WO2011048272A1

WO2011048272A1 - A disc filter, as well as a method and a system for adjusting a disc filter

Info

Publication number: WO2011048272A1
Application number: PCT/FI2010/050826
Authority: WO
Inventors: Tuomo Aho; Harri Kesseli
Original assignee: Metso Paper, Inc.
Priority date: 2009-10-21
Filing date: 2010-10-20
Publication date: 2011-04-28
Also published as: FI20096086A; FI20096086A0

Abstract

The invention relates to a disc filter (1), which disc filter comprises a disc (3) provided with a collection trough (8), a vat (7) of the disc filter, and means for feeding a flow to said disc filter (1). Furthermore, the disc filter comprises means for providing a by-pass flow, which means are placed at least partly in connection with the vat (7) of said disc filter (1), and which said means for providing the bypass flow comprise at least either means for implementing an overflow from the vat (7) of the disc filter, or at least one discharge channel (10), such as a pipe, a tubular structure, and open channel, or a chute. Furthermore, the invention relates to a system and a method for adjusting a disc filter (1).

Description

A DISC FILTER, AS WELL AS A METHOD AND A SYSTEM ADJUSTING A DISC FILTER

Field of the invention

The invention relates to a disc filter. The invention also relates to a method and a system for adjusting a disc filter.

Background of the invention

Fibrous pulp suspension is normally led to a disc filter in such a way that the pulp suspension is fed through a feed vessel to the vat of the disc filter. In practice, this can be done, for example, either across or through the wall between the feed vessel and the vat of the disc filter. The fibres contained in the pulp suspension introduced in the vat form a fibre layer on the perforated surface of the disc, inside the sectors of the disc, that is, inside the areas delimited by the sector edges.

The fibre layer formed on the surface of the disc is normally formed by the effect of suction pressure generated by means of a suction leg. Furthermore, water contained in the fibrous material is filtered through the fibre layer formed by the suction effect on the surface of the disc, to form a filtrate inside the sectors of the disc. Thus, said fibre layer acts as a filter to remove particles from the water flowing through said fibre layer.

The fibre layer formed on the surface of the disc is removed after the disc has revolved above the fluid level in the vat of the disc filter. This is normally done so that the suction pressure is dropped before the removal of the fibre layer, that is, the precipitated mass, formed on the surface of the disc, and furthermore, the removal of the mass is intensified by means of jets. The removed fibre layer drops into collection troughs between the discs, on the bottom of which there is normally a screw conveyor for removing this precipitated mass from the disc filter. The aim is to keep the level of the pulp suspension in the vat of the disc filter normally constant so that said level would remain constantly at about 95% of the maximum. The maximum of the level in the vat of the disc filter refers to the level of the upper edge of the collection trough of the disc filter, in other words, the lowest height of the upper edge in the vat of the disc filter.

The level adjustment is normally performed by adjusting the rotating speed of the discs of the disc filter. If the level in the vat of the disc filter rises, the rotating speed is increased. This change in the rotating speed may be, for example, in the order of 0.2 to 0.4 rpm, that is, 0.2 to 0.4 rotations per minute. Said increase in the rotating speed of the discs will reduce the thickness of the fibre layer forming on the surface of the discs and being used as a filter, wherein the flow of the filtrate through the disc will increase and the level in the vat will become lower. In a corresponding manner, if the level in the vat becomes lower than desired, the rotating speed of the discs is reduced, wherein the flow rate of the filtrate through the discs is reduced and the level in the vat rises.

The level in the vat of the disc filter is normally measured by a pressure sensor which can be calibrated by means of the pressure value caused by the maximum level in the vat of the disc filter. The level can also be determined by means of ultrasonic or microwave radar, or by visual methods, such as with a glass tube connected to a tub, inside which tube the variation in the level of the filtrate follows the variation in the level of the filtrate in the tub. The determination of the variation in the level of the disc filter in a tub or in said glass tube can be automated, for example, by utilizing a camera technique or inductive measurement.

If the adjustment of the rotating speed does not satisfy to keep the level in the vat of the disc filter sufficiently high, it is possible to increase the inlet flow and/or to reduce the suction pressure. In a corresponding manner, if the level in the vat of the disc filter cannot be kept sufficiently low, the situation can be corrected by reducing the inlet flow and/or by increasing the suction pressure. The level of the disc filter should not rise too high, because if the level of the disc filter rises too high, the vat of the disc filter will start to overflow into the pulp vat. The above-mentioned operation of the disc filter of prior art involves several drawbacks. The control of the disc filter does not influence the quality of the pulp to be filtered or of the filtrates, wherein the quality of both the pulp and the filtrate may vary to a great extent. These variations in the quality of filtrates may affect substantially the quality and efficiency of the product and may cause various disturbances, because for example clear filtrate is used elsewhere in the process after the purification by the disc filter. Possible variations in the level of the disc filter are also harmful because differences in the level of the feed vessel and the vat have a substantial significance on the injection of air in the pulp drifting onto the disc surface. When the inlet flow into the vat takes place with a great height difference, it causes strong swirling in the pulp flowing in the vat and, on the other hand, sinking of the pulp supplied into the vat and the air carried by it, below the level of the vat. As a result, so much air can be injected in the pulp to be filtered that it will disturb the operation of the disc filter.

In disc filters of prior art, problems are also caused because, for example, the pulp in disc filters of prior art often tends to precipitate in those sections of the disc filter in which the flow in the vat is decelerated. Such undesired precipitation takes place, for example, close to the end margins of the vat of the disc filter. Precipitation may also take place in the upper part of the feed vessel of the disc filter when running at a low flow rate, because in this case the inlet flow does not flow in the normal way primarily as an overflow across the wall between the feed vessel and the vat of the disc filter.

The precipitation of pulp in the vat of the disc filter may cause various problems in the process. For example, the behaviour of the mass may change in the disc filter so that the precipitated mass no longer drops normally into the collection trough of the disc filter but remains as a mass at the edge of the collection trough. This, in turn, may prevent the pulp, dropping further from the disc, from entering the collection trough, wherein structural damage may result. Such possible structural damage includes, for example, the bending of the edges of the collection trough, collapsing of the filter surfaces inwards, and even disc damage.

Document EP 0 686 419 mentions a disc filter according to prior art, in which the aim is to direct the inlet flow of the disc filter towards the most precipitated mass in the vat of the disc filter, between the discs. Even this document does not, however, disclose a satisfactory solution for optimizing the operation of the disc filter. In industry, there would thus be a need for a construction, by which the disc filter could be used in a more efficient way than in prior art, and process a larger quantity of pulp suspension, simultaneously producing homogeneous, sufficiently pure clear and superclear filtrates for the further use of filtrates. Furthermore, in industry, there would be a need for a construction, by which the operation of a disc filter could be secured in such a way that the precipitation of pulp in the vat of the disc filter could be reduced or prevented so that, for example, the probability of structural damage of the disc filter would be reduced.

Brief summary of the invention

It is an aim of the present invention to disclose a solution to the above- mentioned problems, resulting in a disc filter having a good capacity and producing substantially homogeneous filtrates. The disc filter according to the solution operates advantageously to prevent the precipitation of mass in the vat of the disc filter and to reduce the probability of structural damage of the disc filter. For this purpose, a novel disc filter is disclosed, which contains means for providing a by-pass flow. Furthermore, a method and a system are disclosed for adjusting the operation of the disc filter.

According to an advantageous example, part of the material fed into the vat part of the disc filter is guided as a by-pass flow away from the vat of the disc filter. According to an example, this by-pass flow is implemented as an overflow. According to another example, the by-pass flow is implemented by using discharge channels or corresponding arrangements suitable for the purpose. According to an advantageous example, the pulp suspension collected as a by-pass flow from the disc filter can be at least partly recirculated to the inlet flow in the vat part of the disc filter. The purpose of this is to control the level in the vat of the disc filter and to minimize quality variations. According to another example, the pulp suspension collected as a by-pass flow from the disc filter can be guided, for example, to a suitable pulp container. According to an advantageous example, the by-pass flow of the disc filter is taken from the area of one end or both ends of the vat of the disc filter. Thus, precipitation possibly occurring in the areas of said ends and disturbing the process will be reduced in the vat of the disc filter.

According to an advantageous example, at least one essential property representing the operation of the disc filter is measured from the material supplied to the disc filter and/or from the by-pass flow of the disc filter. Such properties may include, for example, the quantity of the inlet flow or the by- pass flow, as well as the consistency/consistencies of said flows. According to an advantageous example, the measurement values are used to control, for example, the quantity and/or quality of the material supplied to the disc filter, and/or the rotating speed of the disc of the disc filter. A disc filter according to an advantageous embodiment of the invention is presented in the appended independent claim 1. A system according to an advantageous embodiment of the invention for stabilizing the operation of the disc filter is presented in the appended independent claim 12. A method according to an advantageous embodiment of the invention for controlling the operation of the disc filter is presented in the appended independent claim 17.

Description of the drawings In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a disc filter according to an example, seen in a slanted view from above,

Fig. 2 shows an example of a by-pass flow vat in a disc filter according to Fig. 1

Fig. 3 shows an example of a disc filter according to Fig. 1 in a cross- sectional view seen from the end of the disc filter, Fig. 4 shows an example of a disc filter according to Fig. 1 seen from above, and

Figs. 5 to 6 show some examples of control diagrams for a by-pass flow in a disc filter according to Fig. 1.

Detailed description of the invention Figure 1 shows an example of a general view of a disc filter 1 , seen in a slanted view from above. The figure shows, among other things, the disc filter 1 , a by-pass flow vat 2, a disc 3, disc sectors 3a, the central axis 4 of the disc filter, the feed vessel 5 for the disc filter, and the vat 7 of the disc filter. The disc filter 1 normally comprises a central axis 4 arranged to be rotatable, to which so-called sectors 3a are fixed radially so that each single rotating disc 3 comprises several sectors 3a. The disc filter 1 comprises discs 3, whose number is typically from a few to several tens. The number of discs 3 in the disc filter normally depends on the capacity desired for the disc filter 1.

The feeding means typically supply the disc filter 1 with a fibre-containing pulp suspension that typically comprises at least so-called circulating water coming to the disc filter for filtering, as well as so-called auxiliary pulp that forms a pulp mat on the disc. Said pulp suspension is fed to the vat 7 of the disc filter normally through the feed vessel 5 of the disc filter.

The disc filter 1 according to the invention further comprises means for providing a by-pass flow. These means may comprise the by-pass flow vat 2 shown in Fig. 1 , but the by-pass flow can also be provided in another suitable manner. The concept of the by-pass flow vat 2 refers, in the present application, to a construction in which at least part of the pulp suspension is collected to be removed as a by-pass flow from the vat 7 of the disc filter. The by-pass flow vat may thus comprise, for example, a vat-like construction and/or, for example, a tubular construction.

Figure 2 shows an example of a by-pass flow vat of a disc filter shown in Fig. 1 , in a cross-sectional view in the longitudinal direction. The figure shows, among other things, the by-pass flow vat 2 of the disc filter, the bottom 2a of the by-pass flow vat, the ends 2b of the by-pass flow vat, as well as an overflow threshold 6. In Fig. 2, the bottom 2a of the by-pass flow vat is shown to be inclined downwards. Thus, the pulp guided into the by-pass flow vat tends, already by the effect of gravity, to flow in the direction of the outlet 11 from the by-pass flow vat. However, the bottom 2a does not need to be inclined in any direction, as shown in the figure, but instead the bottom 2a of the by-pass vat may also be essentially horizontal or, for example, irregularly inclined. For removing the pulp suspension from the by-pass vat 2, it is also possible to use some means suitable for the purpose, instead of or in addition to the downwards inclined bottom. Advantageously, the by-pass flow can also be implemented, for example, as an overflow across an overflow threshold 6. In the present application, overflow refers to the removal of pulp suspension in a controlled manner from the vat 7 of the disc filter, at the level of the liquid volume therein. The overflow must take place at a predetermined location of overflow in such a manner that the flow is then guided in an expedient way forward, for example to a container, a discharge channel, or a corresponding means for collecting the mass being discharged.

The overflow threshold 6 possibly placed between the by-pass vat 2 and the vat 7 of the disc filter (shown, for example, in Fig. 3) may consist of several parts, as shown in Fig. 2, such as a first overflow threshold 6a, a second overflow threshold 6b and a third overflow threshold 6c. The different parts 6a, 6b, 6c of the overflow threshold 6 are preferably separately adjustable. Thus, for example, in the area of the end 7b of at least one disc filter vat (corresponding to the ends 2b in the situation of Fig. 2), it is possible to lower the overflow threshold 6a, 6c lower, if necessary, wherein the flow can be intensified in the area of said at least one end 7b of the vat 7 of the disc filter 1. In a corresponding manner, if the flow situation tends to slow down in another section along the length of the vat 7 of the disc filter 1 , the desired overflow threshold 6 can be lowered in such a section along the length of the vat 7, to increase the flow. By operating in this way, it is possible to prevent the formation of precipitations in the vat 7 of the disc filter. If the overflow is arranged at least partly by using overflow thresholds 6, the number of overflow thresholds is advantageously at least one, or at least two, more preferably at least three or at least four. It is also possible that only some of the overflow thresholds 6 are adjustable. In this case, there is advantageously at least one adjustable overflow threshold 6, more advantageously at least two, often more preferably at least three or at least four adjustable overflow thresholds. The adjustment of the overflow threshold can be implemented, for example, so that the overflow threshold is formed by a baffle plate fastened by a screw joint or another suitable joint to the wall between the vat of the disc filter and the by-pass vat, the height of the baffle plate being adjusted by opening the joint keeping the baffle plate in place, by altering the height of the baffle plate and by fastening the joint of the baffle plate back to the closed position. Furthermore, it is possible that the overflow threshold/s is/are arranged to be stationary so that their height is not intended to be adjustable. There may also be more than four adjustable or non-adjustable overflow thresholds along the length of the vat of the disc filter.

The overflow threshold or thresholds may also be such that their height changes in the longitudinal direction of the overflow threshold, for example, in such a way that one edge is lower than the other edge, wherein a gradually increasing volume flow is formed over the overflow threshold, seen in the longitudinal direction of the overflow threshold, and sharp angles of the overflow threshold are avoided, in which the mass could stick and gradually accumulate. A gradual change in the height position of the overflow threshold can be provided as a linear change or to follow a convex or concave change in the height position, or as a combination of said different changes in height.

As mentioned earlier, the by-pass flow is preferably taken from that part or those parts of the vat 7 of the disc filter, in which the flow most typically slows down. In this way, it is possible to prevent the precipitation of the pulp in the vat 7 of the disc filter. In Fig. 2, there are three overflow thresholds in such a way that in said example, the overflow threshold is lower at the ends 2 of the by-pass vat (that is, close to the area of the ends 7b of the disc filter) than in the centre. Thus, the flow in the vat 7 of the disc filter increases particularly in the area of the ends 7b of the vat 7. Figure 3 shows an example of a disc filter shown in Fig. 1 , in a cross- sectional view seen from the end of the disc filter. Figure 3 shows, among other things, the disc filter 1 , the by-pass vat 2 of the disc filter, the overflow threshold 6 of the disc filter, the disc 3 of the disc filter, the disc sectors 3a, the central axis 4 of the disc filter, the vat 7 of the disc filter, the pulp collection trough 8, and the feed vessel 5 of the disc filter.

The by-pass flow of the disc filter 1 is arranged advantageously at such a great distance from the flow coming from the feed vessel 5, to avoid such a direct flow of the inlet flow to the by-pass flow, by which no remedy is achieved to the tendency of precipitation in the vat 7 or in the feed vessel 5 of the disc filter. Normally, this is realized when the by-pass flow is arranged so that it is taken, at the earliest, from half-way of the vat 7 of the disc filter in relation to the feed vessel 5. In other words, the location of taking the by- pass flow is suitably closer to the opposite edge of the vat 7 of the disc filter with respect to the feed vessel 5, that is, in the half 7d of the vat of the disc filter, than in the half 7c of the vat of the disc filter on the side of the feed vessel 5. The by-pass flow can be led, for example, seen from the feed vessel 5 of the vat 7 of the disc filter, to a by-pass vat 2 formed on the opposite edge, where the by-pass flow is led advantageously as an overflow. Alternatively, or in addition, a by-pass vat or vats 2 may also be provided at the ends 7b of the vat 7 of the disc filter, to receive the overflow particularly through the ends 7b of the vat 7 of the disc filter.

Further, the by-pass flow of the disc filter 1 may be implemented advantageously not only as an overflow across the overflow threshold 6 but also, for example, by removing pulp suspension via discharge channels 10 arranged in suitable locations in the vat of the disc filter 1. The discharge channels 10 may be, for example, pipes or tubular structures under the fluid level, or pipes or tubular structures or open channels or chutes extending from below the fluid level to the fluid level or over it. As mentioned above, the by-pass flow may, in some cases, be advantageously taken from below the level of the vat of the disc filter. A suitable depth location for taking the by-pass flow may vary even in such a way that the by-pass flow can also be taken through the bottom of the vat 7 of the disc filter, for example, through an opening or a channel provided in the wall of the vat 7 of the disc filter. It is thus possible to intensify the flow in the vat in the depth direction and to avoid precipitation in such areas of the vat 7 of the disc filter.

The by-pass flow can be taken by using the overflow threshold 6, the discharge channels 10 and any other similar arrangements, for example close to the ends 7b of the vat 7 of the disc filter (shown in Fig. 1 ), wherein the by-pass flow can be advantageously taken from at least one end 7b of the vat of the disc filter, through openings or channels 10 provided in the end surface 7b of the vat 7 of the disc filter, and/or an overflow threshold 6 formed in the end surface. The overflow can also be taken not only at the ends 7b of the vat 7 of the disc filter but also, for example, from the side opposite to the feed vessel 5 of the vat 7 of the disc filter. This can be implemented in a corresponding manner, for example, as a flow through said discharge channels 10, or as an overflow across the overflow threshold 6.

Figure 4 shows an example of a disc filter shown in Fig. 1 , seen from above. The figure shows the feed vessel 5, the vat 7 of the disc filter, the by-pass flow vat 2, the outlet pipe 11 of the by-pass flow vat, and the discharge channels 10 for the by-pass flow.

As shown in Fig. 4, the disc filter 1 may comprise both at least one by-pass flow implemented advantageously as an overflow to the by-pass flow vat 2, and at least one by-pass flow discharge channel 10. It is also possible that the disc filter 1 comprises only either at least one by-pass flow implemented ^preferably as an overflow to the by-pass flow vat 2, or at least one by-pass flow discharge channel 10. A suitable number of discharge channels 10 for the by-pass flow as well as overflow arrangements can be added in suitable locations in the vat 7 of the disc filter to achieve a sufficient flow throughout the vat 7 of the disc filter.

Figures 5 and 6 show some advantageous examples for controlling the disc filter 1 comprising by-pass flow means, advantageously on the basis of measurements. In the example of Fig. 5, the whole by-pass flow is returned to the pulp suspension entering the disc filter 1. In the example of Fig. 6, the whole by-pass flow is led to the pulp precipitated in the disc filter.

Figures 5 and 6 show, among other things: the disc filter 1 , the by-pass flow vat 2 of the disc filter, the feed vessel 5 of the disc filter, the pulp collection trough 8, control valves (HC-4) 22a, (HC-5) 22b for the by-pass flows of the disc valve, and a circuit for the combined by-pass flow (FC-3) 22c, a circuit (SC-7) 23 for controlling the rotating speed of the disc of the disc filter, a circuit (QI-1 ) 24 for measuring the material to be fed into the disc filter, a circuit (QI-10) 25 for measuring the cloudy filtrate leaving the disc filter, a circuit (QI-11 ) 26 for measuring the clear filtrate leaving the disc filter, a circuit (QC-6) 27 for the moment control of the disc filter, a circuit (QC-9) 28 for measuring the pulp leaving the disc filter; and, among other things, the following containers: cloudy filtrate container 30, clear filtrate container 31 , superclear filtrate container 32, and circulating water container 33; and, among other things, the following systems of pipelines: pulp suspension to the disc filter 40, pulp from the collection trough 41 , pulp suspension from the by-pass flow 44, filtrates from the disc filter 43a cloudy, 43b clear, 43c superclear, and clear filtrate to the disc filter 44.

The pulp suspension is led to the disc filter 1 advantageously along the systems of inlet lines 40 shown in Figs. 5 and 6. The pulp suspension in the system of inlet lines 40 may comprise, for example, circulating water to be purified, cloudy filtrate isolated from the disc filter, and suitable pulp mixture. Part of the pulp suspension entering the disc filter 1 is removed as an overflow through the vat of the disc filter, suitably along the system of pipes 42. Most of the fibre mixture entering the disc filter is removed via the collection trough 8 along the system of pulp lines 41 to the subsequent process steps. The filtrates filtered by the disc filter 1 are guided via systems of filtrate lines 43a, b,c to filtrate containers, such as a cloudy filtrate container 30, a clear filtrate container 31 and possibly also a superclear filtrate container 32. The clear filtrates are normally led to further use in process steps which require relatively pure water, such as spray water. The cloudy filtrate is typically used, for example, as dilution water.

In order that the function of the disc filter 1 could be adjusted efficiently by means of the overflow, it is advantageous to take some suitable measurements, on which the controls of the function of the disc 1 are preferably based. Advantageously, such measurements include, for example, the measurement of at least one property of any of the following lines: the bypass flow 42 of the disc filter, the inlet flow 40 of the disc filter, the precipitated pulp 41 , and the filtrate/filtrates 43.

The efficient function of the disc filter 1 normally refers to the fact that the disc filter 1 produces, in operation, substantially constantly a high quantity of filtrates in relation to its filtering surface area. At the same time, it is typically required that the quality of the filtrates is acceptable. The quality of the filtrates can be evaluated by measuring, for example, the solids content, the relative particle size, i.e. turbidity, the conductivity, or the cloudiness of the filtrate. Some advantageous points for measuring the filtrates are indicated in the drawings with the reference numerals 25 (QI-10), 26 (QI-11 ) and 29 (Ql- 12).

By the control parameters of the disc filter 1 , it is possible to control the quantity and quality of the filtrate 43 and/or the quantity and quality of the precipitated pulp 41 , which can be done, for example, by multivariable control. The control parameters may include, for example, the quantity of the by-pass flow 42, the rotational speed of the discs 3, the torque or power required for rotating the discs 3, the quantity of the inlet flow 40, the quality of the material to be fed, the fluid level in the vat 7 of the disc filter 1 , or the suction pressure of the discs 3.

For example, on the basis of measuring the quantity of the by-pass flow 42 of the disc filter 1 , it is possible, among other things, to adjust the quantity of the inlet flow 40 of the disc filter 1 and/or the quality of the pulp to be fed. Measurements on the solids content and/or the freeness of the inlet flow, in turn, are suitable for adjusting, for example, the quantity of said inlet flow 40 and the quality of material, for adjusting the quantity of filtrates 43 and for adjusting the rotational speed and/or moment of the disc 3 of the disc filter 1.

The by-pass flow 42 can be conveyed not only to the inlet flow 40 but also, either in part or in whole, to other process applications, such as the dilution of pulp in pulp flotation, screening or centrifugal cleaning. Furthermore, it has been found the by-pass flow 40, particularly as a flow discharged from the surface of the vat 7 of the disc filter, may contain surfaced light reject, for example a concentrated level of expanded polystyrene, wherein in one advantageous embodiment of the invention, the by-pass flow 42 can be guided to the removal of light reject. The light reject can be removed from the by-pass flow 42, for example, in a screen, by means of a reject drum, or by separating devices based on centrifugation or the centrifugal force, or by utilizing settling by gravity. The by-pass flow 42, from which the light reject has been removed, can be advantageously returned to the inlet 40 of the disc filter, or it can be led to another process device, of which some examples have been listed above.

Thanks to the by-pass flow 42, the above-described quantity and quality of the filtrates 43 of the disc filter 1 , and/or the quantity and quality of the precipitated pulp 41 can be adjusted in a controlled manner by various combinations of control alternatives, because the precipitation in the vat 7 of the disc filter or in the area of feeding the pulp can be avoided.

From at least one flow of the disc filter, such as the inlet flow 40 of the disc filter, it is possible to measure, among other things, the pH, the temperature, the ash and/or solids content, and the flow rate. One aim of the disc filter 1 is to separate or recover the solids in the form of precipitated pulp from the aqueous fibre suspension processed in the disc filter 1. The quality of the precipitated pulp 41 can be defined by measuring, for example, the solids content, the brightness and/or the ash content of the pulp 41.

The properties of the pulp to be precipitated and/or the filtrate can be controlled, among other things, on the basis of the above-mentioned measurements, for example in the following ways: 1 ) control of the quantity of the by-pass flow

The by-pass flow 42 can be changed, for example, by adjusting (either manually or automatically) the height of the overflow threshold 6 of the disc filter 1. This can be done either on the whole length of the overflow threshold 6 or in only a part of the length of the overflow threshold 6. If the overflow threshold is divided, for example, in at least three pieces 6a-c (shown in Fig. 2), it is possible, for example, to lower the outermost overflow thresholds lower than the other thresholds, wherein the flow in the edge area of the precipitator increases, keeping the fluid in the vat of the precipitator substantially totally in motion. Instead of or in addition to adjusting the overflow threshold 6, the by-pass flow can be controlled by changing the flow of the discharge channels 10 possibly extending from opposite sides of the disc filter 1 , either manually or automatically. This can be done, for example, by means of control valves 22a (HC-4), 22b (HC-5) and 22c FC-3) for the bypass flow, shown in Figs. 5 and 6. There may be several discharge channels 10 per side, and these may be placed in different points.

2) Controlling the rotational speed of the disc filter

The control of the rotational speed of the disc filter 1 with the circuit 23 (SC-7) can be performed, for example, according to an instruction by the operator. Thus, the operator enters the setting value for the rotational speed to the circuit 23 (SC-7). The operator may monitor the other measured values and change the rotational speed with the circuit 23 (SC-7) until the desired measurement value is achieved. The control circuit may also be controlled automatically or connected to a cascade. When the control circuit is controlled automatically, the control circuit controls, for example, a valve position or the rotational speed independently, trying to maintain, for example, a flow rate or a rotational speed according to a set value. A control circuit connected to a cascade, in turn, receives a new setting value or new setting values, for example, from another control circuit or the operator, for adjusting the position of a valve or a rotational speed or another adjustable control variable of an actuator to a new setting value. In cascade control, that is control outside the control circuit, the control of the control circuit is thus performed on the basis of a change or a cause measured or detected outside the control circuit. For example, the following circuits shown in Figs. 5 and 6 may give the setting value for the rotational speed of the disc 3 when the control circuit is automatically controlled or connected to a cascade in the above-described manner, either

1. according to the load or moment of a motor,

o The load and/or moment can be determined, for example, by means of a frequency converter. The operator gives the setting value for the load or the desired moment to the circuit of the motor/frequency converter of the disc filter, according to which the circuit 23 (SC-7) of the rotational speed receives the setting value.

2. under control on the basis of measured values, such as ash content, turbidity, or solids content;

o The operator thus gives the desired setting value to at least one of the following circuits:

^■ the circuit 24 of the inlet of the disc filter (QI-1 ) ■ the circuit 25 of the cloudy filtrate from the disc filter

(QI-1 )

^■ the circuit 26 of the clear filtrate from the disc filter (QI-11 )

o Thus, the circuit 23 (SC-7) controlling the rotational speed of the disc filter receives its setting value according to this data.

3. under control by a separate moment measurement of the disc filter, that is, under control by the circuit 27 (QC-6);

o The operator gives the desired setting value to the circuit 27 (QC-6) for measuring the moment of the disc filter, according to which the circuit 23 (SC-7) for the rotational speed of the disc of the disc filter receives its setting value.

4. under control by the incoming and/or outgoing CSF and/or SR value.

o Thus, the operator gives the desired setting value to the circuit 28 (QC-9) which measures the pulp discharged from the disc filter. The circuit 23 (SC-7) controlling the rotational speed of the disc 3 of the disc filter receives the setting value on the basis of these measurements.

According to an advantageous example, part of the incoming inlet flow is led via the by-pass flow back to the suction of the pump feeding the disc filter 1 and/or to the pumping container of said pump. Normally, it is advantageous to utilize the closest preceding or following pump and/or containing in the process order. The preceding container may be, for example, the container for fine screening or centrifugal cleaning, or a suction pipe of a pump relating to these devices. Consequently, the by-pass flow can also be led to a process after the disc filter, wherein the by-pass flow can be led, for example in the process of recovering fibre, to the container of pulp precipitated with the disc filter, to dilute the precipitated pulp.

The pulp suspension removed as a by-pass flow 42 from the disc filter 1 is advantageously conveyed at least partly back to the disc filter 1 , as the inlet flow 40. The pulp suspension removed as a by-pass flow 42 from the disc filter 1 can also be led at least partly forward in the process, for example to the pulp suspension container following the disc filter 1. More advantageously, the pulp suspension discharged as a by-pass flow is returned substantially in whole as an inlet flow 40 to the disc filter 1. When the by-pass flow 42 is returned substantially in whole in the inlet flow 40, or when the by-pass flow 42 is led in whole into a pulp suspension container or the like after the disc filter 1 , the installation constructions required by the bypass flow 42 become simpler when process arrangements required by partitions do not need to be implemented in the by-pass flow 42.

Taking the by-pass flow 42 from the disc filter 1 , for example via the discharge channels 10 or as an overflow over the overflow threshold 6, is preferably implemented as a free flow by the effect of hydrostatic pressure. If necessary, said by-pass flow 42 can also be taken from the disc filter 1 by intensifying the flow with a suitable pump of prior art. In a corresponding manner, possible returning of the by-pass flow 42 taken from the disc filter 1 to the inlet flow 40 of the disc filter, preferably to the feed pump of the inlet flow, is also provided advantageously either as a free flow or intensified with a pump.

The quantity of the pulp suspension to be discharged by the by-pass flow 42 is suitably selected so that by means of the by-pass flow, the pulp in the vat 7 of the disc filter 1 can be made both homogeneous and moving, wherein the precipitation of the pulp in the vat 7 of the disc filter 1 is avoided. The quantity of the overflow 42 must be selected so great that a situation is achieved, in which the disc filter 1 functions with an optimal efficiency.

The by-pass flow 42 is advantageously arranged to function as a continuous flow so that at least part of the by-pass flow 42 is fed back to the inlet flow 40 of the disc filter. Thus, the operation of the disc filter 1 is assisted, among other things, in such a way that variations in the consistency in the vat 7 are reduced, and simultaneously disturbance situations caused by said variations in the consistency in the vat 7 are reduced or eliminated. The by-pass flow 42 can be provided not only as a continuous flow but also as an intermittent flow.

Preferably, the disc filter 1 is controlled by means of measurements taken from the inlet flow 40 or the disc filter and/or the by-pass flow 42 of the disc. Properties to be adjusted by the measurements may include, for example, the rotational speed or moment of the disc 3, the quantity of the inlet flow 40 of the disc filter 1 , and the quality of the pulp fed. By means of these measurements and controls, it is possible to control the quantity of the material throughput in the disc filter 1 , the thickness of the fibre layer formed on the disc 3, the level in the vat 7 of the disc filter 1 , as well as the quality and quantity of the precipitated pulp and the filtrates produced.

In the following examples 1 and 2, some advantageous examples will be presented, relating particularly to the advantages in the adjustment of the disc filter 1 obtained from measurements taken in the by-pass flow 42 and the inlet flow 40 of the disc filter.

According to an advantageous example, the disc filter 1 is controlled by utilizing the moment control, for example, in the control of the filtrate 43 of at least one disc filter 1 and/or the quality of the pulp to be precipitated. One example of this is presented in the example 3 below.

Example 1. Advantages of measurements of the quantity of the bv-pass flow in the vat of the disc filter

When the quantity of the by-pass flow 42 of the vat 7 of the disc filter 1 is measured by a measuring means of prior art, it is possible to adjust, for example, the rotational speed or the moment of the disc filter 1 , on the basis of the measurement results. Thus, it is possible to maximize the production capacity of the disc filter 1 or to otherwise optimize the operation of the disc filter 1 , for example to achieve homogeneous filtrates 43a-c. The controls to be made on the basis of the by-pass flow 42 of the disc filter 1 are based on the fact that when the measurement results show a small quantity of the bypass flow 42, it can be concluded that the fluid level has sunk in the vat 7 of the disc filter 1. Thus, correcting measures can be taken, such as, for example, lowering the rotational speed of the disc 3 of the disc filter 1. In a corresponding manner, when the measurement results show that the by-pass flow 42 is greater than usual, the fluid level in the vat 7 of the disc filter 1 has presumably risen, wherein this can be corrected, for example, by increasing the rotational speed of the disc 3 of the disc filter 1.

On the basis of measuring the quantity of the by-pass flow 42, it is also possible to adjust not only the rotational speed or the moment of the disc 3 of the disc filter 1 but also the quantity of the inlet flow 40, for example. By adjusting the quantity of the inlet flow 40, it is possible to maximize the production capacity of the disc filter 1 , because by adjustments made on the basis of the measurements, it is possible to stabilize the quality of the pulp to be fed into the disc filter 1. Furthermore, the efficiency of the disc filter 1 may increase, because thanks to the invention, it is possible to keep the fluid level in the vat 7 of the disc filter 1 substantially constant, wherein the filtering area used by the disc filter 1 remains large.

On the basis of measuring the quantity of the by-pass flow 42, it is also possible to adjust the quality of the material of the inlet flow, wherein it is further possible to optimize the production or production capacity of the disc filter 1. This is based on the fact that when the quantity of the by-pass flow 42 changes, the change in the quality of the material of the inlet flow 40 can be used to increase or decrease the capacity of the disc filter 1. It is thus possible to secure that the requirements for the filtrates 43 of the disc filter 1 , particularly the clear filtrate 43b and the superclear filtrate 43c, are met simultaneously when the quantity of the precipitated pulp is maximized.

Example 2. Advantages of measurements of the solids content and the freeness of the inlet flow of the disc filter The solids content of the inlet flow 40 of the disc filter 1 can be measured by a measuring means of prior art. Thus, on the basis of said solids content measurements, it is possible to adjust the quantity of the inlet flow 40, thanks to which the production capacity of the disc filter 1 can be maximized so that the quality of the filtrates 43 is simultaneously kept sufficiently good and homogeneous. This is based on the fact that when the solids content of the inlet flow 40 is low, the quantity of the inlet flow 40 of the disc filter 1 can be increased, because the disc filter 1 can thus, in most cases, process a greater material flow. When the solids content is high, it is, in turn, usually advantageous to reduce the quantity of the inlet flow 40 correspondingly, because the capacity of the disc filter 1 is thus sufficient to process a smaller material flow than before. In addition to or instead of the solids content of the inlet flow 40, the freeness (CSF) of the inlet flow can be measured for this adjustment.

On the basis of measuring the solids content and/or the freeness of the inlet flow 40 of the disc filter , it is possible to adjust, in addition to or instead of the inlet flow 40, the rotational speed and/or moment of the disc 3 of the disc filter. Thus, it is possible to further maximize the production capacity of the disc filter 1 , because when the solids content is low, the rotational speed or moment of the disc 3 can be usually increased. This is based on the fact that when the solids content decreases, the disc filter 1 can normally process a greater material flow, and an increase in the rotational speed and/or moment of the disc 3 will increase the quantity of the material flow filtered by said disc 3. In a corresponding manner, when the solids content of the inlet flow 40 of the disc filter 1 is particularly high, the disc filter 1 is normally capable of processing a smaller material flow than before, wherein it is advantageous to reduce the rotational speed or moment of the disc 3. Thanks to these adjustments, the operation of the disc filter 1 can be intensified to be better than before in such a way that the quality of the filtrates 43 obtained is simultaneously kept substantially uniform. On the basis of measuring the solids content of the inlet flow 40, it is also possible to adjust the quality of the material of the inlet flow 40, wherein it is possible to maximize and/or optimize the production or production capacity of the disc filter 1 further. This is done so that when the solids content of the inlet flow 40 is too low or too high, a change in the quality of the material of the inlet flow 40 can be used to increase or decrease the solids content of said inlet flow 40. By this, it is possible to influence the production capacity of the disc filter 1 and/or to adjust the meeting of the requirements of quality (clarity) of the filtrates 43 of the disc filter 1.

The quality of the material to be filtered by the disc filter 1 can be changed, in the simplest way, by adjusting the content of dilution water in the inlet flow 40. This influences directly the solids content in the inlet flow 40 of the disc filter 1 so that the flowing ability of the material to be processed by the disc filter 1 can be increased or reduced linearly. The quality of the material to be processed by the disc filter 1 can also be adjusted by changing the raw material for the material to be processed to have a different composition and/or different properties.

Example 3. Controlling the quality of at least one filtrate of the disc filter and/or the pulp by measuring the moment

The quality of the filtrate of the disc filter 1 and/or the quality of the pulp to be precipitated can be adjusted by means of measuring the moment of the disc filter 1. By adjusting the thickness of the layer of the fibre mass forming on the disc 3 of the disc filter by measuring the moment, it is possible to adjust the properties affecting the quality of the fractions discharged from the disc filter 1 in a desired direction. Such properties may include, for example, the solids content of one or more filtrates 43, the solids content, ash content and/or the drainage resistance or so-called freeness (CSF) value or Schopper-Riegler (SR) value of the pulp to be precipitated. A thicker mass layer on the surface of the disc 3 typically indicates a greater moment caused by the mass, which can be both measured and adjusted. The adjusting of the moment can be taken care of, for example, by changing the rotational speed of the disc 3 of the disc filter 1. When the rotational speed of the disc 3 of the disc filter increases, a thinner mass layer is precipitated on said disc than before. In a corresponding manner, when the rotational speed of the disc 3 decreases, the mass layer precipitating of the disc 3 becomes thicker. A change in the solids content of the filtrate 43 of one or disc filter 1 can therefore be implemented, for example, in the following way:

- When it is detected that the solids content of the filtrate 43 to be controlled exceeds a setting value, a thicker mass layer on the disc 3 of the disc filter 1 is needed for reducing the solids content of said filtrate 43. As a solution, it is possible to increase the moment by reducing the rotational speed of the disc 3 of the disc filter 1 , until a predetermined moment and/or a solids content of the filtrate 43 is achieved.

A change in the moment caused by the change in the rotational speed of the disc 3 of the disc filter 1 is advantageously taken into account by defining (for example, by calculating) a moment curve for said disc 3.

Stabilizing and adjusting the disc filter, implemented by means of the by-pass flow, may have several advantages, because thanks to the by-pass flow and the measurements advantageously relating to it, it is possible to avoid disturbance situations caused by variations in the consistency in the disc filter and/or to avoid the precipitation of pulp in the vat. Furthermore, by the arrangement according to the invention, it is possible to achieve a substantially more uniformly operating disc filter, wherein both the consistency in the vat part of the disc filter and the level of the vat can be kept substantially more stable than before. When the level of the vat of the disc filter remains constant, the filtering surface of the filter used in the disc filter can be kept constantly as large as possible. Furthermore, the injection of air caused by height differences of the level in the vat of the disc filter in the pulp conveyed to the disc surface is reduced or eliminated. An additional advantage in the stabilizing of the disc filter is normally also the fact that the filtrates remain constantly sufficiently clean and homogeneous in view of further use. Thanks to these features, the production capacity of the disc filter increases. The invention is suitable both for maximizing the capacity of the disc filter and for optimizing the operation of the disc filter. The presented construction is low-cost, and further, it is simple to test and optimize in practice. The invention is not limited solely to the examples presented in Figs. 1 to 6 and in the above description, but the invention is characterized in what will be presented in the following claims.

Claims

Claims:

1. A disc filter (1 ) comprising

a disc (3) provided with a collection trough (8),

- a vat (7) of the disc filter, and

means for feeding a flow to said disc filter (1 ),

characterized in that the disc filter further comprises means for providing a by-pass flow, which means are placed at least partly in connection with the vat (7) of said disc filter (1 ), and which means for providing the by- pass flow comprise at least either

means for implementing an overflow from the vat (7) of the disc filter, or

at least one discharge channel (10), such as a pipe, a tubular structure, an open channel, or a chute.

2. The disc filter according to claim 1 , characterized in that the means for implementing the overflow comprise at least one adjustable overflow threshold (6).

3. The disc filter according to claim 1 or 2, characterized in that the means for implementing the overflow comprise at least one stationary overflow threshold (6).

4. The disc filter according to any of the claims 1 to 3, characterized in that at least one set of means for providing an overflow are placed at least partly in the area of one end (7b) or both ends (7b) of the vat (7) of the disc filter (1 ).

5. The disc filter according to any of the claims 1 to 4, characterized in that the height of the overflow threshold (6) is adjustable in the longitudinal direction of the overflow threshold so that the overflow threshold may be substantially at a different height at a first point of the overflow threshold than at a second point of the overflow threshold.

6. The disc filter according to any of the claims 1 to 5, characterized in that the overflow threshold is formed of at least two parts (6a, 6b, 6c) which can be adjusted at different height levels with respect to each other.

7. The disc filter according to claim 6, characterized in that the overflow threshold (6, 6a, 6b, 6c) is lower on the side of at least one end (7b) of the disc filter than the overflow threshold (6b) in the central part of the disc filter.

8. The disc filter according to claim 6 or 7, characterized in that the overflow threshold (6a, 6b, 6c) is stepped in its structure.

9. The disc filter according to any of the claims 4 to 8, characterized in that at least one discharge channel (10) of the disc filter is provided in the area of at least one end (7b) of the vat (7) of the disc filter (1 ).

10. The disc filter according to any of the claims 1 to 9, characterized in that the system comprises at least one discharge channel (10) below the level of the vat of the disc filter.

11. The disc filter according to any of the claims 1 to 10, characterized in that the system comprises both at least one discharge channel and at least one overflow threshold.

12. A system for adjusting a disc filter (1 ), which system comprises a disc filter (1 ), the disc filter comprising

a disc (3) provided with a collection trough (8),

- a vat (7) of the disc filter, and

means for feeding an inlet flow to said disc filter (1 ),

characterized in that the disc filter (1 ) further comprises means for providing a by-pass flow from said vat (7) of the disc filter, wherein at least one set of means for providing a by-pass flow is placed at least partly in at least one such point in the vat (7) of the disc filter (1 ), where the flow of the pulp suspension in said vat (7) is substantially slower than the average flow of pulp suspension in the vat (7), which means for providing the by-pass flow comprise at least either

means for implementing an overflow from the vat (7) of the disc filter (7), or

one or more discharge channels (10), such as a pipe, a tubular structure, an open channel, or a chute.

13. The system according to claim 12, characterized in that the means for implementing the overflow comprise at least one adjustable overflow threshold (6).

14. The system according to claim 12 or 13, characterized in that the means for implementing the overflow comprise at least one stationary overflow threshold (6).

15. The system according to any of the claims 12 to 14, characterized in that at least one set of means for providing an overflow are placed at least partly in the area of one end (7b) or both ends (7b) of the vat (7) of the disc filter (1 ).

16. The system according to any of the claims 12 to 15, characterized in that the system comprises measuring means arranged to measure at least one property of at least one of the following lines:

- the pulp fed to the disc filter (1 ),

- any filtrate of the disc filter (1 ),

- pulp precipitated by the disc filter (1 ),

- by-pass flow of the disc filter (1 ),

and that the system further comprises control means arranged to adjust, on the basis of the measurement results, at least one of the following:

- the rotating speed of the disc (3) of the disc filter (1 ),

- the moment of the disc (3) of the disc filter (1 ),

- the inlet flow of the disc filter (1 ), or

- the quality of pulp fed to the disc filter (1 ).

17. A method for adjusting a disc filter, which method comprises a disc filter (1 ) comprising

a disc (3) provided with a collection trough (8), and

a vat (7),

and in which method fibre-containing material is fed into the vat (7) of the disc filter (1 ), characterized in that the vat (7) is provided with a by-pass flow, by means of which by-pass flow part of the pulp suspension in the vat (7) is led out of the disc filter (1 ), the by-pass flow being arranged in at least one such location in the vat (7) of the disc filter (1 ), where the flow of the pulp suspension in said vat (7) is substantially slower than the average flow of pulp suspension in the vat (7), and that the by-pass flow is arranged at least either

- as an overflow from the vat (7) of the disc filter (7), or

- through one or more discharge channels (10), such as a pipe, a tubular structure, an open channel, or a chute.

18. The method according to claim 17, characterized in that at least part of the pulp fed into the vat (7) of the disc filter (1 ) is recirculated to the inlet flow of the disc filter by means of the by-pass flow.

19. The method according to claim 18, characterized in that the by-pass flow of the vat (7) of the disc filter (1 ) is recirculated substantially in whole to the inlet flow of the disc filter (1 ).

20. The method according to any of the claims 17 to 19, characterized in that the by-pass flow of the disc filter (1 ) is kept substantially continuous when the disc filter (1 ) is in operation.

21. The method according to any of the claims 17 to 19, characterized in that the by-pass flow is taken from the vat (7) intermittently when the disc filter (1 ) is in operation.

22. The method according to any of the claims 17 to 21 , characterized in that in said method, at least one property is measured from at least one of the following lines: