Dissolution leaching

For chlorine, low results obtained with HNO, addition were suspected to be due to an incomplete dissolution (leaching of Cl rather than attack) which justified the rejection of one set of results. Chlorine was certified on the basis of 10 sets of results (8 IC, 1 SPEC and 1 argentometric titration). The certified value is (323 22) mg kg. ... 

The analysis of the reaction mixture indieated that no catalyst dissolution/leaching occurred during the reaction, i.e. the liquid phase sample from the reactor did not contain any ionic liquid [NB4MPy [BF4 ] within the detection limit of ca. 2 10 g/ml (8.4 10 mol/dm ). According to ICP-MS measurements the reaction mixture contained 45 pg dm (58 ppb) Pd proving that the metal leaching was very low. 

In general, the foUowing steps can occur in an overall Hquid—soHd extraction process solvent transfer from the bulk of the solution to the surface of the soHd penetration or diffusion of the solvent into the pores of the soHd dissolution of the solvent into the solute solute diffusion to the surface of the particle and solute transfer to the bulk of the solution. The various fundamental mechanisms and processes involved in these steps make it impracticable or impossible to describe leaching by any rigorous theory. 

The rate of dissolution is limited by oxygen availabiUty rather than by cyanide concentration. When oxygen solubiUty is reduced by water salinity or by consumption by ore constituents such as sulfide minerals, enrichment of the air with oxygen or addition of hydrogen or calcium peroxide improves leaching kinetics and decreases cyanide consumption (10). 

Bacterial leaching is another example of oxidizing dissolution whereby specific bacteria either directiy attack the sulfide mineral or indirectiy enhance the regeneration of the oxidant. 

AH operating facilities shear the spent fuel elements into segments several centimeters long to expose the oxide pellets to nitric acid for dissolution. This operation is often referred to as chop-leach. The design and operation of the shear is of primary importance because (/) the shear can be the production botdeneck, and (2) the shear is the point at which tritium and fission gases are released. 

Syneresis of sodium silicate gels may occur under some conditions, eg, in pure gels or coarse formations. Cement grouting should then precede chemical grouting. Leaching that results from dissolution under water-saturated conditions may be eliminated by use of proper reagent proportions. 

In most ores, sufficient Fe is already present. For some ores, it is necessary to add metallic iron. In practice, the oxidation potential of the solution can be monitored and controlled using the Fe /Fe ratio. Very high leaching efficiencies with H2SO ate common, eg, 95—98% dissolution yield of uranium (39). If acid consumption exceeds 68 kg/1 of ore treated, alkaline leaching is preferred. The comparative costs of acid, sodium hydroxide, and sodium carbonate differ widely in different areas and are the determining factor. 

Acid Leaching. DHect acid leaching for vanadium recovery is used mainly for vanadium—uranium ores and less extensively for processing spent catalyst, fly ash, and boiler residues. Although 20 in spent catalysts dissolves readily in acid solutions, the dissolution of vanadium from ores and... 

Australian Vanadium—Uranium Ore. A calcareous camotite ore at YeeHrrie, AustraHa, is iU-suited for salt roasting and acid leaching. Dissolution of vanadium and uranium by leaching in sodium carbonate solution at elevated temperature and pressure has been tested on a pilot-plant scale... 

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