Industrial process for the production of artificial anhydrite

Abstract

The invention relates to a process for the industrial production of artificial anhydrite 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 in a longitudinal direction inside the heating element (4), the process comprising the following steps: a) hydrated calcium sulfate is introduced into the heating element (4), b) the hydrated calcium sulfate introduced into the heating element (4) is exposed to microwaves emitted by said at least one microwave generator (24a, 24b) and carried into the heating element (4) by said at least one waveguide (22), so as to dehydrate at least a part of the hydrated calcium sulfate introduced and produce an artificial anhydrite, c) the artificial anhydrite produced is extracted from the heating element (4). Figure for the summary: Fig. 2

Description

Description Title of the invention: Industrial production process for anhydrite artificial

[0001] — The present invention relates generally to the field of production of artificial anhydrite.

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

[0003] — Anhydrite is a chemical compound of anhydrous calcium sulfate whose characteristics- Crystallochemical risks can vary depending on the temperature range at in which it was produced. Thus, four types of anhydrite are usually distinguished. Main crystallographic phases: anhydrite I, anhydrite II or P, anhydrite III or a and anhydrite in its natural form.

[0004] = Artificial anhydrite is obtained by heating hydrated calcium sulfate, by An example of natural gypsum, for the purpose of its dehydration. Natural gypsum is a species mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO₄, 2 H, O. This heating process is also called "calcination".

[0005] = Anhydrite I is obtained by calcination above 700°C. Anhydrite IT is obtained by calcining gypsum between 350 °C and 700 °C, ideally 500 °C, followed thermal quenching is used to stabilize this metastable phase. Anhydrite III is obtained by calcining gypsum between 200 °C and 350 °C, ideally 250 °C, followed thermal quenching to stabilize this metastable phase.

[0006] In the following, all three of these forms, anhydrite I, II and III, will be referred to as the term "artificial anhydrite". Figure [1] shows the phase diagram cor- responding to anhydrite.

[0007] — The different forms of anhydrite grouped under the name "anhydrite" "Artificial" ones are distinguished from each other by their crystalline structure and by their difference solubility in water.

[0008] — The solubility of anhydrite III is, for example, almost three times greater than that of anhydrite II at room temperature.

[0009] = Artificial anhydrites are potentially used in a field of various applications, such as the production of hydraulic binders, plasters, of mortars, liquid screeds, etc.

[0010] — The processes currently used for the production of artificial anhydrite use one of the following heating devices: a rotary kiln using a fuel fossil fuel (gas or fuel oil) with a calcination time of 1 or 2 hours and a calciner flash using a fossil fuel (gas or fuel oil) with a calcination time of a few seconds. Because these devices use fossil fuels, the associated production processes generate combustion gas emissions into the atmosphere, including carbon dioxide (CO2) and other air pollutants. For long-burning processes, CO2 emissions per ton of calcined gypsum are estimated at between 100 and 250 kg of CO2 equivalent, depending on the temperatures and heating times. For short-burning processes using a flash calciner, CO2 emissions per ton of calcined gypsum are on the order of 50 to 150 kg of CO2 equivalent, depending on the heating temperatures used. In order to limit the release into the atmosphere of combustion gases related to the use of fossil fuels, the present invention proposes a process for the industrial production of artificial anhydrite using a microwave oven equipped with a heating element, at least one microwave generator and at least one waveguide extending in a longitudinal direction inside the heating element, said process comprising the following steps: a) hydrated calcium sulfate is introduced into the heating element, b) the hydrated calcium sulfate introduced into the heating element is exposed to microwaves emitted by said at least one microwave generator and carried into the heating element by said at least one waveguide, so as to dehydrate at least part of the hydrated calcium sulfate introduced and produce an artificial anhydrite, c) the artificial anhydrite produced is extracted from the heating element. Thus, thanks to the process of the invention, hydrated calcium sulfate is at least partially calcined by an electrically powered microwave oven. Flue gas emissions are reduced compared to a conventional process. To dehydrate the introduced calcium sulfate, it is heated to a suitable temperature for a sufficient duration to achieve this dehydration. The suitable temperature for dehydrating at least a portion of the introduced calcium sulfate is, for example, between 150 and 800 degrees Celsius. The sufficient heating time required to dehydrate this at least a portion of the introduced hydrated calcium sulfate depends on the temperature used, as well as the composition and form of the material containing the calcium sulfate. This sufficient time is typically between 5 and 240 minutes. Dehydration is also referred to here as "calcination." The process according to the invention also has the advantage of concentrating microwaves onto a bed of moving materials to homogenize heating. The heating temperature of hydrated calcium sulfate is particularly well controlled, which ensures satisfactory and uniform properties for the artificial anhydrite. produced by this process. Advantageously, the method according to the invention allows for the continuous production of artificial anhydrite. In practice, production is described as "continuous" because it is possible to carry out step c) of extracting the produced artificial anhydrite over a period of time exceeding 4 hours, without an interruption of more than 1 minute. In other words, the flow of artificial anhydrite extracted from the microwave oven does not have any interruption of more than one minute during an extraction time of at least 4 hours. Other non-limiting and advantageous features of the process according to the invention, taken individually or in all technically possible combinations, are as follows: - the heating element of the microwave oven extending along a longitudinal axis between two opposite longitudinal ends, calcium sulfate is introduced, in step a), at one of the two longitudinal ends of the heating element, in step b) the hydrated calcium sulfate and / or artificial anhydrite contained in the heating element is advanced in a direction parallel to the longitudinal axis of the heating element, from the first longitudinal end towards a second longitudinal end, and the artificial anhydrite is extracted, in step c), at the second longitudinal end of the heating element; - the production of artificial anhydrite 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), hydrated calcium sulfate 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), artificial anhydrite is extracted by a drain system opening into the heating body at the second longitudinal end thereof, and in step b), said heating body is rotated about its longitudinal axis, the longitudinal axis of the heating body is inclined with respect to the horizontal so as to regulate the advance rate of the hydrated calcium sulfate in the heating body; - Microwaves are trapped inside the oven by means of two fixed covers, each cover being mounted on one of the longitudinal ends of the heating body with interposition of annular wave-trapping seals; - in step b), the gases released during the heating of the hydrated calcium sulfate are evacuated by an evacuation device ensuring a vacuum in the heating element of the furnace and / or gases are injected inside the heating element; - the oven comprising two microwave generators and two corresponding waveguides and the heating element 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 hydrated calcium sulfate passes successively through the first part and then the second part of the heating element, said first and second parts of the heating element having different temperature profiles; - the first microwave generator and the first waveguide being configured to produce a gradual increase in temperature along the first part of the heating body and the second microwave generator and the second waveguide being configured to maintain a constant target temperature along the second part of the heating body, in step b) the hydrated calcium sulfate is subjected to the gradual 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; - in step a), the hydrated calcium sulfate introduced into the heating element comprises at least one of the following components; natural gypsum, industrially sourced calcium sulfates, phosphogypsum, desulfogypsum, fluorogypsum, borogypsum or a mixture of these components; - in step b) the hydrated calcium sulfate is heated to a temperature between 150 and 800 degrees Celsius to achieve dehydration of at least part of this hydrated calcium sulfate; - the hydrated calcium sulfate is dried using a drying device before being introduced into the heating element; - the hydrated calcium sulfate is ground before being introduced into the heating element; - after grinding, the hydrated calcium sulfate is filtered and / or stored before being introduced into the heating element; - before its introduction into the heating element, the hydrated calcium sulfate is preheated in a preheating device; - before preheating, the hydrated calcium sulfate 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 hydrated calcium sulfate in the furnace are re-circulated to the drying device; - the gases released during heating for the calcination of hydrated calcium sulfate in the furnace are recirculated to the preheating device; - after step c), the artificial anhydrite is calcined again using a cal- fossil fuel flash cinator; - after step c), the artificial anhydrite is conveyed through a cooling system to be cooled; - before its introduction into the heating element, the hydrated calcium sulfate is preheated in a preheating device and according to which the heat from the artificial anhydrite recovered by the cooling system is reused by the preheating device; - The artificial anhydrite is ground after its extraction from the heating element; - The ground artificial anhydrite is filtered and / or stored; - before step a), the hydrated calcium sulfate is preheated by conveying it through a fossil fuel-powered heat treatment device; - before preheating, the hydrated calcium sulfate is dried using a drying device, whereby the gases heated in the fossil fuel heat treatment device and / or in the furnace are recirculated to the drying device; - the heating element being internally fitted with stirring blades which extend at an angle to the longitudinal axis of the heating element and are rotationally fixed to it, and in step b), the stirring blades are rotated with the heating element while tilting its longitudinal axis to advance the hydrated calcium sulfate and / or artificial anhydrite contained in the heating element; - Hydrated calcium sulfate is calcined without producing carbon dioxide. The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out. Regarding the attached drawings: [Fig.1] is a schematic view of a phase diagram of anhydrite, [Fig.2] is a schematic view of a first industrial assembly implementing a first embodiment of the process according to the invention, [Fig.3] is a schematic view of a second industrial assembly implementing a second embodiment of the process according to the invention, [Fig.4] is a schematic view of a third industrial assembly implementing a third embodiment of the process according to the invention, [Fig. 5] is a schematic view of a fourth industrial assembly implementing a fourth embodiment of the process according to the invention, [Fig.6] is a graph representing the variation of the temperature of a G1 gypsum sample as a function of time when it is calcined in a microwave oven with a set temperature of 250°C for 2 minutes, G1 gypsum being a natural gypsum of granular class 0 / 10 mm with 12% moisture, [Fig.7] is a graph representing the variation in temperature of a G1 gypsum sample as a function of time when it is calcined in a microwave oven with a set temperature of 350°C for 2 minutes, [Fig.8] is a graph representing the variation in temperature of a G1 gypsum sample as a function of time when it is calcined in a microwave oven with a set temperature of 500°C for 2 minutes, [Fig.9] is a schematic longitudinal cross-sectional view of a microwave oven equipped with a rotating cylindrical heating element used for implementing the embodiments of the process shown in Figures 2 to 5, [Fig.10] is a cross-sectional view along plane II-II of [Fig.9], [Fig.11] is a schematic view showing an example of a wave trapping seal from the microwave oven of [Fig.9], [Fig.12] is a schematic view showing an example of a slotted waveguide from the microwave oven of [Fig.9], [Fig.13] is a schematic cross-sectional view of an early example of a microwave oven heating element from [Fig.9], [Fig.14] is a schematic cross-sectional view of a second example of a microwave oven heating element from [Fig.9], [Fig.15] is a schematic cross-sectional view of a third example of a microwave oven heating element from [Fig.9]. In the description that follows, by convention, the terms "upstream" and "downstream" will be used with reference to the direction of the flow of materials introduced into the microwave oven, and more generally with reference to the direction of the flow of materials and then products in the industrial assembly used for the implementation of the process according to the invention. As a preliminary point, it should 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. The invention relates to a process for the industrial production of artificial anhydrite from hydrated calcium sulfate. The following table summarizes the main properties of the compounds in this family. The aim is to promote the production of soluble artificial anhydrite, that is, anhydrite (II) or (IID). [Table 1] 64 | 84 ! 8 rt: rt re Fast Fast i- Ï tent Ï Fast PL Ve | Name Sulphate Sulfate of Sulphate of Anhydrite 1} | Anhydrite or a) | Anhydrite: calcium calcium valiumg. : | moB) dihydrate or hemihydrate | Notation S 2H 8H LA LAUR [AG | Formula CaSO4-2H2O | CaSO4-0.5H2O CaSO4-0.5H2O i Casos Caso4 Caso4 Ï Monokinic Form = Rhombohedral | Rhombohedral i Rhombohedral Ï Orthorhombic | Hexagonal | crystalline î H | Substance 24 63 85 © 127 les | in 'water at ' °Cengl | Average Rate Fast Fast {- | Slow | Fast | solubilization * : : fCnaleurge [5710 5.706 1 i- [- [- | solution in; Figures 2 to 5 show four embodiments of an industrial assembly 300, 400, 500, and 600 adapted to implement the process according to the invention. Each of these industrial assemblies 300, 400, 500, and 600 notably comprises a microwave oven 2 used according to the process of the invention to produce artificial anhydrite. This microwave oven 2 is equipped with a heating element 4 extending along a longitudinal axis XX between two opposite longitudinal ends, at least one microwave generator 24a, 24b and at least one waveguide 22 extending inside the heating element 4 (see figures 2 and 9). This heating element 4 is here cylindrical and rotating. According to the method of the invention, the following steps are carried out: a) Hydrated calcium sulfate is introduced into the heating element, b) the hydrated calcium sulfate introduced into the heating element is exposed to microwaves emitted by the microwave generator and transmitted into the heating element by the waveguide, so as to dehydrate at least part of the hydrated calcium sulfate introduced and produce artificial anhydrite, c) the artificial anhydrite produced is extracted from the heating element. Generally, the waveguide 22 is mounted inside the heating element 4. It extends along a longitudinal direction parallel to the longitudinal axis X-X of the heating element 4 and is connected at one of its two ends F, F' to the microwave generator 24a, 24b (Figures 1 to 5, 9 and 12). The heating element 4 includes an internal cavity that accommodates the materials introduced into it. An example of a microwave oven adapted for implementing the process according to the invention will be described in more detail later with reference to figures 9 to 15. Step a) In step a), the hydrated calcium sulfate introduced into the heating element 4 of the furnace 2 comprises one or more components. These components have different chemical compositions and / or different crystallographic structures. In particular, the hydrated calcium sulfate may contain at least one of the following components: natural gypsum, industrially produced calcium sulfates, phosphogypsum, desulfogypsum, fluorogypsum, borogypsum, or a mixture of these components. Referring to the preceding table, the hydrated calcium sulfate introduced into the heating element 4 may contain, for example, dihydrated calcium sulfate (gypsum) or semihydrated calcium sulfate: a-hemihydrate or b-hemihydrate. The material is preferably introduced into the heating element 4 of the furnace 2 in a divided form, for example, as a powder. The particles of the material are preferably smaller than 80 microns. This divided form may be natural or obtained by grinding or micronization. In practice, at step a), the hydrated calcium sulfate is introduced into the furnace 2 by means of a feed system 34 opening into the heating element 4 at one of its two longitudinal ends. An example of such a feed system will be described later. In practice, calcium sulfate is introduced, in step a), at one of the two longitudinal ends of the heating element. Step b) The material introduced into the heating element 4 of the furnace 2 is heated by microwaves. This heating allows for various simultaneous heat treatments of the material, including drying, heating, and dehydration. Drying here refers to a superficial dehydration of the material. The dehydration of the hydrated calcium sulfate under high temperature is carried out to the core of the material, by volume, and will also be referred to as "calcination" hereafter. Preferably, the hydrated calcium sulfate is heated to a temperature between 150 and 800 degrees Celsius to perform dehydration by calcination of this compound. Alternatively, the hydrated calcium sulfate is heated to a temperature, for example, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800 degrees Celsius.Hydrated calcium sulfate is preferably heated for a time between 5 minutes and 240 minutes, depending on the material and the temperature used. These heat treatments can cause the release of gas. The dehydr- Dilation of calcium sulfate produces water vapor. During the heating of the hydrated calcium sulfate, the gases released are evacuated by a gas evacuation device 42 (figures 2 to S and 9) ensuring a depressurization of the heating body 4 of the furnace 2. The hydrated calcium sulfate is thus heat-treated in furnace 2 under a "closed" atmosphere. It is also possible to inject gases inside the heating element 4. This injection can, for example, be carried out at the waveguide in order to prevent dust from entering it. Step c) In step c), the artificial anhydrite produced by the calcination of the hydrated calcium sulfate in step b) is removed from the heating element 4 of the furnace 2. In practice, the artificial anhydrite is removed from the furnace 2 via a venting system 36 opening into the heating body 4 at a second longitudinal end opposite the first longitudinal end. Thus, the artificial anhydrite is extracted, in step c), at the second longitudinal end of the heating body. An example of a venting system will be described later. The advantages of the process according to the invention are numerous. In particular, it allows for the concentration of microwaves onto a bed of moving materials, thus homogenizing the calcination. It also allows for satisfactory control of the temperature of the hydrated calcium sulfate in the furnace, and therefore of the calcination temperature. Finally, the emissions of combustion gases and associated pollutants are reduced, or even eliminated. It should also be noted that one of the distinctive features of microwave heating is that it heats the core of the material without directly heating the heating element. The heating element is heated indirectly by the rise in the material's temperature. Consequently, the temperature of the heating element is lower than that observed in conventional fossil fuel-based heat treatment devices. This extends the furnace's lifespan. In general, the fossil energy used comes from the combustion of fossil fuels, such as gas, fuel oil, coal or coke. Advantageously, according to the invention, the process for producing artificial anhydrite is carried out continuously. This means that the hydrated calcium sulfate is not heated in the microwave oven in separate, successive batches. In other words, an initial quantity of calcium sulfate is not introduced, heated, and calcined in the microwave oven to produce artificial anhydrite, and then completely removed from the oven before the introduction of a second quantity of hydrated calcium sulfate, as would be the case in a batch process. After a transient regime corresponding to the initiation of the process, the calcium sulfate is introduced into the microwave oven, calcined and extracted from the oven continuously. In practice, production is considered "continuous" because it is possible to carry out step c) of extracting the produced artificial anhydrite over a period of more than 4 hours without an interruption exceeding 1 minute. In other words, the flow of artificial anhydrite extracted from the microwave oven does not experience any interruption of more than one minute during an extraction time of at least 4 hours. Interruptions of 5, 10, 20, 30, 40, or 50 seconds are possible. Preferably, when implementing such a method continuously, steps a), b), and c) can be carried out simultaneously. This is the case, for example, when the microwave oven 2's feed system includes a metal gate valve for regulating the flow of hydrated calcium sulfate introduced into the oven, as described later. The oven's supply of hydrated calcium sulfate and the extraction of artificial anhydrite from the oven can then be carried out continuously, without interruption, over the aforementioned time period. Alternatively, steps a) and / or c) are performed at time intervals. This is the case, for example, when the microwave oven's feed system includes an inlet chamber that can be created by two consecutive guillotine valves. The feed is then delivered in bursts. Similarly, the extraction of artificial anhydrite can be carried out in bursts. Alternatively, we can consider that only feeding or only extraction is carried out by jerk. To ensure continuous operation, in step b), the hydrated calcium sulfate and / or artificial anhydrite contained in the heating element is advanced in a direction parallel to the longitudinal axis XX of the heating element 4, from the first longitudinal end to the second longitudinal end. This advancement of the materials contained in the heating element allows for the continuous implementation of the method according to the invention. Preferably, these materials are advanced continuously throughout the implementation of the method. The oven is equipped for this purpose with feeding means allowing the transport of hydrated calcium sulfate and artificial anhydrite from the first to the second longitudinal end of the microwave oven's heating element. Here, for example, the heating element 4 is rotated around its inclined longitudinal axis in order to advance the material contained in the furnace from the longitudinal end of the heating element, to which the feeding system introduces the hydrated calcium sulfate, towards the longitudinal end of the heating element. The heating element is heated by the extraction system, which extracts the artificial anhydrite. The supply means therefore include a device for rotating the heating element and an inclined support for this heating element. The arrangement of microwave oven 2, an example of which is detailed later, ensures airtightness as well as trapping microwaves inside the heating element. These operations are carried out continuously in order to maintain all production equipment at a stable temperature. The process according to the invention can also be carried out discontinuously: a batch of hydrated calcium sulfate is introduced into a furnace. The introduction of hydrated calcium sulfate ceases, and the introduced hydrated calcium sulfate is heated and then extracted from the furnace during a draining operation before the introduction of another batch of hydrated calcium sulfate. Furnace emptying operations lead to production stoppages during which temperatures drop sharply. This discontinuity results in increased energy consumption. Furthermore, in the production of artificial anhydrite, precise control of temperatures and calcination times is essential to control the crystalline form of the predominantly produced artificial anhydrite and to avoid under- and over-firing. Continuous heating therefore offers a dual advantage: firstly, improved quality of the artificial anhydrite produced through the standardization of its characteristics, and secondly, a reduction in the process's energy consumption. A batch process can nevertheless be considered and implemented using the industrial systems described. Advantageously, in the microwave oven 2 used to implement this process, the waveguide 22 comprises a slotted waveguide mounted inside the heating element 4 between its two longitudinal ends and connected to one of these longitudinal ends of the microwave generator 24a. The waveguide preferably extends from one longitudinal end of the heating element to the other. Microwaves are thus transmitted from one longitudinal end of the heating element to the other. Each microwave generator 24a, 24b is located outside the heating element 4, at one of its longitudinal ends. Each waveguide is preferably connected to a microwave generator. However, it is possible to connect each waveguide to several microwave generators in order to increase the power output. Furthermore, during steps a), b), and c), the heating element 4 is rotated about its longitudinal axis XX, and the longitudinal axis XX of the heating element 4 is inclined relative to the horizontal in order to regulate the advance rate of the hydrated calcium sulfate. artificial anhydrite is introduced into the heating element 4 (see figures 2 to 5). This allows the introduced hydrated calcium sulfate and the produced artificial anhydrite to be advanced through the microwave oven 2. As mentioned in the detailed example below, the heating element can also be fitted with stirring blades (not shown). The stirring blades extend inside the heating element. They extend parallel to the longitudinal axis or at an angle to the longitudinal axis of the heating element. They are rotationally fixed to the heating element, and, in step b), the stirring blades are rotated with the heating element while its longitudinal axis is tilted. The material contained within the heating element is then stirred during the rotation of the heating element. This homogenizes the mixture of materials present in the heating element and may also help to control the advance of this mixture of materials within the heating element. In addition, microwaves are trapped inside the oven 2 by means of two fixed hoods 14, each hood being mounted on one of the longitudinal ends of the heating body 4 with interposition of annular wave-trapping seals 16 (figures 9 and 11). A detailed example of a furnace describing the means of implementing these characteristics of the process according to the invention is described further on (with reference to figures 9 to 15). According to a particular embodiment of the process, 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]). The two waveguides may comprise two independent waveguides, each connected to one of the two microwave generators, or two waveguide sections 22a, 22b 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 then extend along the same axis parallel to the longitudinal axis XX of the heating element, as shown in [Fig. 9]. In this case, one of the two microwave generators 24a, 24b is located at one of the two longitudinal ends of the heating element 4, and the other microwave generator is located at the second longitudinal end of the heating element. Alternatively, two separate waveguides can be provided extending side by side, for example in two directions parallel to the longitudinal axis of the heating element. Of course, one can consider a number greater than two waveguides extending into the heating body, each waveguide being connected to a separate microwave generator or to the same microwave generator. Then, in step b), the hydrated calcium sulfate passes successively through the first part and then the second part of the heating element, the said first and second parts of the heating element having different temperature profiles. Advantageously, it can be provided that, with the first microwave generator and waveguide configured to produce a gradual temperature increase along the first section of the heating element, and the second microwave generator and waveguide configured to maintain a constant target temperature along the second section of the heating element, in step b), the hydrated calcium sulfate is subjected to the gradual temperature increase along the first section of the heating element and then maintained at the target temperature along the second section of the heating element. This first section constitutes an upstream heating zone, while the second section constitutes a downstream maintenance zone at a target temperature. The gradual increase can bring the calcium sulfate up to the said target temperature. Other temperature profiles can obviously be envisaged, with two or more waveguides defining two or more parts of the heating body having different temperature profiles. Furthermore, two or more microwave ovens arranged in series can be used to calcine hydrated calcium sulfate. For example, a first oven can be used to heat the hydrated calcium sulfate with a first temperature profile, such as a ramp-up, and a second oven can be used to heat the hydrated calcium sulfate and / or artificial anhydrite with a second temperature profile, such as maintaining a calcination temperature. The second oven is fed with the heated hydrated calcium sulfate and / or artificial anhydrite produced in and extracted from the first oven. In practice, the process of producing artificial anhydrite involves other steps occurring before or after heating in the microwave oven 2. In general, the process according to the invention may include one or more of the following additional steps: - the hydrated calcium sulfate is dried using a drying device 120 before being introduced into the heating element 4 (figures 2 to 5); - the hydrated calcium sulfate is ground before being introduced into the heating element 4, for example using a grinder 130 (figures 2 and 4); - After grinding, the hydrated calcium sulfate is filtered, for example using a dynamic selector 140 and / or a sleeve filter 150 and / or it is stored in a storage device 170 before carrying out step a) (figures 2 and 4); - before its introduction into the heating body 4, the hydrated calcium sulfate is preheated in a preheating device 180; 185 (figures 2, 4 and 5), the preheating device being able to include a fossil energy heat treatment device 185; - after step c), the artificial anhydrite produced is calcined again using a 230 fossil fuel flash calciner ([Fig.4]); - after step c), the artificial anhydrite produced is conveyed through a cooling system 210 to be cooled (figures 2 and 4); - artificial anhydrite is ground after its extraction from the heating body 4, for example using a grinder 240 (figures 3 and 5); - the crushed artificial anhydrite is filtered, for example through a dynamic selector 250 and / or a bag filter 260 (figures 3 and 5); - the crushed and filtered artificial anhydrite is stored, for example in a 220 storage hopper (figures 2 to 5); - the temperature of the hydrated calcium sulfate of the artificial anhydrite present inside the microwave oven 2 is measured, for example using an infrared pyrometer. The drying of hydrated calcium sulfate by the drying device is a surface drying of hydrated calcium sulfate, carried out at temperatures lower than those used during heating for the dehydration of hydrated calcium sulfate by volume or calcination. In general, it is also expected that the gases heated in the preheating device when it is in use and / or the gases released during the heating of the hydrated calcium sulfate in the furnace are recirculated to the drying device 120. It can also be expected that, when the hydrated calcium sulfate is preheated, the gases released during the heating of the hydrated calcium sulfate in the furnace are re-circulated towards the preheating device 180; 185. Furthermore, in the case where the artificial anhydrite is cooled by passing through a cooling system 210, the heat from the artificial anhydrite recovered by the cooling system 210 is reused by the preheating device 180; 185. In practice, the gases heated by the artificial anhydrite in the cooling device 210 are recirculated to the preheating device. In the following, various embodiments of the process according to the invention are described with reference to Figures 2 to 5. In these figures, material transfers before and after calcination are represented by solid arrows. The transfers of Hot gases are represented by dotted lines. Arrow R indicates a supply of fresh air. According to the first embodiment of the process according to the invention shown in [Fig.2], the material feeding the industrial assembly 300 and containing hydrated calcium sulfate is introduced into a drying device 120 or dryer. The material is then conveyed through the crusher 130. The crushed material passes through the dynamic selector 140, allowing the selection of material particles with predetermined particle size characteristics. Particles of calcium sulfate smaller than 80 microns are preferably selected. The particle size characteristics of the calcium sulfate are determined here, for example, using a sieve column or a laser particle size analyzer. 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 whole assembly is brought under negative pressure by means of a fan. The material particles are stored in a storage device 170, for example in a hopper. The material stored in the storage device 170 is then conveyed through a preheating tower 180 which gradually heats the material to a temperature between 50 and 400°C. At the exit of the preheating tower, the material containing hydrated calcium sulfate is introduced into the microwave oven 2. It is introduced into the heating element 4 through the feed system 34 and heated to a set temperature between 200 and 650°C. The heating element 4 is mobile in rotation around its longitudinal axis XX. It rotates continuously during the heating of the material. It is supported by a platform 12 that is adjustable in inclination. The longitudinal axis XX of the heating body 4 being inclined with respect 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. An example of a microwave oven 2 will be described in more detail later. Upon exiting furnace 2, the calcined material, which includes artificial anhydrite, is conveyed through a cooling tower 210 to progressively lower its temperature to between 50 and 150 °C. The cooled calcined material is then stored in a storage unit 220. such as a hopper. As schematically represented in [Fig. 2], the gases heated in the preheating tower 180 are preferably recirculated to the drying unit 120 in order to reduce the energy consumption of this drying unit. These gases have a temperature between 100 and 250°C. Similarly, the gases heated in the cooling device 210 and / or the gases heated in the microwave oven 2 and evacuated by the gas evacuation 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. These gases here have a temperature between 150 and 500°C. According to the second embodiment of the process according to the invention shown in [Fig. 3], the material feeding the industrial assembly 400 and containing hydrated calcium sulfate is introduced 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. The drying process heats the material to a temperature between 50 and 100°C. According to this embodiment, the material is not ground before being introduced into the furnace 2. After drying, the material is introduced into the microwave oven 2. It is introduced into the heating element 4 through the feed system 34 and calcined at a set temperature preferably between 200 and 650°C. As described previously, the material is gradually conveyed to 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. At the exit of furnace 2, the calcined material which includes artificial anhydrite is conveyed through a crusher 240. The crushed material passes through a dynamic selector 250 allowing selection of the calcined material particles having predetermined granulometric characteristics. Preference is given to artificial anhydrite particles with sizes less than 80 microns. The calcined material particles selected in the dynamic selector 250 are recovered and the accompanying hot gases are filtered in a bag filter 260. The hot gases are recirculated to the drying device 120. These gases have a temperature between 150 and 250 °C. The particles of calcined material are stored in the storage device 220, for example in a hopper. As schematically represented in [Fig.3], the gases heated in the microwave oven 2 and evacuated by the gas evacuation device 42 are preferably recirculated to the drying device 120 in order to reduce its energy consumption. Depending on the desired flow rate of artificial anhydrite, the required electrical power can be very high. Therefore, given the locally available capacity on the electrical distribution networks, microwave oven 2 can be combined with a fossil fuel-powered heat treatment device, such as a continuous rotary kiln using fossil fuel or a flash calciner also using fossil fuel, to pre-calcine or complete the calcination of hydrated calcium sulfate. In this case, it will be a hybrid process. Two examples of hybrid processes are shown in Figures 4 and 5 and described below. The third embodiment of the process according to the invention shown in [Fig.4] is identical to the embodiment of [Fig.2], except that at the exit 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. Microwave oven 2 partially calcines the material here before it is introduced into flash calciner 230. The microwave oven's set temperature is, for example, fixed between 150 and 400°C. The flash calciner completes the calcination by heating the material to between 200 and 650°C. The fourth embodiment of the process according to the invention, shown in [Fig. 5], is identical in all respects to the embodiment of [Fig. 3], 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 fuel-powered heat treatment device 185 from the industrial assembly 600. The fossil fuel-powered heat treatment unit 185 heats the material before it is introduced into the furnace. It can also perform partial calcination of hydrated calcium sulfate. The setpoint temperature of the fossil fuel-powered heat treatment unit 185 is, for example, set here to a value between 150 and 400°C. The microwave oven 2 carries out or completes the calcination of the material at a setpoint temperature between 200 and 650°C. Feasibility tests of the process for calcining hydrated calcium sulfate using a microwave oven were carried out on a C400 type laboratory microwave oven with a power rating of 6000 W and a frequency of 2450 MHz. This microwave oven comprises a rotating cylindrical cavity with a diameter of 400 millimeters (mm) and a height of 400 mm. This cavity is driven in rotation by a motor. Variable speed. The temperature of the materials inside the oven is recorded by an infrared pyrometer. Such a microwave oven is described, for example, in document EP2530059. A sample of GI gypsum was tested in this laboratory microwave oven. G1 gypsum is a natural gypsum with a 0 / 10 mm grain size and 12% moisture content. The particle size class is indicated according to the d / D formula, where d is the smallest representative diameter and D is the largest representative diameter of the material grains determined by particle size analysis. This analysis is performed here using a sieve column or a laser particle size analyzer. The tests were carried out according to the experimental protocol summarized in the following table: [Tables 2] |Test / Graph |Sample |Temperature [Calcination Time | Humidity 500°C [2 min 12% rage |a1 |250 °C |? min [12% 6 P MW / figure |G1 Jso°e [pin [12% 7 E3 MW / figure [G1 50°C [emim 12% 8 These feasibility tests were carried out without optimization of the process or the results obtained, starting from the characteristics of the flash calcination process, except for the calcination time, given the different nature of this calcination method. The graphs shown in Figures 6 to 8 illustrate the temperature rise curves and plateaus measured over time for each of the tests performed. The power in watts (W) is indicated on each graph for reference. Figures 6 to 8 also show the cooling times of the different samples after calcination. These different curves correspond to different calcination temperatures: 250, 350 and 500°C which theoretically frame the production of anhydrite III, the most soluble anhydrite. These curves exhibit similar plateaus, particularly between 140 and 150°C, which correspond to the transformation of gypsum, i.e., hydrated calcium sulfate, into artificial anhydrite, i.e., anhydrous calcium sulfate, through the loss of its water content. This water loss was observed during the test as the release of water vapor. These tests therefore confirm that the calcination of gypsum into artificial anhydrite was carried out in the microwave oven. For comparison, some of these different tests were compared to tests carried out with the same natural gypsum G1 calcined in a flash calciner at the following temperatures: [Tables 3] Sample Test [Temperature | Calcination Time | Humidity] 1F Gl 250 °C 2 seconds 12% EF Gl 350 °C 2 seconds 12% The analysis of artificial anhydrites obtained by calcining G1 gypsum aims to determine the relative quantities of the different chemical compounds produced, for example, the percentage of anhydrite II, III, and even I, the presence of any traces of unprocessed gypsum, as well as their characteristics, such as their crystallographic forms. Such an analysis was carried out for the artificial anhydrites produced during each reference test El MW, E2 MW, El F, and E2 F in Tables 2 and 3. The results of these analyses are presented in Table 4. Comparison of the two processes shows better control of dehydration with the microwave oven compared to a flash calcination oven, which is confirmed by the following markers: - No residual gypsum with the microwave oven (traces with the flash calcination oven) - Production of anhydrite III at 250°C with the microwave oven (not possible with the flash calcination oven) - Production of anhydrite II at 350°C with the microwave oven (less with the flash calcination oven). [Tables 4] 05% 34% Reference test EL MW E2MW ELF EZF Thermogravimetric analysis Free water 25°C to 80°C 0.3% 0.0% 0.5% 0.3% Water 630°C to 1000°C 2.2% 2.85% 3.3% 3.8% Gypsum 0% 0% 2% 1% Bassanite 48% 41% 87% 62% (Anhydrite | 386 1 0 8 06 0) ofome ee 0e IE Phyllosilicates and / or micas 0 109% 11% Amorphous or poorly crystallized phases 2% 5% 40% 3% Other 2 4% 4% In general, the lump breaker, drying device, grinder, dynamic separator, baghouse filter, storage devices, preheating device, cooling device, fossil fuel-powered heat treatment device, and other components of the industrial assembly can be of any type known to those skilled in the art and adapted to the processing of hydrated calcium sulfate described above. They will not be described in further detail here. Thanks to the process according to the invention, the hydrated calcium sulfate is heated at least partially without producing carbon dioxide. Subject to the use of decarbonized electrical energy, the process according to the invention will significantly reduce carbon dioxide (CO2) emissions from the calcination of hydrated calcium sulfate. Depending on the embodiment used (100% microwave or hybrid), CO2 emissions per ton of calcined gypsum will be between 10 and 80 kg of CO2 equivalent per ton, representing a minimum reduction of 40% compared to the lowest-emitting technology currently available, namely flash technology. According to one embodiment of the process according to the invention, the microwave oven 2 is used as shown in Figures 9 to 15. This oven 2 can be used for example in one of the industrial sets 300; 400; 500; 600 described previously and shown again in Figures 2 to 5. The furnace can perform continuous heat treatment of the materials introduced into it. During such continuous treatment, the heat-treated material is removed from the furnace over a period of time exceeding 4 hours, without interruption. interruption of the flow of artificial anhydrite for more than 1 minute within this time range. As shown in Figures 9 and 10, the furnace 2 includes in particular the rotating cylindrical heating element 4 which is centered on the longitudinal axis XX positioned so as to form an angle & between 0.1 and 10° with respect to the horizontal. The heating element 4 is a tube made, for example, of refractory steel and open at each of its longitudinal ends. It is fitted with an external layer of thermal insulation 6 wrapped around its outer diameter. Different design configurations of the heating element will be described later in connection with Figures 13 to 15. At each of its longitudinal ends, the heating element 4 is fitted with an annular disc 7 extending from the outer layer of thermal insulation 6, creating a controlled heating volume based on its height. Lifting blades 9 positioned inside the heating element ensure complete drainage of the heat-treated material by passing it over the annular disc 7 located at the outlet of the heating element 4. At its longitudinal ends, the heating element 4 further includes bearing 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 element 4 around its longitudinal axis XX at a speed typically between 0.1 and 40 revolutions per minute. Of course, other mechanisms for rotating the heating element could be considered (gear motor, rack and pinion, etc.). The LO rotary rollers are mounted on a platform 12 adjustable in inclination relative to the horizontal by an angle & between 0.1 and 10° in order to regulate the advance speed of the hydrated calcium sulfate introduced into the heating body 4. The heating element may also include stirring blades to promote and control the advance of the hydrated calcium sulfate within the heating element 4 and to improve the temperature homogenization of the hydrated calcium sulfate and the artificial anhydrite contained within the heating element. The arrangement and geometry of these stirring blades can vary considerably and depend on the material (powder, granules, inlet dryness, etc.). For example, the stirring blades may be arranged in sections along the longitudinal axis of the furnace or be inclined relative to this axis. The lengths may be identical for all blades or vary. The blade profile may be triangular, straight, inclined, or curved, for example. Thus, according to the process of the invention, the feed rate of the hydrated calcium sulfate in the heating element is controlled. Homogeneity of temperature of the hydrated calcium sulfate and artificial anhydrite contained in the heating element. The furnace 2 also includes two fixed hoods (or flanges) 14 which are mounted on each longitudinal end of the heating body with interposition of annular wave-trapping seals 16. These hoods 14 are for example made of refractory steel. More specifically, each hood 14 consists 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. The wave-trapping seals 16 are positioned between the collar 14b of each cover 14 and the thermal insulation layer 6 of the heating element. They ensure a microwave-tight seal between the fixed covers and the rotating heating element despite thermal stresses and expansions. As shown in [Fig.11], each wave trapping joint 16 comprises at least one pair of discs 18 centered on the longitudinal axis XX of the heating body and fixed to the collar 14b of the hood extending radially towards the thermal insulation layer 6 of the heating body. The 18 discs are made of electrically conductive material and are thermally resistant to high temperatures (on the order of 1000°C). For example, they can be made of refractory steel. Furthermore, the disks 18 of the same pair are spaced longitudinally apart from each other 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 a multiple of the wavelength (that is: p =} / 4 + k A with A for the wavelength and k an integer). Preferably, each wave-trapping joint 16 comprises several pairs of disks 18 (five pairs in the example of [Fig.11]) which are spaced longitudinally apart from each other by a distance d corresponding to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: d = 1 / 4 + n A with A for the wavelength and n an integer). This particular geometry of the wave-trapping joints 16 largely cancels any wave that might pass between the collar 14b of the cover 14 and the thermal insulation layer 6 of the heating element. Indeed, part of the incident wave passes directly along the thermal insulation layer (in a longitudinal direction), and another part enters one of the pairs of disks 18. The distance traveled (in a radial direction) by this wave portion in the groove 20 is such that a round trip of the wave results in a phase shift of Δ / 2 per relative to the incident wave, this has the effect of canceling out the incident wave. The combination of several pairs of disks makes it possible to obtain zero leakage. Furthermore, the longitudinal spacing d between the pairs of disks 18 represents a second type of wave trapping (in the longitudinal direction). Indeed, part of the incident wave escaping under the first pair of disks encountered is reflected by the disk of the next pair on its path and returns to the first pair of disks with a phase shift of M2. As shown in Figures 9 and 10, the furnace 2 includes the slotted waveguide 22 which is mounted inside the heating body 4 extending longitudinally between each end thereof. The waveguide 22 is connected at one end to a microwave generator 24a, 24b (or to several microwave generators) and runs longitudinally through the heating element between its two longitudinal ends. It is designed to distribute the microwaves in a controlled manner along the entire length of the heating element directly onto the hydrated calcium sulfate 26 to be treated. Typically, the microwave generator 24a, 24b comprises a magnetron with a unit power that can vary between 1kW and 10MW, coupled to a frequency generator that can vary from 200MHz to 4000MHz. The waveguide 22 can be made 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 embodiment shown in [Fig. 9], 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. Similarly, it is possible to provide for a plurality of waveguides which are mounted 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 made in one or more sections. Furthermore, the waveguide 22 is preferably wrapped with a thermal insulator 28 that is transparent to the microwaves generated by the wave generator. For example, this thermal insulator is made of silica and alumina fibers or quartz. The waveguide 22 can also be isolated from dust by protective windows 30 (see [Fig. 10]) which are transparent to the microwaves generated by the wave generator and resistant to high temperatures. These protective windows are made, for example, of quartz or ceramic and have a thickness of between 5 and 20 mm. They can be attached to the waveguide by means of tabs or any other fastening system. Alternatively, or in addition to protective windows, the guide The wave can be put under slight overpressure (relative to the inside of the heating body) by injecting air at one of its longitudinal ends in order to limit dust entry. Alternatively, a dust cleaning system can be provided, comprising a perforated cylinder placed on the upper surface of the waveguide. The cleaning phases are achieved by injecting pulsed air. Furthermore, as shown in [Fig.12], each waveguide 22 comprises a plurality of slots 32 which are positioned opposite the hydrated calcium sulfate 26 introduced into the heating element. In the embodiment shown in [Fig. 12], the waveguide slots 32 are substantially rectangular in shape, with their length aligned with the longitudinal axis A of the waveguide. Furthermore, the slots 32 in this embodiment are arranged on either side of the longitudinal axis A of the waveguide. In general, the arrangement and geometry of the 32 slots of the waveguide are designed on the one hand to be compatible with the microwave frequency used, and on the other hand to ensure that the heating of the hydrated calcium sulfate is carried out optimally with one or more specific heating zones depending on the desired temperature profile. For example, it is possible to provide an upstream temperature rise zone and a downstream maintenance zone at a target temperature. In this example, in the upstream temperature rise zone (see [Fig.12]), the slots 32 of the waveguide are spaced, for example, 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 a multiple of the wavelength (i.e.: e = 2 + m A with A for the wavelength and m an integer), and on the other hand transversely from the longitudinal axis A of the waveguide by a distance h. 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. Furthermore, the slot 32a, which is furthest downstream of the waveguide, is located at a distance £ from the downstream end F of the waveguide which is equal to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: f = 1 / 4 + p A with A for the wavelength and p an integer). Similarly, the slot 32b, which is furthest upstream of the waveguide, is located at a distance g from the upstream end F' of the waveguide which is equal to the half the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: g = 1 / 2 + q A with A for the wavelength and q an integer). The furnace 2 used in the process according to the invention also includes the hydrated calcium sulfate supply system 34 which passes through the upstream hood 14 and opens into the heating body 4 at its first longitudinal end (or upstream end). Alternatively, the power supply can be achieved by gravity using tubes whose diameter and length are determined so as not to exhibit wave leakage. For products in powder form, a metal gate valve type supply system will be chosen to regulate the flow, prevent microwave leaks and prevent the heating element from being exposed to air. For larger granular products, it is preferable to feed the furnace in bursts through an inlet chamber, which can be created by two consecutive guillotine valves. This inlet chamber ensures a seal against waves and air. Similarly, the furnace includes the artificial anhydrite exhaust system 36, which passes through the downstream hood 14 and opens into the heating element 4 at a second longitudinal end (or downstream end) opposite the first longitudinal end. This exhaust system 36 can be coupled to a flow control valve 38 and a temperature probe 40 inside the heating element ([Fig. 9]). Furnace 2 also includes the gas evacuation device 42, which here comprises a flue gas exhaust tube. This tube can also be used for gas injection into the heating element. In conjunction with figures 13 to 15, we will now describe different possible configurations for the construction of the furnace heating element. In the embodiment of [Fig.9], the heating body 4-1 includes 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 X-X of the heating body. The plates 46 overlap in pairs in the circumferential direction, each plate being able to slide tangentially on the two plates directly adjacent to it in order to allow the inner tube 44 to absorb thermal expansions. The heating element 4-1 also includes an outer tube 48 which is arranged coaxially around the inner tube 44. Studs 50 extending in radial directions connect the outer tube 48 to the plates of the inner tube 44. Finally, a thermal insulator 52 is positioned in the annular space formed between the inner tube 44 and the outer tube 48. In the embodiment of [Fig.14], the heating body 4-2 comprises an inner tube 54 which is made of refractory steel and is monobloc, and an outer tube 48 which is arranged around the inner tube coaxially with it. In addition, a plurality of spring blades 56 (or braces) extending in directions tangential to the inner tube connect the outer tube 48 to the inner tube 54. Finally, as with the previous embodiment, a thermal insulator 52 is positioned in the annular space formed between the inner tube 54 and the outer tube 48. In the embodiment of [Fig. 15], the heating element 4-3 comprises an inner tube 58 made of refractory steel, one-piece and longitudinally slotted to allow it to expand circumferentially. 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. The heating body 4-3 also includes an outer tube 48 which is arranged around the inner tube 58 in a coaxial manner, and a thermal insulator 52 positioned in the annular space formed between the inner tube and the outer tube. Furthermore, as is known, the rotary kiln also includes devices (not shown in the figures) for gas sealing and thermal insulation between the heating element and the fixed covers. Typically, these devices can be thermal fabric gaskets. The present invention is not limited to the embodiments described and represented in the various figures.

Claims

Demands

1. Process for the industrial production of artificial anhydrite using a microwave oven (2) equipped with: - of a heating element (4), - at least one microwave generator (24a, 24b) and at least one waveguide (22) extending along a longitudinal direction at the interior of the heating element (4), the process comprising the steps following: a) Hydrated calcium sulfate is introduced into the body of heater (4), b) the hydrated calcium sulfate introduced into the body is exposed heats (4) by microwaves emitted by said at least one generator of microwaves (24a, 24b) and conveyed into the heating element (4) by said at least one waveguide (22), so as to dehydrate at minus a portion of the hydrated calcium sulfate introduced and produce a artificial anhydrite, , (c) The artificial anhydrite produced is extracted from the heating element (4).

2. A method according to claim 1, wherein the heating element (4) of the microwave oven (2) extending along a longitudinal axis (XX) between two opposite longitudinal ends, - calcium sulfate is introduced, in step a), into the first of the two longitudinal ends of the heating element (4), - in step b) the hydrated calcium sulfate and / or the artificial anhydrite contained in the heating element according to a direction parallel to the longitudinal axis (XX) of the heating element (4), of the first longitudinal end towards a second longitudinal end- student - artificial anhydrite is extracted, in step c), at the second end longitudinal of the heating element.

3. A method according to claim 2, wherein the production of Artificial anhydrite is produced continuously.

4. A method according to any one of claims 2 and 3, according to which - said waveguide (22) comprising a slotted waveguide (32) mounted inside the heating element (4) extending along a longitudinal direction parallel to the longitudinal axis (XX) of the body of heater (4) and connected at one of its ends (FF) to said generator microwave (24a, 24b), - in step a), hydrated calcium sulfate is introduced into the body of heated (4) by a supply system (34) opening into the body heating element (4) at the first of the said two longitudinal ends of this one, - in step c), artificial anhydrite is extracted by a system exhaust vent (36) opening in the heating body (4) at the second longitudinal end thereof, - during step b), the said heating element (4) is rotated around its longitudinal axis (XX), the longitudinal axis is inclined {(XX) of the heating element (4) relative to the horizontal so as to regulate the rate of advance of hydrated calcium sulfate in the body heating (4).

5. A method according to claim 4, in which microwaves are trapped inside the oven (2) by means of two fixed hoods (14), each hood (14) being mounted on one of the longitudinal ends of the body of heating with interposition of annular wave-trapping seals (16).

6. A method according to any one of claims 1 to 5, wherein, in step b), The gases released during the heating of calcium sulfate are vented. hydrated by a drainage device (42) ensuring a de- pressure of the heating element (4) of the furnace (2) and / or gases are injected at the inside of the heating element (4).

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

8. A method according to claim 7, wherein the first generator of microwave (24a) and the first waveguide (22a) being configured of in order to produce a gradual increase in temperature along the first part of the heating element and the second generator microwave (24b) and the second waveguide (22b) being configured to maintain a constant target temperature along the second part of the heating element, in step b) the sulfate of Hydrated calcium is subject to a progressive increase in temperature. temperature along the first part of the heating element then is maintained at the target temperature along the second part of the body heating.

9. A method according to any one of claims 1 to 8, wherein, in step a), hydrated calcium sulfate introduced into the heating element includes at least one of the following components: natural gypsum, industrially sourced calcium sulfates, phosphogypsum, and desulfogypsum, fluorogypsum, borogypsum or a mixture of these components.

10. A method according to any one of claims 1 to 9, wherein in step b) one heats the hydrated calcium sulfate to a temperature between 150 and 800 degrees Celsius to achieve the dehydration of at least one part of this hydrated calcium sulfate.

11. | A method according to any one of the preceding claims, in which one dries the hydrated calcium sulfate using a drying device (120) before its introduction into the heating element.

12. A method according to any one of the preceding claims, in which one grinds the hydrated calcium sulfate before its introduction into the body heating.

13. A method according to the preceding claim, wherein, after grinding, The hydrated calcium sulfate is filtered and / or stored before its in- introduction into the heating body (4).

14. A method according to any one of claims 1 to 13, wherein, before its Introduced into the heating element, hydrated calcium sulfate is preheated in a preheating device (180; 185).

15. Method according to the preceding claim, wherein, before its pre- heating, the hydrated calcium sulfate is dried using a device drying (120), and according to which the gases heated in the device preheating (180; 185) and / or gases released during heating of the hydrated calcium sulfate in the oven is recirculated to the device drying (120).

16. | A method according to any one of claims 14 and 15, wherein the gases released during the heating of hydrated calcium sulfate in the oven are recirculated to the preheating device (120).

17. | A method according to any one of claims 12 to 16, wherein, after Step c), the artificial anhydrite is calcined again using a flash calciner (230) using fossil fuel.

18. A method according to any one of claims 12 to 17, wherein, after Step c), the artificial anhydrite is conveyed through a system of cooling (210) to be cooled.

19. A method according to claim 18, in which, before its introduction In the heating element, the hydrated calcium sulfate is preheated. in a preheating device (180; 185) and according to which the heat artificial anhydrite recovered by the cooling system (210) is reused by the preheating device (180; 185).

20. A method according to any one of claims 1 to 11, wherein the following are ground artificial anhydrite after its extraction from the heating element.

21. | A method according to claim 20, wherein artificial anhydrite The crushed material is filtered and / or stored.

22. A method according to claim 21, wherein, before step a), the Hydrated calcium sulfate is preheated by passing it through a fossil fuel heat treatment device (185).

23. A method according to the preceding claim, in which, before its pre- heating, the hydrated calcium sulfate is dried using a device drying (120), and according to which the gases heated in the device fossil fuel heat treatment (185) and / or in the furnace (2) are recirculated to the drying device (120).

24. A method according to any one of the preceding claims, according to which, the heating element (4) being internally fitted with stirring blades which extend at an angle to the longitudinal axis of the body of heating and are rotationally fixed to it, and in step b), we rotates the stirring blades with the heating element (4) while tilting its longitudinal axis to advance the sulfate of hydrated calcium and / or artificial anhydrite contained in the body of heating (4).

25. | A method according to any one of the preceding claims, in which one calcines hydrated calcium sulfate without producing dioxide carbon.