Patent Publication Number: US-6907995-B1

Title: Chip screening method and plant

Description:
BACKGROUND AND SUMMARY 
   This invention relates to the screening of wood chips and, especially, to the removal of the pin chips and to the dosing of them back into the process. 
   The objective of the chip screening is to free the chips from such shares of small-size and large-size fractions that could have a detrimental effect on the pulp making process. In the screening process, the large-size fractions (too big and too thick chips) are usually treated by means of a rechipper, for example, into smaller particles or crushed by means of a roller so press into a form more suitable for the process. The finest fraction (the sawdust) causes problems in most processes, which is why every effort is made to purify the chips from it as well as possible. The next biggest fraction, the so-called pin chips, as such, constitutes quite a good raw material in terms of fibres, but a high content of pin chips is disadvantageous to some processes, e.g. to continuously operating digesters, as it may cause malfunctions in the process (e.g. blocks). 
   For the removal of pin chips, the same kind of devices are used as for the removal of sawdust, i.e. flat screens, vibrating screens, roll screens, disc screens and so forth. To the screening elements themselves, however, changes are made because of the larger particle size. Screening plants known in the prior art are described, for example, in Finnish patent specifications 79251 and 90019. If the subsequent process (a chemical or a mechanical pulp making process) sets strict restrictions on the maximal amount of pin chips, and if it is possible that the pin content of the material to be fed is high, the pin chips are usually separated by screening and stored in a separate bin. Then, the pin chips are dosed into the accepted fraction fed to the subsequent process, using a constant ratio. The plants designed for this purpose are reliable as such but represent, from an economical point of view, a considerable extra investment. On an average, the amount of pin chips among the chips to be screened is usually larger than the allowable amount, especially when the chips are bought from sawmills. The method according to the invention provides the same result at substantially lower costs. The pin chips are separated in the screening process but a maximum desired amount is dosed back among the chips due for the process immediately after the separation. Thus, no intermediate storage of the pin chips, and, consequently, no pin bin with auxiliary devices are needed in the method according to the invention. The screening can be optimised. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and the details thereof will now be described in more detail with reference to the following drawings wherein 
       FIG. 1  shows the traditional way of separating and dosing pin chips using a flat screen, 
       FIG. 2  shows a method of separating and dosing pin chips in connection with thickness screening, 
       FIG. 3  shows the method of separating and dosing pin chips according to the invention using a flat screen, 
       FIG. 4  shows the method of separating and dosing pin chips according to the invention in connection with thickness screening, 
       FIG. 5  shows the principles of the pin chip dosing method according to the invention in connection with a sectional feeder, 
       FIG. 6  shows a dosing screw and 
       FIG. 7  shows the use of a scraper conveyor for the dosing. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a screening system for separating pin chips and for re-dosing them so that the amount of pin chips among the chips that are to be led to the subsequent process remains constant. A flat screen  1  divides the input  2  into four parts. The large-size fraction  3  is led to a rechipper from which, once the re-chipping has been completed, it is conducted back to screen  1  via an air separating cyclone  5 . The accepted fraction  6  is led to the subsequent process  7 . The finest material  8 , i.e. the sawdust, is usually led to a burning process. The pin chips  9  are conducted into a pin bin  10  from which a pin chip flow  11  of desired size is dosed back into the subsequent process  7 . When the pin bin  10  becomes full, the pins are discharged from the bin among the sawdust  12 , for example, to be burned. 
   In  FIG. 2  is shown a thickness screening system wherein the large-size fraction  3 , i.e. the chips that have crossed the first thickness screen  13 , is led to a roller press  14 . The chips  15  treated by the roller press are led directly to the subsequent process  7 . The chips  16  that have penetrated the end part of the thickness screen are led directly to the subsequent process. The chips  17  that have penetrated the front part fall onto a sawdust screen  18 . The particles  8  that have penetrated the sawdust screen are sawdust and are led to a burning process. The chips that have crossed the sawdust screen are pin chips  9  that are led into a pin bin and treated in the same way as in FIG.  1 . 
     FIG. 3  shows the pin chip dosing method according to the invention in connection with a flat screen. The pin chips  9  are led from the screen directly to a dosing apparatus  19  that doses only a certain amount  20  of pin chips into the subsequent process. The dosing apparatus  19  leads the surplus  21  of the pin chips among the sawdust  8 . Alternatively, the surplus  21  of the pin chips is transported away from the screening plant, back to the chip pile, for example (if the variation in chip quality is only temporary), or, to a separate sawdust digesting process, for which the relevant pin fraction is excellent. 
   In  FIG. 4 , the pin chip dosing method according to the invention is used in a thickness screening system. Pin chips  9  coming from a sawdust screen  18  are led to a dosing apparatus  19 ′ which, in this case, is constituted by two screw conveyors. The dosing apparatus  19 ′ operates in the same way as the dosing apparatus shown in FIG.  3 . 
   Said dosing can be performed by means of many kinds of devices that feed only a certain amount of pin chips among the accepted chips at a certain rate. The rest of the pin chips are led among the sawdust or away from the screening process, as desired. It is essential that the dosing apparatus is set for dosing the appropriate chip amount. Some of the most common ways of performing this kind of dosing are described in the following. In a socalled sectional feeder, for example, the pin chips are led into the feeding chute  22  of the sectional feeder  19  (FIG.  5 ). The dosing apparatus can be set to dose a desired amount  20  of pins into the subsequent process by adjusting the size of the sections  23  and the rate of rotation of the rotor  24 . The rest  21  of the pin chips fall over the lower edge  25  of the feeding chute  22 . 
   Correspondingly, dosing can be performed for example by means of two screw conveyors (FIG.  6 ). A desired amount of pin chips  20  is extracted by means of adjusting the speed of rotation of the lower screw conveyor  26 . The upper screw conveyor  27  transports the rest  21  of the pin chips to a separate discharge opening  30  from which they are conducted, for example, among the sawdust. Dosing can be also performed by means of a scraper conveyor (FIG.  7 ), for example by adjusting the rate of speed of the lower scraper conveyor  28  to control the amount of pin chips  20  led into the subsequent process. It is also possible to place a screw conveyor under the scraper conveyor  29 , to operate according to FIG.  6 . 
   If the input of the screening process varies in quantity, the input or the output of the screening process can be measured and, correspondingly, the amount of pin chips led to the subsequent process can be adjusted. Thus, the ratio of the pin chips to the total amount of chips led to the subsequent process remains constant all the time. If the input of screening plant is constant, a constant amount of pin chips can be dosed. The term “pin chip” is defined, for example, in standard SCAN-CM 40:94.