Leaching of brannerite in the ferric sulphate system — Part 1: Kinetics and reaction mechanisms

Abstract

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.

Leaching of brannerite in the ferric sulphate system. Part 2: Mineralogical transformations during leaching

“…Analyses of leaching kinetic data relating to this investigation reported in Gilligan and Nikoloski (2015b) showed that the rates of uranium and titanium dissolution were strongly dependent on the temperature, with the final uranium extraction ranging from 30% up to 99%. Similarly, titanium extraction ranged from 25% to 89%.…”

“…Each leaching experiment was run for 5 hours and the ferric concentration was kept constant at 2.8 g/L. For the details of leaching experiments the reader is referred to Gilligan and Nikoloski (2015b).…”

“…This becomes of particular interest in view of the latest knowledge on the different reaction mechanisms that take place in the acidic ferric sulphate system as a function of temperature and acid concentration. To provide this information, the brannerite sample and residues from the leaching experiments reported in Gilligan and Nikoloski (2015b) were subjected to further detailed characterisation by surface imaging and X-ray diffraction techniques. …”

“…In order to develop effective processes for the extraction of uranium from brannerite dominant ores, it is necessary to improve the understanding of the leaching reaction chemistry. A recent study of the leaching reaction chemistry of brannerite in ferric sulphate media (2.8 g/L Fe 3+ ) at different temperatures (25-96°C) and acid concentrations (10-200 g/L H 2 SO 4 ) (Gilligan and Nikoloski, 2015b) has shown that the dissolution process follows two different leaching reaction mechanisms. At lower temperatures, uranium is dissolved at higher rate than titanium initially, leaving titanium rich areas on the brannerite particles which suggest incongruent dissolution, while at higher temperatures, uranium and titanium dissolve at similar rates suggesting congruent dissolution.…”

“…Older specimens were more disordered and metamict, but even the most recently formed specimens obtained from Alpine pegmatites (10-20 million years old) were observed to be metamict. The age of brannerite from Sierra Albarrana in Spain which was studied in Gilligan and Nikoloski (2015b), was determined to be between 350 and 390 million years old, altered and metamict (Lumpkin et al, 2012).…”