FI97983C - Screening method and sieve - Google Patents

Description

97983

The present invention relates to a method according to the preamble of claim 1 for screening coating compositions and slurries used in the paper industry.

The invention also relates to a screen based on the method.

The paper industry uses a variety of pigment slurries and coating compositions to coat papers and board. These blends are blends of different types of solids and water that are made in a paper mill according to the needs of the application. In order to maintain the quality of the coated paper, the blends must be sieved. The sieving is generally carried out on the freshly prepared slurry or coating mixture, but in addition the sieves are placed in the feed circuit of the coating station. The purpose of the screens is to separate the dry matter deposits, the fibers detached from the paper, the felt hairs detached from the paper machine and other impurities that could cause coating defects. Two different screens are used for screening, a pressure screen and an open vibrating screen. The open vibrating screen is commonly used for screening sludges and is screened through a planar wire cloth using a hydrostatic pressure of a few centimeters of sludge. The wire is kept clean by vibration, which at the same time increases the capacity of the device because the vibration: ‘i’; improves sludge fluidity. The advantage of vibrating screens is good resolution, compared to pressure screens. Because the wire is used as the separating element, in which the openings are square, the * · .3 * 0 sieve effectively separates, in particular, elongate impurities such as fibers detached from the web. Used for slit: *. sizes are 50 μm upwards.

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In pressure sieves, the screening element is usually a 35 - drum drum. The drum is typically made of a wire about 1 mm '' thick which is wound into a spiral and the turns are suitably fastened at a constant distance of 2,97983 from each other. is elongated.For example, the drum described above has only one helical slit.

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Pressure screens are used to screen both the sludge and the machine circuit to screen the coating. The sealing surface is kept clean by mechanical patterning or countercurrent washing. The sieve is washed or the accumulated reject is removed by dropping a suitable amount of liquid to be sieved from the sieve, usually every few hours. The capacity of the pressure screen is many times greater than the vibrating screen compared to the vibrating screen, because thanks to the strong-structured drum, many times the pressure difference can be used than in the vibrating screen. The open area of the drum is usually less than 10% of the open area. Because the screening slot is elongated, long and thin fibers may pass through the screen slot.

One problem with vibratory screening is the small capacity in relation to: the area of 20 wires and especially the open screen surface, which is up to 30-40% of the total area of the wire. The durability of the wire limits the capacity and it is not possible to maintain a large pressure difference across the wire, which could enhance the operation of the wire. Typically, about 1 m2 of wire surface is screened by 1 1 / s of sludge. "From low efficiency to screening area, it follows that the space requirement of the vibrating screens is large. When sifting pigment sludge, typically up to ten screens are required. When using a number of screens, it is very difficult to place them in a factory building, so in all cases, vibrating screens cannot be used.

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The biggest problem with both vibrating screens and pressure screens is clogging of the screen during use. Despite the vibration and mechanical patterning, the screens must be disassembled periodically for cleaning and maintenance, and clogging causes a decrease in screening capacity.

3 97983 The screening devices described in DE-A-30 10 952 and EP-A-0 616 072 can be considered as vibrating screens. In these devices, a routed disk is used as a separating element and the disk is kept clean and the transmission is enhanced by a rotating wing in the vicinity of the disk. Since it is difficult to make holes in a rigid plate so that the hole size is small enough and at the same time the proportion of holes and permeable surface is large relative to the surface area of the plate, these devices have poor resolution and poor efficiency due to small permeable surface area. Usually, the proportion of the permeable surface of the routed plate is about 10%, and since the penetration resistance increases rapidly as the hole size decreases, the hole size of the plate is usually 0.5 mm or more. These devices are intended for the separation of paper fibers and cellulosic fibers and the removal of impurities from recycled paper pulp. The device described in the application EP 0 616 072 is intended for screening and cleaning recycled paper pulp, so it is clear that a screening plate is required. 20 be perforated and highly permeable to paper fibers. Equipment for screening suspensions containing fiber pulp is not suitable for screening that requires good resolution, such as screening of coatings or filler slurries. In these devices, the screening is not carried out with an actual overpressure: ·· 25 ti overpressure, but the pumping of the substance to be screened • l * · a small pressure difference over the screening member may occur. However, this does not have a large effect on the operation of the device because: j the screening members are very permeable due to the large hole size.

• · «•« .. ’30 The pressure screen has a large capacity compared to a vibrating screen, but its resolution is poor. In particular, the separation of elongated ·; · * or plate-like impurities from the sieve-liquid to be sieved is difficult due to the geometry of the gap. The sealing surface is kept clean by mechanical patterning or frequent countercurrent washing. Formulation consumes both drum and scrapers. The fibrous impurities tend to accumulate and clog the sieve slits and also form large deposits inside the sieve, which cannot be removed during rejection but the sieve has to be opened and washed by hand. Countercurrent washing causes large 5 losses and a wastewater problem. Counter-current washing is also not so effective that the screens often do not have to be opened and washed. A further weakness of countercurrent washing is the change in the composition of the material to be treated, which may result in coating defects.

The object of the present invention is to provide an efficient screening method with a resolution better than conventional pressure filters.

The invention is based on the fact that the screening is performed under the effect of a pressure difference through a separating member having a hole size of less than 300 μπι and the permeability of the substance to be treated is improved by creating a flow on the supply side of the separating member.

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The separation side of the separating member is preferably formed; 'the turbulent flow and the surface of the wire are kept clean by means of a rotating rotor fitted on the supply side.

More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

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The screen according to the invention is in turn characterized by what is set forth in the characterizing claim 6. part of the independent.

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The invention provides considerable advantages. The screen according to the present invention is a pressure screen which combines the advantages of a vibrating screen and a conventional pressure screen. A wire similar to that of a vibrating screen can be used as the screen element, so that the resolution is good. The wire is supported by a grooved plastic sheet and a support net or fabric, which provides a much better pressure resistance than a vibrating screen. 5 A perforated sheet can also be used as a screen, but the production of a perforated sheet with a small hole and a sufficiently open surface is expensive compared to the cost of a wire element with current manufacturing methods. Sludges used in papermaking, such as coatings and filler sludges, are thixotropic and pseudoplastic.

Such substances are characterized by a large increase in fluidity when the substance is mixed. This feature is exploited by creating a turbulent flow on the feed side of the screen separating member by means of a rotor. The mixing of the sieve-15 material considerably improves its flowability and the mixing allows the permeability of the small-hole sieve to be improved. In addition, the cleaning of the wire is arranged by a mixing rotor and not by vibration, so that the vibration does not strain the wire. Because no vibration is used, the wire .20 will not break at its attachment points. Thanks to the wire support structures, the same pressure difference can be used in the sieves as in traditional pressure sieves.

Since the wire has up to 4-5 times more open screen -:.: 215 surfaces than the wire drum of pressure screens, the mu-V of the invention is; the capacity of the sieve relative to the area of the separating member is higher than in previous pressure sieves with a corresponding pressure difference. On the other hand, because of the traditional vibratory • · ·. * ·. Compared to this, a substantially larger ./30 pressure difference can be used, the capacity is proportionally higher • ·: · per same open area. Because the capacity of the sieve * ··. ' is proportional to the pressure difference, the capacity can be easily increased tenfold compared to a conventional vibrating screen, requiring only one screen in sites where multiple parallel screens have previously been used. The mixing of the sieve material 6 97983 increases the capacity in particular, so that the sieving costs can be significantly reduced with the sieve according to the invention, because fewer and smaller sieves are required.

5 The wire is kept open and clean by a rotor, the turbulence of which raises the impurities from the surface of the wire. The movement of the rotor in the direction of the wire plane enhances the separation of elongate impurities such as fibers. The separation of particles of other shapes is enhanced because the root-10 tori causes a flow transverse to the wire and its openings, making it difficult for the separable bodies to pass through the wire. Turbulence also prevents the accumulation of material to be deposited on the inner surfaces of the screen.

The invention will now be described in more detail with reference to the accompanying drawings.

Figure 1 shows a cross-section of one device according to the invention.

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Figure 2 shows one separation element used in the device of Figure 1.

• Figure 3 is a partial magnification of Figure 2.

Figure 4 is an assembly view of the member of Figure 2.

: # The present invention essentially relates to the hole size of a separator member «« · ·. ·; . ko, which means the largest diameter of the hole, for example * • 30 ksi diameter. The shape of the holes can be quite diverse, depending on the method of manufacture of the separator member used.

* ... · To achieve sufficient resolution, the hole size must not exceed 300 μπι and normally a maximum hole size of 200 μπι must be used. The most preferred hole size is between '35 50 and 120 μπι, but the hole size to be selected varies depending on, among other things, the shape of the holes.

7 97983

The screen shown in Fig. 1 is assembled in a base and frame plate 1. The screening takes place in a screening chamber 2, which is a rotationally symmetrical flat chamber divided into two sub-chambers by a separating member 3. The upper part of the chamber is closed by a lid 5. the feed channel 5 and the outlet channel 6 surrounding the feed channel 5. The feed channel 5 leads to the upper sub-chamber of the screening chamber 2, i.e. the inlet chamber 8 bounded above by the lid 4 and below the separating member 3. Below the separator-10 member 3 there is an outlet chamber outlet duct 6. The center line of the screening chamber 2 further has a degassing channel 11 leading to the inlet chamber 8. The degassing channel 11 is attached to a grid 10 supporting the lid. The outer edge of the screening chamber 15 has a reject outlet 24. A shaft 7 extends into at the end is a rotor 12, already ka is spaced from the surface of the separating member 3. The shaft 7 is fixed to the frame 1 by bearings 13 and is used. 20 by an electric motor 14 attached to the frame 1 via a V-belt transmission.

The separating member 2 is shown in more detail in Figures 2, 3 and 4. The member consists of five parts. Its body is formed by a perforated support plate 17. The support plate 17 is a round, substantially planar plate with a round hole 21 in the center.

At the edges of the circular hole 21 and at the outer edge of the plate 17 there are:: holes 22 and 23 for fixing screws and holes through the support plate 17 are placed in the bottom of the machined circular grooves. A support net 19 is arranged on the support plate 17, the mesh size of which is quite large, and on the support net * ... · 19 there is an actual sieve net 20 formed of a woven material similar to a wire of a vibrating screen, where: the wires form openings through which the material to be screened can pass through the screen net.

The support net 19 and the sieve net 20 are fixed to the support plate 8 97983 17 at the edge and from the edge of the central hole 21 by tensioning straps 16 and 18 and the function of the support net 19 is to prevent the wire from penetrating the grooves of the support plate 17.

5 A device like the one described above works as follows. The material to be screened is fed to the screen through the feed channel 5 under overpressure, whereby the material first enters the inlet chamber 8, from where it passes through the separating member 3 to the outlet chamber 9 and further through the outlet channel 6 to the coating machine. The substance entering the inlet chamber 8 passes mainly through the screen net 20, whereby the actual separation of oversized particles and lumps and fibers takes place. The material passing through the screen 20 then flows freely through the holes in the sparse support 19 and the support plate 17 into the out-15 chamber.

Since a considerable amount of material to be sieved passes through the sieve, the Sieve Mesh 20 would easily become clogged if it were not kept clean. The sieve according to the invention is in principle a pressure sieve in which the liquid passes through the overpressure through the wire. The overpressure is easily achieved by means of a feed pump, keeping the volume flow sufficient. The substance to be sieved is fed into the space above the wire in the inlet chamber and the feed takes place in the middle of the sieve around the axis of the root 1.25, whereby the flow direction of the sieve t-'' of the sieve is radially towards the outer circumference. The cleaning of the female arm of the wire is done by means of a rotating rotor 12.

•,: The rotor 12 rotating at a circumferential speed of about 5-20 m / s acquires turbulence, by means of which the oversized particles>% 30 detach from the surface of the screen net 20. Driven by the feed method and the pumping effect caused by the rotor »*‘ · ’12, the contaminants travel towards the outer perimeter with the reject outlet 24 and the reject is removed either in a continuous flow or at certain intervals. The reject is treated with a secondary separator, for example a vibrating screen. Usually, the amount of reject relative to the material flow to be handled is quite small, so periodic removal of 9 97983 reject is often sufficient.

However, the sole function of the rotor 12 is not to keep the separating member 3 clean but also to facilitate the flow. Since the hole size of the separator member 5 must be quite small, it is poorly permeable to difficult-to-flow substances such as coating and filler slurries. The pressure difference across the separating member increases the capacity of the device, but in fact the sieving according to the invention with these substances makes it possible to vigorously mix the substance to be sieved with a rotor. When mixed, the viscosity of the substances decreases considerably, so that they flow more easily through the separating member and the device achieves a high screening capacity without compromising the resolution. Thus, in this screening method, the mixing has partly the same function as the vibration of the vibrating screen, but the mixing combined with a pressure difference considerably higher than the vibrating screen is considerably more efficient in capacity than the vibration alone. 1

In addition to the above, the present invention has other embodiments. A variety of mesh structures can be used as the target mesh or wire, but it is important that the mesh consists of intersecting yarns to provide a mesh with a large permeable area and good resolution at the same time. Commonly used

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*, · '· The mesh size of nets and wires is about 50 to 120 μπι, but the mesh size is naturally chosen according to the application. The support plate and support network should pass through possible- »* * *, 'y. and at the same time support the wire against the stress caused by the differential pressure used in the sieve. Without the support, the wire would not withstand screening over the pressure difference, but with the supported structure at least the same pressure difference, typically about 1 bar, over the screen can be used as with conventional pressure screens.

35 A perforated plate made of steel or other suitable material can also be used as the separating member, but 9 79 c 3 10 with current methods it is relatively expensive to produce a small perforated plate with a permeable surface area of 20-30%. However, such a sheet can be made, for example, by laser machining and has the advantage of better mechanical strength than wire. The permeable area of the separator member affects the capacity of the device, so if the capacity requirements are otherwise met, a separator member with a smaller permeable area can be used.

10 The operation of the device can be controlled by changing the rotor profile and speed as well as the pressure difference used for screening. The shape of the rotor can be very diverse, but most importantly, it provides a sufficiently turbulent flow to lift the separable particle-15 coils from the surface of the wire. The rotor drive motor can be positioned so that the shaft 7 does not pass through the separator member, in which case it would be above the device in the example device. Because the device uses a rotating rotor, the shape of the screening chamber is rotationally symmetrical. If the mixing and cleaning of the differential member and the supply of the substance to be treated are arranged differently, the shape of the device can, of course, be changed. The supply of the material to be targeted can be arranged from the side, whereby large and hard objects which may pass between the rotor and the separating member cannot damage the rotor. For example, cross-sectional * T; in a square device, the feed could be at the short side of the square and the turbulent flow could be provided by high flow velocity or by different vane or propeller solutions. Instead of a planar separator member 30, it is conceivable to use a slightly curved J. a separator plate or a curved support plate on which the wire • i or other Target Mesh is placed. In this way, the transport of the reject to the edges of the device can be enhanced, but on the other hand, the structure becomes more complex.

Claims (15)

11 97983 A method for screening paper filler and pigment sludges and coating compositions, in which method 5 - the material to be screened is fed into a chamber (8), at least part of the wall of which is formed by a separating member (3) with apertures which pass through smaller particles; ingredient is passed on and impermeable portion is removed as reject, and 15 in the pressure element (3) on the supply side is increased to be higher than the output-side pressure, characterized in that 20 of the straining element (3) is used to perforate the kappa piece with a hole size of less than 300 μηη, and - the substance to be screened is mixed on the feed side of the separator member (3) to reduce its viscosity. kb; * * *: Method according to Claim 1, characterized in that a separating element (3) is used, in which the transmission! The openings are formed by openings for forming intersecting wires. A method according to claim 2, characterized in that the separating member (3) is supported on the lower pressure side. Method according to one of Claims 1 to 3, characterized in that the surface of the separator element (3) is kept clean by arranging a turbulent flow in the vicinity of the surface. Method according to Claim 4, characterized in that the surface of the separating element (3) is kept clean and the viscosity of the substance to be sieved is reduced by means of a rotating rotor (12). Method according to one of the preceding claims, characterized in that the substance to be screened is fed into the screening chamber (8) via a feed channel (5) surrounding the shaft (7) of the rotor (12) and the reject is removed from the outer edge of the screening chamber (8). Method according to one of the preceding claims, characterized in that the pressure difference across the separating element is about 1 bar (100 kPa). A screen for screening paper filler and pigment slurries and coating compositions, the screen comprising - an inlet chamber (8), the wall of which is at least partially formed by a separating member provided with openings. (3) with permeable holes, «· 25« «. - means (5) for supplying the substance to be deposited into the support chamber (8), • · · • · · - means (6) for introducing the component 30 which has passed through the separating member (3) and means (24) for impermeable group removing a portion of the reject, and • · • »· • · · ··, - means strained material supplying inlet chamber (8) such that the inlet chamber (8) pressure: 35 higher than the separation means (3) to the outlet side (9) to the pressure , 13 97983 characterized in that - the separator member (3) is a perforated body with a hole size of less than 300 μιπ, and 5 - means (12) are arranged in connection with the inlet chamber (8) for mixing the target material and forming a flow in the vicinity of the separator member (3) . 10 Sieve according to Claim 8, characterized in that the separating element (3) is a net-like body in which the penetration openings are formed by openings formed by intersecting wires. 15 Sieve according to Claim 9, characterized by means (17, 19) for supporting the separating element (3) on the lower pressure side. · 20. Sieve according to Claim 8, characterized in that the separating element (3) is a rigid perforated plate. Screen according to one of Claims 8 to 11, characterized in that the means for mixing the material to be screened comprise a rotating rotor (12). Screen according to Claim 12, characterized by: a feed channel (5) and an outlet channel (24) surrounding the shaft (7) of the rotor (12) for removing rejects from the outer edge of the screening chamber (8). A strainer according to claim 9, characterized by ·; that the separating member (3) comprises a wire (20) or the like: a corresponding net, a round and planar support plate (17) perforated behind the wire, and a wire (20) and a support plate (19) of the support net (19); 17) in between. 3 14 97983 Sieve according to one of Claims 8 to 14, characterized in that the hole size of the separating element (3) is less than 200 μπκ Sieve according to one of Claims 8 to 14, characterized in that the hole size of the separating element (3) is 50 to 120 μπι. Screen according to one of Claims 8 to 16, characterized in that the separating element (3) has more than 20% open permeable surface. * k t: • »• # · 1 ·« t I i • · · 15 97983