GB2054406A - Comminuting apparatus - Google Patents

Abstract

A device comprises a longitudinal shaft carrying a co-axial helical blade 3 thereon, the shaft being journalled in a vessel which has a multi-apertured wall co-operating with the helical blade. Preferably, the vessel is a trough 1 with a screen 2 apertured base which encompasses at least 120 DEG of the circumference of the helical blade. The device finds particular use in the milling and/or classification of peat. <IMAGE>

Description

SPECIFICATION Particle classification The present invention relates to a device and method for classifying and comminuting particulate materials, notably peat.

It is often desired to comminute materials, e.g.

peat, and to select a product of a given particle size.

This would normally require two separate operations, comminution (e.g. milling) and classification.

There have been proposals for carrying out the two operations in a single machine. For example, peat has been milled and classified in an horizontal cylindrical screen in which a cruciform paddle is rotated to sweep the peat particles over the surface of the screen. However, such a machine suffers from the major disadvantages of erosion of the moving parts, excessive vibration, jamming of peat between the paddle blades and the screen and a high power requirement.

Surprisingly, we have now found that the above problems can be reduced if the straight longitudinal paddles are replaced by an helical blade or worm screw. We believe that the helical blade moves the peat particles more slowly across the surface of the screen and that, in addition to reducing the above problems, this reduces the compaction of the peat which is of advantage agronomically.

Accordingly, the present invention provides a device for comminuting and/or classifying a particulate material which comprises a longitudinal shaft carrying one or more helically wound blades thereon, the shaft being mounted in a vessel having at least part of a longitudinal wall thereof formed from a multi apertured member co-operating with and generally co-axial with the shaft, the shaft and apertured member being rotatable relative to one another about the axis of the shaft.

The invention also provides a method for comminuting and/or classifying a particulate material which comprises feeding it to a device of the invention, rotating the shaft relative to the apertured member and recovering particulate material from the apertured member.

The invention can be applied to the milling and classification of a number of materials, eg peat, ash, chalk or charcoal, but is of especial use in the milling of peat, notably sedge peat. For convenience the invention will be described in terms of this use and, to aid understanding, with respect to the accompanying drawings in which Figure 1 is a side elevation of a preferred form of device for use in the method; Figure 2 is a plan view from above of the device of Figure 1; and Figure 3 is a vertical section through the device of Figure 1.

Peat, for example sedge peat which has a moisture content of 55 to 80% w/w, is fed to one end of an open trough 1 which is fitted with a U-shaped or semi-circular bottom screen member 2. Trough 1 may be inclined downwards from the feed end, e.g.

at up to 100 to the horizontal if desired, to aid flow of peat along the screen. The trough can be made with the screen member extending circumferentially wholly around a worm screw 3 longitudinally located in the trough. However, we prefer to form trough 1 as an open topped trough with screen member 2 forming the bottom and side walls thereof as shown in Figure 3. The size of the apertures in screen 2 is selected according to the desired product but is usually within the range 15 to 25 mms for a peat product. If desired, the size of the apertures can be varied along the length of screen 2, eg be finer at the feed end to remove excessively fine material before the desired product is allowed through the screen. Screen 2 is preferably made adjustable so that the clearance between the screen and the screw can be varied.If desired screen 2 can be readily removable for replacement by a screen of different aperture size, eg by bolting it in place on the trough structure as shown in Figure 3. Alternatively, screen 2 could be provided with means (eg an adjustable obturator plate) by which the size of the apertures can be varied with the screen 2 in situ.

Journalled within trough lisa worm screw 3 having its longitudinal axis substantially co-axial with the axis of screen 2. Screw 3 can take the form of a series of helical blades mounted on a shaft or a single helical screw flight or true worm screw. The screw 3 rotates about its longitudinal axis within trough 1 and is driven by any suitable means.

Preferably, screen 2 is substantially co-axial with screw 3 over at lest 900, preferably at least 1200, at the bottom of trough 1 to provide an area over which the crown of the helix of screw 3 co-operates with screen 2. Whilst screw 3 can be a close fit to screen 2, eg have a clearance of less than 0.1 mms between them over this area, and act by cutting material held in the apertures as the leading edge of the screw passes over the aperture; we prefer that the screw 3 be a loose fit within the trough 1 and screen 2 and act by gently pushing material through the apertures rather than by a shearing/cutting action. We prefer that there be a clearance of 1 to 10 mms between the crown of the helix of screw 3 and the co-operating portion of screen 2.

The screw 3 acts both to tumble the particles across the face of the screen 2 and to move particles along the length of trough 1. We have found that the action of the screw results in less compaction of the product than where a straight longitudinal beater blade is used to push material through the screen apertures. We prefer that the helix of the screw have a pitch of at least 20 , eg 20 to 70 , to the axis of rotation of the screw.

Whilst comminution of material occurs throughout the length of screen 2 where it co-operates with screw 3, the majority of this comminution should have occurred before the end of portion, say the last quarter, of trough 1 is reached. The particles carried by the screw 3 at this portion will be oversize particles and it may be desired to restrict the travel of particles along the trough so that they can undergo prolonged comminution and thus reduce the amount of oversize material. This is conveniently achieved by bridging the gap between adjacent flights of the screw with one or more longitudinal barriers. These have the effect of hindering free travel of the particles along the screw.The particles are thus held at one area of the trough until they are either passed out through the screen or until sufficient particles accumulate against the barrier to overflow the barrier and continue along the trough to be discharged as oversize material. Suitable barriers are provided by longitudinal bars 5 extending from 1/5 to of the depth of the screw between adjacent flights. The bars 5 can extend longitudinally over several flights and are preferably located in the first 2/3rds of the length of screen 2. The bars are conveniently in the form of angle irons mounted with their apex directed radially outward towards the screen. It is also preferred that the bars be inclined to the longitudinal axis of the screw in the reverse senseto the pitch of the screw so thatthe bars tend to throw particles in the opposite direction to the screw.

The above device can be constructed from any suitable material, eg mild, high tensile and/or stainless steel, depending upon the material to be comminuted. The material to be comminuted is fed to one end of the screw 3, eg by a conventional hopper and chute. Product size material is collected by any suitable means, eg by means of one (or more when different fractions of product are being produced) collection hopper, conveyor or chute located under screen 2. Oversize material is conveniently discharged from the end of trough 1, eg by an opening in the base thereof under the end of screw 3, and can be recycled for retreatment if desired.

Whilstthe invention has been described above by way of a shaft rotating within a fixed screen, it will be appreciated that where screen 2 is of tubular form, the shaft can be fixed and the screen can rotate about the shaft.

Claims (12)

1. A device for comminuting and/or classifying a particulate material which comprises a longitudinal shaft carrying one or more helically wound blades thereon, the shaft being mounted in a vessel having at least part of a longitudinal wall thereof formed from a multi-apertured member co-operating with and generally co-axial with the shaft, the shaft and apertured member being rotatable relative to one another about the axis ofthe shaft.

2. A device as claimed in claim 1 wherein the shaft is rotatable about its longitudinal axis.

3. A device as claimed in either of claims 1 or 2 wherein the vessel comprises a U or semi circular crosssection trough having a screen member as a wall co-operating with the helical blade.

4. A device as claimed in claim 3 wherein the screen member extends over at least 120 of the circumference ofthe helical blade.

5. A device as claimed in any one of claims 1 to 4 wherein there is a clearance of from 1 to 10 mms between the helical blade and the apertured member.

6. A device as claimed in any one of claims 1 to 5 wherein longitudinal barrier members bridge the gap between adjacent flight of the helical blade.

7. A device as claimed in claim 6 wherein the barrier members extend radially from one-fifth to one-half of the depth of the helical blade.

8. A device as claimed in either of claims 6 or 7 wherein the barrier members are inclined to the longitudinal axis of the helical blade in the reverse sense to the pitch of the helical blade.

9. A device as claimed in claim 1 substantially as hereinbefore described.

10. A device substantially as hereinbefore described with respect to and as shown in the accompanying drawings.

11. Amethod of comminuting and/or classifying particulate material which comprises feeding the material to the vessel of a device as claimed in claim 1, rotating the shaft relative to the apertured member and recovering particulate material from the apertured member.

12. A method as claimed in claim 11 wherein the particulate material is peat.