SCREENING OF FIBROUS SUSPENSIONS

Abstract: A method of separating a fiber fraction by screening from a liquid containing fibers and small particles, where the separation takes place in that the ingoing liquid is allowed to flow along a screen member with screen apertures from an inlet (2) to a first outlet (3) for a first fraction which passes through the screen apertures, and a second outlet (4) for a second fraction, which does not pass through the screen apertures. 50-80 % of the flow of the second fraction is recycled from the second outlet (4) to the inlet (2) for increasing the concentration of network forming fibers in the ingoing liquid.

Description: Screening of fibrous suspensions This invention relates to separation of a fiber fraction from a liquid containing fibers and small particles. At the manufacture of papermaking pulp, both from wood- and recycled fiber raw material, the treatment is carried out in several steps, at which the process water is supplied and drained. At the manufacture of paper in an integrated pulp- and paper mill, the papermaking pulp is transported as fiber suspension at a pulp concentration of 3 to 4 % from the pulp mill to the paper mill. The process water in the paper mill, thus, originates mainly from the pulp mill. The surplus water from the papermaking is re-used later on at the pulpmaking. This implies that at the making of pulp and paper very great amounts of water are used which mainly circulate in the manufacturing process. From this complicated process results totally a small surplus of water, which must be cleaned before it can be discharged to the recipient. In order to reduce the need of external water cleaning and to decrease the pressure on the environment, the water flow in the mills for the making of pulp and paper has been closed to the greatest possible extent. As a result thereof, disintegrated (dispersed) impurities, fiber fragments and loose substance circulate about to an ever increasing extent and thereby disturb the production process. The normal way of reducing the build-up of internal circulation is at the discharge of the surplus water of the process to choose a heavily contaminated water. This water is led to external water cleaning, for example microflotation or membrane filtration. The problems with heavily contaminated circulating water flows are especially great at the manufacture of paper products based on recycled fibers. It becomes increasingly more difficult from an environmental and economic point of view to handle contaminated water flows from recycled fiber plants and paper mills. Inreased re-circulation of water in the mill results in an increased build-up of contaminations in the process. These contaminations, such as size lumps, particles of printing ink, ash and other matter (dispersed or solved) affect the process negatively. Even if it was possible to remove a large portion of the heavily contaminated process water, it is normally not defensible economically, because it often contains too many prime fibers. It is one object of the precent invention to solve the asforesaid problems by making it possible to effectively recover usable fibers from contaminated process water in connection with the making of paper-making pulp. Another object of the present invention is to increase the fiber separation efficiency of liquor screens in chemical pulp mills. These screens separate fibers from the waste liquor, before it is to be combusted. The separation efficiency of these screens is often much too low, especially in the case of short-fibrous hardwood pulps. The characterizing features are apparent from the attached claims. The method according to the invention is described in greater detail in the following with reference to the Figures, of which Fig. 1 is a flow chart showing an embodiment of the invention; Fig. 2 shows the effect of re-circulation of fibers according to the invention. The embodiment shown in Fig. 1 comprises a conventional screen means 1 with an inlet 2 and a first outlet 3 for a first fraction and a second outlet 4 for a second fraction. There is further a return line 5 with a central valve 6 between the second outlet 4 and inlet 2. In Fig. 1 the volume flow is indicated by the following definitions: Nf= inject flow Nr = recovered flow V c = return flow A liquid, which can be process water containing fibers and contaminations, is fed as inject (Vj) through the inlet 2 to the screen means 1. The liquid is allowed to flow along a screen member with screen apertures whereby a first fraction (V f - V r ) passes through the screen apertures and is taken out through the first outlet 3, and a second fraction (V r - V c ), which does not pass through the screen apertures, is taken out through the second outlet 4. The first fraction (V f - V r ) contains mainly small particles in the form of dispersed contaminations and fine material from the papermaking pulp, and the second fraction (V r - V c ) contains mainly long usable fibers. At this fractionation the fiber concentration of the ingoing liquid is very low and thereafter increases along the screen member during the course of fractionation. This implies that the strength of the fiber network increases much along the screen member, from the inlet to the outlet for concentration fibers. The energy supply from the rotor of the screen produces turbulence, which breaks up the weak fiber network at the beginning of the screening zone, which results in deteriorated separation of fibers. At the end of the screening zone the strength of the fiber network is much higher, because the fiber concentration has been multiplied, and the efficiency of the separation process is high. The fiber network formed on the surface of the screen member due to the flocculation tendency of the fibers has a very important role in the separation mechanism. It is, thus, not only the apertures of the screen member which separate the fibers from the liquid. By increasing according to the invention the flow of the second fraction (V r - V c ) and returning a portion (V c ) of the second fraction from the second outlet 4 via the line 5 to the inlet 2, the concentration of this fraction can be increased at the inlet end of the screen member. The flow of the second fraction preferably is increased 2-5 times, and 50 - 80% of this fraction is recycled to the inlet. The re-circulation factor (CF = V < /V r ) varies in these cases from 1-4. By this circulation of mainly long fibers the concentration gradient of this fiber fraction decreases along the length of the screening zone. Instead of that the concentration of the actual fraction to be separated varies with a power often along the length of the screening zone, the differences in concentration decrease strongly along the length of the screening zone with the help of the long fiber re-circulation according to the invention. This renders it possible to optimize the supplied de-flocculation energy along the screening zone so that the efficiency of the process increases, i.e. it is thereby possible to substantially decrease the share of long usable fibers in the first fraction. An increased fiber concentration in the ingoing liquid increases the network formation in the ingoing fraction, and the probability that network forming fibers shall pass through the apertures of the screen barrier decreases. Example Fig. 2 shows by means of a theoretical model based on mill data the effect of recirculation of a varying portion (V c ) of the second fraction (V r + N c ) to the inlet when the totally separated flow is 5 and 10 %, respectively. The flow division (FS = V/ V f ) is 0,05 and 0,10, respectively. It appears from Fig. 2, that it is possible to increase the efficiency of the recovery of usable fibers from about 80 % to about 90 % by using a re-circulation factor (CF) of 3-4. The fiber concentration in the inlet has simultaneously trebled from 0,2 to 0,6. The invention can also be used for increasing the efficiency of liquor screens in processes for the production of chemical pulp. Liquor screens are used, for example, in a sulphate mill for recovering fibers from black liquor before its combustion. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the claims.

Claims: Claims 1. A method of separating a fiber fraction by screening from a liquid containing fibers and small particles, where separation takes place in that the ingoing liquid is allowed to flow along a screen member with screen apertures from an inlet (2) to a first outlet (3) for a first fraction, which passes through the screen apertures, and a second outlet (4) for a second fraction, which does not pass through the screen apertures, characterized in that 50 - 80% of the flow of the second fraction is recycled from the second outlet (4) to the inlet (2) for increasing the concentration of network forming fibers in the ingoing liquid. 2. A method as defined in claim 1 , characterized in that the ingoing liquid consists of contaminated process water, from which usable fibers are recovered in the second fraction. 3. A method as defined in claim 1 or 2, characterized in that the ingoing liquid consists of fibrous black liquor from pulpmaking, whereby usable fibers in the black liquor are recovered in the second fraction.

Inventors: FREDRIKSSON BOERJE, FREDRIKSSON, BOERJE
Applicants: FREDRIKSSON BOERJE, SUNDS DEFIBRATOR IND AB, FREDRIKSSON, BOERJE, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1999
Main classification: D21D5
IPC codes: B01D29, D21C11, D21D5