Powder

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Iron powder

A powder is a dry, bulk solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material. In particular, powders refer to those granular materials that have the finer grain sizes, and that therefore have a greater tendency to form clumps when flowing. Granulars refer to the coarser granular materials that do not tend to form clumps except when wet.

Contents

Types

Many manufactured goods come in powder form, such as flour, sugar, ground coffee, powdered milk, copy machine toner, gunpowder, cosmetic powders, and some pharmaceuticals. In nature, dust, fine sand and snow, volcanic ash, and the top layer of the lunar regolith are also examples.

Because of their importance to industry, medicine and earth science, powders have been studied in great detail by chemical engineers, mechanical engineers, chemists, physicists, geologists, and researchers in other disciplines.

Mechanical properties

Typically, a powder can be compacted or loosened into a vastly larger range of bulk densities than can a coarser granular material. When deposited by sprinkling, a powder may be very light and fluffy. When vibrated or compressed it may become very dense and even lose its ability to flow. The bulk density of coarse sand, on the other hand, does not vary over an appreciable range.

The clumping behavior of a powder arises because of the molecular Van der Waals force that causes individual grains to cling to one another. This force is present not just in powders, but in sand and gravel, too. However, in such coarse granular materials the weight and the inertia of the individual grains are much larger than the very weak Van der Waals forces, and therefore the tiny clinging between grains does not have a dominant effect on the bulk behavior of the material. Only when the grains are very small and lightweight does the Van der Waals force become predominant, causing the material to clump like a powder. The cross-oversize between flow conditions and stick conditions can be determined by simple experimentation. [1]

Many other powder behaviors are common to all granular materials. These include segregation, stratification, jamming and unjamming, fragility, loss of kinetic energy, frictional shearing, compaction and Reynolds' dilatancy.

Transport

Powders are transported in the atmosphere differently from a coarse granular material. For one thing, tiny particles have little inertia compared to the drag force of the gas that surrounds them, and so they tend to go with the flow instead of traveling in straight lines. For this reason, powders may be an inhalation hazard. Larger particles cannot weave through the body's defenses in the nose and sinus, but will strike and stick to the mucous membranes. The body then moves the mucous out of the body to expel the particles. The smaller particles on the other hand can travel all the way to the lungs from which they cannot be expelled. Serious and sometimes fatal diseases such as silicosis are a result from working with certain powders without adequate respiratory protection.

Also, if powder particles are sufficiently small, they may become suspended in the atmosphere for a very long time. Random motion of the air molecules and turbulence provide upward forces that may counteract the downward force of gravity. Coarse granulars, on the other hand, are so heavy that they fall immediately back to the ground. Once disturbed, dust may form huge dust storms that cross continents and oceans before settling back to the surface. This explains why there is relatively little hazardous dust in the natural environment. Once aloft, the dust is very likely to stay aloft until it meets water in the form of rain or a body of water. Then it sticks and is washed downstream to settle as mud deposits in a quiet lake or sea. When geological changes later re-expose these deposits to the atmosphere, they may have already cemented together to become mudstone, a type of rock. For comparison, the Moon has neither wind nor water, and so its regolith contains dust but no mudstone.

The cohesive forces between the particles tend to resist their becoming airborne, and the motion of wind across the surface is less likely to disturb a low-lying dust particle than a larger sand grain that protrudes higher into the wind. Mechanical agitation such as vehicle traffic, digging or passing herds of animals is more effective than a steady wind at stirring up a powder.

The aerodynamic properties of powders are often used to transport them in industrial applications. Pneumatic conveying is the transport of powders or grains through a pipe by blowing gas. A gas fluidized bed is a container filled with a powder or granular substance that is fluffed up by blowing gas upwardly through it. This is used for fluidized bed combustion, chemically reacting the gas with the powder.

Some powders may be dustier than others. The tendency of a powder to generate particles in the air under a given energy input is called "dustiness". It is an important powder property which is relevant to powder aerosolization process. It also has indications for human exposure to aerosolized particles and associated health risks (via skin contacts or inhalation) at workplaces. Various dustiness testing methods have been established in research laboratories, in order to predict powder behaviors during aerosolization. [2] [3] [4] [5] These methods (laboratory setups) allow application of a wide range of energy inputs to powdered materials, which simulates different real-life scenarios.

Explosion risk

Many common powders made in industry are combustible; particularly metals or organic materials such as flour. Since powders have a very high surface area, they can combust with explosive force once ignited. Facilities such as flour mills can be vulnerable to such explosions without proper dust mitigation efforts.

Some metals become especially dangerous in powdered form, notably titanium.

Comparison with other substances

A paste or gel might become a powder after it has been thoroughly dried, but is not considered a powder when it is wet because it does not flow freely. Substances like dried clay, although dry bulk solids composed of very fine particles, are not powders unless they are crushed because they have too much cohesion between the grains, and therefore they do not flow freely like a powder. A liquid flows differently than a powder, because a liquid cannot resist any shear stress and therefore it cannot reside at a tilted angle without flowing (that is, it has zero angle of repose.) A powder on the other hand is a solid, not a liquid, because it may support shear stresses and therefore may display an angle of repose.

See also

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<span class="mw-page-title-main">Granular material</span> Conglomeration of discrete solid, macroscopic particles

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Aerosolization is the process or act of converting some physical substance into the form of particles small and light enough to be carried on the air i.e. into an aerosol. Aerosolization refers to a process of intentionally oxidatively converting and suspending particles or a composition in a moving stream of air for the purpose of delivering the oxidized particles or composition to a particular location.

<span class="mw-page-title-main">Dust explosion</span> Rapid combustion of fine particles suspended in the air

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<span class="mw-page-title-main">Particle-size distribution</span> Function representing relative sizes of particles in a system

In granulometry, the particle-size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. Significant energy is usually required to disintegrate soil, etc. particles into the PSD that is then called a grain size distribution.

In particle segregation, particulate solids, and also quasi-solids such as foams, tend to segregate by virtue of differences in the size, and also physical properties such as volume, density, shape and other properties of particles of which they are composed. Segregation occurs mainly during the powder handling and it is pronounced in free-flowing powders. One of the effective methods to control granular segregation is to make mixture's constituents sticky using a coating agent. This is especially useful when a highly active ingredient, like an enzyme, is present in the mixture. Powders that are inherently not free flowing and exhibit high levels of cohesion/adhesion between the compositions are sometimes difficult to mix as they tend to form agglomerates. The clumps of particles can be broken down in such cases by the use of mixtures that generate high shear forces or that subject the powder to impact. When these powders have been mixed, however, they are less susceptible to segregation because of the relatively high inter-particulate forces that resist inter-particulate motion, leading to unmixing.

Moving Bed Heat Exchangers are widely used in industry, on applications involving heat recovery and filtering.

<span class="mw-page-title-main">Granulation</span> Forming grains or granules from a powdery or solid substance

Granulation is the process of forming grains or granules from a powdery or solid substance, producing a granular material. It is applied in several technological processes in the chemical and pharmaceutical industries. Typically, granulation involves agglomeration of fine particles into larger granules, typically of size range between 0.2 and 4.0 mm depending on their subsequent use. Less commonly, it involves shredding or grinding solid material into finer granules or pellets.

<span class="mw-page-title-main">Dilatancy (granular material)</span> Volume change of a granular material under shearing

In soil mechanics, dilatancy is the volume change observed in granular materials when they are subjected to shear deformations. This effect was first described scientifically by Osborne Reynolds in 1885/1886 and is also known as Reynolds dilatancy. It was brought into the field of geotechnical engineering by Peter Walter Rowe.

Dustiness is the tendency of particles to become airborne in response to a mechanical or aerodynamic stimulus. Dustiness is affected by the particle shape, size, and inherent electrostatic forces. Dustiness increases the risk of inhalation exposure.

References

  1. ↑ Smalley, I.J. 1964. Flow-stick transition on powders. Nature 201, 173–174. doi:10.1038/201173a0
  2. ↑ Ding, Yaobo; Stahlmecke, Burkhard; Jiménez, Araceli Sánchez; Tuinman, Ilse L.; Kaminski, Heinz; Kuhlbusch, Thomas A. J.; Van Tongeren, Martie; Riediker, Michael (2015). "Dustiness and Deagglomeration Testing: Interlaboratory Comparison of Systems for Nanoparticle Powders" (PDF). Aerosol Science and Technology. 49 (12): 1222–1231. Bibcode:2015AerST..49.1222D. doi:10.1080/02786826.2015.1114999. S2CID   53998736.
  3. ↑ Schneider, T.; Jensen, K. A. (2007). "Combined Single-Drop and Rotating Drum Dustiness Test of Fine to Nanosize Powders Using a Small Drum". The Annals of Occupational Hygiene. 52 (1): 23–34. doi: 10.1093/annhyg/mem059 . PMID   18056087.
  4. ↑ Ding, Yaobo; Riediker, Michael (2015). "A system to assess the stability of airborne nanoparticle agglomerates under aerodynamic shear" (PDF). Journal of Aerosol Science . 88: 98–108. Bibcode:2015JAerS..88...98D. doi: 10.1016/j.jaerosci.2015.06.001 .
  5. ↑ Morgeneyer, Martin; Le Bihan, Olivier; Ustache, Aurélien; Aguerre-Chariol, Olivier (2013). "Experimental study of the aerosolization of fine alumina particles from bulk by a vortex shaker". Powder Technology. 246: 583–589. doi:10.1016/j.powtec.2013.05.040.
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