Phosphoric Acid Recovery System of the
Detroit Gasket and Manufacturing Co.
C. BUCKINGHAM, Metallurgist
Extruded Metals Division
Detroit Gasket and Manufacturing Company
Belding, Michigan
Detroit Gasket and Manufacturing Company's Extruded Metals Division is
located on the Flat River in Belding, Michigan.
In an average year it uses approximately 485 tons of 85 per cent phosphoric
acid in the chemical brightening of the fabricated aluminum parts it produces.
Since none of this acid is an integral part of the finished product, the entire
amount, as phosphates of one type of another, is either discharged into the Flat
River thereby creating a serious pollution problem or it is recovered to the extent
that phosphate pollution is kept well within acceptable limits.
Since continued pollution was out of the question, the company sought an
economical and efficient method of recovery. After considering a number of
possible approaches, the present system, suggested by H. R. Tyler of Mobil
Chemical Company, was adopted as the most practical for our requirements. The
system has been in operation since April 14, 1966.
The essentials of the system are 1) a means for maintaining and collecting a
rinse solution of approximately 30 per cent phosphoric acid equivalent content, 2)
a means for concentrating the 30 per cent rinse to a solution of 70 per cent phosphoric acid equivalent, and 3) storage facilities for holding the 70 per cent solution until it is shipped from the plant.
Figure 1 is a schematic arrangement of the equipment for maintaining and
collecting the 30 per cent phosphoric acid rinse. This arrangement of rinse tanks
is known as a cascading or counterflow system; the rinse waters flowing or cascading from tank to tank in one direction and the work loads moving from tank to
tank in the opposite (or counter) direction. The arrangement allows work to travel
through successively more dilute (cleaner) rinses while the more dilute rinses
overflow into rinses of higher acid concentration, thus the phosphoric acid in the
rinses is always moving toward the most concentrated or first rinse tank. The
source of all the phosphoric acid and phosphates involved is the bright dip solution
(not shown in Figure 1). This solution contains, by weight, approximately 68 per
cent phosphoric acid, 3 per cent nitric acid, 45 g/1 of aluminum as aluminum
phosphate and sufficient water to maintain the specific gravity at about 1.74.
Even at a working temperature of 210 F the solution is somewhat viscous.
As the work load of bright dipped parts is transferred from the bright dip to
the first (or 30 per cent) rinse the surfaces of the parts are coated with the viscous
solution containing about 68 per cent phosphoric acid.
To prevent "back etching" of the chemically brightened work pieces,
the transfer is effected in less than 20 seconds, allowing only limited
drainage of solution. The first rinse, operated at 85 to 90 F readily removes the
dragged out solution. The danger of "back etch" is eliminated and ample time
for drainage of parts is allowed before transfer to the second and third rinses.
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