PROCESS FOR THE PURIFICATION OF HYDROCHLORIC ACID 5 FIELD OF THE INVENTION 10 The present invention relates to a process for the purification of hydrochloric acid and to a plant suitable for the implementation of this process. The invention can in particular be used in the context of the treatment of effluents resulting from a catalytic fluorination reaction. TECHNICAL BACKGROUND It is known to produce fluorinated compounds, such as hydrofluorocarbons, by fluorination of chlorinated compounds, such as hydrochlorocarbons in particular. This fluorination is generally a catalytic 15 fluorination using hydrofluoric acid (HF) as fluorinating agent. 20 During this type of reaction, hydrochloric acid (HCI) is coproduced. It is known to separate the HCI from the other gases produced via a distillation column and then to absorb the HCI in an adiabatic absorption column in order to generate an HCI solution of commercial type. The document FR 1507252 describes stages of treatment on active charcoal, at high temperature and in the presence of water, and of washing with concentrated aqueous hydrochloric acid. The document US 3 353 911 describes the purification of an HF/HCI gas mixture by bringing into contact with an absorbent solution which is a 25 saturated boric acid solution. 30 However, the known techniques for the purification of hydrochloric acid do not make it possible, in some cases, to achieve the required HCI purity. There thus exists a need to provide an improved process for the purification of hydrochloric acid in a gas stream. SUMMARY OF THE INVENTION The invention relates first to a process for the treatment of a gas stream comprising hydrochloric acid, hydrofluoric acid and fluorinated/oxygenated compounds, in which the gas stream is successively 35 subjected to: a stage of catalytic hydrolysis; a stage of washing with an acid solution; a stage of adsorption of impurities by active charcoal; 2 a stage of adiabatic or isothermal absorption of the hydrochloric acid in an aqueous solution, making it possible to collect hydrochloric acid solution. According to one embodiment, the stage of catalytic hydrolysis is 5 carried out on a bed of active charcoal. According to one embodiment, the acid solution used during the washing stage is a hydrochloric acid solution afld preferably originates from the hydrochloric acid solution collected on conclusion of the adiabatic or isothermal absorption stage. 10. According to one embodiment, the process additionally comprises: a stage of bringing the hydrochloric acid solution into contact with a silica gel. According to one embodiment, the fluorinated/oxygenated compounds comprise carbonyl difluoride, carbonyl chloride fluoride, trifluoroacetyl fluoride 15 and/or trifluoroacetic acid and, preferably, the gas stream comprises at least 50 mg/1, in particular at least 100 mg/1, indeed even at least 200 mg/1, of trifluoroacetyl fluoride and/or trifluoroacetic acid. 20 Accordirrgto one embodiment, boric acid is added to the acid solution used for the washing stage. According to one embodiment, the gas stream is a stream resulting from a catalytic fluorination reaction of at least one chlorinated compound to give at least one fluorinated compound, said stream preferably being collected at the outlet of a distillation of a stream of products from the catalytic fluorination reaction. 25 According to one embodiment, the catalytic fluorination reaction is 30 35 carried out in the presence of oxygen. According to one embodiment: the chlorinated compound is a chlorocarbon, a hydrochlorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a chloroolefin, a hydrochloroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin and the fluorinated compound is a fluorocarbon, a hydrofluorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a fluoroolefin, a hydrofluoroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin; preferably, the chlorinated compound is chosen trichloroethane, 1,1, 1 ,2,3-pentachloropropane, from 1,1,2- 1 '1 '1 ,3,3- pentachloropropane,. 1,1 ,2,2,3-pentachloropropane, 2,3-dichloro- 1, 1, 1-trifluoropropane, perchloroethylene, 1 ,2-dichloroethylene, 5 10 15 20 3 1,1 ,2,3-tetrachloropropene, 2,3,3,3-tetrachloropropene, 1,1 ,3,3- tetrachloropropene, 1 ,3,3,3-tetrachloropropene, 2-chloro-3,3,3- trifluoropropene, mixtures; and preferably, the pentafluoroethane, tetrafluoropropene, trifluoropropene, mixtures. 1-chloro-3,3,3-trifluoropropene and their fluorinated compound is chosen from 1-chloro-2,2-difluoroethane, 1 ,3,3,3- 2,3,3,3-tetrafluoropropene, 2-chloro-3,3,3- 1-chloro-3,3,3-trifluoropropene and their The invention also relates to a process for the preparation of a fluorinated compound, comprising: the provision of a chlorinated compound and of hydrofluoric acid; the catalytic reaction of the chlorinated compound with the hydrofluoric acid and the collection of a stream of crude products; the separation of the stream of crude products, preferably by distillation, making it possible to recover, on the one hand, a stream of fluorinated compound and, on the other hand, a gas stream mainly comprising hydrochloric acid and also minor amounts of hydrofluoric acid and of fluorinated/oxygenated compounds; the treatment of the gas stream comprising hydrochloric acid, hydrofluoric acid and fluorinated/oxygenated compounds as described above. According to one embodiment, the catalytic reaction is carried out in 25 the presence of oxygen. 30 35 According to one embodiment: the chlorinated compound is a chlorocarbon, a hydrochlorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a chloroolefin, a hydrochloroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin and the fluorinated compound is a fluorocarbon, a hydrofluorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a fluoroolefin, a hydrofluoroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin; preferably, the chlorinated compound is chosen trichloroethane, 1,1, 1 ,2,3-pentachloropropane, from 1,1 ,2-· 1 '1, 1,3,3- pentachloropropane, 1,1 ,2,2,3-pentachloropropane, 2,3-dichloro- 1, 1, 1-trifluoropropane, perchloroethylene, 1 ,2-dichloroethylene, 1,1 ,2,3-tetrachloropropene, 2,3,3,3-tetrachloropropene, 1,1 ,3,3- 5 4 tetrachloropropene, 1 ,3,3,3-tetrachloropropene, 2-chloro-3,3,3- trifluoropropene, 1-chloro-3, 3,3-trifluoropropene and their mixtures; and preferably, the pentafluoroethane, tetrafluoropropene, trifluoropropene, mixtures. fluorinated compound is chosen from 1-chloro-2,2-difluoroethane, 1 ,3,3,3- 2,3,3,3-tetrafluoropropene, 2-chloro-3,3,3- 1-chloro-3,3,3-trifluoropropene and their The invention also relates to a plant for the treatment of a gas stream 10 comprising hydrochloric acid, hydrofluoric acid and fluorinated/oxygenated 15 20 25 compounds, comprising: a pipe for introducing a gas stream comprising hydrochloric acid, hydrofluoric acid and fluorinated/oxygenated compounds, feeding a catalytic hydrolysis unit; a washing unit, fed on the one hand by a pipe for collecting the hydrolyzed gas stream resulting from the catalytic hydrolysis unit and on the other hand by a pipe for introducing an acid solution; a unit for adsorption of impurities comprising a bed of active charcoal, which unit is fed by a pipe for collecting the washed gas stream resulting from the washing unit; a unit for adiabatic or isothermal absorption, fed on the one hand by a pipe for collecting the purified or isothermal stream resulting from the adsorption unit and on the other hand by a pipe for introducing an aqueous solution; a pipe for collecting a hydrochloric acid solution at the outlet of the adiabatic or isothermal absorption unit. According to one embodiment, the catalytic hydrolysis unit comprises a bed of active charcoal. According to one embodiment, the pipe for introducing an acid solution 30 does or does not result directly from the pipe for collecting a hydrochloric acid solution. 35 According to one embodiment, the plant additionally comprises: an additional adsorption unit comprising a silica gel, which unit is fed by the pipe for collecting a hydrochloric acid solution; and a pipe for collecting a purified hydrochloric acid solution resulting froni the additional adsorption unit. According to one embodiment, the plan comprises a contribution of boric acid solution to the washing unit. 5 10 15 20 25 5 According to one embodiment, the pipe for introducing a gas stream comprising hydrochloric acid, hydrofluoric acid and fluorinated/oxygenated compounds results from a distillation unit, the distillation unit preferably being fed by a pipe for collecting crude products at the outlet of a catalytic reactor. According to one embodiment, the catalytic reactor is fed by a pipe for introducing a chlorinated compound and a pipe for introducing hydrofluoric acid, and the pipe for collecting crude products transports a stream comprising a fluorinated compound, and preferably: the chlorinated compound is a chlorocarbon, a hydrochlorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a chloroolefin, a hydrochloroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin and the fluorinated compound is a fluorocarbon, a hydrofluorocarbon, a chlorofluorocarbon, a hydrochlorofluorocarbon, a fluoroolefin, a hydrofluoroolefin, a chlorofluoroolefin or a hydrochlorofluoroolefin; more particularly, the chlorinated compound is chosen from 1,1 ,2- trichloroethane, 1,1, 1 ,2,3-pentachloropropane, 1,1, 1 ,3,3- pentachloropropane, 1,1 ,2,2,3-pentachloropropane, 2,3-dichloro- 1, 1, 1-trifluoropropane, perchloroethylene, 1 ,2-dichloroethylene, 1,1 ,2,3-tetrachloropropene, 2,3,3,3-tetrachloropropene, 1,1 ,3,3- tetrachloropropene, 1 ,3,3,3-tetrachloropropene, 2-chloro-3,3,3- trifluoropropene, 1-chloro-3,3,3-trifluoropropene and their mixtures; and more particularly, the fluorinated compound is chosen from pentafluoroethane, 1-chloro-2,2-difluoroethane, 1 ,3,3,3- tetrafluoropropene, 2,3,3,3-tetrafluoropropene, 2-chloro-3,3,3- trifluoropropene, mixtures. 1-chloro-3,3,3-trifluoropropene and their According ·to "'ne embodiment, the plant comprises a contribution of 30 oxygen to the catalytic reactor. The present invention makes it possible to overcome the disadvantages of the . state of the art. It more particularly provides an improved process for the purification of the hydrochloric acid in a gas stream. This is accomplished by virtue of the use of three successive stages of 35 treatment of the gas stream, namely a stage of catalytic hydrolysis, a stage of washing and a stage of adsorption on active charcoal, this taking place before the stage of adiabatic or isothermal absorption which generates a concentrated hydrochloric acid solution. 6 The invention is based on the identification, by the present inventors, that the gaseous HCI recovered at the distillation column top may be contaminated by a small amount of free HF (entrained due to azotropes with light fluorinated compounds) but also by fluorinated/oxygenated compounds, 5 such as carbonyl difluoride (COF2), carbonyl chloride fluoride (COFCI) and trifluoroacetyl fluoride (CF3COF). 10 15 These compounds are generated in particular when the gas stream to be treated results from a fluorination reaction carried out in the presence of oxygen. These compounds are highly toxic and are hydrolyzable. They are thus capable of releasing Hr, in particular during the stage of adiabatic or isothermal absorption in water, thus contaminating the HCI solution obtained with HF. In addition, the trifluoroacetyl fluoride generates trifluoroacetic acid (or TFA) during its hydrolysis, which compound is harmful. The invention makes it possible to separate the HCI from the HF with which it is mixed but also to separate it from the abovementioned fluorinated/oxygenated compounds. The invention is also base-a on the identification, by the present inventors, that the gaseous HCI recovered at the distillation column top may 20 be contaminated by light organic compounds. The invention also makes it possible to satisfactorily remove these light organic compounds during the treatment. 25 BRIEF DESCRIPTION OF THE FIGURES Figure 1 diagrammatically represents an embodiment of a plant according to the invention. DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention is now described in more detail and without implied 30 limitation in the description which follows. The invention applies in particular to the treatment of a gas stream resulting from a catalytic fluorination reaction of at least one chlorinated compound to give at least one fluorinated compound. The gas stream treated according to the invention can be directly the gas stream of products 35 resulting from a reactor or more preferably a stream resulting from a distillation (carried out in a distillation column or in a series of several successive distillation columns) on conclusion of the catalytic fluorination. In this case, the gas stream to be treated according to the invention has been 7 essentially separated beforehand from the fluorinated compound and/or from the unreacted chlorinated compound and/or from the byproducts of the reaction. The gas stream to be treated according to the invention preferably 5 predominantly comprises HCI, with minor amounts of contaminants, such as HF and the abovementioned fluorinated/oxygenated compounds (and in particular trifluoroacetyl fluoride). The term "chlorinated compound" (with represents the main reactant of the catalytic fluorination reaction) is understood to mean an organic 10 compound comprising one or more chlorine atoms and the term "fluorinated compound" (which represents the desired product from the catalytic fluorination reaction) is understood to mean an organic compound comprising one or more fluorine atoms. It is understood that the chlorinated compound can comprise one or 15 more fluorine atoms and that the fluorinated compound can comprise one or more chlorine atoms. Generally, the number of chlorine atoms of the fluorinated compound is less than a number of chlorine atoms of the chlorinated compound and the number of fluorine atoms of the fluorinated compound is greater than the number of fluorine atoms of the chlorinated 20 compound. The chlorinated compound can be an alkane or an alkene optionally having substituents chosen from F, Cl, I and Br (preferably from F and Cl) and comprising at least one Cl substituent. The fluorinated compound can be an alkane or an alkene optionally 25 having substituents chosen from F, Cl, I and Br (preferably from F' and Cl) and comprising at least one F substituent. The chlorinated compound can in particular be an alkane with one or more chlorine substituents (hydrochlorocarbon or chlorocarbon) or an alkane with one or·more chlorine and fluorine substituents (hydrochlorofluorocarbon 30 or chlorofluorocarbon) or an alkene with one or more chlorine substituents (chloroolefin or hydrochloroolefin) or an alkene with one or more chlorine and fluorine substituents (hydrochlorofluoroolefin or chlorofluoroolefin). The fluorinated compound can in particular be an alkane with one or more fluorine substituents (fluorocarbon or hydrofluorocarbon) or an alkane 35 with one or more chlorine and fluorine substituents (hydrochlorofluorocarbon or chlorofluorocarbon) or an alkene with one or more fluorine substituents (fluoroolefin or hydrofluoroolefin) or an alkene with one or more chlorine and fluorine substituents (hydrochlorofluoroolefin or chlorofluoroolefin). 8 The chlorinated compound and the fluorinated compound can be linear or branched, preferably linear. According to one embodiment, the chlorinated fluorinated compound comprise just one carbon atom. compound and the 5 According to one embodiment, the chlorinated compound and the fluorinated compound comprise two carbon atoms. According to one embodiment, the chlorinated fluorinated compound comprise three carbon atoms. According to one embodiment, the chlorinated compound and the compound and the 10 fluorinated compound comprise four carbon atoms. According to one embodiment, the chlorinated fluorinated compound comprise five carbon atoms. compound and the The invention finds in particular an application in the following fluorination reactions: 15 - fluorination of perchloroethylene (PER) to give pentafluoroethane (HFC-125); - fluorination of 1,1, 1 ,2,3-pentachloropropane (HCC-240db) to give 2-chlor