Key points
In material handling processes, electrostatic charging of powders becomes common when powders get smaller, hotter or transported at a high speed. For instance, in pneumatic transport lines, the accumulated charge on particles can potentially lead to a risk of dust explosions due to electric discharge. High levels of charge in powder may also lead to extra powder coating on surfaces or severe powder agglomeration. Engineers must understand these hazards and know how to mitigate them.
Electrostatic charge on solid particles is commonplace in many industries such as pharmaceuticals and food, where powders used in the process are handled through different processes in contact with surface materials which have different work functions.
Electrostatic charge on a surface is generated whenever any two surfaces are in contact and then separate afterwards (see Figure 1).
Most static charges on powders are generated when fast movement between particles is presented between the contacting surfaces and then electrons are transferring from one surface to the other. In many common processing operations, such as when conveying, grinding or sieving, static charge is generated and remains on the surface of solids whereas the equipment is likely to be earthed.
FIGURE 1. Electrostatic charging whenever any two surfaces are in contact and then separate.
The main concern posed by static electricity in an industrial environment is the risk of fire and explosion due to ignition of flammable atmospheres by electrostatic discharge.
Additionally, other common hazards such as extra agglomeration, strong segregation or surface coating in the process can be caused by the electrostatic charge.
Precautions on such charge hazards are often ignored until the static charge creates issues to the process such as dust accumulation, or downtime of the process increased due to agglomeration. Static charge on powders is normally invisible in itself but is noticeable when the hazard is created.
To control static charge on powders, first we have to understand how static charge is generated and accumulated, as well as at what level it produces the hazards, even if there is not a potential risk on fire ignition and dust explosion.
Reading on, we will discuss how electrostatic charge is generated in the material handling process, why it accumulates to produce recognisable hazards and what practical measures can be taken to control the electrostatic hazards in material handling.
Electrostatic charging of powders
There are several different mechanisms of charge generation on powders including contact charging, charging by induction, corona charging, etc. Contact electrification and induction charging are the most commonplace for powder charging.
Contact electrification:
Contact charging or frictional charging is a common thing when two different materials are brought into contact and a rearrangement of charges on the two surfaces in contact occurs. It is also known as triboelectrification.
In the contacts, electrons at the contacting surfaces transfer from one surface to the other surface and the surface left with a deficit of electrons becomes positively charged.
Contact charging of powders frequently occurs, for example, solid particles collide or rub against the blades and wall of a blender during mixing.
The level of charge generated by contact electrification is influenced by many factors, such as the work functions of the two contacting surfaces, the true contact area, contact pressure and surface conductivity.
The handling and processing of bulk solids and powders typically results in the development of relatively high levels of static charge because many powders are electrically insulating (high resistivity), and powders have a relatively large surface area per unit mass available for contact electrification to take place.
On the other hand, factors in the working environment also can have a big impact on the charging, such as temperature and humidity.
Induction charging:
Induction charging occurs when a conductive object that is electrically insulated from ground leaves (or enters) an electric field held on an adjacent charged object. Typically, the conductive object may gain or lose charge because of a charged object approaching it.
If the conductor becomes sufficiently charged by induction, there arises a risk of electric discharge. Commonly powders are good insulating each other during transport even if they are made of good conductors.
The process equipment is often made of metals so have very good conductivity, and are usually well earthed. The charge appearing on the conductive objects due to induction charging may pass to the other insulators such as powders and cause the powders to be charged.
The charged particles often will adhere to the conductive surfaces because of the forces generated by the electrostatic charge. In the presence of any high voltage electrical field, induction charging can happen.
Particularly with insulated highly conductive objects, the objects are charged once the objects are contacting an earthed surface.
Electrostatic hazards in material handling process
The generation of electrostatic charge on powders in material handling processes can be hazardous.
Hazards to plant, equipment and personnel:
Where charging is happening, charge can accumulate on plant facilities, equipment or human bodies if the charge cannot be dissipated (as shown in Figure 2).
Because the particles in a powder are good insulators and take charge easily, charge on equipment or personnel can build-up over the time due to the accumulation of the powders.
This charge build-up results in a high voltage, which can break down the insulating properties of air leading to a spark that can ignite a dust cloud or flammable atmosphere that may be present.
Additionally, the presence of static charges can produce other hazards even where the explosion risk is low – troublesome agglomeration of powders, material build-up in the equipment, blockage in filters, etc.
FIGURE 2. Static charge generated in material handling processes.
Hazards to powders:
Compared to electric charging on equipment or personnel, electric charging on powders can be more complicated.
Appearance of powder charging can be either significant or insignificant, overall, but quite often the powders can be significantly charged but with a balance between the oppositely charged particles.
The first case is called unipolar charge and the second one is called bipolar charge. For the unipolar charged particles, it appears either positively charged or negatively charged as a bulk.
Instead of charge polarity, magnitude of the charge level for powders is important. Because of the large surface areas of solid particles, static charge has much stronger influences on the powders compared to the process equipment.
Whatever the powders are made of, the static charge effects on powders can be significant and problematic, when the powders are transported in an insulated media such as pneumatic conveyors and silos, or even in an open space (see in Figure 3).
FIGURE 3. Static charge on powders can accumulate in a process.
Hazards to processes:
Unipolar charging in particular can create large quantities of charge and voltages up to 20,000 volts, and the substantial hazard of ignition and dust explosion have already been mentioned.
However, the accumulated charge on powders can also generate other hazards in the processes. One of the charge hazards is extra agglomeration of powders if the powder is likely to be charged bipolarly.
In most cases, particles with smaller sizes can suffer more charge issues. In the processes, if powders are moving fast through a hot environment such pneumatic conveying or rubbing against each other or a moving surface, such as in a sieving process for example, more charge on the powders will be generated.
The charged powders may face strong agglomeration or segregation, making it very hard to get them through the sieve in the example given; also, the powders can coat onto equipment surfaces, which will alter the friction coefficient between the powders and the equipment surface. Therefore, charged materials may need extra time in material processing.
In summary, the risk of electrostatic hazards in material handling processes depends both on the chargeability of the powders and on the process conditions.
Even if the charge on powders is not enough to cause a dust explosion, the charge on powders can lead to other problems in processing and handling.
Control of electrostatic hazards in material handling
The approaches to control electrostatic hazards in bulk solids handling processes, not only for fire and dust explosion but also to avoid negative impact on efficiency of the processes, involves both preventing ignition and protecting plant and people from the accumulation and consequences of static charges.
The control of static charges for powders requires a special focus since it is not always easy to spot where the generation of charge is likely to cause a hazard or problem. A four-step approach is advocated for the control of electrostatic hazards.
- Collect electrostatic characteristics of powders and evaluate it if it indicates any likely susceptibility to hazards.
- Evaluate the charge hazards in process including process equipment and environment.
- Assess the hazards for processes, dust generation and charge accumulation.
- Take measures to control static electricity and minimise the risks of the static charge.
Environmental effects on charge accumulation:
Electric charging on powders can be significantly affected by the temperature and the humidity in the operation environment.
The electron transfer rate from one surface to another depends on the nature of the surface and the humidity of the atmosphere.
Hot air and less humidity will increase the chance of powder charging. In other words, the charge on powders can be easier to dissipate if the humidity in the environment is high – although beware that high humidity can have other negative effects on powder behaviour.
Control powder charging in handling process:
There are many methods that can be used for the control of powder charging hazards in materials handling process.
Firstly, minimising the charging level in powders is essential, although sometimes it is hard to reduce the charge build-up on powders due to the process. However, this can be an effective way to reduce the hazards by changing the process or using ironising if it is possible.
Secondly, equipment earthing is critical for control. Earthing of all conductive parts of plant and equipment must be enforced, which is the most important safety measure to reduce electrostatic hazards.
Earthing the equipment helps to reduce the charge build-up and to prevent dangerous charge accumulations on conductive equipment in all situations.
“Isolated conductors”, especially, must be avoided at all costs – for example a length of metal pipe suspended between two rubber sleeves.
These can act as a capacitor, accumulating very large amounts of charge at high voltage creating the danger of a high energy incendive spark to an adjacent earthed surface.
However, earthing cannot be achieved for the particles within the powders, because of ineffectiveness of charge dissipation through the contacts between powders and the low conductivity of most powders.
It is very difficult to predict or to control the static charge level of powders in industrial operations. Thus, hazard assessment for powders needs to be used to evaluate the likelihood of the charge accumulating and the potential incendivity of discharges.
Concentration of powders in the environment is a key factor to the incendivity. Minimising dust emission can also be an effective way to reduce electrostatic hazards in material handling process.
Remarks and suggestions
Electrostatic charging of powders can be common in many industrial processes, especially for mineral, pharmaceutical and food industry where multiple ingredients have different surface work functions and can be highly chargeable.
Electrostatic charges are generated whenever materials come together and separate particularly for powders.
In industrial processes, electric charging of powders can lead to severe hazards including dust explosion and some other common problems in handling process such as extra agglomeration, material coating on the equipment, flow issues and segregation.
Control of electrostatic hazards in material handling is essential. A four-step approach is suggested for control of electrostatic hazards of powders. Minimising the level of powder charging is a better way but insufficient on its own for powders.
Earthing of the equipment is essential if powders are chargeable. Preventing dust explosion and controlling dust concentration is key.
The essential starting point is that staff need to be trained to understand electrostatic hazards and the measures in place to control static electricity.
Contact tong.deng@gre.ac.uk +44 20 8331 8646 www.bulksolids.com
