SE528924C2 - Method of treating a fiber suspension - Google Patents

Abstract

Methods for treating a fibrous suspension are disclosed, including cleaning the fibrous suspension in a screening room with a plurality of cleaning stages and including a final cleaning stage, and in which the cleaning takes place at a low fiber concentration, and recycling at least a partial flow of the accept from the last cleaning stage to a position before or in the screening room after it passes through a thickener, where the concentration of the accept is increased to a concentration which minimizes dilution of the concentration of the fiber suspension, to which the accept is supplied.

Description

io 15 25 30 'fiber concentration 52š8 9224 2 the cleaning device in the form of, for example: a final stage screening or twig washing important. The last cleaning device can also consist of cyclones or vortex cleaners. This last purification step is run at a relatively low fiber concentration to obtain an efficient separation. Typically, such a final purification step is run at fiber concentrations of the order of 1.1%. The acceptance from such a purification device has a concentration of the order of 0.5% f The acceptance from the last purification step can be returned in the process to a mixing tank located before the silage. The mixing tank can be, for example, a storage tank or a blowing tank.

A "problem with returning fiber suspension" from a Vsista. purification step is that the recycling can cause the fiber concentration to be lowered throughout the silage. With too low a fiber concentration, problems arise with limitations in hydraulic capacity, especially in subsequent steps - such as, for example, washing equipment. ' Hydraulic capacity refers to the amount of liquid per unit time that can pass through an equipment. A low fiber concentration means relatively larger amounts of liquid and the capacity of the equipment is then limited by the possibility for the liquid to enter and through the equipment in question. In addition, with a larger amount of liquid, a forcing of the fiber suspension through the silage can be forced forward. Such an increased passing speed_ entails a poorer screening result. Even a subsequent washing equipment can have a worse washing result.

At present, it is difficult to cascade; i.e. return the acceptance from Å a last purification device which is "run on to the purification device because the liquid volumes obtained at the low fiber concentration the nearest lower fiber concentration become so large that a cascade connection is practically impossible. The only possible feedback position was previously 10 15 20 25 30 528 924 3 becomes a mixing tank with a large volume capacity.

The present invention has for its object to solve the problems of capacity limitation in a treatment line for a fiber suspension.

A further object is to solve problems with fluid balances in a system for treating a fiber suspension.

¶ Another purpose is to enable cascade coupling from a final purification device running at low fiber concentration.

According to the invention, the system consists of a silage which consists of, for example, being connected in parallel or cascaded. How number _, purification devices. The purification devices can the silage is built up depends i.a. on which production volumes - to be handled. According to the invention, a thickener is placed in the process line so that at least a partial stream of the acceptor from a last purification device run with low fiber concentration passes to a mixing tank located before the screen or alternatively to one of the thickener before the acceptance is returned to the screen stages. Preferably, the acceptor passes an intermediate storage tank located in a position before the thickener step »before at least a partial stream of the acceptor passes the thickener.

The method according to the invention consists in passing at least a partial stream of the acceptor from a purification device which has been run at a low fiber concentration before a thickener before the fraction is returned to a position located before or in the silage.

In an embodiment of the method according to the invention, the following reference is made to the figures which show different embodiments of the concentration of the fiber suspension in the thickener suitably to a concentration which minimizes a decrease of the concentration in the mixing tank. According to one embodiment of the method, the concentration of the partial current passing through the thickener is increased to »a 5%. any equipment concentrating a fiber suspension. interval of 1% -_ The thickener edge be which the invention will be described in more detail with the invention.

Figure 1 shows a process diagram of a system according to the invention. From a mixing tank (1) a fiber suspension with a concentration of 1-5% is passed through a sieve (2). The strainer possibly consists of a plurality ((21, '22, 23) embodiments are connected in parallel without feedback, the acceptance strainer steps which in this are carried from the strainer steps to a subsequent washing equipment (5).

The reject from the silage is taken to a purification device (3) which is run at a low fiber concentration (of the order of 1%). The acceptance that leaves the purification device has a concentration in the order of 0.5%. From the purification device (3) the acceptance ~ is transferred to an intermediate storage tank (6). After the intermediate storage tank, a partial current of the acceptance passes through a thickener (4). The thickener is arranged in a side line (9) to the main line (8). The filtrate from the thickener is taken to a filtrate tank (7) where the filtrate from the washing equipment (5) is also taken. The thickened fiber suspension is returned to the mixing tank (1).

Figure 2 shows a process diagram of a system according to another embodiment of the invention. In the embodiment shown, the screen consists of three cascade-connected screens (24, 25, 26).

The acceptance from the third screen (26) is returned to the second screen (25) and the acceptance from the second screen (251 is returned to the first screen (24). The acceptance from the purification device ~ (3) is passed via an intermediate storage tank (6) to a thickener (4). * 'The thickened fiber suspension is returned' to the Intermediate storage tank (6) From the intermediate storage tank the wine-thickened fiber suspension is returned to the third sieve ¶ (26) .

The filtrate from the thickener is fed to the ifiltrat tank of the washing equipment (7). to those embodiments but can be varied within the scope of the claims.

The invention is of course not limited, ViSadê

Claims (5)

Accepted claims in a fiber suspension where a Method for the fiber suspension is purified in silage (2) with a, final purification step (3) performed at low fiber concentration, characterized by the thickener (4) before it is returned to a position located before the purification step (3) passes at least one of the sieve steps included in the sieve (2), the concentration. on the acceptance is raised to a concentration that minimizes a dilution of the concentration in the fiber suspension to which the acceptance is returned. Method according to claim 1, characterized in that the acceptance is returned to a mixing tank (1) the silage (2). located before Method according to claim 1, characterized in that the acceptance is returned to a screening device included in the silage (21, 22, 23, 24, 25, 26). Method according to claim 1, characterized in that a partial current of the acceptance passes the thickener (4) which is placed to a main line (8) the acceptance to a position located before or in the silage (2). in a side line (9) that returns Method according to one of the preceding claims, characterized in that in the last cleaning step (3) one of the cleaning devices is used, final stage screen, twig washing, vortex cleaner and cyclone.i