Pyrometallurgical Process to Recover Lead and Silver from Zinc Leaching Residue

Abstract

During the roasting, leaching, and electrodeposition of zinc ores, lead–silver residues are produced. These residues contain valuable metals (Pb, Zn, and Ag) and toxic metals (Cd and As). In this study, a pyrometallurgical process is proposed for treating Pb-Ag residues, consisting of drying, roasting, and reduction steps to recover valuable metals, such as silver in a metallic Pb phase, while converting the waste into an environmentally friendly slag. First, the Pb-Ag residue is dried at 100 °C, then roasted at 700 °C, and finally reduced at a high temperature, with Na₂CO₃ as a flux and CaSi as a reducing agent, rather than carbon-based reducing agents (carbon or carbon monoxide), to minimize greenhouse gas production. The effects of the reduction temperature and the mass of the reducing agent were investigated on a laboratory scale. The metallic phase and slag obtained in the reduction step were characterized by their chemical composition and mineralogy via chemical analysis, X-ray diffraction, and SEM-EDS. The results showed that silver and lead formed a metallic phase, and that silver content decreased from 1700 ppm in the Pb-Ag residue to 32 ppm in the final slag at 1300 °C. The Pb-Ag residue and final slag were leached with an aqueous acetic acid solution to evaluate their chemical stability.

1. Introduction

Most zinc production occurs hydrometallurgically through the roast–leach–electrolysis (RLE) process, which produces two main residues: a Pb-Ag residue with up to 0.2% Ag and a jarosite residue [1,2,3]. The Pb-Ag residue is produced in the leaching step using high-temperature acid and contains most of the lead and silver metal from the raw materials. Pb-Ag residues and jarosite have low stability and poor storage options. In addition to zinc, these waste materials also contain Cu, Pb, Ag, Au, In, and some undesirable toxic metals, such as As, Cd, and Sb, in non-negligible quantities [4]. The possibility of recycling these residues is continually being examined due to their negative environmental impact and the constant demand for metals.

Many efforts have been made to treat jarosite residues and recover valuable metals from the waste [5,6,7,8,9,10]; however, few studies have examined the treatment of Pb-Ag residues to recover metals. Hellgren et al. [11] characterized RLE residues and proposed selectively dissolving PbSO₄ using triethylenetetramine (TETA) to recover Pb. Jiang et al. [12] combined sulfate roasting and water leaching to recover valuable metals from zinc leaching residue. The residue was first roasted with ferric sulfate at 640 °C, and the valuable metals were converted to water-soluble sulfate, while iron remained as ferric oxide. Water leaching was then conducted to extract the valuable metal sulfate for recovery.

Wang et al. [13] analyzed and characterized Pb-Ag and jarosite residues from zinc hydrometallurgy wastes using bioavailability tests, sequential extraction procedures, and standard leaching toxicity methods. They conclude that Pb-Ag and jarosite residues are hazardous wastes with substantial leaching toxicities, metal mobilities, and significant biological hazards. Du et al. [14] reported the recovery of zinc and silver from zinc-leaching residue wastes using the flotation method. The optimized parameters of this method achieved a silver recovery grade of 80.32%. Turan et al. [15] increased the reactivity of solid waste from zinc plant residues by mechanical activation using a high-energy ball mill, followed by two-stage leaching with a hydrochloric acid solution to recover Zn, Pb, and Ag.

Xing et al. [16] reported the thorough cleaning of zinc leach residue and the recovery of valuable metals through a hydrometallurgical route. The purification process involved acid leaching, followed by calcium chloride leaching, silver and lead extraction, and recovery of the lead as electrolytic lead through electrolysis. Wang et al. [17] investigated a method for recovering Pb and Zn from an acidic leach residue produced using conventional Zn hydrometallurgy. This process included sulfation roasting, water leaching, and chlorination leaching. Zheng et al. [18] proposed converting PbSO₄ and ZnSO₄ into their respective sulfides via reduction roasting with coal powder, followed by flotation treatment.

We previously reported on the recovery of lead and silver from jarosite waste using a three-stage process—drying, roasting, and reduction [19,20]—where we tested solid (C and CaSi) and gaseous (CO) reducing agents. In those previous studies, we used CaO and SiO₂ as fluxes because a large amount of hematite (Fe₂O₃) was produced during the roasting of jarosite. The amount of CaO and SiO₂ was estimated using the CaO-SiO₂-FeO ternary phase diagram. However, in the present work, we use soda ash (Na₂CO₃) as a flux because the main components of the Pb-Ag residue are calcium and lead sulfates.

The present work proposes a pyrometallurgical process for recovering valuable metals from Pb-Ag residues, converting the residues into an environmentally friendly slag product. First, the samples were dried and then roasted to remove moisture and some OH⁻ and sulfate groups via thermal decomposition. The roasted material was melted at a high temperature using Na₂CO₃ as a flux and CaSi as a reducing agent. CaSi is proposed in this work as an alternative reducing agent to carbon-based compounds, aiming to reduce the carbon footprint and mitigate greenhouse gas emissions. The phases of the reducing process were analyzed using X-ray diffraction (XRD), scanning electron microscopy, and an energy-dispersive spectrometer (SEM-EDS). The chemical stability of the Pb-Ag residue and the final slag was evaluated using the procedure outlined in the Mexican environmental regulations [21].

2. Materials and Methods

2.1. Sample Preparations

The Pb-Ag residue used in this study was obtained from one of the largest zinc smelters in Mexico. This residue was first processed using the ASTM C702/C702M-11 [22] standard to obtain a representative sample. The residue was heated at 100 °C for 8 h to remove moisture, and then a chemical analysis was performed using the X-ray fluorescence technique (RIGAKU Model Primus II, RIGAKU, Tokyo, Japan).

Table 1. Chemical composition of the Pb-Ag residue after drying (wt%).

Element	O	Ca	Zn	Fe	S	Si	Na	Mg	Pb	As	Cd	Ag	Others
Before roasting	42.99	15.76	6.04	9.89	16.23	2.86	0.99	0.18	3.24	0.12	0.17	0.15	1.38
After roasting	39.56	19.70	7.1	11.4	13.03	3.2	0.01	0.24	4.01	0.14	0.09	0.17	1.35

shows the elemental composition of the chemical analysis of the residue after drying. Pb-Ag residues are classified as hazardous waste due to their high lead concentrations. As shown in Table 1, other heavy metals, such as As and Cd, were also observed. Therefore, when recycling valuable metals from this residue, the impact of heavy metals should also be considered due to the environmental activity and potential ecological risks associated with the zinc leaching residue.

2.2. Roasting Treatment

The mineralogical species in the dry Pb-Ag residue mainly consisted of gypsum (CaSO₄·2H₂O), plumbojarosite (PbFe₆(SO₄)₄(OH)₁₂), and blende (ZnS). After drying, the residue was roasted in a muffle furnace at 700 °C to decompose the plumbojarosite and remove OH⁻ and sulfate groups via thermal decomposition and eliminate structural water from the gypsum species. The heating rate was fixed at 15 °C/min. Table 1 shows the chemical composition of the roasted residue. The mineralogical species in the roasted residue mainly consisted of anhydrite (CaSO₄) and franklinite (ZnFe₂O₄). Figure 1 shows the XRD pattern of the materials that were heated at 100 °C to remove moisture and the material that was roasted at 700 °C. X-ray diffraction (XRD) analysis was performed using an XRD Bruker D8 Focus (Bruker Coorporation, Madison, WI, USA) and an X-ray tube with a copper anode. The recording conditions were as follows: angular range (2 θ): 10°–90°; increment: 0.02°; recording rate: 10°/min.

2.3. Methods

The Pb-Ag residue was prepared according to ASTM C702/C702M-11 and then heated at 100 °C for 8 h to eliminate moisture. The roasting process was carried out at 700 °C for 1 h in a laboratory muffle furnace. This temperature was chosen based on work by Zhu et al. [23], who reported that SO₄²⁻ is the major S-containing component of jarosite. Due to its high thermal stability, a high temperature is required to remove sulfur. The roasted material was then ground to a size of 74 μm (−200 mesh) and mixed with Na₂CO₃ as a flux and a reducing agent (CaSi). The reduction step was carried out at 1300 °C (1573 K) and 1400 °C (1673 K) for 2 h in an aluminum oxide crucible in a laboratory muffle furnace equipped with an off-gas cooling system and a bag filter. The roasting and reducing processes were performed in air. The reduction experiments were carried out with 10 g of roasted residue, 10 g of Na₂CO₃, and the reducing agent (CaSi) in amounts ranging from 1 to 5 g. The treated samples were removed from the furnace before being cooled in air. We chose CaSi as a reducing agent instead of solid C or CO gas because it contains two metals, Ca and Si, which may improve the efficiency of the process and reduce the generation of greenhouse gases.

2.4. Characterization

The composition of the samples was determined via chemical analysis using atomic flame absorption. Species morphology and elemental distribution were analyzed using scanning electron microscopy (SEM) in conjunction with an energy-dispersive spectrometer, SEM–EDS, Jeol 6300, (JEOL, Peabody, MA, USA) and X-ray diffraction analysis (XRD Bruker D8 Focus) to identify crystalline compounds.

The raw Pb-Ag residue sample and the final slag from the reduction process were ground into fine powder. The chemical stability of these materials was evaluated under Mexican environmental regulations [21] using the leaching technique. It states that the material must be crushed below −200 mesh (74 μm) and brought into contact with 500 cm³ of an aqueous acetic acid solution with a pH of 2.88 ± 0.05 in a rotary system for 20 h at 30 ± 2 rpm. Then, the solid waste was filtered through ashless filter paper, Whatman 542 (Merck, Rockville, MD, USA), and the liquid solution was analyzed for Ag, Pb, As, Cd, and Zn using atomic absorption spectrometry, Perkin Elmer Analyst 200 (PerkinElmer Co., Houston, TX, USA).

2.5. Thermodynamic Modeling

A thermodynamic analysis was performed using the software FactSage 8.3 [24] to determine the effects of temperature and the amount of reducing agent (CaSi) on the equilibrium phase composition of the system. This software considers the initial mass of the chemical species, the temperature, and the system pressure. The program then calculates the most stable species using the Gibbs free energy minimization method. The amount (in g) of the initial species considered in this system is as follows:

4 CaSO₄ + 4 ZnFe₂O₄ + 1 PbO + 0.017 Ag + 0.8 CaFeSi₂O₆ + 10 Na₂CO₃ + x CaSi

(1)

where “x” ranged from 0 to 10, and pressure was 1 bar. FactSage contains models in its database for slags with sulfur as sulfide or sulfate, as well as matte and bullion, which is a Pb liquid phase.

Figure 2 shows that a critical amount of the reducing agent is required to form the metallic phase, which mainly consists of molten lead with small amounts of silver, iron, and zinc, while the slag contains Na₂SO₄ as its main component. As the amount of the reducing agent increases, a matte phase consisting mainly of Fe and S is formed.

Figure 3 shows the calculated effect of temperature and the amount of reducing agent (CaSi) on lead recovery (bullion), starting with 10 g of roasted Pb-Ag residue and 10 g of Na₂CO₃. At all temperatures, a significant increase in the mass of the bullion is observed when the mass of CaSi is between 4 and 5 g; however, with between 5 and 10 g of CaSi, only a small increase in the metallic phase is observed. Also, as the temperature increases, a smaller amount of the metallic phase is recovered. However, at lower temperatures below 1200 °C, a semi-liquid system is experimentally produced. Based on these results, we chose 1300 °C and 1400 °C as the temperatures for the reduction experiments.

3. Results and Discussion

3.1. Results of the Reduction Experiments

According to the results of the thermodynamic analysis, a series of experiments were conducted at different temperatures and varying amounts of the reducing agent (CaSi).

Table 2. The parameters used in the reduction experiments.

Test	Reduction Time (min)	Temperature (°C)	Mass of CaSi (g)
1	120	1300	1
2	120	1300	2
3	120	1300	3
4	120	1300	4
5	120	1300	5
6	120	1400	2
7	120	1400	4

shows the experimental parameters selected for this study. The experiments were carried out at 1300 °C and 1400 °C in an aluminum oxide crucible, with a reduction time of 120 min in a laboratory muffle furnace. The mass of roasted Pb-Ag residue remained constant at 10 g with 10 g of flux (Na₂CO₃), and the reducing agent ranged from 1 to 5 g. It is worth mentioning that some preliminary experiments were also carried out at 1200 °C; however, the system was only partially melted.

The iron content, expressed as FeO or Fe₂O₃ in the roasted material, is less than 15%. According to the CaO-SiO₂-FeO or CaO-SiO₂-Fe₂O₃ diagrams [25], the slag would have a high melting point (above 1450 °C) in systems with such a low iron oxide content. For this reason, soda ash (Na₂CO₃) was selected as the flux.

Table 3. The composition of the metallic (bullion) and slag phases (wt%).

Test	Ag Bullion	Zn Bullion	Fe Bullion	Ag Slag	Zn Slag	Fe Slag	Pb Slag	CD Slag	As Slag
1	---	---	---	---	---	---	---	---	---
2	0.72	0.305	0.43	0.0184	1.72	5.20	0.57	0.06	0.05
3	1.06	0.230	0.35	0.0074	1.71	4.58	0.56	0.04	0.05
4	1.58	0.127	0.27	0.0039	1.66	4.52	0.43	0.02	0.03
5	1.59	0.103	0.31	0.0032	1.46	4.68	0.45	0.02	0.03
6	0.59	0.298	0.28	0.0028	1.65	4.96	0.46	0.03	0.04
7	0.92	0.216	0.095	0.0095	1.58	5.31	0.41	0.01	0.03

shows the percentages by mass of silver, zinc, and iron in the Pb phase (bullion) and the slag composition obtained in each experiment, with varying amounts of reducing agent and temperatures. The results show that test number 1 did not produce a metallic phase due to the small amount of CaSi used in this test. The results show that the maximum silver yield was obtained in test number 5, with 5 g of CaSi; however, the silver content was very similar in tests 4 and 5. The silver yield of the bullion obtained at 1400 °C (samples 6 and 7) was lower than that obtained at 1300 °C with the same amount of reducing agent. Table 3 shows that Fe and Zn are almost absent from the Pb phase, thus indicating nearly complete concentration of these metals in the slag. The amount of Fe and Zn ranged from 4.52 wt% to 5.31 wt% and 1.46 wt% to 1.72 wt% in the final slags, respectively. These results are consistent with the results of the thermodynamic analysis.

Figure 4 compares the silver content in the bullion as a function of the reducing agent and temperature. Table 3 shows that the silver content was reduced from 0.17 wt% (1700 ppm) in the roasted residue to 0.0032 wt% (32 ppm) in the final slag in test 5 and 0.028 wt% (280 ppm) in test 6, which means that the silver content in the residue was reduced by 98% and 86% in tests 5 and 6, respectively.

The XRD patterns of the slags obtained in all tests conducted at 1300 °C are shown in Figure 5. All slag samples are mainly composed of sodium sulfate (Na₂SO₄), sodium zinc silicate (Na₂ZnSiO₄), and calcium and iron silicates (Ca₃SiO₅ and FeSiO₃). The formation of Zn- and iron-based species observed in the final slags is consistent with the chemical analysis presented in Table 3. The metallic phase (bullion) contains a small amount of Zn and Fe, and the XRD patterns of the slags show the presence of Zn- and Fe-based species. Therefore, we concluded that Zn and Fe are almost always in the slag.

Given the presented results, it can be concluded that sulfur forms sulfate species (Na₂SO₄) that are incorporated into the slag, and a matte sulfide-based phase is not detected in the DRX patterns; however, thermodynamic analysis indicates that increasing the reducing agent may form a matte rich in iron and sulfur.

Figure 6 shows the SEM micrograph and the EDS analysis of the metallic phase obtained in experiment number 5. According to these results, the slag contains mineralogical species with Ca, Fe, and Zn. Figure 7 shows the SEM micrograph and the EDS analysis of the metallic phase obtained in test 5. According to these results, the metallic phase mainly consists of lead with free silver-containing particles. The immiscible silver particles in bullion can be explained with the help of the binary Pb-Ag phase diagram [26]. This diagram shows that lead and silver are completely miscible in the liquid phase at high temperatures; however, when the system cools to a solid state, silver and lead become immiscible. Table 3 shows that the Ag content in test 5 was 1.59 wt%. This chemical analysis, together with the SEM-EDS results, confirms that Ag was collected in the metallic phase, whereas zinc was present in the slag as Na₂ZnSiO₄, as shown in the XRD patterns of Figure 5.

Figure 8 shows the XRD pattern of the metallic phases from experiment number 5. The metallic phase mainly contains lead and a small amount of silver. No zinc or iron could be detected in the XRD pattern. However, the chemical analysis shows that the bullion contains a small amount of zinc and iron.

The results obtained in this work show that the majority of Fe and Zn is concentrated in the slag phase. The lead phase was formed, comprising Pb with collected Ag and small amounts of Fe and Zn. The matte phase was not observed in the experiments. The input material represents Zn-residue with a high sulfur content and the presence of contaminants (As), requiring a specific off-gas treatment, which could include quenching, baghouse filter, scrubbing, and optional absorption.

3.2. Leaching Results

Table 4. The leaching results of the Pb-Ag residue and slag from test number 5 (mg/L).

	Ag	Pb	As	Cd	Zn
NOM-053-SEMARNAT-1993	5 max.	5 max.	5 max.	1	100
Pb-Ag residue	7.5	27.3	2.6	3.2	210
Slag (Test No. 5)	0.16	1.2	0.32	0.5	35.6

shows the results of the chemical analysis of the liquid solutions obtained after the leaching experiment in an acidic solution for both the dry Pb-Ag residue and the final slag obtained after the reduction process in test number 5. The concentrations of Ag, Pb, and Zn in the dry Pb-Ag residue are 7.5, 27.3, and 210 mg/L, respectively, which are above the regulatory thresholds. This indicates that the residue has high environmental toxicity and poses a serious potential ecological risk. However, the Ag, Pb, and Zn concentrations in the final slag residue drop sharply after the reducing pyrometallurgical process to 0.16, 1.2, and 35.6 mg/L, respectively, well below the limits. This is attributed to the extraction of silver into a lead-rich metallic phase, and zinc and iron form stable silicate compounds in the slag. Additionally, the concentrations of other toxic elements, such as arsenic and cadmium, are below the limit values. Although the slag meets the Mexican environmental standard, long-term durability tests will be necessary to assess its resistance to carbonation and microbiological activity, thereby fully evaluating its environmental stability. It must be mentioned that the leaching test was only performed on test number 5, as it gave the highest silver recovery. In addition, future work should take into account the possible volatilization of toxic elements during the reducing step.

4. Conclusions

In this work, a process for recovering lead and silver from Pb-Ag residues is proposed. The process steps include drying, roasting, and reducing at high temperatures using Na₂CO₃ as a flux and CaSi as a reducing agent. CaSi is used as an alternative reducing agent to carbon-based compounds, reducing greenhouse gas emissions. The conclusions from this work are as follows:

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The mineralogical species of the Pb-Ag residue were plumbojarosite, gypsum, and blende, which transformed into anhydrite and franklinite after the roasting process at 700 °C.

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There is a critical amount of reducing agent (CaSi) to obtain a metallic phase containing lead. Beyond this level, lead and silver were almost completely recovered in this process, while zinc and iron remained in the slag.

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The silver content was reduced from 1700 ppm in the roasting residue to 32 ppm in the final slag in test 5 at 1300 °C and to 280 ppm in test 6 at 1400 °C. This means that the silver content in the residue was reduced by 98% and 86% in tests 5 and 6, respectively.

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The slags resulting from the reduction mainly consist of sodium sulfate (Na₂SO₄), sodium zinc silicate (Na₂ZnSiO₄), and calcium and iron silicates (Ca₃SiO₅ and FeSiO₃). This final slag meets the Mexican environmental norm; however, long-term durability tests will be necessary to fully evaluate the slag’s environmental stability.