KMG•GOLD Waste Rock and Acid Mine Drainage

Tailings and waste rock piles at existing mine sites and abandoned mine sites are evidence of the destruction that mining operations can bring upon the environment. In the past, non-environmentally friendly methods were the method of the day - profit at all costs being the main motivator.

The geochemical reactions that take place within a tailings deposit are interdependent on the geotechnical, surface water, and ground water properties. The major cause of tailings acidification is the oxidation of sulphides, particularly pyrite, (FeS2). Under natural, undisturbed conditions, this oxidation process occurs very slowly, limited by the amount of oxygen within the voids or the degree of saturation within the vadose zone. However, bacteria can speed up the process so that surface oxidation can occur within weeks. When in contact with oxygen and water, a series of geochemical reactions take place resulting in a net production of sulphuric acid (H2SO4). Production of sulphuric acid contributes to the precipitation or leaching of heavy metals from the tailings into the surface and groundwater regimes.

Waste rock and tailing dumps can leach hundreds of tons of sulfuric acid and metals into the environment, contaminating aquifers and aquatic habitat alike.

Recycling gold that has already been mined and processed will not add to the huge stockpiles of waste rock and tailings dumps.

There is a process ongoing and in the recent past whereby old tailings dumps and waste rock piles are re-processed to extract as much of the remaining gold reserves as possible. This method is called heap-leaching. Waste rock or tailings are irrigated with a cyanide solution which percolates through the heap and leaches out the gold. This process can take several weeks. The solution dissolves the gold and continues percolating through the rock until it reaches a liner at the bottom of the heap where it drains into a storage pond. After precipitating the gold from the cyanide solution, the cyanide can be re-used in another heap-leach process or sent to a water treatment facility where the residual cyanide is treated and residual metals removed. The water is then discharged to the environment, posing possible further water pollution.