A Spectroscopic and Electrochemical Study of Molybdenum(IV) and Tungsten(IV) Species in Alkali Chloride Melts
Authors: Ivanov Alexander B. , Volkovich Vladimir A. , Pukhov Mikhail A. , Podlasova Kristina O. , Poskryakov Dmitry A. , Vasin Boris D. , Griffiths Trevor R. 
Published: February 2020

Alkali chloride based melts can be employed as the working media for electrowinning and electrorefining refractory metals, including molybdenum and tungsten. In addition, chloride melts are suitable for manufacturing various items of these metals and recycling molybdenum and tungsten containing scrap. Knowing electrochemical properties and speciation of the metals of interest in the molten electrolyte is essential for designing a feasible process.

Molybdenum and tungsten are the polyvalent metals and follow the general trend of increasing the lowest stable oxidation state in the high temperature chloride ionic media as one moves down the group from 3d to 5d transition metal. In the group 6 of the periodic table the lowest oxidation state of chromium in fused chlorides is +2, and this increases to +3 for molybdenum and +4 for tungsten.

The oxidation state +4 is unusual for molybdenum in molten chlorides and the aim of the present study was finding the conditions of Mo(IV) species formation, determining their stability and comparing molybdenum(IV) and tungsten(IV) species behavior and electrochemical properties.

The experiments were performed between 500–900 oC in the melts based on LiCl–KCl–CsCl, NaCl–KCl–CsCl and NaCl–CsCl eutectic mixtures.

Anodic dissolution of metallic tungsten results in the formation of predominantly W(IV) ions. Increasing temperature causes slight increase of the average oxidation state of tungsten in the melt. The electrode potentials of tungsten were determined by potentiometry and the speciation characterized by electronic absorption spectroscopy measurements. Prolonged holding of tungsten-containing chloride melts in contact with silica results in partial oxidation of W(IV) ions.

Red-ox processes involving Mo(III) and Mo(IV) ions were studied by cyclic voltammetry and potentiometry combined with in situ electronic absorption spectroscopy measurements. Anodic oxidation of Mo(III) chloro-ions on a glassy carbon electrode results in formation of Mo(IV) species. Mo(III)-Mo(IV) oxidation-reduction is reversible. Molybdenum(IV) chloro-species, MoCl6
2– are stable at 550-600 oC. At higher temperatures a disproportionation reaction occurs:

2 MoCl6
2– → MoCl6
3– + MoCl6
–.

Molybdenum(V) ions decompose and molybdenum pentachloride sublimes from the melt:

2 MoCl6
– → MoCl5↑ + Cl–.

Reactions of molybdenum and tungsten dioxides with hydrogen chloride in fused alkali chlorides were also investigated by in situ spectroscopy measurements. Depending on temperature and cationic composition of the melt the following reactions take place in the case of molybdenum:

MoO2 + 4 HCl + 2 Cl– → MoCl6
2– + H2O

MoO2 + 2 HCl → MoOCl2↑ + H2O

Decreasing temperature or increasing mean alkali cation radius favors MoCl6
2–formation.

Reaction of MoO2with HCl is a convenient method of preparing Mo(IV) containing chloride melts.

Molybdenum and tungsten dioxides can be prepared by hydrogen reduction of the respective trioxides and the reduction processes were also studied in the course of the present work.

The authors are grateful to Dr. A.V. Chukin for the help with XRD measurements.