Industrial production process for calcined clays

Abstract

The invention relates to a process for the industrial production of calcined clays using a microwave oven (2) equipped with: - a heating element (4), - at least one microwave generator (24a, 24b) and at least one waveguide (22) extending longitudinally inside the heating element (4), the process comprising the following steps: a) clay is introduced into the heating element (4), b) the clay introduced into the heating element (4) is exposed to microwaves emitted by said at least one microwave generator (24a, 24b) and transmitted into the heating element (4) by said at least one waveguide (22), so as to dehydroxylate at least a portion of the introduced clay and produce calcined clay, c) the calcined clay produced is extracted from the heating element (4). Figure for the abstract: Fig. 1

Description

Title of the invention: Process for the industrial production of calcined clays

[0001] The present invention relates generally to the field of the production of calcined clays.

[0002] It relates more particularly to an industrial production process for calcined clays using a microwave heat treatment device.

[0003] The term "calcined clay" is used to designate a set of phyllitic minerals treated thermally in order to give them pozzolanic properties, that is to say an ability to form, by mixing with lime or any other strong base, at ordinary temperature, compounds having hydraulic properties. These pozzolanic properties allow the consumption of portlandite and / or activation in a strong basic medium.

[0004] Among the calcined clays, metakaolin is a particular calcined clay, ultrafine, reactive, having a field of varied applications, such as the production of hydraulic or pozzolanic binders, the use in addition for the production of ready-mixed concrete for the improvement of mechanical performance, durability and resistance to aggressive environments, the production of mortars, the production of coatings, the production of remediation binders, the treatment of ultimate waste, the addition as an active charge in the field of bituminous coatings, etc.

[0005] The processes currently used for calcining clays use one of the following heating devices: a continuous rotary kiln using a fossil fuel (gas or fuel oil) with a calcination time of 1 or 2 hours, a vertical kiln, a batch kiln, a tunnel kiln, a flash calciner using a fossil fuel (gas or fuel oil) with a calcination time of a few seconds.

[0006] Since all these devices use fossil fuels, the associated processes are the source of combustion gas emissions into the atmosphere, including carbon dioxide CO2 and other atmospheric pollutants. In long calcination processes, CO2 emissions per tonne of calcined clay are of the order of 250 kg of CO2 equivalent according to the DIOGEN database. In the short calcination process with a flash calciner, CO2 emissions per tonne of calcined clay are of the order of 140 kg of CO2 equivalent according to this DIOGEN database.

[0007] In order to limit the releases into the atmosphere of combustion gases linked to the use of fossil energy, the present invention proposes a method for producing calcined clays using a microwave oven equipped with a heating body, at least one microwave generator and at least one waveguide extending along a longitudinal direction inside the heating body, said method comprising the following steps: a) clay is introduced into the heating body, b) the clay introduced into the heating body is exposed to microwaves emitted by said at least one microwave generator and conveyed into the heating body by said at least one waveguide, so as to dehydroxylate at least part of the clay introduced and produce the calcined clay, c) the calcined clay produced is extracted from the heating body.

[0008] Thus, thanks to the method according to the invention, the clay is calcined at least partially by an electrically powered microwave heat treatment device. The combustion gas emissions are reduced compared to a conventional method.

[0009] To dehydroxylate the introduced clay, the introduced clay is heated to a suitable temperature for a sufficient time to carry out this thermochemical reaction. The temperature suitable for dehydroxylating at least a portion of the introduced clay is, for example, between 550 and 850 degrees Celsius. The sufficient heating time to dehydroxylate said at least a portion of the introduced clay depends on the temperature used, the composition of the introduced clay, and its shape, for example the size of the introduced clay particles. This sufficient time is typically between 5 and 240 minutes. The hydroxylated clay is referred to herein as “calcined clay”.

[0010] The method according to the invention also has the advantage of concentrating the microwaves on a bed of moving materials to homogenize the heating. The calcination temperature of the clay is particularly well controlled, which ensures satisfactory and uniform properties for the calcined clay produced by this method.

[0011] Advantageously, the method according to the invention makes it possible to carry out the production of calcined clays continuously.

[0012] In practice, the production is said to be "continuous" because it is possible to carry out step c) of extracting the calcined clay produced over a time range of more than 4 hours, without interruption of more than 1 minute. In other words, the flow of calcined clay extracted from the microwave oven does not have an interruption of more than one minute during an extraction time of at least 4 hours.

[0013] Other non-limiting and advantageous characteristics of the method according to the invention, taken individually or in all technically possible combinations, are the following: - the heating body of the microwave oven extending along a longitudinal axis between two opposite longitudinal ends, the clay is introduced, in step a), at a first of the two longitudinal ends of the heating body, in step b) the clay and / or the calcined clay contained in the heating body is advanced according to a direction parallel to the longitudinal axis of the heating body, from the first longitudinal end towards a second longitudinal end of the heating body, the calcined clay is extracted, in step c), at the second longitudinal end of the heating body; - the production of calcined clays is carried out continuously; - said waveguide comprises a slotted waveguide mounted inside the heating body extending in a longitudinal direction parallel to the longitudinal axis of the heating body and connected at one of its ends to said microwave generator; in step a), the clay is introduced into the heating body by a feed system opening into the heating body at the first of said two longitudinal ends thereof, in step c), the calcined clay is extracted by an evacuation system opening into the heating body at the second longitudinal end thereof, and during step b), said heating body is rotated about its longitudinal axis, and the longitudinal axis of the heating body is inclined relative to the horizontal so as to regulate the speed of advance of the clay in the heating body, - the microwaves are trapped inside the oven using two fixed covers, each cover being mounted on one of the longitudinal ends of the heating body with the interposition of annular wave-trapping seals; - in step b), the gases released during heating of the clay are evacuated by an evacuation device ensuring a depression of the heating body of the furnace and / or gases are injected inside the heating body; - the oven comprising two microwave generators and two corresponding waveguides and the heating body of the oven comprising a first part in which extends a first of said two waveguides associated with a first of said two microwave generators and a second part in which extends the second waveguide associated with the second microwave generator; in step b), the clay successively passes through the first part then the second part of the heating body, said first and second parts of the heating body having different temperature profiles; - the first microwave generator and the first waveguide being configured to produce a gradual increase in temperature up to a target temperature along the first portion of the heating body and the second microwave generator and the second waveguide being configured to maintain a constant target temperature along the second portion of the heating body, in step b); the clay is subjected to the gradual increase in temperature as it advances along the first portion of the heating body and is then maintained at the target temperature as it advances along the second portion of the heating body; - in step a), the clay introduced into the heating body of the furnace comprises a single component or several components, among the following components: kaolinite, illites, palygorskites, smectites, halloysites; interstratified, micas; - in step a), a material comprising said clay is introduced into the heating body, said material comprising between 25 and 75% by weight of one or more of said components; - in step b), the clay is heated to a temperature between 550 and 850 degrees Celsius to carry out the dehydroxylation of at least part of this clay; - the clay is dried using a drying device before being introduced into the heating body; - the clay is crushed before being introduced into the heating body; - after grinding, the clay is filtered and / or stored before being introduced into the heating body; - before its introduction into the heating body, the clay is preheated in a preheating device; - before preheating, the clay is dried using a drying device, and according to which the gases heated in the preheating device and / or the gases released during the calcination of the clay in the kiln are recirculated to the drying device; - gases released during heating for calcination of clay in the microwave oven are recirculated to the preheating device; - after step c), the calcined clay is calcined again using a fossil-fuelled flash calciner; - after step c), the clay is conveyed through a cooling system to be cooled; - before its introduction into the heating body, the clay is preheated in a preheating device and according to which the heat of the calcined clay recovered by the cooling system is reused by the preheating device; - the calcined clay is crushed after its extraction from the heating body; - the crushed calcined clay is filtered and / or stored; - before step a), the clay is preheated by passing it through a fossil-fuelled heat treatment device; - before preheating, the clay is dried using a drying device, and according to which the gases heated in the fossil energy heat treatment device and / or in the kiln are recirculated to the drying device; - the heating body is internally provided with stirring blades which extend inclined relative to the longitudinal axis of the heating body and are integral in rotation with it, and, in step b), the stirring blades are rotated with the heating body while tilting the longitudinal axis thereof to advance the clay and / or calcined clay contained in the heating body; - the clay is calcined without producing carbon dioxide.

[0014] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0015] In the attached drawings:

[0016] [Fig-1] is a schematic view of a first industrial assembly implementing a first embodiment of the method according to the invention,

[0017] [Fig.2] is a schematic view of a second industrial assembly implementing a second embodiment of the method according to the invention,

[0018] [Fig.3] is a schematic view of a third industrial assembly implementing a third embodiment of the method according to the invention,

[0019] [Fig.4] is a schematic view of a fourth industrial assembly implementing a fourth embodiment of the method according to the invention,

[0020] [Fig.5] is an informative schematic view representing the chemical and crystallographic transformations occurring in different types of clays (kaolinite and halloysite) during heating of these clays,

[0021] [Fig.6] is a differential thermal analysis diagram of kaolin showing the chemical and crystallographic transformations of kaolin upon heating of this clay,

[0022] [Fig.7] is a graph representing the variation of the temperature of a sample of Kl clay as a function of time when it is calcined in a microwave oven with a set temperature of 650°C for 60 minutes, the Kl clay corresponding to raw kaolin clay, of granular class 0 / 20 mm and a water content of 13%, containing approximately 30% of kaolin, the rest of the minerals being mainly made up of quartz,

[0023] [Fig.8] is a graph representing the variation of the temperature of a sample of Kl clay as a function of time when it is calcined in a microwave oven with a set temperature of 700°C for 60 minutes,

[0024] [Fig.9] is a graph representing the variation of the temperature of a sample of K2 clay as a function of time when it is calcined in a microwave oven with a set temperature of 650°C for 60 minutes, K2 corresponding to dried ground kaolin clay, of granular class 0 / 60 microns and a water content of 0%, containing approximately 45% of kaolin, the rest of the minerals being mainly quartz,

[0025] [Fig. 10] is a graph representing the variation of the temperature of a sample of K2 clay as a function of time when it is calcined in a microwave oven with a set temperature of 650°C for 10 minutes,

[0026] [Fig. 11] is a graph representing the variation of the temperature of a sample of K2 clay as a function of time when it is calcined in a microwave oven with a set temperature of 700°C for 60 minutes,

[0027] [Fig. 12] is a graph representing the variation of the temperature of a sample of K3 clay as a function of time when it is calcined in a microwave oven with a set temperature of 650°C for 60 minutes, the K3 clay corresponding to dried ground kaolin clay, of granular class 0 / 60 microns and a water content of 0%, containing approximately 70% of kaolin, the rest of the minerals being mainly quartz,

[0028] [Fig. 13] is a schematic longitudinal sectional view of a microwave oven provided with a rotating cylindrical heating body used for implementing the embodiments of the method shown in Figures 1 to 4,

[0029] [Fig. 14] is a sectional view along plane II-II of [Fig. 13],

[0030] [Fig. 15] is a schematic view showing an example of a wave trapping joint of the microwave oven of [Fig. 13],

[0031] [Fig. 16] is a schematic view showing an example of a slotted waveguide of the microwave oven of [Fig. 13],

[0032] [Fig. 17] is a schematic cross-sectional view of a first example of a heating body of the microwave oven of [Fig. 13],

[0033] [Fig. 18] is a schematic cross-sectional view of a second example of a heating body of the microwave oven of [Fig. 13],

[0034] [Fig. 19] is a schematic cross-sectional view of a third example of a heating body of the microwave oven of [Fig. 13].

[0035] In the description which follows, by convention, the terms "upstream" and "downstream" will be used with reference to the direction of the flow of the materials introduced into the microwave oven, and more generally with reference to the direction of the flow of the materials and then the products in the industrial assembly used for the implementation of the method according to the invention.

[0036] As a preliminary point, it will be noted that, from one figure to another, the identical or similar elements of the different examples of implementation of the invention will be referenced by the same reference signs and will not be described each time.

[0037] The invention relates to a process for the industrial production of calcined clays.

[0038] In Figures 1 to 4, four embodiments of an industrial assembly 300; 400; 500; 600 are shown, adapted to implement the method according to the invention.

[0039] Each of these industrial assemblies 300; 400; 500; 600 remarkably comprises a microwave oven 2 used according to the method of the invention to at least partially calcine the clay treated according to this method.

[0040] This microwave oven 2 is provided with a heating body 4, at least one microwave generator 24a and at least one waveguide 22 extending in a longitudinal direction inside the heating body 4 (see figures 1, 13 and 14).

[0041] According to the method of the invention, the following steps are carried out: a) clay is introduced into the heating body 4 of the microwave oven 2, b) the clay introduced into the heating body 4 is exposed to microwaves emitted by the microwave generator 24a and conveyed into the heating body 4 by the waveguide 22, so as to dehydroxylate at least part of the clay introduced and produce calcined clay c) the calcined clay produced is extracted from the heating body 4,

[0042] Calcined clay is clay that has been heated to a predetermined temperature and for a predetermined time period to allow a dehydroxylation reaction of the clay to form the calcined clay. Heating to a temperature that allows this dehydroxylation reaction, without overfiring, is referred to herein as "calcination."

[0043] The heating body 4 is for example of the rotating cylindrical type and extends along a longitudinal axis XX between two opposite longitudinal ends.

[0044] Generally, said waveguide 22 is mounted inside the heating body 4. It extends in a longitudinal direction parallel to the longitudinal axis XX of the heating body 4 and is connected at one of its ends F, F' to said microwave generator 24a, 24b (Figures 1 to 4, 13 and 16). The heating body 4 comprises an internal cavity which accommodates the materials introduced into this heating body. An example of a microwave oven suitable for implementing the method according to the invention will be described in more detail later with reference to Figures 13 to 19.

[0045] Step a)

[0046] In step a), the clay introduced into the heating body of the furnace comprises a single component or several components. These components have different chemical compositions and / or different crystallographic structures.

[0047] In particular, the clay may contain different components, for example among the following components: kaolinite, illites, palygosrkytes, smectites, halloysites, interstratified, micas,.....

[0048] The clay may be introduced pure or mixed with other elements. In step a), a material comprising said clay is introduced into the heating body, said material comprising between 25 and 75% by weight of one or more of said components. The other elements forming said material comprise, for example, quartz.

[0049] The material is introduced into the heating body 4 of the furnace 2 preferably in a divided form, for example in the form of a powder. The particles of this material preferably have a size of less than 80 microns. This divided form can be natural or obtained by grinding.

[0050] Primary kaolin or kaolinic clay is preferably introduced into the furnace so as to obtain, after calcination, metakaolin having pozzo-lanic properties. Other clays (illites, palygosrkytes, smectites, halloysites or mixtures thereof) can be used. They are generally calcined at higher temperatures and do not always have the same level of reactivity as calcined kaolinites.

[0051] The material introduced into the furnace is, for example, raw clay or washed clay containing a proportion of kaolin or other clays which can vary between 25% and 75%.

[0052] Clay contents of less than 25% or greater than 75% nevertheless remain acceptable for the process according to the invention.

[0053] In practice, in step a), the clay is introduced into the heating body of the furnace 2, at a first of said two longitudinal ends thereof. This is achieved by means of a feed system 34 opening into the heating body 4 at the first longitudinal end thereof. An example of such a feed system will be described later.

[0054] Step b)

[0055] The material introduced into the heating body 4 of the furnace 2 is heated by microwaves. This heating allows various simultaneous thermal treatments of the material, in particular its drying, its temperature rise and its calcination, that is to say its dehydroxylation under high temperature.

[0056] Drying here corresponds to a superficial dehydration of the material.

[0057] The clay is preferably heated to a temperature of between 550 and 850 degrees Celsius to carry out the dehydroxylation of this clay. Alternatively, the clay is heated to a temperature of between 600 and 800°C, between 650 and 800°C, between 700 and 800°C, between 750 and 850°C, between 600 and 750°C, between 650 and 750°C, between 600 and 700°C or between 650 and 700°C. The clay is preferably heated for a time of between 5 and 240 minutes, preferably between 10 and 240 minutes, depending on the material and the temperature.

[0058] These heat treatments can cause the release of gases, in particular water vapor.

[0059] During the heating of the clay, the released gases are evacuated by a gas evacuation device 42 (figures 1 and 13) ensuring a depression of the heating body 4 of the furnace 2.

[0060] The clay is thus heat treated in furnace 2 under a “closed” atmosphere.

[0061] It is also possible to inject gases inside the heating body 4. This injection can, for example, be carried out at the right angle of the waveguide in order to avoid the entry of dust into the latter.

[0062] Step c)

[0063] In step c), the calcined clay produced by heating and calcining the clay in step b) is removed from the heating body 4 of the furnace 2.

[0064] In practice, the calcined clay is removed from the furnace 2 at a second longitudinal end of the heating body 4 opposite the first longitudinal end. It is extracted from the furnace by a calcined clay discharge system 36 opening into the heating body 4 at the second longitudinal end thereof. An example of such a discharge system will be described later.

[0065] The advantages of the method according to the invention are numerous. In particular, it allows the microwaves to be concentrated on a bed of moving materials, which makes it possible to homogenize the calcination. It also allows satisfactory control of the temperature of the clay in the kiln, and therefore of the calcination temperature. Finally, the discharge of combustion gases and associated pollutants is reduced.

[0066] It should be noted that one of the particularities of microwave heating is that it heats the core of the material, without directly heating the heating body. The latter is heated indirectly by the increase in the temperature of the material. As a result, the temperature of the heating body is lower than that observed in conventional fossil fuel heat treatment devices. The lifetime of the furnace is thus extended.

[0067] Generally speaking, the fossil energy used comes from the combustion of fossil fuels, such as gas, fuel oil, coal or coke.

[0068] Advantageously, according to the invention, the process for producing the calcined clay is carried out continuously. This means that a first quantity of clay is not introduced, heated and calcined in the microwave oven to produce calcined clay, which is completely extracted from the oven before the introduction of a second quantity of clay, as would be the case in a batch process. After a transient regime corresponding to the initiation of the process, the clay is here introduced into the microwave oven, calcined and extracted from the oven continuously.

[0069] In practice, the production is said to be “continuous” because it is possible to carry out step c) of extraction of the calcined clay produced over a time range of more than 4 hours without interruption of the flow of calcined clay for more than 1 minute.

[0070] Thus, the calcined clay produced can for example be extracted either without interruption over a time range greater than 4 hours, or over short time intervals, separated for example by 0 to 1 minute or by 5, 10, 20, 30, 40, 50 seconds, over this time range.

[0071] Preferably, when implementing such a continuous method, steps a), b) and c) can be carried out simultaneously. This is the case, for example, when the microwave oven 2 feed system used comprises a metal sluice gate valve for regulating the flow rate of clay introduced into the oven, as described later. The supply of clay to the oven and the extraction of calcined clay from the oven can then be carried out continuously, without interruption, over said time range.

[0072] Alternatively, steps a) and / or c) are carried out at time intervals. This is the case, for example, when the microwave oven's power supply system includes an inlet airlock that can be achieved by two consecutive guillotine valves. Feeding is then carried out in jerks. Similarly, the extraction of calcined clay can be carried out in jerks.

[0073] It is also possible to envisage, as a variant, that only the feeding or only the extraction is carried out by jerks.

[0074] In order to ensure this continuous operation, in step b), the clay and / or calcined clay contained in the heating body is advanced in a direction parallel to the longitudinal axis XX of the heating body 4, from the first longitudinal end to the second longitudinal end. This advance of the materials contained in the heating body allows the continuous implementation of the method according to the invention.

[0075] Preferably, these materials are advanced continuously throughout the duration of implementation of the method. For this purpose, the oven has supply means allowing the clay and calcined clay to be transported from the first to the second longitudinal end of the heating body of the microwave oven.

[0076] Here, for example, the heating body 4 is rotated about its inclined longitudinal axis in order to advance the clay from the longitudinal end of the heating body at which the feed system introduces the clay towards the longitudinal end of the heating body at which the discharge system extracts the calcined clay. The feed means therefore comprise a device for rotating the heating body and an inclined support for this heating body.

[0077] The arrangement of the microwave oven 2, an example of which is detailed later, ensures airtightness as well as trapping of the microwaves inside the heating body.

[0078] These operations are carried out continuously in order to maintain all production equipment at a stable temperature.

[0079] In a batch calcination process, a batch of clay would be introduced into the kiln. The introduction of clay would cease and the introduced clay would be calcined and then completely extracted from the kiln during emptying before the introduction of another batch of clay.

[0080] The furnace emptying operations lead to production stoppages during which temperatures drop sharply. This discontinuity induces overcon energy consumption. In addition, during the production of calcined clays, control of calcination temperatures and times is essential to avoid under- and over-firing. Continuous calcination therefore has a double advantage: on the one hand, an improvement in the quality of the calcined clay produced by standardizing its characteristics, and on the other hand, a reduction in the energy consumption of the process.

[0081] A discontinuous process can nevertheless be envisaged and implemented using the industrial assemblies described.

[0082] Particularly advantageously, in the microwave oven 2 used for implementing this method, said waveguide 22 comprises a slotted waveguide mounted inside the heating body 4 between the two longitudinal ends thereof and connected to one of these longitudinal ends to said microwave generator 24a. The waveguide preferably extends from one longitudinal end of the heating body to the other. The microwaves are therefore routed from one longitudinal end of the heating body to the other. Each microwave generator 24a, 24b is arranged outside the heating body 4, at one of the longitudinal ends of the heating body.

[0083] Each waveguide is preferably connected to a microwave generator. However, it is possible to envisage connecting each waveguide to several microwave generators in order to increase the power delivered.

[0084] Furthermore, during steps a), b) and c), said heating body 4 is rotated about its longitudinal axis XX and the longitudinal axis XX of the heating body 4 is inclined relative to the horizontal so as to regulate the speed of advance of the clay in the heating body 4 (see figures 1 to 4). This makes it possible to advance the clay through the microwave oven 2.

[0085] As mentioned in the example detailed below, the heating body may also be provided with stirring blades (not shown). The stirring blades extend inside the heating body. They extend parallel to the longitudinal axis or inclined relative to the longitudinal axis of the heating body. They are integral in rotation with the latter, and, in step b), the stirring blades are rotated with the heating body while tilting the longitudinal axis thereof. The material contained in the heating body is then stirred continuously during the rotation of the heating body. This makes it possible to homogenize the mixture of materials present in the heating body and to contribute, possibly, to controlling the advance of this mixture of materials in the heating body.

[0086] In addition, the microwaves are trapped inside the oven 2 by means of two fixed covers 14, each cover being mounted on one of the longitudinal ends of the heating body 4 with the interposition of annular wave-trapping seals 16 (figures 9 and 11).

[0087] A detailed example of an oven describing the means of implementing these characteristics The characteristics of the method according to the invention are described further below (with reference to Figures 9 to 15).

[0088] According to a particular embodiment of the method, which will be described in more detail later, the oven comprises two microwave generators 24a, 24b and two corresponding waveguides 22 and the heating body 4 of the oven 2 comprises a first part in which extends a first of said two waveguides associated with a first 24a of said two microwave generators and a second part in which extends the second waveguide associated with the second microwave generator 24b ([Fig.9]).

[0089] The two waveguides may comprise two independent waveguides each connected to one of the two microwave generators or two sections 22a, 22b of waveguide 22 connected to each other, each section 22a, 22b being connected at one end to one of the two microwave generators 24a, 24b. The two sections 22a, 22b may extend along the same axis parallel to the longitudinal axis XX of the heating body, as is the case in [Fig. 9]. In this case, one of the two microwave generators 24a, 24b is arranged at a first of the two longitudinal ends of the heating body 4 and the other microwave generator is arranged at the second longitudinal end of the heating body.

[0090] Alternatively, two separate waveguides may be provided extending side by side, for example in two directions parallel to the longitudinal axis of the heating body.

[0091] Of course, it is possible to envisage a number greater than two of waveguides extending in the heating body, each waveguide being connected to a separate microwave generator or to the same microwave generator.

[0092] Then, in step b), the clay successively passes through the first part then the second part of the heating body, said first and second parts of the heating body having different temperature profiles.

[0093] In a particularly advantageous manner, it can be provided that the first microwave generator and the first waveguide being configured so as to produce a progressive increase in temperature along the first part of the heating body and the second microwave generator and the second waveguide being configured so as to maintain a constant target temperature along the second part of the heating body, in step b), the clay is subjected to the progressive increase in temperature along the first part of the heating body and then is maintained at the target temperature along the second part of the heating body.

[0094] Said first part constitutes an upstream temperature increase zone, while the second part constitutes a downstream zone for maintaining a target temperature.

[0095] Other temperature profiles can obviously be envisaged, with two or more waveguides defining two or more parts of the body of heaters with different temperature profiles.

[0096] Furthermore, it may be contemplated to use two or more microwave ovens arranged in series to calcine the clay. For example, a first oven may be used to heat the clay with a first temperature profile, e.g., a temperature ramp-up, and the second oven may be used to heat the clay with a second temperature profile, e.g., maintaining a calcination temperature. The second oven is fed with clay heated in and extracted from the first oven.

[0097] In practice, the process for producing calcined clay comprises other steps occurring before or after the calcination of the clay in the microwave oven 2.

[0098] Generally speaking, the method according to the invention may comprise one or more of the following additional steps:

[0099] - the clay is dried using a drying device 120 before being introduced into the heating body 4 (figures 1 to 4);

[0100] - the clay is crushed before being introduced into the heating body 4, for example at using a 130 grinder (figures 1 and 3);

[0101] - after grinding, the clay is filtered, for example using a dynamic selector 140 and / or a bag filter 150 and / or it is stored in a storage device 170 before carrying out step a) (figures 1 and 3);

[0102] - before its introduction into the heating body 4, the clay is preheated in a preheating device 180; 185 (figures 1, 3 and 4), the preheating device possibly comprising a fossil energy heat treatment device 185;

[0103] - after step c), the calcined clay is calcined again using a calciner flash 230 with fossil energy ([Fig.3]);

[0104] - after step c), the clay is conveyed through a cooling system 210 to be cooled (figures 1 and 3);

[0105] - the calcined clay is ground after its extraction from the heating body 4, by example using a 240 grinder (figures 2 and 4);

[0106] - the crushed calcined clay is filtered, for example through a dynamic selector 250 and / or a bag filter 260 (figures 2 and 4);

[0107] - the crushed and filtered calcined clay is stored, for example in a hopper of storage 220 (figures 1 to 4);

[0108] - the temperature of the calcined and uncalcined clay present inside is measured of the microwave oven 2, for example using an infrared pyrometer.

[0109] Drying of the clay by the drying device is carried out at temperatures lower than those used during heating for calcining the clay.

[0110] Generally, it is further provided that the gases heated in the preheating device when the latter is in use and / or the gases released during the heating of the clay in the kiln is recirculated to the drying device 120.

[0111] It may also be provided, when the clay is preheated, that the gases released during the heating of the clay in the furnace are recirculated to the preheating device 180; 185.

[0112] Furthermore, in the case where the calcined clay is cooled by passing through a cooling system 210, the heat of the calcined clay recovered by the cooling system 210 is reused by the preheating device 180; 185. In practice, the gases heated by the calcined clay in the cooling device 210 are recirculated to the preheating device.

[0113] In the following, different embodiments of the method according to the invention are described with reference to Figures 1 to 4. In these figures, the material transfers before and after calcination are represented by solid arrows. The hot gas transfers are represented by dotted lines. The arrow R indicates a supply of fresh air.

[0114] According to the first embodiment of the method according to the invention shown in [Fig.l], the material feeding the industrial assembly 300 and containing the clay is first introduced into a lump breaker 110. The material then passes into the drying device 120 or dryer. The drying heats the material to approximately 250°C.

[0115] The material is then conveyed through the grinder 130. The ground material passes through the dynamic selector 140 allowing the selection of material particles having predetermined particle size characteristics.

[0116] Clay particles with sizes less than 80 microns are preferably selected. The granulometric characteristics of the clay are determined here, for example, using a sieve column or a laser granulometer.

[0117] The material particles selected in the dynamic selector 140 are recovered and the air is filtered in the bag filter 150. This is connected to a chimney 160. The assembly is put under depression using a fan.

[0118] The material particles are stored in a storage device 170, for example in a hopper. This material is partially cooled.

[0119] The material stored in the storage device 170 is then conveyed through a preheating tower 180 which gradually heats the material to 200°C, then 300°C and 400°C.

[0120] At the outlet of the preheating tower, the material containing the clay is introduced into the microwave oven 2. It is introduced into the heating body 4 through the feed system 34 and heated to a set temperature of 650°C.

[0121] The heating body 4 is rotatable about its longitudinal axis XX. It rotates continuously while the material is being heated. It is supported by a platform 12 with an adjustable inclination.

[0122] The longitudinal axis XX of the heating body 4 being inclined relative to the horizontal such that the first longitudinal end through which the material is introduced is higher than the second longitudinal end of the heating body 4 along a vertical axis, the material is progressively conveyed towards the evacuation system 36 located at the second longitudinal end of the heating body by the effect of gravity and the rotation of the heating body 4 around its longitudinal axis XX.

[0123] An example of a microwave oven 2 will be described in more detail later.

[0124] At the outlet of the kiln 2, the calcined material which comprises the calcined clay is conveyed through a cooling tower 210 so as to gradually lower the temperature of the calcined material to 150°C. The cooled calcined material is stored in a storage device 220 such as a hopper.

[0125] As schematically shown in [Fig.l], the gases heated in the preheating tower 180 are preferably recirculated to the drying device 120 in order to reduce the energy consumption of this drying device.

[0126] Similarly, the gases heated in the cooling device 210 and / or the gases heated in the microwave oven 2 and discharged by the gas discharge device 42 are preferably recirculated to the preheating tower 180 and / or to the drying device 120 in order to reduce the energy consumption of these elements of the industrial assembly 300.

[0127] According to the second embodiment of the method according to the invention shown in [Fig.2], the material feeding the industrial assembly 400 and containing the clay is first introduced into the lump breaker 110. The material then passes into the drying device 120 or dryer. This is connected to a chimney 160 allowing the gases released during the drying of the material to escape. Drying heats the material to approximately 100°C.

[0128] According to this embodiment, the material is not crushed before its introduction into the furnace 2.

[0129] After drying, the material is introduced into the microwave oven 2. It is introduced into the heating body 4 through the feed system 34 and is calcined at a set temperature of 650°C.

[0130] As described previously, the material is progressively conveyed towards the evacuation system 36 located at the second longitudinal end of the heating body by the effect of gravity and the rotation of the heating body 4 around its longitudinal axis XX.

[0131] At the outlet of the furnace 2, the calcined material which comprises the calcined clay is conveyed through a grinder 240. The ground material passes through a dynamic selector 250 making it possible to select the particles of calcined material having characteristics predetermined granulometric characteristics.

[0132] Preferably, calcined clay particles of sizes less than 80 microns are selected.

[0133] The calcined material particles selected in the dynamic selector 250 are recovered and the hot gases accompanying them are filtered in a bag filter 260. The hot gases are recirculated to the drying device 160.

[0134] The particles of calcined material are stored in the storage device 220, for example in a hopper.

[0135] As schematically shown in [Fig.2], the gases heated in the microwave oven 2 and discharged by the gas discharge device 42 are preferably recirculated to the drying device 120 in order to reduce its energy consumption.

[0136] Depending on the desired flow rate of calcined clay, the electrical power required can be very high. Therefore, taking into account the capacities available locally on the electrical distribution networks, the microwave oven 2 can be associated with a fossil energy heat treatment device, for example a continuous rotary kiln using a fossil fuel or a flash calciner also using a fossil fuel, to pre-calcine or complete the calcination of the calcined clays. In this case, it will be a hybrid process. Two examples of hybrid processes are shown in Figures 3 and 4 and described below.

[0137] The third embodiment of the method according to the invention shown in [Fig. 3], is identical to the embodiment of [Fig. 1], except that at the outlet of the microwave oven 2, the material calcined in the microwave oven 2 is introduced into a flash calciner 230 of the industrial assembly 500.

[0138] The microwave oven 2 here partially calcines the material before its introduction into the flash calciner 230. The set temperature of the microwave oven is here, for example, set at 550°C. The flash calciner completes the calcination by bringing the clay to 650°C.

[0139] The fourth embodiment of the method according to the invention shown in [Fig. 4], is similar in all respects to the embodiment of [Fig. 2], except that the material dried in the drying device 120 is not introduced directly into the microwave oven but into a preheating device. The preheating device here is a conventional fossil energy heat treatment device 185 of the industrial assembly 600.

[0140] The fossil energy heat treatment device 185 heats the material before it is introduced into the furnace. The set temperature of the fossil energy heat treatment device 185 is here, for example, set at 400°C. The microwave oven 2 calcinates the material at a set temperature of 650°C.

[0141] Feasibility tests of the clay calcination process using a microwave oven were carried out on a C400 type laboratory microwave oven, with a power of 6000 W and a frequency of 2450 MHz. This microwave oven comprises a rotating cylindrical cavity with a diameter of 400 mm and a height of 400 mm. This cavity is rotated by a variable speed motor. The temperature of the materials inside the oven is recorded by an infrared pyrometer.

[0142] Such a microwave oven is for example described in document EP2530059.

[0143] Three different kaolin clays were tested in this laboratory microwave oven:

[0144] Kl: raw kaolin clay, granular class 0 / 20 mm and a water content of 13%. This clay contains approximately 30% kaolin. The rest of the minerals are mainly quartz.

[0145] K2: Dried ground kaolin clay, granular class 0 / 60 microns and a water content of 0%. This clay contains approximately 45% kaolin. The rest of the minerals are mainly quartz.

[0146] K3: Dried ground kaolin clay, granular class 0 / 60 microns and water content of 0%. This clay contains approximately 70% kaolin. The rest of the minerals are mainly quartz.

[0147] The granular classes are indicated according to the formalism d / D where d is the smallest representative diameter, D the largest representative diameter of the grains of the material determined by granulometric analysis using a sieve column or a laser granulometer.

[0148] The tests were carried out according to the experimental protocol summarized in the following table:

[0149] [Tables 1] Test / Graph Sample Temperature Calcination time Humidity El / figure 7 Kl 650 °C 60 min 13% E2 / figure 8 Kl 700 °C 60 min 13% E3 / figure 9 K2 650 °C 60 min 0% E4 / figure 10 K2 650°C 10 min 0% E5 / figure 11 K2 700 °C 60 min 0% E6 / figure 12 K3 650 °C 60 min 0%

[0150] These feasibility tests were carried out without optimization of the process or the results obtained, in comparison with the characteristics obtained on these same clays with the flash calcination process.

[0151] The graphs shown in Figures 7 to 12 show the curves and temperature rise levels measured as a function of time for each of the tests carried out. On each of the graphs and for each of the calcination stages, the power in watts (w) is given for information purposes. Figures 7 to 12, with the exception of [Fig. 10], also show the cooling times of the different samples after calcination.

[0152] These different curves present thresholds which may be offset depending on the types of materials; however, if we take material E3 as a reference, we can distinguish: - A high temperature rise rate between 20 and 500°C, - An inflection of this curve between 500 and 550°C, - A resumption of a higher temperature rise rate between 550 and 650°C,

[0153] These three main phases are to be compared to the thermal phenomena which induce the modification of kaolin clay, as shown schematically in [Fig.5], namely: - Delamination between 475 and 540°C, - Dehydroxylation between 540 and 650°C, - Formation of metakaolinite beyond the temperatures mentioned above.

[0154] These tests therefore confirm that the calcination of the clay was indeed carried out in the microwave oven.

[0155] The differential thermal analysis diagram of kaolin in [Fig.6] shows the same phenomenon in the RI rectangle which covers the entire endothermic peak, a marker of the transformation of kaolin into metakaolin. In this rectangle, the kaolin undergoes dehydroxylation marked by a weight loss of 11.98% between 457 and 711°C. The maximum thermal emission is recorded at the temperature 519°C.

[0156] Rectangle R2 covers an exothermic peak at 991°C. This is the tracer of the transformation of metakolin into mullite, with loss of the amorphous characteristic of metakolin, and therefore of its reactivity.

[0157] The temperature difference between these two peaks is comfortable in the case of metakaolin, but it is lower in the case of meta-illites and other meta-clays, making their calcination more difficult to control because the temperature must be controlled more precisely.

[0158] Generally speaking, the lump breaker, the drying device, the grinder, the dynamic selector, the bag filter, the storage devices, the preheating device, the cooling device, the fossil energy heat treatment device and the other elements of the industrial assembly may be of any type known to those skilled in the art and suitable for the treatment of the clay described above. They will not be described in more detail here.

[0159] Thanks to the method according to the invention, the clay is heated at least partially without producing carbon dioxide.

[0160] Subject to the use of carbon-free electrical energy, the use of this kiln technology will make it possible to considerably reduce CO2 emissions from the calcination of calcined clays. Depending on the process used (100% microwave or hybrid), CO2 emissions per tonne of calcined clay will be between 10 and 80 kg of CO2 equivalent per tonne, i.e. a minimum reduction of 40% compared to the technology emitting the least CO2 to date, flash technology.

[0161] According to one embodiment of the method according to the invention, a microwave oven 2 is used as shown in Figures 13 to 19. This oven 2 can be used for example in one of the industrial assemblies 300; 400; 500; 600 described previously and shown in Figures 1 to 4.

[0162] The furnace can allow continuous heat treatment of the materials introduced into this furnace. During such continuous treatment, the heat-treated material is extracted from the furnace over a time range of more than 4 hours, without interruption of the flow of calcined clay for more than 1 minute in this time range.

[0163] As shown in Figures 13 and 14, the furnace 2 comprises in particular the rotating cylindrical heating body 4 which is centered on the longitudinal axis XX positioned so as to form an angle δ of between 0.1 and 10° relative to the horizontal.

[0164] The heating body 4 is a tube made for example of refractory steel and which is open at each of its longitudinal ends. It is provided with an external layer of thermal insulation 6 wrapped around its external diameter. Different design configurations of the heating body will be described later in connection with Figures 17 to 19.

[0165] At each of its longitudinal ends, the heating body 4 is provided with an annular disc 7 which extends from the external thermal insulation layer 6 and which makes it possible to create a controlled heating volume as a function of its height. Lifting blades 9 positioned inside the heating body make it possible to ensure complete emptying of the heat-treated material by passing it over the annular disc 7 located at the outlet of the heating body 4.

[0166] At its longitudinal ends, the heating body 4 further comprises rolling rings 8 which are mounted around the thermal insulation layer 6 and which rest on rotating rollers 10 so as to be able to rotate the heating body 4 around its longitudinal axis XX at a speed typically between 0.1 and 40 revolutions per minute.

[0167] Of course, other mechanisms for rotating the heating body could be considered (gear motor, rack, etc.).

[0168] The rotating rollers 10 are mounted on a platform 12 adjustable in inclination relative to the horizontal by the angle δ between 0.1 and 10° so as to regulate the speed of advance of the clay introduced into the heating body 4.

[0169] The heating body may also include stirring blades in order to promote and control the advance of the clay in the heating body and to improve the homogenization of the temperature of the clay contained in the heating body. The arrangement and geometry of these stirring blades may be very variable and depend on the material (powder, granulate, dryness at the inlet, etc.). For example, the stirring blades may be arranged in sections in the direction of the longitudinal axis of the furnace or be inclined relative to this axis. The lengths may be identical for all the blades or variable. The profile of the blades may be triangular, straight, inclined or curved for example.

[0170] Thus, according to the method of the invention, the speed of advance of the clay in the heating body is controlled. Temperature homogeneity in the clay contained in the heating body is also ensured.

[0171] The furnace 2 also comprises two fixed covers (or flanges) 14 which are mounted on each longitudinal end of the heating body with the interposition of annular wave trapping seals 16. These covers 14 are for example made of refractory steel.

[0172] More precisely, each cover 14 is composed of a disc 14a closing one of the longitudinal ends of the heating body and an annular collar 14b partially covering the thermal insulation layer 6 of the heating body.

[0173] The wave trapping seals 16 are positioned between the collar 14b of each cover 14 and the thermal insulation layer 6 of the heating body. They ensure microwave sealing between the fixed covers and the rotating heating body despite thermal stresses and expansions.

[0174] As shown in [Fig. 15], each wave trapping seal 16 comprises at least one pair of discs 18 centered on the longitudinal axis XX of the heating body and fixed on the collar 14b of the cover while extending radially towards the thermal insulation layer 6 of the heating body.

[0175] The discs 18 are made of electrically conductive material and thermally resistant to high temperatures (of the order of 1000°C). For example, they can be made of refractory steel.

[0176] Furthermore, the discs 18 of the same pair are spaced longitudinally from one another so as to form an annular groove 20 having a depth p corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: p = X / 4 + k X with X for the wavelength and k an integer).

[0177] Preferably, each wave trapping joint 16 comprises several pairs of discs 18 (five pairs in number in the example of [Fig. 15]) which are spaced longitudinally from each other by a distance d corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: d = X / 4 + n X with X for the wavelength and n an integer).

[0178] This particular geometry of the wave trapping joints 16 makes it possible to largely cancel any wave which would pass between the collar 14b of the cover 14 and the thermal insulation layer 6 of the heating body. Indeed, a part of the incident wave passes directly along the thermal insulation layer (in a longitudinal direction) and another part is introduced into one of the pairs of discs 18. The length traveled (in a radial direction) by this part of the wave in the groove 20 is such that a round trip of the wave makes it possible to obtain a phase shift of X / 2 with respect to the incident wave, which has the effect of canceling this incident wave. The accumulation of several pairs of discs makes it possible to obtain zero leakage.

[0179] Furthermore, the longitudinal spacing d between the pairs of discs 18 represents a second type of wave trapping (in the longitudinal direction). Indeed, a part of the incident wave escaping under the first pair of discs encountered is reflected by the disc of the next pair on its path and returns towards the first pair of discs with a phase shift of X / 2.

[0180] As shown in Figures 13 and 14, the furnace 2 comprises the slotted waveguide 22 which is mounted inside the heating body 4 extending longitudinally between each end thereof.

[0181] The waveguide 22 is connected at one end to a microwave generator 24a, 24b (or to several microwave generators) and passes longitudinally through the heating body between its two longitudinal ends. It is designed to distribute the microwaves in a regulated manner along the entire length of the heating body directly onto the clay 26 to be treated.

[0182] Typically, the microwave generator 24a, 24b comprises a magnetron having a unit power which can vary between 1kW and 10MW, coupled to a frequency generator which can vary from 200MHz to 4000MHz.

[0183] The waveguide 22 can be produced in a single section or in several sections connected to each other, each section being connected at one end to a microwave generator. Thus, in the exemplary embodiment of [Fig. 13], the waveguide 22 comprises two sections 22a, 22b which are connected to each other and each connected to one of the two microwave generators 24a, 24b.

[0184] Similarly, it is possible to provide a plurality of waveguides which are mounted at inside the heating body and which extend parallel to each other at each end of the heating body, each of these waveguides being able to be produced in one or more sections.

[0185] Furthermore, the waveguide 22 is preferably wrapped with a thermal insulator 28 which is transparent to the microwaves generated by the wave generator. For example, this thermal insulator is made of silica and alumina fibers or quartz.

[0186] The waveguide 22 can also be isolated from dust by protective windows 30 (see [Fig. 14]) which are transparent to the microwaves generated by the wave generator and resistant to high temperatures. These protective windows are for example made of quartz or ceramic and have a thickness of between 5 and 20 mm. They can be fixed to the waveguide by means of tabs or any other fixing system.

[0187] Alternatively, or in addition to the protective windows, the waveguide can be placed under slight overpressure (relative to the interior of the heating body) by injecting air at one of its longitudinal ends so as to limit the entry of dust.

[0188] Alternatively, a dust cleaning system may be provided comprising a perforated cylinder placed on an upper face of the waveguide. The cleaning phases are carried out by injecting pulsed air.

[0189] In addition, as shown in [Fig. 16], each waveguide 22 comprises a plurality of slots 32 which are positioned opposite the clay 26 introduced into the heating body.

[0190] In the embodiment of [Fig. 16], the slots 32 of the waveguide have a substantially rectangular shape whose length is aligned with the longitudinal axis A of the waveguide. In addition, the slots 32 of this embodiment are arranged on either side of the longitudinal axis A of the waveguide.

[0191] Generally speaking, the arrangement and geometry of the slots 32 of the waveguide are designed on the one hand to be compatible with the frequency of microwaves used, and on the other hand so that the heating of the clay is carried out optimally with one or more specific heating zones depending on the desired temperature profile.

[0192] For example, it is possible to provide an upstream temperature rise zone and a downstream zone for maintaining a target temperature. In this example, in the upstream temperature rise zone (see [Fig. 16]), the slots 32 of the waveguide are for example spaced, on the one hand longitudinally from each other by a distance e corresponding to half the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: e = X / 2 + m X with X for the wavelength and m an integer), and on the other hand transversely to the longitudinal axis A of the waveguide by a distance h.

[0193] Still in this example with two specific heating zones, the slots of the waveguide corresponding to the downstream temperature maintenance zone (not shown in the figures) are spaced longitudinally from each other by a distance different from the distance e, and / or transversely from the longitudinal axis A of the waveguide by a distance different from the distance h.

[0194] Furthermore, the slot 32a which is furthest downstream of the waveguide is located at a distance f from the downstream end F of the waveguide which is equal to a quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: f = X / 4 + p X with X for the wavelength and p an integer).

[0195] Similarly, the slot 32b which is the most upstream of the waveguide is located at a distance g from the upstream end F' of the waveguide which is equal to half the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: g = Ul + q X with X for the wavelength and q an integer).

[0196] The furnace 2 used in the method according to the invention also comprises the clay supply system 34 which passes through the upstream cover 14 and which opens into the heating body 4 at the first longitudinal end (or upstream end) thereof.

[0197] Alternatively, the feed can be carried out by gravity using tubes whose diameter and length are determined so as not to present wave leaks.

[0198] For products in powder form, a feed system using a metal lock valve type valve will be chosen to regulate the flow rate, prevent microwave leaks and prevent the heating element from being exposed to air.

[0199] For granular products with a larger particle size, it is preferable to choose to feed the furnace in bursts via an inlet airlock which can be made by two consecutive guillotine valves. This inlet airlock guarantees a seal against waves and air.

[0200] Similarly, the furnace comprises the calcined clay discharge system 36 which passes through the downstream cover 14 and which opens into the heating body 4 at a second longitudinal end (or downstream end) thereof opposite the first longitudinal end. This discharge system 36 can be coupled to a flow control valve 38 and to a probe 40 for measuring the temperature inside the heating body.

[0201] The furnace 2 also comprises the gas evacuation device 42 which here comprises a smoke evacuation tube. This tube can also be used for the injection of gas inside the heating body.

[0202] In connection with figures 17 to 19, we will now describe different possible configurations for the production of the heating body of the furnace.

[0203] In the embodiment of [Fig. 17], the heating body 4-1 comprises in particular an internal tube 44 which is composed of a plurality of angular plates 46 made of refractory steel and distributed angularly around the longitudinal axis XX of the heating body.

[0204] The plates 46 overlap two by two in the circumferential direction, each plate being able to slide tangentially on the two plates which are directly adjacent to it in order to allow the internal tube 44 to be able to absorb thermal expansions.

[0205] The heating body 4-1 also comprises an outer tube 48 which is arranged around the inner tube 44 and is coaxial therewith. Studs 50 extending in radial directions make it possible to connect the outer tube 48 to the plates of the inner tube 44.

[0206] Finally, a thermal insulator 52 is positioned in the annular space formed between the inner tube 44 and the outer tube 48.

[0207] In the embodiment of [Fig. 18], the heating body 4-2 comprises an internal tube 54 which is made of refractory steel and which is in one piece, as well as an external tube 48 which is arranged around the internal tube and is coaxial with it.

[0208] In addition, a plurality of spring blades 56 (or braces) extending in directions tangential to the internal tube connect the external tube 48 to the internal tube 54.

[0209] Finally, as for the previous embodiment, a thermal insulator 52 is positioned in the annular space formed between the internal tube 54 and the external tube 48.

[0210] In the embodiment of [Fig. 19], the heating body 4-3 comprises an inner tube 58 which is made of refractory steel, in one piece and split longitudinally to allow it to expand in the circumferential direction. For this purpose, the inner tube 58 has a discontinuity 60, the two angular ends of the inner tube delimiting this discontinuity being connected to each other by a plate 62 screwed onto the inner tube.

[0211] The heating body 4-3 also comprises an outer tube 48 which is arranged around the inner tube 58 while being coaxial therewith, and a thermal insulator 52 positioned in the annular space formed between the inner tube and the outer tube.

[0212] Furthermore, in a known manner, the rotary kiln also comprises devices (not shown in the figures) for gas sealing and for thermal insulation between the heating body and the fixed hoods. Typically, these devices may be thermal fabric seals.

[0213] The present invention is not limited to the embodiments described and shown. on the different figures.

Claims

Claims

1. A method for the industrial production of calcined clays using a microwave oven (2) provided with: - a heating body (4), - at least one microwave generator (24a, 24b) and at least one waveguide (22) extending in a longitudinal direction inside the heating body (4), the method comprising the following steps: a) clay is introduced inside the heating body (4), b) the clay introduced into the heating body (4) is exposed to microwaves emitted by said at least one microwave generator (24a, 24b) and conveyed into the heating body (4) by said at least one waveguide (22), so as to dehydroxylate at least a portion of the introduced clay and produce a calcined clay, the waveguide distributing the microwaves directly onto the clay, c) extracting from the heating body (4) the calcined clay produced.

2. Method according to claim 1, according to which, the heating body (4) of the microwave oven (2) extending along a longitudinal axis (XX) between two opposite longitudinal ends, - the clay is introduced, in step a), at a first of the two longitudinal ends of the heating body (4), - in step b) the clay and / or the calcined clay contained in the heating body is advanced in a direction parallel to the longitudinal axis (XX) of the heating body (4), from the first longitudinal end towards a second longitudinal end - the calcined clay is extracted, in step c), at the second longitudinal end of the heating body.

3. A method according to claim 2, wherein the production of calcined clays is carried out continuously.

4. Method according to one of claims 2 and 3, according to which - said waveguide (22) comprising a slotted waveguide (32) mounted inside the heating body (4), extending in a longitudinal direction parallel to the longitudinal axis (XX) of the heating body (4) and connected at one of its ends (F, F) to said microwave generator (24a, 24b), the slotted waveguide (32) comprising slots positioned opposite the clay in order to direct the microwaves towards the clay, - in step a), the clay is introduced into the heating body (4) by a feed system (34) opening into the heating body (4) at the first of said two longitudinal ends thereof, - in step c), the calcined clay is extracted by an evacuation system (36) opening into the heating body (4) at the second longitudinal end thereof, - during step b), said heating body (4) is rotated around its longitudinal axis (XX), the longitudinal axis (XX) of the heating body (4) is inclined relative to the horizontal so as to regulate the speed of advance of the clay in the heating body (4).

5. Method according to one of claims 1 to 4, according to which the microwaves are trapped inside the oven (2) by means of two fixed covers (14), each cover (14) being mounted on one of the longitudinal ends of the heating body (4) with the interposition of annular wave trapping seals (16).

6. Method according to one of claims 1 to 5, according to which, in step b), the gases released during the heating of the clay are evacuated by a gas evacuation device (42) ensuring a depression of the heating body (4) of the furnace (2) and / or gases are injected inside the heating body (4).

7. Method according to one of claims 1 to 6, according to which, - the oven (2) comprising two microwave generators (24a, 24b) and two corresponding waveguides (22a, 22b) and - the heating body (4) of the oven (2) comprising a first part in which extends a first (22a) of said two waveguides (22) associated with a first (24a) of said two microwave generators and a second part in which extends the second waveguide (22b) associated with the second microwave generator (24b), in step b), the clay successively passes through the first part then the second part of the heating body (4), said first and second parts of the heating body (4) having different temperature profiles.

8. A method according to claim 7, wherein the first microwave generator (24a) and the first waveguide (22a) are configured to produce a gradual increase in temperature along the first portion of the heating body (4) and the second microwave generator (24b) and the second waveguide (22b) are configured to maintain a constant target temperature along the first portion of the heating body (4). along the second part of the heating body (4), in step b), the clay is subjected to the progressive increase in temperature as it advances along the first part of the heating body (4) and is then maintained at the target temperature as it advances along the second part of the heating body (4).

9. Method according to one of claims 1 to 8, according to which in step a), the clay introduced into the heating body (4) of the furnace (2) comprises a single component or several components, among the following components: kaolinite, illites, palygosrkytes, smectites, halloysites, interstratified, micas.

10. Method according to claim 9, according to which, in step a), a material comprising said clay is introduced into the heating body (4), said material comprising between 25 and 75% by weight of one or more of said components.

11. Method according to one of claims 1 to 10, according to which, in step b), the clay is heated to a temperature between 550 and 850 degrees Celsius to carry out the dehydroxylation of at least part of this clay.

12. Method according to one of the preceding claims, according to which the clay is dried using a drying device (120) before its introduction into the heating body (4).

13. Method according to one of the preceding claims, according to which the clay is crushed before its introduction into the heating body (4).

14. Method according to the preceding claim, according to which, after grinding, the clay is filtered and / or stored before its introduction into the heating body (4).

15. Method according to one of claims 13 and 14, according to which, before its introduction into the heating body (4), the clay is preheated in a preheating device (180).

16. Method according to the preceding claim, according to which, before its preheating, the clay is dried using a drying device (120), and according to which the gases heated in the preheating device and / or the gases released during the heating of the clay in the kiln (2) are recirculated to the drying device (120).

17. Method according to one of claims 15 and 16, according to which the gases released during the heating of the clay in the kiln (2) are recirculated to the preheating device (180).

18. Method according to one of claims 12 to 17, according to which, after step c), the calcined clay is calcined again using a fossil-fuelled flash calciner (230).

19. A method according to one of claims 12 to 18, wherein, after step c), the clay is conveyed through a cooling system (210) to be cooled.

20. Method according to claim 19, according to which, before its introduction into the heating body (4), the clay is preheated in a preheating device (180; 185) and according to which the heat of the calcined clay recovered by the cooling system (210) is reused by the preheating device (180; 185).

21. Method according to one of claims 1 to 12, according to which the calcined clay is ground after its extraction from the heating body (4).

22. A method according to claim 21, wherein the ground calcined clay is filtered and / or stored.

23. The method of claim 22, wherein, prior to step a), the clay is preheated by conveying it through a fossil fuel heat treatment device (185).

24. Method according to the preceding claim, according to which, before its preheating, the clay is dried using a drying device (120), and according to which the gases heated in the fossil energy heat treatment device and / or in the furnace (2) are recirculated to the drying device (120).

25. Method according to one of the preceding claims, according to which the heating body (4) is provided internally with stirring blades which extend in an inclined manner relative to the longitudinal axis of the heating body and are integral in rotation with the latter, and, in step b), the stirring blades are rotated with the heating body (4) while inclining the longitudinal axis thereof to advance the clay and / or the calcined clay contained in the heating body (4).

26. A method according to any preceding claim, wherein the clay is calcined without producing carbon dioxide.

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