NP calculated based on carbonate content (“carbonate NP”)
Päivi M. Kauppila, Geological Survey of Finland, P.O. Box 1237, FI-70211 Finland; e-mail: paivi.kauppila(at)gtk.fi
Acid base accounting (ABA) provides a screening tool for the prediction of potential of waste material to produce acid mine drainage. It is calculated based on the neutralising potential (NP) and the acid production potential (AP) of the waste. AP is measured based on the sulphide sulphur or total sulphur content of the waste and NP is typically determined using static tests (Price et al. 1997, White et al. 1999). However, NP can also be calculated based on the carbonate carbon content of the waste material (Lawrence & Wang 1996). This approach is presented here.
The most typical carbonate minerals that contribute to the acid buffering capacity in the mining wastes include calcite (CaCO3) and dolomite [CaMg(CO3)2]. Other potential carbonate minerals in wastes include e.g. siderite (FeCO3), magnesite (MgCO3), ankerite [Ca(Fe,Mg,Mn)(CO3)2] and rhodochrosite (MnCO3).
In the method, NP is calculated based on the carbonate carbon content. Carbonate content is measured e.g. as a difference between the total carbon content measured using a high temperature combustion method and the non-carbonate carbon content, which is measured with combustion method after removing carbonate carbon by hydrochloric acid (EN13137:2001; Räisänen et al. 2010). Subsequently, carbonate NP is calculated by multiplying the carbonate carbon concentration (%) by 83.34 to yield the NP in units of kg CaCO3/t (e.g. Räisänen et al. 2010).
Carbonate NP represents the neutralization potential of the readily available carbonate minerals in the waste material (Lawrence & Wang 1997).
The method is suitable for NP and ABA calculations of mining wastes, particularly, when there are slowly weathering carbonate minerals in the waste, which may not dissolve during the titration in the typical static tests.
- Method allows simple, rapid and low-cost screening of waste samples to assess the neutralization potential of waste materials.
- Carbonate minerals containing iron, in particular siderite (FeCO3), do not necessarily contribute to neutralisation. Siderite is known to consume acid when it dissolves, but the subsequent hydrolysis of the leached iron will release protons and produces a neutral to slightly acid solution (Lawrence & Wang 1997, Haney et al. 2006). Therefore, carbonate carbon based NP overestimates the neutralization potential of the waste for materials containing siderite.
- Method does not take into account the reactive non-carbonate minerals that may contribute to the acid neutralization potential in the waste. The method may thus underestimate the NP if there are e.g. easily dissolving silicate minerals with high acid buffering capacity in the wastes.
EN13137:2001. Characterization of waste – Determination of total organic carbon (TOC) in waste, sludges and sediments.
Haney, E.B., Haney, R.L, Hossner, L.R. & White, G.N. 2006. Neturalization potential determination of siderite (FeCO3) using selected oxidants. Journal of Environmental Quality 35 (3), 871-879.
Lawrence R.W. & Wang, Y. 1996. Determination of neutralization potential for acid rock drainage prediction. MEND Project 1.16.3. 38 p. http://mend-nedem.org/wp-content/uploads/2013/01/1.16.3.pdf. Accessed 6thNovember 2014.
Price, W.A. 1997. Draft Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia. Reclamation Section, Energy and Minerals Division; British Columbia Ministry of Employment and Investment. 159 p. http://mend-nedem.org/wp-content/uploads/2013/01/1.20.1-Ref.pdf. Accessed 6th November 2014.
Price, W.A., Morin, K., Hutt, N. 1997. Guidelines for prediction of acid rock drainage and metal leaching for mines in British Columbia: Part II. Recommended procedures for static and kinetic tests. Fourth International Conference on Acid Rock Drainage. Vancouver, B.C. Canada, May 31 – June 6, 1997. Proceedings, vol. I, 15-30.
Räisänen, M.L., Kauppila, P.M. & Myöhänen, T. 2010. Suitability of static test for acid rock drainage assessment of mine waste rock. Bulleting of the Geological Society of Finland 82, 101-111. www.geologinenseura.fi/bulletin/Volume82/Raisanen_2010.pdf
White III, W.W., Lapakko, K.A. & Cox, R.L. 1999. Static-test Methods most commonly used to Predict Acid Mine Drainage: Practical Guidelines for Use and Interpretation. In: Plumlee, G.S. & Logsdon, M. (Eds): The Environmental Geochemistry of Mineral Deposits. Part A: Processes, Techniques, and Health Issues. Reviews in Economic Geology, Vol. 6A, 325-338.