Waste Management 23 (2003) 173–182
www.elsevier.com/locate/wasman
Preparation of aluminium–magnesium alloys and some valuable
salts from used beverage cans
Mahmoud A. Rabah*
Central Metallurgical R&D Institute (CMRDI), Industrial Wastes Laboratory, PO Box 87, Helwan 11421, Cairo, Egypt
Abstract
The purpose of this work is to recover standard aluminium–magnesium alloy(s) and some valuable salts from used beverage cans
(UBCs). The suggested method updated the current recycling technology by augmenting removal of the coating paint, decreasing
magnesium loss during melting process and improving hydrochloric acid leaching of the formed slag. Iron impurity present in the
leaching solution, was removed by oxidation using oxygen gas or hydrogen peroxide and filtered as goethite. Results obtained
revealed that a mixture of methyl ethyl ketone/dimethyl formamide entirely removes the paint coating at room temperature. The
process compares favorably to the current methods involving firing or swell peeling. The coating decomposes to titanium dioxide by
heating at 750 C for 30 min. Standard compositions of Al–Mg alloys are formulated using secondary magnesium. The extent of
recovery (R̆) of these alloy(s) is a function of the melting time and temperature and type of the flux. The maximum (R̆) value
amounts to 94.4%. Sodium borate/chloride mix decreases magnesium loss to a minimum. The extent of leaching valuable salts from
the slag increases with increasing the molarity, stoichiometric ratio and leaching temperature of the acid used. Removal of iron is a
function of the potential of the oxidation process. Stannous chloride has been recovered from the recovered and dried salts by
distillation at 700–750 C.
# 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
The recycling of aluminium beverage cans reduces
waste, saves energy, conserves natural resources, lessens
use of municipal landfills and provides recyclers and
municipalities with considerable revenue. The energy
needed to produce each tonne of primary new metal
from raw materials was reduced by over 30% in the last
35 years (RecycleNet Corporation, 2000).
Conventional methods for reproduction of UBCs
included crushing, roasting, grinding and press forming.
However, the overall losses were not given in details but
the economic effects reached more than 30% (Arikata,
1997). Litalien et al. (1997), showed a recovery process
of wrought alloys from mixed alloy aluminium scraps.
The authors described two newly developed enabling
technologies aiming to recover the full value of the aluminium from scrap containing varying amounts of contamination. They agreed with Oosumi (1995) that paints
applied to aluminium beverage cans were the source of
titanium impurity in the recycled aluminium alloys.
* Tel.: +20-2-5010642; fax: +20-2-5010639.
E-mail address: rucmrdi@rusys.eg.net (M.A. Rabah).
Takahashi et al. (1997) and Fujisawa et al. (1998)
showed that paints occurred as unfavorable molten
metal compositions (primarily titanium) and thus lower
the metal yield. They applied a swell-peeling method to
remove paints and reported that the method improved
the molten metal yield and prevented titanium from
contaminating the molten metal simultaneously. In
Finland, Worden (1999) recovered about 2200 tons of
aluminium from food and drink packaging. Foster
Wheeler Service Oy supplied a bubbling-bed gasifier and
gas boiler for the recycling project. The project would
be the first of its kind in the world, Foster Wheeler said.
The product was a primary aluminium alloy.
In 1998 Thomas et al. (1998) recycled used beverage
cans (UBCs) using a closed loop recycling system. In
that process the key element in the recycling process was
thermal removal of organic coatings. The organic concentration in UBCs was around 4%. The cans were
melted with manufacturing scrap and some prime metal.
Such thermal de-coating allowed efficient removal of
organic materials from scraps with up to 50% organic
concentration, the authors said.
Sahai et al. (1998) showed that sulfate salt was a
harmful flux in terms of their chemical interaction with
aluminium metal. The authors avoided such effect by
0956-053X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
PII: S0956-053X(02)00152-6
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M.A. Rabah / Waste Management 23 (2003) 173–182
adding carbon and soda ash. However, the authors had
seen some industrial implications involving extra ordinary quantity of aluminium that went to slag. Roy (1998)
showed that the molten salt flux became progressively
viscous as the oxide films stripped from metal were suspended in the flux. Implications such as coalescence of
aluminium and UBC alloy drops were claimed. A model
explaining the process of coalescence was developed. A
ternary interfacial energy diagram was constructed for
Al–Al2O3–salt system and its relation to the removal of
oxide film from the metal was discussed. Rabah and ElSayed (1994) reported that ammonium chloride promoted the highest recovery of secondary zinc as compared with both sodium and potassium chlorides. On
the other hand, sodium borate and carbon mix look
better compared to ammonium chloride for the recovery
of secondary copper at 1150–1300 C (Rabah, 1998).
Rao and Finch (1992) and Sole and Hiskey (1991)
studied the recovery of metal values from effluent solution and slag formed during melting processes respectively by hydrometallurgical methods. The recovery of
metals from weakly or strongly acidic chloride solutions
could be performed by means of neutral or anionexchange extract and, after stripping, the metals were in
chloride medium again (Dalton et al., 1991). Alex et al.
(1995), showed that, in the presence of oxygen, iron
impurity precipitated from acid copper chloride solution
as goethite (FeO.OH). Abdul Basir and Rabah (1990)
and Rabah (2000) reported that addition of hydrogen
peroxide to the acidified chloride solution of the leached
metals from slag helped oxidation of iron to goethite.
The objective of this work is to recover standard aluminium–magnesium alloy(s) and some valuable salts
such as sulfate and chloride from used beverage cans
(UBCs). The method focused on updating the current
recycling technology by augmenting the removal of the
paint coating, decreasing magnesium loss during melting and improving hydrochloric acid leaching of the
formed slag. For refining purposes, iron present in the
leaching solution was removed by oxidation using oxygen gas or hydrogen peroxide and filtered as goethite.
Parameters affecting the extent of recovery and quality
of the products were investigated.
1.1. The proposed flow sheet
The proposed flow sheet is shown in Fig. 1. The
UBCs were first de-coated using different ways including firing, swell peeling, sand blasting and organic solvents. After filtration, the cans were rinsed with tap
water and left to natural drying. They were then pressed
into blocks prior to melting. The required flux dose was
loaded and melted in a silicon carbide crucible placed in
a crucible furnace maintained at the required temperature. The UBCs blocks were fed stepwise and stirred.
The molten alloy produced was poured in a graphite
receiver. Metals that went to slag were extracted by
leaching in HCl acid.
2. Experimental
2.1. Materials
An Egyptian company concerned with marketing
spent artifacts supplied the used beverage cans (UBCs)
sample weighing 50 kg from different beverage producers. The UBCs were in different states of deformation.
The sample was washed using a detergent solution,
rinsed with tap water and dried before use. The chemicals used in analysis were of pure grade. Commercial
grade of ammonium chloride, sodium borate, carbonate, chloride, molybdate, pyrophosphate, potassium
chloride, sulfide, and molybdate were separately or a
mixture of them used as flux or as de-oxidizing agent.
Spent active carbon powder, 76 mm (from beverage
industry) and hydrogen gas were used for thermal
reduction. Twice distilled water was used for chemical
analysis whereas tap water was used for other purposes.
2.2. De-coating of the used beverage cans (UBCs)
The UBCs are usually coated with a painting of four
colors, 4–6 mm in thickness. Removal of this coating
was carried out either by solvent extraction, firing or by
mechanical sand blasting.
2.2.1. De-coating by solvent extraction
A solvent mixture consisted of dimethyl formamide,
HCON(CH3)2 (one part), and methyl ethyl-ketone (one
part), in different parts of water was used for removing
the coating. The UBCs were placed in a basket with a
perforated bottom. The basket with the UBCs was then
immersed in the solvent. The basket revolved for 5–10
min after which it stopped rotating and the solvent was
drained. After draining the solvent, the cans were brushed under a circulating water shower. At the end, the
UBCs were unloaded and left to dry. The eluted coating
was separated from the solvent by filtering using a cloth
filter. The solvent and the rinsing water were separately
recycled.
2.2.2. De-coating by sand blasting
Sand blasting technique for 15 min was also tested to
remove the paint coating from the UBCs. Sand having
grain size < 0.5–0.25 mm in diameter was used. The
used sand was leached with HCl acid to recover titanium oxide.
2.2.3. De-coating by firing
The UBCs were placed on a belt chain of a conveyor
furnace and fired using two oppositely mounted natural
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M.A. Rabah / Waste Management 23 (2003) 173–182
Fig. 1. The process flow sheet for recycling the used beverage cans (UBCs).
gas blowers. Firing was effected at about 350 C for
3–5 min. Coating after firing escapes as fugitive emissions.
2.3. Pressing of the cleaned UBCs into solid blocks
The cleaned cans were rolled using a double rolling
machine to form plain sheets. The sheets were then
pressed under a pneumatic pressure of 100 t/cm2 into
blocks using a hydraulic press type M-200, Erhardt,
germany, before melting.
2.4. Melting of the pressed UBCs blocks
Blocks weighing 5 kg each, were fed into a crucible
furnace preheated at the required temperature up to
1100 C. A silicon carbide crucible, 2 l capacity, was
loaded with the salt flux before feeding the blocks. After
melting, the metal alloy(s) was stirred and soaked for
3–5 min before pouring. The slag that was formed was
regularly skimmed off. A sample of the molten alloy was
analyzed and the standard composition was adjusted by
addition of secondary or primary magnesium before
pouring.
2.6. Analytical methods
Analysis of the input UBCs, intermediate and end
products was carried out by a chemical method and
with the help of an atomic absorption spectrometer type
Perkin Elmer 2380 and a laser induced optical emission
spectrophotometer type 3460-880 ARL-Seuces. The
weight percentage of slag (Ws) and the recovered metal
ingot (Wr) were determined gravimetrically. Loss in
weight percentage due to escape of metal(s) vapor or
other volatile (WV) was computed from the relation: WV
=Ws (Wsm+ Ws) where Wsm is the weight of the input
scrap material(s). The weight of metal in the leached
components (W1) was computed from (Wm Wu) where
Wm and Wu are the weight of the metal in the input
material and unleached part respectively. The extent of
recovery of the method ("r) was determined from the
relation: "r=[(Wr+ W1)/Wsm]100.
3. Results
The lid and the body of the UBCs sample are separated and analyzed. Table 1 shows the chemical composition of these parts. The major metals are aluminium,
2.5. Preparation of some valuable metal salts
The metals went to the slag (Al, Sn, Fe) and were
leached in 1–6 M hydrochloric or sulfuric acid under
different conditions of time, temperature and stoichiometric ratio. The produced salts were freed from iron
impurities by oxidation to geothite applying the method
given by Rabah (2000). Aluminium chloride was separated by distilling the dried salts under vacuum at
200 C. Stannous chloride was distilled at 650–700 C.
Table 1
Chemical composition of the used beverage cans (UBCs)
UBCs Weight (g)
Metal content (wt.%)
Coated Decoated Al
Lid
3.45
Body 28.18
Total 31.63
3.40
28.0
31.40
Mg Sn Zn
Fe Ni
Si
Others
92.87 5.31 1.28 Nil
0.30 Nil
0.13 0.10
92.88 3.12 0.95 0.16 2.52 0.12 0.07 0.18
92.98 3.35 0.97 0.18 2.27 0.17 0.06 0.02
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M.A. Rabah / Waste Management 23 (2003) 173–182
magnesium and tin. Other metals are present in minor
concentrations. Fig. 2 shows the effect of concentration
of methyl ethyl ketone/dimethyl formamide mixture in
water on the extent of de-coating the UBCs. Experiments were conducted at room temperature for different
periods up to 30 min. It is seen that the extent of
removing the paint coating increases with increasing
both the solvent concentration and time of treatment.
The recommended method for completely removing the
print coating from the UBCs surface is the use of
590% solvent concentration for 515 min at room
temperature. However, more time would be practically
needed when diluted solvent is used. Solvent concentration of < 85% is found poor to de-coat the UBCs.
Fig. 3 shows the extent of decoating the UBCs by
blasting sand as a function of grain size. It is seen that the
removal of the coating increases gradually with increasing the grain size approaching a maximum with sand
grains > 63–106 mm in diameter. Coarse particles promote incomplete de-coating whereas de-coating the
UBCs surface by firing at 400 C in air has been tested. It
is found that such treatment causes harmful oxidation of
the UBCs elements to their respective oxides. Table 2
Table 2
Properties of the used methods for de-coating the used beverage cans
(UBCs)
Property
De-coating efficiency
Weight lossa
Recyclability
a
Method used for de-coating UBCs
Solvent
Sand blasting
Thermal 400 C
100%
–
Recyclable
92-95%
26–38%
Recyclable
95–97
7–9%
Non-recyclable
Due to abrasion.
shows the data obtained with the three methods of decoating. It is seen that removal of paint coating using
solvent is recommended.
Results given in Fig. 4 show the apparent density of
the pressed UBCs as a function of the pressing load. It
is seen that the density increases with increasing load,
approaching a constant value of 2360 kg/m3. The density of aluminum metal is 2700 kg/m3. Consequently,
the void fraction is 12.6% on a volume basis.
Fig. 5 shows the extent of recovery of aluminium–
magnesium alloy (Wr) in weight percentage, by melting
the pressed UBCs samples at 800 C, as a function of
density. It is seen that the extent of recovery increases
with increasing the density approaching a maximum
Fig. 2. Effect of solvent concentration in water on the extent of decoating the used beverage cans (solvent: dimethyl formamide/methyl
ethyl ketone 1:1).
Fig. 4. The apparent density of the pressed block of rolled used beverage cans as affected by the pressing load.
Fig. 3. Effect of the particle size of the sand grains on the extent of decoating the used beverage cans (t=30 min).
Fig. 5. The effect of density of the pressed used beverage cans on the
extent of recovery of aluminium alloy without using flux salt.
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M.A. Rabah / Waste Management 23 (2003) 173–182
recovery of 82.5 and 71% with the de-coated and coated
cans respectively.
Fig. 6 shows the effect of using different flux salts on
the Wr of the Al–Mg alloys. It is seen that, the use of a
flux salt increases the extent of recovery and such effect
is in the order: borax–NaCl mixture, borax, molybdate,
pyrophosphate and sodium carbonate. Sodium salts are
more effective as compared to the potassium salts. For
example, using 10% by weight of sodium borate/chloride mixture increases the extent of recovery of Al–Mg
alloys from 82.5 to 96.6%. However, ammonium chloride is found less effective. Fig. 7 shows the effect of
melting temperature on the extent of recovery using 10%
by weight of different flux salts. It is seen that the extent
of recovery increases with the increase in temperature
passing through a maximum at 800 C. Table 3 shows
the weight and composition of the slag obtained within
the applied temperature range.
Fig. 8 shows the extent of leaching of aluminium,
magnesium and tin from the slag using HCl acid.
Leaching experiments are conducted at 75 C for 4 h. It
is seen that the leaching efficiency value, W1, increases
with increase in the acid molarity attaining its maximum
with 5 M acid. The leaching efficiency decreases in the
order magnesium (99.4%), aluminium (94%) and tin
(75%). Fig. 9 represents the results of leaching the same
Table 3
The weight and composition of the slag obtained from melting of 100
kg of the used beverage cans (UBCs) at different temperatures
Melting
temperature
( C)
750
800
850
900
950
1000
Slag
weight
(kg)
4.0
3.5
3.4
4.2
5.6
6.8
Metal content in the slag
Al (g)
%
Mg (g)
%
Sn (g)
%
2.64
1.53
1.55
2.18
2.45
3.70
66
43.7
45.5
51.9
43.7
54.4
1.28
1.94
1.82
1.99
3.00
3.05
32
55.4
53.5
47.3
53.7
44.8
0.08
0.03
0.03
0.03
0.05
0.05
2
0.8
0.8
0.7
0.9
0.7
Fig. 6. The effect of density of the pressed used beverage cans (UBCs)
on the extent of recovery of aluminium alloy using different flux salts.
Fig. 8. The extent of leaching of Al, Mg and Sn as affected by HCl
acid concentration at 75 C.
Fig. 7. The effect of melting temperature on the recovery efficiency of
aluminium alloy from used beverage cans using different flux salts.
Fig. 9. The extent of leaching Al and Mg from the melting slag of
used beverage cans in 1–6 M sulphuric and HCl acids.
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M.A. Rabah / Waste Management 23 (2003) 173–182
metals using 1–6 M HCl and sulfuric acids under the
same conditions. It is seen that the extent of leaching
using HCl acid is higher than with sulfuric acid. The
maximum W1 value attained with sulfuric acid amounts
to 95% with magnesium and 88% with aluminium.
Leaching efficiency using hydrochloric acid is higher
than using sulfuric acid. Fig. 10 shows that increasing
the leaching temperature causes an increase of the
extent of leaching. The maximum extent of leaching is
achieved at 575 C.
Fig. 11 illustrates the effect of stoichiometric ratio of 5
M HCl and 5 M sulfuric acids on the extent of leaching
the melting slag of the UBCs. It is seen that the W1
value increases gradually with increasing the stoichiometric ratio towards a maximum plateau attained with 2
and 2.25 stoichiometric ratio for the two acids respectively. Fig. 12 shows the Arrhenius plot of the HCl acid
leaching process for Al, Mg and Sn. The activation
energy value for these elements has been computed. It
Fig. 10. The effect of leaching temperature on the extent of leaching
Al, Mg and Sn in 5 M HCl.
Fig. 11. The effect of stoichiometric ratio of the acid used on the
extent of leaching Al, Mg and tin from the melting slag of the used
beverage cans (T=75 C).
amounts to 317 kJ/mol with magnesium, 405 kJ/mol
with aluminium and 601 kJ/mol with tin.
Fig. 13 shows the increase in redox potential achieved
by blowing atmospheric air at various flow rates into
the leached acid solution to oxidize the ferrous ions to
insoluble basic ferric oxide. It is seen that the potential
increases with an increase in air flow-rate and time of
blowing. However, the measured potential is lower than
the standard potential for the oxygen-water couple
(1.229 V). The effect of addition of hydrogen peroxide
to the acid solution on the redox potential was studied.
Experiments were performed under air blowing conditions at a flow rate of 5 ml/s. It is found that the
potential shifts to a more positive value, 1.76 V and then
decreases to a steady-state potential of 0.8 V. potential
on oxidation of ferrous ion impurity to goethite. Fig. 14
shows the purity of titanium dioxide obtained by roasting the printed coating of the UBCs. It is seen that
heating at 5800 C only removed the organic component present in the coating. Titanium dioxide is thermally stable.
Different valuable salts can be prepared from the
leaching solutions. Aluminium ions are precipitated by
Fig. 12. The Arrhenius plot of the HCl acid leaching process of aluminium, magnesium and tin from slag.
Fig. 13. The redox potential as affected by the flow rate of blown air
into the leached acid solution of the slag.
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M.A. Rabah / Waste Management 23 (2003) 173–182
Table 4
The physical constants of aluminium, magnesium and tin (Weast and
Astle, 1980)
Boiling
Molecular Crystalline Density Melting
wt.
form
(g/cm3) point ( C) point ( C)
Element
Aluminium 26.89
Magnesium 24.31
Tin, white 118.69
Fig. 14. The purity of TiO2 as a function of roasting temperature.
ammonium hydroxide to form Al(OH)3. Addition of
0.05% polyacrylamide A100 solution improves the filtration of the Al(OH)3 gel. Aluminium acetate
[Al(C2H3O2)3], boride [AlB2], oxalate [Al(C2O4)2.4H2O],
salicylate [Al (C7H6O3)3] stearate [Al (C18H36O2)3] as
well as other salts can be prepared by reacting the
Al(OH)3 gel with the respective acids under the proper
conditions. Heating the Al(OH)3 gel in ammonia produces aluminium nitride [AlN]. After separation of aluminium from the leaching solution, magnesium ions are
precipitated as carbonate by addition of equimolar
sodium carbonate in presence of ammonium chloride (a
buffer medium) and filtered. Stannous chloride remains
in the filtrate.
4. Discussion
Recycling of the used beverage cans, UBCs, would
conserve the national resources, improve economy and
control pollution of environment. Different technologies
are described including direct re-melting but the yield is
low and the quality of the product is rather poor.
Advanced methods of recycling the UBCs focused on
the removal of the coating and use of sodium chloride–
potassium chloride mixture as a flux during the melting
step. In this study, the suggested method includes
removal of the printed coating by solvent extraction,
rolling of the UBCs to form plates prior to pressing into
solid blocks as a precondition to melting under salt flux.
The salt flux is composed of 1:1 by weight of sodium
borate–chloride salt mixture. The solvent removes the
coating without corrosive action on the UBCs alloy and
is recyclable. The removed coating is easily filtered and
roasted to titanium dioxide. The decoating method
using sand blasting is unacceptable due to its highly
abrasive nature. De-coating by firing the UBCs is very
pollutant due to the escape of the pyrolysis products as
fugitive emissions with subsequent oxidation of the
UBCs body.
The alloy composition of the starting UBCs is shown
in Table 1. Aluminium, magnesium and tin are the
major elements. Other minor elements such as zinc,
iron, silica and manganese are detectable. Table 4 shows
Cubic
2.702
Hexagonal 1.74
Tetragonal 7.28
660.37
648.8
231.88
2467
1107
2260
that the melting point of aluminium, magnesium and tin
is 660, 648 and 118 C, respectively (Weast and Astle,
1980, B-51, B-93 and B-136). The melting point of the
UBCs material is 615 C. Melting of such alloy at temperatures slightly higher than 650 C, may cause partial
oxidation of some of these metals to their respective
oxides. Table 5 shows the heat of formation (Ho) of
these oxides (Weast and Astle, 1980, D-45, D-47 and
D-50). It is seen that magnesium would be more readily
oxidized than aluminium. It becomes legitimate to presume that the weight and composition of the produced
alloy as well as the slag would be affected by the physical properties of he metals involved and by the (Ho)
assigned for their oxidation reactions. Based on these
assumptions, one would expect that aggressiveness of
atmospheric oxygen to oxidize the available metals in
the molten UBCs alloy is in the order magnesium, tin
and aluminium. The data given in Table 3 shows that
the weight percentage of aluminium in the slag is higher
than that of tin.
The role displayed by the flux sodium borate/sodium
chloride mixture has been studied. The molten flux salt
floats on the surface of the molten metals. It smothers
the atmospheric oxygen from diffusing into the molten
UBCs alloy and minimizes the prospective oxidation
reactions. In this context, the salt flux is to be chemically inert and thermally stable under the experimental
conditions of the method. In a previous work, lowering
the viscosity of the molten flux may help free movement
Table 5
Heat of formation of aluminium, magnesium and tin oxides
Reaction
1
2
2Al(l) +
Al(l) +
Temperature
range ( C)
1
2
O2
O2
(g)=Al2O
(g)=AlO
Al(l) + 1 12 O2
Ho
(g cal/mol)
500–1093
38,670
500–1093
+ 8,170
(g)=Al2O3
500–1093
407,950
Mg(l) +
1
2
O2
(g)=MgO
(periclase)
495–756
145,810
Mg(l) +
1
2
O2
(g)=MgO
(periclase)
756–1093
180,700
Sn(l) +
1
2
O2
Sn(l) + O2
(g)
(g)
=SnO
=SnO2
260–756
69,670
260–1036
143,080
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M.A. Rabah / Waste Management 23 (2003) 173–182
of the metallic species to the surface (Fujisawa et al.,
1998; Thomas et al., 1998). This property may minimize
impurity contamination in the produced alloy. In the
present work, sodium borate/sodium chloride mixture
(1:1 by weight) satisfies these requirements.
As far as the quality and yield of the product are
concerned, the recycling method under discussion
includes pressing of the de-coated UBCs to solid blocks
prior to melting. This step would achieve partial deaeration of the input scrap. The surface area of the
input UBCs susceptible to oxidation and the weight of
the formed slag would decrease. From the foundry
point of view, melting of pressed blocks in hot crucible
furnace containing molten flux is much easier than
feeding UBCs one by one.
The increase in leaching efficiency of the slag with
increasing the molarity and stoichiometric ratio of the
used acids is rather interesting. Results are explained
from the fact that acid–MOx reactions take place in a
multi-step sequence. Metal oxides with high state of
oxidation are firstly reduced to a lower state whereby
the latter is further reduced to form the salt. The ratedetermining step of the overall acid leaching process is a
chemically controlled reaction. It is directly related to
the acid concentration and temperatures up to 75 C.
The departure from linearity (Fig. 9) can best be
explained by postulating that acid leaching at the oxide
surface is high. Leaching may be limited by the rate of
diffusion of the acid through a stagnant boundary film
established by the reaction products. Explanation of the
effect of leaching temperature on the extraction percentage is in line with this model. Increasing temperature
would provide the necessary energy to accomplish or
enhance one or more of the steps of the leaching
sequence. However, the activation energy E of the
acid leaching processes for the metals concerned has
been computed from the Arrhenius plot (Fig. 12). The
higher E value obtained with tin as compared to
magnesium and aluminium indicates that oxide transformation step would take place more readily with Al
and Mg. The leaching efficiency amounts to 99.4%.
Iron impurity was detected in the leaching solution as
ferrous chloride. Passing oxygen gas in the leaching
solution forms insoluble goethite (iron oxyhydroxide,
FeO.OH), as identified by X-ray diffraction analysis in
agreement with the findings of Alex et al. (1995) and
Beutier et al. (1986). This helps separation of iron
impurity from the spent acidic solution. Measurement
of the potential of the oxidation process of ferrous ions
with oxygen-water couple (Fig. 13) suggests that the
process is directly related to the dissolved oxygen concentration in solution, which is a function of flow rate
and time of air blowing. The addition of H2O2 shifts the
potential of the system towards a more positive direction followed by a decrease. Not only is peroxide
unstable with respect to the oxidation of water, but also
with respect to its own oxidation and reduction in both
acid and alkaline solutions. Peroxide decomposes to O2
and water and thus provides the system with dissolved
oxygen that enhances iron oxidation. The observed
steady-state rest potential is 0.8 V for the peroxidewater couple approached with 55% H2O2 by volume
to the spent solution. Such a potential is accounted for
by a local cell composed of H2O2 oxidation to O2
through the O2/H2O2 reaction and the reduction of
H2O2 to O2 through the H2O2/H2O reaction. The overall process is the decomposition of hydrogen peroxide.
Oxidation of the ferrous ions is a quantitative measure
of the oxygen concentration in the system. An increase
in temperature enhances the rate determining step
(reaction 2) and hence, the oxidation of ferrous ions.
The activation energy of the iron oxidation process
amounts to 9.2 and 3.3 kJ/mol without and with air
blowing, respectively.
Table 6 summarizes the mass balance and composition of the end products. Table 7 shows the details of
the different salts obtained. It is worth noting that
valuable salts of aluminium, magnesium, tin and the
other alloying metals can be produced from the UBCs
when they are directly leached with mineral acids in the
same manner. In this work, aluminium alloys together
with different optional valuable salts are prepared.
Table 6
The mass balance and composition of the end products (Recycling
conditions; 750 C, block density, 2.36 t/m3)
Input materials
Output products (kg)
UBCs Acid (kg) Alloy composition (wt.%)
Slag
(kg)
Weight Al Mg Sn
Other Weight Salts
(kg)
(kg)
(kg)
(option)
100
96.0
95.9 2.72 0.455 0.925 4.0
Sulphuric
HCl
Nitric
Oxalic
Stearic
52.97
41.07
50.69
49.97
132.8
Table 7
The weight of the different hydrated salts obtained by acid leaching of
slag
Metal
Salt form
Sulphate
Chloride
Nitrate
Oxalate
Stearate
Aluminium
Magnesium
Tin
33.07
19.66
0.244
23.62
19.22
0.23
36.70
13.59
0.40
38.14
11.83
–
85.77
47.18
–
Total
52.97
41.07
50.69
49.97
132.85
M.A. Rabah / Waste Management 23 (2003) 173–182
181
5. The process economics
6. Conclusion
A preliminary cost estimates the suggested combined
hydro-pyrometallurgical method. Working capital is
defined as funds in addition to fixed capital investment
(for 30 days, one shift/day): raw materials and supplies,
product and in-process inventory, accounts receivable
and available cash (direct expenses). Cost estimate is
based on the following assumptions: (1) significant
quantity of the used beverage cans are annually recurring (2) the metallic contents of this scrap material may
only be subject to a minor change in the near future; (3)
the annual demand of aluminium-magnesium alloy(s) is
liable to maintain. Table 8 presents the economic study.
Used beverage cans are an endless waste that can be
recycled to obtain valuable products. The suggested
method combined hydro-pyrometallurgical treatments
and provides a suitable way to remove the printed
coating using solvent extraction technique. The clean
cans are then pressed to blocks prior to re-melting in a
crucible furnace using sodium borate-sodium chloride
mixture as a flux. The optimum recovery efficiency of
aluminium alloy(s) amounts to 96.6% at 800 C.
Leaching of the slag using different mineral acids
produces pure valuable salts and the leaching efficiency is 99.4%. The cost price of the products is
competitive to the local market price for the same
primary products.
Table 8
Preliminary cost estimate on 100 kg used beverage cans
Item
Quantity Unit
(kg)
price
($)
Subtotal Total
price
price
($)
($)
(1)-Capital expenditure
Equipment:
Consuming rate
Spare parts and accessories
Maintenance and service, ..etc.
9.5
12.6
4.0
26.1
(2) Running costs and utilities
beverage cans
100
expenses against transportation
28.9
17
28.9
17
Chemicals: (consumable)
Commercial sulphuric acid, kg
or commercial HCl acid, kg
23.8
58
0.15
0.2
3.5
11.6
Energy, light fuel oil,
(kerosene), kg
10
0.15
15
Natural gas, kg
25
0.13
3.3
Power, kWh
32
0.1
3.2
(3) Subtotal (1+2)
(4) Labour1 day
losses
95.1
1
8
20
8
20
(5) Overhead charges, 8% ( 3+4)
Banking rate 9%pa, (3+4+5,
1 week)
Others, 38% (4)
27.8
2.7
Subtotal (4+5)
69.1
10.64
Grand total costs (3+4+5)
Products:
Option 1 Al–Mg alloy
+ Al sulphate (hydrated)
Mg sulphate (hydrated)
Sn sulphate
Option 2: Al–Mg alloy
+ Al Cl3 (hydrated)
MgCl2 (hydrated)
SnCl2
Local market price of
products (mean)
Product price cost price
164.2
94.4
33
19.6
0.24
94.4
23.6
19.2
0.23
1.65 155.7
0.5
16.3
0.75 14.7
3.3
0.8
1.65 155.7
0.6
19.8
0.45
8.6
3.18
0.7
187.5
184.8
186.2
22
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