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Lecture 6: Uranium Chemistry

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1 Lecture 6: Uranium Chemistry
From: Chemistry of actinides Nuclear properties U purification Free atom and ion property Metallic state Compounds Chemical bonding Structure and coordination chemistry Solution chemistry Organometallic and biochemistry Analytical Chemistry

2 Nuclear properties Fission properties of uranium
Defined importance of element and future investigations Identified by Hahn in 1937 200 MeV/fission 2.5 neutrons Natural isotopes 234,235,238U Ratios of isotopes established 234: ±0.001 235: ±0.001 238: ±0.002 233U from 232Th

3 Uranium Minerals 200 minerals contain uranium Bulk are U(VI) minerals
U(IV) as oxides, phosphates, silicates Classification based on polymerization of coordination polyhedra Mineral deposits based on major anion Secondary phases may be important for waste forms Incorporation of higher actinides Pyrochlore A1-2B2O6X0-1 A=Na, Ca, Mn, Fe2+, Sr,Sb, Cs, Ba, Ln, Bi, Th, U B= Ti, Nb, Ta U(V) may be present when synthesized under reducing conditions XANES spectroscopy Goes to B site

4 Polyhedra classification U(VI) minerals
Linkage over equatorial position Bipyramidal polyhedra Oxygens on uranyl forms peaks on pyramid Different bond lengths for axial and equatorial O coordinated to U Method for classification Remove anions not bound by 2 cations, not equatorial anion on bipyramid Associated cation removed Connect anions to form polyhedra Defines anion topology Chains defined by shapes P (pentagons), R (rhombs), H (hexagons), U (up arrowhead chain), D (down arrowhead chain)

5 Uranium purification from ores
Common steps Preconcentration of ore Based on density of ore Leaching to extract uranium into aqueous phase Calcination prior to leaching Removal of carbonaceous or sulfur compounds Destruction of hydrated species (clay minerals) Removal or uranium from aqueous phase Ion exchange Solvent extraction Precipitation Leaching with acid or alkaline solutions Acid solution methods Addition of acid provides best results Sulfuric or HCl (pH 1.5) U(VI) soluble in sulfuric Oxidizing conditions may be needed MnO2 , chlorate, O2, chlorine Generated in situ by bacteria High pressure oxidation of sulfur, sulfides, and Fe(II) sulfuric acid and Fe(III) Carbonate leaching Formation of soluble anionic carbonate species Somewhat specific for uranium Use of O2 as oxidant Bicarbonate prevents precipitation of Na2U2O7 OH-+HCO3-CO32- + H2O

6 Recovery of uranium from solutions
Ion exchange U(VI) anions in sulfate and carbonate solution UO2(CO3)34- UO2(SO4)34- Load onto anion exchange, elute with acid or NaCl Solvent extraction Continuous process Not well suited for carbonate solutions Extraction with alkyl phosphoric acid, secondary and tertiary alkylamines Chemistry similar to ion exchange conditions Chemical precipitation Older method Addition of base Peroxide Ultimate formation of (NH4)2U2O7 (ammonium diuranate), then heating to form U3O8 or UO3 Contaminates depend upon mineral V, Mo TBP extraction Based on formation of nitrate species UO2(NO3)x2-x + (2-x)NO3- + 2TBP UO2(NO3)2(TBP)2

7 Uranium atomic properties
Ground state electron configuration [Rn]5f36d17s2 Term symbol 5L6

8 cm-1


10 Metallic Uranium Three different phase a, b, g phases
Dominate at different temperatures Uranium is strongly electropositive Cannot be prepared through H2 reduction Metallic uranium preparation UF4 or UCl4 with Ca or Mg UO2 with Ca Electrodeposition from molten salt baths

11 Metallic Uranium phases
a-phase Room temperature to 942 K Orthorhombic U-U distance 2.80 Å Unique structure type b-phase Exists between 668 and 775 ºC Tetragonal unit cell g-phase Formed above 775 ºC bcc structure Metal has plastic character Gamma phase soft, difficult fabrication Beta phase brittle and hard Paramagnetic Temperature dependence of resistivity a‐phase U-U distances in layer (2.80±0.05) Å and between layers 3.26 Å b-phase

12 Resistivity–temperature curve for a-U along the [010] axis

13 Intermetallic compounds
Wide range of intermetallic compounds and solid solutions in alpha and beta uranium Hard and brittle transition metal compounds U6X, X=Mn, Fe, Co, Ni Noble metal compounds Ru, Rh, Pd Of interests for reprocessing Solid solutions with: Mo, Ti, Zr, Nb, and Pu

14 Uranium-Titanium Phase Diagram. Uranium-Aluminum Phase Diagram.

15 Chemical properties of uranium metal and alloys
Reacts with most elements on periodic table Corrosion by O2, air, water vapor, CO, CO2 Dissolves in HCl Also forms hydrated UO2 during dissolution Non-oxidizing acid results in slow dissolution Sulfuric, phosphoric, HF Exothermic reaction with powered U metal and nitric Dissolves in base with addition of peroxide peroxyuranates

16 Uranium compounds Uranium-hydrogen b-UH3 from H2 at 250 ºC
a-UH3 prepared at -80 ºC from H2 at 250

17 Uranium hydride compounds
Uranium borohydride UF4 + 2Al(BH4)3U(BH4)4 + 2Al(BH4)F2 U(BH)4 is tetragonal U(BH4)3 forms during U(BH4)4 synthesis Vapor pressure log p (mmHg) = T-1 UXAlHy compounds UXAl absorbs hydrogen upon heating X=Ni, Co, Mn y = 2.5 to 2.74 TGA analysis evaluates hydrogenation

18 Uranium-oxygen UO Solid UO unstable, NaCl structure
From UO2 heated with U metal Carbon promotes reaction, formation of UC UO2 Reduction of UO3 or U3O8 with H2 from 800 ºC to 1100 ºC CO, C, CH4, or C2H5OH can be used as reductants O2 presence responsible for UO2+x formation Large scale preparation UO4, (NH4)2U2O7, or (NH4)4UO2(CO3)3 Calcination in air at ºC H2 at ºC UO2has high surface area

19 Uranium-oxygen U4O9 UO2 and U3O8 5 UO2+ U3O8 2 U4O9
Placed in evacuated ampoule Heated to 1000 ºC for 2 weeks Three phases a-U4O9 up to 350 K b-U4O9 350 K to 850 K g-U4O9 above 850 K Rearrangement of U4+ and U5+ forces disordering of O U3O7 Prepared by oxidizing UO2 below 160 ºC 30 % of the oxygens change locations to new positions during oxidation b phase prepared by heating at 200