BACKGROUND TO THE INVENTION The production of halocarbon components in the above manner has been proposed and in general the methods which utilise electric arc plasmas to obtain the high temperature zone necessary to obtain the reactive fluorine-containing and reactive carbon-containing precursors all suffered some or other disadvantage.

These disadvantages include high energy consumption, high carbon electrode consumption, difficulties in quantitative control of reactants, and fouling of equipment to name but a few. However, the principe of selectively forming desired fluorocarbons utilising a high temperature reaction process with controlled recombination quenching remains very attractive for commercial exploitation.

OBJECT OF THE INVENTION It is the object of the present invention to provide a process for the production of desired fluorocarbons, of which tetrafluorethylene (C2F4), hexafluoroethane

(C2Fg), hexafluoropropane (C3F6) octafluoropropane (C3F8) tetrafluoromethane (CF4) and cyclic octafluorobutene (c-C4Fg) are particularly important, using a chemical flame reaction electrically assisted to raise its energy level which will avoid at least some of the above disadvantages.

SUMMARY OF THE INVENTION According to this invention there is provided a process for the selective production of fluorocarbons comprising chemically reacting fluorine with a small excess of finely divided carbon in a chamber, heating the chemical flame generated by the reaction by means of electric discharges to the temperature necessary for precursor release to obtain an appropriate gaseous mixture of reactive fluorine-containing precursors and reactive carbon-containing precursors and obtaining a predetermined fluorocarbon by the controlled quenching of the gaseous mixture.

The invention also provides for the electric discharge to be obtained between carbon electrodes, for the reaction to be contained in a graphite lined chamber and for the carbon to be fed as a shroud around a central fluorine feed for the chemical reaction.

Further features of this invention provide for the quenching to be effected at a rate and to a temperature which will allow the optimum production of the predetermined fluorocarbons, for the required control of the gaseous carbon to fluorine molar ratio to be effected by the sublimation of part of the carbon electrodes used for the electric discharge and for the quenching to be effected in a cold walled chamber into which a fairly coarse feed of granular carbon is introduced at the tail flame from the electrically assisted chemical reaction.

Still further features of this invention provide for the carbon introduced into the quenching chamber to be used to keep the walls of and passages from the chamber free of solid contaminants by an abrasive blasting effect and for the carbon to be recycled at a controlled temperature in a dense phase transfer to assist in the controlled quench of the reactor gas.

The invention also provides for a large proportion of the heat in the reaction chamber to be obtained from at least one low energy level plasmatron utilising non-consumable electrodes and fed with fluorocarbon gas to discharge into the

reaction chamber. The fluorocarbon gas can conveniently be recycled product of the process.

Still further features of this invention provide for the gaseous C: F molar ratio in the reaction chamber gas to be controlled to between 0.2 to 2 and for the electrical energy input to be controlled at a value below 3 kWh/kg of feed for a time sufficient to obtain the reactive carbon-containing precursors.

The fluorine is obtained as fluorine gas or from the electrolysis of anhydrous hydrofluoric acid and, apart from the required range of fluorocarbons, waste material is produced only in small quantities as harmless effluent fit for waste disposal or land filling. This waste may include some carbon fines, alkaline calcium fluoride sludge with traces of silicates and metal oxides mainly derived from contaminants to the carbon used.

Preferably the pressure in the reaction chamber will be maintained at between 0.01 and 2 bar absolute and the temperature of the electrically assisted flame is maintained at between 2000K and 4500K and quenching reduces the temperature to below 800K in less than 0.05 seconds. The size of carbon particles introduced into the reaction chamber range from about 1 x 10-9 m to 150 x 104m and those in the quenching stream from about 200 x 104 m to 2 x 10-3 m.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of this invention for the production of mainly tetrafluoroethylene is described below with reference to the accompanying drawings in which: FIG 1 shows a diagrammatic layout of the chemical flame reactor used; and FIG 2 a flow diagram of the plant to produce a polytetrafluoroethylene basic end product.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS As shown, in Fig 1 the fluorocarbon reactor (1) consists essentially of a graphite lined chemical reaction chamber (2) into the top of which are introduced a central

fluorine feed at (3) and finely divided partides of carbon ranging in size from 1 x 10-9m to 150 x 10-m preferably towards the lower limit of this range at (4). The carbon is supplied in small excess of the requirement for the natural exothermic reaction with fluorine and is fed as a shroud around the central supply of fluorine gas This is done to protect the graphite lining of the chemical reaction chamber from direct attack by the fluorine feed.

The combustion products of this reaction are passed from the reaction chamber (2) into a heating zone (5) where the temperature of the products is raised by means of electrical discharge generated between graphite electrodes (6). The gaseous carbon is also adjusted at this point to achieve the C: F molar ratio referred to below.

From the heating zone (4) the gases pass into an extended reaction zone (7) where a plasma flame is introduced into the reactor (1) from a pair of oppositely disposed plasmatrons (8) which are fed with fluorocarbon gases recycled from the reactor output.

The temperature in the heating zone (4) and extended reaction zone (7) is raised to obtain an appropriate mixture of fluorine-containing precursors and carbon-containing precursors in the gaseous state so that desired fluorocarbon reaction products will be formed on quenching of the heated gases.

A quench unit (9) extends below the extended reaction zone (7). At the junction between the zone (7) and the unit (9) provision is made for the introduction of carbon partides.

These carbon particles are in the size range of 200 x 104 to 2 x 10-3 and preferably of a size at the lower end of this range.

The quench unit (9) is a cold walled chamber with cold water reticulation indicated through inlet (10) and outlet (11).

The carbon particles act to quench the hot gas and also under the influence of the stream of hot gases emanating from the extended heating zone (7) act to blast the surfaces of the quench unit (9) free of condensation products which

would otherwise accumulate thereon over periods of use and in the outlet passages from the reactor.

These carbon quench particles are discharged from the quench unit and recyded along with any excess small carbon particles which have not taken part in the reaction. Separation of the small particles from the longer quench particles can be effected from time to time as they accumulate with extended operation of the process. These partides can be recycled as feed to the chemical reaction.

The larger carbon partides are recyded to the quench unit at a controlled temperature in a dense phase transfer to assist in the controlled quench of the reactor gases.

The gaseous products from the quench chamber are passed through a ceramic filter (12) and separated to remove the desired fluorocarbons required. The remainder are recycled and introduced into the plasmatrons to form the plasma flame generated by the non-consumable electrodes and discharged into the reactor as part of the reaction products.

The rate of cooling and the temperature to which the gases from the heating zone are cooled determines the range and quantities of products produced.

It will be appreciated by those skilled in the art that the carbon electrodes used to obtain the electrical discharge at the heating zone can be controlled to ensure the sublimation of carbon in gaseous form from the electrode to ensure the correct gaseous C: F molar ratio in the heated gases is controlled to between 0.2 and 2. The small excess of fine particles through the reaction chamber acts to reconstitute the sublimated electrodes to some extent and also the graphite linings which will inevitably be attacked by the fluorine gas.

The external electrical energy input necessary for the heating is controlled to below 3 kWh/kg of feed and the pressure within the reactor is controlled to between 0.01 and 2 bar absolute.

The operational pressure of the system will be chosen to suit individual applications of the process. Conversion of the feed to desired fluorocarbons takes place more effectively at pressures below atmospheric but this requires secure sealing of the reaction chamber and the costs between greater efficiency

of operation must be balanced against costs of sealing and costs of energy supplied to the process.

The temperatures of the exiting gases from the heating zone is maintained at between 2000K and 4500K with quenching to below 800K in less than 0.05 seconds. This can give a fluorine carbon recovery of approximately the following :- Fluorocarbon Percentage CF4 52.1 C2F6 11. 28 C3F6 3.06 C3Fg 1.91 C2F4 31.65 Optimum recovery rates will be obtained by one skilled in the art through the appropriate control in a particular production plant of the various parameters but it will be clear that there are cost savings in the electrical energy input and the production of initial input materials over those encountered in existing installations.

It will be appreciated that the introduction of fluorine gas into the reaction chamber has an added advantage over existing plants producing fluorocarbons.

While cleaning effect of the quenching carbon particles tends to prevent the build up of condensation products on the walls of the quenching unit, extended use of the plant and process will result in some contamination causing a reduction in operating efficiency. To rectify that situation fluorine gas alone will be passed through the reaction chamber and quenching unit with a very effective purging action. Thus the downtime of the plant for cleaning purposes will be short.

In Fig 2 the flow diagram for the process shows the introduction of fluorine and carbon at (20) and (21) into the reactor (22). The fluorine may be used as purchased or anhydrous hydrofluoric acid may be used to generate the required fluorine. It will depend on the cost and availability of these products as to which will be used. Derived from the reactor (22) is a flow line for the carbon used to quench the reaction gases and for recirculation to the reaction chamber.

Because carbon available will always include some contaminants there will need to be separation of these contaminants as they build up due to the recycling.

The small amounts of waste products which can readily be disposed of as harmless effluent will include alkaline calcium fluoride and silicates and metal oxides.

Using the process as above described the output from the reactor is indicated at (23) and these products are pumped at (24) to a distillation plant (25).

From the distillation plant are first recovered the useful and saleable gaseous products other than tetrafluoroethylene indicated at (29). Also removed are the other fluorocarbons produced from the reactor (22) used for recycling and feed gas to the plasmatrons. This is shown as flow path (26). A make-up supply of tetrafluoromethane indicated at (27) will ensure there is an adequate supply of gas for the reaction process through the plasmatrons.

The distillation will also result in some losses as wastage of which sulphur is a notable cause and these products including reactive fluorides will be scrubbed and inert degassing will take place along the line indicated at (28).

The required tetrafluoroethylene is passed to the polymerisation process at (30) for conversion to polytetrafluoroethylene (PTFE), which is one of the desired end products (31) of the process. The unconverted tetrafluoroethylene is added to the recycled fluorocarbons along the flow path (32).

The process above described is economically viable and conversion of the feed materials to the reactor can be expected to be greater than 99%.

The constructional details of the plant are not given in the specification as they are all, including the plasmatron design, well within the abilities of competent personnel skilled in the art of fluorocarbon production.