is present in apatites, because the ionic radius of uranium(Ⅳ) is cornparable to that of the calcium ion. In the wet process the apatite is digested with dilute sulfuric acid producing wet process phosphoric acid. Raw phosphoric acid produced by the wet process contains, depending upon the ore used, between 50 and 200 ppm U₃O₈. The uranium can be recovered from wet process acid by extraction. The extraction agents used are preferably a mixture of trioctylphosphine oxide (TOPO) and di-(2-ethylhexyl)-phosphate (DEHPA) in kerosene (process developed by the Oak Ridge National Laboratory) or a mixture of mono- and dioctylphenylesters of phosphoric acid. Further developments of the Oak Ridge National Laboratory process are the reduction stripping and oxidation stripping processes.

In the reduction stripping process uranium(Ⅳ) in the raw wet process acid is oxidized to uranium(Ⅵ) by treatment with sodium chlorate, hydrogen peroxide or air at 60 to 70°C, the uranium(Ⅵ) formed being extracted with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate in kerosene and the resulting solution finally reductively stripped repeatedly with aqueous iron(Ⅱ) solutions. This results in an enrichment by a factor of 40. After oxidation of the stripped solution with sodium chlorate or ambient oxygen and renewed extraction of the uranium(Ⅵ) formed with trioctylphosphine oxide/di-(2-ethylhexyl)phosphate, the phosphoric acid is removed from the organic phase by washing. The uranium(Ⅵ) is then stripped with ammonium carbonate and precipitated as the ammonium diuranyl-tricarbonato-complex. This is filtered off, washed and calcined.

In the oxidation stripping process the uranium(Ⅵ) in the raw wet process acid is initially reduced to uranium(Ⅳ), which is extracted with a mixture of mono and dioctylphenyl esters of phosphoric acid in kerosene. Oxidation with sodium chlorate in phosphoric acid transfers the uranium to the aqueous phase and it is then extracted with trioctylphosphine oxide/di-(2-ethylhexyl)-phosphate, as in the reduction stripping process.