Uranium in phosphate rock

Uranium is a trace constituent of all apatites in amounts that typically range from <0.001 to 0.003 percent for guano and guano-derived deposits; from 0.001 to 0.010 percent for igneous apatites; from 0.005 to 0.030 percent for marine phosphorites. Uranium may be enriched to as much as 0.05 percent in phosphorites reworked in a marine environment, and isolated bones and concretions may contain as much as 0.8 percent uranium as a result of enrichment by ground water.

Uranium as U(IV) replaces calcium in the apatite structure. Possibly some U(VI) occurs as U0₄^-2 groups. Uranium is readily removed from apatite by weathering, and enrichment of phosphate after deposition may be entirely residual, as in the case of the brown rock deposits of Tennessee, or uranium may be added as in the case of the lateritic weathering of the deposits in Florida.

Uranium has been recovered as a byproduct of the manufacture of phosphoric acid by the wet process. In 1972, about 15 million tons of phosphate rock was used to make phosphoric acid in the United States. This phosphoric acid probably contained about 1500 tons of uranium. An additional 10 million tons, worldwide, was made into phosphoric acid, and thus, 1000 tons of uranium was in solution. Increased demand for phosphatic fertilizers and increases in the amount of phosphate rock used to make phosphoric acid makes it likely that tonnages of uranium in phosphoric acid will reach 5000 tons per year within the next 25 years. Thus, marine phosphorites are a significant source of uranium for the future.