Abstract
Low ore-grade waste samples from the Codelco Andina mine that were analyzed in an environmental and mineralogical test program for acid rock drainage prediction, revealed inconsistencies between the quantitative mineralogical data (QEMSCAN®) and the results of geochemical characterizations by atomic absorption spectroscopy (AAS), LECO® furnace, and sequential extractions). For the QEMSCAN® results, biases were observed in the proportions of pyrite and calcium sulfate minerals detected. An analysis of the results indicated that the problems observed were likely associated with polished section preparation. Therefore, six different sample preparation protocols were tested and evaluated using three samples from the previous study. One of the methods, which involved particle size reduction and transverse section preparation, was identified as having the greatest potential for correcting the errors observed in the mineralogical analyses. Further, the biases in the quantities of calcium sulfate minerals detected were reduced through the use of ethylene glycol as a polishing lubricant. It is recommended that the sample preparation methodology described in this study be used in order to accurately quantify percentages of pyrite and calcium sulfate minerals in environmental mineralogical studies which use automated mineralogical analysis.
Highlights
Accurate mineral quantification is key in order to predict the behavior of a rock sample in different geochemical environments, due to potential dissolution or oxidation reactions, which may lead to the release of elements into the environment and/or generation of acidity, as for example during sulfide oxidation and the subsequent formation of acid rock drainage (ARD)
In order to confirm the presence of particle segregation and evaluate the degree to which it was occurring in the previously analyzed polished sections, one of the sections was selected for vertical slicing and preparation of a transversely-mounted polished section
It can be seen that the particles with greater mass are concentrated near the center of the section, which represents the bottom of the original polished section, which was the original analysis surface
Summary
Accurate mineral quantification is key in order to predict the behavior of a rock sample in different geochemical environments, due to potential dissolution or oxidation reactions, which may lead to the release of elements into the environment and/or generation of acidity, as for example during sulfide oxidation and the subsequent formation of acid rock drainage (ARD).There is a tendency, which is often due to reduced budgets for waste rock and tailings characterization testwork, to only perform a suite of relatively inexpensive geochemical “environmental”tests on these materials, which may include acid base accounting (ABA), paste pH, net acid generation (NAG), and typically in fewer cases, humidity cell tests [1]. Accurate mineral quantification is key in order to predict the behavior of a rock sample in different geochemical environments, due to potential dissolution or oxidation reactions, which may lead to the release of elements into the environment and/or generation of acidity, as for example during sulfide oxidation and the subsequent formation of acid rock drainage (ARD). It is most critical to accurately quantify these minerals in order to be able to predict the formation of ARD or the release of hazardous elements in solution. The scope of a study executed between 2008 and 2012 for the Andina Division, CODELCO, Chile, included geochemical and mineralogical characterizations for 360 samples in order to evaluate their potential for acid rock drainage formation and liberation of toxic constituents. The analytical approach included sequential extractions (Table 1) [24] and automated quantitative mineralogical characterization by
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