In order to transfer material from storage to process, it is important to know how the particulate material will flow. If the particles tend to agglomerate, poor flow properties may again be expected. Agglomeration arises from interaction between particles, as a result of which they adhere to one another to form clusters.
Mechanical interlocking. If particles are long and thin in shape large masses may become completely interlocked. Surface attraction. Surface forces, like van der Waals’ forces, may give rise to substantial bonds between particles, particularly where particles are very fine (<10 μm), their surface per unit volume is high. freshly formed surface, such as that resulting from particle fracture, gives rise to high surface forces.
Plastic welding. When irregular particles are in contact, the forces between the particles will be on extremely small surfaces and the very high pressures are developed Electrostatic attraction. Particles may become charged significant electrostatic charges may be built up, particularly on fine solids.
Effect of moisture. Moisture will tend to collect near the points of contact between particles and give rise to surface tension effects. It may dissolve a little of the solid, which then acts as a bonding agent on subsequent evaporation. Temperature fluctuations give rise to changes in particle structure and to greater cohesiveness.
“Glidants” are added Very fine powders which reduce interparticle friction by forming surface layers on particles Thus combat effect of friction arising from surface roughness Also reduce effect of electrostatic charges But Optimization of particle size is most important to improve flow properties
Fine particles may be difficult to discharge from hoppers as particles may cling to the walls. Though can be minimised by vibration or mechanical stirring, but very difficult to overcome them entirely only satisfactory solution is to increase the particle size by forming them into aggregates. In addition, very fine particles give rise to serious environmental and health problems, particularly as they may form dust clouds during loading in windy conditions disperse over long distances.
A desired particle size may also be achieved by building up from fine particles Such as production of fertiliser granules by agglomeration. Formation of pellets or pills for medicinal purposes by the compression of a particulate mass, often with the inclusion of a binding agent that will impart the necessary strength to the pellet.
Prilling Urea and ammonium nitrate production Cascading conc sol of urea from the top of the tower Air used to cool the sol to form granules Seed granules of <0.5mm dia can be built to product size of 2-3 mm
Pelletizing is the processApplications of Size of compressing or molding aEnlargement material into the shape of a pellet. Such as in medicines (pharmaceuticals) etc. Pressure Compaction with or without the addition of a binder
A dust explosion is the fast combustion of dust particles suspended in the air in an enclosed location. Coal dust explosions are a frequent hazard in underground coal mines, but dust explosions can occur where any powdered combustible material is present in an enclosed atmosphere.
There are four necessary conditions for a dust explosion or deflagration:1. A combustible dust2. The dust is suspended in the air at a high concentration3. There is an oxidant (typically atmospheric oxygen)4. There is an ignition source
electrostatic discharge Friction hot surfaces, including e.g. overheated bearings fire However it is often difficult to determine the exact source of ignition post-explosion.
Many materials which are commonly known to oxidize can generate a dust explosion E.g. coal, sawdust, and magnesium. Many mundane materials can even lead to a dangerous dust cloud such as grain, flour, sugar, powdered milk and pollen. Powdered metals (such as Al and titanium) can form explosive suspensions in air. The dust can arise from activities such as transporting grain Grain silos do have dust explosions. Mining of coal leads to coal dust and flour mills likewise have large amounts of flour dust as a result of milling.
For combustion, the dust must consist of very fine particles with a high surface area to volume ratio, Thus making combined surface area of all the particles very large Dust is defined as powders with particles less than about 500 micrometres in diameter but finer dust will present a much greater hazard than coarse particles by virtue of the larger total surface area of all the particles.
Below a certain value, the lower explosive limit (LEL), there is simply insufficient dust to support an explosion. A figure 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit (UEL) there is insufficient oxidant to permit combustion to continue at the necessary rate.
Bulk Density: mass of many particles of the powdered material divided by the total volume they occupy Dusts have a very large surface area compared to their mass. Since burning can only occur at the surface of a solid or liquid, where it can react with oxygen, this causes dusts to be much more flammable than bulk materials. For example, a 1 kg sphere of a material with a density of 1g/cm3 has a surface area of 0.3 m2.
However, if it was broken up into spherical dust particles 50µm in diameter (about the size of flour particles) it would have a surface area of 1600 m² This greatly increased surface area allows the material to burn much faster and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material, (as there is no heat loss to conduction within the material). When this mixture of fuel and air is ignited, especially in a confined space such as a warehouse or silo, a significant increase in pressure is created, often more than sufficient to demolish the structure.
Even materials that are traditionally thought of as non-flammable, such as Al or Fe, or slow burning, such as wood, can produce a powerful explosion when finely divided, and can be ignited by even a small spark. Such metal powders are widely used in fireworks for their dramatic effects.
Diluting the dust – such as coal dust by stone dust to the point where it cannot burn Use of inert gases (N2, Ar, CO2) instead of O2 in some industries Spraying water on area such as mines. Good housekeeping
Oxidant Concentration Reduction Deflagration venting Deflagration pressure containment capable of withstanding the maximum pressures resulting from an internal deflagration. Deflagration suppression Technique of detecting and arresting combustion in a confined space while the combustion is still in its incipient stage Deflagration venting through a dust retention and flame- arresting device The dust retention device is for retaining dust when particulate material is dumped through the device into a container