WO2009013427A2

WO2009013427A2 - Method for treating the white water from a machine of the paper machine type

Info

Publication number: WO2009013427A2
Application number: PCT/FR2008/051319
Authority: WO
Inventors: Sébastien CHARIGNON; Cyril Blois
Original assignee: Arjowiggins Licensing
Priority date: 2007-07-13
Filing date: 2008-07-11
Publication date: 2009-01-29
Also published as: WO2009013427A3; FR2918678A1

Abstract

The invention relates to a method for treating the white water from the production of fibre sheets with a machine of the paper machine type, comprising at least one step of recovery of the solid material contained in said white water (E, Y) from said machine, characterised in that said at least one recovery step for the material in suspension is achieved by means of centrifugation of said white water by means of a compact device (14, 17) forming at least two fractions from said white water, a fraction (Z) of clarified water and a fraction (T) with concentrated material in suspension.

Description

Process for treating white water from a paper machine

The present invention relates to the treatment of water from the manufacture of fibrous sheets by draining a suspension of material including fibers in aqueous medium, then pressing and drying on a machine type paper machine.

The invention relates to a process for the treatment of white water, and more particularly to the recovery of suspended matter in the pulp and water circuit of the machine and a method for producing fibrous sheets in which the white water is treated in accordance with The invention The manufacture of a fibrous sheet is divided into two main stages. suspending and purifying the pulp through the dough circuit and forming the fibrous sheet by draining, pressing and then drying the fiber pulp suspension obtained in the previous step.

In order to carry out suspension of the paper pulp, each paper pulp constituting the fibrous composition of said fibrous sheet is introduced into a pulper.

The suspensions thus obtained are called thick pastes because generally at a concentration calculated according to ISO 4119 (NF Q 50-006) higher than 30 g / L. They are then optionally refined, before being mixed with the bonding agents, the fillers, the pigments and any adjuvants, in particular such as pH or cationicity regulators, and then sent to the leading vat, also called vat cell. walk.

The thick paste thus obtained is optionally refined before being conveyed to a system allowing the stabilization of its flow rate, in particular such as a "paste castle".

The thick paste is then injected via a weighting valve or "dough valve" at the bottom of a water column, generally a "pit", this weighting valve allowing in particular the control of the final basis weight of said fibrous sheet.

The portion of the dough circuit previously described and located upstream of said water column is referred to as the "thick dough circuit" as opposed to the "diluted dough circuit" described hereinafter.

The diluted dough located after said previously described water column undergoes several purification steps to remove contaminants, such as metal particles, sand or plastic. Vortex cleaning is usually used followed by pressure basket treatment. According to the type machine paper machine used, flat or round-shaped machine, the diluted dough is then respectively injected by a headbox on the forming web of said flat table or poured into a shaped tank in which said round shape is partially immersed . Draining said diluted dough is then carried out respectively through said forming web or on the web of said round shape. A fibrous sheet is thus obtained which must still undergo a pressing step.

The sewage and pressing waters of the fibrous sheet, which are commonly called "white water" because having elements such as fibers, pigments or fillers that have not been retained during the dewatering of the dough to paper, are recovered. A large fraction of the white water is then mixed with the thick paste at the bottom of the aforementioned water column, upstream of the different purification steps.

Due to the introduction of water into the paper circuit, especially such as dilution waters of raw materials or adjuvants, rinsing water or water intended to prevent the formation of foam in the cuviers of the circuit, there is an excess of white water carrying more or less large quantities of matter. This excess of white water can be rejected but it is preferably treated so as to recover the material it conveys. In fact, if the excess of white water is sometimes voluntary because it makes it possible to limit the enrichment of the circuits with disturbing substances, in particular such as bacteria or colloids which may for example lead to the formation of fiber agglomerates and / or However, the papermakers wish, especially for ecological and economic reasons, to recover the matter contained in this excess of white water.

The treatment of the excess of white water may in particular be carried out by the various material recovery systems described below.

Simple filtration, carried out by devices such as the "Gravity Strainer" from Kadant Lamort, is a very efficient system for recovering fibers which has a short stabilization time of the material to be treated in said apparatus. However, this system has the drawback of not recovering small particles, in particular such as fillers and pigments.

The term "stabilization time" of a device is defined as the time at which the device under consideration subjected to an input variation has 100% output. the response due to said input variation. For practical reasons, it is later considered that stabilization is achieved when 95% of the response is reached.

In practice, one can for example measure a stabilization time by injecting at time t0 a tracer at a given mass flow rate Q in a device in operation. According to the preceding paragraph, the stabilization time reached is then considered when a mass flow rate of said tracer equal to 0.95 Q is recovered as output. In particular, care will be taken generally to select said tracer so that it is measurable directly or indirectly at the output of said device, and that it behaves in the same way as the element whose stabilization time in said device is to be measured.

The "cake" filtration system is used with disk-type or drum-type filters, such as those supplied by the Laperrière & Verreault Group (GL & V). It allows the fibers and small particles to be efficiently recovered. size, but because of its operation by filtration through a "cake" of dough, it is a bulky device and requires a long time of stabilization of the material to be treated. These devices also require extra dough to form the "cakes" of dough necessary for their operation.

Simple decantation is very effective for dense particles, but it does not allow effective recovery of fibers and light particles. The stabilization times of the material to be treated in this type of device are very long because the sedimentation rates are low. In order to increase the sedimentation rate, it is in particular possible to add a flocculant to the suspension in decantation.

For example, catiomques aniomques or polymer emulsions such as Floclair STF references ^® company or Nalco-Chemie Munzmg 74648 ^® of Nalco can be used as a flocculant, especially in systems such as "Turbodrain ^® " described below.

As a remark, additives called "retention agents" are commonly used in paper milling to improve the retention of the various components during the formation of the paper sheet on the paper machine, especially during dewatering. Although causing some flocculation, these agents are not considered in the present application as flocculants. Indeed, their addition is not intended to improve the efficiency of the recovery system of the present invention. exhaustive, these retention agents can be polyelectrolytes, cationic, anionic or nonionic polymers or be rather systems involving several products to better control the type of action generated.

There are systems combining filtration and the use of a flocculant. These systems such as the one developed by the company Bellmer through the "Turbodrain ^® " make it possible to recover fibers and small particles with short stabilization times and require significant flocculation.

Dissolved air fiotation is also used to recover the material. This system shows efficient operation for recovering fibers and small particles. The necessary stabilization time is relatively long and the addition of flocculants is essential for the proper functioning of said system.

Table 1: Advantages and disadvantages of the prior art, given as an indication and taking orders of magnitude common in the paper industry

The object of the present invention is to propose a process for recovering suspended matter in white water, in particular in order to reintroduce said suspended matter in the circuit of a paper machine-type machine, this method preferably being effective to recover the fibers and said small elements, requiring no addition of chemical, in particular flocculant in the sense of the invention, and having a short stabilization time of the material to be treated.

Note in Table 1 above that no system mentioned above is entirely satisfactory because if certain systems allow a efficient recovery of fibers and small elements, or they require the addition of flocculants, thus impairing the efficiency of the recovered elements and may disrupt the physico-chemical balance of the rest of the circuit of said machine-type paper machine either they implement white water stabilization times too long and thus disrupt the speed of stabilization of the machine type paper machine.

In addition, flocculation is undesirable because it affects the effectiveness of the material, especially if it is made of fillers and pigments added to improve the opacity of the fibrous sheet that is manufactured. Flocculants in effect affect the material by changing its state and therefore lead to a decrease in its effectiveness. It is therefore sought to recover said material without its effectiveness in view of the properties for which it is used during the manufacture of a fibrous sheet (retention, opacity, coloring, ...) is reduced, and this to allow the reuse of said material.

In addition, a system implementing short stabilization times is preferable. Indeed, because of the complexity of the material flows implemented by the paper machine-type machine, starts, fractures of the fibrous sheet (or "breakages") and changes in raw materials (paste, admixtures, fillers, pigments, ...) cause a destabilization of said machine which will take even longer to stabilize that the recovery system will have a long stabilization time.

Thus, on certain machines of the paper machine type subjected to frequent changes of raw materials or "changes of sort" (one kind being a particular type of paper, in particular defined by a grammage, a hue, a fibrous composition, a recipe for additives, ...), and by the systems currently used, it is sometimes even impossible to recover said suspended matter. Due to a long residence time of the white water in said material recovery systems, the recovered fibers, fillers and pigments would indeed be reintroduced into the dough circuit long after a change of kind, thus delaying the start-up of the manufacture or even "pollute" the second kind whose fibrous compositions and adjuvants are for example different from those of the first kind. The time required to put the good, that is to say to the compliance of different characteristics, in particular optical and mechanical, of said fibrous sheet, would then be considerably increased

The Applicant therefore proposes a white water treatment process allowing the efficient and rapid recovery of suspended solids without affecting the efficiency of said suspended solids.

To this end, the invention relates to a method for treating white water originating from the manufacture of fibrous sheets with a paper machine type machine, comprising at least one step for recovering the solids contained in the white water coming from said machine, characterized in that said at least one step of recovery of suspended matter is carried out by centrifugation of said white water through a device, in particular a compact device, forming at least two fractions from said white water: a fraction of clarified water, and a concentrated matter fraction in suspension whose concentration is greater than that of said fraction of clarified water. It is described as "compact" any device whose operating volume is relatively low. Thus, we can qualify as "compact" any device whose operating volume is of the order of magnitude of the cubic meter. However, this order of magnitude being a function of the flow rate of the material to be treated, preference will be given to a stabilization time of less than 15 minutes, preferably less than 7 minutes and more preferably less than 5 minutes.

"By the order of magnitude of the cubic meter" means a volume of less than 10 m ³ , for example between 0.1 and 10 m ³ , better still 0.1 and 5 m ³ , more preferably 0.1 and I m ³ .

According to a particular case of the invention, said solids recovery step is carried out without the addition of a chemical product, and in particular without flocculating agent within the meaning of the invention

Centrifugal devices are known devices and used in several industrial fields such as water treatment, in particular for the thickening of sludge produced by sewage treatment plants or as the treatment of filtrates from the manufacture of pulp. paper, in particular for the separation of solid matter and lipophilic extracts as described in patent application WO 02/090647. The use of such devices in the field of water treatment white not only clarifies the white water, but also to quickly and efficiently recover the material without impairing its effectiveness, so that it now appears possible to reintroduce said recovered material in the diluted pulp circuit thereby reducing considerably the time necessary for the stabilization of said paper machine

According to a particular case of the invention, said recovery of solids is carried out by at least two centrifugal devices such as that described above. Said at least two centrifugal devices can in particular be associated in series or in parallel. Their parallel association can in particular make it possible to treat all the white waters of the circuit of said paper machine while benefiting from the efficiency and speed of the machine. recovering solids inherent in the operation of said centrifugal devices.

According to a particular case of the invention, said solids recovery is carried out by at least one centrifugal centrifugal separation type centrifugal device. This type of device consists of a "bowl" in rotation in the center of which the suspension to be treated is introduced. The term "bolus" designates the element within which the centrifugation process takes place. This is generally a hollow element of cylindrical or conical shape, or consisting of the combination of at least one cylinder and at least one end-to-end cone. Under the action of bowl rotation, particles having a higher density than the transport liquid (in this case water) are subjected to a centrifugal force of up to 10,000 times the acceleration of gravity but which will be more conventionally of the order of 1000 to 5000 times the acceleration of gravity, and pressed against the wall of the cylinder. In general, the rotation speed of the bowl and consequently the centrifugal force will preferably be adapted to ensure good recovery of the particles, that is to say so as to obtain a fraction of clarified waters whose concentration the lowest possible. They are then discharged at one end of said bowl by means of a rotating conveyor inside the bowl. Said conveyor is a worm screw adjusted to a diameter slightly smaller than that of the bowl so as to allow rotation at a different speed from that of the bowl. It ensures the evacuation of said denser particles constituting the concentrated matter fraction in suspension. The clarified water fraction is extracted from the center of said bowl, at one of ends of the bowl, said bowl end may or may not be opposite the end where takes place the extraction of said concentrated matter fraction in suspension.

Centrifugal devices of decanter centrifugal separation type are manufactured by Flottweg company under the name "Decanter", "Sedicanter ^®", "Tricanter ^®" or "Sorticanter ^®" and by Alfa Laval Corporate and

Andritz under the name "Centrifugal Decanter" including tri-phase, co-current or against the current according to the provision of food and extractions.

Said recovery of suspended solids may also be carried out by at least one centrifugal device of the disc separator type. This type of device consists of a stack of rotating conical disks disposed within the "bowl" and at the base of which the suspension to be treated is introduced. The densest particles are pressed against the wall and said fraction of clarified water is extracted by the center of the device. Said fraction of concentrated material in suspension consisting in particular of said denser particles is extracted at the periphery of the device. In particular, said extraction is carried out continuously or intermittently.

Centrifugal devices of the disc separator type are in particular manufactured by the Flottweg companies under the name of "Plate Centrifuges" and Alfa Laval under the name of "Centrifugal disc separators" or "Disc stack separator". According to a particular case of the invention, said white water to be treated is dewatering water, in particular water of first or second dripping, and / or press water, in particular wastewater at the level of the zones. pressing, water from the suction boxes of felts or water from the suction cylinders. During the manufacture of said fibrous sheets, said white water is water removed by dripping or pressing. They are generally recovered at several points of said machine of the paper machine type, said points being generally organized according to the waters they collect and which can be: a. the dewatering water, which may in particular comprise the first drain water removed, depending on the type of machine considered paper machine, or on the flat table through the action of dewatering elements such as dewatering fingers , more commonly known as "foils", either by suction through the web of a round shape, and / or - The second drainage water removed by the action of dewatering elements such as suction boxes combining a succession of foils and a suction device, and / or b. the pressing water which may especially comprise the water removed by pressing of said fibrous sheet and recovered by bacholines situated at the pressing zones,

water removed by pressing of said fibrous sheet and recovered by suction through hole presses, more commonly called "suction presses", and / or water removed by suction of the water contained in the felts or wrappings of the presses through elements called "suction box" or "filler box".

Said white water may in particular be stored before being centrifuged. In fact, according to a common method for producing fibrous sheets on a paper machine type machine, the storage of said white water has been done in vats, and more particularly in vats called "Pit" and "foot of column" recovering the white water for draining and / or pressing

According to a particular case of the invention, said fraction of suspended concentrated material coming from the recovery system is introduced into said diluted dough circuit of said paper machine-type machine in particular, as explained above, in order to minimize the time necessary for the stabilization of said machine-type paper machine. In particular, this decrease in said time required for stabilization is possible thanks to the relatively short stabilization times implemented by said compact centrifugal devices. The place of introduction of said fraction of concentrated material suspended in said diluted dough circuit may in particular be a vat such as the "PIT" or the

"Foot of column".

Thus, on machines of the paper machine type subject to changes of raw materials and / or of frequent kinds, the present invention makes it possible to recover and reuse the said raw materials necessary for the manufacture of the said fibrous sheets, and thus to achieve savings. financial. According to another particular case of the invention, said fraction of concentrated material in suspension is reintroduced into the thick paste circuit of said paper machine type machine. In particular, it is reintroduced into the vat of the head, in a vat of broken wet or very near upstream of the dough valve. "Broken wet" is the broken up recovered upstream of the dryer and mainly consist of clippings of the fibrous sheet during manufacture. In addition, and according to the current papermaking process, when said paper machine-type machine is "broken", that is to say when said fibrous sheet in the course of manufacture has been broken, a cleaning of said paper machine-type machine is carried out. . During this cleaning, in particular rolls and rolls of said paper machine type machine located downstream of the place where said rupture occurred, said paper machine type machine continues to produce said fibrous sheet on all the part situated upstream of the place of said rupture. This fibrous sheet is recovered in vats at the site of said rupture, generally for use later, after resuspension, as a raw material. When said rupture takes place upstream of the dryer, that is to say in the wet part of said machine-type paper machine, said fibrous sheet during manufacture which is recovered is part of, with said trimmings, said broken wet.

According to another particular case of the invention, said fraction of concentrated material is stored, in particular in a vat, before being used later. In particular, said fraction of concentrated material is introduced, when it is subsequently used, into said diluted dough circuit or said thick dough circuit of said paper machine.

According to a particular case of the invention, said fraction of clarified waters is used for rinsing felts, cloths, presses, squeegees, rollers and / or rolls, or according to another particular case for dilution, in particular adjuvants and / or pulp.

In particular, the centrifugal devices, in particular compact, make it possible to obtain a fraction of clarified waters whose material concentrations and particle size distributions are such that said fraction of clarified water used in said rinsers does not lead to obstruction of the nozzles of said rinsers, nor fouling of said rinsers too fast. According to a particular case of the invention, said suspended materials comprise, in particular, cellulose fibers, in particular wood or cotton, and / or natural organic fibers other than cellulosic and / or synthetic fibers, for example such as polyester fibers. or polyamide, and / or optionally mineral fibers, for example such as glass fibers.

According to a particular case of the invention, said suspended materials comprise fillers and / or pigments, in particular inorganic or organic pigments.

According to a particular case of the invention, said suspended materials comprise organic fillers and / or pigments of small size. In particular, the expression "charges and / or small organic pigments" denotes fillers and / or pigments of which at least one of the dimensions is less than 1 μm.

According to a particular case of the invention, said suspended materials comprise low density fillers and / or organic pigments. In particular, the term "low density organic fillers and / or pigments" is intended to mean fillers and / or pigments whose density is less than 1.5 and preferably close to 1.

If the water and the material it contains are, for economic and environmental reasons, routinely recovered in the paper industry, this recovery is often difficult or impossible on machines subject to frequent changes of a kind. The main reasons leading to this situation are as follows: most of the time, the use of the devices of the prior art is satisfactory insofar as the reuse of the recovered material is not called into question by changes in so frequent, some charges and some pigments used in the paper industry are low density and small particles which are therefore difficult to recover

When such changes are frequent and the recovered material can not be reused from one kind to the other, the reduction of the stabilization time of the machine becomes paramount. The difficulty and the long time required to recover the material using the methods of the prior art therefore lead a number of paper manufacturers not to recover the material contained in their white water.

The invention makes it possible to recover small and / or low density particles. None of the devices of the prior art makes it possible to obtain, in a short time, low solid concentrations, for example less than 0.15 g / L, from white water at a concentration calculated according to ISO 4119 (NF Q 50-006) of approximately 6 g / L and having a of fillers (ratio of the mass of fillers and pigments on the mass of fibers, fillers and pigments) determined by filtration then by application to the retentate of the ISO 1762 (NF T12-027) higher than 86%.

As a remark concerning the determination of the charge rate and according to the composition of the suspension whose load ratio is to be determined, more or less porous filters may be used to carry out said filtration. In particular, the use of glass fiber filters may be advisable when the particles contained in said suspension are of small size.

This effect, combined with the efficiency of said centrifugal devices, therefore allows the recovery of solids when changes of raw materials and / or so are common. In addition, papermakers using low density and / or small particles can, thanks to the present invention, recover said low density particles and / or small size which becomes all the more interesting that said particles have a high cost.

The present invention is therefore particularly interesting when said white waters contain low density particles and / or small.

"Small dimension" particles are defined here in that at least one of their dimensions is less than 10 μm, preferably less than 1 μm. In particular, the fillers and pigments commonly used in the paper industry, the sizes of which are generally of the order of magnitude of a micrometer or a tenth of a micrometer, are targeted. However, said fillers and said pigments are in particular present in the papermaking circuits in their fioculated form. Indeed, said fillers and said pigments react in particular with certain adjuvants commonly used in the field of papermaking such as polyamido-amine epichlorohydrin (PAAE). Said aforementioned elements thus flocculated thus reach dimensions smaller than 10 μm, hence the previously mentioned range of value.

The term "order of magnitude" of a unit means a value less than ten times that unit. As a remark, these dimensions may correspond to unit particles (that is to say considered in their non-flocculated state) of which at least one of the dimensions is of the order of magnitude of the tenth of a micrometer.

Similarly, "low density" particles are defined here by densities less than 1.5 and preferably close to 1.

According to a particular case of the invention and according to orders of magnitude encountered in the paper industry, said white water has a concentration calculated according to ISO 4119 (NF Q 50-006) of between 0.1 and 25 g / L, and preferably between 0.5 and 12 g / l. According to a particular case of the invention, said white waters have a charge rate determined by filtration and then by application to the retentate of the ISO 1762 standard (NF T 12-027) of between 60 and 95%, preferably between 80 and 95%, so as to ensure optimum operation of said compact centrifugal devices.

Centrifugation of particles such as fillers or pigments is more difficult than centrifugation of the fibers, especially when said particles are of small size and compact form as opposed to the elongated form of said fibers. However said centrifugal devices implemented in the context of the invention are effective for the recovery of materials (fibers and particles), suspended in said white water, and this in particular because the fibers present in the white water cause said particles to said fraction of concentrated material in suspension. In order to optimize the process that is the subject of the present invention, it may be desirable to use white waters comprising a certain amount of fibers compared with the quantity of pigments and / or fillers.

Still with the aim of guaranteeing efficient operation of the process which is the subject of the present invention, it may be advantageous to know the concentrations of the said two fractions formed by the said centrifugal device. These concentrations may in particular be measured automatically, in particular continuously by concentration measuring apparatus or manually, in particular by sampling on said fractions formed by said centrifugal device. Thus, according to two particular cases of the invention, said fraction of concentrated suspension material has a concentration calculated according to the ISO 4119 standard (NF Q 50-006) of between 0.2 and 500 g / l, preferably between 2 and 100 g / L, and said clarified water fraction has a concentration calculated according to ISO 4119 (NF Q 50-006) less than 5 g / l, preferably less than 0.5 g / l and more preferably less than 0.15 g / l.

In particular, obtaining a clarified water fraction at concentrations calculated according to the ISO 4119 standard (NF Q 50-006) of less than 0.15 g / L makes it possible to use said fraction of clarified waters in said rmceurs without causing fouling or obstruction of the rinsing nozzles and especially as the non-centrifuged particles are the smallest and therefore those with the least tendency to lead to obstruction. As a remark and although the concentration of said clarified water below which said rmeur nozzles are obstructed, depends on the diameter of said nozzles of rmceurs used, the value of 0.15 g / L was determined according to the characteristics of rmceurs commonly used in the paper industry.

According to a particular case of the invention, the volume flow rate of said fraction of concentrated suspended matter represents at most 30% and preferably less than 15% of the volume flow rate of said white water at the inlet of said compact centrifugal device.

According to another of its aspects, the invention also relates to a process for producing fibrous sheets by draining a suspension of material, including fibers in an aqueous medium, and then pressing and drying on a machine such as a paper machine, implementing a method for treating white water as defined above.

The invention also relates to an installation comprising a white water treatment circuit from a paper machine type machine, characterized in that the circuit comprises a device for recovering suspended matter by centrifugation of said white water through said device forming at least two fractions from said white water: a fraction of clarified water and a fraction of concentrated material in suspension. Said device may in particular have characteristics similar to those mentioned above.

The invention will be better understood on reading the nonlimiting example and FIGS. 5 and 6 which follow, FIGS. 1 and 2 corresponding to the prior art. EXAMPLE ACCORDING TO THE INVENTION

Considering the production, on a flat-table paper machine, of a fibrous sheet according to one aspect of a process forming the subject of the present invention. The material balance corresponding to this example is shown in FIG. 7. A mixture of two paper pulps is introduced into a head chest: 20% of softwood pulp and 80% of both hardwood pulp previously refined, as well as 61 % relative to the mass of titanium dioxide fibers and water so as to obtain a suspension of thick paste of total concentration calculated according to ISO 4119 (NF Q 50-006) equal to 45 g / L. Once mixed, said slurry of thick paste undergoes refining said "head" then it is injected at a rate of 59 m3 / h and diluted at the bottom of the "PIT" by adding white water first dripping said paper machine, so as to obtain a suspension of diluted concentration paste calculated according to the standard ISO 4119 (NF Q 50-006) equal to 12 g / L.

Said slurry of diluted pulp of said "PIT" then undergoes a first purification through swirling scrubbers enabling contaminants to be eliminated mainly from the sand-type particles present in said diluted pulp suspension. These contaminants are discharged to the purification plant where they They will be treated moreover constitute a negligible flow compared to the flows put stake in the present example. A second purification through pressurized basket scrubbers retains the last fibrous clumps present in said diluted pulp suspension. As a remark, the two purifications described above are grouped together in FIG. 7 and thus appear as a single step: "Purifications".

In order to improve the retention of the fillers and pigments on the forming wire, 5 m 3 / h of a dilute cationic starch solution are added to the said suspension of diluted pulp before being injected onto the formation web. by the headbox of said paper machine at a rate of 395 m3 / h. The suspension of diluted dough thus injected onto said forming wire is at a concentration calculated according to the ISO 4119 standard (NF Q 50-006) of 12 g / l. By a first draining thanks to ceramic blades located under the forming fabric and more commonly called "foils", 285 m3 / h are recovered of white water called first dewatering having a concentration calculated according to the ISO 4119 standard. (NF Q 50-006) of 5 g / L and a charge rate determined by filtration and then by application to the retentate of ISO 1762 (NF T 12-027) equal to 80%. The first dewatering water is recovered in said "PIT".

The second dewatering zone consisting of "foils" as described previously coupled to an aspiration system makes it possible to recover 100 m3 / h of white waters called second dewatering having a concentration calculated according to the ISO 4119 standard (NF Q 50 -006) of 5 g / L and a charge rate determined by filtration and then by application to the retentate of ISO 1762 (NF T 12-027) equal to 85%. These second dewatering waters are recovered in the "foot of column". At the exit of said flat table, a fibrous sheet with 18% dryness is obtained which is transferred from the forming fabric to the pick-up felt of the section of the presses. The different pressing waters described above are recovered at said section of the presses and then sent to the "foot of column". Their flow rate is 13.5 m3 / h, they have a concentration calculated according to the ISO 4119 standard (NF Q 50-006) of 11.5 g / L and have a degree of charge determined by filtration and then by application to the retentate of the ISO 1762 standard (NF T 12-027) equal to 90%. Said fibrous sheet leaves the press section for drying at a dryness of 45%. By drying said pressed fibrous sheet on drying cylinders fed with steam, a 97% dryness sheet at the machine outlet. Finally, the dry fibrous sheet thus obtained is rolled up before undergoing any subsequent transformations, in particular such as leaf cutting, impregnation or even coating.

The level of white water in the "PIT" and in the "foot of column" is regulated by a pump system. The excess of white water is used to feed two 250 L centrifugal decanters in parallel. It represents a flow rate of 79 m3 / h at a concentration calculated according to the ISO 4119 standard (NF Q 50-006) of 5.6 g / L and with a rate of charge determined by filtration and then by application to the retentate of the ISO standard. 1762 (NF T12-027) of 86%.

During manufacture, a quantity of clear water and clarified water of 18 m3 / h is introduced throughout the circuit, at different points: with a cationic starch used as a retention agent, to "destroy Mechanically the foam forming in the "foot of column"("offalfoam") and in the rinsers (headbox, rinsing the cloth and rinsing felts). This water flow rate is taken into account in the flow and concentration values mentioned previously This flow rate of 18 m3 / h consists of 12 m3 / h of fresh water and 6 m3 / h of clarified water leaving said settlers centrifuges, said clarified water being reused to feed the "foam offal" (3 m3 / h) and the fabric rmceurs (3 m3 / h).

Said centrifugal device consisting of said two centrifugal decanters placed in series provides two fractions: a fraction of 71 m / h of clarified water at a concentration calculated according to the ISO 4119 standard (NF Q 50-006) of 0.12 g / L and having a degree of charge determined by filtration and then by application to the retentate of ISO 1762 (NF T12-027) equal to 86%, and a fraction of 7.9 m3 / h of concentrated material suspended at a concentration calculated according to the ISO standard 4119 (NF Q 50-006) of 55 g / L and having a degree of charge determined by filtration and then by application to the retentate of the ISO 1762 standard (NF T 12-027) equal to 85%.

Said fraction of clarified water is used to dilute the broken wet as well as to feed the rmceurs of said formation web while said fraction of suspended concentrate material is reintroduced in said "PIT" as a priority, that is to say to say in the vicinity of the extraction of said diluted dough of said "PIT" towards the purification. As a remark, the use of clarified white water to feed rmceurs is made possible by the efficiency of said compact centrifugal device which provides clarified water whose material concentrations calculated according to ISO 4119 (NF Q 50- 006) are very low, that is, less than 0.15 g / L. In addition, the non-sedimented particles in said slurry concentrate fraction are the finer particles and thus do not clog the flushing nozzles commonly used on paper machine type machines. DESCRIPTION OF THE FIGURES

For the sake of clarity, the relative proportions of the various elements represented have not always been respected, the views being schematic. Figures 1 and 2 show schematically the wet portion and the diluted pulp circuit of two paper machines according to the prior art. FIG. 3 represents the operating principle of a compact centrifugal device of the centrifugal separation settling type useful for the invention.

FIG. 4 represents the operating principle of a compact centrifugal device of the disc separator type, useful for the invention. Figures 5 and 6 show schematically the wet portion and the diluted pulp circuit of two paper machines according to the invention.

FIG. 7 represents a material balance of the process according to the invention described in the example according to the invention.

In Figures 1, 2, 5 and 6, schematically showing the wet portion and the diluted dough circuit of a flat-table paper machine, the letters W represent the main possible points of introduction of material such as fillers, pigments, fibers and adjuvants commonly used in the paper industry, the numbers 20 and 21 represent the main possible water entrances, respectively for dilution, in particular of adjuvants, and for rinsing, in particular felts 8d and training fabric 7c.

A process according to the prior art is shown in FIG. 1. It comprises in particular the following stages: the mixture of water A, fibers B and adjuvants, fillers and pigments C originating from the thick paste circuit 1 arrives via a dilution valve 2 in the "PIT" 3 where the white water is also recovered, said dilution valve for regulating the concentration of the diluted dough obtained at the outlet of the "PIT", the diluted dough then undergoes purification swirling in hydrocyclones 4 allowing the removal of contaminants X, then a pressure treatment in one or more scrubber (s) basket 5, the diluted and purified pulp is then injected by the headbox 6 on the training table 7 of the paper machine 10, it then undergoes a first dewatering 7a and a second dewatering 7b through the forming fabric 7c which make it possible to obtain a fibrous sheet, - said fibrous sheet is tr anferred from the training fabric 7c to a press felt 8d, then it undergoes at least one pressing through the section of the presses 8, before being conveyed to dryer 9, thereby leaving the wet part of said paper machine 10.

FIG. 1 shows the recovered white water: the first drainage water E, the second drainage water F, the water G of the suction cylinder of fabric 7d and the suction cylinder "Pick-up" 8a, the pressing water H recovered in bacholles 8b and the pressurized water J extracted from the felts 8d of the section of the presses 8 by suction boxes of felts, such as for example the suction box 8c shown in FIG.

These white waters are conveyed either to the "PIT" 3 or to the "foot of column" 30, pumps providing communication between these two Cuviers K.

A portion L of the excess of said white water is used for the dilution of the wet broken M in the wet-broken vat 31, and the other part Y is sent directly to the treatment plant without prior treatment.

A second method according to the prior art is shown in FIG. 2 which takes the same notations as FIG. 1. This process commonly used in the papermaking field differs from the prior art represented in FIG. 1 by the following steps: excess of white water Y is enriched in matter through the U filling from the thick paste circuit, then treated by a disk filter 11. This method makes it possible to obtain a clarified fraction stored in a vat 32 and a fraction of concentrated material suspended T reintroduced upstream of the diluted pulp circuit. Said clarified fraction stored in the vat 32 is used in part for the dilution 21 of admixtures introduced into the pulp W, the other part being discharged to a purification plant.

A compact centrifugal device of the decanter type with centrifugal separation is shown in section in FIG. 3. The aqueous suspension to be treated 1 is introduced into said compact centrifugal device by an axial duct 5 which supplies the bowl 12 with a diffuser 7.

The motor 9 and the decoupling housing 13 allow the rotation of the cylindro-conical bowl 12 and the conveyor screw 6 at two different speeds. The rotation of the bowl 12 allows the formation, under the action of the centrifugal forces, of a layer of material 10 against the wall 4 and the rotation of the conveyor screw makes it possible to convey said layer of material 14 to an outlet cone 8 then to the outlet 11 of concentrated material 3 located opposite the supply. The residence time of said material in the bowl 12 of said compact centrifugal device is determined by the difference in speed of rotation between the bowl 12 and the conveyor screw 6.

The fraction of clarified water is extracted from the feed side 1. A compact centrifugal device of the disc separator type is shown in section in FIG. 4. It is a so-called discontinuous discharge device. The suspension to be treated 1 is introduced into said compact centrifugal device by the axial duct 5 of annular section which feeds the bowl 12 at the diffuser 7 located at the base of said bowl 12.

The motor 9 allows the rotation of the stack of disks, thus causing, due to the shape of said bowl 12, the material 10 against the inner edge of the plates 6. The extraction of the concentrated material 3 is done by opening said bowl 12 at regular intervals. The upper part 12a of said bowl 12 is rapidly raised from the lower part 12b so as to allow the extraction of said concentrated material 3 at the points 11. The fraction of clarified water is extracted 2 at the top of the bowl 12.

A first method according to the invention is shown in FIG. 5 which takes the same notations as FIG. 1. This method differs from the prior art shown in FIG. 1 by the following steps: the excess of white water Y feeds a compact centrifugal device of disc-type separator type 14. Said device 14 makes it possible to obtain a clarified fraction Z and a fraction of concentrated suspension material T stored in the vat 33 and reintroduced into the "PIT" 3 of the diluted dough circuit. As shown in FIG. 5, said fraction of concentrated suspension material T is reintroduced in priority in the "PIT" 3, that is to say, to avoid an endless recirculation of said fraction of concentrated matter in suspension T between the compact centrifugal device 14 as described in FIG. 4, the "PIT" 3 and the "column foot" 30, this fraction T is introduced into the "PIT" 3 near the point of extraction towards the purification 4.

Said clarified fraction Z is especially used for dilutions of adjuvants introduced into the paper pulp W. A second method according to one of the aspects of the invention is represented in FIG. 6 which takes the same notations as FIG. method differs from the prior art shown in Figure 1 by the following steps: all of the first water draining E as well as the excess of white water Y from the "bottom of column" feeds a compact centrifugal centrifugal separator type centrifugal device 17 as described in FIG. 3 Said device 17 makes it possible to obtain a clarified fraction Z and a fraction of concentrated material in suspension T. Said fraction of concentrated material suspended T is mixed with the thick paste from the thick paste circuit 1 via the dilution valve 2. The diluted dough thus obtained is then purified 4 and 5 before to be injected on the flat table 7 by the headbox 6.

Said clarified fraction Z is especially used for dilutions of adjuvants introduced into the paper pulp W. This second method according to the invention has the particular advantage of allowing the treatment of all the white water. The material is thus efficiently and quickly recovered, the material to be treated being deaerated at the same time during the passage of the centrifugal decanter.

By way of remark, and although only one compact centrifugal device of the centrifugal separation decanter type 17 is shown in FIG. 6, the flow of white water to be treated can in particular require the parallel implementation of several centrifugal devices.

Of course, the invention is not limited to the implementation examples which have just been described. The characteristics of the various examples described can in particular be combined within non-illustrated variants.

The expression "comprising a" or "comprising a" must be understood as being synonymous with "comprising at least one" or "comprising at least one" unless the contrary is specified.

Claims

Process for the treatment of white water originating from the production of fibrous sheets with a machine of the paper machine type, comprising at least one step for recovering the solids contained in the white water coming from said machine, characterized in that said at least one step of recovering suspended solids is carried out by centrifugation of said white water through a compact device forming at least two fractions from said white water: a fraction of clarified water and a fraction of material concentrated in suspension.

2. Method according to the preceding claim, characterized in that said solids recovery step is carried out without the addition of chemical, in particular such as a flocculant.

3. Method according to the preceding claim, characterized in that the stabilization time of said white water in said centrifugal device is less than 15 minutes.

4. Method according to the preceding claim, characterized in that the stabilization time of said white water in said centrifugal device is less than 7 minutes.

5. Method according to the preceding claim, characterized in that the stabilization time of said white water in said centrifugal device is less than

5 minutes.

6. Method according to at least one of the preceding claims, characterized in that said recovery of suspended matter is performed by at least two centrifugation devices.

7. Method according to at least one of the preceding claims, characterized in that said centrifugation is performed by at least one device of the decanter type with centrifugal separation.

8. Method according to one of claims 1 to 6, characterized in that said centrifugation is performed by at least one device of the type disc separator.

9. Process according to at least one of the preceding claims, characterized in that said white water is dewatering water, in particular water of first or second dripping, and / or press water, in particular water of recovery at the level of pressing zones, water from suction boxes felts or water from the suction cylinders.

Process according to at least one of the preceding claims, characterized in that the slurry concentrate fraction is reintroduced into the diluted pulp circuit of said paper machine.

Process according to at least one of Claims 1 to 9, characterized in that the slurry concentrate fraction is reintroduced into the thick slurry circuit of said paper machine, in particular in the sluice chamber. , in a chest of broken wet or upstream of the dough valve.

12. Method according to at least one of claims 1 to 9, characterized in that the fraction of concentrated material in suspension is stored in a vat before further use.

13. Method according to at least one of the preceding claims, characterized in that said fraction of clarified water is used for rinsing felts, fabrics, presses, squeegees, rollers and / or cylinders.

14. Process according to at least one of claims 1 to 12, characterized in that said clarified water fraction is used to carry out at least one dilution, in particular of at least one adjuvant or at least one pulp. paper.

15. Method according to at least one of the preceding claims, characterized in that said suspended materials comprise fibers, in particular cellulose fibers, in particular wood or cotton fibers, and / or natural organic fibers other than cellulosic fibers and / or or synthetic fibers and / or optionally mineral fibers.

16. Process according to at least one of the preceding claims, characterized in that said suspended materials comprise fillers and / or pigments, in particular inorganic or organic pigments.

17. Method according to the preceding claim, characterized in that said charges and / or said pigments have, in the flocculated state or not, at least one of their dimensions of the order of magnitude of the micrometer, preferably of the order of a tenth of a micrometer.

18. The method of claim 16, characterized in that said fillers and / or said pigments have a density of less than 1, 5 and preferably close to 1.

19. Method according to at least one of the preceding claims, characterized in that said white water has a concentration calculated according to ISO 4119 (NF Q 50-006) of between 0.1 and 25 g / L.

20. Method according to at least one of the preceding claims, characterized in that said white water has a charge rate determined by filtration and then by application to the retentate of ISO 1762 (NF T 12-027) between 60 and 95 %.

21. Method according to at least one of the preceding claims, characterized in that said fraction of concentrated material in suspension has a concentration calculated according to ISO 4119 (NF Q 50-006) of between 0.2 and 500 g / L. and preferably between 2 and 100 g / L

22. Method according to at least one of the preceding claims, characterized in that said fraction of clarified water has a concentration calculated according to ISO 4119 (NF Q 50-006) less than 5 g / L, preferably less than 0.5 g / l and more preferably less than 0.15 g / l.

23. A method according to at least one of the preceding claims, characterized in that the volume flow rate of said fraction of suspended concentrate material is at most 30% and preferably less than 15% of the volume flow of said inlet white water.

24. A process for producing fibrous sheets by draining a suspension of material including fibers in an aqueous medium, and then pressing and drying on a machine of the paper machine type, implementing a white water treatment process as defined in any one of the preceding claims.

25. Installation comprising a white water treatment circuit from a machine of the paper machine type, this circuit being characterized in that it comprises a compact device for recovering suspended matter by centnfugation of said white water through said device, this device forming at least two fractions from said white water: a fraction of clarified water and a fraction of concentrated material in suspension