Rio Tinto | Minerals and Metals for the world

Within mine waste dumps, where piles of broken rock are built during a mine's life as permanent stores of uneconomic materials, the risks may outlast the productive life of the mine. Unless acid generation is stifled in some way it will continue for as long as the dump remains permeable to rainwater and contains "culprit" minerals.

There are similar risks with accumulations of ground-up ore in the form known as "tailings", and with mine openings including open pit walls. All of this explains why there is such a woeful legacy of noxious effluents from abandoned, ownerless mines.

ARD is often referred to as acid mine drainage, which suggests to some people that it is invariably triggered by mining. Miners do take a particular interest in sulphides, and the rustiness of many of the places that attract them clearly signal a connection between mining and acid generation.

Obviously there is a connection. Mining generally involves fragmenting rock, which creates a vast number of artificial channelways for water, air and bacteria. (Bacteria are not essential to sulphide oxidation, but the strains that promote it are voracious and very difficult to exclude from open air rock piles.) Mining brings within the reach of oxidation huge volumes of rock that might otherwise have remained fresh until doomsday.

But in fact ARD needs no help from miners to get started and keep going. It is happening all the time, wherever sulphide-bearing rocks are reacting freely with rainwater and neutralizing agents are in short supply. In fact, you could say the boot's on the other foot: natural ARD effects have been a great help to miners in leading them towards undiscovered orebodies.



Within mine waste dumps, where piles of broken rock are built during a mine's life as permanent stores of uneconomic materials, the risks may outlast the productive life of the mine. Unless acid generation is stifled in some way it will continue for as long as the dump remains permeable to rainwater and contains "culprit" minerals. There are similar risks with accumulations of ground-up ore in the form known as "tailings", and with mine openings including open pit walls. All of this explains why there is such a woeful legacy of noxious effluents from abandoned, ownerless mines. ARD is often referred to as acid mine drainage, which suggests to some people that it is invariably triggered by mining. Miners do take a particular interest in sulphides, and the rustiness of many of the places that attract them clearly signal a connection between mining and acid generation. Obviously there is a connection. Mining generally involves fragmenting rock, which creates a vast number of artificial channelways for water, air and bacteria. (Bacteria are not essential to sulphide oxidation, but the strains that promote it are voracious and very difficult to exclude from open air rock piles.) Mining brings within the reach of oxidation huge volumes of rock that might otherwise have remained fresh until doomsday. But in fact ARD needs no help from miners to get started and keep going. It is happening all the time, wherever sulphide-bearing rocks are reacting freely with rainwater and neutralizing agents are in short supply. In fact, you could say the boot's on the other foot: natural ARD effects have been a great help to miners in leading them towards undiscovered orebodies.
PAGE 2 OF 8

Review is published by Rio Tinto, 6 St James' Square, London SW1Y 4LD, England Telephone +44 (0)20 7930 2399 Editor: Cherry DeGeer