Chemical concentration in Uranium Leaching Methods

4.3 Chemical concentration.
Solutions from leaching have a content 0.3 to 1.0 g/l U₃O₈ and dissolved gangue. This content is low and be increased by ion exchange, and solvent extraction.

Both processes make ion exchange or reactions of double decomposition between the liquor and resins or organic solvent that are selective for uranium and can be loaded until saturation. This is the loading stage.

The reaction is reversible and allows a second stage (elution) where uranium is recovered in a solution with 10 to 30 g/l U₃O₈ and few impurities.

Resins employed for uranium recovery are anionic with a base of tertiary or quaternary ammonium. They are insoluble in water and have two important parts: matrix (R) and functional group (X) that has an exchange ion. The matrix is styrene divynil benzene polymer. The functional groups are tertiary or quaternary amines, and the exchange ion can be Cl^- or SO₄^2- that is linked to a methyl group.

The reactions involved are:

Acid leaching.

4RX + [UO₂ (SO₄)₃]^{4 -} = R₄UO₂ (SO₄)₃ + 4X^-

Alkaline leaching.

4RX + [UO₂(CO₃)₃]^{4 -} = R₄UO₂(CO₃)₃ + 4X^-

For acid leaching pH value is 1.5 to 2.0, and for alkaline leaching is 10.0.

Contact time is 3 minutes, but for practical application 10 to 15 minutes.

There are some interferential anions such as sulphate, bisulphate, chloride, nitrate, and phosphate. The latter can precipitate uranium like uranyl phosphate at pH 2.

Elution can be done employing sodium nitrate and nitric acid or with a solution 10% sulphuric acid.

Elution of alkaline solution is done with alkalis or neutral compounds such as chlorides or nitrates (1M) and carbonate or bicarbonate (0.5M).

The equipment used with resins consists in several columns filled with resin. The contact between resin and solution can be downstream or upstream in three or more stages.

Solvents are liquid mixtures formed by three components: extractant, diluent, and modifier.

The extractant is an active substance with high strength of capturing uranium, and is 2 or 5% of the mixture. Extractants can be classified in three categories:

Formation of addition compounds.

Ionic exchange like resins. Some commercial products are Alamine 336 and Adogen 368.

Formation of chelates.

Ionic exchange extractant is more selective. The reactions involved are:

Solvent activation.

2R₃N_(org) + H₂SO₄ = (R₃NH)₂SO_{4 (org)}

Loading.

2 (R₃NH)₂SO_4(org) + UO₂(SO₄)₃^{4 -} = (R₃NH)₄UO₂(SO₄)₃ + 2SO₄^2-

Re-extraction.

(R₃NH)₄UO₂(SO₄)_3(org) + 5Na₂CO₃ = 4R₃N_(org) + Na₂UO₂(CO₃)₃ + 3Na₂SO₄

The Diluent dissolves extractant and constitutes 90 to 98% of the mixture. The more common is kerosene. Others diluents are gasoline, chloroform, and carbon tetrachloride.

The modifier dissolves all the salts formed in the interphase in order to avoid emulsions. A modifier can be an alcohol with large chain such as decanol and isodecanol.

The equipments employed for solvent extraction are mixer-settlers working with aqueous and organic phases in countercurrent. Usually are necessary 3 to 5 stages so that uranium can be transferred to the organic phase obtaining a sterile aqueous phase and a loaded organic phase.

From loaded organic phase, uranyl is re-extracted using sodium carbonate (8%) or sodium chloride (1M). it is possible use sulphuric acid (0.05M).

The main interferences are Mo, Fe⁺³, and Al.

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