Anodizing Solution
Regeneration by Ion Exchange
Durward T. Armstrong—Chemical Engineer
Grumman Aircraft Engineering Corporation
Bethpage, New York
The chromic acid disposal problem can be handled quite economically through the use of ion exchangers. Such a system has been
used quite effectively on a 3600 gallon production anodizing installation of the Grumman Corporation.
In the first stage the chromic acid is reclaimed in the anodizing
bath proper, thereby eliminating completely the necessity for destroying the concentrated parent solution. The anodizing solution is passed
directly through 10 cubic feet of a strong cation resin where the combined aluminum is exchanged for hydrogen ions and free hexavalent
chromium is then returned to the bath. The removal of the cations
from the processing tank is accomplished at such a rate as to maintain
the pH between 0.7 and 0.9.
This operation as used in lieu of making additions of chromic
acid to the tank when the bath requires replenishing. In this way
additions are made only to replace the chromic acid actually consumed
in forming the anodic films. In the North Anodize Room at Plant
2 of the Grumman Corporation more than 12,500 pounds of chromic
acid per shift per year is tied up.
Approximately one-half pound of aluminum must be removed
for every thousand square feet of material processed. The anodizing
cycle is about forty minutes. However, it is not necessary to keep
pace with this as the process is not critical provided the pH range
can be maintained. In the particular case being considered, about
20,000 square feet of material is anodized before a service cycle is
run. During that time, between 8 and 10 pounds of alumina will
be thrown into the bath. In the conventional 10 per cent bath, over
80 per cent of the chromic acid is tied up with alumina. Although
the principal constituent is aluminum, other cations are present including trivalent chromium and traces of alloying elements.
It is no longer necessary to take periodic draw-off increments every
three or four months. The usual draw-off used to be between 25
and 50 per cent of the bath. These portions were sent to the disposal
house and destroyed. It is interesting to note that if a 50 per cent
increment were diluted to 50 p.p.m. the resulting volume would be
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