Aqueous leaching

An alternative process for opening bastnasite is used ia Chiaa high temperature roastiag with sulfuric acid followed by an aqueous leach produces a solution containing the Ln elements. Ln is then precipitated by addition of sodium chloride as a mixed sulfate. Controlled precipitation of hydroxide can remove impurities and the Ln content is eventually taken up ia HCl. The initial cerium-containing product, oace the heavy metals Sm and beyond have been removed, is a light lanthanide (La, Ce, Pr, and Nd) rare-earth chloride. 

AH commercial copper extractants selectively extract copper from weakly acidic aqueous leach solutions by the general reaction... 

At low (>450° C) temperatures, the presence of these materials, particularly the oxides, leads to simple masking or fouling. In some cases, a catalyst that shows reduced activity beheved to be from poisoning may simply be masked, and activity can be rejuvenated by cleaning with aqueous leaching solutions (21). 

Table 5.5 Typical pyrometallurgical treatments prior to aqueous leaching.

The Relative Retention of Biocide after Aqueous Leaching... 

Figure 10 Effect of carrier after 24 hours aqueous leaching on emulsion paint containing 1200 ppm OIT without carrier, with zeolite and with silica (left to right).

Another useful approach to this problem has been made by Whistler and coworkers,23a who have presented electron micrographs of the outside structure and internal structure of the granule. Studies of the effect of aqueous leaching and of preferential hydrolysis by acids20 and enzymes23b on the granule are also important. 

Meyer31 has separated maize and potato amyloses into two fractions each, by (1) aqueous leaching, followed by (2) dissolution and precipitation of the remainder. His results of colorimetric, end-group determinations of size were ... 

A 0.5-g aliquot is analysed from the homogenized sample using an aqueous leach to target and extract weakly bound hydrocarbons from the surface of the sample particles or grains. 

It is the surface of the particle that is directly accessible to extraction by aqueous leaching in the environment or by body fluids following inhalation or ingestion (11, 12, 14). 

Vance, E. R., Watson, J. N. et al. 2000. Crystal chemistry, radiation effects and aqueous leaching of brannerite, UTi206. Ceramic Transactions, 107, 561-568. 

Soroczak, M. M., Eaton, H. C. Tittleb aum, M. E. 1987. An ESCA and SEM study of changes in the surface composition and morphology of low-calcium coal fly ash as a function of aqueous leaching. Materials Research Society Symposium Proceedings, 86, 37-47. 

Fresh glass sample surfaces were produced immediately prior to leaching by fracture. These were exposed to static aqueous leaching at 25°C in a Teflon vessel for periods of 1 minute to 3 days. At several reaction times, samples were also leached in an ice-water mixture ( 0°C). Blank specimens were fractured at the same time and left in contact with the ambient atmosphere. 

Figure 1. SIMS depth profiles in a simulated nuclear waste glass. Major and minor elemental profiles are shown for fractured surfaces exposed to 25° C aqueous leaching for 2 d (a) and 50 d (b). (Reproduced, with permission, from Ref. 4. Copyright 1980, North-Holland Publishing Co.)...

In Figure 2 the positive ion intensities are plotted as a function of time for several major elements in the glass. These depth-composition profiles are compared for unleached and 30 minute aqueous leaching conditions. For each case, the results of profiles on two separate samples have been shown (solid and dashed lines), so that the reproducibility of the method can be judged. 

In Figure 4, the changes in ion intensity for two different reaction times are plotted for 0°C aqueous leaching. The ion... 

Figure 2. SIMS depth profiles for a sodium borosilicate glass blank sample, air-exposed only (a), and sample exposed to aqueous leaching at 25°C for 30 min (b).

Figure 3. Comparison of SIMS depth profiles of aqueous leaching of a sodium borosilicate glass, for 30 min (a). Key ----------, 0°C and--------, 25°C. Error func-...

Figure 4. SIMS depth profiles for a borosilicate glass exposed to 0°C aqueous leaching for periods of 5 (-----------------------) and 30 (------) min.

All the by-products of the reaction, and the excess reactants, are soluble in water or dilute acid thus, if the refractory metal is unreactive toward water, aqueous leaching provides a good method for isolating the product. If appreciable sodium remains, however, it should be killed with alcohol before attempting the aqueous leaching. 

Alcohols, biochemical production from carboxylic acids, IV, 108, 109 effect on aqueous leaching of starch, I, 263... 

Chemical stabilization is the alteration of the chemical form of the contaminants to make them resistant to aqueous leaching. Solidification/Stabilization processes are formulated to minimize the solubility of metals by controlling pH and alkalinity. Entrapment or microencapsulation may immobilize anions, which are more difficult to bind in insoluble compounds. Chemical stabilization of organic compounds maybe possible, but the mechanisms involved are poorly understood [22]. 

Gold leaching, Acidified organic solvents. Dissolution rate, Non-aqueous leaching... 

By /3-amylase conversion studies, R. W. Kerr of this laboratory has reached similar conclusions regarding the effectiveness of aqueous leaching methods. 

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