WO1989004220A1

WO1989004220A1 - Method and apparatus for classifying particles

Info

Publication number: WO1989004220A1
Application number: PCT/GB1988/000965
Authority: WO
Inventors: Alan Davis
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-11-09
Filing date: 1988-11-09
Publication date: 1989-05-18
Anticipated expiration: 1990-05-09
Also published as: AU2625788A; GB2212075A; JPH03500741A; EP0395675A1; GB2212075B; GB8726170D0

Abstract

A method of classifying particles or powders (2) into separate groups according to their mass or size comprises the steps of: passing convection currents of air (16) through a length of ducting (11), the upper wall (12) of which extends at an angle to the horizontal; passing the particles into the flow of air (16) within the ducting, and allowing or causing the particles to fall into a plurality of separate receptacles (13) longitudinally spaced apart along the floor of the ducting.

Description

METHOD AND APPARATUS FOR CLASSIFYING PARTICLES

The invention relates to the classification of particles for example powders. The invention is of especial use in the classification of metal or alloy powders in substantially dry form.

Metal powders are used for example in metal spraying of eroded components to build up the component. During the manufacture of such powders, there is a need to classify the powder according to size and to remove hollow particles and ceramic inclusions which are introduced into the powder during melting, atomizing and the other processes.

It is known to sieve the powders, but this method cannot remove, eg, ceramic particles of a similar size to the desired powder, or remove very fine dusts.

According to a first aspect of the invention there is provided a method of classifying particles into separate groups, eg, according to their mass or size, the method comprising the steps of; - passing heated gas through a length of ducting, the upper wall of which extends upwardly at an angle to the horizontal,

- passing the particles from above the ducting into the flow of heated gas within the ducting; and,

- allowing or causing groups of particles of different mass or size to settle in a plurality of separate receptacles longitudinally spaced apart along the floor of the ducting.

Preferably the upper wall of the ducting extends upwardly between a minimum angle of about 20° and a maximum angle of about 30°, in order that the apparatus is compact and yet still provides efficient and finely divided classification.

Preferably the heated gas is air at a uniform pressure and velocity and caused to flow in laminar convection currents through the ducting. The gas may be instead a gas inert with respect to the particles, eg, nitrogen or a noble gas.

The particles to be treated are preferably substantially dry so that their mass remains constant during travel through the apparatus. As a result of the shape of the upper wall, the thermal currents tend to rise against the gravitational fall of the particles and the particles are suspended in the currents for a short time and have a short vertical and horizontal trajectory only. The particles are subject to a small force and the air turbulence is low. The classification is very efficient and the different classes of particles can be finely divided.

In another aspect the invention provides apparatus for use in the method. The invention also extends to particles that have been classified by the method.

In order that the invention may be better understood, it will now be described by way of example with reference to the accompanying diagramatic drawings, in which

Figure 1 is a schematic cross sectional view of apparatus according to the invention; and

Figure 2 is a view to an enlarged scale along line A-A on Figure 1.

In the drawings, a hopper 1 contains a mixture of metal particles 2 of varying sizes together with ceramic inclusions and the like. There is an outlet 3 in the bottom of the hopper 1 through which the particles pass into a rotary screw feed 4 which is driven by a motor (not shown) . The screw feed 4 supplies the particles 2 through an inlet 5 at the top of a vertical length of ducting 6.

A powder divider 7 is mounted within the ducting 6 and comprises a first generally triangular sheet 8 mounted vertically within the ducting. Two smaller triangles 8 are mounted underneath the first, and four yet smaller triangles

8 are mounted beneath these.

In use, particles 2 are fed to the apex of the first triangle 8 and flows down the diverging sides thereof. The two streams of particles then pass down the diverging sides of the second row of triangles 8 and then finally the third row so that eight sub-streams of particles flow onto a chute

9 mounted at the bottom of the ducting 6. The chute 9 extends across path of the streams of particles at an angle of 60° to the horizontal and is oscillated in its own plane by a motor and springs or the like.

In this way, a "curtain" which is substantially one particle thick is passed into the classifying chamber 10.

The classifying chamber 10 comprises a length of ducting 11 formed from steel sheeting or the like. The ducting 11 includes an upper wall 12 which extends at an angle of betv/een 20° and 30° to the horizontal.

The floor of the ducting 11 defines four longitudinally spaced apart receptacles or collecting bins 13 which have valves 14 by which the bins may be emptied.

An electric heater 15 is mounted down stream of the chamber 10 and further ducting 11 leads from the heater 15 towards the chamber 10, the ducting 11 being of sufficient length to allow the thermal currents 16 produced by the heater 15 to settle into a laminar flow at a constant velocity and pressure. A baffle plate 19 is located on the floor of the ducting 11 adjacent the entrance to the chamber 10 to direct the flow of gas towards the upper wall 12 of the chamber. A cyclone 17 or pair of electrostatic plates (not shown) may be disposed adjacent the outlet end 18 of the apparatus to collect the very fine dusts that have passed straight through.

In use, the temperature of the thermal currents may be preset to a value dependent on the type of particles to be classified, eg, according to range of sizes, mass and shape. The temperature is preferably maintained at: + 1°C.. In tests, I have found that a temperature of about 30°C is sufficient for fairly light particles such as aluminium, while a higher temperature such as 70°C is required for heavier particles.

The particles 2 are fed by the screw drive 4 from the hopper 1 and a "curtain" of particles is passed from the chute 9 into the laminar air flow 16. The angle of the chute 9 ensures that air does not convect upwards and disrupt the particle flow. The larger or heavier particles fall into the first collecting bin 11 while the smaller or lighter particles are carried along within the convection currents to fall within the further collecting bins 11.

A test was conducted on a sample of dry substantially spherical metal alloy powder having a range of particle diameters of between 300 microns and 6.6 microns. The apparatus was according to that shown in the drawings but including three spaced collecting bins 11.

The diameter of particles within each of the three bins were measured and the results are summarised in table 1 below. For example, in bin 1_Λ 90% of the sample had a diameter of less than 167 microns and only 10% had a diameter of less than 56 microns, whereas in bin 3, 90% of the sample were less than 44 microns. Maximum measured diameter (Microns) of particles within each percentage group

Claims

1. A method of classifying particles into separate groups according to their mass or size, the method comprising the steps of:

- passing heated gas through a length of ducting, the upper wall of which extends upwardly at an angle to the horizontal;

- passing the particles from above the ducting into the flow of gas within the ducting; and

2. A method according to Claim 1, characterised in that the heated gas comprises air at uniform pressure and velocity and caused to flow in convection currents through the ducting.

3. A method according to Claim 1 or 2, characterised in that the upper surface of the ducting extends upwardly at an angle of between about 20° to about 30° to the horizontal.

4. A method according to any preceding Claim, characterised in that the particles are caused or allowed to fall into the ducting as a curtain of particles, substantially one particle thick.

5. A method according to any preceding Claim, characterised in that the particles are fed into the ducting from an oscillating chute arranged relative to the ducting so that the heated gas cannot enter the chute.

6. A method according to any preceding Claim, characterised in that the particles are allowed to fall into a plurality of collecting bins longitudinally spaced apart along the floor of the ducting.

7. A method according to any preceding Claim, characterised in that a cyclone is disposed beyond the outlet of the ducting and particles not settling in the receptacles are received in the cyclone.

8. A method according to any preceding Claim, characterised in that the particles comprise substantially spherical particles of metal having a range of diameters of between 6.6 microns and 300 microns.

9. Apparatus for use in a method of classifying particles according to any preceding Claim, comprising a length of ducting having an upper surface which in use extends upwardly at an angle to the horizontal and a floor which has longitudinally spaced apart receptacles, means for passing the particles into the ducting from above the ducting and gas heating means arranged in use to pass thermal currents of gas at uniform pressure and velocity along the ducting.

10. Apparatus according to Claim 10, characterised in that the inlet for the particles is located in the upper wall of the ducting a short horizontal distance in advance of the first of the receptacles in the floor.

11. A supply of particles each of substantially the same mass or size, the particles having been classified from other particles of different mass or size by a method comprising the steps of:

- passing the particles from above the ducting into the flow of gas within the ducting; and allowing or causing groups of particles of different mass or size to settle in a plurality of separate receptacles longitudinally spaced apart along the floor of the ducting.