18.08.2015   NUCLEAR CHEMICAL ENGINEERING

URANIUM SOLUTION CHEMISTRY

1.11Oxidation States of Uranium in Aqueous Solution

In aqueous solution, uranium may occur as trivalent U3+, the tetravalent uranous ion U4+, pentavalent Uv02V or the hexavalent uranyl ion UVI022+. However, U3+ is unstable, reducing water with production of hydrogen, and Uv02+ is unstable, disproportionating into U,+ and UV1022+:

2Uv02+ + 4H+ -+ U4* + UVI023+ + 2H20 Thus, only the uranous and uranyl ions are of practical importance.

1.12 Uranium(IV) Solutions

Solutions of tetravalent uranium salts are usually prepared by reduction of the corresponding uranyl compounds. Reduction may be effected by metallic zinc or at the cathode of an electrolytic cell:

U022+ + 4H+ + 2e — -* U4* + 2H20

Table 5.12 Thermodynamic properties of UF6 in ideal gas state at 1 atm pressure

Temperature T, К

Heat capacity Cp, cal/(g-mol’°C)

Free-energy function

-(С0-й?)/Г,

cal/(g-mol‘°C)

Entropy 5°, cal/(g-mol’ C)

Enthalpy relative to solid at 298.15 K,

H°G — He,298. cal/g-mol

0

5,420

100

18.93

50.91

62.99

6,628

150

23.45

56.41

71.58

7,695

200

26.74

61.14

78.81

8,954

273

30.13

67.08

87.65

11,139

298

31.00

68.92

90.34

11,807

323

31.75

70.69

92.89

12,594

348

32.38

72.33

95.22

13,389

373

32.93

73.93

97.47

14,204

400

33.46

75.65

99.86

15,104

500

34.80

81.25

107.45

18,520

750

36.35

92.99

122.37

27,455

1000

36.94

101.37

132.58

36,630

Pure uranous compounds may be prepared by precipitating U(OH)4 from aqueous solution with ammonium hydroxide and dissolving the precipitate in the appropriate acid. Uranous sulfate, the most common salt, is soluble in water, as are the chloride, bromide, and iodide. Uranous nitrate is unstable, gradually undergoing oxidation to uranyl nitrate with liberation of oxides of nitrogen.

Tetravalent uranium can be precipitated from aqueous solution as the insoluble oxalate, fluoride, or phosphate. UF4 precipitated from aqueous solution contains water of crystal­lization. When this compound is heated to drive off the water, it is partially hydrolyzed to an oxyfluoride. The phosphate U3(P04)4 is soluble in hot, concentrated phosphoric acid and appears in this form when uranium in phosphate rock, Ca3(P04)2, is dissolved in sulfuric acid.