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Metals and Minerals Processing News
News Release from: Parker Hannifin | Subject: Alumina smelter crust breakers
Edited by the Processingtalk Editorial
Team on 23 January 2007
Reducing anode effects in aluminium
smelting
Intelligent crust breaking cylinders from Parker have helped cut anode effects by 75% in aluminium smelting plant, increasing efficiency and reducing greenhouse emissions of CF4 and C2F6 as well
Parker, the world's largest manufacturer of motion and control technology, is helping the Elkem Aluminium plant in Mosjoen, Norway to streamline processes and significantly increase efficiency through the use of its intelligent pneumatic crust breaking cylinders As well as being able to withstand the extreme conditions within an aluminium smelter, the cylinders feature leading edge closed loop control, to improve the performance of the feeders and enable the plant cell operators and maintenance crew to respond to problems quickly, reducing the number of anode effects* (and thereby greenhouse emissions) and also increasing productivity
This article was originally published on Processingtalk on 19 Jun 2008 at 8.00am (UK)
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The successful smelting of aluminium relies both on a controlled supply of aluminium oxide, also known as alumina, in order for production to be optimised and for anode effects to be prevented, to minimise greenhouse emissions and potline disturbances .
Prior to implementing the Parker technology, the Elkem plant was operating conventional crust breaking cylinders with fixed dwell times, regularly attempting to break the crust which forms on top of the alumina and cryolite mixture in a smelting pot, as a part of the alumina feed operation.
The dwell time selected was a compromise between having sufficient time to achieve enough plunger force, and a short enough time to avoid electrolyte deposits sticking to the plunger.
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There was no feedback on whether the crust was successfully broken, often resulting in alumina being poured on top of the crust and not entering the mixture, resulting in anode effects.
In response to these problems, manual checks were frequently required, with process staff 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.
Roald Hvidsten, Process Control Manager at the Elkem plant, 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 CF gases into the environment (CF4 and C2F6).
They have a greenhose equivalent of 6,500 and 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; Parker offered us the cylinder that is the key component in our closed-loop feeder control.
Parker experience, as the leading supplier of pneumatics to the aluminium industry, made it clear that they were best suited to supplying us with the crust breakers.
Its products have proven ability to withstand the high temperatures, strong magnetic fields, electrical arcing, and abrasive environments typical of aluminium smelters, which reassured us that we would end up with a robust solution that would last.
An important consideration in the project was to not introduce more critical components.
The 'intelligent' parts added to a standard cylinder would not be able to stop the feeding in case of a fault".
In particular, the Parker intelligent crust breaking cylinders have been one of the key enablers for Elkem to reduce the rate of anode effects per pot from one every second day to approximately one every 10 days.
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 build-up 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 preventing 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 (e.g stuck chisel, hard crust) are sent to the pot operators via a PDA in dedicated small crust-breaking vehicles, allowing them 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 fall-back mode, operating as a "dumb" feeder, while alarms are given to the maintenance group.
These problems can wait until next workday, since the feeder continues to work (although with reduced functionality).
Product development for the cylinders was carried out in an environment matched to the harsh conditions found inside the plant reduction cells, with high temperatures, heavy abrasive dust, and strong magnetic fields present, resulting in cylinders that are able to operate continuously at 150C or intermittently at 200C, 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, delivering repeatability, accuracy, and rugged dependability for high volume production requirements.
Roald Hvidsten is clear about the benefits that the Parker cylinders are offering the Elkem plant, "The intelligent crust breaking cylinders have greatly increased efficiency levels at the plant, significantly cutting the number of anode effects that occur.
The new feeders give a better mechanical performance, and also replace the earlier 3,300 routine inspections per day with around 80 actions to feeder alarms on our 404 cells.
Through the automated closed loop control that they offer, we are able to utilise our resources much more effectively, and increase productivity as part of a complete solution".
Note * : When the alumina concentration falls below an acceptable level, fluorides from the electrolyte and carbon from the anode form CF4 and C2F6.
These gases are not absorbed in the gas scrubber, but pass through. Request a free brochure from Parker Hannifin ...
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