Electrodialysis in the recovery and reuse of chromium from industrial effluents

Abstract

One of the major technical requirements of the leather and other industries employing large quantities of chromium salts in wet processing is the need to minimise the concentration of the metal ion in the effluents. International specifications for permissible levels of chromium stipulate values less than 0.4 ppm. Although several recycling techniques have been recommended in the past, a true technical solution to the problem of tanneries is yet to be forthcoming. In this work, a new possibility to selectively remove neutral salts contained in spent chromium tanning solutions has been presented with a view to achieving more efficient recycling of unused chromium as well as water. An electrodialysis technique using Selemion AMV and CMV membranes and a prototype electrodialysis cell has now been standardized. The application of 4000 mA of current over a period of, typically, 5-6 hr achieved the reduction of sodium chloride concentrations from 3.3 to 0.3% in 5 dm 3 of spent chrome tanning solutions. Simultaneously, sulphate concentrations in the spent solutions were reduced from 3.5 to 1.7 ± 0.1% over the same period. The membrane separation procedure described here offers a new possibility to reuse water, neutral salts and chrome without any attendant problems in process control or effluent treatment.