EP1232047B1 - Method and device for treating wood and similar materials - Google Patents

Abstract

The invention concerns a device and a method for heat treatment and impregnation, essentially by immersion, of wood or similar ligneous materials. The treatment method comprises a heating step for obtaining sudden evaporation of the water contained in the piece to be treated, followed by a step which consists in rapidly cooling the piece by contacting it with a liquid medium (6) containing wood preserving agent(s).

Description

The present invention relates to a method and a device for heat treatment and impregnation, essentially by immersion, wood or similar woody materials.

There are many wood preservation processes in sight to improve its resistance to attack by biological agents such as termites or other wood-eating insects and rot (cubic, fibrous or soft). They are distinguished on the one hand by the agent or the solution impregnation used, and on the other hand by the process allowing to introduce and fix this agent (or the solution to be impregnated) inside pieces or structures of wood to be treated.

Patent application EP-A-956 934 describes a process impregnating wood with a termite preservative and arylpyrazole type mushrooms, in which the mixture is heated to 350 ° C. wood to remove moisture and oils, placing it in an oven under an inert gas atmosphere for 5 minutes; the wood is then cooled to 20 ° C and immersed in an aqueous solution containing the agent of preservation for one hour.

Another wood impregnation process is mentioned in patent application CA-A-976 813.

The impregnation of wood in preservatives depends on the composition and structure of the latter. The impregnation capacity is very variable from one species to another; beech, maple and charm soak up completely while the heart of other species is difficult accessible and only the sapwood can be significantly impregnated with preserving substances. Finally, some species, such as fir, spruce or Douglas fir can be very difficult to impregnate.

The effectiveness of an impregnation treatment can be assessed at once by the quantity and depth of penetration of the preservative absorbed in the piece of wood. Among the processes ensuring low penetration depth, let us quote in particular: the external application of the preservative using a brush, by sprinkling or spraying, by direct immersion in the treatment solution containing the preservation. Increasing penetration depth and impregnated quantities is generally obtained from a treatment in autoclave such as the process described in the document "Guide to the wood preservation ”, Michel RAYZAL, March 1998, Center technique du wood and furniture (CTBA) - 75012 PARIS, pages 62-67.

The major drawback of these treatments is the use enclosure and pumps allowing the realization of vacuum - pressure cycles including investment and operating costs (energy essentially) are high. In addition, achieving quality given impregnation requires pre-drying the wood - expensive energetically and long - as well as delicate steering and control of pressure and vacuum cycles.

The autoclave treatment times are also long (several hours) and require long equipment downtimes.

The object of the present invention is to provide an improved method wood treatment and a device for its implementation.

The present invention also aims to remedy, in part at least, to the disadvantages of known methods.

In a method of treating a piece of wood according to the invention, said part is heated in order to obtain an intense vaporization of a part water contained in the material. Before the water content at the heart of the part is below the fiber saturation point (i.e. either located in a range from 20% to 60%, in particular between 30% and 40% mass / dry mass), said part is suddenly cooled by placing contact with a "cold" liquid medium (the temperature of which is very below the boiling temperature of water at working pressure) containing the preservative (s). This last step is carried out preferably by immersion in a (liquid) bath of the solution of which seeks to permeate the room.

It has been found experimentally that, during this cooling, the treatment liquid penetrates to a significant depth in the piece of wood. This penetration results from the strong depression created by the brutal condensation of the residual vapor previously created. The power of suction (or aspiration), thus generated, allows impregnation significant of the initial voids or those left by the departure of the water during the first stage.

In other words, the invention is based on the generation of a strong pressure drop in wood fabrics (induced by intense vaporization) in the presence of high heat flux densities and a temperature of the material greater than or equal to the boiling point of water at the working pressure. This first step is followed by contacting the as fast as possible with the treatment solution at a very high temperature below the boiling point of water. To the internal overpressure of the first stage follows a depression during the second stage which is used for the impregnation of the pores and vessels of the piece of wood by capillary flow of the treatment solution in contact with its surface exterior.

To this end, it is essential to avoid prolonged contact of the heated room, when transferred to the cooling bath containing the preservative (s), with an intermediate phase non-treating (gaseous, in particular with ambient air), which would likely to "fill" the wood vessels in places and place of the product treatment. Preferably, this transfer must be made in less than 30 seconds, especially in less than 10 seconds, for example in less than 3 or 5 seconds. This effect can, if necessary, be limited or even avoided by waterproofing or covering the ends of the part to be treated where the longitudinal fibers of the wood are exposed and which offer low pressure losses with gases and liquids; this further promotes the implementation internal removal of the piece of wood by heating. Likewise, the maintaining the temperature of the atmosphere traversed by the material, during its transfer, at a temperature higher than the boiling point of water (at working pressure) by spraying liquid, hot air from steam or by additional heating (by infrared, high frequencies, microwave) will prevent early condensation of the vapor trapped in the porous matrix. The latter can finally be prevented by a transfer of the material with the atmosphere used during the first stage by means of a nacelle, a lock or any other adapted device.

Unlike autoclave processes which use, during cycles vacuum-pressure, the variation of specific volume of gases initially present in the material, and therefore prior drying, it is essential, in a process according to the invention, that the parts to be treated are not dry; it is essential that the parts to be treated have humidity greater than 25%, in particular of the order of 40% to 80% (mass / mass dry); for this purpose the parts can if necessary be re-humidified before treatment by prolonged immersion in a tank of liquid water at ambient temperature.

In order to obtain rapid cooling, the latter is carried out in liquid medium in the treatment solution at a very less than the surface of the heated piece of wood. The gap between these last two temperatures are generally chosen above 40 ° C, in particular above 60 ° C. In the first step, we heat to this effect the piece of wood up to a temperature higher than the temperature boiling water on the surface (around 110 to 180 ° C if the operation is performed at atmospheric pressure) and at a core temperature close to the boiling point of water (which depends on the working pressure and internal pressure). In the second step, we maintain the bath temperature, thanks to a cooling unit, a temperature generally below 90 ° C at atmospheric pressure, plus precisely located in the range of 10 ° C to 70 ° C. These operations, in particularly that of cooling, are preferably carried out at atmospheric pressure or at a pressure close to it. In the case particular where the heating is carried out in a closed enclosure, the steam of water coming out of the piece of wood can be used to induce a increase in working pressure in the enclosure and thus increase overheating in the room to be treated; vapor condensation during the second step will then result from the simultaneous action of cooling and return to atmospheric pressure (isenthalphic condensation of the steam in the latter case).

The treatment bath, in which the cooling is carried out quick immersed parts to be treated, can essentially consist an aqueous phase containing hydrophilic preservatives (such as than Copper - Chromium-Arsenic), by creosotes diluted in petroleum, by an emulsion or a dispersion of these same agents in phases aqueous or hydrophobic, by one or a mixture of organic solvents. However, the use of an oil bath (s) (mineral oils, oils vegetable or animal fat) added with preservatives hydrophobic is preferred (for example pyrethrins and pyrethroids). The cooling in an oil bath can greatly reduce costs of the treatment solution and ensuring rapid cooling, about fifty degrees in less than 30 minutes, especially less than 10 minutes, under natural convection conditions (the oils are very thermodilatable fluids). In order to facilitate the circulation of solution and homogenize the treatment between the pieces of wood (treatment by batch), we induce a tangential movement of the cold medium treating by relation to the parts is achieved, either by moving the parts, or by flow of the bath, either by a combination of these means (displacement pieces of wood on rails or carpets combined with a recirculation of the treatment solution in the cooling tank).

According to one characteristic of the invention, the total duration of the treatment does not exceed four hours; for example the duration is of the order of 30 minutes at 1 hour (depending on the dimensions and the initial water content of the parts to be treated. However, depending on the desired quality objective (quantity impregnated and / or penetration depth), it is possible to repeat several times the heating-cooling sequence. We will be able to also, depending on the anisotropy of the treated wood and the objective sought (a impregnation by one or the ends, impregnation only surface), possibly use partial waterproofing or total of the ends of the piece of wood (chemical, physical or mechanical).

According to another characteristic of the invention, the heating is carried out in the presence of high flux densities and must lead to a boiling of the water in the material (ie vaporization of the water at the heart of the material under the effect of strong heat flux) ; the flux densities are generally at least equal to 10 kW / m ² or 2 MW / m ³ , and in particular situated in the range from 10 kW / m ² to 300 kW / m ² (power related to the external surface of the parts treated), and / or in the range from 2 MW / m ³ to 20 MW / m ³ (power based on the volume of parts treated). Heating can be obtained by various processes which can be combined if necessary: radiation (infrared, high frequencies, ultra high frequencies), exposure to very hot air (eg combustion gas) or superheated steam , immersion in a fluid at high temperature (between 110 and 300 ° C at atmospheric pressure) whose boiling temperature is much higher than that of liquid water; in the latter case, the parts are preferably immersed in a bath of a similar nature to that used for cooling, in particular an oil bath maintained at a temperature between 110 and 250 ° C. at atmospheric pressure.

For the heating oil bath, an oil (or a mixture of oils) of animal, vegetable or mineral origin, in particular used vegetable oils, by-products of the food industry such as used frying oils. These inexpensive products are thermodegraded and therefore rich in surfactants (free fatty acids); they allow the use of preservatives hydrophilic or hydrophobic in the cooling bath.

Likewise the use, for the cooling step, of an oil or of a treatment solution having the same hydrophobic character, allows significantly reduce the costs of the impregnation solution and protect wood treated with a subsequent rewetting and its effects during a outdoor use (drying-rewetting cycles). These effects are responsible for the appearance and propagation of cracks in the periphery of the treated wood and constitute preferential routes of penetration of agents biological degradation of wood.

The oil is used for its excellent fluid properties coolant in a two-phase medium (i.e. in the presence of steam) during the first stage. At high temperature, this first step can be accompanied by the strong fixation of fatty acids from the bath on celluloses and wood hemicelluloses, a very favorable reaction to the preservation of wood. During the cooling step, the impregnation of the material in thermodegraded substances such as frying oils reduces very strongly the biological availability of the wood substrate for microorganisms and insects. In addition, the use of oils offers outlets for low-valued agro-food industry by-products and allows the material to be impregnated with biodegradable substances which will allow better compatibility of the materials produced with the environment during their use or recycling.

• un ressuyage, voire un séchage complémentaire du matériau,
• un léger traitement thermique conduisant à une relaxation des contraintes mécaniques dans le matériau et à une légère torréfaction du bois (permettant d'obtenir une réduction partielle des réserves alimentaires que recherche les organismes responsables de la dégradation du matériau ligneux),
• une imprégnation en profondeur ne nécessitant pas l'usage d'un cycle de variation de pression du milieu entourant la pièce à traiter, comme c'est le cas dans un autoclave.

The use of a step of "frying" the wood (so named by analogy with the use of oils in the food sector) followed quickly by immersion in the medium (the solution) with which one seeks to impregnate the material ( wood) allows both:

• drying, or even additional drying of the material,
• a slight heat treatment leading to a relaxation of the mechanical stresses in the material and to a slight roasting of the wood (making it possible to obtain a partial reduction of the food reserves that the organisms responsible for the degradation of the woody material are looking for),
• deep impregnation not requiring the use of a pressure variation cycle of the medium surrounding the part to be treated, as is the case in an autoclave.

• de forts flux de chaleur absorbés par le matériau, (qui permettent des débits évaporatoires élevés et d'une forte surpression (transitoire) dans le matériau,
• un traitement rapide (quelques minutes à dizaines de minutes),
• une mise en oeuvre aisée,
• le traitement direct de bois verts (i.e. humides et non imprégnables en l'état par les procédés précédemment décrits),
• le traitement thermique et les contraintes générées sont susceptibles d'améliorer l'imprégnabilité du matériau par l'ouverture continue du réseau cellulaire (ouverture d'opercules...),
• de faibles coûts d'investissement et de fonctionnement.

Compared to other surface (infrared, superheated steam) or volume (microwave, high frequency) heating modes, the immersion step in a bath brought to a temperature above the boiling temperature of water is interesting for the impregnation process proposed by what it allows:

• strong heat fluxes absorbed by the material, (which allow high evaporative flows and a strong (transient) overpressure in the material,
• rapid processing (a few minutes to tens of minutes),
• easy implementation,
• direct treatment of green woods (ie moist and not impregnable in the state by the methods described above),
• the heat treatment and the stresses generated are likely to improve the impregnability of the material by the continuous opening of the cellular network (opening of lids ...),
• low investment and operating costs.

• l'utilisation d'une dépression dynamique du fait de la modification de l'équilibre liquide-vapeur lors du refroidissement,
• une imprégnation profonde en présence de forts gradients de pression difficiles à générer par les procédés classiques,
• une réduction de la durée de traitement,
• un investissement réduit, une mise en oeuvre aisée,
• l'utilisation d'une grande variété de solutions d'imprégnation (aqueuses ou hydrophobes) dont les huiles et les alcools qui peuvent être difficilement utilisés dans les procédés classiques de vide-pression en autoclave (dans le cas de l'utilisation d'alcool additionné de substances de préservation, le solvant vecteur peut être secondairement éliminé par séchage à l'air).

The step of immersion in the solution to be treated after partial vaporization of the water contained in the material allows:

• the use of a dynamic vacuum due to the modification of the liquid-vapor balance during cooling,
• a deep impregnation in the presence of strong pressure gradients difficult to generate by conventional methods,
• a reduction in the duration of treatment,
• reduced investment, easy implementation,
• the use of a wide variety of impregnating solutions (aqueous or hydrophobic) including oils and alcohols which can be difficult to use in conventional vacuum-pressure processes in an autoclave (in the case of the use of alcohol added with preserving substances, the carrier solvent can be secondarily removed by air drying).

• des moyens de chauffage de pièces de bois en présence de fortes densités de flux de chaleur, dans une première enceinte ou conteneur,
• des moyens de refroidissement rapide de pièces de bois réalisé dans une (deuxième) enceinte ou conteneur, de préférence distincte de la première enceinte ou conteneur,
• des moyens de transfert (déplacement) rapide des pièces de bois de l'enceinte (ou du conteneur) où est réalisé chauffage vers le l'enceinte (ou le conteneur) où est réalisé le refroidissement.

According to another aspect, the invention consists of a wood treatment device which comprises:

• means for heating pieces of wood in the presence of high densities of heat flow, in a first enclosure or container,
• means for rapidly cooling pieces of wood produced in a (second) enclosure or container, preferably separate from the first enclosure or container,
• means for rapid transfer (movement) of the wooden pieces of the enclosure (or of the container) where heating is carried out towards the enclosure (or the container) where cooling is carried out.

In the case where the heating and / or cooling are obtained by soaking the pieces of wood in a bath, each container receiving the bath includes means for holding pieces of wood in immersion total (the buoyancy of the material depending on the composition of the material and in particular its water content). These devices can be fixed or mobile, in the latter case they are also used for moving pieces in the bath. The two containers (or bins) must be contiguous preference; they can be obtained by equipping a tank with a partition wall physically and thermally insulating the bath heating the cooling bath. This separation can be total or partial. In the latter case, the transfer of parts from the first to second bath can be obtained without contact with the gas atmosphere present above the baths; for this purpose, the device may include a airlock (or "lock") fitted with devices for holding parts in immersion during their transfer. The bins are preferably covered, at least in part, by a cover making it possible to reproduce an enclosure of confinement under slight overpressure. This allows i) to maximize impregnation during the cooling step, ii) limit the risks of projection, iii) to increase the temperature of the atmosphere traversed by the material during its transfer (if it takes place in the gas phase) and to limit air circulation that could induce premature condensation of the vapor contained in the material, iv) to carry out a possible spraying material by superheated steam, very hot air, oil at high temperature. In all cases the devices and their conduct must allow the overpressure in the wood material to be maintained and thus limit the filling of the vessels with a gas phase during the transfer..

According to another aspect, the invention consists of a piece of wood, which is likely to be obtained, directly or indirectly, by a process according to the invention, part of which (or peripheral layer) at less is impregnated with a preservative; said part or layer is impregnated with an impregnation vector chosen from mineral oils, animal oils, vegetable oils, solvents (creosotes, tars, mineral oils from the fractionation of petroleum products, extracts vegetable in mineral solvents or not); in addition, a part (layer) can be roasted (partial charring), as a result of surface heating, especially in a high oil bath temperature (between 110 and 250 ° C at atmospheric pressure). Usually, the average water content of the part (in particular of said layer in peripheral part) will be less than 0.3 kg or 0.4 kg of water per kg of non-oily dry matter, in particular less than 0.1 kg / kg. Content average in impregnation solution (in particular oil) of said impregnated layer will generally be of the order of 0.05 to 2 kg of oil per kg non-oily dry matter. The thickness of the impregnated layer is in the order of 1 to 100 mm depending on the direction in which the pieces of wood are cut by relation to the orientation of the cellular elements of the wood, in particular of around 5 to 50 mm for a log 200 mm in diameter. The thickness mean of the roasted surface layer, characterized by alteration hemicellulose and amorphous cellulose molecules (decrease significant of the number of hydroxyl groups associated with a partial crosslinking) and the appearance of pyrolysis products, may be about 0.5 to 2 cm (it depends very much on time and temperature treatment).

Other advantages and characteristics of the invention will be understood through the following description which refers to the accompanying drawings, which illustrate without any limiting character preferential modes of realization of the invention.

Figures 1 to 6 schematically show 6 examples of embodiments of the method according to the proposed invention, FIG. 7 presents a schematic cross section of a piece of wood treated according to the invention.

Each of the devices 1 comprises a tank 2 containing the bath 3 of heating of pieces 4 of wood, and a tank 5 containing the bath 6 of cooling and processing these same parts.

For this purpose the tank 2 is equipped with a connected heat exchanger 7 by conduits 50 to a heating device (not shown) of a heat transfer fluid circulating in the exchanger 7 and the conduits 50.

Similarly, the tank 5 is equipped with a heat exchanger 8 connected by conduits 51 to a cooling unit (not shown) of a coolant circulating in the exchanger 8 and the conduits 51.

In order to hold the pieces of wood (such as planks, beams, stakes or posts) immersed in each bath 3, 6, each tray 2, 5 is respectively equipped with a member 10, 12 ensuring immersion.

This member 10, 12 can be fixed and be in the form of a rail flush with or immersed in the surface of the bath; alternately this member can be mobile and can be in the form of a conveyor belt, chain or cable; in this case, it can be equipped with organs protrusions forming fingers capable of moving parts 4 in motion in the baths, as schematically represented by the arrows 11, 13.

One or two conveyor (s) 14a, 14b, 14c (figures 1 and 3) of structure similar can be used to quickly transfer parts 4 from bath 3 to bath 6: each of these conveyors has a cable 52 fitted of fingers 53 for driving parts 4; conventionally, each cable shaped in a closed loop, extends between two pulleys or rollers rotated by an actuator along perpendicular axes in terms of figures.

In the embodiment illustrated in FIG. 1, such a conveyor 14a partially immersed in bath 3, is used to move (arrow 55) the pieces 4 along an inclined wall 54 of the tank 2, so as to cause them to fall (arrow 56) into the bath 6.

In the embodiment illustrated in FIG. 3, two conveyors 14b, 14c similarly transfer the pieces 4 from tray 2 to tray 5 by sliding and / or rolling on the inclined walls 54, 57 (respectively up and down) of these 2 tanks; the bins are joined by the edge common superior of the two walls 54, 57.

In the variant illustrated in Figure 4, a single conveyor 14, the central part of the longitudinal profile is curved to extend substantially parallel to the inclined walls 54, 57 through which the trays are connected, fulfills both the function of drive in motion parts 4 in the baths 3, 6, for keeping these parts immersed in these baths, and transfer of the parts from the first to the second bath.

According to a variant not shown, one or more pumps and / or agitators can contribute to the relative movement of the pieces of wood with the treatment solution.

As illustrated in FIGS. 1 to 4 in particular, a cover 16 equipped with a nozzle 18 for injection (arrow 58) of a hot gas, covers zone 9 of transfer between the tanks.

The heating of rooms 4 can be obtained and / or supplemented by the action of a device as marked 17 which diffuses into the enclosure 59 of processing delimited by the cover 16 and the tanks 2, 5, microwaves or infrared radiation.

In the variant illustrated in Figure 2, the transfer from one bin to the other is obtained by pivoting 60 along the axis 61 (perpendicular to the plane of the figure), of a basket 62 mounted at the end of an arm 63.

In the variant illustrated in FIG. 5, this transfer is obtained by a similar pivoting of a partially submerged airlock with four flaps 64 whose transverse profiles form a cross; the extremities of the two flaps located in the lower part slide, during the pivoting of the airlock, along a wall 65 in the form of a cylinder portion with a horizontal axis, which wall extends at the upper end of a partition 66 separating the trays 2, 5; the space 67 delimited by the two lower flaps and the wall 65 is adapted to receive one or more pieces 4; flap rotation of the airlock according to the arrow (anti-clockwise) causes the transfer of the parts of the bath 3 to bath 6 without emersion.

The device illustrated in FIG. 6 has two structures similar to that of Figure 3, which are arranged end to end and connected by a additional transfer device capped with a structure 16, 18 of protection; this device allows a treatment comprising successively a first heating, a first cooling, a second heating and a second cooling.

The cross section of a treated axis 4b post which is illustrated Figure 7 shows the impregnated layer 4c of thickness 4d, the part of which 4th roasted device of thickness 4f.

Example 1:

To treat a batch of 3 stakes of debarked Douglas-fir 0.15 m diameter and 1.5 m long with an initial water content close to 100% (kg / kg dry mass), two rapeseed oil baths were used maintained respectively at 180 ° C and 35 ° C at atmospheric pressure. We have submerged the stakes, the ends of which had been previously waterproofed by soaking in tar, in the heating bath for 60 minutes taking care to allow easy circulation of the oil between the posts by natural convection and ensuring a effective compensation for the drop in bath temperature. The stakes clamped at their ends by metal frames have been kept immersed in the bath by chains and removed from the bath by the same device. Transfer to the cooling bath was carried out with a passage in the air of the order of 5 seconds. The material remained submerged in the treatment solution for about 30 minutes, taking care to limit the heating of the cooling bath below 50 ° C. This bath colored red to red Sudan contained pyrethrums as a preservation.

After leaving the bath and draining (without drying), it was found, in a cross section of the stake, an impregnation of the wood with oil colored and the preservative to a depth of 3 to 6 cm (in in all cases the sapwood is completely soaked in colored oil) depending on the state of the outer surface and a light roast to a depth of 5 mm maximum.

Example 2:

• un bain de refroidissement contenant une solution aqueuse de chrome, cuivre arsenic colorée au bleu de méthylène,
• un bain de refroidissement maintenu à 20°C.

Example 1 was repeated with the following differences:

• a cooling bath containing an aqueous solution of chromium, arsenic copper colored with methylene blue,
• a cooling bath maintained at 20 ° C.

In this case, a depth of impregnation in solution has been observed. of preservation of the order of 2 to 6 cm, and a superficial roasting on a thickness of the same importance.

Claims (18)

1. A method of treating pieces (4) of wood such as planks, beams, stakes, or posts by means of a hydrophobic wood-preserving agent, the method comprising a step of heating said pieces by immersing them in a bath of oil, followed by a step of cooling the pieces by putting them into contact with a bath of oil (6) containing said agent, in which method the pieces for treatment present a moisture content of more than 25% by weight, and for this purpose the pieces (4) are moistened, where appropriate, prior to being heated, the method being characterized in that:

   the intensity of the heating of said bath of heating oil is not less than 10 kW/m² of outside surface of the treated pieces or 2 MW/m³ of volume of the treated pieces, so as to cause the water contained in the pieces to vaporize suddenly;

   the pieces are transferred into a said bath of cooling oil where they are confined in an enclosure (16, 59) so as to maintain the pieces at raised pressure, while avoiding prolonged contact between the heated pieces and a non-treating medium such as ambient air; and

   in that said heating and cooling baths are of a composition based on vegetable oil.
2. A method according to claim 1, in which the temperature of the environment through which the pieces pass on being transferred into said bath is maintained at a temperature above the boiling temperature of water, by applying additional heating.
3. A method according to claim 1 or claim 2, in which the pieces are transferred with the environment used during the heating step by means of a boat, a lock, or any other suitable device.
4. A method according to any one of claims 1 to 3, in which the pieces (4) of wood that are treated have a moisture content of about 40% to 80% by weight (relative to dry weight).
5. A method according to any one of claims 1 to 4, in which the heating and the cooling are obtained by successive immersion in two baths (3, 6) of oil, the pieces (4) being transferred from the first bath to the second bath in less than 30 seconds.
6. A method according to claim 5, in which the first bath is maintained at a temperature lying in the range 110°C to 250°C, and in which the second bath is maintained at a temperature below 90°C.
7. A method according to any one of claims 1 to 6, in which prior to heating, the ends of the pieces (4) where the longitudinal fibers of the wood are flush with the surface are made liquid-proof or are covered so as to enhance the rise in pressure inside the pieces (4) by the heating operation, and so as to limit the contact between the pieces and a non-treating substance.
8. A method according to any one of claims 1 to 7, in which the duration of the treatment is of the order of 30 minutes to 4 hours, and in which the duration of the cooling is longer than the duration of the heating.
9. A method according to any one of claims 1 to 8, in which the heating power is delivered per unit area with flux densities lying in the range 30 kW/m² to 300 kW/m² by making contact with a hot liquid, by infrared radiation, by making contact with superheated steam (at 140°C to 300°C) or with air at high temperature (150°C to 300°C), or in bulk by high frequency or very high frequency radiation at flux densities lying in the range 2 MW/m³ to 20 MW/m³.
10. A method according to any one of claims 1 to 9, in which the changeover from heating to cooling is made when the ratio of the weight of water contained in the pieces over the dry weight thereof lies in the range 60% to 30%.
11. Apparatus for treating pieces (4) of wood, the apparatus comprising:

    a heater device (7) for heating the pieces (4); and

    a cooler device (8) for cooling the pieces (4) by contact with a bath of liquid (6) containing a wood preservation agent,

       which treatment apparatus is characterized in that:

    said heater device (7) is suitable for delivering a flux of not less than 10 kW/m² of outside area of the treated pieces or of 2 MW/m³ of volume of the treated pieces, so as to obtain sudden vaporization of the water contained in a piece; and

    in that it includes a device (9) for moving the pieces (4) relative to the heater and cooler devices (7, 8), and a confinement enclosure (16, 59) enabling the pieces (4) to be maintained at high pressure and prevent prolonged contact between the heated pieces and a non-treating medium such as ambient air.
12. Apparatus according to claim 11, which comprises:

    a vessel (2) suitable for receiving a bath (3);

    a member (7) for heating the bath (3), such as a heat exchanger;

    a member (10) for keeping the pieces (4) immersed in the bath; and

    a member (11) for moving the pieces (4) relative to the bath (3), such as a pump for circulating the bath or a conveyor for moving the pieces;

       and which further comprises:

    a vessel (5) suitable for receiving a bath (6);

    a cooling member (8) for cooling the bath (6), such as a heat exchanger;

    a member (12) for keeping the pieces (4) immersed in the bath (6); and

    a member (13) for moving the pieces (4) relative to the bath (6), such as a pump and/or a conveyor;

       and in which the displacement device (9) comprises a conveyor (14) or a transfer device (15) of the airlock type.
13. Apparatus according to claim 11 or claim 12, which further comprises:

    a cover (16) for at least partially confining the treatment enclosure (2, 5) and the displacement device (9);

    a member (17) for heating the pieces (4) without contact, such as a microwave generator or an infrared radiation generator; and

    a member (18) for introducing a hot gas into the treatment enclosure (2, 5, 16).
14. Apparatus according to claim 12 or claim 13, the apparatus comprising at least two heater vessels (2) and at least two cooling vessels (5) disposed in alternation, each pair of contiguous vessels (2, 5) being provided.with means for transferring pieces (4) from one vessel to the other.
15. A piece (4) of wood suitable for being obtained by a method according to any one of claims 1 to 11, in which at least a portion of the piece is impregnated with a preservative agent and with used vegetable oil.
16. A piece (4) according to claim 15, in which the used oil content in the impregnated portion of the piece (4) is of the order of 0.05 to 2 kilograms of oil per kilogram of non-oily dry matter.
17. A piece (4) according to claim 15 or claim 16, including a roasted surface layer of thickness of the order of 0.5 centimeters to 2 centimeters.
18. A piece according to any one of claims 15 to 17, in which the thickness of the impregnated layer relative to the half-diameter (or equivalent diameter) of a cross-section of the piece is 10% to 100%, and in which the water content of said impregnated layer is less than 0.4 kilograms per kilogram of dry matter.

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