Method for producing tungsten hexachloride

In a method for producing tungsten hexachloride which comprises reacting tungsten with chlorine gas at a temperature sufficient to result in the conversion of the tungsten to tungsten hexachloride, the improvement comprising reacting tungsten in the form of pressed pieces to convert the pieces to tungsten hexachloride having an oxygen content of less than about 0.5% by weight with the rate of conversion to tungsten hexachloride being at least about 2.3 times greater than the rate of conversion when the tungsten is in an unpressed form.

This invention relates to a method for producing tungsten hexachloride from 
tungsten billets as opposed to using tungsten metal powder as the starting 
material. This results in a higher rate of reaction and a lower oxygen 
content in the tungsten hexachloride as opposed to the tungsten 
hexachloride produced from tungsten powder. 
BACKGROUND OF THE INVENTION 
Tungsten hexachloride is used as a catalyst, in the chemical vapor 
deposition of tungsten, and in the production of high purity fine particle 
size tungsten metal powder. 
Tungsten hexachloride (WCl.sub.6) is typically produced by passing chlorine 
(Cl.sub.2) over a quartz boat filled with tungsten powder condensed and 
collected as powder at the exit end of the reaction furnace. When the 
tungsten powder in the boat is completely consumed by the reaction, the 
quartz furnace tube must be opened to replace the empty boat with a filled 
one. There are several disadvantages in using tungsten powder for the feed 
material. First, the powder must be placed in a suitable container in 
order to be effectively stoked into the hot zone of the furnace tube. 
Quartz boats are typically used since quartz resists attack by chlorine at 
high temperature. However, the boats are fragile and will gradually 
deteriorate so that they must be frequently repaired or replaced. A second 
disadvantage is that the reaction rate is slow. The production rate for 
tungsten hexachloride seems to be primarily dependent on the amount of 
tungsten surface area exposed to the flowing chlorine in the furnace tube. 
Since the powder is contained in a boat, only the top surface is available 
for reaction. Finally since the furnace tube must be opened frequently to 
replace empty boats, the tungsten hexachloride product can easily be 
contaminated by oxygen, forming tungsten oxychloride. To reduce the oxygen 
content of the product it must be boiled and recrystallized. This reduces 
the yield since some tungsten oxychloride escapes during the boiling of 
pure tungsten hexachloride. Minimizing the oxygen contamination increases 
the formation of pure tungsten hexachloride. 
It would be desirable to produce tungsten hexachloride without the above 
disadvantages. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of this invention, there is provided an 
improvement in a method for producing tungsten hexachloride which 
comprises reacting tungsten with chlorine gas at a temperature sufficient 
to result in the conversion of the tungsten to tungsten hexachloride, the 
improvement comprising reacting tungsten in the form of pressed pieces to 
convert the pieces to tungsten hexachloride having an oxygen content of 
less than about 0.5% by weight with the rate of conversion to tungsten 
hexachloride being at least 2.3 times greater than the rate of conversion 
when the tungsten is in an unpressed form. 
DETAILED DESCRIPTION OF THE INVENTION 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above description of some of the aspects of the invention. 
The present invention provides a method for producing tungsten hexachloride 
from tungsten in the form of billets as opposed to tungsten metal powder. 
This affords a number of advantages. The use of costly boats is eliminated 
because the billets can be introduced directly into the furnace without a 
container. Also, without a boat, the billets have nearly their entire 
surfaces exposed to the chlorine gas so that the rate of reaction is more 
than doubled. Still another advantage in using billets is that since the 
density of the billets is several times greater than powder, more tungsten 
can be loaded into the furnace at one time and therefore the number of 
reloadings is reduced. As a result less air is introduced into the furnace 
and therefore the tungsten hexachloride product is significantly lower in 
oxygen than if tungsten powder were used as the starting material. This 
eliminates the need for further purification steps of boiling and 
recrystallization. 
The starting materials for this procedure are pressed tungsten pieces or 
billets. In general, these can be of any convenient shape and size. For 
example, solid rectangular bars work well in horizontal tube reactors. 
Other compacted forms such as cylinders, pellets, rods, etc. can work 
equally well depending on the reactor configuration. The pressure required 
to form the pieces from tungsten powder must be sufficient to produce 
pieces strong enough to be loaded into the reactor without breaking. This 
pressure will vary depending on the type of press and the morphology of 
the tungsten powder. Anyone skilled in the art would know how to press 
parts so that they are suitable for handling in this operation. Preferably 
the pieces are pressed without additives such as wax to minimize 
contamination in the tungsten hexachloride product. Also the pieces should 
be large enough so that no boat is required to contain them in the 
reactor. 
The formed billets or pieces are loaded into a reactor and reacted with 
chrlorine gas to form the tungsten hexachloride product. Because of the 
increase in effective tungsten surface area of the pieces exposed to the 
chlorine gas the reaction rates are greater than if tungsten powder is 
used. 
The reaction conditions depend on the design and size of the equipment and 
on the size of the charge. As a non-limiting illustrative example, in a 2" 
diameter reactor having an 18" heated zone and containing 5 kg of tungsten 
billets, the temperature is typically free from about 800.degree. C. to 
about 900.degree. C. with from about 860.degree. C. to about 880.degree. 
C. being preferred. Higher temperatures result in faster tube corrosion. 
Lower temperatures result in longer reaction times. The reaction times are 
typically from about 20 to about 30 hours with from about 22 hours to 
about 26 hours being preferred. The flow rate of chlorine gas is typically 
from about 0.8 to about 1.0 SLPM, (standard liters per minute). These 
conditions result in essentially all of the tungsten being converted to 
tungsten hexachloride. The oxygen content of the resulting tungsten 
hexachloride product is less than about 0.5% by weight and more typically 
from about 0.1% to about 0.5% by weight as opposed to typical levels of 
from about 0.5% to about 1.0% by weight in material produced from tungsten 
in the unpressed form, most typically, in powder form. 
To more fully illustrate this invention, the following nonlimiting example 
is presented.

EXAMPLE 
Tungsten powder is mechanically compacted in a dual acting opposed ram and 
die type press using about 600 tons of force. The resulting bars are about 
1/2".times.1/2".times.about 18" long and weigh about 2.5 kg each. No 
binder is used. Two bars are loaded directly into a 50 mm quartz tube 
which is resting in a horizontal tube furnace. The 24" hot zone of the 
furnace is set at from about 860.degree. C. to about 880.degree. C. The 
tube is sealed and after purging with nitrogen gas, chlorine gas is 
allowed to flow through the tube at a rate of about 1 liter/minute. As the 
chlorine gas flows past the hot tungsten bars, tungsten hexachloride vapor 
is formed which is pushed out of the furnace where it is condensed and 
collected. After about 26 hours the bars are completely consumed which 
equates to a tungsten hexachloride production rate of about 415 g/hr. 
Previously a 500 g boatload of tungsten powder subjected to the same 
temperature and chlorine rate would last about 6 hours for a tungsten 
hexachloride formation rate of about 180 g/hr. The reaction rate for the 
bars is about 2.3 times greater than for the powder. Also the tungsten 
hexachloride product from the bars has little or no contamination oxygen 
present. Since tungsten hexachloride powder is black, the presence of 
brown tungsten oxychlorides is easily detected by visual inspection. 
Typically the tungsten hexachloride product from tungsten powder in boats 
contains much more off-color material. The difference in the oxygen 
containing species between material from tungsten powder and material from 
tungsten bars is that the reactor is opened and reloaded when the powder 
in boats is used. For example, the furnace of the present example 
accommodates a 500 g load when tungsten is in the form of powder and it 
accommodates a 5,000 g load when the tungsten is in the form of pressed 
bars. Therefore in order to process the same amount of tungsten in powder 
form, the furnace would have to be opened ten times more, and therefore 
the potential for contamination with oxygen is ten times greater with 
powder than with pressed bars. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.