Mineralogical calculation of AP and NP

Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi

Introduction

The commonly used acid rock drainage (ARD) prediction methods (ABA, NAG) have some known mineralogy related limitations, e.g. (White III et al. 1999, Jambor 2003, Parbhakar-Fox & Lottermoser 2015, Dold 2017):

  • AP may be overestimated if there are other sulphide or sulphur containing minerals than rapidly acid producing ones
  • AP may be underestimated if waste contains much easily dissolvable and acid generating iron sulphate minerals or siderite
  • NP may be underestimated if the weathering of silicate minerals is not considered in APP estimations
Scanning electron microscope (SEM) based automated mineral quantification methods have been considerable developed in the last decades, increasing the available mineralogical data, which could and should be utilized in ARD prediction (Dold 2017).
ARD prediction by mineralogy based calculation of acid production potential (AP) and neutralization potential (NP) has been presented by Lawrence and Scheske (1997), utilizing relative reactivity and acid-neutralization capacity of minerals presented by Sverdrup (1990).

Description of the method

AP and NP can be determined by calculations based on mineralogy (Dold 2017, Lawrence & Scheske 1997), as follows.

Mineralogical NP for carbonates: Wt%C in the mineral * wt% of the mineral to get the wt%C * 83.3.

Mineralogical NP calculations for non-carbonate minerals can be calculated by adding together the weighted NP values for each (significant) mineral present in the sample. To simplify the mineralogical NP calculations, it is not necessary to take into account the minerals in slow weathering, very slow weathering and inert groups, as their NP contribution is insignificant (Sverdrup 1990).
Sverdrup (1990) states that the relative reactivity of minerals change depending on the total percentages of different mineral groups in the sample (Table 1). To simplify calculations, the relative reactivities of the average mineral class content of 30 % can be used, as this is probably close to average realistic mineral class contents.
Calculated mineralogical NP values can be converted to kg CaCO3 by using the molecular weight of CaCO3 to the mineral.
NP contribution kg CaCO3–equivalent/tonne = (mineral mass % / 100) x (1000 kg / 1 tonne) x (mol. wt. CaCO3/ mol. wt. mineral) x relative reactivity of the mineral.
For example NP of 30 wt% of diopside would be: 30/100 * 1000 kg/t * (100.09 g/mol / 216.55 g/mol) * 0.67 = 92.9 kg CaCO3/t.
Mineralogical AP of sulphides: Wt%S in the sulphide mineral * wt% of the mineral to get the wt% S, which can be multiplied by 31.25 (commonly used factor) or in some cases by 62.5 (as proposed by Dold 2017).
An example for mineralogical calculation of NNP and NPR of an imaginative sample is presented in the table 2.

Table 1. Relative reactivity in acid-neutralization capacity of selected minerals (after Sverdrup 1990).

Table 2. Example calculation for mineralogical NNP and NPR.

Appropriate applications

ARD prediction based on SEM mineralogy and mineralogical calculations can be done If sufficient mineralogical data is available.

References

Chen, Y. & Brantley, S.L. 1997. Diopside and anthophyllite dissolution at 25° and 90°C and acid pH. Chemical Geology 147, pp. 233-248.
Dold, B.  2017. Acid rock drainage prediction: A critical review- Journal of Geochemical Exploration 172, pp. 120-132.
Jambor JL (2003) Mine-waste mineralogy and mineralogical perspectives of acid-base accounting. In: Jambor JL, Blowes DW, Ritchie AIM (Eds.) Environmental aspects of mine wastes. Mineralogical Association of Canada, Short Course Series 31, p.117-145.
Lawrence, R.W. & Scheske, M. 1997. A method to calculate the neutralization potential of mining wastes. Environmental Geology, 32, pp. 100-104.
Parbhakar-Fox, A. & Lottermoser, B.G. 2015. A critical review of acid rock drainage prediction methods and practices. Minerals Engineering 82 197–124.
Rozalen, M., Ramos, M.E., Gervilla, F., Kerestedjian, T., Fiore, S. & Huertas, F.J. 2014. Dissolution study of tremolite and anthophyllite: pH effect on the reaction kinetics. Applied Geochemistry 49, pp. 46-56.
Schweda, P. & Kalinowski, B. 1994. Dissolution rates and alteration of muscovite, phlogopite and biotite at pH 1 to 4, room temperature. Goldschmidt conference proceedings, Edinburgh 1994. pp. 817-818
Sverdrup, H.U. 1990. The Kinetics of Base Cation Release Due to Chemical Weathering. Lund University Press, Lund.
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.