Lay Summary

of my thesis:

Crystallisation pathways of REE carbonates and the hydrothermal wall-rock mineralisation of economic REE fluorcarbonate and REE phosphate minerals

Rare earth elements (REEs) are a series of 15 metals that are prized for their unique chemical and physical properties - properties that make them indispensable to many modern-day items such as batteries, digital screens, wind turbines or catalytic converters just to name a few. Despite the name, rare earths are not particularly rare in terms of abundance on earth. They are more abundant than precious metals such as gold or silver, and some REEs even match copper or lead abundances. What makes them so valuable is that, unlike copper or gold, REEs rarely occur together in a clump or confined area. REE atoms are relatively plentiful on earth but seldom occur in a consolidated area.

There are of course exceptions and REE concentrations in natural environments can be increased by various naturally occurring processes. A commonly suggested mechanism to enrich an environment with REEs involves a fluid carrying REEs and fluoride coming into contact with a rock that is rich in carbonate and/or phosphate. Upon contact, the fluid dissolves parts of the rock, releasing the carbonate and/or phosphate which in turn binds with the REE to form REE fluorcarbonate and/or REE phosphate minerals (these minerals are the ones most commonly targeted in mining operations). As this REE-rich fluid is near-impossible to collect and many of these mineralisation events happened millions of years ago, the next best thing involves collecting the REE-rich minerals that formed from these fluids. Some of the world’s largest REE deposits are thought to be created from this process.

To investigate this mineralisation model practically, we created an experimental environment approximating this natural process. We explored how the chemistry of the REE-bearing fluid as well as the composition of the rock affected the REE mineralisation. Other parameters such as temperature, experiment duration, reactant quantities etc. were also varied to study their effects. Experiments using similar fluids and rock compositions as natural environments resulted in a suite of minerals typically observed in nature. These experimental results validate many of the proposed mineralisation models. In addition to the experiments using natural fluid compositions, some experiments performed with unnatural REE distributions also yielded these economic REE minerals. This uncovered the ability of these minerals to form under conditions beyond those currently noted in nature and their resilience to less-than-ideal mineralisation environments.

To complement the knowledge of the overall mineralisation controls, there also needs to be an understanding of the intrinsic crystallisation mechanisms of REE carbonates. REE minerals tend to naturally incorporate multiple REEs. Isolating and purifying the 15 individual REEs are key challenges in commercial REE mineral processing. A important component to fine tuning this process lies in the improved understanding of how REEs crystallise. Many bodies of work have studied the crystallisation of single-REE carbonates. However, the crystallisation process of multi-REE carbonates has not been studied before. Given that natural REE carbonate minerals contain multiple REEs, we deemed it important to understand how (or indeed if) the crystallisation of multi-REE carbonates differed from that of the pure single REE carbonates reported in the scientific literature. Results showed that the presence of multiple REEs in the crystallisation environment altered the crystallisation timings and end-products, compared to the same experiments with single REEs. From these differences, we refined previously proposed models of REE carbonate crystallisation pathways.

Full thesis here