Among sulfide minerals, molybdenite is a relatively stable mineral, and other sulfides are generally susceptible to oxidation. According HH Playa Corzine reported sequential oxidation rate of sulfide minerals: pyrrhotite> galena> brass ore> pyrite> molybdenite. Therefore, in the long-term accumulation of ore in the plant, the oxidation rate of molybdenum ore is much lower than that of copper, lead, zinc and iron.
Molybdenum ore can be slowly oxidized under a weak oxidizing atmosphere at normal temperature and normal pressure. Its oxidation mainly occurs on the "edges" of polarity, while the non-polar "faces" are rarely oxidized.
The atomic arrangement of the "edge"-S-Mo-SS-Mo-S- is also only present in the vacuum. In air, water, steam or aqueous media, it will react with oxygen in the medium and is preferably oxidized to MoO 2 2+ :
MoS 2 + | 1 | O 2 + H 2 O | → | MoO 2 ·SO 4 + H 2 SO 4 |
2 |
A series of oxidized intermediates are formed on the surface of molybdenite: MoO 2 , MoO 2.5~3 , MoO 3 . The most important one is MoO 2.5~3 .
The state of the molybdenum compound varies with the oxidation-reduction potential (Eh) and alkalinity (pH) in the medium. State diagram of the ore-forming conditions (with Fe 3+ present) at normal temperature (25 ° C) and atmospheric pressure (0.1 MPa) (see Figure 1). Chand also introduced a state diagram of molybdenum ore with Eh-pH in an aqueous medium with Na 2 MoO 4 (see Figure 2).
Figure 1 Eh-PH diagram of Mo-Fe-SH 2 O system
Figure 2 Molybdenum changes with Eh-PH state in aqueous medium at 25 ° C, 0.1 MPa
It can be seen from the figure that between the lower limit of water stability (Eh 0 = 1.23V) (Eh 0 =0V), with the change of Eh and PH, molybdenum MoS 2 , MoO 2 2+ , HMoO 4 - , MoO 4 2- A variety of phases. According to JA Hansal (see Figure 1), the pH of HMoO 4 - , sapphire or hydromolybdenum ore converted to MoO 6 2- ion is 6.2.
Whether it is JA Hansal's geochemical learning from molybdenum, or S. Chander's state diagram from the molybdenite in the water medium, they are consistent with the inferences of S. Chand and DW Fuerstinau. : Molybdenum ore has a thiomolybdate charged surface consistent in the aqueous medium.
[MoS x O y ] -n + H+ â†â†’ [MoS x O y H] -n+1
or
MoO 4 2- (mineral surface) + H + (solution) â†â†’ HMoO 4 - (mineral surface), pK=5.95
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