Refining Disc or Refining Disc Segment

Abstract

The present disclosure provides a refining disc or a segment thereof, comprising a base composed of a cast iron and teeth composed of a metal carbide composite, which teeth protrude from the base, wherein the teeth are partially embedded in the base. A disc refiner comprising a first and a second disc composed of such discs or segments, said first and second discs being arranged such that the teeth of the first disc face the teeth of the second disc as well as a method of disintegrating a material using such as disc refiner are also provided.

Description

TECHNICAL FIELD

The present invention relates to the field of refining, and in particular to refining discs or refining disc segments for disc refiners.

BACKGROUND

Disc refiners (sometimes referred to as double-disc refiners) are traditionally used in the pulp- and paper industry for dispersing pulp of recycled paper and thereby improve the appearance of the paper product (e.g. reduce the impurities in the recycled paper pulp to an invisible size). The disc refiners comprise refining discs or refining disc segments. The refining disc segments are arranged to form full refining discs in the disc refiner. Toothed refining discs are sometimes referred to as dispersing discs.

U.S. Pat. No. 4,116,392 discloses a pulp refining disc composed of an annular base and sets of radially oriented, tempered knife blades projecting from the base. The knife blades are made of a relatively high temperature resistant material.

U.S. Pat. No. 5,868,330 discloses another pulp refining disc comprising a base member and a plurality of refiner bars integrally informed with the base member and extending upwardly from it. Portions of the bars are formed of a material having gains of abrasive imbedded therein such that surface portions of the uppermost surface of each bar wear rough.

SUMMARY OF THE INVENTION

The inventors have realized that it is desirable to find a structure of the refining discs or refining disc segments that improves their performance and/or resistance to wear. In particular, they have found that disc refiners may have new applications, such as the disintegration of wood chips or wood pellets and that the new structure is particularly important in such cases.

A benefit of using disc refiners is such applications is that the disc gap normally may be adjusted, also during operation, in response to the properties of the supplied material or a product specification, such as a certain particle distribution.

Further, the inventors have found that the combination of a cast iron base and metal carbide composite teeth provides the desired refining disc structure.

The present disclosure thus provides a refining disc or a segment thereof, comprising a base composed of a cast iron and teeth composed of a metal carbide composite, which teeth protrude from the base, wherein the teeth are partially embedded in the base. Preferably the teeth are cast in the base, i.e. fixed in the base by means of casting. The present disclosure also provides a disc refiner comprising a first and a second disc composed of such discs or according, said first and second discs being arranged such that the teeth of the first disc face the teeth of the second disc.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a top view of a refining disc segment according to an embodiment of the present disclosure.

FIG. 2 is a simplified radial cross section view of a refining disc segment according to an embodiment of the present disclosure.

FIG. 3 is a cross-section view of a disc refiner.

FIG. 4 shows various examples of refining discs segments (a-c), of which four are needed to form a full disc (d). The teeth of 4a are widely spaced. The teeth of 4b are medium spaced. The teeth of 4c are closely spaced.

FIG. 5 shows various examples of refining discs segments (a-c), of which six are needed to form a full disc (d). The teeth of 5a are widely spaced while the teeth of 5b are closely spaced.

FIG. 6 shows various examples of refining discs segments (a-c), of which eight are needed to form a full disc (d). The teeth of 6a are widely spaced. The teeth of 6b are medium spaced. The teeth of 6c are medium spaced.

FIG. 7 shows the result a disintegration of pellets using segments having a tooth geometry and a teeth pattern according to embodiments of the present disclosure. The diagrams shall be interpreted as the proportion of disintegrated material that is larger than a cut-off determined by a sieve. The numbers on the x-axis are in millimeter (mm).

FIG. 8 shows the energy consumption in a pellets-to-powder application using white iron discs (the “a” curve) and discs having metal carbide teeth (the “b” curve”), respectively.

FIG. 9 shows the share of particle sizes below 2 mm and 1 mm for the white iron discs (white bars) and the discs having metal carbide teeth (black bars), respectively, in the pellets-to-powder application.

DETAILED DESCRIPTION

There is thus provided a refining disc or a segment thereof, comprising a base composed of a cast iron and teeth composed of a metal carbide composite, which teeth protrude from the base, wherein the teeth are partially embedded in the base. To cast prefabricated teeth in the base has proven to be a particularly suitable fixation technique.

The metal carbide composite of the teeth is hard and resistant to wear. Typically, the hardness of the metal carbide composite is at least 1000 Vickers (HV10). The metal carbide of the metal carbide composite may for example be WC (tungsten carbide), TiC (titanium carbide), TaC (thallium carbide) or a metal carbonitride, such as TiCN. Further, the metal carbide composite may comprise a metal alloy binder, such as Co (cobalt), Ni (nickel) or Iron (Fe). The metal carbide particles of a metal carbide composite may be evenly distributed in the metallic alloy binder. Thus, the hardness and strength of the metal carbide is combined with the toughness and plasticity of the metallic alloy binder. Such a material is frequently referred to as a cemented carbide. Cemented carbide is normally produced by powder metallurgy.

The process of casting cemented carbide teeth into cast iron follows largely common sand molding methods. To produce discs or segments of the present disclosure, the teeth may be arranged in their pattern in a sand core and the molding process may be performed in accordance with what is described in the patent application EP 0 374 116 A1 (see e.g. column 3, lines 9-26 and the example). Further information on casting cemented carbide in cast iron is found in the patent application WO 92/13651 (however, in the normal case centrifugal casting is not performed in connection with the present disclosure).

The hardness requirements for the base material are much lower than for the teeth. For example, the base may be composed of a material having a hardness of HRC 30-70, such as HRC 50-60 (Rockwell scale). Thus, a cheaper and in at least in some aspects more workable material may be used for the base. However, the base must be capable of fixing the teeth, and cast iron generally meets this requirement. The cast iron may for example be ductile iron, grey cast iron or white cast iron. Ductile iron, such as nodular cast iron, may be preferred as it is impact resistant and has a relatively high tensile strength. The grey cast iron may be preferred as it is relatively easy to form in a casting process. The white cast iron may however also be preferred as it is relatively hard (compared to other types of cast iron, such as grey cast iron) and thus relatively resistant to wear during use. The cast iron may for example comprise 1-30% Cr (chromium), such as 5-30% Cr, such as 10-30% Cr, such as 15-30% Cr. A higher Cr content makes the cast iron harder. White cast iron is one example of a cast iron that may have high Cr content.

The cast iron of the present disclosure may also have a lower carbon content and be classified as steel.

In commercial disc refiners, the refining discs are normally ring shaped. Thus, the refining disc of the present disclosure may have such a shape. Sometimes, a plurality of refining disc segments are arranged to form a refining disc in a disc refiner. Thus, the refining disc segment of the present disclosure may have the shape of a ring segment. Examples of segments having such a curved shape are shown in FIGS. 4-6. Thus, each segment may have a shape corresponding to ¼^th, ⅙^th, ⅛^th or 1/16^th of a ring. The skilled person understands that the shape may also correspond to half of a ring, a third of a ring, a fifth of a ring or any other fraction of a ring within practical limits.

In disc refiners, the material to be disintegrated is normally fed centrally, i.e. to the center of the refining discs (see FIG. 3). As the material is disintegrated/refined/dispersed, it moves outwards towards the outer circumference of the refining discs. Thus, the teeth closest to the inner edge of the refining disc/segments work the biggest pieces of material. Therefore, it may be beneficial if the teeth arranged closer to the inner edge are bigger and/or more widely spaced than the teeth arranged closer to the outer edge. This is exemplified in FIGS. 1 and 4-6.

The teeth may have the shape of a pyramid or truncated pyramid (see e.g. FIGS. 1 and 2). Thus, the base area of a tooth according to the present disclosure may be larger than its top area. The base area of a typical tooth may be 50-400 mm², such as 75-250 mm². Further, the height of a typical tooth may be 5-30 mm, such as 10-20 mm. Normally, at least 5%, such as at least 10%, such as at least 15% of the height of a tooth according to the present disclosure is embedded in the base. Further, less than 50% of the height is normally embedded. It is generally preferred that at least 2 mm of the height of a tooth according to the present disclosure is embedded in the base to obtain a sufficient fixation. The base are of a tooth according to the present disclosure may for example have the shape of a quadrilateral (four-sided polygonal). The top of a tooth of according to the present disclosure may for example be flat, and optionally have the shape of a quadrilateral. A tooth of the present disclosure may have one or two slits as in the refining disc segments marketed by Andritz. The person of skill in the art understands that the embodiments related to the tooth characteristics described in this paragraph not necessarily concern all teeth of the disc or segment. A disc or segment having a few teeth of differing characteristics is still encompassed by the embodiments. It is however preferred that at least the majority, such as at least 75%, of the teeth have the characteristics of a certain embodiment.

A refining disc segment of the present disclosure typically weighs 1-15 kg and has an area of 100-1000 cm².

In some disc refiners, two discs facing each other are arranged such that a row of teeth of one disc run between rows of teeth of another disc when the one disc rotates relative the other. To facilitate such rotation of one disc relative another, the teeth of the discs may be arranged in concentric rows (see the rows in FIGS. 1 and 4-6).

The present disclosure also provides a disc refiner comprising a first and a second disc composed of discs or segments according to any one of the embodiments described above, wherein said first and second discs are arranged such that the teeth of the first disc face the teeth of the second disc. In such a disc refiner, two whole refining discs may thus be arranged to face each other. Alternatively, a plurality of refining disc segments may be arranged to form two full discs that face each other. In the disc refiner construction, the second disc may be fixed and the first disc may be rotatably arranged such that it may rotate in respect of the first disc. The rotatable disc may be connected to a rotatable shaft. The shaft may be movable in the direction of its elongation such that the gap between the teeth of the first disc and the teeth of the second disc may be adjusted. Accordingly, the degree of disintegration may also be adjusted. An embodiment of a disc refiner is illustrated in FIG. 3.

The present disclosure also provides a method of disintegration of a material, wherein a disc refiner according to any one of the embodiments described above is used for disintegrating the material. The disc refiner is particularly suitable for the disintegration of a material comprising cellulose fibers. An example of such a material is pulp. The paper pulp may for example be the pulp of recycled paper, which is the type of material traditionally disintegrated with a disc refiner. Other examples of pulp are reject pulp and virgin pulp.

Wood-based material such as saw dust, cutter dust, wood chips and, in particular, wood pellets have been found to be difficult to disintegrate. As an example, disc refining using discs having teeth of cast iron (e.g. the base and the teeth cast in one piece) disintegrate wood pellets with a satisfactory result (see FIG. 7) and low energy consumption, but to the surprise of the inventors, the wear of the discs during such refining is unacceptable. Thus, the inventors found that the disc refining technique works surprisingly well with a wood-based material, such as wood pellets, and the introduction of the teeth of harder material solves the wear problem. Here, it may also be noted that the disintegration of wood pellets with alternative machinery, such as hammer mills, has been found to be associated with various problems and safety issues.

The purpose of disintegrating (raw) wood chips may be to set free drying surfaces or to produce a material having a particle size suitable for wood pellets production.

Other examples of material comprising cellulose fibers are straw, straw briquettes (pucks), agro chips form one-year plants and bagasse.

The material may also be torrefied material (torrefied biomass), other types of bio-coal, pellets of torrefied material or pellets of other types of bio-coal.

The material may also be pellets of organic material different from the above, such as pellets of an organic compound.

Further examples of materials are organic sludge, household waste, rejects from sewage treatment plants, cotton, tobacco, grain, manure and slaughtering residues.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIGS. 1 and 2 show a refining disc segment 1 according to an embodiment of the present disclosure. Six such segments may be arranged to form a full refining disc having an annular shape as shown in FIGS. 5a and 5c. The segment 1 has an inner edge 2 and an outer edge 3. The segment 1 comprises a base 4 and teeth 5, which are embedded and fixed in the base 4. In this particular embodiment, the teeth 5 have the shape of truncated pyramids. In the context of the present disclosure, the shape is considered to be pyramidal even though the surface plane of one side of the teeth 5 is perpendicular to the plane of the base 4. The teeth 5 are arranged in concentric rows, which allow the teeth 5 of a full disc to run between the teeth of an opposed disc when rotating in respect of the opposed disc. The distance between teeth 5 in the rows of an inner portion 6 of the segment 1 is generally greater than the distance between the teeth 5 in the rows of an outer portion 7 of the segment 1. Further, the teeth 5 closer to the inner edge 2 are bigger, i.e have a lager base area and/or a greater height and thus a bigger volume, than the teeth 5 closer to the outer edge 3. The inner portion 6 of the segment 1 may be referred to as the pre-milling zone, while the outer portion 7 may be referred to as the milling zone. The segment 1 has two holes 8 which allow it to be fixed in a disc refiner apparatus. FIG. 3 show part of a disc refiner 30. The disc refiner 30 comprises a rotor 32 onto which refining disc segments 1a are attached. The rotor 32 is connected to a rotating shaft 33. Refining disc segments 1b are also fixedly attached opposed to the refining disc segments 1a on the rotating body 32. The shaft 33 may be moved in the direction of its elongation so as to adjust the width of the gap between the segments 1a, 1b facing each other. Material for disintegration, such as pellets, is fed into the disc refiner 30 through its inlet 31. The material moves outwards (radially) as it is disintegrated, and the disintegrated material is collected in the collecting zone 34, from where is may be transported further (in the case of disintegrated pellets, to combustion or intermediate storage).

FIG. 4 shows three examples 4a, 4b, 4c of refining disc segments according to embodiments of the present disclosure. The teeth closer to the inner edges of the segments are generally bigger and more widely spaced than the teeth closer to the outer edges of the segments. Four segments according to FIG. 4 may be arranged in a disc refiner to form one full refining disc 4d. A disc refiner having two opposing discs thus need eight segments according to FIG. 4. A segment according to FIG. 4 typically weighs 1.5-3 kg.

FIG. 5 shows two examples 5a, 5b of refining disc segments according to embodiments of the present disclosure. The teeth closer to the inner edges of the segments are generally bigger and more widely spaced than the teeth closer to the outer edges of the segments. Six segments according to FIG. 5 may be arranged in a disc refiner to form one full refining disc 5c. A disc refiner having two opposing discs thus need twelve segments according to FIG. 5. A segment according to FIG. 5 typically weighs 5-7 kg.

FIG. 6 shows three examples 6a, 6b, 6c of refining disc segments according to embodiments of the present disclosure. The teeth closer to the inner edges of the segments are generally bigger and more widely spaced than the teeth closer to the outer edges of the segments. Eight segments according to FIG. 6 may be arranged in a disc refiner to form one full refining disc 6d. A disc refiner having two opposing discs thus need 16 segments according to FIG. 6. A segment according to FIG. 6 typically weighs 7-10 kg.

16 disc segments similar to those shown in FIGS. 4-6 may also be arranged to form one full disc in a disc refiner.

EXAMPLES

Example 1

A power plant requested a wood powder (disintegrated wood pellets) according to the following specification:

Particle size Proportion
<2.0 mm       >100%
<1.5 mm       >99%
<0.5 mm       >55%
<0.25 mm      >20%

The wood pellets are less bulky and more suitable for transport and storage than the wood powder. Thus, the wood powder is preferably produced at the site of the power plant.

The wood pellets, which had a density of 0.670 kg/dm³ and a dry matter content of 91%, were disintegrated with a disc refiner (KRIMA Disperser, marketed by Cellwood Machinery) having refining disc segments with widely spaced teeth (marketed by Cellwood Machinery) using the following conditions:

			Density	Motor	Aver-
Sam-		Disc	of	load	age	Flow	Specific
ple	Temp	gap	product	(Gross	load	(dm3/	energy (net
no	(° C.)	(mm)	(kg/dm3)	kW)	(kW)	min)	kWh/ton dry)
1	25	0.5	0.33	10-15	13	25	9
2	30	2.0	—	15-23	18	50	7

The run time was two minutes for both samples.

The results are presented in FIG. 7. FIG. 7b represents the result of sample no 1, and FIG. 7a represent the result of sample no 2.

The results were considered to meet the specification to a sufficient degree. Thus, disc refiners having toothed disc segments were found to successfully disintegrate the wood pellets. At a later time, it was however found that the disc segments were worn down considerably during disintegration of wood pellets. The wear problem is solved by the introduction of teeth of harder material embedded in the base of the disc or segment as described herein.

Example 2

A mill study was carried out, first using standard refining discs (base and teeth made of white iron) and then refining discs having metal carbide teeth cast in an iron base.

The following conditions applied:

Equipment: Krima Disc Refiner, type KR-710

Application: Wood Pellets to Powder

Production: 10 Ton/h

As shown in FIG. 8, the white iron disc initially functioned well but worn down quickly, which resulted in a dramatic increase in energy consumption. After 100 h, the trial with the white iron discs was aborted. In contrast, no significant wear problems had been observed after about 1100 h of operation when metal carbide teeth discs were employed. Further, the initial energy consumption was about 50% lower for the metal carbide teeth discs.

As explained above in Example 1, it is generally desirable that the share of powder particles being smaller than 2 mm is as high as possible in this pellets-to-powder application. As shown in FIG. 9, this share is significantly higher for the metal carbide teeth discs than for the white iron discs even though the energy consumption was about 50% lower for the metal carbide teeth discs. Further, it is shown in FIG. 9 that the share of powder particles being smaller than 1 mm is higher for the metal carbide teeth discs, which is also beneficial. Thus, the refining efficiency is improved with the metal carbide teeth.

Claims

1-15. (canceled)

16. A refining disc or a segment thereof, comprising a base having a cast iron and teeth that protrude from the base, wherein the teeth comprise a metal carbide composite and are partially embedded in the base.

17. The disc or segment according to claim 16, wherein the cast iron is white cast iron or grey cast iron.

18. The disc or segment according to claim 16, wherein the metal carbide composite comprises a metal alloy binder.

19. The disc or segment according to claim 18, wherein the metal alloy binder is Co, Ni or Fe.

20. The disc or segment according to claim 1, wherein the metal carbide of the metal carbide composite is WC, TiC and/or TaC.

21. The disc or segment according to claim 16, wherein the metal carbide composite is cemented carbide.

22. The disc or segment according to claim 16, wherein the hardness of the metal carbide material is at least 1000 Vickers.

23. The disc or segment according to claim 16, having the shape of a ring or a ring segment.

24. The disc or segment according to claim 16, having a radially inner edge and a radially outer edge, wherein the teeth arranged closer to the inner edge are bigger than the teeth arranged closer to the outer edge.

25. The disc or segment according to claim 16, wherein a tooth base area of a majority of the teeth is 50-400 mm2.

26. The disc or segment according to claim 25, wherein a tooth base area of the majority of the teeth is 75-250 mm2.

27. The disc or segment according to claim 16, wherein the tooth height of a majority of the teeth is 5-30 mm.

28. The disc or segment according to claim 27, wherein the tooth height of the majority of the teeth is 10-20 mm.

29. The disc or segment according to claim 16, wherein the teeth are arranged in concentric rows.

30. A disc refiner comprising:

a first and second refining disc or a segment thereof, each of the first and second refining disc comprising a base having a cast iron and teeth that protrude from the base, wherein the teeth comprise a metal carbide composite and are partially embedded in the base said first and second discs being arranged such that the teeth of the first disc face the teeth of the second disc.

31. The disc refiner according to claim 30, wherein the second disc is fixed and the first disc is rotatably arranged such that it may rotate in respect of the second disc.

32. A method of disintegration of a material, wherein a disc refiner according to claim 30 is used for disintegrating the material.

33. A method according to claim 32, wherein the material comprises cellulose fibers.