Leaching of brannerite in the ferric chloride system

Squire

et al. 2019

Metals

In this study, synthetic zirconolite samples with a target composition Ca0.75Ce0.25ZrTi2O7, prepared using two different methods, were used to study the stability of zirconolite for nuclear waste immobilisation. Particular focus was on plutonium, with cerium used as a substitute. The testing of destabilisation was conducted under conditions previously applied to other highly refractory uranium minerals that have been considered for safe storage of nuclear waste, brannerite and betafite. Acid (HCl, H2SO4) leaching for up to 5 h and alkaline (NaHCO₃, Na2CO3) leaching for up to 24 h was done to enable comparison with brannerite leached under the same conditions. Ferric ion was added as an oxidant. Under these conditions, the synthetic zirconolite dissolved much slower than brannerite and betafite. While the most intense conditions were observed previously to result in near complete dissolution of brannerite in under 5 h, zirconolite was not observed to undergo significant attack over this timescale. Fine zirconolite dissolved faster than the coarse material, indicating that dissolution rate is related to surface area. This data and the long term stability of zirconolite indicate that it is a good material for long-term sequestration of radioisotopes. Besides its long term durability in the disposal environment, a wasteform for fissile material immobilisation must demonstrate proliferation resistance such that the fissile elements cannot be retrieved by leaching of the wasteform. This study, in conjunction with the previous studies on brannerite and betafite leaching, strongly indicates that the addition of depleted uranium to the wasteform, to avert long term criticality events, is detrimental to proliferation resistance. Given the demonstrated durability of zirconolite, long term criticality risks in the disposal environment seem a remote possibility, which supports its selection, above brannerite or betafite, as the optimal wasteform for the disposition of nuclear waste, including of surplus plutonium.

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Section: Introductionmentioning

confidence: 99%

Chemical Stability of Zirconolite for Proliferation Resistance under Conditions Typically Required for the Leaching of Highly Refractory Uranium Minerals

Squire

et al. 2019

Metals

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“…Brannerite dissolves under oxidizing conditions in the conventional acidic ferric sulphate system, releasing uranium into solution as uranyl sulphate complexes such as UO 2 (SO 4 ) 2 2-(Equation [1]) and forming secondary titanium oxide through the reversible hydrolysis of titanyl ions and complexes (Equation [2]) (Gilligan and Nikoloski, 2015b;Gogoleva, 2012;Smits, 1984). This process is driven by the presence of ferric ions in solution which oxidize uranium to the hexavalent state, sulphate ions which complex uranium, and acid which attacks the titanium oxide.…”

mentioning

confidence: 99%

“…This process is driven by the presence of ferric ions in solution which oxidize uranium to the hexavalent state, sulphate ions which complex uranium, and acid which attacks the titanium oxide. Gilligan and Nikoloski (2015b). At higher temperatures and under more strongly acidic conditions, the uranium and titanium in brannerite dissolve congruently through Equation [3] (Gilligan and Nikoloski, 2015b).…”

mentioning

confidence: 99%

“…Gilligan and Nikoloski (2015b). At higher temperatures and under more strongly acidic conditions, the uranium and titanium in brannerite dissolve congruently through Equation [3] (Gilligan and Nikoloski, 2015b).…”

mentioning

confidence: 99%

“…We performed all ferric sulphate leaching experiments using the procedure described by Gilligan and Nikoloski (2015b). Similar procedures were used for the leach tests with alternative lixiviants, with some minor changes, such as the substitution of FeCl 3 /HCl for Fe 2 (SO 4 ) 3 /H 2 SO 4 in the chloride leaching experiments, or the addition of 10 g/L CaF 2 or Ca 5 (PO 4 ) 3 F in the gangue interaction tests.…”

mentioning

confidence: 99%

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The process chemistry and mineralogy of brannerite leaching

J. South. Afr. Inst. Min. Metall.

2017

Brannerite (UTi 2 O 6 ) is present as a major uranium mineral phase in many uranium and rare earth element (REE) deposits around the world. It is the most important uranium mineral after uraninite and coffinite (Finch and Murakami, 1999). Brannerite is a refractory uranium mineral and dissolves slowly compared to other uranium minerals under typical processing conditions (Gilligan and Nikoloski, 2015a;Lottering et al., 2008).Brannerite is typically metamict, rendered amorphous by self-irradiation. A study of brannerite specimens from different locations and of varying ages showed that this process takes less than 10 million years (Lumpkin, Leung, and Ferenczy, 2012), much less than the age of most uranium ores. Altered and metamict brannerite is more reactive than crystalline brannerite. Recrystallizing brannerite by heating it in a furnace greatly reduces the rate of uranium extraction during leaching (Charalambous et al., 2014).Brannerite dissolves under oxidizing conditions in the conventional acidic ferric sulphate system, releasing uranium into solution as uranyl sulphate complexes such as UO 2 (SO 4 ) 2 2-(Equation [1]) and forming secondary titanium oxide through the reversible hydrolysis of titanyl ions and complexes (Equation [2]) (Gilligan and Nikoloski, 2015b;Gogoleva, 2012;Smits, 1984). This process is driven by the presence of ferric ions in solution which oxidize uranium to the hexavalent state, sulphate ions which complex uranium, and acid which attacks the titanium oxide. The process chemistry and mineralogy of brannerite leaching by R. Gilligan*, and A.N. Nikoloski* Brannerite (UTi 2 O 6 ) is the most important uranium mineral after uraninite and coffinite, and the most common refractory uranium mineral. As the more easily leachable uranium ores are becoming exhausted, it is necessary to process the complex and refractory ores in order to meet the growing demand for uranium as an energy source. This typically requires either more intense leaching conditions or a better-designed process based on sound understanding of feed mineralogy and reaction chemistry. The present study was carried out to provide information that will enable the development of a more effective processing strategy for the extraction of uranium from ores containing brannerite. The leaching behaviour of brannerite in sulphate media under moderate temperature conditions was investigated and compared with its relative leachability in alternative acid and alkaline systems. The feed and the leached residues were characterized by XRD and SEM-EDX techniques. Brannerite dissolutions of up to 95% after 5 hours of leaching in ferric sulphate media, up to 89% in ferric chloride media under similar conditions, and up to 82% in 24 hours in sodium carbonate media were obtained. Since alkaline leaching was considered promising for acid-consuming ores, leaching was repeated with a high-carbonate brannerite-bearing ore, with comparable extractions. Mineralogical characterization showed that altered and amorphous regions are a regular feature o...

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The Effects of Phosphate Gangue on the Leaching of Uranium from Brannerite

Mining, Metallurgy & Exploration

2021