Isolation of Chemical Substances from Dilute Solutions

Isolation of Chemical Substances from Dilute Solutions 

Ion-exchange membranes may be used in a process aimed at isolation of a valuable chemical substance from dilute solutions. For example, a dye can be extracted from a dilute solution [2]. The process is based on the reversible precipitation of the chemical compound on the surface of a polarized ion-exchange membrane. Electromembrane apparatus with only the one type of ion-exchange membrane (anion - for anion dyes and cation - for cation dyes) are used for these processes. 

The ions are delivered to the membrane by diffusion and electromigration. Because the membrane is not permeable for large dye molecules the concentration of dye increases near the membrane surface and reaches the limit of its solubility. Then, dye crystallization occurs and microcrystals get induced dipole moments. 

The first crystal layer is held by electrostatic interaction. The second and following layers are held by dipole-dipole interaction. After the electric current is turned off dipole-dipole interaction disappears, the precipitate is resuspended and it can been removed with a small volume of water. This method allows not only dye concentration but also dye purification from substances nonperceptible on membranes (for example, readily soluble nonorganic and organic compounds) occurs. 

The following parameters of the process are described flow velocity 0.6 cm/s, C.I. Direct Black 3 concentration -29 mg/1, energy consumption 0.6-0.9 kWh/g for complete dye extraction, current density 50-100 mA/sq. cm, and capacity 430 mg/sq. m membrane surface. The dye concentration after desorption was 1-2 g/1. The highest degree of purity is confirmed by the fact that the actual specific conductivity of dye produced is only a fraction of that reported in the literature. Purification of colloids and ionized substances nonpermeable for an ion-exchanger membrane may be done using this method. 

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