Halogenated hydrocarbons or "halocarbons" are alien to our natural environment. They have varying effects on plant and animal metabolism, and little is known of their ultimate effect on biological life in general. This lack of knowledge is partly because halocarbons have been used in quantity only during this century, most notably since World War II. Virtually all halocarbons are man-made. They have many applications where their unique chemical and physical properties are desirable: lack of flammability and chemical reactivity, high density, and volatility.
Halocarbons are the basis of remarkably strong and inert plastics; Teflon® and polyvinyl chloride are two familiar examples. Another type of halocarbon, the chlorofluorocarbons (CFCs), were essentially an outgrowth of our early nuclear industry. Materials were sought that were inert to uranium hexafluoride, and wartime research and production led to the manufacture of large quantities of CFC-based plastics and refrigerants. Highly volatile and quite chemically inert, CFCs were expected to last indefinitely once produced, and these properties were to eventually cause the difficulties of cleaning up CFC-contaminated materials.
Most halocarbons originate from reaction of a halogen (chlorine, bromine, fluorine) and a hydrocarbon. Industrially, chlorine and bromine are obtained by the electrolysis of brine. The most reactive halogen, fluorine, is obtained from reacting fluorine-containing minerals with sulfuric acid. The resulting hydrogen fluoride (HF) is then used "as is" or electrolyzed to produce fluorine. Chlorine reacts with hydrocarbons to form chlorocarbons and hydrogen chloride (HCl). Totally chlorinated hydrocarbons are common starting species for CFCs. For example, carbon tetrachloride (CCl4) reacts with pure HF in the presence of a catalyst to produce refrigerant-11 and refrigerant-12.
Today, many halocarbons are listed as hazardous or as ozone-depleting compounds. Biologically, several chlorocarbons are suspect carcinogens. CFCs supply chlorine atoms to the stratosphere, and such atoms have been shown to disrupt the ozone production cycle. Moreover, chlorinated hydrocarbons have contaminated thousands of sites both above and below ground throughout the world.
The general chemical inertness of these compounds precludes an easy solution to their cleanup and destruction.
In nuclear technology, the presence of these compounds in nuclear waste classifies the entire lot as "mixed waste" (waste containing both radioactive and hazardous materials, e.g., plutonium and CCl4). The work described in this article presents a simple solution to the halocarbon destruction problem and promises many attractive advantages in dealing with mixed wastes.
At TA-55 and other places, soda lime has been used to scrub fluorine gas from the fluorination systems. Similar scrubbing of chlorine, carbon dioxide, HCl, and phosgene has also been done by soda lime. Prior research in plutonium chlorination revealed that soda lime, when added to a chlorination flow loop using CCl4, apparently trapped or destroyed the CCl4 along with the acid gases. Later, when heat was applied to a similar soda lime reactor, CCl4 was completely and reproducibly destroyed. Subsequently, other compounds similar to CCl4 were successfully destroyed in this fashion. As experiments progressed, it appeared that CFCs were also destroyed completely by this method at temperatures somewhat higher than those for the chlorocarbons.
In all cases, the reactions are exothermic. If the halocarbon vapor is sufficiently concentrated and is delivered constantly, the reaction self-sustains. Reactor core temperatures rose to 600°C during runs with vapor that was continuously fed into the reactor. This phenomenon is very controllable and can maximize process efficiency by using the latent heat produced.
Figure 4. The system uses soda lime to convert hazardous halocarbons to nonhazardous gases and solids.
The advantage of this process is particularly noteworthy in the nuclear industry; it has great potential in destroying the hazardous components of mixed waste. Halocarbons are particularly tough to remediate because of their resistance to conventional means of treatment. For example, burning or incineration of halocarbons is difficult to perform totally, and it is necessary to scrub the off-gases of the acids produced, such as HCl and HF. Thermal desorption of the hazardous component is the preferred method to use in tandem with the soda-lime-based system.
The destruction of halocarbons in a soda lime system may be applied in many industries: automotive, aviation, semiconductor, dry cleaning, refrigeration, waste treatment, and environmental restoration. The system can not only destroy halocarbons from liquid inventories and hazardous waste forms, it can be used as a scrubber for off-gases from semiconductor etching and cleaning and for destroying residual gases from dry cleaning and refrigeration systems. For waste solvents and the like, the soda lime system may treat vapors directly from a storage vessel. There is a patent pending for this destruction system, and an industrial partner is being sought to complete development and marketing.
Jerry Foropoulos, Jr., NMT-6, is the principal researcher for this project.
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