Handling Fine Powder - Problems & Solutions

INTRODUCTION

Many industries handle fine powders. Few, if any filters, transfer positions, ship loading chutes or conveyors are designed to eliminate fugitive material. Successful conveying transferring dust management and all other operations with fine particulates require a complete understanding of Stokes Law. Fine powders are easily disturbed during all operations where air or gas movement relative to the powder exists.

VERTICLE TRANSFERS

Powders falling from a conveyor entrain air and eventually the powder streams are completely dispersed. The powder particles fall at their terminal velocities relative to the surrounding air. If the stream is confined in a tube as in a ship loader the air is accelerated as the powder falls if air is admitted. The maximum velocity is limited by friction losses and distance dropped. The resultant stable operation may result in very significant air flows and nuisance dusting as the powder and air are discharged. A conservatively high estimate of air flow can be made considering free fall due to gravitational acceleration. A free fall of 30 meters in a 500 mm diameter pipe would result in an air flow of approximately 4.7 m^3/sec. Entrainment will cause a cloud of powder to escape with air from ships holds. Ship loading with Alumina is usually a very dusty operation because the loading chutes are often not well designed. Silos may be enclosed and fitted with dust collectors but ships holds are usually open.

Preventing air from entering loading chutes will limit the product velocity to near the terminal velocity and significantly reduce the emission of particulate from hold of a ship or an open container.

DUST COLLECTERS

Dust collectors must be designed to ensure total freedom from up flowing air in the dirty air regions of the collector equipment. Air moving upwards entrains fine particulates and eventually a fluidized bed is established. The mass of the fluidized bed equals the pressure drop times cross sectional area of the bed. The free cloth area of filter surface for filtration is reduced so clear filter surface has to pass increased quantities of air. Localized cloth failure is very likely. Few commercially manufactured filters work well with fine powders.

TRANSFERRING FROM ONE CONVEYOR TO ANOTHER CONVEYOR.

Discharge from the feeding conveyor should be via scraper and slide contacting the belt. The contacting scraper edge should be pivoted such that it will swing out from the belt. Following the scraper a “vacuum box’ should be positioned to draw powder from surface cracks on the belt and to remove material retained by static electricity effects.  Material retained by static electricity is often rejected in part by take-up rollers. Slides should confine the material and feed to next conveyor at a speed matching that conveyor. Fall heights should be kept to a minimum. Dust extraction positioned well down from the depositing position should be effected by minimum air circulation at very low velocities through a generously sized hood. Powder may be returned to the destination conveyor up stream of the transfer point. Air induced to match extracted flow must be at low velocities to minimize pick-up of fines.

This article was written by Tony Lumley, an independent industry consultant based in Gladstone, Queensland. If you would like more information, Tony can be contacted on 07 4979 3901 or 0451 010 001.