The powder is dry, consisting of a large number of very fine particles that can flow freely when shaken or tilted. Powder is a special sub-class of granular material, although the terms powder and granular are sometimes used to distinguish separate material classes. In particular, powder refers to granular materials that have finer grain size, and therefore have a greater tendency to form clumps when flowing. Granulars refers to coarse granular materials that do not tend to form clumps except when wet.
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Many manufactured goods come in powder form, such as flour, sugar, coffee powder, milk powder, toner copiers, gunpowder, cosmetic powder, and some medicines. In nature, dust, fine sand and snow, volcanic ash, and lunar regolith layers are also examples.
Because of their importance to industry, medicine, and earth sciences, the powder has been studied in great detail by chemical engineers, mechanical engineers, chemists, physicists, geologists, and researchers in other fields.
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Mechanical properties
Typically, powders can be compacted or loosened into a much larger bulk density range than can be rough granular materials. When stored with sprinkling, the powder may be very light and smooth. When vibrated or compressed, it becomes very crowded and even loses its ability to flow. Mass density of coarse sand, on the other hand, does not vary within a considerable range.
The powder agglomeration behavior arises because of the Van der Waals force of the molecule that causes individual grains to cling to one another. Actually, this style is present not only in powder, but also in sand and gravel. In heavy grain material, however, individual inertial grains are much larger than the very weak Van der Waals forces, and therefore the small grain attachment has no dominant effect on the behavior of most materials. Only when the grain is very small and light, Van der Waals force becomes dominant, causing the material to clot like a powder. Cross-over size between flow conditions and stick conditions can be determined by simple experiments.
Many other powder behaviors are common to all granular materials. These include segregation, stratification, jamming and unjamming, fragility, loss of kinetic energy, frictional friction, compaction and blurring of Reynolds.
Powder transport
Powders are transported in different atmospheres from coarse grain materials. For one thing, small particles have a small inertia compared to the gas drag forces that surround them, and so they tend to follow the current instead of traveling in a straight line. For this reason, the powder may be an inhalation hazard. Larger particles can not weave through the body's defenses in the nose and sinus, but will attack and attach to the mucous membranes. The body then moves out the mucus from the body to dissipate the particles. The smaller particles on the other side can travel to the lungs from which they can not be expelled. Serious and sometimes fatal illnesses such as silicosis are the result of working with certain powders without adequate respiratory protection.
Also, if the powder particles are small enough, they can become suspended in the atmosphere for a very long time. Random movements of air molecules and turbulence provide an upward force that can counteract the downward force of gravity. The coarse granules, on the other hand, were so heavy that they fell straight back to the ground. Once disturbed, dust can form a massive dust storm that crosses continents and oceans before returning to the surface. This actually explains why there is a bit of harmful dust in the natural environment. Once higher, the dust is very likely to remain high until it meets with water in the form of rain or water bodies. Then stick and washed downstream to settle as a mud deposit on a calm lake or sea. When the geological changes then recast this precipitate into the atmosphere, they may have been cemented together to become a mudstone, a rock. By comparison, the Moon has neither wind nor water, and thus its regolith contains dust but no mud.
The cohesive forces between the particles tend to reject them into air, and the movement of winds on the surface is less likely to disturb the lowland dust particles from larger grains of sand that jutle higher into the wind. Mechanical agitation such as vehicle traffic, digging or skipping animals is more effective than strong winds when stirring powder.
The aerodynamic properties of powders are often used to transport them in industrial applications. Pneumatic conveying is the transport of powder or grain through a pipe by blowing gas. A fluidized gas bed is a container filled with a granular powder or granular substance which is patted by blowing the gas onto it. It is used for fluidized bed burning, chemically reacting gas with powder. '
Some powders may be more dusty than others. The tendency of the powder to produce particles in the air below the given energy input is called "dustiness". This is an important powder property relevant to the powder aerosolization process. It also has indications for human exposure to aerosol particles and associated health risks (through skin contact or inhalation) in the workplace. Various dustiness testing methods have been established in the research laboratory, to predict the behavior of the powder during aerosolization. This method (laboratory setting) allows the application of various energy inputs to powder materials, which simulate different real-life scenarios.
Powder fire powder
Many common powders are made in flammable industries; especially metals or organic materials such as flour. Because the powders have very high surface area, they can burn with explosive strength after being ignited. Facilities such as flour mills may be susceptible to such explosions without proper dust mitigation efforts.
Some metals become very dangerous in powder form, especially titanium.
Comparison with other substances
Pasta or gel can be powdered after being completely dry, but not considered powder when wet because it does not flow freely. Substances such as dry clay, although dry solids composed of very fine particles, are not powders unless they are destroyed because they have too much cohesion among the grains, and therefore they do not flow freely like powder. The liquid flows differently from the powder, because the fluid can not withstand the shear stress and therefore can not be at an inclined angle without flowing (ie, it has zero resting angle. ) The powder on the other side is solid, not liquid , because it can support the shear stress and therefore can display the rest angle.
See also
References
- Duran, J., Reisinger A., ​​â € <â € << b> Sand, Powders, and Grains: Introduction to Granular Physics Materials . November 1999, Springer-Verlag New York, Inc., New York, ISBN 0-387-98656-1.
- Rodhes, M (editor), Powder technology principle , John Wiley & amp; Sons, 1997 ISBNÃ, 0-471-92422-9
- Fayed, M.E., Otten L. (editor), Handbook of powder & amp; technology , second edition, Chapman & amp; Hall, ISBN 0-412-99621-9
- Bagnold, R.A., Impenetrable Sand Physics and Desert Deserts , First Springer edition, 1971, ISBN 0-412-10270-6.
- Fundamentals of Particle Technology - free books
External links
- powder filling machine
Source of the article : Wikipedia
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