(Thi Thuy Nhi Nguyen)
1
(Man Seung Lee)
*1
Copyright © The Korean Institute of Metals and Materials
Key words(Korean)
black dross, ball milling, NaOH leaching, alumina, ultrasound wave
1. INTRODUCTION
Aluminum dross is produced by the treatment of secondary resources containing Al metal.
This dross consists of a mixture of free Al metal and nonmetallic substances such
as aluminum oxide, nitride, carbide, salts, metal oxides, and other elements [1]. The small amount of Al metal present in dross is recovered from the dross, and
the residue after this treatment is called black dross. However, some Al metal and
several metal oxides are still present in black dross. Therefore, the recovery of
valuable components in black dross is important and necessary.
In the hydrometallurgical process, alumina and other oxides are dissolved from the
black dross by employing either acid or alkaline solutions [2-4]. Alkaline leaching offers the selective leaching of alumina over other oxides from
black dross, but the leaching percentage of alumina is generally low [4,5]. In order to improve the leaching percentage of alumina without affecting the purity
in the leaching solution, the choice of chemicals and the selection of effective methods
are key to developing a new process.
In recent years, the use of ultrasound has become more common in metallurgy as an
auxiliary means during the ore leaching process. Ultrasound consists of a series of
longitudinal waves with differences in density that spread through the medium [7]. Irradiating ultrasonic energy in solution causes bubble formation and subsequent
violent collapse, in a process known as ‘‘acoustic cavitation” [8]. This may cause direct erosion on the surface of the particle, and the de-aggregation
of particles, which can hinder agglomeration [9]. The use of ultrasonic energy has a number of benefits, including optical, electrical,
mechanical, thermal, chemical and biological effects [10,11].
Many researchers have employed ultrasound in the heavy metal leaching process to increase
the extraction of metals [12,13]. The application of ultrasonic energy in hydrometallurgy has primarily concentrated
on the treatment of heavy metals and precious metals, but little research has been
performed on alumina production. In particular, no work has been reported on the selective
dissolution of alumina from black dross employing ultrasound.
Since alumina is amphoteric, alumina can be dissolved in either acidic or alkaline
solutions. In general, the leaching percentages of alumina and silica from black dross
in NaOH solution are 80% and 68%, respectively [14]. This makes it necessary to remove a large amount of dissolved silica from the leaching
solution to obtain a pure Al(III) solution. Mechanical activation of black dross by
ball milling followed by NaOH leaching leads to a leaching solution of Al(III) with
98% purity [15]. However, the leaching percentage of alumina was around 35% [15], and thus further work is needed to improve the leaching of alumina from the mechanically
activated dross.
The use of auxiliary energies is one way of increasing the leaching efficiency, by
boosting transport phenomena. Ultrasound has proven to be a very effective tool for
enhancing reaction rates by inducing a number of chemical and mechanical effects in
the reaction medium [16]. It can improve the traditional leaching process and become a costefficient method
for heavy metal recovery.
In this study, the effect of ultrasound on the alkaline leaching of alumina from mechanically
activated black dross was investigated by varying leaching temperature and time and
stirring speed. The combined effect of ultrasound, and the addition of different chemicals
(CaCl2 and organic acids) to the NaOH solution, on the leaching of alumina and silica was
also investigated.
2. EXPERIMENTAL
2.1. Materials
The salts present in black dross were dissolved in water at 90 °C [14]. After drying the residues in an oven, the dried residues with an average particle
size <150 μm were employed in the leaching experiments. A vertical planetary ball
mill (Fritsch Pulverisette 7 Bead Mill, Fritsch, Idar-Oberstein, Germany) with a rotation
speed up to 800 rpm was employed in the mechanical activation of the black dross.
Mechanically activated samples were prepared as follows: the black dross after water
leaching (8 g) was added into a vessel with 40 g agate balls (a ball was 6 mm in diameter)
with the weight ratio of ball/black dross of 5:1 and then milled for 1 h at 400 rpm.
The mixture of mechanically activated dross and CaCl2 was prepared by varying the weight ratio of CaCl2 and black dross (mass of CaCl2/mass of black dross) as 0.5, 1, 1.5, 2, 2.5. The mixture was transferred to a corundum
crucible and heated in an SX-GD7123 muffle furnace. The temperature protocol was 30
to 500 °C with a heating rate of 10 °C/min, followed by continuous heating under 500
°C for 2 hours. The mechanically activated dross was used in these experiments. The
heat treated mixtures were then allowed to naturally cool to room temperature.
The chemical composition of the black dross after water leaching was determined using
X-ray fluorescence spectrophotometry (XRF, Hitachi, Tokyo, Japan). Point analyses
were taken with the XRF operated at 15 kV. Table 1 shows the chemical composition of the black dross after water leaching. The structure
of the raw black dross, the residues after water leaching and the ball-mill-treated
black dross were characterized by X-ray diffraction (XRD) (D8 Advance (Bruker AXS,
Karlsruhe, Germany)) with Cu Ka (40 kV/40 mA, γ = 0.15406 nm) radiation (Fig 1).
Figure 1 shows that the as-received black dross contained significant amounts of KCl and NaCl
(see Fig 1a) which disappeared after water leaching (see Fig 1b). Twice leaching with water at 90 °C led to the complete removal of NaCl and KCl
from the black dross. The leaching solution was prepared by dissolving extra pure
sodium hydroxide (Duksan Co.) in doubly distilled water.
2.2. NaOH leaching
The leaching experiments with the mechanically activated dross were carried out in
a 500 mL three-neck round bottom flask (Duran Triple Neck Flask, Germany) which was
submerged in an ultrasonic bath (WUC-A02H, 40kHz, Ultrasonic Cleaner, 40 kHz, 1.8
Lit., USA). The frequency of the ultrasonic waves was 40 kHz and power was kept at
a maximum of 100W. Teflon tape was used to seal the outside joints of the glassware
to avoid evaporation loss. The slurry was stirred at 200 rpm and at the temperature
of 50 °C except for experiments on the effect of stirring speed and temperature.
At desired time intervals slurry samples were taken and filtered using vacuum filtration.
The filtered residues were washed several times with distilled water until the filtrate
became completely clean and then dried at 60°C for 48 h. The morphologies of the leaching
residue of the mechanically activated dross was investigated by Field Emission scanning
electron microscopy (FE-SEM) (S-4800, Hitachi, Tokyo, Japan). The weight percentage
of alumina in the residue after NaOH leaching, and the concentration of minor elements
in the solution, was measured by XRF, and inductively coupled plasma optical emission
spectrometers ICP-OES (Spectro Arcos), respectively. The leaching percentage of alumina
was calculated using Eq. (1), while that of the minor elements was calculated with Eq. (2).
where WAl(initial) and WAl(residue) represent the weight of alumina in the samples before and after the leaching experiments.
CA and V refer to the concentration of minor elements in the leaching solution and
the volume of leaching solution after filtration, respectively.
3. RESULTS AND DISCUSSION
The optimum conditions for the selective dissolution of alumina from the mechanically
activated dross were obtained from a previous work: 5 M NaOH, 50 °C, 2 h, pulp density
of 100 g/L [15]. At this leaching condition, the purity of Al(III) in the leaching solution was
higher than 98% but the leaching percentage of alumina was only around 35% [15]. No Ca, Fe, Mg and Ti oxides were dissolved, except for alumina and silica, at every
NaOH concentration studied.
Therefore, to find the optimum conditions to improve the leaching of alumina from
the mechanically activated dross, the effect of ultrasound was investigated. The dross
which had been mechanically activated at 400 rpm for 1h was employed for all experiments
in this work.
3.1. Effect of temperature
In order to investigate the effect of temperature on the dissolution of metal oxides
in the presence of ultrasound, the mechanically activated dross was leached at varying
temperatures from 30 to 100 °C for 2 h. In these experiments, the concentration of
NaOH and pulp density were the same as the optimum conditions obtained from the previous
work: 5 M NaOH and the pulp density of 100 g/L [15]. In these experiments, the ultrasound power was fixed at 100 W.
The effect of temperature on the leaching of the oxides with ultrasound of 100 W is
illustrated in Fig 2. The leaching percentage of alumina slightly increased from 38 to 44% as the temperature
increased from 30 to 50 °C, and then was constant, irrespective of the temperature.
When applying ultrasound to an alkaline leaching process, ultrasonic waves create
liquid cavitation that can blow up the surface of the solid. This generates highly
reactive surfaces causing transient high temperature and pressure at the surface,
which produces the formation of surface defects and deformations, and thus increases
the surface area of friable solid [17-19]. Accordingly, the impact of ultrasound may accelerate the leaching of components
which have higher solubility, and thus result in an increase in leaching efficiency
for the metal. At the same time, ultrasonic waves may also enhance the settling of
metal colloid particles with lower solubility and thus reduce their leaching efficiency
[20].
The SEM images reveal the differences in the microstructure of the leaching residues
of the mechanically activated dross in the presence and absence of ultrasonic treatment
under the optimum conditions. Comparing Fig. (3a) and (3b), the particle size produced by ultrasonic treatment was finer than that produced
by regular treatment. These particles became smaller and looser with the ultrasonic
leaching. These phenomena confirm that the ultrasonic energy can shatter solid particles
and hinder the growth of solid grains, which is beneficial for the solidliquid reaction,
and thus promotes the leaching of the components [10].
3.2. Effect of leaching time
In order to investigate the effect of leaching time, experiments were conducted with
varying times from 2 h to10 h. The mechanically activated dross was leached using
5 M NaOH solution at a pulp density of 100 g/L at 50 °C. Within our experimental range,
the leaching percentage of alumina was about 60%, while that of other oxides was nearly
zero at any leaching time (see Fig 4).
The cavitation effect of ultrasound was significant within the leaching time up to
4h. When leaching time was longer than 4h, there was little change in the leaching
percentage of alumina. Unlike alumina, the leaching percentage of silica was very
low and became reduced as leaching time increased. This might be attributed to the
precipitation of sodalite which occurred between aluminate and silicate ions in the
leaching solution [21]. In other words, dissolved silicate ions can form a precipitate with aluminate ions
during the leaching reaction.
Eqs. (3) and (4) represent the leaching of alumina and the precipitation reaction of sodalite in our
experiments.
Similar observations were obtained by SEM in Fig 5. The mechanically activated black dross after alkaline leaching without ultrasonic
wave contained angular particles with different sizes but was broken into much smaller
particles after treatment by ultrasonic waves. Large clusters with a typical size
of several micrometers were observed, due to the aggregation of smaller nanoparticles
[22].
3.3. Effect of stirring speed
Stirring of the leaching systems facilitates the intimate mixing of black dross and
reagents as well as mass transfer. In the absence of stirring, the turbulence caused
by ultrasound is not enough to make the leaching reaction proceed [23]. Therefore, the effect of stirring speed in the presence of ultrasound was investigated
in this work.
The effect of stirring speed was investigated using 5 M NaOH at a pulp density of
100 g/L and 50 °C, ultrasonic power of 100 W. In these experiments, the mechanically
activated dross was leached by varying stirring speeds from 200 to 600 rpm. Within
our experimental range, the stirring speed did not show any effect on the leaching
of the oxides in the mechanically activated dross (see Fig 6). The leaching percentage of alumina was about 60%, while that of other oxides was
nearly zero under any conditions. The purity of Al(III) in the leaching solution was
99% at the leaching time of 4 h and speed at 200 rpm. The composition of the leaching
solution and purity of Al(III) in the solution after NaOH leaching is shown in Table 2.
3.4. Combination of ultrasound and different chemical additives in NaOH leaching
3.4.1. CaCl2 roasting combined with NaOH ultrasound-assisted leaching
In black dross, alumina might exist as either α-Al2O3 or γ-Al2O3. Some literatures have reported that α-Al2O3 is difficult to dissolve in alkaline or acid solution [24]. The combined effect of high temperature and salt addition was investigated [25]. Unlike alumina, CaO does not dissolve in strong alkaline solution. Therefore, the
addition of CaCl2 as a calcium ion source was tried. These samples were used for ultrasound-assisted
leaching in a 5 M NaOH at 50 °C for 2 hours, ultrasonic power of 100 W. However, the
results showed that the presence of Ca(II) can lead to the precipitation of dissolved
alumina and silica in the NaOH solution [26]. As the weight ratio of CaCl2 to black dross increased, the leaching percentage of silica decreased, and no silica
was dissolved when the weight ratio was higher than 1.5 (see Fig 7). The leaching percentage of alumina decreased rapidly when the weight ratio of CaCl2 to black dross was increased. No oxides of Ca, Fe, Mg, and Ti were dissolved in these
experiments.
The predominant species of dissolved alumina and silica in 5 M NaOH solution were
Al(OH)-4 and H3SiO42-4, respectively [27]. In the presence of calcium ions, the silicate and aluminate can form precipitates
with the calcium ions, and the reactions are represented as Eqs. (5) and (6) [27]
Moreover, the reaction product of Eq. (6) can further react with silicate ions through Eq. (7) on the surface of the precipitates, to form a hydrated Ca-Al-Si-O compound [28].
where x depends on the experimental conditions.
Although a pure Al(III) solution can be obtained by leaching the mixture of black
dross and CaCl2 in NaOH solution in the presence of ultrasonic waves, the low leaching percentage
of alumina under these conditions indicates the unsuitability of adding CaCl2 to the black dross.
3.4.2. The presence of reducing agent in NaOH ultrasound-assisted leaching
Some literatures have reported that the addition of organic reducing agents such as
oxalic acid, tartaric acid and citric acid can improve the leaching percentage of
several metals [29,30]. The effect of reducing agent on the leaching of alumina and silica from the mechanically
activated black dross was investigated by using 5 M NaOH at a pulp density of 100
g/L and 50 °C, ultrasonic power of 100 W. The effects of some organic reducing agents
are shown in Table 3. Clearly, organic reducing agents increased the solubility of aluminum in an alkaline
solution [31].
The addition of a carboxylic acid to the solution causes competition between ligands
for coordination sites on the metal ions [32]. This creates competition between OH and other ligand groups with metal ions. From
the structural configurations of these acids (scheme 1), deprotonation in the hydroxyl
groups of organic acids can occur. Alumina atoms can form coordination bonds between
these deprotonated groups and the free electron pair of the oxygen atom on the CO
double bond. The relative positions of hydroxide and carboxylate in the 5- or 6-bong
ring structure with Al(III) might enhance the dissolution of alumina in an NaOH solution
containing organic acid (see scheme 2) [33,34]. Thus, the enhancement of the complexation of the dissolved metal ion with the organic
ligands and hydroxide ion may be responsible for the increased leaching efficiency
of alumina and silica in NaOH solution.
Although the leaching percentage of alumina increased, some of the silica was also
leached. Therefore, the addition of organic reducing agents is not recommended for
the dissolution of alumina in terms of the purity of Al(III) in the leaching solution.
4. CONCLUSIONS
To recover the alumina present in black dross in a previous work, leaching of the
mechanically activated black dross in NaOH solution was performed. Although the purity
of Al(III) in the leaching solution was 98% under the optimum conditions, the leaching
percentage of alumina was only 35%. Under most leaching conditions, only alumina and
a small amount of silica were dissolved, while the oxides of Ca, Fe, Mg, and Ti were
not dissolved. Among the leaching variables, leaching time had a remarkable effect
on the dissolution of alumina as the reaction time increased to 4 h. The use of ultrasound
waves increased the leaching percentage of alumina to 60% and the purity of aluminum
to higher than 99%. Therefore, ultrasound waves were confirmed to have a favorable
effect on the separation of alumina from silica during leaching in NaOH solution.
The effect of the combination of ultrasound and the addition of CaCl2 and organic acids to the NaOH solution on the separation of alumina from silica by
was also investigated. The addition of organic reducing agents to the NaOH solution
resulted in an increase in the leaching percentage of alumina and silica, and thus
was not favorable to obtaining a pure aluminum solution from the black dross. Although
a pure Al(III) solution was obtained from the black dross by roasting the black dross
with CaCl2 followed by NaOH leaching in the presence of ultrasonic waves, it did not improve
the leaching percentage of alumina.
Acknowledgements
This work was supported by the Global Excellent Technology Innovation of the Korea
Institute of Energy Technology Evaluation and Planning (KETEP), granted financial
resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No.20165010100880).
We express sincere thanks to the Korea Basic Science Institute (KBSI), Gwangju branch
for providing ICP-OES data.
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Figures and Tables
Fig. 1.
XRD patterns of black dross before and after treatment. (a) raw black dross; (b) pretreated
black dross with water (c) mechanically activated black dross.
Fig. 2.
Effect of reaction temperature on the ultrasound-assised leaching of the oxides from
the mechanically activated dross in 5 M NaOH solution. (reaction time: 2 h, stirring
speed: 200 rpm, pulp density: 100 g/L, ultrasonic power 100 W).
Fig. 3.
SEM images of mechanically activated black dross after alkaline leaching: (a) without
ultrasonic wave; (b) with ultrasonic wave. (reaction time: 2 h, reaction temperature:
50 °C stirring speed: 200 rpm, pulp density: 100 g/L, ultrasonic power 100 W).
Fig. 4.
Effect of leaching time on the leaching of the oxide from mechanically activated black
dross by ultrasound-assisted leaching in 5 M NaOH solution at 50 °C. (pulp density:
100 g/L, stirring speed: 200 rpm, ultrasonic power 100 W).
Fig. 5.
SEM images of mechanically activated black dross after alkaline leaching: (a) without
ultrasonic wave (reaction time: 2 h); (b) (c) (d) with ultrasonic wave. (reaction
time: 2 h, 4 h, 6 h).
Fig. 6.
Effect of stirring speed on the leaching of the oxide from mechanically activated
black dross by ultrasound-assisted leaching in 5 M NaOH solution at 50 °C. (pulp density:
100 g/L, leaching time: 4 h, ultrasonic power 100 W).
Fig. 7.
Effect of ratio of mass CaCl2: black dross on the leaching of the oxide from mechanically
activated black dross by ultrasoundassisted leaching in 5 M NaOH solution at 50 °C.
(pulp density: 100 g/L, leaching time: 2 h, ultrasonic power 100 W).
Table 1.
The chemical composition of black dross after water leaching. (unit: wt%)
|
Element
|
Al
|
Ca
|
Fe
|
Mg
|
Si
|
Ti
|
|
Residue after water leaching
|
40.52
|
3.55
|
8.34
|
2.86
|
15.13
|
12.01
|
Table 2.
The composition of the leaching solution and the purity of Al in the solution after
NaOH leaching of the mechanically activated dross at the optimum condition.
|
Composition
|
Al
|
Si
|
Purity of Al (%)
|
|
Concentration (ppm)
|
20695.8
|
35.95
|
99.8
|
Table 3.
The leaching percentage of Al after NaOH Ultrasoundassisted leaching without and with
presence of organic from black dross at 50 oC, 2h.
|
Element
|
Leaching percentage (%)
|
|
|
|
Without organic
|
With Oxalic acid
|
With Tartaric acid
|
With Citric acid
|
|
Al
|
44
|
49.3
|
45.5
|
43.6
|
|
Si
|
0.23
|
7.56
|
6.72
|
5.28
|