The term "powder" encompasses a broad range of dry organic or inorganic material in the form of fine particles ranging from spherical to cuboid, within a relatively uniform size range. Since most materials do not exist naturally in powder form, they are typically reduced by crushing, grinding or other size reduction methods, which may additionally include washing and screening. The material may also undergo atomization when exceptionally fine particle sizes are specified. In the case of metallic materials, electrolysis may be employed to improve the purity of the product in powdered form.

Bulk material in powder form is utilized across the food, dairy, pharmaceutical, nutritional, chemical, paint/pigment, powdered metal, munitions and other industries. It is widely used in the manufacture of such every day products as drink mixes, cocoa, dried milk, whey, vitamins, excipients, ceramic tile, glass, plastics, paper, construction and roofing materials, paint, toothpaste, cosmetics and batteries. In addition, powder can be employed to contain surface spills, to filter impurities from water and air, and as an abrasive to clean or cut metal surfaces and shapes.

Match powder characteristics to equipment capabilities

The flow characteristics of powders are affected by the bulk density of the material and the size, shape and cohesiveness of individual particles. Blends of multiple powders may include particles having various origins, sizes, shapes and bulk densities. Ambient moisture, temperature, vibration, or the introduction of dissimilar powders, can alter the characteristics of a powder or powder blend, causing it to aerate, fluidize, agglomerate or degrade.

Softer powders, such as flour or hydrated lime, may move sluggishly through the production line and have a tendency to "bridge" or "rat hole," impeding flow in process vessels. Light, fluffy powders such as fumed silica or talc tend to aerate, blinding filters and seeping into the plant environment. Carbon black and titanium dioxide powders are difficult to handle as they pack or smear, bridging or "rat holing" in the process stream, greatly reducing efficiency; both are also
notorious for potentially binding or jamming moving components, and incurring labor-intensive and time-consuming cleanup efforts. Milk powder and other types of diary powder, and powders containing high fat content may leave a buildup on the conveying line, which raises the risk of product contamination while causing flow problems. Micronized salt or sand can be corrosive and abrasive, damaging conveying lines and other powder handling equipment. Blends of powders comprised of disparate particle sizes, shapes and/or bulk densities are prone to separation, reducing product quality and generating waste.

Conveying, filling, discharging, blending and other processes in which powder is put into motion, tend to generate dust that can contaminate the plant environment and pose a health risk. Containing dust generated during the handling of powders is therefore a priority to most processors. In the United States, the Occupational Safety and Health Administration (OSHA) has identified dust as one element in the Explosion Pentagon.1 In most industrialized nations, regulations require that additional steps be taken during the handling of powder to prevent it from escaping into the outside atmosphere, where it can settle on motors, lamps, or other heat sources, increasing the likelihood of an incident. In addition, escaping dust can also contaminate other materials and pose a health risk for nearby operators. Some powders are water reactive, meaning exposure to moisture may trigger a chemical reaction that leads to unexpected heat generation releasing toxic or flammable gas. The heat generated from such a reaction can be sufficient to cause either spontaneous combustion or an explosion. For most other types of powder, the addition of moisture will cause particles to agglomerate, altering flow characteristics.

A fundamental safeguard against these and other powder-related problems is to utilize dust-tight processing equipment and connections to upstream and downstream process equipment and storage vessels, preventing the escape of dust, and the incursion of excess moisture or contaminants.

For powder being unloaded from bulk bags, Flexicon Bulk Bag Dischargers form a secure, dust-tight connection
between the bag spout and the equipment inlet, creating a sealed system. Available flow promotion devices promote evacuation of powder from the bag. These include spring-loaded frames that stretch the top of the bag upward as it empties and elongates, causing powder to flow from corners of the bag, and pneumatically actuated flow promotion devices that massage the bag and direct non-free-flowing powders into the bag spout. To prevent the escape of dust in the event of a leak and/or when incoming powders displace air, the sealed system can be vented to a dust collector that puts the entire system under vacuum. This provides the added benefit of collapsing empty bags prior to tie-off and removal, preventing dust generated when empty bags are flattened manually.

When filling bulk bags with powders, dust can be contained by making secure connections between the material feed source and the bulk bag filler inlet. The inflatable spout seal of Flexicon Bulk Bag Fillers creates a secure, dust-tight connection to the bulk bag inlet. The sealed system is vented to a filter sock or optional dust collection system to prevent displaced air and dust from entering the plant environment. To stabilize bulk bags being filled with powders, Flexicon Bulk Bag Fillers are also available with vibratory decks that deaerate and densify the material.

When dumping powders manually from bags, sacks and other hand-held packaging, a Flexicon Manual Dumping Station with a dust collection hood is recommended. Dust generated during dumping activities is drawn away from the worker, onto filter cartridges by a vacuum. Reverse-pulse cleaning of each filter on an alternating cycle returns accumulated dust to the hopper, preventing waste while maintaining continuous operational efficiency.

When dumping powders received in larger containers, Flexicon Drum/Box/Container Dumpers hydraulically raise
and rotate the container, creating a dust-tight seal against a discharge hood that mates with a gasketed inlet port fitted to any receiving vessel. A slide gate valve at the discharge hood outlet allows the flow of powder into the receiving vessel to be started and stopped as needed, allowing total or partial discharge of the container's contents, all dust-free.

How best to convey powders hinges on application parameters too numerous to cover here. However, Flexicon Dilute-phase Pneumatic Conveying Systems and Flexicon Tubular Cable Conveyors are both generally recommended for dry, free-flowing powders that do not tend to pack, cake or smear, whereas Flexicon Flexible Screw Conveyors are suitable for both free-flowing and non-free-flowing powders, including those that pack, cake or smear.

In the case of dilute-phase pneumatic conveying of powders, especially toxic powders, vacuum systems may be specified in lieu of pressure systems to contain dust in the event of a leak in the system.

Trial run powder handling equipment in full-scale test laboratories

Due to the highly variable properties of powders, and the limitless permutations of equipment available to load, unload, fill, condition, dump, weigh and convey them, testing the users' actual material in full-scale test laboratories is highly recommended, following a detailed analysis of the application in question by a Flexicon Applications Engineer.

1 Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions
Sources: Where noted. All other information courtesy of Flexicon Corporation.