Development of prevention methods to guarantee the reduction of contaminants in acid mine water.
The presence of acid waters becomes a potential risk to health of populations living near mining projects, as well as to the biodiversity of the area and the stability of the ecosystem in general. For this reason, the Chamber of Mines of Peru promotes systems for prevention of contaminant effects in order to reduce the impact of exploitation.
In addition, remediation ends up representing a heavy investment for the companies. Therefore, it is important to optimize the effectiveness of preventive methods to reduce possible formation of these contaminating substances.
In relation to mining activity, there are four main sources of acid water. In the first place, we have the abandoned mines. The problem is that many mining operations end up being decommissioned without the development of an adequate mine closure plan or other environmental care program. This generally results in permanent leach of non-waste rock, which causes contamination of receiving water bodies, sediments and soils used for agricultural purposes by farming communities.
An example of the danger abandoned deposits pose to ecosystem stability is tailings spill that occurred in August 2015. When a federal cleanup team of abandoned deposits accidentally dumped almost four million wastes in the southwestern part of the state of Colorado (United States). This situation ended up coloring the waters of the Colorado River a yellow-orange color.
The second source of acid water is the ore stockpiles, because there is a possibility that they contain sulfide minerals, depending on the existing water. The third source is waste rocks and finally, tailings, as waste from flotation operations.
According to Patricio Navarro Donoso, a specialist from the Chamber of Mines of Peru, «Characterization is one of the most important tasks in the prediction of acid water formation. This task allows early identification of potential acid drainage generators and definition of the way in which they will be treated afterwards. Prevention of contaminant effects is important since necessary actions can be taken at the right time, without developing reactive measures later. This helps to reduce long-term environmental effects and contributes to reducing the costs associated with corrective measures.”
Among the signs that help to recognize the formation of acid waters are: reduction of pH, increase of specific conductivity, increase of mineral acidity or sulfate content of water, absence or low amount of alkalinity, colored precipitates at the banks of water bodies, presence of visible hydroxides or sulfates, and finally, dead vegetation and fauna.
In order to eliminate or reduce the appearance of these contaminating substances, the following design and risk management criteria must be taken into account. First, the activity and surroundings must be adequately planned, characterizing the possible effluents and their effects on the environment. Another alternative is to detect and characterize flows and concentrations, both possible sources of contamination and discharge points; to act quickly and efficiently in the construction of barriers in order to concentrate the effluents and isolate them from the environment; to control the hydraulic network continuously; to carry out the treatment of acid waters, regenerating the environment. Finally, to quantify the effects.
Secondly, the so-called «ABA» test (Acid Base Accounting) is implemented. This is a procedure that consists of two parts: determining the acid generating potential and the neutralization capacity of a given sample; in order to determine the net neutralization potential (NNP) and the PN/PA ratio based on both values. This is one of the most commonly used techniques in the prediction of acid mine drainage.
Third, the acid-base technique calculates acid potential from the difference between total sulfide and hydrochloric acid soluble sulfide sulfate. Finally, there are the geochemical, kinetic or dynamic tests, which serve to simulate the behavior of weathering in tailings. The aim is to predict characteristics such as the following: the rate of sulfide oxidation, the level of acid generation and the capacity for metal dissolution. The main kinetic tests include the Cell Test.
As an example of the application of these predictive techniques, we can mention the case of the Inmaculada mine in Peru (Ayacucho), a silver producer, which began operations in June 2015. Before starting with its mining operations, the deposit managed by the British company Hochschild Mining, carried out in December 2014, the respective analyses to know the physical and chemical stability of the waste dump. This was carried out through acid drainage prediction tests.
Among the most commonly used techniques to prevent contaminant effects, we have the water cover, where the idea is that oxidation of sulfide minerals is inhibited by a water cover, which acts as a kind of barrier against the diffusion of oxygen from the atmosphere to submerged sulfides. Saturation, which refers to the exclusion of oxygen by saturating pores with moisture, is also employed. This process can be achieved through tailings thickening (thickened or paste).
Finally, the other technique is the design of dry covers that are mostly used to close ballast dumps and tailings dams. Its basic objective is to provide a barrier that minimizes the entry of atmospheric oxygen into the mine waste and to limit moisture infiltration. Other methods employed include co-disposal of tailings and ballast, separation and segregation, permafrost and freezing, and backfilling.
