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Abstract

A method for the recovery of acids from dilute aqueous acid solutions containing at least one metal cation such as Zn, Ca, Mg, Na, K, Mn, Al, Fe, Ni and Co comprises feeding solution to the dialyzate compartments of a dialysis unit. The dialyzate, substantially containing any metal cations, is removed from the process. The diffusate, containing the acid from the feed solution and being substantially free of metal cations, is fed into the dilute stream circulating through the diluate compartments of an electrodialysis unit. A portion of the circulating diluate is returned to the diffusate compartments of the dialysis unit. The concentration gradient of acid anions across the dialysis membranes is increased by adding a quantity of acid-receiving solution to the returned diluate portion. The feed rates of solutions to the compartments of the dialysis unit are at least double those in processes using only dialysis. A concentrate stream is recirculated through the concentrate compartments of the electrodialysis unit. The applied current causes an increase in the acid concentration in the concentrate. A portion of the concentrate is withdrawn as a concentrated acid product substantially free of metal cations. If desired, the acid concentration in the product is controlled by feeding a quantity of the diffusate from the dialysis into the circulating concentrate stream, as necessary. The dialysis is carried out at ambient temperatures up to 45.degree. C., and electrodialysis at from 0.degree. to 60.degree. C. and at 100 to 1000 A/m.sup.2.

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