ANALYSIS AND TREATMENT OF REACTIVE WASTE: A CASE STUDY
IN THE DUCTILE IRON FOUNDRY INDUSTRY
Thomas R. Stolzenburg, Water Chemist
Lane D. Tickanen, Project Engineer
Bruce E. Dudzik, Project Engineer
James E. Vondracek, Project Engineer
Themis Flores de Pierquet, Lab Analyst
Jitendra T. Radia, Chief
Air Pollution and Industrial Hygiene Engineering
Mark L. Holcomb, Project Engineer
William A. Stephens, Manager
Industrial Waste Treatment and Reclamation Group
Robert K. Ham, Senior Technical Consultant
Residuals Management Technology
Madison, Wisconsin 53703
INTRODUCTION
Solid wastes can be classified as hazardous under the federal Resource Conservation and Recovery
Act (RCRA) as either "listed" or "characteristic" wastes. One of the characteristics of a hazardous
waste, as defined under 40 CFR Part 261.23, is reactivity. Calcium carbide desulfurization slag generated at ductile iron foundries may be considered hazardous by this definition. Specifically, the
calcium carbide slag reacts with water to form acetylene gas which may give rise to explosive mixtures
as defined in 261.23 (3). Also, at neutral to acidic pH values, the calcium carbide desulfurization
slag may emit hydrogen sulfide gas which may result in its definition as hazardous under Part 261.12
(5). Other gases which are potentially toxic at high concentrations may also be generated during
treatment of the slag (Part 261.23 (4)). For these reasons, analysis and treatment of this type of waste
is of great concern to the ductile iron foundry industry.
CHEMISTRY OF CALCIUM CARBIDE DESULFURIZATION
In the production of ductile iron, it is often necessary to add a desulfurizing agent in the melt
because the charge materials contain excessive amounts of sulfur. One desulfurization agent commonly used is solid calcium carbide (CaC2). Based on investigations by Talballa et al. [1], the calcium
carbide is thought to decompose to calcium and graphite. The calcium then reacts with sulfur in the
melt to form calcium sulfide (CaS) (Equation 1).
CaC2 + 2CaO + 3FeS - 3CaS + 2CO + 3Fe (1)
The calcium carbide desulfurization slag is generally removed from the molten iron in the ladle
and placed into a hopper. To ensure adequate sulfur removal, calcium carbide must be added in
slight excess. Therefore, the slag contains both CaS and CaC2.
One common practice is to treat the calcium carbide desulfurization slag with water. This is done
to generate and release acetylene gas from the unreacted calcium carbide (Equation 2).
CaC2 + 2H20 -» Ca(OH)2 + C2H2 (2)
The other major reaction product is solid calcium hydroxide or lime which can be disposed as a
nonhazardous waste or reused.
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