Many School Physics Teachers love to demonstrate the Van de Graaff generator – a chrome dome on a stick, which makes your hair stand up! However, few people are aware that a hopper full of moving powder can be doing exactly the same thing, albeit on an industrial scale and rather less exciting to the human eye.
In process plants, powder is often moved around using vacuum conveyors, which form a dust cloud that can easily catch fire. The charge on the individual particles of powder causes the major problems. The process by which the powder becomes charged is called triboelectric charging. As described earlier, this is a process where two dissimilar materials are rubbed together and electrons are transferred from one material to another. In the case of a hopper full of moving powder, this is the surfaces of individual particles of powder rubbing against each other and against the surfaces of pipes and other equipment. A well known example of triboelectric charging is when a balloon is rubbed against a woollen jumper, in which case the balloon becomes charged with a large amount of static electricity (which causes the hair on your head to stand up) and the jumper becomes charged with a large amount of positive static electricity. However, the amount of static charge that can be generated by rubbing a balloon on a jumper is nothing compared to the amount of static charge that can be generated by rubbing huge numbers of tiny particles of powder against each other and against the surfaces of pipes and other equipment in a hopper full of moving powder.
This series of materials is ranked according to their tendency to donate and to accept electrons, therefore materials become charged when in contact with other materials. The triboelectric series is often used by process engineers to select appropriate materials for the surfaces of contact with powders, i.e. the internal surfaces of hoppers, pipes and chutes. For example, nylon coated particles are very triboelectrically charged when passing through a PTFE lined pipe, whereas two nylon parts would be charged to a much lesser degree. A list of some common materials and their place in the triboelectric series is given below.
The two critical parameters which make matters worse are particle size and moisture. The smaller the particle the greater the surface area per unit mass. Therefore there are more points of contact and therefore more transfer of charge. Also in dry situations there is little moisture in the air to conduct the charge away slowly. Some milled powders are notorious for this reason i.e. some pharmaceuticals and some foodstuffs. There is a useful background explanation of electrostatic ignition risks in industry.
But don’t just assume that a few static eliminating wipes to your hands will be enough to prevent any damage. so it is clearly a phenomenon that needs to be treated with serious consideration. For Vacuum Conveyor, see www.aptech.uk.com/pneumatic-conveying-systems/vacuum-conveying.
The Van de Graaff generator is typically a school physics demo tool which is bolted to a bench in a dry classroom. Powder in process is a very different matter.