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National Analytical Management Program (NAMP) U.S. Department of Energy Carlsbad Field Office Radiochemistry Webinars Actinide Chemistry Series • Uranium Chemistry – General Properties of Uranium In Cooperation with our University Partners Meet the Presenter… Dr. Mikael Nilsson Dr. Mikael Nilsson is an assistant professor in the Department of Chemical Engineering and Materials Science at the University of California, Irvine. Before 2009, Dr. Nilsson was a post-doctoral research associate at Washington State University. He graduated with an M.S. in chemical engineering in 2000, and with a Ph.D. in nuclear chemistry in December 2005 from Chalmers University of Technology, Gothenburg, Sweden. During his M.S. and Ph.D. studies, Dr. Nilsson was one of the Swedish contributors to the European projects for partitioning of spent nuclear fuel, PARTNEW and EUROPART. He was also a member of the Swedish reference group on partitioning and transmutation research during this time . In 2004, he was a visiting scientist at the research center in Jülich, Germany under the supervision of Dr. Giuseppe Modolo. Between September and November 2008, Dr. Nilsson was a visiting scientist at Pacific Northwest National Laboratory. In October 2010, he became a certified senior reactor operator for the UC Irvine TRIGA nuclear reactor. At UC Irvine, Dr. Nilsson teaches courses in Advanced Chemical Engineering Thermodynamics, Chemical Engineering Unit Operations (laboratory course), The Nuclear Fuel Cycle, Introduction to Nuclear Chemical Engineering, and Radioisotope Techniques, and is involved in summer programs for nuclear reactor operators. His research interests include actinide chemistry, solvent extraction fundamental chemistry and process development, extraction and detection equipment development, radiolysis and phase composition of organic solvents. Dr. Nilsson’s research group currently comprises six graduate student researchers and four undergraduate students. He is active in the Industrial and Engineering Chemistry Division of the American Chemical Society. Dr. Nilsson is author of over 20 papers in peer-reviewed journals and proceedings, ten publicly-released reports, several papers in conference proceedings, and numerous oral conference presentations. Tel: 949-824-2800 E-mail: [email protected] Uranium Chemistry – General Properties of Uranium Dr. Mikael Nilsson National Analytical Management Program (NAMP) U.S. Department of Energy Carlsbad Field Office TRAINING AND EDUCATION SUBCOMMITTEE 4 Outline • • • • • • • History The element uranium The uranium ion Natural occurrence and minerals Complexes with uranium Solution behavior Recovery and purification of uranium 4 5 Definitions • Ions: – Ions are charged atoms, molecules or complexes. They are either negative (anions), UO2(CO3)34-, or positive (cations), U4+. • Minerals: – Minerals are naturally occurring inorganic materials that usually have a defined chemical composition and structure. They are formed in nature usually over long time periods, and the surrounding chemical environment governs what mineral is formed. Examples: graphite, diamond, quartz, pyrite (fool’s gold). 5 6 Definitions (cont.) • Chemical Reactions : – Chemicals (reactants) react together to form other chemicals (products). The reaction can be irreversible, shown by a single arrow, often for redox reactions: X+Y Z+W – Or reversible, shown by a double arrow, for chemical equilibria: X+Y Z+W • Chemical complexes : – Stable complexes formed by a chemical reaction. The complexes can bind either covalently (strong, permanent) or electrostatically (weaker). – Large molecules often form ring structures when binding to metal ions, so called chelates (from Greek for claw). 6 7 History • 1781, The planet Uranus was discovered. • 1789, Klaproth dissolves pechblende (pitchblende) in nitric acid. After addition of potash (K2CO3), a yellow precipitate was formed containing a “new” element, uranium. • 1841, Eugene-Melchior Pelgiot showed that Klaproth’s uranium was really UO2. The same year, Pelgiot prepared metallic uranium by heating UCl4 with potassium metal (reducing U(IV) to U(0)). 8 History (cont.) • 1896, Becquerel discovers that natural uranium emits radiation. Radioactivity is discovered. • 1898, the Curies discover polonium by isolating it from natural uranium. • 1935, Dempster discovers a lighter isotope in natural uranium; 235U is found by mass spectroscopy. • 1938, Hahn, Meitner and Strassmann discover nuclear fission. 9 Bucholz, C.F.: Neues allgem. J. der Chemie, 4, (1805), 157. Beiträge zur chemischen Kenntnis des Urans Verhalten des Schwfeläthers zu einer gesättigten Auflösung des salpetersauren Urans und des Wassers zu dem uranhaltigen Schwefeläther. Acht un funfzigster und neun un funfzigster Versuch: Zu einem Theil einer mässig concentrirten Auflösung des salpetersauren Urans in Wasser (zwei Theile Wasser, ein Theil Salz) wurden nach und nach sechs Theile Schwefeläthers gegossen und geschüttelt. Die Auflösung schien bei Hinzusetzung der erstern drei Theile nach dem Umschütteln jedes Mahl blässer zu sein, die folgenden drei Theile aber schwächten die Farbe der Unten stehenden wässrigen Auflösung gar nicht weiter, und es schien jetztein Gleichgewicht zwischen der Anziehung des Wassers und des Aethers zu dem aufgelösten Salze eingetreten zu sein. Jetzt wurde ein Theil der obigen Auflösung des Salzes in Schwefeläther nach und nach mir vier Theilen Wasser geschüttelt, wodurch solcher immer mehr und zuletzt gänzlich entfärbt erschien. Aus dieser Versuchen ergibt sich, dass die wechselseitige Anziehung des Äthers und Wassers zum salpetersauren Uran von ihren relativen mengen abhängt, und dass der Äther um so weniger davon der wässrigen Auflösung entzieht, je verdünnter diese ist. Übringens ist es offenbar, das letzteres überhaupt nur geschehen könne, wenn das trockne Salz in Äther auflöst. 10 Contributions for the chemical knowledge of the uranium behavior in sulfur ether in contact with a saturated solution of uranium nitrate in water. The 58th and 59th experiment: To one part of a moderately concentrated solution of uranium nitrate in water (two parts water, one part salt) gradually six parts of sulfur ether were added and the two phases contacted by shaking. The ether seemed to be more colorful with the first three additions after contact; the following three additions, however, did not weaken the color of the (lower) aqueous phase at all and it seemed that an equilibrium was established for the dissolved salt between the water and the ether. Now one part of the upper organic phase containing the salt was gradually contacted with four parts of water, whereby the organic phase was gradually decolorized and finally completely colorless. From these experiments the results indicate that the affinity of uranium nitrate to water or ether depends on their relative volumes, and that the ether extracts uranium nitrate from the aqueous phase less the more diluted the aqueous phase is. It is also obvious that the extraction can only take place if the dry salt is soluble in ether. 11 Contributions for the chemical knowledge of the uranium behavior in sulfur ether in contact with a saturated solution of uranium nitrate in water. The 58th and 59th experiment: To one part of a moderately concentrated solution of uranium nitrate in water (two parts water, one part salt) gradually six parts of sulfur ether were added and the two phases contacted by shaking. The ether seemed to be more colorful with the first three additions after contact; the following three additions, however, did not weaken the color of the (lower) aqueous phase at all and it seemed that an equilibrium was established for the dissolved salt between the water and the ether. Now one part of the upper organic phase containing the salt was gradually contacted with four parts of water, whereby the organic phase was gradually decolorized and finally completely colorless. From these experiments the results indicate that the affinity of uranium nitrate to water or ether depends on their relative volumes, and that the ether extracts uranium nitrate from the aqueous phase less the more diluted the aqueous phase is. It is also obvious that the extraction can only take place if the dry salt is soluble in ether. 12 Contributions for the chemical knowledge of the uranium behavior in sulfur ether in contact with a saturated solution of uranium nitrate in water. The 58th and 59th experiment: To one part of a moderately concentrated solution of uranium nitrate in water (two parts water, one part salt) gradually six parts of sulfur ether were added and the two phases contacted by shaking. The ether seemed to be more colorful with the first three additions after contact; the following three additions, however, did not weaken the color of the (lower) aqueous phase at all and it seemed that an equilibrium was established for the dissolved salt between the water and the ether. Now one part of the upper organic phase containing the salt was gradually contacted with four parts of water, whereby the organic phase was gradually decolorized and finally completely colorless. From these experiments the results indicate that the affinity of uranium nitrate to water or ether depends on their relative volumes, and that the ether extracts uranium nitrate from the aqueous phase less the more diluted the aqueous phase is. It is also obvious that the extraction can only take place if the dry salt is soluble in ether. 13 Contributions for the chemical knowledge of the uranium behavior in sulfur ether in contact with a saturated solution of uranium nitrate in water. The 58th and 59th experiment: To one part of a moderately concentrated solution of uranium nitrate in water (two parts water, one part salt) gradually six parts of sulfur ether were added and the two phases contacted by shaking. The ether seemed to be more colorful with the first three additions after contact; the following three additions, however, did not weaken the color of the (lower) aqueous phase at all and it seemed that an equilibrium was established for the dissolved salt between the water and the ether. Now one part of the upper organic phase containing the salt was gradually contacted with four parts of water, whereby the organic phase was gradually decolorized and finally completely colorless. From these experiments the results indicate that the affinity of uranium nitrate to water or ether depends on their relative volumes, and that the ether extracts uranium nitrate from the aqueous phase less the more diluted the aqueous phase is. It is also obvious that the extraction can only take place if the dry salt is soluble in ether. 14 Contributions for the chemical knowledge of the uranium behavior in sulfur ether in contact with a saturated solution of uranium nitrate in water. The 58th and 59th experiment: To one part of a moderately concentrated solution of uranium nitrate in water (two parts water, one part salt) gradually six parts of sulfur ether were added and the two phases contacted by shaking. The ether seemed to be more colorful with the first three additions after contact; the following three additions, however, did not weaken the color of the (lower) aqueous phase at all and it seemed that an equilibrium was established for the dissolved salt between the water and the ether. Now one part of the upper organic phase containing the salt was gradually contacted with four parts of water, whereby the organic phase was gradually decolorized and finally completely colorless. From these experiments the results indicate that the affinity of uranium nitrate to water or ether depends on their relative volumes, and that the ether extracts uranium nitrate from the aqueous phase less the more diluted the aqueous phase is. It is also obvious that the extraction can only take place if the dry salt is soluble in ether. 15 Significance of U-extraction • Over 200 years ago, Bucholtz was able to show that: – Ether extracts uranyl nitrate by itself. – An equilibrium will establish between uranyl nitrate in the water (aqueous) phase and the ether (organic) phase. – The extraction is concentration-dependant. – One can strip the uranium from the ether using water. • Almost 150 years later, this would be used again for large production of pure U. 16 The Element Uranium •Uranium is a dense metal; its appearance resembles that of steel. •It is by far the most studied element in the actinide series. 16 17 The Element Uranium (cont.) • Uranium is the heaviest naturally occurring element. (Pu may be found in nature, but is from fallout and other man-made sources.) U Atomic Number: 92 Atomic Weight: 238.03 u Boiling Point: 3818 C Melting Point: 1132 C Density: 19.07 g/cm3 17 18 Uses of Uranium • • • • • As a fuel for nuclear power reactors As a counter weight (ballast), much like lead As radiation shielding In armor-piercing projectiles For coloring glass (vaseline glass) http://www.orau.org/ptp/collection/consumer%20products/vaseline.htm 18 19 Important Isotopes of U Natural Abundance (%) Mass Number Interval Half-life 234 0.0059 – 0.0050 2.48*105 years 235 0.7202 – 0.7198 7.13*108 years 238 99.2752 – 99.2739 4.5*109 years 238U is the mother in the 4n+2 decay series (238U→…→206Pb) 235U is the mother in the 4n+3 decay series (235U→…→207Pb) 234U is obtained by decay of 238U in the 4n+2 decay series (238U →234Th + α → 234Pa + β- → 234U + β- → …→206Pb) 19 20 Nuclear Reactions with U • Can fission by irradiation of thermal neutrons yielding 2*107 kWh/kg 235U. • Irradiation of uranium results in important nuclides (99Mo, 239Pu, 137Cs, etc.). • Nuclear reactions: • 239Pu + n →FP + energy +2.5n 238U + n → 239U 239U → 239Np → 239Pu 235U can fission by thermal neutrons (MOX fuel). 20 21 Isotopic Separation • Used to produce material of higher concentration of 235U. • A sample deviating from natural ratios of isotopic compositions suggests anthropogenic sources. – Enriched uranium, higher 235U/238U ratio – Depleted uranium, lower 235U/238U ratio – Samples containing 236U or 233U suggest neutron irradiated material: 235U + n → 236U 232Th + n → 233Th → 233Pa + β- → 233U + β21 22 The Uranium Ions • U3+ is a very strong reducing agent that is oxidized by water (reduces water to hydrogen). Hence, no stable complexes can be maintained in aqueous solution. • Aqueous solutions containing U4+ are stable in the absence of oxidation agents like dissolved oxygen. Complexes with tetravalent uranium have some similarity with those of zirconium and thorium, except for being prone to oxidation. • The only known uranium ion in the +5 oxidation state in aqueous solution is the linear dioxouranium (V) ion, UO2+. This will disproportionate in water. • Most stable is +6: UO22+. 22 23 Uranium as Water Color • U3+ : Red (pink) • U4+ : Green (olive) • UO2+ : Colorless • UO22+ : Yellow http://www.chemie-master.de/ NOTE: Indoor lighting sometimes distorts the color of actinide and lanthanide solutions. It is important to use incandescent lighting to observe the correct colors. 23 24 Redox • The redox pairs of U4+/U3+ and UO22+/UO2+ can be rapidly altered. • The redox pairs of UO2+/U4+ and UO22+/U4+ are much slower reactions and require the oxygen bonds to be broken/formed. • The odd oxidation number (V) is the most difficult oxidation state to stabilize in solution. Some stability has been observed around pH 2-2.5. • Disproportionation will turn U(V) to U(IV) and U(VI), may be accelerated by radiation. 2UO2+ + 4H+ UO22+ + U4+ + 2H2O 24 25 Uranium Occurrence in Nature • Most important oxidation states: 4+ and 6+. • Compounds with U4+ are insoluble in mild acidic to alkaline conditions. • Compounds with linear uranyl (O=U=O)2+ are highly soluble and mobile. • In solution, UO22+ forms soluble complexes with carbonate, oxalate, hydroxide. • UO22+ is susceptible to adsorption by organic matter, Fe-hydroxides or by precipitation with hydroxide, silicate, vanadate, arsenate, phosphate. 25 26 Uranium Occurrence in Nature (cont.) • In groundwater systems U(VI) is reduced to U(IV) if an effective reductant is present (H2S). • Uranium minerals display an extraordinary wide structure and chemical variability, resulting from the different chemical conditions of U-mineral formation. • There are about 200 minerals that contain uranium as an essential component. • The U(VI) minerals constitute the largest portion. • Understanding of uranium mineralogy gives insight into the possible mechanism for uranium and radionuclide retardation following the weathering of nuclear waste materials. Dark uranium minerals form a roll-front deposit in Colorado 26 27 Uranium Minerals • Uranium is present in nature in many different minerals, divided into two separate classes: – Primary minerals: • Uraninite, pechblende, etc. • The composition varies from UO2 to UO2.67. • Often U(III,IV), black or dark minerals. – Secondary minerals: • Produced by hydration or oxidation of the primary. The new mineral is often transported away from the source mineral and redeposited. • Often U(VI), light yellow-green or red. 27 28 Uranium Minerals (cont.) • Primary minerals, uraninite (pechblende) Uraninite in coaly wood http://webmineral.com Uraninite encased in gold http://www.mineralatlas.com 28 29 Uranium Minerals (cont.) • Secondary minerals, uranophane, torbernite, etc. Uranophane (CaO*2UO3*2SiO2*6H2O) deposited on calcite http://www.mineralatlas.com Torbernite (Cu(UO2)2(PO4)2*8-12H2O) deposited on quartz 29 30 Radiation from Uranium in Nature 30 Nuclear Chemical Engineering, 2nd ed. 31 Uranium Metal • Uranium is strongly electropositive, and uranium and its compounds are difficult to reduce. • At elevated temperatures, uranium reacts with most common metals, such as iron. Any crucible used for molten uranium must be lined with CaO or other inert material. • Finely divided uranium metal reacts at room temperature with all components in air (save for noble gases), i.e., it may combust in air. • To make metal, halide complexes of uranium are often used since they are easier to reduce and have a low melting point. 32 Uranium Metal (cont.) • Uranium tetrafluoride is the common starting material for U-metal production. It is a green salt stable at room temperature. UCl4 can be used, but is less stable and more difficult to use. • Magnesium metal is often used to reduce the uranium from U(IV) to U(0). This is done in a so-called bomb. • The product is often a flat uranium slab (pancake) that needs to be further processed. Chemistry of the Actinide Elements, 2nd ed. 33 Uranium Metal (cont.) • Metallic uranium has some plasticity, is malleable, and and can be extruded. • The electrical conductivity is similar to iron. Uranium is weakly paramagnetic and has been shown to have superconductive properties at low temperatures. • The natural radiation of uranium affects the metal and may cause phase transformations in the material. • Uranium can form intermetallic compounds with a range of different metals, for example: Al, As, Au, Bi, Cd, Cu, Fe, Ge, Hg, Mn, Mi, Pb, Pd, Pt, Rh, Ru, Sn, Re, Tc. – Some of these are of importance for pyroprocessing of metallic spent nuclear fuel. 33 34 Uranium Metal (cont.) • Uranium oxidizes readily in air at room temperature. Color changes from silver to gold/yellow to black oxide/nitride (in 3-4 days). • For uranium oxidation in air, the following reactions are of importance: O2(g) O2(absorbed) U U4+ + 4e- 2e- + O O2- 2O(dissociated) • After this we consider the following: U+ O UO UO+O UO2 3UO2 + 2O U3O8 U3O8 + O 3UO3 http://www.chemie-master.de/ 35 Uranium Metal (cont.) • Metallic uranium will also oxidize in water according to: U + H2O UO2 + 2H2 2U + 3H2 2UH3 2UH3 + 4H2O 2UO2 + 7H2 and 4H+ + U U4+ + 2H2 • The oxidation of uranium metal in water is one of the reasons why metallic nuclear fuel is required to be encased in cladding when used in water-cooled reactors. • The last reaction indicates that uranium is dissolved by acid. 35 36 Uranium Metal (cont.) Acid Reaction Rate Reaction products Remarks HF Slow U(IV) fluoride HCl* Rapid Varying amounts of U(III), U(IV), a black residue HNO3 Medium H2SO4 Slow Acid sulfate of U(IV) No reaction with dilute H2SO4 H3PO4 Slow Acid phosphate of U(IV) Reaction proceeds rapidly in hot H3PO4 HClO4 Rapid** An insoluble film is formed Uranyl nitrate Insoluble in dilute HClO4 *As uranium is dissolved in hydrochloric acid, the black residue formed contains complexes of uranium hydrides. ** Fine shavings of uranium metal react very violently with concentrated perchloric acid. Table reproduced from “Analytical Chemistry of Uranium” 37 Uranium Metal (cont.) • Metallic uranium undergoes a number of phase transformations if its temperature increases. During these transformations it can undergo drastic density changes. • At ~662 °C metallic uranium changes from the so called α-phase to β-phase, and the density changes from ~19.07 kg/L to 18.37 kg/L. • This large decrease in density results in swelling of the metal. Uranium metal used as fuel in nuclear reactors is canned in a non-reactive metal, such as stainless steel, aluminum or zirconium. The drastic change in density can cause damage to the fuel elements and therefore metallic fuel is not used in reactors operating above ~650 °C. 38 Uranium Complexes • Uranium forms a number complexes of varying stability with a range of different elements: – Hydrides, UHx – Halogens, UZx (Z is F, I, Br, Cl) – Nitrides, UNx – Carbides, UCx – Oxides, UOx – Phosphates, UPx 39 REACTIONS OF URANIUM WITH NONMETALS Reacting substance Temperature of reaction, °C Reaction products compact uranium powdered uranium 250 25 UH3 1800-2400 800-1200 UC, U2C3, UC2 - 600-1000 U3P4 700 500 UN, UN1.75, UN2 150-350 Pyrophoric UO2, U3O8 Fluorine 25 - UF6 Chlorine 500-600 150-180 UCl4, UCl5, UCl6 Bromine 650 210 UBr4 Iodine 350 260 UI3, UI4 Water 100 25 UO2 Hydrogen fluoride - 200-400 UF4 Hydrogen chloride - 250-300 UCl3 700 400 UN1.75 400-500 - U3O8 - 900 UC Hydrogen Carbon Phosphorus Nitrogen Oxygen Ammonia Nitrogen oxide Methane 40 Halides • The uranium halides constitute an important group of compounds because some of them are used extensively in the uranium industry. • The most important compounds from a nuclear energy perspective are the U-fluoride complexes, UF4 (tetrafluoride) and UF6 (hexafluoride). • Bromide, iodine and chloride also form complexes of the general form UX4 (X= Br, Cl, I), but they are less stable than UF4. Higher halogens (except UF6) of uranium are quite unstable and the stability decreases as: UF6>>UCl6>UBr6≈UI6 41 Halides (cont.) • UF4 is an emerald green solid salt at room temperature. It is usually formed by reacting uranium oxide with hydrofluoric acid: UO2 + 4HF UF4 + 2H2O (at 550 °C) • It is possible to produce it by reacting with elemental F (F2 gas), but this is more expensive and complicated. • UF6 is a white compound. It is solid in room temperature, but will sublime at 56.5 °C. It is commonly formed by reacting dry UF4 with F2 gas at elevated temperatures. UF4,(s) + F2,(g) UF6,(g) 42 Uranium Hexafluoride • Because of its usefulness in the nuclear industry, uranium hexafluoride has been extensively studied. • It is used as the primary feed for uranium enrichment plants separating 235UF6 from 238UF6. • UF6 becomes a liquid at elevated pressure and temperature (1137 torr, 64 °C), and the liquid is used for pumping/pouring in process. • UF6 is hygroscopic and reacts quickly with water, and it is necessary to keep water or air out of any process. UF6,(g) + 3H2O UO2F2 + H2O + 4HF • UF6 is very corrosive and pipes may be lined with teflon to avoid degradation. 43 Hydrides • Uranium reacts with hydrogen to form hydrides: U + 3/2 H2 UH3 • This is a reversible reaction and the rate is fastest around 225 °C (for 1 atm of H2). • Uranium hydride is very reactive and normally ignites spontaneously in air. This may be prevented by coordinating the hydride with various electrondonor compounds. • Of the different hydrides, uranium-borohydride is of special interest, U(BH4)4. This is one of the few volatile uranium compounds (no comparison to UF6) and it can be formed by reacting UF4 with aluminum UF4 + 2 Al(BH4)3 U(BH4)4 + 2Al(BH4)F2 44 Hydrides (cont.) • Uranium-borohydride, U(BH4)4. • This compound can form large networks, crystalline structures of borohydride. • It will only form in vacuum or in inert atmosphere since air (oxygen or moisture) will quickly decompose it. • In this example, the uranium is coordinated by 12 hydrogen atoms. Chemistry of the Actinide Elements, 2nd ed. 45 Oxides • Uranium has been reported in a large range of different oxides. Examples are UO, UO2, UO3, as well as non-stoichiometric complexes such as: U3O8, U12O35, U16O37, etc. • All types of oxides might not exist separately but may be found as a mixture of different oxides. • UO2 has been extensively studied as it is the ceramic material used in most of the conventional nuclear power reactors worldwide. • UO3 and U3O8 also play a large role in uranium production (yellowcake). 46 Mixed Oxides • Uranium can also form mixed oxide complexes with a number of metals. – Examples include: MgO, CaO, SrO, BaO and rare earth oxides. – Other examples are systems with TiO2, ZrO2, CeO2. • Especially important are ThO2 and PuO2 since these can be used as a mixed-oxide fuel material in nuclear reactors (thorium for potential breeding of 233U and Pu in conventional MOX-type fuel). • Pu-U fuel (MOX) is used in a number of reactors in the US today. 47 Reactions with U-Oxides • Reactions between uranium oxides and some nonmetal complexes may form mixed oxides (UO2F2) or other compounds (halides, U-ions, etc.). Reagent Temperature Product with following oxide (°C) UO2 U3O8 UO3 CO 350 n.a. n.a. UO2 750 n.a. UO2 UO2 HF(g) 550 UF4 UO2F2 + UF4 UO2F2 CCl4(g) 400 UCl4 UCl4 + UCl3 UCl4 + UCl3 S2Cl2 450 UCl4 UCl4 UCl4 H2SO4(aq) 25 n.a. n.a. UO22+ HNO3(aq) 25 UO22+ UO22+ UO22+ n.a. = no reaction or very slow. Table reproduced from “Analytical Chemistry of Uranium” 48 Organometallics of U • A large range of organometallic complexes have been synthesized, stabilized, and studied in nonaqueous solvents. Too many to review here! • Some of the classics are uranocene, a sandwich structure of U between two cyclooctane rings. • Other common structures are cyclopentadienylcontaining molecules with U. Pictures from Wikipedia 49 Uranium in Solution • When working with uranium in solution, it is important to consider the oxidation state. • For most practical purposes, only U(IV) and U(VI) are important for solutions containing uranium ions. • In aqueous solutions, U(IV) is very prone to undergo hydrolysis, much like Th4+ or Pu4+, and the free U4+ ion is only observed at fairly high acidity. • The general formula for hydrolysis reactions with U4+: U4+ + xH2O U(OH)x(4-x) + xH+ Where x = 1 to 5 has been observed Polynuclear species of the general formula U(UOOH)n(n+4) have been suggested as well to form during U(IV) hydrolysis. 50 Complexes with U(IV) UClO43+ U4+ + Cl- UCl3+ U4+ + 2Cl- UCl22+ U4+ + CNS- UCNS3+ U4+ + 2CNS- U(CNS)22+ U4+ + HSO4U4+ + 2HSO4U4+ + HPO42U4+ + HF U4+ + 2HF USO42+ + H+ U(SO4)2 + 2H+ UHPO42+ UF3+ + H+ UF22+ + 2H+ Stronger U4+ + ClO4- Weaker • In aqueous solutions, U(IV) also forms complexes with halogens, F-, Cl-, Br-. Charged complexes (ionic compounds) may also be formed with sulfate, carbonate, thiocyanate, oxalate, etc. 51 Hydrolysis of U(VI) • U(VI) will also hydrolyze, but it is less prone to do so than U(IV). Extensive hydrolysis is not observed until neutral or higher pHvalues. Chemical Thermodynamics of Uranium, OECD-NEA 52 Hydrolysis of U(VI) (cont.) Structure and schematic of hydrolysed uranyl, UO2(OH)42-, UO2(OH)53-. Clark et al. Inorg. Chem. 1999, 38, 1456-1466 Structure of UO22+ coordinated to 5 water molecules, UO2(H2O)52+ Vallet et al. J. Am. Chem. Soc. 2001, 123, 11999-12008 53 Hydrolysis of U(VI) (cont.) • If other complexing reagents such as carbonate are included, the behavior of uranyl in solutions may quickly become complicated. Mixed (UO2)x(OH)y(CO3)z complexes may form. 54 Complexes with UO22+ UO22+ + ClO4- UO2ClO4+ UO2Cl+ UO22+ + NO3- UO2NO3+ UO22+ + HF UO2F+ + H+ UO22+ + SO4- UO2SO4 UO22+ + 2SO4- UO2(SO4)22- Stronger UO22+ + Cl- Weaker • In general, aqueous complexes with UO22+ are much better studied because of the higher stability of U(VI) in solution and the ease of hydrolysis of U(IV). 55 Complexes with Uranyl • Uranyl forms aqueous complexes with simple organic compounds such as acetate, gluconate, carbonate, citric acid, hydroxamic acid, etc. • Several studies exist where stability constants for such complexes have been found. • Such complexes are important for environmental reasons (carbonate especially), and for mining purposes using carbonate leaching. • Many studies to obtain stability constants were done by solvent extraction techniques. This is a useful and relatively simple method, but one must be careful if the ionic strength and ionic medium change significantly as this might introduce large errors. 56 Uranyl Complexes • Di- and poly-carboxylic acids can form strong complexes with uranium. Specifically, chelating complexes form stable ring structures with the metal ion. UO2-tris-oxalate ionic complex. Most stable state (ground state). Vallet et al., Inorg. Chem. 2003, 42, 1982-1993 57 Uranyl-chelates • a through f: different isomers of UO2(oxalate)34- that can exist in solution. Vallet et al., Inorg. Chem. 2003, 42, 1982-1993 58 Uranium in Non-aqueous Solvents • Since Bucholtz’s discovery in 1805, numerous reports have been written on different options for extracting uranium, as uranyl, into different organic solvents. • This often requires assistance of a lipophilic complexing agent, extraction reagent, to coordinate with the uranyl, resulting in a lipophilic complex. • Examples of extracting reagents are tributyl-phosphate, tetraalkylamines, phosphoric acids (DEHPA, HDBP). • Some reagents are acidic and extract by an ion-exchange mechanism, others are neutral and require anions to be present in the final complex (to balance the +2 charge or uranyl). 59 Extracting Reagents for UO22+ • Solvating complex with tributyl phospate, UO2(NO3)2TBP2 H3C O O O U O P O O CH3 O O O N - O O H3C 2+ O P O O H3C N O O CH3 - CH3 • Chelating complex with dibutyl phosphoric acid, UO2DBP2HDBP2(*2H2O) H3C O H3C O O H O P O • One (HDBP)(DBP) dimer is missing in this figure - 2+ U O CH3 O O P O O CH3 60 Analytical Tools for Uranium • Mass spectrometery, ICP-MS – ICP-MS gives no information of speciation, destructive technique. • Neutron Activation Analysis – This gives no information of speciation. Sample might be recovered, but will need to be decontaminated. • UV-VIS spectrometry – Can show difference in speciation and oxidation of uranium. • Fluorescence – Can show difference in speciation. Can be very sensitive. 61 UV vis Spectra of U U(VI) Spectra of U(III), U3+, U(IV), U4+ and U(VI), UO22+ in hydrochloric acid ~420 nm 521 nm U(III) 648 nm U(IV) The chemistry of the Actinide elements, 1st ed. 62 UV vis Spectra of U (cont.) Spectra of uranyl nitrate (UNH) in: I: Methyl ethyl ketone II: Acetone III: Diethyl Ether IV: Water Analytical Chemistry of Uranium 63 UV vis Spectra of U (cont.) Spectra of uranyl nitrate (UNH) in: I: Formamide II: Tributyl Phosphate III: Dioxane Analytical Chemistry of Uranium 64 Uranium Fluorescence This technique can be very sensitive, but is also highly dependant on the chemical environment and what uranium complex is measured. 65 Uranium Chemistry on a Commercial Scale www.world-nuclear.org 66 Uranium Mining • Because natural uranium is always accompanied by its daughter products, mining can be potentially hazardous in terms of radiation levels. High concentrations of radon may exist. • This is also a problem after the first treatment steps of the ore. Ponds of leaching solutions and heaps of mill tailings can be quite radioactive and need to be monitored and covered to reduce the radiation to the environment. 67 Uranium Mining (cont.) • Mining and recovery of uranium can be divided into three principal steps: – Concentration – Purification – Conversion • In the first step, concentration, the ore particles containing the valuable material (uranium) are concentrated from a few % up to 85%-95%. 68 Uranium Mining (cont.) • In the second step, purification, the last small impurities are removed to obtain a pure uranium compound. This is done at a uranium refinery and is a liquid process. Any solid uranium mineral still present must be dissolved. • In the third step, conversion, the uranium in solution is chemically converted to uranium hexafluoride for enrichment. 69 Methods for Concentration • Gravity concentration – This is usually not used because it requires a very special ore type with large uranium grains, only present in a few mines. • Flotation – This method does not work well with uranium. It does not float, or it does not float selectively. • Leaching – This is by far the most common method used for concentration of uranium ore. 70 Recovery after Leaching • Once the uranium is in the leach solution, it can be recovered by precipitation, ion exchange, or solvent extraction. For sodium carbonate, leach solutions precipitation with NaOH can work. • Acid leach has a tendency to include other materials, so selective precipitation of U only is difficult. Solvent extraction or ion exchange works better. 71 Leaching • In the leaching process, chemicals are used for selectively leaching out uranium from the solid ore. Common leaching agents are sulfuric acid or alkaline carbonate solution. The choice of leaching agent depends on the mineral. For minerals that consume acid, such as limestone, sodium or ammonium carbonate is good. For example: UO2 + ½ O2 UO3 (oxidation of uranium) UO3 + Na2CO3 + 2 NaHCO3 Na4UO2(CO3)3 +H2O 72 Leaching (cont.) • Although carbonate leaching is a more gentle method, leaching with sulfuric acid is still very common. Again, uranium is in the +4 oxidation state in the ore so it needs to be oxidized to +6 UO22+. Solvent extraction is more common than ion exchange for the recovery of UO22+ and can be done by using lipophilic phosphoric acid compounds or trialkyl amines. UO22+ + SO42- + 2(R3NH)2SO4,org UO22+ + 2(HDEHP)2,org (R2NH)4UO2(SO4)3, org UO2(HDEHPDEHP)2, org + 2H+ 73 Leaching (cont.) • There are, in principle, two methods for leaching of uranium ore: – “Heap” leaching: The ore is excavated and crushed and the material is contacted with a leaching solution for some time. – In situ leaching: This method is becoming more common since it does not require open mines. Wells are drilled and the leach solution is pumped underground and recovered downstream. 74 Uranium Mining • In open-pit mines, the excavated ore is taken to a nearby plant for concentration and purification. Rössing mine, Namibia. (www.world-nuclear-news.org) • For in situ leaching, the leachate is recovered directly and can be further concentrated and purified. Beverly wellfield, Australia. (www.world-nuclear.org) 75 Uranium Mining (cont.) www.world-nuclear.org 76 Uranium Refining • This is the overall process for purifying the concentrate and converting it to uranium hexafluoride. • In this process, yellowcake is formed (UO3) or (U3O8). • In this form, the uranium can be shipped or handled. Since this is a heavy metal powder, care must be taken not to ingest/inhale it. The radiation levels are low since all daughters have been removed. Nuclear Chemical Engineering, 2nd ed. 77 Uranium Refining • This is the overall process for purifying the concentrate and converting it to uranium hexafluoride. • In this process, yellowcake is formed (UO3) or (U3O8). • In this form, the uranium can be shipped or handled. Since this is a heavy metal powder, care must be taken not to ingest/inhale it. The radiation levels are low since all daughters have been removed. Nuclear Chemical Engineering, 2nd ed. 78 Uranium Refining • This is the overall process for purifying the concentrate and converting it to uranium hexafluoride. • In this process, yellowcake is formed (UO3) or (U3O8). • In this form, the uranium can be shipped or handled. Since this is a heavy metal powder, care must be taken not to ingest/inhale it. The radiation levels are low since all daughters have been removed. Nuclear Chemical Engineering, 2nd ed. 79 Uranium Refining • This is the overall process for purifying the concentrate and converting it to uranium hexafluoride. • In this process, yellowcake is formed (UO3) or (U3O8). • In this form, the uranium can be shipped or handled. Since this is a heavy metal powder, care must be taken not to ingest/inhale it. The radiation levels are low since all daughters have been removed. Nuclear Chemical Engineering, 2nd ed. 80 Uranium Ore Concentration • Ore is first crushed and then ground with hot water until most of it passes a fine mesh. The resulting fine slurry goes into the leaching step. The first tank dissolves most uranium. Particles that are more resistant are dissolved in the second step, where sodium chloride is added to increase the reduction potential. This causes iron to act as a catalyst for uranium oxidation. • The next set of steps separates the leach liquor, now containing the uranium, from the remaining suspended solids. Cyclone separators, rakes, and thickeners are used to remove all solids. • The leach liquor is taken to solvent extraction purification. Nuclear Chemical Engineering, 2nd ed. 81 Uranium Ore Concentration • Ore is first crushed and then ground with hot water until most of it passes a fine mesh. The resulting fine slurry goes into the leaching step. The first tank dissolves most uranium. Particles that are more resistant are dissolved in the second step, where sodium chloride is added to increase the reduction potential. This causes iron to act as a catalyst for uranium oxidation. • The next set of steps separates the leach liquor, now containing the uranium, from the remaining suspended solids. Cyclone separators, rakes, and thickeners are used to remove all solids. • The leach liquor is taken to solvent extraction purification. Nuclear Chemical Engineering, 2nd ed. 82 Uranium Ore Concentration • Ore is first crushed and then ground with hot water until most of it passes a fine mesh. The resulting fine slurry goes into the leaching step. The first tank dissolves most uranium. Particles that are more resistant are dissolved in the second step, where sodium chloride is added to increase the reduction potential. This causes iron to act as a catalyst for uranium oxidation. • The next set of steps separates the leach liquor, now containing the uranium, from the remaining suspended solids. Cyclone separators, rakes, and thickeners are used to remove all solids. • The leach liquor is taken to solvent extraction purification. Nuclear Chemical Engineering, 2nd ed. 83 Uranium Ore Concentration • Ore is first crushed and then ground with hot water until most of it passes a fine mesh. The resulting fine slurry goes into the leaching step. The first tank dissolves most uranium. Particles that are more resistant are dissolved in the second step, where sodium chloride is added to increase the reduction potential. This causes iron to act as a catalyst for uranium oxidation. • The next set of steps separates the leach liquor, now containing the uranium, from the remaining suspended solids. Cyclone separators, rakes, and thickeners are used to remove all solids. • The leach liquor is taken to solvent extraction purification. Nuclear Chemical Engineering, 2nd ed. 84 Uranium Ore Concentration • Ore is first crushed and then ground with hot water until most of it passes a fine mesh. The resulting fine slurry goes into the leaching step. The first tank dissolves most uranium. Particles that are more resistant are dissolved in the second step, where sodium chloride is added to increase the reduction potential. This causes iron to act as a catalyst for uranium oxidation. • The next set of steps separates the leach liquor, now containing the uranium, from the remaining suspended solids. Cyclone separators, rakes, and thickeners are used to remove all solids. • The leach liquor is taken to solvent extraction purification. Nuclear Chemical Engineering, 2nd ed. 85 Uranium Concentrate Purification • This is a liquid-liquid extraction process for purification of uranium using tributyl phosphate. • This particular process was developed by Mallinckrodt. Similar flow-sheets are used by other refineries. • The solvent extraction is carried out using mixer-settler batteries and the organic to aqueous ratio is 13:1 in the extraction step. • Scrubbing is done to remove all unwanted (non-U) metal ions that were extracted. Nuclear Chemical Engineering, 2nd ed. 86 Uranium Concentrate Purification • The uranium concentrate from the leach step enters the process and the acidity is adjusted. In this example the liquid-liquid extraction reagent is tributylphosphate (TBP). • The adjusted concentrate (the aqueous feed ) is contacted with an organic solvent containing TBP to extract the uranium . • Following the extraction are scrub stages to remove unwanted impurities and a uranium strip stage to recover the uranium from the organic solvent • Finally, the organic solvent is washed for reuse in the extraction stage. Nuclear Chemical Engineering, 2nd ed. 87 Uranium Concentrate Purification • The uranium concentrate from the leach step enters the process and the acidity is adjusted. In this example the liquid-liquid extraction reagent is tributylphosphate (TBP). • The adjusted concentrate (the aqueous feed ) is contacted with an organic solvent containing TBP to extract the uranium . • Following the extraction are scrub stages to remove unwanted impurities and a uranium strip stage to recover the uranium from the organic solvent • Finally, the organic solvent is washed for reuse in the extraction stage. Nuclear Chemical Engineering, 2nd ed. 88 Uranium Concentrate Purification • The uranium concentrate from the leach step enters the process and the acidity is adjusted. In this example the liquid-liquid extraction reagent is tributylphosphate (TBP). • The adjusted concentrate (the aqueous feed ) is contacted with an organic solvent containing TBP to extract the uranium . • Following the extraction are scrub stages to remove unwanted impurities and a uranium strip stage to recover the uranium from the organic solvent. • Finally, the organic solvent is washed for reuse in the extraction stage. Nuclear Chemical Engineering, 2nd ed. 89 Uranium Concentrate Purification • The uranium concentrate from the leach step enters the process and the acidity is adjusted. In this example the liquid-liquid extraction reagent is tributylphosphate (TBP). • The adjusted concentrate (the aqueous feed ) is contacted with an organic solvent containing TBP to extract the uranium . • Following the extraction are scrub stages to remove unwanted impurities and a uranium strip stage to recover the uranium from the organic solvent. • Finally, the organic solvent is washed for reuse in the extraction stage. Nuclear Chemical Engineering, 2nd ed. 90 Uranium Conversion • For the conversion of uranium concentrates to UF6, the UO22+ is first reduced to UO2. HF is added to convert the UO2 to UF4: UO2 + 2HF UF4 + 2 H2O. This is often done in fluidized beds. • The UF4 solid is fed into a tower reactor together with F2 gas under high temperature, and UF6 gas is formed. • The gas mixture, UF6, F2, and some diluent gases, are separated to produce pure UF6 for the enrichment. Nuclear Chemical Engineering, 2nd ed. 91 Uranium Conversion • For the conversion of uranium concentrates to UF6, the UO22+ is first reduced to UO2. HF is added to convert the UO2 to UF4: UO2 + 2HF UF4 + 2 H2O. This is often done in fluidized beds. • The UF4 solid is fed into a tower reactor together with F2 gas under high temperature, and UF6 gas is formed. • The gas mixture, UF6, F2, and some diluent gases, are separated to produce pure UF6 for the enrichment. Nuclear Chemical Engineering, 2nd ed. 92 Uranium Conversion • For the conversion of uranium concentrates to UF6, the UO22+ is first reduced to UO2. HF is added to convert the UO2 to UF4: UO2 + 2HF UF4 + 2 H2O. This is often done in fluidized beds. • The UF4 solid is fed into a tower reactor together with F2 gas under high temperature, and UF6 gas is formed. • The gas mixture, UF6, F2, and some diluent gases, are separated to produce pure UF6 for the enrichment. Nuclear Chemical Engineering, 2nd ed. 93 Uranium Conversion • For the conversion of uranium concentrates to UF6, the UO22+ is first reduced to UO2. HF is added to convert the UO2 to UF4: UO2 + 2HF UF4 + 2 H2O. This is often done in fluidized beds. • The UF4 solid is fed into a tower reactor together with F2 gas under high temperature, and UF6 gas is formed. • The gas mixture, UF6, F2, and some diluent gases, are separated to produce pure UF6 for the enrichment. Nuclear Chemical Engineering, 2nd ed. 94 Uranium Conversion • For the conversion of uranium concentrates to UF6, the UO22+ is first reduced to UO2. HF is added to convert the UO2 to UF4: UO2 + 2HF UF4 + 2 H2O. This is often done in fluidized beds. • The UF4 solid is fed into a tower reactor together with F2 gas under high temperature, and UF6 gas is formed. • The gas mixture, UF6, F2, and some diluent gases, are separated to produce pure UF6 for the enrichment. Nuclear Chemical Engineering, 2nd ed. 95 Final Steps of U-production • The UF6 is taken to an enrichment plant where enriched and depleted uranium is produced. • Depending on the final use of the uranium, different methods exist for reducing the uranium hexafluoride and converting it back to a condensed phase (solid metal, oxide ceramic). 96 Current and Future U-Chemistry • Although uranium has been studied for over 200 years, there are still discoveries to be made. • Recent work by J. Kiplinger from Los Alamos National Laboratory explores the possibility of preparing new U-compounds. • The 1,4-dioxane derivatives of uranium iodides were prepared in glove boxes and can be converted into a range of other uranium compounds, including alkoxides, halides, amides, nitrides and carbides. Many of these may be important for future advanced nuclear fuel cycles. http://www.rsc.org/chemistryworld/News/2011/March/28031101.asp 97 References • Analytical Chemistry Uranium, 1st ed. Pavi. Ann ArbourHumphrey, 1970. • The Chemistry of the Actinide Elements, 1st ed. Katz, Seaborg. Methuen & co, 1957. • The Chemistry of the Actinide Elements, 2nd ed. Katz, Seaborg, Morss. Springer, 1986. • The Chemistry of the Actinide Elements, 3rd ed. Katz, Morss, Edelstein, Fuger. Springer, 2007. • Nuclear Chemical Engineering, 2nd ed. Benedict, Pigford, Levi. McGraw-Hill, 1981. • Chemical Thermodynamics of Uranium, Grenthe, Konings. OECD-NEA, 1992. • www.world-nuclear.org (world nuclear association) 98 Future Webinars in Actinide Chemistry Series • • • • • Plutonium Chemistry Environmental Chemistry of Uranium and Plutonium Analytical Chemistry of Uranium and Plutonium Source Preparation for Alpha Spectroscopy Sample Dissolution • NAMP website www.inl.gov/namp