Recycling of distributed aluminium turning scrap
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Minerals Engineering 15 (2002) 963–970
This article is also available online at:
www.elsevier.com/locate/mineng
Recycling of distributed aluminium turning scrap q
Y. Xiao *, M.A. Reuter
Raw Materials Technology, Delft University of Technology, Mijnbouwstraat 120, 2628 RX Delft, Netherlands
Received 7 May 2002; accepted 6 July 2002
Abstract
The relationship between scrap type and its recoverable metal content can play a crucial role in industrial recycling processes. In
this paper, the recyclability of different aluminium turnings has been experimentally studied. Various categories of scrap were melted
at 800 C to recover aluminium metal with the protective salt flux of NaCl–KCl–Na3 AlF6 under nitrogen atmosphere. In order to
understand the melting behaviour, thermo-gravimetric analysis was applied to investigate the weight loss during the melting process.
It is shown that the difficulty of recycling the selected aluminium scrap depends on scrap type, scrap size distribution, contaminant,
and the ratio of surface area to body volume. Lower distribution mean value, more non-metallic contaminates (oil, plastics), smaller
size and higher ratio of surface area to body volume generally led to a lower metal yield. The effect of cryolite addition on the metal
yield was also studied, especially for the scrap with higher ratio of surface area to body volume. It was shown that the accumulation
of the metal beads was improved with higher amount of cryolite addition. A simple statistical approach is presented to correlate
metal yield to scrap properties for inclusion in process optimisation and control models.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: Recycling; Pyrometallurgy; Mass balancing; Classification
1. Introduction to aluminiumÕs high reactivity, metal yield of aluminium
is a function of numerous parameters such as surface
The recycling of aluminium scrap has significant area to volume ratio (due to oxidised surface), shape of
economic, energy, environmental and resource savings the scrap, type of alloy, scrap history, contaminants (e.g.
implications. Comparing to the primary aluminium pro- oxides, water, oil and paint) and amount of required flux
duction, aluminium recycling has a great advantage due additives in the melting process. For example, any in-
to lower production cost (Campbell, 1996; Henstock, creased level of contamination on scrap reduces metal
1996). In order to efficiently recycle metals the industry recovery due to reaction with aluminium, and further
is faced by various issues which include scrap sampling, lowers the metal yield.
scrap purchasing, metal recovery (based on recoverable In the scrap yard, materials are usually identified by
metal in scrap) and yield (based on total mass of scrap), experienced sorters based on the object recognition,
production cost and hence profit margins, product knowledge of use, colour, and apparent density, and last
quality, environmental issues and regulation. but not least by proper sampling. In the secondary
The chemical composition of the molten aluminium smelting furnace, metal recovery and metal yield may
product is controlled not only by the process operation, vary according to the quality of the charge material, and
but also to a large extent by proper selection of charged the poorly sorted charge may to a certain extent widen
aluminium scrap. As the real metal content of the scrap the predicted range of metal yield. The metal recovery is
remains unknown, metal yield becomes a crucial factor defined as the percentage of metal gained from the metal
for the recycling of aluminium scrap. Unfortunately, due content of the scrap. However, the metal yield represents
the percentage of metal gained from the total mass of
scrap. Normally, the metal yield is lower than the metal
q recovery, due to the various contaminants and losses
Presented at Pyromet Õ02, Cape Town, South Africa, March 2002.
*
Corresponding author. Tel.: +31-15-2785580; fax: +31-15-
during melting. In the secondary aluminium production,
2782836. it is very important to emphasize these terms in order
E-mail address: y.xiao@ta.tudelft.nl (Y. Xiao). to obtain a precise interpretation of results and the
0892-6875/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 8 9 2 - 6 8 7 5 ( 0 2 ) 0 0 1 3 7 - 1964 Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970 modelling of them for optimising the furnaces and the was closed, and nitrogen gas was flushed through to complete plant. protect the system from oxidation. Through reviewing the information in the literature In addition, a few tests with thermo-gravimetric for the aluminium recycling, it was clear that most ef- analysis (TGA) were conducted in a resistance tube forts were laid on the economics, process analysis and furnace. The scrap was melted in an alumna crucible at industrial technology. For the fundamental research, 800 C with salt flux protection under a nitrogen at- several papers were found for discussing the recycling mosphere. of used beverage cans and aluminium dross (Peterson, The salt flux composition for all the experiments was 1990; Roy and Sahai, 1998; Utigard et al., 1998; Ye and 70 wt.% NaCl–30 wt.% KCl with additional different Sahai, 1996). In the authorsÕ previous work, various amounts of Na3 AlF6 . aluminium scrap were characterized and melted under Four types of aluminium turnings were investigated: different experimental conditions, including rolling mill cuttings, cast ingots, margarine foils, bottle caps (Xiao • the aluminium turning scrap A is characterised by a et al., 2000). small turning with included plastic pieces; Turnings from machining various aluminium prod- • sample B is characterised by small cuttings with a ucts, the focus of this paper, are recycled in a conven- wider size distribution, which was classified by sieving tional rotary salt slag melting furnaces. The surface area the scrap into different categories, to examine the ef- of this scrap is relatively high and covered with an alu- fect of turning sizes on the metal yield; minium oxide layer and machining oil. Depending on • sample C is a widely distributed cutting with varied the products, some turnings even contain significant width and length; amount of plastics, which are unfavourable for metal • sample D is oil contaminated characterised by smaller recovery and yield. The oxide film on the turnings, the turning chips. new oxide skin formed during the melting process and the related chemical reactions have a significant influ- After melting, the crucibles with the samples were ence on metal losses, and lead to a skim formation washed with water, and the metal beads and the pre- during melting. To facilitate handling of these alumin- cipitates were filtered, dried and sieved. The filtered ium turnings and to reduce the metal loss in an indus- precipitates were prepared for XRF and XRD analysis. trial process, it is essential to investigate the melting After mass balancing, the fraction of metal recovered behaviour of these different aluminium turnings. with respect to the total scrap (in other words the metal In the present paper, the melting experiments were yield) was calculated and the size distribution of recov- carried out in an electrical resistance chamber furnace in ered metal beads was measured. The metal beads are a laboratory scale with controlled nitrogen atmosphere created due to the high surface tension created by the at 800 C. The NaCl–KCl–Na3 AlF6 system was used as oxidic and other products that collect on the surface of salt flux for protecting metal from oxidation, absorbing the molten aluminium and the fact that no stirring oc- contaminates, and promoting coalescence of aluminium curs in the crucible. Note that due to unknown quan- droplets. The recoverable metal contents in different tities of oxide material entering, oxidation and side scrap are discussed. The main objective is to study the reactions considerable care must be taken to produce a recyclability of different turning scrap and its relation good mass balance. This in addition to careful experi- with turning size and melting conditions and also find a mentation is of utmost importance to produce good model to predict the metal yield. yield data. 2. Experimental 3. Results and discussion The experiments were carried out in a high temper- Various turning scrap were melted under the same ature Carbolite chamber furnace equipped with a re- experimental conditions. In addition, the effect of turn- movable stainless steel retort to ensure that the N2 ing size and cryolite addition on the melting behaviour atmosphere can be controlled. The temperature was was investigated. controlled within an accuracy of 5 C. The gas flow rate was controlled at 2 l/min. Alumina crucibles were 3.1. Melting of various Al turnings used in the experiments. Sufficient amount of salt was added to cover the scrap, to minimise oxidation and to The experimental results for scrap A and B before and collect the various contaminants from the scrap. To after melting are shown in Figs. 1 and 2. In order to ensure a good fluidity of the molten flux, a weight ratio better evaluate the metal content and the recyclability of of salt to scrap was controlled to be 2. After the crucible different turnings, the metal recovered from all four scrap was charged and placed within the furnace, the system types and the amount of metal beads in the size bigger
Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970 965
Fig. 1. Scrap A (a) before melting and (b) after melting.
Fig. 2. Scrap B (a) before melting and (b) after melting.
than 2 mm are shown in Fig. 3. From these results, it can 3.2. Effect of scrap distribution on metal yield
be seen that the recovered metal amount from turning
scrap A is 84.3 wt.%, and that from scrap B, C and D are From the above results it is suggested that both the
95.3, 94.5 and 91.8 wt.%, respectively. This represents the metal yield and the product (metal beads) size distri-
metal content in the scrap if disregarding the possible bution are highly affected among other factors by the
reactions of salt flux with metal. Under similar condi- size distribution of the charging scrap. In order to
tions, the metal beads with bigger size are easier to co- quantify this correlation and to gain insight into the
alescence and settle from flux to metal. melting behaviour in relation to the scrap size, the
The scrap with more percentage of bigger metal beads asymmetrical Weibull distribution function (Eq. (1)) was
(>2mm) would be considered to have better recyclabil- applied to characterise the size distribution of different
ity. Therefore, it can be concluded that scrap B and C scrap before melting and the metal product leads after
produce a higher yield and could therefore be easier to melting (Figs. 1 and 2).
recycle than scrap A and D. ða1Þ
a x a
f ðxÞ ¼ eðx=bÞ ð1Þ
b b
In Eq. (1) the a value defines the spread of the data
while a larger b value leads essentially to a higher
mean value.
In order to investigate the scrap distribution on metal
yield, representative amount of samples from different
scrap and the recovered metal beads were sieved into six
size fractions viz. 12 mm.
Fig. 4 depicts the measured and calculated size distri-
butions of the scrap before and after melting.
Fig. 4 depicts the size distributions of the feed and
the final product. It is clear that the product of scrap A
and D are similar, while that of B and C also, in spite
Fig. 3. Metal yield from melting different Al turnings. of different feed size distributions.966 Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970
Fig. 4. Probability distribution function f ðxÞ with size x for feed material and final product beads.
Fig. 5. Effect of turning distribution on metal beads distribution and metal yield: (a) effect of a and (b) effect of b.Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970 967
In order to formalise the results in Fig. 4, the rela- However, the slightly lower yield for the melting of the
tionship between the a and b values before and after turning with size 8–12 mm may be caused by the loose
scrap melting and their effect on the metal yield are compacting of the scrap in the crucible and possible
depicted by Fig. 5. It is known that the metal yield is not aluminium being lost due to oxidation since it was not
only affected by the scrap distribution, but also to a completely covered by salt.
great extent influenced by the contaminants in the scrap.
As is clear from Fig. 5, a lower a and higher b value for 3.4. Effect of cryolite on the melting behaviour
the scrap will result in higher metal yield. This does not
hold for scrap D, however the result for scrap D can be The salt composition was selected based on the pre-
explained by its high oil contamination. ferred European melt salt composition, considering that
In summary, therefore, for the melting product, it is KCl is more expensive than NaCl. Theoretically, the salt
clearly shown that a higher a value (narrow distribution) flux on equimolar NaCl–KCl composition (i.e. 44 wt.%
and b (larger average particle size) value always corre- NaCl–56 wt.% KCl) would give better melting results,
spond to a higher metal yield. which corresponds to the eutectic temperature of about
In general, to obtain more constructive relations for 650 C. Addition of cryolite to the mixture of NaCl and
establishing a statistical model, further research is nec- KCl was to increase the interfacial tension between the
essary, and more experimental data are required. The salt and the molten metal, enhance the stripping of oxide
results in this study show a realistic path for research to film from metal droplets, favour the agglomeration of
follow in the future in order to understand the governing the metal drops and reduce the aluminium loss by en-
parameters, which influence metal recovery and yield in trapped metal into salt slag.
the industrial process. Samples from scrap A and scrap B with size 2–4 mm
were melted at 800 C for 2 h with the salt flux com-
position of 70 wt.% NaCl, 30 wt.% KCl and additional
3.3. Melting of scrap B with different sizes
5, 10, 15 and 20 wt.% cryolite, respectively, under N2
atmosphere. It proves that cryolite addition has positive
Each of the five categories of scrap B was melted at
effect on the metal coalescence. The results for scrap A
800 C for 2 h with the salt flux composition of 70 wt.%
are illustrated in Figs. 7–10.
NaCl, 30 wt.% KCl and additional 5 wt.% cryolite under
nitrogen atmosphere. As with the previous experiments,
the metal beads were recovered from the salt in the
crucibles after melting by dissolving the salt in water.
The recovered metal was measured and classified. The
amount of metal recovered after melting is shown in Fig.
6 for beads larger than 2 mm.
It shows that the smallest size (968 Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970
coalesced. Cryolite addition at the same time increases
the density of the salt flux, thus reduce the density dif-
ference between the salt and the metal. This discourages
the metal separation from the salt, and further the metal
phase down settling and coalescing.
3.5. TGA experiments
The weight losses recorded from TGA experiments
are demonstrated in Fig. 12. It is seen that the weight
loss of the scrap during heating began at about 150 C
Fig. 9. Scrap A with 15 wt.% cryolite.
and took place in stages. In the first stage, there is a
sharp drop of the weight for all three tested scrap, due
to the volatilisation and decomposition of the organic
contaminants. It is also clear that scrap A has the highest
weight loss compared to scrap B. For scrap B melting
smaller size results in higher weight loss. When the
temperature increases to around the melting points of
the scrap, there exists a transient stage for all three tests.
Further increasing the temperature, the weight is drop-
ped linearly with time, and scrap types do not affect the
rate of the weight loss. This may be contributed mainly
by volatilisation of the salt. When temperature lowered
to the solidifying temperature, the weight was further
decreased but with slower rate.
Fig. 10. Scrap A with 20 wt.% cryolite.
The main products from the decomposition of or-
ganic materials are normally gases and fine carbon
precipitates. For the melting of the scrap A, due to the
A thin black layer was formed on the top of the cru- significant amount of plastics, a thick black layer was
cible. The precipitated black powder (carbon) amount is formed on the top of the crucible after melting. With
increased with cryolite addition. The size distributions increasing the amount of cryolite in the salt flux, more
of the recovered metal beads after melting with the effect black (carbon) was precipitated out. For melting of
of scrap size and cryolite addition are represented in Fig. scrap B, a thinner black layer was observed.
11. In general, higher concentration of cryolite in the Based on the scrap analysis, both scrap A and B
salt flux and bigger size of scrap give better coalescence contains magnesium as alloying element. Scrap A (0.11%
of the metal droplets. For the scrap A, if further in- Mg) contains more magnesium than scrap B (0.02% Mg),
creasing cryolite to 20 wt.%, the metal droplets were less and scrap B contains higher silicon (10.2%). Mg acting
Fig. 11. Size distribution of the metal beads from aluminium turning scrap with different cryolite additions and scrap sizes.Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970 969
Fig. 12. Weight changes during scrap melting.
as a surface-active element (Roy and Sahai, 1993) in the 3.6. Mass balance
aluminium melt can lower the surface tension of the melt.
During melting process, if the protective oxide layer on A good mass balance could be attained in the melting
the scrap (molten metal) is ruptured by the salt flux, the experiments. The weight of charged crucible before and
metal phase will be exposed to the flux, and magnesium after the experiment was measured for the various alu-
as the surface-active element will react with salt flux. minium scrap type investigated in this study. The rela-
Through analysing the residual of salt slags, neighborite tionship between the total weight loss of the charged
(NaMgF3 ) was determined to be present in the salt slag, crucible and the recovered metal is given in Fig. 13.
and the amount in the salt slag from melting scrap A is Usually a lower weight loss of the crucible set will result
significantly higher than that from melting scrap B. This in a higher percentage of metal yield. The total weight
has been proved to be feasible according to thermody- loss is contributed mainly by volatilisation and burning
namic calculations. For the scrap B containing higher (decomposition) of organic contaminants in the scrap.
silicon, it was found that there existed certain amount of The amount of salt added may affect the metal loss.
silica in the salt slag residue. Inorganic impurities from the scrap are dispersed in the
It is also clear from Figs. 3 and 6, the metal yield for salt-phase and may change the density and viscosity of
scrap B is higher than for scrap A. This can be attrib- the molten salt slag. If the amount of salt is not enough,
uted to the weight loss depicted by Fig. 12 indicating the the high concentration of oxides and other contami-
effect of organic compounds, oils, etc. on metal yield. nants in the salt may lead to a high viscosity of the
Fig. 13. The relationship between the total weight loss of charged crucible and the percentage of metal yield.970 Y. Xiao, M.A. Reuter / Minerals Engineering 15 (2002) 963–970
molten salt. The metal drops distributed in the salt may derstanding of the melting behaviour of the distributed
be difficult to settle down, and the more viscous slag will turning scrap.
keep the metal droplets entrapped and lead to a signif- The distributed metal yield as a function of scrap type
icant metal loss. and size is the basis for establishing a future statistical
As it is well known, there is always an oxide film on model to ensure better product quality in the recycling
the surface of aluminium scrap. Usually, the thickness of industry. Some indications are given here how such a
the oxide film depends on the scrap compositions and its statistical model could look like. In addition, the data
history (van Linden and Reavis, 1986). The identical presented here for use in population balance models are
materials with different using history may enter the melt currently being developed by the authors for aluminium
with different oxide contents. Oxides on the aluminium scrap melting.
turnings may account for a significant amount. During
the melting, usually the metal phase will melt first due to
the lower melting point, and settled down on the bottom Acknowledgements
of the crucible. Small metal beads were entrapped into
salt flux even with the addition of cryolite, possibly due The aluminium turning scrap and the analysis sup-
to the looser compactness. The major metal phase plied by Brinker Aluminium Schmelzwerk GmbH, Ger-
formed a ball with less adhesion to the flux layer. many, are acknowledged.
4. Conclusions References
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turning scrap was investigated. The melting experiments primary metals production. International Council on Metals and
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Peterson, R.D., 1990. Effect of salt flux additives on aluminum droplet
have significant effect on the melting behaviour. The coalescence. In: Second International Symposium––Recycling of
metal recovered from the turning scrap ranges from 84 Metals and Engineering Materials. The Minerals, Metals and
to 95 wt.%, representing the metal content of the scrap if Materials Society, pp. 69–84.
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The Minerals, Metals and Materials Society, pp. 1067–1072.
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metal) is better than scrap A (84.3 wt.% recovered me- aluminum. In: Welch, B. (Ed.), Light Metals. The Minerals, Metals
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creasing cryolite from 5 to 15 wt.%. The accuracy of the Boin, U., 2000. Experimental study on aluminium scrap recycling.
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