Synthesis, characterisation and dissolution studies of the uranium mineral coffinite
Creators: name: Reynolds, Hailey
Publisher: name: RMIT University
Year: 2024

description: To meet potential increases in demand for uranium, lower grade and/or “difficult to process uranium minerals” will need to be mined and processed. To efficiently process the aforementioned a greater understanding of the chemistry involved is essential. The chemistry involved in the processing of the uranium bearing mineral coffinite is of particular interest as this mineral is found in both low grade ores and is considered difficult to process. This thesis presents detailed studies on the composition, structure and dissolution chemistry of natural coffinte and studies on the synthesis and characterisation of synthetic coffinite. <br><br>Two natural uranium bearing samples were analysed and were found to contain non stoichiometric coffinite which varied in the uranium:silicon ratio across the coffinite grain. <br><br> Synthetic coffinite was successfully prepared when 10mL of 5mmol UCl<sub>4(aq)</sub> was added to 25mL of equimolar Na<sub>2</sub>SiO<sub>3</sub> and NaOH was added until gelling occurred. The solution was buffered and placed into a sealed in a Teflon lined autoclave vessel, which was heated to 250°C for 24Hr. The synthetic coffinite produced was consistent with other reported anyalysis of synthetic coffinite and the patchy, polycrystalline aggregates with heterogeneous BSE contrast was similar to that of natural coffinites. The unit cell parameters were calculated (a=b 6.99795±0.0002 Å and c 6.27197±0.0002 Å) and the coffinite found to contain between 1-1.5moles of water in a number of different varieties. As with the natural coffinites, the amount of uranium and silicon varied throughout the sample (62.72–65.44wt% and 8.93–11.33wt% respectively), the overall U:Si was 0.73, although low compared to the theoretical value (1.0), it was higher than that reported previously for synthetic coffinite (0.6)<br><br>The dissolution studies carried out on the prepared synthetic coffinite (similar to natural coffinite samples) resulted in trends that were different than what has been observed for the most common uranium mineral, uraninite. <br>Temperature was found to have a significant effect at or above 80°C. The activation energy was calculated as 64.6kJ/mol and an Arrhenius parameter of 220. Increasing the concentration of sulfuric acid resulted in an unusual increase and subsequent decrease in the extent of uranium dissolution, with maximum uranium dissolution occurring after 90 minutes when 1.5g/L of sulphuric acid was used. Increasing the [Fe]Total had little effect on the extend or rate of uranium dissolution from coffinite when the Fe<sup>2+</sup>:Fe<sup>3+</sup> ratio is maintained, however it is highly dependent on the solution ORP. Increasing the solution ORP resulted in an increase in the dissolution of uranium from coffinite. Fe<sup>2+</sup> was found to have a significant effect on the uranium dissolution, where even trace amounts of Fe<sup>2+</sup> present in the system resulted in a significant depression of the amount of uranium dissolved. The Fe<sup>2+</sup> effect was greater in systems containing the lowest amounts of Fe3+ . Under relatively mild conditions (1g/L of Fe<sup>3+</sup>; 3 g/L of Fe<sup>2+</sup>; 15 g/L of sulphuric acid and at 50 °C) 100 % uranium dissolution from can be achieved between 36 and 48 hours.<br>