Anna Tornivaara, Geological Survey of Finland, P.O. Box 1237, FI-70211 FINLAND, e-mail: anna.tornivaara(at)gtk.fi
The control of solution pH (e.g. effluents from the waste facility) by the addition of alkaline materials is a relatively common approach to mitigate acid rock drainage (ARD) (INAP 2009). It can be done in several methods. One option is to use alkaline material as a neutralising basal layer underneath the acid producing waste.
Description of the technology
In the method, alkaline material is placed underneath the acid producing waste material. The objective of a neutralising basal structure is to neutralise the seepage from the waste material and to reduce AMD impacts on the surrounding environment. Neutralising reactions will also promote precipitation of harmful elements and compounds in the waste. (INAP 2009, Kauppila et al. 2013).
Neutralising basal layer can be used for both waste rock and tailings to reduce AMD.
- Can be very eco-efficient, if there is suitable acid consuming/neutralising material available in-situ (or limestone)
- Easy to implement (INAP 2009)
- Basal structure has already been done in the construction stage of the waste facility making preparation or modifications of the structure impossible after the disposal has begun
- Limestone doesn’t have cementing properties and cannot be used as a barrier or low-permeability material by itself
- Secondary precipitates may form passivating coating on the surface of the alkaline particles thus reducing the effectiveness of the basal layer (INAP 2009)
- May require additional liner layer/s to limit spreading of drainage from the waste
Neutralising capacity of the basal layer has to be calculated to equate the acid producing capacity of the whole life cycle of the waste facility.
If the basal structure works like designed, it has no special maintenance needs.
Neutralising base can be used with other methods of alkaline addition (blending, layering), especially if neutralising capacity of the base is fading out.
Site specific data needs
Full waste characterisation is essential to meet the demands of the neutralising basal structures.
Requires also proper characterisation of the material to be used as a neutralising layer (including e.g. neutralisation potential, leaching potential of harmful substances).
Requirements for the materials and appliances
Alkaline material has to fulfil the quality and quantity requirements to meet the demands of certain waste. Commonly used neutralising alkaline materials include limestone, lime, kiln dust, steel slags and different ashes. Before the use, the alkaline material should be tested with a complete set of analysis to evaluate the possible leaching and the effects on drainage quality (INAP 2009).
Minimisation / treatment of potential discharges
Accurate design and delicate construction stage are essential to prevent for example leaking risks.
Monitoring / control needs
Normal monitoring of drainage water is advisable, to ensure the neutralising base is working as designed.
INAP 2009. The GARDGuide. The Global Acid Rock Drainage Guide. The International Network for Acid Prevention (INAP). http://www.gardguide.com/
Kauppila, P., Räisänen, M.L. & Myllyoja S. (Eds) 2013. Best environmental practices in metal mining operations. The Finnish Environment 29en/2011, Environmental Protection, Finnish Environment Institute (SYKE). The publication in English is available only on the internet: www.syke.fi/publications (available also in Finnish). 219 p.