WO2010055099A1

WO2010055099A1 - Three roll mill

Info

Publication number: WO2010055099A1
Application number: PCT/EP2009/065060
Authority: WO
Inventors: Alexandre Claudon; Laurent Salvignol
Original assignee: BTG Eclépens S.A.
Priority date: 2008-11-17
Filing date: 2009-11-12
Publication date: 2010-05-20
Also published as: MY163761A; JP3175274U; TR201104646U2; DE212009000145U1; AU2009101359A4; RU112074U1; FIU20114030U0; FI9344U1; ES1076457Y; ES1076457U; PL66119Y1; CN202356130U; PL120076A1; AU2009315631A1; ES1076457U9

Abstract

The present invention relates to a three roll mill comprising a feed roll, a center roll, and a discharge roll arranged to rotate around their respective longitudinal axes, and a discharge blade arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll, characterized in that the discharge blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll, to a method of mixing, refining, dispersing and/or homogenizing a material in such a three roll mill, and to a material obtainable by such a method. The invention further relates to a discharge blade comprising a wear resistant deposit for use in a three roll mill.

Description

THREE ROLL MILL

Field of the invention

The present invention relates to three roll mills having a feed roll, a center roll, a discharge roll and a discharge blade for scraping off finished product from the discharge roll, to a method for grinding, milling, mixing, refining, dispersing and/or homogenizing viscous and/or particulate materials in such a mill, and to the use of a discharge blade for scraping off finished product from the discharge roll in a three roll mill.

Background

Three roll mills are used in a wide field of applications for grinding, milling, mixing, refining, dispersing and/or homogenizing viscous and/or particulate materials. Examples of materials often processed in three roll mills include electronic thick film inks, high performance ceramics, cosmetics, plastisols, carbon or graphite containing compositions, paints, printing inks, pharmaceuticals, chemicals, glass coatings, dental composites, pigments, coatings, adhesives, sealants and foodstuff.

Roll mills are based on the general principle of subjecting a material to high shear force between two rolls rotating in opposite directions and at different speeds. A three roll mill generally comprises a feed roll, a center roll, and a discharge roll arranged to rotate around their respective longitudinal axes, and a discharge blade arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll. In a three roll mill, the feed roll and the center roll are arranged to rotate in opposite directions, such that material fed into the mill is subjected to a first nip between the feed and center roll. The center roll and the discharge roll are arranged to rotate in opposite directions, such that material remaining on the center roll after the first nip is fed into a second nip between the center roll and the discharge roll. The discharge blade is generally arranged such that material remaining on the discharge roll after the second nip is scraped off from the discharge roll by the discharge blade. An increasing interest in the use of milled materials in emerging technical fields such as electronics and high performance materials have lead to higher demands on the quality and purity of materials processed in three roll mills. This, in turn, has led to higher demands on the performance of three roll mills.

Summary of the invention

It is an object of the present invention to meet at least some of the above mentioned demands.

It is another object of the present invention to provide a three roll mill which may improve the quality of the finished product.

A further object of the present invention is to improve safety for workers operating a three roll mill.

Yet a further object of the invention is to reduce the costs for operating a three roll mill.

These objects, and other objects that will be apparent to a person skilled in the art when presented with the disclosure of the present invention, are accomplished by the different aspects of the present invention.

A three roll mill generally comprises a feed roll, a center roll, and a discharge roll arranged to rotate around their respective longitudinal axes, and a discharge blade arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll. The term three roll mill, as used herein, is intended to encompass roll mills having at least three rolls, such as for example three, four, five, six, seven rolls or more. The invention generally will be described herein with reference to a mill roll mill having three rolls, but is equally applicable to a roll mill having more than three rolls. In a three roll mill, the feed roll and the center roll are arranged to rotate in opposite directions, such that material fed into the mill is subjected to a first nip between the feed and center roll. The center roll and the discharge roll are arranged to rotate in opposite directions, such that material remaining on the center roll after the first nip is fed into a second nip between the center roll and the discharge roll. The feed, center and discharge roll may also be arranged to rotate at different speeds. The rolls are generally made of chilled cast iron, but other materials are also used. The discharge blade is generally arranged such that material remaining on the discharge roll after the second nip is scraped off from the discharge roll by the discharge blade. The three roll mill may further comprise some type of means for collecting the finished product, for example an apron or a slide arranged to receive material scraped off from the discharge roll by the discharge blade.

When material is passed through the first nip, between the feed roll and the center roll, it is subjected to high shear force resulting in mixing, refining, dispersing and/or homogenizing of the material. When the material having passed through the first nip is subjected to the second nip, between the center roll and the discharge roll, it is again subjected to high shear force resulting in further mixing, refining, dispersing and/or homogenizing of the material. Treated material remaining on the discharge roll after the second nip is scraped off from the roll by a discharge blade, also referred to herein as "blade" or "scraper blade". Applications in which products processed in three roll mills are used have developed quickly over the past year. Use of milled materials in technical fields such as electronics and high performance materials have lead to higher demands on the quality and purity of the formed products. The present invention is based on the realization that the objects referred to above can be achieved by the introduction of a wear resistant coating covering a working edge of the discharge blade which is used for scraping off the treated material from the discharge roll.

In a first aspect thereof, the present invention provides a three roll mill comprising a feed roll, a center roll, and a discharge roll arranged to rotate around their respective longitudinal axes, and a discharge blade arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll, characterized in that the discharge blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll. The conventional discharge blades used in three roll mills are generally steel based and the wear from use in the mill may result in abrasion of metal residues from the blade that end up as particulate or ionic contamination in the processed material. The applicant has realized that the quality of the milled material can be improved by the introduction of a wear resistant deposit covering at least the working edge of the discharge blade which is adapted for contact with the roll. The wear resistant deposit of the present invention significantly reduces contamination of the milled material relating to wear of the discharge blade.

Conventional blades are worn down quickly during processing of abrasive particles, e.g. colored pigments. Many times, the wear of the blade will not be exactly uniform along the length of the blade, i.e. certain portions of the working edge of the blade may be worn faster than others. This may result in a worn blade having a non-uniform shape along the edge section adapted for contact with the discharge roll. This, in turn, may result in a deviation, e.g. in the particle size distribution, in the formed product over the life span of the discharge blade. The present invention may reduce or substantially eliminate this effect, since the wear resistant deposit helps the working edge to retain its original shape. A blade having a wear resistant deposit thereby provides a more consistent and uniform milling process resulting in improved consistency and quality of the formed product. Reduced wear also has the additional advantages of reducing production down-time due to blade change, reducing labor, and ultimately reducing production costs.

The present invention also provides a significant improvement in safety for workers operating three roll mills. The scraper blades used in conventional three roll mills are generally steel based and the wear from use in the mill results in the formation of a very sharp edge on the blade. Since the blades are generally replaced by hand the risk of injury in this work is considerable. The blade of the present invention, having a wear resistant deposit at the working edge, reduces the risk of injury in two ways. Firstly, the wear resistant deposit may help the working edge to retain its original shape so that no sharp edge is formed, and secondly, the wear resistant deposit will increase the life span of the blade, resulting in less frequent replacement of blades. Both the exposure to the blades, and the risk associated with exposure to the blades is thereby reduced. The discharge blade may be any shape suitable for scraping off material from the discharge roll. Generally, the blade will have a longitudinal extension substantially parallel to the longitudinal axis of the discharge roll, and have a working edge adapted for contact with the discharge roll. The blade will generally have a top side, a bottom side and two lateral portions extending along the length of the blade. The top and bottom sides of the blade are generally formed in substantially parallel planes. The thickness of the blade may generally be in the range of 0.5-2 mm, or in the range of 0.5-1 mm. The width of the blade between the lateral portions of the blade will generally be in the range of 10-100 mm. The length of the blade in the longitudinal direction may vary depending on the intended use, normally within the range of 0.1 -3 m. The blade further comprises an edge section arranged along at least one lateral portion of the blade. The edge section comprises the working edge adapted for contact with the discharge roll and generally at least a portion of each of the sides adjacent to the working edge.

The exact geometry of a blade cross section orthogonal to the longitudinal extension of the blade may vary. To reduce the risks associated with handling the blades, it is preferred that the blade does not comprise unnecessarily sharp edges. In order to reduce the risks of injuries from manual handling of the blade, preferably no two adjacent sides or bevels of an edge section of the blade should form an angle of less than 50°. To even further reduce the risks of injuries, preferably no two adjacent sides or bevels of an edge section of the blade should form an angle of less than 70°.

Therefore, in an embodiment of the present invention the discharge blade has a top side, a bottom side and an edge section, wherein no two adjacent sides of an edge section of the blade form an angle of less than 50°. In another embodiment, no two adjacent sides of an edge section of the blade form an angle of less than 70°.

In an embodiment, the discharge blade comprises an edge section having a back side facing and/or contacting the roll and a front side adapted for collecting scraped off material, wherein the working edge of the blade is formed by an angle β between said back side and said front side, and said angle is in the range of 50-130° or in the range of 60-120° or in the range of 70-110°. Preferably, in this embodiment, an angle α between the top side of the blade and the front side of the edge section may be in the range of 50- 180°, and an angle Y between the bottom side of the blade and the back side of the edge section may be in the range of 50-180°, and the angles α, β, and Y may preferably be selected such that the sum of the angles α, β, and Y is approximately 360°, such as in the range of 355-365° or in the range of 359- 361 °. More preferably, the sum of the angles α, β, and Y is 360°.

In an embodiment, the discharge blade comprises an edge section having a back side facing and/or contacting the roll and a front side adapted for collecting scraped off material, wherein the working edge of the blade is formed by an angle β between said back side and said front side, and said angle is in the range of 50-130° or in the range of 60-120° or in the range of 70-110°. Preferably, in this embodiment, an angle α between the top side of the blade and the front side of the edge section may be in the range of 50- 130°, and an angle Y between the bottom side of the blade and the back side of the edge section may be in the range of 130-180°, and the angles α, β, and Y may preferably be selected such that the sum of the angles α, β, and Y is approximately 360°, such as in the range of 355-365° or in the range of 359- 361 °. More preferably, the sum of the angles α, β, and Y is 360°. In other words, in an embodiment, the blade comprises both a top bevel and a bottom bevel at the edge section. In an embodiment comprising both a top bevel and a bottom bevel, the working edge of the blade is formed by the angle β between the top bevel and the bottom bevel. In this embodiment, the working edge angle should preferably be in the range of 50-130°, for example in the range of 60-120° or in the range of 70-110°. An angle α between the top bevel and the top side of the blade may preferably be in the range of 50-130°. An angle Y between the bottom bevel and the bottom side of the blade may preferably be in the range of 130-180°. The angles α, β, and Y may preferably be selected such that the sum of the angles α, β, and Y is approximately 360°, such as in the range of 355-365° or in the range of 359-361 °. More preferably, the sum of the angles α, β, and Y is 360°. In another embodiment, the top side of the blade and the front side of the edge section are the same, i.e. α=180°, and the working edge of the blade is formed by an angle β between said back side of the edge section and the top side of the blade, and said angle is in the range of 50-90°. In other words, in an embodiment, wherein the blade comprises a top side and a bottom side, the blade comprises a bottom bevel at the edge section. In an embodiment comprising only a bottom bevel, the working edge of the blade is formed by the angle between the bottom bevel and the top side of the blade. In this embodiment, the working edge angle β should preferably be in the range of 50-90°. An angle Y between the bottom bevel and the bottom side of the blade may preferably be in the range of 90-130°. The angles α, β, and Y may preferably be selected such that the sum of the angles α, β, and Y is approximately 360°, such as in the range of 355-365° or in the range of 359- 361 °. More preferably, the sum of the angles α, β, and Y is 360°.

In another embodiment, the bottom side of the blade and the back side of the edge section are the same, i.e. γ=180°, and the working edge of the blade is formed by an angle β between said front side and the bottom side of the blade, and said angle is in the range of 50-90°. In other words, in an embodiment, wherein the blade comprises a top side and a bottom side, the top side comprises a top bevel at the edge section. In an embodiment comprising only a top bevel, the working edge of the blade is formed by the angle between the top bevel and the bottom side of the blade. In this embodiment, the working edge angle β should preferably be in the range of 50-90°. An angle α between the top bevel and the top side of the blade may preferably be in the range of 90-130°. The angles α, β, and Y may preferably be selected such that the sum of the angles α, β, and Y is approximately 360°, such as in the range of 355-365° or in the range of 359-361 °. More preferably, the sum of the angles α, β, and Y is 360°.

The blade may also comprise further sides or bevels at the edge section resulting in other angles suitable for use as the working edge of the blade.

In order to reduce the risks of injuries from manual handling of the blade, preferably no two adjacent sides or bevels of an edge section of the blade should form an angle of less than 50°. Having angles of less than 50° between two adjacent sides at the edge section of the blade will make the blade unnecessarily sharp. For the same reason, the width of sides or bevels at the edge section of the blade should preferably be 100 μm or higher, since sides or bevels having a width of less than 100 μm may be very sharp. In an embodiment, the width of the front side of the edge section between angles α and β is 100 μm or higher, preferably 200 μm or higher, more preferably 300 μm or higher.

The discharge blade will generally be arranged in sliding contact with the discharge roll, at an angle in the range of 10-80° relative to a tangential direction of the discharge roll.

Due to the nature of the application, even a discharge blade having a wear resistant deposit will be subject to a certain degree of wear over time. When the blade is worn, a wear face is formed at the working edge of the blade. In order to avoid undue sharpening of the blade during use, it should be arranged such that a wear face formed by wear of the blade does not form sharp angles with adjacent sides or bevels. This may be accomplished in a three roll mill according to the invention by arranging the discharge blade at an angle against the discharge roll, such that when the blade becomes worn during use in the three roll mill, a wear face formed at the working edge of the blade does not form an angle of less than 50° with any surface adjacent to it. In this respect, the blade of the present invention having a wear resistant deposit has an important advantage over conventional steel blades in that wear occurs much more slowly. The wear resistant deposit helps the blade retain its original edge shape for a significantly longer period of time, whereas the edge of a conventional steel blade may quickly lose its original shape and receive a shape determined by the positioning and angle of the blade against the discharge roll. The discharge blade may preferably be arranged at an angle against the discharge roll, such that when the blade becomes worn during use in the three roll mill, the wear does not result in the formation of a side of the edge section of the discharge blade having a width of less than 100 μm, preferably not less than 200 μm.

In an embodiment, when the blade is worn, the width of the front side of the edge section between angle α and the wear face is 100 μm or higher, preferably 200 μm or higher. In another embodiment, when the blade is worn, the width of the back side of the edge section between the angle Y and the wear face is 100 μm or higher, preferably 200 μm or higher. In a preferred embodiment, the width of the front side of the edge section between angles α and the wear face, and the width of the back side of the edge section between the angle Y and the wear face, are both 100 μm or higher, preferably 200 μm or higher. The blade will generally comprise a core or substrate made of an iron based material, such as carbon steel or stainless steel. Other suitable core or substrate materials, e.g. other metals or alloys, ceramics, polymers or composites, may also be used in the discharge blade.

The blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll. The wear resistant deposit preferably comprises a material having a wear resistance substantially higher than that of carbon steel or stainless steel. The scope of the present invention is not limited to the specific examples of wear resistant materials mentioned hereinbelow. Other suitable high wear resistance materials suitable for use with the present invention may be identified by a person skilled in the art of materials technology and include materials commonly used in tool making and processing of hard materials such as stone or metals.

The wear resistant deposit of the invention may for example comprise a material selected from the group consisting of wear resistant carbides, oxides, nitrides, borides, silicates, and mixtures thereof. The wear resistant deposit may further be a mixture, or a composite, of any one of the above mentioned wear resistant materials with a further material, such as a metal.

In an embodiment, the wear resistant deposit is a cermet. A cermet is a composite material comprising ceramic particles dispersed in a metal matrix. In an embodiment, the wear resistant deposit comprises tungsten carbide. It has been found that materials within the tungsten carbide cobalt family are particularly useful in the present invention. In an embodiment, the wear resistant deposit comprises a tungsten carbide cobalt cermet. A cermet may for example be deposited onto the blade substrate using thermal spraying HVOF (high velocity oxygen fuel) technology.

A cermet deposit also has the additional advantage of reducing the friction between the discharge blade and the discharge roll, e.g. a chilled cast iron roll, as compared to a conventional steel blade, resulting in less wear on blades and rolls, reduced heat of friction, and reduced power consumption due to less resistance between the blade and the roll. A tungsten carbide cobalt cermet provides especially low friction. For example, a deposit of tungsten carbide cobalt cermet covering the working edge of the blade allows the blade pressure to be reduced from 15 bars for conventional steel blades, to 8 bars. Reduced blade pressure results in reduced blade wear, reduced discharge roll wear, and reduced power consumption due to less resistance between the blade and the roll. Without being bound to any specific scientific theory, it is believed that running steel on cast iron counter-part may induce adhesive phenomenon that increases friction. Moreover, effect of temperature at contact due to high load may soften and deform the metallic material. As a consequence, adhesive wear can be promoted and friction will increase further. This phenomenon is far less likely to occur with the wear resistant deposit of the present invention, due to the poor chemical compatibility with the counter-part and the general material stability.

The thickness of the wear resistant deposit may be varied depending on the specific material, application and other requirements, e.g. the abrasiveness of the material to be processed. Generally, the thickness of the wear resistant deposit may be less than 500 μm. In an embodiment, the thickness of the wear resistant deposit is less than 300 μm. In another embodiment, the thickness of the wear resistant deposit is in the range of 50- 300 μm. The thickness may also be less than 50 μm depending on the properties of the wear resistant material and the method of applying the wear resistant deposit. The wear resistant deposit preferably covers at least the working edge of the blade adapted for contact with the discharge roll since this is the portion of the blade subject to the most wear. In certain cases, e.g. for wear resistance or manufacturing reasons, it may be advantageous to also cover other surfaces of the blade with the wear resistant deposit. Therefore, in different embodiments, the entire edge section, the top side of the blade, the bottom side of the blade or the entire surface of the blade may be fully or partially covered by the wear resistant deposit.

In a second aspect thereof, the present invention provides the use of a discharge blade for scraping off treated material remaining on a discharge roll of a three roll mill, wherein said discharge blade is provided with a wear resistant deposit covering at least a working edge thereof adapted for contact with said discharge roll.

Generally, a three roll mill is useful for grinding, milling, mixing, refining, dispersing and/or homogenizing viscous and/or particulate materials. In an embodiment, the material which is processed in the three roll mill of the invention comprises a viscous liquid or a paste. In another embodiment, the material comprises solid or semi solid particles. The material may also comprise a viscous liquid or a paste and solid or semi solid particles. A wide range of materials may be processed in a three roll mill according to the invention, although the blade of the invention comprising a wear resistant deposit is especially useful in the processing of materials comprising abrasive particles that would quickly wear down a conventional steel blade. Examples of materials that may be processed in a three roll mill according to the invention include electronic thick film inks, high performance ceramics, cosmetics, plastisols, carbon or graphite containing compositions, paints, printing inks, toner particles, liquid crystal dispersions, pharmaceuticals, chemicals, glass coatings, dental composites, pigments, coatings, adhesives, sealants, and foodstuff.

Examples of materials that often comprise abrasive particles for which the present invention is especially useful include printing inks, paint pigments, toner particles, cosmetic pigments, liquid crystal dispersions, and ceramic pastes. In a third aspect thereof, the present invention provides a method of mixing, refining, dispersing and/or homogenizing a material in a three roll mill having a feed roll, a center roll, and a discharge roll, comprising the steps: a) feeding a material to be treated into a feed zone of the three roll mill, b) grinding, milling, mixing, refining, dispersing and/or homogenizing the material in a first treatment step between the feed roll and the center roll and in a second treatment step between the center roll and the discharge roll, and characterized by the step c) scraping off treated material remaining on the discharge roll after step b) using a discharge blade having a wear resistant deposit covering at least a working edge thereof, before collecting the finished product.

The product obtained using a three roll mill, according to the above mentioned aspects of the present invention has improved quality and homogeneity, in terms of metal contamination and particle size distribution, as compared to products obtained with similar three roll mills using conventional steel discharge blades. The enhanced properties of the materials obtainable by a method of the present invention are not easily described in terms of physical parameters. Therefore, in a fourth aspect thereof, the present invention provides a ground, milled, mixed, refined, dispersed and/or homogenized material obtainable by the method according to the third aspect of the invention. In a fifth aspect thereof, the present invention provides a discharge blade for scraping off finished product from a discharge roll in a three roll mill, said blade having top side, a bottom side and an edge section comprising a back side adapted for contact with the discharge roll and a front side adapted for collecting scraped off product, wherein the working edge of the blade is formed by an angle β between said back side and said front side in the range of 70-110°, and no two adjacent sides of an edge section of the blade form an angle of 70° or less, characterized in that the discharge blade comprises a wear resistant cermet covering at least a working edge thereof.

The discharge blade of the fifth aspect of the invention may further be defined as described above in respect of the first aspect of the invention.

A blade according to this aspect of the invention is especially useful in three roll mills for processing of materials comprising abrasive particles. The inventive blade provides several advantages over steel blades conventionally used in three roll mills, e.g. reduced metal contamination of the treated product as wear is reduced by the wear resistant deposit, reduced friction against the discharge roll of the mill, and improved worker safety as the absence of sharp angles in combination with the wear resistant deposit highly reduces the risks associated with handling of the blades.

Brief description of the drawings

Fig 1 is a schematic side view of a three roll mill according to the invention.

Fig 2 is a schematic cross section of a discharge blade comprising a wear resistant deposit covering a working edge thereof.

Fig 3 a-d represents a schematic view of different blade cross section geometries.

Fig 4 represents a schematic view of the blade profile of an embodiment of the invention.

Fig 5 illustrates a prior art blade when new (a) and when worn (b).

Fig 6 illustrates an embodiment of the blade of the invention when new (a) and when worn (b).

Fig 7 a-c are photographs of blade profiles. In the photographs, the blades are presented with the bottom side of the blade facing upward. The edge section of the blades is to the right. A scale showing the distance of 1 mm provided for reference. Fig. 7a shows an image of an embodiment of the inventive unworn (new) blade, wherein the darker triangular area on the top right of the blade is the wear resistant deposit. Fig. 7b shows an image of the inventive blade of Fig 7a after being worn for 250 hours on a three roll mill. The darker triangular area on the right side of the blade is the remaining wear resistant deposit. This blade has no angles of 50° or less. Fig. 7c shows an image of the worn steel blade reference after 2 hours on a three roll mill. The tip angle is about 25°. Detailed description of preferred embodiments

In an embodiment of the invention illustrated in Fig 1 , the three roll mill comprises a feed roll (1 ), a center roll (2), and a discharge roll (3) arranged to rotate around their respective longitudinal axes, and a discharge blade (4) arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll. In the three roll mill of this embodiment, the feed roll and the center roll are arranged to rotate in opposite directions, such that material fed into the feed section of the mill is subjected to a first nip (5) between the feed and center roll. The center roll and the discharge roll are arranged to rotate in opposite directions, such that material remaining on the center roll after the first nip is fed into a second nip (6) between the center roll and the discharge roll. The discharge blade (4) comprises a wear resistant deposit covering a working edge thereof adapted for contact with the discharge roll. The three roll mill may further comprise a slide or apron (7) for collecting treated material

In an embodiment of the invention illustrated in Fig 2, the discharge blade of the three roll mill comprises a carbon steel band having a main longitudinal extension, and having a top side (8), a bottom side (9) and two longitudinal lateral portions (only one shown). The thickness of the blade is less than 2 mm and the width of the blade is less than 100 mm. Along one of the lateral portions, the blade is provided with an edge section (10) comprising a working edge (11 ) formed by an angle between the bottom side of the blade and a lateral side of the blade at the edge section. The angle between the bottom side and said lateral side is in the range of 50-130°. The edge section (10) comprises the working edge and at least a portion of each of the sides adjacent to the working edge (11 ). The working edge is adapted for contact with the discharge roll, such that when the roll rotates, treated material on the envelope surface of the roll is scraped off from the roll and onto the top side of the blade. The blade comprises a tungsten carbide cobalt cermet deposit (12) covering the edge section of the blade. The thickness of said deposit is in the range of 50-300 μm from the bottom side of the blade. Fig 3 a-d shows the blade profile of four different embodiments of the invention. Fig 3 a shows an embodiment having a top bevel and a bottom bevel, wherein the edge angle β between the top and the bottom bevel is in the range of 50-130°, the angle α between the top bevel and the top surface is in the range of above 90° and below 180° and the angle Y between the bottom bevel and the bottom surface is in the range of above 90° and below 180°. The angles α, β, and Y are preferably selected such that the sum of the angles α, β, and Y is 360°. Fig 3 b shows an embodiment having a top bevel, but no bottom bevel, wherein the edge angle β is in the range of 50-90°. In this case, Y is 180°. The angles α and β are preferably selected such that the sum of the angles α, β, and Y is 360°. Fig 3 c shows an embodiment having a bottom bevel, but no top bevel, wherein the edge angle β is in the range of 90-130°. In this case, α is 180°. The angles β, and Y are preferably selected such that the sum of the angles α, β, and Y is 360°. Fig 3 d shows an embodiment having a substantially rectangular profile, in which the both the top angle α and the edge angle β are about 90°. In this case, Y is 180°. The angles α and β are preferably selected such that the sum of the angles α, β, and Y is 360°.

In another embodiment, the discharge blade has a cross section geometry as illustrated in Fig 4. In this embodiment, the discharge blade of the three roll mill comprises a carbon steel band having a main longitudinal extension, and having a top side (13), a bottom side (14) and two lateral portions (only one shown). The top and bottom side of the blade are formed in substantially parallel planes. The thickness of the blade is less than 2 mm and the width of the blade is less than 100 mm. Along one of the lateral portions, the blade is provided with an edge section. The edge section comprises a back side (15) facing and/or contacting the discharge roll and a front side (16) adapted for collecting scraped off material, and the working edge (17) of the blade is formed by an angle β between said back side and said front side. The angle β between said back side and said front side is in the range of 50- 130°. The angle α between the front side of the edge section and the top side of the blade is in the range of 90-130°, and the angle Y between the back side of the edge section and the bottom side of the blade is in the range of 130- 180°. The angles α, β, and Y are selected such that the sum of the angles α, β, and Y is 360°. The width d of the front side of the edge section between angle α and angle β is at least 300 μm. The working edge is adapted for contact with the discharge roll, such that when the roll rotates, treated material on the envelope surface of the roll is scraped off from the roll and onto the top side of the blade. The blade comprises a wear resistant tungsten carbide cobalt cermet deposit covering the edge section of the blade. The thickness of said deposit is in the range of 50-300 μm. Fig 7a shows a photographic image of a cross section of an embodiment of the discharge blade of the invention, wherein the width d of the front side of the edge section between angle α and angle β is about 390 μm. Fig 5 a and b show the effect of wear on a conventional prior art steel blade. The profile of the blade quickly changes during use from the shape of the new blade (18) in Fig 5a, to the used blade (19) shape of Fig 5b having a sharp top angle, generally in a range of 20-45°, and having no front side of the edge section. Fig 7c shows a photographic image of a cross section of a worn conventional prior art steel blade. This blade has a very sharp angle of about 25° and no front side of the edge section.

Fig 6 a and b show the effect of wear on a blade according to an embodiment of the present invention. The profile of the blade changes very slowly during use because of the wear resistant deposit covering the working edge of the blade. Fig 6 a represents a new blade (20) and Fig 6b represents the same blade when worn out (21 ). Wear does not result in the formation of any sharp angle at the blade, and a width d of the front side of the edge section, between angle α and angle β, of at least 100 μm is retained in the worn blade. Fig 7b shows a photographic image of a cross section of a worn discharge blade according to the invention. This blade has no sharp angles, i.e. 50° or less, and the remaining width d of the front side of the edge section is about 250 μm.

Examples

Example 1. Preparation of a discharge blade comprising a wear resistant tungsten carbide cobalt deposit.

A steel substrate having a thickness of 0.600 mm and a width of 50 mm was first ground at one longitudinal side to shape the edge section and to allow the wear resistant deposit to be sprayed on. The edge section was then sandblasted using F60 corundum to increase the bond strength between the steel substrate and the wear resistant deposit. The sandblasted edge portion was then sprayed with the wear resistant tungsten carbide cobalt cermet WC- Co (88-12 weight %) using HVOF (High Velocity Oxygen Fuel) technology. The top and bottom sides of the blade were then ground to obtain the desired blade shape. Example 2. Blade life-time increase using a wear resistant tungsten carbide cobalt deposit.

Example 2a.

In a three roll mill (security ink provider, Bϋhler SDVE 1300 CLS), trials were performed using two different discharge blades.

The first blade type was a standard sharpened steel blade A used as a reference (prior art blade). The second blade type was an improved blade B according to the present invention having a wear resistant tungsten carbide cobalt deposit covering the edge section of the blade.

The following operating conditions were used in these trials:

- Type of pigmented pastes: transparent white, high solid white, red paste.

- Roller surface: cast iron

- Roller temperature: 30⁰C

- Discharge blade dimensions: 0.635 x 40 x 1345 mm (thickness x width x length).

- Blade holder angle: 55° (sliding contact with the discharge roll relative to the tangential direction of the discharge roll).

- Discharge roll speed: 522 rpm

- Blade pressure: 8 bars

The working life-time for the reference blade A (steel blade) is on the average about 2 hours. For producing a batch of 500 kg of pigmented paste, this blade had to be taken off 4 times in order to be sharpened. The risk of injuries is high and the production down-time is relatively consequent: the change of blade takes about 10 minutes.

In comparison, the second blade B tested on the same machine with the same type of pigmented pastes lasted about 250 hours. For the following and different batches, the blade was left inside the blade holder and just cleaned. Based on these trials, the inventive blade B can potentially produce over 100 times more of the same type of product before changing the blade. The wear of blade B was very low and even, so there was no need for the operator to change the blade holder angles to improve performance. Accordingly, the gain in productivity (reduction of production down-time) was considerable using blade B.

Example 2b.

A second trial was performed with a very abrasive pigmented paste. Comparative tests were carried out with the two different types of discharge blades A (standard steel blade) and B (inventive wear resistant blade) mentioned in Example 2a. The operating conditions used were also the same as those mentioned in Example 2a.

For producing a batch of 500 kg of a very abrasive pigmented paste, 10 blades of type A were required during 4 hours of production. Each reference blade A lasted about 20 to 30 minutes, after this time the wear at the blade edge was so severe that the scraping off of finished product was less homogeneous.

The under corresponding conditions, blade type B lasted 15 hours and produced 4 batches of 500 kg of abrasive pigmented paste. Again, the gain in productivity is considerable.

Further to the trials of Examples 1 a and 1 b, different measurements have been carried out on the discharge roll diameter in order to check and confirm that the blade having the wear resistant deposit (tungsten carbide cobalt deposit) had no negative impact on the surface of the roll and also on the dimensions of the roll diameter. The following information was obtained:

- no significant surface wear of the discharge roll after 4000 hours operating time,

- after 2000 hours, there was a sort of micro-abrasion of the cylinder which was similar for the both type of blades: the wear was in the range of 0.015 to 0.045 mm.

- Figure 7a shows an image of the inventive unworn (new) blade

- Figure 7b shows an image of the worn inventive blade after 250 hours on a three roll mill.

- Figure 7c shows an image of the worn steel blade reference after 2 hours on a three roll mill. Example 3. Friction and blade pressure reduction using a wear resistant tungsten carbide cobalt deposit.

Friction and blade pressure of the two different types of discharge blades A (standard steel blade) and B (inventive wear resistant blade) mentioned in Example 2a in a three roll mill were studied. The operating conditions of the three roll mill were also the same as those mentioned in Example 2a.

In order to ensure a constant and homogeneous scraping off of the finished product, the pressure of the reference blade A had to be increased from 10 bars up to 15 bars during a short period of 1 hour. Direct consequences of such hard working conditions include rapid wear of the steel blade leading to metallic contamination inside the processed material.

In comparison, the blade having the wear resistant deposit worked well at a lower and constant pressure of 8 bars.

Claims

1. A three roll mill comprising a feed roll, a center roll, and a discharge roll arranged to rotate around their respective longitudinal axes, and a discharge blade arranged substantially parallel to the longitudinal axis of the discharge roll for scraping off finished product from the discharge roll, characterized in that the discharge blade has a top side, a bottom side and an edge section comprising a back side facing and/or contacting the roll and a front side adapted for collecting scraped off material, and wherein the working edge of the blade is formed by an angle between said back side and said front side, and said angle is in the range of 50-130°, wherein the front side of the edge section has a width of more than 100 μm, and the discharge blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll.

2. A three roll mill according to claim 1 , wherein the discharge blade has a top side, a bottom side and an edge section, and wherein no two adjacent sides of an edge section of the blade form an angle of less than 50°.

3. A three roll mill according to according to any one of the preceding claims , wherein the angle between said back side and said front side is in the range of 60-120°.

4. A three roll mill according to according to any one of the preceding claims , wherein the angle between said back side and said front side is in the range of 70-110°.

5. A three roll mill according to any one of the preceding claims, wherein each of the sides of the edge section has a width of more than 100 μm.

6. A three roll mill according to any one of the preceding claims, wherein the discharge blade is arranged at an angle against the discharge roll, such that when the blade becomes worn during use in the three roll mill, a wear face formed at the working edge of the blade does not form an angle of less than 50° with any surface adjacent to it.

7. A three roll mill according to any one of the preceding claims, wherein the discharge blade is arranged at an angle against the discharge roll, such that when the blade becomes worn during use in the three roll mill, the wear does not result in the formation of a side of the edge section of the discharge blade having a width of less than 100 μm.

8. A three roll mill according to any one of the preceding claims, wherein the discharge blade has a thickness of less than 1 mm.

9. A three roll mill according to any one of the preceding claims, wherein the wear resistant deposit comprises a material selected from the group consisting of wear resistant carbides, oxides, nitrides, borides, silicates, and mixtures thereof.

10. A three roll mill according to any one of the preceding claims, wherein the wear resistant deposit is a cermet.

11. A three roll mill according to any one of the preceding claims, wherein the wear resistant deposit comprises tungsten carbide.

12. A three roll mill according to any one of the preceding claims, wherein the wear resistant deposit comprises a tungsten carbide cobalt cermet.

13. A three roll mill according to any one of the preceding claims, wherein the wear resistant deposit has a thickness of less than 300 microns.

14. Use of a discharge blade for scraping off treated material remaining on a discharge roll of a three roll mill, wherein said discharge blade has a top side, a bottom side and an edge section comprising a back side facing and/or contacting the roll and a front side adapted for collecting scraped off material, and wherein the working edge of the blade is formed by an angle between said back side and said front side, and said angle is in the range of 50-130°, wherein the front side of the edge section has a width of more than 100 μm, and wherein the discharge blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll.

15. Use according to claim 14, wherein the discharge blade is further defined as in any one of claims 2-13.

16. Use according to any one of claims 14-15, wherein the material comprises a viscous liquid or a paste.

17. Use according to any one of claims 14-16, wherein the material comprises solid or semi solid particles.

18. Use according to any one of claims 14-17, wherein the material comprises a printing ink, a paint pigment, a toner particle, a cosmetic pigment, a liquid crystal dispersion, or a ceramic paste.

19. A method of mixing, refining, dispersing and/or homogenizing a material in a three roll mill having a feed roll, a center roll, and a discharge roll, comprising the steps:

a) feeding a material to be treated into a feed zone of the three roll mill, b) grinding, milling, mixing, refining, dispersing and/or homogenizing the material in a first treatment step between the feed roll and the center roll and in a second treatment step between the center roll and the discharge roll,

and characterized by the step

c) scraping off treated material remaining on the discharge roll after step b) using a discharge blade, wherein said discharge blade has a top side, a bottom side and an edge section comprising a back side facing and/or contacting the roll and a front side adapted for collecting scraped off material, and wherein the working edge of the blade is formed by an angle between said back side and said front side, and said angle is in the range of 50-130°, wherein the front side of the edge section has a width of more than 100 μm, and wherein the discharge blade comprises a wear resistant deposit covering at least a working edge thereof adapted for contact with the discharge roll.

20. Method according to claim 19, wherein the three roll mill is further defined as in any one of claims 1 -13.

21. Method according to any one of claims 19-20, wherein the material to be treated is further defined as in any one of claims 16-18.

22. A ground, milled, mixed, refined, dispersed and/or homogenized material obtainable by the method according to any one of claims 19-21.

23. A discharge blade for scraping off finished product from a discharge roll in a three roll mill, said blade having top side, a bottom side and an edge section comprising a back side adapted for contact with the discharge roll and a front side adapted for collecting scraped off product, wherein the working edge of the blade is formed by an angle between said back side and said front side in the range of 70-110°, and no two adjacent sides of an edge section of the blade form an angle of 70° or less, characterized in that the discharge blade comprises a wear resistant cermet covering at least a working edge thereof.

24. A discharge blade according to claim 23, further defined as in any one of claims 2-13.