The ore extracted by open cast or underground mines is coarsely ground in crushers and then finely ground, the thereby released radon being sucked off as a radiation prevention measure. The content is extracted with the help of hydrometallurgical processes, in which the enrichment of uranium via ion exchange or extraction processes and a precipitation process follow the leaching step.

Leaching Processes

The leaching of uranium ores proceeds with acid or alkali, depending upon the composition of the ore. Ores with alkaline gangue are preferably leached with alkali. If uranium(Ⅳ) compounds are present, the leaching is carried out under oxidative conditions. Uranium is leached from low grade uranium ores by trickling the leaching medium through heaps of roughly crushed ore or into the seam itself (in situ leaching). Heap leaching is currently only used sporadically. In-situ leaching is industrially important particularly in Kazakhstan, Uzbekistan and the USA.

In-situ leaching in the seams themselves proceeds with sulfuric acid or carbonate solutions. The leaching agent is fed in via injection tubes into the rock seam and brought to the surface via a central tube. In situ uranium leaching efficiency is 60 to 85%. Currently ca. 5000 t of uranium are extracted in this way.

Leaching with Acid

Dilute sulfuric acid is generally used as the acidic leaching agent. This digests the uranium(Ⅵ) oxide, which mainly occurs in secondary deposits, to uranyl sulfate:



The uranium in primary pitch blend is mainly present as uranium(Ⅳ) oxide and must first be oxidized to hexavalent uranium. This is most easily achieved with the Fe³⁺ ions, which come from the ore itself:

If insufficient trivalent iron is present, it is produced from divalent iron by blowing in air or oxygen or by adding chlorate solution continuously to the leaching sludge.

Between 20 and 1100 kg-of sulfuric acid is used per t ore. Acid leaching generally proceeds at atmospheric pressure.

Bacterial leaching with thiobacillus thiooxidans is also an acid leaching process. Sulfidic sulfur, e.g. in pyrites, is oxidized to sulfate and iron(Ⅱ) is oxidized to iron(Ⅲ), which itself oxidizes uranium(Ⅳ) to uranium(Ⅵ). This process has not yet been operated industrially.

Leaching with Alkali

Leaching with alkali always takes place at high temperatures, either under pressure (5 to 6 bar, 95 to 120°C) or at atmospheric pressure (75 to 80°C). The leaching agent used is sodium carbonate, sodium hydrogen carbonate or ammonium carbonate. Uranium(Ⅵ) oxide is converted in this process into uranyl tricarbonato-complexes:

The ur-anium(Ⅳ) is first oxidized in the alkaline medium by ambient oxygen to uranium(Ⅵ). Leaching with alkali under pressure has been carried out for a long time.

Separation of Uranium from the Leaching Solutions

Ion exchange and extraction with organic solvents have proved effective in separating uranium from the leaching solutions. Combinations of the ion exchange process with solvent extraction are also known. This is normally preceded by separation of the leaching solution from solids by multistage filtration or countercurrent decantation followed by clarification e.g. over a bed of sand.

Separation by ion exchange: Strongly basic or weakly basic ion exchange resins are used to separate selectively the uranium from the weakly acidic or alkaline solutions from the leaching step. The uranium is eluted from the ion exchange resins with nitrate or chloride solutions as anionic (carbonato- or sulfato-) complexes.

The ion exchange is carried out according to different processes. Separation from solid-free solutions can, in addition to conventional fixed bed processes, be carried out continuously with cylindrical columns’ mounted in series. Ion exchange can also be carried out on unclarified turbid leaching solutions, if baskets with ion exchange resin are moved in the turbid leaching solution or by continuous ion exchange. The advantage of the ion exchange process is that uranium can be extracted from very dilute solutions as well as from turbid unclarified solutions.

Sepurution by solvent extruction: Uranium can be extracted from aqueous solutions using extraction agents into the solvent phase, from which it can be stripped. The extraction agents used are phosphorus compounds such as di-(2-ethylhexyl)-phosphate, tri-n-butyl-phosphate and tri-n-octylphosphine oxide as well as primary, secondary and tertiary amines in salt form or as quaternary ammonium salts. The extraction agents are diluted with inert hydrocarbons, preferably kerosene, to concentrations of 4 to 10% by volume. The solubility of the amine salts, particularly the hydrogen sulfates, chlorides and nitrates is increased by adding long chain alcohols (e.g. isodecanol).

The extraction processes, which are mainly carried out in mixer-settler plants, are preferably carried out with acidic leaching solutions. For the different processes see the marginal notes.

Stripping of the uranium compounds proceeds by mixing the separated uranium-containing organic phase with an aqueous solution of sodium chloride, sodium carbonate or ammonium sulfate at a slightly acid or alkaline pH. The uranium present in the organic phase, e.g. as (R₃NH)₄[UO₂(SO₄)₃), goes into the aqueous phase, e.g. in the form of a carbonato-complex: