J. Cent. South Univ. (2017) 24: 2542-2549

DOI: https://doi.org/10.1007/s11771-017-3667-6

Volatilization behaviors of molybdenum and sulfur in vacuum decomposition of molybdenite concentrate

ZHOU Yue-zhen(周岳珍)^{1, 2}, LU Yong(卢勇)^{1, 2}, LIU Da-chun(刘大春)^{1, 2, 3},

CHEN Xiu-min(陈秀敏)^{1, 2, 3}, LI Hui(李慧)^{1, 2}, LI Wei(李玮)^{1, 2}

1. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology,Kunming 650093, China;

2. Key Laboratory of Vacuum Metallurgy of Non-ferrous Metals of Yunnan Province, Kunming 650093, China;

3. State Key Laboratory of Complex Non-ferrous Metal Resources Clear Utilization in Yunnan Province,Kunming 650093, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract: Thermodynamic calculation, ab initio molecular dynamics (AIMD) and vacuum decomposition experiments were performed to study the volatilization behaviors of Mo and S from molybdenite concentrate by vacuum decomposition. In thermodynamic calculation, starting decomposition temperatures of reactions were calculated, and saturated vapor pressures of Mo, S and MoS₂ were also analyzed. In AIMD, geometries of the S_n (n≤8), Mo_m (m≤8) and Mo_mS_n (m+n≤8) clusters have been optimized using density functional theory (DFT) with generalized gradient approximation (GGA). And these clusters were simulated in DFT with Cambridge Sequential Total Energy Package (CASTEP) code of Material Studio software. Structures and stabilities of these clusters before and after molecular dynamics simulations were discussed, and diffusion coefficients were also calculated. In vacuum decomposition experiments, relationship between heat preservation time and volatilization rate of Mo and S was obtained, while the constant temperature and chamber pressure were 1823 K and 5–35 Pa, respectively. Above all, both the theoretical and experimental results showed that volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate were as follows: Mo could partly evaporate into the condensate in the form of clusters, and S could easily evaporate into the condensate.

Key words: volatilization behavior; vacuum decomposition; thermodynamics; ab initio molecular dynamics; clusters

1 Introduction

Molybdenite is the major industrial mineral for molybdenum [1]. At present, molybdenite concentrate is converted to molybdenum oxide by oxidizing roasting firstly, and then the resultant oxidized concentrate is purified by conversion of the molybdenum oxide to ammonium molybdate, finally commercial molybdenum metal can be obtained by reduction of molybdenum oxide with H₂ in two or more stages [2]. However, there are several serious problems in processing molybdenite by this method: valuable elements in the concentrate such as rhenium and selenium cannot be recovered easily to high degrees and air pollution may occur due to the emission of SO₂ gas [3].

Comparing to traditional metallurgical process, vacuum metallurgy owns many advantages, such as high metal recovery rate, less pollution, less energy consuming. Vacuum distillation and decomposition are the important parts of vacuum metallurgy, the former is regarded as one of the most effective and environment- friendly methods for metal separation, preparation of high purity metal and recycling of secondary metal resources，and the latter is mainly applied to thermal decomposition of compounds [4–7].

Many scholars made several meaningful researches in vacuum decomposition of molybdenite concentrate. On the view of experiments, CHEN [8] investigated vacuum decomposition process of analytic grade molybdenum disulfide and molybdenite concentrate respectively, and useful experimental parameters were obtained. WANG et al [9] studied the key steps of thermal decomposition process of molybdenum concentrate in vacuum and verified it by vacuum decomposition experiment. On the view of molecular dynamics simulation, structural and other properties of Mo_n(n=2–55) [10], Mo_n(n=2–8) [11], S_n(n=2–8) [12], Mo₅S_n(n=5–15) [13], Mo_mS_n(n=1–6, m=n–3n) [14] and MoS₆ clusters [15] were calculated based on DFT. And LIU et al [16] optimized MoS₂ crystal structure and simulated the thermal decomposition of MoS₂ by DFT.

The main purpose of this research is to study the volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate, from the view of both thermodynamics and molecular dynamics simulation, and further verify it through vacuum decomposition experiments.

2 Theoretical analysis of volatilization behaviors

2.1 Thermodynamic calculation

The possible mechanism of thermal decomposition of MoS₂ under vacuum was as follows [17], and the reaction’s Gibbs free energy at different pressures were shown in Table 1 [9].

4MoS₂=2Mo₂S₃+S₂                                 (1)

Mo₂S₃=2Mo+1.5S₂                            (2)

Table 1 Starting decomposition temperature of MoS₂ and Mo₂S₃ at different pressures

As seen in Table 1, the starting decomposition temperature kept decreasing as pressure fell. And comparing to reactions happened under condition of atmospheric pressure, whose starting decomposition temperatures were at least above 2000 K, the temperatures were 1491 K and 1575 K while the pressure fell to 10 Pa. Thus, it is feasible to separate Mo and S in molybdenite concentrate by vacuum decomposition.

In order to investigate the volatilization behaviors of Mo and S during vacuum decomposition process, saturated vapor pressure of Mo, S and MoS₂ were calculated with the van LAAR equation expressed as follows [18]:

lgP^Θ=AT^–1+BlgT+CT+D                       (3)

lgP^Θ=–AT^–1+B                               (4)

where P^Θ was the saturated vapor pressure, Pa; A, B, C and D were evaporation constants [19]; T was the thermodynamic temperature, K. The melting points and saturated vapor pressure of Mo, S and its compound were shown in Table 2 and Fig. 1.

Table 2 Melting points of Mo, S and its compound

Fig. 1 Relationship between temperature and saturated vapor pressure

As seen in Table 2 and Fig. 1, saturated vapor pressure of S was much larger than 10 Pa as temperature rose, which indicated that S can easily evaporate into the condensate during the whole vacuum decomposition process. While the temperature was below starting decomposition temperature (1491 K or 1575 K), the saturated vapor pressure of MoS₂ was smaller than the pressure in furnace. Namely, Mo could not evaporate into the condensate in the form of MoS₂ before the vacuum decomposition reactions started. While the temperature was above starting decomposition temperature, Mo was still in solid-state. Namely, Mo could not evaporate into the condensate in the form of pure substance after reactions started.

According to thermodynamic calculation results, volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate were as follows: Mo nearly enriched in the residual, and S could easily evaporate into the condensate.

2.2 AIMD

2.2.1 Computational methods

Based on the authors’ previous theoretical calculation, ground-state structures of S_n(n≤8) clusters, Mo_m(m≤8) clusters and Mo_mS_n(m+n≤8) clusters were obtained [20]. The computational details were as follows: calculations were performed with DFT implemented in the Dmol3 package in the Materials Studio software [21]. In the geometric optimizations, the exchange correlation interaction was treated within the GGA using the PW91 functional [22]. The convergent threshold was set to 10^–5Ha for energy change, 0.002 Ha/ for forces and 0.005 for displacement. In the electronic structure calculations, the double numerical basis set including d-polarization function (DND) was utilized to describe the valence electrons and the core electrons were described with effective core potential. Self-consistent field (SCF) calculations were done with a convergence criterion of 10^–6 Ha on the total energy. The direct inversion in an iterative subspace (DIIS) approach was used to speed up SCF convergence. A 0.005 Ha of smearing was applied to the orbital occupation.

The dynamics simulation calculations of the optimized S_n clusters, Mo_m clusters and Mo_mS_n clusters were carried out with DFT by using the program package CASTEP [23]. Exchange and correlation effects were treated with GGA implemented in PW91 as exchange- correlation functional. It was simulated by NVT system, with the simulation time 10.0 ps and time step size 1.0 fs, total 10000 step. And the simulation temperature was 1823 K. The Nosé thermostat [24] and Andersen barostat [25] were used in the dynamics simulation. Brillouin- zone sampling of 1×1×1 k-points and cutoff energy of 180.0 eV in plane wave basis sets were verified to be sufficient to obtain good numerical accuracy with reasonable computational costs.

2.2.2 Structures and stabilities of clusters

Ground-state structures and simulated structures of Mo_m clusters are shown in Fig. 2. Structural changes were as follows: 1) Comparing to the ground-state structures of Mo_m (m=2, 5, 6) clusters, structures after dynamics simulation were basically unchanged. Though bond length changed to a certain degree, Mo–Mo bonds did not break in these clusters; 2) Due to the relatively great changing bond length, several Mo–Mo bonds broke or formed, and structures of Mo_m (m=3, 4, 7, 8) clusters after dynamics simulation changed obviously.

Thus, the structural stability of Mo_m (m=2, 5, 6) clusters was larger than it of Mo_m (m=3, 4, 7, 8) clusters. Namely, Mo_m (m=2, 5, 6) clusters were more likely to steadily exist under the condition of vacuum and high temperature.

Ground-state structures and simulated structures of S_n clusters were shown in Fig. 3, structural changes were as follows: 1) Comparing to the ground-state structures of S_n (n=2, 3, 4, 6) clusters, structures after dynamics simulation were basically unchanged. 2) Comparing to the ground-state structures of S_n (n=5, 7, 8) clusters, structures after dynamics simulation changed obviously. The spatial structure of S₅ cluster transformed from cyclic structure to chain structure because several S—S bonds in this cluster broke. S_n (n=7, 8) clusters’ structures changed greatly, and they broke into several parts which consisted with less atoms.

Thus, the structural stability of S_n (n=2, 3, 4, 6) clusters was larger than it of S_n (n=7, 8) clusters. Namely, S_n (n=2, 3, 4, 6) clusters were more likely to steadily exist under the condition of vacuum and high temperature.

Part ground-state structures and simulated structures of Mo_mS_n clusters were shown in Fig. 4, structural changes were as follows: 1) To a certain degree, all kinds of the Mo_mS_n (m+n≤8) clusters’ structures changed, and only 10 kinds of these clusters were basically unchanged after dynamics simulation. These Mo_mS_n clusters were MoS, MoS₂, MoS₃, MoS₄, Mo₂S₂, Mo₂S₄, Mo₃S₂, Mo₄S₄, Mo₅S₂ and Mo₆S; 2) Due to the conspicuous changing bond of Mo—Mo, S—S and Mo—S, the other 18 kinds of Mo_mS_n clusters’ structures changed obviously, such as Mo₂S, Mo₂S₅, Mo₃S₄ and Mo₇S.

Fig. 2 Structures of Mo_m clusters before (ground-state) and after (1823 K) simulations

Fig. 3 Structures of S_n clusters before (ground-state) and after (1823 K) simulations

Thus, the structural stability of the former 10 kinds of Mo_mS_n clusters was larger than that of the latter 18 kinds of Mo_mS_n clusters. Namely, the former 10 kinds of Mo_mS_n clusters were more likely to steadily exist under the condition of vacuum and high temperature.

In conclusion, Mo_m (m=2, 5, 6) clusters, S_n (n=2, 3, 4, 6) clusters and the 10 kinds of Mo_mS_n clusters mentioned before were more likely to steadily exist under the condition of vacuum and high temperature.

2.2.3 Diffusion property of clusters

In order to investigate the volatilization behaviors of the 17 kinds of clusters which could steadily exist under the condition of vacuum and high temperature, diffusion property was calculated based on the Einstein law as follows [26, 27]:

          (5)

                          (6)

where d_MS is the mean square value of particles’ displacement; r_i(t) is location of particle i while time equaled to t; N is the sum of particles; < > was the ensemble average of all particles during the dynamics simulation process; D is diffusion coefficient. The diffusion coefficients of clusters under the condition of vacuum and high temperature are shown in Tables 3 and 4.

As seen in Tables 3 and 4, diffusion coefficients of these clusters kept decreasing with the increasing atomic number of the clusters. And while the ratio between atomic number of molybdenum and sulfur increasing, diffusion coefficients kept decreasing too.

According to the diffusion coefficients of the 17 kinds of clusters, which were more likely to steadily exist under the condition of vacuum and high temperature, several rules were found as follows: 1) Diffusion coefficient of Mo₂ cluster was relatively larger than that of other clusters in Mo_m clusters; 2) Diffusion coefficient of S₂, S₃ and S₄ clusters were relatively larger than that of other clusters in S_n clusters, and their values were about the double of Mo₂ cluster’s. 3) Diffusion coefficients of MoS, MoS₂, MoS₄ and Mo₂S₄ clusters were relatively larger than those of other clusters in Mo_mS_n clusters, and their values were approximately between the values of Mo_m clusters and S_n clusters.

Fig. 4 Structures of Mo_mS_n cluster before (ground-state) and after (1823 K) simulations

Table 3 Diffusion coefficients (D/(²·s^–1)) of Mo_m clusters and S_n clusters after simulations

Table 4 Diffusion coefficient (D/(²·s^–1)) of Mo_mS_n clusters after simulations

Thus, among all kinds of Mo_m, S_n and Mo_mS_n clusters simulated here, 8 kinds of clusters’ structures were basically unchanged after dynamics simulation, and the diffusion coefficients of these clusters were relatively large. Namely, these clusters could steadily exist under the condition of vacuum and high temperature, and could easily evaporate into the condensate relatively easily. These clusters were Mo₂, S₂, S₃, S₄, MoS, MoS₂, MoS₄ and Mo₂S₄ clusters.

According to AIMD results, volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate were as follows: Mo and S could evaporate into the condensate in the form of clusters, and the volatilization ability of Mo was much weaker than that of S.

In conclusion, theoretical analysis of volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate was as follows: 1) Thermodynamic calculation results demonstrated that Mo nearly enriched in the residual, but AIMD results indicated that Mo could evaporate into the condensate in the form of clusters, the conclusions draw here were contradictory; 2) Both thermodynamic calculation and AIMD results proved that S could easily evaporate into the condensate.

Thus, it was necessary to perform experimental study to further investigate the volatilization behaviors of Mo and S during vacuum decomposition process.

3 Vacuum decomposition experiments of molybdenite concentrate

3.1 Experimental conditions and materials

3.1.1 Experimental conditions

In order to make experimental results as a reliable evidence to further prove the volatilization behaviors of Mo and S during vacuum decomposition process, appropriate experimental conditions were very important.

We selected 1823 K as the constant temperature while studying the influence of heat preservation time on the volatilization behaviors of Mo and S, and the chamber pressure was 5–35 Pa. Firstly, the starting decomposition temperatures of vacuum decomposition reactions were 1491 K and 1575 K in thermodynamic calculation, and the simulation temperature was set 1823 K in AIMD. Thus, it was appropriate to select 1823 K as the constant temperature. Secondly, the pressure obtained in our experimental equipment was 5–35 Pa, and this narrow range of pressure was basically consistent with the theoretical analysis.

3.1.2 Experimental materials

The chemical composition of molybdenite concentrate used in this experiment was shown in Table 5.

Table 5 Chemical composition of molybdenite concentrate (mass fraction, %)

As seen in Table 5, the main chemical components were Mo and S. And several impurities, such as O, Al, Si, Fe and Cu, were found in molybdenite concentrate. Thus, the experimental materials used here was high grade molybdenite concentrate, and its main compound was MoS₂.

Here, we selected molybdenite concentrate as experimental materials for the following reasons: Firstly, the experimental results could be a reliable evidence to further prove the theoretical analysis results because the molybdenite concentrate mainly contained Mo and S; Secondly, both experimental and theoretical results could further provide heat preservation time selection in vacuum decomposition experiments of molybdenite concentrate.

3.2 Experimental equipment and methods

Vacuum decomposition experiments were carried out in a laboratory-scale vertical vacuum distillation furnace, as shown in Fig. 5. It mainly consists of a chamber, condensing zone, heating zone and vacuum pump systems [28].

A cylindrical material, with diameter and thickness of 23 mm and 18 mm, respectively, was obtained through tableting. Then the material was removed into crucible and put into the vacuum furnace. The feeding material was heated to different temperatures at a heating rate of 15 K/min and each temperature was kept for different time when the chamber pressure was (5–35) Pa. Then, the heater power was switched off after the end of heating period and vacuum was unnecessary to maintain while the temperature in furnace below 323 K. In the end, residual and condensate were gathered, weighed and chemically analyzed, respectively.

Fig. 5 Schematic diagram of vertical vacuum distillation furnace

3.3 Experimental results and discussion

Vacuum decomposition experiments were carried under the pressure of (5–35) Pa at 1823 K for different heat preservation time, from 15 to 120 min. Volatilization rate of Mo or S was calculated as follow:

                           (7)

where r is the volatilization rate, %; m₀ is the mass of Mo or S in molybdenite concentrate, g; m_r is the mass of Mo or S in residual, g. The effect of heat preservation time on volatilization rate of Mo and S is shown in Fig. 6.

Fig. 6 Relationship between heat preservation time and volatilization rate

As seen in Fig. 6, volatilization rate of S increased from 58.75% to 99.82% while the heat preservation time extended from 15 to 120 min. Namely, S could easily evaporate into the condensate and its volatilization rate nearly reached to 100%. Thus, experimental results were consistent with the thermodynamic calculation and AIMD results about the volatilization behavior of S during vacuum decomposition process. While extending the heat preservation time, volatilization rate of Mo increased from 0.93% to 6.42%. Namely, Mo could partly evaporate into the condensate and its volatilization rate was much less than that of S. Although thermodynamic calculation results indicated that Mo nearly enriched in the residual, it still could safely draw the conclusion that Mo could partly evaporate into the condensate, according to both AIMD and experimental results.

According to theoretical and experimental results, volatilization behaviors of Mo and S during vacuum decomposition process of molybdenite concentrate were as follows: Mo could partly evaporate into the condensate in the form of clusters, and S could easily evaporate into the condensate.

Volatilization rate of S increased from 58.75% to 99.30% while the heat preservation time extended from 15 to 60 min, and it slightly increased to 99.82% while doubled the heat preservation time. In the meantime, volatilization rate of Mo still increased from 5.96% to 6.42%. Thus, on the premise of desulphurization, it was reasonable to select 60 min as the heat preservation time while the vacuum decomposition experiments were carried at 1823 K.

4 Conclusions

Thermodynamic calculation results showed that Mo nearly enriched in the residual, and S could easily evaporate into the condensate. However, AIMD results indicated that Mo and S could evaporate into the condensate in the form of clusters, and the volatilization ability of Mo was much weaker than that of S. In order to further investigate the volatilization behaviors of Mo and S, relationship between heat preservation time and volatilization rate of Mo and S was obtained through vacuum decomposition experiments of molybdenite concentrate. Taking both theoretical and experimental results into consideration, volatilization behaviors of Mo and S were as follows: Mo could partly evaporate into the condensate in the form of clusters, and S could easily evaporate into the condensate. And heat preservation time selection in vacuum decomposition experiments was also investigated as follows: On the premise of desulphurization, it was reasonable to select 60 min as the heat preservation time while the vacuum decomposition experiments were carried at 1823 K.

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(Edited by HE Yun-bin)

Cite this article as: ZHOU Yue-zhen, LU Yong, LIU Da-chun, CHEN Xiu-min, LI Hui, LI Wei. Volatilization behaviors of molybdenum and sulfur in vacuum decomposition of molybdenite concentrate [J]. Journal of Central South University, 2017, 24(11): 2542–2549. DOI:https://doi.org/10.1007/s11771-017-3667-6.

Foundation item: Projects(1202271, 51104078) supported by the National Natural Science Foundation of China; Project(IRT1250) supported by the Program for Innovative Research Team in University of Ministry of Education of China

Received date: 2015-10-10; Accepted date: 2017-01-20

Corresponding author: LIU Da-chun, PhD; Tel: +86–871–65114017; E-mail: lcd_2002@sina.com