WO1992018248A1

WO1992018248A1 - Milling process and apparatus

Info

Publication number: WO1992018248A1
Application number: PCT/GB1992/000679
Authority: WO
Inventors: Terrance John Coles; Linda Patricia Peet
Original assignee: Terrance John Coles; Linda Patricia Peet
Priority date: 1991-04-18
Filing date: 1992-04-13
Publication date: 1992-10-29
Also published as: JPH04334559A; KR920019420A; JPH04334560A; AU1582992A; NZ237880A

Abstract

A mill (10) for grain or other particulate matter is provided with a grinding chamber (12a) which has a stationary grindstone (23) forming a cylindrical wall of the grinding chamber, and rotating fan blades (20) in the chamber to blow grain against the grindstone (23) and around the chamber (12a). By gravitational separation, smaller particles rise upwards in the chamber, while larger, heavier particles sink lower. A plate (19) creates back pressure in the chamber (12a), and allows only a thin stream of fine grain particles to escape from the top of the grinding chamber into a chamber (12b) above. In this chamber, fan blades (21) create a light draft from the edges of the chamber towards the centre, which pulls the finest particles towards a central exit (29), while larger particles are held away from the centre by centrifugal force. Above the central exit (29), a blower (30) blows particles from the exit (29) out an outlet (31) for collection.

Description

Title: MILLING PROCESS AND APPARATUS

FIELD OF THE INVENTION

The present invention relates to milling apparatus and processes for producing powders, including wholemeal flour.

PRIOR ART

A wide variety of machines and methods have been used for grinding granular substances such as rocks, grain and chemical products into powders. Many use a centrifugal impact system, as shown in Japanese Laid Open Patent Application 1987 (Sho-62) No 117642 : Bryan A. Bartley (equivalent to US Patent No. 4,921,173). In that specification, apparatus is described which flings granular material outwardly from a central rotor, to collide with more material inside a surrounding cylinder. These collisions cause the granular material to break into smaller particles. Another system commonly used is shown in US Patent No. 4,747,550 (1988) : Gunther Jackering. In that specification, apparatus is described which includes a milling chamber with a large number of rotating plates and vanes mounted in it. Granular material is blown through the chamber on an air current, and turbulence created by the plates and vanes causes the granules to collide with other granules, and with the plates, vanes and walls of the chamber. These collisions cause the granular material to break into smaller particles.

PROBLEM TO BE SOLVED

A problem found with prior art apparatus of the type described above, has been that the collisions caused by the apparatus tend to break the granules unevenly, into a number of particles of different sizes and weights. The product therefore comprises a mixture of particles of different sizes, which must then be sifted, so that smaller particles can be collected, and larger particles can be put through the mill again. Sifting apparatus is prone to clogging, particularly when used on wet particles, and still allows a wide range of particle sizes through, beneath a limit set by screen size.

OBJECT

It is an object of the present invention to go at least partway towards providing novel or improved apparatus and/or processes for producing powders, or at least to provide the public with a useful choice.

STATEMENT OF INVENTION

The present invention can provide apparatus which allows particles to be graded during the grinding process, so that particles are abraded for different periods of time, depending at least in part on their size or weight

In one aspect the invention provides apparatus for reducing material from granules to a powder, including a chamber which is circular in cross-section about a central axis, having a curved side wall and two ends, the chamber comprising first and second sections in series along the axis; one or more inlets for air or other gas into the first section at or near a first end of the chamber; one or more inlets for granular material into the first section at or near the first end of the chamber; a wall at the second end of the chamber extending inwardly towards the central axis from the curved side wall; and an arrangement of parts to retard air or gas flow in an axial direction from the first section to the second section; characterised in that: first impellor means is provided to move air or other gas and the material around the first section and outwardly against the curved side wall; second impellor means is provided to move air or other gas and the material around the second section of the chamber, and inwardly towards the central axis; an outlet is provided through the wall at the second end of the chamber at or near the central axis, positioned inwardly away from the curved side wall; and an inside surface of the curved side wall is at least in the first section of the chamber comprised of a grindstone having a substantially smooth, abrasive inner surface.

In another aspect the invention provides a method for grinding a granular material to a powder, comprising the step of feeding a stream of the granular material and air or other gas into a mill, characterised in that the mill comprises a chamber which is circular in cross-section about an elongate central axis, having a curved side wall and two ends, the chamber comprising first and second sections in series along the axis, having an arrangement of parts to retard air or gas flow in an axial direction from the first section to the second section, an end wall at an end of the chamber adjacent the second section and an outlet through the end wall positioned inwardly away from the curved side wall, and the curved side wall having an abrasive inner surface in at least the first section; and the method further comprises the steps of blowing the granular material and air or other gas around the first section of the chamber and outwardly against the abrasive inner surface of the curved side wall, so that the granular material is abraded; and blowing air or other gas around the second section of the chamber and inwardly towards the outlet through the end wall, so that abraded material passing from the first section to the second section can be separated, by heavier particles being held outwardly towards the curved side wall by centrifugal force, and lighter particles being blown inwardly towards the outlet by the inward flow of air or other gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a description of one preferred form of the present invention, given by way of example only, with reference to the accompanying drawings in which:

Figure 1: shows preferred apparatus of the present invention in side view and cross- section.

Figure 2: shows the apparatus of Figure 1 in plan view and partial section.

Figure 3: schematically illustrates the flow of grain and flour though the apparatus of Figure 1, in the preferred process of the present invention.

Figure 4: shows possible modifications to the apparatus of Figures 1 and 2.

Figure 5: shows further possible modifications to the apparatus of Figures 1 and 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred form of the present invention provides means for grinding foodstuffs such as legumes, seeds and grain to a consistent fine grade powder, and particularly for producing flour from whole grain such as wheat or rice, comprising a mill in which the grain or other matter is blown onto and around a cylindrical stationary grindstone by a central fan arrangement. The ground particles and powder rise up the cylindrical grindstone as they becomes finer and lighter, and are further separated and collected with blowing and channelling means at the top.

As illustrated in the drawings, the apparatus of the present invention comprises a mill 10 for grain, including an inlet channel 11, a cylindrical grinding chamber 12 divided into a lower part 12a and an upper part 12b, and a collection chamber 13. The mill as illustrated may be relatively small, the grinding chamber 12 being approximately 50cm in diameter, and is powered by a 7 horsepower electric motor 14. The general flow of granules and powder through the mill during processing is upward, the grain being gravity-fed through the inlet 11 to the lower grinding chamber 12a, and rising upward from there to the collection point

The inlet 11 comprises a hopper 15 and tube 16, and may be stationary. The grain falls through the tube 16 to a plate 17 in the lower grinding chamber 12a, which is connected to the motor 14 by a shaft 18, and in normal use is spinning. Mounted over the plate 17 is a further plate 19 which divides the lower chamber 12a from the upper chamber 12b, on which are mounted a number of fan blades 20 and 21. Preferably six or more large, curved blades 20 are mounted on the underside of the plate 19, and twelve of more smaller flat blades 21 are mounted on top. More blades are preferred in larger apparatus. The shape and configuration of the blades is best seen in Figures 1 and 2. A collar 22, rotatable with the plates 17 and 19, extends up from the plate 19 around the tube 16, and further fan blades 30 are mounted on this, as described hereafter.

The cylindrical wall of the grinding chamber 12 is comprised of a single annular grindstone 23, clamped between upper and lower plates 24 and 25 by bolts 26. A metal shield 26a may also be provided around the grindstone 23, to protect it, and prevent fragments from flying outward if the grindstone is ever cracked or broken. The grindstone 23 is fixed in place, and does not rotate. An air intake 27 is provided in the center of the lower plate 25, under the plate 17, and a further port 28 is provided at the base of the lower chamber 12a, through which solid detritus may be expelled in use.

While the chamber 12 is illustrated as a cylinder, it might alternatively be made slightly conical, or otherwise shaped to alter the resulting product while still remaining circular in horizontal section. In particular, the grindstone 23' may be concave as shown in Figure 5, with the fan blades 20' and 21' shaped to match.

From the point of view shown in Figure 2, the fan blades 20, 21 and 30 spin clockwise in use. As shown in Figure 2, the lower fan blades 20 extend radially outwards from the centre, and curve back at the ends. When spinning, they create a clockwise, outward air flow from the central inlet 11 out to the grindstone 23. As shown in Figure 4, the blades 20' may have a straight trailing wall 40 covering the pocket 41 behind the curved leading edge, to prevent material from being caught up in the pocket 41. The blades 20 might also extend horizontally forwards at the bottom to provide a scoop, which will catch granules or particles which fail to be entrained in the general air flow when first coming off the plate 17.

A clearance of approximately 10mm is provided between the ends of the blades 20 and the inner surface of the grindstone 23. In use, grain landing on the plate 17 from the inlet 11 is carried outward with the rush of air created by the fan blades 20. It is propelled onto the inner surface of the grindstone 23 and around the chamber 12 at speed, grinding and wearing away against the grindstone 23 as it does so.

As the grinding action continues, the particles naturally begin to separate, with larger, heavier particles sinking to the bottom of the chamber 12, and smaller, lighter particles rising upwards in the chamber 12. Eventually, particles become light enough to rise through the small gap 19a between the grindstone 23 and the periphery of the plate 19.

The gap 19a is approximately 10mm wide, and is narrow enough to restrict airflow from the lower chamber 12a to the chamber 12b above, creating back-pressure in the chamber 12a and reduced pressure in the chamber 12b.

In the chamber 12b the blades 21 are angled forward, as shown in Figure 2, rather than backward like the blades 20 in the chamber 12a below. Consequently, the air flow is clockwise and inwards, towards the centre, rather than outwards as in the chamber below.

This inward airflow is relatively weak however, because of the reduced air pressure in the chamber 12b, and because the spinning action creates an outward centrifugal force, contrary to the inward airflow. Lighter particles are less strongly affected by the centrifugal force, and tend to be entrained by the inward airflow. Heavier particles are more strongly affected by the centrifugal force, and less affected by the weak airflow, and therefore tend to remain at the periphery of the chamber 12b.

A secondary separation process occurs among the fine particles-^*which rise into the chamber 12b. The most fine particles are drawn into the centre of the chamber and out through an aperture 29, and the less fine particles remain at tfae perimeter of the chamber 12b and are further abraded against the grindstone 23. The size of the aperture 29 influences the amount of time for which particles remain in the chamber 12, and the fineness of the final product. The aperture 29 may be adjusted to suit a particular purpose by removing the annular plate 35 between the chamber 12 and the collection chamber 13 above it, and replacing it with another plate having a central aperture of different dimensions. Alternatively, an iris shutter 35' as schematically shown in Figure 5 or other means could be used to provide a variable aperture 29, which can be adjusted without opening the mill, and can be operated even while the mill is in use, to adjust the milling speed and quality.

Above the aperture 29, the collection chamber 13 has two or more fan blades 30, which blow the particles out of the apparatus through an outlet 31. The chamber 13 and outlet 31 have a rounded "snailshell" form in plan view (as shown in Figure 2) to assist in providing a smooth flow of air and flour out of the apparatus, and to avoid a buildup of compacted flour on the chamber walls. An air inlet 32, preferably adjustable, is provided in the top of the chamber 13, to assist the smooth flow of powder from the chamber 13 into the outlet channel and reduce protracted circulation around the chamber 13, by increasing the quantity, pressure and flow rate of air in the chamber 13, compared to the chamber 12b. As in the lower chamber 12a, air flow and centrifugal force in the chamber 13 work together to throw the particles outward. The collection chamber 13 is preferably constructed as a separate side wall 33 clamped between an upper plate 34 and a lower plate, the lower plate being the top plate 24 of the grinding chamber. The floor of the collection chamber comprises a separate annular plate 35 held in place with bolts 36 as shown. The rate of throughput and the grade of particles produced by the mill can be adjusted for different types of grain and for different purposes, by unbolting the plate 35 and replacing it with a plate having a different size of aperture 29. If the aperture 29 is smaller, particles must be ground more finely to escape out of the chamber 12b, and remain longer in the mill

The flour passing out through the outlet 31 may be collected and packaged by any known means, bearing in mind that it is carried out in an airstream, rather than pouring out fluidly. Grain or other matter which (for any reason) passes into the grinding chamber but can not be ground to a sufficiently fine, light powder, will eventually be carried downward by gravity and pass out through the outlet port 28 at the bottom of the chamber, or will simply remain in the grinding chamber 12, unable to pass the plate 19 or to be lifted out through the aperture 29 into the collection chamber 13. By this means, the flour coming out through the final outlet 31 is automatically and consistently graded to a given level of fineness and lightness. Those parts of the grain which are heavier or more difficult to reduce to a powder will automatically be retained longer in the grinding chamber than the lighter elements. Furthermore, the relatively high speed, low pressure grinding possible with the use of a single grindstone and air pressure is particularly suitable for grinding the bran or outer coating of the grain, so that this can be included along with the germ and endosperm. By this means, flour of consistently fine grade containing all parts of the grain can be produced, and this not only provides nutritional advantages but also increases the overall volume of useful flour produced from a quantity of grain by approximately 20 to 30%. There is increased moisture loss from the ground product as a result of being blown through the grinding apparatus on an air current, and this means that the weight of the flour is less than an equivalent volume of flour produced by a conventional mill.

There is a high concentration of fatty acids in the germ of the grain, and consequently the end product may need to be chilled or refrigerated to prevent these from becoming rancid. The food value of the flour is however significantly increased by retaining this part of the grain, without altering the texture or baking properties of the flour to any substantial extent although more water may be required in working the flour than with flours produced by conventional mills.

The grade of flour produced can be altered by manipulation of the air inlet vents, alteration of fan sizes and configurations, or by altering the dimensions of the apparatus - it will be appreciated that if the height of the grinding chamber is greater, the flour must be ground to a finer grade for particles to be light enough to be carried up and out

Such grinding will generally take more time, and consequently a balance should be reached between production quality and quantity over time. The embodiment illustrated above has a grinding chamber height of approximately 20cm, and may have an output of approximately 20kg per hour, with harder grains such as rice being quicker than soft grains. The particle size is found to be more consistent than flour produced by conventional mills, and may be of smaller size than the average for white flour. Using wheat, the mill of the present invention can produce flour with 95% of particles below 50 microns in diameter, and 75% in the range between 30 microns and 20 microns in diameter. By contrast, white flour produced in a conventional mill might have only 30% of particles less than 50 microns in diameter, and not more than 40% of particles within any single 10 micron range. Production rates may be altered by changing the grinding chamber diameter. This may range from 10cm in a domestic model, through to 50cm or more in commercial models.

An advantage of the preferred embodiment illustrated above may be reduced wear and fatigue in the mill apparatus. It will be appreciated that wear on the grindstone is minimal, because no moving parts of the mill are ever in contact with i the grain being blown across its abrasive surface rather than being crushed or rolled against it. Furthermore, the grain is reduced to a powder by scraping or rubbing, rather than by impact as in centrifugal or turbulent mills. Hard particles such as stones accidentally put into the mill tend to drop out fairly quickly, and do not scour the abrasive surface, or strike forceably against it. Similarly, the fans are not in contact with other parts of the apparatus, and are not subjected to any notable wear or strain.

Although a stone or stone composite grindstone 23 may be used with apparatus of the present invention, it may be found preferable to use a ceramic grindstone 23. Such materials can have a disadvantage when used with conventional mills in that they may wear relatively quickly. The reduced wear on the grinding surface facilitated by the apparatus and processes of the present invention may reduce this disadvantage to the extent that such materials are significantly more preferable than others conventionally used.

The preferred ceramic composition comprises a clay mixture including silicon carbide (carborundum) particles, as may be used in the production of refractory bricks, crucibles or the like. While this composition is used in refractory applications because of its resistance to cracking or crumbling at exceedingly high temperatures, the silicon carbide in the present application acts as an abrasive. The proportion of silicon carbide to clay used in the composition can be varied to alter the hardness and texture of the abrasive surface, and other materials may also be included in the composition to vary the abrasive surface. The composition preferred at present is approximately 75% silicon carbide to 25% ball clay, fired at 1300". The clay component may however comprise anything more than 10% of the total composition, depending on the strength and abrasiveness required. Preferably the clay includes a relatively high proportion of silicon dioxide. Other types of clay might also be used. The grindstone 23 is preferably formed by slip casting in a mould, such that the ceramic is fired having a shape specifically suited to the mill apparatus for which it is intended. It may alternatively be pressed or otherwise formed, and larger grindstones might be manufactured as a number of interlocking segments, or bricks. These may be held together by interlocking clips, a form of cement, or by a strap or casing around the outside.

An advantage of a ceramic grindstone 23 over stone or stone resin composites is the rapid dissipation of heat through the ceramic material. This may be further enhanced by the moulded shape of the grindstone 23, which can include vanes projecting from the back or sides of the stone, behind or away from the abrasive surface, to radiate heat rapidly to the surrounding air. As shown in Figure 5, the grindstone 23' may be enclosed in a water jacket 43, through which water may be circulated to an external heat exchanger if necessary.

Preferably the overall thickness of the grindstone 23 between the abrasive surface and surfaces in contact with water or air is small enough that the heat at the abrasive surface is kept below a level which will cause oxidation or denaturing of the oils and other substances in the germ of grain, cause damage to the starch in the endosperm, or similarly damage other foodstuffs. Heat can also bring moisture out of the grain, to an extent that may cause blockage in the apparatus. In a preferred embodiment of the present invention it is envisaged that this distance may be approximately two or three centimetres, although it will be appreciated that this will vary considerably depending on the form of the mill, the usual speed and period of operation and the product intended to be ground. A compromise may be necessary in some cases to reach a workable balance between heat radiation and structural strength. Similarly, silicon carbide tends to store heat rather than transmitting it, and a balance should be met in the ceramic composition between the abrasive benefits of the silicon carbide particles and the heat transmission properties of the ceramic substrate. It should be noted that this arrangement can work equally well for heating the product during the milling process, if desired, with heat passing rapidly through the grindstone 23 into the chamber 12.

INDUSTRIAL APPLICABILITY

The illustrated embodiment is intended for the commercial production of flour from grain, apparatus of the present invention may also be used for reducing a variety of other substances to a powder, whether in a commercial or domestic situation. In particular, mills of the type illustrated in this example may be used for grinding legumes, seeds, and other plant or animal products, although some modifications to dimensions or details of form may be made to suit specific applications. The mill might also be used in the production of paint powders, pharmaceutical or other chemical powders, and in grinding powders to a more fine and/or consistent grade. The mill of this invention is particularly suited to the grinding of wheat to produce wholemeal flour to a consistency and fineness that has not been possible with prior art grinding or milling processes or equipment

VARIATIONS

A variety of other changes and modifications might be made to the above example within the scope of the present invention, which may be characterised by the following claims.

Claims

1. Apparatus for reducing material from granules to a powder, including a chamber which is circular in cross-section about a central axis, having a curved side wall and two ends, the chamber comprising first and second sections in series along the axis; one or more inlets for air or other gas into the first section at or near a first end of the chamber; one or more inlets for granular material into the first section at or near the first end of the chamber; a wall at the second end of the chamber extending inwardly towards the central axis from the curved side wall; and an arrangement of parts to retard air or gas flow in an axial direction from the first section to the second section; characterised in that: first impellor means is provided to move air or other gas and the material around the first section and outwardly against the curved side wall; second impellor means is provided to move air or other gas and the material around the second section of the chamber, and inwardly towards the central axis; an outlet is provided through the wall at the second end of the chamber at or near the central axis, positioned inwardly away from the curved side wall; and an inside surface of the curved side wall is at least in the first section of the chamber comprised of a grindstone having a substantially smooth, abrasive inner surface.

2. Apparatus as claimed in claim 1, characterised in that the central axis is substantially vertical and the chamber is circular in horizontal section; the first section is a lower section, the second section is an upper section, and the wall at the second end of the chamber comprised a lid.

3. Apparatus as claimed in claim 1, characterised in that the arrangement of parts to retard air or gas flow in an axial direction from the first section to the second section comprises a plate between the first, lower section and the second, upper section, with a narrow gap provided between an outer edge of the plate and the curved side wall, through which a restricted flow of air or gas can pass from the first section to the second section.

4. Apparatus as claimed in claim 3, characterised in that the first impellor means comprises a radial fan in the lower section, rotatable about the central axis of the chamber, with a plurality of blades extending generally radially outwards but inclined backwards relative to the direction of rotation towards the outer end, to push air or other gas around the lower section and outwardly against the abrasive inner surface.

5. Apparatus as claimed in claim 4, characterised in that the second impellor means comprises a radial fan in the upper section rotatable about the central axis of the chamber, with a plurality of blades extending generally radially outwards, inclined forwards relative to the direction of rotation towards the outer end, to push air or other gas around the upper section and inwardly towards the outlet in the lid.

6. Apparatus as claimed in any one of claims 1 to 5, characterised in that a second chamber is included, wherein the outlet through the wall at the second end of the first chamber opens into the second chamber; and the second chamber includes a collection duct and impellor means to draw air or other gas through the outlet into the second chamber, and move air or other gas out of the second chamber along the collection duct

7. Apparatus as claimed in claim 6, characterised in that there are one or more further inlets for air or other gas, opening into the second chamber.

8. Apparatus as claimed in any one of claims 1 to 7 characterised in that the grindstone is ceramic, formed from a composition including silicon carbide.

9. Apparatus as claimed in any one of claims 1 to 7 characterised in that the grindstone is formed from stone.

10. A method for grinding a granular material to a powder, comprising the step of feeding a stream of the granular material and air or other gas into a mill, characterised in that the mill comprises a chamber which is circular in cross- section about an elongate central axis, having a curved side wall and two ends, the chamber comprising first and second sections in series along the axis, having an arrangement of parts to retard air or gas flow in an axial direction from the first section to the second section, an end wall at an end of the chamber adjacent the second section and an outlet through the end wall positioned inwardly away from the curved side wall, and the curved side wall having an abrasive inner surface in at least the first section; and the method further comprises the steps of blowing the granular material and air or other gas around the first section of the chamber and outwardly against the abrasive inner surface of the curved side wall, so that the granular material is abraded; and blowing air or other gas around the second section of the chamber and inwardly towards the outlet through the end wall, so that abraded material passing from the first section to the second section can be separated, by heavier particles being held outwardly towards the curved side wall by centrifugal force, and lighter particles being blown inwardly towards the outlet by the inward flow of air or other gas.

11. A method as claimed in claim 10, characterised in that the mill is arranged so that the central axis is substantially vertical, the chamber is circular in horizontal section, the first section is a lower section, the second section is an upper section, the end wall at an end of the chamber adjacent the second section comprises a lid, and the abrasive inner surface of the curved side wall comprises a grindstone having a substantially smooth abrasive inner surface, so that in use the granular material can be abraded without being deflected away from the curved wall; the method further including the step of maintaining a smooth continuous flow of material around and against the grindstone, at least until partial stratification occurs, with heavier material generally sinking towards the bottom of the first, lower section, and lighter material generally rising towards the top of the first, lower section; so that the abraded material passing into the second, upper section of the cylindrical chamber comprises generally lighter material rather than heavier material.

12. A method as claimed in claim 11, characterised in that greater pressure of air or other gas is maintained in the first, lower section of the chamber than in the second, upper section, so that the outward pressure of air or other gas in the first, lower section is greater than the inward pressure of air or other gas in the second, upper section.

13. A method as claimed in claim 12, characterised in that the outlet from the upper section of the cylindrical chamber leads into a further chamber; the method further including the steps of providing further air or other gas to the further chamber and creating an flow of air or other gas from the further chamber to powder collecting means, so that abraded material which passes through the outlet is blown to the powder collecting means.

14. A method for grinding a granular material to a powder substantially as hereinbefore described with reference to or as illustrated in any of the accompanying drawings.

15. Apparatus for reducing material from granules to a powder substantially as hereinbefore described with reference to or as illustrated in any of the accompanying drawings.