Phase Separation and Leaching

The soluble phase which consists primarily of Na20 and B2O3 can be leached out by a leaching agent. If larger pores are desired, additional leaching steps can be taken. 

The pore size of the membrane can also be enlarged by partially dissolving the silica structure through the use of an alkali. Membranes with a pore diameter of 2 to 300 nm can be prepared this way. However, such a porous glass, due to the presence of the rather 

Porous metal membranes similarly can be prepared by this technique of heat treatment followed by leaching with a strong acid, base or hydrogen peroxide [Croopnick and Scruggs, 1986]. 

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An inorganic membrane can be prepared by various methods such as sol-gel, phase separation and leaching.2,3 The sol-gel process is considered the most practical method among those used to prepare inorganic membrane. Sol-gel processing is a simple technology in principle but requires considerable effort to become of practical use. The advantage of this... 

One simple way to increase the solubility of organic substrates in aqueous solution is the addition of co-solvents to an aqueous two-phase system. This increases the reaction rate of the Rh/TPPTS catalyst, but at the same time the selectivity drops [7]. Furthermore, it can result in troublesome phase separation and leaching of the co-solvent into the product phase [7]. Instead of the addition of co-solvents triphenylphosphine can be added to the two-phase Rh/TPPTS system. The local concentration of the active rhodimn complex at the interface will be increased, resulting in a rate enhancement in the hydroformylation of 1-octene by a factor of 10-50 [8]. The active species at the interface contains both TPPTS and triphenylphosphine hgands. Test reactions performed in methanol confirmed that this increase in reaction rate was due to promotion of interfacial catalysis. However, after recycling the catalyst Rh/TPPTS was found in the aqueous phase whereas the PPhs was in the organic phase and in the subsequent reaction with freshly added organic phase the activity of the catalyst is similar to that of the biphasic reaction in the absence ofPPhs. 

A variety of techniques have been developed to produce various porous membranes - such as particle dispersion and dip-coating, sol-gel processing, chemical vapor deposition, phase separation and leaching, pyrolysis, anodic oxidation, and so on [6, 7]. A good method should be of simple operation, good control of membrane quality (no cracks and pinholes, homogeneous thickness, narrow pore size distribution, and sufficient mechanical strength), as well as low cost. 

Over the last 40 years, a significant amount of research has been devoted to the use of PGMs for gas separation. Most of this research was carried out on commercial PGMs (e.g. Vycor). ° However, gas separation membranes should have smaller pores than the Vycor glass membranes, in order to work efficiently. The preparation of such membranes requires optimal phase separation and leaching conditions, as well as the possibility of post-synthetic modification. ... 

The contactor finds extensive use where high performance phase separation and countercurrent extraction or washing in the one unit are required. Particularly important applications are the removal of acid sludges from hydrocarbons, shown in Figure 13.40, hydrogen peroxide extraction, sulphonate soap and antibiotics extraction, the extraction of rare earths such as uranium and vanadium from leach liquors, and the washing of refined edible oils. 

SCCO2 has also found an interesting application not as the solvent for a hydroformylation reaction but as a switch to halt the reaction, induce a phase separation and separate products with negligible metal leaching. Briefly, the hydroformylation of 1-octene was conducted neat in the presence of a catalyst... 

Use a glass composition that phase separates and then leaches out (e.g., using an acid) one of the phases to produce porous glasses. 

Cai Q, Yang J, Bei J, Wang S. A novel porous cells scaffold made of polylactide-dextran blend by combining phase-separation and particle-leaching techniques. Biomaterials 2002 23 4483-92. 

PIMs have been modelled after the P-diketone-containing membranes used by Sugiura (67-69) and are formed by the polymerization of cellulose triacetate (CTA) to form a thin film. Polymerization takes place in the presence of a macrocyclic carrier, and as the thin sheet forms, carrier molecules are trapped within the CTA matrix. The resultant membrane is then placed between aqueous source and receiving solutions and selectively mediates transport of a desired species from one phase to the other. While PIMs can effectively separate two aqueous phases, they are not dependent upon organic solvents to maintain phase separation and allow transport. Thus they are simpler to use than SLMs and do not suffer from loss of organic solvent nor as much leaching of carrier into the aqueous phases. 

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

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