[0001] The present invention relates to zeolites, more precisely the field of industrial synthesis of zeolite crystals and more particularly that of the synthesis of crystal zéolilthe FAU type X. [0002] The synthesis of FAU type zeolite X (or more simply zeolite X in the remainder of this description) is conventionally carried out in the industry in stirred batch reactor with heating of the reaction mixture by steam injection and / or using a jacket. [0003] The preparation of the synthesis gel for zeolite crystal formation commonly comprises mixing a sodium aluminate solution with sodium silicate solution, this mixture can be carried out either in an installation upstream of the crystallization reactor either directly in the crystallization reactor. [0004] In the case of zeolite X, which is a metastable crystalline phase, it is known to necessarily carry out a low-temperature curing phase, generally below 40 ° C to form seeds that give their crystal growth zeolite X after the crystallization stage carried out at higher temperature. [0005] In contrast, zeolite preparation highest temperature X can lead to other crystalline forms of training such as zeolites A and P, as taught for example in the book "Zeolite Molecular Sieves" by DW Breck (1974) page 273, published by John Wiley & Sons, New York. [0006] However, the document EP0149929 discloses a method for obtaining a zeolite, particularly zeolite A, continuously feeding an aqueous solution of sodium silicate and an aqueous solution of sodium aluminate in a non-stirred tubular reactor, so that the mixing time is less than the gelation time. [0007] BE869156 discloses in turn an industrial process for the continuous production of zeolite A which comprises first preparing a gel instantaneously, with a turbine and continuously from a sodium silicate solution and a sodium aluminate solution at temperatures between 40 ° C and 90 ° C. The thus prepared continuously gel is then transferred in crystallizers trays to be heated between 70 ° C and 100 ° C to effect crystallization batch. [0008] In order to significantly reduce the synthesis time, it could be particularly advantageous to operate at higher temperatures than conventional temperatures usually used for the synthesis of X zeolite crystals, and thus be able to consider even more profitable industrial syntheses, especially industrial syntheses continuously, zeolite crystals X. [0009] However, as taught in the book "Zeolite Molecular Sieves", ibid., X zeolite crystallized between 25 ° C and 120 ° C, with all the variables crystallization of time shorter than the crystallization temperature is high see for example "Zeolite Molecular Sieves" by DW Breck (1974), pp. 333-334, published by John Wiley & Sons, New York. [0010] Thus a first object of the present invention to provide a synthesis of X zeolite crystals, and more specifically to a continuous synthesis of zeolite crystals X. Another object is to provide a synthesis, typically a continuous synthesis, crystals of zeolite X with formation of crystals of high purity, for example with a crystallinity equal to or greater than 98% and a purity equal to or greater than 98%. Yet another object is to provide a synthesis, typically a continuous synthesis, crystals of zeolite X with formation of high purity crystals, with a reduced synthesis time, particularly reduced relative to what is known in the art. [0011] The Applicant has now unexpectedly discovered that by practicing seeding by addition of agent (s) seeding in the synthesis gel, it is possible to form zeolite crystals X to a temperature above 120 ° C while reducing drastically the time of synthesis of zeolite crystals X. Moreover, crystals X zeolite obtained under these conditions have a degree of crystallinity entirely satisfactory, most often greater than 98%, specifically including between 98% and 100%. [0012] Without being bound by theory, it has been observed that the addition of agent (s) seeding in the synthesis gel enables to dispense with the curing phase which is generally carried out at low temperature . Under these conditions, it is possible to consider introducing the reagents in synthesis reactor heated to a temperature above the temperature normally required for the curing phase, which is typically at most 40 ° C. The fact of introducing hot reactants, typically at a temperature above 40 ° C allows to further reduce the overall synthesis time. [0013] Thus, the present invention firstly provides a process for synthesizing zeolite X crystals, said process comprising at least one step of adding agent (s) seeding in a synthesis gel and at least one step of forming zeolite crystals X to a temperature (crystallization temperature) strictly greater than 120 ° C, preferably equal to or greater than 130 ° C. [0014] Specifically, the method of the present invention for the synthesis of zeolite crystals X, comprises at least the following steps: a) preparation of a synthesis gel by mixing at least one silica source, at least one source of alumina and optionally but preferably at least an aqueous solution of alkali metal hydroxide or alkaline earth, b) mixing said synthesis gel with at least one seeding agent to obtain a reaction medium, c) feeding of the reaction medium in the reactor, d) carrying out the crystallization reaction at a temperature above 120 ° C, e) filtering the reaction medium to recover zeolite crystals X products. [0015] The synthesis gel mixed in step b) with at least one seeding agent is typically the synthesis gel obtained in step a) of the process of the present invention. [0016] According to one embodiment, the crystallization temperature of the process of the present invention is equal to or less than 200 ° C, more or less 160 ° C. According to a preferred aspect, the crystallization temperature (Tc) of the process of the present invention is such that 120 ° C