Uranium Ores Processing · removed by various methods • Uranium repreciptated with ammonia •...
Transcript of Uranium Ores Processing · removed by various methods • Uranium repreciptated with ammonia •...
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Uranium Minerals
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Mining Methods
https://geoinfo.nmt.edu/resources/uranium/mining.html
• Open Pit • Underground • In-Situ Recovery or In-Situ Leaching
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Open Pit Uranium Mine
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Reclaimed Tailings Piles
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Uranium ore processing
• Drying • Crushing • Blending • Fine Ore Storage • Roasting
to remove carbonaceous matter to convert vanadates to soluble form to alter clay minerals
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Processing Methods
• Agitation Leaching (Selective Dissolution) – Acid leach – Basic Carbonate Leach
• Percolation Leach • Pressure Leach • Heap Leaching • In-Situ Leach
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Solubility of Uranium Minerals
• Hexavalent uranium oxides dissolve in acids UO3 + 2 H+ = UO2
++ + H2O
• Tetravalent Oxides require an oxidant 2Fe3+ + 2e = 2Fe++
UO2 = UO2++ + 2e
Overall reaction:
UO2 + 2Fe3+ = UO2++ + 2Fe++
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Stability domain of water
2H2O = O2(g) + 4H+ + 4e E = E0 + {RT/nF}ln{[O2][H+]4 / [H2O]2} E = 1.23 – 0.059 pH (b)
H2(g) = 2H+ +2e (a) E = 0.0 – 0.059 pH
H2O = H2O2 + 2H+ + 2e E = 1.77 – 0.059 pH + 0.0295 log (H2O2)
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Oxidation of Ferrous Ions
• 6Fe++ + NaClO3 = Fe3+ + NaCl + 3H2O • 2Fe++ + MnO2 + 4H+ = 2 Fe3+ + Mn++ + 2H2O
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Oxidizing Agents • H2O2 + 2 H+ + 2e = 2 H2O E° = 1.77 V • MnO4¯+ 8 H+ +5e = Mn2+ + 4 H2O E° = 1.52 V • ClO3¯ + 6 H+ + 5e = ½ Cl2 (g) + 3 H2O E° = 1.47 V • MnO2 (s) + 4 H+ + 2e = Mn2+ + 2 H2O E° = 1.23 V • O2 (g) + 4 H+ + 2e = 2 H2O E° = 1.23 V • NO3¯ + 4 H+ + 3e = NO (g) + 2 H2O E° = 0.96 V • ClO¯+ H2O + 2e = Cl¯ + 2 OH¯ E° = 0.89 V • Fe3+ + e = Fe2+ E° = 0.77 V • Cu2+ + 2e = Cu (s) E° = 0.34 V • 2 H+ + 2e = H2 (g) E° = 0.0 V • Fe2+ + 2e = Fe E° = -0.44
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• Acid Leaching – Low carbonate ores
UO3 + 2H+ = UO2++ + H2O
UO2 + 2Fe+++ = UO2++ + 2Fe++
Uranyl ion forms complexes with sulfate ions UO2
++ + SO4¯ ¯ = UO2 SO4
UO2 SO4 + SO4¯ ¯ = [UO2 (SO4)2]2¯
[UO2 (SO4)2]2¯ + SO4¯ ¯ = [UO2 (SO4)3]4¯
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Acid Consuming Reactions
• Carbonates • Sulfides • Phosphates • Vanadates • Phosphates • Some oxides • Clays
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Autoclave Leaching
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Pressure Oxidation of Pyritic Ores
• 2FeS2 + 7.5 O2 + H2O = Fe2(SO4)3 + H2SO4
• UO2 + Fe2(SO4)3 = UO2SO4 + 2FeSO4
• 4FeSO4 + 2H2SO4 + O2 = 2Fe2(SO4)3 + 2H2O
• CuFeS2 + 2H2SO4 + O2 = FeSO4 + CuSO4 + 2H2O
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Heap Leaching
• FeS2 + H2O + 3.5O2 = FeSO4 + H2SO4
• 2FeSO4 + H2SO4 + ½ O2 = Fe2(SO4)3 + H20 – Thiobacillus Ferrooxidants – Thiobacillus Thiooxidants – Ferrobascillus Ferrooxidants Bacteria oxidize Ferrous ions to Ferric and help
dissolve uraninite
UO2 + 2Fe3+ = UO2++ + 2Fe2+
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Abandoned Injection Well
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Recovery of Uranium from Acidic Solutions
• Solvent Extraction with both cationic (alkyl phosphoric acids) or anionic (tertiary amine) liquid ion exchangers:
• UO2++ + 2(RH)2(org) = UO2R4H2(org) + 2H+
• 4R3N(org) + 2H2SO4 + UO2(SO4)34- =
(R3NH)4UO2(SO4)3(org) + 2SO42-
• Uranium stripped with Na2CO3 solution and precipitated
as Na2U2O7
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Carbonate Leaching
• 2UO2 + O2(dissolved) = 2UO3 (or H2O2)
• UO3 + Na2CO3 + 2NaHCO3 = 4Na+ + UO2(CO3)3
4- + H2O
Sodium carbonate hydrolyzes in water to produce highly basic solutions. Bicarbonate added to neutralize excess OH¯ to prevent precipitation of sodium diuranate (Na2U2O7 - yellowcake).
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Ion Exchange Resins
• Both acidic or basic carbonate solutions
4RX + [UO2(SO4)3]4- = R4UO2(SO4)3 + 4X¯
4RX + [UO2(CO3)3]4- = R4UO2(CO3)3 + 4X¯
X : NO3¯>CN¯>HSO4¯>Cl¯>HCO3¯>OH¯
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Ion Exchange Resin
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Uranium Leaching, Counter Current Decantation, Solvent Extraction, Precipitation
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Uranium Precipitation and Purification
• Yellow cake (>70% U3O8) precipitated from carbonate solutions by MgO, NaOH or ammonia is dissolved
• Phosphates, arsenates, molybdates, vanadates etc. removed by various methods
• Uranium repreciptated with ammonia • Ammoniun diuranates decomposed • Oxides reduced by H2(g) to UO2
• UO2 converted to UF4 then UF6 (g) • U235 enriched from U238 by gaseous diffusion or
centrifuge
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Uranium Enrichment Centrifuges