Ultrasound-Assisted Selective Leaching of Arsenic from Copper Smelting Flue Dust

Abstract

Copper smelting flue dust (CFD) is a byproduct of pyrometallurgical copper production, containing valuable metals like lead, zinc, and copper, but also hazardous arsenic, which complicates its handling and recycling. Traditional methods for arsenic removal from CFD, such as pyrometallurgical and hydrometallurgical processes, are often inefficient or result in the loss of valuable metals. This study explores the efficacy of ultrasound-assisted leaching for selective arsenic extraction from CFD, offering a potentially more efficient and environmentally friendly alternative. We employed a combination of sodium hydroxide and sodium sulfide in an aqueous solution, enhanced by ultrasonic waves, to selectively recover arsenic into solution. The optimal leaching conditions were determined to be 0.4 M NaOH, 0.2 M Na₂S, a liquid-to-solid ratio of 50 mL/g, a temperature of 80 °C, an ultrasound power of 150 W, and an ultrasound frequency of 100 kHz, under which up to 99% of arsenic was extracted within 45 min. The kinetic analysis conducted suggests that the leaching process is controlled by the chemical reactions occurring at the surface of the particles.

1. Introduction

When smelting copper concentrates in copper pyrometallurgical production, part of the fine particles (copper smelting flue dust, CFD) passes into the gas phase and is captured by electrostatic precipitators. According to some estimates, for one ton of copper produced, about 0.2 tons of dust are generated [1]. CFD contains significant amounts of valuable metals such as lead, zinc, copper, and others [2,3,4,5,6]. For example, CFD produced at metallurgical plants in Kazakhstan (Balkhash and Dzhezkazgan copper smelters) contains 30%–50% lead, 5%–7% copper, and 6%–9% zinc [7]. This circumstance makes it attractive to use CFD as a raw material for the production of the mentioned metals. However, this is prevented by a very high arsenic content; about 15% of the arsenic contained in the concentrate goes into matte, about 22% into slag, and the remaining arsenic into dust [8]. The arsenic content in CFD can reach half the total mass of dust [9]. With sizes ranging from tens of microns to hundreds of nanometers, CFD poses a serious threat to the environment, and its storage is highly undesirable [10]. However, direct CFD processing is not possible due to the arsenic contamination of metallurgical products. Therefore, it is necessary to remove arsenic from dust.

There are two main methods for removing arsenic from CFD: pyrometallurgical (roasting) and hydrometallurgical [11,12,13,14]. High temperatures promote the volatilization of most arsenic-containing compounds, including sulfides and oxides, into the gas phase; meanwhile, arsenates remain in the original material [15,16,17,18,19]. The high capital and operating costs reduce the efficiency of pyrometallurgical arsenic removal from CFD. Hydrometallurgical methods appear more attractive. Processing the dust with a water solution of inexpensive and readily available sulfuric acid allows for the extraction of over 90% of the arsenic into the solution; however, valuable metals, primarily zinc and copper, also transition into the solution [20,21,22]. This circumstance not only leads to an excessive consumption of acid but also complicates the subsequent production of copper- and zinc-containing commercial products.

The selective extraction of arsenic into solution can be achieved by alkaline leaching of the dust in the presence of sodium sulfide [23,24,25]. The essence of the method lies in leaching arsenic together with other dust components, such as lead, zinc, and copper, in a sodium hydroxide solution. The presence of sulfide ions in the solution (for example, by using sodium sulfide) leads to the precipitation of lead, copper, and zinc as sulfides, while arsenic remains in solution [23,24,25]. However, the widespread adoption of this method is hindered by the increased consumption of sodium hydroxide and relatively low arsenic extraction into solution compared to sulfuric acid leaching.

In CFD, arsenic occurs in the oxidation states As³⁺ (arsenite) and As⁵⁺ (arsenate). In alkali media, the possible reactions of arsenic leaching could be represented as follows:

$${\mathrm{As}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+2{\mathrm{OH}}^{-}\to 2{\mathrm{AsO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}$$

(1)

As₂O_5(s) + 6OH⁻ = 2AsO₄³⁻+ 3H₂O

(2)

The Gibbs free energy of reaction (1) is −46.24 kJ/mol, while the Gibbs free energy of reaction (2) is −282.24 kJ/mol [1]. Thus, from the perspective of chemical thermodynamics, arsenates dissolve better than sodium arsenites, and the oxidation of arsenites to arsenates will promote more complete alkaline leaching of arsenic [26]. Oxidative pressure leaching was applied to promote arsenic leaching efficiency in an alkali medium [27]; mechanoactivation was used to enhance the reactivity of arsenic-containing particles [28].

Recently, a microwave-assisted leaching process for arsenic removal from CFD was developed [1]. Microwave treatment significantly reduced the duration of leaching and extracted about 98% of arsenic in an alkaline solution. However, the use of microwaves is associated with difficulties in scaling up the leaching process; the cost of specialized equipment is quite high. An attractive alternative appears to be the use of ultrasound in leaching. Ultrasonic intensification of the leaching process is based on the principle of cavitation, which occurs when ultrasonic waves act on a liquid [29,30,31,32]. Cavitation causes the formation and subsequent collapse of microscopic bubbles in a solution, leading to local extreme conditions such as high temperature and pressure, as well as high fluid jet speeds. These conditions help improve mass transfer, accelerate chemical reactions, and increase the efficiency of extracting target components from a solid substrate. Ultrasound-assisted leaching has been successfully used in leaching arsenic from contaminated soils [33], gypsum [34], coal-associated rock samples [35], iron slag [36]. However, to our knowledge, ultrasound has not been used in the leaching of metals from CFD, including arsenic. To fill this gap, we present, for the first time, the results of ultrasound-assisted selective leaching of arsenic from CFD. Optimal conditions, such as ultrasonic frequency, temperature, liquid-to-solid ratio, leaching time, and leaching agent concentration, are explored through experiments. The kinetics of the ultrasound-assisted leaching process are studied. It is suggested that ultrasound-assisted leaching is a promising technique for efficient and selective arsenic removal from CFD.

2. Materials and Methods

2.1. Materials

A sample of copper flue dust (CFD) was received from the “Kazakhmys Smelting” copper plant (Balkhash, Kazakhstan). Sodium hydroxide (≥97% NaOH) and sodium sulfide (32%–38% Na₂S) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without further purification.

2.2. Methods

An ultrasonic instrument (KQ-300VDE, Kunshan, China) with a maximum ultrasonic power output of 300 W was used for the experiments. About 4500 mL of water was placed into the ultrasonic instrument and heated to a pre-determined temperature (40, 50, 60, 70, and 80 °C). The beaker, filled with a solution of NaOH and Na₂S, was placed into the ultrasonic instrument to reach the desired temperature. When the desired temperature in the beaker was reached, a pre-determined amount of CFD was added to the solution, and the formed mixture was stirred by using a mechanical stirrer (SCI40-S, Berlin, CT, USA). Simultaneously, an ultrasonic treatment was carried out at pre-determined values of frequency and power. The duration of leaching was 90 min; every 15 min, liquid samples were taken from the beaker with a micropipette to determine arsenic, copper, zinc, and lead ions.

Leaching parameters varied within the following limits:

[NaOH]: 0.1, 0.2, 0.3, 0.4, and 0.5 mol/L;

[Na₂S]: 0.10, 0.15, 0.20, and 0.30 mol/L;

Temperature: 40, 60, and 80 °C;

Liquid-to-solid ratio: 20, 30, 40, and 50 mL/g;

Leaching duration: 15, 30, 45, 60, 75, and 90 min;

Ultrasound frequency: 0, 45, 80, and 100 kHz.

Ultrasound power was kept at 150 W (50% of maximum power) in all experiments.

To evaluate the efficiency of leaching, the recovery of abovementioned elements (α) was determined using the following equation:

$$\alpha =\frac{m_x}{m_0}\times 100\%$$

(3)

where $m_x$ and $m_0$ are masses of element in the solution and the initial CFD sample, respectively.

Once the leaching procedure was completed, solid residue was separated from the liquid, washed with distilled water, and dried in the oven. Dried material was weighted and subjected to elemental and XRD analysis.

The elemental composition of the CFD and leaching residue was determined by using atomic absorption spectrometry (AAS) on an AA-6200 spectrometer (Shimadzu, Japan); preliminary microwave decomposition of the solid sample with concentrated nitric acid at 90–95 °C by using the system for microwave digestion of samples, Tank-eco (Hanon, Jinan, China), was performed prior to elemental content determination in a resulting solution.

Scanning electron microscopy (SEM) imaging was performed by using a Quanta 200i 3D (FEI Company, Hillsboro, OR, USA) electron microscopy.

The XRD patterns of the CFD and leaching residue were recorded by using a D8 Advance diffractometer (Bruker, Bremen, Germany) with CuKα (40 kV, 40 mA) radiation.

Particle size distribution was determined by using a particle size laser diffraction analyzer Mastersizer 2000E (Malvern, Worcestershire, UK).

Specific surface area (SSA) of the CFD sample was determined by using nitrogen adsorption technique (NOVA 1200e, Quantachrome Instruments, Boynton Beach, FL, USA).

3. Results and Discussion

3.1. Characterization of CFD Sample

Copper flue dust appeared as a fine, grey powder (Figure 1). The XRD pattern of the initial CFD sample is presented in Figure 2.

XRD analysis of the CFD showed the presence of anglesite (PbSO₄), galena (PbS), zinc ferrite (ZnFe₂O₄), and cuprospinel (CuFe₂O₄) as lead, zinc, and copper-containing minerals in the crystalline part of the sample. Interestingly, no arsenic-containing minerals were detected in the diffraction pattern; this fact may indicate that arsenic is concentrated in the amorphous part of the sample. In general, the studied dust sample was characterized by a high degree of amorphy (more than 73% of the amorphous part). The analysis of the particle size distribution of CFD particles, as illustrated in Figure 3, demonstrates that 80% of the particles were finer than 38 µm (P80 value).

Elemental composition of the dust sample is presented in

Table 1. Elemental composition of the dust sample.

Element (wt.%)
Cu	S	Pb	Fe	Zn	As
3.76	14.11	24.27	1.94	6.17	12.35

.

A scanning electron microscope (SEM) image of the CFD sample is shown in Figure 4.

The SEM image showcases the textured surface of a CFD sample. The image reveals fine particles unevenly distributed with varied morphologies. Some particles exhibit spherical shapes, while others appear more agglomerated with irregular forms. The surfaces of these particles are coated with fine grains and pores.

3.2. Leaching Experiments

Figure 5 shows the dependence of the fraction of arsenic recovery into solution on the concentration of sodium hydroxide and the duration of leaching under the following conditions: 0.30 mol/L Na₂S; liquid-to-solid ratio 50 mL/g; 80 °C; ultrasound frequency 100 kHz.

Both NaOH concentration and leaching duration have a significant effect on arsenic recovery. As the NaOH concentration increases from 0.1 M to 0.5 M and the leaching time increases from 15 to 90 min, there is a noticeable increase in the proportion of arsenic recovered into solution. At a NaOH concentration of 0.1 M and a leaching time of 15 min, the reduction fraction is 0.15, whereas under the same conditions, except for a NaOH concentration of 0.5 M and a leaching time of 90 min, the reduction fraction increases to 0.99. The maximum degree of arsenic recovery (0.99) was achieved at 0.4 M NaOH and 45 min of leaching; further increases in NaOH concentration and leaching duration did not affect the degree of arsenic recovery.

The effect of leaching duration and sodium sulfide concentration on arsenic recovery into solution is presented in Figure 6; other parameters were maintained at the following levels: 0.50 mol/L NaOH; liquid-to-solid ratio 50 mL/g; 80 °C; ultrasound frequency 100 kHz.

Arsenic recovery increases with both the duration of leaching and the concentration of Na₂S. Specifically, at the 45 min mark, a significant jump in arsenic recovery is observed at higher Na₂S concentrations (0.2 and 0.3 mol/L), reaching up to 0.95. As the leaching continues up to 45 min and beyond, the recovery rates at these higher concentrations nearly plateau at about 0.98.

Figure 7 shows the effect of liquid-to-solid ratio and leaching duration on the extraction of arsenic into solution under the following conditions: 0.50 mol/L NaOH; 0.3 mol/L Na₂S; 80 °C; ultrasound frequency 100 kHz.

The initial phase of leaching (0 to 15 min) is characterized by significantly lower recovery rates, highlighting the time required for the solution to penetrate the solid matrix of CFD and initiate arsenic dissolution. Increasing the liquid-to-solid ratio enhances arsenic recovery at every time interval. At 15 min, recovery at a 50 mL/g ratio is 0.41 compared to just 0.06 at 20 mL/g. This pattern holds consistently through the 90 min mark, where the recovery reaches 0.99 at 50 mL/g compared to 0.34 at 20 mL/g. As the duration of leaching increases, there is a corresponding increase in the recovery rate across all liquid-to-solid ratios. By the 90 min mark, the higher L:S ratios (40 mL/g and 50 mL/g) achieve near-complete recovery (0.98 and 0.99, respectively). However, with a liquid-to-solid ratio of 50 mL/g, the value of 0.99 was achieved much faster, namely after 45 min of leaching.

Thus, at an ultrasound frequency of 100 kHz and 80 °C, the following conditions provide the maximum (0.99) extraction of arsenic into solution: [NaOH] = 0.4 mol/L, [Na₂S] = 0.2 mol/L, liquid-to-solid ratio 50 mL/g, leaching duration 45 min.

Using the same conditions, but in the absence of ultrasonic exposure, made it possible to extract only 64% of arsenic within 4.5 h.

The next series of experiments was aimed at identifying the influence of ultrasound frequency and temperature on the extraction of arsenic into solution. Figure 8 shows the dependence of arsenic extraction into solution on ultrasound frequency.

It can be seen that with traditional leaching (without ultrasonic exposure), only one third of the arsenic went into solution. The use of ultrasound significantly increased the degree of arsenic extraction; 45 kHz led to the extraction of more than two-thirds of all arsenic contained in the dust, the degree of extraction at 80 kHz was 0.85, and at 100 kHz, almost all of the arsenic passed into the solution (extraction degree 0.99).

Figure 9 demonstrates the effect of temperature and leaching duration on the recovery of arsenic into solution under the following conditions: 0.5 mol/L NaOH; 0.30 mol/L Na₂S; liquid-to-solid ratio, 50 mL/g; ultrasound frequency, 100 kHz.

After 45 min, recovery at 80 °C is almost complete at 0.99, while at 60 °C and 40 °C, the recovery becomes lower at 0.42 and 0.15, respectively. By the 60th minute, recovery at 80 °C maintains almost the full 0.99 level, showing a plateau. At temperatures of 60 °C and 40 °C, there is a continued increase in recovery over time, reaching 0.61 and 0.24, respectively, over the same time interval.

Thus, the following conditions for ultrasound-assisted extraction of arsenic into solution during alkaline leaching are optimal: [NaOH] = 0.4 mol/L, [Na₂S] = 0.2 mol/L, liquid-to-solid ratio, 50 mL/g, leaching duration, 45 min; ultrasound frequency, 100 kHz; temperature, 80 °C. These conditions make it possible to extract 99% of arsenic into solution.

Under these conditions, the extraction of zinc, copper, and lead into the solution is less than 1%.

The solution after leaching under optimal conditions had the following composition, mg/L: As—2453, Cu—57, Pb—35, Zn—9, Fe—2. After precipitation of arsenic and adjustment of the NaOH and Na₂S content, the solution can be reused for leaching a new portion of dust. A number of works are devoted to the precipitation of arsenic from alkaline solutions, for example, Refs. [37,38]. In [38], Yang et al. used hydrothermal precipitation of arsenic from NaOH solutions in the form of practically insoluble Ca₅(AsO₄)₃OH. The residual arsenic content in the solution was only 2.1 mg/L, which is below the limit of that set by the U.S. Environmental Protection Agency (5 mg/L). This route ensures almost complete recovery of arsenic from solution in the form of a stable solid material subject to burial.

3.3. Leaching Kinetics Analysis

When studying the kinetics of leaching, the shrinking core model is most widely used [39,40,41]. According to the model, leachable particles are represented as regular spheres. Over the course of leaching, the radius of the particles decreases due to the transition of part of the solid into solution, and the size of the spheres decreases. The dependence of the proportion of solid that has passed into solution and the leaching time is described by different equations, depending on which stage of leaching is limiting [42].

$$1-\frac{2}{3}{X}_{Me}-{(1-X_{Me})}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}=k\mathsf{\tau }$$

(4)

$$1-{(1-X_{Me})}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}=k\tau$$

(5)

$$\frac{1}{3}\ln \left(1-X_{Me}\right)-1+{(1-X_{Me})}^{\raisebox{1ex}{$-1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}=k\tau$$

(6)

where $X_{Me}$ is the fraction of solid recovered, $k$ is the rate constant of chemical reaction of leaching, and $\tau$ is the time in which the recovery of $X_{Me}$ is achieved.

The equations provided model the processes governed by different rate-limiting steps—diffusion (Equation (4)), chemical reaction occurring at the particle’s surface (Equation (5)), and both diffusion and the chemical reaction (Equation (6)).

To identify the limiting stage of the arsenic alkaline leaching by using ultrasound, the dependencies of the left sides of Equations (4)–(6) on the leaching duration at three temperatures were plotted for the data shown in Figure 9. The coefficients of determination are given in

Table 2. Determination coefficients (R²) for linear dependencies according to Equations (4)–(6), obtained based on the data in Figure 9.

Equation	Temperature, °C (K)
	40 (313)	60 (333)	80 (353)
(4)	R2 = 0.7971	R2 = 0.9268	R2 = 0.9055
(5)	R2 = 0.9655	R2 = 0.9939	R2 = 0.9877
(6)	R2 = 0.7318	R2 = 0.7267	R2 = 0.6777

.

Based on the coefficients of determination of linear dependencies, Equation (5) most closely describes the process of arsenic leaching, and the limiting stage is the stage of the chemical interaction of the leaching agent with arsenic compounds on the surface of a solid particle.

To determine the kinetic parameters of the arsenic leaching process, the data from Figure 9 were used in combination with Equation (5), and the resulting plots of $1-{(1-X_{Me})}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}$ vs. leaching duration at three different temperatures are presented in Figure 10.

The rate constants of the chemical reaction of arsenic leaching corresponding to the slope coefficients of the equations of the straight lines are equal to 0.0017 min⁻¹ (40 °C), 0.0032 min⁻¹ (60 °C), and 0.0071 min⁻¹ (80 °C).

The Arrhenius plot for arsenic recovery from CFD was created by using these calculated rate constants (Figure 11). The linear fitting confirms that the arsenic recovery into the solution is controlled by the chemical reaction on the CFD surface.

The activation energy (E_a) of the overall chemical reaction determining the arsenic recovery into solution was determined using Arrhenius’s law (Equation (7)) [42]

$$lnk=-\frac{E_a}{RT}$$

(7)

where k is the rate constant of chemical reaction (min⁻¹), R is the universal gas constant (J/(mol × K), and T is absolute temperature (K).

The calculated E_a and A was 36.43 kJ/mol. This value practically corresponds to that obtained with microwave-assisted arsenic alkaline leaching from CFD and is lower than for conventional alkaline leaching, for which the activation energy is about 43 kJ/mol [1]. Reducing the activation energy of the target arsenic leaching process by 1.2 times compared to conventional leaching leads to an increase in the reaction rate and a decrease in energy consumption, which can have a positive effect on the technical and economic indicators of production.

3.4. Leaching Residue Analysis

The XRD pattern of the leaching residue is presented in Figure 12, showing that the diffraction pattern of the leach residue retained all the peaks from the original dust, while also exhibiting new peaks corresponding to covellite (CuS) and sphalerite (ZnS).

Elemental composition of leaching residue was, wt.%, as follows: Cu—5.03; S—19.76; Pb—32.54; Fe—2.73; Zn—8.26; As—0.16.

An SEM image of the residue after CFD leaching showed significant changes compared to the pre-leaching sample (Figure 13).

The particle morphology was altered, with a reduction in particle size due to the dissolution of soluble components. Spherical particles became more fragmented and showed signs of surface erosion. Residual deposits were observed on the surfaces of the particles, potentially from the leaching agents or undissolved material left after the process, appearing as new phases or coatings on the particle surfaces.

3.5. Possible Processes during Ultrasound-Assisted Selective Leaching of Arsenic from CFD

Despite the fact that the results of XRD analysis did not reveal crystalline arsenic minerals in the CFD, the presence of arsenic (III) oxide As₂O₃ can be assumed, as well as arsenates of lead (Pb₅(AsO₄)₃OH, Pb₂As₂O₇), zinc (Zn₃(AsO₄)₂), and copper (Cu₃(AsO₄)₂) [43,44]. The possible chemical reactions of original arsenic, copper, lead, and zinc compounds with sodium hydroxide could be expressed as Equations (8)–(12) [45,46,47], as follows:

As₂O₃ + 2NaOH →2NaAsO₂ + H₂O

(8)

Pb₅(AsO₄)₃OH + 19NaOH → 5Na₂PbO₂ + 3Na₃AsO₄ + 10H₂O

(9)

Pb₂As₂O₇ + 10NaOH → 2Na₂PbO₂ + 2Na₃AsO₄ + 5H₂O

(10)

Zn₃(AsO₄)₂ + 12NaOH → 3Na₂ZnO₂ + 2Na₃AsO₄ + 6H₂O

(11)

Cu₃(AsO₄)₂ + 12NaOH → 3Na₂CuO₂ + 2Na₃AsO₄ + 6H₂O

(12)

Besides, zinc ferrite, as well as cuprospinel can react with sodium hydroxide:

ZnFe₂O₄ + 2NaOH →Fe₂O₃ + Na₂ZnO₂ + H₂O

(13)

CuFe₂O₄ + 2NaOH → Fe₂O₃ + Na₂CuO₂ + H₂O

(14)

For reactions (13) and (14) to occur more or less completely, high concentrations of sodium hydroxide and a long time are required [48,49,50]; therefore, under the conditions under consideration, these reactions occur partially, as evidenced by the detection of ZnFe₂O₄ and CuFe₂O₄ during XRD analysis of the leach residue.

Double oxides of sodium (Na₂PbO₂, Na₂ZnO₂, Na₂CuO₂) react with sodium sulfide in solution to form the corresponding sulfides of lead, zinc, and copper, as indicated here:

Na₂PbO₂ + Na₂S +2H₂O → PbS + 4NaOH

(15)

Na₂ZnO₂ + Na₂S +2H₂O → ZnS + 4NaOH

(16)

Na₂CuO₂ + Na₂S → CuS + 4NaOH

(17)

Under the conditions under consideration, arsenic remains in solution, which ensures its selective extraction from CFD.

Arsenic(III) oxide reacts with sodium sulfide in solution to form sodium thioarsenite [51], as indicated here:

4Na₂S + As₂O₃ → 2NaAsS₂ + 6NaOH

(18)

NaAsS₂ + Na₂S → Na₃AsS₃

(19)

Thioarsenites have better solubility than arsenites and arsenates [52]; this may explain the positive effect of sodium sulfide on arsenic extraction.

4. Conclusions

For the first time, a feasible ultrasound-assisted leaching process was developed for selective arsenic recovery from copper smelter flue dust during alkaline leaching in the presence of sodium sulfide. About 99% of arsenic was leached from the dust, while copper, zinc, and lead remain in solid residue. The activation energy of the arsenic extraction process was 36.43 kJ/mol, which is 1.2 times lower than that for conventional leaching. The kinetic analysis using the shrinking core model revealed that the process is controlled by the chemical reactions occurring at the surface of the CFD particles. The optimal conditions for maximum arsenic extraction were identified as a combination of specific concentrations of sodium hydroxide (0.4 M) and sodium sulfide (0.2 M), an appropriate liquid-to-solid ratio (50 mL/g), and a control of temperature (80 °C), ultrasound power (150 W), and ultrasound frequency (100 kHz).