Method for making amalgam pellets

A method for forming sodium-mercury amalgam pellets of predetermined size. The amalgam is heated to a liquid state in a reservoir and discharged from an orifice at a controlled rate in droplet form onto a rotating disc. The droplets solidify into pellet form as the disc rotates and are automatically dislodged and discharged from the disc into a storage vessel. The method is preferably practiced in a controlled inert atmosphere.

BACKGROUND OF THE INVENTION 
This invention relates to a method for making sodium-mercury amalgam 
pellets for use as a part of the discharge sustaining fill of a 
high-pressure sodium (HPS) discharge lamp. 
In the prior art practice the sodium-mercury amalgam used as the discharge 
sustaining fill in a high-pressure sodium discharge lamp has been 
delivered to the discharge lamp arc tube as fine salt like granules 
introduced through the exhaust tubulation at one end of the arc tube body. 
In newer versions of the high-pressure sodium discharge lamp, the arc tube 
body is now constructed without exhaust and fill tubulation and the end 
closures are ceramic buttons instead of refractory metal end caps with 
their associated exhaust and fill tubulation. With this configuration, 
liquid amalgam has been introduced into the open end of an arc tube that 
has had one end previously sealed. Amalgam heated to a liquid is retained 
in a reservoir that is pierced by a plunger which has been suitably 
notched to form a cavity for the required charge of amalgam. Motion of the 
cavity into and out of the liquid measures the droplet that enters the arc 
tube. The introduction of amalgam is more readily and reliably preformed 
if the amalgam is in the form of a pellet of predetermined weight. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a method has been developed for 
forming sodium-mercury amalgam pellets of predetermined size. The 
apparatus used in this invention includes a dispensing means comprising an 
amalgam reservoir, a discharge tube extending from the reservoir and an 
orifice at the end of the discharge tube remote from the reservoir. The 
orifice is situated a short distance above a rotatable disc mounted for 
rotation beneath the orifice means for receiving droplets therefrom. Drive 
means is connected to the disc for rotating the disc at a predetermined 
speed whereby droplets of sodium-mercury amalgam falling from the orifice 
deposit on the rotating disc and solidify into pellets during less than a 
single rotation of the disc. Means are provided for dislodging the pellets 
from the rotating disc after they solidify which directs the pellets to a 
collector means in which they are retained until needed for use in 
providing a specific predetermined sodium-mercury amalgam dose for the arc 
tube of a high-pressure discharge lamp. Heater means are associated with 
the reservoir, the discharge tube and the orifice to maintain the amalgam 
at a predetermined temperature and valve means is provided in the 
discharge tube adjacent to the orifice to control the flow rate of the 
amalgam into the orifice and hence the spacing between droplets on the 
disc. 
The method of the invention involves the heating of a predetermined ratio 
of sodium and mercury to a predetermined temperature to form a solution, 
releasing droplets of predetermined volume of the solution from an orifice 
onto a rotating disc, rotating the disc at a predetermined speed, whereby 
the droplets solidify into pellet form on the disc during rotation of the 
disc and automatically dislodging the pellets from the disc and delivering 
the pellets into a container. The entire process is performed in an inert 
atmosphere preferable in an atmosphere of argon.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the drawing wherein like reference 
characteristics represent like parts throughout the several views, there 
is illustrated in FIGS. 1 and 2 the apparatus used in the invention and 
which must be employed in an inert atmosphere, and preferably in a dry 
box. The apparatus is disposed on a separate base plate 10. A mounting 
stand 12 is provided to support the droplet dispensing means 14 which 
includes a sodium-mercury amalgam reservoir 16 having connected thereto at 
its lower end a discharge tube 18 which discharge tube 18 at its other end 
terminates in a closure member 20 which includes a nozzle-receiving 
orifice 24 at its end or bottom surface. The nozzle-receiving orifice 
includes an O-ring seal 26 which is adapted to receive and retain a 
square-cut hypodermic needle 28 forming the nozzle. 
The reservoir 16 is provided with a heating coil 30 and the discharge tube 
18 is also provided with a heating coil 32 which serve to maintain the 
sodium-mercury amalgam in a liquid state within the reservoir and 
discharge tube. The heating coil 32 also serves to heat the closure member 
20 and the orifice 28. 
At the upper end of the reservoir 16 a valve-mounting structure 34 is 
provided which includes an elongated threaded aperture 35 through which an 
elongated valve shaft 36 extends. The valve shaft 36 has at its lower end 
a needle valve 38 which extends into a valve seat 39 in the bottom end of 
the discharge tube 18 and at its upper end includes a flow-rate adjusting 
knob 40. A threaded portion 37 of the shaft 36 within the valve-mounting 
structure 34 defines the position of the needle valve 38 with respect to 
the valve seat 39 thereby controlling the flow rate of the liquid 
sodium-mercury amalgam into the nozzle 28. Situated about 1/4 inch below 
the end of the nozzle 28 is a stainless steel disc 42 which is mounted on 
the drive shaft 44 of a motor 46. The disc 42 is caused to rotate at a 
predetermined speed by the motor 46 and as drops of sodium-mercury amalgam 
fall from the orifice 28, they are deposited on the disc near its 
periphery at 48 in the form of a droplet 50. As the disc 42 rotates, the 
droplets remain for almost one full revolution on the surface of the disc 
where they solidify to pellet form 51 and slightly adhere to the stainless 
steel surface. 
The droplets now in pellet form proceed to removal means in the form of a 
blade member 52 which overlies the upper surface of the disc 42 and is 
disposed at an angle to the path of the pellets by a mounting bracket 54. 
As the pellets encounter the blade 52 they are dislodged from the surface 
of the stainless steel disc 42 and move outwardly from their original 
position due to the angle of the blade with respect to their travel path 
until they fall off the edge of the disc into a funnel 56 which directs 
the pellets to a storage container 58. 
In accordance with the preferred embodiment, the heater coils 30 and 32 
preferably maintain the sodium-mercury amalgam within the reservoir 16, 
discharge tube 18 and nozzle 28 at about 130.degree. C. The sodium-mercury 
amalgam is preferably about 25 wt.% sodium, but in some applications it is 
desirable that other ratios be used such as 21% or 19% depending upon the 
characteristics of the lamp in which the amalgam is to be used. The needle 
valve 38 controls the frequency with which droplets leave the nozzle 28 
and in combination with the speed of rotation of the disc 42 determines 
the spacing between the droplets on the disc surface. The size or weight 
of the droplet is a function of the size and shape of the orifice, the 
surface tension and density of the liquid. Square cut hypodermic tubing, 
as the nozzle, has worked well for this purpose. The size of the pellets, 
and hence the preselected dose size for an arc tube, may be changed as 
desired simply by interchanging the size of the hypodermic tube 26 
inserted into the opening 24 in the bottom face of the closure member 20. 
In order to prevent splattering of the droplet it is preferred that the 
end of the hypodermic tube 28 be situated about 1/4 inch above the 
rotating disc 42. 
With a 10 inch diameter disc and a disc speed of 6 RPM, the valve 38 can be 
set to deposit droplets at 3/8 inch intervals and will provide 
approximately 30,000 sodium-mercury amalgam pellets per hour of 
approximately 50 milligrams in weight. The approximately 50-milligram 
pellets can be produced from a square cut standard 19-gauge stainless 
steel hypodermic tube. 
The entire operation must be performed in an inert atmosphere because of 
the hygroscopic nature of sodium. Preferably, the operation is performed 
in a dry box having an argon atmosphere therein which dry box 57 is shown 
schematically in FIG. 2. Such a dry box is readily available, for example, 
from Laminar Flow Inc., 102 Richard Road, Ivyland, Pa. 
As will be apparent from the foregoing, the method of this invention 
provides a large number of sodium-mercury amalgam pellets of uniform size 
in a form which is readily suited for use as the sodium-mercury amalgam 
dose for a high-pressure sodium discharge lamp arc tube.