Large Continuous Electrostatic Powder Container Mixer

 Background to Dust Explosions in Powder Coatings

Manufacturers that are considering manufacturing powder coatings will need to consider
the process risks. The process will involve weighing/loading and mixing polymers,
hardeners, extenders and pigment into containers / mixers. Some Powder coating raw
materials themselves are potentially flammable as dusts some would inhibit and act as
suppressers. Therefore the handling and processing of these materials needs to be
performed in a way which minimises the risk of a dust explosion.

In order to define a 'basis of safety' for operation of a process, it is necessary to consider
the level of risk reduction afforded by any prevention or protection methods proposed. This
enables a judgement to be made on the adequacy of any proposed system for reducing
the risk to tolerable levels. Key information required for assessing risk in this situation is
explosion data for the materials being handled.

Some powder coatings raw materials may be potentially explosive if dispersed in air within
a certain concentration range. These would be the resin polymer fines that will only be in
very small amounts an are considered insignificant.

Even with an ignition source of sufficient igniting power was simultaneously present would
unlikely result in a dust explosion. Powder Coatings themselves are potentially explosive
dusts. Minimum ignition energy is dependent upon particle size and to a lesser extent, on
formulation. Fine powder particles (dv,50 3-4 microns) have been measured as 1-3 mJ.
Powder Coating particles are susceptible to certain types of electrostatic discharges. In
terms of explosion severity, powder coatings typical have a range of Kst values from 100-
200 bar.m/s, 200 being only for very fine psd material. This would only be present with the
addition of rework material or fines from the milling process in large quantities.

 Pre-mixing process route

1 Pre-mixing

In order to produce powder coatings pre-mix, a process is required that will be mechanical
energy from medium/high speed mixing tools. The mixing speed will be fixed, as generally
high speed is required to mix within a reasonable time scale, but low speed is preferable
for mixing in the fine raw materials and achieving the adhesion. Low speed minimises
damage to the pigment structure and flushing in the mechanical seals.

It is important to keep the pre-mix cool to avoid excessive agglomeration and lump
formation. In the extreme, some raw materials can melt and solidify on the mixer head and
agitator if overheated in premixing. This can happen if repeated additions are made to a
mix followed by repeated mixing there will be a build up of bulk temperature in the pre-mix.
Cooling of the mixer head can be achieved by introducing chilled water into the mixer head
jacket this is only necessary if extended mix times are required for the formulations or
operation are in areas of high ambient temperature.

2 Cleaning

The amount of time required to clean the mixer will vary and it may be necessary to
remove the mixing tools to clean properly or lift them from the mixing head to check
clenliness. Solvent may be needed to remove ‘stubborn’ stains. Use of polished stainless
internals can help reduce build up and cleaning time, although care must be taken not to
scratch the polished surface as this will promote adhesion and extend cleaning time.

Use of internal cooled mixing tools can help prevent adhesion to the tools. However,
experience has shown that cooled tools are more complex to remove for cleaning.

Experience has shown that the cleaning regime does depend on the colour and chemistry
changes and can be back to bare metal with solvent cleaning in one extreme or just a
superficial brush or wash down in the other.