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Industrial Processes
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Cutting the cost of recycled aluminium

   
 
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The competitiveness of recycled aluminium is likely to improve, thanks to a new low-cost re-melting technique developed by a small group of industrial and research partners. They have discovered, optimised and patented a means of recovering aluminium from scrap using phase separation, allowing greatly reduced consumption of expensive chemical salts. For some applications, a completely salt-free recovery process is possible. The cost-cutting technology has already been proven on an industrial scale. The progressive introduction of more modern plants will see its economic and environmental benefits spread over the coming years.

The 1.6 million tonnes of secondary aluminium recovered from recycled scraps in Europe each year supplies as much as 35% of the continent's aluminium requirements.
The traditional re-melting process makes extensive use of costly chemical salts to separate the metal from its oxides. World-wide, producers have for some time been looking for ways to reduce the cost of recovery.
This project has made an important contribution to this effort, which will help to maintain European competitiveness in this market. Industrial companies and research institutions in France, Belgium and Spain have developed a new method which allows aluminium scrap to be re-melted entirely without salt.

A low-salt diet

The central problem in the recovery of aluminium from scrap is to separate the metal from its coating of oxides. This is achieved by melting the scrap with salts capturing the oxides in a liquid slag which can then be poured from the furnace, leaving the metal behind.
The greater the proportion of oxides in the scrap, however, the larger the quantity of salt needed to collect them, and the less cost-effective the recovery of the aluminium becomes. Not only is the salt itself expensive, resources are also consumed in heating it and in reclaiming it from the slag. If the metal content of the scrap is low, recovery is simply becoming uneconomical.
Using a conventional aluminium salt bath rotary furnace, any reduction in the amount of salt simply decreases the efficiency of the process, producing less reusable metal. The goal of the project was to improve the cost-effectiveness of re-melting by reducing the quantities of salt used, without impairing the rate of recovery.

Happy accident

The project was led by the French secondary smelter Affimet, a subsidiary of the Aluminium Pechiney group, which recovers aluminium for use in the production of casting alloys. Affimet and its partners set out to investigate ways of reducing salt consumption through the adjustment of process parameters and the development of new equipment.
Their initial approach was to look for material properties which could be used to control the behaviour of the liquid slag inside the furnace. The project's academic partner, the Université Libre de Bruxelles (ULB), undertook a series of dynamic viscosity measurements, but decomposition of the salt slag was found to make such measurement impossible.
The partners' disappointment was balanced by their discovery that the viscometer could be used to separate the solid and liquid phases of a multi-phase molten slag. Following this insight, the work plan was reoriented to explore the potential of phase separation as a method for drawing liquid salt and oxides off the aluminium granules.
ULB designed special laboratory scale devices, of three litre and ten litre capacities. These were used in a series of experiments designed to evaluate the effectiveness of the new method as a means of recovering aluminium, both with and without salt, from different types of scrap. The laboratory also sought to optimise the process by precise control of furnace temperature, air intake, and other parameters.

From laboratory to industry

Affimet itself carried out the first small-scale industrial test, and succeeded in confirming the laboratory results. The industrial potential of the approach was then assessed in a further series of tests, on a progressively larger scale.
The first study was conducted by research laboratory Inasmet in Bilbao, using a special one tonne rotary-tilting furnace of the kind already widely employed in the copper and steel industries. Affimet carried out a further series of tests, using a 20 tonne conventional rotary salt bath furnace.
Taken together, the results were conclusive. By carefully controlling process parameters it was possible either to reduce the quantity of salt or, under certain conditions, and only in the rotary-tilting furnace, to re-melt without salt altogether.

Commercial application of a patented technique

Affimet is already beginning to apply the project's results within its own operations. The optimisation and more accurate control of the re-melting process, designed to minimise salt consumption, has been supported by a programme of operator re-training. The next step will be to invest in an industrial-scale rotary-tilting furnace. The Pechiney Research Centre, also part of the Aluminium Pechiney group, has built a small rotary-tilting device as a way of developing the company's own technical expertise, and of generating data about the new process.
However, Aluminium Pechiney has no ambition to produce its own full-scale plant, and devices of this type are in any case already available on the market. Competitors in Europe, and in Canada and the United States, have also been working towards the application of this technology in the field of secondary aluminium smelting. However, there is no indication that they have succeeded in matching Affimet's salt-free technique, for which a European patent is pending.
Affimet's objective now is to apply the project results in order to improve its own cost structure, however, those results will eventually be of benefit to all secondary aluminium producers. ULB is pressing forward with further research studies, and the partners are convinced that before long the industry will employ a mixture of traditional rotary salt slag furnaces, alongside newer rotary-tilting furnaces.
Rotary-tilting plants will use the Affimet techniques to keep down costs by reducing, or cutting out altogether, the use of salt. Conventional furnaces, and the older salt slag technique, on the other hand, will continue to be used for the re-melting of scrap such as lathe turnings, in order to avoid the burn-out of fine particles of aluminium.

 

 

Project Title:  
Procede avance d'affinage d'aluminium avec reduction des couts et reduction des flux polluants

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: PL-20

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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