Aluminium Industry Research Papers - Academia.edu (original) (raw)

SUMMARY
Primary aluminium production is an energy intensive process. With an increase of the
aluminium use in car manufacturing a recycling technological process is imperative. The
energy use for recycling is only a fraction of the energy needed to produce primary
aluminium. To this end a joint project with Corus plc in IJmuiden, TNO-MEP in Apeldoorn
and TNO Institute of Industrial Technology in Eindhoven was carried out as a part of the
research project Sustainable Technology for the Reclamation of High-grade Aluminium
from Scrap, which is undertaken in the context of the Dutch R&D stimulation programme
on Economy, Ecology and Technology (contract number EETK98026). While our partners
worked on a suspension based fractional crystallisation process, our research at Delft
University of Technology was concentrated on layer based fractional crystallization
technique for aluminium scrap purification.
Historically speaking, three stages can be recognized in the development of fractional
crystallization processes for the purpose of metal refining. A first milestone was the
invention of the Pattinson process for the extraction of silver from lead in 1833, the second
was the development of the zone-melting technique in 1952, and the third one is the socalled
Yunnan crystalliser for refining of tin as of 1975, which is probably the most
advanced application for metals at the moment. Fractional crystallization, widely known in
the metals world as fractional solidification, is a separation technique applied for
purification of metals and organic melts. While most of the currently available processes
are mainly developed for the production of high-purity metals (99.99 wt % and more), the
emerging technologies appear to be exploring the possibilities for aluminium scrap
recycling. For the latter application, proposed methods have not left the laboratory stage yet
with research in this field still ongoing. General restrictions are that the processes are quite
slow and of limited production capacity. Also, the different requirements on the formation
of the crystals (concerning crystal growth rates and stirring) and the separation of the solid
from the liquid fraction are in most cases not fully met. Consequently, there still is room for
further progress.
Fractional crystallization technique is much more advanced for organic compounds than for
metals. This is mainly due to the better process conditions, such as low melting temperature
and lower reactor attack, of organic materials. However, a careful study of possibilities of
adopting these processes for metals will open new horizons. Prospects are good for both
modes of operation: suspension and layer crystallization. The most important metals that
could use this technique are aluminium, silicon, magnesium, lead, and precious metals.
Zone melting is a relatively simple and useful experimental technique to assess the ability
to refine distinct alloying constituents from complicated alloy systems (like industrial kinds
of scrap aluminium). Regarding the obtained purification in the experiments, results
basically comply with the estimates based on the binary alloy phase diagrams, although
there are known distinct deviations from other experimentally achieved results. Chemical
analyses reveal that purity of the samples on the head and on the middle positions are
almost the same, which reveals that purification takes place in more than the half of the
original samples.
Microscopic examination of the AlSi6 alloy reveals that while aluminium cells grow, the
impurities that rich eutectic concentration are trapped between them. The AlSi0.4Fe1.6
alloying system micrographs show a flat interface till about 40 per cent of the sample is
crystallized and later a eutectic growth resumes. The other two systems studied in this
thesis (AlCu4Mg1.5 and AlSi1.5Mn1.1) grow with flat interface and no change in their
morphology was observed by microscopy.
In chapter 5 a fractional crystallization process for aluminium recycling based on static
layer growth is described. Static layer crystallization experiments showed that for AlSi7.2
alloying systems, typical growth rates of 1 to 2 μm/s are optimal for aluminium scrap
purification. In order to achieve the preset purification criteria for the pure product, one or
more re-crystallizations steps are required. Depending on the impurity concentration, the
crystallization temperature varies from 600 to 660 °C.
Refining aluminium by means of layer growth fractional crystallisation is technically a
viable option in developing a sustainable technology for recycling of a high-grade
aluminium from scrap. Chemical analyses reveal that significant purification is achieved.
However, if an effective removal of the alloying elements from aluminium scrap is to be
achieved a repetition of the crystallisation process has to be performed. Microscopic
examinations of the crystal samples, cut at different directions of crystal growth, reveal that
while crystals are growing as primary aluminium cells, an impurity enriched melt is
entrapped in the intercellular space. Line scans reveal that concentration of impurities
within cells is in line with thermodynamic predictions while impurity rich area is detected
in the intercellular space. In the intercellular space a number of intermetalic compounds
were found to be present. This leads to decreased purification efficiency of the layer grown
crystals.
Application of the post-purification processes, such as washing or sweating, may increase
the purification efficiency significantly. During layer growth purification has taken place
and during the sweating stage additional purification occurs. With the increasing amount of
sweating the intercellular eutectic material is partially removed, while the impurities within
the grains are not affected. Sweating is based on the partial melting of the crystal layer by
gentle heating close to the melting point of the pure substance. The temperature during the
sweating process is higher than in the crystallization stage. Although during the sweating
process about 10 - 20 % of the crystals are removed, sweating is effective and avoids an
additional crystallization step in a fraction of the time. The amount of material sweated out
is higher than the amount of the eutectic present, which means that also some crystals of the
layer are drained off. For an effective purification two strategies may be considered. The
first is only layer crystallization and the second is layer crystallization at a higher growth
rate, followed by a post-purification step such as sweating. The later saves costs and energy.
Falling film crystallization process proved to have several advantages over the static
crystallizer. As a result of the flowing film the liquid becomes turbulent. The turbulence
has positive effect on enhancing both energy and mass transfer of the crystallizer. Due to
this effect falling film crystallizer for organic compounds achieves the same purification
efficiency, as for static crystallizer, with four to five times higher growth rate.
Theoretical analysis of the falling film process for aluminium scrap refining reveals that the
optimum growth rate of such crystallizer is within the order of 10-5 m/s, which is about 10
times faster than optimum growth rate for static process. This increase in the optimum
growth rate is attributed to the improved mass and heat transfer due to the turbulence that
appears as a result of the melt flow down the heat exchanger wall. The higher growth rates
compared to static crystallizer make this process suitable for a process that requires high
production rates, which with aluminium scrap recycling is the case. Further (experimental)
studies are needed, whereby several technical challenges associated with falling film
crystallization of aluminium are addressed, before the techno-economic potential of this
new technology can be established unambiguously.