FR2534850A1 - Process and plant for the manufacture of fibreboards by wet route. - Google Patents

Abstract

This plant is characterised in that the defibration section comprises a lagged vessel 6 in which is housed the preheater 4 for the wood waste (chipping) and which is traversed, under the control of a pump 9, by a heat-transfer fluid coming from a heat source 8, a constant-level vat 16 fed with process water, a soda tank 17 for feeding the constant-level vat 16 with soda and for neutralising the process water, and a high-pressure pump 22 for feeding the preheater 4 with neutralised process water.

Description

 The present invention relates to a method and an installation for the manufacture of wet-agglomerated wood fiber panels.

 Currently, for the manufacture of agglomerated wood fiber panels, hardwood logs are used which are shredded in the form of chips, these chips then being washed, preheated and mechanically defibrated in the presence of live steam. The wood fiber thus obtained is then mixed with manufacturing water to form a whore. This paste is conveyed on a shaping machine where, after draining and wringing, a mattress or "boat" of wet fibers is formed. This cake is then deposited on a trellis and a metal rod and the whole is conveyed in a press. multiple stages-s ob, under the combined action of temperature and pressure, the wood fibers are welded and form a homogeneous whole called a panel.

 The traditional method of manufacturing panels, as described above, has the major disadvantage of being polluting because the circuit of recirculating water is only partially closed. The volumes are in fact surplus and are included in a range ranging from 0 to 5 m3 / h.

 Currently the total elimination of polluted water in the outside environment is only possible if additional treatments are implemented (purification system, concentration with combustion, evaporation, osmosis etc). Unfortunately, all these systems consume a large amount of energy, the investments which they support are expensive and increase the cost price of the panels produced.

 To remedy these drawbacks, it is possible to envisage making a complete closure of the manufacturing water circuit, which avoids using the abovementioned purification processes. This recirculation of water inside the factory must however be total because partial it does not in any way solve the problem of pollution, quite the contrary because the polluting loads calculated by twenty four hours, that is to say the flow multiplied by the value of the measurement considered (DB05-DC0), remain in too large values. In addition, the closure of the manufacturing water circuit in the heart of the factory creates serious problems at the production level. In fact when the recirculation waters reach high concentrations from 70 to 90 grams per liter of total materials, the drainage of the wood pulp on the shaping machine becomes very difficult and the production rates must be reduced.

 According to the traditional process, the defibration of the chips is carried out by preheating them in the presence of saturated steam, the pressure inside the preheater varying from 3 to 7 bars and the preheating time from 3 to 5 minutes. At this stage there is a significant degradation of the wood with the formation of hemicellulose, lignin, more or less degraded. The quantity of dissolved matter depends on the temperature and the preheating time. This amount varies from 5 to 10S compared to dry fiber; more the dryness between the entry of the chips in the preheater and the fiber at the exit of the cyclone located downstream remains constant and it is between 55 and 65%. It is absolutely impossible to vary this dryness.

 In addition, according to the traditional method, during the hot pressing of the wet cake with a dryness of 28 to 35%, the production water inside the cake is in two forms, namely in the liquid state, start of pressing, water which is recycled in the circuit, and in the vapor state which is evacuated to the open air. The volume of water supplied by the fresh wood before defibering is equal to the volume of water evaporated by the press. Pressing is carried out at constant temperature and the pressures applied vary during the pressing cycle.

 To increase the production rates it is logical to increase the temperature of the press which would reduce the drying time, but this is impossible because the very high concentration of recirculation water causes very significant surface defects (burns, stains) caused by bad calorie transmissions. In addition, the stainless steel sheets of the press quickly clog up. Furthermore, the retention of colloids and resin at the heart of the cake is insignificant.

 The present invention aims to remedy these drawbacks by providing a method and an installation making it possible to completely eliminate the aqueous pollutant source without using additional purification systems, to achieve significant energy savings and to considerably reduce the time. drying panels.

 To this end, this process for manufacturing wet-agglomerated wood fiber panels in which wood chips are washed, preheated and mechanically defibers, then the wood fiber thus formed is mixed with manufacturing water to constitute a stinking, we form from this leg, after draining and spinning, a cake of wet fibers and this cake is subjected to hot compression to weld the wood fibers together and form a homogeneous whole constituting the panel, is characterized in that the chips are preheated, in the presence of manufacturing water, by means of a heat transfer fluid coming from an external heat source, and the wet fiber cake is hot pressed for at least a first phase at a temperature of 260-2800C, under a slight pressure so that there is evaporation without water flow.

 According to an additional characteristic of the invention following the first low-pressure pressing phase, a second compression phase is carried out at a higher pressure of the order of 40La0 bars, at a lower temperature of 160-1800C, so as to extract the rest of the water and vapor present inside the fiber cake, then a third pressing phase is carried out at a lower pressure of 10-20 bars, at constant temperature of the order from 170-1800C.

The subject of the invention is also an installation for the manufacture of wet-agglomerated wood fiber panels comprising a defibration section of
wood chips, then a shaping section for
sea cakes of wet fibers then a section of prss-
sage comprising a multi-stage press, characterized
in that the defibration section comprises an enclosure
insulated in which is housed the preheater
wood chips and which is traversed under the command of a
pump, by a heat transfer fluid coming from a source of
heat, a constant level tank supplied with fabric water
cation, a soda tank to supply soda to the tank
at constant level and neutralize the manufacturing water, and a
high pressure pump to supply water to the preheater
neutralized manufacturing.

According to a complementary characteristic of the in
vention the pressing section comprised of the first and
heat accumulators containing heat transfer fluid
connected to a common heat source and in the
which are permanently maintained res temperatures
pectives of 260-2800C and 170-1800C, these first and second
accumulators being connected in parallel and connected by
through valves to a supply circuit of the
multi-stage press in heat transfer fluid, valves
being connected to a control circuit so that ali
ment the multistage press in heat transfer fluid to
high temperature, from the first accumulator during
a first phase of pressing the cycle, and supplying it with
coolant at lower temperature, from
second accumulator, during the rest of the pressing cycle.

The process and the installation according to the invention
eliminate the aqueous polluting source that cons
tituates manufacturing water, eliminating most of the dice
surface defects of the panels produced, improve the
press energy balance and give prospects
development of new products.

Because the chips are preheated
in the presence of manufacturing water, this water transformed into vapor is eliminated in a cyclone at the outlet of the shredder
and this results in deficit volumes included in
a negative range of 0 to - 5 m3 / h, which represents a fairly large safety margin. In other words, the quantity of manufacturing water decreases and fresh water should be added periodically to keep a balance, avoiding an excess of polluted water. The physico-chemical parameters of the manufacturing water remain stable (pH and acidity). There is no need to vary these two parameters: they directly influence the fouling of the fixed stainless steel sheets of the press. colloids, the bonding of fibers using polyelectrolytes. Starches, especially those with a cationic character, have very positive effects on water. By improving the turbidity of the waters, therefore the retention 9 the dripping speeds are increased and the problems of sticking of the panels in the press are eliminated.

 The method and the installation according to the invention make it possible to save on the order of 10 to 20% of heavy fuel oil compared to the traditional method. We can thus save 15 to 30 liters of heavy fuel oil per tonne of panels produced.

 Due to the fact that during the pressing of the panels, a first prepressing phase at 270-280 ° C., the bonding of the wood fibers is improved by a concentration and prepolymerization of the resin at the heart of the panel. On the contrary in the traditional process, the quantities of glue or resin are strongly entrained by the press water. Due to the concentration and the pre-polymerization of the resin obtained with the process according to the invention, it is possible to significantly reduce the amounts of resin to be added to the wood pulp.

 The process according to the invention lends itself particularly well to the production of panels of variable density. Indeed If we limit the pressing to the first phase, at low pressure and at a temperature of 260-280 C, by extending the duration of this phase, we can obtain panels of lower density than those manufactured using the three phases successive aforementioned.

A form of embodiment of the present invention will be described below, by way of non-limiting example, with reference to the appended drawing in which
Figure 1 is a diagram of an installation implementing the method of manufacturing agglomerated wood fiber panels according to the invention.

 Figure 2 is a schematic sectional view of part of a fiber cake placed between the sheets of a multi-stage press.

FIG. 3 is a diagram illustrating the variation of the temperature in the press as a function of time during the successive pressing phases
FIG. 4 is a diagram illustrating the variation of the pressure in the press during the successive pressing phases.

 FIG. 5 is a diagram illustrating the variation of the dryness of the fiber cake as a function of time during the first phase of the pressing cycle.

 FIG. 6 is a diagram illustrating the variation of the thickness of the fiber cake as a function of time during the first phase of the pressing cycle.

 FIG. 1 shows a diagram of the assembly of an installation according to the invention for the manufacture of agglomerated wood fiber panels.

 This installation comprises a first section 1 for defibrating the chips which is followed by a second section 2 for shaping the cakes of wet fibers, which is in turn followed by a third and last section 3 for hot pressing of the wet cakes.

 The first chip defibration section includes a preheater 4 at the top of which are introduced the raw chips 5 resulting from the shredding of hardwood logs, after washing. The preheater 4 is housed inside a heat-insulated enclosure 6 containing a heat transfer fluid 7. This heat transfer fluid circulates in a closed circuit between a heat source B and the heat-insulated enclosure 6, this circuit comprising a pump 9 and a valve. 11.

 The preheating of the chips inside the preheater 4 is carried out in the presence of manufacturing water.

This manufacturing water came from a central vat 12 collecting the recirculation water and forming part of the second shaping section 2. The recirculation water is pumped, by a circuit comprising a pump 13 and a pipe 14, into a tank. constant level 15 which feeds by gravity another constant level tank 16.

The manufacturing water which is returned to the constant level tank 16 with a pH equal to approximately 5.0, is neutralized in this tank with sodium hydroxide, until its pH becomes equal to 7.0. The soda is contained in a reservoir 17 supplying by gravity the constant level tank 16, by means of a valve 18. The pH of the production water being in the constant level tank 16 is controlled by a pH meter 19 which controls the opening of the valve - 18.

 The preheater 4 is supplied with the recirculating water thus neutralized, from the constant level tank 16, by a pipe 21 to which a high pressure pump 22 (10 bars maximum) is connected.

 The neutralization of the recirculation water can be varied before injecting it into the preheater 4, in order to avoid premature corrosion of the preheater 4 and the defibrator.

 The quantity of water to be injected for a temperature of 1500C at a pressure of 4 to 6 bars inside the preheater 4 is 1 m3 for 5 to 6 tonnes of wet chips.

 Chemicals can optionally be injected into the recirculation water contained in the constant level tank 16, in order to facilitate subsequent defibration of these chips.

 The defibration process according to the invention makes it possible to control three important parameters, namely the temperature and the pressure in the defibrator 4, as well as the quantity of dissolved material.

 The shavings thus preheated are taken up at the outlet of the shredder 4, in the lower part, and are transmitted by a pipe 23 to the upper part of a cyclone 24.

 At the upper part 25 of the cyclone 24 steam escapes which can optionally be extracted thereafter by-products such as aldehydes, ketones, furfural etc. At the bottom of the cyclone 24 flows a mixture of fibers and recirculating water which falls into a pit tank 26 in which is housed an agitator 27. The quantity of steam exiting at the top 25 of the cyclone 24 and the dryness of the fibers leaving the lower part of this cyclone gives information on the quantity of water to be injected into the pre-heater 4. A dryness point gained corresponds to ten liters of water per tonne of wet wood per hour.

Ten dry points for ten tonnes of wet wood per hour correspond to 1 m3 of water to be injected into the preheater.

 If the quantity of dissolved material is still too large, it is possible to preheat under vacuum, the temperature lowering by 20 to 30 C compared to the initial temperature. It also avoids the formation of organic acids, aldehydes which distill from 100 1100C, the preheating time being greater6.

 An energy balance is required between the electrical power dissipated by the discs and the calories required for preheating the chips.

 The dough in the vat 26 is transferred through a pipe 28 to which a pump 29 is connected, to a shaping machine 31 of any known type.

This machine comprises a movable canvas on which the dough is deposited, the water of which drips to give a fiber cake 32 at the end. The water dripping from the dough falls into the central collecting tank 12.

 The wet cake 32 leaving the shaping machine 31 is then transmitted to a multi-stage press 33 which is supplied with a heat transfer fluid or superheated water by means of a pipe 34 on which the is mounted. feed pump 35. The heat transfer fluid or the superheated water is supplied by a heating installation comprising a heat source 36 which supplies two heat accumulators 37 and 38 in parallel. The heat source 36 is connected to a coil 39 housed in the first accumulator 37 via a circuit comprising a pipe 41, a valve 42 and a pump 43. In the same way the heat source 36 is connected to a coil 44 housed in the second accumulator 38 by l intermediate of a circuit comprising a pipe 45, a valve 46, and a pump 47. The first accumulator 37 is connected by a pipe 48 and a valve 49 to the pump 35 and, by a pipe 51 and a valve 52 to a canalisati the output of the multi-stage press 33 is 53. In the same way, the second accumulator 38 is connected, by a pipe 54 and a valve 55, to the pump 35 and, by a pipe 56 and a valve 57, to the pipe 53 from press outlet 33.

 The installation is adjusted so that the temperatures prevailing in the two accumulators 37, 38 are different, the temperature in the first accumulator 37 being the highest and of the order of 260 b 28O0C while the temperature in the second accumulator 38 is maintained at around 170 - 1800C.

 The pressing cycle of the wet fiber cakes in the press 33 is preferably carried out in several.

phases.

 During the first phase I of the pressing cycle, the wet fiber cakes located in the press 3 undergo a prepressing at a temperature of 260-280 ° C. (FIG. 3) and at low pressure. For this purpose the valves 49 and 52 are open while the valves 55 and 57 are closed. The pump 35 then supplies the press 33 with heat transfer fluid coming from the first accumulator 37, through the pipes 48 and -34, and the heat transfer fluid is brought back to the accumulator 37 by the pipes 53 and 51. The temperature of the press 33 is kept constant by an automatic regulation system.

 During this first phase I, a slight pressure is applied (FIG. 4) to the fiber cakes so that there is evaporation without water flowing. The dryness s of the fiber cake varies as a function of time t as indicated by the curve of the diagram in FIG. 5 (FIG. 3). The duration of the first phase I can range from 1'30 to 2'30 for standard panel thicknesses from 3 to 3.3 mm (density 0.9-1.0). The manufacturing water trapped inside the wet cake 32 gradually vaporizes (Figure 2). The fixed press stainless steel sheet 58, the fabric 59 and the movable sheets 61 remain clean in contact with the steam which easily escapes. The manufacturing water inside the wet cake is concentrated and distributed evenly inside the fiber cake. The distribution of calories throughout the fiber cake is homogeneous. The dryness and thickness of the fiber cake varies. The dryness s goes from 28 - 30% to 50 - 60% after 2 ', as we can see in figure 5. Furthermore, if we refer to figure 6 which gives the variation of the reduction x, in for hundred, of the thickness of the wet cake as a function of the time t in minutes of the first phase I, it can be seen that the thickness of the wet cake is reduced by more than 50 m after one minute.

 One of the advantages provided by this first phase of prepressing at low pressure and at relatively high temperature is that the bonding of the fibers is improved by a concentration and prepolymerization of the resin used at the heart of the panel. On the contrary in the traditional process, the quantities of glue or resin which are used as a binder are strongly entrained by the pressing waters. The method according to the invention therefore makes it possible to substantially reduce the amount of resin to be added to the wood pulp.

 Once the first phase I of the pressing cycle is completed, we pass to a second phase II during which the valves 49, 52 are closed and on the other hand the valves 55, 57 are open. The second accumulator 38 then supplies the multi-stage press 33 with heat transfer fluid at a temperature of 160 - 1800C (FIG. 3), the supply circuit being established by the pipe 54 up to the pump 35 and by the pipe 56 until 'to 1-a return line 53.

 During this second phase II, a strong pressure is exerted in the press 33 (FIG. 4) on the cakes of fibers which are still wet. The pressure used is then 50 -70 bars so as to extract the rest of the water and steam present inside the fiber cake. The retention of colloids and binding substances is improved.

 The duration of the second phase II of the pressing cycle is at most 1'9 1'30 for standard thicknesses of panels from 3.0 to 3.3 mm (density 0.9).

 During the third phase III of the pressing cycle which follows the previous one, the pressure in the press 33 is lowered from 50 - 70 bars to 10 - 20 bars (Figure 4).

This pressure is kept constant for a period of time ranging from 2 'to 3'. The temperature is kept constant at 170 - 180 C but it can vary at the end of the pressing cycle.

 The drying speed of the fibreboards can be monitored by means of a micrometer 62 of thickness measurements which provides information on the drying time. When this micrometer 62 stabilizes, the press 33 is again fed from the first accumulator 37, while isolating the second accumulator 38. The press 33 then stabilizes at a temperature of 260 - 270go during the loading and unloading operations .

 Valves 49, 52, 55 and b7 are automatically controlled and controlled by a timer system 63.

 Tests carried out with the installation according to the invention have shown that a very significant reduction is obtained in the volume of manufacturing water recovered at the outlet of the press 33. For example, with the traditional process, a set of wet cakes of 'A total weight of 3.3 tonnes, subjected to pressing in the press 33, gives a volume of steam of 850 liters and a volume of recycled de-fabr-ication of 1450 liters. On the contrary, the process according to the invention gives a volume of steam of 1,700 liters and a volume of recycled fabication water of 600 liters. This avoids recycling of 850 liters of manufacturing water per tonne of dry panels produced. To this value should be added the volume of 285 liters per tonne of panels which is evacuated to the shredder.

 It can be seen from the above description that the manufacturing water circuit is completely closed and as the volume of manufacturing water becomes in deficit, it is necessary to make a controlled supply of fresh water to make up for this deficit.

 As previously seen, the process according to the invention comprises three pressing phases I, II and III during which the pressures and temperatures are applied, the values of which have been indicated, when it is desired to obtain relatively high density panels, in the range of 0.9 to 1.0. However, according to an alternative embodiment, the method can also be used to manufacture panels having lower densities.

Thus if we remove phases II and III and if we extend the first phase I at a temperature of 260-2800C and at low pressure, we can manufacture panels whose density is close to 0.5. A reduction in drying time is obtained, compared to the traditional process, in the ratio of 6 to 1.

 One can also manufacture, by the process according to the invention, panels of variable density, ranging from 0.6 to 0.8, by modifying the durations and other parameters of phases II and III.

 Naturally during the manufacture of panels having a lower density, the volume of production water becomes even more deficient and it is necessary to correlatively increase the addition of fresh water.

Claims (7)

 1- Method for manufacturing wet-agglomerated wood fiber panels in which wood shavings are washed, preheated and mechanically defibers, then the wood fiber thus formed is mixed with manufacturing water to form a paste, a cake of wet fibers is formed from this paste, after draining and spinning, and this cake is subjected to hot compression to weld the wood fibers together and form a homogeneous assembly constituting the panel, characterized in that the '' the chips are preheated, in the presence of manufacturing water, by means of a heat transfer fluid from an external heat source, and the wet fiber cake is hot pressed for at least a first phase at a temperature of 2g0-2800C, under slight pressure so that there is evaporation without water flowing.  2- A method according to claim 1 caractXrisé in that following the first phase of pressing at low pressure a second compression phase is carried out at higher pressure of the order of 50-70 bars, at a temperature time 160-180DC lower, so as to extract the rest of the water and vapor present inside the fiber cake, then a third pressing phase is carried out at a lower pressure of 10-20 bars, at constant temperature of the order of 170-1800C.  3- A method according to any one of the preceding claims characterized in that neutralizes, by means of soda, the manufacturing water used during the preheating phase of the chips.  4.- Method according to claim 3 characterized in that during the preheating phase controls the pH of the water -fabrication and regulates the sodium hydroxide supply according to the measured pH.  5- Installation for the manufacture of wet-agglomerated wood fiber panels comprising a section for defibering wood chips, then a shaping section to form cakes of wet fibers, then a pressing section comprising a multi-stage press multiple, characterized in that the defibration section comprises a thermally insulated enclosure (6) in which is housed the preheater (4) of wood chips and which is traversed, under the control of a pump (9), by a heat transfer fluid from a heat source (8), a constant level tank (16) supplied with process water, a soda tank (17) for supplying soda to the constant level tank (16) and neutralize the water from manufacturing a high pressure pump (22) to supply the preheater (4) with neutralized manufacturing water.  6.- Installation according to claim 5 characterized in that it comprises a pH meter (19) controlling the opening of a valve (18) connected to a pipe supplying soda to the constant level tank (16).  7- Installation according to claim 5 characterized in that the pressing section (3) comprises first and second heat accumulators (37, 38) containing a heat transfer fluid, connected to a common heat source (36) and in which are permanently maintained at respective temperatures of 260-2800C and 170-1800C, these first and second accumulators (37, 38) being connected in parallel and connected via valves (49, 52; 55, 57) to a circuit ( 35, 34, 53) for supplying the multi-stage press (33) with heat transfer fluid, the valves (49, 52, 55, 57) being connected to a control circuit (63) so as to supply the press to multiple stages (33) in heat transfer fluid at high temperature, from the first accumulator (37), during a first phase of pressing of the cycle, and in supplying it with heat transfer fluid at lower temperature, from the second accumulator (38 ), during the rest of the pressing cycle.