New Developments in the
Treatment of Metal Finishing Wastes by
Ion Exchange of Rinse Waters
FRIEDICH K. VON AMMON, Senior Chemist
Bavarian Bureau of Water Supply and Pollution Control
Munich, Germany
INTRODUCTION
The waste waters of the electroplating and metal finishing industries play an
important role in the pollution of surface waters - - fish kills and retarded self-
purification -- and the toxic influence on sewage treatment plants -- biological
treatment, sludge digestion. The main reasons are that the number of such plants
has increased rapidly, with a tendency for very big shops for mass production, and
that the wastes contain highly toxic substances like cyanide, chromic acid and
metal ions as copper, nickel, zinc and chromium. The authorities responsible
for pollution control therefore have to enforce very strict standards on the effluents. Desirable are values under 0.1 mg/l CN and one mg/1 or 10_5 mol/1 of
total metals. When the waste flow is large even with these standards the load of
pollution in kg/day may be too high for a small watercourse.
During the past 10 to 15 years there has been a large development of waste
treatment in continuous flow plants by oxidation of cyanide with chlorine, reduction of chromates, neutralization to a pH for best possible precipitation of the
metals and retention of the metal salts by sedimentation or filtration. Nevertheless even good plants are often not able to meet the desirable standards. Furthermore they rely on precarious electronic devices for measurement and regulation,
which are endangered by the complex waste waters and need qualified and regular
attendance. In case of misfunction it may be necessary to shut down the whole
plant, or there will be a high hazard for the receiving water. For the factories
these plants afford costs for investment and operation which are totally unproductive.
Another possibility of waste treatment with greater safety and lower costs
opened up with the recent development of ion exchange resins. It depended on a
deeper knowledge of polymerization and resin production and led to a broad application in purification of boiler feed water and many fields of the chemical industry.
When applied to wastewater treatment one must consider clearly that ion ex -
change is not a process of detoxication but of concentration. Indeed one of the
problems with plating wastes is that a big quantity of rinse water with low concentrations has to be treated. With the aid of ion exchange the rinse waters are
not only treated to be safe for discharge but the demineralized water is better
suited tor rinsing than most natural water supplies, and therefore recirculated. The
former impurities accumulated on the resins are recovered during regeneration.
These liquids from regeneration together with other concentrated wastes constitute
the actual waste water to be discharged after treatment. The advantages and
problems of this process shall be discussed from the experience with three new
plants in Germany.
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