Science Blog: Acid rock drainage prediction by SEM mineralogy

GTK has a long history in mining environmental investigation. A recently published study (Karlsson et al. 2018) examined the use of SEM mineralogy in acid rock drainage (ARD) prediction. The results were promising and indicate that mineralogical calculation can be an efficient tool in mine drainage quality assessment.

ARD with high concentrations of harmful elements is one of the main concerns in management of mining wastes. This applies particularly to deposits containing sulphide minerals, e.g. pyrite (FeS2) and pyrrhotite (Fe1-xS), which are prone to oxidisation under the influence of atmospheric conditions. Since ARD plays a major role in causing environmental issues, accurate ARD prediction is of the utmost importance. However, prediction is sometimes challenging, as ARD generation depends on various mineralogical, chemical, hydrological and microbiological factors.

The most commonly used ARD prediction test is acid-base accounting (ABA), which includes determination of the sulphur content of rock material in order to calculate the acid potential (AP) and determination of the neutralisation potential (NP) (Price 2009). The AP is usually calculated based on the total sulphur content of the sample and expressed as pyrite equivalents, since it is assumed that pyrite is the most common sulphide mineral and that 4 moles of protons are produced during the oxidation process (Dold 2017). Another commonly used test is net acid generation (NAG). It is based on the reaction of a sample with hydrogen peroxide, which accelerates the oxidation of sulphide minerals in the sample.

The widely used ABA and NAG tests have known limitations related to the mineralogy of the sample material (Parbhakar-Fox and Lottermoser 2015; Dold 2017). For example, the AP may be overestimated if there are other sulphide or sulphur-containing minerals than acid-producing pyrite. The NP may be underestimated if the weathering of silicate minerals is not considered. In addition, the commonly used tests do not indicate the source minerals of NP and AP. Therefore, some mineralogical-based approaches have been proposed, e.g. by Lawrence and Scheske (1997) and Parbhakar-Fox and Lottermoser (2015).

In this study, we compared the ABA test as presented in the standard EN 15875, the NAG test as presented in the AMIRA guidebook (Smart et al. 2002) and SEM mineralogy-based ARD predictions based on Lawrence and Scheske (1997) and Dold (2017). The objective of the study was to assess the functionality of standard and mineralogical ARD prediction methods and determine whether the commonly used laboratory ARD tests could be replaced in some cases by ARD prediction based only on SEM mineralogy.

According to the results, pyrrhotite appears to be the most commonly occurring sulphide at Finnish mine waste sites. Pyrrhotite also seems to be responsible for most of the acid production potential. Therefore, the standard ABA test often results in overly pessimistic ARD predictions, as the AP is overestimated by assuming that all sulphur is pyritic.

At most of the mine sites investigated, silicate minerals (especially biotite), rather than carbonates, were found to be the most important contributors to the neutralisation potential. As silicate minerals appear to have significant importance for the neutralisation capacity and therefore ARD prediction, the behaviour, relative reactivities and dissolution rates of these minerals should be more thoroughly investigated.

SEM mineralogy-based AP and NP calculation appears to be an efficient tool for ARD prediction. As an enhancement to the common laboratory tests, mineralogical investigation reveals the actual minerals corresponding to AP and NP. If sufficient suitable mineralogical data are available, there is less need to perform other ARD tests, which reduces the analytical costs. Instead, funding could be diverted to investigating the mobility of potentially harmful elements. The mineralogical ARD prediction calculations could also be integrated with the mineralogical results provided to customers by GTK.
The study is described in more detail in: Karlsson, T., Räisänen, M. L., Lehtonen, M. & Alakangas, L. 2018. Comparison of static and mineralogical ARD prediction methods in the Nordic environment. Environmental Monitoring and Assessment 190:719.

Teemu Karlsson

Teemu Karlsson completed his MSc degree on Quaternary geology at the University of Turku in 2008. Since 2016, he has been working on a PhD project at Luleå University of Technology. The working title of his thesis is: “Evaluating the performance of geochemical methods in assessing the short- and long-term behaviour of waste rocks”. He has worked at GTK since 2011, mostly carrying out research and coordinating projects on mining environmental subjects.