a chromium - based fluorination catalyst is involved in the process of the invention . the catalyst used preferably comprises oxides , halides , oxyhalides or mineral salts of chromium optionally doped with a metal element , for instance nickel , cobalt , magnesium or zinc . it is preferentially a chromium oxide , a chromium fluoride or a chromium oxyfluoride , or alternatively chromium doped with a metal element , for instance nickel or magnesium . the catalyst may undergo activation by a heat treatment and / or a fluorination treatment . in particular , the activation may take place during the fluorination . the temperature is advantageously chosen between 100 ° c . and 400 ° c ., preferably between 200 ° c . and 300 ° c . the chromium is used in particular in the form of oxides with different degrees of oxidation and / or in the form of hydroxides in powder or gel form . it is possible to use an activated chromium iii oxide prepared , for example , by precipitation of water - soluble chromium iii salts , for example chlorides , nitrates , acetates or sulfates using an alkali metal hydroxide , preferably sodium or potassium hydroxide , or ammonium hydroxide . the precipitate is dried at about 110 ° c . and calcined at a temperature below 700 ° c ., preferably between 400 and 600 ° c . anhydrous chromium oxide may be obtained by calcination of inorganic chromium salts such as ammonium chromate or chromium nitrate or by calcination of organic chromium salts , for instance chromium oxalates or formates at 350 ° c ., under a nitrogen atmosphere . use may be made especially of a catalyst of cr — ni type , with a chromium valency of between 2 and 3 and a nickel valency of between 0 and 2 , the amount of nickel expressed as an atomic percentage representing from 0 . 1 % to 10 %. one method for preparing this catalyst consists in performing a thermal decomposition , separately or as a mixture of one or more organic chromium salts ( for example oxalate ) and of one or more nickel salts ( for example oxalate ), forming the mixture and then fluorinating the formed catalyst . the thermal decomposition generally takes place between 370 ° c . and 400 ° c ., under an atmosphere of inert gas , for example nitrogen . the forming of the catalyst obtained may be performed , under non - oxidative conditions , for example by extrusion , and the formed product is then dried at about 120 ° c .- 130 ° c ., and then calcined at 370 ° c .- 400 ° c ., under an inert atmosphere . the catalyst is heated to between 100 ° c . and 500 ° c ., under hydrofluoric acid , for between 1 and 12 hours . it may be obtained especially by mixing a chromium salt ( for example nitrate ) in solution with a magnesium oxide or hydroxide , prolonged drying for between 12 and 24 hours , for example at 100 ° c ., and then activation with hydrofluoric acid , for example at 200 ° c . the active phase may be provided in a finely divided form , or alternatively formed or deposited on a support . examples of supports that may be mentioned include silica , alumina , zirconia and titanium oxide . preferably , the chromium is deposited on a support in a proportion of 0 . 5 % to 5 % of the weight of the catalyst . the catalysts may be present in various forms in the process of the invention : powder , formed products such as granules ( for example extrudates or beads ), pellets , which are obtained by extrusion , molding , compacting or any other type of known process . in practice , at the industrial level , it is the granule or bead forms that offer the greatest advantages , both as regards the efficacy and as regards the ease of use . in accordance with the process of the invention , a fluorination reaction is performed in a first step by reacting dichloroacetyl chloride and hydrofluoric acid , in the gas phase , in the presence of the fluorination catalyst . the ratio between the hydrofluoric acid and the dichloroacetyl chloride may vary widely . generally , the hydrofluoric acid is in excess amount . thus , the ratio between the number of moles of hydrofluoric acid and the number of moles of dichloroacetyl chloride usually ranges between 1 and 30 . it is advantageously chosen between 6 and 10 . the process in accordance with the invention is performed at a high temperature , generally above 200 ° c . it is recommended to work at temperatures of between 250 ° c . and 400 ° c . and preferably between 250 ° c . and 300 ° c . for reasons of simplicity , the process of the invention is performed at atmospheric pressure . however , it is also possible to work at lower or higher pressures . from a practical point of view , the process may be performed in batch or continuous mode . to begin with , the dichloroacetyl chloride and the hydrofluoric acid are mixed together in any manner . said reagents may thus be mixed together in a mixing zone , and the mixture obtained may then be conveyed onto the catalytic bed . when the process is performed in batch mode , the amount of fluorination catalyst used , expressed as weight of catalyst per weight of dichloroacetyl chloride , may vary , for example , between 0 . 5 % and 20 % and preferably between 0 . 5 % and 5 %. the other variant of the invention consists in performing the reaction in continuous mode , in a tubular reactor comprising the solid catalyst arranged as a fixed bed . the dichloroacetyl chloride and the hydrofluoric acid may be introduced separately or as a mixture into the reactor . as mentioned previously , they may be mixed together in a mixing zone , and the mixture obtained may then be conveyed onto the catalytic bed . the reaction mixture passes through the catalytic bed , preferably from the bottom upward . the contact time , which is defined as the ratio between the apparent volume of catalyst and the flow rate of the gas stream , may vary widely , and is usually between 0 . 2 and 100 seconds . the contact time is preferably chosen between 5 and 50 seconds . the weight of substrate used per weight of catalyst and per hour generally ranges between 0 . 01 h − 1 and 2 h − 1 and preferably between 0 . 05 h − 1 and 0 . 5 h − 1 . at the end of the reaction , a gas phase comprising the difluoroacetyl fluoride , the excess hydrofluoric acid and the hydrochloric acid formed by the reaction is recovered . in accordance with the process of the invention , a step of hydrolysis of the difluoroacetyl fluoride to difluoroacetic acid is then performed . to this end , the gas stream is placed in contact with water . the amount of water used is at least equal to the stoichiometric amount . generally , the operation is performed by sending , the gas stream into a hydrolysis column : the water being sent in counter - currentwise relative to the gas stream which rises from the bottom upward in the column . it is also possible to perform an acidic hydrolysis , for example by using a solution of a strong mineral acid , for example hydrochloric acid at 30 % by weight . thus , the difluoroacetic acid is recovered at the bottom of the column and hydrogen chloride gas is recovered at the top of the column . difluoroacetic acid salts may be readily manufactured from the acid , especially by reaction with a base , preferably sodium hydroxide or potassium hydroxide . the process of the invention is advantageously performed in apparatus capable of withstanding the corrosion caused by the hydrofluoric acid . to this end , materials such as , for example , vitrified steels or hastelloy ® steels are chosen . according to one variant of the invention , the difluoroacetyl fluoride is recovered from said gas stream comprising the difluoroacetyl fluoride , the excess hydrofluoric acid and the hydrochloric acid formed by the reaction , by condensing said gas stream by lowering its temperature to between − 40 ° c . and 10 ° c . and preferably between − 20 ° c . and 0 ° c . and then by distilling the condensed stream . the process of the invention is particularly advantageous since it uses a cheap starting material and has good production efficiency . the examples that follow illustrate the invention without , however , limiting it . in the examples , the degree of conversion and the yields obtained are defined . the degree of conversion ( dc ) corresponds to the ratio between the number of moles of dcac substrate converted and the number of moles of dcac substrate employed . the yield ( ry ) corresponds to the ratio between the number of moles of dfaf product formed and the number of moles of dcac substrate employed . the yield ( cy ) corresponds to the ratio between the number of moles of dfaf product formed and the number of moles of dcac substrate converted . the operating protocol that is followed in the various examples is given below . the reaction is performed in a reactor made of hastelloy c276 , which is a nickel - based alloy ( 57 % by weight ) comprising 16 % chromium , 16 % molybdenum , 5 % iron , 4 % tungsten , 2 . 5 % cobalt , 1 % manganese , 0 . 35 % vanadium , 0 . 08 % silicon and 0 . 01 % carbon . predistilled dichloroacetyl chloride ( dcac ) is introduced at a flow rate of 0 . 11 mol / h (˜ 15 g / h ) and anhydrous hf is introduced at a flow rate of 1 . 1 to 2 . 6 mol / h ( 22 to 52 g / h ) into a hastelloy c276 reactor formed of a tube 60 cm long with an outside diameter of 2 . 5 cm , filled with a catalyst based on chromium iii oxide (˜ 150 g ) predried to constant weight and fluorinated ( 24 hours at 250 ° c .). according to the test , the temperature is set from 200 ° c . to 350 ° c ., as an isotherm or gradient . under these conditions , the residence time t r ranges between 10 and 20 seconds . after reaction , the difluoroacetyl fluoride ( dfaf ) is assayed by converting it into the form of the potassium salt of difluoroacetic acid . the exiting stream is hydrolyzed in potassium hydroxide spargers mounted in series , and the various acids are assayed in the form of potassium salts by ion chromatography . the examples that follow illustrate the effect of the temperature , the hf / dcac ratio and the residence time .