Special Maintenance Procedures When Burning Off Pvc Or Other Halogenated Polymers - Cole Parmer FSB-200 Series Instruction Manual

All articles should be completely cleaned and removed from the bed before shutdown. Corrosion of
processed parts could be seriously increased if they are left immersed overnight. Furthermore, residual
plastic, instead of being burned off in a fluidised state, could percolate down through a static bed and
settle on the porous plate causing blockage and poor fluidization.
When parts are removed from the bed, they should be allowed to cool in the air and, while still warm,
wiped with on oily cloth to prevent rusting. If the bath is left unused for long periods of time, empty the
aluminium oxide and store it in a separate container. Keep the inside of the bath clean and dry.
The fume extraction system (if used) should be regularly maintained. The cyclone trap collection bin should
be emptied at the end of each day's use; the collected fluidising medium being returned to the fluidised
bath. It is important to ensure that the airtight seal between the cyclone and collector bin is re-made.
SPECIAL MAINTENANCE PROCEDURES WHEN BURNING OFF PVC OR
OTHER HALOGENATED POLYMERS
Burning off PVC (polyvinyl chloride) in a fluidised bath offers one of the most severe conditions of operation.
Hydrogen chloride (HCl) liberated on the breakdown of PVC is absorbed by the bed medium creating an
acidic environment within the bed. This happens especially when the bed also absorbs water from the
atmosphere or when the fluidising air is cold. HCl is extremely corrosive, especially when it is aerated and
wet. In addition, in water it produces chloride ions which, even in neutral or alkaline solutions, promote
corrosion and rusting in steel.
Consequently, fluidising beds used for burning off PVC require strict supervision to minimize corrosion of
the bed itself and of parts cleaned in it, especially if these are of un-coated steel. The purpose of most of
the recommended maintenance procedures is aimed at keeping the bed medium clean, free-flowing, free
of gums, acids, agglomerates, partly decomposed plastic, char and larger particles. These cleaning
processes have the additional benefit of ensuring good fluidization and thus good heat transfer throughout
the bed and through immersed parts. This, in turn, reduces burn-off time, uneven heating of parts and
thus distortion, increases heater life by eliminating localized hotspots and makes cleaning easier on a regular
basis.
The following instructions apply generally to halogenated polymers and specifically to PVC. They are aimed
at minimising corrosion of the bath and of immersed metals.
DAILY INTERVALS - at the end of each working day
1. Maintain the working temperature (about 400°C) for half an hour after the last processed batch, to
assist removal of corrosive acids from the bath.
2. If possible, maintain the temperature of the bath at about 100°C overnight and over weekends and
holidays to reduce absorption of water vapour into the batch. Overnight, the residual heat in the bath
will usually ensure this.
3. Scoop out charred plastic residues, clods of media and articles being processed.
WEEKLY INTERVALS - at the end of each working week and before prolonged shut-downs
1. Ensure compliance with daily instructions above.
2. Add about 50g (2oz) of powdered mild alkali to the bath and mix in well by maintaining fluidisation
for 5 minutes. Suitable alkalis are limestone, dolomite, hydrated or slaked lime, sodium bicarbonate,
sodium carbonate, magnesia. The addition of corrosive alkalis such as caustic soda (sodium hydroxide)
and quick lime (calcium oxide) should be avoided.
MONTHLY INTERVALS
1. Empty the bath and inspect it for signs of corrosion, especially pitted surfaces and flaking scale. The
inside should be wiped out with a rag wetted with 5% washing soda (sodium carbonate) solution.
26