Extraction hydro-diffusion

In membrane extraction of metals, the mass transport of solute from one phase to another occurs by diffusion. It is controlled by phase equilibrium and the resistances of boundary layers in two phases and the membrane material. Both types of materials are used for membrane extraction and stripping, hydrophilic and hydrophobic, and composite hydro-philic/hydrophobic barriers are also developed to avoid the membrane solubilization [122,123]. To enhance separation, the reactive liquids that induce chemical reaction with one of the separated species can be used. In membrane SX of metals, extracting agents, such as tri- -octylphosphine oxide (TOPO), di(2-ethylhexyl)phosphoric acid (D2EHPA), and n-octyl(phenyl)-A,A-diisobutylcarbamoylmethylphosphine oxide (CMPO), and commercial reagents like CYANEX 301, CYANEX 923, LIX622, and LIX622N are applied. 

In many cases, the concentration level obtained by evaporation is relatively high, but the use of the OMD technique avoids any thermal damage or loss of the solutes. The OMD process works in this way a porous hydro-phobic membrane (generally PTFE, PVDF, or PP), is in contact with two different non-wetting aqueous solutions for example a juice solution and a concentrated salt solution (stripping solution or extractant). The different water activity of the two solutions corresponds to two different water vapour pressures. Due to the combination of hydrophobicity and the narrow pore size of the membrane, neither the first solution nor the extractant passes through the membrane pores. Only water vapour is allowed to diffuse, and the water vapour pressure difference between the two sides of the membrane constitutes the driving force of the process. 

Simulation results for the hydrodynamic contribution, Du = D-Do/Am, to the diffusion coefficient are plotted in Fig. 10 as a function of the hydrodynamic radius (85). In the limit Am > 1, the diffusion coefficient D is dominated by the hydro-dynamic contribution Du, since Dh Am. For shorter chains, Do/Am cannot be neglected, and therefore has to be subtracted in order to extract the scaling behavior of Dh. The hydrodynamic part of the diffusion coefficient Z>h exhibits the dependence predicted by the Kirkwood formula and the Zimm theory, i.e., Dh l/f H-The finite-size corrections to D show a dependence D = D , — const./L on the size... 

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