WO2018117849A1

WO2018117849A1 - Pellet mill

Info

Publication number: WO2018117849A1
Application number: PCT/NO2016/050265
Authority: WO
Inventors: Fredrik Bing BUCK
Original assignee: Buck Technology
Priority date: 2016-12-21
Filing date: 2016-12-21
Publication date: 2018-06-28

Abstract

Pellet mill comprising an annularly shaped rotary die (defining a ring wall having an internal annular surface (11B) and an outer circumferential surface (11A), said ring wall (13/11C) being provided with numerous through bores (12) defining ejector channels for compressed powdery raw material for the production of pellets. A pressure roller assembly (20) is arranged within the space confined by the internal annular die surface (11B), arranged adjacent to the internal die surface (11B). The respective pressure roller exhibits numerous elongate grooves (21) having closed ends and extending substantially along the rotational axis of the die (11), defining numerous elongate ridges (22) between adjacent grooves (21). The apex of the respective ridge (22) exhibits a width (W_R) viewed perpendicular to its longitudinal axis, the bore exhibits a diameter (D_B) and a bore inlet diameter (D_I), wherein (D_I > D_B ≥ W_R). The elongate grooves (21) may be slightly inclined with regard to the axis of rotation. The ridges exhibit a height (H_R) which is at least about 100 % larger than prior art ridges.

Description

Pellet mill

The present invention is related to a pellet mill as described in the preamble of claim 1. Background

There are numerous different types of additional machines or equipment with the aim of producing improved physical quality pellets from bulk material, such as double pelleting, expanders, and pressure roller adjustment assemblies. A common feature of machines like this is that the bulk material, e.g. powdery raw material, is processed or machined at high pressure before being pressed into bores in a die, thus obtaining an improved pellet quality. However, the equipment or machines is/are expensive, require additional energy and do not have any substantial potential for increase of capacity.

The present invention is related to a pellets mill comprising a rotatable circular die provided with radially extending bores extending from an inner rim of the die to an external rim of the die, and a pressure roller assembly arranged rotary within the internal space of the die. Each pressure roller is provided with numerous grooves extending substantially along the axis of rotation of the pressure roller/die. The die is driven in a rotating manner by a motor, whereas each roller in the roller assembly is arranged on bearings and is driven by frictional force from contact with a gear wheel operation with gears made of the material to be processed, e.g. feed, and located between the die and the respective pressure roller.

In a pelleting process, e.g. powdery feed is optionally treated in a conditioner unit and supplied with steam or other additives to improve the properties of the pellet product and the process it self. Then, the powdery feed is ejected into the internal space of the annular die and distributed evenly along the inner surface of the die. Supply rate, humidity content and uneven distribution of raw material have an impact upon the effectivity of the process and the lifetime of the components because of excessive wear. For example an excessive content of humidity will decrease the viscosity of the raw material, whereupon the respective pressure roller fails to roll over the bulk raw material and prevent the same from being compressed and inserted into the bores of the die. Uneven distribution of raw material along the inner surface of the die may result in a build-up of powdery raw material, in front of the nip between the pressure roller and die, thus may result in clogging and failure. Increased production rate may also result in clogging and failure, since the air located in the raw material in front of the nip only can escape in the front of the respective press roller, thus entraining powdery raw material in front of the nip back into the space ahead of the respective press roller. As a result there will be a build-up of powdery raw material that in the end may result in clogging and failure.

The prior art as suggested some solutions these problems. One approach has been to decrease the distance between the respective pressure roller to increase friction at the nip adjacent to the die. This will result in a metal-to-metal contact. However, the improvement is not going to last for long and the pressure roller and the die will become worn-out rapidly. The apertures into the bores in the die will become rolled down over time with a decreased aperture area as a result. In other words, metallic deformation will make the bore inlets narrower and narrower.

US patent 4,983,343 to Lund describes a pressure roller in a pellet mill of this type. In order to solve the air problem outlined above, Lund suggested including air draining channels in the surface of the respective pressure roller. In further detail, the axially extending grooves milled in the surface of the respective pressure roller have open ends, and one or more circumferential grooves are milled in the pressure mill surface. The open ends and the circumferential grooves assist in draining air entrapped in the grooves. However, this approach has several disadvantages. The circumferential groove(s) is/are not contributing in the pelletizing process. Moreover, the axially extending open-ended grooves do not contribute in the pelletizing process either since the air being drained from the grooves also will squeeze powdery raw material toward the open groove ends. As a result, only a limited part of available surface is being used in effective pelletizing process, e.g. 50%. From a constructional point of view, the dimensions of the pellet mill will have to be increased in order to obtain a desired production rate.

DE 40 10 936 Al by Muller is related to a pellet mill of the type described above. Contrary to the Lund pressure roller discussed above, the grooves of the Muller pressure roller exhibit closed ends, thus leaving an even surface at the rims of the pressure roller. In order to increase friction, the surfaces at the rims of the pressure roller are provided with numerous closed recesses or holes. The purpose of these recesses or holes is to accommodate powdery raw material to establish a contact between raw material in the recesses and the raw material upon the inner annular surface of the die to reduce squeezing or slipping of raw material out of the recesses, instead of metal-to-bulk material contact, thus reducing wear of the mill components.

US 2006/0027174 Al discloses a pelletized feed supplement composition to help prevent dehydration during extreme environmental conditions. An improved pellet mill is also described, but not claimed. The improved pellet mill exhibits matrix channels with enlarged inlet and outlet to reduce friction between compressed bulk material therein. US 3,485,186 discloses a pellet mill with slots or grooves in the roller surface is provided with greater depth at the center of the roller and shallower depth in the outer positions thereof. This groove configuration is described to provide a smoother compression of bulk material and hence reduce increased wear of the rollers at the center thereof compared to its outer portions. Numerous groove shapes are suggested, such as curved, inclined and discrete, including open- ended grooves as well as grooves with closed ends.

Object

An object of the present invention is to provide a pellet mill of the type having a rotary annularly shaped die with radially extending bores, and a pressure roller assembly arranged in a rotary manner inside the die, as described above, that enable production of pellets having improved quality. Another object is to provide a pellet mill of this type that enables increased production rate for a given pellet quality, which can allow a total shutdown of the complete pellet production line and hence save energy. Another related object is to provide a pellet mill that reduces the risk of clogging and failure. Yet another object is to provide a pellet mill with reduced power demand. Another object of the invention is to enable use of raw materials having poorer pelletizing properties and at the same time obtain pellets having the same acceptable pellet quality as pellets produced from raw materials having acceptable pelletizing properties.

The invention

The objects above are achieved by a pellet mill in accordance with the invention as set forth in the characterizing part of patent claim 1. Additional advantageous features appear from the dependent claims.

Definitions

In the following, the terms "bulk material", "raw material" and "powder" have been used interchangeably to denote any discrete material that can be compressed in a press mill to produce products in the form of pellets.

The term "feed" has also been used. This material is only one out of numerous materials that may be pressed in a pellet mill according to the present invention and is meant to be included by the terms above.

The term "closed ends" as used herein in connection with the elongate grooves formed in the surface of pressure roller, is meant to include a groove which is terminated by a wall in the pressure roller surface at the respective short ends of the elongate groove, thus leaving a continuous uninterrupted surface at the respective rims of the pressure roller. The width of the uninterrupted surface of the pressure roller may be for example about 5 mm. In operation, the closed ends prevent compressed air with entrained bulk material, which may have a pressure of several tenths of bar or more, to escape from the grooves and away from the pressure roller.

Summary of invention

The pellet mill in accordance with the invention is related to a pellet mill of the type comprising a circular die defining a ring wall having an internal annular surface and an outer circumferential surface. Numerous bores are provided in the ring, defining ejector channels for compressed powdery raw material for the production of pellets. The bores preferably exhibit a conical inlet. The die is arranged to rotate by a rotary drive. A pressure roller assembly is arranged within the space confined by the internal annular die surface, consisting of two or more pressure rollers arranged close to the internal die surface and supported by bearings or similar in a rotary manner about the rotational axis of the die. Typically the assembly contains two or three separate pressure rollers. Each separate pressure roller exhibits elongate grooves extending substantially along the rotational axis of the pressure roller/die, defining numerous elongate ridges or teeth between the grooves. The apex of the respective ridge exhibits a width W_R viewed perpendicular to the axis of rotation, whereas the inlet of the respective bore in the die exhibits a diameter D|. The bore itself exhibits a diameter D_B. In accordance with the invention the grooves exhibit closed ends, and the ridge width W_R, viewed perpendicular to the longitudinal axis of the ridge is less than the diameter D| of the bore inlets, and at maximum equals the bore diameter D_B. Accordingly, the contact area of a tooth or ridge is not large enough to cover the bore inlet area, and as a result the teeth will not be able to press powdery material directly down into the bores. According to the invention, the height H_R of the respective ridges is substantially larger than the prior art ridge height, particularly at least about 100% larger. As an example, the ridge height H_R may be in the range from 6 to 12 mm, particularly 7.5-8 mm, particularly 7.8 mm. This height provides a layer of compressed bulk material of about 4 mm. As an example, the distance between adjacent teeth or ridges may be within the range from 8 to 10 mm, such as 7.5-8.5 mm, particularly about 8 mm. However, these figures are examples only and may vary by desired mill dimensions and the consistence of the bulk material to be processed, to have the best result from the invention. It should be noted that the volume between adjacent ridges or teeth should be sufficiently large to prevent bulk material from being left and captured between the ridges after the ridges have left the engagement zone involved in the compression.

The higher volume between the ridges, the thicker layer of compressed bulk material is left on the die. Thus, the inventive dimensioning of the ridges allows for an increased retention time of the powdery raw material and hence increased kneading and processing, producing pellets with better quality. Moreover, the inventive design prevents air from being return back to the space ahead of the respective pressure roller (inlet side). Instead, air is being entrained in the grooves and ejected at the opposite side (outlet side), leaving a layer of compressed powdery raw material distributed on the internal surface of the die. No powdery raw material is left in the ridges between the teeth in the pressure roll. This is described in further detail below.

The present invention provides a pellet mill that is able to produce pellets at a substantially increased rate compared to prior art pellet mills. This has been verified by experiments of the present inventor. Having a given pellet mill dimension, the present invention enables increased production rate and hence decreased specific energy consumption per weight unit pellet produced. On the other hand, having a fixed production rate, the present invention enables the use of a pellet mill having smaller dimensions compared to prior art pellet mills, thus decreasing production costs of the mill itself, transportation cost and space requirement at the pellet production site. Having a fixed production rate, the present invention enables a substantially increased production rate where the pellet mill production line can be shut down completely between productions. This saves electricity and provides more time for maintenance. It should be mentioned that the inventor of the present invention revealed that an idle running pellet mill production line consumed more electricity than electricity consumed during pelletization, which can provide huge savings in energy consumption.

In the following, the course of a part of the pelletizing process involving compression and ejection of compressed powdery raw material is described. Details about dimensioning of die, die bores, pressure roll and pressure roll grooves per se from a prior art view, has been omitted since it is assumed that a person having ordinary skill in the art will be able to design and construct pressure rollers having the present disclosure in hand, and end up with a pellet mill having the desired technical effect as outlined above:

Having powdery raw material supplied and distributed evenly along the inner surface of the die, the respective pressure rollers rotates with the die (the pressure rollers are not driven by any external drive means) and rolls over the powdery raw material. At the same time, air is entrained with the powder and then trapped inside the elongate grooves (having closed ends) between adjacent ridges or teeth. The powder is compressed and kneaded, together with the entrained air. The trapped air together with bulk material is being compressed to a maximum pressure when the groove/teeth are located a little ahead of their closest position to the co-rotating die. Here, the air pressure is increased to a level equal to or higher than the slip resistance of the compressed material inside the bores. This typically occurs at a point a little ahead or in front of the nip between the roller and die. At this stage, a ridge/groove section is formed by the compressed powder, in engagement with the correspondingly shaped pressure roller. This gear-like inter engagement between the compressed powder and pressure roller is in fact the driving force that brings the pressure roller to rotate. To the contrary, prior art pressure rollers are driven by friction between the roller and a thin layer of compressed bulk material deposited upon the inner die surface.

When the pressure within a groove overcomes the frictional resistance force between bulk material residing in a die bore and the die bore wall, the compressed bulk material starts to move rapidly in the bore since the frictional resistance of "resting" bulk material is higher than the frictional resistance of moving compressed bulk material. During this course, the die and pressure roller continue to move and the bulk material is pressed into the bores, whereupon the air volume within the grooves increases, thus pressing bulk material out of the grooves. At the end of this zone, the apex part of the teeth formed by the bulk material is partly disintegrated and distributed along the inner die surface level of bulk material. In this way, no bulk material is left within the groove leaving its engagement with bulk material and die, and all the entrained air is ejected at the outlet side of the engagement zone between die and pressure roller. The material to be processed obtains an increased retention time and hence kneading and processing, resulting in improved pellets quality. Without wish of being bound to any particular theory, the inventor believes that the reduced energy consumption observed is caused by delayed pressing or ejection of raw material into the die bores, starting at a later point in time, compared to prior art pellet mills. Air is not being forced back into the space at inlet side of the engagement zone, but is entrained with the bulk material, compressed and then expanded and ejected at the opposite, outlet, side of the engagement zone. The risk of raw material build-up, clogging and mill failure can be reduced substantially. The air still locked inside the grooves behind the nip at the "outlet" side of the compression zone contributes as a driving force to the rotation of the pressure roller.

In a preferred embodiment, the longitudinal axis of the grooves (substantially along the axis of rotation) is slightly inclined. With a pellet mill having a pressure roll assembly consisting of two pressure rolls, the grooves of the first pressure roll are slightly inclined in a first angle with the axis of rotation of the roll, e.g. about 1-5 degrees with the axis of rotation, whereas the grooves of the second pressure roll are slightly inclined in a second angle, resulting in a mutual inclination angle of the respective grooves of between 2 and 10 degrees. In a particularly preferred embodiment, the first and second angle is about 3 degrees, respectively. The effect of the inclination is that there is an engagement between die and pressure roller at all times, thus decreasing wear and preventing air from moving within the grooves in engagement. The compression proceeds in a smooth manner, contrary to grooves extending in parallel with the axis of rotation of the roller, which result in a compression at numerous bores at the same time, resulting in a more noisy operation. This may during contact between pressure rolls and die result in fatigue fracture in the pellet mill because the the pressure rolls appear with a multi-angular surface. There is also a risk that the pressure foils may proceed in their respective footprints and cause reduced kneading and poorer pellet quality. With a pellet mill having a pressure roll assembly consisting of three pressure rolls, the grooves of the first and second pressure rollers can be slightly inclined as described above, whereas the groove angle of the third pressure roller is different from the groove angle of the preceding roller with respect to the direction of operation, such as 1-2 degrees lesser or greater. It should be emphasized that the angle of inclination quantified above vary with the pressure roll diameter, pressure roll (or groove) axial extension and open distance between the top of adjacent ridges. The figures should for that reason only be interpreted as guidelines and not as limiting to the invention. Dimensioning of the groove inclination is expected to be within the reach of a person skilled in the art.

Drawings

In the following, a short description of the invention is provided with reference to drawings, in order to simplify the interpretation of the present invention. In the drawings: Fig. 1 illustrates a part of a prior art pellet mill, and

Fig. 2 is a cross-section through a part of a die and one pressure roll in accordance with the invention, taken perpendicular to the axis of rotation.

Fig. 1 has been included to clarify the type of pellet mill the present invention is related to. Figure 1 illustrates a die and pressure roller assembly indicated generally at reference numeral 10. An annularly ring-shaped die 11 exhibits an outer or external circumferential surface 11A and an inner or internal circumferential surface 11B, thus defining a die wall 11C. Numerous bores are provided throughout the die wall 11C, defining passages for ejection of compressed powdery material, such as feed, to be ejected at the external die surface 11A and cut by a knife or similar (not shown) to produce discrete pellets indicated at reference numeral 40.

Moreover, a first and second pressure roll 20 and 30 are arranged rotary within the space confined by the internal die wall 11B. The direction of rotation of the die and the pressure rolls are indicated by the arrows.

Fig. 2 shows a strongly schematic radial cross-section through a part of the die and one pressure roller. The die is provided with numerous through bores 12 where the solid part of the die wall is indicated at reference numeral 13. Here, compressed powdery material is indicated at reference numeral 50. The engagement section of the area between the pressure roller and inner die surface is represented by reference numerals 23A through 23G, discussed in further details below. At the inlet side of the engagement zone, a layer of fresh powdery material to be kneaded, compressed and finally ejected and cut into discrete pellets, is indicated at reference numeral 52. A "stationary" layer of powdery raw material is indicated at reference numeral 51A. A similar layer "stationary" is indicated at the opposite side of the engagement zone at the left hand side of the drawing by reference numeral 51B, deposited upon the inner surface of the die. As explained above and in further details below, the composition of powder layer 51B leaving the engagement zone is a mix of powder present in layer 51A entering the engagement zone and "fresh" powder introduced into the engagement zone in layer 52. The remaining part of the powder has been pressed into the respective bores 12 of the die. At reference numeral 23A, air has been entrained with the powder and trapped inside the groove. As the pressure roller and die are co-rotating, powder is being pressed into the groove and the air compartment compressed and its volume decreased. This is indicated at reference numerals 23B and 23C. At 23C the bulk material is about to be pressed down into the bores. At reference numeral 23E, the compression is substantially at its maximum and material starts to move rapidly into the bores 12. At reference numeral 23F the air has expanded and is assisting rotation of the rollers 20, 30. At reference numeral 23G, the tooth-shaped compressed powder starts to disintegrate and mix with air whereupon the powder is distributed along the layer 51 and the air released at the outlet side of the engagement zone at the left hand side of the drawing.

As can be seen from Fig. 2, the width of the ridge W_R is less than the diameter D| of the conical bore inlet, whereas the width W_R of the ridges at maximum equals the bore diameter D_B and the bore inlet diameter D| is greater than the bore diameter D_B Accordingly, the ridge top is not able to completely cover the inlet area of the bores 12 and is not effecting the pressing action. Instead, it is the compressed powder and the air that make the compressed powder to move outwards in the bores 12 and to the external surface 11A of the die 11 (Fig. 1). The bore diameter is indicated at D_B, whereas the ridge height is indicated at H_R. The direction of rotation of the roller and die are indicated by the reference signs l and R2, respectively.

Example The inventor of the present invention performed tests of the pellet mill according to the present invention in a full-scale production plant producing chicken feed. In a first run, pelletizing was performed with a prior art roll having a ridge height of about 3-4 mm and with raw materials having normal pelletizing properties. The resulting production rate was 8-9 tons pellets per hour. Then, in a second run pelletizing was performed with raw materials having poor pelletizing quality. In order to obtain satisfactory pellets quality, the production rate had to be reduced to 6 tons per hour. Then, in a third run the prior art rollers were replaced by rollers in accordance with the present invention having a ridge height of 7.8 mm. The resulting production rate was surprisingly 13 tons pellets per hour, limited by the feeding equipment, and the pellet quality was still better than in the second run with a production rate of 6 tons per hour. At production rates higher than this, the pellets started to exhibit chips on the surface, and the production rate was therefore limited to 13 tons pellets per hour. Accordingly, the present invention provided a production rate increase of more than 117 %.

Claims

1. Pellet mill comprising an annularly shaped rotary die (11) defining a ring wall (11C) having an internal annular surface (11B) and an outer circumferential surface (11A), said ring wall (11C) being provided with numerous through bores (12) defining ejector channels for compressed powdery raw material for the production of pellets, wherein a pressure roller assembly is arranged within the space confined by the internal annular die surface (11B), consisting of two or more rotary pressure rollers (20, 30) arranged adjacent to the internal die surface (11B), wherein the respective pressure roller exhibits numerous elongate grooves (21) extending substantially along the rotational axis of the die (11), defining numerous elongate ridges (22) between adjacent grooves (21), characterized in that the grooves (21) exhibit closed ends, and wherein the apex of the respective ridge (22) exhibits a width (W_R) viewed perpendicular to its longitudinal axis, which at maximum equals the bore diameter (D_B), wherein the bore inlet exhibits a diameter (D|) which is larger than the bore diameter (D_B), and that a height (H_R) of the respective ridges is at least about 100 % larger than the ridge height of prior art pressure rollers (20, 30).

2. The pellet mill of claim 1, characterized in that the pressure roller assembly comprises a first (20) and second (30) pressure roller, and that the grooves of the first pressure roller (20) are slightly inclined in a first direction with regard to the axis of rotation, and that the grooves of the second pressure roller (30) are slightly inclined in a second direction with regard to the axis of rotation, substantially opposite the first direction, resulting in a mutual angle of inclination angle between said first (20) and second (30) pressure roller.

3. The pellet mill of claim 2, characterized in that the mutual angle of inclination is between 2 and 10 degrees.

4. The pellet mill of claim 2 or 3, characterized in that the mutual angle of inclination is about 6 degrees.

5. The pellet mill of any one of claim 1 to 4, characterized in that the height (H_R) of the respective ridges is in the range from 6-12 mm, preferably 7.5 to 8 mm, particularly 7.8 mm.