|The conveying and handling characteristics of a solid material are characterised by such factors as:
- chemical-physical properties;
- rheological properties;
All bulk solid materials have spaces between the particles, filled with air. The percentage volume relative to the total volume not occupied by particles, the fractional free volume, is known as voidage fraction e . The percentage volume actually occupied by the solid is (1- e ).
The value of this parameter varies greatly from one material to another, and largely depends on the shape of the particles. For spheres of uniform diameter e = 0.48.
This is generally indicated by the symbol r ; the density is defined as the ratio of the product mass and the volume it occupies. Bulk and vibrated density are only two values of a range that varies as a function of consolidating pressure.
The methods for expressing the dimensions of the particles depend on the measuring device employed; the most common one uses standard screens. The data obtained from the granulometric analysis are usually presented as integral distribution curve - in X-co-ordinate the diameter of the particles and in ordinate the material¿s percentage in weight. From the integral distribution curve it is possible to obtain the percentile P50, the value assumed as the average diameter of the solid concerned.
SHAPE of the particles: the shape factor most widely adopted for classifying particles is known as the sphericity factor y ; for spherical particles y =1.
Flowability is the more or less distinct tendency of solid particles constituting the material to flow:
- with respect to one another, evaluated in terms of internal friction angle;
- with respect to a surface, evaluated in terms of wall friction angle.
The internal friction angle indicates the tendency of the material to flow on itself in the presence of a compression force applied perpendicularly to the shearing stress direction; usually, it is evaluated using the Jenike's test, and depends on various factors:
- pressure to which the material is subjected;
- humidity content of the material;
- temperature: the flow of many plastic materials tends to be reduced when heated;
- particles size and shape.
The wall friction angle indicates the tendency of the material to flow along a wall in the presence of compression forces applied perpendicularly to the shearing stress direction; it is evaluated using the Jenike's test and, in addition to the factors described above, it also depends on:
- wall surface, mainly in terms of degree of roughness.
The flowability of a material is also influenced by the time factor: when a material remains stored for a long time in a silo without being subjected to movement, when it is unloaded from the silo it is often much less free-flowing than would be expected.
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.
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.
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.
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 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.
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.
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.