The abrasiveness of a material is determined by its hardness factor and the shape of its particles. For instance, a material which has a high Moh's hardness factor and has sharp, angular-shaped particles will be considered highly abrasive.
Adhering of the material to a surface or particle other than itself. (After material has been squeezed in your hand for one minute, observe the adhesive characteristic by noting particles sticking to your extended fingers.)
Adhesiveness can be described as "external cohesiveness" which is the ability of material to adhere to other surfaces.
The action of introducing air (or gas) to a bulk material by any means. Aeration may cause the material to become fluidized or agitated.
- Air Retention:
Air retention is the ability of a material to retain air (or other gas) in the void spaces of the material after the air (or gas) supply to it has been terminated. Air retention capability can vary between almost zero and several days, depending on the material's other physical characteristics.
- Angle of Repose:
The angle of repose of a bulk material is the angle formed between the horizontal and sloping surface of a piled material, which has been allowed to form naturally without any conditioning.
- Bulk Density (Fluidized):
Fluidized bulk density is the apparent bulk density of a material in its fluidized state. It is generally lower than either the packed or loose bulk density due to the air absorbed into the voids.
- Bulk Density (Loose):
The loose bulk density (sometimes called the poured bulk density) of a bulk material is the weight per unit of volume (usually pounds per cubic foot) that has been measured when the sample is in a loose, non-compacted or poured condition. The loose bulk density may be close to the "as conveyed" bulk density and is preferred for the purposes of pneumatic conveying system design.
- Bulk Density (Packed):
The packed bulk density of a bulk material is the weight per unit volume (usually pounds per cubic foot) that has been measured when the sample has been packed or compacted in. For instance, a silo or bin or after containerized transportation. The packed bulk density does not compare to the conditions that the loose bulk density is preferred for the purposes of conveying system design.
- Bulk Material Composition:
The following table describes the general compositions that may be found in a bulk material.
A single bulk material whose particles possess the same size and shape.
A single bulk material whose particle size and shape may vary.
A bulk material comprised of individual particles which can be visibly discerned.
A bulk material comprised of individual particles which cannot be visibly discerned.
Two or more different bulk materials which have been combined.
Cohesiveness describes the tendency of a material to adhere to itself. The Cohesiveness of a bulk solid material can be caused by any and sometimes by all of several factors. These include electrostatic charging, surface tension effects, and interlocking of certain particle shapes, particularly fibrous types of materials. Cohesiveness in bulk solids causes erratic flow from bins, pipeline feeding problems, and adverse effects in certain kinds of valves.
Ability of a material to compress under head load (its own weight). Squeezing material in hand will reveal this characteristic.
Some materials have chemical properties which will, when combined with other materials such as moisture and air, cause chemical deterioration to materials of construction.
In certain conditions, some bulk materials can form potentially explosive mixtures when combined with air. These conditions depend on (a) the nature of the material itself, which would include its ignition temperature, its chemical reaction with oxygen, its particle size distribution, and so on; and (b) the nature of the operation involving the material. Details of explosion risk, reactivity, and fire hazard are not required by law in most states in the U.S. to be listed on the Material Safety Data Sheet (MSDS). The MSDS must accompany any material which is transported, stored or tested. In all cases involving explosive materials, reference should be made to NFPA classifications.
Reflects a material's tendency to aerate and act as a fluid. (Squeeze material quickly in your fist. If it squirts through your fingers, then it is floodable.) Floodable materials are difficult to restrain in controlled feeding applications, and deserve conscientious sizing.
Flowability is the ease with which a bulk material flows under the influence of gravity only.
Fluidized describes the state some bulk materials achieve when a gas has been entrained into the void spaces between the particles of the material. Material in a highly fluidized state tends to behave more like a fluid (as the term implies) than a solid bulk material.
Friability describes a bulk material where particles are easily crumbled or pulverized.
A property of a solid material which contributes to its overall abrasiveness. The harder a material is, generally, the greater the erosion for a given velocity on a pipeline. Hardness is difficult to quantify and is somewhat subjective when described. Moh's Scale of Hardness is used to describe the materials when designing pneumatic conveying systems.
The ability of a material to absorb moisture from its surroundings. Moisture may be absorbed from either the ambient air (especially during high humidity conditions) or the conveying air of the pneumatic system. (After squeezing material in hand, it will show a thin layer coating on opened fingers). Sugar is a good example.
- Internal Friction Angle:
This is the internal first favored vertical flow channel angle the material makes in flowing downward through itself. It identifies the first favored flow stream. It can be approximated by drawing your finger through a mound of the material. If the angle is 45 degrees or less, no flow inducement is needed. If angle is 60 degrees or more (with horizontal), the material will probably "core" — i.e. plug flow, and will require "cycling" if accompanied by a feeder.
- Material Temperature:
Generally, most bulk materials are handled at ambient temperature conditions. However, in some cases, the material may be at an elevated temperature. Elevated temperature can affect both the condition of the material itself and its surroundings - particularly the equipment that is being used to convey it. Care should be taken, when considering high temperature materials that the temperature range is clearly and accurately stated, and any effects on the material (particularly its handling characteristics) should be noted. The temperature of the bulk material, measured in either F. or C., for purposes of pneumatic conveying design, is the material temperature taken at the point of entry to the system.
- Material Temperature Sensitivity:
The temperature at which a bulk material changes its characteristics.
- Maximum Particle Size:
Maximum particle size is the maximum dimension in inches (in the case of lumpy materials) or the maximum sieve size (in the case of powders and granules) of the largest lump or particle in the bulk material. Maximum particle size can be critical in the design of pneumatic conveying systems.
- Median Particle Size:
The median size or mid-point of the particle size distribution.
- Particle Density:
The mass of a particle divided by its volume. For a bulk material, average particle density is used, found by dividing the mass of the material by its volume, excluding the voids.
- Particle Shape:
The shape and form of the particles of a bulk material can vary considerably. The following table specifically describes the individual particle shape only and not the bulk material as a whole.
Long. thin, rigid, straight, and pointed.
Sharp-edged or having a multi-faced, irregular shape.
Of geometric shape or multi-faced regular shape.
Having a branched, crystalline shape with the branches extending from the faces of the body.
Regularly or irregularly thread-like with a flexible structure.
Similar to spherical but being somewhat deformed or elongated.
Several individual particles bonded together.
- Particle Size Distribution:
The particle size distribution of a bulk material is a tabulation of the percentage of particles by mass in each particle size range. The percentage described is either that passing or being retained on a specific screen size. In the United States, the screens used are "U.S. Standard Screens" or "Tyler Test Screens". Other methods of size analysis may be used, particularly in the case of very fine and/or cohesive powders. These methods include photo sedimentation, optical microscopy, electrical sensing zone techniques (such as the Coulter counter), and laser diffraction spectrometry.
The permeability of a bulk material is the degree to which air (or other gas) may be passed through the void spaces between the particles of the material.