Chlorine trifluoride (ClF3), for example, has many uses such as a specialised fluorinating agent, etching material and high performance cleaning agent, due to its extremely reactive nature. It may also be used as a dry cleaning agent in the electronics industry.

However, it is a highly dangerous substance and consequently there are exceedingly strict regulations governing its use and particularly its transportation. The dangers with ClF3 are that at room temperature and pressure ClF3 is an extremely reactive gas. Organic substances, for example, will react immediately with ClF3 and ignite. ClF3 also reacts violently with water giving an emission of toxic fumes.

Additionally ClF3 reacts with any moisture in air, and on contact with body tissue produces substances which are both toxic and corrosive to the body. ClF3 is not only a toxic hazard, but also a fire hazard.

As a consequence there are substantial requirements to be met in disposing of ClF3 and/or other accompanying materials following use. The present invention aims to provide a more effective extraction system for such materials following use, as well as offering possibilities for safely storing at least some of the materials following recovery and the potential for their re-use.

According to a first aspect of the invention we provide a method of scrubbing a gas stream containing XFn where X is C1, Br or I and n is 1, 3, 5 or 7, preferably containing Cur3, comprising contacting the gas stream with alkali metal fluoride.

The gas stream may contain one or more gas species, potentially including more than one XFn species.

Preferably the alkali metal fluoride is KF. The use of such scrubbers is advantageous over the prior art systems due to the lower heat generation. In the prior art systems, which utilise soda lime and alumina there are problems with heat generation and thus risk of fire.

The alkali metal fluoride may be provided in powder or pellet form. The alkali metal fluoride may be mixed with a support, for instance calcium fluoride. Alternatively or additionally the alkali metal fluoride maybe supported on a series of trays or filter elements in the scrubber. These elements maybe constructed of nickel or monel.

Preferably the alkali metal fluoride is employed dry.

The alkali metal fluoride maybe heated prior to use.

Preferably a temperature in excess of 1000C and more preferably in excess of 150C is employed in drying. Alternatively or additionally a dry nitrogen gas stream maybe used to purge the alkali metal fluoride prior to use.

Preferably the XFn species, preferably ClF3, containing gas stream is contacted with the scrubber at a temperature most preferably less than 100"C, optionally less than 2000C. A temperature gradient may exist along the flow path of the gas through the scrubber. The temperatures used may range between 0 and 2000C.

Preferably the XFn, preferably ClF3, is absorbed by the scrubber in the form of alkali metal fluorohalate, preferably tetrafluorohalate.

A plurality of scrubbers may be provided in series.

These scrubbers may each comprise alkali metal fluoride scrubbers. Alternatively a scrubber downstream relative to the alkali metal scrubber may be provided as an activated alumina or soda lime scrubber.

The alkali metal fluoride scrubber or scrubbers may be used to remove at least 75k and more preferably at least 90% of the XFn, and or ClF3 present.

One or more activated alumina and/or soda lime scrubbers may be used to remove XF, and/or ClF3 down to a level of less than 5% and more preferably 1W, of the initial level, in the exit stream. The level may be reduced to Sppm or less of XFn and/or ClF3 in the exit stream.

Additional scrubbers, or additional materials within the alkali metal fluoride, alumina or soda lime scrubber may be provided to remove one or more other components of the waste stream.

The scrubber may be provided as a cylinder. Valves maybe provided at opposing ends of the scrubber. The pellets or powder may fill up to 60 or even up to 80% of the scrubber volume.

The scrubber may be provided with integral heaters. The heaters may be provided in the form of heat transfer fins. The fins may extend radially outwards from the centre of the vessel provided. During scrubbing these fins serve to remove heat from the scrubbing vessel.

The scrubbed XFn, preferably ClF3, may be reused as a source of active fluorine or stored for later use. The scrubber unit may also act as a transportation or storage device or be chargeable to such a device, for instance as a cartridge. The device may be heated to generate the active fluorine when next required. The heat transfer fins may be employed in aiding the heating process.

The transportable vessel is preferably a cylinder which is adapted to be able to contain the solid alkali metal fluorohalate, such as tetrafluorohalate. The fluorohalate, such as tetrafluorohalate may be contained directly or in cartridges which can be introduced into the vessel. A nickel or monel lined cylinder may be used. A filter may be provided, most preferably at the outlet, to help retain the solids. A sintered filter may be preferred.

The scrubber may be regenerated by heating to greater than 150"C. The scrubber may be heated to greater than 3000C periodically to remove any HF build up. The alkali metal fluoride in the scrubber may be periodically replaced to maintain scrubbing efficiency.

According to a second aspect of the present invention there is provided a method of reusing a XFn, and preferably ClF3, containing gas stream, where X is Cl, Br or I and n is 1, 3, 5 or 7, by scrubbing the stream using alkali metal fluoride, reusing the XFn or ClF3 absorbed by the alkali metal fluoride by generating active fluorine at the point of use by heating the alkali metal fluorohalate or tetrafluorohalate.

Active fluorine consists of highly energetic fluorine species, such as a fluorine radical.

Other possibilities for the scrubbing are provided in the first aspect of the invention.

According to a third aspect of the invention we provide a method of forming alkali metal fluoride pellets, the method comprising mixing alkali metal powder with a lubricant, introducing the mixture to a pellet forming location and applying pressure to form the pellet.

Preferably the alkali metal desired in the pellet is potassium fluoride.

The lubricant may be or include zinc stearate.

The lubricant may be or include a bisstearamide, such as ethylene bisstearamide, and particularly N,N' ethylene bisstearamide.

The lubricant may be in particulate form, for instance with particles of between 3 microns and 25 microns as their average size.

Preferably the method includes providing at least 5 wt% and more preferably at least 10 wt% of lubricant in the mixture. Preferably the method includes providing at most 30 wtt of lubricant in the mixture.

Where the alkali metal fluoride is potassium fluoride preferably the method includes providing a less deliquescent material in the mixture, such as sodIum fluoride. The method may include the provision of at least 10 wtt and more preferably at least 15 wtt of such materIal, such as sodium fluoride.

The method may include the provision of a mixture containing between 40 and 8G wt% potassium fluoride, between 10 and 30 wtt sodium fluoride and between 5 and 25 wt% of lubricant, the three components, with or without other components adding to 100 wt%.

The method may include the provision of a mixture containing between 60 and 80 wtt potassium fluoride, between 20 and 30 wt% sodium fluoride and between 3 and 8 wtt of zinc stearate, the three components, with or without other components adding to 100 wt%.

The method may include the provision of a mixture containing between 50 and 70 wt% potassium fluoride, between 20 and 30 wt% sodium fluoride and between 10 and 20 wt% of bisstearamide, the three components, with or without other components adding to 100 wt%.

The method may include drying the feed materials prior to, and/or during and/or after mixing. Preferably the alkali metal fluoride, for instance potassium fluoride, is heated to at least 10000, and more preferably at least 1400C. Preferably heating is maintained for at least 1 hour and more preferably at least 5 hours.

Preferably the method includes the step of removing the lubricant from the pellet and/or rendering the lubricant chemically inert within the pellet. The lubricant may be removed from the pellet by contacting the pellet with XF3, where X is a halogen other than fluorine, and/or contacting the pellet with fluorine. A temperature of greater than 200"C may be employed. This may serve to increase the porosity and specific surface area of the resulting pellets. The alkali metal fluoride may be mixed with a support, such as calcium fluoride.

The lubricant may be substantially removed by thermal decomposition. The pellet may be heated to at least 2000C and more preferably to between 200"C and 400"C. The heating is preferably applied under vacuum. The heating may be applied for at least 10 hours, and more preferably at least 50 hours, to remove the lubricant.

The method may include chemically treating the pellet, before or after heat treatment, or instead of, to render the lubricant or components or residues thereof chemically inert to XFn. The treatment may comprise contacting the pellets with fluorine. Preferably contact is provided for at least 5 hours and more preferably at least 15 hours. Contact may be provided under at least atmospheric pressure of fluorine and more preferably a pressure of 1.2 bar or greater.

According to a fourth aspect of the present invention we provide alkali metal fluoride in pellet form.

Preferably the alkali metal fluoride pellets are dried.

Preferably the alkali metal fluoride in the pellet is potassium fluoride.

The pellets may be formed by cold compaction.

The pellets may comprise alkali metal fluoride and a lubricant. The lubricant may comprise between 0.1 and 15wt% and most preferably 0.5 and 7 wt%.

The lubricant may be or include zinc stearate.

The lubricant may be or include a bisstearamide, such as ethylene bisstearamide, and particularly N,N' ethylene bisstearamide most preferably in atomised form. The lubricant may also act as a binder.

The quantities of lubricant should desirably be sufficient to provide good pellet quality, but not so much as to be detrimental to the purity of XF3, regeneration.

Where the alkali metal fluoride is potassium fluoride preferably the pellet also includes a less deliquescent material, such as sodium fluoride. The pellet may include at least 10 wtk and more preferably at least 15 wtt of such material, such as sodium fluoride.

The pellet may include between 40 and 80 wt% potassium fluoride, between 10 and 30 wt sodium fluoride and between 5 and 25 wt% of lubricant, the three components, with or without other components adding to 100 wtt.

The pellet may include between 60 and 80 wt% potassium fluoride, between 20 and 30 wt sodium fluoride and between 3 and 8 wts of zinc stearate, the three components, with or without other components adding to 100 wit%.

The pellet may include between 50 and 70 wtt potassium fluoride, between 20 and 30 its sodium fluoride and between 10 and 20 wtt of bisstearamide, the three components, with or without other components adding to 100 wt%.

According to a fifth aspect of the invention we claim the use of alkali metal fluorohalate, preferably tetrafluorohalate, as a transportable and/or storable source of active fluorine following scrubbing of a waste gas stream containing XFn, preferably CiF3.

According to sixth aspect of the invention we provide the use of active fluorine as a cleaning agent and/or a method of cleaning by using active fluorine, where ClF3 is scrubbed or obtained using the method of the first aspect and/or device of the second aspect and/or the use of pellets produced according to the method of the third aspect of the invention and/or involves the use of pellets. of the fourth aspect and/or involves the use of the fifth aspect.

The invention will now be described in detail, by way of example only, and with reference to the following figure in which Figure 1 is an illustration of a scrubber assembly; Figure 2 is an illustration of a further scrubber assembly; and Figure 3 is a schematic illustration of the removal and reuse of C1F3 in accordance with one embodiment of the invention.

At the desired location of use ClF3 may be produced by one of a number of routes, including its direct provision from a metal pressurised cylinder. Following its use, however, steps must be taken to ensure that it is not released into the environment.

Figure 1 illustrates an embodiment of a scrubber assembly for removing the ClF3 contained in a waste stream following its use. The waste stream may for instance follow the use of ClF3 in cleaning chemical vapour deposition equipment.

The scrubber consists of a cylinder 50 enclosed within an exterior safety and mounting jacket 52. Thermal insulation 54 is provided between the two, with a series of heating elements 56 provided on the outside of the cylinder 50. The heater elements 56 are controlled and powered by lead 58 which also carries a thermocouple to monitor the heating.

The cylinder 50 is provided with pipes 60 and 62, each end of the pipes 60, 62 are connected to valves (not shown).

The interior of the cylinder 50 is provided at each end with a sintered filler 68 to retain the cylinder contents. The KF 70 which acts as the scrubber is provided in pellet form supported on nickel elements 72. A series of such elements 72 are provided.

The pellets, may by way of example consist of potassium fluoride in dry form mixed with 7wt% N,N' ethylene bisstearamide in atomised form (Acrawax obtained from Lonza Speciality Chemicals a CAS NollO-30-5) , with an average particle size of 6jim. The N,N' ethylene bisstearamide acts as a binder and lubricant during the dry compaction of the pellets.

Heating the formed pellets drives off the lubricant producing a porous, high specific surface area pellet. Typical properties for the KF include bulk density of powder 0.3kg/l; bulk density of pellets 0.26kg/l.

In use the scrubber is connected to the waste stream from a process and the cylinders contents are controlled to a temperature of between 0 and 2000C. With both valves open the gas enters the cylinder 5C through valve connected to pipe 60 and contacts the KF 70. By virtue of the reaction the ClF3 is absorbed and retained within the scrubber cylinder 50. The gas exiting the cylinder 50 through the valve connected to pipe 62 is substantially cleaned of hazardous ClF3.

The concentration can be reduced still further by employing a polishing scrubber 80 as illustrated in Figure 2 also. In this case the scrubber 80 is also provided in the form of a cylinder with an entry valve (not shown) connected to pipe 82 and exit valve (not shown) connected to pipe 84, heater elements 86 and scrubbing material 88. The scrubbing material can be further pellets of KF or activated alumina or soda lime or NaF.

As a consequence of the further scrubbing substantially all the ClF3,in the waste gas stream is removed. The, or some of the, scrubbers can be used to remove other chemical vapour deposition of gases such as HF, SiF4 etc.

Due to the nature of the absorption reaction used in the scrubbing process the scrubbing device itself can be used as a source for generating active fluorine at a desired location of use at a subsequent time.

Prior to this generation the active fluorine is safely stored once more.

The overall nature of this process is illustrated in Figure 3. In step A the waste gas containing ClF3 and other gases is scrubbed to extract the ClF3 by its reaction with KF to give KClF4. In step B KClF4 can be stored or transported safely. In step C the KClF4 is heated to produce active fluorine.

The production of alkali metal fluoride pellets, and particularly potassium fluoride pellets with optimum properties, is important in ensuring the best absorption / desorption characteristics for the scrubber system and/or for the generator system if the recovered material is reused.

Whilst potassium fluoride is a white, free flowing powder, it has a deliquescent nature. The absorption of even small quantities of water results in the formation of a solid layer on the exposed surface and gives rise to lumps in the feed material. This can give rise to blocking in any feed hoppers for a pelletising process and also can result in sticking of the potassium fluoride material to the punch surfaces giving rise to pellet disintegration when the pellet press is open cnce more.

To remove water as far as possihie, potassium fluoride fed to the pelletising process was always dried at above 1000C and in the case of the tests which follow at 150°C.

The feed materials for the pelletising process were fed via a hopper provided with a vibratory feed mechanism to a 16 position rotary press.

A variety of test runs were undertaken with potassium fluoride in conjunction with Acrawax from Lonza Inc, 17-17 Route 208, Fairlawn, New Jersey 07410 as a lubricant and/or with sodium fluoride present and/or with zinc stearate present as a lubricant. The results from the tests are presented in Table 1.

TABLE 1 Pellet Composition (%w/w) Summary of Results Trial no. KF NaF Acrawax Zinc Stearate A1 95.5 0.5 Failed due to feeding problems. A2 97 3 Failed due to feeding problems. A3 93 7 Feeding problems after only a few pellets. B1 60 30 10 Failed due to feeding problems. B2 60 20 20 Successful production. B3 70 15 15 Failed due to feeding problems. B4 60 25 15 Successful production. B5 70 20 10 Successful production. B6 80 10 10 Failed due to feeding problems. B7 75 15 10 Successful production. B8 75 20 5 Successful production.

High levels of lubricant, such as acrawax or zinc stearate, gave a freer flowing feed material improving the filling of the pellet forming location. The presence of NaF as a free flowing powder of far lower hydroscopic nature than potassium fluoride also improved material flow and gave a tougher pellet as a result.

Pellets formed of the mixture of 60% KF, 25% NaF and 15% acrawax or of 75% KF, 25% NaF and 5% zinc stearate were found to give the best flow characteristics whilst obtaining a high loading of potassium fluoride within the pellet. Over 100 pellets were produced in the runs using these mixtures without any production problems.

Once pelletised, to improve the characteristics of the pellets in scrubbing ClF3 and also to reduce the reactive nature of the ClF3 the lubricants present were removed.

Lubricant removal by heating the pellets at between 200 and 4000C under vacuum was found to be sufficient to remove the lubricants to the substantial degree.

The pellets produced from zinc stearate as a lubricant were found to retain better structural integrity following lubricant removal than those produced using acrawax.

Further investigations into lubricant removal for 75t KF, 20% NaF and 5% zinc stearate pellets involving heating them to 400"C for in excess of 60 hours indicated a 5.5h weight reduction. This loss was determined to be due to the thermal decomposition of the lubricant present and also to the loss of a small amount of absorbed moisture. With the zinc stearate decomposing to zinc oxide, a theoretical mass production of 4.5% was anticipated. The additional 1% is attributable to the water loss.

Investigation of the pellet composition indicated that some carbon material remains within the pellet following lubricant removal. To present this carbon in an inert state so as to avoid any undesirable or unknown reactions during scrubbing, the pellets were exposed to fluorine at a pressure of 1.2 bar for a 20 hour period to fluorinate the remaining organic and zinc compounds. During this process the pellets gained a further 1.3% on the original weight indicating that fluorination had occurred.

To illustrate the effectiveness of the scrubber produced in removing ClF3 from a waste gas stream, a cylinder containing liquid ClF3 was heated to generate a pressure of 2 bar. The pellets were exposed to this ClF3 source at 1150C for 20 hours.

At the end of this period the pellets has increased in mass by about 100%, equating to a 65 molk of the KF present absorbing ClF3 to give KClF4.

Re-heating the KF / KC1F4 produced in this way gave rise to ClF3 production.