Nitric acid leach liquors

Uranium Extraction from Ore Leach Liquors. Liquid—Hquid extraction is used as an alternative or as a sequel to ion exchange in the selective removal of uranium [7440-61-1] from ore leach Hquors (7,265,271). These Hquors differ from reprocessing feeds in that they are relatively dilute in uranium and only slightly radioactive, and contain sulfuric acid rather than nitric acid. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

The leach liquor is first treated with a DEHPA solution to extract the heavy lanthanides, leaving the light elements in the raffinate. The loaded reagent is then stripped first with l.Smoldm nitric acid to remove the elements from neodymium to terbium, followed by 6moldm acid to separate yttrium and remaining heavy elements. Ytterbium and lutetium are only partially removed hence, a final strip with stronger acid, as mentioned earlier, or with 10% alkali is required before organic phase recycle. The main product from this flow sheet was yttrium, and the yttrium nitrate product was further extracted with a quaternary amine to produce a 99.999% product. 

Recovery from Nitric Acid Leach Liquors... 

Under certain conditions, there are definite advantages in using hydrochloric, nitric, or other acids to carry out a dissolution step. In their evaluation of proposed processes for the recovery of alumina, Peters ei al. (P8) cited earlier experimental work which showed that both hydrochloric and sulfuric acid are equally good in extracting alumina from calcined clay (TIO). In the separation of the leach liquor from the silica residue by filtration, the chloride solution rapidly separated, while the sulfate solution did not separate easily. In addition to ease of filtration, the hydrochloric acid leach also made the later removal of iron easier. The insolubility of titanium dioxide in hydrochloric acid also eliminated another separation problem. Under this particular situation, hydrochloric acid was the natural choice. As in most large leaching operations, the acid would be recovered and recycled. 

Sheared fuel is fed to a receiving basket held in one compartment of a three-compartment semicontinuous dissolver. Nitric acid is fed continuously to the compartment and leach liquor is discharged continuously from it to the feed adjustment tank. When the basket is filled with fuel, the compartment holding it is rotated to a second position in which dissolution of the remaining oxide fuel in additional acid is completed. The basket and fuel hulls are finally rotated to a third position where water washes the residual leach liquor into the feed adjustment tank. The nitric acid contains 5.6 g of gadolinium as nitrate, to prevent criticality. 

This sludge contains 2-8 % rare earths. As large amounts of this sludge are produced, they are a valuable source of rare earths. The sludge is washed and leached with dilute nitric acid, to which calcium nitrate has been added. From the leach liquor, the rare earths are recovered by solvent extraction and finally precipitated as rare earth oxalates. The oxalates are calcined in a rotary furnace to yield mixed rare earth oxides of 89-94 % purity. 

Solvating extractants include the substances such as trialkyl phosphates, ketones and carbitols the most well known example being tributyl phophate which was originally used for the extraction of uranyl nitrate from leach liquors containing nitric acid. A number of other organophosphates have proved of value in the concentration and separation of the lantanide and actinide elements (46). 

The leaching operation is carried out fairly rapidly in a semi-continuous manner. The vessel is first filled with 2 per cent nitric acid and then further acid and coarsely ground reaction cake are added simultaneously over a period of time. The bottom valve is opened sufficiently to allow the salt solution to drain away at the same rate as fresh acid is added, so maintaining a constant liquor level in the vessel. The titanium metal remains behind on the filter until the conclusion of the run. It is then taken away as a slurry, via an exit pipe just above the filter level. The slurry is finally dewatered in a rubber-lined batch centrifuge and dried in air at a moderate temperature. 

The recovery of uranium from ores uses SX to reject impurities and concentrate the uranium in solution so that it can be economically recovered (Gupta and Singh 2003 Lloyd 1983). The choice of extractant depends on the lixiviant used in the upstream leaching operation, which, in turn, depends on the type of ore in which the uranium is found. Most nranium-bearing ores are readily leached in sulfuric acid and the uraninm is recovered by SX using amines or dialkylorganophosphorus acids. Phosphate ores (snch as those in Florida) are leached in a mixture of sulfuric and phosphoric acids or in phosphoric acid alone. Hot nitric acid has also been used as a lixiviant for nraninm ores (as at Phalaborwa, South Africa). The two common extraction systems for the recovery of uranium(VI) from sulfate leach liquors are compared in Table 5.6. 

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