Sulfates leaching

For extraction of uranium from sulfate leach Hquors, alkyl phosphoric acids, alkyl phosphates, and secondary and tertiary alkyl amines are used in an inert diluent such as kerosene. The formation of a third phase is suppressed by addition of modifiers such as long-chain alcohols or neutral phosphate esters. Such compounds also increase the solubihty of the amine salt in the diluent and improve phase separation. 

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

Young et al. (in press) conducted related experiments. They examined the Cu isotope compositions of the solutions produced during acid sulfate leaching experiments aimed at extracting Cu from ore minerals (chalcopyrite, chalcocite, djurleite, bomite). The leaching experiment for chalcopyrite reads ... 

Ardizzone et al. used the esterification of benzoic acid with methanol to test the catalytic performance of different SZ catalysts. " Water had to be continuously removed from the reaction medium to shift the reaction equilibrium to product formation and to avoid catalyst deactivation by sulfate leaching. According to these authors, catalysts with a higher density of acid sites with KdL values in the range —14.2 to -5.6 performed better. Acid sites with pKa. of... 

Amine salts represent the only commercially important class of extractants of the anion-exchange type. Their most widespread use is in the extraction of uranium from sulfate leach liquors, but they have found application in the recovery of cobalt, zinc and copper from chloride solutions, as well as in the extraction of metals that readily form oxyanions, such as vanadium, molybdenum and tungsten. 

The sorption of uranium from acid sulfate leach liquors by strong base anion exchange resins is unusual since complexes of the type [U02(S04) ] " may be sorbed by both ion exchange and addition mechanisms. High concentrations of other species are present in the leach solution due to dissolution of pyritic and siliceous components of the ore, but, apart from iron, they do not interfere with the sorption of uranium as a complex anion. Iron(III) also forms an anionic sulfate complex, but is only weakly held by the resin and is displaced ahead of the uranium. The sorption of uranium may be represented by equations of the type ... 

The alkyl amines offer greater selectivity than organophosphorus compounds in many applications, particularly in uranium hydrometallurgy. Amine extraction is typified by the Amex process, which uses a tertiary or branched secondary amine to extract uranium from sulfate leach liquors (11). A similar process based on the use of a primary or straight-chain secondary amine (sometimes modified with an organic-soluble alcohol) has given good results in thorium recovery (12). 

Cobalt is produced as a coproduct of nickel or copper refining. Copper-cobalt sulfide concentrates can be processed by the RLE process. Mixed cobalt-nickel sulfides can be precipitated from ammoniacal leach solutions and as mixed nickel-cobalt hydroxide or carbonate from acid sulfate leach processes. From chloride leach solutions, cobalt can be separated by solvent extraction. Most cobalt production is associated with nickel production from sulfide and laterite ores. Pressure leaching, solvent extraction followed by the electrowinning of... 

Sayed, M.S.E. (2003). Uranium extraction from gattar sulfate leach liquor using aliquat-336 in a liquid emulsion membrane process. HydrometaUurgy, 68, 51-6. 

Source D. J. Crouse and K. B. Brown, Recovery of Thorium, Uranium, and Rare Earths from Monazite Sulfate Leach Liquors by the Amine Extraction (Amex) Process, Report ORNL-2720, July 16, 1959. 

Using the galvanic stripping process, it is possible to remove iron from an acidic zinc sulfate leach electrolyte and to produce a ferrous sulfate solution having an iron concentration as high as 130 g/L. A zinc concentration as low as 250 mg/1 is possible, but the pH of the strip... 

Ferric-ion complexes are important in acid-sulfate leaching because ferric ion can be generated from fenous ion using air or oxygen in situ. The reduction of ferric iron to ferrous occurs as the ferric-ion complex diffuses through fluid-filled pores and channels in the rock matrix and encounters reactive metals or sulfides. In most instances, as already discussed, the rate of ferric ion reduction is a diffusion-limited process. The oxidation of ferrous iron to ferric in aqueous solution becomes of primary importance because of its in situ regeneration capacity under appropriate oxidation potentials. 

Copper Recovery from Sulfate Leach Liquors.148... 

Ketoximes also operate efficiently in circuits in which colloidal silica is present (they have low tendencies to form crud), have low aqueous solubility (low losses to the raffinate), and good phase separation. However, ketoximes do not generally allow the extraction of copper at pH values below 1.8, and have slow extraction kinetics (particularly in colder climates). Ketoximes exhibit copper selectivity over iron of about 300 1. Their selectivity as a function of pH for other cations commonly found in sulfate leach liquors is shown in Figure 5.5. 

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. 

Today, amines are more widely used for the recovery of uranium from sulfate leach liquors, and uranium recovery is one of the most important commercial uses of amines (Nicol et al. 1987). Plants in the United States (Merritt 1971), Canada (Rit-cey and Ashbrook 1972), Australia (Bellingham 1961), South Africa (Boydell and Viljoen 1982 Finney 1977), and Namibia (Sole et al. 2005 Lewis and Kesler 1980) have been described. 

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