organic wastes will be useful for the mining industry only if they provide a permanent, socially-acceptable and cost-effective solution to tailings abandonment. Even if the municipalities provide MSW compost at no charge or with subsidization, transportation costs from the major sources in the south to tailings sites in the north may be prohibitive for a mining company. Ontario government policy and regulations that currently prevent the export of waste from a municipality should be examined with respect to low-grade compost use for tailings reclamation.

Recommendations

1. Laboratory work using instrumented leaching columns should be conducted to test the physical aspects and optimization of the compost cover layer models.
2. Laboratory analyses of leachate composition from unsaturated and saturated compost layers, containing compost of various degrees of maturity, are required to establish environmental risk.
3. Laboratory studies of chemical processes within tailings under the optimal cover layer design will confirm whether reducing conditions are present and chemical amelioration is occurring.
4. Field lysimeter studies of the optimal compost cover layer design, as determined by laboratory experiments, should be conducted at the Nickel Rim tailings site.
5. Discussions should be initiated by Falconbridge Ltd. with provincial government ministries, municipal government and the community concerning the use of MSW compost in tailings reclamation.

Last Modified: 2003-11-26		Important Notices

Français | Contact Us | Help | Search | Canada Site
Home | What's New | CANMET-MMSL | MMS Site | NRCan Site