Alumina

Alumina is one of the few powders where fluidisation is the primary transport principle rather than a flow aid.

Air slides and fluidised transport

A gently inclined air slide moves large tonnages with little energy and almost no moving parts, which is why smelters are built around them.

The requirements are dry, clean air at the right pressure and a fabric in good condition; a blinded or torn membrane stops the flow without warning.

Wear in pneumatic lines

Where pneumatic conveying is used, velocity decides the service life of bends and valve seats.

• Dense phase conveying at low velocity for long transfers.
• Hardened or ceramic-lined bends at every change of direction.
• Routine wall thickness measurement at known wear points.

Moisture

Alumina takes up moisture from humid air, and damp material neither fluidises nor discharges properly.

Dry conveying and fluidising air is a process requirement, and a failed air dryer shows up quickly as a flow problem.

Dust and exposure

Loading and open transfers generate respirable dust, and smelter environments add fluoride-bearing dust from the process side.

Enclosed chutes with extraction and sealed loading interfaces are the usual controls.

What to establish before buying equipment

• Particle size and the fines fraction, which drive both fluidisation and wear.
• Air quality and dew point available for slides and conveying.
• Design velocity and the resulting wear allowance.
• Dust control at loading and in the potroom interface.

Alumina stores well and discharges deceptively well, which is where the trouble starts.

The powder aerates readily and behaves almost like a liquid while air is still entrained. A silo that has just been filled pneumatically will flood through an open outlet, overrun the feeder below and make a weighing system useless for the first minutes after filling.

• Allow deaeration time between filling and extraction, and design for it rather than relying on the operator.
• Give the outlet a volume that can absorb a surge instead of passing it on.
• Expect the apparent density to change between a freshly filled and a settled silo.

Smelter grade alumina also takes up moisture from humid air, which affects both its flow and its behaviour in the process that follows.

Alumina is the textbook case where velocity decides the maintenance budget.

It is hard enough to erode steel bends in months at dilute phase velocities, and the wear is concentrated exactly where the flow changes direction. Dense phase conveying at low velocity is the usual answer for any line that runs continuously.

The second decision is the feeder. Because the powder aerates, a volumetric device loses accuracy as soon as the density moves, and the density moves every time the silo is filled.

• Dense phase for continuous duty, dilute phase only for short transfers.
• Wear-resistant liners or ceramic-backed bends where the line cannot be taken out of service.
• Gravimetric feeding wherever the rate matters.

Alumina is not combustible and not toxic, which removes two of the usual concerns and leaves two others.

The first is respirable dust. Fine alumina is a nuisance dust with an occupational exposure limit, and capture at the source is the practical control.

The second is the physical hazard of a flooding powder. A vessel that discharges uncontrolled through an open outlet can bury equipment and block access, and aerated powder behaves like a fluid when it does.

• Dust capture at every transfer point, sized for a fine, dense powder.
• Isolation that can be closed against a flowing column, not only against a settled one.
• Protection against inhalation during filter changes, where the collected dust is finest.