Title : Supercritical fluid extraction of uranium
Abstract:
Supercritical Fluid Extraction (SFE) employing CO2 is Generally Regarded As Safe (GRAS). SFE has gained tremendous popularity owing to its inherent potential to minimize liquid waste generation, which is of paramount significance in the nuclear industry. In this talk I am going to summarise the research work carried out at Bhabha Atomic Research Centre, Mumbai, India on SFE of uranium. Supercritical Fluid (SCF) is the phase of a substance above critical point, where solid and liquid merge into a single one. SCF possesses hybrid properties of both liquid as well as gas. Liquid like solvation and gas like diffusivity enable SCF to penetrate deep inside the solid matrix causing extraction. For metal SFE, due to the charge neutralization requirement and no-polar nature of CO2, metal ion needs to be complex with a suitable organic compound, which is soluble in SC CO2. On depressurisation, CO2 escapes as gas; recyclable on industrial scale. In SFE of uranium from nitric acid medium using tributyl phsophate (TBP), operating parameters affection extraction were identified and optimised. Ligand solubility and stability in SCF, extent of metal-complex formation, solubility of metal-complex in SCF, pH, matrix type, extraction time, static /dynamic extraction mode etc., were found to affect extraction efficiency. Crown Ethers due to size selective cation binding nature were investigated, HPFOA (Pentdedecfluoro-n-octanoic acid ) for counter ion production and the observed extraction efficiency trend was Ditertbutldicyclohexano-18-crown-6> Ditertbutldibenzo-18-crown-6> dicyclohexano-18-crown-6> Dibenzo-18-crown-6. The electron donating/withdrawing group affects the basicity of oxygen atoms, influencing binding with metal ions. For same substituent, extraction efficiency trend was Dibenzo-24-crown-8> Dibenzo- 21-crown-7> Dibenzo-18-crown-6, increase with ring size. SFE of uranium from U3O8 powder and UO2 powder, granules, green pellets and sintered pellets was performed by single medium TBP–HNO3 dissolution as well as complexation. With SC CO2 alone, uranium extraction efficiency was ~ 70%, which increased to ~90% with 2.5% TBP in SC-CO2 stream. Nearly complete uranium extraction (~99%) was observed with 20% TBP, also with 2.5% TTA in methanol. For extraction and purification of uranium, from crude sodium diuranate (SDU) ~ 95% extraction efficiency was observed. From rock phosphate ores containing lean uranium content (~50-100 ppm), adducts of trialkyl phosphate (TBP, TiAP and TEHP) were investigated. From yellow cake, uranium extraction efficiency >90% was achievable. N,N-Dialkyl aliphatic amides such as DBEHA, DBDMBA, DBOA were studied for uranium extraction from nitric acid medium and tissue paper matrix. Straight chain DBOA was found suitable for uranium extraction whereas branched chain amide DBEHA was suitable for separation of uranium and thorium.
