Electric lamp and discharge devices – Having vapor generating material – Mercury vapor material
Utility Patent
1999-03-09
2001-01-02
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
Having vapor generating material
Mercury vapor material
C313S546000, C313S485000, C313S639000
Utility Patent
active
06169362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to improvements in fluorescent lamps and is directed more particularly to providing a mercury vapor discharge lamp containing means for reducing leachable mercury.
2. Description of the Prior Art
During the manufacture of a fluorescent lamp, a quantity of elemental mercury is sealed within the lamp envelope. It is known that after a period of operation most of the elemental mercury adheres to a phosphor coating deposited on the inside wall of the lamp envelope, leaving a small portion of the mercury in the form of mercury vapor. During lamp operation, a portion of the mercury may be converted to a water-soluble mercury compound, such as mercuric oxide (HgO).
There is growing concern that a waste stream resulting from the disposal of fluorescent lamps may leach environmentally harmful amounts of this soluble form of mercury. One method of measuring the amount of soluble mercury which may leach from the waste stream resulting from the disposal of fluorescent lamps is described in a Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987-26998 of volume 55, number 126 of the Jun. 29, 1990 issue of the Federal Register. According to the procedure, the lamp being tested is crushed, ground, cut, or otherwise particulated, into granules having a “surface area per gram of material equal to or greater than 3.1 cm
2
or having a particle size smaller than 1 cm in its narrowest dimension, i.e., is capable of passing through a 9.5 mm (0.375 inch) standard sieve.” Following particulation, the granules are subjected to an extract solution comprising a sodium acetate buffer solution having a pH of approximately 4.93 and having a weight twenty times the weight of the granules.
The Environmental Protection Agency (EPA) defines a regulatory threshold for mercury at 0.2 milligram leachable mercury per liter of extract solution when the TCLP is applied. According to present standards, a fluorescent lamp is considered “nonleachable”) when less than 0.2 milligram of leachable mercury per liter of extract solution results from a TCLP extraction.
Various methods have been proposed which attempt to treat or process burned-out discharge lamps, or scrap lamp exhaust tubing, containing mercury, in order to reclaim the mercury and thereby reduce the amount of mercury-contaminated scrap. In U.S. Pat. No. 5,229,686, issued Jul. 20, 1993 to Richard A. Fowler et al.; and U.S. Pat. No. 5,229,687, issued Jul. 20, 1993 to Richard A. Fowler et al., there are described methods by which to render a mercury vapor lamp “nonleaching”) upon disposal without the use of expensive treatment processes to reclaim the mercury. The method described in '686 employs a chemical agent enclosed within the lamp, and which is suitable for chemically combining with a substantial portion of the soluble mercury as a sparingly soluble salt when the lamp is pulverized. The method described in '687 employs a chemical agent enclosed within the lamp, suitable for electrochemically reducing a substantial portion of the soluble mercury to elemental mercury when the lamp is pulverized. Preferably, this chemical agent is an element which has an electrode potential for oxidation reactions higher than mercury, but which is not sufficiently active to displace hydrogen from acidic aqueous solutions. In a preferred embodiment, the chemical agent is sealed within an enclosure (e.g., glass) which is rupturable upon pulverization of the lamp. In another embodiment, the chemical agent is mixed with the basing cement used to secure the lamp bases to the glass envelope. The chemical agent acts to reduce soluble mercury produced during lamp operation to elemental mercury, which is not leachable as measured by the TCLP.
The chemical agent used in '687 may be used in various forms, e.g., as a powder, dust, wire mesh, or metallic foil. The amount or size of the chemical agent is directly related to the surface area and surface condition, finely divided metallic powders being preferred over a solid mass because of their relatively large effective surface areas. Because of their availability and inexpensive cost, iron and copper, in the form of a powder or dust, are preferred. The amount of chemical agent present should be sufficient to electrochemically reduce the amount of soluble mercury within the lamp which is leached, at the time of disposal, to less than 0.2 milligram per liter of extract solution, as prescribed in the TCLP.
However, there are several disadvantages to the methods described in U.S. Pat. Nos. '686 and '687. In '686, the quantity of chemical agent required to chemically combine with nearly all of the mercury within a fluorescent lamp may be so large as to be inconvenient or impossible to contain within a standard lamp envelope. In '687, the metallic copper or iron reduces the amount of leachable mercury via a surface redox reaction between adsorbed mercury ions and zero-valent metal atoms. In order for this reaction to occur, the dissolved ionic mercury must first find its way to, and become adsorbed upon, the metal surface. Thus, the effectiveness of a metallic element as a means of reducing leachable mercury ultimately will be limited by the rates at which mercury ions diffuse into the metal surface and become adsorbed thereon. A means of reducing leachable mercury which does not depend upon chance contact between dissolved mercury ions and a metal surface, followed by the adsorption of the mercury into that surface, is deemed beneficial.
It is on occasion difficult or impossible to incorporate a sufficiently large quantity of a finely divided metal within a fluorescent lamp, particularly in the smaller or more compact lamps. In a small lamp, the only convenient way to introduce the metal often is as a component of the basing cement. However, the electrical conductivity of the metal may prevent its incorporation into the basing cement inasmuch as the cement may come into contact with internal electrical leads. On the other hand, electrically insulating materials easily are added to basing cement in addition to, or in place of, the normal CaCO
3
cement filler, without risk of creating electrical short circuits within the lamp.
In U.S. Pat. No. 5,736,813, issued Apr. 7, 1998 to Donald F. Foust et al., it is disclosed that “the formation of leachable mercury upon disposal, or during TCLP testing, of mercury vapor discharge lamps is substantially prevented by incorporation of a pH control agent in the lamp structure, or in the test solution, to provide a pH of about 5.5 to 6.5.” A low pressure mercury discharge lamp is disclosed which includes about 5-15 grams of a pH control agent (generally a water-soluble base) which, it is suggested, is sufficient to substantially prevent formation of ferric and cupric compounds which oxidize elemental mercury to a soluble form. The primary disadvantage of this method of reducing mercury leaching is that it is difficult or, depending upon the lamp type, practically impossible to package the relatively large amounts of the required pH control agent (5-15 grams) within the structure of a typical mercury vapor lamp.
Recently, there has become known an improved mercury vapor discharge lamp in which is disposed an effective amount of a nonmetallic copper-containing compound which, when the lamp is pulverized to granules and subjected to a suitable aqueous acid extract solution, dissolves in the acid solution, resulting in a concentration of extracted mercury less than 0.2 mg per liter of extract solution. The effective amount of soluble copper is relatively small (between 0.1 and 4 mg per gram of total lamp weight, depending upon lamp type and size, total mercury loading, etc.); however, copper in the environment may be toxic to certain marine invertebrates. In order to eliminate the possibility of damage to ecological systems, the EPA has placed a limit of 25 mg/l for copper levels in discharges from nonferrous operations to lakes and streams. It is desirable, therefore, to min
Fowler Richard A.
Klinedinst Keith A.
Shinn Dennis B.
Bessone Carlo S.
Day Michael H.
Orsam Sylvania Inc.
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