Geochemical and mineralogical laboratory methods in waste rock drainage quality prediction

Harmful substances containing acid or neutral rock drainages (ARD and NRD) are a major challenge related to the management of extractive industry wastes. This issue is particularly related to deposits containing sulphide minerals, which are prone to oxidization under the influence of atmospheric oxygen and water. The drainage quality depends mainly on the mineralogical and chemical composition of the extractive wastes, and especially on the ratio of acid-producing and neutralizing minerals, combined with reactions catalysed by microbes. Since harmful drainages play a major role in the generation of environmental issues for extractive industry, the accurate prediction of the drainage quality is of utmost importance. To design appropriate extractive waste facilities and drainage management, the characterisation of extractive wastes and assessment of the behaviour of the waste material is essential already before the actual mining activities start. Several methods have been developed to characterize extractive waste materials and to predict their short and long term behaviour, including e.g. geochemical laboratory tests, static tests and longer term kinetic tests, and geochemical modelling. The characterisation methods for assessing the ARD risk can be divided into static and kinetic tests. Static tests are short term laboratory analyses, usually used for preliminary investigation and screening. Kinetic tests are longer term tests, revealing information on the time scale of drainage events. Commonly used static tests for ARD prediction include acid–base accounting (ABA) tests and the net acid generation (NAG) test. Since acid and neutralisation potential largely depend on the ratio and quality of acid-producing and neutralizing minerals, mineralogical calculations could also be used for ARD prediction. The mobility of potentially harmful substances from extractive waste can be preliminary assessed using different geochemical laboratory tests, including selective extraction and leaching methods. The most commonly used selective extraction method in Finland is the aqua regia (AR) extraction. In addition to some silicates and secondary precipitate minerals, it is intended to dissolve elements bound especially to sulphide phases. A less commonly used method for element mobility prediction is the analysis of the single addition NAG test leachate. In this study, several Finnish waste rock sites were investigated and the performances of different preliminary drainage quality test methods evaluated and compared. The assessed acid production potential methods included the ABA test as presented in the standard EN 15875, the single addition NAG test as presented in the AMIRA guidebook, and a SEM mineralogy-based calculation. The assessed methods for element mobility prediction included the single addition NAG test leachate analysis and the AR extraction. According to the results, pyrrhotite seems to be the main mineral contributing to acid production, and the silicate minerals the main contributors to the neutralisation potential at the most Finnish waste rock sites. Since silicate minerals appear to have a significant role in ARD prevention, the behaviour of these minerals in mining environment should be more thoroughly investigated. In the investigated Finnish waste rocks, Co, Cr, Cu and Ni often occurred as elevated concentrations, and the most widely abundant harmful elements in the waste rock drainages were Co, Cu, Ni and Zn. The results suggest that an acid production prediction based on SEM mineralogical calculation is at least as accurate as the commonly used static laboratory methods. The AR extraction indicates well which elements might occur as elevated concentrations in the drainage. Also the NAG test leachate analysis performed well in element mobility assessment, but only when the NAG test leachate was sufficiently acidic, the leachate pH being below of 3-6, depending on the element of interest