J. Cent. South Univ. Technol. (2010) 17: 480-484
DOI: 10.1007/s11771-010-0510-8 
Electric-oxidation extraction of molybdenite concentrate in
alkaline NaCl electrolyte
CAO Zhan-fang(曹占芳), ZHONG Hong(钟宏), LIU Guang-yi(刘广义),
FU Jian-gang(符剑刚), WEN Zhen-qian(闻振乾),WANG Shuai(王帅)
School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract: The dissolution of molybdenite concentrate in NaCl electrolyte was investigated. The results show that the dissolution rate increases with the increase in liquid-to-solid ratio, stirring speed, NaCl concentration and temperature. When the liquid-to-solid ratio is 30:1, stirring speed is 400 r/min, concentration of NaCl is 4 mol/L at pH=9 and room temperature, the leaching efficiency of molybdenite concentrate will reach 99.5% in 240 min. Molybdenite concentrate cannot be electro-oxidized directly on the anode. The kinetic studies show that the dissolution of molybdenite concentrate is represented by shrinking core model with diffusion through a porous product layer of element sulfur, and the apparent activation energy for the dissolution reaction is 8.56 kJ/mol.
Key words: wet leaching; molybdenite; electric-oxidation; kinetics; dissolution
1 Introduction
The commercial route for the extraction of molybdenum from its sulphide mineral molybdenite involves roasting of the concentrate, purification of the resultant calcining, either by distillation of MoO3, or by a hydrometallurgical route, and, finally, hydrogen reduction of the trioxide to the metal. During roasting, much rhenium is lost due to volatilization and SO2 generated is a source of environmental pollution [1-2].
With the increasingly stringent environmental requirements, the traditional technology gradually shows its shortcomings. Hence, hydrometallurgical processing becomes more attractive especially for low grade and complex ores. Since 1970s, researches on molybdenite wet leaching have attracted more attention, and the technologies on the oxygen pressure process, nitric acid decomposition, sodium hypochlorite, electro-oxidation and bio-leaching methods have been developed one after another [3-9]. Cheap oxidants, air or pure oxygen, are consumed in oxygen pressure process and nitric acid oxidation. However, not only high temperature and high pressure are needed in this process but also high demands are required for chemical reactor. The leaching process is difficult to control, and there are certain security risks in the production [3]. Compared with the above two processes, NaClO method has moderate reacting conditions, low equipment investment, and is easy to control [10-11]. Electric-oxidation is actually the development of NaClO, which is to combine NaClO and oxidation of MoS2 in electrolysis bath, thus significantly reducing production cost [2].In this investigation, molybdenite concentrate oxidation in alkaline NaCl electrolyte as well as its kinetics analysis was focused on.
2 Experimental
The molybdenite concentrate (in mass fraction, Molybdenite 40.22%, S 31.40%, Cu 2.70%, P 0.04%, Fe 3.50%, As 0.10%, CaO 0.17%, SiO2, 4.35%, Sn 0.10%) was taken from Dexing Copper Mine, Jiangxi Province, China, of which the particle size ranges from 10 to 100 ?m, accounting for 80% with particle size less than 40 ?m. XRD pattern (Fig.1) of the ore shows that molybdenite is the major phase, and minor amounts of tugarinovite (MoO2) and molybdite (MoO3) are also present.
All chemicals used in the experiments were of analytical regent grades (Changsha, China) that were used directly as received from the manufacturer.
A certain amount of molybdenite, NaCl and some water were added into the slurry. Then, this pulp was put into self-made electrolytic bath. The septum-free electrolysis bath, dimensionally stable anode (DSA) and iron cathode were chosen, the electrode was fixed in bath by monopolar type, and the electrode space between each of the adjacent anode and cathode was 10 mm. The mechanical stirring is used for pulp dispersion [12].
Fig.1 XRD pattern of raw ore
The concentration of molybdenum was determined by colorimetry using thiocyanate. The concentrations of NaClO and NaClO3 were determined by titration using sodium thiosulfate and potassium dichromate, respectively.
3 Results and discussion
3.1 Anode reaction
Oxidation of molybdenite concentrate on the DSA was researched under the conditions that mass of ore sample, stirring speed, operating potential, pH, and leaching temperature were 5 g, 400 r/min, 3.0 V, 9, and room temperature, respectively. For comparison, the direct leaching in the water was also studied.
Fig.2 shows that, molybdenum dissolution rate in the electrolysis bath when cell voltage is 3.0 V and direct dissolution rate in the water are completely coincident. Molybdenum leached should be MoO3. Mo(Ⅵ) was not deoxidized in the cathode (MoO42-+4H2O+6e=Mo+ 8OH-, E0=-1.05 V) [12], so cathode reaction was not investigated.
Fig.2 Direct leaching and direct electric-oxidation leaching of molybdenite concentrate in water
3.2 Dissolution reactions
Under the conditions that concentration of NaCl, the liquid-to-solid ratio, stirring speed, operating potential, anode current density, pH, leaching temperature, and leaching time were 4 mol/L, 30:1, 400 r/min, 3.0 V, 800 A/m2, 9, room temperature, and 240 min, respectively, the contents of NaClO and NaClO3 were measured. The results are shown in Fig.3. Seen from Fig.3, as the time grows, the content of NaClO firstly increases and then decreases, and the content of ClO3- will gradually increase in the process of electrolysis when ore sample is not added into the electrolyte; if ore sample exists in the electrolyte, Cl- ions are mostly oxidized to NaClO, part of which are farther oxidized in the form of NaClO3 existing in the solution. In the electrolysis process, when pH is 9, NaClO plays the leading role of oxidant.
Fig.3 Contents of NaClO and NaClO3 under different conditions
NaCl electrolysis effectively resolved media recycling issues, and its principle is shown as follows:
2NaCl+2H2O=2NaOH+H2↑+Cl2↑ (1)
2NaOH+Cl2=NaClO+NaCl+H2O (2)
MoS2+6NaClO+4NaOH=
Na2MoO4+Na2SO4+S+6NaCl+2H2O (3)
6NaClO+6OH-=2NaClO3+4NaCl+3/2O2+3H2O (4)
3.3 Influence of liquid-to-solid ratio
Under the conditions that mass of ore sample, concentration of NaCl, stirring speed, operating potential, anode current density, pH, leaching temperature, and leaching time were 5 g, 4 mol/L, 400 r/min, 3.0 V, 800 A/m2, 9, room temperature, and 240 min, respectively, the impact of liquid-to-solid ratio on molybdenite leaching is shown in Fig.4.
Seen from Fig.4, with the increase in liquid-to-solid ratio, the leaching efficiency of molybdenite also gradually increases in molybdenite leaching process, which is advantageous to leaching, because during the oxidation, increasing liquid-to-solid ratio is good for reactive medium diffusion, making the reaction more efficient; but when liquid-to-solid ratio comes to 30:1, increasing the ratio is not helpful for leaching efficiency, and the molybdenite leaching efficiency reaches 99.5% in 240 min. So liquid-to-solid ratio of 30:1 is selected.
Fig.4 Influence of liquid-to-solid ratio on molybdenite leaching efficiency
3.4 Influence of stirring speed
Under the conditions that mass of ore sample, concentration of NaCl, the liquid-to-solid ratio, operating potential, anode current density, pH, and leaching time were 5 g, 4 mol/L, 30:1, 3.0 V, 800 A/m2, 9, and 240 min, respectively, the effect of stirring speed on molybdenite leaching was investigated in the range of 200-500 r/min. The results in Fig.5 show no significant difference in molybdenite extraction when stirring speed is equal to or higher than 400 r/min.
Fig.5 Influence of stirring speed on molybdenite leaching efficiency
Seen from Fig.5, agitation has slight effect on integrated leaching efficiency, as lower as 10%. Therefore, leaching steps are controlled by solid film diffusion rather than fluid film diffusion. In investigations of the effect of other parameters, 400 r/min is selected as the optimum operating stirring speed.
3.5 Influence of concentration of NaCl
Under the conditions that mass of ore sample, liquid-to-solid ratio, stirring speed, operating potential, anode current density, pH, leaching temperature, and leaching time were 5 g, 30:1, 400 r/min, 3.0 V, 800 A/m2, 9, room temperature, and 240 min, respectively, the effect of concentration of NaCl on molybdenite leaching was investigated. When the concentration of NaCl changes from 2 to 6 mol/L, it can also obviously impact leaching speed (Fig.6), and molybdenite leaching efficiency increases with increasing NaCl concentration. No significant difference exists in molybdenite extraction when NaCl concentration is equal to or higher than 4 mol/L. In investigations of the effect of other parameters, 4 mol/L is selected as concentration of NaCl.
Fig.6 Influence of concentration of NaCl on molybdenite leaching efficiency
3.6 Influence of temperature
The effect of temperature on molybdenite dissolution is given in Fig.7. Temperature cannot obviously impact the leaching efficiency, and reaction heat is produced in the process of electrolysis and oxidation. The temperature of electrolyte can reach 42 ℃, so the optional temperature is chosen as room temperature.
3.7 Kinetic analysis
In hydrometallurgy, most of reactions are non- reversible, and the slowest stage controls overall speed of process. In order to determine the rate-controlling step of molybdenite leaching, in heterogeneous solid-liquid reactions, the soluble reactants diffuse across the interface and/or through the porous solid layer. Afterwards, chemical reactions occur. The reaction rate is controlled either by the diffusion of reactant through the solution boundary layer, or through a solid product layer, or by the chemical reaction rate on the surface of the core of unreacted particles.
Fig.7 Influence of temperature on molybdenite leaching efficiency
.The simplified equation of the shrinking core model when diffusion is the slowest step can be expressed as follows [13-16]:
1-2/3a-(1-a)2/3=kt (5)
where k is the leaching rate constant, a is the leaching rate, and t is the time.
After the experimental data are analyzed, it is determined that the shrinking core model is suitable for this leaching reaction, and the diffusion rate of the reactants through the particle pore alone is rate limiting. Eq.(5) reveals that if the diffusion through the product layer controls the leaching rate, there must be a linear relationship between the left side of the equation and time. The slope of the line is the apparent rate constant k. After electrolysis for 30 min, enough oxidant is produced. Suppose that the oxidant concentration is steady, 2 g ore sample is added to make sure that leaching kinetics analysis is processed under conditions that concentration of NaCl, stirring speed, operating potential, anode current density, and pH are 4 mol/L, 400 r/min, 3.0 V, 800 A/m2, and 9, respectively, and the effect of leaching temperature on molybdenite leaching was investigated in the range of 20-60 ℃. The results are shown in Fig.7. Leaching rate of molybdenite increases with the time; apparent rate constants k obtained for both equations and the correlation coefficients were calculated from the plots at each temperature in Fig.7. The results obtained by Eq.(5) are given in Fig.8. As a porous elemental sulphur layer occurs on the molybdenite surface during the process of electrolysis in chloride media, a similar situation should be valid for the application of Eq.(5). Arrhenius plot is obtained (Fig.9), molybdenite leaching process is controlled by diffusion control, and its relation can be expressed as k= Aexp(-Ea/(RT)). In this formula, A is the frequency factor, Ea is the apparent activation energy, R is the ideal gas constant, and T is the thermodynamic temperature. The apparent activation energy is calculated as 8.56 kJ/mol. This value clearly confirms that the process is controlled by diffusion. According to the above kinetic analysis, it can be judged that the leaching process is mainly controlled by solid diffusion, part of which is possibly controlled by liquid film, because of the interruptive role played by reticulation electrode of both sides. Agitation plays a weak role in diffusion mass transfer when stirring speed is equal to or higher than 400 r/min.
Fig.8 Plot of Eq.(5) vs t for different temperatures
Fig.9 Arrhenius plot for molybdenite electric-oxidation leaching
4 Conclusions
(1) Under conditions of the liquid-to-solid ratio of 30:1, stirring speed of 400 r/min, pH=9, room temperature, concentration of NaCl of 4 mol/L, the leaching efficiency of molybdenite can achieve 99.5% in 240 min.
(2) Molybdenite concentrate cannot be electro- oxidized directly on the anode.
(3) A shrinking core model is presented to describe the dissolution and to analyze the data. The leaching process is mainly controlled by the diffusion through an element sulfur layer, and the apparent activation energy of this dissolution process is 8.56 kJ/mol.
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Foundation item: Project(2007BAB22B01) supported by the 11th Five-Year Plan of National Science and Technology of China; Project(50704036) supported by the National Natural Science Foundation of China
Received date: 2009-06-04; Accepted date: 2009-11-26
Corresponding author: ZHONG Hong, PhD, Professor; Tel: +86-731-88836603; E-mail: zhong@mail.csu.edu.cn
(Edited by CHEN Wei-ping)
