Extraction of Pb, Cu, Zn and As from Fine Dust of Copper Smelting Industry via Leaching with Sulfuric Acid

Abstract

Fine dust from copper smelting plants is an important source of raw materials for the extraction of various valuable metals. A specific feature of dust from copper smelting plants in Kazakhstan is their high arsenic content of up to 15%. This work shows the possibility of hydrometallurgical processing of fine dust from copper smelters, obtained during the converting of copper mattes through the Vanyukov process, via direct leaching with sulfuric acid. The influence of temperature, process time and the S:L (solid/liquid) ratio on the selective extraction of Pb, Zn, Cu and As into targeted products under leaching conditions is studied. The results of the test work show that with the optimal process parameters of S:L = 1.5, t = 60 °C, τ = 60 min, the extraction of copper and zinc into solution is achieved as 89% and 96%, respectively, and lead into cake by up to 97%. The relatively low extraction of copper and zinc into the solution is explained by the transition of copper and zinc ferrites that are insoluble in sulfuric acid into the lead cake. The redistribution of arsenic between the leaching products established in this case also affects the reduction in copper and zinc in the solution. The extraction of arsenic into the solution is 49.32%. More than half of the arsenic from the dust is left in the lead cake. The concentration of arsenic in lead cake will lead to its transition into circulating dust during smelting. This will increase the accumulation of arsenic in the overall process flow. Therefore, when organizing dust-processing technology, it is necessary to provide measures for the preliminary removal of arsenic.

1. Introduction

The linear model of producing metals from low-quality primary sulfide raw materials, which dominates in copper production, does not meet modern requirements anymore.

Taking into account the problems in the field of waste processing, the presence of a large volume of generated technogenic waste has led to a significant deterioration of the environment, and consequently to a decrease in the standard of living of the population.

The traditional economy, based on a linear “take–consume–throw away” model of metal production, now relies on the production of large quantities of readily available materials and energy. With this production model, the consumer receives a final product made from a certain amount of raw materials, which is subsequently disposed when fulfilling its purpose. At the same time, the amount of unjustifiably discarded and underused products is increasing, displacing the natural human habitat [1].

The highly developed horizontal connections between enterprises that previously existed in Kazakhstan made it possible to process significant volumes of substandard intermediate products and waste, unsuitable for processing in one enterprise, by transferring them to others, where they were used as a source of raw materials for the production of metals. The disruption of these connections led to large distortions. The observed significant increase in the volume of substandard intermediate products and waste at enterprises, the amount of which is increasing exponentially due to the lack of rational technology for their processing, is of great concern and requires immediate action.

Today, the task of primary importance for Kazakhstan is obvious: the transition to a circular economy model using resource-saving and energy-efficient technologies. A circular economy is a model of production and consumption that involves reusing, recovering and recycling existing materials and products. The point of the circular economy is primarily to stop producing waste as much as possible and rely on the renewability of everything around, trying to restore nature.

Waste management systems in many countries already meet the principles of a circular economy [2]. In recent years, the circular economy model within the framework of the "zero waste” concept has become widespread in the global economy.

In general, in Kazakhstan, a state policy for a clear accounting and regulation of intermediate products and industrial waste from industrial plants should be developed, and a unified policy in this area should be formed. All initiatives on this issue are in their infancy, although in world practice, new accessible technologies are actively being developed, which operate successfully in developed countries within the framework of the circular economy.

The increase in the content of toxic and carcinogenic arsenic [3,4,5,6] in copper concentrates [7,8,9] significantly affected its distribution between the gas, slag and matte phases in smelting processes [9]. As a result, large volumes of substandard intermediate products, recycled materials and technogenic waste with a high content of arsenic were formed. This has increased their negative impact on the environment and public health, and restrains their further processing. Arsenic emissions, especially As₂O₃, cause serious environmental pollution and significant harm to human health. Arsenic control has become one of the important issues for all copper and lead smelting industries [10].

For the economy of Kazakhstan, a task of primary importance is the transition of the activities of the mining and metallurgical complex of the republic to a circular economy within the framework of the zero waste concept using resource-saving and energy-efficient technologies. The development and design of new technologies aimed at the comprehensive processing of multi-component raw materials—intermediate products and industrial waste from copper and lead production—seem to be very relevant.

Materials characterized by a complex chemical composition containing a significant amount of arsenic include fine dust from electrofilters obtained from the converting process of copper matte. An important prerequisite for organizing a separate production for processing fine dust from copper smelting production is their multicomponent nature (%): 35–50 Pb; 5–7 Cu; 6–9 Zn; 4–6 Fe; 8–13 As; others. The value of dust is enhanced by the presence of dual-use metals in them: rhenium, osmium, selenium, etc. An increase in the arsenic content (up to 13%) in the dust of copper smelting industry in Kazakhstan at some point restricts the development of new highly efficient technologies.

The chemical composition of fine dust from the copper smelting industry depends mainly on the smelting process conditions and the type and composition of the concentrate. Minor deviations of the furnace operating modes, the chemical composition of the concentrate and other factors from the specified parameters can lead to changes in the metal content in the dust [11,12,13,14,15,16]. Dust from copper smelters is characterized by a heterogeneity of particle size, chemical and mineralogical composition [17,18,19], which must be taken into account when choosing a technology for processing the dust of a specific composition.

S&P Global Market Intelligence forecasts that the annual global demand for copper will nearly double from 25 million tons to approximately 50 million tons by 2035 [20]. Consequently, this production will be accompanied by an increase in the amount of dust, sludge, slag and wastewater, which poses a serious threat to the environment and human health. It seems that this trend will remain unchanged in the coming years. The global demand for copper is expected to continue to grow in the future due to the indispensable role of copper in modern technology. Increasing copper production and reducing copper content in ore will increase energy consumption and have a greater impact on the environment [21]. Traditional primary copper production uses chalcopyrite–pyrite concentrates as raw materials, and lead production uses galena–sphalerite concentrates. The main route of primary processing is the smelting of sulfide concentrate into matte, followed by its processing into blister copper, which is subjected to electrorefining. Dust is generated at all stages of primary copper production, and the mass of dust can reach 10% of the mass of the original concentrate. This dust is collected by waste heat boilers, bag filters and electrofilters. A common practice is to process this dust in a smelter to extract heavy non-ferrous metals. Recycling dust reduces furnace power and overall performance. Dust from concentrate smelting typically contains high concentrations of copper (20%) and sulfur (10%) and low levels of lead, antimony and arsenic: 3% Pb, 0.05% Sb and 1.88% As. On the other hand, converter dust has a lower copper content (8.3% Cu) and a higher content of lead, antimony and arsenic (20–40% Pb, 0.21% Sb and up to 13% As). As a result, the direct recycling of metallurgical dust becomes more feasible, and incentives to clean converter dust are increased [22].

As it is well known, the process of the smelting of copper concentrates and then the process of converting the copper matte takes place under conditions of autogenous smelting using blast as a main driver of the process. As a result, the volume of the blast and the composition of the supplied air or oxygen directly affect both the removal of concentrates as a dust from the units during the loading and the content of the dust. Fine dust after electrofilters mainly contains lead, which is not sent to the existing process for further processing, since it contains a high amount of arsenic. The processing of this type of intermediate product requires the careful selection of separate technology to avoid the production of a carcinogenic product containing arsenic.

For dust processing, various options for pyro- and hydrometallurgical methods have been developed, a number of which have been successfully used in practice in recent years [23,24,25,26,27,28,29,30,31,32,33,34,35].

The most common pyrometallurgical process is the method of melting dust in an electric furnace with the addition of sodium sulfate and soda in a reducing atmosphere [23]. The main products of such smelting are metallic lead, cadmium sublimates and sodium slag, accumulating zinc, arsenic, selenium and other trace elements. The average composition of dust, which is sent for processing (%): 52–60 Pb; 11–19 Zn; 0.8–1.5 Cd; 5–6 S; Sb and As 0.1–1.5. The consumption of soda and reducing agent is 20–25% and 8–12% of the weight of the feed charge, respectively. Exhaust gases (826–1026 °C) after the afterburning chamber are sent to bag filters for cleaning.

The work [24] proposed a combined method for processing fine dust from electrofilters of the converter stage of copper smelters in the Ural region (Russia), including their reduction smelting with lead sulfate cake, sodium sulfate, soda and coke. The advantages of the technology include: the complexity of the use of raw materials by increasing the extraction of lead, bismuth and precious metals into rough lead; the possibility of the joint processing of converter dust and sulfate intermediate products (cake, sludge); and favorable conditions for the removal of arsenic from the technological process of copper smelting plants in the form of a low-toxic compound—arsenic trisulfide (As₂S₃).

Pyrometallurgical methods of dust processing, despite the high specific productivity and relatively low cost of the used reagents [25,26], are accompanied by an intense release of secondary dust. This leads to additional losses of valuable metals and requires additional costs for gas purification. The low quality of the resulting products requires additional operations to refine them, which significantly reduces the efficiency of the overall technology as a whole [27,28].

In recent years, for the production of non-ferrous, rare earth metals from copper smelting dust, special attention has been paid to hydrometallurgical technologies. The main operation when processing dust via hydrometallurgical methods is through leaching using acidic solutions (H₂SO₄, HNO₃, HCl), alkalis (NaOH, NH₄OH) or acidified salts (FeCl₃, Fe₂(SO₄)₃). All of the above methods are described in sufficient detail in the technical literature [29,30,31,32,33,34,35,36] and do not require special comment.

Hydrometallurgical methods of dust processing have a number of technological and environmental advantages over pyrometallurgical methods. They do not require large capital costs, ensure a high complexity of the use of raw materials, selectivity in the extraction of valuable components and the absence of dust and gas emissions [29,30]. However, during the hydrometallurgical (sulfuric acid) processing of copper smelting dust, the problems of leaching poorly soluble compounds of non-ferrous metals (ferrites, sulfides) and purifying the resulting sulfate solutions from iron and other impurities, in particular, arsenic [30,31,32] have to solved. Therefore, in recent years, intensive research has been carried out on the use of other solvents for dust processing [31,32,33,34].

The most common hydrometallurgical method for processing dust is sulfuric acid leaching, which is mostly due to the low cost and availability of sulfuric acid. The lower solubility of lead sulfate compared to sulfates of other non-ferrous metals provides greater selectivity for hydrometallurgical schemes based on sulfuric acid.

The purpose of this work is to study the recovery of Pb, Cu, Zn and As from copper smelter fine dust via direct leaching with sulfuric acid.

To achieve this goal, the work studied the influence of temperature, duration and ratio of the solid phase (dust) to the liquid phase (sulfuric acid) S:L on the completeness of the extraction of Pb, Cu, Zn and As into the targeted products under conditions of fine dust leaching. A distinctive feature of this work is the application of sulfuric acid leaching to fine dust containing high arsenic up to 13%, with an assessment of its distribution among the leaching products, and the possibility of using the known method for multicomponent arsenic-containing dust from copper smelting production.

2. Experimental Section

2.1. Experimental Procedures

In the conducted experiments, the influence of temperature (20, 40, 60, 80 °C), different S:L ratio (0.5; 1.0; 1.5; 2.0) and the duration of the process for the extraction of lead and copper into the targeted products were studied. Particular attention was paid to study the distribution of arsenic between leaching products.

The original dust was pre-ground to obtain a homogeneous fine fraction < 200 mesh (74 microns) and subjected to leaching on the device shown in Figure 1.

The initial sample of dust in all experiments was 200 g. The experimental procedure was as follows. The dust was loaded into a heat-resistant glass beaker, and a solution of sulfuric acid was added to it. Next, the glass with the contents was placed on a thermostat and leaching began using a mixing device (Figure 1). The required leaching temperature was set using a thermostat, the stirrer speed was 300 rpm. The acidity of the solution was pH = 0.8–1.0.

After the required time, the suspension was filtered. The solid sediment (cake) and solution obtained after the experiment were weighed and subjected to elemental and phase analyses.

To obtain reliable accurate results, each experiment was repeated three times. Based on the results of product yield and elemental composition, the material balance was calculated for each experiment. The obtained data were used to select and justify optimal process parameters that ensure the maximum extraction of lead into the cake, and copper and zinc into the solution.

2.2. Research Methods

Preparation of the samples for chemical analysis was carried out as follows: 10 mg of the material was dissolved in 65% (vol.) nitric acid and then analyzed. Material composition was characterized using an atomic absorption spectrophotometer equipped with a graphite combustion chamber (Perkin Elmer 5100, PerkinElmer Inc., Waltham, MA, USA). Powder X-ray diffraction (XRD) was performed on an Ultima III diffractometer (Rigaku Corporation, Tokyo, Japan) with quantitative phase analysis accomplished using Jade_10 512i (MDI, Cal.) software and the ICSD database, and energy-dispersive X-ray fluorescence spectroscopy was performed on a LEO Supra scanning electron microscope (SEM).

Additional studies of the chemical and phase analyses of the original dust and leaching products were carried out using an X-ray diffractometer Bruker D8 ADVANCE, α-Cu radiation, tube voltage—40 kV, current 40 mA. Processing of the obtained diffraction pattern data and calculation of interplanar distances were carried out using EVA (V3.1) software. Sample interpretation and phase identification were carried out using the Search/match program using the PDF-2 Powder Diffractometric Database.

The micromorphology of the materials was characterized by the results of SEM-EDS analysis obtained using a JED-2300 energy dispersive X-ray spectrometer (JEOL).

Each sample was subjected to elemental composition twice. The final elemental composition was determined based on the average value obtained from the results of two independent measurements.

To better understand the mechanism of the dust leaching process, a detailed thermodynamic analysis of the reactions of interaction of dust components with sulfuric acid was carried out. Thermodynamic calculations were carried out using Outotec HSC Chemistry 8.1.5 software.

2.3. Feed Material

In this study, the fine dust from one of the copper smelters (Kazakhstan) obtained after the converting process of copper mattes of autogenous smelting (Vanyukov furnaces) was used. The original dust samples were subjected to elemental composition twice in independent studies: using a Perkin Elmer 5100 graphite combustor atomic absorption spectrophotometer, an Ultima III diffractometer (Rigaku Corporation, Japan) and a Bruker D8 ADVANCE. The final elemental composition was determined based on the average value obtained from the results of two measurements.

The elemental analysis of the original dust is shown in

Table 1. Elemental composition of fine dust from copper smelter.

Elements	Cu	Pb	Zn	Fe	As	Si	S	O	Others
Content, %	3.04	25.06	5.91	1.35	13.0	0.3	8.19	22.25	20.23

.

A specific feature of the dust is its high content of non-ferrous metals and arsenic—25% Pb, 3% Cu, 5% Zn, 13% As, respectively.

The results of XRD analysis of dust obtained using an Ultima III diffractometer (Rigaku Corporation, Japan) are presented in Figure 2.

As can be seen in Figure 2, the main components in dust are lead (PbSO₄) and zinc (ZnSO₄) sulfates. Due to the low content of other components in the dust, they are not visible on the XRD pattern. Dusts have a complex composition, so during research, it is necessary to take into account the presence of accompanying elements in them.

The results of studies of the microstructure and elemental content of dust, obtained using SEM-EDS analysis [37] on a JED-2300 apparatus (Figure 3), showed the presence of various compounds present in typical forms: light gray (point 1, the main components are lead, zinc, arsenic oxides As³⁺, As⁵⁺, zinc and copper ferrites, silicates), the presence of the sulfide phase of non-ferrous metals is minimal; black (point 2, the main components in this area are lead sulfate and oxide, minor amounts of copper and zinc oxide); gray (point 3, the main components are oxides of lead, copper, zinc and arsenic, a significant amount of aluminum oxide). Sulfur is mainly present in the form SO₃²⁻ in lead and zinc sulfates. Iron was found in small quantities in the form of wüstite (FeO), hematite (Fe₂O₃) and magnetite (Fe₃O₄).

The results of mapping dust samples (Figure 4) show that arsenic oxides (arsenic is green in the graphs) are scattered among the oxides of lead (lead is blue), copper (copper is red) and their sulfates in greater or lesser quantities. This is confirmed by the chemical composition of the detected phases. The established total content of arsenic oxide in dust in the form of As³⁺, equal to approximately 8% (Figure 3), indicates the presence of arsenic oxide in the form of As⁵⁺ in the feed dust. Moreover, if we proceed from the total arsenic content in dust (13%, Table 1), we can assume that in the feed dust, the proportion of the As₂O₃/As₂O₅ ratio is ~60% to 40%. The obtained result seems fundamental and requires taking into account the influence of As₂O₅ on the distribution of metals between the leaching products.

3. Results and Discussion

3.1. Thermodynamics of Sulfuric Acid Leaching of Fine Dust

Thermodynamic analysis of the leaching process was carried out based on the summarized results of the elemental and phase composition of dust without taking into account the physical dissolution of lead and zinc sulfates and the behavior of sulfides, due to their insignificant content in the original dust.

The behavior of copper, lead, zinc, iron and arsenic during leaching can be described by the reactions of non-ferrous metal oxides, iron and arsenic with sulfuric acid:

CuO(s) + H₂SO₄(l) = CuSO₄(l) + H₂O(l),

(1)

PbO(s) + H₂SO₄(l) = PbSO₄(s) + H₂O(l),

(2)

ZnO(s) + H₂SO₄(l) = ZnSO₄(l) + H₂O(l),

(3)

FeO(s) + H₂SO₄(l) = FeSO₄(l) + H₂O(l),

(4)

Fe₂O₃(s) + 3H₂SO₄(l) = Fe₂(SO₄)₃(l) + 3H₂O(l),

(5)

Fe₃O₄(s) + 4H₂SO₄(l) = Fe₂(SO₄)₃(l) + FeSO₄(l) + 4H₂O(l),

(6)

As₂O₅(s) + H₂O(l) = 2HAsO₃(l).

(7)

A detailed thermodynamic analysis was carried out using Outotec HSC Chemistry 8.1.5 software.

The change in the Gibbs free energy (∆G°_T, kJ/mol) of reactions (1)–(7) from temperature is shown in Figure 5.

As can be seen from Figure 5, changes in ∆G°_T of the leaching of non-ferrous metals’ components (reactions (1)–(3)), iron (reactions (4)–(6)) and arsenic (reaction (7)) depending on temperature have a general character: ∆G°_T with increasing temperature decreases linearly in absolute value. Based on the values of ∆G°_T of reactions (1)–(7), the components can be arranged in a series according to their decreasing dissolution in a sulfuric acid medium: Fe₃O₄ > PbO > Fe₂O₃ > FeO > ZnO > CuO > As₂O₅.

Ferrites of copper and zinc, present in dust, do not dissolve in a solution of sulfuric acid, and when leached, they stay in lead cake. This is indicated by the calculated positive values of the Gibbs free energy of the interaction reactions of copper and zinc ferrites with sulfuric acid over the entire studied temperature range. The transition of ferrites into the lead cake increases the loss of copper and zinc during leaching, which reduces their overall recovery into the final product.

In the entire studied temperature range of 298–373 K (25–100 °C), the Gibbs free energy for the reaction of As₂O₃ with sulfuric acid is positive. This indicates that during leaching, the transition of arsenic in the form As³⁺ into solution is impossible. Consequently, most of the arsenic is concentrated in the solid phase—in lead cake. The transition and final content of arsenic in the solution will be determined by the dissolution of pentavalent arsenic oxide (As₂O₅), according to reaction (7).

3.2. Extraction of Copper, Zinc, Lead and Arsenic into Targeted Products

In the conducted experiments, the influence of temperature, leaching duration and S:L ratio on the recovery of copper, lead and arsenic was studied. Each experiment was carried out twice. The average values were taken as the final result. For each experiment, a material balance of the process was compiled, based on the results of which the extraction of metals into the leaching products was calculated: solution and lead cake.

The extraction of metals into solution (Ɛ_s) was determined by the equation:

Ɛ_s = (γ_s · β_Me/γ_feed · α_Me) · 100, %,

where γ_s—mass of solution, g; β_Me—metal content in solution, %; γ_feed—mass of initial dust, g; α_Me—metal content in dust, %.

The results for the extraction of metals into solution depending on temperature, leaching duration and S:L ratio are shown in Figure 6.

Negative values ∆H < 0 kJ/mol for reactions (1)–(7) characterize leaching as an exothermic process and show that the efficiency of a particular reaction is related to the temperature. To assess the effect of temperature on the extraction of Zn, Cu and As into solution, a series of dust leaching experiments were carried out at temperatures of 20, 40, 60 and 80 °C. The feed sample of dust in all experiments was constant and amounted to 200 g. The duration of the experiments was 60 min. The acidity of the solution was maintained at pH = 0.8.

The effect of temperature on the extraction of Zn, Cu and As is shown in Figure 6a. As can be seen, a sharp increase in the extraction of Zn into the solution from 84.56 to 93.5% is observed in the temperature range from 20 to 60 °C. In the temperature range from 60 to 80 °C, the extraction of zinc into the solution, although slows down, increases slightly from 93.5 to 95.21%. The established high extraction of zinc into the solution, in comparison with the extraction of copper, is quite explainable by the increased content of zinc in the original dust (Table 1). During leaching, along with the physical dissolution of zinc sulfate, the transition of zinc into solution will occur due to reaction (3). The rate of reaction (3) with increasing temperature strongly shifts toward the formation of zinc sulfate. This makes a significant contribution to the total extraction of zinc into the solution.

Similar processes occur in copper. The curve of copper extraction versus temperature shows a similar pattern as for zinc: copper extraction into solution shows a slight increase with increasing temperature. In the temperature range from 60 to 80 °C, copper extraction increases from 85.4% to 87.54%. The low extraction of copper into the solution compared to the extraction of zinc is explained by the fact that the copper content in the original dust is two-times lower than the zinc content. In addition, the course of reaction (3), which describes the transition of zinc into solution, is more preferable compared to reaction (1). High values of the Gibbs free energy of reaction (3) indicate a high possibility of completeness of its occurrence. This increases the extraction of zinc into the solution, as observed in Figure 6a. The established low values for the extraction of zinc and copper into solution are associated with the direct transition of their acid-insoluble sulfides and ferrites into lead cake.

During leaching, the insignificant iron content in the original dust (Table 1) cannot be neglected. Iron in dust is presented in the form of ferrites, higher oxides (hematite, magnetite) and wüstite. The occurrence of reactions (4)–(6) with their participation have more negative values of the Gibbs free energy than reactions (1) and (3) for copper and zinc, respectively. As a result, the concentration of iron oxides in the solution will decrease due to the formation of iron sulfate. With increasing temperature, the rate of reactions between iron oxides and sulfuric acid increases. The more preferable nature of reactions (4)–(6) will inhibit the transition of zinc and copper into solution. However, due to the low iron content in the initial dust, the occurrence of reactions (4)–(6) will not have a significant effect on the transition of zinc and copper into solution.

The distribution of arsenic between the liquid (solution) and solid phase (lead cake) is also important during the leaching process. The maximum transfer of arsenic into solution provides an answer to the main problem: to minimize its transfer into lead cake in order to obtain arsenic-free circulating dust during smelting. The complete transfer of arsenic into solution will create the opportunity for its removal from the technological scheme and disposal in the form of a safe compound—calcium arsenate.

The extraction of arsenic into solution increases slightly with increasing temperature (Figure 6a). However, despite its high content in the original dust (Table 1), its extraction into solution, in comparison with the extraction values of zinc and copper, is very low: the maximum extraction, equal to 40.72%, is achieved at a temperature of 80 °C. Moreover, in the temperature range from 40 to 80 °C, the extraction does not change and remains at the level of ~40%. This indicates that a significant part of arsenic, in the form of As₂O₃, insoluble in a solution of sulfuric acid, passes from dust into lead cake. The transition of arsenic into solution occurs mainly due to reaction (7). This is quite consistent with the results of the elemental and phase composition of the original dust, and the established pattern shown in Figure 6a. Based on the obtained results, the optimal temperature for the dust leaching process is 60 °C. At this temperature, the extraction of zinc, copper and arsenic into the solution reaches maximum values and amounts to (%): 93.5; 85.4 and 39.5, respectively.

Figure 6b shows the effect of leaching time on the extraction of Zn, Cu and As into the solution. The experiments were carried out at a temperature of 60 °C with different leaching times from 20 to 80 min. The acidity of the solution was maintained at pH = 0.8.

It can be seen that in the range of 20–60 min, an increase in leaching time leads to an increase in the extraction of Cu, Zn and As into the solution. By increasing the leaching time from 20 to 60 min, the extraction of copper into the solution increases from 70.8% to 85.4%; zinc—from 72.04% to 93.5%—and arsenic—from 31.7% to 39.5%. The choice of the optimal leaching time should be chosen based on ensuring the completeness of reactions (1)–(7), which describe the interaction of copper, zinc and arsenic oxides with sulfuric acid with the formation of their sulfates. As can be seen in the graph, the optimal process time, which ensures the maximum extraction of metals into the solution, is achieved with a leaching time of 80 min. However, the increase in metal recovery achieved at 80 min is slightly higher than the values for the recovery of metals into solution obtained by leaching dust for 60 min (absolute increase in copper recovery—2.4%, zinc—2.03% and arsenic—2.01%). Based on economic feasibility, the most optimal leaching time can be taken to be 60 min, which is quite sufficient for the maximum transfer of Cu, Zn and As into solution.

A comparative analysis of the dependence of the influence of temperature and time on the extraction of Cu, Zn and As into solution (Figure 6a,b) shows an identical course of the curves. This indicates that the effects of temperature and time during leaching have an equal impact on the extraction of metals from dust into solution. Consequently, one of the main parameters that has a strong impact on the extraction of Cu, Zn and As into solution will be the S:L ratio.

To determine the effect of S:L ratio on the extraction of Cu, Zn and As into the solution, a series of experiments were carried out under the following conditions: t = 60 °C, τ = 60 min. The amount of the initial sample, as in previous experiments, was constant and amounted to 200 g. The experiments were carried out at various values of S:L ratio—0.5; 1.0; 1.5.

The effect of S:L ratio on the extraction of Cu, Zn and As into the solution is shown in Figure 6c. It can be seen that high values for the extraction of metals into solution are achieved even at low S:L = 0.5. An increase in the S:L ratio from 0.5 to 1.5 leads to a significant increase in the extraction of metals: copper, from 75.6% to 88.67%; zinc, from 86.23% to 95.21% and arsenic, from 34.6% to 49.32%. A further increase in S:L seems inappropriate due to the increased consumption of sulfuric acid. The established ratio S:L = 1.5 seems to be optimal, which ensures the maximum extraction of Cu, Zn and As from dust into solution. A decrease in the S:L ratio from 1.5 to 1.0 has an insignificant effect on reducing the extraction of copper and zinc into solution; however, for arsenic, this effect is significant. Arsenic recovery leads to a sharp decline in recovery from 49.32% to 39.5% when S:L ratio decreases from 1.5 to 0.5. This seems natural. To completely transfer arsenic into solution, it is necessary to create conditions for the reaction to occur (7). The increase in the rate of reaction (7) will shift toward the formation of arsenous acid as the acid concentration increases. Moreover, the rate and completeness of reaction (7) will be significantly influenced by the occurrence of reactions (1) and (3). Therefore, the target to simultaneously achieve high extractions of Cu, Zn and As into solution by increasing the S:L ratio is not entirely effective. In addition, this will lead, as shown in Figure 7, to a decrease in the concentrations of copper, zinc and arsenic in the solution due to an increase in the volume of liquid, which will subsequently require additional costs for their extraction.

Figure 8 also shows that with an increase in the S:L ratio from 0.5 to 1.5, the lead content in the cake increases from 38.7% to 50.6%. The maximum extraction of lead in the cake is achieved at S:L = 1.5, which is 96.1%. The beneficial effect is that when the S:L ratio increases from 1.0 to 1.5, the arsenic content in the cake drops from 16.4% to 13.5%.

Thus, the results of the experimental studies show that the main factor influencing the selective extraction of Cu, Zn, As and Pb into the targeted products during leaching is the S:L ratio. To complete the picture of the effect of the S:L ratio on the extraction of metals into the targeted products during leaching, the extraction of metals into the lead cake and into the solution was calculated based on the average material balances of each series of experiments. The results of the obtained data are presented in Figure 8, in the form of the dependence of metal extraction on the S:L ratio.

The obtained results of experimental studies show the fundamental possibility of selective extraction of Cu, Zn and Pb into targeted products by direct leaching of fine dust from a copper smelting industries with sulfuric acid and to establish optimal leaching parameters: S:L = 1.5; t = 60 °C; τ = 60 min.

Figure 9 shows the results for the extraction of metals into targeted products obtained under optimal leaching parameters.

The maximum extraction of metals into the targeted products was: copper and zinc in solution at 89% and 96%, respectively, and lead in cake up to 97%. The recovery of arsenic into the solution was 49.32%. More than half of the arsenic (51%) of its total content in the dust is concentrated in lead cake.

The elemental compositions of the leaching products—lead cake and solution—were determined using a Bruker D8 ADVANCE diffractometer and chemical analysis. The results showed good convergence. The average value was taken as the final result. The reliability of the obtained results was confirmed by the reproducibility of the analysis results, the coincidence of the obtained data via independent research methods and the positive results of laboratory experiments.

The composition of lead cake obtained under optimal leaching conditions (wt. %): 0.92 Cu, 47.1 Pb, 0.8 Zn, 16.36 As, 0.4 Fe, others. The composition of the solution (g/L): 15.31 Cu, 14.34 Pb, 32.59 Zn, 30.26 As, 6.81 Fe, others.

4. Conclusions

The article presents the process of the complex extraction of Pb, Cu, Zn and As from fine dust from copper smelters in Kazakhstan via direct leaching with sulfuric acid. The lead cake and solution obtained by leaching are intermediate products for obtaining pure commercial copper and lead, which are widely used in electrical engineering, electronics, military–industrial and other sectors of the national economy.

Based on the results of the test work performed, the influence of various factors on the mechanism of the leaching process and on the extraction of Pb, Cu, Zn and As into the targeted products, the following conclusions can be drawn.

The results of thermodynamic calculations showed that the method of the direct leaching of dust with sulfuric acid can provide a selective release of lead—into lead cake, copper and zinc—into solution with their further extraction to a marketable product.

Experimental results showed that when dust is leached with sulfuric acid, the influence of temperature and process duration on the extraction of metals into the targeted products is of an equivalent nature. It has been established that a sharp increase in the extraction of Zn into the solution is observed in the temperature range from 20 to 60 °C: from 84.56 to 93.5%. In the temperature range from 60 to 80 °C, the extraction of zinc into the solution slows down, varying within insignificant limits: from 93.5 to 95.21%. A similar nature of the curve of extraction versus temperature is preserved for copper: the extraction of copper into solution with increasing temperature shows a slight increase, and in the temperature range from 60 to 80 °C, it changes from 85.4 to 87.54%. Losses of zinc and copper during leaching are mainly formed due to the transition of their ferrites, which are sparingly soluble in sulfuric acid (∆G° > 0) into lead cake. The increased distribution of arsenic during leaching into lead cake (up to 60%, Figure 9) is characterized by the direct transfer into the cake of a significant proportion of arsenic oxide insoluble in sulfuric acid in the form of As₂O₃. The transfer of arsenic from dust into solution occurs due to reaction (7)—the interaction of pentavalent arsenic oxide (As₂O₅), present in the original dust, with sulfuric acid to form arsenous acid.

Reducing the process time below 60 min will lead to an incomplete transition of copper and zinc due to reactions (1) and (3), respectively. This will reduce their extraction into solution. Increasing the leaching time will lead to an increase in the extraction of copper, zinc, and arsenic into the solution; however, the productivity of the process will decrease.

The most significant factor influencing the extraction of metals is the S:L ratio. High values for the extraction of metals into solution are achieved even at low S:L = 0.5. An increase in the S:L ratio from 0.5 to 1.5 leads to a significant increase in the extraction of metals: copper—from 75.6% to 88.67%, zinc—from 86.23% to 95.21% and arsenic—from 34.6% to 49.32%. A further increase in S:L seems inappropriate due to the increased consumption of sulfuric acid. The established ratio S:L = 1.5 seems to be optimal, which ensures the maximum extraction of Cu, Zn and As from dust into solution. While a decrease in the S:L ratio from 1.5 to 1.0 has an insignificant effect on reducing the extraction of copper and zinc into solution, for arsenic, this effect is significant. Arsenic recovery when S:L ratio decreases from 1.5 to 0.5 leads to a sharp decline in recovery from 49.32% to 39.5%.

The obtained results allow us to formulate a number of important conclusions for practice: (i) with the direct leaching of fine dust from copper smelters in Kazakhstan using sulfuric acid, it is impossible to achieve the maximum transfer of arsenic into solution for the purpose of its further removal from the general technological scheme and disposal; (ii) a high concentration of arsenic in lead cake leads to the transition of arsenic during smelting into circulating dust and (iii) the recycling of dust will increase the accumulation of arsenic in the overall process flow.

The drawn conclusions indicate the economic instability of production in the case of using the known technology of dust leaching with sulfuric acid containing high arsenic. Despite the positive possibilities of the selective extraction of Pb, Cu and Zn into the targeted products, they are not commercial and contain a significant amount of arsenic. At the same time, there is a significant accumulation of arsenic in the technological chain, which does not meet the requirements and approaches of the circular economy. One of the most important tasks is to ensure not only the technological and economic component, but also the creation of environmentally friendly technology. This approach is one of the key principles of economic sustainability of production [38,39].

The formation of arsenic-containing waste during the leaching of fine dust with sulfuric acid, with its further storage and the creation of any form of storage for it, poses a serious danger to the environment. During storage in the dump mass, chemical interactions occur with the formation of water-soluble arsenic phases. The latter are concentrated in pore solutions and subsequently migrate from the dumps into the environment with drainage waters. The toxic danger of dumps to the environment increases sharply over time. Therefore, any storage of arsenic-containing waste is temporary and does not eliminate the danger to the environment.

The use of this technology from a technological point of view is quite possible, but only after measures have been taken to first remove arsenic from dust. At the same time, it can be reasonably assumed that the obtained products will have high quality, and the extraction of valuable metals into marketable products will increase. The economic efficiency of the technology will increase by increasing the productivity of the leaching process, reducing the consumption of sulfuric acid, minimizing the consumption of reagents for arsenic disposal and the release or absence of arsenic-containing waste.

The purpose of this work is to rethink the concept of arsenic-containing management in dust, in the context of the transition to a circular economy, to assess and establish features as well as determine directions for testing foreign experience in the field of waste management in Kazakhstan.

The work is aimed at expanding economic, social development and environmental protection, which are three interconnected and mutually reinforcing pillars of sustainable development in a society.

The theoretical approach is based on an examination of existing studies on dust recycling, and on the content analysis of existing modern sources in the field of the circular economy. The study was carried out by selecting and systematizing facts and data, by abstracting and examining them to identify problems of the separate complex processing of non-ferrous metallurgy dusts in Kazakhstan as well as finding their solutions.

The management system for industrial waste, including dust, is one of the conditions for the implementation of the “green economy”, which is due to the insufficient degree of the development of practical approaches to the topic of their processing. One of the driving factors that contribute to the effectiveness of the dust management system is the development and design of “green” categories, such as circular production model, recycling technologies, financing tools, etc. All this is aimed at solving problems associated with environmental pollution. The main factors hindering the development of a dust management system are internal factors, including personal ones.

The solutions proposed in the work are aimed, first of all, at the removal of toxic arsenic and its compounds at the initial stage of the technological scheme with a complete disposal of toxic substances. This approach makes it possible in the future to build an environmentally friendly, waste-free combined technology that makes it possible to comprehensively extract a whole range of valuable metals into targeted commercial products. For example, using the technology we are developing, lead, copper, zinc and rhenium are extracted. Further development of technology helps to eliminate the formation and accumulation of fine dusts with a high content of toxic substances, significantly improving the state of the environment. The organization of the separate processing of dust will significantly affect the improvement of the social sphere in the region: additional production and additional jobs will appear.

The positive results achieved in the work can be used in various regions of Kazakhstan to attract the attention of local representative authorities and the public to the problem of fine dust management, which is one of the important components of the circular economy in the context of the development of the “green economy”.

Achieving certain results in the field of rational and efficient, waste-free recycling of dust will lead to minimizing their negative impact on the environment.

The originality of the article lies in the fact that it more systematically examines the problems of the poor functioning of the system of education and management of non-ferrous metallurgy dusts, and shows the shortcomings in organizing their further rational processing based on the latest achievements of world experience and practice.

Prospects for further research are related to the continuation of the analysis of the problems of fine dust accumulation from non-ferrous metallurgy and the search for ways to rationally solve them based on “green technologies”.

The significance of the work on an international scale lies in solving two key problems: environmental and technological. The solution to the first problem is to involve non-ferrous metallurgy dust in the processing of arsenic-containing dust—in reducing the burden on the environment due to the removal of arsenic and its disposal in the form of non-toxic calcium arsenate, thereby reducing the volume of waste gases into the environment. The solution to the second problem is to expand the range of commercial products through the integrated extraction of non-ferrous metals and rhenium.

The proposed technology may become an alternative for the production of non-ferrous metals and rhenium in countries where there is no primary raw material for their production.

The use of this method will significantly improve the technological performance of leaching and will create the opportunity to use it for processing other substandard arsenic-containing materials. Preliminary studies on fine dust roasting have shown positive results, which will be published in the near future.