FI112256B - Pulp thickener for use in pulp treating, i.e. for fiber suspensions of a paper and wood processing industry has a valves in the discharge conduits for controlling the operation of the pre-thickener - Google Patents

Abstract

A pulp thickener has an elongated outer casing, an inlet conduit, and discharge conduits for the thickened fiber suspension and for filtrate out. A filter surface is arranged inside the casing between the inlet conduit and discharge conduit. The conduits are provided with valves to control the operation of the pre-thickener. The pulp thickeners (10) has an elongated outer casing (12). The first end is closed with an end plate. The inlet conduit (18) is arranged at the first end of the casing for fiber suspension to be treated. The other end of the casing is closed with an end plate. The discharge conduit (20) is arranged at the other end for discharging the thickened fiber suspension. The outer casing is provided with a discharge conduit (26) for the filtrate (F). The filter surface (22) has a round cross section arranged inside the casing. The filter surface has a cleaning member comprising a rotating shaft (30). The shaft has a fixed screw thread (32) for keeping the filter surface clean. The discharge conduits (20, 26) are provided with valves (40, 46) to control the operation of the pre-thickener. An Independent claim is also included for a method of treating pulp i.e. fiber suspensions of the paper and wood processing industry by feeding a low consistency pulp into the pre-thickener. The liquid is removed from the pulp in the pre-thickener. A layer of thickened pulp is allowed to form on the filter surface and the thickened pulp is pushed by a cleaning member along the filter surface to the discharge end of the apparatus in an axial direction. The non-thickened pulp is allowed to flow through the apparatus from the inlet end to the discharge end via the space between the cleaning member and the shaft. Part of the non-thickened pulp flow is guided to the filter surface portion being wiped by the cleaning member.

Description

112256

Method and apparatus for treating pulp

The present invention relates to a method and apparatus for treating pulp.

Preferably, the method and apparatus of the invention are suitable for the precipitation of fiber suspensions in the wood processing industry. The method and apparatus of the invention are particularly useful in applications where the fiber suspensions need to be dewatered with relatively low energy consumption, in particular pre-precipitators used in connection with various known precipitators and the like.

10

Sorting of fiber suspensions has traditionally been carried out at a consistency of about 1-2% in chemical and other pulping. At this consistency, it is easy to sort the fiber suspensions or pulp and obtain a good pulp purity. After sorting, the pulp is normally precipitated by suction or disk filters to a consistency of about 8 to 16%. This technology is quite functional in itself, but low sorting consistency raises pumping costs and suction drum and disc filters require high building volume.

'New technology has introduced screening plants in which the screen feed pump of the screening unit 2 0 generates a differential pressure, which transfers the pulp through the screeners and further to a hydraulic precipitator closed by the overpressure prevailing in the screeners.

Such technology is described in patent application EP-A-0390403. The process described in said '·' · 1 publication has the advantage of removing the expensive bulky * 1 1 '·' suction drum and disc filters. A disadvantage of the described process has been that it has had to increase the sorting consistency to a range of 3-5%, which has led to * 1 1 T, 'running difficulties and sometimes also problems with the purity of the pulp. Closed hydraulic t · '!' the action of the precipitators has required a feed consistency of at least 3 to 5%, which has limited '·': the freedom to choose the sorting consistency.

'. It is an object of the present invention to make it possible to build and run. 1 1 · sorting plants so that the consistency in the sorting itself is lower than the feed consistency of the precipitator to which the pulp is eventually fed. The present invention makes it possible to perform low density screening of 112256 2 while still using new efficient closed hydraulic precipitators. Typical sorting densities are 2 to 4% and typical deposition feed densities are 3 to 6%. Thus, it is typical that the consistency difference between sorting and feeder feed is 1 to 3%, most often 1 to 2%.

5

The consistency difference between the sorting event and the precipitation event is achieved by using a pre-precipitator as shown in Figure 1 before the actual precipitator. Preferably, the prepreg is pressurized and hydraulically filled with liquid. In this case, the entire screening plant, consisting of screeners, pre-precipitator and actual precipitator, operates in a closed space of 10 mm. the amount of odor compounds released into the air remains low. The sorting consistency is 2 to 4%, the consistency after the pre-precipitator is 3 to 6% and the consistency after the precipitator is 8 to 40%, preferably 10 to 16% for the precipitator type and 25 to 40% for the precipitator type.

Hydraulic thickeners suitable for increasing the consistency of the pulp have been previously described. Patent application EP-A-0 298 499 discloses a precipitator solution for increasing the consistency of a fiber suspension from a feed consistency of 0.3 to 1.0% to a range of 1.0 to 5.0% or a feed consistency of 3 to 10% to a range of 10 to 25%. It is therefore a very efficient precipitator, which can make large changes in consistency. However, this device is:. unnecessarily expensive and its operating costs, mainly energy consumption, renders it virtually unusable for the present purpose.

I · »: 'i Simple precipitators are disclosed in the literature which consist solely of a perforated tube in which the mass flows. Such are disclosed, for example, in EP-B-0274690 and SE-C-227 590. However, practice has shown that such simple devices are not suitable for industrial use. Their problem is' *. ,,: that even though they work momentarily, their screen surfaces tend to get clogged: \ · from time to time, and reopening or keeping them clean doesn't want to succeed, • e; · ·; that they have some kind of rotor. Thus, in the context of the present invention: v. It has been decided to use another type of device, although the goal of increasing the consistency is so. : 30 as low as 1-4%. The sensitivity of the screen surface to clogging in the present case is increased by the presence of pressure pulses in both the screening plant and its equipment which tend to force the fibers into the screen surface openings, which in turn results in screen screen clogging unless a screen cleaning element is used.

Also known in the art is a continuous filter device according to U.S. Patent 4,085,050, consisting of a vertical cylindrical screen surface, a filtration chamber provided outside the screen surface, a central open screw outlet and a filter screen and an inlet screen. The device operates to form, or alternatively to form, a screen surface 10. precoat, which acts as the actual filtering material. As the material to be filtered precipitates on this precoat layer, a screw thread wipes away the precipitated layer, allowing new material to precipitate on the precoat layer. Purification of said precoat layer is accomplished by supplying washing water through the axis of the device which, under pressure, cleans the precoat 15 layer.

U.S. Patent 4,464,253 discloses a device for elevating the dry matter content of a substance to a high level and removing the thick area through the cone. Such an operation cannot be performed on the fiber suspension because it refuses to flow in a tapered cone when viscous. That patent teaches that the difference in pressure required for filtration is created either by the feed pump of the device alone or by one of those mentioned. '. ·. feed pump and vacuum in the filtrate spaces. The apparatus described in this patent is intended to be used in such a way that the material to be filtered is fed from above and the precipitated material is removed from the bottom end of the apparatus. The device consists of cylindrical and conical parts: '·. · 25 and is most obviously intended for large sludges.

Further, U.S. Patent No. 5,034,128 discloses a device of the same type for increasing consistency in the range of 5 to 30% of low feed consistency. In this case it is. '. this is a device specifically designed to remove liquid from the cellulosic industry, ·,; 30 fiber suspensions, but aim at high consistency and high final consistency.

The device is characterized in that its screw is solid, i.e. the screw thread is fixed 112256 4 directly to the cylindrical or conical shaft core. Further, the device is characterized in that the screw thread is arranged so close to the screen surface that it keeps the screen surface clean. In other words, the device operates without a precoat layer. However, it is our understanding that the apparatus cannot function as described in the publication, but that the apparatus screw, like a press 5, must be used to achieve high consistency.

U.S. Patent 4,582,568 further discloses a device for precipitating fiber suspensions using a screw press. However, unlike the three devices mentioned above, this device is characterized in that the differential pressure 10 required for precipitation is generated by the device's own screw.

The prior art devices described above have a number of weaknesses, of which at least the following are worth noting: - In the case of a downward flow device with open screw thread 15 (US 4,085,050), it is difficult to adjust the device to cellulose, i.e. pulp. Moving down the pulp so that it could be substantially precipitated at a consistency of less than 8% is not possible due to the properties of the pulp stock.

We believe that a device with a sealed screw does not work at a dilute mass, i.e. at a consistency of 1 to 5%, because it generates a helical flow of · ·, 2 0 screw threads, which flushes the cake accumulated on the screen surface and. : thus prevents precipitation, furthermore, a device with a closed screw stops the whole. '. *. process, for example, in the event of a drive failure or the like.

»· ·

The method and apparatus for treating pulp in accordance with the present invention remove 2 to 5 mm. the above-mentioned problems with prior art devices. The features and characteristics of the method and apparatus of the invention will be apparent from the appended claims. The method and apparatus of the invention have e.g.

; Characterized in that the mass of Λ is fed to a device according to a preferred embodiment of the invention, preferably by using the outlet pressure of the sorters as an inlet pressure, the feed consistency of the device according to a preferred embodiment is 2 -4%, preferably 2 to 3 %, the consistency is increased by 1 to 4%, preferably 1 to 2%, 5 to 5, the consistency of the device according to a preferred embodiment is 3 to 6%, preferably 4 to 6%, the device according to a preferred embodiment is connected between the precipitator, where it operates so that when the pulp pressure in the screen exceeds the atmospheric pressure, the precipitator is also pressurized, whereby the pressure in the screen 10 pushes the filtrate through the screen surface of the precipitator and preferably in the device to feed the pulp to a precipitator located downstream of the precipitation device.

In the following, the method and apparatus for treating pulp according to the invention will be explained in more detail with reference to the accompanying figures, in which Figure 1 shows the apparatus according to the invention in a process, and Figure 2 illustrates a device according to a preferred embodiment of the invention. in more detail.

\ 20. Fig. 1 shows very schematically the positioning of the device 10 according to the invention in a preferred embodiment of the invention, i.e. after the screening plant 2; The actual precipitator 4. When using the device 10 according to the invention, the sorting can be carried out at an optimum consistency for the result of the sorting, which is mainly between 2% and 4%, depending on the mass and the type of separator used. By using the device 10 according to the invention, the consistency of the pulp is increased by a few percentage points to a range of 3-6%, followed by the following actual precipitator to increase the consistency according to the process needs:: · * · depending on either MC range 10-16%;

Figure 2 shows a device 10 according to a preferred embodiment of the invention.

30 112256 6

This device, i.e., in the embodiment of Figure 1, the pre-precipitator 10 consists of a substantially elongated outer jacket 12 having a first end sealed by an end plate 14 and a first end provided with an inlet 18 of the an outlet conduit 20 for discharging the fiber suspension Pout to be precipitated from the device. An outer conduit 12 is further provided with an outlet conduit 26 for filtrate Fout. A screen surface 22 is provided substantially inside the outer jacket 12 at least between the inlet 18 and the outlet 20. The screen 22 preferably has a circular cross-section. The end plates 14 and 16 of the device 10 are provided with bearings 28 on which the shaft 30 is supported.

The shaft 30, in turn, is secured with at least one screw thread 32 so that, according to a preferred embodiment of the invention, it is centrally located inside the screen surface 22 and extends over substantially the entire length of the screen surface. It is essential for the screw thread 32 according to the invention to be located at a distance from the shaft member 30 via the fixing rods. One reason for arranging the screw to be open is to substantially increase the reliability of the device 15. In the event of a hardware failure, the fiber suspension flowing into the device can flow through the open hub from the inlet to the outlet substantially unobstructed. The only drawback to the process in such a situation is that the consistency of the fiber suspension no longer decreases as desired but remains substantially the same as the consistency of the pulp fed to the device. Another reason for arranging the device to be open is that the open screw controls the non-fibrous mat better than the closed screw. In a closed screw, flowing faster than the feed speed of the screw. the fiber suspension circulates a helical path along the screw thread of the device, whereby ·. the flow is substantially detrimental to the formation of the carpet. The low consistency fiber suspension in the open screw can flow through the open center of the device without adversely affecting carpet formation. Valves 40 and 46 are provided for the operation of the apparatus at both the precipitated outlet 20 and the filtrate outlet 26. : to adjust. The valves control not only the precipitation rate, i.e. pulp consistency ..., but also, especially the precipitation valve, the 40 pulp flow rate in the device.

• '. According to a preferred embodiment of the invention, the surface of the screen used in the device is internally grooved substantially in the axial direction of the device in order to cause the deposited fiber mat accumulating on the screen surface 112256 7 to slide along the grooves directly to the outlet of the device. This ensures that the fiber mat does not get caught in the screw and thus cannot rotate with the screw. It is, of course, also possible to use other guides arranged in a substantially axial direction, such as, for example, strips attached to the screen surface 5 or the like. If the fiber mat were to rotate with the screw, the screw would not push the deposited fiber layer into the outlet of the device, but would get into the non-deposited mass.

The device 10 shown in Fig. 2 operates in such a way that the mass Pjn is supplied to the device 10 under pressure from the connection 18 at a pressure of 1 to 5 bar, preferably 1 to 3 bar. The precipitated pulp Pout exits the device 10 through 20 under a pressure of 0-4 bar, preferably 1-3 bar. At its most typical, the feed consistency is 2.5%, i.e. 40 tonnes of water per tonne of pulp. A typical removal consistency of 4% is 25 tonnes of water per tonne of pulp. In other words, even though the consistency has been increased by only 1.5%, almost 15 half of the liquid in the pulp has been removed and the actual precipitate to which the pulp is introduced can be dimensioned for a much smaller amount of water. Thus, surprisingly, a small increase in consistency (as a percentage of consistency) solves the problems associated with the large amounts of water in the actual precipitator. The consistency of the pulp discharged from the device is easily adjusted by changing the position of either the precipitation 40 or filtrate valve 46 or both 20. Simply closing the precipitation valve will increase the pressure inside the strainer, thereby. : most of the water in the suspension is eliminated in the filtrate. Opening; V; the filtrate valve facilitates this event, which increases the consistency of the pulp more.

The filtrate removal can be further enhanced by providing a vacuum in the filtrate chamber, which naturally increases the pressure difference across the screen surface.

25 I »t · ·

In the device according to the invention, a screen surface 22 is used, which is preferably torn; with a hole size, depending largely on the actual application of the device, *: ··· 0.1-3 mm in diameter, preferably LO - 2.0 mm. The apertures of the screen surface may also be; ·. ·. Slots slightly smaller than the diameter of the hole sieve used for a similar purpose: 30 diameters. In addition, in some applications, it has been found advantageous to use screen apertures smaller in size than other devices in the inlet end of the device, i.e. near the end where the pulp is fed to the 112256 8 device, to prevent fibers of low consistency from being entrapped.

A pressure difference of less than 1.0 bar, preferably less than 0.5 bar, more preferably about 0.3 bar is maintained over the screen surface. At higher pressure differences, the risk of screen surface clogging increases as high pressure tends to press the fibers into the screen surface openings. For example, the desired pressure difference can be adjusted such that when the device 10 is pressurized from 1 to 5 bar in the interior of the device, the filtrate outlet flow is choked by a valve to obtain the desired pressure difference over the screen surface 22. The pressure difference between the filtration chamber 24 and the interior 10 of the device is critical for the operation of the device, i.e., the opening of the screen surface 22.

The opening of the screen surface 22, as already mentioned, is facilitated by a mechanical member 30, 32 which is preferably a screw bearing at both ends 28 on the end plates 14 and 16 of the device. The screw thread / threads 32 are spaced from the screen surface 22 from the screen surface of the pulp and leads to as uniform a "sock" as possible, without spinning, to remove the precipitate. A suitable distance is less than 5 mm, preferably less than 3 mm and suitably 0.2 to 2 mm from the screen surface. In other words, the screw rotates to prevent a permanent mass layer from forming on the screen surface 22.

20th : Also, the screw thread width, which must be individually determined for each application, is essential for optimum operation of the device, since it is affected by '; '; of course, both yield and precipitation requirements for the device.

The pitch of the screw thread 32 and the speed of rotation of the screw are selected such that the desired ",,," for each type of pulp is optimally formed, i.e. deposited.

', | Practice has shown that the equipment used in our experiments has a fiber suspension; ·· the residence time in the equipment should be less than five seconds, since then no significant precipitation occurred in the equipment used in our experiments. It is possible, though. ; 3 0 that by modifying the equipment we use, much longer residence times can be used. Thus, the rotation speed of the screw is selected such that the feed speed it produces (more specifically, the lifting speed if the device is vertical) is less than 3 m / s, preferably between 0-2 and 1.0 m / s, and most preferably about 0.5 m / s. However, this is not an actual supply of pulp, since the screw does not completely feed the pulp through the device, but merely inserts a portion deposited on the screen surface 5 of the pulp into the outlet of the device. Factors limiting this feed rate include: the rate of fluid drainage from the fiber suspension; and the development of turbulence between the fiber mat and the screen surface.

In a device according to the invention, the rotation speed and pitch of the screw were selected such that at the desired degree of deposition and yield at the outlet end of the device, the flow rates of both the precipitated pulp cake fed therein In other words, the flow rate of the fiber suspension fed into the device and in this case at the feed end of the device was higher than the feed rate of the screw. This difference in velocity then equalized as the liquid drained through the screen surface from the fiber suspension.

As will be seen from the foregoing, it has been possible to develop substantially previously known precursor solutions either simpler and / or at least more reliable! a solution with a very different level of reliability and reliability compared to prior art devices.

Claims (14)

1. A method of treating pulp, viz. the fiber suspensions of the paper and wood processing industry 5, in which process mass of layers of consistency is introduced into a pre-watering device, liquid is removed from the pulp in the pre-watering device substantially by means of the influence of the pre-watering device's feed pressure, a layer of thickened pulp of thickened pulp pulp is wiped off as the filter surface of the pre-watering device with a cleaning means, and the thickened pulp and filtrate is removed from said device, characterized in that the layer of thickened pulp is displaced with the cleaning means along the filter surface 15, to the discharge end of the device, which is substantially axially oriented, flowing through the device from the feed end to the discharge end through the space between. the cleaning means and the shaft of the device. Method according to claim 1, characterized in that pulp is introduced into; the pre-watering device (10) from a screen (2), whose screen consistency is about 2 - 4%. *. Method according to claim 1, characterized in that the pulp thickened in the pre-watering device (10) is fed to a filter (4), whose feed consistency is 3 to 6%. j 25 The method of claim 2 and? 3, characterized in that the pulp consistency is increased: with the pre-watering device (10) about 1 - 4% between the screen (2) and the filter (4). Method according to Claim 1, characterized in that the rotational speed of the cleaning means is such that it develops for the thickened pulp layer a speed below 3 m / s towards the outlet end of the device. 112256 Method according to claim 5, characterized in that said speed is 0.2 - 1.0 m / s, preferably about 0.5 m / s. 5 7. Method according to claim 1, characterized in that the feed rate of the screw and the flow rate of the non-thickened pulp are substantially identical at the outlet end of the device. 8. A method according to claim 1, characterized in that the supply pressure 10 of the device is developed by means of a pump. 9. A method according to claim 1, characterized in that the dilution of the pulp is controlled with valves for the filtrate and / or the thickened material. 15 Method according to claim 1, characterized in that the flow rate of the pulp in the device is controlled by means of valves for the filtrate and / or the thickened material. 11. Pulp treatment device, i. the paper and wood processing industry; Fiber suspensions, the device (10) comprising a substantially elongated outer sheath (12), the first end of which being closed by an end plate (14); at the first end of which jacket is provided an inlet channel (18) for the fiber suspension to be treated, Pjn; the other end of which jacket is closed with an end plate (16); at the other end on which sheath is provided an outlet channel (20) for the thickened: Pout fiber suspension removed from the device; which jacket (12) is provided *: with an outlet channel (26) for the filtrate Fout; within which sheath (12) substantially at least between the inlet duct (18) and the outlet duct (20) is provided a filter surface in *; (22) preferably having a circular cross-section and a cleaning means arranged on the inside of the filter surface, said cleaning means comprising a rotating shaft (30), to which shaft at least one screw thread (32) is attached to keep the filter surface (22) clean, Characterized in that the screw thread (32) is attached to the shaft (30) by means of connecting rods leaving a free space between the shaft and the screw thread and that the screw thread (32) distance from the filter surface (22) is less than 3 mm. 12. Device for treating pulp, viz. the fiber suspensions of the paper and wood processing industry 5, which device (10) comprises a substantially elongated outer sheath (12), the first end of which is closed by an end plate (14); at the first end of which jacket is provided an inlet channel (18) for the fiber suspension to be treated Pin; the other end of which jacket is closed with an end plate (16); at the other end on which sheath is provided an outlet channel (20) for the thickened fiber suspension Pout which is removed from the device; which sheath (12) is provided with an outlet channel (26) for the filtrate Fout; within which sheath (12) is substantially at least between the inlet passage (18) and the outlet passage (20), a filter surface (22) preferably having a circular cross-section and a cleaning member arranged on the inside of the filter surface, which includes a rotating shaft (30). , to which shaft at least one screw thread (32) is attached to keep the filter surface (22) clean, characterized in that the screw thread (32) is attached to the shaft (30) by means of connecting rods leaving a free space between the shaft and the screw thread and the screen surface (22) is provided with substantially axial rafters or corresponding conduits which prevent the fiber mat from rotating on the inside of the filter surface (22). ;;;; 20 Device according to claim 11 or 12, characterized in that the screw thread (32) * '·. · ·. distance from the filter surface (22) is preferably 0.2 - 2 mm. • * t • * * * Device according to claim 11 or 12, characterized in that both the filtrate outlet duct 25 (26) and the thickened fiber suspension outlet duct (20) are "provided with a control valve (46, 40)."