By Dr Michael Anenburg, Australian National University
A collaboration between the Australian National University, BHP Olympic Dam and the Australian Research Council is testing a novel molten salt process that could transform low-grade tailings into valuable sources of rare earth elements.
Geological study of ore deposit formation using geochemistry, petrology, and mineralogy is not always a first priority for chemical engineers working to extract metals from ore. Extractive ore processing typically follows established protocols and flow sheets, and does not concern itself with how the ore got to the rock in the first place. This approach works well for time-tested metals such as copper or gold, but the recent push for critical metal production requires new methods. Often, critical metals occur as unextracted by-products that remain in waste material or tailings, demonstrating that any processes currently in place were unable to extract them.
A notable example is Olympic Dam in South Australia. The mine is operating within an iron-oxide copper-gold (IOCG) deposit site, with current production primarily consisting of copper, gold, silver, and uranium; however, Olympic Dam is a massive geochemical anomaly containing elevated concentrations of essentially half the periodic table. Despite potential economic possibilities, these metals are not currently produced due to various difficulties. Of particular importance are the lanthanides, or rare earth elements, a group of technologically vital metals. Olympic Dam contains an enormous rare earth element resource, with estimates up to 50 million tonnes total rare earth oxide (TREO). This resource is comparable in size to Bayan Obo in China, the largest currently producing rare earth element deposit in the world, and about 10 times larger than Mount Weld in Western Australia, the only currently producing Australian deposit. Despite the vast rare earth element quantities in Olympic Dam, ore grades are low (0.1–0.5 per cent TREO), roughly an order of magnitude lower than any standalone rare earth element deposit. Combined with the unusually large diversity of rare earth element mineral hosts, and their haphazard spatial and grain size distribution, an economically viable extraction process is not presently available. Instead, the rare earth elements accumulate in the flotation tailings waste site.
The rare earth element minerals of Olympic Dam are very small, often smaller than 20 micrometres, necessitating prohibitively expensive comminution just to liberate them and make them available for further acid treatment. Moreover, there is no one single mineral host for the rare earth elements. They are distributed among many minerals, with the most important being florencite, bastnaesite, synchysite, xenotime, and monazite. Some rare earth elements are even hosted as nanoinclusions within hematite, one of the most abundant gangue minerals. Even if one were to physically separate these minerals from the gangue, each mineral requires different chemical processing. While monazite and bastnaesite have well-established processing methods, florencite does not. This will detrimentally impact rare earth element recovery, even if mineral concentrate is obtained.
Geological knowledge of Olympic Dam’s formation may hold the key to unlocking this resource. When Olympic Dam formed, more than one billion years ago, nature did not use acids or organic solvents to transport rare earth elements from the mineralising fluid to their deposit site. The past decade of research on Olympic Dam and other IOCG deposits worldwide reveal that highly saline hydrothermal fluids (also known as salt melts) carried the metals. Would it be possible to use similar molten salts to take the rare earth elements back into a fluid, from which it could be much easier to extract them?
This question is currently explored at the Research School of Earth Sciences of Australian National University in Canberra. In a project co-funded by BHP Olympic Dam and the Australian Research Council, student Kathryn Keane and I are experimentally testing the reaction of rare earth elements in the Olympic Dam tailings to different molten salt combinations.
Essentially, this involves putting the minerals back into a synthetic ‘magma chamber’, similar to the one that the deposit originally formed from. The main salt ingredients are sodium and potassium carbonates, and phosphates. These salts are widely available and relatively cheap. They are unhazardous and environmentally friendly; their industrial uses include food additives. If they’re safe enough to eat, there should be no issues using them for rare earth element extraction. The addition of phosphate solves the mineral diversity problem of the Olympic Dam tailings. Its abundance triggers reaction of all rare earth element–bearing mineral hosts to monazite. Importantly, the grain size of this new monazite is much larger than any of the pre-existing rare earth element minerals. Instead of being surrounded by difficult-to-handle gangue minerals such as quartz and hematite, this monazite now sits inside a matrix of soluble sodium and potassium salts. Liberating the monazite is as simple as rinsing the salt. The leftover solution can be evaporated to recover and recycle the salts. Recovered monazite can then potentially be sold as a commercial product. Preliminary work shows that this process happens effectively in the lab, and current research focuses on tweaking salt compositions, melting temperature and duration, and other parameters.
This project is part of a wider trend in recent years to improve and understand geometallurgy, and provide real-world applications to mineralogical knowledge. Much work has been done across Australian universities and government agencies such as CSIRO and ANSTO to leverage thorough knowledge of the mineral constituents of ore elements in a rock and their interrelations, to improve recovery during beneficiation. This is achieved using modern imaging techniques that allow automated mineral identification and mapping. Typically, this is done on scanning electron microscopes, but recent technological advances improve the speed and accuracy of other methods that require less sample preparation – such as microscale X-ray fluorescence, infrared reflectance, laser-induced breakdown spectroscopy, and more. Based on this data, innovative visualisation brings to life the spatial and mineralogical distribution of various critical elements in ore samples, facilitating optimal extraction techniques.
Photograph courtesy of Jamie Kidston, Australian National University.
