Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J61/00—Gas-discharge or vapour-discharge lamps
  • H01J61/70—Lamps with low-pressure unconstricted discharge having a cold pressure < 400 Torr
  • H01J61/72—Lamps with low-pressure unconstricted discharge having a cold pressure < 400 Torr having a main light-emitting filling of easily vaporisable metal vapour, e.g. mercury

Definitions

• This invention relates to mercury vapor discharge lamps and more particularly to fluorescent lamps. Still more particularly it relates to lamps that can be landfilled without leaching potentially damaging mercury into the environment.
• the invention relates for example to linear fluorescent lamps, including large and small diameter fluorescent lamps corresponding for example to 30 to 40 mm diameter (large) or below 30 mm (small). Typical diameters are T12 (about 38 mm, large) and T8 (about 25 mm, small) and even smaller ones.
• Fluorescent lamps contain elemental mercury. During lamp operation, chemical reactions take place that convert some of the elemental mercury to salts or compounds, such as mercuric oxide (HgO), that are water soluble. There is a growing concern that a waste stream resulting from the disposal of fluorescent lamps may leach excessive amounts of this soluble form of mercury (Hg) into the environment.
• An acceptable 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 the Toxicity Characteristic Leaching Procedure (TCLP) prescribed on pages 26987 - 26998 of volume 55, number 126 of the June 29, 1990 issue of the Federal Register.
• TCLP Toxicity Characteristic Leaching Procedure
• the lamp to be tested is pulverized into granules having a surface area per gram of materials equal to or greater than 3.1 cm 2 or having a particle size smaller than 1 cm in its narrowest dimension.
• the granules are then subject to a sodium acetate buffer solution having a pH of approximately 4.9 ands a weight twenty times that of the granules.
• the buffer solution is then extracted, and the concentration of mercury is measured.
• the United States Environmental Protection Agency (EPA) defines a maximum concentration level for mercury to be 0.2 milligram of leachable mercury per liter of leachate fluid when the TCLP is applied.
• U.S. Patent No. 5,998,927, Foust, et al. teaches a method for inhibiting the formation of leachable mercury associated with a mercury arc vapor discharge lamp when the mercury is in elemental form.
• the method comprises providing high-iron content metal components in the lamps, at least one of the high-iron content metal components having an amount of oxidizable iron of at least about 1 gm per kilogram of lamp weight.
• the addition of the substantially un-doped stannous oxide provides a totally unexpected, synergistic effect between the stannous oxide and the oxidizable iron to inhibit mercury leaching when the mercury is present in an ionic form.
• a first embodiment is dealing with large diameter T12 lamps without excluding smaller diameters.
• Table I Shown in Table I are the results of a series of TCLP tests carried out with F40T12 lamps in which all of the mercury (5.0-5.5 mg) was initially present in the soluble ionic form (added as HgO).
• the first test was run without the addition of any metallic iron.
• the second and third tests were run with the inclusion of 4.3 cm 2 of 0.15 mm thick metallic iron foil, while the forth and fifth tests were run with the inclusion of 6.4 cm 2 of 0.15 mm thick iron foil.
• These quantities of metallic iron correspond to approximately 1.8 and 2.7 grams of oxidizable iron per kilogram of lamp weight, well within the range prescribed by Foust, et al in the referenced patent.
• SnO 2 In order to reduce the voltage necessary for ignition of certain fluorescent lamps (in particular, certain T12 lamp types that are no more than 1,3 m in length), it is known in the art to deposit transparent and electrically conductive, doped SnO 2 upon the inside surfaces of the cylindrical glass lamp envelopes.
• the SnO 2 is typically doped with fluorine (F) or antimony (Sb) (most typically fluorine), which dopants have the effect of greatly increasing the electrical conductivity of the material.
• the first test was run with a lamp which did not contain an SnO 2 coating on the glass (similar to the first test listed in Table I above), while the second test was run with glass that had been coated on the inside surface with F-doped SnO 2 . As shown, the presence of the conductive, F-doped SnO 2 coating had essentially no effect upon the result of the TCLP test.
• the toxic effect is due to the precipitation of the available bodily calcium by the fluoride. This typically leads to a drastic drop of the calcium level, essential for most vital functions. If not promptly treated, often-fatal complications may follow (e.g., cardiac arrest). It would obviously be advantageous if the fluoride content of the SnO 2 coating could be eliminated without significantly altering the beneficial effects of the coating.
• the presence of the fluoride dopant in the SnO 2 coating also exacerbates the formation of a type of lamp defect often referred to as 'black spot patches' or 'measles' which develop during lamp operation as a result of an interaction involving the conductive layer and the mercury in the arc discharge.
• the mercury penetrates the phosphor layer, leading to conditions that allow buildup of charge and subsequent discharge, which result in the 'measle' defect by disrupting the phosphor layer and generally forming a small crater in the glass tube.
• What is disclosed herein is a method for inhibiting the leaching of mercury from mercury-containing fluorescent lamps, as determined by the TCLP.
• the method comprises providing a transparent, substantially undoped (and, therefore, effectively nonconducting) tin-oxide coating on the inside surfaces of the glass envelopes of said lamps, in combination with high-iron content metal components at least one of which contains an amount of oxidizable iron of at least about 1 gram per kilogram of lamp weight.
• the inside surfaces of two groups of standard T12 lamp envelopes about 38 mm (or 1.5 inches) in diameter and about 1,22 m (4 feet) in length were coated with tin oxide (SnO 2 ) using the standard spraying method with standard SnCl 4 and solvent concentrations used in each case.
• the sprayed solution also contained the standard concentration of hydrofluoric acid (HF).
• the tubes were coated using an SnCl 4 solution which did not contain any HF.
• Both groups of lamp envelopes were coated with the standard thickness of SnO 2 .
• the relative resistivities of the undoped and F-doped coatings were determined using point probes positioned close to the ends of the coated surfaces of each tube.
• the relative end-to-end film resistance of the undoped SnO 2 coating was found to be between 3 and 4 times that of the F-doped coating.
• the coated surfaces also were examined analytically by two methods: energy dispersive spectroscopy (EDS) and Rutherford backscattering spectroscopy (RBS). Taken together, the results of these measurements indicated an average film thickness of about 50 nm (corresponding to a coating density of approximately 40 micrograms/cm 2 ).
• EDS energy dispersive spectroscopy
• RBS Rutherford backscattering spectroscopy
• the improved method for the control of leachable mercury in a fluorescent lamp is based upon the surprising synergy that exists between substantially undoped (and, therefore, effectively nonconducting) SnO 2 (deposited upon the inside surface of the lamp's glass envelope) and a relatively small amount of oxidizable metallic iron or other high-iron content metal, to inhibit mercury leaching.
• SnO 2 deposited upon the inside surface of the lamp's glass envelope
• oxidizable metallic iron or other high-iron content metal to inhibit mercury leaching.
• the high-iron content metal could be included within the lamp in a variety of ways, as suggested by the prior art.
• This method for controlling the amount of leachable mercury in fluorescent lamps with diameters less than 40 mm (or 1.5 inches) is based upon the surprising synergy that exists between SnO 2 deposited upon the inside surface of the glass envelope and a relatively small amount of oxidizable iron or other high iron content metal contained with the lamp.
• the high iron content metal can be included within the lamp in a variety of ways, as is known.

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• Vessels And Coating Films For Discharge Lamps (AREA)

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Citations (5)

• 2003
  • 2003-06-17 DE DE2003600468 patent/DE60300468T2/de not_active Expired - Fee Related
  • 2003-06-17 EP EP20030013718 patent/EP1376654B1/fr not_active Expired - Fee Related

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