Thermodynamic analysis and experimental rules of vacuum decomposition of molybdenite concentrate

doi:10.1016/j.vacuum.2015.08.009

Vacuum

Volume 121, November 2015, Pages 166-172

https://doi.org/10.1016/j.vacuum.2015.08.009 Get rights and content

Highlights

•
Feasibility of vacuum decomposition was proved by small and kilo-scale experiments.
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Experimental rules of vacuum decomposition were investigated in small experiments.
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Crude molybdenum and sulfur products were obtained from kilo-scale experiments.

Abstract

Vacuum decomposition process of molybdenite concentrate was investigated under pressure of 5–35 Pa for 15–120 min at the temperature range 1473 K–1973 K. The theoretical and experimental results showed that Gibbs free energy of vacuum decomposition reactions and evaporation rate of pure sulfur provided the theoretical calculation basis for temperature and heat preservation time selection in vacuum decomposition experiments of molybdenite concentrate. Melting points and saturated vapor pressures of pure substances and compounds predicted the evaporation behavior of impurity elements and its compounds during the experimental process. Both Cu and Fe could partly evaporate into condensate and Cu had better evaporation ability than Fe. MoO₃ could easily and Al₂O₃, SiO₂ could partly evaporate into the condensate. Kilo-scale experiment was performed based on the small experiments and its results showed that the Mo content of molybdenum metal product was 92.38% and the S content of sulfur product was 96.28%, and the molybdenum recovery rate reached to 95.94%. Both the theoretical and experimental results proved that it was feasible to produce crude molybdenum and sulfur from molybdenite concentrate through vacuum decomposition.

Introduction

Molybdenite is the major industrial mineral to produce molybdenum. Due to the excellent flotability, molybdenite can be treated by flotation method to produce molybdenite concentrate. Molybdenite concentrate treatment can be classified into two major categories, including hydrometallurgy and pyrometallurgy. The common feature of these two methodologies is the conversion of sulfide ores to oxide or its salts, then the further purification of intermediates and finally the reduction to molybdenum metal. This long process will cause SO₂ emission and other environmental pollution [1].

Compared to traditional metallurgical process, vacuum metallurgy owns many advantages, such as high metal recovery rate, less pollution, less energy consuming, etc. Vacuum distillation and decomposition are the important parts of vacuum metallurgy, the former is regarded as one of the most effective and environment-friendly method 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 [2], [3], [4], [5].

And many scholars made several meaningful researches in vacuum decomposition of molybdenite concentrate, theoretically and experimentally. On the view of molecular dynamics simulation, Lei Tian-min et al. [6] calculated the electronic structure and optical properties of monolayer MoS₂ based on density functional theory(DFT). Liu Da-chun et al. [7] optimized MoS₂ crystal structure and simulated the thermal decomposition of MoS₂ by DFT, which indicated the feasibility of producing crude molybdenum and sulfur from molybdenite concentrate through vacuum decomposition. On the view of thermodynamics and kinetics, WANG Lei et al. [8], [9] studied the vacuum decomposition process of molybdenite concentrate and verified it by vacuum decomposition experiments. Firstly, the feasibility of vacuum decomposition process of molybdenite concentrate was proved, while temperature range 1773 K–2073 K and pressure is less than 100 Pa. Secondly, the influence of temperature and heat preservation time was further investigated, while temperature range 1423 K–1773 K and pressure is less than 10–200 Pa. Finally, molybdenite concentrate was proved to be completely decomposed, while temperature was 1773 K and pressure was 20 Pa. On the view of kilo-scale experiments, Scholz W.G. et al. [10] presents a description of the stages in scaling-up of the dissociation equipment and an account of attempts to evaluate the molybdenum produced by thermal dissociation.

The main purpose of this research is to investigate the influence of temperature and heat preservation time on the evaporation behavior of Mo and S, and evaporation behavior of the main impurities (including Al₂O₃, SiO₂, Fe and Cu) was also studied, while temperature was 1473–1973 K and pressure was 5–35 Pa. And kilo-scale experiment was also performed to further prove the feasibility of vacuum decomposition of molybdenite concentrate.

Section snippets

Experimental materials

The chemical composition and XRD pattern of molybdenite concentrate used in this study were shown in Table 1 and Fig. 1, respectively.

As seen in Table 1 and Fig. 1, the main chemical components were Mo and S. And several impurities, such as O, Al, Si, Fe and Cu, were found in molybdenite concentrate. The XRD pattern showed that Mo in molybdenite concentrate mainly existed in the form of MoS₂, and oxide of molybdenum also was found slightly.

Experimental equipment and methods

Vacuum decomposition experiments were carried out in a

Effect of temperature

Buker D.O. [12] studied the mechanism of thermal decomposition of MoS₂ under vacuum, and did not think it was the direct decomposition reaction:4MoS₂ = 4Mo + 4S₂

He believed that the mechanism was as follows4MoS₂ = 2Mo₂S₃ + S₂2Mo₂S₃ = 4Mo + 3S₂

And both the simulated thermal decomposition process of MoS₂ by DFT [7] and phase evolution analysis by XRD pattern [9] demonstrated that molybdenite concentrate decomposed step by step, which further proved the mechanism mentioned above.

In order to

Conclusions

According to the thermodynamic analysis and experimental results, experimental rules were obtained as follows: Gibbs free energy of vacuum decomposition reactions, evaporation rate of pure sulfur, melting points and saturated vapor pressures of pure substances and its compounds provided the theoretical calculation basis for temperature selection, heat preservation time selection and evaporation behavior of impurity elements in vacuum decomposition experiments of molybdenite concentrate,

Acknowledgment

The authors acknowledge the financial support from National Natural Science Foundation of China (No. U1202271), Youth Fund of the NSFC (No. 51104078), and the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT1250).

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Thermodynamic analysis and experimental rules of vacuum decomposition of molybdenite concentrate

Highlights

Abstract

Introduction

Section snippets

Experimental materials

Experimental equipment and methods

Effect of temperature

Conclusions

Acknowledgment

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