WO2023242288A1

WO2023242288A1 - Method for the industrial production of quicklime

Info

Publication number: WO2023242288A1
Application number: PCT/EP2023/065998
Authority: WO
Inventors: Sébastien Langlois; Frédéric Vandenbussche
Original assignee: Colas
Priority date: 2022-06-17
Filing date: 2023-06-14
Publication date: 2023-12-21
Also published as: FR3136758A1

Abstract

Method for the industrial production of quicklime using a microwave furnace (2) equipped with a heating unit (4), at least one microwave generator and at least one waveguide extending in a longitudinal direction inside the heating unit, the method comprising the following steps: a) introducing limestone inside the heating unit, b) exposing the limestone introduced into the heating unit to the microwaves emitted by the microwave generator and transported in the heating unit by the waveguide, so as to produce the quicklime, c) extracting from the heating unit the quicklime produced.

Description

Title of the invention: Process for the industrial production of quicklime

The present invention generally relates to the field of quicklime production.

It relates more particularly to a process for the industrial production of quicklime using a microwave oven.

Lime or calcium oxide is a mineral with the formula CaO.

It is known to produce lime by calcining limestone in different types of fossil fuel kilns. Here, calcination means heating the limestone for a sufficient time and at a temperature suitable for its decarbonation and the production of quicklime.

Lime is used in construction, soil treatment, in the steel and metallurgy industries and in a large number of diverse and varied applications, particularly in the chemical industry.

There are different types of lime, including quicklime and slaked lime.

Quicklime is obtained at the exit of a calcination kiln. The main constituent of quicklime is calcium oxide, which has the formula CaO. It is a very reactive compound in the presence of water which requires precautions for use. It is mainly used in industry and agriculture.

Slaked lime is obtained after the complete reaction of quicklime with water. It consists mainly of calcium hydroxide (CafOH). It is called aerial lime or hydraulic lime according to its capacity to set under water, i.e. its hydraulicity corresponding to its compressive strength (after having set) expressed in MPa or in kg/cm2. Its hydraulicity makes it possible to define different grades intended for different uses.

Air lime (hydrated lime) is obtained by hydrating quicklime from very pure limestone. The higher the calcium oxide content, the “greasier” the lime is. Air lime has been used since Antiquity to make mortars for construction, coatings and whitewashes on walls. It is also used to protect fruit trees, or fight against the rotting of corpses in the event of an epidemic. Hydraulic lime (lean lime) comes from clayey limestone containing a proportion of 10 to 20% clay, which upon calcination produces calcium silicates and aluminates. It sets in a few hours on contact with water, hence its name.

The processes currently used for the calcination of limestone and the manufacture of quicklime use continuously rotating drum kilns using a fossil fuel (gas or fuel oil) or continuous vertical kilns using a fossil fuel (coal or coke). All of these processes use fossil fuels and cause emissions of combustion gases into the atmosphere, including carbon dioxide (CO2) and other air pollutants. The production of quicklime emits large quantities of CO2, coming on the one hand from the decarbonation reaction of the limestone and, on the other hand, from the use of fossil fuels for heating the limestone.

CO2 emissions from quicklime production vary between 800 and 950 kg of CO2 equivalent per tonne of calcined limestone, depending on the type of kiln used. The share linked to the decarbonation of limestone varies between 600 and 700 kg of CO2 equivalent per tonne. The share linked to the use of fossil energy varies between 200 and 250 kg of CO2 equivalent per tonne of calcined limestone.

The following documents are known concerning such processes: CN102745914 which mentions a process for the industrial production of calcined lime using a furnace equipped with a heating body, and CN102519247 which mentions indirect heating of sintered materials by microwaves emitted in a ceramic tube, 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 quicklime using a microwave oven provided with a heating body, at least one microwave generator and at least one wave guide extending in a longitudinal direction inside the heating body, the process comprising the following steps: a) limestone is introduced into inside the heating body, b) the limestone introduced into the heating body is exposed to the microwaves emitted by the microwave generator and conveyed into the heating body by the wave guide, so as to produce quicklime, c) the quicklime produced is extracted from the heating body.

Thus, thanks to the process according to the invention, the limestone is heated at least partially by an electrically powered microwave oven. Combustion gas emissions are reduced compared to a conventional process.

Thus, CO2 emissions linked to the production of quicklime can be reduced by around 20% to 25%.

The other advantage of using a microwave oven is to avoid mixing combustion gases with the CO2 emitted by decarbonation. The reprocessing of these gases is made easier. 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 limestone calcination temperature is particularly well controlled, which ensures satisfactory and uniform properties for the quicklime produced by this process.

To decarbonate the limestone introduced, the limestone is brought to a suitable temperature, for a sufficient time to carry out this chemical reaction. The temperature suitable for decarbonating at least part of the limestone introduced is for example between 900 and 1200 degrees Celsius. The sufficient heating duration to dehydroxylate said at least part of the limestone introduced depends on the temperature used, the composition of the material containing the limestone used and its shape, for example the size of the limestone particles. This sufficient duration is typically between 150 and 300 minutes.

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 limestone is introduced, in step a), at a first of the two longitudinal ends of the heating body, at step b) the limestone and/or quicklime contained in the heating body is advanced in a direction parallel to the longitudinal axis of the heating body, from the first longitudinal end towards a second longitudinal end and the quicklime is extracted, in step c), at the second longitudinal end of the heating body; the production of quicklime is carried out continuously;

- said waveguide comprising a slotted waveguide mounted inside the heating body and connected at one of its ends to said microwave generator, in step a), the limestone is introduced in 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 quicklime is extracted by an evacuation system opening in the heating body at the second longitudinal end thereof, and during step b), said heating body is rotated around its longitudinal axis, the longitudinal axis of the heating body is inclined by relation to the horizontal so as to regulate the speed of advance of the limestone in the heating body;

- the microwaves are trapped inside the oven using two fixed covers, each cover being mounted on one of the longitudinal ends of the heating body with the interposition of annular wave trapping seals;

- in step b), the gases released during the heating of the limestone are evacuated by an evacuation device ensuring a depression of the heating body of the oven and/or gases are injected inside the heating body ; - the oven comprising two microwave generators and two corresponding wave guides and the heating body of the oven comprising a first part of the heating body in which extends a first of said two wave guides associated with a first of said two microwave generators and a second part of the heating body in which the second wave guide associated with the second microwave generator extends, in step b), the limestone successively passes through the first part of the heating body 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 waves being configured so as to maintain a constant target temperature along the second part of the heating body, in step b) the limestone is subjected to the progressive increase in temperature along the first part of the heating body heats and maintains at the target temperature along the second part of the heating body;

- in step a), the limestone introduced into the heating body is crushed and contains a proportion of calcium carbonate varying between 60% and 95%;

- in step b) the limestone is heated to a temperature between 900 and 1200 degrees Celsius; the limestone is crushed before its introduction into the heating body;

- before its introduction into the heating body, the limestone is preheated in a preheating device;

- the gases released during heating of the limestone in the oven are recirculated to the preheating device;

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

- before its introduction into the heating body, the limestone is preheated in a preheating device and according to which the heat of the quicklime recovered by the cooling system is reused by the preheating device;

- before step a), the limestone is preheated by routing it through a fossil energy heat treatment device;

- before preheating in the fossil energy heat treatment device, another preheating is carried out in a preheating device different from the fossil energy heat treatment device, and according to which the gases heated in the fossil energy heat treatment device and /or in the oven are recirculated towards the preheating device;

- the heating body being provided internally with stirring blades which extend so inclined relative to the longitudinal axis of the heating body and are integral in rotation with it, in step b), the stirring blades are rotated with the heating body while tilting the longitudinal axis of this to advance the limestone and/or quicklime contained in the heating body);

- the limestone is heated without producing carbon dioxide other than that produced by the decarbonation reaction of the limestone.

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 a first industrial assembly implementing a first embodiment of the method according to the invention,

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

[Fig. 3] 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 1 and 2,

[Fig. 4] is a sectional view along the plane ll-ll of Figure 3,

[Fig. 5] is a schematic view showing an example of a wave trapping joint of the microwave oven of Figure 3,

[Fig. 6] is a schematic view showing an example of a slotted waveguide of the microwave oven of Figure 3,

[Fig. 7] is a schematic cross-sectional view of a first example of a heating body of the microwave oven of Figure 3,

[Fig. 8] is a schematic cross-sectional view of a second example of a heating body of the microwave oven of Figure 3,

[Fig. 9] is a schematic cross-sectional view of a third example of the heating body of the microwave oven of Figure 3.

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 implementing the process according to the invention. Preliminarily, it will be noted that, from one figure to another, the identical or similar elements of the different examples of implementation of the invention will be referenced by the same reference signs and will not be described each time.

The invention relates to a process for the industrial production of quicklime.

In Figures 1 and 2, two embodiments of an industrial assembly 300 are shown; 400; suitable for implementing the method according to the invention.

Each of these industrial units 300; 400 remarkably comprises a microwave oven 2 used according to the process of the invention to calcine at least partially the limestone treated according to this process.

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 and with at least one guide. waves 22 extending in a longitudinal direction inside the heating body 4 (see Figures 1 to 4). This heating body 4 is here cylindrical and rotary.

According to the method of the invention, the following steps are carried out: a) limestone is introduced inside the heating body 4, b) the limestone introduced into the heating body 4 is exposed to the microwaves emitted by the microwave generator 24a and conveyed into the heating body by the wave guide 22, so as to produce the quicklime, c) the quicklime produced is extracted from the heating body.

Quicklime is the name for calcium oxide with the chemical formula CaO. Limestone contains calcium carbonate with the chemical formula CaCOs. Calcination, that is to say heating to high temperatures, of the calcium carbonate contained in limestone produces calcium oxide or quicklime by decarbonation.

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 and 13).

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 4 to 9.

Step a) In step a), the limestone introduced into the heating body 4 of the oven 2 may comprise a single component or several components. These components have different chemical compositions and/or different crystallographic structures.

Limestone is a sedimentary rock composed mainly of calcium carbonate with the chemical formula CaCOs.

In particular, the limestone used preferably comprises at least 60% calcium carbonate, for example between 60% and 95% calcium carbonate.

The limestone is introduced into the heating body 4 of the oven 2 preferably in divided form. For this purpose, the limestone is for example crushed before its introduction into the oven 2. The size of the limestone particles introduced into the oven is preferably less than 50 millimeters (mm).

For example, we use limestone with a granular class of 20/50 mm. The limestone can also be ground more finely before its introduction into the heating body, for example a granular class 0/80 microns.

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 for example using a sieve column or a laser particle size analyzer. In practice, in step a), the limestone 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. The limestone is thus introduced at a first of the longitudinal ends of the heating body. An example of such a power system will be described later.

Step b)

The material introduced into the heating body 4 of the oven 2 is heated by microwaves. This heating allows its calcination, that is to say here its decarbonation at high temperature.

The limestone is preferably heated to a temperature between 900 and 1200 degrees Celsius to carry out decarbonation. Alternatively, the limestone is heated to a temperature for example equal to 900, 950, 1000, 1050, 1100, 1150 or 1200 degrees Celsius. The limestone is preferably heated for a time of between 150 minutes and 300 minutes depending on the material, the temperature and power of microwave generators.

These heat treatments can cause the release of gases, in particular carbon dioxide CO2. During heating of the limestone, the gases released are evacuated by a gas evacuation device 42 (figures 1 and 13) ensuring a depression of the heating body 4 of the oven 2.

The limestone 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 quicklime produced by heating the limestone in step b) exits the heating body 4 of the oven 2, at the second longitudinal end of the heating body.

In practice, the quicklime leaves the oven 2 via a quicklime evacuation system 36 opening into the heating body 4 at a second longitudinal end thereof opposite the first longitudinal end (figure 3) . An example of such 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 limestone in the oven, and therefore the calcination temperature. Finally, emissions of combustion gases and associated pollutants are reduced.

Note 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.

The quicklime production process is preferably carried out continuously

This means that the limescale is not heated in the microwave oven 2 in successive separate batches. In other words, a first quantity of limestone is not introduced, heated in the microwave oven to produce quicklime, then completely extracted from the oven before the introduction of a second quantity of limestone, like this would be the case in a batch process.

After a transient regime corresponding to the initiation of the process, the limestone is introduced, heated and extracted from the furnace continuously. In practice, production is called "continuous" because it is possible to carry out step c) of quicklime extraction over a time range of more than 4 hours without interruption of the quicklime flow for more than 1 minute. .

Thus, the quicklime produced can for example be extracted either without interruption over a period of time greater than 4 hours, or over short time intervals, separated for example from 0 to 1 minutes, for example by 5, 10, 20, 30, 40 or 50 seconds, over this time range.

Preferably, when implementing such a continuous method, 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 includes a metal sluice valve type valve making it possible to regulate the flow of limestone introduced into the oven, as described later. The supply of limestone to the kiln and the extraction of quicklime from the kiln 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 quicklime 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 limestone and/or quicklime contained in the heating body is advanced in a direction parallel to the longitudinal axis X-X of the heating body 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 limestone and quicklime 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 limestone towards the the longitudinal end of the heating body at which the evacuation system extracts the quicklime. The means of delivery include therefore 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 limestone is introduced into the oven. The introduction of limestone stops and the limestone introduced is heated, then extracted from the oven during emptying before the introduction of another batch of limestone.

Furnace emptying operations lead to production stoppages during which temperatures drop sharply. This discontinuity induces excess energy consumption. In addition, during the production of quicklime, controlling temperatures and cooking times is useful to limit undercooking and overcooking and ensure uniform production quality. Continuous calcination therefore presents a double advantage: on the one hand the assurance of constant and uniform quality, and on the other hand the reduction of the energy consumption of the process.

Particularly advantageously, in the microwave oven 2 used for implementing this method, said waveguide 22 comprises a slotted waveguide mounted inside the heating body 4 between the two longitudinal ends thereof and connected to one of these longitudinal ends to said microwave generator 24a. The waveguide preferably extends from one longitudinal end of the heating body to the other. The microwaves are therefore 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 limestone in the heating body 4 (see figures 1 to 4). This helps move the limescale 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 axis longitudinal 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 process 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 2 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 of said two waveguides associated with a first 24a of said two microwave generators and a second part in which extends the second waveguide 22b associated with the second microwave generator 24b (figure 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 limestone successively passes through the first part then the second part of the heating body, said first and second parts of the heating body having different temperature profiles. 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 limestone is subjected to the progressive increase in temperature along of 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 upstream zone can bring the limestone 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.

In addition, one can consider using two or more microwave ovens arranged in series to heat the limestone and ensure its decarbonation. For example, a first oven can be used to heat limestone with a first temperature profile, for example a rise in temperature, and the second oven can be used to heat limestone and/or quicklime with a second temperature profile , for example maintaining a decarbonation temperature. The second kiln is powered by the heated limestone not decarbonated in the first kiln and the quicklime produced in the first kiln.

In practice, the quicklime production process includes other steps occurring before or after the calcination of the limestone in the microwave oven 2.

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

- the limestone is crushed before its introduction into the heating body 4 of the oven 2;

- the limestone is dried before its introduction into the heating body 4 of the oven 2;

- before its introduction into the heating body 4, the limestone is preheated in one or more preheating devices 110, 130, 185 which may include a fossil energy heat treatment device 185 (Figures 1 and 2);

- after step c), the quicklime is routed through a cooling system 210 to be cooled (Figures 1 and 2); - before preheating in the fossil energy heat treatment device 185, another preheating is carried out in a preheating device 110, 130 different from the fossil energy heat treatment device 185 (figure 2),

- before its introduction into the heating body 4, the limestone is preheated in a preheating device 110 and the heat of the quicklime recovered by the cooling system 210 is reused by the preheating device (Figures 1 and 2);

- the temperature of the material present inside the microwave oven 2 is measured, for example using an infrared pyrometer.

Preheating the limestone by the preheating device(s) brings the limestone to a temperature lower than that used during heating for calcination of the limestone. The limestone is, for example, preheated to a temperature between 350 and 500°C.

Generally, it is further provided that the gases heated in the cooling device 210 and/or the gases released during the heating of the limestone in the oven 2 are recirculated towards the preheating device(s) 110 and 130 different from the treatment device. fossil fuel thermal energy 185.

It can also be provided, when the limestone is preheated in a fossil energy heat treatment device 185, that the gases released during the heating of limestone in the fossil energy heat treatment device 185 are recirculated towards the other preheating device(s). 110; 130.

Furthermore, in the case where the quicklime is cooled by passing through a cooling system 210, the heat of the quicklime recovered by the cooling system 210 is reused by the preheating device(s) 110 and 130 different from the device. of fossil energy heat treatment 185. In practice, the gases heated by the quicklime in the cooling device 210 are recirculated towards the preheating device(s) 110 and 130 different from the fossil energy heat treatment device 185.

In the following, two embodiments of the process according to the invention are described with reference to Figures 1 and 2. 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 1, the material supplying the industrial assembly 300 is first conveyed through a device preheating 110 which heats the material to a temperature between 350 and 400°C. The material here is for example crushed limestone with a granular class of 20/50 mm.

The air heated (between 80 and 100°C) in the preheating device 110 is exhausted to the outside through a chimney 120 using a fan 120A.

At the outlet of the preheating device 110, the material containing the limescale is introduced into the microwave oven 2. It is introduced into the heating body 4 through the supply system 34 and heated to a set temperature included between 900 and 1200°C.

The heating body 4 is movable in rotation around its longitudinal axis X-X. It rotates continuously while heating the material.

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 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 X-X.

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

At the exit from oven 2, the calcined material which includes quicklime is conveyed through a cooling device 210 so as to gradually lower the temperature of the calcined material to a temperature between 100 and 120°C. The cooled calcined material is stored in a storage device 220.

The cooling device 210 is for example here a forced air cooler. It is supplied with fresh air (arrow R) by a 230 fan.

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 device 110 in order to reduce its energy consumption. These gases have a temperature between 550 and 600°C.

Depending on the desired quicklime flow rate, 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 185, for example a rotary kiln using a fossil fuel, to preheat the limestone. In this case, it will be a hybrid process. An example of a hybrid process is shown in Figure 2 and described below. Heating by a fossil energy heat treatment device is preferably carried out upstream of the calcination with the microwave oven. This first heating step brings the limestone to a temperature lower than that required for decarbonation, for example between 600 and 650°C. It is preferable to carry out the decarbonation of the limestone with the microwave oven to avoid mixing the carbon dioxide CO2 resulting from the decarbonation with the combustion gases of the fossil energy heat treatment device. The treatment of gases discharged from the microwave oven is simpler than that of gases discharged from the fossil fuel heat treatment device because the gases leaving the microwave oven do not include pollutants and particles from fuel combustion fossil.

According to the second embodiment of the method according to the invention shown in Figure 1, the material supplying the industrial assembly 400 is first conveyed through a first preheating device 110 which heats the material to a temperature between 350 and 400°C. The material here is for example crushed limestone with a granular class of 20/50 mm.

The air heated in the first preheating device 110 (here between 80 and 100°C) is evacuated to the outside through the chimney 120 using the fan 120A, as described in the first embodiment.

At the outlet of the first preheating device 110, the material containing the limestone is introduced into a second preheating device 130 which heats the material to a temperature between 450 and 500°C.

At the outlet of the second preheating device 130, the material containing the limestone is introduced into a third preheating device in the form of a fossil fuel heat treatment device 185 which also heats the material to a temperature between 600 and 650 °C.

At the outlet of the heat treatment device 185, the material containing the limestone is introduced into the microwave oven 2. It is introduced into the heating body 4 through the supply system 34 and heated to a set temperature between 900 and 1200°C.

The microwave oven 2 is identical to that described in the first embodiment.

The cooling device 210 is for example here a forced air cooler. It is supplied with fresh air (arrow R) by a 230 fan. The gases heated (in the example between 550 and 600°C) 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 first preheating device 110 in order to reduce its energy consumption.

The gases heated in the fossil fuel heat treatment device 185 are preferably recirculated to the second preheating device 130 in order to reduce its energy consumption. The second preheating device 130 receiving gases from the fossil energy heat treatment device 185, the gases heated in the second preheating device 130 are filtered by a filter 140 before being evacuated to the outside by a chimney 150 at using a 150A fan. They have a temperature for example between 80 and 100°C.

In general, the filter, the storage devices, the preheating device(s), the fossil energy heat treatment device, the cooling device and the other elements of the industrial assembly can be of any type known to the Skilled in the trade and suitable for the treatment of limestone described above. They will not be described in more detail here.

Thanks to the process according to the invention, the limestone is heated at least partially without producing carbon dioxide.

Provided that carbon-free electrical energy is used, the use of this kiln technology will significantly reduce CO2 emissions from the production of quicklime. Depending on the process used (100% microwave or hybrid), CO2 emissions per tonne of quicklime produced will be between 750 and 850 kgCC, i.e. a reduction of around 15 to 25% compared to existing processes. .

According to one embodiment of the method according to the invention, a microwave oven 2 is used as shown in Figures 3 to 9. This oven 2 can be used for example in one of the industrial assemblies 300; 400 described previously and shown in Figures 1 and 2.

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

As shown in Figures 3 and 4, the oven 2 notably comprises the rotating cylindrical heating body 4 which is centered on the longitudinal axis XX 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 7 to 9.

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 limestone introduced into the heating body 4 .

The heating body can also include stirring blades in order to promote and control the advance of the limestone in the heating body and to improve the homogenization of the temperature of the limestone 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 limestone in the heating body is controlled. We also ensure temperature homogeneity in the limestone 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 5, 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 1200°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 = X/4 + k À with X 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 5) 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 = X/4 + n À with X 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 X/2 relative 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 mirrored 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 3 and 4, 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 limestone 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 3, 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 4) which are transparent to 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 6, each waveguide 22 comprises a plurality of slots 32 which are positioned facing the limestone 26 introduced into the heating body.

In the exemplary embodiment of Figure 6, 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.

In general, the arrangement and geometry of the slots 32 of the wave guide 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 limestone is carried out in a manner optimal 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 6), 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 = X/2 + m À with X 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 = X/4 + p À with X 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 = X/2 + q X with X for the wavelength and q an integer). The oven 2 used in the process according to the invention also comprises the limestone supply system 34 which passes through the upstream cover 14 and which opens into the heating body 4 at the first longitudinal end (or upstream end) thereof .

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 quicklime 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) of the latter opposite 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.

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 7 to 9, we will now describe different possible configurations for producing the heating body of the oven.

In the embodiment of Figure 7, 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 X-X 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 8, 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 9, 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

Claims Process for the industrial production of quicklime using a microwave oven (2) provided with a heating body (4), at least one microwave generator (24a, 24b) and at least one waveguide (22) extending in a longitudinal direction inside the heating body (4), the method comprising the following steps: a) limestone is introduced inside the heating body (4) , b) the limestone introduced into the heating body (4) is exposed to the microwaves emitted by the microwave generator (24a, 24b) and conveyed into the heating body (4) by the wave guide ( 22), so as to produce the quicklime, the waveguide distributing the microwaves directly to the limestone, c) the quicklime produced is extracted from the heating body (4). 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 limestone is introduced, in step a), at a first of the two longitudinal ends of the heating body (4),

- in step b) the limestone and/or quicklime contained in the heating body is 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 quicklime is extracted, in step c), at the second longitudinal end of the heating body. Process according to claim 2, according to which the production of quicklime is carried out continuously. 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) and connected at one of its ends (F, F') to said generator microwave, the wave guide having slots positioned facing the limestone,

- in step a), the limestone 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 thereof,

- in step c), the quicklime 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 axis longitudinal (XX), the longitudinal axis of the heating body (4) is inclined relative to the horizontal so as to regulate the speed of advance of the limestone in the heating body (4). Method according to one of claims 1 to 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 ends longitudinal of the heating body (4) with the interposition of annular wave trapping joints (16). Method according to one of claims 1 to 5, according to which, in step b), the gases released during the heating of the limestone are evacuated by a gas evacuation device (42) ensuring a depression of the body of heats (4) of the oven (2) and/or gases are injected inside the heating body (4). 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 of the heating body (4) in which extends a first of said two waveguides (22a) associated with a first of said two micro-generators waves (24a) and a second part of the heating body (4) in which extends the second wave guide (22b) associated with the second microwave generator (22b), in step b), the limestone successively passes through the first part of the heating body (4) then the second part of the heating body (4), said first and second parts of the heating body (4) having different temperature profiles. Method according to claim 7, according to which 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 heating body (4) 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 (4), at step b) the limestone is subjected to the gradual increase in temperature along the first part of the heating body (4) and maintained at the target temperature along the second part of the heating body (4). Method according to one of claims 1 to 8, according to which in step a), the limestone introduced into the heating body (4) is crushed and contains a proportion of calcium carbonate varying between 60% and 95%. Method according to one of claims 1 to 9, according to which in step b) the limestone is heated to a temperature between 900 and 1200 degrees Celsius. Method according to one of the preceding claims, according to which the limestone is crushed before its introduction into the heating body (4). Method according to one of claims 1 to 11, according to which, before its introduction into the heating body (4), the limestone is preheated in a preheating device (110; 130). Method according to the preceding claim, according to which the gases released during heating of the limestone in the oven (2) are recirculated towards the preheating device (110). Method according to one of claims 1 to 13, according to which, after step c), the quicklime is conveyed through a cooling system (210) to be cooled. Method according to claim 14, according to which, before its introduction into the heating body (4), the limestone is preheated in a preheating device (110) and according to which the heat of the quicklime recovered by the cooling system (210 ) is reused by the preheating device (110). A method according to claim 15, wherein, before step a), the limestone is preheated by conveying it through a fossil fuel heat treatment device (185). Method according to the preceding claim, according to which, before preheating in the fossil energy heat treatment device, another preheating is carried out in a preheating device (110; 130) different from the fossil energy heat treatment device (185), and according to which the gases heated in the fossil fuel heat treatment device (185) and/or in the oven (2) are recirculated to the preheating device (110; 130). 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 this, in step b), the stirring blades are rotated with the heating body (4) while tilting the longitudinal axis of the latter to advance the limestone and/or quicklime contained in the heating body (4) Method according to one of the preceding claims, according to which the limestone is heated without producing carbon dioxide other than that produced by the decarbonation reaction of the limestone.