A discussion of uranium control in phosphogypsum

Abstract

Wet-process phosphoric acid plants produce about 4.5 tons of gypsum for each ton of fertilizer (P 2O 5 equivalent). About 33 million tons of gypsum are produced at phosphoric acid plants in the United States each year, and about 330 million tons are contained in existing stockpiles. Although most of the uranium initially present in phosphate rock is dissolved when the rock is acidulated to produce phosphoric acid, a fraction of it remains undissolved and the gypsum contains 0.03 to 0.08 lb of uranium per ton. Uranium concentrations in filter cakes produced by the hemihydrate process are significantly higher (as much as 0.2 lb/ton). This material hydrates to gypsum in the wastepile and most of the uranium becomes readily soluble in the process; the concentration of insoluble uranium in the hydrated cake is about the same as that in filter cake produced by the dihydrate process. Even at these low concentrations, the gypsum produced at phosphoric acid plants each year contains 1 to 2 million lb of uranium, and the stockpiled gypsum may contain a total of 10 to 20 million lb. Economical recovery of uranium from gypsum appears very unlikely in the foreseeable future. The losses to the filter cake, however, can possibly be reduced by operating the phosphoric acid plant under conditions that maximize uranium dissolution during rock acidulation. Studies of this problem showed that the major variable affecting the distribution of uranium between phosphoric acid and gypsum during phosphoric acid manufacture was the redox potential of the slurry during acidulation. Other variables affecting the uranium distribution included rock size, digestion temperature, and concentrations of excess sulfuric acid and fluoride.