CHEMISTRY OF LEACHATES FROM
RAW AND STABILIZED COAL FIRED WASTES
Lane D. Tickanen, Graduate Student
Joseph A. FitzPatrick, Associate Professor
Department of Civil Engineering
Northwestern University
Evanston, Illinois 60201
INTRODUCTION
In order to meet federal clean air standards, electric power plants have increased their use of flue
gas desulfurization (FGD) processes which serve to remove or "scrub" sulfur oxides (SOx) from stack
exhaust gases. The scrubbing process produces large quantities (8 million tons in 1977) [1] of semisolid sludge (primarily CaS03 and CaS04) which requires disposal. One common disposal alternative
is landfilling. The physical, engineering, and chemical properties of the sludge dictate whether direct
landfilling is suitable or whether "fixation" or "solidification" need be employed. Many FGD
systems produce wastes that contain primarily calcium sulfite hemihydrate, sludges which characteristically have very low strength and strong thixotropic tendencies [2]. The thixotropic character of
the sludges could give rise to future landfill stability problems unless the sludges were stabilized prior
to disposal.
One candidate for fixation may be the fly ash which is also generated through coal combustion
(36 million tons in 1975) [3] and is removed from stack gases via electrostatic precipitation or bag-
house filtration. Fly ash is a good material for co-mixing because it is readily available, is also a waste
which requires disposal, and has chemical and physical properties that make it a good adsorbent. In
the presence of sufficient lime, fly ash and scrubber sludge undergo pozzolanic reactions [2], forming
ettringite-type and tobermorite-type crystals which bind the mixture in a monolithic low-permeability
mass.
The main environmental concern regarding scrubber sludge disposal or sludge-fly ash co-disposal
is the potential for contamination of surface or ground water. Leaching of such landfilled wastes
could ultimately result in the release of hazardous levels of metals to aquifers. Through the combined
efforts of mathematical modeling and laboratory experimentation, we can develop the means to
understand and predict the magnitudes of these releases. Furthermore, we can hope to offer some
answers to some additional disposal-related questions. What role does pH play in the release of
hazardous quantities of trace metals? Does forced oxidation of FGD sludges reduce or enhance metals
releases? Does the presence of organic ligands, such as acetate ions, enhance metals leachability?
Finally, does the co-mixing of FGD sludge and fly ash provide improved metals fixation or present
increased environmental hazard?
In this work, two sludges from dual alkali scrubber systems and two fly ashes were extracted
separately using various extraction procedures. In addition, sludge-fly ash blends and lime stabilized
mixtures were extracted. The results of chemical analyses were then compared to computer predictions of leachate quality. The computer code MINEQL [4] was modified and used to predict specia-
tion and concentrations of various major and trace metallic constituents as a function of pH and
redox intensity, p«, using thermodynamic data for precipitation and complex-formation reactions.
The variables pH and pt; were chosen to examine the effects of pH adjustment and forced oxidation on metals leachability. The pH, in practice, can be controlled by adding alkalinity, usually in the
form of lime. The redox intensity (pe) can be altered in a real situation by oxidizing sulfite to sulfate
in the scrubber sludge. In dual alkali scrubber systems, such as the ones studied here, this could
be achieved by aerating the scrubber liquor or operating the system at lower pH. The effect of the
presence of one organic ligand, acetate, was examined both through laboratory leaching testing and
computer modeling.
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