REMOVAL OF CYANIDES FROM COKE PLANT WASTEWATERS BY
SELECTIVE ION EXCHANGE-RESULTS OF PILOT TESTING PROGRAM
Robert A. Bessent, Environmental Engineer
Paul A. Luther, Senior Environmental Engineer
Carl W. Eklund, Associate
Envirodyne Engineers
12161 Lackland Road
St. Louis, Missouri 63141
INTRODUCTION
Selective ion exchange treatment technology was chosen to remove cyanides from coke
plant wastewaters at a midwest steel manufacturer.   Due to limited information on the
effectiveness of ion exchange treatment technology to coke plant wastewaters, a pilot study
was conducted to determine its applicability.   Cyanide limitations to be met by a full-scale
ion exchange treatment system effluent, as dictated by the receiving metropolitan sewage
authority, are 2 mg/1 free cyanide and 10 mg/1 total cyanide.   In addition to cyanide limitations, total iron is limited to 50 mg/1.
The pilot study program was initiated based on the information outlined by Avert and
Waitz [1],   This paper presents the results of a pilot program based upon selective ion
exchange treatment of complex cyanides.   Preliminary design parameters and flow diagrams
resulting from the pilot study are also presented.
PROCESS DESCRIPTION
Rohm and Haas Amberlite IRA 958 (formerly XE-279) ion exchange resin was chosen
for its ability to specifically remove complexed cyanides present in the coke plant wastewater.   Amberlite IRA 958 resin is made of an acrylic material and is a strongly basic anionic
exchanger with a published capacity ranging between 1.2 to 2.3 pounds of complexed
cyanide per cubic foot [1].   Free cyanides are not effectively removed by the resin; therefore, ferrous sulfate is added to the coke plant wastewater to convert free cyanides to
ferrocyanides.   Ferrocyanides can then be removed in the ion exchange columns.  The equation for the conversion of free cyanide to ferrocyanide is [1]:
Fe2+ + 6CN"  P10^9   FefCN)t 0)
Under alkaline conditions, excess iron introduced to this reaction precipitates as ferrous
hydroxide.   The Fe(OH)2 precipitate, conditioned by cationic and anionic polymers, is
settled following a flocculation period.   Insoluble complex cyanides also settle out during
this operation.   The clarified coke plant wastewater proceeds to a filtration system followed
by the ion exchange columns.   The purpose of the filtration system is to protect the resin
in the ion exchange columns from excess solids carried over from the clarifier.   The ion
exchange columns remove cyanides as follows [ 1 ]:
4 Resin-N-R3+cr + Fe(CN)«~ -»   (Resin-N-R3+)4 + (Fe(CN)J")    + 4Cl~ (2)
Once the ion exchange resin is exhausted, it is regenerated with a 15% solution of NaCl [ 1 ]:
(Resin-N-R3)4 + (FefCN/e" )    + 4NaCl -+4 Resin-N-R3+Cr+ Na4Fe(CN)6 (3)
The regenerant stream contains a less toxic concentrated form of cyanide which will be
treated by ultraviolet ozonation.   After adequate regeneration, the exchange resin is rinsed
with water, and then backwashed for reclassification and cleansing purposes.   Coke plant
wastewater can then be reintroduced to the ion exchange column to begin a new treatment
cycle.
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