Separation and Enrichment of Au and Ag from Lead Anode Slime by a Selective Oxidation–Vacuum Volatilization–Carbon Reduction Process

Abstract

Huge amounts of Au and Ag are recovered from the hazardous waste lead anode slime. The conventional extraction of precious metals from lead anode slime is based on pyrometallurgical and electrolytic processes, which are seriously conditioned by the separation of harmful elements As and Sb. In this paper, an innovative and efficient oxidation–vacuum volatilization–carbon reduction process was proposed to separate and enrich Ag and Au from lead anode slime. Before vacuum volatilization, selective oxidation of the lead anode slime was performed. Then, vacuum volatilization and vacuum carbon reduction were used to obtain a gold- and silver-rich alloy. The feasibility of the process was verified experimentally and theoretically. The effects of temperature and time on vacuum volatilization separation and reduction enrichment were investigated. The experimental results showed that the Ag content in the resulting gold- and silver-rich alloy was as high as 67.58%, Au was as high as 4287 g/t, and the efficiencies for the recovery of Ag and Au from the lead anode slime were 99.25% and 99.91%, respectively. The gold- and silver-rich alloy can be directly used to produce Ag ingots. Moreover, no gas or wastewater was discharged in this process, so Ag and Au were recovered in a sustainable and cleaner manner.

1. Introduction

Lead anode slime (LAS) produced from the crude Pb electrolytic refinement process is a significant resource for recovering precious metals [1,2]. In 2022, the world’s refined Pb production reached 12.29 million tons [1,2], and more than 20% of refined Pb was produced by electrolytic refining [3]. The output of LAS is generally 1.2–2% of that of refined Pb [4,5]. According to these industrial data, at least 0.2 million tons of lead anode slime was produced in 2022 [6]. LAS contains 0.2 to 10% Ag by weight, and each ton contains 20 to 600 g of Au [7]. In 2022, 4775 tons of Au were produced worldwide [8], and the average price of Au was as high as USD 55.96 per gram [9]. Global Ag output was as high as 29,208.5 tons, and the average price was USD 0.94 per gram in the same year [10]. Compared to 2020 and 2021, the outputs of Au and Ag rose in 2022. This can mainly be attributed to the abatement of the COVID-19 epidemic and the revival of global industries [11]. Because of their high corrosion resistance and stability, good electrical and thermal conductivity, and excellent plasticity, Au and Ag have been widely used in modern high-tech industries, including electronics, communication, aerospace, chemicals, medicine, and other fields, in addition to their use in reserves, investments, and jewelry [12]. Improving the utilization of high-value wastes containing Au and Ag could have significant economic and industrial implications, such as relieving the demand for precious metals. The amount of Ag recycled from LAS accounts for more than 70% of the total Ag output, and this proportion is still rising [13,14]. The amount of Au recovered from LAS represents more than 8% of the annual gold output [15]. With the increasing scarcity of precious metal-rich ores, clean and efficient recovery of Ag and Au from LAS is gaining traction. Lead anode slime has emerged as a critical raw material for the extraction of Ag, Au, and other valuable metals.

However, LAS contains precious metals, heavy metals such as Pb, Sb, Cu, and Bi, and the toxic element As. LAS has As levels of 0.6–20 wt% [16,17]. Therefore, the Ministry of Ecology and Environment of China has included lead anode slime (Waste Code 321-019-48) in the National Hazardous Waste List and strictly manages its storage and treatment [18]. To obtain the precious metals, the other elements present in LAS must be removed [19,20]. Conventional treatment of LAS involves pyrometallurgical processes and electrolytic refining. First, LAS is oxidized and reduced in a reverberatory or converter. The dust absorbs over 85% of the As and Sb from LAS, and Au and Ag are enriched in noble Pb [21]. The furnace temperature must be kept between 1200 and 1300 °C during the oxidation reduction blowing process, and the coal or coke powder reductant, the As removal agent soda (Na₂CO₃), the Pb replacement agent (Fe filings), the slagging agent limestone, and fluorite must all be present [22]. Then, the noble Pb is sent to a converter for oxidative refining at temperatures between 800 and 900 °C, compressed air is introduced to oxidize As and Sb into oxides, low-valent oxides are volatilized into secondary dust, and oxidation dross containing As, Sb, and Pb is generated [23]. After preliminary oxidative refining, the furnace temperature is raised to 1000–1100 °C, and blowing is resumed to oxidize and slag the remaining As, Sb, and Pb in the melt [21]. The lead anode slime can sometimes contain high concentrations of Se and Te, and it is difficult to completely oxidize these two rare metals by oxidative refining. To enhance the oxidation process, nitrate (NaNO₃) must be added as an oxidant, and soda (Na₂CO₃) must be added to reduce Te volatilization [24,25]. Finally, the crude Ag–Au alloy produced by oxidative refining can be electrolyzed to produce Au and Ag ingots. The Au recovery rate in this traditional precious metal recovery process is no more than 96.8%, the Ag recovery rate is less than 95.6%, and 0.3 tons of high-arsenic dust is produced for every ton of lead anode slime [24], Which are dispersed in a wide range of products and must be removed through a series of metallurgical steps. In view of this, it is of great significance to develop a sustainable and technically feasible process for effectively separating Ag and Au from lead anode slime.

Traditional pyrometallurgical LAS treatment necessitates multiple oxidation and reduction processes that consume large amounts of energy and generate large amounts of slag and metal dust. Even if a dust collection system is in place, dust and other toxic substances containing heavy metals inevitably contaminate the local environment via precipitation, which results in air pollution and an excess of heavy metals in the water and soil environment [26,27]. At present, industrial activities are also considered to be the major source of As pollution [27]. Vacuum volatilization, a clean metallurgical method, has been widely applied in alloy separation, metal purification, and the recycling of precious metals from solid waste residues because of advantages such as efficient metal recovery, short processing time, low pollution generation, and low energy consumption [28,29,30]. Vacuum volatilization is considered a simple process and a sustainable physical metallurgical technology, and it is based on the higher saturation vapor pressures of substances that are easily evaporated under vacuum conditions relative to substances with lower saturation vapor pressures [31,32]. Given these reasons, we investigated this technology. In this study, a selective oxidation–vacuum volatilization–carbon reduction process is proposed to separate and enrich Ag and Au from lead anode slime. Au and Ag were enriched in a gold- and silver-rich alloy. Sb and As, which were combined with the precious metals, were removed into volatiles. The new method achieved the goal of efficient and green recovery of precious metals in a cleaner manner.

2. Materials and Methods

2.1. Materials and Chemicals

The LAS used in this study was generously provided by a lead smelting plant in Henan Province, China. Before the experiment, the initial LAS had to be dried and ball milled. LAS consisted of a black powder. The dried LAS was analyzed by chemical titration, flame atomic absorption spectrometry (AAS, WFX-320, Beifen-Ruili, Beijing, China), and inductively coupled plasma∓atomic emission spectrometry (ICP–AES, OPTIMA 8000, PerkinElmer, Waltham, MA, USA) (

Table 1. Main chemical composition of the dried LAS.

Element	Ag	Au	Cu	Te	As	Sb	Pb	Bi
Content (wt %)	8.90	561 g/t	3.19	0.29	4.92	34.62	17.22	17.08

). The determination of elements present in the lead anode slime was based on the Chinese Nonferrous Metal Industry-Standard YS/T 775.5-2011 [33], which specified the methods and steps. The dried LAS powder contained 8.90% Ag and 561 g of Au per ton of LAS, as well as 37.5% Cu, Pb, and Bi. The high As (4.92%) and Sb (34.62%) concentrations were noteworthy. In addition, the nonmetallic elements F, Cl, S, and O were found in the lead anode slime by X-ray fluorescence analysis (XRF, XRF-1800, Shimadzu, Kyoto, Japan).

The phases were identified by X-ray diffraction (XRD) using a Rigaku/D-MAX/2600 diffractometer with Cu Kα (40 kV) radiation. XRD patterns were collected over the range of 10 to 90°. The X-ray diffraction pattern is depicted in Figure 1a. Most of the Ag in the lead anode slime was present as elemental Ag and silver antimonide (Ag₃Sb). The Au content of the lead anode slime was lower than that of the other metals, and there were no obvious X-ray diffraction peaks for elemental Au or Au compounds. The two main phases of As in the lead anode slime were arsenic trioxide (As₂O₃), arsenic pentoxide (As₂O₅), and copper arsenide (Cu₃As). Sb was present as antimony trioxide (Sb₂O₃), silver antimonide (Ag₃Sb), and elemental Sb. LAS also contained lead monoxide (PbO), lead telluride (PbTe), and elemental Pb. In addition, XPS was used to examine the valence states of Ag and Au in LAS (Figure 1b). As shown in Figure 1b, Ag had two valence states in LAS, 0 and +1; Au had a low content and only existed as an elementary substance. These findings are in agreement with the given XRD and elemental distribution analysis of the dried LAS.

2.2. Extraction Process of Ag and Au

The integrated recovery process for Ag and Au was systematically designed, as shown in Figure 2. First, the lead anode slime was vacuum dried for 24 h at 393 K and milled to a 200-mesh size with a ball crusher. Second, the pretreated lead anode slime and oxidant were uniformly mixed. The oxidant was a mixture of lead trioxide, lead tetroxide, and lead dioxide, with a Pb content of 88.59%, which was made by desulfurizing the lead paste of a waste lead–acid battery and then calcining the desulfurization product at 653 K to prevent the introduction of other impurities. According to SEM surface morphology analysis, in addition to Pb, there is only an element of O in the oxidant. The core apparatus was a homemade vertical vacuum furnace. Then, a vacuum pump was turned on to extract air. When the pressure in the furnace was pumped below 10 Pa, the temperature was raised to 653 K and held for 120 min to selectively oxidize LAS.

Following the oxidation stage, the mixed material was heated to the volatilization temperature and kept there. Volatile substances evaporated into the vapor phase, while low-volatility substances remained. Ag and Au were separated from Sb, As, and other heavy metals by this vacuum volatilization step. Both volatiles and residues were collected after the furnace had cooled to room temperature. Finally, vacuum carbon reduction enrichment at a predetermined reduction temperature and time interval was carried out with an appropriate amount of high-purity carbon particles to enrich the precious metal-containing residues obtained from the separation stage. The condenser and crucible were cleaned and weighed after the experiment. The volatiles and residues were randomly sampled at six different locations for further analysis.

3. Results and Discussion

Ag and Au were the elements targeted for recycling in this study. In the separation and enrichment stages, the effects of the vacuum distillation temperature and time on the recovery of precious metals were investigated. The following formulas were used to calculate the removal rate for As and the efficiency for the recovery of Ag or Au:

$$\mathit{Removal}\mathit{rate}=\frac{m_1\times {\omega }_1}{m_0\times {\omega }_0}\times 100\%$$

(1)

$$\mathit{Recovery}\mathit{efficiency}=\frac{m_2\times {\omega }_2}{m_0\times {\omega }_0}\times 100\%$$

(2)

where m₀ is the mass of the dried LAS, g; m₁ is the mass of the volatile component, g; ω₀ is the content of As (wt%), Ag (wt%), or Au (g/t) in the dried LAS; ω₁ is the content of As in the volatile component, wt%; m₂ is the mass of the residues, g; and ω₂ is the content of Ag (wt%) or Au (g/t) in the residues.

3.1. Regulation of LAS by Selective Oxidation

According to the analysis of elements present in Section 2.1, there were intermetallic compounds Ag₃Sb, Cu₃As, and PbTe in LAS, which would affect the separation of precious metals from other elements and reduce the enrichment effect of Au and Ag. Therefore, before vacuum volatilization, selective oxidation was used to regulate the lead anode slime. The oxidant was a mixture of lead trioxide, lead tetroxide, and lead dioxide, which was made by desulfurizing the lead paste from a waste lead acid battery and then calcining the desulfurization product at 653 K for 6 h. LAS was oxidized at an oxidant ratio of 0.45, an oxidation temperature of 653 K, and an oxidation time of 120 min. The phases of the oxidation product were tested by XRD and XPS. There were no intermetallic compounds Ag₃Sb, Cu₃As, and PbTe in the selective oxidation product, and Au and Ag were all in an elemental state (Figure 3). LAS pretreatment with selective oxidation lays the foundation for the efficient separation of other heavy metals, such as Sb and As, and for the enrichment of Au and Ag.

3.2. Separation of Ag and Au by Vacuum Volatilization

The oxidized LAS was treated by vacuum volatilization to separate the precious metals Ag and Au from As. To study the separation effect in the oxidized LAS, vacuum volatilization experiments were carried out with a pressure of 10 Pa, a volatilization temperature in the range of 1023–1173 K, and a volatilization time in the range of 60–105 min. The experimental results are shown in Figure 4.

The effect of volatilization temperature on the recovery of Ag and Au was studied under the gasification time of 60 min. As shown in Figure 4a, the Ag content in the residues increased from 12.82% to 14.32% as the volatilization temperature was increased from 1023 K to 1173 K; Au content showed a similar tendency and increased from 807.9 g/t to 902.8 g/t. Nevertheless, upon increasing the volatilization temperature from 1023 K to 1173 K, the efficiency for the recovery of Ag decreased from 99.99% to 99.95%, and the efficiency for the recovery of Au decreased from 99.99% to 99.96%. Given the recovery efficiencies and evaporative losses for Ag and Au, the appropriate volatilization temperature for Ag and Au recovery was 1123 K. Figure 4b summarizes the relationship between Ag and Au recovery and volatilization time.

As shown in Figure 4b, the Ag and Au contents in the residues increased with the extension of volatilization time with a constant gasification temperature of 1123 K. When the volatilization time was increased from 60 min to 105 min, the Ag content in the residues increased from 13.70% to 14.34%; Au content increased from 863.3 g/t to 903.9 g/t. When the volatilization time was 90 min, the efficiencies for the recovery of Ag and Au were 99.97% and 99.98%, respectively; when the volatilization time was further extended to 105 min, the contents of Ag and Au increased slightly, but so did the loss of Ag and Au. As a result, the optimal volatilization time was determined to be 90 min. With a volatilization temperature of 1123 K and a time of 90 min, the efficiencies for the recovery of Ag and Au in this separation stage were 99.97% and 99.98%, respectively, and the contents of Ag and Au in the residues reached 14.18% and 893.7 g/t (

Table 2. Performance comparison for various steps in each process.

Comparison	Conventional Pyrometallurgical Oxidation Reduction [26,27]	Novel Selective Oxidation–Vacuum Volatilization–Carbon Reduction
Direct recovery rate of Ag	88.12% to 95.6%	99.25%
Direct recovery rate of Au	90%–96.8%	99.91%
Product	Noble Pb	Au- and Ag-rich alloy
Additives	Oxidant (O2, etc.), slagging agent (Na2CO3, CaF4, CaO, etc.), reductant (coke, Fe filings, etc.), pulverized coal, natural gas, nitrogen	(NH4)2CO3, waste lead paste, pulverized coal
Energy consumption	Petrochemical resources (natural gas, heavy oil, etc.)	Electricity < 7.23 kW·h/kg
By-products	High As dust, washing water, smelting slag, and oxides of carbon, sulfur, and nitrogen (SO2 content of 1% to 4%)	High Pb volatiles returned to lead smelting, high As and Sb volatiles returned to further extract As and Sb
Separation degree of As	3% to 10% As remained in noble Pb	95.13% As in LAS entered into high As and Sb volatiles
As-containing products	High As dust, noble Pb	High As and Sb volatiles

). At the same time, under the above experimental conditions, the content of As in the residues was only 0.38%, and 95.13% of the As in LAS was removed as volatiles.

3.3. Enrichment of Ag and Au by Vacuum Reduction

To further enrich the Ag and Au contents, the precious metal-containing residues from the separation stage were vacuum distilled with an appropriate amount of deoxidizing carbon particles. The effects of temperature and time on the enrichment results were investigated at a pressure of 10 Pa, a reduction temperature range of 973–1123 K, and a reduction time range of 60–105 min. The results are given in Figure 5.

Firstly, the effect of volatilization temperature on the recovery of Ag and Au was investigated across a temperature range of 973–1123 K, maintaining a constant volatilization duration of 60 min. As observed in Figure 5a, the contents of Ag and Au in the residues increased as the reduction temperature was increased from 973 K to 1123 K; the content of Ag in the residues increased from 47.36% to 75.32%, and the Au content increased from 2990 g/t to 4806 g/t. This tendency can be explained by the fact that the vapor pressures of Ag and Au rose with increasing temperature. When the reduction temperature was 1073 K, the contents of Ag and Au in the residues were 67.58% and 4287 g/t, respectively. As the reduction temperature continued to increase, the efficiencies for the recovery of Ag and Au decreased, resulting in a loss of precious metals. Thus, 1073 K was selected as the volatilization temperature at this enrichment step.

At a set volatilization temperature of 1073 K, the retention times explored were 60 min, 75 min, 90 min, and 105 min. Figure 5b shows that as the reduction time was increased from 60 min to 105 min, the content of Ag in the residues increased from 58.51% to 70.57%, and the Au content increased from 3770 g/t to 4493 g/t. When the reduction time was 90 min, the efficiencies for the recovery of Ag and Au were 99.28% and 99.93%, respectively. As the reduction time was increased further from 90 min to 105 min, the recovery efficiency of Ag decreased from 99.28% to 98.89%, and Au from 99.93% to 99.91%. Because the reduction temperature was high, the volatilization reduction process was completed quickly. In addition, prolonging the reduction time led to an increase in energy consumption. Therefore, the optimized reduction time was determined to be 90 min. After vacuum reduction-based enrichment, the contents of Ag and Au in the precious metal-rich residues reached 67.58% and 4287 g/t, respectively; the experimental conditions included heating to 1073 K and maintaining that temperature for 90 min.

3.4. Characterization of the Gold- and Silver-Rich Alloy

Separation and enrichment of Ag and Au from LAS by selective oxidation–vacuum volatilization–carbon reduction was used to obtain residues rich in precious metals, and the content of Ag was as high as 67.58%, and that of Au was as high as 4287 g/t. The residues looked like metals or alloys, and their XRD patterns are given in Figure 6. Figure 6 shows that the precious metal-rich residues contained various phases, mainly elemental Pb, Bi, Cu, and Ag, intermetallic compounds Ag₃Sb, Cu₃Sb, Au₂Sb, and alloys PbBi and Pb₇Bi₃. In addition, there were no oxides in the precious metal-rich residues, indicating that they can be designated as alloys with high Au and Ag contents.

4. Theoretical Analysis

4.1. Determination of Saturated Vapor Pressure

Each substance in the lead anode slime and each element in the precious metal-rich alloy has a different saturated vapor pressure at the same temperature, which was the basic principle used to separate the constituents’ vacuum volatilization. Based on the relationship between the temperature and saturation vapor pressure [34,35], Figure 7 was obtained.

The saturation vapor pressures of pure substances are proportional to the temperature, as shown in Figure 7. The higher the saturation vapor pressure of a substance at a specific temperature, the easier it evaporates. The saturation vapor pressures of As, Te, Sb, Pb, and Bi and their oxides, As₂O₃, Sb₂O₃, TeO₂, PbO, and Bi₂O₃, are much higher than those of Ag, Cu, and Au at the same temperature. In addition, the saturated vapor pressures of As₂O₃ and Sb₂O₃ are much higher than those of As and Sb. Therefore, selective oxidation can be used to convert elements, including Sb and As, bound to Au, Ag, and Cu, into oxides, and such phase regulation facilitates the separation of Sb, As, and other heavy metals and the enrichment of Au and Ag. To separate the precious metals, Ag and Au, from LAS and to recover and enrich these two precious metals, vacuum distillation was carried out twice, and the vacuum distillation temperature of the two stages was kept below 1173 K. Therefore, Ag, Au, and other substances with low saturated vapor pressure could be enriched in the residues under specific temperature and time intervals during vacuum volatilization.

4.2. Gibbs Free Energy Calculation

4.2.1. Selective Oxidation of Intermetallic Compounds

To efficiently separate Ag and Au from elements, including Sb and As, and avoid the loss caused by the volatilization of Ag in the form of Ag₃Sb, a carbonation desulfurization product obtained from waste lead paste by calcination at 653 K was used to oxidize Sb, and As easily formed the intermetallic compounds, Ag₃Sb and Cu₃As, in LAS. The oxidant was a mixture of Pb₃O₄, PbO₂, and Pb₂O₃. The feasibility of the selective oxidation reactions of the intermetallic compounds was determined by the Gibbs free energy (Δ_rG_T) of the oxidation reactions at 653 K, as shown below:

$$aP{b}_3{O}_4+b{M}_{x}M{e}_y=cPbO+d{M}_m{O}_n+eM{e}_l{O}_p$$

(3)

$$aPb{O}_2+b{M}_{x}M{e}_y=cPbO+d{M}_m{O}_n+eM{e}_l{O}_p$$

(4)

where a, b, c, d, and e represent the stoichiometric number of each substance participating in the reactions, and x, y, m, n, l, and p refer to the number of corresponding atoms in a single cell of a substance. When Δ_rG_T is negative, the reaction is completed spontaneously [36,37]. Conversely, when Δ_rG_T is positive, the reaction cannot occur. Equation (5) represents the Gibbs free energy formula, in which Δ_rG^Θ represents the standard Gibbs free energy at atmospheric pressure, R represents the universal gas constant, T represents the temperature, and Q represents the reaction quotient.

$${\Delta }_r{G}_T={\Delta }_r{G}^{\mathsf{\Theta }}+RT\ln Q$$

(5)

As determined with HSC CHEMISTRY 6.0 software, the Δ_rG_T values for the oxidation reactions at 653 K are given in Figure 8a. As shown in Figure 8a, all of the Δ_rG_T values for the oxidation of Sb and As in the intermetallic compounds by Pb₃O₄ and PbO₂ were negative; that is, Pb₃O₄ and PbO₂ can oxidize Sb and As into high- or low-valent oxides. Therefore, the addition of a mixture of PbO₂, Pb₃O₄, and Pb₃O₄ could realize the oxidation of Sb and As in the intermetallic compounds at 653 K without introducing other impurities. The high-valence oxide Sb₂O₅ that may be generated during selective oxidation can decompose into Sb₂O₃ at 923 K, As₂O₅ decomposes into As₂O₃ at 588 K, and low-valence oxides Sb₂O₃ and As₂O₃ have a high saturated vapor pressure. Given the above analysis, the selective oxidation treatment of LAS is conducive to the further adoption of vacuum volatilization to separate those elements that combine with Au and Ag, reducing volatilization losses of precious metals.

4.2.2. Vacuum Reduction in Oxides

In general, O readily combines with metallic elements to form oxides or salts at high temperatures, which is not conducive to the volatilization of elements with high saturated vapor pressures. To remove O in precious metal-containing residues and realize further enrichment of Ag and Au, an appropriate amount of high-purity carbon particles was added during the volatilization-based enrichment stage. Since it is difficult to determine the activity value of oxygen in a molten alloy, the Δ_rG_T (Gibbs energy per mole of the reaction, kJ/mol) values for the reactions of carbon with the various oxides were used to determine whether the reduction processes would occur. The Δ_rG_T values for these reduction reactions were also calculated with Equation (5) and HSC Chemistry 6.0. As presented in Figure 8b, with a reduction temperature of 1073 K and an oxygen pressure of 10 Pa, all of the Δ_rG_T values for the reactions of the different oxides with carbon were negative; that is, C could remove oxygen from the precious metal-rich alloy in the form of CO. After the reduction in these oxides, the heavy metals, including Pb, Bi, Sb, and As volatilized under a vacuum, and the precious metals, Au and Ag, were strongly enriched in the form of the alloy.

5. Process Economical and Environmental Assessment

This work indicates that the novel selective oxidation–vacuum volatilization–carbon reduction process greatly affects the enrichment of Au and Ag from hazardous LAS; elements, such as As and Sb, which are easily bound to precious metals, are effectively removed. Table 2 compares the economic and environmental performance of the traditional pyrometallurgical Au and Ag extraction process and the selective oxidation–vacuum volatilization–carbon reduction process.

The traditional oxidation-reduction process for recovering gold and silver not only consumes a large number of auxiliary materials, such as reduced coal, coke powder, soda ash, iron filings, lime, fluorite, nitrate, oxygen-enriched gas, heavy oil, and natural gas, but also produces high As content dust, SO₂ gas, and other by-products, which are harmful to the environment. The new method of selective oxidation–vacuum volatilization–carbon reduction considerably shortens the treatment cycle of LAS and improves the recovery efficiency for Au and Ag. Au and Ag are enriched in the form of alloys, and the amount of As and Sb in gold- and silver-rich alloys that can easily lead to smelting dust does not exceed 5%, facilitating further separation and purification of Au and Ag. Novel technology does not consume fossil fuels, does not involve toxic and harmful chemical reagents, such as CaF₂, and does not generate harmful waste. The use of waste lead paste as a raw material for the preparation of selective oxidizers not only does not introduce impurity elements but also results in the comprehensive recovery of valuable metals. It is encouraging to note that this innovative process reduces auxiliary material and energy consumption, reduces the economic cost of separating and enriching precious metals from LAS, and has favorable environmental benefits.

6. Conclusions

Herein, a selective oxidation–vacuum volatilization–carbon reduction process was proposed for the first time to separate and recover Ag and Au from LAS. The feasibility of the process was confirmed experimentally and theoretically. In this overall process, first, the lead anode slime was selectively oxidized at a temperature of 653 K for 120 min using a mixture of PbO₂, Pb₃O₄, and Pb₃O₄, which was an oxidant obtained by calcining a waste lead paste desulfurization product at 653 K for 6 h. Then, the precious metals were separated from oxidized LAS by vacuum volatilization for 90 min at a temperature of 1123 K. The precious metal-containing residues obtained in the separation stage were enriched by vacuum reduction at 1073 K for 90 min with the addition of an appropriate amount of carbon powder, and finally, a gold- and silver-rich alloy with a Ag content of 67.58% and a Au content of 4287 g/t was obtained. Ag and Au were effectively enriched, and the efficiencies for the recovery of Ag and Au were 99.25% and 99.91%, respectively. The gold- and silver-rich alloy can be returned to a dore furnace and cast into silver ingots. The process effectively separates Ag and Au and avoids the continuous accumulation of the hazardous element As in metallurgical systems. This study provides a new solution for the highly effective and green recycling of precious metals from secondary resources comprising nonferrous metals.