GB2212075A

GB2212075A - Thermal classification system for dry powder

Info

Publication number: GB2212075A
Application number: GB8726170A
Authority: GB
Inventors: Alan Michael Davis
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-11-09
Filing date: 1987-11-09
Publication date: 1989-07-19
Anticipated expiration: 2007-11-09
Also published as: GB8726170D0; JPH03500741A; WO1989004220A1; AU2625788A; GB2212075B; EP0395675A1

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

THERMAL DRY POWDER CLASSIFICATION SYSTEM This invention relates to the classification of metal powder alloys and dry powders in general.

Dry powders, after manufacture or processing, contain a size range of particles and foreign inclusions which in certain applications can be detrimental to the product or process for which it is to be used.

For example, certain types of metal powdered alloys which are used for the manufacture of sintered metal powdered components, and also general purpose spray powders, contain fine dust particles, hollow particles and ceramic inclusions which are unavoidably created or introduced into the powder during the melting atomizing and processing of the powder manufacture.

To improve the quality of these powders, new methods are required to remove these unwanted components from the powder.

The demand for closer tolerances in the size distribution range, and a reduction in the quantity of non-metallic inclusions in the powder, has been achieved with the thermal dry powder classification system.

Important technical features relating to the Thermal Dry Powder Classification System are as follows:- i) The duct carrying the thermal currents along the heat stabilizing unit and the classifier chamber is set at an angle between 200 - 300 to the horizontal to allow the thermal currents to convect along the top underside surface as shown in Figure 1.

ii) The heat stabilizing chamber is of sufficient length to allow the thermal currents to stabilize themselves into a uniform pressure at a constant velocity at the point of entry of the powder into the classification chamber, as shown in Figure 2.

iii) Before the powder is fed into the classifier, the thermal current temperature is pre-set and its value is dependent on the type of powder to be classified, i.e. size range, mass and shape and is maintained at -1 C during the process.

iv) The powder, on entry into the classification chamber, is in a mono-particle formation to allow the thermal currents to displace the individual particles into their respective collector bins, relative to the particles' diameter and mass respectively.

v) The discharge points of the collector bins are air-locked to prevent surrounding ambient air currents from being drawn into the classification chamber and disturbing the normal classification process.

vi) The oscillating feed chute is set at an angle of 600 to the horizontal to prevent thermal currents entering the powder feed system.

A test was conducted on a sample of spherical metal powder having size distribution range of 300 microns down to 6.6 microns.

The powder sample was passed through the Thermal Dry Powder Classification Unit and was segregated into three different size fractions, i.e. coarse, medium and fine.

The 'microtrack' results obtained from the test are shown in table form at Figure 3 and graphically at Figure 4 which illustrates the weight percentage/size distribution curve of the fines after being classified from the original sample.

Also shown at Figure 5 is a photograph (magnification x24) of the classified powders.

A description of the Thermal Dry Powder Classification System is as follows, referring to the drawing at Figure 6.

The powder storage bin 1, from where the powder is fed by way of a variable speed screw feed 2, into a three stage powder divider 3, and then on to an oscillating chute 4 which is set at an angle of 60 to the horizontal to prevent the thermal currents from convecting vertically into the feeder system and impeding the gravitational fall of the powder.

On leaving the oscillating chute 4 in a mono-particle formation, the particles enter the classification chamber 5, at which point they are impinged upon by the thermal currents whose force moves the particles into a horizontal/vertical trajectory, the distance displaced by each individual particle being proportional to its diameter and mass respectively, i.e. coarse powder falling into collector bin 6, with a graduation down to the fine powder in collector bin 7.

The thermal currents are generated at 8 by means of a heating element, from where they convect into the heat stabilizing chamber 9 which allows the thermal currents to form themselves into a uniform pressure at a constant velocity prior to entry into the classification chamber 5.

The temperature of the thermal currents, hence the force applied to the powder particles, is accurately monitored and controlled by a heat sensing thermocouple 10 which is mounted at a point at which the powder enters the classification chamber 5.

Any sub-micron particles that fail to be liberated from the thermal currents within the normal range of the collection system are extracted from the thermal currents by means of a cyclone unit 11 which is mounted at a given distance above the outlet point of the classifier.

Claims

1 A Thermal Dry Powder Classification System comprising angled ducting along which thermal currents are convecting. Dry powder is introduced into the classification section of the ducting at the point where a uniform pressure at a constant velocity is achieved on the underside of the ducting. The thermal currents are continually rising against the gravitational fall of the powder particles and only for a relatively short period of time are the particles in suspension within these thermal currents. This leads to a very short horizontal/vertical trajectory for the powder particles to be liberated from the thermal currents, the distance travelled by individual particles being dependent upon their ballistic properties.

2 A Thermal Dry Powder Classification System as claimed in Claim 1 wherein a feeder system allows the powder to fall in a mono-particie formation on entry into the classification chamber.

3 A Thermal Dry Powder Classification System as claimed in Claim 1 wherein a thermal current stabilizing unit of sufficient length to allow the thermal currents to form themselves into a uniform pressure.

4 A Thermal Dry Powder Classification System as claimed in Claim 1 wherein the discharge points of the collection bins are air-locked.