Industrial process for the production of quicklime

Abstract

A process for the industrial production of quicklime using a microwave furnace (2) equipped with a heating element (4), at least one microwave generator, and at least one waveguide extending longitudinally inside the heating element, the process comprising the following steps: a) limestone is introduced into the heating element; b) the limestone introduced into the heating element is exposed to microwaves emitted by the microwave generator and transmitted into the heating element by the waveguide, so as to produce quicklime; c) the quicklime produced is extracted from the heating element. Figure for the abstract: Fig. 1

Description

Description Title of the invention: Industrial lime production process long live

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

[0002] = It relates more particularly to an industrial lime production process vivid using a microwave oven.

[0003] — Lime or calcium oxide is a mineral with the formula CaO.

[0004] It is known to produce lime by calcining limestone in different types of fossil fuel furnaces. Here, calcination refers to the heating of limestone for a sufficient time and at a suitable temperature for its decarbonation and the production of quicklime.

[0005] — Lime is used in construction, soil treatment, in the steel industry and metallurgy and a large number of diverse and varied applications, including in the chemical industry.

[0006] There are different types of lime, notably quicklime and slaked lime.

[0007] — Quicklime is obtained at the outlet of a calcination kiln. The main A constituent of quicklime is calcium oxide, which has the formula CaO. It is a This compound is highly reactive in the presence of water and requires precautions for use. It is mainly used in industry and agriculture.

[0008] — Slaked lime is obtained after the complete reaction of quicklime with Water. It is mainly composed of calcium hydroxide (Ca(OH)₂). It is called aerial lime or hydraulic lime depending on its ability to set underwater, either its hydraulic properties corresponding to its compressive strength (after having done pressure) expressed in MPa or kg / cm². Its hydraulic properties allow for the definition of different grades intended for different uses.

[0009] = Air lime (hydrated lime) is obtained by hydrating quicklime from of very pure limestone. The higher the calcium oxide content, the more the lime is called "fatty". Aerial lime has been used since Antiquity to make mortars for the construction, plastering and whitewashing on walls. It is also used for protecting fruit trees, or combating the putrefaction of corpses in case of epidemic. Hydraulic lime (lean lime) is derived from clayey limestone containing a proportion of 10 to 20% clay, which during calcination produces calcium silicates and aluminates. It sets within a few hours upon contact with water, hence its name.

[0010] — The processes currently used for calcining limestone and manufacturing Quicklime uses continuous rotary drum kilns using fossil fuel (gas or fuel oil) or continuous vertical kilns using fossil fuel (coal or coke). All these processes, using fossil fuels, generate emissions of combustion gases into the atmosphere, including carbon dioxide (CO₂) and other air pollutants. Quicklime production emits large quantities of CO₂, partly from the decarbonation reaction of the limestone and partly from the use of fossil fuels to heat the limestone. CO₂ emissions from quicklime production vary between 800 and 950 kg CO₂ equivalent per tonne of calcined limestone, depending on the type of kiln used. The portion related to limestone decarbonation varies between 600 and 700 kg CO₂ equivalent per tonne. The portion related to fossil fuel use varies between 200 and 250 kg CO₂ equivalent per tonne of calcined limestone. In order to limit the release into the atmosphere of combustion gases related to the use of fossil fuels, the present invention proposes a process for the industrial production of quicklime using a microwave oven equipped with a heating element, at least one microwave generator and at least one waveguide extending in a longitudinal direction inside the heating element, the process comprising the following steps: a) Limestone is introduced inside the heating element, b) The limestone introduced into the heating element is exposed to microwaves emitted by the microwave generator and transmitted into the heating element by the waveguide, so as to 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 related to the production of quicklime can be reduced by around 20% to 25%. Another advantage of using a microwave oven is that it avoids mixing combustion gases with the CO2 emitted during decarbonation. This makes the reprocessing of these gases easier. The process according to the invention also has the advantage of concentrating microwaves on a bed of moving materials to homogenize heating. The calcination temperature of the limestone is particularly well controlled, which ensures satisfactory and uniform properties for the quicklime produced by this process. To decarbonate the introduced limestone, the limestone is heated to a suitable temperature for a sufficient duration to allow this chemical reaction to occur. The appropriate temperature for decarbonating at least some of the introduced limestone is, for example, between 900 and 1200 degrees Celsius. The sufficient heating time required to dehydroxylate at least some of the introduced limestone depends on the temperature used, the composition of the material containing the limestone, and its form, for example, the size of the limestone particles. This sufficient time is typically between 150 and 300 minutes. Other non-limiting and advantageous features of the process according to the invention, taken individually or in all technically possible combinations, are as follows: - the heating element of the microwave oven extending along a longitudinal axis between two opposite longitudinal ends, the limestone is introduced, in step a), at one of the two longitudinal ends of the heating element, in step b) the limestone and / or quicklime contained in the heating element is advanced in a direction parallel to the longitudinal axis of the heating element, from the first longitudinal end towards a second longitudinal end and the quicklime is extracted, in step c), at the second longitudinal end of the heating element; - 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), limestone is introduced into the heating body by a feed system opening into the heating body at the first of said two longitudinal ends thereof, in step c), quicklime is extracted by an evacuation system opening into the heating body at the second longitudinal end thereof, and during step b), said heating body is rotated around its longitudinal axis, the longitudinal axis of the heating body is inclined relative to the horizontal so as to regulate the advance speed of the limestone in the heating body; - Microwaves are trapped inside the oven by means of two fixed covers, each cover being mounted on one of the longitudinal ends of the heating body with interposition of annular wave-trapping seals; - in step b), the gases released during the heating of the limestone are evacuated by an evacuation device ensuring a depressurization of the heating element of the oven and / or gases are injected inside the heating element; - the oven comprising two microwave generators and two corresponding waveguides, and the oven heating element comprising a first part of the heating element in which extends a first of said two waveguides associated with a first of said two microwave generators, and a second part of the heating element in which extends the second waveguide associated with the second generator. microwave nerator, in step b), the limescale passes successively through the first part of the heating element and then the second part of the heating element, said first and second parts of the heating element having different temperature profiles; - the first microwave generator and the first waveguide being configured to produce a gradual increase in temperature along the first part of the heating element and the second microwave generator and the second waveguide being configured to maintain a constant target temperature along the second part of the heating element, in step b) the limestone is subjected to the gradual increase in temperature along the first part of the heating element and maintained at the target temperature along the second part of the heating element; - in step a), the limestone introduced into the heating element 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 ground before being introduced into the heating element; - before being introduced into the heating element, the limestone is preheated in a preheating device; - the gases released during the heating of the limestone in the oven are recirculated to the preheating device; - after step c), the quicklime is conveyed through a cooling system to be cooled; - before its introduction into the heating element, the limestone is preheated in a preheating device and according to which the heat from the quicklime recovered by the cooling system is reused by the preheating device; - before step a), the limestone is preheated by conveying it through a fossil fuel-powered thermal treatment device; - before its preheating in the fossil fuel heat treatment device, another preheating is carried out in a preheating device different from the fossil fuel heat treatment device, and according to which the gases heated in the fossil fuel heat treatment device and / or in the furnace are re-circulated to the preheating device; - the heating body being internally equipped with stirring blades which extend in an inclined manner with respect to the longitudinal axis of the heating body and are rotationally fixed to it, in step b), the stirring blades are rotated with the heating body while inclining the longitudinal axis of the latter to advance the limestone and / or quicklime contained in the heating body); - We heat the limestone without producing any carbon dioxide other than that produced by the limestone decarbonation reaction. The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out. Regarding the attached drawings: [Fig.1] is a schematic view of a first industrial assembly implementing a first embodiment of the process according to the invention, [Fig.2] is a schematic view of a second industrial assembly implementing a second embodiment of the process according to the invention, [Fig.3] is a schematic longitudinal cross-sectional view of a microwave oven equipped with a rotating cylindrical heating element used for implementing the embodiments of the process shown in Figures 1 and 2, [Fig.4] is a cross-sectional view along plane II-II of [Fig.3], [Fig.5] is a schematic view showing an example of a wave trapping seal from the microwave oven of [Fig.3], [Fig.6] is a schematic view showing an example of a slotted waveguide from the microwave oven of [Fig.3], [Fig.7] is a schematic cross-sectional view of an early example of a microwave oven heating element from [Fig.3], [Fig.8] is a schematic cross-sectional view of a second example of a microwave oven heating element from [Fig.3], [Fig.9] is a schematic cross-sectional view of a third example of a microwave oven heating element from [Fig.3]. In the description that follows, by convention, the terms "upstream" and "downstream" will be used with reference to the direction of the flow of materials introduced into the microwave oven, and more generally with reference to the direction of the flow of materials and then products in the industrial assembly used for the implementation of the process according to the invention. As a preliminary point, it should be noted that, from one figure to another, the identical or similar elements of the different examples of implementation of the invention will be referenced by the same reference signs and will not be described each time. The invention relates to an industrial production process for quicklime. Figures 1 and 2 show two embodiments of an industrial assembly 300; 400; adapted to implement the process according to the invention. Each of these industrial sets 300; 400 includes in a remarkable way 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 equipped with a heating element 4 extending along a longitudinal axis XX between two opposite longitudinal ends, at least one microwave generator 24a and at least one waveguide 22 extending in a longitudinal direction inside the heating element 4 (see figures 1 to 4). This heating element 4 is cylindrical and rotatable. According to the process of the invention, the following steps are carried out: a) Limestone is introduced inside the heating element 4, b) the limestone introduced into the heating element 4 is exposed to microwaves emitted by the microwave generator 24a and transmitted into the heating element by the waveguide 22, so as to produce 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 CaCO₃. Calcination, that is, heating to high temperatures, of the calcium carbonate contained in limestone produces calcium oxide, or quicklime, through decarbonation. In general, said waveguide 22 is mounted inside the heating body 4. It extends in a longitudinal direction parallel to the longitudinal axis X-X of the heating body 4 and is connected at one of its two ends F, F' to said microwave generator 24a, 24b (figures 1 and 13). The heating element 4 comprises an internal cavity that accommodates the materials introduced into this heating element. An example of a microwave oven adapted to implement the process 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 element 4 of the furnace 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 CaCO₃. 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 element 4 of the furnace 2 preferably in a divided form. For this purpose, the limestone is for example crushed before being introduced into the furnace 2. The size of the limestone particles introduced into the furnace is preferably less than 50 millimeters (mm). For example, limestone with a granular class of 20 / 50 mm is used. The limestone can also be ground more finely before being introduced into the heating element, for example a granular class of 0 / 80 microns. The particle size class is indicated according to the formalism d / D, where d is the smallest representative diameter and D is the largest representative diameter of the material grains, determined, for example, using a sieve column or a laser particle size analyzer. In practice, in step a), the limestone is introduced into the kiln 2 by means of a feeding system 34 opening into the heating element 4 at one of its two longitudinal ends. The limestone is thus introduced at one of the longitudinal ends of the heating element. An example of such a feeding system will be described later. Step b) The material introduced into the heating element 4 of the oven 2 is heated by microwaves. This heating allows its calcination, that is to say, in this case, its decarbonation under high temperature. Limestone is preferably heated to a temperature between 900 and 1200 degrees Celsius for decarbonation. Alternatively, the limestone can be heated to a temperature of, for example, 900, 950, 1000, 1050, 1100, 1150, or 1200 degrees Celsius. The limestone is preferably heated for a time between 150 and 300 minutes, depending on the material, the temperature, and the power of the microwave generators. These heat treatments can cause the release of gases, including carbon dioxide (CO2). During the heating of the limestone, the gases released are evacuated by a gas evacuation device 42 (figures 1 and 13) ensuring a depressurization of the heating body 4 of the furnace 2. The limestone is thus thermally treated in kiln 2 under a "closed" atmosphere. It is also possible to inject gases inside the heating element 4. This injection can, for example, be carried out at the waveguide in order to prevent dust from entering it. Step c) At step c), the quicklime produced by heating the limestone in step b) is removed from the heating body 4 of the kiln 2, at the second longitudinal end of the heating body. In practice, the quicklime is removed from the kiln 2 by a quicklime evacuation system 36 opening in the heating body 4 at a second longitudinal end of the latter opposite the first longitudinal end ([Fig. 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 for the concentration of microwaves onto a bed of moving materials, which allows It homogenizes the calcination process and also allows for satisfactory control of the limestone temperature in the kiln, and therefore the calcination temperature. Finally, emissions of combustion gases and associated pollutants are reduced. It should be noted that one of the distinctive features of microwave heating is that it heats the core of the material without directly heating the heating element. The heating element is heated indirectly by the rise in the material's temperature. Consequently, the temperature of the heating element is lower than that observed in conventional fossil fuel-based heat treatment devices. This extends the furnace's lifespan. In general, the fossil energy used comes from the combustion of fossil fuels, such as gas, fuel oil, coal or coke. The production process for quicklime is preferably carried out continuously. This means that the limestone is not heated in the microwave oven in separate, successive batches. In other words, a first quantity of limestone is not introduced, heated in the microwave oven to produce quicklime, and then completely removed from the oven before the introduction of a second quantity of limestone, as 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 said to be "continuous" because it is possible to carry out step c) of quicklime extraction over a period of time of more than 4 hours without interruption of the quicklime flow of 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 minute, for example by 5, 10, 20, 30, 40 or 50 seconds, over this period of time. Preferably, when implementing such a method continuously, steps a), b), and c) can be carried out simultaneously. This is the case, for example, when the feeding system of the microwave oven 2 used includes a metal gate valve for regulating the flow of limestone introduced into the oven, as described later. The feeding of limestone into the oven and the extraction of quicklime from the oven can then be carried out continuously, without interruption, over the aforementioned time period. Alternatively, steps a) and / or c) are performed at time intervals. This is the case, for example, when the microwave oven's feed system includes an inlet chamber that can be created by two consecutive guillotine valves. The feed is then carried out in bursts. Similarly, the extraction of quicklime can be carried out in bursts. Alternatively, we can consider that only feeding or only extraction is carried out by jerk. To ensure continuous operation, in step b), the limestone and / or quicklime contained in the heating element is advanced in a direction parallel to the longitudinal axis XX of the heating element 4, from the first longitudinal end to the second longitudinal end. This advancement of the materials contained in the heating element allows for the continuous implementation of the method according to the invention. Preferably, these materials are advanced continuously throughout the implementation of the method. The oven is equipped for this purpose with feeding means allowing the transport of limestone and quicklime from the first to the second longitudinal end of the microwave oven's heating element. Here, for example, the heating element 4 is rotated around its inclined longitudinal axis to advance the material contained in the kiln from the longitudinal end of the heating element, where the feeding system introduces the limestone, to the longitudinal end of the heating element, where the discharge system extracts the quicklime. The feeding means therefore include a device for rotating the heating element and an inclined support for this heating element. The arrangement of microwave oven 2, an example of which is detailed later, ensures airtightness as well as trapping microwaves inside the heating element. These operations are carried out continuously in order to maintain all production equipment at a stable temperature. The process according to the invention can also be carried out discontinuously: a batch of limestone is introduced into the kiln. The introduction of limestone ceases, the introduced limestone is heated, and then removed from the kiln during a draining operation before the introduction of another batch of limestone. Kiln emptying operations lead to production stoppages during which temperatures drop sharply. This interruption results in increased energy consumption. Furthermore, in quicklime production, controlling temperatures and firing times is essential to minimize under- and over-firing and ensure consistent production quality. Continuous calcination therefore offers a dual advantage: firstly, it guarantees consistent and uniform quality, and secondly, it reduces the process's energy consumption. In a particularly advantageous manner, in the microwave oven 2 used for implementing this process, said waveguide 22 comprises a slotted waveguide mounted inside the heating element 4 between its two longitudinal ends and connected to one of these longitudinal ends of said waveguide. Microwave generator 24a: The waveguide preferably extends from one longitudinal end of the heating element to the other. Microwaves are therefore transmitted from one longitudinal end of the heating element to the other. Each microwave generator 24a, 24b is located outside the heating element 4, at one of its longitudinal ends. Each waveguide is preferably connected to a microwave generator. However, it is possible to connect each waveguide to several microwave generators to increase the power output. Furthermore, during steps a), b), and c), the heating element 4 is rotated about its longitudinal axis XX, and the longitudinal axis XX of the heating element 4 is inclined relative to the horizontal to regulate the rate at which the limescale advances through the heating element 4 (see Figures 1 to 4). This allows the limescale to advance through the microwave oven 2. As mentioned in the detailed example below, the heating element can also be fitted with stirring blades (not shown). The stirring blades extend inside the heating element. They extend parallel to the longitudinal axis or at an angle to the longitudinal axis of the heating element. They are rotationally fixed to the heating element, and, in step b), the stirring blades are rotated with the heating element while its longitudinal axis is tilted. The material contained within the heating element is then stirred during the rotation of the heating element. This homogenizes the mixture of materials present in the heating element and may also help to control the advance of this mixture of materials within the heating element. In addition, microwaves are trapped inside the oven 2 by means of two fixed hoods 14, each hood being mounted on one of the longitudinal ends of the heating body 4 with interposition of annular wave-trapping seals 16 (figures 9 and 11). A detailed example of a furnace describing the means of implementing these characteristics of the process according to the invention is described further on (with reference to figures 9 to 15). According to a particular embodiment of the process, which will be described in more detail later, the oven 2 comprises two microwave generators 24a, 24b and two corresponding waveguides 22 and the heating body 4 of the oven 2 comprises a first part in which extends a first of said two waveguides associated with a first 24a of said two microwave generators and a second part in which extends the second waveguide 22b 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 Waveguides 22 are connected to each other, with each section 22a, 22b connected at one end to one of the two microwave generators 24a, 24b. The two sections 22a, 22b then extend along the same axis parallel to the longitudinal axis XX of the heating element, as shown in [Fig. 9]. In this case, one of the two microwave generators 24a, 24b is located at one of the two longitudinal ends of the heating element 4, and the other microwave generator is located at the second longitudinal end of the heating element. Alternatively, two separate waveguides can be provided extending side by side, for example in two directions parallel to the longitudinal axis of the heating element. Of course, one can consider a number greater than two of waveguides extending into the heating body, each waveguide being connected to a separate microwave generator or to the same microwave generator. So, in step b), the limescale passes successively through the first part and then the second part of the heating element, the said first and second parts of the heating element having different temperature profiles. In a particularly advantageous manner, it can be provided that, with the first microwave generator and waveguide configured to produce a gradual increase in temperature along the first part of the heating element and the second microwave generator and waveguide configured to maintain a constant target temperature along the second part of the heating element, in step b) the limescale is subjected to the gradual increase in temperature along the first part of the heating element and then maintained at the target temperature along the second part of the heating element. The first part constitutes an upstream temperature-raising zone, while the second part constitutes a downstream temperature-maintenance zone. The upstream zone can bring the limestone to said target temperature. Other temperature profiles can obviously be envisaged, with two or more waveguides defining two or more parts of the heating body having different temperature profiles. Furthermore, two or more microwave ovens connected in series can be used to heat the limestone and ensure its decarbonation. For example, a first oven can be used to heat the limestone with an initial temperature profile, such as a rapid increase in temperature, and a second oven can be used to heat the limestone and / or quicklime with a second temperature profile, such as maintaining a decarbonation temperature. The second oven is fed with the limestone heated without decarbonation in the first oven and the quicklime produced in the first oven. In practice, the process of producing quicklime involves other steps occurring before or after the calcination of the limestone in the microwave oven 2. In general, the process according to the invention may include one or more of the following additional steps: - the limestone is ground before being introduced into the heating element 4 of the furnace 2; - the limestone is dried before being introduced into the heating element 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 fuel heat treatment device 185 (figures 1 and 2); - after step c), the quicklime is conveyed through a cooling system 210 to be cooled (figures | and 2); - before its preheating in the fossil fuel heat treatment device 185, another preheating is carried out in a preheating device 110, 130 different from the fossil fuel heat treatment device 185 ([Fig.2]). - before its introduction into the heating body 4, the limestone is preheated in a preheating device 110 and the heat from the quicklime recovered by the cooling system 210 is reused by the preheating device (figures 1 and 2); - the temperature of the material inside the microwave oven 2 is measured, for example using an infrared pyrometer. Preheating the limestone using the preheating device(s) lowers the temperature of the limestone to a level below that used for calcination. For example, the limestone is preheated to a temperature between 350 and 500°C. In general, it is also provided that the gases heated in the cooling device 210 and / or the gases released during the heating of the limestone in the furnace 2 are recirculated to the different preheating devices 110 and 130 of the fossil fuel thermal treatment device 185. It can also be foreseen that, when limestone is preheated in a fossil fuel thermal treatment device 185, the gases released during the heating of limestone in the fossil fuel thermal treatment device 185 are recirculated to 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 from the quicklime recovered by the cooling system 210 is reused by the different preheating device(s) 110 and 130 of the fossil fuel thermal treatment device 185. In practice, the gases heated by the quicklime in the cooling device 210 are recirculated to the different preheating device(s) 110 and 130 of the fossil fuel thermal treatment device 185. Two embodiments of the process according to the invention are described below with reference to Figures 1 and 2. In these figures, material transfers before and after calcination are represented by solid arrows. Hot gas transfers are represented by dashed lines. Arrow R indicates the supply of fresh air. According to the first embodiment of the process according to the invention shown in [Fig. 1], the material feeding the industrial assembly 300 is first conveyed through a preheating device 110 which heats the material to a temperature between 350 and 400°C. The material here is, for example, crushed limestone of granular class 20 / 50 mm. The heated air (between 80 and 100°C) in the preheating device 110 is vented to the outside through a chimney 120 using a fan 120A. At the outlet of the preheating device 110, the material containing the limestone is introduced into the microwave oven 2. It is introduced into the heating element 4 through the supply system 34 and heated to a set temperature between 900 and 1200°C. The heating element 4 is mobile in rotation around its longitudinal axis XX. It rotates continuously during the heating of the material. The longitudinal axis XX of the heating body 4 being inclined with respect to the horizontal such that the first longitudinal end through which the material is introduced is higher than the second longitudinal end of the heating body 4 along a vertical axis, the material is progressively conveyed towards the evacuation system located at the second longitudinal end of the heating body by the effect of gravity and the rotation of the heating body 4 around its longitudinal axis XX. An example of a microwave oven 2 will be described in more detail later. Upon exiting kiln 2, the calcined material, which includes quicklime, is conveyed through a cooling device 210 to gradually lower its temperature to between 100 and 120°C. The cooled calcined material is then 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 fan 230. The gases heated in the cooling device 210 eV / or the gases heated in the microwave oven 2 and vented through the gas venting 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 output, the required electrical power can be very high. Therefore, considering the available capacity Locally on electrical distribution networks, the microwave oven 2 can be combined with a fossil fuel-powered 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 [Fig. 2] and described below. Heating with a fossil fuel-based heat treatment device is preferably carried out before calcination with a microwave oven. This initial heating stage raises the limestone to a temperature lower than that required for decarbonation, for example, between 600 and 650°C. It is preferable to perform limestone decarbonation with a microwave oven to avoid mixing the carbon dioxide (CO₂) produced during decarbonation with the combustion gases of the fossil fuel-based heat treatment device. Treating the gases expelled from the microwave oven is simpler than treating the gases from the fossil fuel-based heat treatment device because the gases exiting the microwave oven do not contain pollutants and particles from fossil fuel combustion. According to the second embodiment of the process according to the invention shown in [Fig.2], the material feeding 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 of granular class 20 / 50 mm. The air heated in the first preheating device 110 (here between 80 and 100°C) is exhausted to the outside through the chimney 120 using the fan 120A, as described in the first embodiment. At the exit 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 exit 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-powered heat treatment device 185 which also heats the material to a temperature between 600 and 650°C. At the exit of the heat treatment device 185, the material containing the limestone is introduced into the microwave oven 2. It is introduced into the heating element 4 through the feed system 34 and heated to a set temperature between 900 and 1200°C. Microwave oven 2 is identical to that described in the first method of implementation. Upon exiting kiln 2, the calcined material, which includes quicklime, is conveyed to 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, a forced-air cooler. It is supplied with fresh air (arrow R) by a fan 230. 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 to the first preheating device 110 in order to reduce its energy consumption. The gases heated in the fossil fuel-fired heat treatment unit 185 are preferably recirculated to the second preheating unit 130 in order to reduce its energy consumption. The second preheating unit 130 receives gases from the fossil fuel-fired heat treatment unit 185. The gases heated in the second preheating unit 130 are filtered by a filter 140 before being vented to the outside through a chimney 150 with the aid of a fan 150A. They have a temperature, for example, between 80 and 100°C. In general, the filter, storage devices, preheating device(s), fossil fuel-powered heat treatment device, cooling device, and other components of the industrial assembly can be of any type known to those skilled in the art and adapted to the limestone treatment described above. They will not be described in further detail here. Thanks to the process according to the invention, the limestone is heated at least partially without producing carbon dioxide. Provided that decarbonized electricity is used, this kiln technology will significantly reduce CO₂ emissions from quicklime production. Depending on the process used (100% microwave or hybrid), CO₂ emissions per ton of quicklime produced will be between 750 and 850 kgCO₂, representing a reduction of approximately 15 to 25% compared to existing processes. According to one embodiment of the process according to the invention, a microwave oven 2 as shown in Figures 3 to 9 is used. This oven 2 can be used for example in one of the industrial sets 300; 400 described previously and shown in Figures 1 and 2. This kiln can perform continuous heat treatment of the materials introduced into it. During such continuous treatment, the heat-treated material is removed from the kiln over a period of more than 4 hours, without any interruption of the quicklime flow exceeding 1 minute within this period. As shown in Figures 3 and 4, oven 2 includes, in particular, the body of 4 rotating cylindrical heater which is centered on the longitudinal axis XX positioned so as to form an angle & between 0.1 and 10° with respect to the horizontal. The heating element 4 is a tube made, for example, of refractory steel and open at each of its longitudinal ends. It is fitted with an external layer of thermal insulation 6 wrapped around its outer diameter. Different design configurations of the heating element will be described later in connection with Figures 7 to 9. At each of its longitudinal ends, the heating element 4 is fitted with an annular disc 7 extending from the outer layer of thermal insulation 6, creating a controlled heating volume based on its height. Lifting blades 9 positioned inside the heating element ensure complete drainage of the heat-treated material by passing it over the annular disc 7 located at the outlet of the heating element 4. At its longitudinal ends, the heating element 4 further includes bearing rings 8 which are mounted around the thermal insulation layer 6 and which rest on rotating rollers 10 so as to be able to rotate the heating element 4 around its longitudinal axis XX at a speed typically between 0.1 and 40 revolutions per minute. Of course, other mechanisms for rotating the heating element could be considered (gear motor, rack and pinion, etc.). The rotating rollers 10 are mounted on a platform 12 adjustable in inclination relative to the horizontal by an angle of between 0.1 and 10° in order to regulate the advance speed of the limestone introduced into the heating element 4. The heating element may also include stirring blades to promote and control the flow of limestone within the heating element and to improve the temperature uniformity of the limestone contained within it. The arrangement and geometry of these stirring blades can vary considerably and depend on the material (powder, granules, inlet dryness, etc.). For example, the stirring blades may be arranged in sections along the longitudinal axis of the kiln or be inclined relative to this axis. The lengths may be identical for all blades or vary. The blade profile may be triangular, straight, inclined, or curved, for example. Thus, according to the process of the invention, the rate of advance of the limescale in the heating element is controlled. Temperature homogeneity is also ensured within the limescale contained in the heating element. The furnace 2 also includes two fixed hoods (or flanges) 14 which are mounted on each longitudinal end of the heating body with interposed annular wave-trapping seals 16. These hoods 14 are, for example, made of re- steel fractary. More specifically, each hood 14 consists of a disc 14a closing one of the longitudinal ends of the heating body and an annular collar 14b partially covering the thermal insulation layer 6 of the heating body. The wave-trapping seals 16 are positioned between the collar 14b of each cover 14 and the thermal insulation layer 6 of the heating element. They ensure a microwave-tight seal between the fixed covers and the rotating heating element despite thermal stresses and expansions. As shown in [Fig.5], each wave trapping joint 16 comprises at least one pair of discs 18 centered on the longitudinal axis XX of the heating body and fixed to the collar 14b of the hood extending radially towards the thermal insulation layer 6 of the heating body. The 18 discs are made of electrically conductive material and are thermally resistant to high temperatures (on the order of 1200°C). For example, they can be made of refractory steel. Furthermore, the disks 18 of the same pair are spaced longitudinally apart from each other so as to form an annular groove 20 having a depth p corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength (that is: p =} / 4 + k A with A for the wavelength and k an integer). Preferably, each wave-trapping joint 16 comprises several pairs of disks 18 (five pairs in the example of Fig. 5) which are spaced longitudinally apart from each other by a distance d corresponding to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: d = 1 / 4 + n A with A for the wavelength and n an integer). This particular geometry of the wave-trapping seals 16 largely cancels any wave that might pass between the collar 14b of the cover 14 and the thermal insulation layer 6 of the heating element. Indeed, part of the incident wave passes directly along the thermal insulation layer (in a longitudinal direction), and another part enters one of the pairs of discs 18. The distance traveled (in a radial direction) by this wave portion in the groove 20 is such that a round trip of the wave results in a phase shift of Å / 2 relative to the incident wave, which cancels the incident wave. The combination of several pairs of discs ensures zero leakage. Furthermore, the longitudinal spacing d between the pairs of disks 18 represents a second type of wave trapping (in the longitudinal direction). Indeed, part of the incident wave escaping under the first pair of disks encountered is reflected. by the disk of the next pair on its path and returns to the first pair of disks with a phase shift of M2. As shown in Figures 3 and 4, the furnace 2 includes the slotted waveguide 22 which is mounted inside the heating body 4 extending longitudinally between each end thereof. The waveguide 22 is connected at one end to a microwave generator 24a, 24b (or to several microwave generators) and runs longitudinally through the heating element between its two longitudinal ends. It is designed to distribute the microwaves in a controlled manner along the entire length of the heating element directly onto the limescale 26 to be treated. Typically, the microwave generator 24a, 24b comprises a magnetron with a unit power that can vary between 1kW and 10MW, coupled to a frequency generator that can vary from 200MHz to 4000MHz. The waveguide 22 can be made in a single section or in several sections connected to each other, each section being connected at one end to a microwave generator. Thus, in the embodiment shown in [Fig. 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. Similarly, it is possible to provide for a plurality of waveguides which are mounted inside the heating body and which extend parallel to each other at each end of the heating body, each of these waveguides being able to be made in one or more sections. Furthermore, the waveguide 22 is preferably wrapped with a thermal insulator 28 that is transparent to the microwaves generated by the wave generator. For example, this thermal insulator is made of silica and alumina fibers or quartz. The waveguide 22 can also be isolated from dust by protective windows 30 (see [Fig. 4]) which are transparent to the microwaves generated by the wave generator and resistant to high temperatures. These protective windows are made, for example, of quartz or ceramic and have a thickness of between 5 and 20 mm. They can be attached to the waveguide by means of tabs or any other fastening system. Alternatively, or in addition to the protective windows, the waveguide can be slightly overpressurized (relative to the inside of the heating body) by injecting air at one of its longitudinal ends in order to limit dust entry. Alternatively, a dust cleaning system can be provided, comprising a perforated cylinder placed on one upper face of the waveguide. Cleaning phases are ensured by injection of pulsed air. Furthermore, as shown in [Fig.6], each waveguide 22 comprises a plurality of slots 32 which are positioned opposite the limestone 26 introduced into the heating body. In the embodiment shown in [Fig. 6], the waveguide slots 32 are substantially rectangular in shape, with their length aligned with the longitudinal axis A of the waveguide. Furthermore, the slots 32 in this embodiment are arranged on either side of the longitudinal axis A of the waveguide. In general, the arrangement and geometry of the waveguide slots 32 are designed on the one hand to be compatible with the microwave frequency used, and on the other hand to ensure that the heating of the limestone is carried out optimally with one or more specific heating zones depending on the desired temperature profile. For example, it is possible to provide an upstream temperature rise zone and a downstream maintenance zone at a target temperature. In this example, in the upstream temperature rise zone (see [Fig.6]), the slots 32 of the waveguide are spaced, for example, on the one hand longitudinally from each other by a distance e corresponding to half the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength (i.e.: e = 2 + m A with A for the wavelength and m an integer), and on the other hand transversely from the longitudinal axis A of the waveguide by a distance h. Still in this example with two specific heating zones, the slots of the waveguide corresponding to the downstream temperature maintenance zone (not shown in the figures) are spaced longitudinally from each other by a distance different from the distance e, and / or transversely from the longitudinal axis A of the waveguide by a distance different from the distance h. Furthermore, the slot 32a, which is furthest downstream of the waveguide, is located at a distance f from the downstream end F of the waveguide that is equal to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e.: f = 1 / 4 + p A with A for the wavelength and p an integer). Similarly, the slot 32b, which is furthest upstream of the waveguide, is located at a distance g from the upstream end F' of the waveguide that is equal to half the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength (i.e., g = M2 + q A with A for the wavelength and q an integer). The kiln 2 used in the process according to the invention also includes the limestone feeding system 34 which passes through the upstream hood 14 and opens into the heating element 4 at its first longitudinal end (or upstream end) Alternatively, the power supply can be achieved by gravity using tubes whose diameter and length are determined so as not to exhibit wave leakage. For products in powder form, a metal gate valve type supply system will be chosen to regulate the flow, prevent microwave leaks and prevent the heating element from being exposed to air. For larger granular products, it is preferable to feed the furnace in bursts through an inlet chamber, which can be created by two consecutive guillotine valves. This inlet chamber ensures a seal against waves and air. Similarly, the kiln includes the quicklime discharge system 36, which passes through the downstream hood 14 and opens into the heating chamber 4 at a second longitudinal end (or downstream end) opposite the first longitudinal end. This discharge system 36 can be coupled to a flow control valve 38 and a temperature probe 40 inside the heating chamber. Furnace 2 also includes the gas evacuation device 42, which here comprises a flue gas exhaust tube. This tube can also be used for gas injection into the heating element. In conjunction with figures 7 to 9, we will now describe different possible configurations for the construction of the furnace heating element. In the embodiment of [Fig.7], the heating body 4-1 includes in particular an internal tube 44 which is composed of a plurality of angular plates 46 made of refractory steel and distributed angularly around the longitudinal axis X-X of the heating body. The plates 46 overlap in pairs in the circumferential direction, each plate being able to slide tangentially on the two plates directly adjacent to it in order to allow the inner tube 44 to absorb thermal expansions. The heating element 4-1 also includes an outer tube 48 which is arranged coaxially around the inner tube 44. Studs 50 extending in radial directions connect the outer tube 48 to the plates of the inner tube 44. Finally, a thermal insulator 52 is positioned in the annular space formed between the inner tube 44 and the outer tube 48. In the embodiment of [Fig.8], the heating body 4-2 comprises an inner tube 54 which is made of refractory steel and is monobloc, and an outer tube 48 which is arranged around the inner tube coaxially with it. In addition, a plurality of spring blades 56 (or braces) extending in directions tangential to the inner tube connect the outer tube 48 to the inner tube 54. Finally, as with the previous embodiment, a thermal insulator 52 is positioned in the annular space formed between the inner tube 54 and the outer tube 48. In the embodiment of [Fig. 9], the heating element 4-3 comprises an inner tube 58 made of refractory steel, one-piece and longitudinally slotted to allow it to expand circumferentially. For this purpose, the inner tube 58 has a discontinuity 60, the two angular ends of the inner tube delimiting this discontinuity being connected to each other by a plate 62 screwed onto the inner tube. The heating body 4-3 also includes an outer tube 48 which is arranged around the inner tube 58 in a coaxial manner, and a thermal insulator 52 positioned in the annular space formed between the inner tube and the outer tube. Furthermore, as is known, the rotary kiln also includes devices (not shown in the figures) for gas sealing and thermal insulation between the heating element and the fixed covers. Typically, these devices can be thermal fabric gaskets. The present invention is not limited to the embodiments described and represented in the various figures.

Claims

Demands

1. Industrial production process for quicklime using a kiln (2) at microwave equipped with a heating element (4), with at least one generator microwave (24a, 24b) and at least one waveguide (22) extending along a longitudinal direction within the body of heating (4), the process comprising the following steps: a) Limestone is introduced inside the heating element (4), b) the limestone introduced into the heating element (4) is exposed to micro- waves emitted by the microwave generator (24a, 24b) and transmitted in the heating element (4) by the waveguide (22), so as to to produce quicklime, (c) The quicklime produced is extracted from the heating element (4).

2. Method according to claim |, wherein the heating element (4) of the microwave oven (2) extending along a longitudinal axis (XX) between two opposite longitudinal ends, - limestone is introduced, in step a), to the first of the two ex- longitudinal openings of the heating element (4), - in step b) the limestone and / or quicklime contained are advanced in the heating element along a direction parallel to the longitudinal axis gitudinal (XX) of the heating element (4), from the first longitudinal end- tubular towards a second longitudinal end - quicklime is extracted, at step c), at the second longitudinal end- heating element tube.

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

4. A method according to any one of claims 2 and 3, according to which - said waveguide (22) comprising a slotted waveguide (32) mounted inside the heating element (4) and connected to one of its ex- terminals (F, F') of the microwave generator, - in step a), the limescale is introduced into the heating element (4) by a supply system (34) opening into the heating body (4) at the first of the said two longitudinal extremities thereof, - in step c), the quicklime is extracted by means of an evacuation system(36) opening in the heating element (4) at the second longitudinal end- study of this one, - during step b), the said heating element (4) is rotated around its longitudinal axis (XX), the longitudinal axis of the heating element (4) relative to the horizontal so as to regulate the limestone advance rate in the heating body (4).

5. A method according to any one of claims 1 to 4, in which the microwave inside the oven (2) by means of two fixed covers (14), each hood (14) being mounted on one of the longitudinal ends of the heating element (4) with interposition of annular trapping seals of waves (16).

6. A method according to any one of claims 1 to 5, wherein, in step b), The gases released during the heating of the limestone are evacuated by a device gas evacuation (42) ensuring a depressurization of the body of heating (4) of the oven (2) and / or gases are injected inside the body of heating (4).

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

8. A method according to claim 7, wherein the first generator of microwave (24a) and the first waveguide (22a) being configured of in order to produce a gradual increase in temperature along the first part of the heating element (4) and the second part microwave generator (24b) and the second waveguide (22b) being configured to maintain a constant target temperature along the second part of the heating element (4), at step b) the limestone is subject to a progressive increase in temperature. along the first part of the heating element (4) and maintained at the temperature target temperature along the second part of the heating element (4).

9. A method according to any one of claims 1 to 8, wherein in step a), the limestone introduced into the heating element (4) is crushed and contains a proportion of calcium carbonate varying between 60% and 95%.

10. A method according to any one of claims 1 to 9, wherein in step b) one heats the limestone to a temperature between 900 and 1200 degrees Celsius.

11. | A method according to any one of the preceding claims, in which one grinds the limestone before its introduction into the heating element (4).

12. A method according to any one of claims 1 to 11, wherein, before its Introduction into the heating element (4), the limescale is preheated in a preheating device (110; 130).

13. A method according to the preceding claim, wherein the gases released During the heating of the limestone in the oven (2) are recirculated to the preheating device (110).

14. A method according to any one of claims 1 to 13, wherein, after Step c), the quicklime is conveyed through a cooling system- dissement (210) to be cooled.

15. A method according to claim 14, wherein, before its introduction in the heating element (4), the limescale is preheated in a device of preheating (110) and according to which the heat of the quicklime recovered by the cooling system (210) is reused by the preheating device (110).

16. A method according to claim 15, wherein, before step a), the limestone is preheated by being conveyed through a device fossil fuel heat treatment (185).

17. | Method according to the preceding claim, wherein, before its pre- heating in the fossil fuel-based thermal treatment device, on performs another preheating in a preheating device (110; 130) different from the energy-efficient heat treatment device fossil (185), and according to which the gases heated in the device of fossil fuel heat treatment (185) and / or in the furnace (2) are recirculated to the preheating device (110; 130).

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

19. A method according to any one of the preceding claims, in which one heats the limestone without producing any carbon dioxide other than that produced by the first decarbonation