Pneumatic cylinders are used in many general industrial applications but perhaps one of the most unusual involves the production of primary aluminum in the metal industry.
Tapped from electrolytic reduction cells (pots) during the electrolytic reduction of aluminum oxide, primary aluminum contains no alloying additives or recycled material metal. Production plants called smelters produce the material in lines comprising hundreds of pots. Each pot contains a liquid “bath,” which is mainly cryolite, and acts as the system cathode.
Placing anodes (carbon blocks) in the bath and connecting the cathodes to an electric current starts the production process. The temperature difference between the bath and the surrounding setting causes a crust to form on top of the bath.
Smelters must somehow poke a hole through the through the crust so the system dispenser can periodically dose the bath with alumina. Here is where a chisel-edged pneumatic cylinder known as a “crust breaker” comes in handy. The device hammers a hole in the crust every two or three minutes. Large production facilities can contain over 300 pots each of which has four or five crust breakers.
Early crust breaker design challenges
A problem with early cylinder designs is they give no feedback whether the chisel successfully broke the crust. The dispenser might therefore pour alumina on the top of the crust, lowering the bath concentration. This hurt the environment by causing anode effects and a corresponding release of carbonyl fluoride gases, which have a greenhouse equivalent 9,500 times larger than that of CO2.
To try and counter this effect, smelter employees periodically lifted the pots’ covers to manually inspect the condition of the crust. Plants contain multiple alumina feeders that need inspecting every shift so this took a lots of man hours. In addition, the inspections were inefficient because anode effects sometimes happen shortly after a feeder stops working.
Older crust breakers also operated with a fixed dwell time, which caused the chisel to get warmer and warmer during the process, eventually becoming too hot. Electrolyte deposits from the bath would then built-up on the chisel, creating a large deposit known as an “elephant foot.” When the foot got too large, the chisel could get stuck in the crust. Employees had to remove the foot manually or with a jack hammer. This often damaged the crust breaker and lowered productivity.
The best way to prevent elephant foot is to stop the chisel from immersing in the liquid bath, so a closed-loop control was necessary to tell the chisel where the bath sat beneath the crust.
Pneumatic cylinder streamlines critical step
A new-generation Intelligent Crust Breaker (ICB) “knows” this information thanks to an advanced continuity monitor. Basically, an electrical signal travels through the cylinder, into the rod. When the rod touches the molten metal, it completes the circuit. The cylinder senses continuity, causing the piston to pull out immediately. Only the tip of the chisel touches the bath thereby eliminating build up. The monitor ensures that the crust has been successfully broken.
Fig 1 Thanks to an advanced continuity monitor, the Intelligent Crust Breaker pneumatic cylinder (chisel not shown) provides system feedback in the primary production of aluminum.
The ICB includes other feedbacks as well.
- The force needed for breaking the crust is determined by its hardness, so using the piston’s full force is not always necessary.
- The ICB is smart enough to distinguish between different crusts.
- An integrated sensor detects when the chisel has retracted, which causes the air supply to shut off.
- Thin crusts might therefore need only about 30 percent of the available air pressure. The air pressure goes up again when the chisel encounters a harder crust.
- For a large plant, the ICB can result in an annual air savings of about 70 percent.
Cylinders boost uptime and production
The upshot is the cylinder helps streamline a critical step in the production of primary aluminum. The device withstands the harsh conditions found in a plant’s reduction cells, such as high temperatures, heavy abrasive dust and strong magnetic fields, and it has a service life of 20 years. The ICB’s quality and longevity boost a plant’s uptime and therefore its production. And most importantly, the device helps plants keep greenhouse gas emissions within government guidelines.
This article was shared by Nic Copley, Vice President Technology and Innovation, Automation Group, Parker Hannifin. To learn more about available technology for Alumninum Crustbreakers download this brochure