<PICT:0573295/IV/1> <PICT:0573295/IV/2> A process for the removal of combined fluorine from a mixture of a hydrocarbon and a fluorinated hydrocarbon comprises supplying a stream of said mixture to a fractionating zone having an increasing temperature gradient from top to bottom and therein fractionating the mixture under reflux conditions and during the fractionation decomposing the fluorinated hydrocarbon by contacting at least a portion of the mixture with a dehydrofluorinating agent selected from the group consisting of metals, their oxides and salts, into hydrogen fluoride and hydrocarbon, fractionating the decomposition products in said fractionation zone during the fractionation of said mixture and separately removing from the fractionation zone a vapourous stream comprising hydrogen fluoride resulting from the decomposition and a liquid hydrocarbon stream substantially free of combined fluorine. The process is particularly applicable to the treatment of the hydrocarbon products of the alkylation of isoparaffins with olefines in the presence of an active fluoride catalyst including hydrogen fluoride hydrofluoric acid and mixtures of hydrogen fluoride and boron fluoride, and in particular to the treatment of hydrocarbons containing four carbon atoms in the molecule having fluorinated hydrocarbons present as impurity. In an embodiment, a hydrocarbon mixture synthesised in the presence of a fluoride catalyst and containing dissolved hydrogen fluoride and a small amount of a fluorinated hydrocarbon as impurity is purified by fractionating under reflux conditions in the fractionation zone so that at least some of the dissolved hydrogen fluoride is removed from the mixture and subjecting the remainder of the mixture to the dehydrofluorinating action of a dehydrofluorinating agent and separating the hydrogen fluoride so formed commingled with the hydrogen fluoride originally dissolved in the mixture by fractionating the decomposition products in the aforesaid fractionation zone. The dehydrofluorinating agents employed may be metals such as aluminium, iron and lead, preferably in a porous form, metal oxides such as alumina which may be essentially pure or which may comprise aluminous minerals such as bauxite, and metal salts, particularly the fluorides, such as aluminium and calcium fluorides. Some of these agents act like a catalyst, for example aluminium, while others appear to react with hydrogen fluoride during the initial stages, but release the hydrogen fluoride during the latter stages of the treatment, for example calcium fluoride. The temperatures employed range generally from 38 DEG to 205 DEG C. and the pressure should be sufficient to liquefy the bottom product in the fractionation. The drawings illustrate embodiments of the invention. Referring to Fig. 1 which shows the preferred embodiment wherein the hydrofluorinating agent is disposed within the fractionation zone, a hydrocarbon fraction obtained as alkylate layer in the alkylation of isobutane with olefines in the presence of a hydrogen fluoride catalyst is introduced via line 1 to a fractionation zone 2 which here comprises a column having the usual fractionating means such as bubble trays disposed in its upper portion and a bed of dehydrofluorinating agent disposed in the mid-portion. A reboiler 3 having heating means 6 is in communication with the fractionation zone 2 by means of lines 4 and 5. Vaporized lighter products comprising mainly hydrogen fluoride and propane and butane pass as overhead products from the zone through a condenser 8 to a receiver 10, under sufficient pressure to condense an acid layer and a hydrocarbon layer mainly butane, while lighter hydrocarbons are released through vent 11. The hydrocarbon layer is returned as a reflux medium to the top of the fractionating zone through lines 12, 14 and 1 by pump 13. The hydrogen fluoride acid layer is removed through line 15 and may be re-employed in the alkylation process. The dehydrofluorinated hydrocarbon mixture is withdrawn from reboiler 3 through line 16 and may be subjected to distillation if desired. The embodiment of Fig. 2 is similar, but the dehydrofluorinating agent is disposed in contact zones external to and adjacent the fractionation zone. The hydrocarbon fraction is supplied via line 21 to a fractionation zone 22 which may be similar to that described in relation to Fig. 1, but does not contain the dehydrofluorinating agent. A side cut is withdrawn from plate 41 in zone 22 and directed to one or other of the contact zones 46 or 48 by way of line 42, pump 43, line 44 and lines 45 or 47. The decomposition products from the contact zones are then returned to the fractionation zone through line 51. Although two contact zones are shown, one or a plurality of three or more may be used depending on the dehydrofluorinating agent employed; only one such zone is required when a catalytic type dehydrofluorinating agent is used, but at least two are required when the agent is the type which combines at least in part with the hydrogen fluoride so that whilst one zone is in service, the agent in the other may be regenerated or replaced. The remaining description of this Figure is similar to that of Fig. 1. Thus, the overhead products leave the fractionation zone via line 27, are condensed in a condenser 28 and collected in receiver 30 from which the lighter products are released by vent line 31, the hydrogen fluoride layer removed by line 35 and the hydrocarbon layer returned to the fractionation zone by means of lines 32, 34 and 1. The dehydrofluorinated hydrocarbon fraction is removed from the reboiler 23 by means of line 36. In a modification, a series of contact zones may be disposed at different intervals along the line of the hydrocarbon mixture in the fractionation zone. Thus, a system of two contact zones as in Fig. 2 may be disposed about one-third from the top of zone 22, another such system disposed at the mid-section of the zone and a third such system disposed two-thirds down from the top of zone 22. In an example, a hydrocarbon layer separated from the catalyst layer in the alkylation of isobutane by butenes in the presence of hydrogen fluoride catalyst at 38 DEG C. was introduced into a vertical stripping zone containing a heating coil in its lower portion and a bed of aluminium turnings in its upper portion. The hydrocarbon layer was passed downwardly through the stripping chamber, under reflux conditions, the liberated hydrogen fluoride was removed from the top and liquid hydrocarbons from the bottom of the zone and the final alkylate product had a fluorine content reduced from 0.01 per cent to 0.0001 per cent. The Specification as open to inspection under Sect. 91 comprised the treatment of mixtures of hydrocarbons and an organic halogen compound with a dehydrohalogenating agent and the separation of the products into hydrogen halide and hydrocarbon. This subject-matter does not appear in the Specification as accepted.