Filter for isotopic alteration of mercury vapor

A filter for enriching the .sup.196 Hg content of mercury, including a reactor, a low pressure electric discharge lamp containing a fill of mercury and an inert gas. A filter is arranged concentrically around the lamp. The reactor is arranged around said filter, whereby radiation from said lamp passes through the filter and into said reactor. The lamp, the filter and the reactor are formed of quartz, and are transparent to ultraviolet light. The .sup.196 Hg concentration in the mercury fill is less than that which is present in naturally occurring mercury, that is less than about 0.146 atomic weight percent. Hydrogen is also included in the fill and serves as a quenching gas in the filter, the hydrogen also serving to prevent disposition of a dark coating on the interior of the filter.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to equipment for use in a photochemical 
process, and more particularly to equipment for use in a photochemical 
process for altering the isotopic composition of mercury. 
BACKGROUND OF THE INVENTION 
The excitation of specific mercury isotopes by photochemical means is well 
known to the art. For example, the paper by Webster and Zare, 
"Photochemical Isotope Separation of .sup.196 Hg 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 to 3 torr, 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 
five hyperfine components, principally the result of isotope shifting. As 
is known, the .sup.196 Hg 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 .sup.196 Hg 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 .sup.202 Hg(6.sup.3 P.sub.1) atoms that are contained in 
natural mercury with hydrogen chloride with a photochemical reaction in 
which the .sup.202 Hg atoms are excited during the reaction to precipitate 
a .sup.202 Hg.sub.2 Cl.sub.2. 
As described in a paper delivered by Mark Grossman and Jakob Maya at the 
International Quantum Electronics Conference, June 1984, very high 
enrichment of .sup.196 Hg can be achieved in a photochemical reaction 
using a natural mercury vapor filter. When radiation from a microwave lamp 
containing mercury enriched to 35% .sup.196 Hg is used in a filter, the 
filter eliminates substantially all of the non-.sup.196 Hg component 
radiation permitting an isotopically selective primary excitation of the 
.sup.196 Hg isotope. Selective excitation of .sup.196 Hg(6.sup.3 P.sub.1) 
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. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a filter for 
enriching the content of .sup.196 Hg in a mercury feedstock which flows 
through a reactor, the filter comprising: an enclosure, at least a portion 
of which is transparent to the passage of ultraviolet light; a fill of 
mercury vapor in said enclosure, said mercury vapor having quantities of 
.sup.196 Hg less than that which is found in naturally occurring mercury.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a novel composition of matter housed 
in a UV-transparent filter for use in a system to enrich the .sup.196 Hg 
content of mercury by a photochemical reaction process. The reaction is 
between "natural" mercury, or possibly previously .sup.196 Hg-enriched 
mercury, and hydrogen chloride. The filter is particularly useful with a 
single pass reaction vessel that surrounds the filter, which filter in 
turn, surrounds a low pressure electric discharge lamp having a fill 
comprising mercury and an inert gas. The elements of the system, the lamp, 
the filter, and the reactor are formed of a material which is transparent 
to radiation at 253.7 nm, such as, for example, quartz or silica. Examples 
of a low pressure (e.g., about 2.5 torr) electric discharge lamp that are 
suitable for use in the system can be, for example, 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. 
The filter of the present invention is disposed around the lamp and is 
formed of a pair of concentric cylindrical (or tubular) members that are 
sealed from the atmosphere. The concentric cylindrical members of the 
filter are preferably sealed at their ends with a pair of spaced-apart end 
members, so as to form a cylinder with an axial passageway that receives 
the low pressure discharge lamp. The filter contains a fill of mercury 
which is depleted in .sup.196 Hg so that the concentration of .sup.196 Hg 
is below the quantity that is usually present in naturally occurring 
mercury, viz. below 0.146 atomic weight percent. 
In another aspect of the invention, hydrogen is included in the filter as 
quenching gas. The inclusion of the hydrogen has been found to inhibit the 
darkening of the interior of the filter. There may be several reasons for 
this. As an example, the hydrogen atoms formed during the quenching of 
excited mercury atoms may react with mercury oxide molecules that could 
form Hg and H.sub.2 O. The result is a reduction of the oxide back to 
elemental Hg. Otherwise the mercury oxide would eventually form an opaque 
film on the inner filter wall. 
The reactor is disposed about the filter and may take the form of a pair of 
concentric cylinders with inlet and outlet ports through which the mercury 
feedstock, HCl gas, and other carrier gases can flow. The exciting 
.sup.196 Hg radiation passes through the lamp envelope, through the filter 
and into the reactor to produce an isotopic-specific reaction to the 
.sup.196 Hg in the reaction vessel, whereby .sup.196 Hg.sub.2 Cl.sub.2 is 
formed. 
Referring to FIG. 1 there is shown a low pressure mercury lamp 1 comprising 
silica or a quartz discharge tube, equipped with microwave cavities 
disposed at one end thereof. For isotope separation of .sup.196 Hg, the 
inner diameter of the tube is preferably approximately 5 mm. The discharge 
lamp 1 typically contains an inert gas, e.g., argon, at a pressure of 
approximately 2.5 torr and a mercury pressure of approximately 1 to 1.5 
millitorr at about 20.degree. C. Although argon is preferred as an inert 
gas in the lamp, other gases such as neon may be used also. 
The filter 3 includes a pair of concentric cylindrical members 3a and 3b 
spaced from each other at a distance about 1.0 centimeters. The filter 3 
is sealed from the atmosphere by 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. 
Reference is herein made to the co-pending application, filed on even date 
herewith, Ser. No. 947,217, entitled "Apparatus For Isotopic Alteration Of 
Mercury Vapor" (Attorney Docket No. 85-1-122) and assigned to the same 
assignee as the present application. In a preferred embodiment disclosed 
therein a means for controlling mercury pressure in the filter comprises a 
tube 5, which is sealed at one end and in communication with the interior 
of the filter 3 through port 5a, is disposed on the lower end member 3d. 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. Most 
preferably, a means for maintaining the mercury at a predetermined 
temperature is further described in the co-pending application. The 
temperature maintaining means includes 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 heat 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 maintain a 
predetermined quantity of mercury vapor 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 port and 
outlet port 21a and 21b are disposed on the top and bottom of the reactor 
20 to allow for the passage of the mercury feedstock. 
As shown in FIG. 2A, the radiation from the low pressure mercury lamp 1, in 
the selected 253.7 nm area, has a principal emission peak and five 
hyperfine peaks to the left of the principal peak. In FIG. 2B, the 
emission after the light has passed through the filter 3, the hyperfine 
emissions (peaks to the left of the .sup.196 Hg peak) have been 
suppressed, thereby reducing the interfering excitation peaks which enter 
the reactor 20, thereby in turn, reducing the chemical reactions between 
the HCl and non-.sup.196 Hg isotopes. 
The filter 3 is formed with at least a portion of its structure having a 
glass that is transparent to radiation at 253.7 nm, such as quartz, and is 
preferably entirely formed of quartz or silica, such that ultraviolet 
light can pass from the lamp 1 to the reactor vessel 3. The filter 3 
contains the previously described fill of depleted .sup.196 Hg-mercury, as 
contrasted to "natural" mercury that contains 0.146 atomic weight percent 
.sup.196 Hg isotope. When the .sup.196 Hg concentration is reduced below 
the levels present in natural mercury, the hyperfine emissions of a 
mercury isotope other than .sup.196 Hg are reduced. 
The absorption coefficient for mercury 196 component is given by the 
expression: 
##EQU1## 
where A=0.146.times.10.sup.-2 for (.sup.196 Hg) in natural Hg 
T.sub.k =vapor temperature in .degree.K. 
N.sub.tot =total Hg density (cm.sup.-3) 
In the model to be considered, radiation is assumed to pass normally from 
the lamp through a one cm. filter. 
At 50.degree. C., N.sub.tot =4.4.times.10.sup.14 cm.sup.-3 : 
For natural Hg[.sup.196 Hg]=0.146%, K.sub.196 =0.376, and e.sup.-kx =0.69; 
For mercury reduced in .sup.196 Hg to 0.063% (one-half the natural mercury 
concentration), [.sup.196 Hg]=0.073%, K.sub.196 =0.184, and e.sup.-kx 
=0.83; and 
For [.sup.196 Hg]=0.0%, K.sub.196 =0.0, and e.sup.-kx =1.00. 
The expression e.sup.-kx represents the relative transmitted radiation or 
I/I.sub.o as given by equation for Beer's law. This approximates the 
attenuation and together with the assumption of normal transmission 
through the reactor gap, forms the "slab model". 
TABLE I 
______________________________________ 
Calculation Of Improvement In Transmission Of 
.sup.196 Hg 253.7 nm Component Through A Filter 
Depleted In .sup.196 Hg Using A Slab Model At 50.degree. C. 
% Improvement 
.sup.196 Hg Absorption Coefficient 
in Transmission 
Concentration 
K.sub.196 
Exp (-K.sub.196 X) 
(I) 
______________________________________ 
Natural Hg 0.376 0.690 0.0 
50% Depleted .sup.196 Hg 
0.184 0.830 20.0 
100% Depleted .sup.196 Hg 
0.00 1.000 45.0 
______________________________________ 
As shown in the above Table and with a 1 cm. thick filter, a 20% 
improvement in transmission of narrow band radiation for .sup.196 Hg 
excitation can be achieved for a 50% depletion of .sup.196 Hg in natural 
mercury (0.073% .sup.196 Hg) and a 45% improvement in transmission can be 
achieved if all the .sup.196 Hg is removed. 
The filter is filled with a mixture of quenching gas and mercury vapor and 
as previously mentioned, the filter contains the depleted .sup.196 Hg and 
hydrogen, the latter serving as a quenching gas. As mentioned above, the 
use of hydrogen reduces the disposition of a dark coating on the inside of 
the filter, which has been observed to occur with other quenching gases. 
Heretofore, nitrogen has been used to act as a quenching gas. When 
compared to using hydrogen as as quenching gas, a nitrogen quenching gas 
is less than fully satisfactory. 
The mercury filter needs to absorb non-.sup.196 Hg radiation which could 
excite non-.sup.196 Hg isotopes, and the hydrogen serves as a quenching 
gas. Quite unexpectedly, it has been found that the hydrogen does not form 
stable by-products in the herein described mercury filter environment, 
which appears to be the case with other quenching gases previously used. 
When using hydrogen as a quenching gas, radiation of the mercury is 
converted to non-radiative energy through collisions. 
As can be seen in FIG. 3B, with a filter having 10 torr H.sub.2, there is a 
significant depression of the emission lines to the left of the principal 
.sup.196 Hg emission line in the curve. With a nitrogen fill, as shown in 
FIG. 3C, emission lines attributable to non-.sup.196 Hg isotopes occur. 
Such is also the case with the curve shown in FIG. 3A which illustrates 
the lamp emission after passing through a filter containing only mercury. 
Without being limitative on the scope of the present application, the 
following specific example is offered: 
A filter having a thickness of 10 mm. with a fill of H.sub.2 as a buffer 
gas at a pressure of 10 torr was used. The excitation lamp diameter was 5 
mm. and had a fill of argon and mercury and operated at a gas fill 
pressure of 2.5 torr at 20.degree. C. The filter contained mercury that 
was 50% depleted in .sup.196 Hg (approximately 0.073 atomic weight percent 
.sup.196 Hg). The filter was operated at a temperature of approximately 
40.degree. C. which provides substantial non-.sup.196 Hg isotope emission 
suppression. The suppression of the non-.sup.196 Hg isotopes is shown in 
that in FIG. 4C at a filter temperature of 8.degree. C., hyperfine 
emissions to the left of the principal .sup.196 Hg peak are less 
suppressed than at 16.degree. C., as shown in FIG. 4B, and less suppressed 
than at 24.degree. C. as shown in FIG. 4C. 
The feedstock into the reactor was natural mercury which flowed at a rate 
of 19 milligrams per hour, together with HCl at a rate of 100 sccm 
(standard cubic centimeters per minute) and argon at a rate of 80 sccm. 
Upon measurement, the mercury was enriched from the 0.146 atomic weight 
percent .sup.196 Hg to 1.6 atomic weight percent. The enrichment of 
.sup.196 Hg can be accomplished as disclosed in a co-pending application 
entitled: "High Feedstock Utilization Process for .sup.196 Hg Enrichment 
in a Photochemical Flow Reactor" (Attorney Docket No. 86-1-010), Ser. No. 
917,218, filed on even date herewith, and assigned to the same assignee as 
the present application. 
It is apparent that modifications and changes can be made within the spirit 
and scope of the present invention, but it is our intention, however, only 
to be limited by the scope of the appended claims.