Mine water treatment technologies have been improving for over 40 years. Mine water from both active and abandoned mining sites has posed a lot of problems to the environment and a variety of treatment methods have been developed to solve this problem.
Cause of contamination
A lot of things contaminate mine water. Apart from solids, the main contamination from most mines is the oxidation of pyrite bearing minerals. This results in three main products, all of which are soluble in water and need treatment: acidity, ferrous iron, and sulphate. In addition to iron, there are some other metals such as manganese, zinc, aluminum and depending on the geology of the rock – arsenic, cadmium, copper and other metals can also be released into solution.
While significant contamination of mine water can be generated during mining, it tends to peak after closing a mine, as accumulated oxidation materials are flushed out of the rock on flooding the mine. These decline over some years to a residual value. This remains stubbornly higher than concentrations in the pre-mining phase. While the flushing period can be short, the long-term legacy needs treatment for some decades to come. With the length of the residual contamination concentration and the flushing period hard to predict, it is advisable to operate a temporary mine water treatment system after the closure of a mine until when a long-term treatment strategy can be determined. Different mine water treatment solutions may need to be used during the active, immediately after closure and long-term post-closure phases.
Solving the problem
The main water treatment strategy is to reduce the amount of contaminated water generated. This involves exclusion of oxygen from reactive substances by sealing and flooding excavations – placing waste products underwater and covering rock dumps with low permeability materials to prevent oxygen and water ingress or mixing pyritic materials with acid-neutralizing/buffering materials to reduce the acidity generated.
Mine water treatment solutions
How the contaminated water residual is treated depends on the load of contamination and the required treated standard. Treatment has focused on pH correction to removing dissolved metals and circa neutral values. Where there is a relatively low contamination concentration, passive solutions can be deployed. These mostly rely on naturally occurring processes to buffer the pH and remove the metals. For instance, iron oxidation to insoluble ferric hydroxide by the use of aerobic wetlands – metals precipitation as sulphide by the means of sulphate minimizing bacteria or adding alkalinity to the water.
For water that has either high concentrations of dissolved metals or low pH, an active treatment that makes use of engineered treatment systems is required. Such systems involve the continual input of reagents to get rid of contaminants and the ongoing sludge disposal by plant continuously. Active treatment plants either use biological activity to reverse the oxidation of pyrite or lime to raise the ph.
Mine water treatment technologies still face a lot of challenges including the treatment of water containing low levels concentration of cadmium, zinc, tightening of the treatment standards for selenium and arsenic and reduction of concentrations of sulphate to low concentrations.