Speciation of the most soluble phases in a sequential extraction procedure adapted for geochemical studies of copper sulfide mine waste

Abstract

Sequential extractions are widely used for exploration purposes and to study element speciation in systems such as soil and sediments, and more recently to understand the complex biogeochemical element cycling in mine waste environments. This method is however often the focus of criticism due to uncertainty in the selectivity of specific leaches utilized. In this study, a procedure is presented how sequential extractions can be adapted to specific mineralogy in order to increase the selectivity and the accuracy of geochemical data interpretation. The application of dissolution kinetic tests and the control of dissolved phases in sequential extractions by X-ray diffraction (XRD) and differential X-ray diffraction (DXRD) indicate which minerals are dissolved in each leach. This information is crucial for the interpretation of geochemical data obtained from sequential extractions and enables to increase the selectivity of the sequence applied. A seven-step sequence was adapted to the specific secondary and primary mineralogy of mine tailings from Cu-sulfide ores, both from porphyry copper and from Fe-oxide Cu–Au deposits. As result of the study, the following seven-step sequence shows best selectivity for the aim of the study of Cu-sulfide mine waste: Step 1 liberates the water-soluble fraction (1.0-g sample into 50-ml deionized H 2O shake for 1 h at room temperature [RT]) dissolving gypsum and metal salts (e.g., chalcanthite (CuSO 4·5H 2O), pickeringite (MgAl 2(SO 4) 4·22H 2O)). Step 2 liberates the exchangeable fraction (1 M NH 4-acetate, pH 4.5, shaken for 2 h, RT) as adsorbed ions, but also dissolves calcite and breaks down a typical secondary vermiculite-type mixed-layer mineral from the low pH oxidation zone. Step 3 addresses the Fe(III) oxyhydroxides fraction (0.2 M NH 4-oxalate, pH 3.0, shaken for 1 h in darkness, RT) and dissolves schwertmannite, two-line ferrihydrite, Mn-hydroxides, secondary jarosite partially, as well as goethite acid mine drainage formed. Step 4 dissolves the Fe(III) oxides fraction (0.2 M NH 4-oxalate, pH 3.0, heat in water bath 80 °C for 2 h) and removes all secondary ferric minerals occurring as higher ordered ferrihydrite (e.g., six-line), goethite, primary and secondary jarosite, natrojarosite, and primary hematite. Step 5 consists of a change from reducing to oxidizing condition and is performed by a H 2O 2 leach (35% H 2O 2 heat in water bath for 1 h), which dissolves organic matter and supergene Cu-sulfides such as covellite and chalcocite–digenite. Step 6 (KClO 3 and HCl, followed by 4 M HNO 3 boiling) dissolves primary sulfides and Step 7 (HCl, HF, HClO 4, HNO 3) the residual fraction (silicates). The application of this extraction sequence to 5 Cu-sulfide mine tailings have shown that sequential extractions can be a powerful tool for detection of element mobilization and retention processes. This is especially the case in dry climates, where it is difficult to obtain pore-water geochemistry. A detailed mineralogical study should however go hand in hand with every geochemical study of mine waste to ensure the accuracy of the geochemical interpretations.