Recovery of Silver and Lead from Jarosite Residues by Roasting and Reducing Pyrometallurgical Processes

Abstract

This work focuses on the recovery of lead and silver from jarosite waste by a three-stage process: drying, roasting and reduction at 100 °C, 700 °C and 1400 °C, respectively. A flux mixture with 48 mass% CaO and 52% SiO₂ was used for the reduction stage. A gas consisting of 70 vol% CO and 30 vol% CO₂ was used as a reducing agent. To select the temperatures and the amount of flux and reducing agent, a characterization of the jarosite waste was carried out using thermogravimetry and high-temperature X-ray diffraction, as well as a thermodynamic study of the effects of the process parameters. The lead-rich metallic and slag phases were characterized by chemical analysis, SEM-EDS and XRD. In addition, the jarosite residue and the final slag were leached with an aqueous acetic acid solution to estimate their chemical stability. The results show a recovery of over 95% of the lead and silver and the formation of an environmentally friendly residual slag.

1. Introduction

The roast-leach process for zinc production involves roasting zinc sulfide concentrate to produce calcine-containing zinc oxide. The roasted zinc concentrate usually contains iron, predominantly converted into zinc ferrite (ZnFe₂O₄) during roasting. After the leaching process, a precipitation step is carried out to remove the co-dissolved iron impurities from the solution [1,2]. In the “jarosite” process, the iron is removed from the leach solution by precipitation as jarosite, a basic iron (III) sulfate complex XFe₃(OH)₆(SO₄)₂ with X = Na, K, H₃O, NH₄ [3]. However, in addition to iron, some lead, zinc, and valuable metals such as indium and silver precipitate during jarosite formation [4]. One of the major disadvantages of the jarosite process for iron separation is the large amount of waste. Therefore, jarosite not only requires a lot of space for storage, but a large amount of valuable metals are also lost and do not return to the value chain. It is estimated that for every ton of zinc produced, 0.5 tons of jarosite are generated [5,6].

Many efforts have been made to treat and recycle jarosite waste. Hydrometallurgical, pyrometallurgical and integrated hydro- and pyrometallurgical processes have been reported in the literature to recover valuable metals such as Fe, Zn, In and Ag and to obtain an environmentally friendly final waste [7,8,9]. Pyrometallurgical treatment of jarosite–leach residue is the most promising processing route, given its capability to generate clean slag and recover the most valuable metals [10,11,12].

Han et al. [13] proposed a roasting process combined with sulfidation flotation to recover anglesite and silver from jarosite residues of zinc hydrometallurgy, with lead and silver recoveries of 66.86% and 81.6%, respectively. Ahamed et al. [6] designed a process with three steps: (i) leaching of jarosite by sulfuric acid solutions, producing Fe(III) and Zn(II) species; (ii) reduction of Fe(III) to Fe(II) with the addition of industrial blende; and (iii) electrodeposition of the alloy from the recovered acidic Fe-Zn solution. Mombelli et al. [14] investigated the metallurgical properties of briquettes made from jarosite powder with blast furnace sludge used as a reducing agent, to recover the iron oxide in the form of pig iron and produce an inert slag. Zhu et al. [12] proposed a process in which the jarosite residue is first heated to dehydrate and desulfurize it, followed by a reductive roasting to selectively volatilize Zn and In, and finally smelted to produce Fe and Ga-bearing alloy, as well as clean slag. Ge et al. [15] evaluate the effects of the amount of jarosite residue, roasting time, and addition of CaCl₂ on phase transformation and migration of valuable elements in a cement clinker. Rämä et al. [16] investigated the pyrometallurgical treatment in two steps: first, the material was melted in an oxidizing atmosphere, after which the molten oxide was reduced to produce an inert, clean slag. The valuable metals, such as silver, accumulated in the liquid metal or speiss phase.

This study proposes a pyrometallurgical method to recover silver and lead from jarosite residues and obtain an environmentally friendly final residue. First, the jarosite residues were heated to 100 °C to remove the moisture and then roasted at 700 °C to thermally decompose the material and release the sulfates and OH groups. The roasted jarosite was mixed with fluxing agents (CaO and SiO₂) to melt the system at 1400 °C. A reducing gas mixture of CO and CO₂ was introduced into the melt. A clean slag and a liquid metal alloy are formed. The formation of the metal alloy during the reduction phase plays an important role, as it can collect various low-concentration metals, such as Ag, As and Sb, from the material [17]. The thermal decomposition of jarosite as well as the analysis of the final slag and metallic phases was studied using techniques such as thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy and energy dispersion spectrometer (SEM-EDS). Chemical stability tests, under Mexican environmental regulations [18], were conducted on both the jarosite residue and the final slag.

2. Materials and Methods

2.1. Materials

The fresh sample of jarosite residue was taken from a domestic zinc smelter in Mexico. To obtain a representative sample, this residue was prepared according to the ASTM C702/C702M-18 standard [19]. After drying at 100 °C for 8 h, the chemical analysis of the sample was performed by atomic flame absorption and is shown in

Table 1. Elemental chemical composition of the jarosite residue.

Element	O	Ca	Zn	Fe	S	Si	Na	Mg	Pb	As	Ag (ppm)
mass%	47.75	12.65	4.19	11.05	17.2	1.05	0.93	0.18	3.57	0.23	144

. Figure 1 shows the XRD pattern of the residue. Mineralogical species in the dried residue included ammonium jarosite ((NH₄)Fe₃(SO₄)₂(OH)₆), gypsum (CaSO₄‧2H₂O) and a small amount of anhydrite (CaSO₄), blende (ZnS) and ZnSO₄‧7H₂O.

2.2. Methods

Das et al. [20] reported that, according to TG and DTA results, the decomposition of ammoniojarosite starts at about 370 °C and completely converts to Fe₂O₃ at 800 °C. High-temperature X-ray powder diffractometry (Panalytical Empyrean Diffractometer, Anton Paar, Graz, Austria) was used in the present work to select a roasting temperature and to estimate the jarosite phase transformation, from room temperature to 950 °C (1223 K). The XRD analysis showed that above 650 °C, almost the entire jarosite compound had decomposed. In addition to the high-temperature XRD analysis, a thermogravimetric analysis was performed to determine the phase changes from room temperature to 1600 °C (1873 K). Based on the XRD and TGA results, a roasting temperature of 700 °C was selected in this study for the decomposition of jarosite and the removal of sulfur.

Figure 2 illustrates the schematic flow chart of the jarosite waste treatment process. The jarosite sample was first prepared according to ASTM C702/C702M-18 [19]. The moisture was then eliminated at 100 °C for 8 h. The roasting process was carried out at 700 °C for 1 h in a laboratory muffle furnace. The roasted material was ground to a size of 74 μm (−200 mesh) and mixed with flux reagents (CaO and SiO₂). Then the sample was placed in an alumina crucible in a vertical tube furnace and heated at 1400 °C. A reducing gas mixture (70 vol% CO and 30 vol% CO₂) was injected from the top of the furnace directly onto the sample through an aluminum oxide rod, which served as a gas lance (3 mm internal diameter). Finally, the slag and metallic phases obtained in this process were subjected to XRD, chemical analysis and SEM-EDS characterization. In addition, the chemical stability of the final slag was evaluated according to the Mexican environmental standard NOM-053-SEMARNAT-1993 [18].

It is worth mentioning that the roasting was carried out at 700 °C, for three different time periods: 30, 60 and 90 min. The optimum time was determined based on the mass loss of the roasted material. The results showed that as the roasting time increased, the mass of the material decreased, due to the loss of H₂O and the evolution of SO_x species. However, there was little difference between the results obtained at 60 and 90 min. Therefore, 60 min was chosen as the optimum roasting time at 700 °C.

2.3. Characterization

The thermogravimetric analysis of the jarosite residue was carried out using thermogravimetry (TG-DSC 1, Mettler Toledo, Star v 10.0). The sample was heated from 25 °C to 1600 °C in an argon atmosphere at a rate of 15 °C/min during TG analysis. Mass changes and thermal effects were recorded during the heating process and the TGA curve was obtained. The composition of the samples was determined by chemical analysis using atomic flame absorption. The morphology of the species and the element distribution were analyzed by scanning electron microscopy and energy-dispersive spectrometer (SEM-EDS, Jeol 6300, Jeol Inc., Boston, MA, USA). X-ray Diffraction (XRD Bruker D8 Focus, Bruker, Karlsrushe, Germany) analysis was carried out to identify crystalline compounds.

2.4. Thermodynamic Modeling

The thermodynamic approach is used to determine the type and amount of chemical species under equilibrium conditions of the system over a range of temperatures and compositions. The thermodynamic software FactSage v 8.3 [21] was used to determine the effects of the initial CaSO₄ content in the jarosite residue and the amount of reducing gas mixture (CO-CO₂) on the equilibrium phases of the system. The software considers the initial mass, temperature, and system pressure to calculate the most stable species by the Gibbs free energy minimization method.

2.5. Leaching

The raw jarosite residue sample and the final slag from the reduction process were ground into a fine powder. The chemical stability of these materials was evaluated using a leaching procedure according to Mexican environmental regulations [18]. According to this, 25 g of each material must be crushed below −200 mesh 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 must be filtered through ashless filter paper (Whatman 542), and the liquid solution should be characterized by atomic absorption spectrometry.

3. Results and Discussion

3.1. TGA Results

Figure 3 shows the thermogravimetric pattern of jarosite in the range from 100 °C (423 K) to 1600 °C (1873 K). The jarosite residues passed through six stages during this heating process. The residues lost free and absorbed water in the first stage (100 °C to 160 °C). In the second stage (160 °C to 416 °C) the waterfrom crystallization was removed from the jarosite. In the third stage (416 °C to 576 °C), jarosite decomposed into Fe₂O₃, SO₂ and H₂, iron hydroxide was converted into Fe₂O₃, and anhydrite was also formed. In the fourth stage (576 °C to 780 °C) the anhydrite remains, while hematite is stabilized and some of the OH⁻ and sulfate groups are released by thermal decomposition. Anhydrite and hematite stay in the fifth stage (781 °C to 965 °C). Hematite remained stable in the sixth stage as well (965 °C to 1296 °C). The total weight loss amounted to 46.5%.

Ristic et al. [22] used Mösbauer and FTIR spectroscopy to determine the thermal decomposition of jarosite. They reported that ammonium jarosite decomposed to Fe₂(SO₄)₃, Fe(OH)SO₄ and Fe₂O(SO₄)₂ at 500 °C, while hematite (Fe₂O₃) is formed when it is heated to 600 °C. Frost et al. [23], using thermogravimetry in combination with mass spectrometry, reported that the decomposition of jarosite mainly occurs in two stages, the first between 280 °C and 450 °C with a significant mass loss, and the second between 580 °C and 730 °C released S from sulfate in the form of SO₃ gas. This corresponds to the following reactions:

2[(NH₄) Fe₃(SO₄)₂ (OH)₆] → (NH₄)₂SO₄ + Fe₂(SO₄)₃ + 2Fe₂O₃ + 6H₂O

(1)

Fe₂(SO₄)₃ → Fe₂O₃ + 3SO₃

(2)

3.2. XRD Results at High Temperature

Figure 4 shows the phases obtained with the in situ high-temperature XRD technique. It can be observed that small peaks of jarosite persisted at 440 °C and were partially transformed into Fe₂(SO₄)₃ and Fe₂O₃, while at 650 °C it was completely decomposed into Fe₂O₃ and Fe₃O₄. Increasing the temperature from 440 °C to 650 °C reduces the peak of Fe₂(SO₄)₃ and increases the peaks of Fe₂O₃ and Fe₃O₄. The peak intensity of gypsum (CaSO₄‧2H₂O) decreases with increasing temperature, and at 950 °C gypsum is completely dehydrated and yields anhydrite (CaSO₄). In the XRD pattern below 440 °C, the ZnFe₂O₄ phase is not visible but can be observed when the temperature is increased to 650 °C. The peaks of anglesite (PbSO₄) were only observed at high temperatures (650 °C and 950 °C). The XRD analysis gives the following approximate composition of the roasted jarosite: 16% hematite (Fe₂O₃) and 42% anhydrite (CaSO₄), with some amounts of franklinite (ZnFe₂O₄), iron sulfate (Fe₂SO₄), anglesite (PbSO₄) and magnetite (Fe₃O₄).

3.3. Reduction Experiments

The roasted jarosite contains mainly hematite and anhydrite. To achieve complete melting of this material and to reduce metallic oxide species, a mixture of CaO and SiO₂ was used. The quantities of these oxides were estimated to obtain a silicate slag with a low melting temperature. The CaO-SiO₂-Fe₂O₃ ternary phase diagram [24] shows that, depending on the Fe₂O₃ content in the roasted jarosite, fluxes with a mass ratio of CaO/SiO₂ = 48/52 = 0.92 can be used, which can give a slag with a melting point between 1300 °C and 1350 °C.

The reduction stage was carried out at two different temperatures, 1300 °C and 1400 °C. A temperature of 1300 °C would be the minimum to obtain a liquid system according to the CaO-SiO₂-Fe₂O₃ ternary phase diagram [24]; however, a partially liquid material was obtained experimentally at this temperature. This could be due to the presence of other components in the roasted jarosite, such as anhydrite (CaSO₄). It was experimentally found that by choosing a temperature of 1400 °C, a fully liquid material could be obtained, which was suitable for the injection of the reducing gas.

3.4. Thermodynamic Analysis

A thermodynamic analysis was performed using FactSage v 8.3 software [21] to estimate the effects of the amount of reducing gas mixture (CO-CO₂) and the presence of anhydrite (CaSO₄) in the roasted jarosite on the metallic phase obtained at equilibrium at 1400 °C. The gas mixture considered contained the molar ratio CO/CO₂ = 70/30. The results are shown in Figure 5. The main conclusions of the thermodynamic analysis are:

(a)

The metallic phase consists mainly of molten lead with small amounts of silver, iron, and zinc.

(b)

Most of the zinc and iron remain in the slag phase, which consists mainly of silicate species and magnetite.

(c)

The CaSO₄ content in the roasted jarosite considerably affects the formation of the lead-rich metallic phase (bullion). The increase in the CaSO₄ content led to a decrease in the metallic phase.

(d)

The high CaSO₄ content also required an increased use of reducing agent gas to form the metallic phase.

Figure 6 shows the effect of temperature on lead recovery (bullion), assuming a mass ratio of CaSO₄/Fe₂O₃ = 1 in the roasted jarosite. As the temperature increases, a larger quantity of the metallic phase is recovered. However, a higher temperature also means higher energy consumption. Based on these results, we choose 1400 °C for the reduction experiments.

3.5. Results of the Reduction Experiments

According to the results of the thermodynamic analysis, a series of experiments were programmed with different amounts of reducing gas, whereby the injection time was changed.

Table 2. Parameters used in the reduction experiments.

Test Number	Reduction Time (min)	Temperature (°C)	Reduction Gas Flow (mL/min)
1	40	1400	150
2	60	1400	150
3	80	1400	150
4	100	1400	150

shows the experimental parameters chosen in this study. The experiments were conducted at 1400 °C, with the reduction time varying from 40 to 100 min. The molar ratio of the reducing gas was CO/CO₂ = 70/30. The mass of the roasted jarosite remained constant at 10 g. The mass of the flux mixture (48% CaO, 52% SiO₂) amounted to 3.2 g. This value was chosen considering the CaO-SiO₂-Fe₂O₃ ternary phase diagram [24], which shows that a mixture with 1/3 Fe₂O₃ and 2/3 flux mixture (48% CaO + 52% SiO₂) has a low melting point, between 1300 °C and 1350 °C.

Table 3. Chemical composition of the metallic phase (mass%).

Test Number	Mass Bullion (g)	%Ag	%Fe	%Zn
1	0.31	0.376	0.064	0.053
2	0.43	0.382	0.072	0.065
3	0.51	0.403	0.088	0.092
4	0.51	0.410	0.089	0.091

shows the chemical composition of the metallic phases obtained in each test by changing the flow of the reducing gas mixture. The results show that the maximum silver yield was obtained in test number 4; however, the silver content was very similar between tests 3 and 4. The chemical analysis also shows a small amount of iron and zinc in the bullion in all the tests, which confirms that most of these metals remain in the slag.

Figure 7 shows the SEM micrograph and the EDS analysis of the metallic phase obtained in experiment number 3. According to these results, the metallic phase consists mainly of lead with silver-containing particles. Figure 8 shows the SEM results of the slag produced in test number 3. The angular crystals (F) are franklinite, the dark zone corresponds to hardystonite (H) and the gray zones are andradite (A).

Figure 9 shows the XRD patterns of the slag and metallic phases obtained in experiment number 3. Figure 9a shows that the slag sample consists mainly of silicates: hardystonite (Ca₂ZnSi₂O₇), andradite (Ca₃Fe₂Si₃O₁₂) and Ca₈Si₄O₁₆, as well as magnetite (Fe₃O₄) and franklinite (ZnFe₂O₄). Figure 9b shows that the metallic phase contains mainly lead and a small amount of iron. No silver and zinc could be detected in the XRD pattern. However, SEM-EDS and chemical analysis show that the bullion contains silver and small amounts of zinc and iron.

Previous work has reported that arsenic vaporizes during the roasting process of jarosite at high temperatures (1100–1500 °C) [4]. The arsenic did not vaporize in the present work, and went into the reduction step. The chemical analysis of the metallic phase obtained in the reduction stage showed no evidence of arsenic, so it is assumed that it forms a stable compound in the slag. Park et al. [25] reported that arsenic remains in the slag when the basicity is about CaO/SiO₂ = 1, and iron arsenate (FeAsO₄) is formed, which is consistent with the conditions of the slag of the present work.

3.6. Leaching Results

Table 4. Leaching results of the jarosite residue and slag of test number 3.

	Ag, mg/L	Pb, mg/L	Zn, mg/L
NOM-053-SEMARNAT-1993	5 max.	5 max.	–
Jarosite residue	7.4	16.7	11,300
Slag (test number 3)	0.04	3.2	26

shows the results of the chemical analysis of the liquid solutions obtained after the leaching test in an acidic solution, both for the jarosite residue in its initial state and for the final slag obtained after the reduction process in test number 3. The final slag becomes environmentally friendly as the dissolved metals in the leaching test are below the recommended limits, especially for silver and lead.

The process proposed in this work for treating jarosite residue by roasting and reduction enabled a high recovery of valuable elements, such as lead and silver, in a metallic phase. However, zinc remained in the slag. The results show over 95% of the lead and silver to be recovered. This means that 0.14 kg of silver and 34 kg of lead can be recovered from 1 ton of jarosite, depending on the initial composition of the jarosite residue and the efficiency of the process.

Through the leaching tests, it was possible to prove that the slag is an environmentally friendly waste that can be used as a raw material in other industries such as cement production, due to its high content of calcium, silicon, and iron oxides. In continuation of this work, the separation of zinc during roasting and the effect of ZnO on the slags used in cement production will be evaluated.

4. Conclusions

In the present work, a process for the recovery of lead and silver from jarosite residues by roasting at 700 °C and reducing at high temperature (1400 °C) using CaO and SiO₂ as fluxes and a CO-rich gas mixture as a reducing agent was carried out. The conclusions from this work are as follows:

▪

The jarosite residues were decomposed at 700 °C into Fe₂O₃, Fe₃O₄ and CaSO₄.

▪

A mixture of 48 mass percent CaO and 52 mass percent SiO₂ was used as a flux for the roasted residue to obtain a completely liquid system. In addition to the fluxes, a reducing gas with a molar ratio of 70/30 CO/CO₂ was introduced into the molten system to obtain a slag and a lead-rich metallic phase.

▪

Lead and silver were almost completely recovered in this process, while zinc and iron remained in the slag.

▪

The presence of CaSO₄ in the system influences the amount of the lead-rich metallic phase. A high CaSO₄ content requires the addition of a larger amount of reducing agent to produce the metallic phase.

▪

The slag resulting from the reduction process meets environmental specifications and could be used as a raw material in other industries.