Hazardous or toxic waste destruction or containment – Destruction or containment of radioactive waste – By fixation in stable solid media
Reexamination Certificate
2000-09-28
2001-07-10
Vincent, Sean (Department: 1731)
Hazardous or toxic waste destruction or containment
Destruction or containment of radioactive waste
By fixation in stable solid media
C588S252000
Reexamination Certificate
active
06258994
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to glass compositions suitable for use in stabilization of radioactive, hazardous, or mixed waste, having relatively low melting points and relatively high amounts of lithia (Li
2
O), to methods of making these glass compositions, and to methods for using the compositions to immobilize waste materials.
2. Description of the Related Art
Various hazardous, radioactive, and mixed (both hazardous and radioactive) wastes, including heavy metal wastes such as lead paint and contaminated soils, require stabilization in solid forms that meet regulatory disposal criteria promulgated by government agencies like EPA and NRC. As discussed below, these wastes originate from a variety of sources, and consequently can exist in a variety of forms, including aqueous waste streams, sludge solids, mixtures of aqueous supernate and sludge solids, combinations of spent filter aids from waste water treatment and waste sludges, supernate alone, incinerator ash, incinerator offgas blowdown, or combinations thereof, geological mine tailings and sludges, asbestos, inorganic filter media, cement waste forms in need of remediation, spent or partially spent ion exchange resins or zeolites, contaminated soils, lead paint, etc.
Many industrial processes generate hazardous wastes in the form of aqueous waste streams, sludge solids, aqueous supernate, incinerator ash, incinerator off gas condensate, and so forth. Waste treatment processes may themselves generate secondary hazardous wastes. For example, solids can be filtered from an aqueous waste stream by passing the stream through filter aids, such as perlite (PERFLO) or diatomaceous earth filters. The spent filter medium is impregnated with the materials removed from the waste stream, such as heavy metals and other hazardous or radioactive substances. The spent filtration wastes must themselves be treated and stabilized before disposal. As used herein, the term “hazardous waste” includes wastes containing substances commonly recognized as hazardous, including but not limited to, chemical wastes, radioactive wastes, mixed chemical and radioactive wastes, heavy-metal-containing wastes, and hazardous organics.
Stabilizing hazardous wastes using currently available technology is expensive and requires enormous resources of equipment and personnel. Stabilization processes must be operated within guidelines established under the Resource Conservation and Recovery Act (RCRA), and the stabilized product must meet stringent state and federal standards. In the case of radioactive or mixed wastes, the stabilized wastes must often be stored for long periods of time waiting for decay of the radioactive components before transportation to an approved underground repository. Minimizing the waste volume is important in minimizing storage, transportation, and final disposal costs.
Incinerators are often used to destroy the hazardous constituents of solid and liquid wastes, as well as municipal garbage. Byproducts of incineration include bottom ash, aqueous incinerator offgas condensate (blowdown), or mixtures of ash and offgas condensate, all of which may contain residual hazardous and/or radioactive substances.
Radioactive waste may be further categorized into high level waste (HLW) and low level waste (LLW). High level waste is generally generated by reprocessing of spent nuclear fuel and other irradiated material, weapons production, research and development, etc. High level waste resulting from fuel reprocessing generally is an acidic, highly radioactive, and heat producing liquid that is generally either calcined to a dry, granular solid or neutralized, dehydrated, and stored as a damp salt, sludge, and supernate liquid. Low level waste generally contains more radioactivity than is allowed for municipal disposal, but are not sufficiently radioactive to produce substantial amounts of heat. Low level waste typically includes contaminated soil, clothing, gloves, resins, waste sludges, etc.
Hazardous wastes may be solidified by vitrification (incorporation into a glass matrix) or cementation. In typical cementation processes, cement-forming materials are added to the waste; any water in the waste solution remains in the solidified product. Therefore, the solidified product has a larger volume than the original waste solution. Also, water, including groundwater, can leach compounds out of cement over time and cement is naturally porous, so the cement-stabilized product must be stored in leak-proof containers to prevent leaching.
Glass is the most long-term environmentally acceptable waste form. Glass is stable and extremely durable. Moreover, the hazardous species are chemically bonded in the glass structure, forming a substantially nonleachable composition. A number of vitrification processes for hazardous wastes have been described. Wheeler (U.S. Pat. No. 4,820,325) stabilizes toxic waste using a glass precursor material such as diatomaceous earth mixed with a compatible glass precursor material such as soda ash, lime (CaO) and alumina. The normally leachable toxicant becomes fixed within the glass when the mixture is vitrified. Hayashi, et al. (U.S. Pat. No. 4,725,383) add ZnO, or a mixture of ZnO with Al
2
O
3
and/or CaO, to a radioactive sodium borate waste solution. The resulting mixture is dehydrated and melted to produce a vitrified solid solution. Schulz, et al. (U.S. Pat. No. 4,020,004) vitrify radioactive ferrocyanide compounds by fusion with sodium carbonate (Na
2
CO
3
) and a mixture of basalt and B
2
O
3
, or silica (SiO
2
) and lime (CaO).
As discussed above, solidification of these and other wastes by glassification or vitrification is known, and generally involves combining glass forming compounds and/or natural rock, such as basalts or nepheline syenite, with the waste materials, and melting this mixture at temperatures sufficient to vitrify the mixture and immobilize the waste species in the resulting glass. The waste materials become dissolved in the melt and atomically bonded to the glass matrix that forms when the melt is cooled.
However, vitrification processes are not perfect, and problems are sometimes experienced due to the often limited solubility of the waste materials in the melt during glassification and/or due to the volatility of some or all of the waste species at the relatively high temperatures reached during the vitrification. Jantzen, “Systems Approach to Nuclear Waste Glass Development,”
J. Non
-
Cryst. Solids,
84 (1-3), 215-225, 1986. As a result, a glass forming additive that lowered the melting temperature of the mixture of waste and glass formers, thereby decreasing the amount or likelihood of waste volatilization, would be very desirable. In addition, a glass forming additive that increased the solubility of waste materials in the glass forming mixture, thereby increasing the amount of waste atomically bonded in the glass, increasing the waste loading capacity of the glass, and decreasing the disposal volume, would also be very desirable.
Lithia (Li
2
O) has been disclosed to accelerate the dissolution of sand grains and increase melt rate. R. M. Wiker, Glass Industry, 37, 28, (1956). Lithia has also been disclosed to have a lower volatility than soda or potash at equal molar concentrations at 1400° C. Volf,
The Chemical Approach to Glass,
1984. Lithia has been disclosed to increase the viscosity of glass at low temperatures, N. W. Taylor et al., J. Amer. Ceram. Soc., 20, p. 296 (1937) and 24, p. 103 (1942), and to decrease the viscosity of glass at high temperatures, G. Heidtkamp et al., Glastechnick Bericht, 14, p. 99 (1936), as compared to soda (Na
2
O) and potash (K
2
O) containing glasses. Lithia is also disclosed to increase the modulus of elasticity in glasses compared to soda and potash. The mobile Li
+
ion has also been disclosed to facilitate transmission of electric current, so that glasses containing Li
+
melt at a faster rate in Joule heated melters than in gas fired commercial melters. Small amounts of Li
+
hav
Cicero-Herman Connie A.
Jantzen Carol M.
Marra James C.
Pickett John B.
Gray, Esq. Bruce D.
Kilpatrick & Stockton LLP
Mallatt, Esq. Kristen D.
Vincent Sean
Westinghouse Savannah River Company
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