the process according to the invention is based on the finding that it is possible to achieve destruction of the cyanides and insolubilization of the impregnated fluorine - containing compounds by chemical recombination , particularly with the additive , without agglomeration by abruptly placing the solid particles of the spent pot lining mixture in contact for a very short time , less than 2 seconds and preferably between 0 . 3 and one second , with a gaseous flow at a temperature t of between 700 ° c . and 1100 ° c . and preferably between 800 ° c . and 1000 ° c . the hot gaseous flow is the product of the combustion of a carburizing gas such as methane , butane or natural gas , in the presence of air , oxygen , or a mixture thereof . the thermal shock created thereby permits the decomposition of cyanides and the recombination of the fluorine - containing impregnants , substantially instantaneously while the temperature of the solid particles remains less than that of the gaseous flow , and therefore between 400 ° c . and 750 ° c ., and preferably between 450 ° c . and 700 ° c . the possible softening of the solid particles by partial melting , in conjunction with the formation of meltable eutectic compounds , is not bothersome to the extent that all prolonged contact , and therefore all bonding between particles , is prevented by generating sufficient agitation in the reaction chamber . this agitation is achieved by regulating the speed of the gaseous flow to a sufficient value , at least 1 m / sec and preferably about 6 m / sec , at the respective treatment temperature . in addition to the temperature t and the speed of the gaseous flow determined by regulating the flow of oxidizing and carburizing gases , it is also advisable to set the average holding time of the particles in the reaction chamber . the accepted range of 0 . 3 to 1 second is the best compromise for effective treatment of pot linings of very different compositions , within the maximum acceptable limits of contents ( na ≦ 20 % and si ≦ 25 %), necessitating a heating of the particles in the mixture at a temperature t , preferably of between 450 ° c . and 700 ° c ., in order to insure sufficient kinetics to produce recombination reactions without risking either combustion of the particles of carbon - containing products or pronounced melting of the particles resulting in agglomeration . this holding time of the particles is a function not only of the speed of the circulating gaseous flow , but also of system engineering parameters , such as the dimensions of the reaction chamber , generally in conical or cylindro - conical form , as well as the path of the circulating gaseous flow , which is associated with the chosen mode of injection , and hence with the characteristics of the injector . the adaptation of the characteristics of the mixture , in particular , its composition and its particle size , is also essential for the implementation of the process . thus the spent pot linings are ground in order to obtain particles of a dimension of less than 5 mm , and preferably less than 3 mm . furthermore , the pulverulent mineral additive , mixed with the ground pot linings in a proportion of less than 50 % by weight in the final mixture , and capable of combining chemically with the products of impregnation , notably with the fluorides , in order to form stable and insoluble compounds , is anhydrous or hydrous caso 4 , or a mixture thereof , that is , reactive materials that are relatively inexpensive and readily available . to increase the reactivity of the pulverulent additive , with a particle size of less than 1 mm , it is advisable to mix it intimately into the ground spent pot linings , and one variant in the preparation of the mixture can consist of introducing the additive into the coarsely ground pot linings and terminating the grinding of the mixture once the required particle size for the pot lining particles is achieved . all these parameters having been examined and established through experimentation , it has proven to be possible , in making use of the invention , to control , continuously and with precision , the heat treatment of major quantities of spent pot linings , giving the process great reliability and great flexibility for functioning on an industrial scale . it is indeed possible , by means of the gaseous flow / solid matter exchange system , to stabilize reactor operating conditions between the entry and exit points of the reactor , in order to maintain a set value t o for the temperature of the gaseous flow , measured before its exit from the reaction chamber at the bottom of the reactor . to do so , it is sufficient to modify the gravimetric output of the pulverulent mixture injected into the reactor , or possibly the temperature of the gaseous flow in the upper part of the reactor , as a function of the temperature variations in relation to the control temperature t o , in order to control the flow of combustible gas . these modes of continuous control and regulation of the heat treatment of spent pot linings are effective when used with a suitable reactor with injector , in particular the device made by vicarb that is used for implementing the process in its preferred version . finally , the gaseous flow , laden with solid particles , is extracted at the base of the reactor , where , after cooling , the gaseous flow is separated from the solid particles , forming an insoluble residue , while the dust - free gaseous flow is discharged directly into the atmosphere in the absence of any trace of fluorine - containing gas . the effectiveness of the insolubilization treatment is monitored by taking samples of the residue and leaching in accordance with french standard x31 - 210 . quantitative analyses of cn - and f - are performed on the leaching juices ; the cyanide and solubilized fluorine contents in proportion to the weight of the residue must remain less than 0 . 001 % and 0 . 05 %- 0 . 3 %, respectively , depending on the national standards currently in force . the process will be better understood from the description of its implementation in its preferred version , as shown in the flow chart of fig1 . the pulverulent mixture , containing at least 50 % by weight of spent pot linings 1 of a particle size of less than 3 mm , added to mineral additive 2 , anhydrous or hydrous caso 4 or to a mixture thereof , is extracted from the base of a storage hopper by a transporter / metering screw with a variable rotation speed . the metering screw is connected 3 to the upper part of a vicarb - type cylindrical reactor equipped with a gas injector , as disclosed in ep 0 171 316 , which serves as a generator of gaseous flow at a set temperature t , preferably between 800 ° c . and 1000 ° c ., by initial control of the combustion of a carburizing gas mixture 4 and air 5 and completed by the adjustment of the output of pulverulent mixture , which in the present case can vary , by means of the controllable - speed screw , between 50 kg / hour and 600 kg / hour , in order to form a suspension with the gaseous flow , the concentration of which is between 0 . 2 and 3 kg / nm 3 , and preferably between 0 . 3 and 2 kg / nm 3 ( nm 3 = normal cubic meter ; m 3 at 20 ° c . and 760 mm hg ). the metered pulverulent mixture 3 is injected into the center of a vortex formed by the hot gas flow 5 arriving tangentially at the top of the cylindrical reaction chamber and driven in a whirling movement . in order to achieve a good dispersion of the particles of the mixture in the gaseous flow in the upper part of the reactor and good agitation , the speed of the flow is regulated at 6 m / second and taking into account the dimensions of the reaction chamber ( 1 meter in diameter and 3 meters in height ), the corresponding average holding time is 0 . 5 seconds . at the temperature t set at the entrance of the reactor , generally between 800 ° c . and 1000 ° c ., and for an average holding time on the order of 0 . 5 seconds for the solid particles , the gravimetric output of which can vary from 50 to 600 kg / hour , these solid particles reach a temperature of between 400 ° c . and 750 ° c ., and preferably between 450 ° c . and 700 ° c ., before departure from the reactor and cooling . parallel monitoring of the temperature t o of the gaseous flow , measured in the same zone , shows that t o varies , like t , between 400 ° c . and 750 ° c ., depending on the thermal range chosen and in conformity with the t / t o / t relationship . once the value of t o is fixed , for example at 550 ° c ., the differences in temperature δt in relation to this set value are corrected by adjusting the rate at which the pulverulent matter is injected by means of the feed screw with a variable rotation speed controlled by a device which measures and records the temperature variations to better than ± 5 ° c . in relation to the set temperature t o = 550 ° c . in order to limit the risk of the particles bonding to the interior wall of the cylindrical reaction chamber , in which the temperature , in case of a breakdown in regulation , can locally exceed 1100 ° c ., it can be advantageous to perform cooling of these walls , for example by sweeping air past the external wall . the gaseous flow 7 at the exit point of the reactor is separated from the solid particles which are cooled and trapped , falling into a water - filled holding tank 8 placed under the reactor in a extension of the reaction chamber . after extraction from the tank , a first solid residue 9 is obtained . the gaseous flow 10 is channeled toward a device for cooling by vaporization of water 11 , where it is cooled 12 to about 200 ° c . before entering a filter 14 to be cleaned of its dust 13 , which constitutes a second solid residue . the gas is then discharged into the atmosphere 15 with hf and dust contents of less than 1 mg / nm 3 and 2 mg / nm 3 , respectively . the two solid residues , stemming from the double gas / solid separation combined with a double cooling , are mixed , and the final residue is subjected to analysis , that is , an inspection of the contents of cn - and f - from residue samples taken and leached according to the french standard x 31 - 210 . this inspection indicates a rate of leachable residual cyanide systematically less than 0 . 0005 % and a rate of leachable fluoride not exceeding 0 . 03 %, which can be lowered , depending on the additive used in the mixture , to 0 . 015 %. the examples pertain to four compositions of homogeneous mixtures of 1000 kg each , with a particle size of less than 1 mm , using spent pot linings with the following gravimetric contents : ______________________________________na % = 14 . 0 . sup . f % = 9 . 14 c % = 29 . 2 si % = 11 . 0 cn % = 0 . 08______________________________________ these pot linings are mixed with following additives , with the exception of example 3 used as a reference : ______________________________________example 1 anhydrous caso . sub . 4 ( anhydrite ) example 2 limeexample 3 no additiveexample 4 caso . sub . 4 . 2h . sub . 2 o ( gypsum ) ______________________________________ the principal operating characteristics and the results of the pollution - control tests are indicated in table 1 below . it can be seen that , for the same compositions of the mixture as those disclosed by the closest prior art ( ep - a - 0465388 and u . s . ser . no . 07 / 716104 ), a better rate of pollution control of the residue occurs with mixtures 1 and 4 using anhydrous caso 4 , and caso 4 . 2h 2 o , respectively , as additives , even despite substantially doubling of the rates of feeding of the mixture and thus the quantities of impurities . it should be noted that the thermal treatment of pot linings under the conditions of the invention but in the absence of an additive ( example 3 ), leads to a rate of pollution control of the residue that is clearly insufficient , since the output of leachable f - remains between 1 . 5 and 2 %, that is to say at least five times more than the least restrictive national standards . finally , in the presence of lime as an additive ( mixture 2 ), contrary to what is observed with anhydrous or hydrous caso 4 , the output of leachable f - ( 0 . 20 %) is not as good as that recorded at the lowest rate ( 0 . 17 %) in the closest prior art . table 1__________________________________________________________________________ examples no . 1 2 3 4__________________________________________________________________________initial total mixture , kg 1 , 000 1 , 000 1 , 000 1 , 000comprising : kg 600 600 1 , 000 600old pot linings , additive : kg 400 0 0 0anhydrous caso . sub . 4 , lime , kg 0 400 0 0gypsum ( caso . sub . 4 2h . sub . 2 o ), kg 0 0 0 400heat treatmentmixture output , kg / h 272 428 109 240gas flow temperature , c .° high t 900 1 , 000 1 , 050 900low t . sub . o 550 550 620 510gas flow speed , m / s 6 6 8 6average holding time , seconds 0 . 5 0 . 5 0 . 35 0 . 5rate of pollution abatement of residuetotal leachable cn . sup .-, % & lt ; 0 . 0005 & lt ; 0 . 0005 & lt ; 0 . 01 & lt ; 0 . 0005total leachable f . sup .-, % 0 . 015 0 . 20 1 . 55 0 . 03__________________________________________________________________________