Excessive charging of the powder leads to loss of powder due to deposition during transport and the flow of powders. Dr Tong Deng reports.
Electrostatic charges generated during industrial powder handling processes may give rise to unwanted particle cohesion or adhesion, which in turn results in difficulties such as particle agglomeration, segregation, material build-up on equipment and ignition of explosion, etc. Its impact is felt in terms of lower production yields and increased process downtime, or high risks to safety in manufacture.
The charges are most often generated by tribo-charging, which arises from particle-particle and particle-wall contacts.
The charging mechanisms are characterised by frictional charging, tribo-electrification or contact charging.
Generation of electrostatic charges (either unipolar or bipolar) is known in many various material handling processes including powder mixing, blending, pneumatic conveying or fluidisation. This phenomenon appears to be found both in powders having similar or dissimilar chemical composition with the common characteristic being that they consist of differing sizes. [Page Break]
A typical example of the processes which may suffer from electrostatic charge is pneumatic conveying. With the pneumatic conveying of dry particulate materials, the powder moves with boundary layer like motion along the transport tube walls during which random charging of both polarities arises. Excessive charging of the powder leads to loss of powder due to deposition during transport and the flow of powders, particularly in the fine size fraction, becomes difficult to control. This could give rise to a higher risk of explosion due to the charges generated in the system remaining on the equipment.
The charge behaviour and consequent risks are erratic and not easy to predict, nevertheless it is always desirable to characterize the charging tendency of raw materials so that corrective action can be taken prior to full scale manufacture.
In the past, many determination methods of electrostatic charge have been developed. In general the existing techniques of charge measurement can be divided into two major groups: static and dynamic. The static methods involve direct charge measurements, eg when a charged particle is made to contact a conductor and give up its charge. Otherwise the charge methods are called as ‘dynamic methods’. A typical example of the dynamic methods is the electro-dynamic sensor. [Page Break]
In a summary, electrostatic charge can be a serious problem for powder handling and exists in many industrial processes. This, in turn, can create an increased risk of explosion and become a major hazard to product quality control. It is therefore crucial to characterise a powder’s electrostatic properties before putting them into the process. Unfortunately, characterisation is not automatically undertaken in industry due to the limitations of measurement methods.
Dr Tong Deng is with the Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, Kent, UK. www.bulksolids.com
