Aluminum smelters produce primary aluminum in potlines comprising hundreds of reduction cells (pots) using electrolysis to turn alumina powder into aluminum. Each pot contains a liquid “bath” (mainly cryolite) and acts as the system cathode. The smelting process starts when anodes (carbon blocks) are placed in the bath and the cathodes are connected to an electric current.
The temperature difference between the bath (950 degrees C) and the ambient temperature causes a crust to form on top of the bath. Every two or three minutes, a chisel-edged pneumatic cylinder known as a “crust breaker” hammers a hole through the crust so the system dispenser can dose the bath with about 1 kilogram of powder. Large smelters can contain over 300 pots, each of which is equipped with four or five crust breakers and fed by six feeders.
Previous crust breakers provided no feedback on whether the crust was successfully broken. Alumina could therefore get poured on top of the crust, making the concentration in the bath too low. This caused anode effects, and a corresponding release of carbonyl fluoride gases into the environment. These gases have a greenhouse equivalent 9,500 times larger than that of CO2.
To try to control the process, employees periodically lifted covers from the pots to manually monitor the crusts’ condition. Because plants contain multiple alumina feeders that need inspections every shift, this took a significant number of manhours. However, spot inspections are inefficient because anode effects can happen shortly after a feeder stops working.
Beware the Elephant Foot
Another problem with older crust breakers: they operate with a fixed dwell time. The chisel, therefore, gets warmer and warmer over time and eventually becomes too hot. This causes electrolyte deposits from the bath to built-up on the chisel, creating a large clump known as an “elephant foot.”
When the foot gets too large, the chisel can potentially get stuck in the crust. Smelters have to remove the foot manually or with a jack hammer, which can damage the crust breaker and caused productivity losses.
The best way to prevent elephant foot is to prevent the chisel from being immersed in the bath. Closed-loop control could tell the chisel where the liquid bath sits beneath the crust.
A new-generation “Intelligent Crust Breaker” (ICB) from Parker Hannifin “knows” this information thanks to an advanced continuity monitor. Basically, an electrical signal travels through the cylinder, into the rod. As soon as the rod touches the molten metal, it completes the circuit. When the cylinder senses continuity, the piston immediately pulls out. The monitor ensures that the crust has been successfully broken. And because only the tip of the chisel touches the bath, deposit build-up is eliminated.
The new cylinder features other feedback mechanisms as well. Because the force required for breaking the crust is related to the crust’s hardness, using the piston’s full force is not always necessary. The ICB can distinguish between different crusts.
An integrated sensor detects when the chisel has reached the retracted position, which causes the air supply to shut off. A thin crust might require only about 30 percent of the available air pressure. Of course, the air pressure goes up again when the chisel encounters a harder crust. For a large plant, sensing the hardness of the crust and adjusting the breaker’s force can result in an annual air savings of about 70 percent.
The Intelligent Crust Breaker cylinder withstands the harsh conditions found in an aluminum plant’s reduction cells, such as high temperatures, heavy abrasive dust and strong magnetic fields, providing a service life of 20 years. This longevity also can boost uptime.
This article was supplied to Automation World by Nic Copley, vice president of technology and innovation at Parker Hannifin Corporation, www.parker.com.
