Investigations into the Natural Variation of Pyrite Reactivity

Author: Owen David Osborne

Osborne, Owen David, 2013 Investigations into the Natural Variation of Pyrite Reactivity, Flinders University, School of Chemical and Physical Sciences

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Abstract

Pyrite (FeS2) is widely accepted to be the most abundant sulfide mineral on the surface of the planet and its abundance in mining waste is well established. The significance of pyrite lies in its potential to oxidise in the environment and cause a process called acid mine drainage. It has been observed that pyrites from different geographical locations will undergo oxidation at different rates, however, the reasons for this are not well understood. This thesis presents investigations into the proposed variation of pyrite reactivity and the mineral characteristics which may contribute to this variation. A method for the rapid determination of the oxidation rate of naturally occurring pyrite samples was developed. The progress of the oxidation reaction was followed by measurement of the concentration of total dissolved Fe using flow injection analysis. Iron was determined using ultraviolet-visible detection after reaction with the colorimetric reagent 5-sulfosalicylic acid in the presence of ammonia. The calibration function was linear between 5 and 150mg.L-1, and the detection limit was 0.46mg.L-1. The relative standard deviation was typically less than 1% (n=10) and the measurement frequency was 60 per hour. The method was used to quantify the oxidation rate of 40 ground and cleaned pyrite samples from various international locations that were subjected to accelerated oxidation in acidic hydrogen peroxide. Results of these experiments showed over a 6-fold difference in oxidation rates across the pyrite samples. Thirty pyrite samples from a range of geological locations were analysed using relative comparator and k0-NAA (neutron activation analysis) at MURR (University of Missouri Research Reactor, Columbia, Missouri, USA) and ANSTO (Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia) respectively. In general, statistical analysis of the trace element data for the two methods showed a good correlation, with the majority of elemental concentrations of paired data reported by MURR and ANSTO being indistinguishable at the 0.05 significance level. Comparison of results presented here for pyrite from Victoria Mine (Spain) compared well with previously published NAA data. Both methods show applicability to the trace element analysis of pyrite. The trace element data for the samples, collected by NAA was compared to their measured semiconductor types. The results showed that p-type samples generally had an abundance of hole donating impurities while n-type pyrites generally had an abundance of electron donating impurities. Semiconducting type was compared with the reactivity of the samples and no correlation was observed. Interestingly, the inclusion of elements common in clay and silicate layer minerals was found to correlate with sample reactivity. By investigating the geology of samples with large and small measured reactivities a trend emerged indicating that samples from sedimentary origins were significantly more reactive than those from higher temperature hydrothermal origins.

Keywords: Pyrite,Flow Injection Analysis,FIA,Neutron Activation analysis,NAA
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2013
School: School of Chemical and Physical Sciences
Supervisor: Assoc Prof Claire Lenehan