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Pollution Prevention and Control Technologies for Plating Operations
Section 4 - Chemical Solution Maintenance
4.8 DIFFUSION DIALYSIS
4.8.3 Applications and Restrictions
The diffusion dialysis process has been applied for the purification of hydrochloric, nitric, hydrofluoric, phosphoric, and sulfuric acids, including mixed acids; and ferric chloride etchant (ref. 192, 252, 336)
The most common applications of this technology include:
- Recovery of mixed acids of stainless steel pickling baths (ref. 336).
- Recovery of H2SO4/HCl or H2SO4/HNO3 of pickling plants for non-ferrous metals such as zinc and aluminum (ref. 336).
- Recovery of H2SO4 and HCl of steel and non-ferrous metal pickling baths (ref. 336).
- HCl and HNO3 acid rack strips (ref. 192).
- H2SO4 anodizing solutions (ref. 192).
- Regeneration of battery acids (ref. 252).
Like acid sorption, diffusion dialysis is limited in applicability by the strength of acid that it can produce. If highly concentrated acids are processed, the resultant product will not be of sufficient strength for direct reuse and the addition of concentrated acid is not practical because it increases the total volume of the solution beyond the capacity of its tank. A vacuum evaporator could be employed to concentrate the acid to bath strength; however, this would most likely make the overall recovery process uneconomical. Examples of successful applications in the literature are shown in Exhibit 4-35, where the original acid concentrations are in the range of 3.4 to 4.7 N.
The diffusion dialysis membrane material is relatively resistant to chemicals commonly used in the plating shop. However, contact with solvents could cause swelling of the membrane and strong oxidizing agents such as chromic acid can deteriorate the membrane material (ref. 252). The process is tolerant of feed solution temperatures up to 50°C (ref. 252).
A process diagram for diffusion dialysis is shown in Exhibit 4-36. Acid is pumped through one side of an ion exchange membrane stack and water flows through the other side of the membrane in the opposite direction. Prefiltering of the acid is needed to prevent fouling of the membrane. Most of the acid diffuses through the membrane and into the clean water producing a clean acid stream. The metal and a percentage of the acid remains in the original acid stream, which is directed to treatment. The acid product is monitored for strength and adjusted with concentrated acid as necessary.