CHAPTER 7: MINIMIZING THE ENVIRONMENTAL IMPACT FROM SULPHIDIC MINE WASTE
By predicting ARD, the right measures for prevention of AMD can be made. As a mining project begins, it is important to have a proactive strategy for preventing ARD. This strategy will differ between mining projects, but generally begins with separating PAG from NAG, limiting the availability of oxygen and controlling the pH.
Knowledge about local geology, rock types and presence of minerals likely to generate metal bearing and low-pH water is available as soon as geological reconnaissance, mapping and sampling commences. Exploration drilling rapidly increases the available information for predicting and assessing ARD potential at a site. All collection of exploration data must be designed to support environmental studies including ARD prediction. As exploration progresses so must ARD prediction and at the stage when mineral exploration transforms to mine project development a significant ARD database should be available. Environmental studies in general, collection of environmental baseline data and ARD prediction in particular, cannot wait or post-date other economic assessment of a mineralisation or mineral resource. ARD prediction like other environmental factors must be integral to any study of mineral resources in the same way as costs of environmental protection is integral to overall mineral extraction economics.
This chapter includes measures for prevention of AMD that could be taken concerning ore, waste rock and tailings before/during/after mining.
During the exploration phase it is important to define the ore in a proper way to limit future generation of waste rock and tailings. Sulfide minerals are often situated in veinlets, that are more extensively exposed as the ore/waste rock is removed from the bedrock, compared to other non-sulfidic minerals. Some rocks may be composed of high contents of acid-producing sulfides, but also buffering minerals, such as calcite. In the case of waste rock, a separation is often made between potentially acid generating rock (PAG, or acid forming rock, PAF) and non-acid generating rock (NAG, or non-acid forming rock, NAF). Additionally, the PAG can also be mixed with a buffering agent, such as limestone. As limestone is dissolving in the presence of AMD, gypsum and hydroxides that are more voluminous than the original minerals will form. This in turn, will densify the waste rock, turning it to be less permeable concerning water and oxygen. In the operational phase, oxygen availability could also be hindered by applying a soil cover on top of waste rock slopes, or by co-disposal of waste rock/tailings filling the voids. Backfilling (also called cut and fill) could also be used to reduce the generation of AMD. Backfilling of waste rock into mined out underground cavities is primarily used to enhance the mechanical strength of surrounding rock, increasing the amount of ore that could be excavated. However, such a method could also prevent air intrusion, while underground cavities will be flooded with water (while oxygen content in water is much less compared to that in air) and thus lowering the sulfide oxidation rate. Backfilling is used in the operational phase and after mining processes has ceased. Backfilling is preferably done using NAG-waste rock or fresh, unoxidized PAG-waste rock, otherwise acid generating salts (coming from sulfide oxidation) will dissolve in contact with water causing AMD to form.
As mining operations ceases, PAG-waste rock piles could be covered with water (wet cover) or soil (dry cover). A dry cover consists of a sealing layer of soil that could retain and store water (i.e. a clayey soil). The sealing layer is overlaid by a protection layer thick enough to hinder drought or frost from penetrating the sealing layer. This is conducted to decrease oxygen and water transport into the waste rock. A wet cover could be applied by diverting surrounding waters to flood the waste rock, or by disposal of the waste rock into an open pit or underground cavity that is to be filled with water. A sealing layer could also consist of a geotextile. If the waste rock is NAG, a thin layer of soil is often applied on top the pile to avoid erosion and dust.
Tailings is a fine-grained, silty material with a high-water content, from the enrichment process. Tailings will include small amounts of the desirable minerals, as no enrichment processes are 100 % effective. Concerning sulfide deposits, unwanted sulfide minerals (such as pyrite), that has no economic value is almost always present in the ore. As the ore is grinded in the milling process, reactivity of these sulfide minerals increases. This is turn, will cause AMD to form more rapidly if no measures are taken. The amount of unwanted sulfides in the tailings could be reduced by adding a desulphurization step in the flotation process, removing these constituents to be concentrated and isolated from the bulk waste. This will reduce the amount of reactive mine waste to be treated.
As for waste rock, tailings could be backfilled into mined out cavities that are to be filled with ground water. Storing tailings permanently saturated with water will decrease the sulfide oxidation rate. However, this will also mean that TSFs must be constructed to resist high water pressures, otherwise dams could burst and release the tailings into the surroundings. Another method to decrease water percolation through tailings is to construct capillary barrier systems, whereas a finer fraction of the tailings is overlying a coarser in a TSF. This will enforce capillary barriers to form, whereas water is held by capillary forces within the fine fraction, thus decreasing the amount of oxygen that could be transported down through the tailings. Capillary barriers could be generated if i.e. hydro cyclones are used to separate the bulk tailings into different size fractions before deposit.
As mining processes ceases, TSFs are to be covered/treated in the same way as waste rock piles (described above).
Enhancing mine water quality is a lot about minimizing the contact water/mine waste. This could be done by the construction of ditches diverting surrounding water from the mine waste, or by the addition of flocculants that will speed up the dewatering process whereas water is separated from the solid tailings. Increasing water run-off from the mine waste is also desirable, this could be accomplished by depositing the mine waste forming cone-like impoundments.
Mitigation of AMD
The mitigation of AMD is often conducted with methods based on passive or active water treatment, generally to increase pH and reduce the amount of water and metals in the water leaving the mine. It is generally difficult and expensive to stop the generation of AMD. Passive treatment often includes the construction of calcite ponds or barriers in which AMD is treated. Calcite increases the pH upon dissolution and may neutralize acid if enough in abundance. As the pH increases, the most metal ions settle easier.
Active water treatment means that the water is treated within a water treatment facility. The methods for active treatment are several, but often include raising the pH in the water adding limestone or caustic soda, which in turn causes most metals to precipitate and settle. This can be done in certain settling or sedimentation ponds. In some cases, flocculants are added to speed up the process. Treatment can also be complemented with other methods, such as filter systems. Treatment methods is to be adapted to actual metal load and leachate chemistry. In many cases, a sequential treatment is needed, to immobilize metals that are soluble in different situations (i.e. high/low pH, oxidizing/reducing environment, metals adsorbed/precipitated onto different mineral surfaces). A sequential treatment could include several different steps such as: a pre-oxidation step (oxidation agent is added, i.e. hydrogen peroxide or oxygen), a pH-increasing step (lime, calcite), a precipitation/adsorption step (addition of metal hydroxides with suitable chemical properties and large surface areas so that metals of concern could attach onto them).
Passive water treatment means that the water is treated without active involvement, sometimes called a “walk-away-solution”, often using natural physical, chemical or biological processes. Common techniques for passive water treatment are using plants that uptake contaminates or using wetlands to remove metals by microbial processes (i.e. metal sulfide precipitation).
Passive water treatment is cheaper than active treatment, which generally is a large cost within the mine operations. However, there are still difficulties in treating highly acidic waters with passive techniques. The combination of active and passive treatment techniques can be used to lower costs.