WAM Scandinavia - Teknologier

Teknologier

Karakterisering af pulvere og granulat materialer

COHESION AND AGGLOMERATION OF THE PARTICLES
Cohesion may be defined as the strenght exerted by a bulk solid material against a shearing stress in the absence of compression forces applied perpendicolarly to the shearing stress direction. This resistance is generated by a complex set of mechanisms; important factors that influence the behaviour of the material in this way are:
- humidity content: usually, cohesion increases as humidity increases;
- dimensions and shape of the particles: there is no direct correlation between dimension, shape and cohesion; in spite of this, it has been found in many materials that the smaller the dimensions of the particles, the higher the tendency to cohesion.
- the presence of oil which, depending on the quantity, tends to reduce or to increase the phenomenon of particles' cohesiveness.
The tendency to agglomerate is mainly due to particles interactions caused by electrostatic forces, by van der Waals forces and is strongly affected to the presence of water in the form of humidity, which may even lead to the formation of bridges between the particles.

ABRASIVENESS AND CORROSION
The abrasiveness of granular materials depends on the shape, dimensions, hardness (measured with the Mohs scale) and specific weight of particles.
Corrosion may be defined as the process through which a metal tends to degrade, passing from the elementary state to the state of oxidized material, caused by environment conditions. The tendency to corrosion of many metals is influenced by the pH of the substance with which it is in contact. Also temperature influences the corrosion process: an increase in temperature increases the speed of the phenomenon.

STATIC ELECTRICITY
All substances are electrified to a greater or lesser degree when placed in contact with substances of a different nature; this phenomenon, known as tribo-electricity, is caused by the migration of a certain number of free electrons from one body to another during the contact. The electrified material thus acquires an energy state governed by the laws of static electricity. When the voltage of the electrified material exceeds a certain value, an electrical discharge may occur between the material and the nearest conductor having less electric potential. The discharge gives rise to the dissipation, in the form of heat, of most of the electro-static energy and such heat, released at an extremely high temperature may, in the presence of flammable gaseous mixtures, trigger very dangerous explosions or fires.

HYGROSCOPICITY
This is the tendency of a solid material to chemically or physically absorb on its surface the water vapour present in the air or in gaseous currents.

DECOMPOSITION AND CONTAMINATION
As regards decomposition, this property depends on the intrinsic biological characteristics of the product. It may be identified as the tendency to form mould, germs and bacteria that give rise to decomposition of the substance. The determining factor is the residence time.
As regards contamination, this property identifies the risk of contamination of the product due to fluids from the handling system's mechanical parts, or from contact with the handling system's surfaces that are not properly protected or finished.

DEGRADABILITY
This expression describes the risk of fragmenting and breaking the original granulometry of the material due to the conveyor's mechanical actions (i.e.: slipping). This effect causes an increase of the finest granulometric fractions, which may affect the production processes downstream.

VOLATILITY AND FLUIDITY
These terms describe the tendency of a solid material in granule or powder form to remain in suspension if struck by a gaseous flow. These properties can be evaluated by experimentally estimating the permeability of the material to the passage of a gaseous stream.

POWDERINESS
Powderiness is the macroscopic result of Brownian motion, which mainly concerns the material¿s finest fractions, determined by collision of the solid particles with the gas molecules which are in turn subject to disorderly motion of thermal agitation.

FLAMMABILITY AND EXPLOSIVENESS
A powdered material in dense form, if primed, may give rise to a fire.
If a cloud of powder is primed, an explosion may occur which, unlike a fire, is characterised by a sudden increase in pressure and by the instantaneous formation of intense heat.
The lower limit of flammability is defined as the minimum concentration of powder (g/m³) capable of exploding in air when provided with sufficient priming energy.
Handling materials in a controlled environment or in nitrogen represents a valid solution for reducing the risk of fire or explosion.

TEMPERATURE
In some processes it is necessary to handle products at high temperature; this condition influences both the choice of material of construction and the type of handling system to be adopted.

TOXICITY CHARACTERISTICS
Another factor which requires considerable attention is the toxicity of the vapour or gas that may be generated by the material; awareness of this characteristic may considerably influence the type of material handling system to be adopted, in order to ensure that the working environment is as safe as possible.
INTERACTION BETWEEN HANDLING PARTICLES AND THE WALL
The behaviour of a solid in granular or powder form with reference to the material that contains it may be characterised by several phenomena such as: the stratification of the particles on the walls of the circuit; the formation of a crust resistant to mechanical cleaning to varying extents; and the tendency of some materials to plasticize. These phenomena are subject to studies by the branch of science called rheology.

Referencer

Foust A.S. et al., "I principi delle operazioni unitarie"; Casa Editrice Ambrosiana, Milano.
Iinoya K., Gotoh K., Higashitani K., "Powder Technology Handbook"; Marc Dekker, Inc.
Jenike A.W., "Storage and Flow of Solids", Bulletin N° 123, University of Utah Engineering Experimental Station, Salt Lake City (Nov. 1964).
Perry R.H., Green D., "Perry's Chemical Engineers' Handbook, sixth edition"; McGraw-Hill International Editions.
Rizzo R., "La sicurezza degli impianti industriali"; Edizioni Scientifiche Italiane, Napoli.
Ronca A., "La sicurezza nell'industria; organizzazione, gestione, formazione e audit"; E.P.C. Editoria Professionale, Roma.
Smith W.F., "Scienza e tecnologia dei materiali"; McGraw-Hill Libri Italia, Milano.

Forfattere

Dato


	June 1999

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