A simple blender apparatus for charging powders
and other dry granular media is shown in the figure below. The
container can be made out of a
In operation, the blender is filled about halfway with plastic particles (e.g., polystyrene granules sized ~1 mm) and then the blade is spun for approximately 30 seconds. Using an electric drill to spin the blade at ~100 RPM will usually achieve specific charge values of ~1 mC/kg with this simple apparatus. This charging level is quite sufficient to create serious electrostatic ignition hazards in the larger sized tanks and mixers used routinely in the chemical and polymer processing industries [Jones and King, 1991].
demonstrating how plastic powder can become charged, the
blender can be used to show how vital it is to
electrically ground vessels intended for storage of dry
solids. See the figure below. A second container
(another 1 quart paint can resting on an insulating
block) is used to represent a receiving vessel, such as
a silo, surge hopper, etc. The TESV is set up on the tripod to
monitor the magnitude and sign of the electrical charge
on the receiving can. In this demonstration, if the
receiving vessel is ungrounded and the charged powder
is poured in from the blender, then a strong electric
field will be detected outside the vessel, despite the
fact that the vessel is a metallic conductor. This
field is present because the net charge of the
system is non-zero. Once the container is grounded, the
charge on the outside surface of the vessel is conducted away to
ground and the external electric field drops to zero.
Note that a properly grounded container, when holding insulating charged product, is itself electrically charged. This charge resides on the inside surface of the vessel. If this vessel is inadvertently disconnected from ground and then emptied of its charged contents, then the risk of capacitive electrostatic discharge from the outside of the vessel is created. Therefore, it is vital that all plant grounding be maintained continuously during all production operations involving powders or granular materials. Indeed, the single most important industrial electrostatic safety measure in manufacturing is the installation and proper maintenance of all electrical grounds and bonds. Most electrostatically ignited fires and explosions occur because ground connections are disconnected during maintenance but are not then properly reconnected after the work is completed.
A very understandable pictorial explanation of the behavior of electric charges both outside and inside a Faraday pail is provided by Bill Beaty.
T.B. Jones and J.L. King, Powder Handling and
Electrostatics, (Lewis Publishers - CRC Press, Boca
Raton, FL) 1991.