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"Intelligent" Crust-Breaking

Cylinders Help Cut Anode Effects

Alumina feeding becomes more efficient, and more consistent, making the results less hazardous to the environment and more profitable to Elkem.

In Mosjoen, Norway, Elkem Aluminium is getting an assist from Parker Hannifin Corp. to streamline an essential process step, increasing efficiency at the 275,000metric tons/year aluminum smelter. Elkem Aluminium is a partnership of Elkem AS, which is a diversified manufacturer of silicon metal and other specialty products and materials, and Norsk Alcoa AS. Parker Hannifin (www.parker. com) is the world's largest manufacturer of motion and control technology.

The project at Mosjoen involved applying intelligent pneumatic technology to the systems used to break the crust atop the cells producing molten metal in the plant's two potlines. As well as being able to withstand the extreme conditions that prevail in an aluminum smelter, the cylinders' closed-loop control improves the performance of the feeders and enable the plant's cell operators and maintenance crew to respond to problems quickly.

One specific effect of this is to reduce the number of anode effects during the smelting process (i.e., when the alumina concentration gets too low, fluorides from the electrolyte and carbon from the anode form CF4 and C2F6, which are not absorbed in the gas scrubber.) Successful aluminum smelting relies on a controlled supply of alumina (i.e., aluminum oxide) in order for production to be optimized, and for anode effects to be prevented, to minimize greenhouse emissions and potline disturbances. Thus, with the intelligent crustbreaking cylinders in place. the productivity of each cell increases.

Before implementing the Parker technology, the Elkem smelter was operating conventional crustbreaking cylinders with fixed dwell times, regularly attempting to break the crust that forms atop the alumina and cryolite mixture in a smelting pot, as a part of the alumina-feeding operation. The dwell time was a compromise between a time long enough to get sufficient plunger force and short enough to avoid electrolyte deposits sticking to the plunger. There was no feedback as to whether or not the crust was broken successfully — sometimes resulting in alumina being poured on top of the crust and not entering the mixture. This results in anode effects.

Manual checks were required, with potroom attendants having to lift heavy covers from the pots in order to monitor their condition. With 1,100 feeders at the plant, needing one or two inspections every shift, this required a significant amount of man hours. These spot inspections were also relatively inefficient as there can be an anode effect just fifteen minutes after a feeder stops working.

Elkem process control manager Roald Hvidsten, explained: "Previous to integrating the Parker cylinders, we were experiencing 0.4 to 0.5 anode effects per pot-day. As well as the resulting impact on production, these anode effects were causing the release of CFgases into the environment (CF4 and C2F6). They have a greenhouse equivalent of 6,500 resp. 9,500 times compared to CO2, and a lifetime in the atmosphere of 10,000 years.

"We required a solution that could both solve our operational issues and reduce these emissions in line with government guidelines," Hvidsten recalls. Parker recommended the cylinder as part of a closed-loop feeder controls system.

Hvidsten details that Parker's experience as a supplier of pneumatics to the aluminum industry confirmed that the crust-breakers would be able to withstand the high temperatures, strong magnetic fields, electrical arcing, and abrasive environments typical of aluminum smelters, and that the system it developed for Elkem would last.

Parker conducted the product development for the cylinders in an environment matched to the harsh conditions found inside the plant's reduction cells, with high temperatures, heavy abrasive dust, and strong magnetic fields present, resulting in cylinders that are able to operate continuously at 150°C or intermittently at 200°C, with no need for additional lubrication. They are effectively sealed against dust (and in this case, alumina) and offer a service life of 20 years, with maintenance-free operation for five years. They are designed to deliver repeatability, accuracy, ruggedness, and dependability for high volume production.

"An important consideration in the project was to avoid introducing more critical components," Hvidsten adds. "The ‘intelligent' parts added to a standard cylinder would not be able to stop the feeding in case of a fault."

And, according to Elkem, Parker's intelligent crustbreaking cylinders have been one of the critical factors allowing the smelter to reduce the rate of anode effects per pot from one every second day to approximately one every 10 days.

Here's how the process works: The chisel-ended piston of each cylinder is lowered in order to break through the alumina crust, and is given six seconds to do so successfully, as opposed to 2.4 seconds with the previous crust-breaking solution. The extended time allows a full air-pressure buildup and maximum chisel force when there is hard crust. This keeps the feedhole open longer. Otherwise, when the feed hole is open, the chisel retracts immediately before it gets overheated. This prevents the electrolyte sticking to it.

The whole feeder cycle is monitored by checking the two end switches for status and timing. Any malfunction is alarmed, either to the process operators or to the mechanical maintenance group, depending on the type of failure.

Process alarms (stuck chisel, hard crust) are sent via PDAs to the pot operators, who work in dedicated small crust-breaking vehicles, so they're available to go straight to the pot that is reporting a problem. These alarms must be handled within 15-20 minutes in order to avoid anode effects.

In case the feeder control identifies a technical fault (e.g., stuck end switch), the control switches to a fallback mode, operating as a "dumb" feeder. Alarms are sent to the maintenance group, which can follow-up the next workday because the feeder continues to work (although with reduced functionality.) Hvidsten indicates that feeders' mechanical performance are also improved, compared with the 3,300 routine inspections per day with around 80 actions, which had been routine for the smelter's 404 cells.

"The intelligent crustbreaking cylinders have greatly increased efficiency levels at the plant, significantly cutting the number of anode effects that occur," Roald Hvidsten confirms. "Through the automated closed-loop control that they offer, we are able to use our resources much more effectively, and increase productivity as part of a complete solution."

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