Cathode rinse stream

The problem of pH control in the cathode rinse stream (catholyte) is usually handled by acidification. Faraday s constant, 96,500 A-sec/eq, is used to calculate the amount of acid required to neutralize the OH" ions generated at the cathode. While acid addition contributes significantly to operating costs of commercial ED plants, the fact that there are hundreds of cell pairs between each pair of electrodes keeps the catholyte acid costs at tolerable levels. In contrast, acidification of the feed stream to prevent precipitation can increase operating costs substantially. 

Kedem et al demonstrated that gas evolution could be avoided by the use of powdered activated carbon in a common electrode rinse stream.19 The large surface of a carbon particle became charged when it came in contact with the anode or with another positively charged carbon particle. This charge was neutralized by sorption of anions from the rinse solution. When the suspension reached the cathode, the charge on the particle was reversed and the sorbed anions were exchanged for cations. With a 2.5% suspension of carbon in 0.02 to 0.2 normal solution, a current of 10-20 mA/cm2 could be sustained for weeks with no gas evolution. 

The total energy consumption of the process is now given by the contribution of the electrical energy to drive the ionic transfer and by the energy of the pumps to circulate the various solutions (see figure Vin - 36). Generally two or three pumps are required for the concentrated and depleted streams and for the anode- and cathode-rinse solutions. The energy consumption can be calculated from eq. VUI - 78. 

Roll cell. These are sandwich constructions consisting of a packed stainless steel or titanium mesh cathode, separator and a screen anode rolled up like a swiss roll. These cells can operate with fluid velocities of 1-10 cm s and with apparent current densities of 10-200 mA cm at the separator. This type of cell is a concentrator device for metal ions, the metal is recovered from the cell by leaching or by anodic dissolution. An economic analysis [23] showed that waste water treatment with this cell is highly competitive with ion-exchange technology. Typical applications for metal ion removal are recovery of copper and Hg from waste stream recovery of Ag from a used fixer solution down to a silver concentration of 0.1 ppm and the treatment of zinc cyanide plating bath rinse waters which contain the Zn(CN)5 complex ion. 

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