EP0226495A1 - Process and apparatus for bleaching paper pulp - Google Patents

Abstract

1. A multistage process for dispersing a gas containing oxygen within a suspension of cellulose pulp, dissolving it therein and reacting it with the latter for the purpose of delignifying or bleaching the said cellulose pulp whilst causing a flow of the pulp suspension to pass through a feed duct (15) coming from a source and injecting oxygen into it characterized in that, in each stage, it consists in : effecting the feed within an injection and mixing duct forming a passage of restricted cross-section (17, 19, 21) for the pulp and continuing the feed within a reaction tank (C, C1 ), diffusing a gas containing oxygen into the passage via a diffuser (B, B1 , B2 ) establishing an interface with the pulp to form microdimensional gas bubbles therein and causing the suspension stream resulting therefrom and containing the bubbles to pass into the reaction tank ; keeping unobstructed at least the greater part of the space comprised within the passage, to ensure a free linear flow of the pulp suspension flow through the said passage ; causing the pulp suspension to pass through the said passage at a speed appropriate to give rise to a turbulent fluid flow, imparting to it substantially "Newtonian" properties which cause it to behave substantially like water and the gas bubbles formed in the suspension to be exposed to mixing in the said suspension without appreciable coalescence ; causing the suspension containing the bubbles to flow into the reaction tank at a speed such as to establish a plug flow ; and, during the final stage (C2 ), recovering the processed suspension at the outlet of the reaction tank.

Description

The invention relates to the dispersion and dissolution of one or more gases in a pulp suspension.

It relates more particularly to a process in which oxygen is dispersed in a cellulose pulp as a delignification or bleaching agent, in order to reduce the quantity of more expensive chemicals used for these treatments.

Considerable work has already been done on the use of oxygen in conjunction with other chemicals, as is evident from the proceedings of a Symposium of the Technical Association of the Pulp and Paper Industry ( "Technical Association of the Pulp and Paper Industry Inc.") (TAPPI) which was held in November 1984 and was exclusively devoted to the delignification and bleaching of the chemical pulp with oxygen, ozone and peroxide hydrogen.

The president of the Symposium pointed out that the chemistry of this new technology is relatively complex and poorly understood and that it represents fields of research that are extremely interesting from a scientific point of view. The current conventional principles of lignin elimination are no longer valid. Although marketing is underway, the basic chemistry of these processes is not widely known.

One seeks in the use of oxygen, a means of introducing this gas into the pulp suspension to obtain a good dispersion with a view to dissolution leading to delignification or bleaching. According to a mixing device, high speed, complicated and expensive mixers are used, such as those described in US Pat. Nos. 4,295,925, 4,295,926, 4,295,927, 4,298,426 and 4,198,427. Another device, a rotating disc with holes inserted in the circulation path of the suspension is used, as described in US Pat. No. 4,427,489 in the name of Jacobsen (1984).

Another mixing device is described in "EO Mixing Breakthrough", published in the review "TAPPI Journal" August 1984, page 6, on behalf of Komax Systems Inc. Here, a static mixer is used to mix the oxygen in the dough . The bubbles and the dough are mixed by eight internal baffles arranged eccentrically, which are said to fluidize, re-mix and mix the bubbles and the dough so as to produce a mixture which is superior to that of mechanical mixers, ensuring that the bubbles are dispersed in the fibers.

The mixing apparatus described in these patents is relatively complicated and expensive, and has movable parts which can cause breakdowns or static parts located in the flow path, which can lead to blockage.

An object of the present invention is therefore to provide an apparatus capable of efficiently dispersing and kneading oxygen in a pulp suspension, simplified and economical compared to the already existing apparatus.

It has been found that effective mixing of oxygen can be carried out with an aqueous cellulose pulp suspension having a consistency of about 8 to about 16%, preferably 10 to 12%, by defining for the pulp a relatively unobstructed flow path passing through and past an oxygen introduction zone and circulating the suspension over the flow path at a rate at which it is fully fluidized and completely turbulent , so as to present substantially "Newtonian" properties, conditions in which it behaves substantially like water, and which produces a very effective mixing. Preferably, the oxygen is introduced into the suspension through diffusion means which have a fixed porous surface in contact with the rapidly moving suspension, in the oxygen introduction zone, in a finely divided form, of preferably in the form of a mass of fine bubbles having a diameter at most equal to about 2 to about 10 microns, for example. This causes the bubbles to disperse throughout the suspension without noticeable coalescence as it would occur with larger bubbles.

Preferred means for diffusing oxygen includes a sintered metal element of the type normally used for filtration. A typical diffuser consists of a sintered stainless steel element produced by the process of US Pat. No. 2,554,343. According to this process, the alloyed metal powder of a selected particle size is passed through a sintering furnace with controlled atmosphere for produce sheets or shaped bodies made of porous media. The powdery metal particles melt at their contact points, resulting in a bonded mass with a homogeneous crystal structure. No binders or other foreign materials are introduced, which retains the basic properties of the metals or alloys used. The element preferably has a thickness in the range of from about 0.8 mm to about 6.5 mm. The pores should be less than about 10 microns in diameter, preferably in the range of about 2 to about 10 microns, and the porosity should be substantially uniform over the entire surface of the porous wall and preferably in the range of about 40 to about 50%.

Porous metals in which up to 50% of the volume is made of intercommunicating voids or pores from a wide range of materials including nickel, "Monel" (registered trademark), "Inconel" (registered trademark) and a large number of stainless alloys of the 300 and 400 series. A widely used type is 316L stainless steel (0.03% carbon maximum). Variable pore sizes are suitable.

The diffusion device has the action of producing a mass of very fine bubbles in accordance with the principle set out in US Pat. No. 3,545,731, with the exception that, in order to mix oxygen properly in a pulp suspension, one must observe special conditions, which have been discovered and which are described in the present invention.

The diffuser element preferably includes a cylinder through which the suspension passes, and the walls of the cylinder define the flow path. The diffuser element can form part of the cylinder wall, it can take the form of a porous tube placed transversely to the axis of the cylinder, or it can take the form of a porous tube along the axis of the cylinder . In all cases, the suspension path is relatively unobstructed. In the case where the cylinder has a porous wall, the suspension passes directly through the cylinder without any obstruction. In the case of a tube placed in the cylinder, this tube occupies a small space, so that most of the interior space of the cylinder is occupied by the suspension which passes through it while remaining in contact with the surface of the porous diffuser .

According to a preferred embodiment, the diffusing apparatus comprises a duct having a cylindrical wall which defines the external limits of the passage and a closed capacity maintained inside the duct, and having a cylindrical wall which defines the internal limits of the passage . A bubble producing surface having a large number of gas outlets, of microscopic dimensions is arranged on the wall of the duct as well as on the wall of the container. A gas manifold surrounds the outer face of the wall of the duct and is connected to a source of pressurized gas which supplies the gas. Closed capacity is also linked to a gas source. With this arrangement, an annular passage is formed through which a stream of pulp suspension is circulated at the fluidization speed, at the same time as the gas is introduced into the stream by diffusion through the microporous surfaces located on both sides of the annular passage.

The nature of the suspension flow in the passages is described in an article entitled "Medium Consistency Technology" "Journal of the Technical Association of the Pulp and paper Industry", volume 64, n ° 6, June 1981, de 0ullichsen and Haerkoenen. Before movement of the suspension in a tube can occur, the plug of dispersed fibers which is in contact with the wall of the tube must be disintegrated by shearing force. The movement starts when this limit constraint has been exceeded. The suspension first moves in the form of a sheared, visco-elastic body, in direct contact with the wall. As the speed is increased, loose fibers and flakes detach from the plug and enter the annular space between the wall and the plug. The decrease in the friction curve indicates another change in the flow mechanism. A continuous and purely liquid annular body was formed between the stopper and the wall. The flow in this ring is laminar. When the flow increases, a speed is reached beyond which the flow becomes turbulent. The increase in speed intensifies the turbulence and the fiber plug decreases in size. New fibers detach from the plug. The whole suspension becomes turbulent when the speed is increased beyond the point where the flow curves of the fiber suspension and the water, after crossing, become parallel. All speed and consistency gradients have disappeared and the suspension behaves like a turbulent liquid. The shear field imposed in the entire mass of fibers now exceeds the value of shear stress which is necessary to completely disintegrate the fibrous network.

It has also been found that, when the liquid suspension in which the oxygen has been dispersed, reaches directly a reactor in which it collects and moves at a much slower speed, in the manner of a plug, one obtains a good reaction between oxygen and the dough.

The invention lends itself particularly well to a dosage of oxygen in stages in the pulp suspension. This is obtained in introducing a stream of a suspension of cellulose pulp under pressure, supplied by a feed, into a reaction system composed of a plurality of successive stages, in each of which the stream first circulates in a restricted section in which it acquires a appropriate speed to fluidize it and give it a liquid flow and, then, it circulates in an enlarged section in which it acquires a speed such that a plug flow occurs. In each stage, oxygen is injected into the flow of liquid under pressure, in the form of a large quantity of small bubbles to disperse these bubbles throughout the mass of the liquid, and as a result, most of oxygen reacts with the pulp in the plug flow at this point. The treated suspension is then recovered at the exit from the final stage.

Another subject of the invention is another preferred method of kneading the suspension which leaves the oxygen diffuser by passing it through a dough fluidization mixer. This mixer can be placed adjacent to the diffuser or placed some distance downstream. This mixer effectively ensures a new mixing and a new dispersion of the possible agglomerations of oxygen, so that the oxygen remains in a finely divided form and is mixed more intimately with the suspension. A preferred static mixer consists of a tube which narrows to form a constriction, followed by a tube which widens to reach the diameter of the tube in which the dough suspension circulates. The speed through the throttle will be roughly the same as that seen in the diffuser.

The invention also relates to means for coordinating the flow of oxygen with the flow of the pulp suspension so as to maintain the appropriate dosage of oxygen. This can be obtained by an apparatus which comprises a device for measuring the flow rate, for example an orifice plate inserted in the path of the suspension, upstream of the diffuser, the orifice plate being connected to a differential pressure cell which, at in turn, is connected to a controller.

The controller sends a signal to an oxygen flow control valve, which regulates the supply of oxygen to the diffuser. There is an orifice plate or other device in the oxygen supply line, to measure the flow of oxygen. The adjustment mechanism may include a computer which maintains a consistent report. as long, predetermined and fixed, between the oxygen and the dough, so as to increase or reduce the dosage to compensate for the variations in flow rate of the dough suspension.

The injection of oxygen into a suspension of cellulose pulp according to the invention can be used at different stages of the delignification and bleaching processes. The aim is to decrease the Kappa index of the dough. For delignification, the Kappa index can be reduced from the range of 50 to 60 to the interval from 20 to 30. For laundering, the Kappa index can be reduced from the interval from 5 to 10 to the range 2.3 to 3.5.

Usually, the speed of the dough passing through the oxygen introduction zone is 0.5 to 50 meters per second, a preferred interval being 5 to 40 meters per second. The surface of the fixed diffuser can have an area of approximately 0.0033 to 0.00033 dm ² per unit of oxygen gas flow of 1 dm ³ per minute, or a flow of 30 to 300 normal cubic meters of oxygen per meter square of porous surface.

The dosage rate can vary from 2.5 to 25 kg, preferably from 5 to 7.5 kg per tonne of dough, on the basis of dry air (AD) for bleaching and from 10 to 50 kg per tonne for delignification.

The oxygen used can be molecular oxygen, as found commercially, which contains 90% or more and preferably 98% or more oxygen. Alternatively, the oxygen can be in the form of a gas containing more than 50% oxygen. The starting paste suspension can contain about 10 to about 20% air by volume of dispersed paste, which dilutes the added oxygen. For a dough with little or no air, the concentration of the gas containing added oxygen can be at the lower end of the oxygen content range and, for a dough containing a lot of air, the added gas may contain oxygen near the upper limit of the oxygen content range.

To adjust the oxygen dosage to meet the conditions, the oxygen content of the residual gas leaving the vent of the final treatment tank is measured. Based on this content, it can be determined whether a good mixing has been obtained. Adjustments can be made accordingly and the dosage can be changed in stages to obtain the best results.

In a multi-stage process such as that described, it is thus possible to add large total amounts of oxygen, in stages, to a pulp suspension, adding small amounts to each stage according to the following formula:

This formula is calculated at the working pressure of the particular stage considered.

The dosage can be uniformly proportioned at each stage or it can be modified to adapt to variations in conditions such as variations in the nature of the dough, or otherwise.

The invention is particularly flexible with regard to the equipment and the process, so that it can be used to economically equip a stationery after the fact. The diffuser elements are relatively uncomplicated and inexpensive and can be easily installed in the installation.

Each factory has a certain nominal production capacity. The present invention lends itself to being adjusted to suit this nominal capacity. The variations in the flows of the pulp suspension and, consequently, of the oxygen, are in relation with the variations in the demand for pulp, so the flow can be reduced in reason of a stop when the dough tanks are full. .

In a preferred device, the pulp suspension passes through a first oxygen diffuser and a plurality of reactors, each of which is preceded by an oxygen diffuser. A fraction or all of the suspension can be introduced into a reaction tower to allow additional extraction with caustic soda.

• La Fig. 1 est une vue schématique d'une installation type pour la mise en oeuvre du procédé de l'invention ;
• La Fig. 2 est une vue en élévation de côté agrandie, en partie en coupe, qui illustre de façon plus détaillée une forme de diffuseur fluidiseur de pâte tel que celui représenté sur la Fig. 1, en série avec un mélangeur fluidiseur de pâte ;
• Les Fig. 3, 4, 5, 6 et 7 sont des vues représentant des variantes de diffuseurs fluidiseurs de pâte.

The invention having been described in general, reference is made in more detail to the appended drawings, which illustrate preferred embodiments and in which:

• Fig. 1 is a schematic view of a typical installation for implementing the method of the invention;
• Fig. 2 is an enlarged side elevation view, partly in section, which illustrates in more detail a form of dough fluidizer diffuser such as that shown in FIG. 1, in series with a dough fluidizer mixer;
• Figs. 3, 4, 5, 6 and 7 are views showing variants of dough fluidizing diffusers.

The typical installation shown will be understood on reading the description of the path of the pulp suspension which circulates in this installation. As an example, the particular stage represented, in the bleaching process, is the first stage of alkaline extraction.

An aqueous suspension of cellulose pulp containing caustic soda is pumped at a high surface speed, which arrives through the tube 15 from a supply source (not shown), through the pump A, in a line 17 which enters an oxygen diffuser B pulp fluidizer in which oxygen is diffused and dispersed in fine bubbles in the suspension stream. The suspension current is pumped from a power source, via line 17, by pump A, at a speed such that the current behaves like water and that there is a strong turbulence throughout its mass. . The oxygen is diffused in the stream through a diffuser B, in fine bubbles, as described below, in more detail, and it provides rapid mixing of the oxygen throughout the dough stream.

The stream of rapidly moving pulp which circulates in line 17 downstream of the oxygen diffuser, and which now contains dispersed oxygen, is introduced into the bottom of a first vertical reaction chamber C, in which it circulates at a much lower speed in the form of a plug. At this lower speed, the suspension loses its fluidity, trapping the oxygen dispersed in the stopper, which allows the oxygen and the caustic soda to react with the paste during the time when the suspension containing oxygen stay in room C.

The suspension is then rapidly sent, in turbulent flow, through a tube 19, into a second oxygen diffuser B _{1 which} it passes through and into the bottom of a second reaction chamber C ₁ in which it remains for a certain period to allow oxygen and bleaching chemicals to continue to react with cellulose.

From column C ,, the suspension is sent, by a tube 21 and through a third oxygen diffuser B ₂ , to a third reaction chamber C ₂ .

The dough then passes through a pressure adjustment valve 23 and, in a tube 24, to enter the upper end of a reaction tower D. The dough stays in the tower D to give more time to the product bleaching chemical to react with cellulose.

Alternatively, the dough can flow in the opposite direction, that is to say from top to bottom through the chambers to reach the base or at the top of the reaction tower, or some or all of the reaction vessels may be horizontal.

The pulp suspension leaving tower D is then collected at the outlet of an outlet tube 25 and it is washed to remove bleaching chemicals and impurities, or alternatively, depending on the stage of the bleaching process, the suspension processed can be sent to another processing.

A vent 29 for the gaseous effluents makes it possible to monitor the excess oxygen, with a view to regulating the process so as to ensure greater efficiency.

Preferably, as shown in Fig. 2, there is a static dough or throttle mixer E, in series with the diffuser B, between this diffuser and the first reaction column through which the suspension stream is passed. containing oxygen. The constriction has a constricted neck which narrows the current and increases its speed. This ensures better dispersion of the oxygen gas bubbles throughout the mass of pulp suspension.

The dough fluidizing oxygen diffuser B is composed, as shown on a larger scale in FIG. 3, of a cylinder 31 which forms a passage 32 for the suspension of dough. The cylinder has flanges 33, 34 on its ends, for connection to a pipe. A porous metal tube 35 is placed diametrically across the passage 32 which, with this exception, is free; this tube enters an opening in the side of the cylinder 31, through a connector 31a which is fixed and sealed in place by a tube connector 6 of the "SWAGELOK" type (registered trademark). On the opposite side of the cylinder 31, there is an opening closed by a plug 31b into which the lower end of the diffuser is inserted. The wall of the tube 35 is preferably made of sintered metal.

The paste is continuously introduced at high speed through the passage 32 while oxygen is supplied to the porous tube 35 and it diffuses into the paste through the wall of this tube.

A variant of the diffuser is shown in Fig. 4. In this case, the wall of the cylinder 41 has a porous cylindrical central section 45. A manifold 47 encloses this section from the outside and an oxygen connection 48 is provided leading to the manifold. The passage of the paste suspension is thus entirely free.

Another diffuser force is shown in FIG. 5. Here, a device 55 in the form of an annular crown is placed against the cylinder 51 and surrounds the passage 52. The tube 55 is provided with suitable small orifices 56 through which the oxygen is diffused. The passage 52 is entirely free.

Another form of diffuser is shown in F _i g.6. Here, a porous tube 65 extends axially and in a central position along the passage 62 and is held in this passage by suitable supports or cross-pieces 69 which are connected inside the wall of the cylinder 61. An oxygen tube 68 is placed from the outside of the cylinder at the end of the diffusion tube 65.

Figure 7 shows yet another preferred form of diffuser. This diffuser is mounted between mounting flanges 100 and 101, from which start short sections of cylindrical tubes 102 and 103 respectively, which are connected to oblique tapered flanges 104 and 105. A double wall J, composed of a wall cylindrical inner wall 111, is placed between the ends of the flanges 104 and 105, a cylindrical outer wall 107 without perforation and the annular walls of perforated ends 108 and 109. The microporous inner wall 111 is preferably made of sintered metal, as before described.

The tube sections 102 and 103, the flanges 104 and 105 and the porous wall 111 form an enclosure.

In this enclosure, there is axially mounted a cylinder K having a microporous wall 116 (preferably made of sintered metal), capped at its ends with impermeable frustoconical caps 115 and 117. The cylinder K is suitably supported by a cross device S not shown, to maintain its position in the duct. The wall 113 of the cylinder K and the wall 111 of the double wall J form between them an annular passage P. A pipe 119 for supplying oxidizing gas connects a source of oxidizing gas under pressure to the interior space of the double wall J An oxidizing gas supply tube 121 connects a source of oxidizing gas under pressure to the interior volume of the cylinder K.

The diffuser is connected, in the same way as in the connection of the other embodiments described above, by the flange 100, to an inlet tube for the paste suspension and, by the flange 101, to a tube which transports the outgoing dough suspension.

In operation, the pulp suspension is introduced into the left end of the device, at a fluidization speed, and flows at high speed through the passage P between the mioroporous walls 111 and 113, and it exits through the right end. An oxidizing gas under pressure is introduced through the tubes 119 and 121 and diffused in fine bubbles through the opposite porous walls 111 and 113, in the suspension of fluidized paste which circulates rapidly in the passage C, as previously described. The diffusion of the gas in the suspension is extremely efficient due to the large diffusion surface formed at the two limits of the fluidized pulp stream.

The static dough fluidizer mixer E, shown in FIG. 2, connected in series with the oxygen diffuser B, comprises an elongated tube of a single piece of circular section, between the flanges 81 and 82. A first frustoconical part 83 narrows from the flange 81 to connect, at the level of a neck 84, to the small end of a longer frustoconical part 85, which widens, and which ends at the level of the flange 82. The mixer E is connected to a connection piece F which has an upstream flange 90 connected to the flange 34 with the interposition of a seal. The transition piece F has a downstream flange 91 assembled to the flange 81 of the mixer. The paste suspension momentarily circulates in a wider section then in a restricted section substantially to the section of the diffuser tube.

The size (the inside diameter) of the cylinder 31 into which the pulp suspension is pumped and brought into contact with oxygen varies according to the capacity of the pulp mill. Typical diameters range from about 50 to about 300 mm and usually from about 75 to about 150 mm.

Cette expression est calculée à la pression du stade particulier considéré.Three reaction chambers C, C ₁ and C _{2 are shown} . A greater or lesser number of reaction chambers can be used, depending on the material to be treated, the amount of oxygen added and the desired bleaching effect. The effects of several diffusers, each of which is equipped with a reaction chamber, consist in adding in stages large additions of oxygen, by adding small amounts to each diffuser, so as to obtain:

This expression is calculated at the pressure of the particular stage considered.

The speed at which the slurry suspension is discharged through the diffuser cylinder 31 is a high speed which means that the current behaves like water and makes it easier to disperse small bubbles of oxygen which are mixed throughout the mass of the current.

The pressure of the pulp suspension may be about 0.1 to 1 megapascals (Mpa), and the oxygen pressure of about 0.14 to 1.4 Mpa.

The residence time of the paste in the cylinder 31 of the diffuser is usually in the range of about 0.001 to about 0.120 seconds.

The residence time in each of reactors C, C and C ₂ is usually in the range of about 1 minute to about 5 minutes and the residence time in tower D is in the range of about 30 to about 90 minutes.

The pressure in each reactor is approximately 0.1 to 1 Mpa.

The invention does not preclude the use or combination with broadcasters for the introduction of oxygen into the dough, high speed mixers such as, for example, those described in US Patent ^No. 4 295 926, in the name of WEYERHAEUSER, or of a "KAtdYR MC" registered trademark pump).

The nature of the treated pulp can be softwood, hardwood or a mixture having a consistency in the range of 5 to 16%. Pasta can be treated according to the invention at different stages of the delignification-bleaching process. For example, the starting pulp in the caustic extraction stage of a bleaching operation may have a Kappa number in the range of 5.0 to 10.0 and the bleached pulp leaving tank D may have, after washing, an index of 2.5 to 3.5.

As a result of the high speed of the pulp suspension which passes through the passage 32 in contact with the diffusion elements 35, 45, 56 or 65, the suspension takes on the characteristics of water. The suspension passes in close contact with the diffusing element and the oxygen leaves through the pores of this element in the form of bubbles of very small diameter. These bubbles are well dispersed due to the turbulence of the dough and they have a large specific surface, compared to their volume, to ensure mass transfer through the gas / liquid interface. At high oxygen concentration gradients and with a large surface / volume ratio, the mass transfer of oxygen is considerably enhanced by this dispersing process with fluidization.

The suspension can be subjected to a pre and post-treatment before being pushed back into the first diffuser B, the suspension has undergone normal treatments in a delignification-bleaching operation. For example, the suspension contains small concentrations of other bleaching chemicals. In addition, the suspension is provided at a temperature in the range of about 40 ° C to about 90 ° C.

The suspension leaving the bleaching tower D will be washed to remove bleaching chemicals and other impurities.

A typical bleaching facility could have a processing capacity of 1000 tonnes of pulp per day, a consistency of 10% with an amount of oxygen of 5 kg per tonne, or 5000 kg of oxygen. Under these conditions, this flow could be 6.321 liters per minute, with a speed of circulation of the suspension through the diffuser of 40 meters per second, and an oxygen flow of 3.13 kg per minute. The diffusion surface would be around 0.26 square decimetres.

Claims (20)

1. A method of reacting oxygen with a suspension of cellulose pulp having a consistency of 8fl at 16%, with a view to delignification or bleaching, characterized in that it consists to give the pulp suspension an oxygen dosage varying in stages and, for this, passing a stream of the cellulose pulp suspension under pressure arriving from a source in a reaction system composed of a plurality of stages in each of which the stream first circulates in a restricted section in which it acquires an appropriate speed to give birth of a turbulent liquid flow, then it circulates in an enlarged section in which it acquires a speed such as to establish a plug flow, at each stage, inject oxygen into the liquid flow under pressure in the form of a large quantity of small bubbles to disperse these bubbles throughout the mass of the liquid stream, so that the oxygen remains in intimate contact with the dough in the stopper stream, and recover the treated suspension at the end of the final stage. 2. Method according to claim 1, characterized in that, at each stage, the oxygen is injected in an appropriate amount according to the

calculated at the working pressure of this stage. 3. Method according to one of claims 1 or 2, characterized in that the oxygen is injected in the form of a large quantity of bubbles having an initial diameter of at least about 10 microns. 4. Method according to claim 1 or 2, characterized in that the suspension current flowing in the restricted section is at a pressure contained in the interval ranging from about 0.1 to 1 MPa. 5. Method according to claim 1 or 2, characterized in that the oxygen is injected at a pressure of about 0.14 to 1.4 MPa. 6. Method according to claim 2, characterized in that the total dosage rate of oxygen is 2.5 to 25 kg per tonne of dry pulp. 7. Method according to claim 1 or 2, characterized in that the oxygen is introduced through a porous surface having an area such that the oxygen injection rate is in the interval from 30 to 300 normal cubic meters per minute and per square meter of porous surface. 8. Method according to claim 1, characterized in that the speed of the suspension of dough in said liquid flow is 0.5 to 50 meters per second. 9. Process for mixing oxygen in a suspension of cellulose pulp having a consistency of 8% to 16% to delignify or whiten the pulp, characterized in that it consists in define a substantially free flow mode containing a pressurized suspension stream, passing the current from a power source in said flow mode at a surface speed which has the effect of producing a liquid flow at the same time as oxygen is injected under pressure into said current in the form of a large quantity of fine bubbles, so that the oxygen is dispersed throughout the suspension, immediately send the current and the dispersed oxygen to a confined reaction zone of larger section to allow the dough and the suspension to thicken to take the form of a plug and to allow the oxygen time to react with the dough, and recover the dough thus treated. 1.0. Process for dispersing a reactive gas throughout the volume of a slurry suspension, which is a thick suspension at rest, characterized in that it consists of s circulate the dough in a stream of restricted section at a speed such that it behaves like a turbulent liquid at the same time as oxygen is injected into the stream in the form of a large quantity of fine bubbles, so that the latter are dispersed throughout the liquid suspension. and immediately thereafter, by circulating the current in an enlarged section which provides a reduced speed so that the suspension and the entrained gas bubbles take the form of a plug for a period which allows the gas to react with the dough. 11. Method for reacting oxygen with a suspension of cellulosic pulp having a consistency of 8% to 16% to effect delignification or bleaching, characterized in that it consists in: provide the pulp suspension with an oxygen dosage varying in stages and for this, passing a stream of suspension of oellulosic paste under pressure arriving from a feed in a reaction system composed of at least one stage in which the stream first circulates in a restricted section in which it acquires a speed capable of giving rise to to a turbulent liquid flow and it then circulates in an enlarged section in which it acquires a speed capable of giving a plug flow, at each stage, inject oxygen into the liquid flow under pressure in the form of a large quantity of fine bubbles to disperse these bubbles throughout the volume, so that the oxygen remains in intimate contact with the dough in plug flow, and recovering the treated suspension at the exit from said stage. 12. Method according to claim 11, characterized in that the oxygen is injected in said stage in an amount according to the formula:

calculated at the working pressure of said stage. 13. Method according to claim 11, characterized in that the suspension current of restricted section is at a pressure in the range of approximately 0.35 to 1 MPa, the oxygen being injected at a pressure of approximately 0, 14 to 1.4 Mpa. 14. Method according to claim 13, characterized in that the oxygen is injected in said stage in an amount according to the formula:

calculated at the working pressure of said stage. 15. Apparatus for reacting oxygen with a cellulose pulp, characterized in that it comprises: a series of connected units which form a reaction circuit, each unit being composed of a throttled conduit leading to a reaction vessel (C, C ₁ ) having a section equal to several times the cross section of the conduit, and a final unit having an outlet (C ₂ ), and, in each stage, means (B, B ₁ , B ₂ ) making it possible to inject oxygen into the duct in the form of a large quantity of small bubbles, a suspension power source, means (15) for passing said suspension from said pressurized power source in said reaction circuit (17), so that the suspension has a speed in each conduit such that it is liquid when it passes through this conduit, and such that it is under the shape of a plug when it passes into the adjacent reaction vessel, means for regulating the introduction of oxygen into each stage in such a way that a dose of oxygen is dispersed throughout the liquid suspension and that most of the oxygen reacts with the paste in the form of a plug during the passage of this paste in the reaction tank. 16. Apparatus for introducing an oxidizing gas into a paste suspension, characterized in that it comprises: means forming a constricted passage (17,19,21) for the suspension; means (A) for passing the suspension through the passage at a pressure such that it passes through the passage in the form of a current at the fluidization speed; a bubble producing surface (B, BJPB2) having a large number of gas outlets of microscopic dimensions exposed to the current flowing in said passage; means for supplying pressurized gas to said outlets so that the gas bubbles are dispersed in said fluidized paste. 17. Apparatus according to claim 16, characterized in that the passage is formed by a conduit (32) and the bubble producing surface (35) is located on a tube which extends transversely to the conduit (32) on the path of the suspension current. 18. Apparatus according to claim 16, characterized in that the passage is formed by a conduit (42), the bubble producing surface (45) is located on a part of the surface of the conduit, and a collector (47) surrounds said part of the surface for receiving said gas under pressure. 19. Apparatus according to claim 16, characterized in that the passage is formed by a conduit (62) and the bubble producing surface is located on a tube (65) which extends longitudinally in the conduit (62), and means (69) being provided to support the tube and the conduit on the path of said current. 20. Apparatus according to claim 16, characterized in that the path is formed by a conduit having a cylindrical wall (107) and a closed capacity (K) disposed in its center and having a cylindrical wall concentric with the wall of the duct and spaced from this wall, the bubble producing surface is provided both on the surface of the conduit (111) and on the surface of said capacity (113). The porous part of the cylinder (111) being surrounded by a manifold (J) intended to receive and supply the gas through the porous wall of the duct, and means for connecting the manifold and the cylinder to a source of pressurized oxidizing gas (119).

Images