Wet-process phosphoric acid plants produce about 4.5 tons of gypsum for each ton of fertilizer (P2O5 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.