‘Bath’ sensor keeps quality in focus

Video images are transmitted to a handheld LCD video screen, then digitally analysed to quantify bath contamination

Molten metal in aluminium production cells can be contaminated by liquid ‘bath’ – cryolite solvent used in the production process. This contamination problem is one that smelter operators would dearly like to solve.

With the support of Alcoa, Comalco and Tomago, a Light Metals Flagship project team is developing an inline sensor system to reduce such contamination while also enhancing worker safety.

Tapping and contamination
Each production cell or pot contains a layer of bath floating on top of molten aluminium. 

The aluminium is sucked into a tapping crucible through a long metal siphon pushed into each pot. Liquid bath is unavoidably taken out with the molten metal. 

This siphoning operation is performed by tapping crews, who visually assess the molten metal stream through a peephole as it enters the crucible. 

Apart from the safety risk, the method is subject to human error, which may result in excessive bath contamination of molten aluminium.

Million-dollar scale problem
Molten aluminium is extracted at a temperature of around 950°C, well above the melting point of aluminium (660°C).

However, bath melts around 935°C, so when the extracted metal cools by 15°C or so, any bath extracted solidifies and sticks inside the tapping crucible.

Tapping crews monitor the tare weight of the tapping crucible between tapping events to gauge any increase in solidified bath. When the tare weight reaches a certain level, the crucible is taken out of production so that solidified bath can be jack-hammered out.

But not all solidified bath sticks to the crucible. Contamination also crops up – more critically – at the cast house, which has prompted some in the industry to refer to it as a “million-dollar scale problem”. 

Handheld LCD vision
The Light Metals Flagship team – based at CSIRO Minerals’ Lucas Heights Science & Technology Centre and led by David Death – is developing a CCTV camera and periscope system to monitor tapped metal for contamination.

Video images are wirelessly transmitted to a handheld LCD video screen, then digitally analysed to quantify bath contamination.

The system – which has undergone smelter trials – will alert potroom floor operators to excessive bath contamination without requiring them to stand next to the hot crucibles. 

Control of tapping process
“We plan to use image-analysis data to create a ‘bath index’ metric that will enable plant managers to quantify bath extracted per pot or per crucible,” says Mr Death.

“In the longer term, the bath index metric could be used to control the metal tapping process – similar to the vision systems used to control many manufacturing processes.

“Tighter control of the tapping process to minimise bath contamination should also benefit casthouse operations and reduce downtime associated with cleaning of crucibles.”

Contact:
David Death
Ph: 61 2 9710 6724
David.Death@csiro.au

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In brief...

The Light Metals Flagship is a CSIRO initiative and part of the National Research Flagships program that aims to deliver scientific solutions to advance Australia's most important national objectives. One of the largest scientific initiatives ever mounted in Australia, it aligns closely with the Federal Government's National Research Priorities. The initiative brings together our national research resources to deliver breakthroughs in fields ranging from healthcare to light metals and the environment.

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