Background of the Invention

The invention relates to disc filters using doctor blades for scraping off the filter cake accumulated on the disc filter during filtering. Especially when the disc filters are used for filtering out lime mud from process liquors, for example white liquor in the caustizising department of a pulp mill, the doctor blades are subjected to extensive wear and need to be made in specially hardened steel or even ceramics, and preferably tungsten carbide is used.

Prior Art

Doctor blade arrangements are used in a number of different machines. Vaimet AB deliver several different doctor blade holders such as LiteCompact ^" ™; LiteFit Pius™; LiteFit™, LiteAdapt™ and FiberDoc™ for paper and tissue machines and similar applications for scraping off fiber webs and/or fiber residues. In those holders the entire holder profile may be removed from the machine, enabling maintenance work such as knife replacement on the machine floor. Typically these knives are made by wear resistant carbon steel or hard alloys.

In some applications the wear on the knife is extraordinary high and this applies for example in filtering equipment in the caustization area, where dry lime mud is scraped off by the doctor blade. Lime mud is extremely abrasive and behaves almost like grinding paste such as silicon carbide (SiC), also known as carborundum. In order to obtain a reasonable operational lifetime for the doctor arrangement is thus knives with superior hardness needed, and typically are knives made in material with high Tungsten carbide content or better material used. Tungsten carbide knives for lime mud doctoring are presently standard Vaimet selection.

Tungsten carbide is about 2 times stiffer than conventional steel for knives, and has a Young ^' s modulus of over 500 GPa, often between 530-700 GPa, which even is considerably stiffer than commercial steliite alloys, i.e. cobalt-chromium alloys designed for wear resistance, and used by several doctor blade suppliers.

Commercial steliite alloys may have a stiffness about 1 /1000 of that of tungsten carbide, i.e. in the order of 500 MPa (=0.5 GPa). The problem with manufacturing of these tungsten carbide knives is that subsequent machining is almost impossible, and most often the knives are cast to size and possibly only sharpened slightly by grinding or laser machining. Hence, the costs for tungsten carbide doctor blades becomes very high, so during installation and replacement must the doctor blade arrangement be handled with care and with high personal safety. In order to obtain a cost effective solution using these expensive knives is the doctor arrangement designed with a number of knife segments stretching over the effective filtering area of the disc, from the center and out to the outer periphery of the disc. A knife made in one single piece and corresponding to the length of the doctor arrangement would become too expensive, and difficult to produce,

Therefore, in state of the art doctor arrangements for caustization disc filters is the doctor including a number of shorter knife segments, each mounted on the doctor arrangement.

Now this solution makes replacement of entire knife edge, or part of it, time consuming and also difficult due to restricted access in-between the discs,

Summary of the Invention.

The general objective of the invention is to enable a safe and fast installation or removal of the doctor blade arrangement in disc filters by one single service personnel, despite the circumstance that the doctor blade arrangement is long and heavy and extends in between two narrowly mounted filter discs that restrict accessibility. The knives in doctor blades for lime mud removal are typically razor sharp and the handling of the doctor blade arrangement must be handled by the service personnel such that full support is obtained during the entire mounting or removal process, and avoids rupture of the filter cloth on the discs.

Hence, the doctor blade arrangement according the invention is used in a disc filter to filter out fine and highly abrasive particle material in a raw suspension comprising a rotating shaft with a multitude of filter discs rotating in a vat. The doctor blade arrangement including a doctor blade holder parallel with the filter surface of the filter disc and with a length of the doctor blade holder corresponding to the active filtering distance of the disc filter as measured from the center of the filter disc and to the outer radial end of the filter disc. Several knife elements made from hard metal with a Young ^' s modulus higher than 200 GPa, and preferably above 500 GPa, are attached to the blade holder with removable first set of fastening members. Further a collection chute located under the doctor blade holder forms a substantially vertical box like collection chute with an upper opening under the doctor blade holder and a lower outlet in the lower part of the vat. The doctor blade arrangement according the invention comprises a doctor blade holder detachabiy mounted in a stationary cradle arranged in the disc filter with a lower V-shaped guide surface on the doctor blade holder located at the opposite side to the scraping edges of the knife elements mounted on the doctor blade holder. Further the cradle includes a corresponding complementary V-shaped guide surface supporting the lower V-shaped guide surface of the doctor blade holder when mounted in the cradle and located above the chute and in a fixed position relative the chute with a second set of fastening members different from the first set of fastening members. By this design could the doctor blade holder as one unitary member be removed by loosening of the second set of fastening members with ail knife elements firmly attached to the doctor blade holder by the action of the first set of fastening members and extracted from the cradle while sliding the doctor blade holder over the V-shaped surfaces.

A single service personnel could then safely and with load support to the inner end of the doctor blade holder slide the whole doctor blade arrangement in between the narrow lying filter discs, reducing risks of dropping the doctor blade holder and causing dents or burrs to the sharp edges of the knife. The entire procedure of removal and installation of a new sharp doctor blade arrangement will be done in fractions of the time necessary to change doctor knife blades of the prior art design, where each knife blade is secured to a stationary cradle with 2 fastening screws, and where the inner knives are difficult to access as they are at the center of the disc filter and in between 2 filter discs.

According to a preferred embodiment of the doctor blade arrangement is the second set of fastening members for the doctor blade holder including an inner form locking member at the inner end of the doctor blade holder at the center of the filter disc when mounted in the cradle, with a complementary form locking member in the cradle, and at least one removable fastening member closer to the other outer end of the doctor blade holder when mounted in the cradle. With these form locking members at the inner end of the doctor blade holder and cradle, is also this inner end securely fastened without requiring access to screws or similar fastening member at the very center of the disc filter. Preferably is the inner form locking member of the doctor blade holder and the complementary form locking member of the cradle comprising a first finger like member extending in the direction of the doctor blade holder and an aperture for the finger like member in each of the cradle and the doctor blade holder respectively. The locking interaction between the finger like member and the aperture may be obtained in the final sliding phase when pushing the doctor blade holder over the V~ shaped surfaces.

According to the invention may also the doctor blade holder comprise a single fiat base member with the lower guide surface integrated with it, and at least one generally flat clamping member clamping at least one knife element to the flat base member under the action of at least one fastening member in the first set of fastening members. Thus the doctor blade holder forms a sandwiched structure with the single elongated base member and a single elongated clamping member holding the knife blades in between. Different modes of applying the clamping force onto the knife elements may be used, and in the general embodiment may fewer fastening members in the first set of fastening members be used, i.e. fewer than the number of knife elements, if the clamping member is sufficiently stiff. As the overall weight of the doctor blade holder is important to reduce, it may be advantageous to reduce stiffness and weight of the clamping member and instead use more fastening members. Preferably is the clamping member one single damping bar for all knife elements with at least as many fastening members in the first set of fastening members as there are knife elements, and preferably with at least one fastening member located for application of a clamping force directly to each knife element. In a further embodiment with increased clamping pressure on each knife element may also at least two fastening members in the first set of fastening members be located for application of a clamping force directly to each knife element.

In order to reduce time for removal and installation of the doctor blade holder and easy access is the second set of fastening members for the doctor blade holder including at least two removable fastening members, and all of these second set fastening members are arranged within a distance not exceeding 1 .5 meter, preferably not exceeding 1 meter, as measured from the outer radial end of the filter disc and towards the center of the filter disc.

For personal safety and reduced risk for dropping the doctor blade holder is an integrated gripping member at the other outer end of the doctor blade holder when mounted in the stationary cradle. The gripping member is preferably in form of handle with a closed elongated aperture enabling insertion of a hand of service personnel in the aperture and gripping around the handle with a closed handgrip.

The general principle of the invention is the use of complementary V-shaped guide surfaces on the doctor blade holder and the cradle, and in a preferred design of these V-shaped guide surfaces is the V-shaped guide surface on the doctor blade holder are inclined versus a vertical plane and at an angle lying in the range 30-60°, and that the corresponding complementary V-shaped guide surface of the cradle are inclined versus the vertical plane and at the same angle.

List of Drawings

In the following schematic drawings are details numbered alike in figures, and details identified and numbered in one figure may not be numbered in other figures in order to simplify figures.

Figure 1 ; show a cross-sectional side view of a conventional disc filter;

Figure 2; show a cross-sectional end view of the disc filter as seen in section A-A in figure 1 , with conventional components identified;

Figure 2a ; show in detail the interaction between a doctor knife and the precoat PC formed on the disc filter;

Figure 3a ; show a top view a prior art doctor arrangement;

Figure 3b; show a cross-sectional end view of the doctor arrangement as seen in section B-B in figure 3a;

Figure 4a; show a first embodiment of the inventive doctor arrangement;

Figure 4b; show a cross-sectional end view of the doctor arrangement as seen in section B-B in figure 4a;

Figure 5a; show the knife base member being part of the doctor arrangement Figure 5b; show a cross-sectional end view of the knife base member as seen in section B~B in figure 5a;

Figure 6a to 6d, show in a sequence 6a->6b->6c->6d how the knife base member may be entered in between the discs to the final position;

Figure 7; show a final version of a doctor arrangement, with the knife base member out of position from the stationary cradle;

Figure 8 show the final version of a doctor arrangement, with the knife base member mounted in the stationary cradle;

Figure 9 show the final version of a doctor arrangement, with the knife base member slightly retracted from the mounting position in the stationary cradle;

Figure 10a; show a cross-sectional end view of the knife base member as seen over the first set of fastening members; and

Figure 10b; show a cross-sectional end view of the knife base member as seen over the second set of fastening members; and

Detailed Description of the Invention

CONVENTIONAL DISC FILTER FEATURES In figures 1 and 2 are disclosed basic well known details of a disc filter.

The disc filter 1 is used to filter out fine particle material in a raw suspension and comprises a rotating hollow shaft 1 1 with a multitude of filter disc 12 rotating in a vat 10 with at least a semicircular bottom wall 10a located in bottom part of the disc filter. The filter discs drains the filtrate to the hollow shaft via outlets 13. A doctor blade 30a is arranged parallel with the filter surface of the filter disc 12, and a collection chute located under the doctor blade forms a substantially vertical box like collection chute with an upper opening 26 under the doctor blade and a lower outlet 27 in the lower part of the semicircular bottom part 10a of the vat. Said box like collection chute having two walls 29a/29b oriented parallel with the filter surface of the filter disc 12 and two walls 28a oriented along the filter surface of the filter disc 12.

The disc filter 1 includes at least one disc 12. The disci 2 is similar to those described in the prior art in that solid material is removed from raw slurry by forcing the liquid through filtering discs 12, while the solid fine particle material accumulates on the filter. The liquid is forced through the discs 12 by creating a pressure differential between the inside of the filter housing 10 and the outlet. The discs 12 may be a perforated material such as metal and covered with a fine mesh cloth such as a polypropylene cloth. The filter material is referred to generally as the filter cloth. Each disc 12 is divided into disc sectors (not shown) which are individually covered with filter cloth. The discs 12 are positioned within the housing 10 that holds the raw solid/liquid slurry in bottom part at a certain liquid level LiqL. The level of the raw slurry can vary but typically is approximately close to or below the center of the disc 12. As the discs 12 rotate, each sector fully enters the slurry and an amount of solid material is deposited on the surface of each disc sector. The solid material is then removed from each sector to separate the solid from the liquid. When filtering green or white liquor slurries in the recovery island of a pulp mill is the filter cloth as such not sufficient to reach a low particle content in the filtrate. The cloth is more used to be able to form a precoat layer of fine particles on the cloth, and after establishment of the precoat could effective filtration begin obtaining a filtrate with low particle content. When lime mud particles build up on this precoat, a scraper mechanism is used to remove the particles.

The scraper mechanism includes a doctor blade 30a that removes the cake layer from the precoat PC layer as shown in figure 2a. As each disc sector passes through the slurry and emerges from the slurry, additional solid material is deposited on the surface of the disc sector. As the disc sectors pass the doctor blade 30a, the cake layer formed on top of the precoat layer is removed. The removed solid material falls by gravity into a box like chute with an inlet 26 arranged below the doctor blade 30a.

The disc filter may also include wash showers and precoat renewal high pressure jet system, for example the SQUIRT™ system sold by Valmet, but these are not shown as they play no role doctor blade design.

In figure 3 is shown a prior art doctor blade arrangement with a stationary cradle 32 arranged over the filter surface from the outer end 12o of the disc and towards the inner end 12i of the disc close to the shaft, !n figure 3b is this doctor blade

arrangement seen in the cross section view B-B as seen in figure 3a. A number of knife elements 31 are screwed to the cradle 32, with two mounting screws for each knife element. When these knife elements are to be replaced must the service personnel reach all the way into the center of the disc filter for the last knife element located closest to shaft, and the risk is that the screws may be lost and dropped into the vat. This design have set severe restrictions on how close the disc may be mounted, as the space between the disc must be so large that a person may enter into the disc vat via the manhole (9 see figure 2).

In figure 4a is shown a first schematic embodiment of the inventive doctor blade arrangement, using a stationary cradle 32 and one doctor blade holder 30. The

5 doctor blade holder 30 comprises an elongated flat base member 33 with the knife elements clamped to it by a knife clamping bar 34. In this embodiment are 2 screws used to apply a clamping force from the knife clamping bar 34 on each of the knife elements 31 . In figure 5a is only the doctor blade holder 30 shown, seen in section B- B in figure 5b, with a handle 40 located at the outer end, and a form locking member 10 42 on the inner end.

In figures 6a to 6d are shown schematically how the doctor blade holder 30 is introduced into the disc filter and finally mounted in the cradle. In figure 6a is shown how the doctor blade holder 30 is introduced into the disc filter via the manhole 9, In next step, figure 6b, is the doctor blade holder dropped into contact with the cradle

15 such that the V-shaped guide surfaces come into point contact with each other. In this position could the service personnel hold the doctor blade holder in the gripping member 40 while most of the weight load from the doctor blade holder is supported from the cradle. As shown in figure 6c is the doctor blade holder in next phase pushed closer to center of the filter while sliding the point contact between the guide

20 surfaces closer to disc filter center and lowering of the outer end. In the final

mounting position as shown in figure 6d is the guide surface of the doctor blade holder in full line contact with the complimentary guide surface of the cradle. The doctor blade holder is locked in position with dedicated locking screws 42 mounted closer to outer end, and with a distance D shorter than 1 ,5 meter, preferably shorter

25 than 1 meter, from the outer end which enable access from the manhole.

Final production version

In figure 7 is shown a final production version of the inventive doctor blade

arrangement, with the doctor blade holder 30 separated to the left from the cradle 32. In figure 7 is the doctor blade holder 30 mounted in the cradle 32. In figure 9a is

30 shown a detail of the fingeriike form locking member 42b before the doctor blade holder is finally pushed to the mounting position, where the fingerlike form locking member rests in a complementary form locking aperture 42b ^' of the cradle. It should be noted that the fingerlike form locking member may alternatively be integrated with the cradle and the aperture in the knife base member 33 instead. In figure 9b is shown in detail the guide surface 33a of the knife base member 33 as well as the guide surface 33a of the cradle 32, with these parts slightly separated, In figure 10a and 10b is shown a cross section view of the doctor blade holder 30, where figure 10a show a cross section over one of the screws in the first set of fastening members, and figure 10b show a cross section over one of the screws in the second set of fastening members.

Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.