Abstract
A commonly-used method in ore exploration is hydrogen peroxide ammonium citrate (HA) extraction, which has not typically been used in waste rock characterization. In this study, the sulfide specificity and leaching of other minerals in HA extraction was evaluated and its performance was compared with the aqua regia (AR) extraction for preliminary assessment of harmful element mobility. Samples collected from several different mine sites in Finland were utilized. The waste rock sample S contents ranged from 0.3% to 5.3%, and sums of the AR extractable elements As, Cd, Co, Cu, Ni and Zn range from 120 to 8040 mg/kg. The drainage types ranged from acid high-metal to neutral low-metal, with pH’s of 3.3–7.7. Mineralogical changes that took place in the HA solution were investigated by the field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy analyzer (EDS) and X-ray diffraction (XRD) methods. Results of the study showed that the HA extraction appears to be a more specific method for sulfide decomposition compared with AR extraction. Sulfide minerals, especially base metal sulfides pentlandite, chalcopyrite and sphalerite, decomposed efficiently in HA extraction. However, the Fe-sulfides pyrrhotite and pyrite only decomposed incompletely. The study showed that the HA extraction results can be used in the preliminary prediction of element mobility. Based on the results, the elevated As, Cd, Co, Cu, Ni, S and Zn leachability in the HA extraction appears to predict elevated drainage concentrations. If the HA-extractable sum of As, Cd, Co, Cu, Ni and Zn is >750 mg/kg, there is an increased risk of high-metal (>1000 µg/L) drainage. Therefore, the HA extraction data, e.g., produced during ore exploration, can be utilized to preliminary screen the risks of sulfide related element mobilities from waste rock material.
Highlights
Sulfides are essential for the mining industry as important metal ore minerals [1]
As sulfides are the primary sources of contaminants in mine wastes, a sulfide-specific extraction method is useful in environmental studies to assess the mobility of harmful elements [11]
Our results suggest that the incomplete dissolution of pyrite in the hydrogen peroxide ammonium citrate (HA) extraction could be partly caused by its occurrence in inclusions inside quartz, which can be observed in the backscattered electron images of the samples WR1 (Figure 2 and Figure S1) and WR2 (Figure 2 and Figure S2)
Summary
Sulfides are the main source of contaminated rock drainage, acid (ARD) or neutral (NRD), which are considered to be the main environmental concerns related to mine waste management [2,3,4,5,6]. A sulfide specific element extraction method is of importance because, besides being bound to sulfides, valuable elements may be abundant in the less economic silicate plus oxide fraction of a sample [8,9]. Selective extractions are used to study the mobility of harmful elements from mine waste materials [10]. As sulfides are the primary sources of contaminants in mine wastes, a sulfide-specific extraction method is useful in environmental studies to assess the mobility of harmful elements [11]. Short-term laboratory analyses, including geochemical extraction methods, are usually good for preliminary investigation and screening, and the results can be used to select suitable samples for more thorough kinetic testing, e.g., humidity cell testing [12]
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