Deportment of radionuclides in copper concentrate from Olympic Dam

Abstract

The Olympic copper-gold province of South Australia is home to the world’s largest uranium resource, representing at least 23% of the known global uranium budget. The primary commodity produced is copper, with secondary Au, Ag, and U (as U₃O₈) also recoverable in economic quantities. Processing the complex, fine-grained ore from Olympic Dam involves multiple, customisable stages of physical and chemical separation of target elements from non-target elements. During some stages of processing, deleterious components of the ore may behave adversely, affecting recovery efficiency or purity of product. Of particular interest to process engineers are daughter radionuclides (RNs) created from decay of ²³²th, ²³⁵U, and especially ²³⁸U. Specific activities of ²²⁶Ra, ²¹⁰Pb, and ²¹⁰Po are such that even concentrations of parts-per-trillion may be above acceptable levels for marketable copper concentrate. Most conventional analytical platforms emphasise either sensitivity (solution inductively coupled plasma mass spectrometry, thermal ionisation mass spectrometry) or high-resolution imaging (transmission electron microscopy, atom probe tomography). Ultra-trace-level RN concentrations combined with the extremely fine-grained nano-included mineral textures of Olympic Dam ore present an unusual challenge for researchers, in that few analytical techniques excel in both sensitivity and resolution. A comparison of available methods yielded excellent results from the CAMECA nanoscale secondary ion mass spectrometry (nanoSIMS) platform, capable of detecting (for the first time) extremely low RN signals in minerals with sub-micron resolution; consequently, nanoSIMS was chosen as the primary analytical method for this project. Subsequent to extensive method development and testing, over 3200 isotopic spatial distribution maps were collected on mineral samples from raw ore material, flotation concentrate, and acid-leached concentrate provided by BHP Olympic Dam. A qualitative RN budget has been created, prioritising minerals which contain significant RNs but also including unexpected minor host minerals and surficial adherence. Four manuscripts have resulted from these analyses, detailing RN host capabilities of various minerals including uraninite, brannerite, coffinite, thorianite, euxenite, apatite, fluorite, xenotime, monazite, baryte, anglesite, and Sr- and Ca-rich alunite-series phases. Mechanisms for surface-sequestration of RNs in high surface area minerals such as covellite and molybdenite have been identified, as well as electrokinetic effects responsible for substantial uptake of RNs on the surface of sulphide minerals. Additionally, diffusion experiments were performed on synthetic minerals to determine mobility of Sr, Ba, Pb, and Ra under varying conditions common to mineral processing. Two manuscripts have resulted from these investigations, emphasizing that baryte and anglesite are well-suited to remove substantial amounts of Ra and Pb from processing streams, especially during acid-leaching, through coupled dissolution-reprecipitation mechanisms. Laser ablation inductively coupled plasma mass spectrometry and nanoSIMS both provided excellent visual evidence for Ra uptake, well in agreement with published results from other research teams As a whole, the data set produced during this project has greatly increased our understanding of the deportment of radionuclides at Olympic Dam – and in uranium-bearing ore systems in general. Provided these data, process engineers are now better-suited to design and implement efficient RN removal procedures, thereby resulting in cleaner ore concentrates available to the global marketplace.