Placement of alkaline material above or below waste as liner or cover material
Anna Tornivaara & Clayton Larkins, Geological Survey of Finland, P.O. BOX 1237, FI-70211, Kuopio, FINLAND, email: anna.tornivaara(at)gtk.fi, clayton.larksins(at)gtk.fi
Acid rock drainage (ARD) can be treated and controlled with alkaline material placements during waste disposal. If waste materials are expected to generate acid, alkaline materials that have a neutralizing effect can be utilized above or below the acid producing waste as part of a waste deposit cover or liner, respectively. Alkaline cover material can provide a source of alkalinity to the underlying, potentially acid generating (PAG) waste, and can promote the development of a chemical barrier, or hardpan, that limits ongoing sulphide oxidation (Lottermoser 2010). The placement of alkaline material as a liner can limit the release of leached oxidation products by contributing neutralizing potential (NP) to the base of the waste deposit (INAP 2009, WDNR 1997). The effectiveness and feasibility of using alkaline materials in covers and liners for ARD control depends on many site specific variables, including the availability of suitable alkali materials, the properties of the waste deposit, and waste materials. Alkaline materials commonly used for controlling ARD include lime, crushed limestone, or fly ash. Alkaline rich industrial by-products such as ash derived from power generation, sludge, or tailings can provide an innovative source for alkaline additives (INAP 2009). However, such materials must be thoroughly characterized, both for their neutralizing potential, and the presence of harmful elements that could be mobilized in effluent during consumption.
Description of the method
Alkaline cover material has been applied as both a treatment and prevention measure for ARD generation (MEND 2010, Tremblay & Hogan 2001, Miller et al. 2003). Alkaline material can be utilized as part of a dry cover, commonly placed between overlying cover material and PAG waste, to contribute alkalinity to the underlying waste (Kauppila et al. 2013, Lottermoser 2010). Alkalinity contributed from cover material can treat acidic pore water and promote the formations of a passivating chemical cap through the precipitation of metal hydroxides. This chemical barrier can act to reduce the oxygen flux deeper into the waste profile, increase the effective cover thickness and allow longer contact time with the neutralizing source (INAP 2009). MEND report 2.46.1 describes the use of alkaline tailings to construct a simple cover and liner for PAG material (MEND 2010). Alkaline liner material, at the base of a waste deposit, does not prevent sulphide oxidation, but can delay and reduce ARD emissions from oxidized waste by neutralizing effluent that has permeated the PAG fraction of the waste deposit (BCAMDTF 1989). To be suitable as a neutralizing additive in either a cover or a base the alkaline material cannot include potentially harmful soluble compounds.
The effectiveness of the alkaline material placed as a cover or liner depends on its neutralizing potential, rate of alkalinity production, and the nature of water flux between it and the PAG waste (INAP 2009). The importance of these variables is emphasized by studies that have found the application of alkaline cover materials to be unsuccessful at mitigating ARD (MEND 1994, BCAMDTF 1989). As reported in the Acid Rock Drainage Technical Guide, alkalinity introduced by cover material is easily overwhelmed by acidity during transport through the pore space of PAG material, and subsequently does not effectively mitigate ARD generation (BCAMDTF 1989). A similar assessment was given in the MEND 2.20.1 report, in which crushed limestone covers did not prevent ARD emissions (MEND 1994). Measures to increase the effectiveness of alkaline covers have included thorough blending of alkaline and PAG material, incorporating alkaline material as part of multilayer soil covers, incorporating alkaline material as part of a water cover, or by encapsulating PAG between alkaline cover and liner material (Mend 1994, Lottermoser 2010, INAP 2009, MEND 2010).
The required NP and alkalinity production rate in alkaline liner material may be difficult to determine due to the potential development of passivating coatings on the reactive surfaces of the alkaline grains (INAP 2009, BCAMDTF 1989). Generally, to account for liner passivation, a significantly larger NP is required for long term effluent treatment than would be expected based on the acid potential (AP) of the PAG waste (BCAMDTF 1989).
The effectiveness of an alkaline liner is also contingent on water flux. Acidic waters must be transmitted through the alkaline liner for neutralization to occur (BCAMDTF 1989). Therefore, the waste deposit design should include considerations that prevent the development of preferential flow that could bypass the alkaline liner (BCAMDTF 1989). In column experiments, the effectiveness of alkaline liner material has been increased by increasing pore water residence time using thicker alkaline liner layers (MEND 2010).
Examples of the application of alkaline material as part of cover and liner systems are discussed in the following case studies:
- Limestone and sand cover incorporated as part of a wet cover design for the Benambra Mine (INAP 2009)
- Alkaline mining waste applied as an encapsulating liner-cover system to control acid generation at the Pamour mine, Timmins Ontario, Canada. This work has been summarized as a case study in the GARD Guide, and is fully reported on in the MEND Report 2.46.1 (INAP 2009, MEND 2010).
Appropriate applications (suitability, benefits/barriers)
Alkaline material can be incorporated in PAG waste deposit cover systems as a measure to both treat and prevent ARD generation, and in liner systems to slow and reduce ARD related emissions (BCAMDTF 1989, INAP 2009). The effectiveness and feasibility of alkaline material additions is dependent on site specific variables and design considerations. A primary consideration is the availability of suitable alkaline additives in close proximity to the site. The suitability of potential alkaline materials must be characterized based on variables such as chemical purity, homogeneity, consumption rate, NP and rate of alkalinity production with respect to the PAG waste fraction. If the quality of alkaline material varies, quality control must be conducted to ensure the suitability of all materials (MEND 1994). Quality control measures were determined to be cost prohibitive to the study of industrial alkaline sludge additives in the MEND report 2.20.1 (MEND 1994).
Other site specific considerations include the chemical and physical properties of the PAG waste, the geometry of the waste area, and climate (INAP 2009, MEND 2010). Studies such as MEND 2.46.1 and MEND 2.37.3 illustrate the importance of considering the potential for continued metals leaching under the influence of alkaline additions (Morin & Hutt 1997, MEND 2010). For example, pH control is not a common treatment for As. Further, very high pH environments may be required to prevent leaching of certain metals, subsequently dictating what alkaline material should be considered suitable for addition (MEND 2010).
Advantages (INAP 2009, MEND 2010)
- Easy to implement and manage
- Provides waste management solution for alkaline industrial by-products and alkaline mine waste
- Versatile for integration into various ARD prevention and treatment systems (e.g. can be applied as part of wet or dry cover)
- Allows in-situ treatment of ARD in waste deposit leachate
- Can limit sulphide oxidation through pH control and limiting oxygen transmission
Disadvantages (INAP 2009, MEND 2010)
- Long term stability and effectiveness of alkaline layers is difficult to predict
- Ongoing effectiveness of an alkaline layer, especially at the base of the waste deposit, is difficult to monitor
- Basal alkaline layers may develop a passivating coating that reduces their effectiveness
- Use is limited by cost and availability of suitable alkaline material
- Properties of alkaline materials must be assessed, which can be cost restrictive if buffering material is sourced from multiple locations (multiple sources may be required for industrial by-product ash or sludge that is produced in small volumes)
- Alkaline material is consumed by neutral waters
- Alkalinity production may not be sufficient to immobilize all metals in the waste effluent
The feasibility of using of alkaline material in cover and liner systems for ARD control is limited by the availability of suitable material. The amount and quality of the alkaline additive required is determined from the characteristics of the PAG waste (MEND 2010, WDNR 1997). Additionally, the dimensions of the disposal area and cover design geometry must be taken into consideration, as space availability may limit the total possible alkaline material additions.
Maintenance needs are dependent on the system design and post-construction monitoring to assess the method’s ongoing effectiveness.
Environmental cost aspects
With successful application, this method can reduce the environmental cost of mine waste disposal by reducing or preventing harmful emissions, reducing the footprint of waste areas, and utilizing non-PAG waste (INAP 2009, MEND 2010).
To ensure that alkaline material additions will effectively minimize environmental costs, the alkaline material, PAG mine waste, leachate products, and waste deposit site should be thoroughly characterized. Environmental costs could arise from incomplete neutralization, contaminants in solution that are not precipitated by neutralization, or contaminants contributed to solution from the dissolution of the alkaline material (MEND 2010).
Site specific data needs
The following site specific data are utilized for the design and application of alkaline cover and liner materials:
- Characterization of PAG material
- Characterization of alkaline additives
- Evaluation of alkaline material availability
- Site characterization
The characterization of PAG waste material and pore-water provides information on the deposit’s AP, and on what metals are of concern in effluent from the waste (INAP 2009).
As described above, alkaline material must be characterized with regard to chemistry, NP, rate of alkalinity production, and rate of consumption (INAP 2009). As identified in the case study described in MEND report 2.37.3, the consumption of NP may not be equivalent to alkalinity production (Morin & Hutt 1997). The composition of the alkaline material must be characterized to ensure it does not contain contaminants such as heavy metals or radioactivity that could be mobilized into the mine waste leachate upon dissolution. Alkaline material must have sufficient NP to create excess alkalinity within the waste mass, must produce alkalinity at a sufficient rate to create excess alkalinity in pore water, but consumption must occur at a rate that provides a long term source of alkalinity (BCAMDTF 1989, EC 2009, INAP 2009). The availability of suitable alkaline buffer material in relation to the waste deposit site is an important consideration for determining the cost based feasibility of alkaline additions (MEND 1994, EC 2009).
Site environmental conditions such as topography, geology, hydrology, hydrogeology, and climate are critical considerations for the design of closure plans and technologies. These variables are utilized in risk based assessment that dictates the design of waste management strategies. Additionally, they provide insight into water fluxes and interactions within the waste deposit. Water flux through the waste deposit drives the exchange and neutralization of pore water between alkaline layers and the PAG waste. Water flux can deliver buffering water from alkaline covers into the underlying mine waste, and acidic, metal laden effluent from mine waste into alkaline liner material. Pore-water residence times and the potential for preferential pathways through the waste deposit will influence the effectiveness of the design (MEND 2010). Additionally, because even neutral water fluxes consume alkalinity, these variables must be considered with regard to the longevity of alkaline layers (INAP 2009).
Requirements for the materials and appliances
Alkaline additives can include lime, limestone, phosphate rock, high alkalinity mine waste, industrial by-products such as kiln dust, steel slag, ash, or commercial products (INAP 2009, MEND 1994). Important factors dictating the applicability of an alkaline additive are its purity, reactivity, availability and proportions for material addition (INAP 2009). Alkaline additives from industrial processes may require multiple sources to meet the volumes required. All additives, from each source must be characterized for suitability (MEND 1994).
In addition to any material processing requirements such as grain size reduction, alkaline covers and liners can be emplaced with standard earth moving equipment (BCAMDTF 1989).
Minimisation / treatment of potential discharges
Effective application of alkaline material covers and liners can slow and prevent emissions from PAG waste material deposits. Method application requires comprehensive waste characterization, suitable alkaline material, and the application of alkaline materials using the correct proportions and layer depths. Additionally, the alkaline material must be homogenously mixed and distributed to prevent contaminant discharge via preferential seepage pathways (INAP 2009). The effectiveness of alkaline layers may be reduced over time by alkalinity consumption or the development of passivating coatings within alkaline materials. Therefore, ongoing monitoring provides a means of early detection and mitigation of discharge as the result of treatment failure.
Monitoring / control needs
Monitoring is required to evaluate the method’s long term effectiveness. Monitoring of pore water within and below the waste deposit and drainage from the deposit provides information on the method’s effectiveness (MEND 2010). If monitoring reveals unacceptable contaminant emissions, drainage control measures (e.g. stopping, capturing and or treating the drainage) may be required as part of mitigating the release (INAP 2009). Further information on drainage control measures and application can be found in the Finnish Environmental Institute guide titled Best Environmental Practices in Metal Ore Mining (Kauppila et al. 2013) and in the Environmental Techniques for the Extractive Industries: Mine Closure Handbook (Heikkinen et al. 2008).
The application of alkaline materials as covers and liners for controlling PAG material has been recognized for decades (BCAMDTF 1989, INAP 2009). The effectiveness of alkaline covers for controlling ARD emissions has been variable and is dependent on site conditions, design, and material characteristics. MEND report 2.20.1 assessed limestone covers to be ineffective at controlling ARD unless blended with the PAG material (MEND 1994). Through continued research and advances in cover construction techniques, including blending, alkaline materials such as limestone and fly ash have been effectively utilized in reaction inhibiting barriers (Tremblay & Hogan 2001, Miller et al. 2003). The characterization and testing used to inform the applicability of alkaline materials for cover or liner systems is illustrated in the MEND 2.46.1 report (MEND 2010). This report describes the use of laboratory analysis to demonstrate that on-site alkaline tailings have the capacity to treat and prevent ARD emissions if placed as cover and liner material (MEND 2010).
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