FR3134014A1 - microwave extraction oven for the continuous heat treatment of organic products in solid or pasty state - Google Patents

Abstract

Microwave extraction furnace for continuous thermal treatment of solid-state organic products The invention relates to a microwave extraction furnace (2) for the continuous heat treatment of organic products in the solid state, comprising a belt conveyor (4) intended to transport the products (P) to be treated, the belt (6) of the conveyor being porous to the extracts resulting from the processing of the products, a heating tunnel (8) crossed right through by the conveyor belt and comprising a box (10) sealed against microwaves and interior of which opens at least one wave guide (12) connected to a microwave generator (14), and a tank (16) for receiving and condensing extracts resulting from the treatment of the products after their diffusion through the belt . The heating tunnel comprises an inlet seal (20) positioned at the entrance to the heating tunnel above the conveyor and an outlet seal (22) positioned at the outlet of the heating tunnel above the conveyor. conveyor, the entry and exit airlocks each being provided with at least one quarter-wave trap (24) in order to ensure sealing against microwaves at the entry and exit of the heating tunnel. Figure for abstract: Fig. 1.

Description

microwave extraction oven for the continuous heat treatment of organic products in solid or pasty state

The invention relates to the general field of microwave heat treatment of organic products in the solid or pasty state to obtain extracts in the liquid state.

A non-limiting field of application of the invention concerns the microwave-assisted extraction of volatile and non-volatile active ingredients from plant biomass.

Different processes for continuous plant extraction of solid or pasty materials using solvents are known. Generally, these processes for extracting solids from liquids work on the principle of solvent extraction, with co-current or counter-current injections on the product in order to harvest extracts such as essential oils or active ingredients by percolation, diffusion or immersion. The devices allowing the transport and mixing of the material are made, for example, using perforated bucket conveyors, perforated belts, endless screws or vertical honeycomb rotors.

The main disadvantage of these processes is that it requires significant extraction times (of the order of two to eight hours) for the solvent to act on the product and deplete the plant. These processes also require the use of significant quantities of solvents while regulations increasingly restrict their use.

One of the known ways of overcoming this drawback is to use very efficient and neutral solvents, such as carbon dioxide in the supercritical state, or to assist the extraction operation using a secondary energy using, for example, ultrasound or microwaves.

If ultrasound is effective in destructuring the plant matrix, it only exceptionally allows a good extraction yield.

On the other hand, microwave extraction processes allow for very rapid release of extracts, often in the order of ten to thirty minutes. Microwaves are in fact particularly effective because they directly heat the product to the core. For example, for plant materials, microwaves make it possible to entrain the water-soluble active ingredients with the water of the plant, to burst the pockets of essential oils and to vaporize the water of the plant which entrains these volatile elements. (essential oils). Microwaves are preferentially absorbed by materials with high dielectric constants, which is the case for water contained in biological materials and their vascular and glandular tissues.

We thus know from publication FR 3,017,308 a device and a discontinuous process for extraction by microwaves and ultrasound. We also know from publication WO 2016/118034 a continuous extraction process using microwaves and ultrasound.

We still know the publications EP 1,629,725 and FR 2,975,020 which describe solvent-free extraction processes assisted by microwaves in which the vaporization of the water contained in the plant allows the extracts to be carried away. These processes are suitable for discontinuous extractions of small volumes, in batches. However, they are not suitable for fragile products with low value in active ingredients or essential oils.

The main disadvantage of these microwave-assisted processes is that they only allow them to operate discontinuously in batches or continuously with screw drive, which is not suitable for fragile products (for example flowers or with fine leaves), nor those with a low content of materials to be extracted.

To overcome such a drawback, it was proposed in publication FR 3,070,869 a device and method for the continuous extraction of solid or pasty organic materials which consists of placing the product to be treated on a conveyor belt comprising slots making it possible to constitute in extraction chambers that are relatively airtight when exposed to microwaves and which allow the partial pressurization of steam throughout the heat treatment. These slots incorporating metal quarter-wave traps also ensure microwave tightness in the entry and exit airlocks.

In this invention, under the action of the microwave field, a fraction of the water contained in the product to be treated vaporizes and carries away the volatile molecules (essential oils, etc.) while another fraction of the water contained in the product to be treated (for example plant water) is extracted by mechanical effect in liquid form, due to the partial vapor pressure at the heart of the product (for example in the cells of a plant). The process according to this invention makes it possible to evacuate the liquid and vapor fractions of the extracts through the product to be treated and its support to recover all of the extracts towards a condenser at a low point. This process thus makes it possible to recover by percolation and diffusion not only the fraction of volatile molecules, but also those that are non-volatile.

Although effective, this extraction device has the disadvantage of having metal quarter-wave traps all along the conveyor belt, which causes a significant mass of the conveyor and a complex implementation of it. this.

The object of the invention is to propose a microwave extraction oven for the continuous heat treatment of organic products in the solid state which does not present the aforementioned drawbacks thanks to specific sealing chambers.

• un convoyeur à bande destiné à transporter les produits à traiter, la bande du convoyeur étant réalisée dans un matériau transparent aux micro-ondes et poreuse aux extraits issus du traitement thermique des produits,
• un tunnel de chauffe qui est traversé de part en part par la bande du convoyeur et qui comprend un caisson étanche aux micro-ondes et à l’intérieur duquel débouche au moins un guide d’ondes raccordé à un générateur de micro-ondes, et
• un bac positionné sous la bande du convoyeur au niveau du caisson pour réceptionner et condenser des extraits issus du traitement thermique des produits après leur diffusion au travers de la bande du convoyeur,
• et dans lequel, conformément à l’invention, le tunnel de chauffe comprend au moins un sas d’étanchéité d’entrée qui est positionné en entrée du tunnel de chauffe au-dessus du convoyeur et au moins un sas d’étanchéité de sortie qui est positionné en sortie du tunnel de chauffe au-dessus du convoyeur, les sas d’entrée et de sortie étant chacun munis d’au moins un piège quart-d’onde afin d’assurer une étanchéité aux micro-ondes en entrée et en sortie du tunnel de chauffe.

This goal is achieved thanks to a microwave extraction oven comprising:

• a conveyor belt intended to transport the products to be treated, the conveyor belt being made of a material transparent to microwaves and porous to the extracts resulting from the heat treatment of the products,
• a heating tunnel which is crossed from one side to the other by the conveyor belt and which comprises a microwave-tight box and inside which opens at least one waveguide connected to a microwave generator, and
• a tray positioned under the conveyor belt at the level of the box to receive and condense extracts resulting from the heat treatment of the products after their diffusion through the conveyor belt,
• and in which, in accordance with the invention, the heating tunnel comprises at least one entry seal which is positioned at the entrance to the heating tunnel above the conveyor and at least one outlet seal which is positioned at the exit of the heating tunnel above the conveyor, the entry and exit airlocks each being provided with at least one quarter-wave trap in order to ensure tightness against microwaves at entry and exit. exit from the heating tunnel.

The extraction oven according to the invention is remarkable in that the sealing against microwaves is carried out, not at the level of slots positioned on the conveyor belt, but at the level of the entry and exit airlocks of the heating tunnel. The implementation of such an extraction furnace is greatly simplified (in particular due to the conveyor belt which can remain unchanged).

Preferably, each quarter-wave trap of the entry and exit airlocks comprises at least two rigid slots arranged perpendicular to a direction of advance of the conveyor belt and over an entire width thereof, the distance between the two slots corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength.

In this case, each quarter-wave trap of the entry and exit airlock advantageously comprises a substantially horizontal upper plate positioned above the conveyor belt and on which the at least two slots are fixed, the distance separating the strip of a free end of the slots being non-zero. This plate is made of an electrically conductive material (for example stainless steel).

In addition, each quarter-wave trap of the entry and exit airlock advantageously comprises a substantially horizontal lower wall positioned under the at least two slots, said lower wall being made of a material transparent to the microwaves emitted by the guide waves and impervious to vapors and dust.

Each slot of the quarter-wave traps may comprise at one free end a groove having a depth corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength. .

In this case, the groove of the slots may contain an insert made of a material having a relative dielectric permittivity less than 10 F/m and a refractive index greater than 1.

Each slot of the quarter-wave traps can have a V shape with its tip directed towards the conveyor belt.

Each slot of the quarter-wave traps is preferably made of an electrically conductive material.

In one embodiment, the entrance airlock and/or the exit airlock is fixed.

In another embodiment, the entrance airlock and/or the exit airlock is movable vertically between a high position in which it frees a passage for conveying products and a low position in which it closes the passage for conveying of products.

In this case, the entrance airlock can be movable vertically and be coupled to a detector of the presence of products on the conveyor belt making it possible to activate the waveguide of the heating tunnel and to control the raising and lowering of said entry lock.

Still in this case, the exit hatch can be movable vertically and be coupled to a detector of the presence of products on the conveyor belt making it possible to control the raising and lowering of said exit hatch.

In yet another embodiment, the entrance airlock comprises at least two airlock sections independent of each other and each movable vertically between a high position in which it frees a passage for the conveying of products and a position low in which it closes the passage for conveying products, and the exit airlock comprises at least two airlock sections independent of each other and each movable vertically between a high position in which it frees a passage for conveying of products and a low position in which it closes the passage for conveying products.

In this case, the two sections of the entry and exit airlocks can be coupled to product presence detectors on the conveyor belt making it possible to control the rise and fall of said sections.

The conveyor belt may comprise a plurality of extraction chambers delimited by walls arranged perpendicular to a direction of advance of the conveyor belt and over an entire width thereof, said walls being made of a material transparent to micro -waves emitted by the waveguide.

The extraction furnace may also include means for recovering the extracts and water resulting from the heat treatment of the products and reinjecting them into the heating tunnel on the products.

There is a longitudinal sectional view of an extraction furnace according to one embodiment of the invention.

There is a cross-sectional view of the heating tunnel of the extraction furnace of showing one of the airlocks (entrance or exit).

There shows in detail and partially the production of the entry and exit seals according to the invention.

There shows in detail a sealing chamber according to a variant embodiment of the invention.

There shows in detail a sealing chamber according to another alternative embodiment of the invention.

There is a longitudinal sectional view of an extraction furnace according to a variant embodiment of the invention in stopped configuration.

There is a view of the oven of the in production configuration.

There is a longitudinal sectional view of an extraction furnace according to another alternative embodiment of the invention in stopped configuration.

There is a view of the oven of the in production start-up configuration.

There is a view of the oven of the in production configuration.

There is a longitudinal sectional view of an extraction furnace according to yet another embodiment of the invention.

The invention relates to the continuous extraction of organic products in the solid state or in the pasty state by means of a microwave extraction oven such as that shown in Figures 1 and 2.

The oven 2 comprises in particular a conveyor 4 with belt 6 which is intended to transport the products to be treated P in a direction of advance F. The speed of advance of the belt 6 of the conveyor is typically between 0 and 1 m/min , and preferably between 0.1 and 0.5 m/min.

The belt 6 of the conveyor is advantageously made of a material transparent to microwaves, for example glass fibers, so that it does not heat up when exposed to microwaves.

Belt 6 of the conveyor is also porous to the extracts resulting from the heat treatment of the products. For this purpose, it can have perforations typically between 50μm and 20mm, and preferably of the order of 2mm.

The conveyor belt 6 passes right through a heating tunnel 8, the length of which depends on the desired power and the flow rate of the product to be treated (it is generally between 1m and 50m).

The heating tunnel comprises a microwave-tight box 10 and inside which opens at least one wave guide 12 connected to a microwave generator 14.

The box 10 is made for example of conductive material of steel or aluminum type and it has a height typically between 100 and 500mm so as to provide a sufficient volume between the arrival of the waveguides 12 and the product to be treated P present on belt 6 of the conveyor. This configuration allows the electromagnetic field to propagate in numerous modes and reach the product P as homogeneously as possible.

In the exemplary embodiment of Figures 1 and 2, the heating tunnel comprises three wave guides 12 regularly spaced along the box and each connected to a microwave generator 14. Of course, other configurations are possible .

The lower part of the heating tunnel further comprises a tray 16 positioned under the belt 6 of the conveyor at the level of the box 10 to receive and condense the extracts resulting from the heat treatment of the products P after their diffusion through the conveyor belt.

For this purpose, this tank 16 is microwave-tight and is for example provided with a double waterproof skin intended to receive a cold fluid so as to cause the condensation of the vapor phase and the cooling of the extracts collected. The liquid extracts obtained are then evacuated by a collector 16a placed at the low point of the tank 16.

A flat waterproof wall 18 closes the box 10 in its lower part by covering the surface of the belt 6 of the conveyor which is located in the heating tunnel 8.

This wall 18 prevents the steam extracted when heating the products under the action of microwaves from diffusing inside the box 10. The wall is vapor-tight and transparent to microwaves. . It is for example made of PTFE.

According to the invention, sealing against microwave leaks at the entrance and exit of the heating tunnel 8 is ensured by at least one entrance seal 20 which is positioned at the entrance to the heating tunnel above of the conveyor belt and by at least one exit seal 22 which is positioned at the exit of the tunnel above the conveyor belt.

The inlet 20 and outlet 22 seals allow the microwaves to be confined inside the heating tunnel 8.

For this purpose, in the embodiment of Figures 1 and 2, the inlet 20 and outlet 22 sealing chambers are each provided with at least one quarter-wave trap.

There shows an example of production of a quarter-wave trap 24 at one of the sealing chambers 20, 22.

The quarter-wave trap 24 comprises at least two rigid slots 26 which are arranged perpendicular to the direction of advance F of the belt 6 of the conveyor and over an entire width thereof.

More precisely, the slots 26 of the quarter-wave trap are made of an electrically conductive material, for example steel or aluminum, and are fixed at an upper end on a substantially horizontal upper plate 28 positioned above the strip of the conveyor.

The slots 26 are dimensioned so as to maintain a non-zero clearance, on the one hand with the side walls (not shown in the figures) of the sealing chambers, and on the other hand between their free end and the upper surface of the belt 6 of the conveyor.

Furthermore, the distance L between two adjacent slots 26 corresponds to a quarter of the wavelength of the microwaves emitted by the waveguide(s) 12 to a multiple of the wavelength (i.e. -say: L = λ/4 + n λ with λ for the wavelength and n an integer).

For a microwave field having a frequency 2450MHz, the distance L between two adjacent slots is for example 76mm, 138mm, 199mm, 260mm or 320mm.

In addition, the inlet 20 and outlet 22 sealing chambers are closed at their lower part by a substantially horizontal lower wall 30 which is positioned under the slots 26 and which is transparent to the microwaves emitted by the guides waves and impervious to vapor and dust. For example, the bottom wall 30 can be made of PTFE.

It will be noted that the minimum clearance provided between the product P positioned on the belt 6 of the conveyor and the lower end of the slots 26 is formed by the thickness of this lower wall 30.

The operation of the quarter-wave trap 24 thus described is as follows. The waves coming from the heating tunnel 8 and penetrating inside the entry seal 20 or the exit seal 22 will for the vast majority of them be reflected on the first slot 16 encountered in the airlock and return to the heating tunnel cavity.

The fraction of waves which manages to pass through the clearance provided under this first slot encountered is for the most part reflected by the following slot and returns to the level of the first slot.

The distance L between these two slots being equal to a multiple of the wavelength added to a quarter of it, the reflected waves travel a distance 2L and find themselves out of phase by half a wavelength with the wave incident at the level of the first slot. Thus, the incident and reflected waves find themselves in phase opposition and cancel each other out, which ensures tightness to microwaves at the entrance and exit of the heating tunnel.

Of course, each quarter-wave trap can include several pairs of slots 26 as described previously, these pairs being spaced longitudinally from each other by a distance corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide (to a multiple of the wavelength).

In a variant embodiment of the quarter-wave trap represented by the , each slot 26' of the quarter-wave trap comprises at one free end a groove 32 having a depth h corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide at a multiple of the length of waves (that is to say h L = λ/4 + k λ with λ for the wavelength and k an integer).

In this way, the waves reflected at the bottom of the groove 32 travel a distance equal to twice that of the depth h and find themselves out of phase by half a wavelength at the entrance to the groove relative to the incident wave. The incident and reflected waves are thus in phase opposition and cancel each other out. The presence of such a groove therefore constitutes a second quarter-wave trap.

According to an advantageous arrangement of this variant embodiment, the groove 32 of the slots 26' can contain an insert 34 made of a material having a relative dielectric permittivity preferably less than 10 F/m and a refractive index greater than 1 so as to reduce the wavelength of the microwave field, and therefore the path to follow and the depth of the throat.

For example, the insert 34 can be made of high or low density polyethylene, HDPE, polypropylene or any other material of equivalent characteristics.

In another alternative embodiment of the quarter-wave trap represented by the , each slot 26'' of the quarter-wave traps has a V shape whose tip is directed towards the belt 6 of the conveyor. Other forms of slots can of course be considered.

In the embodiments of Figures 1 to 5, the inlet 20 and outlet 22 sealing chambers are fixed.

In another embodiment illustrated by Figures 6A and 6B, these inlet 20' and outlet 22' sealing airlocks are movable vertically between a high position in which they free a passage for conveying the products P and a low safety position in which they block the passage for conveying products.

More precisely, in this embodiment, the entrance seal 20' is coupled to a presence detector 36 of products P on the belt 6 of the conveyor which is capable of activating the waveguides 12 of the tunnel. heater 8 and to control the raising and lowering of said entrance airlock.

Likewise, the exit airlock 22' is coupled to a presence detector 38 of products P on the belt 6 of the conveyor which is able to control the raising and lowering of said outlet airlock.

The operation of this oven is as follows. In its stopped configuration ( ), the entry 20' and exit 22' sealing chambers are in the low safety position and are flush with the upper surface of the conveyor belt 6. The tightness of the heating tunnel is thus ensured at the entrance and exit.

Once the advance of the belt has started, as soon as the products P placed on the conveyor belt reach the presence detector 36 coupled to the entrance airlock 20', the rise of said entry airlock is controlled to free passage to the products and the waveguides 12 of the heating tunnel are activated ( ).

The P products can then enter the heating tunnel where they are subjected to the microwave field. The quarter-wave traps 24 of the entrance airlock 20' ensure tightness to microwaves at the entrance to the heating tunnel, while the lowered outlet airlock 22' ensures tightness to microwaves at the exit of the heating tunnel.

When the products P reach the presence detector 38 located at the exit airlock 22', the presence detector triggers a rise in the exit airlock to allow the products to exit the heating tunnel and to provide microwave sealing thanks to their quarter-wave traps 24.

In yet another embodiment illustrated by Figures 7A, 7B and 7C, the entrance airlock 20'' comprises at least two airlock sections 20a, 20b independent of each other and each movable vertically between a position high in which each airlock section frees a passage for conveying products and a low safety position in which each airlock section closes the passage for conveying products,

Likewise, still in this embodiment, the exit airlock 22'' comprises at least two airlock sections 22a, 22b independent of one another and each movable vertically between a high position in which each airlock section releases a passage for conveying products and a low safety position in which each section of airlock closes the passage for conveying products.

In addition, the two sections 20a, 20b of the entrance airlock 20'' and the two sections 22a, 22b of the exit airlock 22'' are each coupled to presence detectors 40a, 40b, 42a, 42b of products on the conveyor belt making it possible to control the rise and fall of said sections.

The operation of this oven is as follows. In its stopped configuration ( ), all of the sections 20a, 20b, 22a, 22b of the inlet 20'' and outlet 22'' are in the low safety position and come flush with the upper surface of the belt 6 of the conveyor. The tightness of the heating tunnel is thus ensured at the entrance and exit.

At the start of the operation ( ), as soon as the products P placed on the conveyor belt reach the presence detectors 40a then 40b coupled respectively to the sections 20a, 20b of the entrance airlock 20'', these airlock sections are successively mounted to free passage for the products .

Furthermore, the waveguides 12 of the heating tunnel are only activated when the products P reach the presence detector 40b which is closest to the heat treatment zone.

In this configuration, sealing against microwaves is ensured, on the one hand at the entrance to the heating tunnel by the quarter-wave traps of the sections 20a, 20b of the entrance airlock 20'' which are positioned in position high, and on the other hand at the exit of the heating tunnel via the section 22a of the exit airlock 22'' which is in the low safety position.

The two sections 22a, 22b of the exit airlock 22'' remain in the low position until the products P reach the presence detector 42a, then then the presence detector 42b which successively control the raising of the sections ( ). Microwave tightness is ensured at the exit of the heating tunnel by the respective quarter-wave traps of these airlock sections.

Finally, when the presence detector 42a which is closest to the exit of the heat treatment zone no longer detects product at the outlet of the heating tunnel, the waveguides are deactivated, then the sections 22a, 22b of the airlock outlet 22'' are successively lowered into the safety position.

According to an advantageous arrangement of the invention which applies to all of the embodiments previously described and which is illustrated on the , the oven can include a counter-current spray ramp 44 which is arranged above the sealed wall 18.

This spray boom 44 has an inlet 44a which opens first into a first spray nozzle 44b located at the exit end of the belt 6 of the conveyor before the exit airlock 22, then into several successive spray nozzles 44c which are spaced longitudinally from each other towards the entrance to the heating tunnel.

The tray 16 positioned under the belt 6 of the conveyor is formed of several sections of trays which are each arranged under a spray nozzle 44b, 44c. The number of tank sections is advantageously identical to the number of spray nozzles.

The extracts obtained are recovered via the collector 16a of the most downstream bin section (relative to the direction of advance of the conveyor belt) and are reinjected via pumps 46 onto the products P via spray nozzles 44c, upstream of the first injection point and so on countercurrent to the direction of advance of the conveyor belt.

After multiple reinjections, the number of which can for example be between 2 and 100, the concentrated extracts are evacuated by the collector of the most upstream section of the tank (in relation to the direction of advance of the conveyor belt).

This embodiment of reinjections advantageously makes it possible to exhaust the products P from their extracts as much as possible with a minimum of solvent, since the last spraying before the products are released is carried out with fresh solvent. This embodiment thus makes it possible to optimize the extract concentration and minimize its consumption.

According to another advantageous provision of the invention which applies to all of the embodiments previously described and which is illustrated by the , the belt 6 of the conveyor comprises a plurality of extraction chambers 48 which are delimited by walls 50 arranged perpendicular to a direction of advance of the conveyor belt and over an entire width thereof.

These walls 50 extend vertically over almost the entire height between the upper surface of the strip 6 and the sealed wall 18 and are made of a material transparent to the microwaves emitted by the wave guides (for example PFTE or carbon dioxide). silicone). These extraction chambers 48 thus make it possible to obtain better sealing against the extraction vapors and to prevent these vapors from escaping through the entrance and exit airlocks of the heating tunnel.

Claims (16)

Microwave extraction oven (2) for the continuous heat treatment of organic products in solid or pasty state, comprising:

• a belt conveyor (4) intended to transport the products (P) to be treated, the belt (6) of the conveyor being made of a material transparent to microwaves and porous to the extracts resulting from the heat treatment of the products,
• a heating tunnel (8) which is crossed right through by the conveyor belt and which comprises a microwave-tight box (10) and into which at least one connected waveguide (12) opens to a microwave generator (14), and
• a tray (16) positioned under the conveyor belt at the level of the box to receive and condense extracts resulting from the heat treatment of the products after their diffusion through the conveyor belt,
• characterized in that the heating tunnel (8) comprises at least one inlet seal (20) which is positioned at the entrance to the heating tunnel above the conveyor and at least one outlet seal ( 22) which is positioned at the outlet of the heating tunnel above the conveyor, the entry and exit airlocks each being provided with at least one quarter-wave trap (24) in order to ensure tightness against micro -waves entering and leaving the heating tunnel.

Extraction furnace according to claim 1, in which each quarter-wave trap (24) of the inlet and outlet airlocks comprises at least two rigid slots (16; 16'; 16'') arranged perpendicular to a direction advance (F) of the belt (6) of the conveyor and over an entire width thereof, the distance (L) between the two slots corresponding to a quarter of the wavelength of the microwaves emitted by the guide of waves to a multiple of the wavelength. Extraction furnace according to claim 2, wherein each quarter-wave trap (24) of the entry and exit airlocks comprises a substantially horizontal upper plate (28) positioned above the conveyor belt and on which the at least two slots are fixed, the distance separating the strip from a free end of the slots being non-zero. Extraction furnace according to claim 3, in which each quarter-wave trap (24) of the entry and exit airlocks further comprises a lower wall (430 substantially horizontal positioned under the at least two slots, said lower wall being made of a material transparent to the microwaves emitted by the waveguide and impervious to vapors and dust. Extraction furnace according to any one of claims 2 to 4, in which each slot (16') of the quarter-wave traps (24) comprises at one free end a groove (32) having a depth ( h ) corresponding a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength. Extraction furnace according to claim 5, in which the groove (32) of the slots (16') contains an insert (34) made of a material having a relative dielectric permittivity less than 10 F/m and a refractive index greater than 1. Extraction furnace according to any one of claims 2 to 4, in which each slot (16'') of the quarter-wave traps (24) has a V shape whose tip is directed towards the strip (6) of the conveyor. Extraction furnace according to any one of claims 2 to 7, in which each slot (16; 16'; 16'') of the quarter-wave traps (24) is made of an electrically conductive material. Extraction furnace according to any one of claims 1 to 8, in which the inlet airlock (20 and/or the outlet airlock (22) is fixed. Extraction furnace according to any one of claims 1 to 8, in which the inlet airlock (20') and/or the outlet airlock (22') is movable vertically between a high position in which it clears a passage for conveying products and a low position in which it closes the passage for conveying products. Extraction oven according to claim 10, in which the entrance airlock (20') is movable vertically and is coupled to a presence detector (36) of products on the conveyor belt making it possible to activate the waveguide (12) of the heating tunnel and to control the raising and lowering of said entrance airlock. Extraction oven according to one of claims 10 and 11, in which the exit airlock (22') is movable vertically and is coupled to a presence detector (38) of products on the conveyor belt making it possible to control the rise and the descent of said exit airlock. Extraction furnace according to any one of claims 1 to 8, in which the entry airlock (20'') comprises at least two airlock sections (20a, 20b) independent of one another and each movable vertically between a high position in which it frees a passage for the conveying of products and a low position in which it closes the passage for the conveying of products, and the exit airlock (22'') comprises at least two airlock sections ( 22a, 22b) independent of each other and each movable vertically between a high position in which it frees a passage for the conveying of products and a low position in which it closes the passage for the conveying of products. Extraction furnace according to claim 13, in which the two sections (20a, 20b, 22a, 22b) of the entry (20'') and exit (22'') airlocks are coupled to presence detectors (40a , 40b, 42a, 42b) of products on the conveyor belt making it possible to control the rise and fall of said sections. Extraction oven according to any one of claims 1 to 14, in which the belt (6) of the conveyor comprises a plurality of extraction chambers (48) delimited by walls (50) arranged perpendicular to a direction of advance of the conveyor belt and over an entire width thereof, said walls being made of a material transparent to the microwaves emitted by the waveguide. Extraction furnace according to any one of claims 1 to 15, further comprising means (44, 44a, 44b, 46) for recovering the extracts and water resulting from the heat treatment of the products and reinjecting them into the extraction tunnel. heats up on the products.