77    TREATMENT PROCESSES AND ECONOMIC EVALUATION
OF CARBONATE REDUCTION AND CYANIDE
REMOVAL FROM AN ELECTROPLATING SOLUTION
C. Peter Varuntanya, Project Manager
Kathleen M. Lagnese, Project Engineer
Chester Environmental
Pittsburgh, Pennsylvania 15244
INTRODUCTION
In the electrogalvanizing process, cyanide contained in the zinc cyanide plating bath breaks down to
form carbonate which eventually supersaturates the plating solution. This supersaturated carbonate
solution interferes with the plating process and reduces the production efficiency of the electrogalvanizing line. In order to reduce the concentration of carbonates that tend to accumulate in the plating
bath solution, two separate treatment processes are currently employed at the subject facility which
produces electrogalvanized strip steel. One system involves chilling the solution to 58 — 60°F for one
to two days to precipitate the carbonate as sodium carbonate crystals. The crystals, which contain
cyanide within or on the crystalline structure, are collected in baskets at the base of the chiller.
Following the treatment, the baskets are removed and the crystals are stored temporarily in drums
prior to off-site disposal as a hazardous waste. The liquid is recycled to the plating baths.
Because the capacity of the chiller system is limited, another separate treatment system was installed
to treat additional plating bath. This system uses lime to precipitate carbonate as calcium carbonate.
Following precipitation, the solution is pumped through a filter press for dewatering. The filter cake,
which also contains cyanide, is collected in roll-off boxes and sent off-site for treatment and disposal
as a hazardous waste. The filtrate is recycled to the plating baths.
The current cost for disposal varies depending on the cyanide content of these two solid wastes. A
study was conducted to identify alternative methods to minimize the disposal costs. In addition, any
wastewater generated by the proposed treatment methods was required to meet the pre-treatment
standards for discharge to the Publicly Owned Treatment Works (POTW), after combining with the
other wastewaters currently discharged from the electrogalvanizing line. The existing pre-treatment
for the electrogalvanizing line discharge is listed in Table I. As shown, the requirements include a total
cyanide limit of 1 mg/L.
LABORATORY TREATABILITY STUDIES
Laboratory treatability studies were performed in two phases. The Phase 1 studies focused on
treating aqueous solutions of the sodium carbonate crystals to reduce the concentration of cyanide to
an acceptable level for discharge to the POTW. In Phase 2 studies, alternative methods to minimize
cyanide concentrations in the calcium carbonate filter cakes generated by the existing lime treatment
system were performed. Based on the results of treatability studies, two alternative treatment schemes
were developed. The results of the treatability studies are presented below.
Phase 1: Treatment of Sodium Carbonate Crystals
Waste Characterization
Samples from three separate batches of the sodium carbonate crystals generated by the existing
chilling system at the facility were collected and analyzed for total solids, total cyanide, total iron, and
total zinc. The results of the initial analyses are provided in Table II. The cyanide concentrations in
the three samples ranged from 5,040 to 41,000 mg/L. The disparity between the cyanide concentrations in these samples indicates that the quality of the sodium carbonate crystals is highly variable.
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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