GROUNDWORK
Management: Cover Up!
Nick Currey, Senior Environmental Officer at Kidston Gold Mines, looks at cover techniques for controlling acid drainage from waste rock dumps

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Dump cover construction at Kidston mine, showing cover 'B', instrument site
and lysimeters
All Photos: N. Currey, Kidston Gold Mines

To manage acid drainage properly, it is imperative that there is a good understanding of the geochemistry of the ore body and associated waste prior to mining and that a well managed waste rock program to control the potential acid producing rock is established to reduce the long term potential for sulphidic waste rock to oxidise.

Design

Rehabilitation of sulphidic waste rock dumps with covers has been used with varying success in Australia for about 25 years. However, many earlier covers were designed when there was limited understanding of the processes involved in the oxidation of sulphidic waste rock and it is really only in the past 10 years that considerable efforts have been made by the industry to fully understand the chemical mechanisms causing acid drainage and to design covers accordingly. Modern cover design must also address the requirements of long term stability and final land use.

Covers are designed to be placed on the crown and sides of waste rock dumps. Their objective is to reduce the ability of oxygen and/or water to flow into the underlying sulphidic waste rock. By limiting the amount of oxygen, the oxidation reaction can be slowed. Likewise, by reducing the flow of water into the waste rock, oxidation products (low pH water and dissolved metals) can be controlled.

The use of computer modelling has assisted in cover design, and a number of models are being used by consulting engineers. The success of these models depends on good input data covering meteorological data, the physical properties of the soil from which the cover is to be constructed and the characteristics of vegetation proposed for the rehabilitation.

Monitoring of a cover after installation is important to determine if it has achieved the desired outcome. Monitoring may have to be undertaken for a number of years to cover a wide range of climatic variations and to provide calibration of the computer model. The cover’s success can be quantified by the reduction in poor quality water seepage emanating from the waste rock dump.

Most covers are constructed using soil materials available at the mine site and hence are site specific. Composite covers, where there are a number of layers, some compacted to reduce their permeability and others uncompacted to support vegetation, are the most commonly used designs.

A Practical Test

Kidston Gold Mines is currently trialing two covers at its Queensland operation. One uses the traditional approach of a 500 mm compacted soil layer and then has a nominal 1500 mm layer of loose material block dumped on-top of it. The second cover has a loose 2500 mm layer of soil. The objective of the covers is to limit water infiltration into the waste rock below and to provide a stable medium to support a sustainable vegetation cover.

The design uses an ecosystem approach. The extra soil depth is designed to contain all wet season rainfall (like a sponge), with the water driven out of the soil during the dry season by evaporation and plant evapo-transpiration. The cover surface effectively contains all erosion products at the base of the soil piles, traps water, which promotes vegetation growth, and provides different micro-environments for flora and fauna establishment.

Performance monitoring instrumentation and lysimeters installed in and below the cover, indicate very low infiltration rates (maximum of 0.45%) of incident wet season rainfall.

During construction of a cover, quality control targets must be set to ensure a high quality uniform construction. This requires survey control, moisture content and soil compaction testing.

Wet Covers

Work in Canada has demonstrated that oxidation can be controlled by placing the acid producing rock under water, greatly reducing the exposure of the rock to oxygen. This technique can be applied in the design of waste dump covers where annual precipitation exceeds evaporation and the cover can be kept at near saturation point.

Waste rock covers using clay materials have been successfully constructed in New Zealand, where they remain more than 95% saturated due to the high rainfall. This technology is applicable in parts of southern Australia, New Zealand and the equatorial countries of Asia.

Dry Covers

In most of Australia, evaporation exceeds rainfall, dictating the use of a "dry" cover. Typical "dry" cover designs include a compacted soil layer (up to 1500 mm thick), with or without a capillary break to limit the rise of salts from the sulphidic material below, topped with a loose layer needed to promote a vegetation cover.

The long term sustainability of vegetation growth on these thin topsoil layers (approximately 300 mm) remains unproven, especially if the pre-mining land use returns (usually low intensity cattle grazing). A number of these covers are also relatively high maintenance as the thin topsoil layer means trees need to be removed regularly to prevent roots penetrating and damaging the compacted layer.

Cover Research

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Construction of the Kidston south dump with
lysimeter pipes in foreground.

Mining companies are currently supporting a number of research projects to gain a better understanding of oxidation processes and alternative treatment treatments for waste rock dumps. The use of alternative treatment processes such as Neutral Barrier Technology being developed by Earth Systems Pty Ltd and ANSTO, Kaolin Amorphous Derivatives being developed by the University of Queensland and Mine Remediation Services Pty Ltd and the use of oxygen consuming materials within the cover, being developed by Environmental Geochemistry International Pty Ltd, offer the potential for new site specific treatments.

Cover design is a relatively new engineering discipline for the mining industry. In order to meet society’s expectations, cover designs should not only provide a barrier to oxygen and/or water but also embrace vegetation sustainability and meet long term land use goals. Ultimately, an understanding of the oxidation processes, best practice rehabilitation techniques and the sharing of information can best benefit the industry as it strives to manage acid drainage.

For further information, contact Nick Currey on (07) 4062 4225 or email: 100253.1317@compuserve.com

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Copyright 1998 Australian Minerals & Energy Environment Foundation
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Web Posted September 1998