Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solution with ammonium hydroxide

Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solutions on addition of aqueous ammonium hydroxide under various conditions has been examined by thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy and X-ray diffraction study. The TG curves of all precipitates show the weight-loss corresponding to the calculated value as UO₃·NH₃·H₂O. The DTA curves of the precipitates give the endotherms at about 130, 210 and 590 °C and the exotherms at 340–420 °C. As a result, it is found that ammonium uranates thermally decompose to amorphous UO₃ at about 400 °C, and transform to U₃O₈ via β-UO₃.     

Thermal and X-ray diffraction analysis studies during the decomposition of ammonium uranyl nitrate

Two types of ammonium uranyl nitrate (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃, were thermally decomposed and reduced in a TG-DTA unit in nitrogen, air, and hydrogen atmospheres. Various intermediate phases produced by the thermal decomposition and reduction process were investigated by an X-ray diffraction analysis and a TG/DTA analysis. Both (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃ decomposed to amorphous UO₃ regardless of the atmosphere used. The amorphous UO₃ from (NH₄)₂UO₂(NO₃)₄·2H₂O was crystallized to γ-UO₃ regardless of the atmosphere used without a change in weight. The amorphous UO₃ obtained from decomposition of NH₄UO₂(NO₃)₃ was crystallized to α-UO₃ under a nitrogen and air atmosphere, and to β-UO₃ under a hydrogen atmosphere without a change in weight. Under each atmosphere, the reaction paths of (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃ were as follows: under a nitrogen atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → U₃O₈, NH₄UO₂(NO₃)₃ → A-UO₃ → α-UO₃ → U₃O₈; under an air atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → U₃O₈, NH₄UO₂(NO₃)₃ → A-UO₃ → α-UO₃ → U₃O₈; and under a hydrogen atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → α-U₃O₈ → UO₂, NH₄ UO₂(NO₃)₃ → A-UO₃ → β-UO₃ → α-U₃O₈ → UO₂.     

Solid phase decomposition of ammonium uranate

The thermal decomposition of ammonium uranate in air has been studied using TG, DTG, surface area measurements, chemical and X-ray analyses. The effect of washing and calcination at different temperatures is discussed. The optimum conditions for preparing β-UO₃ are chosen to be via ammonium uranate washed by distilled water and calcined at 500°C.The kinetics of the thermal decomposition are studied using Kissinger's shape index method. The thermal decomposition includes dehydration reaction, complicated reactions to form UO₃ and thermal decomposition of UO₃ to U₃O₈. The order of reaction is calculated for each stage.     

UO2, NpO2 and PuO2 preparation in aqueous nitrate solutions in the presence of hydrazine hydrate

It was established that heating to 90 °C of nitrate solutions of U, Np and Pu in the presence of hydrazine hydrate results in the formation of hydrated dioxides of these elements. On ignition under inert or reducing conditions in the temperature range of 280–800 °C hydrated uranium dioxide transmogrify into crystalline UO₂. On ignition in air atmosphere UO₂·nH₂O turns into UO₃ at 440 °C and into U₃O₈ at 570–800 °C. It was shown that thermolysis of the solution containing a mixture of uranium, neptunium and plutonium nitrates at 90 °C in the presence of hydrazine hydrate allows one to prepare hydrated dioxides (U, Np, Pu)O₂·nH₂O which on heating to ~300 °C transmogrify into crystalline product of UO₂, NpO₂ and PuO₂ solid solution. The technique of preparation of solid solutions of U and Pu dioxides is very promising as simple and effective method of production of MOX-fuel for.     

Thermal behaviour of co-precipitated mixtures of chromium(III) and uranium(VI)

In the CrUO system, besides the phases reported earlier, a triuranate CrU₃O_10-x (x ～ 0.3) could be identified. It is unstable above 70o°C and decomposes to a mixture of CrUO₄ and U₃O₈. Under reducing atmospheres up to 1600°C, the uranium—chromium—oxygen system gives a mixture of Cr₂O₃ and UO₂. No new phase could be identified. The compound CrUO₄ is unstable under reducing conditions and decomposes to a mixture of Cr₂O₃ and UO₂.     

Thermal dehydration and decomposition of M[M(C2O4)2]·xH2O (x=3 forM=Sr(II) andx=6 forM=Hg(II))

Strontium(II) bis (oxalato) strontium(II) trihydrate, Sr[Sr(C₂O₄)₂]·3H₂O and mercury(II) bis (oxalato) mercurate(II) hexahydrate, Hg[Hg(C₂O₄)₂]·6H₂O have been synthesized and characterized by elemental analysis, reflectance and IR spectral studies. Thermal decomposition studies (TG, DTG and DTA) in air showed SrCO₃ was formed at ca. 500°C through the formation of transient intermediate of a mixture of SrCO₃ and SrC₂O₄ around 455°C. Sharp phase transition from γ-SrCO₃ to β-SrCO₃ indicated by a distinct endothermic peak at 900°C in DTA. Mercury(II) bis (oxalato) mercurate(II) hexahydrate showed an inclined slope followed by surprisingly steep slope in TG at 178°C and finally 98.66% of weight loss at 300°C. The activation energies (E ^*) of the dehydration and decomposition steps have been calculated by Freeman and Carroll and Flynn and Wall's method and compared with the values found by DSC in nitrogen. A tentative reaction mechanism for the thermal decomposition of Sr[Sr(C₂O₄)₂]·3H₂O has been proposed.     

Defluorination behavior and mechanism of uranium dioxide

The residual fluorine in ammonium uranyl tricarbonate (AUC) cannot be removed, while a large part of residual fluorine in ammonium diuranate (ADU) can be removed, when AUC and ADU are decomposed and reduced under dry hydrogen atmosphere. UO₂ was prepared by decomposing and reducing AUC and ADU in dry hydrogen atmosphere. The defluorination kinetics of UO₂ at 500–700°C in atmosphere of 50% H₂-50% H₂O was investigated. The results show that the defluorination kinetics supports the Lindman's assertion that the residual fluorine forms a solid-solution in UO₂.     

Thallium(I)-Uranates(VI)

The solid state preparation, thermal and hydrolytic characteristics of thallium(I)—uranates(VI) are described. The phases identified were Tl₂UO₄, Tl₂U₂O₇ and a range of solid solution (Tl₂O. 2,33 UO₃? Tl₂O. 6 UO₃). The thallium uranates are isostructural with the corresponding potassium uranates. Tl₂U₂O₇ is the stable phase formed from the other uranates on hydrolytic treatment. The thallium uranates lose thallium(I) oxide on heating to temperatures above 750°C and the order of thermal stability is Tl₂U₆O₁₉～Tl₂U₃O₁₀～Tl₂U₂O₇»Tl₂UO₄.     

The thermal stability of calcium uranates in a hydrogen atmosphere

The stability and decomposition of CaUO₄, Ca₂UO₅, and Ca₃UO₆ on heating in hydrogen were investigated by X-ray powder diffraction and thermogravimetry. Ca₂UO₅ decomposes at 450°C into Ca₂UO_4.5 with a triclinic unit cell. At 850°C, it changes to monoclinic Ca₂(Ca_0.67U_0.33)UO₆ which loses some oxygen up to the composition Ca₂(Ca_0.67U_0.33)UO_5.83. At 1100°C, it decomposs to UO₂ solid solution and CaO. CaUO₄ decomposes at 900°C to Ca₂(Ca_0.67U_0.33)UO_5.83 and CaU₂O₆. The decomposition products of Ca₃UO₆ at 850°C are Ca₂(Ca_0.67U_0.33)UO_5.83 and CaO.     

Chemistry,crystal structure,and structural evolution versus temperature of the uranyl diselenite UO2Se2O5: Structural relationships with (UO2)Se2O5·2H2O,UO2SeO3, and β-U3O8

The chemistry and structural chemistry versus temperature in the system UO₃SeO₂H₂O were determined. A proposal has been presented for the structural transformations of various selenites, i.e., UO₂Se₂O₅·2H₂O, UO₂Se₂O₅, UO₂SeO₃, and the U₃O₈ uranium oxide final product in its β form. The unit-cell of the hydrated uranyl diselenite has been determined from an indexed powder pattern: it crystallizes in the triclinic system with a = 9.40(4)Å, b = 11.85(5)Å, c = 6.69(5)Å, α = 94.3(3)°, β = 90.3(3)°, and γ = 114.5(3)°, V = 676ų. On this basis, a structure derived from that of UO₂Se₂O₅ is proposed, corresponding to a reasonable packing of oxygen, water molecules, and lone pairs.     

Thermogravimetric study of uranium phosphates

The thermal decomposition of (UO₂)₃(PO₄)₂ and U(HPO₄)₂ ·xH₂O in the temperature range 25–1600?, was investigated. (UO₂)₃(PO₄)₂ decomposed first to 1/3[U₃O₈ + 3U₂O₃P₂O₇] and then to U₃O₅P₂O₇ before a loss of phosphorus was observed above 1350?. Decomposition in air and in inert atmospheres was nearly identical. Reduction with H₂ or with carbon black in argon gave U₃O₅P₂O₇ and [UO₂ + + (UO)₂P₂O₇] before pure UO₂ was formed. U(HPO₄)₂ ·xH₂O decomposed to UP₂O₇ in argon. It oxidized partly in air before the same product was obtained. The high temperature stability of UP₂O₇ and U₃(PO₄)₄ was also investigated.     

The first uranyl complexes with valerate ions

FT–IR spectroscopy and single‐crystal X‐ray structure analysis were used to characterize the discrete neutral compound diaquadioxidobis(n‐valerato‐κ²O,O′)uranium(VI), [UO₂(C₄H₉COO)₂(H₂O)₂], (I), and the ionic compound potassium dioxidotris(n‐valerato‐κ²O,O′)uranium(VI), K[UO₂(C₄H₉COO)₃], (II). The U^VI cation in neutral (I) is at a site of 2/m symmetry. Potassium salt (II) has two U centres and two K⁺ cations residing on twofold axes, while a third independent formula unit is on a general position. The ligands in both compounds were found to suffer severe disorder. The FT–IR spectroscopic results agree with the X‐ray data. The composition and structure of the ionic potassium uranyl valerate are similar to those of previously reported potassium uranyl complexes with acetate, propionate and butyrate ligands. Progressive lengthening of the alkyl groups in these otherwise similar compounds was found to have an impact on their structures, including on the number of independent U and K⁺ sites, on the coordination modes of some of the K⁺ centres and on the minimum distances between U atoms. The evolution of the KUO₆ frameworks in the four homologous compounds is analysed in detail, revealing a new example of three‐dimensional topological isomerism in coordination compounds of U^VI.     

Synthesis and characterization of MIIU3O10 · nH2O (MII = Mg, Mn, Co, Ni, Cu, Zn, Cd) triuranates

Individual crystalline phases of composition M^IIU₃O₁₀ · nH₂O were prepared by reacting schoepite UO₃ · 2.25H₂O with aqueous solutions of Mg, Mn, Co, Ni, Cu, Zn, or Cd nitrates under hydrothermal conditions at 200°C. The composition and structure of the resultant compounds were determined by hightemperature X-ray diffraction, IR spectroscopy, scanning calorimetry, and chemical analysis; the dehydration and thermal destruction of the compounds were studied.     

Extraction of pertechnetates from HNO<Subscript>3</Subscript> solutions into ionic liquids

The effect of the oxidation temperature of sintered UO₂ pellets on the powder properties of U₃O₈ was studied in the temperature range 250–900 °C in air. The U₃O₈ was obtained at 450 °C after 180 min and its particle size and surface area are respectively, 35 µm and 0.7 m²/g. The reduction of the U₃O₈ powder resulted in UO₂ after 30 min with a surface area of 0.8 m²/g. This value was improved more than 3.5 times by applying five alternating oxidation–reduction cycles.     

Thermogravimetric study of uranium phosphates

The thermal decomposition of UO₂NH₄PO₄ · 3H₂O and UO₂HPO₄ · 4H₂O was studied in the temperature range 25–1600?C. Both compounds gave U₂O₃P₂O₇ around 900?C after a two step dehydration and an orthophosphate-pyrophosphate transformation. UO₂NH₄PO₄ · 3H₂O did not form any pure intermediates, but (UO₂)₂P₂O₇ could be prepared from UO₂HPO₄ · 4H₂O. In air, U₂O₃P₂O₇ lost phosphorus above 1250?C. In argon, (UO)₂P₂O₇ was first formed between 1000 and 1290?C and this product only lost phosphorus at still higher temperatures. (UO)₂P₂O₇ was also obtained by reduction of (UO₂)₂P₂O₇ or U₂O₃P₂O₇ at 700?C in H₂ or with carbon black in argon above 1000?C. It oxidised in air above 250?C with the formation of U₂O₃P₂O₇.     

Oxygen isotope exchange in oxygen-bearing compounds of uranium

Isotope exchange is reported for gaseous oxygen in contact with the following uranium compounds: -Na₂UO₄, -Na₂UO₄, Na₂U₂O₇, UO₃(A), -UO₃, -UO_2.94 and U₃O₈.; qualitative tests have also been done with UO₂F₂ and Cs₂UO₂Cl₄. The times of half-exchange have been determined as functions of temperature for U₃O₈, -UO_2.94, Na₂U₂O₇ and -Na₂UO₄; diffusion coefficients for oxygen have been calculated for UO₃(A), -UO₃, Na₂U₂O₇, -Na₂UO₄ and -Na₂UO₄. Activation energies have been deduced for diffusion and surface exchange. All the oxygen atoms in these compounds are equivalent as regards isotope exchange; the above activation energies increase with the UO ratio in some cases. Diffusion-limited exchange tends to show periodic oscillations in rate not ascribable to errors of measurement; a mechanism is proposed for this.     

Synthesis,crystal structure and electrical characterization of Cs4[(UO2)2(V2O7)O2], a uranyl divanadate with chains of corner-sharing uranyl square bipyramids

A new cesium uranyl vanadate Cs₄[(UO₂)₂(V₂O₇)O₂] has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the orthorhombic symmetry with space group Pmmn and following cell parameters: a=8.4828(15) Å, b=13.426(2) Å and c=7.1366(13) Å, V=812.8(3) Å³, Z=2 with ρ_mes=5.39(2) g/cm³ and ρ_cal=5.38(1) g/cm³. A full-matrix least-squares refinement on the basis of F² yielded R₁=0.027 and wR₂=0.066 for 62 parameters with 636 independent reflections with I⩾2σ(I) collected on a BRUKER AXS diffractometer with MoKα radiation and a charge-coupled device detector. The crystal structure is characterized by _∞²[(UO₂)₂(V₂O₇)O₂]⁴⁻ corrugated layers parallel to (001). The layers are built up from distorted (UO₂)O₄ octahedra and divanadate V₂O₇ units resulting from two VO₄ tetrahedra sharing corner. The distorted uranyl octahedra (UO₂)O₄ are linked by corners to form infinite _∞¹[UO₅]⁴⁻ chains parallel to the a-axis. These chains are linked together by symmetrical divanadate units sharing two corners with each chain, the two last corners being oriented towards the same interlayer. The cohesion of the structure is assured by interlayer Cs⁺ ions. Their mobility within the interlayer space gives rise to a cationic conductivity with an important jump between 635°C and 680°C. Cs₄[(UO₂)₂(V₂O₇)O₂] is readily decomposed by water at 60°C to form the Cs-carnotite analog Cs₂(UO₂)₂(V₂O₈) compound.     

Synthesis and study of lithium triuranate Li2U3O10 · 6H2O

Li₂U₃O₁₀ · 6H₂O crystal hydrate was synthesized by the reaction between synthetic schoepite UO₃ · 2.25H₂O and aqueous lithium nitrate solution under hydrothermal conditions at 200°C. The composition and structure of the obtained compound were established, and its dehydration and thermal decomposition were studied, by chemical analysis, X-ray diffraction, IR spectroscopy, and scanning calorimetry.     

Determination of specific surface area of uranium oxide powders using differential thermal analysis technique

The two step oxidation of UO_2+x and reduction of U₃O₈ powders observed during Differential Thermal Analysis (DTA) has been exploited to determine their Specific Surface Areas (SSAs). The results obtained by this method have been compared with the Braunauer, Emmett and Teller (BET) method and are found to be in good agreement in the SSA range of 2–4 m²/gm in the case of UO_2+x obtained from ADU route and 4–8 m²/gm in the case of AUC route. A precision of ±0.1 m²/gm is obtained. The maximum temperature of oxidation and reduction of these oxides are dependent upon their preparative routes such as Ammonium Diuranate (ADU) and Ammonium Uranyl Carbonate (AUC).     

Synthesis and study of polyuranates MIIIU3O10.5·6H2O (MIII = La, Ce, Pr, Nd, Sm)

Previously unknown individual crystalline compounds M^IIIU₃O_10.5·6H₂O were obtained by the reaction of synthetic schoepite UO₃·2.25H₂O with aqueous solutions of La, Ce, Pr, Nd, and Sm nitrates in hydrothermal conditions at 200°C. Their composition and structure were determined, and the processes of dehydration and thermal decomposition were studied by the methods of chemical analysis, X-ray diffraction, IR spectroscopy, and scanning calorimetry.