Brannerite, a new uranium mineral

This paper provides a review and synthesis of research literature on the geological, mineralogical and geochemical data to uranyl mineralization of the Arabian Nubian Shield (ANS). In general, the studied uranyl mineralization from the different locations of the ANS is similar from the mineralogical point of view. The mineral assemblages include, not conclusively, uranophane, kasolite, soddyite, boltwoodite, autunite, meta-autunite, liandratite, uranopyrochlore, ishikawaite and fergusonite. The compiled dataset supports a model of two main stages, namely hypogene or high-T alteration stage and supergene or low-T alteration stage. Both stages played an important role in the genesis of uranyl mineralization of the ANS. The late hydrothermal fluids leached uranium from the deep part of the host rocks and transported it in different uranous and uranyl complexes. While moving upward, the fluids gradually cooled to the temperature of meteoric water and probably have mixed with it. At these conditions, the pH of the fluids changes to be more alkaline and uranyl minerals start to develop in the fracture system mainly adsorbed on the clay minerals and probably coprecipitated with iron oxy-hydroxides. The requirement for supergene or low-T alteration stage would be saturation by oxic groundwater, a case implying the availability of meteoric water during the formation of uranyl mineralization in the ANS. Some of these minerals were dated by U-series method as 50–159 ka, and this could be attributed mainly to the Saharan II pluvial period.

The actinide-containing nuclear waste has an extremely long half-life and a large specific activity, whose long-term management via immobilization and, ultimately, geological disposal is essential for the sustainable development of nuclear energy. The immobilization of radioactive nuclear waste within a durable matrix is of particular importance to achieve the safe management of the nuclear cycle. A key aspect for nuclear waste management is the design, manufacture, and performance assessment of ceramics for the immobilization of actinide-based high-level wastes. In this chapter, the basic principle of incorporating actinides within candidate single-phase ceramic hosts is explored for pyrochlore, zirconolite, perovskite, brannerite, and zircon. We provide detailed chemical insight into the processing of the candidate ceramic hosts with a particular focus on actinide incorporation, crystal structure, waste loading, radiation tolerance, and chemical durability. The candidate ceramics are reviewed alongside their equivalent natural minerals, known to retain radioactive elements over geological timescales, which provide vital information on the wasteform longevity with respect to nuclear waste management applications.

Brannerite, UTi₂O₆, is the most common refractory uranium mineral and is the most important uranium ore mineral after uraninite and coffinite. In order to develop an effective treatment method for the hydrometallurgical extraction of uranium from ores containing brannerite, it is necessary to understand the chemistry of the leaching process. Part 1 of this series described the results of a study of the chemical reaction mechanisms of brannerite under conditions similar to those used industrially. In this paper, the mineralogical data obtained from samples collected during the leaching work is used to derive further insight into the transformations that take place during leaching. Detailed characterisation of the brannerite feed specimen and leach residues was carried out using surface imaging and X-ray diffraction techniques. It was shown that the brannerite specimen is heterogeneous, consisting of at least two phases. The brannerite phase was metamict and showed signs of natural alteration to fine-grained (10–20 nm) crystalline anatase. Comparisons between the feed and residues showed that the X-ray amorphous materials, in particular lead and silicon enriched areas identified near the anatase were most susceptible to leaching while the crystalline material was relatively resistant to leaching. These results demonstrate that the extent of brannerite alteration and its texture are important considerations to the hydrometallurgical behaviour of a particular ore along with the typical concerns such as grade, liberation size and gangue mineralogy.

The uranium titanate mineral brannerite, UTi₂O₆ is the most common of the uranium minerals which is considered refractory. Ore containing brannerite mineralisation has been mined and processed in several locations around the world. Under typical uranium ore processing conditions, brannerite is often lost to tailings. In order to design an effective process for the leaching of high-brannerite uranium ores, it is first necessary to understand the mechanism of the chemical processes through which brannerite dissolves in the absence of interferences from the host rock. In the present study, a specimen of brannerite obtained as a single crystal was leached in sulphuric acid (10–200 g/L) and ferric sulphate (2.8 g/L Fe^3 +) solution at 25–96 °C for 5 h. The rate of titanium dissolution was monitored along with uranium. Comparisons between the rates at which these two elements dissolved and the morphological changes that were observed to take place during the dissolution process indicated two different sets of leaching reaction mechanisms. At low temperatures, uranium dissolved at a much higher rate than titanium initially, leaving titanium rich areas on the brannerite particles similar to observations reported in earlier investigations which suggest incongruent dissolution. The calculated activation energies for uranium and titanium dissolution were 36 and 48 kJ/mol respectively. At higher temperatures, uranium and titanium dissolved at similar rates in constant proportions suggesting congruent dissolution. The calculated activation energy for this reaction was 23 kJ/mol. The transition between incongruent and congruent dissolution took place at lower temperatures when the acid concentration was higher. Titanium appeared to undergo hydrolysis after dissolution, forming anatase. This side reaction was most favourable at lower acid concentrations and high temperatures.

Uranium leaching tests were conducted on two naturally occurring, highly metamict brannerite ores from the Crockers Well and Roxby Downs deposits, South Australia. The ores were leached over a range of temperatures and Fe^(III) and H₂SO₄ concentrations. As well, samples of the ores were calcined at 1200 °C in air to investigate the effect of thermally induced recrystallisation on uranium dissolution. For the unheated samples, a maximum of ∼80% U dissolution was obtained using an Fe^(III) concentration of 12 g/L, an acid concentration of 150 g/L H₂SO₄ and a temperature of 95 °C. The heat treated samples performed poorly under identical conditions, with maximum uranium dissolution of <10% recorded. High uranium dissolution from natural brannerite can be achieved providing; (i) acid strength, oxidant strength and temperatures are maintained at elevated levels (compared to those traditionally used for uraninite leaching), and, (ii) the brannerite has not undergone any significant recrystallisation (e.g. through metamorphism).

Australian uranium ores are often composed of complex mineral assemblages. Differences in ore compositions and textures are seen between deposits as well as within a single deposit, which can host a range of ore types. Such a wide variety of uranium ores make it impossible for a single extraction or treatment process to be developed that will accommodate all of the ores. From a mineralogical perspective, key issues confronting the Australian uranium mining industry include: the prevalence of low grade ores; a lack of detailed chemical and mineralogical information (uranium speciation, texture, grainsize) for the various ore deposit types; and the presence of refractory uranium-bearing minerals and highly acid-consuming gangue minerals. This paper reviews some of the main controls on uranium geometallurgy by linking concepts relating to ore genesis and the resulting ore mineralogy, with the processing behaviour of specific Australian uranium ore types. Emphasis is placed on the value of detailed ore mineralogical analysis and the insight this provides into the factors of importance when considering uranium extraction.