TABLE OF CONTENTS Separation of electrolytes based on the difference in chemical potential across the ion exchange membrane can be classified into: (i) diffusion dialysis, (ii) Donnan dialysis, (iii) neutralization dialysis and (iv) up-hill transport. The natural flux of electrolytes through the membrane is low compared with that in the presence of an electrochemical potential.
Diffusion dialysis was initially applied in viscose rayon production to recover caustic soda using parchment paper as a membrane. Today the largest industrial utilization of diffusion dialysis is to recover acids or alkalis from waste acids and alkalis using anion or cation exchange membranes. Figure 6.28 shows the principle of diffusion dialysis for acid recovery from waste acid solution. The process and membrane performance are evaluated on the basis of the flux of acids or alkalis (dialysis coefficient) and the ratio of the flux of metal salt to that of acids or alkalis (separation coefficient). The total dialysis coefficient [mol h ?1 m ?2 (mol 1 ?1)], U 0, is defined by
where ? m is the flux of acid or alkali and metal salt, A is the effective membrane area (m 2), ? ? is the concentration difference of electrolyte across the membrane (mol 1 ?1) and t is the period of diffusion dialysis (h).
