Pollution Prevention and Control Technologies for Plating
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
- 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
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