Leaching alkaline

A.lkaline Solutions. The most important example of alkaline leach is the digestion of hydrated alumina from bauxite by a sodium hydroxide solution at 160-170°C, ie, the Bayer process (see Aluminumand aluminum alloys). 

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. 

Alkaline leaching is carried out by using sodium carbonate solution. In this case any U(IV) present in the ore must also be oxidized to U(VI). The uranium species soluble in carbonate leach solutions in the uranyl tricarbonate ion. The formation of this ion by solubilization of a hexavalent uranium mineral such as camotite, or a tetravalent uranium mineral such as uraninite, may be represented by the following reactions ... 

The normal choice ( 80% of all U.S. ores) for chemical leaching is acid leaching with sulfuric acid (due to its low cost and great availability). For the ores that have high carbonate content, an alkaline leach with Na2CC>3 is used to prevent high acid concentration. 

In alkaline leaching of high carbonate materials, one takes advantage of the unique solubility of [U02(C03)3]4-. One begins with finely divided material and must also deal with the oxidation of any U(IV) that is present. The basic reaction (pun intended) is... 

Alkaline leach mills will use NaOH for final product recovery (in the form of sodium uranate, Na2U207). Often these uranium ore concentrates (yellowcake) are transformed into U3Os by drying at 200°C to remove water and calcining, that is, heating until decomposition. 

To determine the effect of the final temperature during the alkaline leaching of Cherokee coal, several runs were carried out in which different portions of the coal were treated with 1.0 M Na2C03 for 1.0 hr. at various final temperatures. The alkali-treaTed coaT was then leached with HN03. The results of the alkaline leaching step are indicated in Figure 3 and the overall results in Figure 4. 

Start adding the powdered stibnite to the still hot lye solution while stirring with a non-metal rod. The amount of ore added depends on its quality, but it is better to add it in excess to the weight of lye used. We can do this alkaline leaching several times to pull out all of the antimony. The solution can even be heated to near boiling to hasten the dissolution of the stibnite. 

The organic amine extractants are the most commonly used anion exchangers. Secondary amines have been used to recover uranium from leach liquors (GlO) secondary and tertiary amines to recover molybdenum from uranium mill circuits (L13) a primary amine, diethylenetriamine-penta-acetic acid (DTPA) to extract cerium group lanthanides (B6) tri-,V-butylamine-3-methyl-2-butanonc to separate yttrium and rare earth nitrates (G13) tricaprylyl amine (Alamine 336) and methyltrioctyl-ammonium salt (Aliquat 336) to recover vanadium from acidic solutions (A3) and Aliquat 336 to extract vanadium from slightly acidic or alkaline leach liquor (S36). 

Acid leached ore Alkaline leached ore Uranium precipitate... 

Fig. 7. Alkalinity leached from stabilized/solidified hazardous wastes in upflow leaching columns as a function of total and leachant addition...

Fig. 8. The ratio alkalinity leached to acid leachant added as a function of total leachant acidity in upflow leaching columns...

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