Microwave extraction oven for the continuous heat treatment of solid or pasty organic products

Abstract

The invention relates to a microwave extraction oven (2) for the continuous heat treatment of solid organic products, comprising: - a belt conveyor (4) intended to transport the products (P) to be treated, the belt (6) of the conveyor being porous to extracts resulting from the treatment of the products; - a heating tunnel (8) through which the belt of the conveyor passes from one end to the other and comprising a microwave-impermeable enclosure (10) inside which at least one waveguide (12) opens, which waveguide is connected to a microwave generator (14); and - a tray (16) for receiving and condensing the extracts resulting from the treatment of the products after they have diffused through the belt. The heating tunnel comprises an entrance seal (20) positioned at the inlet to the heating tunnel above the conveyor, and an exit seal (22) positioned at the outlet of the heating tunnel above the conveyor, the entrance and exit seals each being provided with at least one quarter wave trap (24) in order to ensure microwave sealing at the entrance and exit of the heating tunnel.

Description

Description Title of the invention: Microwave extraction furnace for the continuous thermal treatment of organic products in solid or paste form Technical Field

[0001] The invention relates to the general field of microwave heat treatment of organic products in solid or paste form to obtain extracts in liquid form.

[0002] A non-limiting application area of ​​the invention relates to microwave-assisted extraction of volatile and non-volatile active ingredients from plant biomass. Previous technique

[0003] Various continuous extraction processes for solid or paste-like plant materials using solvents are known. Generally, these solid-liquid extraction processes operate on the principle of solvent extraction, with co-current or counter-current injections onto the product to collect extracts such as essential oils or active ingredients through percolation, diffusion, or immersion. Devices for transporting and mixing the material are implemented, for example, using perforated bucket conveyors, perforated belts, screw conveyors, or vertical rotary rotors with honeycomb structures.

[0004] The main drawback of these processes is that they require significant extraction times (on 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 large quantities of solvents, while regulations are increasingly restricting their use.

[0005] One known way to overcome this drawback is to use high-performance, neutral solvents, such as carbon dioxide in its pure form. supercritical, or to assist the extraction operation with secondary energy by using for example ultrasound or microwaves.

[0006] While ultrasound is effective in breaking down the plant matrix, it only exceptionally allows for a good extraction yield.

[0007] In contrast, microwave extraction processes allow for very rapid release of extracts, often within ten to thirty minutes. Microwaves are particularly effective because they heat the product directly to its core. For example, with plant materials, microwaves allow water-soluble active ingredients to be carried along with the plant's water, burst essential oil sacs, and vaporize the plant's water, which carries these volatile elements (essential oils) with it. Microwaves are preferentially absorbed by materials with high dielectric constants, which is the case for the water contained in biological materials and their vascular and glandular tissues.

[0008] A device and a discontinuous method for extraction using microwaves and ultrasound are known from publication FR 3,017,308. A continuous method for extraction using microwaves and ultrasound is also known from publication WO 2016 / 118034.

[0009] We are still familiar with publications EP 1,629,725 and FR 2,975,020, which describe microwave-assisted solvent-free extraction processes in which the vaporization of the water contained in the plant allows the extracts to be carried away. These processes are suitable for batch extractions of small volumes. However, they are not suitable for fragile products with low active ingredient content or essential oils.

[0010] The main drawback of these microwave-assisted processes is that they can only operate in batches or continuously with screw drive, which is not suitable for fragile products (e.g., flowers or thin leaves), nor for those with a low content of materials to be extracted.

[0011] To overcome this drawback, a device and method for the continuous extraction of solid or pasty organic materials was proposed in publication FR 3,070,869. This method involves placing the product to be treated on a conveyor belt with slots that create relatively airtight extraction chambers during microwave exposure and allow for partial steam pressure throughout the heat treatment. These slots, incorporating metallic quarter-wave traps, also ensure microwave tightness in the inlet and outlet chambers.

[0012] In this invention, under the influence of a microwave field, a fraction of the water contained in the product being treated vaporizes, carrying with it volatile molecules (essential oils, etc.), while another fraction of the water (for example, plant water) is extracted mechanically in liquid form due to the partial pressure of vapor within the product (for example, in plant cells). The process according to this invention allows the liquid and vapor fractions of the extracts to be evacuated through the product and its support, recovering all the extracts at a condenser at a low point. This process thus allows the recovery, by percolation and diffusion, of not only the volatile molecules but also the non-volatile ones.

[0013] Although effective, this extraction device has the disadvantage of having metallic quarter-wave traps all along the conveyor belt, which results in a significant mass of the conveyor and a complex implementation of it. Description of the invention

[0014] The invention aims to provide a microwave extraction oven for the continuous thermal treatment of organic products in solid state which does not present the aforementioned disadvantages thanks to specific sealing chambers.

[0015] This goal is achieved using a microwave extraction oven comprising: - a conveyor belt designed to transport the products to be processed, the conveyor belt being made of a material that is microwave-transparent and porous to extracts from the heat treatment of the products, - a heating tunnel through which the conveyor belt passes and which includes a microwave-proof enclosure, into which opens at least one waveguide connected to a microwave generator, and - a container positioned under the conveyor belt at the level of the box to receive and condense extracts from the heat treatment of the products after their diffusion through the conveyor belt, - and in which, according to the invention, the heating tunnel comprises at least one inlet sealing chamber which is positioned at the inlet of the heating tunnel above the conveyor and at least one outlet sealing chamber which is positioned at the outlet of the heating tunnel above the conveyor, the inlet and outlet chambers each being equipped with at least one quarter-wave trap in order to ensure microwave sealing at the inlet and outlet of the heating tunnel.

[0016] The extraction oven according to the invention is remarkable in that microwave sealing is achieved not at slots positioned on the conveyor belt, but at the inlet and outlet chambers of the heating tunnel. The implementation of such an extraction oven is thus greatly simplified (particularly because the conveyor belt can remain unchanged).

[0017] Preferably, each quarter-wave trap of the entry and exit airlocks includes at least two rigid slots arranged perpendicular to a direction of advance of the conveyor belt and across its entire width, the distance between the two slots corresponding to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength.

[0018] In this case, each quarter-wave trap of the inlet and outlet chambers advantageously comprises a substantially horizontal upper plate positioned above the conveyor belt and on which at least two slots are fixed, the distance separating the belt from a free end of the slots being non-zero. This plate is made of an electrically conductive material (for example, stainless steel).

[0019] In addition, each quarter-wave trap of the entry and exit airlocks advantageously includes a substantially horizontal lower wall positioned below the at least two slots, said lower wall being made of a material transparent to microwaves emitted by the waveguide and impermeable to vapors and dust.

[0020] Each quarter-wave trap slot may include at one free end a groove having a depth corresponding to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength.

[0021] In this case, the groove of the slots may contain an insert made of a material having a relative dielectric permittivity of less than 10 F / m and a refractive index greater than 1.

[0022] Each slot in the quarter-wave traps can have a V shape with the point directed towards the conveyor belt.

[0023] Each slot in the quarter-wave traps is preferably made of an electrically conductive material.

[0024] In one embodiment, the entry airlock and / or the exit airlock is fixed.

[0025] In another embodiment, the entry airlock and / or exit airlock is vertically movable between a high position in which it opens a passage for the conveyance of products and a low position in which it closes the passage for the conveyance of products.

[0026] In this case, the entry airlock can be vertically mobile and coupled to a product presence detector on the conveyor belt, allowing to activate the waveguide of the heating tunnel and to control the raising and lowering of said entry airlock.

[0027] In this case as well, the exit gate can be vertically mobile and coupled to a product presence detector on the conveyor belt allowing the raising and lowering of said exit gate to be controlled.

[0028] In yet another embodiment, the entry airlock comprises at least two independent airlock sections, each movable vertically between a high position in which it opens a passage for the conveyance of products and a low position in which it closes the passage for the conveyance of products, and the exit airlock comprises at least two independent airlock sections, each movable vertically between a high position in which it opens a passage for the conveyance of products and a low position in which it closes the passage for the conveyance of products.

[0029] In this case, the two sections of the entry and exit airlocks can be coupled to product presence detectors on the conveyor belt, allowing the raising and lowering of said sections to be controlled.

[0030] The conveyor belt may include a plurality of extraction chambers delimited by walls arranged perpendicular to a direction of advance of the conveyor belt and across its entire width, said walls being made of a material transparent to microwaves emitted by the waveguide.

[0031] The extraction furnace may further include means for recovering extracts and water from the heat treatment of the products and reinjecting them into the heating tunnel onto the products. Brief description of the drawings

[0032] [Fig. 1] Figure 1 is a longitudinal cross-sectional view of an extraction furnace according to an embodiment of the invention.

[0033] [Fig. 2] Figure 2 is a cross-sectional view of the heating tunnel of the extraction furnace in Figure 1 showing one of the sealing chambers (inlet or outlet).

[0034] [Fig. 3] Figure 3 shows in detail and partially the realization of the entry and exit sealing chambers according to the invention.

[0035] [Fig. 4] Figure 4 shows in detail a sealing airlock according to an alternative embodiment of the invention.

[0036] [Fig. 5] Figure 5 shows in detail a sealing airlock according to another embodiment of the invention.

[0037] [Fig. 6A] Figure 6A is a longitudinal cross-sectional view of an extraction furnace according to an alternative embodiment of the invention in a stopped configuration.

[0038] [Fig. 6B] Figure 6B is a view of the furnace in Figure 6A in production configuration.

[0039] [Fig. 7A] Figure 7A is a longitudinal cross-sectional view of an extraction furnace according to another embodiment of the invention in a stopped configuration.

[0040] [Fig. 7B] Figure 7B is a view of the furnace in Figure 7A in production start-up configuration.

[0041] [Fig. 7C] Figure 7C is a view of the furnace in Figure 7A in production configuration.

[0042] [Fig. 8] Figure 8 is a longitudinal cross-sectional view of an extraction furnace according to yet another embodiment of the invention.

[0043] [Fig. 9] Figure 9 is a longitudinal cross-sectional view of an extraction furnace according to an alternative embodiment of the invention. Description of the implementation methods

[0044] The invention relates to the continuous extraction of organic products in solid or paste form using a microwave extraction oven such as that shown in Figures 1 and 2.

[0045] The oven 2 includes in particular a conveyor 4 with a belt 6 which is intended to transport the products to be treated P in a direction of advance F. The advance speed 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.

[0046] Conveyor belt 6 is advantageously made of a microwave-transparent material, for example fiberglass, so that it does not heat up when exposed to microwaves.

[0047] Conveyor belt 6 is also porous to extracts from the heat treatment of the products. For this purpose, it may have perforations typically ranging from 50 µm to 20 mm, and preferably around 2 mm.

[0048] The conveyor belt 6 passes through a heating tunnel 8 whose length depends on the desired power and the flow rate of product to be treated (it is generally between 1m and 50m).

[0049] The heating tunnel includes a microwave-proof box 10 into which opens at least one waveguide 12 connected to a microwave generator 14.

[0050] The enclosure 10 is made, for example, of a conductive material such as steel or aluminum, and its height is typically between 100 and 500 mm to ensure sufficient space between the arrival of the waveguides 12 and the product to be treated P on the conveyor belt 6. This configuration allows the electromagnetic field to propagate in multiple modes and reach the product P as homogeneously as possible.

[0051] In the example of the embodiment shown in Figures 1 and 2, the heating tunnel comprises three waveguides 12 regularly spaced along the box and each connected to a microwave generator 14. Of course, other configurations are possible.

[0052] The lower part of the heating tunnel also includes a tank 16 positioned under the conveyor belt 6 at the level of the box 10 to receive and condense the extracts from the heat treatment of the products P after their diffusion through the conveyor belt.

[0053] For this purpose, this tank 16 is microwave-proof and, for example, equipped with a double-walled, sealed lining designed to receive a cold fluid in order to cause condensation of the vapor phase and cooling of the collected extracts. The resulting liquid extracts are then discharged through a collector 16a located at the bottom of the tank 16.

[0054] A flat, sealed wall 18 closes the box 10 in its lower part by covering the surface of the conveyor belt 6 which is located in the heating tunnel 8.

[0055] This wall 18 prevents the steam extracted during the heating of the products by microwaves from diffusing inside the chamber 10. The wall is vapor-tight and microwave-transparent. It is made, for example, of PTFE.

[0056] According to the invention, microwave leak-proofness at the inlet and outlet of the heating tunnel 8 is ensured by at least one inlet sealing chamber 20 which is positioned at the inlet of the heating tunnel above the conveyor belt and by at least one outlet sealing chamber 22 which is positioned at the outlet of the tunnel above the conveyor belt.

[0057] The inlet 20 and outlet 22 sealing airlocks allow the microwaves to be confined inside the heating tunnel 8.

[0058] For this purpose, in the embodiment of figures 1 and 2, the inlet sealing chambers 20 and outlet chambers 22 are each equipped with at least one quarter-wave trap.

[0059] Figure 3 shows an example of the implementation of a quarter-wave trap 24 at the level of one of the sealing chambers 20, 22.

[0060] The quarter-wave trap 24 includes at least two rigid slots 26 which are arranged perpendicular to the direction of advance F of the conveyor belt 6 and over one of its widths.

[0061] More specifically, 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 to a substantially horizontal upper plate 28 positioned above the conveyor belt.

[0062] 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 conveyor belt 6.

[0063] 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 (that is: L = A / 4 + n A with A for the wavelength and n an integer).

[0064] For a microwave field with a frequency of 2450MHz, the distance L between two adjacent slots is for example 76mm, 138mm, 199mm, 260mm or 320mm.

[0065] Furthermore, the inlet 20 and outlet 22 sealing chambers are closed at their lower part by a substantially horizontal lower wall 30 positioned below the slots 26, which is transparent to microwaves emitted by the waveguides and impermeable to vapor and dust. For example, the lower wall 30 can be made of PTFE.

[0066] Note that the minimum clearance between the product P positioned on the conveyor belt 6 and the lower end of the slots 26 is formed by the thickness of this lower wall 30.

[0067] The operation of the quarter-wave trap 24 thus described is as follows. The waves coming from the heating tunnel 8 and entering the inlet sealing chamber 20 or the outlet sealing chamber 22 will, for the vast majority of them, be reflected on the first niche 16 encountered in the sealing chamber and return into the cavity of the heating tunnel.

[0068] The fraction of the waves that manages to pass through the gap e provided under this first slot encountered is mostly reflected by the next slot and returns to the level of the first slot.

[0069] Since the distance L between these two slots is equal to a multiple of the wavelength plus one-quarter of that wavelength, the reflected waves travel a distance of 2L and are phase-shifted by half a wavelength from the incident wave at the first slot. Thus, the incident and reflected waves are out of phase and cancel each other out, ensuring microwave sealing at the inlet and outlet of the heating tunnel.

[0070] Of course, each quarter-wave trap can have several pairs of slots 26 as described above, these pairs being spaced longitudinally apart from each other by a distance corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide (to within a multiple of the wavelength).

[0071] In an embodiment variant of the quarter-wave trap shown in Figure 4, each slot 26' of the quarter-wave trap includes at a free end a groove 32 having a depth h corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength (i.e. h L = A / 4 + k A with A for the wavelength and k an integer).

[0072] Thus, the waves reflected at the bottom of the throat 32 travel a distance equal to twice the depth h and are phase-shifted by half a wavelength at the entrance of the throat relative to the incident wave. The incident and reflected waves are therefore in opposite phase and cancel each other out. The presence of such a throat thus constitutes a second quarter-wave trap.

[0073] According to an advantageous arrangement of this 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 in order to decrease the wavelength of the microwave field, and therefore the path to travel and the depth of the throat.

[0074] For example, insert 34 can be made of high or low density polyethylene, HDPE, polypropylene or any other material with equivalent characteristics.

[0075] In another embodiment of the quarter-wave trap shown in Figure 5, each 26" slot of the quarter-wave traps has a V-shape with its point directed towards conveyor belt 6. Other slot shapes can, of course, be considered.

[0076] In the embodiments of figures 1 to 5, the inlet sealing chambers 20 and outlet chambers 22 are fixed.

[0077] In another embodiment illustrated by figures 6A and 6B, these inlet 20' and outlet 22' sealing chambers are vertically movable between a high position in which they free up a passage for the conveyance of products P and a low safety position in which they close the passage for the conveyance of products.

[0078] More specifically, in this embodiment, the entry sealing airlock 20' is coupled to a presence detector 36 of products P on the conveyor belt 6 which is capable of activating the waveguides 12 of the heating tunnel 8 and of controlling the raising and lowering of said entry airlock.

[0079] Similarly, the exit sealing chamber 22' is coupled to a presence detector 38 of products P on the conveyor belt 6 which is capable of controlling the raising and lowering of said exit chamber.

[0080] The operation of this furnace is as follows. In its stopped configuration (Figure 6A), the inlet 20' and outlet 22' sealing chambers are in the lower safety position and are flush with the upper surface of conveyor belt 6. The heating tunnel is thus sealed at both the inlet and outlet.

[0081] Once the belt advance has started, as soon as the products P placed on the conveyor belt reach the presence detector 36 coupled to the entry gate 20', the rise of said entry gate is commanded to free the passage of the products and the waveguides 12 of the heating tunnel are activated (figure 6B).

[0082] The P products can then enter the heating tunnel where they are subjected to the microwave field. The quarter-wave traps 24 of the inlet airlock 20' ensure microwave sealing at the entrance to the heating tunnel, while the lowered outlet airlock 22' ensures microwave sealing at the exit of the heating tunnel.

[0083] When the products P reach the presence detector 38 located at the exit airlock 22', the presence detector triggers a rise of the exit airlock to allow the products to exit the heating tunnel and to achieve microwave sealing thanks to their quarter-wave traps 24.

[0084] In yet another embodiment illustrated by figures 7A, 7B and 7C, the 20" entry airlock comprises at least two airlock sections 20a, 20b independent of each other and each movable vertically between a high position in which each airlock section provides a passage for conveying products and a low safety position in which each airlock section closes the passage for conveying products,

[0085] Similarly, still in this embodiment, 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 each airlock section opens a passage for the conveyance of products and a low safety position in which each airlock section closes the passage for the conveyance of products.

[0086] In addition, the two sections 20a, 20b of the 20" entry airlock and the two sections 22a, 22b of the 22" exit airlock are each coupled to product presence detectors 40a, 40b, 42a, 42b on the conveyor belt allowing the raising and lowering of said sections to be controlled.

[0087] The operation of this furnace is as follows. In its stopped configuration (Figure 7A), all sections 20a, 20b, 22a, 22b of the 20" inlet and 22" outlet chambers are in the lower safety position and are flush with the upper surface of conveyor belt 6. The heating tunnel is thus sealed at both the inlet and outlet.

[0088] At the start of the operation (figure 7B), 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 entry airlock 20", these airlock sections are successively mounted to free the passage of the products.

[0089] 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 area.

[0090] In this configuration, microwave sealing is ensured, on the one hand at the entrance of the heating tunnel by the quarter-wave traps of sections 20a, 20b of the 20" entrance airlock which are positioned in the high position, and on the other hand at the exit of the heating tunnel by section 22a of the 22" exit airlock which is in the low safety position.

[0091] The two sections 22a and 22b of the exit airlock 22" remain in the lowered position until the products P reach the presence detector 42a, and then the presence detector 42b, which successively trigger the raising of the sections (Figure 7C). Microwave sealing is ensured at the exit of the heating tunnel by the respective quarter-wave traps of these airlock sections.

[0092] Finally, when the presence detector 42a, which is closest to the exit of the heat treatment area, no longer detects any product exiting the heating tunnel, the waveguides are deactivated, and then sections 22a, 22b of the exit airlock 22" are successively lowered into the safety position.

[0093] According to an advantageous arrangement of the invention which applies to all the embodiments described above and which is illustrated in Figure 1, the oven may include a counter-current spray ramp 44 which is arranged above the sealed wall 18.

[0094] This spraying ramp 44 has an inlet 44a which opens first into a first spray nozzle 44b located at the exit end of the conveyor belt 6 before the exit gate 22, then into several successive spray nozzles 44c which are spaced longitudinally from each other towards the entrance of the heating tunnel.

[0095] The tray 16 positioned under the conveyor belt 6 is made up of several tray sections, each arranged under a spray nozzle 44b, 44c. The number of tray sections is advantageously the same as the number of spray nozzles.

[0096] The extracts obtained are recovered via collector 16a from the most downstream section of the tank (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 counter-current to the direction of advance of the conveyor belt.

[0097] After multiple reinjections, the number of which can be between 2 and 100 for example, the concentrated extracts are evacuated by the collector of the upstream section of the tank (relative to the direction of advance of the conveyor belt).

[0098] This method of reinjection advantageously allows for maximum depletion of the P products from their extracts with a minimum of solvent, since the final spraying before product dispensing is performed with fresh solvent. This method thus optimizes extract concentration and minimizes solvent consumption.

[0099] According to another advantageous embodiment of the invention which applies to all the embodiments described above and which is illustrated by Figure 8, the conveyor belt 6 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 across its entire width.

[0100] These walls 50 extend vertically over almost the entire height between the upper surface of the band 6 and the airtight wall 18 and are made of a material transparent to the microwaves emitted by the waveguides (for example, PTFE or silicone). These extraction chambers 48 thus provide better sealing against extraction vapors and prevent these vapors from escaping through the inlet and outlet airlocks of the heating tunnel.

[0101] Figure 9 shows an extraction furnace 2' according to an alternative embodiment of the invention. This extraction furnace 2' differs from the one previously described in that it includes an inlet cladding (or formwork) system 52 which is positioned upstream of the inlet sealing chamber 20 and around the conveyor belt. This inlet cladding system 52 prevents any microwave leakage at the inlet sealing chamber 20 without the need for a vertically movable inlet sealing chamber (as described in particular in connection with Figures 6A and 6B).

[0102] Similarly, the extraction furnace 2' according to this embodiment includes an outlet cladding (or formwork) system 54 which is positioned downstream of the outlet sealing chamber 22 and around the conveyor belt. This outlet cladding system 54 also prevents any microwave leakage at the outlet sealing chamber 22 without the need for a vertically movable outlet sealing chamber (as described in particular in connection with Figures 6A and 6B).

[0103] In this embodiment, the inlet cladding system 52 includes a product inlet 52a which is made microwave-tight by means of, for example, a rotary gate valve 52b. Similarly, the outlet cladding system 54 includes a product outlet 54a which is made microwave-tight by means of, for example, a rotary gate valve 54b.

Claims

Demands

1. Microwave extraction furnace (2; 2') for the continuous heat treatment of organic products in solid or paste form, comprising: - a conveyor belt (4) intended to transport the products (P) to be treated, the conveyor belt (6) being made of a material that is microwave-transparent and porous to extracts from the heat treatment of the products, - a heating tunnel (8) through which the conveyor belt passes and which includes a microwave-tight enclosure (10) into which opens at least one waveguide (12) connected to a microwave generator (14), and - a container (16) positioned under the conveyor belt at the level of the box to receive and condense extracts 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 sealing chamber (20) which is positioned at the inlet of the heating tunnel above the conveyor and at least one outlet sealing chamber (22) which is positioned at the outlet of the heating tunnel above the conveyor, the inlet and outlet chambers each being equipped with at least one quarter-wave trap (24) in order to ensure microwave sealing at the inlet and outlet of the heating tunnel.

2. Extraction furnace according to claim 1, wherein each quarter-wave trap (24) of the inlet and outlet airlocks comprises at least two rigid slots (16; 16'; 16") arranged perpendicular to a feed direction (F) of the conveyor belt (6) and over a full width thereof, the distance (L) between the two slots corresponding to one-quarter of the wavelength of the microwaves emitted by the waveguide to within a multiple of the wavelength.

3. Extraction furnace according to claim 2, wherein each quarter-wave trap (24) of the inlet and outlet chambers comprises a substantially horizontal top plate (28) positioned above the conveyor belt and on which are fixed at least two slots, the distance separating the belt from a free end of the slots being non-zero.

4. Extraction furnace according to claim 3, wherein each quarter-wave trap (24) of the inlet and outlet airlocks further comprises a substantially horizontal lower wall (430) positioned below the at least two slots, said lower wall being made of a material transparent to microwaves emitted by the waveguide and impermeable to vapors and dust.

5. Extraction furnace according to any one of claims 2 to 4, wherein each slot (16 7) quarter-wave traps (24) include at a free end a groove (32) having a depth (h) corresponding to a quarter of the wavelength of the microwaves emitted by the waveguide to a multiple of the wavelength.

6. 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.

7. Extraction furnace according to any one of claims 2 to 4, wherein each slot (16") of the quarter-wave traps (24) has a V shape whose tip is directed towards the conveyor belt (6).

8. Extraction furnace according to any one of claims 2 to 7, wherein each slot (16; 16'; 16") of the quarter-wave traps (24) is made of an electrically conductive material.

9. Extraction furnace according to any one of claims 1 to 8, wherein the inlet airlock (20) and / or the outlet airlock (22) is fixed.

10. Extraction furnace according to any one of claims 1 to 8, wherein the inlet airlock (20') and / or the outlet airlock (22') is vertically movable between a high position in which it opens a passage for conveying products and a low position in which it closes the passage for conveying products.

11. Extraction furnace according to claim 10, wherein the inlet airlock (209) is vertically mobile and is coupled to a product presence detector (36) on the conveyor belt enabling the waveguide (12) of the heating tunnel to be activated and the inlet airlock to be controlled.

12. Extraction furnace according to any one of claims 10 and 11, wherein the exit gate (229) is vertically mobile and is coupled to a product presence detector (38) on the conveyor belt enabling the raising and lowering of said exit gate to be controlled.

13. Extraction furnace according to any one of claims 1 to 8, wherein the inlet airlock (20'9) comprises at least two airlock sections (20a, 20b) independent of each other and movable vertically between a high position in which it opens a passage for conveying products and a low position in which it closes the passage for conveying products, and the outlet airlock (22'9) comprises at least two airlock sections (22a, 22b) independent of each other and movable vertically between a high position in which it opens a passage for conveying products and a low position in which it closes the passage for conveying products.

14. Extraction furnace according to claim 13, wherein the two sections (20a, 20b, 22a, 22b) of the inlet (20'9) and outlet (22") airlocks are coupled to presence detectors (40a, 40b, 42a, 42b) of products on the conveyor belt enabling the raising and lowering of said sections to be controlled.

15. Extraction furnace according to any one of claims 1 to 14, in which the conveyor belt (6) comprises a plurality of extraction chambers (48) delimited by walls (50) arranged perpendicular to a direction of advance of the conveyor belt and across its entire width, said walls being made of a material transparent to microwaves emitted by the waveguide.

16. Extraction furnace according to any one of claims 1 to 15, further comprising means (44, 44a, 44b, 46) for recovering the extracts and water from the heat treatment of the products and reinjecting them into the heating tunnel on the products.

17. Extraction furnace (29 according to any one of claims 1 to 16, further comprising an inlet cladding system (52) positioned upstream of the inlet sealing chamber (20) and / or an outlet cladding system (54) positioned downstream of the outlet sealing chamber (22) in order to prevent microwave leakage.

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