Investigations into the synthesis, characterisation and uranium extraction of the pyrochlore mineral betafite

The usage of nuclear power, to generate base-load electricity, has steadily increased in acceptance as a more environmentally friendly alternative to fossil fuels since its first commercial introduction in the 1950s. Increasing consumption of uranium and the rising demand for nuclear fuel has led to a deficiency of high grade uraninite and pitchblende deposits. This has led to increased attention on the refractory uranium pyrochlore betafite due to its abundance in currently exploited uranium deposits.<br><br>In this document primary work was conducted on natural betafite from Ambatofotsy Madagascar, Miarinarivo Madagascar and Silver Crater Mine, Canada. X-ray diffraction analysis showed that anatase was the only crystalline compound present in each of the unheated samples. After being heated to 1200 °C all samples exhibited diffraction lines characteristic of betafite and rutile, indicating that the betafite present in the preheated samples was amorphous (metamict). Electron probe microanalysis of the samples showed a high degree of compositional heterogeneity throughout each sample, most likely caused by aqueous fluid interactions forming secondary alteration products. All unheated samples were subject to a sulfuric acid leach over 6 h with the extent of uranium dissolution correlated with the degree of alteration. The most highly altered sample had 42% U dissolution while the least-altered exhibited only 7%.<br><br>Due to the differences in the degree of alteration observed in the natural samples it was decided to synthesise a pure betafite sample via a solid state synthesis route. The optimum conditions for the synthesis of betafite were found to be heating the reactants required at 1150 °C for 48 h under an inert gas atmosphere. XPS analysis of the sample showed that the uranium in the synthesised betafite was predominately present in the U5+ oxidation state. A minor amount of U6+ was also detected which was possibly due to surface oxidation.<br><br>Dissolution studies were then conducted on the synthetic betafite to investigate the influence of various parameters on leaching. The results showed the uranium dissolution curves obtained over a range of experimental conditions demonstrated three discrete segments representing significantly different rates of uranium dissolution. The first segment occurred in the initial minute and involved extremely rapid dissolution. This segment made up for the majority of the overall uranium dissolution obtained over the test period (~2.0% U). The second segment between 1 – 120 minutes was characterised by slow uranium dissolution rates which could be partially influenced by altering the experimental conditions. The uranium dissolution rate in the third segment was negligible due to passivation of the sample surface leaving no exposed uranium available for leaching.<br><br>Long term dissolution studies were also conducted on the synthetic betafite sample in order to determine the leach rate over a 90 day leach period. The results of these experiments showed very minor influences were observed when each of the standard parameters (temperature, [H2SO4], [FeTOT] and redox potential) were studied. The influence of fluoride addition showed a slightly greater leach rate than that achieved with high acid concentration, though the main difference between this experiment and other experiments conducted with no fluoride was the solubilisation of the metals Ti, Nb and Ta which were not observed to leach in experiments where fluoride was not added.<br><br>Post leach characterisation of synthetic betafite identified only minor changes occurred in the sample. These changes included the formation of pyrochlore and a greater concentration of rutile was observed to be present. Analysis via XPS of the post leach residues showed the samples contained similar U oxidation states to those observed in the preleached samples. Moreover, significantly greater Ta was observed via XPS in the post leach sample indicating that the Ta concentration had increased on the surface of the mineral. This indicated a possible passivation layer which caused the low rate of dissolution that was observed to occur.