Static electricity influence on powder flow

The impact of electrostatic charges on powder flowability is usually not well known but it can cause bad flowability, segregation, powder build-up that can be troublesome for a company, especially when filling powders. This page is giving information about the root causes of electrostatics phenomena in powder handling, list some problem that can happen and how to solve them.

1. Static electricity and powder flow

The friction of 2 solids is releasing electric charges, think for example of what happens when someone is rubbing a cloth on a glass stick : the stick becomes charged.

It is the same phenomena, called triboelectrification, that happens when bulk solids are moved : the particles are hitting each other, the pipe, a hopper... and then get charged [3].

When charged sufficiently, and when getting in contact with surfaces charged oppositely, then the particles can adhere on the surface. For the phenomena to happen and be significant, the powder must be sufficiently insulating (high resistivity).

Experimental observations show that the particles in a mixture are not all charged the same way : smaller particles tend to be charged the opposite way of the bigger particles. It seems those small particles are also charged more negatively (but not only, some of them are also positive from charge transfer with the process equipment) than the bigger ones [3].

Which unit operations are generating static electricity ?

As mentioned above, triboelectrification is happening when the powder is moving.The friction of the particles with the process equipment are creating charges on the powder. The intensity of the phenomena is actually proportional to the mass flow rate, the velocity of the particles and inversely proportional to the particles diameter :

I = (Ka.M.V^b)/d [1]

With :
I = triboelectric current (A)
Ka = constant dependent on the material
M = mass flow rate (kg/s)
V = velocity (m/s)
d = particles diameters (m)
b = constant (1.4 to 1.9)

This means that static electricity will be generated preferentially for fine particles, conveyed at high velocity and at high flowrate. One process operation fits especially to this definition : pneumatic conveying. The pneumatic conveying of powder is indeed generating a lot of friction and thus charges the particles and the process equipment (thus the need to ground the equipment). Gravity drop over long height can also be another operation creating a strong amount of static electricity.

2. Flow problems created by static electricity

The triboelectrification of the bulk solids can lead to different problems, especially :

• A bad flow of powder out of hoppers or a powder build up
• A segregation of the bulk powder will separation of large and fine particles
• A change in the bulk density of the powder

2.1 Bad flowability

Due to their electrostatic charge, the particles may adhere onto the surfaces of hoppers, or other equipment parts, if they are sufficiently insulating (they will keep their charge for some time even in the contact of a grounded equipment). For large processes this is normally not a big concern, but it can be of consequence for operations requiring more precision, such as volumetric filling (some powder can stay stuck inside the volumetric cups and thus change the actual volume filled, adhering on grounded metal parts or insulated plastic parts).

2.2 Segregation of powder

Similarly to the phenomena described above, the static electricity charging the particles will have a primary effect on small particles, for which the (tiny) electrostatic forces will be enough to keep the particles on the surface of a hopper for instance. The small particles will therefore be preferentially retained while the large ones will be relatively unaffected. It may therefore be possible to observe a segregation effect where large particles will come out first of the hopper.

2.3 Bulk density change

The particles, during the triboelectrification process, have the same charge. As a consequence, they are pushing each other apart which is leading to a looser product having a lower bulk density. This can be an issue especially during bagging or filling process where the volumetric filling is accurate only if the bulk density is well controlled.

3. How to solve flowability problems due to static electricity ?

As seen above, the charging of powders during handling can lead to different issues whose consequences can affect the industrial process, especially in the case of filling operations. It is therefore necessary, when an issue due to electrostatics has been identified, to find ways to control it. The following strategies can be applied :

• Let the time for the powder to discharge : the powder is getting charged during motion, which implies friction, once the powder is stored in a hopper, it will slowly discharge. It can then be interesting  to have a buffer whose residence time is long enough to let the powder release their charges. This method may not be economically viable if the discharge time is too long.
• Control the humidity of the environment : insulating particles will keep their charge for some time which leads to adherence to surfaces, however if the humidity increases (typically RH>65%) then some liquid bridges are formed in between particles which allow to drain the charges and avoid the phenomenon described above. This way of working is however only valid for materials that are not sensitive to humidity (not ok for food for instance), and whose flow properties do not change too much at higher humidity (this can lead to caking).
• Ionize the air [2] : another strategy given by the source [2] is to provide the ions to neutralize the particles. This can be done by blowing ionized air in the product stream for example, just before filling. It is unclear if this way of working has reached industry significance.

4. Static electricity and powders : Safety aspects

It is critical to understand that, if the powder gets charged during transport, getting in contact with pieces of equipment, those pieces of equipment are also getting charged. If the equipment is isolated, there is a risk of static electricity discharge after some time that can lead to a dust explosion. It is therefore mandatory to ensure that all pieces of the processing equipment are grounded. You can have more information about this very important safety requirement in our Safety Handbook : Bulk powder electrostatics hazards

As well, if the particles have been heavily charged during an operation, for example pneumatic conveying, and are stored in a hopper, the differential of potential in between the bulk of particles and the hopper wall can be such that the particles can start discharging, leading to sparks that can trigger an explosion. This phenomena, called cone discharge is explained in the following page : Cone discharge

Source

[1] Triboelectricity : a parameter for solids flow measurement, Dechene and Averdieck, PBE, 1987

[2] Understanding and managing static electricity problems in bagging and packaging, Poidras and Piret, PBE, 1991

[3] Measurement of electrostatic charging of powder mixtures in a free fall test device, Zhang et al, 2015, Procedia Engineering