Treatment of Chromate Waste Using
Liquid Sulfur Dioxide
A.J. D'ORAZIO, Assistant Chief Chemist
Pontiac Motors Division
General Motors Corporation
Pontiac,  Michigan
The chemicals required in electroplating solutions are extremely toxic to
aquatic life and are detrimental to the efficient operation of sludge digesters
in sewage treatment plants. However, more important than the danger to
aquatic life is the danger to human life. All wastes at Pontiac Motor Division of General Motors Corporation are completely treated before being discharged to the sewage treatment plant of the city of Pontiac, Michigan.
The waste treatment plant consists of five separate treatment systems:
cyanide system, metallic acid, alkali acid, alkali oily, and final treatment.
The copper, nickel, and chromium plating wastes are treated in the metallic acid system. This paper will deal mainly with the treatment of hexavalent chromium in the metallic acid system.
The volumes of waste to be disposed of in this system vary from 400,000
to 700,000 gal per day.
Table I shows the volumes and concentrations treated in this system over
a five-day period.
Batch Volume In Concentration in PPM
No. Gals Cu Ni Cr+6
1
2
3
4
5
6
I
9
The table shows that the concentrations of hexavalent chromium are
lower than the concentrations of nickel and copper. This is due mainly to the
fact that the chromium wastes are recovered in the plating plant by means of
ion exchange followed by vacuum evaporation. By this method 550 lbs of
chromic acid are recovered daily. Also sodium hydrosulfite is used in the
plating plant by additions to the rack strip and cleaning solutions in the continuous plating line to reduce whatever chromic acid is trapped by the plating
racks in the chrome plating tanks and which is not removed by cleaning and
rinsing processes.
This usage of hydrosulfite automatically reduces the demand of chromium reduction process in the waste plant.
When waste flows are determined from a plant survey, the use of gaseous
sulfur dioxide on a continuous basis seemed to be the best available method
- 90 -
TABLE I
olume In
Con
Gals
Cu
407,500
2.8
380,300
43
271,700
63
244,500
184
309,700
80
326,000
100
255,400
39
326,000
45
298,300
33
212
4.5
110
6.0
87
24
235
92.5
100
10
165
12
110
40
163
38
100
10