1、英文原文Controlled Precipitation Control of the flow rate to the precipitation circuit is considered to be the key to the optimum metallurgical balance of the plant. In view of the closed circuit operation this flow rate affects the amount of wash solution returned for use in the filter circuit and thus
2、 the wash efficiencyAs a result, decreasing the flow rate of the pregnant solution to precipitation decreases the amount of caustic required for neutralization of bicarbonate，but at the same time，decreases the wash efficiency and results in higher uranium soluble losses to tailingsA compromise based
3、 on a ratio of 0.65 ton of pregnant solution fed to the precipitation circuit per ton of ore has been found to give maximum economy while still providing sufficient solution in the filter circuit for repulping of the filter cakes between stagesTo keep this ratio compatible with the ratio of 1.0 ton
4、of solution per ton of ore required in the leaching circuit requires recycle of part of the pregnant solution to the mill solution tankThis results in a buildup of U3O8 in the pregnant solutions to a level between 3.0 and 3.5 grams per liter at equilibriumDeficiencies in the sodium balance due to de
5、creased caustic addition are compensated by direct addition of soda ash to the preheat tanks preceding the leach circuit Yellow cake is precipitated from the pregnant solution by the addition of caustic soda and this product is purified in a second treatment stage by roasting and water leaching to r
6、emove vanadium In the primary precipitation stage the solution is first heated to l65and recycled yellow cake is added to increase the soluble U3O8 content from an initial value of about 3.5 grams per liter to a final value of about 7.5 grams per literThis technique has been found to improve the eff
7、iciency of the caustic precipitation from 93 percent for the normal solution to better than 98 percent for the enriched solutionHeating of the solution is accomplished by heat exchange with the autoclave discharge slurryYellow cake is recycled in an amount equivalent to 500 percent of the U3O8 in th
8、e feed solution and partially dissolved in a series of two agitated tanks providing 5 hours of residence timeUndissolved yellow cake remaining in the solution has no apparent effect on subsequent precipitationThe caustic precipitation step is accomplished in a series of eight steam-coil-heated and a
9、gitated tanks in series which provide eight hours of residence timeCaustic is normally added to the first tank in the series to obtain an excess of approximately 5.0 grams of NaOH per liter in the discharge from the final tankThe solution temperature remains at 165 Following precipitation the yellow
10、 cake is dewatered in a 40-foot diameter by 12-foot deep thickener to 35 to 40 percent solids in the underflowPart of this underflow is recycled to the dissolution tanks and the remainder further dewatered by a disk filter in preparation for treatment in the purification circuit Barren solution over
11、flowing the yellow cake thickener is clarified by passage through Shriver filter presses and then recarbonated with boiler flue gas in packed towersThe treated solution，containing from 2 to 3 grams of NaHCO3，40 to 45 grams of Na2CO3，and about 0.07 gram of U3O8 per liter，is used to wash the second-st
12、age filter cake in the liquid-solid separation circuit previously describedThe concentration of bicarbonate is purposely maintained at a low value during recarbonation since additional bicarbonate is formed by the oxidation of sulfides in the leaching circuitPurification The yellow cake，as precipita
13、ted with caustic soda，contains 5 to 6 percent V2O5 and 2 to 2.5 percent CO32-，both exceeding specifications and requiring a purification stepTo accomplish this，the disk filter cake is repulped with water to about 70 percent solids in an agitated tank，and 0.75 to 1.0 pound of Na2CO3 is added per poun
14、d of yellow cakeThis slurry is then fed to an 8 1/2-foot diameter，6-hearth roaster，having sufficient capacity to hold the yellow cake for approximately 1/2-hour on the last two hearths at a maximum temperature of 1,580The calcine discharges to a water-cooled screw conveyor which reduces the temperat
15、ure to between 200 and 300 enroute to a water-quenching tank The high vanadium content of the initial caustic precipitate is presumed to be due to coprecipitation with the uraniumBy roasting in the presence of soda ash this vanadium is converted to the water-soluble sodium vanadate which is then dis
16、solved by water leachingCarbonate in the initial caustic precipitate is probably present due to physical occlusion during precipitation and is either decomposed during roasting or is released from the initial crystal structure and subsequently dissolved during water leaching. The discharge from the quench tank is dewatered in a 16-foot diameter by 10-foot deep thickener followed by two 8-foot
