NZ237879A - Apparatus for grinding granular material by moving an entraining gas around the inside of a circular chamber of moulded hardened ceramic composition - Google Patents

Description

?37g7<p) Patents Form # 5 NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION AFTER PROVISIONAL NO: 237879 DAXEI2:18 April 1991 TITLE: MILLSTONE We,: (1) Terrance John Coles (2) Linda Patricia Peet Address: (1) 6/42 Courtney Road, Tauranga, New Zealand (2) 6/42 Courtney Road, Tauranga, New Zealand Nationality: (1) New Zealand (2) United Kingdom hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: v " ,-p JWP FEE CODE -1050 237879 Abstract A mill is provided with a millstone which has consistent, predetermined grinding characteristics, and which allows accurate heat regulation at the grinding surface and in the product being ground.

A millstone (30) is moulded and tired from a ceramic composition including abrasive particles, preferably of silicon carbide. One surface (31) of the millstone is used as a grinding surface, and a second surface (32) is heated or cooled with air, water, or another gas or liquid (33), so that heat is transferred from the grinding surface (31) to the gas or liquid, or vice versa, through the millstone (30). «02CS2.492/to 237879 Field of the Invention The present invention relates to millstones, particularly industrial millstones.

Prior Art Millstones have hitherto been made from a variety of natural rocks, particularly volcanic lava or rhyolite, but also from some types of sandstone or limestone. These types of stone are selected for their rough surfaces and hardness, which make them particularly useful for grinding. Synthetic grindstones are also commonly produced as a composite of rock fragments or particles set in a resin or other base material. A millstone may be made up of a number of cut stone segments, as described for example in US Patent No. 4,474,355 (1984): Allan J. Wildey.

A metal pulverising surface may be used, in roller mills, or centrifugal impact mills as shown in US Patent No. 4,747,550 (1988): Gunther JSckering. These mills do not grind or abrade a product, but instead either crush it or fragment it by impact. In both cases the finished product is different from a ground or abraded product Problem to be Solved Natural rock - under consideration for use as a grindstone- suffers from a number of disadvantages. Suitable rock is only found in some parts of the world, and it is expensive to transport from one country to another. The surface of the rock may be irregular in texture, resulting in corresponding inconsistencies in the powder produced, both within a single batch and over a period of time as the stone is worn away.

The heat generated by the grinding process is stored up in the rock over a period of time, and can have detrimental effects on grain and flour by driving off or denaturing oils in the germ of the grain and/or damaging the starch in the endosperm. Similarly other food products may also be damaged by heat in the grinding process. With regard to some pharmaceutical or other chemical powders, temperature can be critical in determining the nature of the final product, and in such cases it has been found difficult to regulate the temperature in a conventional grinding chamber with sufficient accuracy to allow the mill to be run for extended periods without stopping for temperature adjustment. 4902CS2.492/to 237879 Synthetic millstones made from a composite of rock and resin can be readily manufactured in a wider range of locations, but nonetheless still suffer from irregularities and heat storage problems.

Roller mills are unable to effectively pulverise for example the germ or bran of grain, and these parts must be separated out after grain is first crushed, leaving the endosperm only. The endosperm is then repeatedly crushed, to form a powder. A variety of other fibrous materials are similarly difficult to effectively reduce to a powder in roller or impact mills.

Disadvantages of a metal millstone, particularly stainless steel, may be greater cost, and wearing smooth of the abrasive surface over time - a stone composition including abrasive particles will retain an abrasive surface as it wears away, new abrasive particles being exposed as the material is worn down, and similarly porous volcanic rock will retain a rough texture as it wears down, but a metal millstone with an artificially textured surface will become more smooth over time, losing effectiveness.

Object It is an object of the present invention to go at least partway towards providing a novel or improved millstone, or at least to provide the public and the powder processing industry with a useful choice.

Statement of Invention The present invention can provide a mill with a millstone which has consistent grinding characteristics, and allows heat to be rapidly transferred to or from the grinding surface, allowing effective regulation of the temperature of the product being ground.

In a first major aspect this invention consists of a millstone for use in grinding granular or particulate material to a powder, characterised in that the millstone is formed from a moulded and hardened ceramic composition.

In a subsidiary aspect this invention comprises a millstone as above, characterised in that the millstone is hollow and includes: an internal first surface for grinding, which is abrasive, and circular in cross-section; 237879 and an external second surface; wherein said ceramic composition includes at least one of the following components: silicon dioxide, aluminium oxide; silicon carbide.

In a related subsidiary aspect this invention comprises a millstone as above, characterised 3 in that said ceramic composition includes at least 10% ball clay, and abrasive particles of silicon carbide.

In a further related aspect this invention comprises a millstone as in the first aspect, characterised in that said ceramic composition comprises an alumina refractory composition.

In a still further related aspect this invention comprises a millstone as in the first aspect, characterised in that the millstone is comprised of a plurality of complementary moulded sections.

In a still yet further related aspect this invention comprises a millstone as above, characterised in that the distance between said internal first surface and said external second surface is at least in part less than 50mm.

In a second major aspect, this invention comprises a mill for grinding granular or particulate material to a powder, including a grinding chamber, characterised in that the grinding chamber is comprised of a millstone with a first surface exposed inside said chamber to act as a grinding surface in use; a second surface outside said chamber, and a space containing gas or liquid adjacent with said second surface; wherein said millstone is formed from a moulded and hardened ceramic composition.

In a related aspect this invention comprises a mill as above, characterised in that blowing apparatus is provided in said grinding chamber, arranged to blow said granular or particulate material across said first surface.

In a further related aspect this invention comprises a mill as above, further characterised in that said first surface is substantially circular in cross-section about a central axis; and the thickness of said millstone between said first surface and said second surface is at 237879 least in part less than 50mm.

In a yet further related aspect, this invention comprises a mill as above, characterised in that blowing apparatus is provided in said grinding chamber, arranged to blow said granular or particulate material around said axis, and press said material against said first surface by centrifugal pressure.

In an even further related aspect this invention is further characterised in that said first surface is concave in axial section.

In a third major aspect, this invention comprises a method for grinding granular or particulate matter to a powder, including the steps of: providing a mill, including a millstone having an abrasive first surface and a second surface; moving said granular or particulate material across said first surface under pressure, to abrade said material against said first surface; characterised by exposing said second surface to contact with a liquid or gas; and regulating the temperature of said liquid or gas, to regulate the temperature of the granular or particulate material by heat transfer through the millstone between the first and second surfaces.

In a related aspect this invention comprises a method as above, characterised by moving said material across said first surface on an air current.

Preferred Embodiment The following is a description of one preferred form of the present invention, given by way of example only, with reference to the accompanying drawing in which: Figure 1: shows preferred apparatus of the present invention in side view and section.

Figure 2: shows alternative apparatus in side view and section.

The present invention in its preferred embodiment comprises a ceramic millstone for grinding granular or particulate matter, such as grain, legumes or seeds. The mill might also be used in the production of paint powders, pharmaceutical or other chemical 237879 powders, and in grinding powders to a more fine and/or consistent grade. "Grinding" may be characterised by movement of the particulate matter across an abrasive surface under pressure (or vice versa), as distinct from rolling, impact fracturing or crushing.

As illustrated in the drawings, the apparatus of the present invention is best used in a mill 10 for grain, in which the grain or other material is ground against the millstone by an air current and centrifugal force. The mill 10 may include an inlet channel 11, a grinding chamber 12 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. While 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.

The general flow of grain and flour through the mill during processing is upward, the grain being gravity-fed through the inlet 11 to the bottom of the grinding chamber 12, 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 grinding chamber 12, 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, on which are mounted a number of fan blades 20 and 21. Preferably six large, curved blades 20 are mounted on the underside of the plate 19, and twelve smaller flat blades 21 are mounted on top. A collar 22, rotatable with the plates 17 and 19, extends up from the plate 19 around the tube 16, and further fan blades are mounted on this. 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 periphery of the chamber 12, through which solid detritus may be expelled in use.

The cylindrical wall of the grinding chamber 12 is comprised of a single annular millstone 30, clamped between upper and lower plates 24 and 25 by bolts 26. The millstone 30 is fixed in place, and does not rotate.

A clearance of approximately 10mm is provided between the ends of the blades 20 and the inner surface of the millstone 30. In use, grain landing on the plate 17 from the inlet 237879 11 is carried outward by the centrifugal force of the spinning plate and the rush of air created by the fan blades 20. It is propelled onto the inner surface of the millstone 30 and around the chamber 12 at speed, grinding and wearing away as it does so. As the particles of grain become progressively smaller and finer, they rise up the chamber 12, until they pass through the small gap between the millstone 30 and the periphery of the plate 19. Here the further fan blades 21 continue to carry the particles around the chamber in a cloud, still abrading against the millstone 30, until they are fine enough to be drawn through an upper aperture 29 into the collection chamber 13, by four fan blades. Here the resultant flour is swirled out through an outlet, for collection and packaging.

An advantage of the type of mill described 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 it, 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.

With apparatus of this type, it is preferable to use a ceramic millstone 30. 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 described above reduces 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 237879 than 10% of the total composition, depending on the strength and abrasiveness required. Ball clay is comprised principally of silicon dioxide and aluminium oxide, with small fractions of iron oxide and other compounds. Preferably the clay includes a relatively high proportion of silicon dioxide. The principal clay mineral in ball clay is kaolinite. Other types of clay might also be used, and a glass or porcelain substrate might be used in the same way as the clay composition described above.

It will be appreciated that by manufacturing the millstone 30 from a ceramic composition, the proportion and consistency of distribution of abrasive particles in the millstone can be very accurately controlled. By comparison, the distribution of abrasive particles in natural stone is very difficult to determine or regulate. The millstone 30 of the present invention could if required be manufactured with a gradient of abrasiveness - for example, with more coarse abrasives in one part, and more fine abrasives in another.

The millstone 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. The millstone might be roughly cast or formed, and then cut or ground to an appropriate shape, but it will be appreciated that the silicon carbide baked into the ceramic will be very detrimental to tools used to cut or shape it. It may alternatively be pressed or otherwise formed in a mould, and larger millstones 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.

When grinding grain such as wheat or rice, it is preferred that heat at the grinding surface 31 is kept below a level which will cause oxidation or denaturing of the oils and other substances in the germ of grain, damage 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, and it is preferable that this also be avoided when dealing with such products.

An advantage of a ceramic millstone 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 millstone, 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. Preferably the shape of the millstone 30 is generally cylindrical, with the heat transferring surface 32 around the grinding surface 31. 237 As shown in Figure 2, the inner wall 31' of the millstone 30' may be concave, to assist smooth air and particle flow and distribution. The inner wall 31' as shown in Figure 2 is nonetheless circular in horizontal cross-section, as is the embodiment shown in Figure 1. The outer surface 32' may be correspondingly convex, or flat, or ribbed as shown.

Alternatively or additionally, air, water or another gas or liquid 33 could be circulated through conduits or pockets provided in the body of the millstone 30, or as shown in Figure 1, the millstone 30 may be enclosed in a water jacket 34. The gas or liquid 33 may simply be kept at room temperature, or may be refrigerated or heated, depending on the particular requirements of the product being reduced to a powder. It should be noted that this arrangement can work equally well for heating the product during the milling process, if desired for a particular product, with heat passing rapidly through the grindstone 32 into the chamber 12.

The millstone 30 is relatively thin, at least in part, to allow rapid heat transfer between the grinding surface 31 and the gas or liquid 33. In a preferred embodiment of the present invention as shown in Figure 1, where the millstone 30 is enclosed in a water jacket 34, the thickness of the millstone between the abrasive grinding surface 31 and the surface 32 in contact with the water 33 may be approximately 20 or 30mm, or up to 50mm. 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. Silicon carbide tends to store heat more than clay, 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. Silicon carbide may still transmit heat better than materials such as stone, resin, and some metals, even though it transmits less well than the clay substrate.

An alternative ceramic for use in the present invention may be an alumina refractory composition, which may comprise for example approximately 95% aluminium oxide, 2.9% calcium oxide, and only 0.1% silicon dioxide, together with small fractions of iron, titanium and magnesium oxides, alkalis and other trace elements. Such a composition may be produced as blend of tabular alumina with aluminate cements having a low iron - n - 237879 and calcium content.

An advantage of such a material is that it can be extremely hard, particularly if fired, but also if simply dried. Alumina refractory compositions are usually used in the manufacture of furnace parts or other high temperature applications, and therefore are usually fired so as to resist cracking under extreme temperatures. For use as a millstone however, the dried, unfired composition may be sufficiently hard and strong, and has the advantage of being significandy easier and cheaper to manufacture.

The millstone is preferably hydraulically cast, but could be moulded by any of a variety of know means.

A flat alumina refractory ceramic surface may not be sufficiently abrasive for use as a millstone. The millstone may therefore be cast with a roughened surface, and/or include particles of an abrasive substance such as silicon carbide in the composition, as with the ball clay mixture described above. One means for producing a suitable roughened surface on the milling face may be to include a sheet of coarse grained or textured material, such as a paper oi cloth, in the mould. The ceramic composition will be pressed against this sheet during casting, and will have a correspondingly textured surface as a result Other ceramic compositions and casting methods might similarly be used.

It will be appreciated that a wide range of changes and modifications might be made to the embodiment illustrated above, within the spirit and scope of the present invention, which may be characterised by the following claims: 237879 1. Apparatus for reducing granular material such as grain to a powder, comprising a 5 grinding member in which is formed a chamber defined by a surface of the grinding member which is circular in cross-section normal to a central axis of the chamber, and means for moving a gas such as air around the chamber thereby to cause said granular material which is entrained in the gas to be reduced to powder substantially by abrasion against said surface of the chamber, said surface being 10 formed from a moulded and hardened ceramic composition which includes at least one of the following components: silicon dioxide; aluminium oxide; silicon carbide. -12

Claims (13)

1. WHAT WE CLAIM IS:
2. 2. Apparatus according to claim 1, in which said surface of the chamber is 15 substantially right circular cylindrical.
3. 3. Apparatus according to claim 2, in which the gas is admitted into the chamber at one end thereof and passes out of the chamber at an opposite end thereof, said surface of the chamber being such that it does not decrease in diameter with 20 increasing proximity to said opposite end.
4. 4. Apparatus as claimed in any one of claims 1 to 3, in which said ceramic composition includes at least 10% ball clay, and abrasive particles of silicon carbide. 25
5. 5. Apparatus as claimed in any one of claims 1 to 3, in which said ceramic composition comprises an alumina refractory composition.
6. 6. Apparatus as claimed in any one of claims 1 to 3, in which the grinding member 30 is comprised of a plurality of complementary moulded sections. 35
7. Apparatus according to any one of claims 1 to 6, in which the grinding member includes a second surface outside said chamber, and a space containing gas or liquid adjacent with said second surface for cooling the grinding member, 4902CL1.094/JM/iv ^ 18 OCT 1994 */ -13-
8. 8. Apparatus as claimed in claim 7, in which a blowing apparatus is provided in said grinding chamber, arranged to blow said granular or particulate material across said surface of said chamber. 5
9. 9. Apparatus as claimed in claim 7, in which the thickness of said grinding member between said surface of said chamber and said second surface is at least in part less than 50mm.
10. 10 10. Apparatus for grinding granular or particulate material to a powder, including a grinding member, substantially as herein described with reference to any either of the accompanying drawings.
11. 11. A method for reducing granular material such as grain to a powder, including the 15 steps of introducing the granular material into a chamber formed in a grinding member which chamber is defined by a surface of the grinding member which is circular in cross-section normal to a central axis of the chamber, and moving a gas such as air around the chamber to cause said granular material which is entrained in the gas to be reduced to powder substantially by abrasion against said 20 surface of the chamber, said surface being formed from a moulded and hardened ceramic composition which includes at least one of the following components: silicon dioxide; aluminium oxide; silicon carbide.
12. 12. Apparatus for grinding granular or particulate material to a powder, substantially 25 as herein described with reference to either one of the accompanying drawings.
13. 13. A method for grinding granular or particulate material to a powder, substantially as herein described with reference to either one of the accompanying drawings. 30 JAMES W PIPER & CO. Attorneys for the Applicants TERRANCE JOHN COLES 35 LINDA PATRICIA PEET 4902CL1.094/JW/iv \ ISOcrm i v