Issue
Metall. Res. Technol.
Volume 119, Number 1, 2022
Article Number 105
Number of page(s) 9
DOI https://doi.org/10.1051/metal/2021090
Published online 05 January 2022

© EDP Sciences, 2022

1 Introduction

Lead is a heavy metal which is widely used in daily life and industrial production, and has been industrially produced as early as the 16th century [1]. With the continuous progress of the new energy industry, lead is more and more widely used, and the requirements for lead purity are getting higher and higher, such as the research and development of new energy vehicle batteries, the development of radioactive protection materials in the nuclear industry and so on. These new properties and applications have promoted the rapid development of the lead metallurgy industry, and posed new challenges to lead purification technology [2]. Arsenic is a highly toxic element, often associated with lead ore, and is one of the main hazardous wastes generated in the lead smelting process [3]. In the smelting process, As is usually discharged into the production system in the form of “three wastes” and enters the natural environment, seriously destroying the surrounding air, water and soil, and has a significant impact on human health and ecological environment. The high arsenic content in crude lead not only pollutes the environment, but also brings many difficulties to the electrolytic refining of crude lead [4]. The electrolysis process will further enrich As, resulting in a high amount of As in the electrolyte, and the cathode will still precipitate As, affecting the purity of refined Pb [5]. Therefore, a method is needed to solve the effect of As on the purity of Pb during smelting.

There are many reports about As removal, most of which are the separation of As from secondary resources, adsorption [6] or leaching of arsenic from arsenic containing wastewater [7,8]. There are relatively few reports on As removal and metal refining in pyrometallurgy. The traditional pyrometallurgy includes oxidation refining, alkaline refining and vacuum distillation technology [9]. Lin et al. [10] used a two-step vacuum dynamic evaporation (VDE) and vacuum dynamic flash reduction (VDFR) method to remove As from crude lead, and the As removal rate can reach 99.96%. However, due to the high cost of vacuum distillation equipment, this method has not been widely used. Oxidation refining uses oxygen in the air to remove impurities, while alkaline refining uses sodium nitrate as oxidant to remove impurities [11]. In the oxidation refining process, the Pb loss is large, the fuel consumption is large, and the operation time is long. Especially the oxide of arsenic is volatile, which is easy to cause secondary pollution of the environment, so this method is rarely used [1214]. Compared with oxidation refining and vacuum distillation technology, alkaline refining method has the advantages of short process, simple operation and rapid reaction. Moreover, arsenic is enriched in alkali residue and can be treated intensively. Toxic substances will not volatilize and cause secondary pollution to the environment.

However, there are few reports of alkaline refining treatment of crude lead. The removal rate of As was studied with a mixture of NaOH and Na2CO3 as removal agent. The effects of Na2CO3:NaOH, holding time, reaction temperature and dosage of arsenic removal agent on As removal were investigated, and the effective As removal process was determined. XRD, XPS, FT-IR and SEM-EDS were used to study the behavior of As in alkaline refining, which further clarifies the demoval mechanism of arsenic in the alkaline refining process.

2 Experiment

2.1 Reagents and materials

The crude lead with high arsenic content used in this study was obtained from a lead plant located at Qujing, Yunnan, China. XRF was used to analyze the content of elements in materials, as shown in Table 1, and As content (1.7%) is a result of quantitative analysis. The results in Table 1 show that the Pb count is 98%, and the contents of other elements except arsenic are very low.

Before the arsenic removal experiment, it is necessary to study the distribution and phase composition of arsenic in crude lead. Figure 1 shows the SEM-EDS analysis results and elemental distribution mapping of crude lead. As is not evenly distributed in Pb, because when Pb and As form a solid solution, the amount of arsenic dissolved is very small. When the temperature is 563 K, the arsenic content in the solid solution of As and Pb is only 0.14%. Therefore, in the solidification process of crude lead liquid, As in crude lead will be enriched in different areas, and will not be evenly distributed in the Pb phase. The As is concentrated in the lower-left corner of Figure 1a, and distributed in strips and blocks, as shown in Figure 1b, c and e. Oxygen is almost uniformly distributed on the surface of the sample (see Fig. 1f), which is due to the oxidation of lead on the surface. No element has the same distribution as arsenic, indicating that arsenic does not form intermetallic compounds with other elements.

Table 1

Elemental compositions of crude lead (wt.%).

thumbnail Fig. 1

Elemental distribution mapping of crude lead and EDS spot scan images of crude lead: (b) energy spectrum of point 1 in the image a, (c) energy spectrum of point 2 in the image a, (d) Pb mapping of the image a, (e) As mapping of the image a, (f) O mapping of the image a.

2.2 Experimental procedure

All arsenic removal experiments were carried out in graphite crucible. In the experiment, a certain amount of crude lead was weighed, and the amount of arsenic removal agent was determined according to the weight of crude lead, and then the arsenic removal agent was prepared in proportion to Na2CO3 and NaOH. Put crude lead and arsenic removal agent into the graphite crucible, and the graphite crucible was heated in the resistance furnace to record the insulation time when the temperature of molten lead reaches the specified temperature. During the heat preservation process, the stirring blade speed was set to 150 rpm. After reaching the insulation time, the arsenic slag produced is removed with a graphite sample scoop. Then the lead liquid after arsenic removal is poured into a cast-iron mold, and the As content in it is analyzed after cooling. The experimental equipment diagram is shown in Figure 2.

The As removal rate was calculated using the formula, where m1 refers to the weight of crude lead (g), w1 represents the As content of crude lead (%), m2 denotes the weight of lead after arsenic removal (g), and w2 stands for the As content of lead after arsenic removal (%).

thumbnail Fig. 2

Diagram of the experiment apparatus.

2.3 Analysis techniques

The heating equipment used in this experiment is box resistance furnace (SG2-5-12 1473 K, Xinghua Huasheng Electric Co., Ltd., China). The morphology of the sample was observed using a field emission SEM instrument (Nova-Nano SEM450). The phase analyses of the samples were carried out via XRD (X‘Pert3 powder, PANalytical, Netherlands), using Cu Kα radiation (λ = 1.54056 Å). A scan rate of 4°/min was applied to record a pattern in the 2θ range 5–90°. A Cu target was used as the X-ray excitation source. The negative pressure was 40 kV and the current was 40 mA and the scanning time was 30 min. The contents of As was detected using an AAS instrument (WFX-320, Beijing Beifen-Ruili Analytical Instrument (Group) Co., Ltd., China). The FT-IR spectra are acquired on a Thermo Fisher Scientific Nicolet iS 50 FT-IR spectrometer with a DTGS detector, a KBr beam splitter spread with Ge, an interferometer driven by plane mirrors electromagnetic force and longer lifetime middle/far infrared source.

3 Result and discussion

3.1 Analysis of influence trend of various factors on As removal

Through the single factor experiment, the optimum process conditions and the influence degree of each factor on the arsenic removal effect were obtained. Through the data of each factor, the influence trend of each factor on the arsenic removal effect was analyzed.

3.1.1 Effect of reaction temperature

Because the change of temperature will affect the occurrence of oxidation reaction, in order to study the effect of temperature on the removal rate of As, a temperature single factor experiment was carried out. Under the conditions that the holding time 120 min, Na2CO3:NaOH 1:1, and the refining agent dosage 8%, the experimental results of the change of As removal rate with temperature are shown in Figure 3. It can be seen from the figure that when the temperature increases from 623 K to 723 K, the removal rate increases from 34.88% to 59.94%. Then the trend of arsenic removal rate gradually becomes flat with rising temperature. The reason is that in the process of arsenic removal, As will first react with O2, and the oxidation product (As2O3) is easy to volatilize at higher temperature [15], so high temperature is favorable for the reaction between As2O3 and arsenic removal agent. The concentration of O2 in the air is limited, as well as the dissolved O2 in the lead liquid. Therefore, with the increasing temperature, the removal rate of As will gradually stabilize. Thus, the As in the crude lead will be gradually transferred to the arsenic removal slag, and the As removal rate increased gradually. So the reaction temperature was 823 K in the subsequent single factor experiment.

thumbnail Fig. 3

Effect of reaction temperature on As removal rate.

3.1.2 Effect of holding time

At the reaction temperature of 823 K, Na2CO3:NaOH 1:1 and refining agent dosage 8%, the relationship between As removal rate and holding time was explored. The results are shown in Figure 4. It can be seen from that the As removal rate is gradually increasing, and the change is very sharp in the first 30 min. The As removal rate increased from 36.82% to 56.47% in 30 min, but the change tends to be gentle with the increase of time. This indicates that the reaction speed in the process of As removal is fast, and the reaction tends to be completed in a short time. Although the removal speed of As is very fast, it can be seen that the As removal rate can only reach 66.09% with the increase of time. This reflects that the effect of As removal depends largely on the oxidation degree of As in crude lead [16]. With the process of arsenic removal reaction, a layer of arsenic removal slag is formed on the surface of the crude lead, which will hinder the oxidation of lead and arsenic, resulting in the slow rate of arsenic removal reaction. From the point of view of energy saving, prolonging the holding time will consume more electric energy for liquid lead insulation and stirring, so 60 minutes is defined as the optimized holding time.

thumbnail Fig. 4

Effect of holding time on As removal rate.

3.1.3 Effect of Na2CO3:NaOH

Sodium carbonate and sodium hydroxide are the main components of arsenic removal agents, and their proportion will directly affect the arsenic removal effect. Under the conditions as follows: reaction temperature 823 K, holding time 60 min, and refining agent dosage 8%, the effect of the ratio between Na2CO3 and NaOH on arsenic removal rate is shown in Figure 5. The As removal rate increases with the raise of NaOH ratio, indicating that the increase of NaOH is beneficial to the production of sodium arsenate. The highest arsenic removal rate was 77.25%. The melting point of the NaOH is about 591.5 K, and that of Na2CO3 is 1124 K. In the reaction process, the molten NaOH can fully contact with the molten lead, which is conducive to the As removal reaction. Na2CO3 can not only remove As in the reaction process, but also make the arsenic removal slag more viscous, which is conducive to the fixation of arsenic removal slag [17]. According to the experimental results, the Na2CO3:NaOH of 1:4 was selected as the follow-up single factor test conditions.

thumbnail Fig. 5

Effect of Na2CO3:NaOH.

3.1.4 Effect of dosage of As removal agent

In the refining process, the output of arsenic removal slag depends on the amount of arsenic removal agent. Excessive arsenic removal slag will increase the burden of slag treatment. Under the condition of 823 K, holding time 60 min and Na2CO3:NaOH 1:4, the relationship between arsenic removal rate and dosage of As removal agent was studied, and the results are shown in Figure 6. The increase of the amount of As removal agent is certainly conducive to the removal of As. This is because adding a large amount of arsenic remover can increase the reactants, make the As removal reaction more thorough, and further reduce the arsenic content. Therefore, it can be seen from Figure 6 that the As removal rate has been on the rise, from 48.29% to 79.09%. Taking 8% as the boundary, the increasing trend of As removal rate between 8% and 10% is significantly higher than that of 6% to 8%.

In conclusion, the optimized experimental conditions are as follows: reaction temperature: 823 K, holding time: 60 min, Na2CO3:NaOH is 1:4, refining agent dosage 10%. Under the optimum conditions, the arsenic removal rate can reach 79.09%. Through communication with the technical staff of the lead refinery, the arsenic content in the refined lead is 0.35%, which meets the requirements of subsequent electrolytic refining.

thumbnail Fig. 6

Effect of dosage of As removal agent.

3.2 Characterization of As removal slag

3.2.1 XPS spectra of As removal slag

Table 2 shows the results of element content analysis of As removal slag. It is found that the main elements are Na, As, and Pb. In order to further analyze the main components of As removal slag, XPS analysis is carried out on arsenic removal slag.

Figure 7 shows the XPS survey spectrum (a) and As3d XPS detail spectrum (b) of As removal slag. A full-scan spectrum was adopted for an overall understanding of surface elemental constituents in the tested sample, which shows the existence of Na, As, O, and Pb. The element As was analyzed by the XPS detail spectrum. It is shown in Figure 7b, the XPS detail spectrum of As3d consists of a peak, located at 44.5eV. From previous reports on binding energy values of As, the binding energy values of As3d are in the range of 44–46 eV, suggesting that As might be present as As(V) [18,19]. As in the As removal slag may occur as As(V) bonded to oxygen ligands, that is the [AsO4]3− anion [20,21]. According to the results of the XPS analysis, arsenic oxidation reaction occurs during arsenic removal.

Table 2

Chemical analysis results of arsenic alkali residue (wt.%).

thumbnail Fig. 7

XPS survey spectrum (a) and As3d XPS detail spectrum (b) of As removal slag.

3.2.2 SEM-EDS analysis of As removal slag

To further analyze the micromorphology and chemical composition of As removal slag, the As removal slag was analyzed by SEM-EDS, and the results are shown in Figure 8. It can be clearly observed from the figure that the surface morphology of the arsenic removal slag is unevenly distributed, mostly in a block structure. The energy spectrum of point 1 shows the Na, As, O, and Pb are the main elements in arsenic removal slag, this is consistent with the XPS results. Then the elemental distribution mapping of As removal slag shows the distribution of Na, As and O is highly consistent. Similarly, Pb, As and O also have the same distribution characteristics, it can be inferred that compounds composed of Na, As and O and compounds composed of Pb, As and O in the As removal slag. In the area of partial O and As enrichment, lead is distributed, which indicates that Pb is likely to exist in the form of Arsenate and oxide of lead in the slag. The above results show that the main substance of As removal slag is arsenates.

thumbnail Fig. 8

Elemental distribution mapping and EDS spot scan images of As removal slag.

3.2.3 FT-IR analysis of As removal slag

The FT-IR spectrum of As removal slag is shown in Figure 9. The results are interpreted on the basis of published data. The OH band from NaOH appears at 3259.38 cm−1. The IR spectrum indicated that the As–O covalent bonds had a strong infrared absorption near the wavenumber of 807.44 and 1436.23 cm−1 [22]. Moreover, there are reports pointed out that the IR bands located at about 1655.27 cm−1 and 865.91 cm−1 correspond to the AsO43- ions [23]. From the results, we can see that the As in arsenic removal slag mainly exists in the form of arsenate, which also shows that As was oxidized to As(V) in the process of arsenic removal. The results of FT-IR and XPS analysis show that As is oxidized to high price, and the reagents added during the experiment are not oxidizing. Therefore, the substance that can oxidize As during the experiment is oxygen. To analyze the phase composition of slag accurately, a X-ray diffractometer should be used for further analysis.

thumbnail Fig. 9

FT-IR spectrum of As removal slag.

3.2.4 X-ray diffraction analysis of As removal slag

Figure 10 shows the XRD spectrum of As removal slag. It can be seen that the As removal slag mainly contains Na2CO3, Pb2As2O7, PbO, and Na3AsO4. This suggests that As in crude lead reacts with Na2CO3 and NaOH to form Na3AsO4, which separates As from crude lead. The effect of Na2CO3 is mainly to make slag, which is conducive to the separation of As removal slag and crude lead. A small part of As exists in the form of Pb2As2O7. In the reaction process, Pb will also be oxidized into PbO and enter the As removal slag. Combined with the conclusions in Figures 79 the results show that most of the As in the crude lead forms Na3AsO4 after alkaline refining and enters into the arsenic removal slag, a small part of it forms Pb2As2O7, As a result, arsenic in the crude lead is greatly purified.

Sodium arsenate is relatively stable in physical and chemical properties, not volatile at room temperature, and soluble in water. Therefore, the arsenic removal slag should be stored in a dry environment. The solid arsenic removal slag is also convenient for centralized processing and transportation, which is beneficial to the recovery and utilization of arsenic secondary resources.

thumbnail Fig. 10

X-ray diffraction pattern of As removal slag.

3.3 Reaction mechanism

From the above conclusions, it can be seen that the roasting process of crude lead adding NaCO3 and NaOH is a multiphase reaction in which gas phase, solid phase and liquid phase interact. Increasing the amount of arsenic removal agent slows down the diffusion of gaseous oxygen atoms to the surface of crude lead, resulting in slower reaction and slower change of arsenic removal rate. It can be seen from the experimental results in Figure 6 that the change of rate increases slowly with the increase of dosage. The active oxygen atoms in PbO can also provide oxygen atoms for reacting with as during the reaction process to promote the removal of arsenic. The main chemical reactions that occur during the heating process are shown below. As reacts with O2 and PbO to form As2O3, and then reacts with Na2CO3 and NaOH to form Na3AsO4 and CO2 [24,25]. The principle is that As has a greater affinity for oxygen than Pb. The As is preferentially oxidized to high valence oxides, then it reacts with NaOH and Na2CO3 to form sodium salt to separate from Pb. The process principle of As removal by alkaline refining is shown in Figure 11.(1) (2) (3) (4)

The Gibbs free energy as a function of temperature for the reaction of the heating process is displayed in Figure 12. It clearly shows the possibility of all reactions occurring in the temperature range 273–1273 K. It can be seen that the Gibbs free energies of all reactions are negative, indicating that all reactions can occur in this temperature range.

thumbnail Fig. 11

Process principle of arsenic removal in alkaline refining.

thumbnail Fig. 12

Relationships between Gibbs free energy of As removal reaction and temperature.

4 Conclusion

According to the experimental results, the conclusions are summarized as follows.

  • The optimum conditions for the experiment are as follows: reaction temperature: 823 K, holding time: 60 min, Na2CO3:NaOH is 1:4, refining agent dosage 10%. Under optimized experimental conditions, the arsenic removal rate can reach 79.09%.

  • Arsenic must be oxidized before it can react with Na2CO3 and NaOH. In the refining process, arsenic can be oxidized by O2 in the air and PbO.

  • Arsenic in the crude lead is separated from lead by reaction with Na2CO3 and NaOH to form arsenic alkali residue, which floats on the surface of the lead liquid. Arsenic exists in the form of Na3AsO4 and Pb2As2O7.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgments

Financial aid from the following programs is gratefully acknowledged: Yunan Ten Thousand Talents Plan Young & Elite Talents Project (grant number YNWR-QNBJ-2018-112), and Analysis and Testing Fund of Kunming University of Science and Technology (2019M20182228016).

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Cite this article as: Rong Zhu, Hailin Long, Haoyu Li, Huimin Xie, Shaohua Yin, Yongmi Wang, Libo Zhang, Shiwei Li, Alkaline refining of crude lead: a method of arsenic removal and the behavior of arsenic in the process, Metall. Res. Technol. 119, 105 (2022)

All Tables

Table 1

Elemental compositions of crude lead (wt.%).

Table 2

Chemical analysis results of arsenic alkali residue (wt.%).

All Figures

thumbnail Fig. 1

Elemental distribution mapping of crude lead and EDS spot scan images of crude lead: (b) energy spectrum of point 1 in the image a, (c) energy spectrum of point 2 in the image a, (d) Pb mapping of the image a, (e) As mapping of the image a, (f) O mapping of the image a.

In the text
thumbnail Fig. 2

Diagram of the experiment apparatus.

In the text
thumbnail Fig. 3

Effect of reaction temperature on As removal rate.

In the text
thumbnail Fig. 4

Effect of holding time on As removal rate.

In the text
thumbnail Fig. 5

Effect of Na2CO3:NaOH.

In the text
thumbnail Fig. 6

Effect of dosage of As removal agent.

In the text
thumbnail Fig. 7

XPS survey spectrum (a) and As3d XPS detail spectrum (b) of As removal slag.

In the text
thumbnail Fig. 8

Elemental distribution mapping and EDS spot scan images of As removal slag.

In the text
thumbnail Fig. 9

FT-IR spectrum of As removal slag.

In the text
thumbnail Fig. 10

X-ray diffraction pattern of As removal slag.

In the text
thumbnail Fig. 11

Process principle of arsenic removal in alkaline refining.

In the text
thumbnail Fig. 12

Relationships between Gibbs free energy of As removal reaction and temperature.

In the text

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