GB2024660A - Cleaning screening apparatus - Google Patents

Abstract

A method of screening comprises directing one or more jets of gas onto the surface of a screen and introducing material to be screened between the jet and the screen surface. An apparatus for carrying out the method comprises a screen (2) and adjacent gas jets, e.g. as produced by gas fed through perforated pipes 3,4, directed onto the screen. The screen and the gas jets preferably move relative to one another during a screening operation and thus the screen may vibrate, oscillate, reciprocate or gyrate, or the gas jets may be made to move over the surface of the screen. <IMAGE>

Description

SPECIFICATION Screening method and apparatus The present invention relates to a method of screening and to an apparatus for carrying out the method.

Screening is an operation of separating a mixture of various sized particles by means of a screen or sieve. The screen comprises a surface containing a large number of openings of predetermined sizes through which pass the smaller particles, the particles which are too large to pass through the holes being retained on the screen.

Screening may be carried out on dry material in which the particles of different size are readily separable one from another. Problems arise howeverwhen the material to be screened is damp e.g. if it has stood out in the open in wet weather conditions.

Under these circumstances the fine particles, by virtue of their greater surface area, retain more of the water than do the large particles and because of the surface tension of the film of moisture they tend to adhere together so in effect forming larger particles which will not pass through the holes in the screen.

In addition, damp particles tend to adhere to the mesh, which in the case of lower mesh sizes rapidly becomes "blinded". Eletrically treating the screen can sometimes be used to prevent this blinding - but it has no effect on the "aggregation" of the particles.

Vibrating the mesh may also delay the onset of blinding as may the use of rotary brushes: but again, neither of these methods prevents the aggregation of the particles. The conventional methods for screening this type of damp material involve heating the screen surface to dry the material being treated or slurrying the material in an excess of water so that the particles are washed apart from one another and then screening the slurry so produced. Both of these methods however possess disadvantages, the heating method is expensive and inefficient and wet screening requires a source of water and requires stronger equipment to support the increased weight of the slurry besides calling for the screened products to be dried after separation. We have now devised a method which overcomes these disadvantages and which will readily separate a damp mixture of solid particles of varying sizes..

According to the invention therefore a method of screening comprises directing one or more jets of gas onto the surface of a screen and introducing material to be screened between the jet and the screen surface so that relatively fine material passes through the screen and relatively coarse material is retained on the screen surface.

The gas may be any inert gas e.g. nitrogen but is most suitably air e.g. from a compressed air source or, preferably an air blower. Preferably a plurality of jets of gas are used and are so arranged that a significant part of the screen surface on which the material to be screened is located has a jet of gas impinging upon it. A suitable arrangement of gas jets is in one or more rows, each row comprising a number of spaced jets the rows being located parallel to each other.

The gas jets may be used in conjunction with a conventional screening apparatus and so the invention may readily be utilised by modification of existing screening equipment. Thus the screen may be a revolving screen i.e. a cylindrical frame with access at both ends surrounded by a mesh and inclined at a slight angle so that material moves through the cylinder from the higher to the lower end. In such a screening apparatus the gas jets may be located in rows running lengthwise through, or circumferentially around, the cylinder. The screen may also be a mechanically shaken screen, slightly inclined and suspended by loose rods or cables, the screen being driven by a reciprocating motion.

Similarly, a vibrating screen may be used either mechanically or electrically driven, or an oscillating, reciprocating or gyrating screen. It is preferred that the gas jets and the screen move relative to one another during the screening operation and this may be achieved by the conventional movement of the screen e.g. rotating, vibrating, oscillating or reciprocating or, less preferably, the gas jets may be made to move over the surface of the screen.

The velocity of the gas leaving the jets, the distance of the jets from the screen, the rate of movement of the screen and the optimum rate at which material is fed to the screen varies from screen to screen and depends upon the duty being performed. The best conditions for any given duty may readily be determined by simple tests. Similarly the disposition and inclination of the jets relative to the screen surface may be varied to suit local conditions e.g. in a rotating cylindrical screen the rows of jets may be located at an angle to a longitudinal axis of the cylinder. Again the jets may be inclined at an angle to the screen surface to facilitate movement of material across the surface of the screen. These and other modifications may be made to take account of local conditions appertaining when the method is in use.

The apparatus which may be used to carry out the method of the invention comprises a screen and one or more means located adjacent to the surface of the screen for directing a jet of gas onto the surface in the direction of the passage of particles through the screen.

Suitable gas-jet directing means include a gas pipe perforated at intervals along its length so that a gas under pressure such as compressed air may pass through the perforations in the form of jets.

An embodiment of the invention will now be described in more detail with reference to the attached drawings in which Figure lisa diagrammatic view of a rotating cylindrical screen and Figure 2 is an end view of the same piece of apparatus.

The screen consisted of a cylindrical frame 1 of 18" internal diameter 36" in length supporting a wire sieve cloth 2 of 12 or 14 gauge mesh. Located inside the cylinder were two gas-pipes 3 and 4 perforated along their length with 3/64 inch diameter holes at 9 inch pitch to allow passage of gas in the form of jets.

These two gas pipes were connected to a source of compressed air not shown providing 60 cu. ft. of air/minute/foot of perforated pipe. Each pipe was placed between 8" and 5/8" away from the surface of the sieve cloth with the holes directed so that the gas jets impinged at right angles onto the surface of the sieve cloth.

In operation the cylinder was inclined to the horizontal and material to be sieved introduced at the upper end A and the cylinder rotated so that the material was carried on th surface of the sieve cloth underthe rows of jets 3 and 4 so that fine material was blown through the mesh, coarser particles passing down the cylinder to be collected at B. The compressed air pressure used was 801 b/square inch and damp iron oxide containing 2-6% water was separated into 3% by weight larger particles collected at B and 97% by weight "fines" passing through the screen at C.

In a similar fashion the apparatus was used to screen successfully wet soil, peat and sand.

Claims (8)

1. A method of screening which comprises directing one or more jets of gas onto the surface of a screen and introducing material to be screened between the jet and the screen surface so that relatively fine material passes through the screen and relatively coarse material is retained on the screen surface.

2. A method according to Claim 1 in which the gas is air.

3. An apparatus for use in the method according to Claim 1 which comprises a screen and one or more means located adjacent to the surface of the screen for directing a jet of gas onto the surface in the direction of the passage of particles through the screen.

4. An apparatus according to Claim 3 in which a plurality of gas jets are used arranged in one or more rows each row comprising a number of spaced jets the rows being located parallel to each other.

5. An apparatus according to Claim 3 or Claim 4 in which the screen is a revolving screen, a vibrating screen, an oscillating screen, a reciprocating screen or a gyrating screen.

6. An apparatus according to Claim 5 in which the screen is a rotating cylinder inclined at an angle with gas jets located in rows running lengthwise through, or circumferentially around the cylinder.

7. An apparatus according to Claim 2 substantially as described in Figure 1 and Figure 2.

8. A method of screening substantially as described in Figure 1 and Figure 2.