This invention relates to a waste treatment process utilizing molten metals. More particularly, the invention relates to a treatment process utilizing molten metals to react chemically with certain waste materials in a waste stream and to alloy radioactive isotopes in the waste stream.
Many waste treatment processes utilize thermal energy to break up waste materials into their constituent elements or more desirable compounds. The use of thermal energy to break down materials is referred to generally as pyrolization. Molten metals have also been used to react with certain waste materials in order to produce more desirable compounds or reduce the waste to constituent elements. In particular, molten aluminum has been used to react with halogenated hydrocarbons and produce aluminum salts. U.S. Pat. No. 4,469,661 to Shultz described the destruction of PCBs and other halogenated hydrocarbons by contacting the hydrocarbon vapor with molten aluminum. The aluminum was contained in low-boiling eutectic mixtures of aluminum and zinc or aluminum, zinc, and magnesium. Shultz also suggested eutectic reactant mixtures containing iron, calcium, and other metals. U.S. Pat. No. 5,640,702 to Shultz disclosed a molten metal treatment for wastes containing radioactive constituents. This patent to Shultz disclosed using lead in the molten reactant metal as a chemically active material for reacting with non-radioactive constituents in the waste to be treated.
U.S. Pat. No. 5,000,101 to Wagner disclosed a process for treating hazardous waste material with molten alkaline metal alloys. The molten metal alloy comprised approximately 50% aluminum, 5% to 15% calcium, 5% to 15% copper, 5% to 15% iron, and 5% to 15 zinc. U.S. Pat. No. 5,167,919 to Wagner disclosed a reactant alkaline metal alloy composition comprising between 40% to 95% aluminum, 1% to 25% iron, 1% to 25% calcium, 1% to 25% copper, and 1% to 25 % zinc. The ""919 Wagner patent also disclosed that magnesium could be substituted for calcium. In both of these Wagner patents, the waste material was reacted in the molten alloy held at about 800 degrees Celsius.
In the process disclosed in the above-described Wagner patents, chlorine atoms in the waste material were stripped from the waste compound primarily by the highly reactive aluminum in the molten reactant alloy. The aluminum and chlorine combined to form aluminum chloride. Carbon from the original waste compound was liberated either in elemental form or as char (CH, CH2, or CH3). Both the aluminum chloride and liberated carbon sublimed to a gaseous state at the 800 degree Celsius reaction temperature and were drawn off and separated.
Many hazardous waste sites have different types of wastes mixed together. The mixed waste may include numerous different types of halogenated hydrocarbons, other non-radioactive wastes, and radioactive isotopes. These mixed wastes which include radioactive and non-radioactive materials have proven particularly difficult to treat. Although, many non-radioactive wastes may be treated chemically and broken down into benign or less hazardous chemicals, radioactive constituents of the mixed waste stream cannot be manipulated to reduce or eliminate their radioactive emissions. It is desirable to separate the radioactive constituents from the other materials in the mixed waste and place the radioactive constituents in an arrangement for safe, long term storage.
Storing radioactive waste poses several problems in itself. For a radioactive isotope which has a long half life, a quantity of the material remains radioactive for many years. Thus, a storage arrangement for this long-lived radioactive waste must be capable of securely holding the waste for a very long period of time. However, radioactive emissions, particularly alpha radiation, can interact with the material of a container intended to store radioactive waste. This interaction can cause the container to degrade relatively quickly, long before the radioactive waste itself has degraded.
It is an object of the invention to provide a waste treatment process for treating radioactive waste materials, particularly mixed waste streams which include both non-radioactive wastes and radioactive constituents.
The waste treatment process according to the invention utilizes a molten reactant metal alloy including at least one chemically active metal for reacting with the non-radioactive material in the mixed waste stream being treated. The reactant alloy also includes at least one radiation absorbing metal. Radioactive isotopes in the waste stream alloy with the chemically active and radiation absorbing metals such that the radiation absorbing metals are able to absorb a significant portion of the radioactive emissions associated with the isotopes. Non-radioactive constituents in the waste material are broken down into harmless and useful constituents, leaving the alloyed radioactive isotopes in the molten reactant alloy. The reactant alloy may then be cooled to form one or more ingots in which the radioactive isotopes are effectively isolated and surrounded by the radiation absorbing metals. The ingots may be encapsulated in one or more layers of radiation absorbing material and then stored.
The chemically active metal in the reactant alloy may comprise any metal capable of reacting chemically with one or more non-radioactive constituents in the waste stream. Preferred chemically active metals include magnesium, aluminum, lithium, zinc, calcium, and copper. In the preferred form of the invention, a combination of these metals is included in the reactant alloy. The particular chemically active metal or combination of chemically active metals used in a particular application will depend upon the makeup of the wastes in the waste stream and the reaction products which are desired from the treatment process.
Each radiation absorbing metal included in the reactant alloy is matched with a particular radioactive isotope to be alloyed with the metals in the molten metal bath. That is, for each type of expected radioactive emission associated with a radioactive isotope to be alloyed, a radiation absorbing metal is included in the alloy for absorbing that particular type of emission. A particular radiation absorbing metal for absorbing a particular radioactive emission will be referred to herein as a corresponding radiation absorbing metal for that emission. Similarly, a particular radioactive emission which may be absorbed by a particular radiation absorbing metal will be referred to herein as a corresponding radioactive emission for that radiation absorbing metal. Preferred radiation absorbing metals include particular isotopes of lead, beryllium, cadmium, vanadium, yttrium, ytterbium, zirconium, and tungsten. One or more of these radiation absorbing metals may be used within the scope of the invention depending upon the radioactive isotopes to be alloyed in the molten metal. For purposes of this disclosure and the accompanying claims, a xe2x80x9cradiation absorbing metalxe2x80x9d comprises a metal which is capable of capturing a particular expected radioactive emission, that is, a particular emission at a natural decay energy level.
As used in this disclosure and the following claims, the xe2x80x9ctype of expected radioactive emissionxe2x80x9d associated with an isotope in the waste material to be treated refers to the particular type of both primary and secondary emission (alpha, beta, gamma, or neutron) characteristic of the isotope and any daughter isotope, and the characteristic energy level of each emission. The xe2x80x9cexpected radioactive emissionxe2x80x9d refers to each respective emission within each type of emission. For the purposes of this disclosure and the claims, a xe2x80x9cprimary radioactive emissionxe2x80x9d comprises the emission or emissions directly from the radioactive decay of an isotope. For most radioactive isotopes, the primary radioactive emissions will include either an alpha or beta emission at a characteristic energy level and a gamma emission at a characteristic energy level. A xe2x80x9csecondary radioactive emission,xe2x80x9d for the purposes of this disclosure, comprises a radioactive emission resulting from a primary radioactive emission. A secondary radioactive emission (commonly gamma radiation or a liberated neutron) is generated as a primary radioactive emission is absorbed by an absorbing material or as a primary radioactive emission otherwise interacts with matter.
Although the invention has particular application in treating mixed waste streams which include both radioactive and non-radioactive wastes, those skilled in the art will appreciate that a waste stream made up of only radioactive materials may be treated using the present process. The process according to the invention is useful for diluting and alloying the radioactive isotopes for storage even in the absence of non-radioactive wastes.
Regardless of the particular composition of the reactant alloy according to the invention, the alloy is heated to a molten state for receiving the waste stream. It is typically desirable to use the lowest reactant alloy temperature necessary to react any non-radioactive constituents in the waste stream and to efficiently melt or dissolve the radioactive material into the alloy. For mixed wastes which include organic constituents, a reactant alloy temperature of at least 770 degrees Celsius is generally required to quickly break the organic molecules down into the desired materials. Higher temperatures may be desirable to better dissolve or melt heavier radioactive isotopes such as transuranic elements.
In one preferred form of the invention, the reactant alloy is heated using fossil fuel burners. Other forms of the invention may employ an electrical induction heating system or any other suitable heating arrangement to heat the reactant metal alloy to the desired operating temperature. The waste material is introduced directly into the molten reactant alloy, preferably below the surface of the molten material.
The aluminum, magnesium, or lithium in the reactant alloy chemically strips chlorine or any other halogen atoms from organic molecules in the waste material to form a metal salt. Some of these metal salts may remain in a molten state and separate by gravity separation in the reactant alloy container. Other metal salts such as aluminum chloride, for example, along with carbon freed from the waste material in the form of elemental carbon and char go to a gaseous state at the temperature of the molten alloy. Gas released in the treatment process may be drawn off and scrubbed in an aqueous scrubber/separator to produce a slurry of char and salt solution. The salt solution may then be separated and processed to recover the salts and char. Each material produced in a reaction with a chemically active metal in the alloy will be referred to in this disclosure and the following claims as a reaction product.
In order to produce a mechanically stable ingot for long-term storage, the treatment process preferably includes maintaining a minimum ratio of radiation absorbing metal atoms to expected radioactive emissions. That is, the amount of radiation absorbing metal in the reactant alloy is varied as a function of the number of radioactive isotopes in the resulting alloy or as a function of the corresponding expected radioactive emissions in the volume of the resulting alloy. The preferred ratio comprises 727 or more atoms of radiation absorbing metal to the corresponding radioactive emission. This ratio produces an alloy in which radioactive emissions may be absorbed by the radiation absorbing metals without significantly degrading the mechanical integrity of the ingot.
The process according to the invention includes the step of identifying each type of radioactive isotope in the waste material to be treated and determining the amount of each identified radioactive material in a waste material. This identification step may be performed by any suitable means, preferably through mass spectroscopy performed on one or more samples of the waste material. The treatment process further includes using this information to build a particular reactant alloy for a selected volume of the waste material. Waste material is then metered into the reactant alloy using a suitable metering device to control the volume of waste material added the alloy.
Once the minimum ratio of radiation absorbing atoms to corresponding expected radioactive emissions is reached, the molten reactant alloy (now including radioactive isotopes) may be cooled to a solid form in one or more ingots. These ingots maintain their mechanical integrity and produce relatively few external emissions due to the radiation absorbing material and thus may be stored in relative safety. Each ingot is preferably encapsulated with a suitable radiation absorbing material or combination of materials. This encapsulant material should be capable of absorbing substantially each type of radioactive emission which could be produced within the ingot.
One advantage of the treatment process according to the invention is that it combines the separation of radioactive waste from non-radioactive wastes with the chemical treatment of non-radioactive wastes. Also, the ingots which result from the process are very stable. There is very little chance for release of the alloyed radioactive isotopes from the ingots. Furthermore, radioactive emissions from the ingots are reduced by the radiation absorbing metals which are distributed throughout the matrix of the alloy along with the radioactive isotopes. The radiation absorbing metals also serve to prevent the radioactive emissions from adversely affecting the other metals in the ingots and prevent significant mechanical degradation in the alloy material.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.
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