4.4 Operational management and treatment
Options to prevent or minimise impacts of AMD on streams include management practices to prevent or reduce the formation of AMD, and treatment of AMD. Best management practices to prevent or reduce the formation of AMD will be more cost effective than ongoing treatment of AMD discharge. Management strategies may include proactive treatment of drainage water from acidic waste rocks for logistic and cost reasons. Treatment of the drainage water will likely still be required even where optimal management strategies have been put in place; however, treatment costs will be lower than if no mitigation strategies had been in place and may also allow for the use of passive instead of active treatment.
In situations where AMD is already present from previous mining activities, preventative management techniques are limited and it is much more likely that treatment will be required. A separate section (4.4.3) is included in this chapter for sites with existing AMD.
Once management techniques have been initiated or treatment commenced, monitoring of any discharge from the site is necessary to verify that management/treatment is successful and that unacceptable impacts do not occur. Water quality parameters and frequency of sampling for different treatment systems, as well as biological monitoring, are included in Chapter 8.
4.4.1 Operational management
Prevention and mitigation of AMD can be achieved through risk-based planning and the mine design approach applied throughout the mine life cycle, although prevention is primarily accomplished in the assessment and design phases. In general, more options and more effective options are available earlier in the mine life (Figure 13). The structural nature and physical environment of the AMD source material influences selection of the most appropriate method (or methods) for prevention and mitigation.
By necessity, the methods to prevent or minimise AMD will be site specific, and will typically involve a combination of different methods. A thorough understanding of the site conditions is required to identify site-specific opportunities and constraints.
More than one method sequentially, or a combination of methods at any one time, may be required to achieve prevention or mitigation of AMD, and different methods will be applicable during different stages of the mine life.
As such, continuous geological monitoring during mining is required to ensure that potentially acid-forming rocks are rapidly identified and appropriate management undertaken (see section 8.2.1).
The extent of monitoring required will depend, in part, on how well a site has been assessed initially, and the geology, including the location of potentially acid-forming rocks. Systems for collecting leachate from waste rock or tailings piles should be put in place during pile construction.
|Regular water quality monitoring of the leachate from waste rock or tailings piles is also required to ensure that the selected management techniques are effective.
Figure 13 Options and effectiveness with time (TEAM NT 2004, in INAP 2009).
|Good mine planning is critical for the success of preventative and mitigation measures, which can significantly reduce the formation of AMD.
Waste rock, tailings and coal refuse, open or filled pits with fractured rock, or underground mine structures may all be sources of AMD. Minimising the formation of AMD is achieved by one or all of the following:
- Avoid disturbing the problem materials;
- Prevent or reduce contact of water and/or oxygen with acid-forming materials, both temporally and spatially; and
- Neutralise or reduce the concentration of contaminants present in mine drainage.
Several techniques are available to achieve each of these strategies, and this section provides an overview of best practice management techniques, with more detail provided in INAP (2009), MEND (2001) and Appendix E. In addition to these, many of the strategies as described for minimising total suspended solids (Appendix B) can be used. The degree of success of any minimisation strategy can be measured by ongoing monitoring of mine drainage (Chapter 8).
The most effective management technique is to avoid disturbing problem materials. At the most extreme, this could mean that a particular coal deposit is not mined. However, this does not mean that mining cannot go ahead but rather that the mine plan may be altered. For example, total or partial reduction in excavation or exposure of problematic materials can limit or prevent sulphide oxidation and metal release. Geological survey data and mine planning will determine if avoidance is a practical option. For example, if PAF rocks are present below coal seams, they can be left in place. Alternatively, a decision may be made not to extract a particularly reactive rock type that will be too difficult to manage in the future. This may require changes to the mine design to work around difficult rock types through alteration of mine access, inclines, and open pit designs. Avoidance also involves planning to ensure placement of waste storage facilities avoids sensitive receiving environments. If avoidance is not practical, other mitigation strategies will be necessary to minimise AMD formation.
Water management methods
Water acts as a transport mechanism and a reactant. Water management is often the most cost effective method of minimising AMD. The key aim is to reduce infiltration into areas including waste rock or tailings piles containing acid-forming materials and thereby reduce the volume of contaminated drainage or contaminant load. Water management techniques can be implemented during the construction phase as well as at later stages of mining operations. Techniques that have been specifically developed for controlling AMD include:
- Diversion is one of the easiest and cheapest methods for minimising the volume of acidic leachate. Unpolluted surface runoff is drained away from PAF materials as rapidly as possible via ditches or pumping. Covers may divert water and limit infiltration and are discussed more below;
- Dewatering - involves lowering the water table to reduce the amount of groundwater in contact with in situ PAF materials. Examples include pit dewatering to reduce seepage through pit walls and shallow groundwater collection ditches above tailing ponds and waste rock dumps;
- Flooding - of underground or surface mine voids minimises AMD production by inhibiting the oxygen supply from reaching PAF materials. However, initial flooding may result in the release of acidic discharge arising from the dissolution of stored oxidation products;
- Hydrogeological controls is primarily applicable for controlling groundwater flow. For example, placement of low-permeability materials such as tailings in an open pit with a highly permeable surrounding material creates a large permeability contrast causing groundwater to flow around, rather than through, the low permeability material; and
- Seals are primarily used when decommissioning underground mines to prevent or minimise AMD production. Hydraulic seals limit movement of air and water through mine workings.
Special handling methods
Special handling primarily relates to waste rock and tailings materials and is often the first step in an AMD management plan. Special handling involves the identification, through initial site characterisation (section 3.2) or subsequent rock monitoring (section 8.2.1), and typically segregation of PAF, NAF and, if present, alkaline materials.
|It is essential that mine waste handling is incorporated into the mine plan to ensure that PAF materials are appropriately managed.
There are a number of special handling techniques that can be adopted to minimise acid production from PAF materials, although complete prevention of AMD is unlikely and treatment is likely to still be required. Special handling techniques include:
- Encapsulation and layering involves the placements of PAF and acid-consuming materials (e.g. carbonates) in geometries to control or limit AMD production. Effectiveness is governed by availability, type and reactivity of acid-consuming materials, the balance between acid-forming and acid-neutralising materials, deposit geometry, the nature and flow of water through the deposit, and chemical armouring of alkaline materials.
- Blending - mixing of waste rock types of varying acid-forming and acid-neutralising potential to create a deposit that generates a discharge of acceptable quality. Effectiveness depends on the availability of materials and the mine plan, geochemical properties, reactivity of waste rock types, flow pathways created within the deposit, and the degree of mixing/method of blending with thorough and homogeneous mixing generally required to achieve maximum benefit. Blending is a form of neutralisation (see below) but refers to the use of waste rock materials on-site.
- Co-disposal of tailings and waste rock can take several forms depending on the degree of mixing. This includes the homogeneous mixing of waste rock and tailings, alternate layering of waste rock and tailings, the addition of waste rock to tailings dams and the addition of tailings to waste rock piles. Co-disposal can reduce the formation of AMD by limiting the flow of water and oxygen to reactive surfaces by filling the voids between waste rock particles with finer tailings particles. Whether co-disposal is appropriate includes consideration of the waste production schedule and proportions of waste rock to tailings. Other advantages are physical stability of tailings piles, minimisation of disposal footprint, elevated water table within deposits, and possible elimination of the tailings dam.
Further details are available in INAP (2009) and MEND (2001).
Additions and amendments
Several addition and amendment methods are available to reduce the formation of AMD, although the addition of alkaline materials is the most common approach. This addition of alkaline materials provides some control of the pH of any leachate by neutralising any acid generated by PAF rocks. Alkaline materials or amendments may include waste rock, pH amendment of tailings, placement of alkaline material above or below wastes as liner or cover materials (layering), and alkaline injection. The amount of neutralising (alkaline) material required to fully neutralise the acid materials will depend on the acidity and amount of the waste rock and the neutralising capacity of the alkaline material. Acidbase accounting data (section 3.2.3) in combination with the quantity of the different rock types can be used to determine the amount of neutralising material required (Appendix E.3).
Waste rock pile management
Waste-rock-pile management is an important part of minimising acid discharge primarily through minimising infiltration of water and air. Management may include rock pile design and the use of dry or wet covers. Rock pile design largely relates to the slope angle and length of the rock pile such that it minimises infiltration and erosion and is appropriate for the climatic conditions. Restriction of the airflow into a rock pile can be achieved by co-mingling of waste rock and fine material (see Co-disposal above). Appropriate water collection systems should also be put in place at the base of the rock piles in order to determine the quality of the leachate (see section 8.2.2).
Dry covers are typically earthen, organic, or synthetic materials placed over mine wastes, which control oxygen diffusion and/or water infiltration. Common dry covers are:
- Soil and organic material soil covers are designed to limit infiltration and oxygen ingress, while preserving runoff water quality, while organic materials may also consume oxygen, or chemically promote reducing conditions or bacterial inhibition. Designs are site and climate specific and often limited by availability of materials. For example, store and release covers for infiltration control are often used in Australia, but may have little application under the high rainfall conditions often experienced in New Zealand.
- Alkaline covers and other neutralising material - can increase alkalinity of infiltration, thereby providing pH control, but AMD prevention is unlikely unless the sulphide content is extremely low. The main limitation is the volume of material required to provide adequate retention time, especially in high rainfall climates.
- Synthetic liners - low permeability liners can be used to maintain saturated conditions in the overlying waste or to protect underlying groundwater resources, and can dramatically reduce infiltration.
- Vegetation establishment protects exposed soils from erosion, and promotes evapotranspiration of water retained in the soil cover to reduce infiltration.
Wet covers basically involve the submergence of acid material under water and include:
- Inundation - flooding of underground or surface mine voids with water has the potential to significantly inhibit the supply of oxygen so that AMD production from PAF materials is not a concern. The depth of water over the PAF material must be sufficient to allow for mixing of the water column and to prevent re-suspension of wastes by wind or wave action. Water covers may not be suitable for material that has already appreciably oxidised.
- Partial water cover acid-generating waste is stored at depth and a small pond in the centre of the tailings impoundment maintains saturation through enough of the waste to minimise oxidation. NAF tailings are used as cover above the level of the pond.
- Wetlands - oxygen depleted and reducing conditions at the base of the cover profile are maintained to protect underlying unoxidised material and promote precipitation of existing AMD products as sulphides.
Selection of waste rock management options
Waste rock, tailings and coal refuse, open or filled pits with fractured rock, or underground mine structures may all be sources of AMD. Of these, management of waste rock, both in situ and post-excavation, will be most significant in minimising the formation of AMD. A flow chart outlining the process for selecting appropriate waste-rock management strategies, described above, is shown in Figure 14; the information outlined should be available from initial geological investigation of the proposed mine site. Important factors include local topography, climate, waste-rock volume and composition, groundwater conditions, the position of the waste rock relative to surface water and groundwater, and the presence or absence of neutralising material. A thorough understanding of the site conditions is required to identify site-specific opportunities and constraints.
Figure 14 Prioritisation for consideration of applicability of different waste-rock management strategies. Refer to above text for details on different techniques.
As stated above, tailings may form sources of AMD, thus appropriate management of these materials is required. Mine tailings are residues from any processing of the coal on site, including hydromining and screening or washing to remove coal fines and clay (fine tailings), and screening to remove coarse material. Coarse material is typically dumped back into the mine pit and is dealt with in the same manner as waste rock. Fine tailings may be handled in a number of ways including (Williams 1990; National Research Council 2002):
- Discharged as a slurry into a tailings dam or underground workings; and
- Dewatered and:
- disposed of in tailings piles or mixed with waste rock;
- used to line landfills; and
- used to make low grade fuel such as briquettes.
Details on each of these methods are provided below.
Fine coal tailings are conventionally discharged from a processing plant as a water-rich slurry, and are accumulated behind a dam so that the solids will settle. The coarse tailings are often used in the dam construction with the fine material being pumped in as slurry. Gravity settling of the fine material results in clear water that can be recycled back into the plant, although problems may occur if the fine particles do not settle or settle slowly and additional treatment is required (see Appendix B.4). The design of tailings dams will be specific to each site as design must take into account pathways for water including coal seams, fractures, and old mine workings. Planning and construction of dams require site-specific details and engineering experience and should be undertaken with consultation from geological, geotechnical and engineering specialists.
Problems can occur with tailings dams. Failure of dams can destroy the environment and property and cause loss of life (National Research Council 2002; Appendix G). Groundwater can pass through the tailings and discharge to the surface below the dam. Chemical interaction between water and tailings can lead to high TSS and elevated dissolved solids. Water collection systems are required to intercept any contaminated water from the tailings dam which is then passed through treatment systems for removal of TSS and trace elements and to raise the pH (see below).
Pumping fine tailings as water-rich slurry into underground workings can be a suitable method of disposal (Williams 1990; National Research Council 2002). There are potential benefits and problems with this method, however. Benefits include reduced surface subsidence over old underground workings if the slurry has some intrinsic strength and can provide lateral support to underground pillars. To achieve this, a cementing agent and possibly coarse waste material can be added. Potential problems with underground disposal include: plugging of pumping systems, incomplete knowledge of available storage space, increasing water flow from underground workings which then must be managed, and increase of hydraulic head on bulkheads and other barriers in the workings which can result in blow-outs (National Research Council 2002).
Alternatively, fine coal tailings can be dewatered and disposed of in a number of different ways including tailings piles or mixed with waste rock and placed in waste-rock piles (Williams 1990, 1991; National Research Council 2002). Dewatering can be accomplished by centrifuge, band press filters, or plate and frame filters. Flocculants are often added but the process is sensitive to pH. The dewatering is expensive both in capital and operating costs and therefore likely to be undertaken only when site conditions, such as limited space/volume for tailings dam, dictate.
Where tailings are disposed of in discrete piles, there may be stability issues as there can still be a significant moisture content, in addition to the risk of spontaneous combustion, therefore geotechnical expertise is required to design and manage tailings piles. Coal tailings may contain pyrite that when exposed to air and/or water may result in AMD, and thus needs to be managed accordingly (see earlier this section). To decrease the weathering process the following factors should be minimised: time of near-surface exposure, pyrite content, the volume of air, and permeability (Kolling and Schuring 1994). If adequately dewatered, mixing tailings with waste rock and placing in waste-rock piles can be a suitable method of disposal. However, if pyrite content is high, AMD can be a significant problem. Conditioning to improve the physical properties of tailings through thickening, filtration, compaction, or gradation control can also limit AMD formation.
Alternative uses of tailings include making briquettes and lining landfills, although these practices are not widely used in New Zealand at present.