As an example of uranium concentration by carbonate leaching, a brief description will be given of the uranium mill owned by United Nuclear-Homestake Partners at Grants, New Mexico fJ1, М3]. Mill capacity in 1977 was 3500 short tons ore per day. The ores treated are primarily sandstones, whose silica or limestone grains are cemented by intergranular uranium-bearing minerals such as coffinite, uraninite, tyuyamunite, and carnotite. Average ore composition is 0.21 w/o и308, with small amounts of vanadium, molybdenum, and selenium.

In this mill, uranium is recovered from carbonate leach liquor by precipitation with sodium

1. Alkaline leach

or ammonium hydroxide. Another U. S. mill employing carbonate leaching recovers uranium by anion exchange (Table 5.19).

Crushing and grinding. Figure 5.4 is a schematic flow sheet of the principal process steps in this mill. Ore is crushed dry to particles smaller than 0.5 in. Crushed ore is ground in two conical ball mills with recycled filter washings containing sodium carbonate and bicarbonate and some recycled uranium. Ball mills are operated in closed circuit with a spiral classifier which returns oversize particles to the mill. Classifier overflow is 16 to 20 percent solids, with 95 percent finer than 48 mesh and 65 percent coarser than 200 mesh. This is finer grinding than used in the Kerr-McGee acid-leach mill (see Sec. 8.5).

Leaching. Ground slurry is divided between two similar leaching circuits. The slurry is thickened to 52 to 54 percent solids in 20-in (0.5-m) diameter cyclones and 100-ft (30-m)

Figure 5.4 Schematic flowsheet, United Nuclear-Homestake Partners uranium mill.

diameter settling tanks, with overflow recycled to grinding. The slurry is preheated to 63°C, and sodium carbonate is added up to 35 to 37 g Na2C03/liter, with about 7 g NaHC03 /liter. Slurry is pumped at a pressure of about 4.5 atm in series through two lines of four stirred autoclaves 12 ft (3.7 m) in diameter and 16 ft (4.9 m) high, which are heated to about 95°C, with a residence time of about 4.5 h. Air is pumped through the autoclaves to oxidize tetravalent uranium.

Less soluble ores are leached for an additional 36 h at 85°C and atmospheric pressure in nine large tanks 19 ft (5.8 m) in diameter by 38 ft (11.6 m) deep, stirred, and supplied with additional air. Enough excess sodium carbonate is used to bring the pH up to 11.

Filtration. Uranium-bearing solution is separated from leached solids in three filter stages with countercurrent washing. Each stage consists of five 650-ft2 (60-m2) and two 570-ft2 (53-m2) rotary-drum vacuum filters, operated batchwise. Filter cake from the fust stage is washed with filtrate from the third stage; filtrate and washings from the first stage constitute the leach liquor, which contains 3 to 3.5 g U308/liter. Filtrate and washings from the third stage are used in the first stage to wash the filter cake and then to reslurry it and move it to the second stage. Filter cake from the second stage is washed and reslurried with sodium carbonate solution; the filtrate and washings containing some uranium are returned to the ball mills. Filter cake from the third stage is washed with dilute carbonate solution from the tailings pond; filtrate and washing? are used to wash and reslurry filter cake in the first stage. Washed filter cake from the third stage is reslurried and pumped to the tailings pond.

Leaching and washing reduce the U308 content of tailings to around 0.01 percent.

Precipitation. Leach liquor from the first stage filters is clarified in a thickener, mixed with about five times its content of recycled yellow cake,1*” heated to 74°C, and held about 5 h in stirred tanks. Recycle increases the soluble uranium content from about 3.5 to 7.5 g U3Os/liter and was found needed to improve the completeness of uranium precipitation in the next step. Precipitation of uranium by the reaction

2Na4U02(C03)3 + 6NaOH -* Na2U207 + 6Na2C03 + 3H20

is accomplished by adding sufficient sodium hydroxide to leave an excess of 5 g КаОНДкег after reaction. The mixture flows through eight heated and stirred tanks in series for an 8-h residence time at 74°C. Reaction product is filtered to produce an impure solid Na2U207 and a solution of NaOH and Na2C03. The solution is recarbonated with flue gas and returned to the leaching circuit via the second-stage filter wash.

Purification. The impure Na2U207 contains 5 to 6 percent V205, probably coprecipitated with the uranium. This impurity is converted to soluble sodium vanadate, NaV03, by roasting the impure yellow cake with about an equal mass of Na2C03 at 860°C for a half-hour, cooling the mass, and leaching it with water. The leached product is filtered and washed, producing a solution from which vanadium is recovered, and a yellow cake that, after drying, contains about 85 percent U308, 0.2 to 0.8 percent V205, and up to 7.5 percent Na.