FILTER FOR ISOTOPIC ALTERATION OF MERCURY VAPOR

Abstract: An apparatus for enriching the isotopic Hg content of mercury is provided. The apparatus includes a reactor, a low pressure electric discharge lamp containing a fill including mercury and an inert gas. A filter is arranged concentrically around the lamp. In a preferred embodiment, constant mercury pressure is maintained in the filter by means of a water-cooled tube that depends from it, the tube having a drop of mercury disposed in it. The reactor is arranged around the filter, whereby radiation from said lamp passses through the filter and into said reactor. The lamp, the filter and the reactor are formed of a material which is transparent to ultraviolet light.

Description: The invention relates to a filter for photochemical isotope separation of ¹⁹⁶Hg component radiation from mercury radiation. The excitation of specific mercury isotopes by photochemical means is well known in the art. For example, the paper by Webster and Zare, "Photochemical Isotope Separation of ¹⁹⁶Hg by Reaction with Hydrogen Halides" J. Phys. Chem. 85, 1302 (1981) discloses such excitation. Mercury vapor lamps are commonly used as an excitation source of mercury isotopes for specific photochemical reactions. To be successful, photochemical separation of a single isotope requires that the spectral band width of the exciting mercury radiation must be sufficiently narrow to excite only the isotope of interest. The specificity depends upon the spectral band width of the source. The rate and extent of separation of the particular isotope from the feed stock can be strongly dependent on the intensity of the radiation emitted from the mercury source. A weakly ionized plasma of mercury and rare gases under low pressure, in the order of 1, 3 to 4 mbar, forms the basis of the fluorescent lamp. Electrical energy is converted to natural mercury resonance radiation at 253.7 nm. at an efficiency of 55 to 65%. This radiation, in fluorescent lamps, is converted to visible light by solid phosphors that are coated upon the lamp envelope. The efficiency of the 253.7 nm. resonance radiation emitted from excited mercury atoms in the plasma is absorbed and reemitted many times by ground state mercury atoms during its escape to the walls of the discharge tube. This trapping of resonance radiation prolongs the effective lifetime of the excited atoms and increases the opportunity for radiationless energy conversion which reduces efficiency. It is known that the 253.7 nm. resonance line of mercury is composed of 5 hyperfine components, principally the result of isotope shifting. As is known, the ¹⁹⁶Hg isotope in natural mercury does not contribute substantially to the radiation because of its low concentration, nor does its emission and absorption heavily overlap with the other hyperfine components. Therefore, by increasing its concentration, an additional channel for the 253.7 nm. photons is provided which reduces the average imprisonment time and increases radiation efficiency. Devices have previously been disclosed to enrich the ¹⁹⁶Hg in mercury feed stocks. In the paper of McDowell et al., "Photochemical Separation of Mercury Isotopes" Can. J. Chem. , Vol. 37, 1432 (1959), a disclosure is made of reacting ²⁰²Hg(6³P₁) atoms that are contained in natural mercury with hydrogen chloride with a photochemical reaction in which the ²⁰²Hg atoms are excited during the reaction to precipitate an ²⁰²Hg₂Cl₂. As described in a paper delivered at the International Quantum Electronics Conference, June 1984, very high enrichment of ¹⁹⁶Hg can be achieved in a photochemical reaction using a natural mercury vapor filter. When radiation from a microwave lamp containing mercury enriched to 35% in ¹⁹⁶Hg is used in a filter, the filter eliminates substantially all of the non-196 Hg component radiation permitting an isotopically selective primary excitation of the ¹⁹⁶Hg isotope. Selective excitation of ¹⁹⁶Hg(6³P₁) in natural mercury vapor is obtained by an RF-excited, Hg and rare gas source whose emission is filtered through an atomic vapor filter before it enters into the reaction zone. It is the object of the invention to provide an effective filter for enriching the ¹⁹⁶Hg isotopic content of mercury. This object is solved by a filter having the features of Claim 1. Preferred embodiments are disclosed in the dependent claims. In the following, the invention is explained in more detail in the figures. IN THE DRAWINGS: FIGURE 1 is a perspective view of an apparatus including a reactor vessel, filter means, and lamp according to a preferred embodiment of the present invention. FIGURES 2A and 2B are curves illustrating the relevant portions of the spectral energy distribution curves of, respectively, the emission of a typical low pressure lamp and the emission after the light has passed through the filter of the present invention. For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to an apparatus, or system, for enriching the ¹⁹⁶Hg content of mercury through a photochemical reaction between "natural" mercury, i.e., naturally occurring mercury, and hydrogen chloride. The system comprises a single pass reaction vessel which surrounds a filter, the filter in turn surrounds a low pressure electric discharge tube having a fill comprising mercury and an inert gas. All of the elements of the system, the lamp, the filter, and the reactor are formed of a glass which is transparent to radiation at 253.7 nm., such as, for example, quartz. Examples of low pressure (e.g., about 3,3 mbar) electric discharge tube that are suitable for use with the system of the present invention include any of the known low pressure discharge lamps which transmit radiation in the range of 253.7 nm, especially those using microwave cavities for the excitation of the mercury. A filter is disposed around the discharge tube and is formed of a pair of concentric cylindrical (or tubular) members that are sealed from the atmosphere at their ends. The filter contains a fill of mercury which is preferably depleted in ¹⁹⁶Hg, and most preferably mercury including less than about 0.07 atomic weight percent) at a pressure of between about (8 to 32) · 10⁻³ mbar Hydrogen is also included in the filter and is maintained at a pressure of between about 9,3 to 17 mbar. The concentric cylinders are preferably sealed at their ends with a pair of spaced-apart end members, so as to form a cylinder with an axially passageway that receives the low pressure discharge device. A means for controlling mercury pressure in the filter is in operational communication with the filter. One example of a mercury pressure controlling means includes a tube, sealed at one end and open to the interior of the filter at the other, which is disposed on the lower end member. A bead of mercury, having a weight of, for example, 10 to 20 milligrams, is disposed in the sealed end of the tube so that its vapors are in communication with the interior of the filter. Optionally, the apparatus further includes a means for maintaining the mercury in the filter at a predetermined temperature. An example of such temperature maintaining means suitable for use with the foregoing example of a mercury vapor controlling means includes a heat exchanger disposed around the outside of the end of the tube and the bead of mercury so as to maintain the mercury at a predetermined temperature. Preferably, this heat exchanger is a sleeve with an inlet port and an outlet port, and having a pump and a temperature controller disposed on the line to control the temperature of the mercury bead, whereby to maintain a predetermined amount of mercury vapor in the filter. A reactor is disposed about the filter and may take the form of a pair of concentric cylinders with an inlet and outlet port through which the mercury feed stock and hydrogen chloride can flow. The exciting ¹⁹⁶Hg radiation passes through the lamp envelope and through the filter and into the reactor to produce an isotopic specific reaction to the ¹⁹⁶Hg in the reaction vessel, whereby ¹⁹⁶Hg₂Cl₂ is formed. In a preferred embodiment, a "T" is disposed on the tube at a location between the filter and the end of the tube. The "T" can be used to make gross changes in the Hg pressure in the filter. Referring to FIGURE 1, there is shown a low pressure mercury lamp 1 comprising a silica or quartz discharge tube. A microwave cavity is placed over one end of the lamp in order to create the discharge. For isotope separation of ¹⁹⁶Hg, the inner diameter of the tube in a preferred embodiment is approximately 5 mm. The discharge lamp 1 typically includes argon at a pressure of approximately 3,3 mbar and a mercury pressure of approximately 1,3 to 2 mbar at about 20°C. Although argon is preferred as an inert gas in the lamp, other gases such as neon may alternatively be used. The filter 3 includes a pair of concentric cylindrical members 3a and 3b. In a preferred embodiment, the concentric cylindrical members are spaced from each other at a distance of about 1.0 centimeter. The filter 3 is sealed from the atmosphere by a seal means. In FIGURE 1, the seal means comprises a pair of spaced apart end members 3c and 3d that are fused to the ends of the concentric cylindrical members 3a and 3b. An axial passageway is formed in the filter 3 by the inner cylindrical member 3b and is arranged to receive the lamp 1. The apparatus of the present invention further includes a means for controlling mercury pressure within the filter. In the preferred embodiment shown, the means for controlling mercury pressure in the filter includes a tube 5, sealed at one end, and in communication with the interior of the filter 3 through port 5a is sealed to and projects from the lower end member 3d sealed to the concentric members 3a, 3b. A bead of mercury 6 is disposed at the closed end of the tube 5 and arranged so as to be in communication with the interior of the filter 3. Optionally, the means for controlling mercury pressure further comprises means for maintaining the mercury at a predetermined temperature. In the illustrated embodiment, the mercury temperature maintaining member comprises a sleeve 7 disposed about the end of the tube 5 and around the bead of the mercury 6. A sealing ring 9, such as a conventional O-ring, is disposed between the sleeve 7 and the tube 5 to hold the sleeve 7 in place and prevent leakage of heat exchange fluid (preferably water) which passes through the sleeve 7. The head exchange fluid flows through a "T" connection 11, down sleeve 7 into heater 14 and thence to pump 12 to return to "T" 11. Pump 12 and heater 14 maintain the temperature of the tube at a predetermined level, so as to control the mercury vapor pressure in the filter. This control permits a predetermined quantity of mercury vapor to be maintained in the filter. The reactor 20 is disposed around the filter 3 and includes a pair of spaced-apart concentric sleeves 20a and 20b. A conventional inlet and outlet port 21a and 21b are disposed on the top and bottom of the reactor 20 to allow for the passage of mercury feedstock. The filter 3 is filled with a mixture of hydrogen or nitrogen gas (preferably hydrogen) and mercury vapor, having a reduced ¹⁹⁶Hg content, as disclosed in co-pending application filed concurrently herewith, Serial No. and entitled Filter For Isotopic Alteration of Mercury Vapor (Attorney Docket No. 85-1-121), and assigned to the same assignee as the present application. Turning to FIGURES 2A and 2B, it can be seen in FIGURE 2A that the Hg(6³P₁-6²S O ), viz ¹⁹⁶Hg, has many observable hyperfine emission peaks to the left of the principal peak. In FIGURE 2B, the hyperfine emissions to the left of the principal emission peak are suppressed in the emissions which were measured after the radiation passed through the filter of the present invention, thereby improving the efficiency of the photochemical reaction process. Without being limitative on the scope of the present invention, the following specific example is offered. A low pressure mercury lamp was operated at 20°C and the reactor was operated at 25°C. The filter was operated at 24°C. A flow of gases and vapors (at a total reactor pressure of 1.3 mbar (1 torr)) formed of a mixture of 100 SCCM (standard cubic centimeters per minute) of butadiene and 100 SCCM HCl and mercury in the naturally-occurring state (having ¹⁹⁶Hg of about O.15 atomic weight percent) was passed through the reactor at a flow rate of 10 mg. of Hg per hour. After passing through the reactor and being irradiated with ultraviolet light from the filter, the ¹⁹⁶Hg was enriched from about 0.15 atomic weight percent to about 4.0 atomic weight percent.

Claims: A filter (3) for photochemical isotope separation of ¹⁹⁶Hg component radiation from mercury radiation characterized by a pair of concentrically arranged cylindrical members (3a,3b) formed of a material transparent to ultraviolet emissions, means (3c,3d) for sealing said concentric cylinder members (3a,3b) from the atmosphere, means (5) for controlling mercury pressure in said filter (3), and a fill of mercury vapor which is contained in said filter (3), said mercury vapor having quantities of ¹⁹⁶Hg less than that which is found naturally-occurring mercury. The filter (3) of Claim 1 characterized in that said means for controlling mercury pressure comprises a tube (5) sealed at one end and projecting from the filter (3), the interior of said tube (5) being in communication with the interior of said filter (3), and a predetermined quantity of mercury disposed in said tube (5). The filter (3) of Claim 2 characterized by means to maintain said mercury at a predetermined temperature. The filter (3) of Claim 3 characterized in that said means to maintain the mercury at a predetermined temperature comprises a sleeve (7) disposed about said tube (5) and means (12,14) to circulate a heat exchange fluid over said tube (5) at a predetermined temperature. Use of the filter of any of the Claims 1 to 4 characterized in that an axial passageway is formed in said filter (3) by said inner cylindrical member (3b) being arranged to receive a low pressure electrical discharge lamp (1), said lamp (1) comprising an envelope transparent to ultraviolet radiation and containing a fill comprising mercury and inert gas, and that a reactor (20) is arranged around said filter (3) whereby radiation passes from said low pressure electric discharge lamp (1) through said filter (3) and into said reactor (20), said reactor (20) being transparent to ultraviolet light.

Inventors: GEORGE, WILLIAM A., GROSSMAN, MARK W., MARCUCCUI, RUDOLPH W.
Applicants: GTE PRODUCTS CORPORATION
Application year: 1987
Main classification: B01D59
IPC codes: B01D59