"MERCURY RELEASING METHOD"

The present invention is directed to a method for releasing mercury. Methods and systems for releasing mercury are used particularly in fluorescent lamps.

The method of dosing directly liquid mercury by means of syringe feeders is unable to provide an exact and reproducible dosage of the smaller and smaller amounts of the element which are required by the present lamps.

Some known methods are based on mechanical systems being loaded with metallic mercury. For example US patents Nos. 4,823,047 and 4,278,908 disclose capsules, made of metal or glass, respectively, containing liquid mercury, while

US patent 4,808,136 and patent application EP 568,317 disclose the use of porous pills or spherules (made of metallic or ceramic material, respectively), being impregnated with mercury which is then released by heating. However, also with these methods the released amount of mercury is hardly reproducible and, mainly in the case of capsules, constructional problems may arise.

Other documents disclose the use of mercury compounds, such as US patent 3,657,589 relating to Ti-Zr-Hg compounds (of particular importance being the compound Ti ₃ Hg) or US patent 5,520,560 dealing with the use of compounds according to US patent No. 3,657,589 in admixture with copper-tin alloys having functions of promoting the mercury release. However these compounds require rather high temperatures for the mercury releasing, generally in excess of 500 °C, whereby a specific high temperature thermal process is required in order to produce metallic mercury within the sealed lamp. Finally there is a great number of documents relating to amalgams being employed, such as the international patent application WO 94/18692 about amalgams with zinc or US patent 5,598,069 about amalgams with indium-silver. However the amalgams generally have a mercury content being not particularly important and above all they have a tendency to release mercury already at relatively low temperatures, e.g. of about 100 °C; the amalgams can thus lose not negligible amounts of mercury even during lamp manufacturing steps wherein this

phenomenon is undesirable, with possible pollution of the working environment; for example the lamps may undergo heat treatments to enhance the removal of gaseous impurities being trapped in the phosphors without being yet cooled down to room temperature when the amalgam is introduced, thus starting to release mercury when the lamp is not yet sealed.

Object of the present invention is to provide a method for dispensing mercury that overcomes at least part of the problems mentioned above.

This object is achieved with the present invention by employing manganese- mercury compositions containing between about 30% and 90.1% by weight of mercury.

Among the compositions useful to be employed in the method of the invention, of particular interest are the one comprising about 55% and the one comprising about 75% by weight of mercury.

The invention will be described in detail in the following with reference to the drawings in which:

- Figures Ia to Id show some possible embodiments of mercury dispensers to be used in the method of the invention;

Figure 2 shows a semi-finished product from which mercury dispensers can be obtained, in which the Mn-Hg compositions are mixed with metallic tin;

- Figure 3 graphically shows the mercury yield as a function of the temperature of two compositions according to the invention;

- Figure 4 graphically shows the mercury yields as a function of the temperature of a composition according to the invention being admixed with metallic tin; and

Figure 5 graphically shows the mercury yield as a function of the temperature of a composition according to the invention, after a heating treatment of relatively long duration.

The compositions of the invention comprise several forms of compounds between the two elements. Mercury percentages of 78.5% and 90.1% by weight correspond to two actual intermetallic compounds, MnHg and Mn ₂ Hg ₅ ,

respectively, whereas the intermediate compositions can consist of mixtures between these compounds and possible amalgams.

These compositions can be obtained by reaction of the two metals in the desired weight ratio, e.g. at temperatures of about 500 °C during a time comprised between 1 and 5 hours. The reaction is usually accomplished in a quartz vial, that for safety reasons can be contained in a reactor or steel housing. Mercury is used in liquid form, while manganese is used in powder form to enhance the contact between the two elements; the inside of the vial can be evacuated or filled with an inert gas. Manganese is preferably pre-treated by heating under vacuum, e.g. at 400 °C during 2 hours, in order to remove the trapped gases which, during the reaction, could cause overpressures and breakages of the vial. As manganese is of lower density with respect to mercury, its loose powder floats on the mercury and during the reaction an interface of reacted material can result, which may be of hindrance to a further progress of the reaction; therefore it could be preferable to compress the manganese powders in form of pills to be stacked in the vial until reaching the upper end thereof, whereby mercury can surround them along the whole length of the stack. At the end of the reaction the vial is opened and a single, rather compact body is withdrawn, which can be easily ground to obtain powders of the desired particle size, for example of less than half a millimeter. The last step of the process for manufacturing the compositions according to the invention is a thermal treatment at about 60 °C under suction, such as with a vacuum of about 10 ^'3 hectoPascal (hPa), in order to remove possible traces of non-reacted mercury which otherwise could evaporate at undesired stages of the lamp manufacturing process, or even earlier, during the storage of the composition, with a possible risk of pollution of the working environment.

The compositions of the invention have in practice no mercury emission until about 150 °C, and consequently they can be introduced into lamps resulting from previous hot manufacturing steps without causing the element to be released. Mercury emission can then be caused to occur with a suitable activation treatment at temperatures comprised between about 200 and 450 °C.

Figure 1 shows some possible embodiments of mercury dispensers made

- A -

with the compositions described in the foregoing. The dispensers can be produced with powders of a Mn-Hg composition only, for example by compressing the powders to obtain a pill 10 (fig. Ia) or a spherule 11 (fig. Ib); in alternative it is possible to manufacture dispensers wherein the powders are supported, for example by depositing powders 12 of the Mn-Hg compositions onto a metallic strip 13 and cutting from the strip lengths 14 forming the single dispensers (fig. Ic), or loading the powders of Mn-Hg composition in an open container 16, thus obtaining the dispenser 17 (fig. Id). Other configurations, not shown in the drawings, are possible, such as the shields for cathode lamps carrying a track of a mercury releasing material of US patent 6,107,737, or the elongated bodies filled with powders of a mercury releasing material of US patent 6,679,745 B2 and of US patent 6,680,571 Bl (see in particular fig. 3 of the latter patent).

The inventors have also ascertained that the presence of metallic tin in mechanical admixture with the powdered compositions is able to significantly increase the values of mercury yield of these compositions when the tin melting temperature is reached. The weight ratio between the Mn-Hg composition and tin can vary between about 4:1 and 1:9; with ratios Mn-Hg/Sn higher than 4:1 the tin quantity is too small and the effect of yield increasing is obtained only in a fraction of the powders, thus giving rise to a mercury dispenser of non- homogeneous properties, whereas with ratios of less than 1 :9 there is tin in excess, which involves the problem of low quantities of Hg available in the dispenser.

The mixture between the chosen Mn-Hg composition and tin, taken in the desired weight ratio, can be formed in the shape of pills or spherules, such as by compression. It is however preferable to form bodies of the mixture by extruding the mixed powders of tin and of the Mn-Hg composition, exploiting the plasticity of tin which allows to form extruded bodies with good characteristics of mechanical strength; to ensure the mechanical properties of the system, in this embodiment the weight ratio Mn-Hg/Sn is preferably lower than 2. Figure 2 shows a possible embodiment of an extruded body; the body 20 has circular cross- section (e.g. with diameter between about 1 and 5 mm to obtain mercury dispensers for lamps) and indefinite length; from body 20 it is possible to obtain

by cutting a series of dispensers 21, either immediately downstream of the extrusion or at the location where the lamps are manufactured. By operating correctly the linear loading of mercury in the body 20 is homogeneous throughout its whole length, so that by presetting the distance between two subsequent cuts, and consequently the length of dispensers 21, it is possible to ensure with good reproducibility the amount of mercury present in each dispenser.

The invention will be further described in the following examples.

EXAMPLE 1

This example concerns the production of a first Mn-Hg composition being useful in the method of the invention.

An open quartz vial, having inner volume of about 50 cm ³ , is placed on the plate of a weighing scale; 15 g of liquid mercury are poured into the vial. Separately 5 g of powdered manganese having particle size of less than 60 μm, being previously subjected to a degassing treatment consisting in heating under vacuum at 400 °C during 2 hours, are weighed; the manganese powders are poured into the vial, which is then flame sealed; all the previous operations are carried out in a "glove-box" under atmosphere of argon. The closed vial is placed in an oven while subjecting the mixture to the following thermal cycle: temperature increasing up to 500 ⁰ C in half an hour, keeping this temperature for one hour, cooling at 200 °C, keeping at this second temperature for 4 hours and finally natural cooling until reaching room temperature, which requires about 2 hours. At the end of this thermal treatment the vial is withdrawn from the oven and broken, thus extracting a pulverulent body which is ground to recover the particle size fraction of less than 50 μm. The powder thus selected undergoes a mild thermal treatment at 60 ⁰ C during 3 hours under pumping to remove possible traces of non-reacted mercury.

EXAMPLE 2

This example is directed to the manufacturing of a second Mn-Hg composition which is useful in the method of the invention. The same procedure of example 1 is repeated, starting in this case from H g of mercury and 9 g of manganese.

EXAMPLE 3

This example concerns the measurement of the characteristics of mercury release from the powder obtained in example 1.

With the powder of example 1 three mercury dispensing devices are manufactured by loading for each dispenser 100 mg of powder into a cylindrical container of diameter 6 mm and height 1.5 mm (of the type shown in figure Id), and compressing the powders in the container with a punch by applying a pressure of 700 kg/cm ² ; the three dispensers thus obtained are commonly referred to as sample 1 in the following. Thermocouple wires are welded to each one of the three dispensers to detect the temperature during the subsequent treatment. The first dispenser of sample 1 is weighed, inserted into an evacuated glass bulb, induction heated from the outside of the bulb to 200 ⁰ C in 10 seconds, kept at this temperature during 20 seconds and finally let to cool down to room temperature; the bulb is then opened and the dispenser is weighed. By weight difference the mercury yield of the sample 1 at 200 °C is obtained (as a percentage with respect to the initially contained mercury). The procedure is repeated with the second and third dispensers, brought to 300 and 400 ⁰ C respectively. The three values of mercury yield thus obtained are graphically plotted in figure 3 as curve 1.

EXAMPLE 4 This example concerns the measuring of the characteristics of mercury release of the powder obtained in example 2.

The test of example 3 is repeated on sample 2, formed of three dispensers manufactured starting from powders of example 2. The three values of mercury yield thus obtained are graphically plotted in figure 3 as curve 2. EXAMPLE 5

This example concerns the measurements of characteristics of mercury release of a mixture between powders of tin and of the composition of example 2.

Three mercury dispensers are produced following the procedure of example

4, but employing a mixture formed of 60 mg of powder of manganese-mercury composition with 40 mg of tin powder with particle size lower than 150 μm. The three dispensers are brought to 250, 300 and 400 °C, respectively. The three

values of mercury yield are plotted, as curve 3, in figure 4 which for comparison reasons shows also the curve 2 of figure 3 (relating to the same manganese- mercury composition but without addition of tin).

EXAMPLE 6 This example concerns the measurements of characteristics of mercury release of a mixture between powders of tin and of the composition of example 2, employing a longer activation time that is adopted in the manufacture of neon signs.

The test of example 5 is repeated, with the following differences: the dispensers are loaded with a mixture formed of 50 mg of powder of the Mn-Hg composition of example 2 with 50 mg of tin powder with particle size lower than

150 μm; the three dispensers are brought to 260, 300 and 350 °C, respectively; and, the activation is carried out by heating each dispenser at the test temperature in 10 seconds, keeping it at this temperature for 110 seconds and finally letting the dispenser to cool down to room temperature.

The three values of mercury yield are plotted, as curve 4, in figure 5.

As can be observed from the analysis of the results, the compositions of the invention show good characteristics of mercury yield in the range 200-400 ⁰ C. In addition the mixtures with tin substantially increase the mercury yield.