WO2023242290A1

WO2023242290A1 - Method for the industrial production of synthetic anhydrite

Info

Publication number: WO2023242290A1
Application number: PCT/EP2023/066001
Authority: WO
Inventors: Sébastien Langlois; Philippe Hauza; Frédéric Vandenbussche
Original assignee: Colas
Priority date: 2022-06-17
Filing date: 2023-06-14
Publication date: 2023-12-21
Also published as: FR3136757A1

Abstract

The invention relates to a method for the industrial production of synthetic anhydrite using a microwave furnace (2) provided with: - a heating unit (4), - at least one microwave generator (24a, 24b) and at least one waveguide (22) extending in a longitudinal direction inside the heating unit (4), the method comprising the following steps: a) introducing calcium sulfate hydrate inside the heating unit (4), b) exposing the calcium sulfate hydrate introduced into the heating unit (4) to the microwaves emitted by said at least one microwave generator (24a, 24b) and conveyed in the heating unit (4) by said at least one waveguide (22), so as to dehydrate at least one portion of the calcium sulfate hydrate introduced and produce a synthetic anhydrite, c) extracting from the heating unit (4) the synthetic anhydrite produced.

Description

Title of the invention: Process for the industrial production of artificial anhydrite

The present invention generally relates to the field of artificial anhydrite production.

It relates more particularly to a process for the industrial production of artificial anhydrite using a microwave heat treatment device.

Anhydrite is an anhydrous calcium sulfate chemical compound whose crystallochemical characteristics can vary depending on the temperature range at which it was produced. Thus, we usually distinguish four main crystallographic phases for anhydrite: anhydrite I, anhydrite II or P, anhydrite III or a and anhydrite in its natural form.

Artificial anhydrite is obtained by heating hydrated calcium sulfate, for example natural gypsum, with a view to its dehydration. Natural gypsum is a mineral species composed of calcium sulfate dihydrate, with the chemical formula CaSC 2 H2O. This heating is also called “calcination”.

Anhydrite I is obtained by calcination above 700°C. Anhydrite II is obtained by calcination of gypsum between 350°C and 700°C, ideally 500°C, followed by thermal quenching to stabilize this metastable phase. Anhydrite III is obtained by calcination of gypsum between 200°C and 350°C, ideally 250°C, followed by thermal quenching to stabilize this metastable phase.

We will hereinafter refer to all of these three forms, anhydrite I, II and III under the name “artificial anhydrite”. Figure 1 shows the corresponding phase diagram of anhydrite.

The different forms of anhydrites grouped within the name “artificial anhydrite” are distinguished from each other by their crystal structure and by their difference in solubility in water.

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

Artificial anhydrites are potentially used in a varied field of applications, such as the production of hydraulic binders, plasters, mortars, fluid screeds, etc.

The processes currently used for the production of artificial anhydrite use one of the following heating devices: a rotary kiln using a fossil fuel (gas or oil) with a calcination time of 1 or 2 hours and a flash calciner using a fossil fuel fossil (gas or fuel) with a calcination time of a few seconds. These devices use fossil fuels and the associated production processes cause emissions of combustion gases into the atmosphere, including carbon dioxide CO2 and other air pollutants. On long calcination processes, CO2 emissions per tonne of calcined gypsum are estimated between 100 and 250 kg of CO2 equivalent depending on temperatures and heating times. On the short calcination process with a flash calciner, CO2 emissions per tonne of calcined gypsum are of the order of 50 to 150 kg of CO2 equivalent depending on the heating temperatures used.

The following documents are known concerning such processes: EP0492567 which mentions a process for the industrial production of artificial anhydrites using an oven equipped with a heating body, and CN102519247 which mentions indirect heating of sintered materials by microwaves emitted in a tube ceramic, the sintered materials being outside the tube and being subjected to thermal radiation from the tube.

In order to limit emissions into the atmosphere of combustion gases linked to the use of fossil energy, the present invention proposes a process for the industrial production of artificial anhydrite using a microwave oven provided with a heating body , at least one microwave generator and at least one waveguide extending in a longitudinal direction inside the heating body, said method comprising the following steps: a) sulfate is introduced of hydrated calcium inside the heating body, b) the hydrated calcium sulfate 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 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 body.

Thus, thanks to the process of the invention, the hydrated calcium sulfate is calcined at least partially by an electrically powered microwave oven. Combustion gas emissions are reduced compared to a conventional process.

To dehydrate the calcium sulfate introduced, it is brought to a suitable temperature for a sufficient time to carry out this dehydration. The temperature suitable for dehydrating at least part of the calcium sulfate introduced is for example between 150 and 800 degrees Celsius. The sufficient heating time to dehydrate said at least part of the hydrated calcium sulfate introduced depends on the temperature used, and the composition and shape of the material containing the calcium sulfate used. This sufficient duration is typically between 5 minutes and 240 minutes. Dehydration is also called here “calcination”. 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 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 makes it possible to produce artificial anhydrite continuously.

In practice, the production is called "continuous" because it is possible to carry out step c) of extraction of the artificial anhydrite produced over a time range of more than 4 hours, without interruption of more than 1 minute. In other words, the flow of artificial anhydrite extracted from the microwave oven does not present an interruption of more than one minute during an extraction duration of at least 4 hours.

Other non-limiting and advantageous characteristics of the process according to the invention, taken individually or in all technically possible combinations, are as follows:

- the heating body of the microwave oven extending along a longitudinal axis between two opposite longitudinal ends, the calcium sulfate is introduced, in step a), at a first of the two longitudinal ends of the heating body, in step b) the hydrated calcium sulfate and/or the artificial anhydrite contained in the heating body are advanced in a direction parallel to the longitudinal axis of the heating body, from the first longitudinal end towards a second end longitudinal, and the artificial anhydrite is extracted, in step c), at the second longitudinal end of the heating body; 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 to one of its ends to said microwave generator; in step a), the hydrated calcium sulfate is introduced into the heating body by a supply system opening into the heating body at the first of said two longitudinal ends thereof; in step c), the artificial anhydrite is extracted by an evacuation system opening into the heating body at the second longitudinal end thereof, and during step b), we put in rotating said heating body around its longitudinal axis, the longitudinal axis of the heating body is inclined relative to the horizontal so as to regulate the speed of advance of the hydrated calcium sulfate 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 seals of wave trapping;

- in step b), the gases released during the heating of the hydrated calcium sulfate are evacuated by an evacuation device ensuring a depression of the heating body of the oven and/or gases are injected inside the heater ;

- the oven comprising two microwave generators and two corresponding wave guides and the heating body of the oven comprising a first part in which extends a first of said two wave guides associated with a first of said two microwave generators -waves and a second part in which the second waveguide associated with the second microwave generator extends; in step b), the hydrated calcium sulfate passes successively 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 wave guide being configured so as to produce a progressive increase in the temperature along the first part of the heating body and the second microwave generator and the second guide the waves being configured so as 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 progressive increase in temperature along the first part of the heating body 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 body comprises at least one of the following components: natural gypsum, calcium sulfates of industrial origin, 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 its introduction into the heating body;

- the hydrated calcium sulfate is crushed before its introduction into the heating body;

- after grinding, the hydrated calcium sulfate is filtered and/or stored before its introduction into the heating body;

- before its introduction into the heating body, 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 oven are recirculated to the drying device; - the gases released during heating for the calcination of hydrated calcium sulfate in the oven are recirculated to the preheating device;

- after step c), the artificial anhydrite is calcined again using a fossil fuel flash calciner;

- after step c), the artificial anhydrite is routed through a cooling system to be cooled;

- before its introduction into the heating body, the hydrated calcium sulfate is preheated in a preheating device and according to which the heat of the artificial anhydrite recovered by the cooling system is reused by the preheating device;

- the artificial anhydrite is crushed after its extraction from the heating body; the crushed artificial anhydrite is filtered and/or stored;

- before step a), the hydrated calcium sulfate is preheated by conveying it through a fossil energy heat treatment device;

- before preheating, the hydrated calcium sulfate is dried using a drying device, and according to which the gases heated in the fossil energy heat treatment device and/or in the oven are recirculated towards the heating device drying;

- the heating body being 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 it, and in step b), we put in rotation the stirring blades with the heating body while tilting the longitudinal axis thereof to advance the hydrated calcium sulfate and/or the artificial anhydrite contained in the heating body;

- the hydrated calcium sulfate is calcined without producing carbon dioxide.

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 carried out.

On the attached drawings:

[Fig. 1] is a schematic view of an anhydrite phase diagram,

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

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

[Fig. 4] is a schematic view of a third industrial assembly implementing a third embodiment of the method according to the invention, [Fig. 5] is a schematic view of a fourth industrial assembly implementing a fourth embodiment of the method according to the invention,

[Fig. 6] is a graph representing the variation of the temperature of a sample of gypsum G1 as a function of time when it is calcined in a microwave oven with a set temperature of 250°C for 2 minutes, gypsum G1 being a natural gypsum of granular class 0/10 mm with 12% humidity,

[Fig. 7] is a graph representing the variation in the temperature of a gypsum sample G1 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 the temperature of a gypsum sample G1 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 view in longitudinal section of a microwave oven provided with a rotating cylindrical heating body used for implementing the embodiments of the method shown in Figures 2 to 5,

[Fig. 10] is a sectional view along the plane ll-ll of Figure 9,

[Fig. 11] is a schematic view showing an example of a wave trapping joint of the microwave oven of Figure 9,

[Fig. 12] is a schematic view showing an example of a slotted waveguide of the microwave oven of Figure 9,

[Fig. 13] is a schematic cross-sectional view of a first example of a heating body of the microwave oven of Figure 9,

[Fig. 14] is a schematic cross-sectional view of a second example of the heating body of the microwave oven of Figure 9,

[Fig. 15] is a schematic cross-sectional view of a third example of the heating body of the microwave oven of Figure 9.

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 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. We wish to promote the production of soluble artificial anhydrite, that is to say anhydrite (II) or (III).

[Table 1]

In Figures 2 to 5, four embodiments of an industrial assembly 300 are shown; 400; 500; 600 adapted to implement the method according to the invention.

Each of these industrial units 300; 400; 500; 600 remarkably comprises a microwave oven 2 used according to the process of the invention to produce artificial anhydrite.

This microwave oven 2 is provided with a heating body 4 extending along a longitudinal axis X-X between two opposite longitudinal ends, with at least one microwave generator 24a, 24b and at least one guide of waves 22 extending inside the heating body 4 (see Figures 2 and 9). This heating body 4 is here cylindrical and rotary.

According to the process of the invention, the following steps are carried out: a) hydrated calcium sulfate is introduced inside the heating body, b) the hydrated calcium sulfate introduced into the heating body is exposed to micro- waves emitted by the microwave generator and conveyed into the heating body by the wave guide, so as to dehydrate at least part of the hydrated calcium sulfate introduced and produce an artificial anhydrite, c) extracting from the body of heats the artificial anhydrite produced. 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 to one of its two ends F, F' to said microwave generator 24a, 24b (Figures 1 to 5, 9 and 12). The heating body 4 includes 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 9 to 15.

Step a)

In step a), the hydrated calcium sulfate introduced into the heating body 4 of the oven 2 comprises a single component or several 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, calcium sulfates of industrial origin, phosphogypsum, desulfogypsum, fluorogypsum, borogypsum or a mixture of these components.

With reference to the previous table, the hydrated calcium sulfate introduced into the heating body 4 may contain for example calcium sulfate dihydrate (Gypsum) or calcium sulfate hemihydrate: a-hemihydrate or p-hemihydrate.

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

In practice, in step a), the hydrated calcium sulfate is introduced into the oven 2 using a supply system 34 opening into the heating body 4 at a first of said two longitudinal ends thereof. An example of such a power system will be described later.

In practice, the calcium sulfate is introduced, in step a), at a first of the two longitudinal ends of the heating body.

Step b)

The material introduced into the heating body 4 of the oven 2 is heated by microwaves. This heating allows different simultaneous thermal treatments of the material, in particular its drying, its rise in temperature and its dehydration. Drying here corresponds to superficial dehydration of the material. The dehydration of hydrated calcium sulfate under high temperature is carried out down to the heart of the material, in volume, and will also be called "calcination" in the following.

The hydrated calcium sulfate is preferably heated to a temperature between 150 and 800 degrees Celsius to carry out dehydration by calcination of this compound. Alternatively, the hydrated calcium sulfate is heated to a temperature for example equal to 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750 or 800 degrees Celsius. The hydrated calcium sulfate is preferably heated for a time of between 5 minutes and 240 minutes, which depends on the material and the temperature used.

These heat treatments can cause the release of gas. Dehydration of calcium sulfate produces water vapor.

During heating of the hydrated calcium sulfate, the gases released are evacuated by a gas evacuation device 42 (Figures 2 to 5 and 9) ensuring a depression of the heating body 4 of the oven 2.

The hydrated calcium sulfate is thus heat treated in oven 2 under a “closed” atmosphere.

It is also possible to inject gases inside the heating body 4. This injection can for example be carried out in line with the waveguide in order to avoid the entry of dust into the latter.

Step c)

In step c), the artificial anhydrite produced by the calcination of the hydrated calcium sulfate in step b) exits the heating body 4 of the oven 2.

In practice, the artificial anhydrite leaves the oven 2 via an evacuation system 36 opening into the heating body 4 at a second longitudinal end thereof 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 an evacuation system will be described later.

The advantages of the process according to the invention are numerous. In particular, it allows 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 hydrated calcium sulfate in the oven, and therefore of the calcination temperature. Finally, discharges of combustion gases and associated pollutants are reduced or even eliminated. It should also 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. This is heated indirectly by raising the temperature of the material. As a result, the temperature of the heating body is lower than that observed in conventional fossil energy heat treatment devices. The lifespan of the oven is thus extended.

Generally speaking, 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 successive separate batches. In other words, a first quantity of calcium sulfate is not introduced, heated and calcined in the microwave oven to produce the artificial anhydrite, then completely extracted 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, the production is called "continuous" because it is possible to carry out step c) of extraction of the artificial anhydrite produced over a time range of more than 4 hours, without interruption of more than 1 minute. In other words, the flow of artificial anhydrite extracted from the microwave oven does not present an interruption of more than one minute during an extraction duration 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 for example the case when the supply system of the microwave oven 2 used comprises a metal sluice valve type valve making it possible to regulate the flow rate of hydrated calcium sulfate introduced into the oven, as described later. The supply of hydrated calcium sulfate to the furnace and the extraction of artificial anhydrite from the furnace can then be carried out continuously, without interruption, over said time range.

Alternatively, steps a) and/or c) are carried out at time intervals. This is for example the case when the microwave oven supply system includes an inlet airlock which can be produced by two consecutive gate valves. Feeding is then carried out in jerky fashion. Similarly, the extraction of artificial anhydrite can be carried out by jerking. It is also possible to envisage as a variant that only the feeding or only the extraction is carried out by jerk.

In order to ensure this continuous operation, in step b), the hydrated calcium sulfate and/or the artificial anhydrite contained in the heating body are advanced in a direction parallel to the longitudinal axis XX of the heating body. heater 4, from the first longitudinal end towards the second longitudinal end. This advance of the materials contained in the heating body authorizes the continuous implementation of the method according to the invention.

Preferably, these materials are advanced continuously throughout the duration of the method. For this purpose, the oven has supply means allowing the transport of hydrated calcium sulfate and artificial anhydrite from the first to the second longitudinal end of the heating body of the microwave oven.

Here for example, the heating body 4 is rotated around its inclined longitudinal axis in order to advance the material contained in the oven from the longitudinal end of the heating body to which the supply system introduces the calcium sulfate hydrated towards the longitudinal end of the heating body to which the evacuation system extracts the artificial anhydrite. The supply means therefore comprise a device for rotating the heating body and an inclined support for this heating body.

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

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 an oven. The introduction of hydrated calcium sulfate ceases and the hydrated calcium sulfate introduced is heated, then extracted from the oven during emptying before the introduction of another batch of hydrated calcium sulfate.

Furnace emptying operations lead to production stoppages during which temperatures drop sharply. This discontinuity induces excess energy consumption. In addition, during the production of artificial anhydrite, controlling temperatures and calcination times is essential to control the crystalline form of artificial anhydrite mainly produced and to avoid uncooking and overcooking. Continuous heating therefore has a double advantage: on the one hand an improvement in the quality of the artificial anhydrite produced by standardizing its characteristics, and on the other hand the reduction in the energy consumption of the process. A discontinuous process can nevertheless be considered and implemented using the industrial units described.

Particularly advantageously, in the microwave oven 2 used for the implementation of 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 conveyed 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.

Each waveguide is preferably connected to a microwave generator. However, it is possible to consider connecting each waveguide to several microwave generators in order to increase the power delivered. In addition, during steps a), b) and c), said heating body 4 is rotated around its longitudinal axis X-X and the longitudinal axis X-X of the heating body 4 is tilted relative to the horizontal so as to regulate the speed of advance of the hydrated calcium sulfate then of the artificial anhydrite in the heating body 4 (see figures 2 to 5). This allows the introduced hydrated calcium sulfate and the produced artificial anhydrite to be fed through the microwave oven 2.

As mentioned in the example detailed below, the heating body can 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 it, 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 mixed 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.

In addition, the microwaves are trapped inside the oven 2 using two fixed covers 14, each cover being mounted on one of the longitudinal ends of the heating body 4 with the interposition of annular seals 16 for trapping waves. (figures 9 and 11).

A detailed example of an oven describing the means of implementing these characteristics of the method according to the invention is described below (with reference to Figures 9 to 15).

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 wave guides 22 and the heating body 4 of the oven 2 comprises a first part in which extends a first 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 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 then extend along the same axis parallel to the longitudinal axis X-X of the heating body, as is the case in Figure 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.

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

Of course, it is possible to envisage a number greater than two of wave guides extending in the heating body, each wave guide 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 then the second part of the heating body, said first and second parts of the heating body having different temperature profiles.

In a particularly advantageous manner, it can be provided that the first microwave generator and the first wave guide are configured so as to produce a progressive increase in the temperature along the first part of the heating body and the second microwave generator microwaves and the second wave guide being configured so as 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 progressive increase of the temperature along the first part of the heating body then is maintained at the target temperature along the second part of the heating body. Said first part constitutes an upstream temperature rise zone, while the second part constitutes a downstream zone for maintaining a target temperature.

The gradual increase can bring the calcium sulfate up to said target temperature.

Other temperature profiles can obviously be considered, with two or more than two waveguides defining two or more than two parts of the heating body having different temperature profiles. Furthermore, one can consider using two or more microwave ovens arranged in series to calcine the hydrated calcium sulfate. For example, a first furnace may be used to heat hydrated calcium sulfate with a first temperature profile, for example a temperature rise, and the second furnace may be used to heat hydrated calcium sulfate and/or anhydrite artificial with a second temperature profile, for example maintaining a calcination temperature. The second furnace is powered by the heated hydrated calcium sulfate and/or artificial anhydrite produced in the first furnace and extracted therefrom.

In practice, the process for producing artificial anhydrite includes other steps occurring before or after heating in the microwave oven 2.

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

- the hydrated calcium sulfate is dried using a drying device 120 before its introduction into the heating body 4 (Figures 2 to 5);

- the hydrated calcium sulfate is crushed before its introduction into the heating body 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 bag 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 may comprise a fossil energy heat treatment device 185;

- after step c), the artificial anhydrite produced is calcined again using a flash calciner 230 using fossil fuels (figure 4);

- after step c), the artificial anhydrite produced is routed through a cooling system 210 to be cooled (Figures 2 and 4);

- the artificial anhydrite is crushed 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 storage hopper 220 (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.

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 dehydration of hydrated calcium sulfate by volume or calcination.

Generally speaking, it is further provided that the gases heated in the preheating device when it is used and/or the gases released during the heating of the hydrated calcium sulfate in the oven are recirculated towards the drying device 120.

It can also be provided, when the hydrated calcium sulfate is preheated, that the gases released during heating of the hydrated calcium sulfate in the oven are recirculated 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 of 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, different embodiments of the process according to the invention are described with reference to Figures 2 to 5. In these figures, the transfers of material before and after calcination are represented by arrows in solid lines. Hot gas transfers are represented by dotted lines. The arrow R indicates a supply of fresh air.

According to the first embodiment of the method according to the invention shown in Figure 2, the material supplying the industrial assembly 300 and containing the 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 making it possible to select the particles of material having predetermined particle size characteristics.

Calcium sulfate particles less than 80 microns in size 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 particles of material 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 is placed in depression using 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 passed 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 the hydrated calcium sulfate is introduced into the microwave oven 2. It is introduced into the heating body 4 through the supply system 34 and heated to a temperature setpoint between 200 and 650°C.

The heating body 4 is movable in rotation around its longitudinal axis X-X. It rotates continuously while heating the material. It is supported by a platform 12 adjustable in inclination.

The longitudinal axis X-X 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 gradually 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.

At the exit from oven 2, the calcined material which comprises the artificial anhydrite is conveyed through a cooling tower 210 so as to gradually lower the temperature of the calcined material to a temperature between 50 and 150°C. The cooled calcined material is stored in a storage device 220 such as a hopper.

As is schematically represented in Figure 2, the gases heated in the preheating tower 180 are preferably recirculated towards the drying device 120 in order to reduce the energy consumption of this drying device. These gases here 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 towards the preheating tower 180 and/or or towards 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 of between 150 and 500°C.

According to the second embodiment of the method according to the invention shown in Figure 3, the material supplying the industrial assembly 400 and containing the hydrated calcium sulfate is introduced in the drying device 120 or dryer. This is connected to a chimney 160 allowing the exit of gases released during drying of the material. Drying heats the material to a temperature between 50 and 100°C.

According to this embodiment, the material is not crushed before its introduction into the oven 2.

After drying, the material is introduced into the microwave oven 2. It is introduced into the heating body 4 through the supply 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 from oven 2, the calcined material which comprises the artificial anhydrite is conveyed through a grinder 240. The crushed material passes through a dynamic selector 250 making it possible to select the particles of calcined material having predetermined particle size characteristics.

Particles of artificial anhydrite with sizes less than 80 microns are preferably selected.

The particles of calcined material 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 towards 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 is schematically represented in Figure 3, the gases heated in the microwave oven 2 and evacuated by the gas evacuation device 42 are preferably recirculated towards the drying device 120 in order to reduce its energy consumption.

Depending on the desired flow rate of artificial anhydrite, the electrical power required can be very high. Consequently, 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 continuously rotating kiln using a fossil fuel or a calciner flash also using fossil fuel, to pre-calcinate 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 method according to the invention shown in Figure 4, is identical to the embodiment of Figure 2, except that at the outlet of the microwave oven 2, the material calcined in the oven microwave 2 is introduced into a flash calciner 230 of the industrial assembly 500.

The microwave oven 2 partially calcines the material here before its introduction into the flash calciner 230. The set temperature of the microwave oven is here, for example, set between 150 and 400°C. The flash calciner completes the calcination by bringing the material between 200 and 650°C.

The fourth embodiment of the method according to the invention shown in Figure 5, is similar in every respect to the embodiment of Figure 3, except that the material dried in the drying device 120 is not introduced directly in the microwave oven but in a preheating device. The preheating device here is a classic fossil energy heat treatment device 185 of the industrial assembly 600.

The fossil fuel heat treatment device 185 heats the material before it is introduced into the furnace. It can also carry out partial calcination of hydrated calcium sulfate. The set temperature of the fossil energy heat treatment device 185 is here for example set at a value between 150 and 400°C. The microwave oven 2 carries out or finalizes the calcination of the material at a set temperature of between 200 and 650°C.

Feasibility tests of the hydrated calcium sulfate 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 includes a rotating cylindrical cavity with a diameter of 400 millimeters (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 measured by an infrared pyrometer.

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

A G1 gypsum sample was tested in this laboratory microwave oven. G1 gypsum is a natural gypsum of granular class 0/10 mm with 12% humidity.

The granular class is 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 particle size analysis. This analysis is carried out 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:

[Table 2]

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 represented in Figures 6 to 8 show the curves and levels of temperature rise measured as a function of time for each of the tests carried out. On each of the graphs, the power in watts (w) is mentioned for information purposes only. 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 govern the production of anhydrite III, the most soluble anhydrite.

These curves present similar levels, in particular between 140 and 150°C and which correspond to the transformation of gypsum, that is to say hydrated calcium sulfate, into artificial anhydrite, that is to say, anhydrous calcium sulfate by loss of constitutional water. This loss of water was observed during the test by 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, part of these different tests was compared to tests carried out with the same natural gypsum G1 calcined in a flash calciner at the following temperatures:

[Table 3]

The analysis of artificial anhydrites obtained by calcination of G1 gypsum aims to determine the relative quantities of the different chemical compounds produced, for example the percentage of anhydrite II, III, or even I, the presence of possible traces of untransformed gypsum, as well as their characteristics, for example their crystallographic forms. Such an analysis was carried out to 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 analyzes are presented in table 4.

The 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 (absent with the flash calcination oven)

- Production of anhydrite II at 350°C with the microwave oven (less with the flash calcination oven).

[Table 4]

Generally speaking, the crusher, drying device, crusher, dynamic selector, bag filter storage devices, preheating device, cooling device, fossil energy heat treatment device and others elements of the industrial assembly can be of any type known to those skilled in the art and adapted to the treatment of hydrated calcium sulfate described above. They will not be described in more 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 using carbon-free electrical energy, the process according to the invention will make it possible to considerably 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 tonne, a minimum reduction of 40% compared to the technology emitting the least CO2 at this time, flash technology.

According to one embodiment of the method 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 assemblies 300; 400; 500; 600 described previously and shown in Figures 2 to 5.

The oven can make it possible to carry out continuous heat treatment of the materials introduced into this oven. During such continuous processing, the heat-treated material is extracted from the furnace over a time range of duration greater than 4 hours, without interruption of the flow of artificial anhydrite greater than 1 minute in this time range.

As shown in Figures 9 and 10, the oven 2 notably comprises the rotating cylindrical heating body 4 which is centered on the longitudinal axis X-X positioned so as to form an angle 6 of between 0.1 and 10° relative to the 'horizontal.

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 thermal insulation layer 6 wrapped around its external diameter. Different design configurations of the heating body will be described later in conjunction with Figures 13 to 15.

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 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.

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. heats 4 around its longitudinal axis X-X at a speed typically between 0.1 and 40 revolutions per minute.

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

The rotating rollers 10 are mounted on a platform 12 adjustable in inclination relative to the horizontal of the angle 6 between 0.1 and 10° so as to regulate the speed of advance of the hydrated calcium sulfate introduced into the body heating 4.

The heating body can also include stirring blades in order to promote and control the advance of the hydrated calcium sulfate in the heating body 4 and to improve the homogenization of the temperature of the hydrated calcium sulfate and the artificial anhydrite contained in the heating body. The arrangement and geometry of these stirring blades can be very variable and depend on the material (powder, aggregate, dryness at the inlet, etc.). For example, the stirring blades can be arranged in sections in the direction of the longitudinal axis of the oven or be inclined relative to this axis. Lengths can be the same for all blades or variable. The profile of the blades can be triangular, straight, inclined or curved for example.

Thus, according to the process of the invention, the speed of advance of the hydrated calcium sulfate in the heating body is controlled. We also ensure temperature homogeneity of the hydrated calcium sulfate and the artificial anhydrite contained in the heating body.

The oven 2 also includes two fixed covers (or flanges) 14 which are mounted on each longitudinal end of the heating body with the interposition of annular wave trapping joints 16. These covers 14 are for example made of refractory steel.

More precisely, each cover 14 consists of a disc 14a which closes one of the longitudinal ends of the heating body and an annular collar 14b which partially covers the thermal insulation layer 6 of the heating body.

The wave trapping joints 16 are positioned between the flange 14b of each cover 14 and the thermal insulation layer 6 of the heating body. They ensure microwave tightness between the fixed covers and the rotating heating body despite thermal constraints and expansions.

As shown in Figure 11, each wave trapping joint 16 comprises at least one pair of discs 18 centered on the longitudinal axis X-X of the heating body and fixed on the flange 14b of the cover extending radially towards the layer thermal insulation 6 of the heating body.

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.

Furthermore, the discs 18 of the same pair are spaced longitudinally 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 guide of waves to a multiple of the wavelength (that is to say: p = À/4 + k À with À for the wavelength and k an integer).

Preferably, each wave trapping joint 16 comprises several pairs of discs 18 (five pairs in number in the example of Figure 11) 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 a multiple of the wavelength (that is to say: d = À/4 + n À with À for the wavelength and n an integer).

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

In addition, 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 λ/2.

As shown in Figures 9 and 10, the oven 2 comprises 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 passes longitudinally through the heating body between its two longitudinal ends. It is designed to distribute microwaves in a regulated manner all along the heating body directly onto the hydrated calcium sulfate 26 to be treated.

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.

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 exemplary embodiment of Figure 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.

Likewise, it is possible to provide a plurality of wave guides which are mounted inside the heating body and which extend parallel to each other at each end of the heating body, each of these guides waves that can be produced in one or more sections. Furthermore, the waveguide 22 is preferably enveloped in 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 or quartz fibers.

The waveguide 22 can also be isolated from dust by protective windows 30 (see Figure 10) 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 20mm. They can be fixed to the waveguide using tabs or any other fixing system.

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 into one of its longitudinal ends so as to limit dust entry.

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 injection of pulsed air.

Furthermore, as shown in Figure 12, each waveguide 22 comprises a plurality of slots 32 which are positioned facing the hydrated calcium sulfate 26 introduced into the heating body.

In the exemplary embodiment of Figure 12, 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 exemplary embodiment are arranged on either side of the longitudinal axis A of the waveguide.

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 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 zone to maintain a target temperature. In this example, in the upstream temperature rise zone (see Figure 12), 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 of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength (that is to say: e = À/2 + m À with À for the length d 'waves and m an integer), and on the other hand transversely of 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 of the longitudinal axis A of the waveguide by a distance different from the distance h.

Furthermore, the slot 32a which is the most 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 a multiple of the wavelength (that is to say: f = À/4 + p À with À for the wavelength and p an integer).

Similarly, the slot 32b which is 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 a multiple of the wavelength (that is to say: g = À/2 + q À with À for the wavelength and q an integer).

The oven 2 used in the process according to the invention also comprises the hydrated calcium sulfate 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) of this one.

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

For products in the form of powders, we will choose a supply system using a metal sluice valve type valve making it possible to regulate the flow rate, avoid microwave leaks and prevent the body from being exposed to air. heated.

For granular products of larger particle size, we will preferably choose to feed the oven in jerks via an inlet airlock which can be created by two consecutive guillotine valves. This entrance airlock guarantees airtightness against waves and air.

Likewise, the oven includes the artificial anhydrite evacuation 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 to the first longitudinal end. This evacuation system 36 can be coupled to a flow control valve 38 and to a probe 40 for measuring the temperature inside the heating body (figure 9).

The oven 2 also includes the gas evacuation device 42 which here includes a smoke evacuation tube. This tube can also be used for the injection of gas inside the heating body.

In conjunction with Figures 13 to 15, we will now describe different possible configurations for producing the heating body of the oven. In the embodiment of Figure 9, 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 body heating.

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.

The heating body 4-1 also comprises an external tube 48 which is arranged around the internal tube 44 while being coaxial with it. Studs 50 extending in radial directions make it possible to 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 internal tube 44 and the external tube 48.

In the embodiment of Figure 14, 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 being coaxial.

In addition, a plurality of leaf springs 56 (or struts) extending in directions tangential to the internal tube connect the external tube 48 to the internal tube 54.

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.

In the embodiment of Figure 15, the heating body 4-3 comprises an internal tube 58 which is made of refractory steel, one-piece and split longitudinally to allow it to expand in the circumferential direction. For this purpose, the internal tube 58 has a discontinuity 60, the two angular ends of the internal tube delimiting this discontinuity being connected to each other by a plate 62 screwed onto the internal tube.

The heating body 4-3 also comprises an external tube 48 which is arranged around the internal tube 58 while being coaxial with it, and a thermal insulator 52 positioned in the annular space formed between the internal tube and the external tube.

Furthermore, in known manner, the rotary oven also includes devices (not shown in the figures) for gas tightness and for thermal insulation between the heating body and the fixed covers. Typically, these devices may be thermal fabric seals.

The present invention is not limited to the embodiments described and represented in the various figures.

Claims

Tl Claims

1. Process for the industrial production of artificial anhydrite using a microwave oven (2) equipped 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) hydrated calcium sulfate is introduced inside the heating body (4), b) the hydrated calcium sulfate introduced into the heating body (4) is exposed to the 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 dehydrate at least part of the hydrated calcium sulfate introduces and produces an artificial anhydrite, the waveguide distributing the microwaves directly to the hydrated calcium sulfate, c) the artificial anhydrite produced is extracted from the heating body (4).

2. Method according to claim 1, according to which, the heating body (4) of the microwave oven (2) extending along a longitudinal axis (X-X) between two opposite longitudinal ends,

- the calcium sulfate is introduced, in step a), at a first of the two longitudinal ends of the heating body (4),

- in step b) the hydrated calcium sulfate and/or artificial anhydrite contained in the heating body are advanced in a direction parallel to the longitudinal axis (X-X) of the heating body (4), from the first longitudinal end towards a second longitudinal end

- the artificial anhydrite is extracted, in step c), at the second longitudinal end of the heating body.

3. Method according to claim 2, according to which the production of artificial anhydrite 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 (X-X) of the body heating (4) and connected at one of its ends (F-F) to said microwave generator (24a, 24b), the waveguide having slots positioned facing the hydrated calcium sulfate,

- in step a), the hydrated calcium sulfate is introduced into the heating body (4) by a supply system (34) opening into the heating body (4) at the first of said two longitudinal ends of that -this,

- in step c), the artificial anhydrite 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 hydrated calcium sulfate in the heating body (4).

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

6. Method according to one of claims 1 to 5, according to which, in step b), the gases released during the heating of the hydrated calcium sulfate are evacuated by an evacuation device (42) ensuring a depression of the heating body (4) of the oven (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 wave guides (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 associated with a first (24a) of said two microwave generators and a second part in which the second wave guide (22b) associated with the second microwave generator (24b) extends, in step b), the hydrated calcium sulfate passes successively through the first part then the second part of the body heating body (4), said first and second parts of the heating body (4) having different temperature profiles.

8. Method according to claim 7, wherein the first microwave generator (24a) and the first waveguide (22a) are configured so as to produce a progressive increase in temperature along the first part of the body heating and the second microwave generator (24b) and the second waveguide (22b) being configured so as 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 then is maintained at the target temperature along the second part of the heating body.

9. Method according to one of claims 1 to 8, according to which, in step a), the hydrated calcium sulfate introduced into the heating body comprises at least one of the following components: natural gypsum, sulfates calcium of industrial origin, phosphogypsum, desulfogypsum, fluorogypsum, borogypsum or a mixture of these components.

10. Method according to one of claims 1 to 9, according to which in step b) the hydrated calcium sulfate is heated to a temperature between 150 and 800 degrees Celsius to carry out the dehydration of at least part of this hydrated calcium sulfate.

11. Method according to one of the preceding claims, according to which the hydrated calcium sulfate is dried using a drying device (120) before its introduction into the heating body.

12. Method according to one of the preceding claims, according to which the hydrated calcium sulfate is crushed before its introduction into the heating body.

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

14. Method according to one of claims 1 to 13, according to which, before its introduction into the heating body, the hydrated calcium sulfate is preheated in a preheating device (180; 185).

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

16. Method according to one of claims 14 and 15, according to which the gases released during heating of the hydrated calcium sulfate in the oven are recirculated towards the preheating device (120).

17. Method according to one of claims 12 to 16, according to which, after step c), the artificial anhydrite is calcined again using a flash calciner (230) with fossil energy.

18. Method according to one of claims 12 to 17, according to which, after step c), the artificial anhydrite is conveyed through a cooling system (210) to be cooled.

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

20. Method according to one of claims 1 to 11, according to which the artificial anhydrite is crushed after its extraction from the heating body.

21. Method according to claim 20, according to which the crushed artificial anhydrite is filtered and/or stored.

22. Method according to claim 21, according to which, before step a), the hydrated calcium sulfate is preheated by conveying it through a heat treatment device (185) using fossil energy.

23. Method according to the preceding claim, according to which, before preheating, the hydrated calcium sulfate is dried using a drying device (120), and according to which the gases heated in the energy heat treatment device fossil (185) and/or in the oven (2) are recirculated to the drying device (120).

24. Method according to one of the preceding claims, according to which, the heating body (4) being 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 tilting the longitudinal axis thereof to advance the hydrated calcium sulfate and /or the artificial anhydrite contained in the heating body (4).

25. Method according to one of the preceding claims, according to which the hydrated calcium sulfate is calcined without producing carbon dioxide.