EP0925114B1

EP0925114B1 - Energy efficient centrifugal grinder

Info

Publication number: EP0925114B1
Application number: EP96926296A
Authority: EP
Inventors: Rajendra P. Gupta; Grant W. Wood
Original assignee: Prosoya Inc
Current assignee: Prosoya Inc
Priority date: 1996-08-12
Filing date: 1996-08-12
Publication date: 2001-06-27
Anticipated expiration: 2016-08-12
Also published as: DK0925114T3; EP0925114A1; CA2261665C; WO1998006497A1; DE69613596D1; ES2158331T3; BR9612694A; DE69613596T2; JP2000516134A; CA2261665A1

Description

The present invention relates to a centrifugal grinder for grinding solids in liquids into a resulting solid liquid slurry, of the type comprising a housing having a circular screen wall dividing the housing into a first chamber and a second chamber.
It is often required to disintegrate solids in liquids without incorporating air in certain processes for air could induce undesirable oxidation of the resulting solid-liquid slurry and/or induce foaming. An example is the disintegration of soybeans in water in order to make soymilk as described in US 4 915 972.
The disintegration of solids in liquids if often achieved by high speed rotating hammermills. However, prior art hammermills create extreme vortexes in liquids which induces the suction of air in the comminuting region. A hammermill also requires a very high starting torque if the solids are already in the mill when it is started. The drive motor has to be sized to provide the high starting torque, which is expensive and inefficient for its normal operation. In addition, it yields solid particles with large variations in size which often requires two or more mills in tandem to get a reasonable grind of the solids. Alternatively, the slurry has to be re-circulated many times through the same hammermill. Either of these known approaches results in increased capital cost and reduced energy efficiency. US 2 738 930 and US 2 738 931 disclosed dispersion apparatus in which a preliminary comminuting system is followed by a plurality of dispersion systems. US 2 519 198 describes coffee grinding or comminuting machines having a plurality of rotating elements. US 3 993 791 discloses continuous lautering apparatus in which a series of continuously decanting centrifuges and an equal number of reslurry stations are provided.
FR 2 171 671 discloses a grinder in which hammers are provided on rotatable discs at their perimeters. FR 2 112 030 also discloses a grinder in which cutters are provided on one side of a rotatable disc and hammers are on the other side of said disc. The grinders disclosed in the aforesaid patents have a screened wall or walls to define first and second chambers. Both grinders, however, could create a vortex in liquids and require high starting torques.
According to the present invention, there is provided a centrifugal grinder of the aforesaid type, characterised by a first rotatable circular disc in the first chamber substantially conforming to the size of the first chamber, dividing the first chamber into a first region and a second region and being rotatable about a central axis; cutters located in the first region and attached to a first side of the first rotatable disc near but inside its perimeter; hammers located in the second region and attached to a second side of the first rotatable disc near but inside its perimeter; an inlet connection to the first chamber for introducing the solids in liquids to the first region on the first side of the disc; an outlet connecting to the second chamber for discharging the solid-liquid therefrom; and a motor mechanically connected to the first rotatable disc so as to drive said first rotatable disc.
The present invention eliminates deficiencies of a hammermill and provides a highly cost effective method of grinding for general purpose use, such as dry grinding of grains, spices, minerals and other food and non-food products. It is also suitable for grinding solids in liquids such as ordinary, choked, flooded and airless grinding. This is achieved by locating the hammering elements only in the vicinity of the impacting surface rather than using the whole rotating element as a hammer. The starting and running torque requirement of the drive motor is greatly reduced and energy use efficiency is improved. The motor torque requirement and energy efficiency is further improved by dividing the milling regions into two or more sections. This division also results in good control over the particle size distribution of the grind and eliminates the need for multiple mills or multiple passes to achieve good results.
The invention will now be described with reference to the accompanying drawings, of which;
Figure 1 is a sectional view of the grinder according to one embodiment of the invention; and
Figures 2 to 4 are sectional views of multi-stage grinders according to several embodiments of the invention.
A schematic representation of a grinder according to one embodiment of the present invention is shown in Figure 1. The grinder includes a housing (10) which is provided with an inlet (12) and an outlet (14). The housing is cylindrical in shape in cross section and is divided into two chambers (16), (18), by a screened wall (20). A disc plate (22) is in the first chamber (16) and is attached on an axle (24) which is in turn adapted to be rotated at high speed by a motor (26). The disc plate (22) has two or more cutter elements (28) attached to its upper side and two or more hammer elements (30) attached to its lower side. The cutter and hammer elements are attached to the disc plate (22) near its circumference. In a further embodiment, the disc plate is provided with a few tapered holes (32) which are too small for the unground solids to pass through. These holes improve circulation of ground product below the plate. They are particularly beneficial when solids are to be ground in the presence of a liquid.
Solids or solids in liquids are introduced into the grinder through the inlet (12). When the disc plate (22) is rotated, the solids are centrifugally thrown out towards the screen wall in the path of the spinning cutters (28) which chop down the solids into small pieces. The small solid pieces then enter the lower region where spinning hammers (30) grind them to a particulate size which is a function of the holes size in screen. The ground solids suspended in the liquid are removed through outlet (14).
In this embodiment, a torque requirement and vortex formation are improved over prior art grinders, that consequently improves energy efficiency. These improvements are results from the following features. Instead of one or more vertical bars as hammers, the invention uses the cutter and hammer elements attached on a horizontal circular plate. The milling chamber is divided into two regions.
The improvement can be illustrated as follows. If the thickness of a cutter element is t1 and the height is h1, the thickness of a hammer element is t2 and the height is h2, and the diameter of the disc plate is D, then the total volume V1 swept by the cutter and hammer elements is: V1 = π D(t1*h1+t2*h2) [π=3.14159] Prior art hammer mills have solid metal bars as hammers. Assuming h1+h2 as their height and D as their length, the volume V2 swept by them is: V2 = π D*D(h1+h2)/4 Assuming for simplicity the thickness of the cutter and hammer elements to be the same, t1 = t2 = t (say), and dividing V1 by V2: V1/V2 = 4. t/D. Typically, t = 1/8" when D = 4", giving V1/V2 = 1/8. This ratio then roughly defines the ratio of the strength of the vortex for the two mills without the disc plate. However, the presence of the disc plate greatly reduces the swept volume available for vortex formation for the present invention - the inlet can only see the swept volume V1' above the disc plate. Since h1 is typically 1/3 of h2, V1' = π D*t*h1; or V1'/V2 = t/D, Which has a value 1/32 for the typical dimensions considered here. The grinder of the invention therefore has a very small vortex and as a result extremely small suction for air to get into it. If this small vortex is still problematic, it could be further reduced by inserting a vortex-cross in the inlet of the grinder.
Assuming s to be the specific gravity of the solid-liquid swept by the grinder and r to be the distance of an element thickness dr from the axis, the ratio of the torques (T1 for the present invention and T2 for the prior art hammermill) can be easily determined as follows: T1 = K*mass*distance2 = {2 π R(h1+h2)t*s) R2 [R=D/2] T2 = K*integral(0 to R)[{2 π r(h1+h2)dr*s} r2] =2 π (h1+h2) s*R4/4
T1/T2 = 8 t/D = 1/4 (for t = 1/8" and D = 4" as above). Here K is a proportionality constant.
This is a marked decrease in the torque requirement. The preceding analysis can qualitatively be envisioned from Figure 1. When rotated axially, the area swept by hammer and cutter elements is obviously a fraction of the area swept by the rectangle formed by connecting the elements as shown by the dotted lines 32. It would take a lot more torque to rotate the latter arrangement than the former when the space inside the screen is filled with solids. It is not difficult to see that even the running torque is lower, leading to improved energy efficiency.
Referring to Figures 2-4, three additional embodiments of the invention are also schematically shown.
In Figure 2, the grinder has a single impeller mounted on a drive shaft 50 of a motor and is surrounded by a fixed cylindrical screen 52. The impeller has multiple grinding stages separated by discs 54 and 56 concentric with the shaft. The grinding elements, like cutters and hammers, are symmetrically mounted on the discs. As the solid such as beans gets crushed or chopped to a certain size by one stage of the grinding, they progress to the next finer stage of grinding or chopping through the opening between the discs and the screen. The section of the screen surrounding the final stage of the grinder has perforations that allow the finely ground solids to exit the grinder. Although the grinder screen may be perforated everywhere, it is undesirable to do so for two reasons: a) the slurry flow between stages is enhanced if the water cannot flow out of the screen in the earlier stages of grinding, and b) it costs money to make perforations in any material, i.e., fewer the perforations, cheaper the screen. For some applications, it may be desirable to have openings in the top section of the screen as well to permit local circulation of the liquid and slurry in the grinder. In other embodiments, the discs are provided with holes which are too small for solids larger than a certain size to go through.
In Figures 3 and 4, multistage grinders are shown in which progressively larger discs are used for better grinding performance.
The grinder of the present invention produces a more uniform particle size distribution in the grind than the prior art and reduces energy and power requirement in grinding a material to desired fineness. The grinding elements last longer and are economical to manufacture. The grinder of the invention also produces higher yield and quality of the end product. It is adaptable to dry grinding of grains, spices, minerals, and other food and non-food products; and is suitable for ordinary, choked, flooded, and airless grinding.

Claims

A centrifugal grinder for grinding solids in liquids into a resulting solid-liquid slurry, comprising

a housing (10) having a circular screen wall (20) dividing the housing into a first chamber (16), and a second chamber (18); characterised by

a first rotatable circular disc (22) in the first chamber (16) substantially conforming to the size of the first chamber, dividing the first chamber into a first region and a second region and being rotatable about a central axis (24);

cutters (28) located in the first region and attached to a first side of the first rotatable disc (22) near but inside its perimeter;

hammers (30) located in the second region and attached to the second side of the first rotatable disc (22) near but inside its perimeter;

an inlet (12) connecting to the first chamber (16) for introducing the solids in liquids to the first region on the first side of the disc (22);

an outlet (14) connecting to the second chamber (18) for discharging the solid-liquid slurry therefrom; and

a motor (26) mechanically connected to the first rotatable disc (22) so as to drive said first rotatable disc.
A grinder according to claim 1, further comprising

a second rotatable circular disc (56) parallel with and adjacent to the second side of the first rotatable disc (22) and rotatable about said axis (50);

the second rotatable circular disc (56) substantially conforming to the size of the first chamber and having said hammers connected to its first side near but inside its perimeter; and

fine grinding hammers attached on its second side near the but inside its perimeter.
A centrifugal grinder according to claim 1 or claim 2, wherein the first rotatable circular disc (22) has holes (32) therein of which the size is smaller and that of unground solids.
A grinder according to claim 2, wherein the first and second rotatable discs have holes therein of which the sizes are smaller than that of the unground solids.
A grinder according to claim 4, wherein the screen wall is cylindrical in shape defining the first chamber therein and has screen holes.
A grinder according to claim 5, wherein the screen holes are only near the fine grinding hammers.
A grinder according to claim 5, wherein the second rotatable circular disc is larger in diameter than the first rotatable circular disc.
A grinder according to claim 4, wherein the screen wall is conical in shape defining the first chamber therein and has screen holes.
A grinder according to claim 8, wherein the second rotatable circular disc is larger in diameter than the first rotatable circular disc.
A grinder according to claim 8, wherein the cutters on the first rotatable disc, the hammers on the fist and second rotatable discs and the fine grinding hammers on the second rotatable disc are attached to their respective rotatable circular discs at angles to roughly conform to the conical shape on the first chamber.
A grinder according to claim 10, wherein the screen holes are only near the fine grinding hammers.