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
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.
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
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
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.