Geochemical and Mineralogical Characterization of Waste Rocks for Preliminary Mine Drainage Quality Prediction

Abstract: Acid or neutral rock drainage (ARD or NRD) with its attendant elevated concentrations of harmful elements presents one of the main challenges related to the management of waste rocks. Low-quality drainage is a particular issue with respect to mineral deposits containing sulfide minerals, of which pyrite and pyrrhotite are especially prone to produce acidic drainage when exposed to oxygen and water. The generation of low-quality drainage depends primarily on the composition of mine waste, in particular the proportions of acid-producing and neutralizing minerals, as well as the abundance of harmful elements bound to leachable mineral phases. To mitigate adverse environmental impacts, it is important to characterize waste rocks at an early phase of any given mining project. Early-phase characterization is needed in designing appropriate waste facilities, water treatment and closure techniques, and to investigate the potential possibilities for utilization of waste material. Several methods have been developed for characterizing waste rocks and for predicting their potential for generating low-quality drainage. These methods include static and kinetic testing, geochemical extractions, geochemical modelling, and the use of analogs from similar, older, mine waste sites. Geochemical extractions and static tests, such as acid-base accounting (ABA) and net acid-generation (NAG) tests, are commonly used for preliminary screening, and in selecting suitable samples for further testing. The assortment of these preliminary characterization methods should be expanded and their performance in ARD and element mobility prediction investigated further, to improve the accuracy of drainage quality prediction.The objective of this study has been to enhance waste rock management by developing tools for preliminary waste rock characterization and drainage quality prediction. An additional objective has been to improve the capacity for using geochemical and mineralogical data that have already been obtained during early phases of a mining project, and to provide general information about Finnish waste rock characteristics, so as to highlight the need for regional scale waste rock management and investigations. Accordingly, this study is based on waste rock and drainage samples collected from 19 Finnish active and closed mine sites, with the aim of assessing and comparing the performance of different methods for the preliminary prediction of drainage quality. The investigated acid potential (AP) methods included the ABA test in accordance with the established standard EN 15875, the single-addition NAG test as presented in the AMIRA guidebook, and an additional calculation based on SEM mineralogy. Furthermore, the suitability of seven different sulfur analytical methods for AP assessment was evaluated. The assessed methods for element mobility prediction included single-addition NAG test leachate analysis, as well as aqua regia (AR) and hydrogen peroxide ammonium citrate (HA) extractions, which are commonly used in mineral exploration to determine the concentrations of valuable elements bound to sulfide minerals.Based on the results, pyrrhotite was found to be the main sulfide mineral contributing to AP in the waste rocks at the investigated sites, with pyrite being the next in importance. The abundance of sulfide species other than pyrite in the waste rocks led to the realization suggested that the appropriate factor for defining the AP, based on multiplying the S content, should instead take into consideration the dominant sulfide species, rather than assume that all S is pyritic. Silicate minerals, especially biotite, were found to be important contributors to the neutralization potential (NP). The results suggested that the AP and NP calculations based on the SEM mineralogy serve as a useful tool in the ARD prediction, as they also reveal the source minerals for AP and NP. However, it is recommended to make use of EDS spectral data to verify that the S concentration calculated by modal mineralogy is in accord with total S based on the EDS sum spectra. The AR-extractable S concentration appeared to be a useful discriminant for determining the S concentration for the AP calculation, as it does not leach baryte, thus more accurately representing the S-content in sulfide.The most abundant harmful elements in the investigated waste rocks were Co, Ni, Cu, and Cr, whereas in the waste rock pile drainages the most prominent elements were  Ni, Co, Zn and Cu. Results indicated that the use of the Finnish PIMA values (the threshold values defined in the Government Decree 214/2007 on the Assessment of Soil Contamination and Remediation Needs) in the waste rock characterization should be reassessed, especially for Cr, for which concentrations often exceeded the PIMA threshold values, even though they were not elevated in the corresponding drainage waters. Based on the measured drainage water concentrations, the AR and HA extraction methods appeared to be effective in the prediction of preliminary ARD quality. The AR extraction realistically reflected the abundances of elements that are likely to occur in elevated concentrations in the waste rock drainage water. However, this method overestimates contaminant mobilities in some circumneutral drainage cases, and the mobility of Cr in general. The HA extraction also performed well in the prediction of harmful element mobilities, with the exception of Al mobility in acidic drainage systems. The HA extraction appeared to be a more sulfide specific method compared with AR, which is evident for example, in better prediction of Cr mobility. The single-addition NAG test leachate analysis performed well in assessing the mobility of harmful elements, but only when the test leachate was acidic, as the elements appeared to co-precipitate when the NAG test leachate pH rises above 3-6.