J. Cent. South Univ. (2020) 27: 3259-3268

DOI: https://doi.org/10.1007/s11771-020-4544-2

Sulfidation mechanism of cerussite in the presence of sulphur at high temperatures

GE Bao-liang(戈保梁)^{1, 2}, PANG Jie(庞杰)², ZHENG Yong-xing(郑永兴)^{1, 3},NING Ji-lai(宁继来)¹, LU Jin-fang(吕晋芳)^{1, 2}

1. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China;

2. Faculty of Land Resource Engineering, Kunming University of Science and Technology,Kunming 650093, China;

3. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology,Kunming 650093, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: In this paper, sulfidation mechanism of cerussite in the presence of sulphur at high temperatures was investigated based on micro-flotation, X-ray powder diffractometry (XRD), electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). The micro-flotation test results showed that flotation recovery of the treated cerussite increased to above 80% under a suitable flotation condition. It was found that the S/PbCO₃ mole ratio and pH obviously affected flotation recovery. XRD analysis results confirmed that the cerussite was decomposed into massicot and then was transformed into mainly PbS and PbO·PbSO₄ after sulfidation roasting. EPMA analysis results demonstrated that surface of the obtained massicot was smooth, but surface of the artificial galena was rough and even porous. Content of oxygen decreased, whereas content of sulphur increased with an increase in the S/PbCO₃ mole ratio. XPS analysis results revealed that various lead-bearing species, including mainly PbS, PbSO₄ and PbO·PbSO₄, were generated at the surface. Formation of PbS was advantageous to flotation of the treated cerussite. Based on these results, a reaction model of the cerussite sulfurized with sulphur was proposed.

Key words: cerussite; massicot; roasting; sulfidation; flotation

Cite this article as: GE Bao-liang, PANG Jie, ZHENG Yong-xing, NING Ji-lai, LU Jin-fang. Sulfidation mechanism of cerussite in the presence of sulphur at high temperatures [J]. Journal of Central South University, 2020, 27(11): 3259-3268. DOI: https://doi.org/10.1007/s11771-020-4544-2.

1 Introduction

With the gradual exploitation and processing of lead sulfide ores, lead oxide ore can make up for the resource gap in lead production. One typical lead oxide mineral is cerussite. It contains 83.52% PbO, but the content of lead in the raw ore is usually no more than 5% [1, 2]. Therefore, mineral processing techniques, such as flotation and gravity separation [3, 4], were considered to concentrate the lead oxide mineral.

Flotation of non-ferrous oxide minerals, mostly as semi-soluble salt minerals, is usually difficult, in contrast to non-ferrous sulfide minerals [5-7]. For the cerussite, considerable amounts of lead ions are dissolved from its lattice and then enter the solutions. After direct addition of xanthate as a frequently used collector, the xanthate interacts with the dissolved lead ions to form a lead xanthate precipitate while significant amounts of carbonate ions are released into the pulp solution. As such, the xanthate is continuously consumed until the new solution equilibrium is established [8, 9]. Although multilayer thin films of lead xanthate are generated at the surface of the cerussite during this process, the films readily detach during the movement of the fluid. Another method is addition of sodium sulfide or sodium hydrosulfide, which makes lead sulfide form at the surface of the cerussite, followed by addition of the xanthate [8, 10]. It is known that excess sulfide in solution can inhibit flotation of the cerussite, whereas inadequate sulfide results in a poor sulfidation performance. Therefore, the amounts of the added sulfide agents are difficult to control. Moreover, the generated film of lead sulfide is easily broken away from the surface.

To enhance sulfidation of the cerussite, sulfidation roasting has been proposed to transform the oxide mineral or its surface to sulfide [11-13]. LI et al [11] investigated the sulfidation roasting of a complex low-grade Pb-Zn oxide ore with sulphur. The results showed that the sulfidation extent of lead reached 98% under the optimum conditions. After flotation of the roasted ore, the flotation recovery of lead was close to approximately 80%. However, the process was carried out in an airtight furnace to reduce loss of the sulphur caused by volatilization. To optimize the roasting process, ZHENG et al [14] adopted a two-step heating method to pre-treat the lead oxide material. The first step roasting was performed at 444 °C, causing more than 90% of lead oxide to transform into lead sulfide. The second step roasting was carried out at 750 °C to synthesize perfectly crystalline galena, which is readily floated in a conventional flotation system. It is well known that flotation performance of oxide minerals is mainly affected by formation of the stable sulfide films at the mineral surface. Therefore, the roasting process, which is mainly characterized by sulfidation extent, seems to be unreasonable. Our team [1] proposed to apply flotation response to directly characterize roasting performance of a low-grade Pb-Zn oxide ore. The results showed that the lead recovery reached 92.3% after a closed-circuit flotation test. Moreover, a complex and lengthy process for determining the chemical phase of lead was omitted, which makes the whole process more effective. However,sulfidation mechanism of the cerussite in the presence of sulphur at high temperatures, especially for the information of surface, was not clearly and completely revealed.

The current work is a deep extension of mechanism study on the interaction between the cerussite and sulphur at high temperatures. A series of flotation tests, X-ray powder diffractometry (XRD) analysis, electron probe microanalysis (EPMA) line scanning and X-ray photoelectron spectroscopy (XPS) were carried out to illustrate sulfidation behaviour of the cerussite at high temperatures. It is believed that this research will further enrich basic theory of the sulfidation roasting of refractory non-ferrous oxide resources.

2 Experimental

2.1 Materials and roasting process

The raw cerussite sample was obtained from Yunnan province in China, followed by artificial removal of gangue such as quartz, calcite and dolomite. The selected sample was crushed, dry ground and then sieved using a standard screen to obtain a particle range of 37-74 μm. Chemical analysis results showed that the prepared sample contained 75.83% Pb, which indicated that the sample was of higher purity. Sulphur, assaying 99.5% of sulphur, was used as the sulfidation agent in the roasting experiments. Ethyl xanthate and terpineol were used as collector and frother, respectively, in the flotation experiments. Hydrochloric acid and sodium hydroxide were used as pH regulators.

The roasting tests were carried out in a tube furnace, as reported in our previous study [15]. The cerussite and sulphur were blended in a required mole ratio and the mixture was put into a 50 mL quartz tube equipped with a plug. Then, the tube loading with samples was heated in nitrogen (1.5 L/min). As soon as the holding time of 60 min reached, the heating program was closed and the sample was cooled in nitrogen. In the available literature, the sulfidation roasting of complex Pb-Zn oxide materials was usually performed at a temperature ranging from 450 °C to 650 °C [1, 11, 14], and a good flotation recovery of lead was obtained. Therefore, the moderate temperature was selected as 600 °C in this work.

2.2 Flotation tests

Flotation tests were carried out in a micro-flotation cell with an effective volume of 40 mL. Approximately 2.0 g of the treated sample was placed in the cell and then 35 mL of distilled water was added. After adjusting the pulp, flotation of the treated cerussite was conducted. After flotation, the floated and non-floated products were filtered, dried and weighed to calculate the recovery.

2.3 XRD analysis

The cerussite samples before and after interaction with sulphur at high temperatures were analyzed by XRD (Rigaku, TTR-III) to determine the phases. The analysis was performed on a Germany Bruker-AXS D8 Advance X-ray powder diffractometer with K_α radiation (λ=0.15406 nm) at a scanning rate of 10°/min.

2.4 EPMA line scanning analysis

Surface constituents of the sample were examined by EPMA (JEOL, JXA-8230) in the Modern Analysis and Testing Center of Yunnan University, China. An EPMA line scanning was performed to investigate the surface differences of the cerussite before and after the sulfidation roasting with sulphur. The resolution of secondary electron image and backscattered electron image can reach 5 nm and 20 nm, respectively. The acceleration voltage of the equipment was fixed at 20 kV.

2.5 XPS analysis

XPS analysis was conducted on a K-Alpha system (Thermo Fisher Scientific) with a resolution of 0.2 eV, using Al K_α monochromatic irradiation (1486.6 eV) at a working pressure lower than 2×10^-5 Pa. A survey scan of the analyzed sample was initially performed to detect elemental information. Then, XPS spectra of specific elements, such as S, O, Pb, Fe and C, were obtained, for analysis and fitting by the Thermo Avantage software. The spectrometer was calibrated by fixing the binding energy of C 1s at 284.8 eV.

3 Results and discussions

3.1 Flotation tests of treated cerussite

Figure 1 shows the recovery of the cerussite before and after sulfidation roasting as functions of ethyl xanthate dosage and pH at a terpineol dosage of 5×10^-5 mol/L. Figure 1(a) indicates that the cerussite after roasting without sulphur presented an extremely poor floatability in the presented xanthate dosage range, whereas the recovery of the cerussite after roasting with sulphur increased with the increase in S/PbCO₃ mole ratios. Recovery of the cerussite after roasting increased from 3.5% to 80% at a xanthate dosage more than 5×10^-5 mol/L when the S/PbCO₃ mole ratio increased from 0 to 1.2. With a further increase in S/PbCO₃ mole ratio to 1.8, the recovery of the treated cerussite increased to 88%. In addition, it was observed that recovery of the cerussite after roasting with sulphur increased when xanthate dosage increased from 0 to 5×10^-5 mol/L. Recovery of the cerussite after roasting in an S/PbCO₃ mole ratio of 0.8-1.8 had little changes with a further increase in the ethyl xanthate dosage.

Figure 1 Recovery of cerussite after roasting at different S/PbCO₃ mole ratios as functions of ethyl xanthate dosage and pH at a terpineol dosage of 5×10^-5 mol/L:

Figure 1(b) shows the effect of pH on recovery of the treated cerussite. It indicates that recovery of the cerussite after roasting with sulphur increased with the increase in S/PbCO₃ mole ratio at a pH less than 7.0. Recovery of the treated cerussite decreased with a further increase in pH value. The preferred pH for the recovery of the treated cerussite was determined to be 6-7. At pH 7.0, the recoveries of the treated cerussite reached 76.5%, 81.5% and 87% when the S/PbCO₃ mole ratios were fixed at 0.8, 1.2 and 1.6, respectively.

3.2 XRD and thermodynamics

Figure 2 shows the XRD patterns of the cerussite after roasting at S/PbCO₃ mole ratios of 0, 0.8 and 1.6. According to this figure, peaks of the cerussite completely disappeared, but peaks of massicot (PbO) appeared when there was no addition of sulphur. This result is accounted for by the fact that the cerussite was decomposed into massicot after the roasting, as described by Eq. (1). When the S/PbCO₃ mole ratio increased to 0.8, the massicot was transformed into galena (mainly) and lanarkite, as expressed by Eqs. (2) and (3). With a further increase in the S/PbCO₃ mole ratio, there were no obvious changes to the diffraction peaks.

PbCO₃→PbO+CO₂(g),

△G^Θ=-39.60 kJ, T=600 °C                 (1)

2PbO+3/2S₂(g)→2PbS+SO₂(g),

△G^Θ=-206.73 kJ, T=600 °C                (2)

5PbO+2S₂(g)→3PbS+PbO·PbSO₄,

△G^Θ=-356.09 kJ, T=600 °C                (3)

To gain insight into sulfidation process of the cerussite, the equilibrium constituents of products were calculated using the equilibrium composition module in the Out-okumpu HSC6.0 software package [16], as shown in Figure 3, which indicates that the sulphur dosage significantly affected the condensed phases and their constituents. When the amount of sulphur was less than 1.5 kmol, the obtained lead-bearing species mainly consisted of PbS, PbO·PbSO₄, PbSO₄, PbO and Pb. The metallic Pb and PbO completely disappeared when the amount of sulphur increased to 1.0 kmol. These results further agreed with the results of XRD analysis (Figure 2). With a further increase in the amount of sulphur to 1.5 kmol, the amount of PbS increased to 1 kmol, suggesting that the obtained PbO ((Eq. (1)) was completely transformed into PbS.

Figure 2 XRD patterns of cerussite after roasting at different S/PbCO₃ mole ratios

Figure 3 Equilibrium constituents of roasted products at different sulphur dosage (1 kmol PbCO₃, 600 °C, 1.01×10⁵ Pa)

3.3 EPMA line scanning analysis

To investigate outer layer of the treated cerussite, EPMA analysis was performed to compare the surface differences of the sample roasted without sulphur and with sulphur, as shown in Figure 4 and Table 1. Figures 4(a)-(c) show the morphology of the cerussite roasted at 600 °C. From Figure 4(a), the generated massicot at a particle size less than 10 μm appeared in the form of irregular polyhedron with smooth surfaces. After the introduction of sulphur at an S/PbCO₃ mole ratio of 0.8 (Figure 4(b)), the treated cerussite mainly occurred as large blocks with slightly rough surfaces, which may be attributed to the uneven adsorption of sulphur vapor at the surface of the formed massicot. With a further increase in the S/PbCO₃ mole ratio to 1.6, the surface of the treated cerussite became rough in contrast to the cerussite roasted at an S/PbCO₃ mole ratio of 0.8. Moreover, many pores appeared at the surface, which may be ascribed to the diffusion of sulfur dioxide and carbon dioxide during the formation of lead sulfide at the outer layer of the cerussite. Figures 4(d)-(f) present surface constituents of the treated cerussite. It is observed that the content of oxygen and lead presented slight fluctuation for the sample roasted without sulphur in contrast to the sample roasted with sulphur. This result further confirmed that the obtained massicot had smooth surfaces. However, the content of sulphur presents obvious fluctuations, which further illustrates the uneven adsorption of sulphur at the surface of the obtained massicot.

Figure 4 EPMA images of n(s)/n(PbCO₃)=0 (a), n(s)/n(PbCO₃)=0.8 (b), n(s)/n(PbCO₃)=1.6 (c) and EDS line scan spectra (d), (e), (f) of cerussite roasted

Table 1 Contents of sulphur, oxygen and lead at surface of treated cerussite

Table 1 shows the statistical values of content of sulphur, oxygen and lead at the surface of the obtained samples. It indicates that the content of oxygen decreased, whereas the content of sulphur increased with the increase in S/PbCO₃ mole ratios. In addition, it was calculated that the difference between the maximum and minimum of the same element for the cerussite roasted without sulphur was less than that for the cerussite roasted with sulphur. This result further verified that the surface of the obtained massicot was even, but the surface of the cerussite roasted with sulphur was rough. Moreover, the average content of oxygen and lead in the obtained massicot was nearly close to the theoretical contents of oxygen (7.17%) and lead (92.83%) in PbO, respectively, suggesting that decomposition of the cerussite was complete. The average contents of sulphur and lead in the artificial galena obtained at an S/PbCO₃ mole ratio of 1.6 were nearly close to the theoretical contents of sulphur (13.39%) and lead (86.61%) in PbS, respectively, indicating that the obtained massicot was nearly transformed into galena.

3.4 XPS analysis

XPS, as an effective method, is used to determine the elemental constituents and chemical state of the obtained products. In this study, it was used to reveal the species formed at the surface of the treated cerussite. The results are shown in Figure 5 and Tables 2-5.

Figure 5(a) presents the survey scan spectra of the treated cerussite over a binding energy range of 0-800 eV, and correspondingly, the surface atomic concentration of S, Pb, O, Fe and C are listed in Table 2. From this table, it is obvious that the atomic concentration of S increased, whereas the atomic concentration of O decreased, which was attributed to the addition of sulphur. The atomic concentration of Fe was low and the atomic concentration of Pb has slight fluctuation. The change in the atomic concentration of C was caused by the changes in the atomic concentrations of S and O.

Figure 5(b) shows the high-resolution XPS spectra of S 2p for the cerussite roasted at S/PbCO₃ mole ratios of 0.8 and 1.6. The results show that the S 2p XPS spectra consisted of a doublet structure of S 2p_3/2 and S 2p_1/2 level and the strength of S 2p_3/2 is higher than that of S 2p_1/2. The results also show that the entire S 2p_3/2 peak can be fitted into two peaks and correspondingly, the assignment and properties of the S 2p_3/2 XPS are listed in Table 3. The first peak (S 2p_3/2), which is located at a binding energy of 160.78 or 160.85 eV in Figure 5(b), was assigned to the S^2- state [17, 18], i.e., the S in lead sulfide generated on the surface of the treated cerussite. The second peak (S 2p_3/2 Scan A), which is located at a binding energy of 168.39 or 168. 57 eV, was assigned to the S atom in lead sulfate (PbSO₄) [18].

According to Table 3, the percentage of S in PbS increased, whereas the percentage of S in PbSO₄ decreased when the S/PbCO₃ mole ratio increased from 0.8 to 1.6. Combined with Table 2, it can be deduced that the increase in the atomic concentration of S was mainly contributed by the increase in the atomic concentration of S in PbS. These results explain that the cerussite roasted at an S/PbCO₃ mole ratio of 1.6 presented a better floatability than the cerussite roasted at an S/PbCO₃ mole ratio of 0.8 (Figure 1(a)).

Figure 5(c) shows the high-resolution XPS spectra of Pb 4f for the cerussite roasted at different S/PbCO₃ mole ratios. The Pb 4f XPS spectra are composed of a doublet structure of Pb 4f_7/2 and Pb 4f_5/2 level [17, 19]. The assignment and properties of the Pb 4f_7/2 XPS are shown in Table 4. For the cerussite roasted without sulphur, the Pb 4f_7/2 peak can be divided into two peaks in Figure 5(c). The peak (Pb 4f_7/2) at a binding energy of 137.9 eV with a proportion of 84.31% was related to the Pb atom in PbO [18, 20, 21]. The other peak (Pb 4f_7/2 Scan A) at a binding energy of 138.93 eV with a proportion of 15.69% was related to the Pb atom in the residual PbCO₃ [22, 23]. For the cerussite roasted with sulphur, it was suggested that the Pb 4f_7/2 peak could be resolved into three components. The Pb 4f_7/2 peaks (Pb 4f_7/2) at a low binding energy with proportion of 61.59% and 61.83% were attributed to the Pb in PbS [23, 24]. The Pb 4f_7/2 peak (Pb 4f_7/2 Scan B1) at a high binding energy with proportion of 15.58% and 12.44% was attributed to Pb in PbSO₄ [25, 26]. The Pb 4f_7/2 peaks (Pb 4f_7/2 Scan A1) at a binding energy of 138.58 eV and 138.77 eV were contributed by the residual PbCO₃. Moreover, both the values were less than the binding energy of Pb in PbSO₄. Combined with the XRD analysis results in Figure 2, the Pb 4f_7/2 peak (Pb 4f_7/2 Scan B1) may be also attributed to PbO·PbSO₄.

Figure 5 XPS spectra of sample roasted at different S/PbCO₃ mole ratios (XPS survey scan of obtained sample (a), high-resolution XPS spectra of S 2p (b), Pb 4f (c) and O 1s (d))

Table 2 Atomic concentration of elements as determined by XPS

Table 3 Assignment and properties of S 2p_3/2 XPS

Table 4 Assignment and properties of Pb 4f_7/2 XPS

Table 5 Assignment and properties of O 1s XPS

Figure 5(d) shows the high-resolution XPS spectra of O 1s for the cerussite roasted at different S/PbCO₃ mole ratios and Table 4 presents the assignment and properties of the O 1s XPS. For the obtained massicot, the O 1s peak can be fitted with three components located at 528.89, 531.01 and 532.07 eV. The former peak centred at 528.89 eV with an average proportion of 33.43% was associated with O in PbO [21, 26]. The middle peak centred at 531.01 eV was attributed to oxygen in the carbonate groups [22, 26]. The last peak centred at 532.07 eV was related to oxygen in the O-C functional group [27, 28]. After roasting in the presence of sulphur, the binding energy of O in PbO shifted towards the high binding energy near 530 eV [18]. The binding energies of 531.37 eV and 531.58 eV were mainly related to the O atom in the PbSO₄. These results further agreed with the analysis results of the Pb 4f_7/2 spectra (Figure 5(c)). The atomic concentration ratios of O in the functional group of C-O increased with the increase in the S/PbCO₃ mole ratio. Combined with Table 2, it can be deduced that the decrease in the atomic concentration of O was mainly contributed by the decrease in the atomic concentration of O in PbO.

3.5 Reaction mechanisms

Based on the above results and discussions, a reaction model of the cerussite roasted with sulphur is shown in Figure 6. The decomposition reaction of the cerussite initially occurred at the outer layer of the particle when the roasting temperature increased to approximately 320 °C (△G^Θ=-1.28 kJ), as described in Eq. (1). The cerussite was completely decomposed into massicot (PbO) and CO₂ when the required temperature and reaction time reached. After the introduction of sulphur, the sulphur in the form of sulphur vapor initially interacted with the PbO generated at the surface to form mainly PbS, PbO·PbSO₄ and PbSO₄ (Figures 2 and 3). With proceeding of the reaction, the formed product layer of PbS inhibited diffusion of the generated CO₂, which impeded the decomposition of PbCO₃ to some extent.

Figure 6 Reaction models of cerussite roasted in the presence of sulphur

4 Conclusions

Efficient sulfidation plays an important role in flotation of the refractory lead-bearing materials. In the present paper, interaction mechanism between the cerussite and sulphur at high temperatures was thoroughly investigated and the related conclusions are drawn as follows:

1) The cerussite after roasting with sulphur exhibited a good floatability, and flotation recovery increased to above 80% at a suitable S/PbCO₃ mole ratio and pH. The increase in S/PbCO₃ mole ratio was advantageous for the increase in flotation recovery, but the higher pH was disadvantageous for the flotation.

2) The cerussite was decomposed into the massicot and then was sulfurized by sulphur to form mainly PbS and lead sulfates. The obtained massicot at a particle size less than 10 μm exhibited smooth surfaces. The surface of the treated cerussite became rough and even porous with the introduction of sulphur. The content of oxygen decreased, whereas the content of sulphur increased with the increase in S/PbCO₃ mole ratio.

3) The atomic concentration of O and S atoms at the surface of the cerussite after sulfidation roasting decreased and increased, respectively, with the increase in S/PbCO₃ mole ratio. Various lead- bearing species, including mainly PbS, PbSO₄, PbO·PbSO₄, were generated at the surface. The increase in the percentage of PbS formed at the surface was advantageous to flotation of the treated cerussite.

Contributors

GE Bao-liang designed the experiments; PANG Jie finished most of the experiments; ZHENG Yong-xing wrote the manuscript text; NING Ji-lai assisted the flotation tests of this paper; LU Jin-fang devised scheme diagram in Figure 6 and polished this paper.

Conflict of interest

The authors declare no competing financial interests.

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(Edited by YANG Hua)

中文导读

白铅矿与硫磺在高温下的硫化机理

摘要：本文采用微浮选、X射线粉末衍射(XRD)、电子探针(EPMA)和X射线光电子能谱(XPS)等分析手段，研究了高温下硫磺与白铅矿的硫化机理。浮选试验结果表明，在适宜的浮选条件下，处理后的白铅矿浮选回收率提高到80%以上。S/PbCO₃摩尔比和pH值对浮选回收率有明显影响。XRD分析结果证实，硫化焙烧后的白铅矿分解为氧化铅，然后转化为以PbS为主和PbO·PbSO₄的含铅物种。EPMA分析结果表明，获得的铅黄表面光滑，但合成的人工方铅矿表面粗糙，甚至孔隙较多。随着S/PbCO₃摩尔比的增加，氧含量降低，硫含量增加。XPS分析结果表明，表面生成了以PbS为主以及PbSO₄和PbO·PbSO₄的多种含铅物种。PbS的形成有利于处理后白铅矿的浮选。基于这些研究结果，提出了白铅矿与硫磺在高温条件下的反应模型。

关键词：白铅矿；铅黄；焙烧；硫化；浮选

Foundation item: Project(51964027) supported by the National Natural Science Foundation of China; Project(2017FB084) supported by the Yunnan Province Applied Basic Research Project, China; Project(2019J0037) supported by the Education Department of Yunnan Province, China

Received date: 2019-09-19; Accepted date: 2020-07-06

Corresponding author: ZHENG Yong-xing, PhD, Associate Professor; E-mail: yongxingzheng2017@126.com; LU Jin-fang, PhD; E-mail: jflv2017@126.com; ORCID: https://orcid.org/0000-0003-1032-6146