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Analysis of single-addition NAG test leachate compared with aqua regia extraction for prediction of waste rock drainage quality
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Geological Survey of Finland GTK. (Applied Geochemistry)
Geological Survey of Finland GTK. (Retired from: Industrial Environments and Recycling Unit, Geological Survey of Finland, P.O. Box 1237, 70211 Kuopio, Finland)
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. (Applied Geochemistry)
Geological Survey of Finland GTK. (Industrial Environments and Recycling Unit, Geological Survey of Finland, P.O. Box 1237, 70211 Kuopio, Finland)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Acid rock drainage and neutral rock drainage containing harmful substances originating mainly from extractive waste facilities are a major environmental problem for the mining industry. In order to design appropriate waste facilities and drainage water management, it is important to characterise extractive wastes and assess the behaviour of waste material before actual mining activities begin.

The mobility of potentially harmful substances from extractive waste can be assessed using different selective extraction and leaching methods. Aqua regia (AR) extraction is the most commonly used selective extraction method in Finland.  A less commonly used method for element mobility prediction is analysis of leachate from a single-addition net acid generation (NAG) test, which is primarily designed for assessment of acid production potential.

In this study, we compared the performance of single-addition NAG test leachate analysis and AR extraction results in drainage quality assessment, using waste rock and drainage water samples from several Finnish extractive waste sites. For selected samples, an additional NAG test leachate analysis was performed before the heating phase in the NAG test, as well as at the end of the test, to evaluate whether this modification improved the accuracy of the test in predicting drainage water quality.

The results showed that AR extraction accurately reflected elements that might occur in elevated concentrations in drainage water. The NAG leachate analysis test also performed well in assessing the mobility of harmful elements, but only when the leachate was acidic, as most of the elements of interest started to precipitate at pH of around 3-6. The modified NAG test did not predict drainage water quality reliably. The most widely abundant harmful elements in drainage from Finnish waste rock at the sites investigated were Ni, Co, Zn and Cu. In waste rocks, Co, Ni, Cu and Cr were also often present in elevated concentrations. 

Keywords [en]
Extractive waste characterisation, drainage water, ARD, NRD
National Category
Geochemistry
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-75746OAI: oai:DiVA.org:ltu-75746DiVA, id: diva2:1346845
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2019-08-29
In thesis
1. Geochemical and mineralogical laboratory methods in waste rock drainage quality prediction
Open this publication in new window or tab >>Geochemical and mineralogical laboratory methods in waste rock drainage quality prediction
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Luleå University of Technology, 2019
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Extractive waste characterisation, drainage water, ARD, NRD, ABA test, NAG test, SEM
National Category
Geochemistry
Research subject
Applied Geochemistry
Identifiers
urn:nbn:se:ltu:diva-75747 (URN)978-91-7790-429-8 (ISBN)978-91-7790-430-4 (ISBN)
Presentation
2019-10-23, E246, Luleå University of Technology, SE-97187, Luleå, 11:29 (English)
Opponent
Supervisors
Available from: 2019-08-29 Created: 2019-08-29 Last updated: 2019-08-29Bibliographically approved

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