Conversion of molybdenite by a mineral phase reconstruction method and leaching kinetics of its product

Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Conversion of molybdenite by a mineral phase reconstruction method and leaching kinetics of its product Fei Li a, Xingyu Chen a, Wenjuan Zhang a, Lihua He a, Zhongwei Zhao a,b,⁎ a b

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, PR China Key Laboratory for Metallurgy and Material Processing of Rare Metals, Central South University, Changsha 410083, PR China

a r t i c l e

i n f o

Article history: Received 26 March 2016 Received in revised form 19 September 2016 Accepted 6 October 2016 Available online 8 October 2016 Keywords: Molybdenite Mineral phase reconstruction Ternary sulfide Kinetics Autocatalysis

a b s t r a c t Determined by the unique S-Mo-S sandwich layer structure, molybdenite is so chemical inert and hydrophobic that it is quite refractory in hydrometallurgical route. To improve Mo leaching by changing mineral structure of molybdenite prior to leaching might be a potential alternative. In this work, a mineral phase reconstruction (MPR) method is presented to treat molybdenite. The transformation of molybdenum-bearing sulfides by this method and leaching performance of MPR product were investigated. Results show that molybdenite can be reconstructed to a ternary sulfide FexMo6S8 by smelting with iron. The ternary sulfide can be leached much faster and more complete than molybdenite under the same conditions. Moreover, laboratory research and commercial practice show that sulfur of molybdenite being totally oxidized to sulfate. But as for the ternary sulfide, sulfur was converted into elemental state. An autocatalytic reaction kinetic equation was applied to fit the experimental data of MPR products leaching with HNO3-H2SO4 mixed solution. The apparent activation energy was calculated to be 76 kJ·mol−1 indicating the process being chemical reaction controlled. The reaction is of the 1.5 and 0.6 order related to HNO3 and H2SO4 concentration respectively. Finally, a kinetic equation is established for the leaching reaction. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Molybdenum, as an important strategic metal, is widespread applied in the fields of steel and petrochemical industry, aerospace engineering and national defense. In the earth, molybdenite (MoS2) is the most abundant primary molybdenum ore (approximately 85%) [1], so the extractive metallurgy of molybdenum is based largely on it. Traditionally, pyrometallurgical processes are used to treat MoS2. Through this route MoS2 is oxidized to molybdenum trioxide (MoO3), which can be separated from other concomitant impurities by dissolving in aqueous ammonia [2]. However, in the pyrometallurgical route, high temperature (over 600 °C) will readily result in volatilization loss of MoO3. To avoid such a problem, a large quantity of air is needed to be passed through the furnace to take away excess heat produced. But on account of dilution by flowing air, the concentration of SO2 in off-gas is about 0.8–3% which is too low to produce sulfuric acid economically. If the low concentration of SO2 is released to the atmosphere without any desulfurization treatment, it will cause severe pollution problems. Hydrometallurgical route is a promising alternative for molybdenum extraction. However, molybdenite is one of the most refractory sulfide ores in hydrometallurgy operations because of its chemical ⁎ Corresponding author at: School of Metallurgical Science and Engineering, Central South University, Changsha 410083, PR China. E-mail address: [email protected] (Z. Zhao).

http://dx.doi.org/10.1016/j.ijrmhm.2016.10.010 0263-4368/© 2016 Elsevier Ltd. All rights reserved.

inertness, surface hydrophobicity [3], physical and chemical anisotropy [4]. In hydrometallurgy, MoS2 is oxygen pressure leached in acid or alkaline media, sometimes using a catalyst. This treatment involves high temperatures and high pressures [5,6], which put forward higher requirements for the equipment. Autoclave explosions are reported occasionally. Therefore, metallurgists attempt to develop a leaching process at atmospheric pressure and lower temperatures. Owing to the extreme chemical stability of MoS2, high concentrations of strong oxidants such as sodium hypochlorite [7], sodium chlorate [8] and even sodium dichromate [9] are used. However, these reagents result in environmental problems because of the toxicity of the related compounds and the difficulty of an economic recycling. Additionally, the leaching process can be stimulated by mechanical activation [10,11] and ultrasonic [12] or microwave [13] support, though the latter methods are still immature. Under so severe leaching conditions, MoS2 can be leached thoroughly. But sulfur is completely oxidized to sulfate, which not only consumes more oxidant but also results in harmful emissions. In principle, crystal structure and reactivity of a compound decide the behavior during the leaching process. So, converting the structure of MoS2 into another compound might be an alternative way to improve its leaching performance. Geochemical studies show that molybdenum is a strong sulfide former [14]. Under natural conditions, Mo exists mainly in the form of binary sulfide, MoS2. With the advancements of technology and materials chemistry, a series of artificial M-Mo-S ternary sulfides can be formed. MxMo6S8 (M = Fe, Cu, Ni and so on, x = 0 ~ 4)

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

is one group of such compounds (also known as Chevrel phases). The insertion/deinsertion of Mn+ cation in or out of the Mo6S8-matrix can be carried out in aqueous media by application of a selective cation transfer. In a study on a Chevrel phase being used as an electrochemical transfer junction in a sulfate electrolyte, S. Seghir [15] found that the host structure Mo6S8 was already oxidized at a potential of 0.64 V/SHE (vs. the standard hydrogen electrode), whereas molybdenite did not react even at 0.77 V/SHE [16]. Therefore, apparently, MxMo6S8 − compounds are easier to oxidize than molybdenite, which led us to the idea that conversion of molybdenite into an MxMo6S8 ternary sulfide might enable an improvement of the molybdenum leaching process. Molybdenite, with its layered structure is commonly used as a dry lubricant for various applications, including steel parts. However, at 700 °C it shows limited stability versus iron, forming FeS and a ternary sulfide, called Y-phase [17]. This information and the abundance and cheapness of iron inspired us to carry out a mineral phase reconstruction (MPR) process by converting molybdenite into a FexMo6S8 ternary sulfide thereby improving the molybdenum leaching process. 2. Materials and procedures 2.1. Materials Natural molybdenite was purchased from a domestic concentration plant. The MoS2 content was better than 99%. Nitric acid and sulfuric acid were purchased from Xilong Chemical Co., Ltd. of China. Iron powder reduced, Fe content was better than 98.5%, ws purchased from Xilong Chemical Co., Ltd. of China. 2.2. Transformation and characterization Iron powder reduced and natural molybdenite (with a certain mass ratio of Fe to MoS2) were mixed and then poured into a corundum crucible, which then was inserted into a larger corundum crucible. The surface of the fill was then covered with activated carbon. The crucible-incrucible device was put in a thermostatic muffle furnace and heated to a pre-set temperature. After a selected time it cooled down to room temperature by switching-off the furnace. The resultant was then crushed,

15

ground and sieved to a size range of 38 to 48 μm. The products were characterized by X-ray powder diffraction using Cu/Kα radiation in a D/Max 2500 Rigaku device, and analysed in a JSM-6360LV scanning Electron Microscope. 2.3. Comparative leaching and kinetic experiments Atmospheric comparative leaching was conducted with 2 g of sized and grain size range of 38 to 48 μm MPR product (Fe/MoS2 = 0.5 w%), or molybdenite with 400 mL 0.9 mol·L− 1 H2SO4–1.2 mol·L− 1 HNO3 mixed solution at 75 °C. Leaching media was contained in a 0.5 L three-necked flask. The reaction flask was heated in a thermostatic water bath. The sample was stirred at 300 rpm to avoid solution inhomogeneity. Aliquots (1 mL) of the solution were withdrawn at regular intervals and diluted to 25 mL for determining the Mo concentration using an inductively coupled plasma method (IRIS Intrepid II xsp, Thermo Electron Corporation, U.S.). The leach systems were filtrated after the reaction was completed, and the residues were dried and characterized by X-ray powder diffraction using Cu/Kα radiation in a D/Max 2500 Rigaku device. Operations of the kinetic experiments of MPR product were the same as comparative leaching experiments, but with different temperatures and concentrations of HNO3 and H2SO4. 3. Results and discussion 3.1. Characterization of the MPR product Fig. 1B depicts the XRD diagram of a MPR product obtained at a smelting temperature of 1100 °C, an [Fe] to [MoS2] mass ratio of 1:2 and a holding time of 3 h. The XRD diagram of molybdenite is shown in Fig. 1A for comparison. SEM images of the powders are presented in Fig. 1C and D. Obviously, Fe1.3Mo6S8 was the main phase of the MPR product. Comparing Fig. 1C and D, a complete transformation of the morphology of the molybdenum-bearing compound took place; the layered structure of molybdenite was no longer maintained after reaction. MPR product has a metallic luster, and it is extremely brittle. It is reported [18] that FexMo6S8 phase crystallizes in a rhombohedral lattice and can be

Fig. 1. XRD pattern of (A) molybdenite and (B) MPR product; SEM of (C) molybdenite and (D) MPR product. [Fe]/[MoS2] = 1:2, 1100 °C, 3 h.

16

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

Fig. 2. Leaching comparison of MPR product to molybdenite: [H2SO4] = 0.9 mol·L−1; [HNO3] = 1.2 mol·L−1; 75 °C.

Fig. 4. Effect of change to stirring rate on molybdenum extraction (1.0 mol·L−1 HNO3, 0.9 mol·L−1 H2SO4, temperature 70 °C).

described by a three-dimensional array of Mo6S8 units. Owing to the insertion of M, the Mo-S atoms form distorted octahedral clusters. It is calculated that hexagonal lattice parameters of ternary phases Fe1.3Mo6S8 are a = 9.56 Å, c = 10.27 Å, hexagonal volume of Fel.3Mo6S8 are 813.6 Å3.

residue of the molybdenite leaching (Fig. 3a), which is consistent with the laboratory results and commercial practice that the sulfur of molybdenite is only oxidized to soluble sulfate. The leaching of molybdenite in the media would occur through: MoS2 þ 6HNO3 ¼ 2H2 SO4 þ H2 MoO4 þ 6NO

3.2. Leachability comparison Aiming to evaluate the effect of the MPR treatment on molybdenum leaching quantitatively, comparative leaching experiments of the MPR product and molybdenite were conducted in H2SO4-HNO3 solutions. The data of Mo leaching fraction vs. time were plotted in Fig. 2. As shown in Fig. 2, the characteristic sigmoidal progress in reaction time denotes a typical of autocatalytic reaction path for both materials. The induction time of the reaction was reduced from 18.6 min for molybdenite to 11.3 min for the MPR product. After 40 min, Mo extraction of the MPR product reached 100%. After that time, only 40% of the molybdenum was extracted from molybdenite. This clear increase in reaction rate is also documented by the slope of the reaction curve, indicating about a 4-times faster extraction process for the MPR product (7.52) as compared to the molybdenite (1.96). The XRD patterns of leaching residue of molybdenite and the MPR product are shown in Fig. 3. Only unreacted MoS2 was detected in the

Fig. 3. XRD pattern of HNO3-H2SO4 leaching residue of: (a) molybdenite and (b) MPR product.

  H2 MoO4 þ 2H2 SO4 ¼ H2 MoO2 ðSO4 Þ2 þ 2H2 O In contrast (Fig. 3b), only elemental sulfur is obtained in the MPR leaching residue. This means that less oxidizing agent is needed for the leaching operation, and less sulfates is formed. The dissolution of MPR product can be represented by the following reactions: Fe1:3 Mo6 S8 þ 2:6Hþ þ 0:65O2 ¼ 1:3 Fe2þ þ Mo6 S8 þ 1:3H2 O 12HNO3 þMo6 S8 ¼ 6H2 MoO4 þ 8S0 þ 12NO   H2 MoO4 þ 2H2 SO4 ¼ H2 MoO2 ðSO4 Þ2 þ 2H2 O

Fig. 5. Effect of temperature on molybdenum extraction (stirring rate 300 rpm, 1.0 mol·L−1 HNO3, 0.9 mol·L−1 H2SO4).

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

17

Fig. 6. ln[α/(1 − α)] vs t diagrams plotted with the data displayed in Fig. 3. Fig. 7. Plot of the lnk vs 1/T × 1000.

After leaching Mo present in the pregnant leach liquor in the form of H2MoO4 and H2[MoO2(SO4)2]. The residue was filtered. Elemental sulfur in the residue can be collected by extraction with kerosene. The treatments of pregnant leach liquor (of either molybdenite or MPR product) include roughly the following processes: the solvent extraction is applied to separate and enrich Mo; the Mo in loaded organic solutions can be stripped by ammonia; the loaded strip liquor is acidified with HNO3 to precipitate ammonium molybdate.

Once the nitrous acid present in system the leaching is accelerated, but the amount of NO2 in early stage is negligible. Secondly, the main reactions concerning NO2 are listed as [22]: 2NO þ O2 ¼ 2NO2 NO2 þ e− ¼ NO− 2 þ NO− 2 þ H ¼ HNO2

3.3. Kinetic analysis 3.3.1. Influence of stirring rate Based on the result that the MPR product showed a better leaching performance during atmospheric leaching than molybdenite, the kinetics of the MPR leaching process was studied in detail, by varying the most crucial parameters during leaching in HNO3-H2SO4 solutions. At first, the influence of the stirring rate was investigated, varying the rate from 250 rpm to 350 rpm, up to 450 rpm (at 70 °C; solution containing 1.0 mol·L−1 HNO3, 0.9 mol·L−1 H2SO4). Results are shown in Fig. 4. As shown in Fig. 4, the stirring rate does not significantly influence the dissolution rate of the ternary sulfide in such a dilute leaching system. It can be inferred that the rate of mass transfer at the solid-liquid interface is not involved in the overall leaching rate. At higher pulp densities, applied in industry, the situation is perhaps different. But here, the stirring rate of 300 rpm was chosen for all further experiments. Additionally, the characteristic “S-shaped” trend of the curve shown in Fig. 5, denotes a radical autocatalytic reaction path [19,20]. According to the literature on the leaching of molybdenite [21], the catalysis is caused by NO or NO2, which are present at the particle surface of molybdenite. And the catalytic efficiency of NO2 is 5.5 times as large as NO [21]. V. Lutsik [22] observed that the autocatalytic reaction also occurred in dissolution of sulfides such as pyrite and sphalerite. We assume that leaching of this ternary sulfide shares the same mechanism of an autocatalytic action of NO2 or NO. It can be elaborated from two aspects: Firstly, the NO derived from the reduction of nitric acid will be oxidized to NO2 soon. When NO2 dissolves in water, nitric acid and another stronger oxidant nitrous acid (EΘ = 0.99V/SHE) will be generated.

HNO2 þ HNO3 ¼ H2 O þ 2NO2 In the catalytic reaction, NO2 acts as an electron transfer, by which nitric acid seizes the electrons from the reducing agent (mineral), triggering the leaching reaction. 3.3.2. Influence of temperature The influence of solution temperature on the dissolution of the MPR product was studied at temperatures of 65, 70, 75 and 80 °C in solutions containing initial 1.0 mol·L− 1 HNO3–0.9 mol·L−1 H2SO4. Results are shown in Fig. 5.

Table 1 Data from Fig. 6. T/K

338.15

343.15

348.15

353.15

k lnk

0.109 −2.212

0.148 −1.908

0.242 −1.417

0.332 −1.101

Fig. 8. Effect of nitric acid concentration on molybdenum extraction (stirring rate 300 rpm, 75 °C, 0.9 mol·L−1 H2SO4).

18

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

Fig. 9. ln[α/(1 − α)] vs t diagrams plotted with the data displayed in Fig. 7.

A general form rate expression of autocatalytic reaction can be represented as follows: ‐dmre =dt ¼ k0 mre þ k1 mre mpr ‐dmre =dt ¼ k0 mre þ

k1 mre m0re −mre

ð1Þ


ð2Þ

m0re,

where, mre are the mass of reactants at the start of reaction and at time t, mpr is the mass of product, k0, k1 is the spontaneous and heterogeneous catalysis rate constant. Separation of variables and integration of Eq. (2) yields Eq. (3),

Fig. 11. Effect of sulfuric acid concentration on molybdenum extraction (stirring rate 300 rpm, temperature 75 °C, 1.0 mol·L−1 HNO3).

As we know, reaction fraction α can be expressed as α = 1 −mre/m0re, so
ln ½α=ð1−α Þ þ ln m0re k1 =k0 ¼ k1 m0re t Eq. (7) is then rearranged to Eq. (8) ln ½α=ð1−α Þ þ B ¼ kt





ln m0re =k0 þ ln k0 þ k1 m0re −mre =mre ¼ k0 þ k1 m0re t

ð3Þ

where, B = ln (m0rek1/k0), k = k1m0re Arrhenius equation




ln m0re =k0  k0 þ k1 m0re −mre =mre ¼ k0 þ k1 m0re t

ð4Þ

k ¼ A0 cn expð−Ea =RT Þ





ln k1 m0re =k0  k0 =ðk1 mre Þ þ m0re −mre =mre ¼ k1 m0re þ k0 t ð5Þ In general, k1 and k1mre are much larger than k0, thus Eq. (5) reduces to Eq. (6),     ln m0re k1 =k0 þ ln m0re =mre −1 ¼ k1 m0re t

Fig. 10. Plot of the lnk vs ln[HNO3]0.

ð6Þ

ð7Þ

ð8Þ

ð9Þ

Take the logarithm of both sides of Eq. (9), Eq. (10) is obtained ln k ¼ lnA0 þ n ln c−Ea =RT

ð10Þ

where α is the reaction fraction, k is the apparent reaction rate constant (min−1), t is the reaction time (min), A0 is the pre-exponential factor, c is the concentration of leaching reactant (mol·L− 1), n is the

Fig. 12. ln[α/(1 − α)] vs t diagrams plotted with the data displayed in Fig. 9.

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

19

Seen from the following reactions, the stoichiometric mole of nitric acid is about 12 times of MPR product, implying that concentration of nitric acid should greatly affect leaching process. Fe1:3 Mo6 S8 þ 2:6Hþ þ 0:65O2 ¼ 1:3 Fe2þ þ Mo6 S8 þ 1:3H2 O 12HNO3 þMo6 S8 ¼ 6H2 MoO4 þ 8S0 þ 12NO The ln[α/(1− α)] vs t diagrams (Fig. 9) are plotted on the data presented in Fig. 8. These plots present straight lines with a good approximation; k values of corresponding nitric acid concentration can be calculated from the slopes of straights according to Eq. (8). Based on the calculated k values and in terms of Eq. (10), at 75 °C, reaction order with respect to concentration of HNO3 was calculated as 1.5 from plot of lnk vs ln [HNO3]0 (Fig. 10).

Fig. 13. Plot of the lnk vs ln[H2SO4]0.

reaction order, Ea is the apparent activation energy (kJ·mol−1), R is the universal gas constant (8.314 J·K−1·mol−1), T is the absolute temperature (K). The ln[α/(1− α)] vs t diagrams (Fig. 6) are plotted on the data given in Fig. 5. Clearly, these plots present straight lines with a good approximation. k values can be calculated from the slopes of straights by linear regression according to Eq. (8). They are listed in Table 1 for 65, 70, 75 and 80 °C. At 1.0 mol·L− 1 HNO3 − 0.9 mol·L− 1 H2SO4, in terms of Eq. (10), the apparent activation energy of the leaching reaction was determined from the slop of lnk vs 1/T plot (Fig. 7, data from Table 1) as 76 kJ·mol−1, which is a typical value for a process controlled by surface chemical reaction. It confirms that temperature has an important influence on molybdenum dissolution in the temperature range studied.

3.3.4. Influence of the initial sulfuric acid concentration The experiments were conducted using four different H2SO4 concentrations (0.6, 0.8, 1.0 and 1.2 mol·L−1) at 75 °C in 1.0 mol·L−1 HNO3. Results are summarized in Fig. 11. The results demonstrate that an increase in concentration of H2SO4 decreases the induction time of the autocatalytic action, but seemingly has insignificant influence on the significant influence on the leaching rate. As well, the ln[α/(1 − α)] vs t diagrams (Fig. 12) are plotted on the data given in Fig. 11. The plots present straight lines with a good approximation and k values can be calculated from the slopes of straights according to Eq. (8). Based on k values calculated and according to Eq. (10), at 75 °C, reaction order with respect to concentration of H2SO4 was calculated as 0.6 from plot of lnk vs ln [H2SO4]0 (Fig. 13). 3.3.5. Establishment of kinetic equation for the leaching process A kinetic equation can now be formed for the leaching process, based on Eq. (8) and an apparent activation energy of Ea 76 KJ·mol−1, as well as a reaction order of 1.5 (HNO3) and 0.6 (H2SO4), respectively. This results in Eq. (11). ln ½α=ð1−α Þ þ B ¼ A0  ½HNO3 01:5  ½H2 SO4 0:6 0  expð−76000=RT Þt

3.3.3. Influence of the initial nitric acid concentration The experiments were carried out at various initial HNO3 concentrations (0.6, 0.8, 1.0 and 1.2 mol·L−1) at 75 °C in 0.9 mol·L−1 H2SO4. Results were plotted in Fig. 8.

ð11Þ

A straight line, with slope of A0 and intercept of B, should be yielded in 0.6 the plots of ln[α/(1− α)] vs [HNO3]1.5 0 ×[H2SO4]0 × exp(−76000/RT)t. Although the points in the plot scatter a little (Fig. 14), a straight line can be fitted to the experimental data (from Figs. 5, 8 and 11) with a correlation coefficient over 0.97. A0 and B were calculated to be 4.96 × 1010 and 5.4 respectively. Now, the kinetic equation of MPR product of molybdenite leaching by nitric and sulfuric acid can be written as follows: 0:6 ln ½α=ð1−α Þ þ 5:4 ¼ 4:96  1010  ½HNO3 1:5 0  ½H2 SO4 0  expð−76000=RT Þt

ð12Þ

4. Conclusions

Fig. 14. Relationship between ln[α/(1 − α)] and ([HNO 3 ] 0 ) 1.5 × ([H 2 SO 4 ] 0 ) 0.6 × exp.(− 76,000/RT) t.

The results of this study show that molybdenite can be successfully reconstructed to a ternary sulfide (FexMo6S8) when reacted with iron at 1100 °C (MPR processing). Comparative leaching experiments in HNO3-H2SO4 solutions demonstrated that the Mo-leaching performance of the ternary sulfide is significantly improved as compared to molybdenite, and less oxidizing reagent is necessary for the leaching process. Based on experimental leaching data, an autocatalytic reaction kinetics was confirmed. Curves of leaching fraction vs. time showed a characteristic “S-shaped” trend with an induction time as one would expect from an autocatalytic reaction path. The apparent activation energy was calculated as 76 kJ·mol−1, indicating that leaching is under “chemical reaction control” and thus strongly dependent on temperature. A

20

F. Li et al. / Int. Journal of Refractory Metals and Hard Materials 62 (2017) 14–20

reaction order of 1.5 with respect to the HNO3 concentration revealed that the oxidant concentration is a critical factor in leaching, whereas the concentration of sulfuric acid showed less impact (0.6 order). Finally, an overall kinetic equation was established for the leaching process. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (key program, Project: 51334008) and Program for Changjiang Scholars. References [1] L.I. Hong-gui, A Handbook for Extractive Metallurgy of Nonferrous Metals-Rare Refractory Metal (one), Metallurgy Industry Press, Beijing, 1999 (in chinese). [2] R. Dorfler, J. Laferty, Review of molybdenum recovery process, JOM 33 (5) (1981) 48–54. [3] K. Premachandran, Kinetics of the Aqueous Oxidation of Molybdenite and the Role of Crystal Anisotropy on the Electrochemical Mechanisms of the Process, University of California, Berkeley, Ann Arbor, 1981. [4] S.W. Ching, A Study of the Leaching of Molybdenite in Water Chlorine Solutions, University of Utah; Department of Metallurgy and Metallurgical Engineering, 1987. [5] S. Km, R. Na, M. Shneerson Ya, L.A. Yu, B. Ga, A. Men'shikov Yu, P. Pn, K. Vp, Autoclave leaching of molybdenite concentrates with catalytic additives of nitric acid, Russ. Metall. 7 (2010) 588–595 (Metally). [6] K. Amin, Y. Hosein, M. Mahdi, S.S. Khatiboleslam, Kinetics of pressure oxidative leaching of molybdenite concentrate by nitric acid, Hydrometallurgy 111 (2012) 52–57. [7] I.H. Warren, D.M. Mounsey, Factors influencing the selective leaching of molybdenum with sodium hypochlorite from copper/molybdenum sulphide minerals, Hydrometallurgy 10 (3) (1983) 343–357. [8] C. ZhanFang, Z. Hong, Q. ZhaoHui, L. GuangYi, Z. WenXuan, A novel technology for molybdenum extraction from molybdenite concentrate, Hydrometallurgy 99 (1–2) (2009) 2–6.

[9] M.M. Antonijević, N.V. Pacović, Investigation of molybdenite oxidation by sodium dichromate, Miner. Eng. 5 (2) (1992) 223–233. [10] H. Hu, C. Qiyuan, Y. Zhoulan, Z. Pingmin, G. Guo, G. Günter, Structural change of mechanically activated molybdenite and the effect of mechanical activation on molybdenite, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 35 (6) (2004) 1203–1207. [11] L. Weiping, H. Xu, X. Yang, S. Xichang, Extraction of molybdenum from low-grade Ni–Mo ore in sodium hypochlorite solution under mechanical activation, Miner. Eng. 24 (14) (2011) 1580–1585. [12] J. Lijuan, Z. Hong, F. Jiangang, Electro-oxidation process and mechanism of molybdenite decomposition under ultrasonic effect, Rare Metals 27 (6) (2008) 566–570. [13] Y. Jb, V. Antonucci, Molybdenite concentrate cleaning by copper sulfation activated by microwave, Miner. Eng. 14 (11) (2001) 1411–1419. [14] Z. Zhongwei, L. He, Enlightenment of geochemistry on separation of molybdenum and tungsten, Chin. J. Nonferrous Metals 24 (6) (2014) 1637–1647 (in chinese). [15] S. Seghir, C. Boulanger, S. Diliberto, Selective transfer of cations between two electrolytes using the intercalation properties of Chevrel phases, Electrochim. Acta 55 (2010) 1097–1106. [16] Z. Zhongwei, Discussion on the theoretical problems from the leaching of molybdenite, Rare Metals and Cemented Carbides, 2, 1995, pp. 1–3 (in chinese). [17] G.H. Moh, High-temperature metal sulfide chemistry, Top. Curr. Chem. 76 (1978) 107–151. [18] L. Elena, A. Doron, Chevrel phases, MxMo6T8(M = Metals, T = S, Se, Te) as a structural chameleon: changes in the rhombohedral framework and triclinic distortion, Chem. Mater. 22 (12) (2010) 3678–3692. [19] A. Francesco, G. Roberto, T. Teresa, V. Andrea, P. Adolfo, Activity and resistance to leaching of Cu-based catalysts in the wet oxidation of phenol, Appl. Catal. B Environ. 45 (1) (2003) 51–62. [20] H. Stanko, K.U. Opara, O. Boris, S. Arico Antonino, K. Hasuck, CWO of phenol on two differently prepared CuO–CeO 2 catalysts, Appl. Catal. B Environ. 28 (2) (2000) 113–125. [21] K. Ag, K. On, P. Gl, K. Sv, P. On, K. Op, Molybdenum recovery from mineral raw materials by hydrometallurgical methods, Eur. J. Miner. Process. Environ. Prot. 2 (2) (2002) 82–93. [22] V. Lutsik, A. Sobolev, The investigation of the kinetics of hydrochemical oxidation of metal sulphides with the aim of determination of the optimal conditions for the selective extraction of molybdenum from ores, J. Min. Metall. 41 (B) (2005) 33–45.