METHOD AND APPARATUS FOR GRADING FIBER SUSPENSION

Abstract: The present invention relates to a method and apparatus for grading fiber suspension. The invention especially relates to so called cyclone separators, by means of which particles heavier than the fibers, such as sand and metal particles, are separated from the fiber suspension in the pulp and paper industry. The apparatus according to the invention is characterized in that the reject is removed from the cyclone separator (1) to a reject chamber (10), inside of which a rotor (11) rotates maintaining the pulp in movement and efficiently separating the acceptable pulp flowing into the reject chamber (10) from the heavier reject, whereby the acceptable material can be returned back to the pulp circulation along duct (12). Additionally, ribs may be provided on the walls of the reject chamber (10) to decelerate the rotational speed of the reject, in other words the heavier particles thrown against the wall of the reject chamber have their descent to the bottom of the reject chamber (10) facilitated thereby, from where they are discharged via a duct (14).

Description: The present invention relates to a method and apparatus for separating, by means of centrifugal force, coarse and heavy particles from material to be graded. The method in accordance with the invention and the pulp grader, a so called vortex cleaner or cleaner realizing the method, are especially suitable for sorting fiber suspensions in the pulp and paper industry, for example, to separate sand, stones, metallic impurities and like from fiber suspension. There are many known so called hydrocyclone type cleaners into which pulp to be cleaned is led tangentially into the upper edge of the cylindrical part, whereby the pulp executes a spiral movement circulating downwardly of the wall of the apparatus. At this stage, any heavy impurities, such as stones and metallic particles are drawn under the action of centrifugal force to the outer rim of the turbulent flow against the wall of the apparatus and the lighter material accumulates towards the middle of the apparatus, from where it is discharged most usually via a duct in the upper part of the apparatus having a considerably smaller diameter than the apparatus itself. The heavy material separated from the pulp flow is discharged by means of special arrangements via a discharge opening located in the lower part of the apparatus. From BE-A-537 381 a centrifugal separator ist known which is divided into two chambers. A rejects flow, including acceptable fiber material and coarser rejects, enters from the primary chamber into the secondary chamber and is accelerated by dilution liquid fed from an inlet duct of the secondary chamber to create a liquid vortex flow outside the rejects flow. At the same time the reject material is being diluted, the coarser reject is separated from the accept which remains closer to the center axis of the vortex. The coarser rejects are then finally rejected from the system through a reject outlet in the bottom of the chamber. However, in this system there exists the danger that the reject outlet may become clogged. Furthermore, the acceptable material must rise upwards through both the secondary and primary chambers to be removed through an accepts outlet located above the primary chamber. Additionally, this system is not suitable for treating higher consistency fiber suspensions. A centrigual separator in accordance with the preambles of claims 1 and 4 of the present invention is known from DE-A-2 757 581. In this known separator, the rotational speed of separated coarse fraction is reduced before being carried to the secondary stage on account of the shape of the separator. Subsequently, a rotor is provided in the reject chamber of the secondary stage to maintain the rotational speed of the pulp entering the reject chamber and to convey the accept out of the reject chamber. In other known separators, a chamber is arranged protruding downwardly from the lower part of the conical part of a hydrocyclone apparatus and communicating with a duct, which can be closed by a closure device. In the lower part of the chamber there is another closure valve, the opening of which enables the discharge of the chamber. In the normal condition the upper closure device is open and the coarse material being separated flows into the chamber. When the chamber is almost full, the upper closure device is closed and the lower closure device is opened, whereby the material being separated accumulates above the upper closure device. When the chamber is empty the lower closure device is closed and the upper one opened, whereby the material accumulated thereon falls into the chamber. For example, US Patent Publication 3533506 discloses very specifically problems which are probably encountered when cyclone separators are used. One of the problems, among others, is in the discharge of the cyclone. Particles to be separated by the cyclone can often be of considerable size or otherwise easily clog the discharge opening. When the discharge opening becomes clogged, the rejection or separation of the grader stops and the level of the separated material quickly rises thus filling the cyclone, whereby the particles to be separated are drawn away with the accept pulp. Attempts have been made to prevent or to minimize this clogging of the discharge end of the cyclon, for example, by arrangements in accordance with GB Patent Publication 1249634, which includes both a transverse movable bar extending through a discharge duct into a cyclone cone and an impeller, both of which are used to create movement in the separated pulp layer so as the prevent the pulp from clogging the discharge opening of the cyclone and allow the pulp to flow uniformly to the discharge duct. The impeller is constructed so as to subject the separated pulp to a force component downwardly towards the discharge opening, in other words the impeller tends to pump the separated material from the cyclone. Conventional apparatus in present use also have the disadvantage of the filling of the bottom thereof and the stopping of the flow at the bottom of the apparatus. The fractions being separated do not reach the bottom anymore, but are drawn upwards together with the flow of the accept pulp fractions. For this reason, it has been important with the apparatus in accordance with the prior art to maintain the level of accumulated material in the lower part of the apparatus the same as or at least below a certain limit so as to prevent the disadvantageous filling of the bottom. Additionally, the known hydrocyclones used to grade fiber suspensions of the pulp and paper industry are characterized in that the consistency of the pulp at which they can treat the pulp, is very low, most usually be below 1%, as stated in the above mentioned US Patent Publication. However, the modern tendency is to reach continuously higher consistencies in the manufacture processes of pulp and cellulose, these consistencies being in any case well above 1%. Consequently, the hydrocyclones in their present form make the process more complicated, because all other components connected with the manufacture process can treat thicker pulp but the pulp must be diluted to a consistency of below 1% before feeding it to the hydrocyclone and must be thickened again after the cuclone treatment. This kind of action irrelevant for the actual process involves unnecessary equipment costs and requires treatment, pumping and circulating of considerable amounts of water. The object of the present invention is to eliminate or minimize said problems and disadvantages and in particular to develop a method and apparatus for grading the fiber suspensions in the pulp and paper industry at a consistency which is higher than that conventionally considered normal. The object of the invention is solved by the method and apparatus according to the invention with which it is possible to efficiently separate the heavier impurities from the fiber suspensions up to a consistency of 5%. Thus the apparatus in accordance with the invention is especially suitable, for example, to operate as a presorter for grinders. The method of grading or removing coarse material from a fiber or pulp suspension in accordance with the invention is characterized in that the rotational speed of the coarse fraction carried to the secondary stage is increased by a conical shape of the centrifugal separator with decreasing diameter and the rotational speed is further increased in the secondary stage by the mechanical means, arranged close to the discharge opening, such that the blocking of the reject outlet by the reject fraction is prevented by maintaining the reject fraction in a substantially rotating state by the mechanical means, and that during the secondary stage the coarse fraction is subjected to shear forces, by which any fiber flocks in the coarse fraction are broken into smaller flocks or single fibers which are returned to the pulp feed. The apparatus in accordance with the invention is characterized in that the conical shape of the centrifugal separator has a decreasing diameter towards the reject opening, the rotor is arranged close to the discharge opening in the reject chamber, and ribs are arranged on a wall of the reject chamber, the purpose of which ribs is to increase the turbulence of impurities rotating about the wall of the reject chamber and to accelerate the descent of the impurities to the bottom of the reject chamber. The invention will be further described below, by way of example, and with reference to the accompanying schematic drawings, in which: Fig. 1 is a sectional view of an apparatus in accordance with a preferred embodiment connected to a hydrocyclone, and Fig. 2 is an enlarged and more detailed sectional view of an apparatus according to a preferred embodiment. A pulp cleaner or grader in accordance with the invention comprises according to Fig. 1 a centrifugal separator 1 and a reject chamber 10. The pulp to be treated is led to the centrifugal separator 1 from the tangential conduit 2, whereby the pulp is caused to execute a circulating movement heading spirally downwards along the cylindrical wall 3 of the separator. The wall of the centrifugal separator changes in- to a conical casing 4 below the cylindrical wall, whereby the rotational speed of the pulp increases and also the effect of the centrifugal force on the pulp particles increases. Any heavy impurities are separated onto the inner surface of the conical casing 4 of the centrifugal separator 1 and are drauwn along the surface to the reject opening 5 located at the bottom of the conical part via the reject opening to the reject chamber 10. When so desired, an intermediate valve can be arranged in the reject opening 5, by which the reject chamber may be temporarily closed. The finer pulp fraction accumulates around the axis of the centrifugal separator 1 and rises towards the discharge opening 6 for accept on the cover 7 of the separator. The fraction separated from the pulp fed into the separator and carried to the reject chamber 10 includes both impurities and acceptable pulp, which should be, if possible, separated from the reject. When the pulp fraction arrives in the reject chamber 10 it is still executing rotational movement, which tends to further separate impurities from the acceptable pulp. Because the diameter of the reject chamber is, however, greater than that of the reject opening 5 of the reject, the rotational speed of the pulp in the reject chamber decreases substantially. However, a rotor 11 is arranged in the reject chamber 10 in accordance with the invention, which rotor accelerates the rotational speed of the pulp so that the acceptable material in the pulp accumulates in the middle part of the reject chamber 10, from where it can be removed, for example, and led back to the pumping chamber to the feed or like of the sorter, in other words to the centrifugal separator along the discharge duct 12. The rotor thus prevents the pulp fraction supplied from the separator to the reject chamber from stopping and prevents the pulp from filling the reject chamber. Due to the more powerful centrifugal force caused by the increased rotational speed, any impurities included in the pulp accumulate at the walls of the reject chamber and flow along the walls to the bottom of the reject chamber 10. The reject is discharged via an opening 13 at the bottom part of the chamber 10, which opening has a valve or closure members 15 in a duct extending from the opening. Thus the invention includes an improvement in the operation of the reject chamber of the centrifugal separator by returning acceptable fiber fraction from the flow to the reject chamber back to the feed of the separator and therefore it also includes an increase in the separation efficiency of the device. Fig. 2 discloses in more detail a reject chamber 10 according to a preferred embodiment having the same basic elements as in the apparatus according to Fig. 1, namely: a rotor 11, a discharge duct 12 for the acceptable fiber fraction, a discharge opening 13 for the reject, a duct 14 and a valve or closure member 15. Additionally, Fig. 2 advantageously shows a counter plate 16 of the vortex located below the discharge opening for the reject in the lower part of the centrifugal separator 1. The counter plate 16 prevents the flow rising in the middle part of the reject chamber 10 from rising back into the separator and on the other hand causes the primary vortex arriving at the reject chamber to be rapidly led to the walls of the reject chamber. The discharge duct 12 for the acceptable fiber fraction advantageously opens immediately below the counter plate of the vortex. In the case of the apparatus of Fig. 2, the rotor 11 is considerably smaller than that shown Fig. 1. According to the drawing, rotor 11 comprises a single or multifoil apparatus rotatable about a vertical shaft in the reject chamber. The drive of the apparatus is arranged by means of a shaft 18 extending through a bottom 17 of the reject chamber 10. Additionally, this embodiment has ribs 19 arranged on the walls of the reject chamber, the purpose of which is to increase the turbulence of the impurities rotating about the walls of the reject chamber and thus accelerate their descent to the bottom of the reject chamber 10 adjacent to the discharge opening 13. This accumulation is also intensified by the shape of the bottom 17 of the reject chamber 10 which is inclined towards the discharge opening 13. The reject chamber 10 is emptied by allowing the pulp accumulated in the chamber to flow away from time to time or by washing the bottom of the chamber, for example, with water. This kind of arrangement allows for several possibilities to adjust the separation efficiency. It is clear that a changing of the rotational speed of the rotor affects the separation speed and also the thickness of the reject layer accumulating at the bottom of the reject chmaber. It is further possible for the emptying of the reject chamber to be automated, for example, by TIMER-installations or by measuring of the moment corresponds a certain height of the reject in the reject chamber. Of course, it is possible to measure the power consumption of the motor rotating the motor, which gives exactly the same result. Although the foils 20 of the rotor 11 in Fig. 2 are axial, their direction can deviate therefrom, for example, in such a way that their direction subjects the pulp to a slightly upwards pumping effect, whereby their purpose is not only to provide the pulp flowing to the reject chamber with additional speed, but also to guide the acceptable fraction to rise towards the opening of discharge duct 12. Yet another use for the rotor 11 is to break the pulp flocks drawn into the reject chamber 10. This enables the finer fractionation of pulp and thus a greater amount of the acceptable material flung to the reject can be returned back to the pumping chamber and further back to the circulation. The flocks, which the rotor tends to break by subjecting the separated pulp to shear forces in the reject chamber, are not very large nor the bonds holding them together very strong. Thus, the energy required for breaking them is not relevant compared to the advantage of improved separation capacity gains. Furthermore, the ribs 19 on the walls of the reject chamber 10 can be used, if so required, also to affect the breaking of the flocks, for example, by extending the foils 20 of the rotor 11 and the ribs 19 to the same axial level and also relatively close to each other, whereby a strong turbulence is effected, which efficiently breaks the fiber flocks. On the other hand, it must also be avoided to effect too much turbulence, as this disturbs the separation of pulp particles. Although the more detailed embodiment of Fig. 2 discloses a reject chamber with a particular shape and construction, the reject chamber in accordance with the invention can differ considerably from the aforesaid. Depending on the purity rate of the pulp flowing into the reject chamber, the consistency and like factors the rotor can also be similar to that of the Fig. 1 or something between these two embodiments. Further -more, the ribs on the walls of the reject chamber 10 can be either axial or spiral depending on whether it is desired that they operate neutrally, lead the separated material downwards or decelerate as efficiently as possible the rotating flow of separated pulp. As is to be appreciated from the description, the invention can differ a great deal from the above described embodiments and yet not differ from the inventive concept which is disclosed in the accompanying claims, which alone define the scope of invention.

Claims: 1. A method of grading fiber suspension in which pulp is fed into a centrifugal separator as a primary stage and separated therein into an acceptable fraction and a coarse fraction, whereby the acceptable fraction is removed from the centrifugal separator and the coarse fraction is led to a secondary stage, in which it is separated into a further acceptable fraction by centrifugal force achieved by mechanical means, which acceptable fraction is returned to the pulp feed, and a further coarse fraction, which is removed from the system through a discharge opening provided in the secondary stage, characterized in that the rotational speed of the coarse fraction carried to the secondary stage is increased by a conical shape of the centrifugal separator with decreasing diameter and the rotational speed is further increased in the secondary stage by the mechanical means, arranged close to the discharge opening, such that the blocking of the reject outlet by the reject fraction is prevented by maintaining the reject fraction in a substantially rotating state by the mechanical means, and that during the secondary stage the coarse fraction is subjected to shear forces, by which any fiber flocks in the coarse fraction are broken into smaller flocks or single fibers which are returned to the pulp feed. 2. A method according to claim 1, characterized in that the further acceptable fraction is discharged from the secondary stage as a continous, uniform flow. 3. A method according to claim 1, characterized in that the consistency with which the pulp is fed to the centrifugal separator is 0.5-5%. 4. An apparatus for grading fiber suspension including a centrifugal separator (1) as primary stage having a conical shape and a reject chamber (10) as a secondary stage being connected to a reject opening (5) for reject in the centrifugal separator (1) and having a discharge opening (13) for reject in the reject chamber (10), provided with a rotor (11) for accelerating the rotational speed of the pulp flowing into the reject chamber (10) and having a discharge duct (12) for an acceptabele fraction separated from the pulp and fed back to the centrifugal separator (1), characterized in that the conical shape of the centrifugal separator (1) has a decreasing diameter towards the reject opening (5), the rotor (11) is arranged close to the discharge opening (13) in the reject chamber (10), and ribs (19) are arranged on a wall of the reject chamber (10), the purpose of which ribs (19) is to increase the turbulence of impurities rotating about the wall of the reject chamber (10) and to accelerate the descent of the impurities to the bottom of the reject chamber (10). 5. An apparatus according to claim 4, characterized in that foils (20) of the rotor (11) are directed so as to create an effect which causes a slight pumping effect on the rotating pulp upwards from the middle of the reject chamber (10). 6. An apparatus according to claim 4, characterized in that the rotor (11) and the ribs (19) extend to the same axial zone in the reject chamber (10), whereby turbulence created by the rotor (11) and the ribs (19) efficiently breaks any fiber flocks in the pulp. 7. An apparatus according to claim 4, characterized in that the rotor (11) extends close to the wall of the reject chamber (10). 8. An apparatus according to claim 4, characterized in that the consistency in which the pulp is fed to the centrifugal separator (1) is 0.5-5%.

Inventors: VIKIOE, PENTTI, Vikiö, Pentti
Applicants: A. AHLSTROM CORPORATION
Application year: 1988
Main classification: D21D5
IPC codes: D21D5, B04C5