Recovery of Valuable Metals by Roasting of Jarosite in Cement Kiln

Abstract

The jarosite residue is a harmful byproduct of the zinc hydrometallurgical process and has been classified as hazardous waste due to its high content of heavy metals and poor stability. In this work, the recovery of valuable metals by synergistic disposal of jarosite residue with cement kiln has been investigated. A series of investigations were undertaken to assess the effect of amount of jarosite residue, roasting time, and the addition of CaCl₂ on phase transformation and migration of valuable elements in the cement clinker. When 25% of jarosite residue was added to the raw cement materials and roasted at 1400 °C for 30 min, the silicate content in the cement clinker reached 54.4%. In the presence of CaCl₂ as the mineralizer, the volatilization rates of Pb and Zn were 93% and 75%, respectively. The results indicated that jarosite residue can be used as the cement additive and valuable metals of Pb and Zn can be recovered simultaneously during the roasting process.

1. Introduction

Zinc is one of the most commonly used metals. Currently, more than 85% of zinc is produced by hydrometallurgical process, which consists of three main stages: roasting, leaching, and electrowinning [1]. As the main raw material for zinc extraction, sphalerite usually contains a certain amount of iron, which is dissolved in the leachate during the leaching process. Iron is one of the main impurities in leaching solution, and it can be removed by three main methods: the goethite process, the jarosite process, and the hematite process [2,3,4]. These methods promote the precipitation of iron ions as XFe₃(OH)₆(SO₄)₂ (where X = Na, K, H₃O, NH₄), as shown in Equation (1) [3,5]:

X⁺ + 3Fe³⁺ + 5SO₄²⁻ + 6H₂O→XFe₃(OH)₆(SO₄)₂ + 3H₂SO₄

(1)

The main drawback of the jarosite process is a huge amount of residue production. It is estimated that the jarosite residue production is approximately 0.5 tons for every ton of zinc produced. As a result, jarosite produced from zinc hydrometallurgical process urgently need to be handled [6]. More specifically, the jarosite residue is characterized by strong acidity because the jarosite process is always conducted at low pH (1–2), and it has a high content of heavy metal ions and poor stability. As a result, jarosite is classified as hazardous industrial solid waste [7,8]. Long-term stacking of jarosite residue can cause serious harm to the environment, soil, water sources, and human health. Therefore, it has become an urgent task to treat and utilize the jarosite residue.

Currently, most studies on jarosite residue treatment have focused on the recovery of metals (e.g., Fe, Zn, Pb, In, Ag, and Cu). Previous studies include hydrometallurgy [9,10,11], pyrometallurgy [12,13,14], and hydro-pyro integrated processes [15]. Calla-Choque et al. [8] effectively recovered about 90% of silver and zinc from jarosite residue by using a single thiourea leaching process at pH 1 and 90 °C. De-la-Cruz-Moreno et al. [11] investigated that indium extraction could reach 96.8% with 6.8 M hydrochloric acid as leaching solution and 9.403 g of jarosite residue in 50 mL of leaching solution with 97.7 h of stirring time. Selective precipitation using ammonia is performed to enrich and purify indium, which is recovered in the form of solid indium hydroxide. However, these methods only consider the recovery of valuable metals from jarosite residue, and a large amount of solid residue is still present. Mombelli et al. [13] proposed a process for the recovery of Fe, Pb, and Zn from jarosite residue by an Arc Transfer Plasma (ATP) reactor. The jarosite residue after dehydration and desulfurization is loaded into the ATP reactor together with metallurgical coke and fluxes and used at 1600~1700 °C to produce pig iron, clean slag, and flue dust (mainly containing zinc, lead, and silver). Zhu et al. [14] dehydrated and desulfurized the jarosite residue by preliminary roasting at temperatures above 1200 °C, and then after reduction roasting for 70 min, the volatilization rates of zinc and indium could reach 99% and 85%, respectively, and the slag was recovered for ironmaking. Ju et al. [15] proposed a hydro-pyro integrated process to treat jarosite residue. The jarosite residue was initially roasted at 650 °C for 1 h; subsequently, the roasting residue was leached in NH₄Cl solution. The recovery of zinc, lead, copper, cadmium, and silver was higher than 95%. The NH₄Cl leaching residue was then leached again in NaOH solution to remove approximately 94% of As and 73% of Si.

Considerable research has also been conducted to achieve environmentally sound treatment by using jarosite residue as a construction material [16,17,18]. Mehra et al. [16] conducted a series of laboratory investigations to evaluate the performance of concrete mixtures incorporated with jarosite residue as a partial replacement of fine aggregates. The effect of jarosite residue on the mechanical properties and durability of concrete was investigated. Gared et al. [19] used jarosite residue instead of Portland pozzolana cement for rigid pavement construction. The compressive strength and flexural strength of rigid pavements increased by 17.1% and 23.8%, respectively, when 25% of the cement was replaced by jarosite residue. Previous studies have shown that it is feasible to use jarosite residue as a construction material, but these methods would result in the waste of valuable metal resources.

It is important to take into account the recovery of metals while achieving the harmless treatment of jarosite residue. In this study, jarosite residue is thermal decomposed to Fe₂O₃, which is used as one of the raw materials for silicate cement clinker production. In addition, mineralizer was added to effectively recycle Pb and Zn during the roasting process. The thermal decomposition process, surface morphology, and phase evolution were analyzed by thermogravimetry and differential scanning calorimetry (TG-DSC), scanning electron microscope-energy dispersive spectrometer (SEM-EDS), and X-ray diffraction (XRD). An approach for treating jarosite residue as a replacement for iron in cement and recycling lead and zinc by roasting is proposed.

2. Experimental

2.1. Materials

In this study, the jarosite residue was obtained from a zinc plant in Inner Mongolia, China. The jarosite residue was pre-dried at 105 °C for 6 h, crushed, and ground to a particle size of ≤80 μm. All chemical reagents are analytical purity. The analytical reagents CaO, SiO₂, and Al₂O₃ were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). In addition, the purity of the above reagents was higher than 99%.

2.2. Methods

A schematic flowchart of the jarosite residue treatment process is shown in Figure 1. The dried jarosite residue was mixed well with CaO, SiO₂, and Al₂O₃ and pressed at 5 MPa to form a columnar briquette with a diameter of 30 mm and a height of 20 mm. The columnar briquette was roasted at 1400 °C in a muffle furnace to prepare cement clinker [20,21,22]. The effects of jarosite residue addition, roasting time, and mineralizer CaCl₂ on the phase of cement clinker and volatilization behavior of important elements during roasting was studied. After roasting, the roasted sample was rapidly cooled to room temperature and crushed in a ball mill for further analysis.

The silicate content of these cement clinkers was used to evaluate the product quality and the details of the tests are given below. Ten grams of cement clinker and 360 mL of anhydrous methanol were placed in a 500 mL beaker at 25 °C for 3 min with stirring; subsequently, 72 g of salicylic acid (C₇H₆O₃) was added. Then, the mixture was reacted at a stirring speed of 600 rpm for 1 h. After the reaction, the slurry was separated from the liquor by filtering with medium-speed quantitative filter paper. The filter cake was washed with deionized water and dried in an oven at 80 °C for 2 h. The mass of dried filter cake A was recorded as A. Next, 2 g of the dried filter cake was mixed with 360 mL of 1 mol/L acetic acid (C₂H₄O₂), and the mixture was reacted at a stirring speed of 600 rpm for 1 h at 25 °C. Likewise, after filtering and drying at 80 °C for 2 h, the mass dried filter cake B was recorded as B. The contents of silicate, aluminate + impurity, and ferroaluminate + aluminosulfate in the cement clinker were calculated according to Equations (2)–(4).

ω₁ = (10 − A)/10

(2)

ω₂ = (2 − B) × A/20

(3)

ω₃ = A × B/20

(4)

where ω₁ represents the content of silicate, %; ω₂ represents the content of aluminate + impurity, %; and ω₃ represents the content of ferroaluminate + aluminosulfate, %.

Free-CaO (fCaO) in the cement clinker was determined by the following procedure: 0.4 g of cement clinker was added into 20 mL ethanol-ethylene glycol solution (C₂H₆O-C₂H₆O₂), and then heated with stirring for 20 min before filtered. Add sodium hydroxide-ethanol solution (NaOH-C₂H₆O) to the filtrate until the solution turned purple–red, and then the filtrate was titrated with standard benzoic acid-ethanol solution (C₇H₆O₂-C₂H₆O) until the solution turned colorless and the color remained unchanged. The free-CaO content in the cement clinker was calculated by Equation (5).

$$\begin{array}{c}fCaO\%=\left({ T}_{CaO} \times V \times 100\right)/\left(G \times 1000\right)\\ R=\frac{T \times V \times 100}{G \times 1000} \times 100\%\end{array}$$

(5)

where R represents the content of free-CaO, %; T represents the mass of calcium oxide titrated per ml of benzoic acid standard solution, mg/mL; V represents the volume of consumed benzoic acid solution, mL; and G represents the mass of cement clinker, g.

2.3. Characterization

The thermogravimetric analysis of the jarosite residue was carried out by thermogravimetry and differential scanning calorimetry (TG-DSC, Netzsch STA 449C). In the TG-DSC analysis, the sample was heated in a nitrogen atmosphere at a heating rate of 15 °C/min from 25 °C to 1100 °C. During the heating process, the mass changes and thermal effects were recorded, and TG-DSC curves were obtained. The chemical compositions of the samples were determined by inductively coupled plasma–atomic emission spectroscopy (ICP–AES, PerkinElmer Optima-4300DV). The phases of the jarosite residue and roasting sample were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Germany), and the operating conditions were 10–90° 2-theta and 8°/min with Cu Kα radiation at 40 kV and 40 mA. The micro-morphologies and element distributions were analyzed by scanning electron microscope (SEM) along with an energy dispersive spectrometer (EDS, Zeiss Gemini SEM 300, Carl Zeiss AG Corporation, Oberkochen, Germany).

3. Results and Discussion

3.1. Properties of the Jarosite Residue

The chemical composition and XRD pattern of the jarosite residue are shown in

Table 1. Chemical composition of the jarosite residue.

Element	Fe (%)	S (%)	Zn (%)	Pb (%)	Ag (g/t)
Content	22.42	12.54	5.35	3.17	238

and Figure 2a. The XRD pattern indicates that the main minerals of the jarosite residue are hydronium K-jarosite (KFe₃(OH)₆(SO₄)₂), zinc ferrate (ZnFe₂O₄), and a small amount of zinc sulfide (ZnS). The contents of Fe and S in the jarosite residue were 22.42% and 12.54%, respectively. The residue also contained 5.35% Zn and 3.17% Pb, which are worth recycling. Figure 2b,c shows the SEM micrograph of the jarosite residue sample and the EDS analysis of the selected area. According to the SEM images, most of the jarosite residue is irregular particles with particle sizes less than 20 μm. EDS analysis of areas A, B, and C showed that they corresponded to zinc sulfide, zinc ferrate, and K-jarosite, respectively, which is in agreement with the result of the XRD phase analysis.

Figure 3 shows the TG-DSC result of jarosite residue in nitrogen atmosphere. The mass loss of the jarosite residue can be divided into three stages. The first stage starts at 20 °C and ends at 260 °C, with a weight loss of 4.5%, which might be due to the evaporation of water in the jarosite residue [23]. The second stage begins at approximately 260 °C and ends at 450 °C. In this stage, the mass loss is 10.5%, which is caused by the dehydroxylation of KFe₃(SO₄)₂(OH)₆ in the jarosite residue, as shown in Equations (6) and (7) [23,24]:

KFe₃(SO₄)₂(OH)₆·xH2O = KFe₃(SO₄)₂(OH)₆ + xH₂O

(6)

2KFe₃(SO₄)₂(OH)₆ = 2KFe(SO₄)₂ + 2Fe₂O₃ + 6H₂O

(7)

The third stage begins at 450 °C and ends at 1100 °C, with a weight loss of 30%. As shown in Figure 3, there is a strong endothermic peak at approximately 725 °C, and no significant heat absorption peak is observed when the temperature is higher than 725 °C, but the mass loss continues until the temperature rises to 1100 °C. XRD quantitative analysis was carried out using the XRD result in Figure 4. The results show that the roasted jarosite is composed by ZnFe₂O₄ and Fe₂O₃ with a weight percentage of 56.3% and 43.7%, respectively. As a result, the Zn to Fe ratio in the roasted jarosite is close to that of the jarosite residue, which means that there is almost no Fe and Zn loss during the roasting process. This also indicates that the mass loss at this stage is caused by desulfurization [15]. At this stage, part of the mass loss is assigned to the decomposition of ZnSO₄ and PbSO₄ [23]. The reaction may take place according to Equations (8)–(10).

2KFe(SO₄)₂ = Fe₂O₃ + K₂SO₄ + 3SO₃

(8)

ZnSO₄ = ZnO + SO₃

(9)

PbSO₄ = PbO + SO₃

(10)

3.2. Thermodynamic Analysis

Metallurgical processes under high temperature conditions usually involve complex physical and chemical reactions. The main composition of silicate cement clinker is 62~67% CaO, 20~24% SiO₂, 4~7% Al₂O₃, and 2.5~6.0% Fe₂O₃. As indicated by TG-DSC results, Kfe₃(OH)₆(SO₄)₂ in the jarosite residue can decompose to Fe₂O₃, ZnFe₂O₄, and SO₃ when the roasting temperature is higher than 700 °C. The possible reactions during the roasting process are shown in

Table 2. The main chemical reactions during the roasting process.

Reaction	Equations
Fe2O3 + 2CaO = 2CaO·Fe2O3	(11)
Fe2O3 + CaO = CaO·Fe2O3	(12)
2CaO + 2SO2 + O2 = 2CaSO4	(13)
CaO + SO3 = CaSO4	(14)
PbO + CaCl2 + SO2(g) + 0.5O2(g) = PbCl2(g) + CaSO4	(15)
PbO + CaCl2 + SO3(g) = PbCl2(g) + CaSO4	(16)
PbS + CaCl2 + 2O2(g) = PbCl2(g) + CaSO4	(17)
PbSO4 + CaCl2 = PbCl2(g) + CaSO4	(18)
PbSO4 = PbO(g) + SO3	(19)
ZnSO4 + CaCl2 = ZnCl2(g) + CaSO4	(20)
ZnS + CaCl2 + 2O2 = ZnCl2(g) + CaSO4	(21)
ZnFe2O4 = ZnO + Fe2O3	(22)
ZnFe2O4 + CaCl2 + 2CaO = ZnCl2(g) + 2CaO·Fe2O3	(23)

.

The change of Gibbs free energy (ΔG) of the reactions In Table 2 were calculated at different temperature under one atmosphere pressure, as shown in Figure 5. The Gibbs free energy changes of reactions (11)–(14) in Table 2 are all negative in the temperature range from 600 to 1500 °C, which indicates that it is feasible to fix Fe and S in silicate cement clinker. The reactions of the major Pb and Zn species in the jarosite residue in the presence of the mineralizing agent CaCl₂ are shown in reactions (11)–(14) in Table 2 Figure 5 show that under the action of the mineralizer CaCl₂, Pb, and Zn in the jarosite residue can be volatilized in the form of chloride during roasting.

3.3. Effect of Jarosite Addition

The jarosite residue was used as a replacement for Fe₂O₃ in silicate cement ingredients. The main purpose of treating iron alum slag was achieved while keeping S content in silicate cement clinker within the standard range. The XRD patterns of the roasting cement clinker obtained at different additions of jarosite residue are shown in Figure 6. C₂S, C₂(A,F), and C₄A₃S were all produced in the roasting silicate cement clinker with the addition of different amount of jarosite residue. The phase composition and the free-CaO in the cement clinker was determined using a three-step extraction method, and the results are shown in

Table 3. Content of each component in cement clinker under different jarosite residue additions. (Conditions: Roasted at 1400 °C for 30 min).

Addition Amount	Silicate (%)	Aluminate and Impurity (%)	Ferroaluminate and Aluminosulfate (%)	Free-CaO (%)
15%	65.50	27.83	6.67	0.63
20%	57.58	34.25	8.17	0.55
25%	54.43	35.16	10.41	0.88
30%	41.23	46.42	12.35	0.98

. When the addition of jarosite residue increased from 15% to 30%, the silicate content in the cement clinker decreased from 65.5% to 41.23%. At the same time, the total amount of iron aluminate and sulfate aluminate increased from 6.76% to 12.35%. The result indicates that the increase in jarosite residue suppresses the production of silicate phase but promotes the formation of ferroaluminate and sulfoaluminate phases. In addition, when the addition of jarosite residue was increased from 25% to 30%, the silicate content of the active ingredient in cement clinker plummeted from 54.43% to 41.23%, which affected the quality of cement clinker.

In the calcination process of cement clinker, most CaO can react with acidic oxides (SiO₂, Al₂O₃, Fe₂O₃) to form C₂S, C₃S, C₂(A,F), C₄A₃S, and other minerals. However, due to the composition, particle size, homogeneity, and roasting temperature of the cement raw materials, a small amount of CaO still exists in the form of free oxide (free-CaO). CaO in the free state directly affects the stability of the cement. Therefore, it is important to determine the content of free CaO in cement clinker. Table 3 shows that the free-CaO content raised with the increasing of jarosite residue addition. This may be due to the fact that the addition of jarosite residue deteriorates the homogeneity of the cement raw material, hinders silicate formation while increasing the free-CaO content.

The volatilization rates of S, Pb and Zn during roasting were calculated by examining the elemental content of the sample before and after roasting, as shown in Figure 7. As the addition of jarosite residue increases, the volatilization rate of S increases, which is not conducive to the fixation of S. The volatilization rate of Zn is stable at approximately 20%, which is independent of the amount of jarosite residue added. This is because most zinc in the jarosite residue exists in the form of zinc ferrate, which is structurally stable and does not participate in the reaction during the roasting process at 1400 °C. In addition, the volatilization rate of Pb decreased from 74% to 51% as the addition of jarosite residue increased from 15% to 30%. From the XRD pattern of the roasting product at a jarosite residue addition of 30%, the peaks of C₃SPb (3CaO·SiO₂·PbO) appeared [21]. The formation of C₃SPb occurred during roasting, indicating that the lead combines with C₃S in the form of lead oxide to form C₃SPb, which results in a lower volatility of the lead.

Considering the above, the quality of cement clinker decreases as the addition of the jarosite residue increases and the volatility of lead and zinc decreases. Therefore, the addition of 25% of the jarosite residue to the total mass of cement raw material was chosen as the optimum addition, provided that the quality of cement clinker is guaranteed.

3.4. Effect of Roasting Time

The effect of roasting time on mineral formation and the behavior of important elements in cement clinker was studied. The XRD patterns of the cement clinker roasted for 30, 60, 90, and 120 min at 1400 °C are shown in Figure 8. Under the different roasting times, the roasting cement clinker obtained contains C₂S, C₄A₃S and ferroaluminate. In addition, the formation of C₃SPb was observed in the XRD patterns of the cement clinker obtained for 60 min and 90 min. This indicates that the increase in roasting time favors the reaction between lead oxide and C₃S, which is not conducive to the volatilization of lead.

Table 4. Content of each component in cement clinker at different roasting times. (Conditions: 25% jarosite residue and roasting temperature at 1400 °C).

Roasting Time	Silicate (%)	Aluminate and Impurity (%)	Ferroaluminate and Aluminosulfate (%)	Free-CaO (%)
30 min	54.43	35.16	10.41	0.88
60 min	49.82	36.95	13.23	0.65
90 min	50.6	35.95	13.45	0.84
120 min	47.28	41.50	11.22	0.74

shows that roasting time has almost no effect on the content of various components in the cement clinker and that the active ingredients in the cement clinker can be formed in only 30 min.

Figure 9 shows the volatilization rates of S, Zn, and Pb in the jarosite residue with different roasting times. The volatilization rate of S was reduced from 30% to 19%, and Zn remained stable at approximately 20% with increasing roasting time. This means that the extended roasting time can facilitate the fixation of S but has almost no effect on Zn species. Adequate roasting time promoted the reaction between lead oxide and C₃S, which inhibited the volatilization of Pb, resulting in a reduction in Pb volatilization from 74% to 57%. Based on the above experimental results, to recover as much Pb and Zn as possible by volatilization and ensure the formation of effective components in cement, the optimal roasting time was considered to be 30 min.

3.5. Effect of Mineralizer CaCl₂

When jarosite residue was added for firing cement clinker, the volatilization rates of Pb and Zn were 25% and 74%, respectively, under optimal conditions. To recover as many valuable metals as possible, CaCl₂ was added as the mineralizer to the raw cement to facilitate the volatilization of Pb and Zn during the roasting process. From the reaction Equations (15)–(23), it can be seen that, theoretically, one mole of CaCl₂ is consumed for each mole of Pb or Zn volatilized. The theoretical addition of CaCl₂ was calculated according to the contents of Pb and Zn in Table 1. The theoretical addition of CaCl₂ was used as a benchmark to explore the effects of different CaCl₂ additions (0.5 times, 0.75 times, 1.0 times, 1.25 times) on the generation of phases and the behavior of important elements in cement clinker. The roasting was carried out at 1400 °C for 30 min with 25% jarosite addition.

XRD patterns of cement clinker formed by roasting with different additions of CaCl₂ are shown in Figure 10. The addition of the mineralizer CaCl₂ complicates the composition of the phases in the cement clinker compared with Figure 6. As shown in

Table 5. Content of each component in cement clinker at different CaCl₂ additions. (Conditions: 25% jarosite residue roasted at 1400 °C for 30 min.).

Amount of CaCl2 Added	Silicate (%)	Aluminate and Impurity (%)	Ferroaluminate and Aluminosulfate (%)	Free-CaO (%)
50%	47.65	33.83	18.52	0.54
75%	45.73	38.22	16.05	0.72
100%	40.81	40.95	18.24	0.69
125%	30.51	46.85	22.64	0.29

, the addition of the mineralizer CaCl₂ contributes to the decrease in silicate and free-CaO content and the increase in aluminate content.

The volatilization rates of S, Pb and Zn in the presence of different amounts of the mineralizer CaCl₂ are shown in Figure 11. As the addition of the mineralizer CaCl₂ increased from 50% to 125%, the volatilization rate of Pb increased from 78% to 93%, while that of Zn increased from 39% to 75%. The addition of the mineralizer CaCl₂ can react with Pb and Zn in the jarosite residue to form volatile chlorides, thus favoring the recovery of Pb and Zn. At the same time, the volatilization rate of S slightly increased.

4. Conclusions

In this work, the effects of jarosite residue addition, roasting time, and mineralizer CaCl₂ on the phase generation and behavior of important elements (Zn, Pb, S) in cement clinker were investigated. The results show that the valuable metals Pb and Zn can be recovered simultaneously by synergistic disposal of jarosite residue with cement kilns. The following conclusions have been drawn from this study:

The thermal decomposition process of the jarosite residue was divided into three stages. From 20 °C to 260 °C, the adsorbed water in the jarosite residue started to evaporate. From 260 °C to 450 °C, the KFe₃(SO₄)₂(OH)₆ in the jarosite residue undergoes dehydroxylation to form KFe(SO₄)₂, Fe₂O₃, and H₂O. From 450 °C to 1100 °C, the KFe(SO₄)₂, ZnSO₄, and PbSO₄ in the residue undergo desulfurization to form Fe₂O₃, K₂SO₄, ZnO, PbO, and SO₃.

Increasing the addition of jarosite residue inhibited the formation of the silicate phase while promoting the formation of ferroaluminate and sulfoaluminate and increasing the content of free-CaO. When the addition of jarosite residue was controlled at 25% and kept at a roasting temperature of 1400 °C for 30 min, the silicate content of the cement clinker obtained was 54.43% and the free-CaO content was 0.88%.

The volatilization rate of Zn was stable at approximately 20% for different jarosite residue additions and roasting times, while the volatilization rate of Pb decreased with increasing roasting time and jarosite residue addition. When 125% the theoretical amount of mineralizer CaCl₂ was added, the volatility of Pb and Zn increased from 74% to 93% and from 25% to 75%, respectively.