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INFLUENCE OF ULTRASOUND ON NITRIC ACID LEACHING
OF ALUMINA FROM KAOLIN CLAYS.
Ruziyev Ulugbek Mamarasulovich
Assistant, Karshi State Technical University,
Uzbekistan, Karshi
E-mail:
https://orcid.org/0009-0001-9533-3603
Abstract:
This article presents the results of a comprehensive study on the
application of ultrasound to intensify the nitric acid leaching of alumina from
calcined kaolin clays of the Angren deposit. The propagation of ultrasonic waves
in liquid media induces cavitation phenomena that enhance mass transfer and
reaction kinetics, especially at the solid
–
liquid interface. A specially designed
ultrasonic bath equipped with a PMS-6M magnetostrictive transducer powered by
a UZG-2.5A generator was used to investigate process intensification under
controlled conditions. The experiments were conducted using a statistical design
of experiments (DoE), specifically a fractional factorial design (2⁴⁻¹), to assess the
influence of key parameters: temperature, leaching time, stoichiometric acid
dosage, and nitric acid concentration. The optimization criterion was the yield of
Al₂O₃ in solution. A regression model was developed, and the method of steepest
ascent was applied to identify optimal process conditions. At the optimal point,
alumina recovery reached 93% within a significantly reduced leaching time.
Keywords:
ultrasonic cavitation, alumina leaching, kaolin clay, nitric acid,
Angren deposit, hydrometallurgy, process intensification, magnetostrictive
transducer, factorial design, aluminum extraction.
Introduction.
The application of ultrasound to enhance various processes and
address specific technological challenges in hydrometallurgy has garnered
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significant interest among researchers. This growing attention is largely attributed
to the distinctive effects produced by the propagation of ultrasonic waves through
liquid media. A central mechanism in this context is the formation of cavitation
zones, along with a range of chemical and physicochemical transformations that
occur within the liquid phase. These effects are particularly pronounced at the
interfaces between different phases in heterogeneous, fluid-based technological
systems, where ultrasound can significantly influence reaction kinetics and mass
transfer. Cavitation phenomena
—
specifically the formation of gas-filled voids or
discontinuities within a liquid
—
arise from the inherent asymmetry in a liquid's
mechanical response: while liquids can resist high compressive forces, they are
highly susceptible to rupture when subjected to tensile (negative) pressures, leading
to the emergence of vapor-filled cavities. Building upon this physical property,
B.A. Agranat [2] proposed and provided a theoretical foundation for the
controllability of ultrasonic cavitation processes.
In recent years, a wide range of innovative ultrasonic equipment has been
developed both domestically and internationally. Among the most widely used
configurations are systems incorporating vacuum tube-based generators coupled
with magnetostrictive transducers, which remain prevalent due to their reliability
and effectiveness in industrial applications.
In modern metallurgical practices aimed at extracting metals and other
valuable components from various types of ores, a broad spectrum of mechanical
vibrations
—
spanning diverse frequencies and intensities
—
is actively employed. In
particular, high-energy elastic oscillations with frequencies exceeding 18 kHz (i.e.,
in the ultrasonic range) are increasingly applied to enhance and accelerate a number
of critical technological operations. Among the equipment capable of generating
such oscillations, vacuum tube-based ultrasonic generators equipped with
appropriate transducers are considered highly suitable, as they fulfill key
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performance criteria for industrial implementation in terms of power stability,
frequency range, and operational reliability.
In the processing of low-grade alumina-bearing raw materials, leaching
represents the central stage in the recovery of the target component
—
alumina.
However, when employing nitric acid leaching techniques, particularly agitation or
autoclave methods, the treatment of iron-rich alumina-containing feedstocks often
results in relatively low alumina recovery rates, accompanied by significantly
increased consumption of acid reagents. This not only reduces process efficiency
but also raises concerns regarding economic viability and environmental impact.
Methods and Results.
In order to optimize leaching efficiency, minimize
nitric acid consumption, and reduce processing time, a series of experiments were
carried out involving the ultrasonic-assisted leaching of alumina from kaolin clays
using nitric acid. The application of ultrasound was intended to intensify the
dissolution process, enhance phase contact at the solid-liquid interface, and thereby
improve the overall recovery of aluminum from the raw material.
Fig. 1. Ultrasonic leaching bath apparatus
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1
–
Ultrasonic bath div (housing)
2
–
Structural frame (support)
3
–
Sampler (sampling valve or port)
4
–
Gasket (sealing element)
5
–
Magnetostrictive transducer (ultrasonic source)
6
–
Control thermometer (manual monitoring)
7
–
Heater (thermal unit for solution temperature control)
8
–
Electric motor with mechanical stirrer
9
–
Hinged lid (top cover for sample insertion/removal)
As the source of ultrasonic energy, a PMS-6M type magnetostrictive
transducer was employed. This transducer was powered by an UZG-2.5A
ultrasonic generator, which provided the required high-frequency oscillations to
induce cavitation and enhance mass transfer during the leaching process.
The experimental studies were carried out in a custom-designed ultrasonic
bath fabricated from 1X18H9T-grade stainless steel, where the bottom of the bath
also functioned as a vibrating diaphragm for the magnetostrictive transducer (see
Fig. 1). The apparatus was engineered to ensure continuous mechanical agitation
of the slurry, enable real-time temperature regulation, and allow sampling during
operation without interrupting the leaching process. The frequency of ultrasonic
oscillations was held constant throughout the experiments, in accordance with the
operational specifications of the UZG-2.5A generator. Maximum anode current
was used as a reference point, and oscillation frequency was monitored using an
ICH-7 frequency meter to ensure process stability and reproducibility.
The initial raw material used in the experiments was calcined kaolin clay
sourced from the Angren deposit. The chemical composition of the material was as
follows (in mass %): Al₂O₃ –
24.00; Fe₂O₃ –
2.50; SiO₂ –
73.36; MgO
–
0.17; SO₃
–
0.17; K₂O –
0.85; N
a₂O –
0.21; loss on ignition
–
0.45. The average particle size
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of the clay fraction was 0.5 mm, ensuring sufficient surface area for interaction
with leaching reagents during ultrasonic treatment.
The primary objective of this research was to intensify the leaching processes
through the application of ultrasonic cavitation. The study was specifically directed
at elucidating the influence of ultrasonic energy on the leaching rate of alumina
from kaolin-based raw materials. To identify the optimal operational parameters,
the investigation employed a statistical experimental design methodology.
The following process variables were selected as independent (input) factors
within the experimental matrix:
x
1
–
Temperature (°C);
x
2
–
Leaching time (hours);
x
3
–
Stoichiometric dosage of nitric acid relative to Al₂O₃ content (%);
x
4
–
Concentration of nitric acid in the leaching solution (%).
This structured approach allowed for a comprehensive assessment of both
individual and interactive effects of process variables on alumina extraction
efficiency under ultrasonic activation.
The response variable used for process optimization in this study was the yield
of Al₂O₃ in the leachate (y). In constructing the experimental design, it was
assumed that second-order interactions among the factors would have negligible
influence on the outcome. Based on this assumption, a fractional factorial design
of type 2⁴⁻¹ was employed, which represents a half
-
replicate of a full 2⁴ factorial
experiment. The design was generated using the defining relation
x
4
=
x
₁
x
₂
x
₃,
allowing for an efficient yet informative exploration of the main effects and
primary interactions among the selected variables.
Table-1.
Planning matrix and results of the first series of experiments.
Basic level
x
1
x
2
x
3
x
4
y
80
1.5
80
20
-
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Range of
variation
10
0.5
20
5
-
Upper level
90
2
100
25
-
Lower level
70
1
60
15
-
Experience1
+
+
-
-
68.7
2
-
-
-
-
52.5
3
+
-
-
+
85.4
4
-
+
-
+
93.2
5
+
=
+
+
79.3
6
-
-
+
+
59.2
7
+
-
+
-
80.6
8
-
+
+
-
74.6
Based on the data obtained from the first series of experiments, the regression
equation describing the relationship between the response variable (y
–
Al₂O₃ yield
in solution) and the coded independent variables (x₁, x₂, x₃, x₄) was constructed
using the method of least squares. The following estimates of the regression
coefficients were calculated:
b₀ = 72.
93
–
intercept (mean response),
b₁ =
–
5.56
–
effect of temperature,
b₂
= +3.51
–
effect of leaching time,
b₃ = +0.49 –
effect of stoichiometric dosage,
b₄
= +3.84
–
effect of acid concentration, b
12
=
–
8.01
–
interaction between
temperature and time,
b₁₃ = +0.96 –
interaction between temperature and dosage,
b₁₄ = +0.13 –
interaction between temperature and acid concentration.
The residual standard deviation was calculated as:
𝑆 {
2
𝑦
} = 0.687,
𝑆
2
{𝑏}
= 0.086,
𝑓 = 8;
𝐹
𝑒𝑘
= 0.184;
𝐹
0.05
(3: 8) = 4.1
The resulting equation for the response function (y
–
Al₂O₃ yield in solution,
%) is as follows:
𝑦 = 72.93 + 5.45𝑥
1
+ 3.51𝑥
2
+ 0.49𝑥
3
+ 3.48𝑥
4
− 8.01𝑥
1
𝑥
2
+
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+0.96𝑥
1
𝑥
3
+ 0.13𝑥
1
𝑥
4
.
Since the linear effect estimates in the applied experimental design are not
confounded by interaction terms, it is valid to apply the method of steepest ascent
(also known as the gradient ascent approach) within the linear approximation
region. This method enables the identification of the most efficient direction in the
factor space to increase the response variable
—
in this case, the Al₂O₃ yield in
solution.
Table-2.
Indicators related to determining the direction of vertical lift.
Basic level
x
1
x
2
x
3
x
4
y
80
1.5
80
20
-
Range of variation
10
0.5
20
5
-
Upper level
90
2
100
25
-
Lower level
70
1
60
15
-
Coefficient b
1
5.65
3.51
0.49
3.84
-
The product of b
1
and the
interval of variation
55.6
1.76
9.8
17.4
-
Step size of 5 when measuring
x
1
5
0.168
0.88
1.565
-
Rounding
5
0.2
1
2
-
Experience 9 (Beginner Level)
80
1.5
80
20
90
10
85
1.7
81
22
93
11
90
1.9
82
24
88
12
95
2.1
83
25
83
13
100
2.3
84
28
-
At the zero (center) level of the experimental design, the leaching of alumina
from Angren kaolin clay was conducted under baseline conditions without
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deviation along the steepest ascent path. Under these conditions, the Al₂O₃ yield in
solution reached 93%, indicating a relatively high extraction efficiency.
The quantitative determination of extracted alumina was carried out using
complexometric titration with Trilon B (EDTA). To ensure accuracy, the results
were cross-
validated by solid phase analysis, confirming the consistency of Al₂O₃
recovery measurements.
To evaluate the specific impact of ultrasonic cavitation on the leaching
process, a comparative analysis was performed between experiments conducted
with and without ultrasonic exposure. The results demonstrated a notable increase
in the leaching rate and shortening of the processing time under the influence of
ultrasonic vibrations, confirming the positive intensification effect of ultrasound on
alumina dissolution kinetics.
During the experiments, the pulp volume was kept constant across all trials,
regardless of the variation in nitric acid concentrations. The mass of kaolin clay
samples ranged from 500 to 960 grams, depending on the specific experiment.
In each case, the pulp was first heated to the target process temperature, after
which ultrasonic exposure was initiated. From this moment, the leaching time was
recorded to ensure consistency in comparing kinetic behaviors under identical
thermal conditions.
The results are illustrated in Figure 2, which clearly demonstrates the
acceleration of the leaching process and enhanced alumina recovery rates under
ultrasonic activation compared to conventional agitation-based leaching.
Experiments have established that 30 minutes after the start of ultrasound
exposure on the pulp, an intense reaction occurs in the bath, which almost
concludes after 1.5 hours.
In the case of leaching without ultrasonic activation, a comparable increase in
alumina yield becomes noticeable only after approximately 2.5 hours of
processing. However, despite extending the total leaching time to 3
–
4 hours, the
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overall extraction efficiency of Al₂O₃ under these conventional conditions remains
limited to 35
–
40%.This contrast highlights the kinetic advantage provided by
ultrasonic cavitation, where enhanced mass transfer and intensified solid
–
liquid
interactions significantly accelerate the leaching process and improve overall
recovery.
Fig. 2.
Dependence of Al₂O₃ yield on leaching duration with ultrasonic
exposure (curve 1) and without ultrasonic exposure (curve 2); process
temperature: 85 °C; nitric acid concentration: 22%.
The results of the present study demonstrate that the application of ultrasonic
cavitation significantly enhances the leaching efficiency of alumina from Angren
kaolin clays when treated with nitric acid. The observed intensification of the
process
—
manifes
ted in higher Al₂O₃ yields and reduced processing times—
clearly
underscores the technological potential of ultrasound-assisted hydrometallurgical
methods.
Given these promising findings, it is scientifically and practically justified to
expand and deepen future investigations in this field. However, it should be noted
that the currently available ultrasonic generators and magnetostrictive transducers
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are not yet sufficiently robust or scalable to fully meet the operational requirements
of large-scale industrial alumina production.
Therefore, future research efforts will be directed toward studying the
decomposition of kaolin clays in an ultrasonic field under near-industrial
conditions, with special attention to the application of a rotary-pulsation
apparatus
—
a promising alternative for intensifying solid
–
liquid processes in
continuous-flow systems.
CONCLUSION
The conducted research confirms the high potential of using ultrasonic
cavitation to intensify the nitric acid leaching process of alumina from kaolin clays.
The application of ultrasound significantly accelerates the dissolution of Al₂O₃,
enhances mass transfer, and increases the yield of the target component.
Experimental results showed that under optimized conditions, alumina recovery
reached up to 93%, while conventional leaching without ultrasound yielded only
35
–
40% after a longer processing time.
The developed regression model and gradient ascent method enabled the
identification of optimal parameter combinations, and the effectiveness of
ultrasonic leaching was confirmed by titration and solid-phase analyses. Despite
the positive laboratory-scale results, current ultrasonic equipment (generators and
transducers) requires further adaptation for industrial-scale operations.
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