Extra leaching

In dump leaching of copper, it is not necessary to use extra stages of extraction to recover all the copper from the solution, since the raiSnate will be recycled to the dump. 

The total concentration of OH groups in dealuminated zeolite Y samples was determined using 1H-NMR techniques described (40) and is in reasonable agreement with the concentration of tetrahedrally coordinated extra-framework Al. The concentration of SiOH groups does not change in hydrothermal dealumination, but increases on acid leaching due to removal of framework Al. 

Steaming usually produces extra framework aluminum, but even in the case of leaching some extra framework aluminum may be left on the mordenite. In some cases they can be removed by ammonium ion exchange, if desired, followed by calcination. However, depending on the amount and location of the extra framework aluminum, as the case may be, these species may increase or decrease the catalyst activity. An early review on the dealumination of mordenite was given by Karge and Weitkamp (14). In particular for hydroisomerization of paraffins, this effect has been studied by A. Corma et al. (15, 16). 

Hydrous zirconium oxide and phosphate may be obtained from Bio-Rad Laboratories, Richmond, Calif. As ordinarily supplied, the oxide is severely contaminated with chloride, but at a modest extra cost, Bio-Rad will provide oxide in the nitrate form from which most of the chloride has been leached with 0.1 if nitric acid. This is the material that has been used, but even it requires exhaustive washing to remove the last traces of chloride, t Perchloric add may be substituted for nitric. 

Corrosion of the Pu-doped pyrochlore-based ceramics is by incongruent dissolution and segregation of secondary phases [40], Leach rates in 1-year PCT-B tests were found to be (in g/(m xday) Ca - 10, Gd - 10, Pu - 10, Zr - <10. The leach rate of Pu is reduced by one order of magnitude - from lO" g/(m day) in short tests (1 to 7 days) to 10 g/(m xday) after 112-324 days of interaction with water [105], Introduction of 8 to 15 wt.% of oxides of typical contaminants (F, Cl, Na, Mg, K, Na, Si, Al, Ga, Mo, W, etc.) yielded extra glass, perovskite, and Ca-Al-Ti phase instead of brannerite [117]. This does not affect or even improve the chemical durability with respect to actinides. 

The dissolution rate of zircon in water (Soxhlet tests) decreases from 4.1x10 at 250 to 4.6x10 g/(m xday) at 90 °C [184]. Synthetic ceramics doped with of 6 to 10 wt.% Pu were produced [185]. Leach rates of Pu (MCC-1, 90 °C) were found to be 7x10 (sample with 6 wt.% Pu) and 0.2 g/(m xday). In the sample with 10 wt.% Pu, extra PUO2 was also present. Zircon is amorphized at a dose of 10 a-events/g or 0.6 dpa [72,186,187] and exhibits significant volume expansion (by 18-20%). This leads to microcracking [188] and an increase of the leach rate. Zircon has a rather low capacity with respect to trivalent actinides... 

A commercial H-mordenite (Norton Zeolon 900 H) with a Si/Al ratio of 5.9, determined by chemical analysis, was the starting material. The dealumination was performed with HNO3 at 90°C under different acid concentrations and leaching time (4). The following chemical Si/Al ratios were obtained 5.9, 7.3,11.0 and 16.9. In all the samples (included the commercial H-Z) extra-lattice aluminum was detected by 27a1 MAS NMR (3). All samples were calcined in oxygen at 400°C. From here onwards, catalysts will be designated as HMRF (where R is the Si/Al ratio and F indicates fresh) and HMRD (where D indicates deactivated). 

Encapsulation refers to the confinement of a liquid solution within small capsules enclosed by a polymer or a surfactant. A potentially high interfacial area is thus created and the recovery of the catalyst is facilitated. The selective sorption through the membrane can further increase catalytic performances. Scaling-up is easy, but capsules should be as small as possible to prevent extra resistance to mass transfer in the non-agitated encapsulated volume. A problem associated with such capsules is the fact that there is no way to provide a fresh solution to the inner portion of the capsule or to continuously remove product from that phase. The capsules have to be either leached or broken at the end. 

As previously stated, leaching is another extraction process in which a liquid is used to remove soluble matter from its mixture with an inert solid. With a few extra considerations, the equilibrium analysis of leaching is the same as for liquid extraction. Several assumptions are made in designing leaching processes. These can be rendered correct with the proper choice of solvent. It is assumed that the solid is insoluble in the solvent (dirt will not dissolve in water) and the flowrate of solids is essentially constant throughout the process. The solid, on the other hand, is porous and will often retain a portion of the solvent. 

Numerous discussions by telephone and fax have helped the translator, Mr. R. G. Leach, to import a flavor of my personal writing style into the English edition. The main aim is to prevent fatigue and to inspire the reader to read on. Also, as a small extra , my ideas for two new cartoons have been excellently translated into actual drawings by Norbert Barth. 

After steaming and acid leaching extra-framework alumina has been removed from both the exterior of the crystals and the mesopores as can be deduced from the reconstructed image of XVUSY in Fig. 8b as well as from the Si/Al ratio (Table 1). The light areas in this slice of the 3D reconstruction are interpreted as mesopores whereas the grey zones relate to the crystal lattice. The size of the mesopores (4-40 nm) is in excellent agreement with results from physisorption [27,29]. Please note that these detailed conclusions cannot be obtained reliably from a conventional TEM image (Fig. 8a). 

In agreement with previous studies, microcalorimetry confirms that in steamed products, most of the strong acid sites are poisoned by cationic extra-framework A1 species. These sites can be recovered by an optimized post-steaming acid leaching. Isomorphously substituted HY which is free of extra-framework cationic species possesses more acid sites than conventional dealuminated solids with similar framework Si/Al ratio. 

In a very rough approach, the amorphous phase Si/Al ratios y have been chosen so that the zeolitic fraction (1-z) is equal to the XRD crystallinity of the solids. It appears, from these calculations, that y varies from 0 to about 2 for (HT) and (HTA1) solids, (Table I) which is in qualitative agreement with STEM results (31). Concerning the (HTA2) solids, y can reach very large values, (>20), which is consistent with the fact that the severe acid leaching extracts most of the extra-framework species. With these values for y, only approximate NH /Al ratios can be calculated (Table I). 

However conventional HY, even those which have been submitted to an optimized acid leaching, possess less acid sites than the (IS) solids. This is not surprising since (HT) and (HTA1) HY are not purely zeolitic, they contain variable amounts of extra-framework material. This extra-framework material is mainly responsible for the Lewis acidity of (HT) and (HTA1) solids while the (IS) solids are nearly pure Bronsted acids. 

Extra-framework A1-0H groups. Acidic and non-acidic extra-framework A1-0H groups are evidenced in solids steamed at moderate temperatures, i.e. below about 973 K. Such groups are not detected in solids treated at higher temperatures since under these conditions they are likely to dehydroxylate. The mild acid leaching does not eliminate all the extra-framework OH bands but leads to a severe decrease of the non-acidic contribution. 

The different performances of catalysts (A) and (B) was elucidated by characterization of these materials [29]. Al and Si MAS NMR showed that after treatment with 0.01 M HCl most of the amorphous silica material is removed from the parent catalyst (A), leaving extra-framework aluminum species also created by the steaming procedure [29]. This can be readily understood, because the solubility of silica is maximum at pH 2 [34]. It is believed that the silica species blocked most of the catalytically active centers, i. e. the highly dispersed Lewis acidic, extra-framework alumina sites, which seem to be partly bonded to the zeo-litic framework of the starting material (A). The EFA species are not, therefore, leached out. 

---