THIOCYANATE BIO-OXIDATION KINETICS
Ronald D. Neufeld, Associate Professor
Lorraine Matson, Graduate Student
Patricia Lubon, Graduate Student
Department of Civil Engineering
University of Pittsburgh
Pittsburgh, Pennsylvania 15261
INTRODUCTION
Effluents from coking, coal gasification, and other coal carbonization processes often
contain high levels of polynuclear aromatic hydrocarbons (PAH), phenolics, thiocyanates,
free and fixed ammonia, cyanides, sulfides, sulfates, and alkalinity. Chemical analysis of
such effluents varies from process to process due to differing conditions of coal carbonization, gas scrubbing temperatures, and type of coal feedstock used.
In a coal gasification facility, ground coal, which may be pretreated to destroy its agglomerating properties, is fed to a gasifier where it reacts with steam and oxygen. Bed
temperatures and product gas composition may be varied within limits by the steam-to-coal
and oxygen-to-coal ratios employed. In fixed bed and some fluidized bed gasifiers, much of
the coal carbon is converted to CO, C02, and CH4, however, a small fraction amounting
to 4 to 6% of the feed carbon, appears as liquid effluent tars and potential water pollutants
from the scrubbed product gas [ 1 ].
Little is known of the formation of thiocyanate (SCN) during gasification. Luthy [2]
indicates that one possible gas phase mechanism for the formation of aqueous thiocyanate
is the reaction of carbon disulfide and ammonia. In the coking industry, the formation of
thiocyanate can be explained on the basis of oxidation of coke oven gas constituents
through various complex reactions in the aqueous phase to form thiocyanate and thiocyanate complexes. As indicated by Luthy, such reactions may involved the complexation
of aqueous polysulfides and thiosulfates with cyanide. Similar aqueous phase reactions are
thought to also exist with coal conversion liquid effluents.
Pollutant production data for four different coal gasification processes is given in Table
I as a function of coal feed type on a pound produced per pound coal gasified, moisture-
and ash-free (MAF) basis. Table I indicates the quantity of thiocyanate produced per ton
of coal gasified can vary from about 0.07 to 2.5 lb SCN/ton coal MAF. The quantity of
wastewater production is directly related to the degree of steam decomposition in the
gasifier and moisture content of the coal. By using the data in Table I and assuming that
approximately one pound of wastewater is produced per pound of coal (MAF) gasified,
then the expected effluent thiocyanate concentration will range between 35 to 1250 mg/1
[11.
Research has been underway at the University of Pittsburgh to investigate certain key
aspects of control technology development associated with coal conversion facilities. The
purpose of this chapter is to report on some research efforts involving the biodegradation
of thiocyanate (SCN) as found in soeme coal conversion and coke plant wastewaters.
Klein et al. [3] have indicated that the most probable technologies for coal conversion
aqueous waste treatment include ozonation, adsorption, biological degradation, solvent
extraction, membrane processes, and coagulation-flocculation. In the specific area of thio-
cyanate/cyanide removal, investigated processes include ozonation, adsorption, biological
degradation, and chlorination.
Chlorination has been the traditional approach to thiocyanate and cyanide removal in
the metal plating industry where the end products are first cyanate and then carbon dioxide
and nitrogen gas. This approach however is not applicable to coal conversion processes due
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