|
...conducting innovative research for the Mining Industry! |
The Centre for Environmental Research in Minerals, Metals, and Materials |
|
|
September, 2002.
 |
|
|
Codisposal of Tailings and Waste Rock
|
Characterization of Mine Waste Rock and Tailings - can they be disposed of together?
Ward Wilson, Professor of Mining Engineering and Chair of Mining and the Environment
and
Björn Weeks, Graduate Student, Mining Engineering
Benjamin Wickland, Graduate Student, Mining Engineering
Pamela Fines, Graduate Student, Mining Engineering
Background All mines produce two types of waste material - finely-ground particles of sand called tailings which derive from the processing plant and much coarser-sized waste rock extracted from the mine itself in order to access the ore. Significant advantages in safely storing these materials are derived from the codisposal of the two wastes together.
Two major issues in storing mine wastes relate to:
Traditional methods of mine waste storage have sometimes created environmental problems and some catastrophic failures have taken place in recent years. Coarse waste rock is typically stored in large dumps or piles sometimes over 30 m in height. The stability of these heaps and their response to water and wind erosion are important factors in characterizing the physical safety of the storage dump.
Note the extensive piles of waste rock accumulated around this typical open pit mining operation in Western Australia.
Waste materials may contain sulfide minerals which on exposure to air and water will eventually begin to produce acid and subsequently leach metals into solution to be dispersed into aquatic environments. Methods to minimize the infiltration of air and/or water into a waste pile would be useful in delaying the the onset and reducing the extent of these reactions.
Tailings are generally deposited into a valley fill behind large earth structures or dams. Stability of these dams is just as much an issue as with waste rock piles. As well, the very fine particles of tailings are more erodable than coarse waste, and the material has little ability to stand-up on its own. The impoundment of tailings creates a long term liability that must be managed well into the future. Over the past decade about one tailings dam per year has undergone failure around the world causing major hardship for local communities, extensive damage to adjacent environments, and expensive lessons for mining companies. Approximate clean-up costs, compensation payments and post-failure monitoring costs over the past decade are estimated to be around 1.4 billion dollars.
A characteristic of tailings that can be beneficial is its relatively low permeability. The flow of water, and in particular, air through a tailings pile is several orders of magnitude below that through a waste rock pile. Precipitation runs off or pools to form a lake on the surface of most tailings ponds whereas rain infiltrates quickly through a waste rock pile.
A characteristic of a waste rock pile that could be exploited to deal with safety issues, is the relative strength of the material. Because waste rock is so much coarser in size than tailings, it is much less susceptible to erosion and failure due to water retention. A properly constructed waste rock dump will standup for a long-time except in extreme weather conditions and depends as well on the mineralogy of the waste.
It is proposed that if a method can be found to dispose of these two vastly different materials simultaneously, then the benefits of these two characteristics might blend together to provide a safer and less-reactive total mass. Between 50 to 60% of a rock pile consists of voids, while a fully-compacted tailings deposit contains only about 30 to 40 percent open space. Since the tailing particles are so small relative to the waste rock, it is considered that much of this material could fit within the large voids in the coarse waste rock reducing the total volume required to be stored on surface.
Projects The work is divided into three separate but interlinked, projects. First, Pamela Fines is studying the characteristics of coarse waste rock piles in order to find similarities and differences in their physical and chemical properties. Secondly, Björn Weeks is developing mathematical models of soil covers to be used to establish effective seals for waste rock piles and prevent infiltration of water and/or air. Finally, Ben Wickland is conducting field trials to establish the characteristics of well-blended mixtures of tailings and waste rock at several mines around the world with particular interest in those regions that experience extremely high rainfall.
The waste rock pile characterization work has included excavation of a number of waste dumps at different mines around the world to examine the weathering that has occurred since the material was deposited. Segregation of coarse and fine materials and stratification of chemical products has been observed at virtually every site. The presence of such diversity leads to natural pathways for moisture and air to enhance the continued reactivity of the material in the pile and the generation of acidic and metallic pollution.
End-dumping of waste rock creates enormous structures that actually promote oxidation and acid rock drainage. As an alternative to traditional methods, Ben Wickland's work is examining co-disposal (or the mixing) of waste rock and tailings. The idea is to create an engineered, environmentally-friendly material with properties similar to glacial till - a well-graded, relatively impermeable, cohesive material often used as covering over dams, dumps, roads, etc. Working with Porgera Gold Mine in Papua New Guinea, different mix ratios of waste rock and tailings have been examined in columns under self-weight consolidation. The results show considerable promise.
Ben Wickland (on the left) with Porgera Mine personnel conducting field trials on the codisposal research.
Conclusion The development of a satisfactory technique to blend tailings and waste rock requires considerable research. The vast difference in the size distributions of tailings and waste rock makes it difficult to avoid segregation and obtain consistent, uniform blending. The use of mechanical equipment such as cement mixers may be needed to achieve success. Despite these problems, this is a research program that merits being conducted because of the far-reaching benefits on the sustainability of both the mine operation and the local region where the waste is to be stored.
The benefits include:
|
Email: cerm3@mining.ubc.ca
Copyright © 2002 Centre for Environmental Research in Minerals, Metals, and Materials. All rights reserved.
Permission is given to reproduce all or any of the contained materials provided due credit is given to CERM3 as the source.
Revised: September 28, 2002.
