The present invention relates to a method and a device for the treatment of suspensions of cellulose fibers, which are to be cleaned and concentrated, wherein the cleaning also encompasses the elimination of light impurities.

The cleaning of cellulose suspensions from heavy im¬ purities, such as sand, bark and short shiveβ, is now normally done in hydrocycloneβ, in which the impurities are taken out in the apex fraction and the purified suspension in the base fraction, such cleaning presupposes a low fiber content, 0.1-3*. preferably 0.2-2*.

Cleaning out light impurities by means of hydrocyclone ^β is also previously known, particularly in the treatment of recycled paper, which often contains significant amounts of waste plastic and ink etc. The hydrocyclones normally used in this case are dimensioned and disposed in such a way that the base fraction contains the light impurities and the apex fraction the major portion of the input cellolose fibers. Hydrocyclones for this purpose are also known in which both fractions are taken out at the same end through separate outlets.

The first mentioned type of ydrocyclone is referred to below as an A-cyclone, while the second is referred to as a B-cyclone.

Since hydrocyclones are not perfect separators, recycling units are always required for economic reasons, which recover cellulose fibers from the reject material. Such recovering is previously known. As regards recycling with B-cycloneβ, reference is made to Swedish patent applica-

tion 8802580-4 from 8 July 1988, which describes hydro¬ cyclones particularly suited for such recycling.

in plants presently used for manufacturing paper and market pulp, it is usual that the suspension produced by whatever method (cooking, defibration, bleaching, beating of recycled paper) is first diluted to be able to be screened and/or cyclontreated, whereafter the cleaned suspension is thickened to be able to be intermediately stored. This thickening is normally done in open thick¬ eners. In order to thereafter be able to conduct the pulp to a wet machine it must be diluted once again, as a rule to a concentration which is higher than that used for the cleaning step. This thickening, intermediate storage and dilution involves extra costs, which can be eliminated by concentrating the pulp coming from the cycloning step and directly leading it to the wet machine, particularly by using the same pump for the primary cycloning step and a closed thickener working under pressure, as disclosed in Swedish Lay-Open Print 8305036-9.

As mentioned aboved, however, there is an increasing need for removal of light impurities, especially plastic, and this applies not only to recycled paper but also to wood, particularly from populated areas, which contains in¬ creasing amounts of plastic impurities. Plastic particles can also come in during production in the plant. Even small amounts of these impurities can present difficulties in paper production, stripes in the paper web can appear for example when coating.

one purpose of the present invention is to provide an improved system for manufacturing paper pulp of good quality, especially as regards elimination of impurities, both in the form of heavy and light particles. (Light particles are meant to include not only particles of low

density, but also particles which due to their shape perform as lighter than cellulose fibers in hydrocycloneβ).

Another purpose is to provide a system which eliminates the need for thickening for intermediate storage of al¬ ready cleaned pulp βuβpenβion. instead, the invention provides an essentially continous process which performs, in one sequence, cleaning, concentration and final treatment in a wet machine or paper machine.

These and other purposes and advantageβ are achieved according to the invention by the proceββ according to claim 1 and by a device according to claim 8.

One particular advantage of the invention iβ the βyner- getic effect achieved by the combination of the functions of thickening and cleaning from light impurities, prefer¬ ably whilst simultaneously eliminating an intermediate storage βtep and a diluting step.

What makes the invention possible iβ the fact that the suspension fraction coming from the apex in a hydrocyclone designed for eliminating lighter impurities has a conβiβt- ency which iβ increased in relation to that of the βus- pension supplied, since much water accompanies the light impurities in the base fraction.

A particularly high consistency can be achieved, with very good separation of light impurities, if one can accept that some cellulose fibres will accompany the base fraction, whereby the recovery is simplified by reducing the number of recycling stepβ by uβing hydrocycloneβ of the type described in Swedish patent application 8802580-4 (not yet publicly available).

According to the invention, it is poββible to achieve

a consistency of between 1 and 3* after combined cleaning of the light impurities and thickening, which is a suit¬ able starting consistency for many types of wet machines and paper machines. Hydrocycloneβ must of courβe be ⁵ operated with an eβsentially constant flow of liquid, and the output consistency is determined by the consistency of the pulp flow supply. The flow to the wet machine or paper machine can then be additionally varied by a feedback which feeds a controllable portion of the output flow to -JO the input side of the cleaning system.

It is suitable to provide the white water tank of the wet or paper machine with a spillway to keep the water level constant, and to take the necessary diluting water from 5 there for the process.

As a rule, hydrocyclones function best if the liquid flows and pressure ratios are kept rather constant at the rated values. However, in different operating situations it can o be necessary to vary the flow to a wet machine or paper machine. It is therefore suitable to arrange a diversion of the treated suβpenβion, which returns the unused but cleaned and concentrated βuspension. This recycling can be done to a point immediately in front of the main pump, 5 without changing the fiber flow to the wet machine or paper machine, but with the result that the consistency of the βuspenβion to the headbox will be increased. Alterna¬ tively, the feedback can be to a point prior to the con¬ sistency control in connection with a first dilution, thus 0 keeping the consistency of the βuβpenβion to the headbox constant and reducing the flow to the headbox.

The invention will now be described with reference to a non-limiting example, shown schematically in diagram form 5 in the drawing.

The schematic figure shows an example of a plant function¬ ing in accordance with the invention. The major portion of the fiber material passing through it flows from a conven¬ tional silo l to a wet machine or paper machine 9 along a main path drawn aβ a thick line. The pulp with a conβiβt- ency of 10-12* iβ diluted in the lower portion of the βilo

1 to about 3.5*, and is diluted at the input side of a pump 103 to a controlled consiβtency of 3.0* and iβ then led to a silo 2, where the level iβ kept constant. The diluting water is taken via a pump 12 from the white water of the wet or paper machine. The person skilled in the art will understand how the level and consiβtency in the βilo

2 will be controlled and therefore it iβ βufficient to state that the dilution is controlled by actuating the valves 100 and 101 by the sensor 102 and the level by the level βenβor 200, which controls the speed regulator 104 to the pump 103. 105 iβ a control unit which openβ the valves 100 and 101 in parallel and controls the flow therethrough in a predetermined ratio. In the silo 2, there iβ thereby achieved a constant level and a conβtant conβiβtency for the suspension.

The suction side of the pump 3 iβ connected to the βilo 2. and it receiveβ diluting water from the pump 12 via the valve 300, which iβ controlled by the βenβor 301, which βenβeβ the conβiβtency of the suspension exiting from the pressure side of the pump 3. The conβiβtency iβ controlled thereby to 2.5*. The flow of the suspension supplied iβ led through a valve 400 under the control of a flow βenβor 401 at a deβired level. The flow of fiberβ through the valve 40~ iβ thuβ kept constant.

A controlled flow of suspension with a controlled con¬ βiβtency (fiber concentration) iβ thuβ led to the βuction βide of the pump 4.

For the present, one can disregard the line 13, which refers to a particular aspect of the invention which will be described in more detail below. The pump 4, a centri¬ fugal pump, provideβ an eββentially constant flow, and the 5 presβure on the βuction βide iβ determined by the level in the white water tank 10. the level of which iβ kept con- βtant in a known manner. The pump 4, aβ long as its coun- terpreββure iβ βtable, giveβ a predetermined flow with a conβiβtency determined by the amount of fiberβ per unit of 0 time which passes through the valve 400. With the valueβ according to the example, a conβiβtency of 0.5* iβ attain¬ ed on the preββure βide of the pump 4, which iβ a suitable value for cycloning, even though thinner and thicker con¬ sistencies are possible to work with. 5

The suspension thus diluted is led through a βcreen 5, which iβ primarily intended to eliminate βcrap particleβ and other larger particleβ, which could damage the cycloneβ, to a multi-hydrocyclone unit 6 with A-cycloneβ, o the base fraction of which is conducted further. The conβiβtency of the baβe fraction will be 0.45*, i.e. somewhat thinner than the suspension supplied. This baβe fraction iβ moved on without intermediate pumping to a multi-hydrocyclone unit 7 with B-cycloneβ, where the fiber 5 iβ removed aβ an apex fraction, while the lighter par¬ ticles go into the baβe fraction, in thiβ apex fraction, the cellulose fiberβ are enriched to a higher conβiβtency, in the example to 1.5*, thuβ thickened by a factor of 3.33. This consiβtency is suitable for the following wet 0 or paper machine 9, to the headbox 8 of which the βuβ- penβion iβ conducted via a valve 800, controlled by a βenβor 801, in thiβ caβe a pressure βenβor. The headbox thus receives the suβpenβion at a controlled preββure. If βo deβired, the βenβor 801 can be a level βenβor or a flow βenβor.

Recovery unite are connected to the two multi-hydro-

cyclone unite 6 and 7. The recovery units are shown here a ^β cascade coupled units with individual pumps at the inlets and with the outlets connected to a common level in the tank 10. Although only two stepβ in each unit have been βhown, the number of βteps iβ determined by the deβired degree of recovery and purification. The recovery units to the unit 6 consist of A-cyclones, while the recycling units to unit 7 are B-cyclones. The number of ^β tepβ in the recycling from unit 7 iβ poβitively affected. as iβ the number of individual cyclones in the units, if the latter are constructed in accordance with Swedish patent application 8802580-4.

in the example, the unit 7 provides both a cleaning from light impurities and a thickening of the suspension thus cleaned, which combination iβ the great advantage of the invention. Thiβ thickening can be done in different degreeβ depending on the cyclone dimensions, the preββure drop and the diβtribution of apex and baβe flowβ of the incoming suspension. The thickening factor, which always exceeds 1, and in thiβ example iβ 3.33, can be increaβed to 5 for example, if βo required, and thiβ can be done without negatively effecting the βeparation of light impurities; on the contrary, the separation effect will tend to increaβe. The fiber content in the baβe fraction may thereby increaβe, βo that the proceββing work in the recovery unite increaβeβ, but thiβ can be dealt with by using hydrocyclones according to the Swedish patent application 8802580-4.

The above-mentioned thickening is a result of βeparation of water and fiberβ. The degree of thickening in a hydro- cyclone iβ determined by the effectiveneββ of thiβ βepara¬ tion. in order to achieve a high degree of thickening, the hydrocyclone iβ made with a smaller baβe opening than the apex opening. For the βeparation work, the conical βepara-

tion chamber βhould have a largeβt diameter of leββ than 125 mm and preferably leββ than 100 mm. Furthermore, the diameter of the baβe opening βhould be leββ than a fifth of the largeβt diameter of the separation chamber.

5

The main flow of fiberβ has now been explained. The return flowβ from the recycling βtepβ go through the line 11' back to the suction side of the pump 4. Thiβ eanβ, that if the line 13 were not there, the entire flow of fiberβ, 0 under βtability conditionβ, coming in via the valve 400, would be fed out to the headbox 8. The flow iβ then determined by the capacity of the pump 4 and by the flow-offβ, determined by the normally conβtant preββure ratios, in the apex fraction of unit 6 and in the baβe 5 fraction of unit 7.

For optimal operation, hydrocycloneβ βhould be operated with predetermined liquid flowβ. However, it iβ not certain that thiβ iβ the case with the headbox of a wet o machine or paper machine, where the flow, preββure and consistency may need to be varied. The conβiβtency, at conβtant flow, can be controlled by controlling the flow and consiβtency of the βuβpenβion βupplied via the valve 400. The flow to the headbox 8 can be controlled by a return line 13 with the valve 130 controlled by a preββure βenβor 131. Thiβ return line 13 can, in accordance with the first embodiment, open into the βuction βide of the pump 4, which will cause the fiber flow to the headbox 8 to be unchanged while the conβiβtency iβ changed.

In accordance with a second embodiment, drawn with daβhed lineβ in the Figure, the return flow iβ effected via a line 13' back to a point prior to the pump 3, in thiβ ca ^β e upβtrea of the dilution control valve 300, but in any caβe to a point upstream of the consiβtency and flow control which determineβ the fiber flow βupplied to the βuction βide of the pump 4. controlling the liquid flow to

the headbox 8 by means of the valve 130 does not effect the conβiβtency of the suspension βupply.

The invention haβ now been deβcribed on the basis of a non-limiting example, from which it is clear that βuch a βyβtem can be controlled within wide limite. Even if the flow capacity for the cyclone equipment iβ given by the deβign, it iβ possible to vary within wide limits the capacity of the pulp flow and the conβiβtency of the βus- penβion outflow.

As mentioned, it iβ alβo possible by designing control measures within the scope of the invention to vary for example the degree of thickening in the βyβtem.

One βhould note in particular that a βingle main pump 4 pumpβ the diluted suspension to the units 6 and 7 coupled in series and then to the headbox 8. In a representative example, the presβure drop between the inlet and the baβe in a A-cyclone iβ 1 bar and the preββure drop in a

B-cyclone between the inlet and the apex iβ 1.5 bar. Theβe preββure dropβ are essentially a function of the flow. The pump 4 iβ dimenβioned βo that it can both compenβate theβe preββure dropβ and produce βufficient preββure to the headbox 8.

control of the valves 800 and 130 must be done in βuch a manner that the operating conditionβ for the multi-hydro¬ cyclone unite are not affected negatively. The main pump 4 -β a centrifugal pump, driven at essentially conβtant rotational βpeed, with falling preββure head as flow increases. The βyβtem iβ deβigned βo that at the rated flow the preββure head iβ βufficient for the preββure dropβ in the multi-hydrocyclone units 6 and 7 and the valve 800 at a normal work point, and controlling within reasonable limits of the flow through the valve 800 to

keep the pressure or the level in the headbox 8 does not involve any appreciable disruption. Major changes in the preββure drop over the valve 800 cannot be tolerated, however, βince the working conditions for the unite 6 and 7 would be adversely affected. In order to make signi¬ ficant changes in the feed flow to the box 8, there must therefore be a diverβion via the shunt valve 130 so that the preββure on the βuβpenβion can be kept reasonably constant. The shunt valve 130 can then either be control- led manually with reference to a manometer or can be preββure controlled in the manner βhown in the drawing.

The variouβ controls in the drawing have been βhown aβ mutually independent local controlβ. It will be under- βtood, however, that βuch controlβ can be effected by a common computer.