The present invention relates to a method and an apparatus for separating more efficiently than before liquid from liquid/fiber suspension by dividing the suspension into two portions, of which one portion mainly comprises liquid and the other both liquid and fibers. The method and apparatus in accordance with the invention may well be applied to thickening of low consistency (0.5 to 5 %) fiber suspensions in pulp and paper industry.

The pulp and paper industry has processes which are carried out with a pulp of low consistency, even under 1 %. Such processes are, for example, normal and reverse vortex cleaning. Subsequent to the vortex cleaning the pulp suspension is transferred to the next processing stage, which may be either a thickener or a headbox of a paper or drying machine. In any case, the stage following the vortex cleaning is almost always thickening, whereby a thickener is required for the low consistency zone, which thickener must be able to treat large amounts of fiber suspension and also discharge large amounts of liquid from the suspension, even if the thickening of the consistency were apparently low (cf. 0.5 % -> 3 %).

Even with paper machines the present trend is to use higher and higher headbox consistencies (1 to 3 %) to improve the level of quality and to lower the costs, as a consequence of which the consistency of pulp coming from the vortex cleaners may be too low for the paper machine.

Thickening is carried out according to the modern techniques by means of different types of disc and drum thickeners and curved filters. In conventional disc or drum thickeners the discharge of liquid, in other words thickening, is based on gravity (so called gravity deckers),

on vacuum thickening (so called vacuum filters) or on pressure thickening (so called pressure filters).

In gravity deckers the thickening is carried out by means of a drum made of perforated plate covered by wire cloth. The pressure difference required for the thickening results from the level difference between the pulp in the inlet tower and the pulp in the filtrate chamber. Pulp may be filtered either from the inside of the drum to the outside or from the outside to the inside, which latter direction is the most usual. In practice, the diameter of the drum may be 4 m, of which, for example, 60 % is underwater. In practice also 10 to 15 % of the inner surface of the drum is operating as a filter surface. The maximum pressure difference is about 20 kPa. At the bottom dead center of the drum the pressure difference is zero and it will increase from thereon to its maximum value towards the surface of the inlet tower. This results in that there is hardly any thickening taking place on either side close to the bottom dead center. The situation is similar, of course, in the part of the drum which is not underwater. Thus, a considerable part of the drum surfaces in a gravity decker is inefficiently utilized. The capacity of the part of the drum in efficient use varies according to the pressure difference prevailing relative to the filtering surface. The specific thickening capacity of gravity deckers varies according to the pulp and the running conditions, but is typically 400 - 700 1/m ² /min. These kind of thickeners are used to pre-thiσken low consistency pulp, for example, from 0,5 % to 1,5 - 5 %.

The filtering surface of the drum is kept clean or open to the flow by moving the surface against the filtrate or by using air to clean it. For example, a mill producing 500 tons of 90 % consistency pulp requires a filter with a diameter of 4 m and a length of 7 m and having about 88 m ² of wire surface, to thicken the pulp from 0,5 % to 1,5 %.

The pressure difference required for filtering in vacuum and disc filters is carried out with a suction leg. These kind of thickeners differ from the gravity deckers in that a pulp layer is formed on them. This means that subsequent to thickening the consistency of the pulp is 8 to 14 %. The capacity of a vacuum or disc filter is approximately the same as that of a gravity decker. The difference is that the pulp web tends to be formed by suction on the filtering surface in the pulp suspension when the filtering surface is submerged. Filtrate is removed from the web generated on the part of the drum, risen above the surface of the suspension, so as to achieve the consistency of said 8 to 14 % in the discharge. It is clear that when forming a pulp layer on the filtering surface, liquid discharge through the layer considerably slows down.

It is not advantageous to use these types of thickeners when tending to pre-thicken, but they may be applied when the required discharge consistency is high. The specific thickening capacity varies according to the pulp quality and conditions from 50 to 3000 1/min/m ² . Compared to the previous example two vacuum filters of said size would be required when aiming for the consistency of 10 %. The advantage of a disc filter compared with a vacuum drum filter is that considerably more filter surface can be included in the same volume.

A pressure filter differs from the vacuum drum filter in that the filtering pressure difference is carried out by pressurized air. The filtering capacity does not substantially differ from that of a vacuum drum filter.

The thickening method using a curved filter is based on gravity decker filtering. The suspension being thickened is pumped onto an inclined filtering surface. The thickening capacity is in practice 3 to 5 % and the specific capacity of liquid discharge is approximately the same as that of

the drum filters. However, it has the advantage of not having any mechanically moving members, but it has the drawback of the apparatus being very easy to clog, because it is difficult to arrange for efficient cleaning. Curved filter types of thickeners are used in the pulp industry where relatively minor thickening and pulp capacities are concerne .

The above described conventional thickeners are characterized in that thickening is carried out using a very small pressure difference in more or less open equipment and only part of of the filtering surface is utilized. A result of the small pressure difference and the partial use of the filtering surface is low liquid discharge capability. Due to the open construction and operational principle the pulp and the filtrate may contain air. As is known air in the pulp decisively lowers the infiltration qualities of the pulp.

Recently apparatuses in- accordance with Finnish patent applications 873020 and 874854 of A. AHLSTROM CORPORATION have been marketed, whose operational principle greatly differs from those of the above-mentioned conventional apparatuses. The patent applications teach, for- example, about a thickening method, in which proessurized pulp is fed from the exterior into a closed apparatus which comprises a filter surface and a surface movable relative to it, whereby, due to the speed difference between the surfaces, the filter surface is subjected to pressure/suction pulses, by which the pulp layer thickened adjacent to the filter surface is broken and mixed so as to stabilize the consistency. Most usually the filter surface is a vertically arranged drum and a surface movable relative to it, which is arranged inside the filter surface, is a substantially cylindrical rotor, on the surface of which either spherical protrusions or foil-like members are arranged in order to create local pressure differences. In all cases, the filter

surface has, however, been of smooth surfaced, perforated plate material.

It has been possible to achieve a specific thickening capacity of about 4500 - 5500 l/m ² /min. with this kind of apparatus. However, it has been discovered in experiments that a thin, although harmful, fiber matting layer generated from the thickened pulp is formed adjacent to the filter surface and through which layer the liquid being discharged from the suspension has to be pressed to be able to flow out to the filtrate space. In order to efficiently disperse the fiber matting by the members of the rotors causing pressure differences, more turbulence energy should be directed to the suspension by said members, for example, by redesigning the members or by increasing their speed, whereby fiber matting would be dispersed, but at the same time energy consumption would increase considerably.

The present invention overcomes or minimizes the above mentioned problem in such a way that members creating micro turbulence of high intensity are arranged on the filter surface itself, and that the size of the openings of the filter surface is diminished in such a way that the diameter of the openings (perforations) is about 0.15- 0.2 mm. Thereby the movement of the suspension, which is due to the members of the rotor causing pressure differences and which is parallel to the filter surface, is disturbed by the members of the filter surface only to the extent that the fiber matting moving along the surface of the filter surface and disturbing the thickening is broken and mixed to the rest of the suspension stabilizing the consistency. Yet this is not a complete thickening of the suspension in the thickening space, because there is no need for such due to the fact that it is possible to remove a desired amount of liquid without breaking all rather small fiber floes. Moreover, the micro turbulence adjacent to the filter surface keeps the floe size of the circulating

pulp as small as possible, whereby the "free" liquid between the fibers and the small floes is able to flow more easily and be discharged to the filtrate chamber. It is especially important for the members generating turbulence on the filter surface that they do not substantially affect the main flow direction of the fiber suspension, which still remains parallel to the rim of the filter surface. Thus it is typical of our invention that turbulence of high intensity tends to be generated where it is required, in other words in close proximity to the filter surface. Thus since it has been possible to distribute the operations of the rotor and the filter appropriately, in other words so that the rotor practically speaking merely circulates the pulp along the filter surface, that is to say keeps the rotational speed of the pulp being thickened at a desired level and the filter surface again generates micro turbulence, the intensity of which is at its highest on the surface, it has been possible to diminish the specific energy consumption and raise the capacity.

Naturally there is an apparatus already known, in which members have been arranged on the filter surface to prevent the formation of a fiber matting on the filter surface. Said apparatus is described in detail in FI patent application 771541, which concentrates on treating pulps having the consistency between 5 and 25 %. For example, in Figs. 5 to 8 of said application the apparatus is applied to thickening pulp. The filter surface in accordance with said application is provided with rib-like protrusions arranged distant from each other, the protrusions being advantageously, as they say, approximately 3 to 18 mm high. As it may be noted from the height of the ribs, the purpose is not to generate a slight micro turbulence on the filter surface, but to fluidize the whole suspension in the thickening space between the rotor and the filter surface, as it can be noted from said application. The reason for this is that thickening pulp having the consistency of more than 5 % (so called medium consistency

zone) requires quite different conditions and operations from what is required when operating in the consistency zone of about 1 %, because, for example, the liquid in the suspension may be separated from the fibers in a quite different way at a low consistency compared with operating at a consistency of more than 5 %. A pulp having the consistency of more than 5 % forms especially when being thickened with the consistency increasing thereby, large pulp cakes having strong bonds between the fibers, which bonds must be broken, for one thing just to allow the flow of the pulp further in the thickening apparatus. On the other hand, the pulp tends to be fluidized completely in the whole thickening space, because otherwise it would not be possible to remove liquid from medium or high consistency pulp. Fluidization in the whole space requires, however, a great amount of energy, so it should be avoided, if possible. A filter surface and a rotor provided with above described rather high protrusions generate a strong turbulence extending throughout the whole treatment zone of pulp, the approximate intensity of which turbulence is at the whole treatment zone, however, low.

Also, it has to be noted that in the arrangement in accordance with said FI application the number of the ribs or protrusions on the filter surface is relatively low, in other words they are located far from each other on the filter surface. For example, in an arrangement in accordance with Fig. 6 there are only six ribs on the filter surface. This kind of arrangement generates a strong turbulence, but of low intensity, which dampens rapidly. The dampening of the turbulence is facilitated considerably by the filtration of liquid through the filter surface, because in practice most of the turbulence disappears with the filtrate.

The thickening method in accordance with the invention is characterized in that micro turbulence is generated on the filter surface by means of the filter surface to disperse

the fiber matting thickening tending to form on the surface or to prevent the formation of fiber matting on the filter surface.

The thickening apparatus in accordance with the present invention is characterized in that unevennesses are provided on the filter surface and are shaped and/or spaced and/or arranged and/or dimensioned such that, in operation and together with the kinetic speed of the fiber suspension, they generate micro turbulence of high intensity, so that any fiber matting otherwise tending to form at close proximity to the filter surface is dispersed or prevented from forming so that the thickening process is optimized.

The method and apparatus in accordance with the present invention are described in more detail, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic illustration of the basic construction of a thickening apparatus in accordance with the present invention;

Figs. 2 a...c are schematic illustrations of a preferred embodiment of an apparatus in accordance with the invention;

Fig. 3 is a schematic illustration of a variation of the embodiment in accordance with Fig. 2;

Fig. 4 is a schematic flow diagram of an apparatus in accordance with the invention when testing the used test arrangement; and

Figs. 5 - 11 are illustrations of a group of appropriate groove alternatives to be applied in a filter plate in accordance with the invention.

According to Fig. 1 a thickening apparatus including a device in accordance with our invention comprises a casing

1, a cover 2, a basis 3 and a drive means 4. The casing 1 has a feed conduit 5 for inlet of pulp, a discharge conduit 6 for discharge of filtrate, a discharge conduit 7 for thickened pulp and a discharge conduit 8 for the possible reject. The casing 1 has a stationary filter cylinder 9 in it with the cylinder 9 being mounted at 9' in the casing 1 to form a separated space 9'' for outflow of filtrate. The filter cylinder 9 has inside it a preferably cylindrical rotor 10, rotatably mounted on bearings, on the surface of which rotor there are members 11 (only two indicated) for removing pulp layer. It is also possible to provide discharge means 12 for light impurities, such as plastics or the like, if desired.

Figs. 2 a...c illustrate fragmentary details of the filter cylinder arrangement 9 in accordance with one embodiment, which arrangement is made of substantially planar smooth metal or other material plate 21 prior to forming, which are covered throughout with small apertures or perforations 22 having a diameter of advantageously 0.1 - 0.25 mm, or with perforations of a size having similar thickening qualities, but having a different form. The plate is, after being covered with apertures, rolled to form a cylindrical filter drum. In some cases, mostly when the starting consistency of the suspension being thickened is higher, it is possible to use a larger perforation/ opening size. Regardless of where said perforations/ openings are located the plate 21 has grooves or recesses 24 formed in the surface 23 on the side of the suspension on the plate 21, advantageously low or shallow grooves the shape of which may greatly deviate from what is shown in the drawing. However, it is most important to note that the machined grooves should only be about 0.2 - 1.0 mm. Fig. 2 c is an alternative arrangement with larger grooves 24 which in effect form what appear as projections 23 with sloping walls. These kinds of unevenness of the surface is used to generate sufficient micro turbulence, in particular to close proximity to the filter surface, to break the

relatively thin and, due to low consistency, weak fiber matting otherwise tending to form on the filter surface or on the whole preventing the formation of the fiber matting. It has been discovered in the experiments that by maintaining the thickness of the highly intensive turbulent layer over 0.5 mm, the formation of fiber matting on the filter surface may be prevented. If the thickness of said turbulent layer considerably exceeds 2 mm, the amount of turbulence also increases so that the possibility of the fibers flowing through with the filtrate increases.

Fig. 3 illustrates as a second alternative a part of a plate 26 for use in producing a cylindrical filter arrangement 9 in accordance with the present invention, which plate is provided with filter openings, perforations or passages 27 similar to Fig. 2, but on the surface 28 of which have then been attached members causing unevenness, such as bars 29 or the like. For example, steel rods or other material may be used, which are easily chosen in the way that their durability, is necessarily higher than that of the basic material, because they come into contact with the wearing flow. It is, of course, also possible that said bars 29 or rods are attached to the plate before machining the filter apertures or openings 27, whereby it is possible to have the filter surface also formed by the openings 27 also extending through the bars 29.

The apparatus described above in connection with different embodiments operates in such a way that the fiber suspension of low consistency is fed under pressure to the apparatus, whereby it flows from the direction of one end of the apparatus advantageously to the thickening space between the rotor 10 and the filter cylinder 9 and whereby due to the effect of the advantageously tangential feeding method and rapidly rotating rotor 10 the fiber suspension is caused to effect a circulating movement in the thickening space between the filter cylinder 9 and the rotor 10. The filter cylinder 9 with small perforations allows the

penetration of the filtrate but not the fibers. Consequently, a layer of thickened pulp would otherwise tend to be formed on the surface of the filter plate 9, and this layer is dispersed with the help of the micro turbulence generated in it by the unevennesses 24, 29 on the surface of the filter plate 9. The members operating as rotor 10 or arranged on the surface of rotor 10 create low frequence pressure/suction pulses in the suspension in the thickening space mixing the already thickened suspension, which is dispersed by the micro turbulence, with the untreated suspension thus maintaining the consistency of the suspension even in the whole thickening space. The movement of the rotor 10 also generates in the arrangement in accordance with our invention turbulence of low intensity prevailing almost throughout the whole thickening space, which also facilitates the mixing of suspension described above. Yet, it must be noted that the consistency of the suspension increases evenly the closer the suspension moves to the discharge end of the thickened pulp. Respectively, also the amount of liquid filtering through the filter surface decreases relative to the infeed end of the pulp to be treated.

The method and apparatus have been examined in the laboratory by tests in accordance with Fig. 4. The reference number 30 signifies a circulation tank or tower, from which pulp is pumped by a feed pump 31 to the test, i.e. filter, apparatus 32 though a feed valve 33. The pulp may be, if required, led back to the circulation tower 30 directly past the filter apparatus 32 by opening valve 34 and closing valve 33. A sample of unthickened pulp may be taken from the conduit at 35 and a sample of thickened pulp from the conduit at 36 downstream of the valve 37. A sample of the filtrate may be taken from the conduit 38 downstream of filter valve 39. It is possible to adjust the desired pressure condition in the filter apparatus 32 by valves 37 and 39.

The surface area of the filter cylinder in the testing apparatus was 0.4 m ² , of which 10 % open area when the perforation size was 0.2 mm. The filter surface in accordance with the invention was produced in such a way that substantially axial grooves were machined on the surface of a smooth perforated plate, the depth of the grooves being about 0.5 mm. The measure of the bottom plane of the groove parallel to the rim was about 1.5 mm, one of the side walls of the groove was perpendicular to the filter surface and the other formed a 30° angle with the filter surface. The measure of the ridge remaining between the grooves, parallel to the rim, was about 0.35 mm. Thus the interval between the ridges is slightly over 3 mm. Of importance to the filter plate in accordance with our invention and especially to its unevennesses is that the height of the unevennesses remains 0.2 - 2.0 mm, advantageously between 0.2 - 1.5 mm and that their distance from each other is 1.0 - 10 mm, advantageously 1.0 - 6.0 mm.

Test runs have shown that when using a filter cylinder provided with an uneven surface the power consumption per thickened pulp ton, i.e. specific power consumption, has been less than when using a smooth cylinder. An explanation for said unexpected phenomenon is that the smooth filter surface is continuously covered by a thin thickened fiber matting, which makes it difficult for the liquid to filter through the filter surface. The unevennesses of the surface again when generating micro trubulence of high intensity break the thickened fiber matting and mix also to some extent the pulp, whereby the liquid content of the surface layer is greater. Similar explanation may be given to the fact that in the tests the specific thickening capacity has proved to be more than 80 % higher when using an uneven plate. The "free liquid" between the fibers gets more easily into contact with the filter surface and thus is also more easily filtered through the surface.

When comparing the turbulence level in figures between both smooth and uneven filter surfaces, the following table may be made as a function of flow rate:

Flow rate Turbulence level m/s Smooth surface Uneven surface 5 1.0 2.5

10 1.7 3.5 20 2.7 7.6

It may be noted that a flow rate, which is about four times of that used with the uneven surface, is required with the smooth surface to achieve a certain turbulence level. On the other hand, it is seen that the turbulence level on the uneven surface is at equal speed 2 to 3 times of that on the smooth surface.

In the actual thickening tests following results were received:

Smooth plate Uneven plate

Inflow consistency to the apparatus 0.5 % 0.5 % Consistency of thickened pulp 1.5 % 1.5 % Consistency of filtrate ca. 0.03 % ca. 0.06 %

Liquid discharge capacity 1/m ² /min ca. 5500 ca. 10000 Pressure difference in the test run 20 - 40 kPa 20 - 40 kPa

The tests performed show that first of all it is possible to achieve multiple capacity increases by the pressurized thickening method and apparatus compared with the conventional thickeners. Additionally, it may be seen in the results that, as mentioned above, the specific capacity when using an uneven filter plate is more than 80 % higher

than when using an even plate. When comparing the power consumption it is seen that the power demand when using an uneven plate is about 80 % of the value when using a smooth plate. Thus the use of a thickening plate in accordance with our invention may be said to be almost 130 % more efficient, if the comparison is based on the power used for thickening a certain amount of pulp. Additionally, the arrangement in accordance with our invention has all the advantages of the pressurized treatment of pulp. Thanks to the pressurized operational principle both the filtrate and the thickened pulp are overpressurized, and this gives great freedom of choice in locating the equipment in the mill and an opportunity to save space. Also the entrance of air to the pulp in the thickening is virtually impossible.

The test runs have shown that the consistency of the fiber suspension rises rapidly from the feed end towards the discharge end, whereby it is possible to intensify the thickening process by changing the size of the unevennesses. The reason for this is that the floes formed by the fibers are stronger when the consistency is higher and the single fibers do not get loose from them so easily, whereby, if the unevennesses are of about the same size throughout the whole axial length of the filter surface, they do not disperse so efficiently the thickened fiber matting. Thus a filter plate may be used in an apparatus, which plate has unevennesses of one or more different sizes. The most practical arrangement has been carried out, for example so that the smallest unevennesses are at the feed end, the slightly larger in the middle and the largest at the discharge end for the thickened pulp.

Figs. 5 - 11 illustrate examples of different groove designs that can be applied to to the filter plate in accordance with the present invention. However, it must be noted that the groove designs used in said drawings do not prevent one from using some other kind of groove, the most

important thing is to provide means which generate micro turbulence on the filter surface, whereby the quality of the pulp, the consistency, the purity, and the like properties may require also other kind of surfaces. It is, of course, also possible to machine or otherwise arrange cavities on the surface, for example, by drilling or milling conic or cylindrical recesses or combinations thereof, as is shown in Fig. 12. Respectively also other kind of arrangements affecting the form of the surface may come into question.

As it can be noted from the description above, a new type of thickening method and an apparatus therefor has been developed, with which it is possible to eliminate or minimize the drawbacks of the apparatuses of the prior art technique without creating new problems. It is clear that only a few advantageous apparatus alternatives and points of application have been introduced above, which by no means are intended to restrict the invention from what is defined in the enclosed claims. Thus it is also clear that both the filter surface and the surface movable relative to the filter surface may be of a form different from cylindrical, said members are only characterized in that they are substantially rotationally symmetrical, cylindrical, conical or spherical, or their form is a combination of those, just to mention a few examples. It is also possible to use the grooved filter surface, mentioned as an example, to either direction, in other words the perpendicular side surface of the groove may be either as the front surface or as the back surface of the ridge between the grooves.