A Test of Two Methods for Waste Rock Drainage Quality Prediction: Aqua Regia Extraction and Single-addition Net-acid Generation Test Leachate Analysis

Abstract

The mobility of contaminants from mine waste can be assessed using different extraction methods. Aqua regia (AR) extraction is the most commonly used method in Finland. Another method is the analysis of leachate from net acid generation (NAG) tests, which is primarily designed for acid production potential assessment. We investigated the performance of single-addition NAG test leachate analysis and AR extraction in drainage quality prediction, using waste rock and drainage water samples from several Finnish waste rock sites. Our objective was to improve interpretation of the AR and single-addition NAG test leachate analysis results in drainage quality prediction. The AR extraction effectively reflected elements that occurred in elevated concentrations in drainage water, though it over-predicted Al, As, Cd, Co, Cu, and Ni in some circumneutral drainages, and Cr in general. The single-addition NAG test leachate analysis also performed well in assessing the mobility of contaminants including Al and Cr at acid mine drainage sites. As the contaminants tend to precipitate in neutral NAG test solution, the usability of the method in neutral mine drainage cases should be further investigated. Furthermore, the conclusions presented in this study are limited to waste rock samples collected from the surface of piles; future work will examine waste rock history, dump cores, drainage quality changes, etc. in more detail.

Sulfur analyses and mineralogical data in the preliminary mine waste characterization

“…This could increase the cost-effectiveness of the AMD prediction in an early phase of the mining operations. Furthermore, the AR method is also useful in the mobility assessment of harmful elements (Karlsson, Alakangas, et al, 2021 ). Another sulfide specific extraction method commonly used in ore exploration is the hydrogen peroxide ammonium citrate extraction (Katsnelson & Osipova, 1960 ).…”

Hydrogen Peroxide Ammonium Citrate Extraction: Mineral Decomposition and Preliminary Waste Rock Characterization

“…AR dissolves elements bound especially to the sulfide fraction, but it also decomposes all carbonates and secondary minerals, and partly some silicates [11,13]. In general, the AR-extractable concentrations of waste rocks indicate well which contaminants are most likely to be of concern in waste rock drainage [15][16][17][18][19]. For some circumneutral drainage systems, the AR extraction might be too aggressive for drainage quality prediction [18].…”

“…In general, the AR-extractable concentrations of waste rocks indicate well which contaminants are most likely to be of concern in waste rock drainage [15][16][17][18][19]. For some circumneutral drainage systems, the AR extraction might be too aggressive for drainage quality prediction [18]. Furthermore, as some minerals that are not likely to be soluble in natural waste rock systems are relatively soluble in AR, the method can overestimate the mobility of elements primarily bound to silicates and oxides, e.g., Al and Cr [18].…”

“…Although HA has been noted not to leach sulfides completely [22,24], it is a straightforward and relatively efficient method, and therefore commonly used especially in Ni and Co exploration [25][26][27]. According to Karlsson et al [18], a H 2 O 2 -based extraction method might be more selective for element mobility assessment in circumneutral drainage systems compared with the more aggressive AR extraction. It would be beneficial, if the same methods that provide data for exploration, could be utilized in the environmental characterization of potential waste rocks already early in the mining project.…”

“…Modern mineralogical analysis methods were used to determine mineralogical changes in the HA extraction. Furthermore, we compared the HA extraction results with the existing AR and drainage data presented by Karlsson et al [18].…”