Figure 1. Acid mine drainage from a tailings area (Photo © Teemu Karlsson, GTK).
Acid mine drainage from pyrite oxidation may occur if the acid potential exceeds the acid neutralisation potential of the minerals in the waste. However, kinetics describe the reaction pathways towards equilibrium. Only if the acid production rate exceeds the rate at which the buffering minerals can neutralise the produced acidity or if the buffering minerals simply become exhausted, acid generation reactions will result in a decrease of pH in the drainage that may lead to e.g. high metal leaching from the waste.
Test methods for the determination of acid generation behaviour of sulphide bearing materials can be divided in static and kinetic tests. Static tests are short term, low cost tests developed for estimating a mine waste’s capacity to produce and neutralise acid. A static test gives only indicative information based on total composition of the waste material. Accordingly, static tests are commonly used for screening purposes and to provide an answer to whether the mine waste material has a potential to be an acid producer or an acid neutraliser. These tests do not consider parameters such as the actual availability of acid-producing and acid-neutralising minerals and differences between the respective dissolution rates of acid-producing and acid-neutralising minerals. However, with the help of kinetic testing it is possible to have more detailed information on materials behaviour based on reaction rates under specified conditions.
Acid generation potential test methods include (Technical Committee CEN/TC 292 2012, Kauppila et al. 2013, INAP 2009, European Commission 2009):
The results from static tests are used for the estimation of the need for kinetic procedures. Kinetic tests include a group of leaching tests that can be performed in either laboratory or field-conditions (Technical Committee CEN/TC 292 2012). In comparison to static tests, kinetic tests give more elaborated information on acid generation behaviour of mine wastes (Technical Committee CEN/TR 16363 2012). Kinetic tests are performed for mine waste samples that are characterised as potentially acid generating or whose acid production potential is uncertain according to static tests. (European Commission 2009)
The duration of kinetic tests can be measured in months or even in years. However, it is yet faster to observe the effects of weathering based on these tests than in the natural environment (Lapakko 2002). The most commonly used laboratory scale kinetic test methods include humidity cell testing, column leach testing and lysimeter studies. All test procedures follow two alternate stages; samples are subject to periodic leaching, after which the drainage is collected and analysed (Technical Committee CEN/TR 16363 2012).
Results from the kinetic test procedures indicate the amounts of leached substances from a waste material under the specific test circumstances (Heikkinen et al. 2008). With the help of kinetic testing it is possible to gather information on the weathering rates of the materials to form secondary minerals and the release of harmful substances into the surrounding environment. Tests simulate naturally occurring weathering reactions, and based on the results it is possible for example to estimate oxidation rates of sulphide minerals, depletion rates of carbonate minerals, duration in which acid generation will take place, and leaching rates of different metals. (Tremblay & Hogan 2001)
Methods to analyse AMD quality in long-term include (Technical Committee CEN/TC 292 2012, European Commission 2009):
European Commission 2009. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. January 2009.
Heikkinen, P.M. (ed.), Noras, P. (ed.), Salminen, R. (ed.), Mroueh, U.-M., Vahanne, P., Wahlström, M., Kaartinen, T., Juvankoski, M., Vestola, E., Mäkelä, E., Leino, T., Kosonen, M., Hatakka, T., Jarva, J., Kauppila, T., Leveinen, J., Lintinen, P., Suomela, P., Pöyry, H., Vallius, P., Nevalainen, J., Tolla, P. & Komppa, V. 2008. Mine closure handbook. Espoo: GTK; VTT; Outokumpu Oyj; Finnish Road Enterprise; Soil and Water Ltd. 169 p.http://en.gtk.fi/informationservices/publications/publications/latest/publication/EJ74.html
INAP 2009. The GARD Guide. 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 Ore Mining. Finnish Environment Institute 29en / 2011. 219 p. https://helda.helsinki.fi/handle/10138/40006
Lapakko, K. 2002. Metal Mine Rock and Waste Characterization Tools: An Overview. Minnesota Department of Natural Resources, US. April 2002 No. 67.
Technical Committee CEN/TC 292 2012. Characterization of waste – Overall guidance document for characterization of wastes from extractive industries. CEN/TR 16376:2012.
Technical Committee CEN/TR 16363 2012. Characterization of waste – Kinetic testing for assessing acid generation potential of sulfidic waste from extractive industries. Technical Report. CEN/TR 16363:2012.
Tremblay, G.A. & Hogan, C.M. (Eds.) 2001. MEND Manual, Volume 3 – Prediction. MEND 5.4.2c.