METHOD TO SCREEN PULP AND A PULP SCREEN

Abstract: The invention relates to a method for separating cellulose fibre pulp when the cellulose fibre pulp to be separated forms a mixture with a medium which is substantially in gaseous form, for example with steam or air. The cellulose fibre pulp is separated at a consistency of at least 10 percent.

Description: Method to screen pulp and a pulp screen The present invention relates to a method and an apparatus for separating fibre pulp. The method comprises separation of ground cellulose fibre pulp in such a way that it does not contain large particles, for example fibre bundles. The cellulose fibre pulp to be separated forms a mixture with a medium which is substantially in gaseous state, for example with steam or air. The apparatus is an apparatus for implementing the above-mentioned method. In the following, prior art will be described with reference to the appended Fig. 1 which shows various apparatuses for separating fibre pulp according to US 4,606,789. The method presented in US 4,606,789 comprises at least one apparatus for separating fibre pulp which separates acceptable fibre pulp from heavier pieces, e.g. slivers. The separating force used is high- pressure steam produced in a grinder, the ground fibre being carried with the steam and introduced at a high speed to the separating appa- ratus. The publication also presents three different alternatives for a separating vessel as shown in Fig. 1 : a) The fibre pulp is introduced along a pipe 20 into a separating apparatus 22, wherein the accept is discharged through a cylinder 68 belonging to the separating apparatus, along a pipe 24, and the reject is discharged through a discharge screw 28, along a pipe 30. b) The fibre pulp is introduced into a separating apparatus 70, along a pipe 20, into a cylinder 72 which is closed at its upper end. The fibres are discharged from the cylinder through the open lower end. At a certain distance under- neath the cylinder, there is an upwards tapering cone 74 which is concentric with the cylinder 72. The cone guides the fibre pulp aside, and the accept is discharged through a discharge pipe 76 and the reject is discharged through a discharge screw 28, along a pipe 30. c) The fibre pulp is introduced into a separating apparatus 80 along a pipe 78. The separating apparatus is based on the fact that heavier particles are dropped closer to the opening of the inlet pipe 78 than lighter particles. The bottom of the separating apparatus is provided with inlet screws 82 and 84 which are separated with a partition wall 86. The reject is led to an outlet 88 and the accept is led to an outlet 90. The steam is discharged via an outlet 92 to a steam cyclone 94. One separating apparatus utilizing the centrifugal force is disclosed in German application publication DE 2012797, to which corresponds Finnish patent 55128. Furthermore, methods are known, in which the pulp flow is led to a cyclone in which the fibres are separated. Such a solution is disclosed, for example, in US 4,309,283. Problems in prior art methods include the low consistency used in the separation, about 1 %. Furthermore, long, flexible fibres are easily discharged with the reject. This is a problem particularly in the TMP process (thermomechanical pulp) which produces a relatively large quantity of long fibres compared with other methods of production of mechanical pulp. The aim of the method according to the invention is to avoid the disadvantages of prior art. The method according to the invention is thus characterized in that the separation of cellulosic pulp is performed at a consistency of at least 10 percent. In the method according to the invention, the separating apparatus is characterized in that it is a cylindrical chamber in which the ratio between the diameter and the length of the housing ranges from 1 to 10. The method and the apparatus according to the invention have several advantages. The method works at a high consistency, requiring less pumping of water and water circulation. At the same time, the separating method becomes simpler. The quality of the pulp is improved, because the separating apparatus efficiently separates rough fibres requiring further grinding, into the reject, and flexible fibres are passed to the accept. Resin remains in the fine material, because it cannot spread onto the surface of the fibres due to the high consistency. The separating apparatus is simple and does not contain movable parts, wherein the manufacturing and maintenance costs are low. The size of the separating apparatus is small, because the separation is performed at a high speed. Due to the small size, the manufacturing costs of the separating apparatus are low. The separating apparatus can use the steam produced in a grinder as the separating force, wherein separate power supplies are not necessary. The fibre pulp is separated at a consistency of at least 10 percent, preferably at a consistency of at least 20 percent, and most advanta- geously at a consistency of at least 40 percent. On the other hand, the consistency of the fibre pulp to be separated can be at a maximum of 90 percent, preferably at a maximum of 80 percent, and most advantageously at a maximum of 60 percent. The consistency refers to the quantity of the pulp as weight percent in a mixture of pulp and water. In the method according to the invention, water is in vapourized form. In the following, the method and apparatus for separating fibre pulp will be described in more detail with reference to the appended drawings, in which Fig. 2 illustrates the structure of the separating apparatus in a cross-section, Fig. 3 illustrates a pulp flow chart during the separation, Figs. 4 to 7 show different alternatives for the apparatus in such a way that Figs. 4a, 5a, 6a, and 7a show the separating apparatus in a top view, and Figs. 4b, 5b, 6b, and 7b show the corresponding apparatus in a side view, and Fig. 8 illustrates separating apparatuses coupled one after the other. According to Fig. 2, the separating apparatus is formed by a cylindrical chamber 1 , in which the ratio between the diameter and the length of the housing ranges from 1 to 10. The length of the housing refers to the transverse distance between the side walls 6 and 7 of the cylindrical chamber from each other. Typically, the diameter of the chamber 1 is about 1 m and the length of the housing is about 0.2 m. The separating apparatus is provided with an inlet for the supply pulp. The supply pulp S is led from a grinder 2 tangentially via an inlet pipe 3 to the chamber 1 in which the pulp is brought into a turbulent movement due to a decrease in the pressure. In the pulp flow rotating in a spiral manner, the rate of the pulp passed in the centre is greater than at the edges, wherein the accept is brought into the centre of the separating apparatus, and the reject to the inner circle of the cylinder in the sepa- rating apparatus, or to its close vicinity within the separating apparatus. The separation is performed on the basis of the mass, size and surface area of the wood fibres. The separating apparatus is provided with outlets for the accept and the reject. As shown in Fig. 3, the accept A is discharged from the centre of the cylinder along a discharge pipe 4, and the reject R is discharged from the circumference of the cylinder along a discharge pipe 5. The reject R is ground again in a grinder 2, from which is it recirculated to the separating apparatus 1 via an inlet pipe 3. The inlet and outlet pipes can be installed in different positions in the longitudinal direction of the housing of the cylinder, or there can be several inlet and outlet pipes. It is also possible that a thresher equipped with slots is installed in the separating apparatus, the thresher being rotated at a suitable rate, for example at the rate of 3000 rpm. A motor is not necessarily required for rotating the thresher, but it can be rotated by the force of steam supplied into the separating apparatus. Figures 4 to 7 show some possible structures for the separating appa- ratus. The diameter of the inlet pipe 3 and/or the outlet pipe 5 can be smaller than (Figs. 5 and 7) or equal to (Figs. 4 and 6) the length of the cylindrical housing of the separating apparatus 1. The inlet pipe 3 and/or the outlet pipe 5 can be placed in different ways in relation to each other as shown in Figs. 4 to 7. The separating apparatus can be placed in such a way that the cylindri- cal part of the chamber is either vertical or horizontal. The separating apparatus can be used to separate all cellulose fibre pulps produced by grinding, but it is particularly advantageous in the application of a manufacturing process producing pulp with relatively long fibres, for example the TMP process. The pulp is supplied to the separating apparatus either by means of a separate power supply, for example pressurized air, or by utilizing the grinder output pressure, which is normally from 3.5 to 4.0 bar. The pulp discharged from the grinder is a mixture of steam and fibres having a consistency of 40 to 60 %. The rate of the mixture is preferably accelerated to the rate of 200 to 800 m/s by selecting the diameter of the inlet pipe 3 in a suitable way or by providing the inlet pipe with suitable nozzles to control the flow. At such a rate, coarse particles are forced to the edges of the separating cylinder and flexible, plastic fibres to the centre. As shown in Fig. 8, it is possible to place several separating apparatuses 1 one after the other, wherein the accept of the next step is re- circulated to the supply of the preceding step. In this way, the reject R is carefully separated from the accept A, and only the pulp that needs to be reground is recirculated to the grinder 2. Example. Thermomechanical wood pulp (TMP), in which the fibre length was 1.46 mm and the linear density was 0.299 mg/m, was supplied to a grinder at different pressures. The following results were obtained (fibre length and linear density measured from the accept): The reject ratio refers to the ratio between the dry substance flow of the reject and the dry substance flow of the supply pulp. The linear density refers to the ratio between the mass of the cellulose fibre and the fibre length, the unit being mg/m. The results show that when the supply pressure is increased, the average length of accepted fibres is increased, wherein the accept contains relatively more longer fibres when the supply pressure is increased. At the same time, however, the reject ratio is increased, wherein a larger part of the pulp is rejected when the pressure is increased. Nevertheless, the linear density of the accept is increased, and the separating apparatus can thus separate also other than very fine fibres into the accept. It is obvious that the method and apparatus for separating cellulose fibre pulp can also be implemented in another way within the scope of the claims. There can be only one outlet for the accept. There can be, for example, guides within the cylindrical housing of the separating apparatus, to guide the reject to the outlet. Furthermore, there can also be abrasion surfaces inside the housing, which, for their part, disintegrate the fibre bundles forced onto the inner surface of the housing. The main idea is that the cellulose fibre pulp is separated at a high consistency by a separating apparatus whose diameter is greater than or equal to the length of its housing.

Claims: Claims: 1. A method for separating cellulose fibre pulp, when the cellulose fibre pulp to be separated forms a mixture with a medium which is in a sub- stantially gaseous state, for example with steam or air, characterized in that the cellulose fibre pulp is separated at a consistency of at least 10 percent. 2. The method according to claim 1 , characterized in that the flow rate of the cellulose fibre pulp is adjusted to a suitable value and the pulp is supplied to a separating apparatus (1 ). 3. The method according claim 1 or 2, characterized in that the cellulose fibre pulp is supplied to the separating apparatus (1 ) tangentially by means of the output pressure of a grinder (2). 4. The method according to claim 2 or 3, characterized in that the cellulose fibre pulp to be separated is supplied to a separating apparatus which is a cylindrical chamber in which the ratio between the diameter and the length of the housing ranges from 1 to 10. 5. The method according to claim 4, characterized in that the accept (A) is discharged from the centre of the cylindrical chamber and the reject (R) is discharged from the circumference of the cylindrical cham- ber and is led to be either reground or to the broke. 6. The method according to any of the preceding claims, characterized in that several separating apparatuses (1) are coupled one after the other in such a way that the accept (A) from the next step is supplied to the input of the preceding step. 7. A separating apparatus for separating cellulose fibre pulp, characterized in that it is a cylindrical chamber in which the ratio between the diameter and the length of the housing ranges from 1 to 10. 8. The separating apparatus according to claim 7, characterized in that a tangential inlet is provided for the supply of the pulp, an outlet for collecting the accept is provided at the centre of the cylindrical chamber, and an outlet for collecting the reject is provided on the circumference of the cylindrical chamber.

Inventors: TIENVIERI TAISTO, TIENVIERI, TAISTO
Applicants: TIENVIERI TAISTO, UPM KYMMENE CORP, TIENVIERI, TAISTO, UPM-KYMMENE CORPORATION
Application year: 2000
Main classification: D21D5
IPC codes: D21D5