Electrochemical Method and Apparatus For Removing Oxygen From a Compound or Metal

Abstract

A cathode comprising an oxygen-containing compound, or a metal containing dissolved oxygen, is arranged in contact with a melt comprising a hydroxide of an alkali metal. An inert anode, advantageously comprising nickel, is also arranged in contact with the melt and a potential is applied between the anode and the cathode such that oxygen is removed from the compound or the metal.

Description

[0001]The invention relates to a method and an apparatus for removing oxygen from a compound, comprising a metal and oxygen, or from a metal or alloy, by an electrolytic process.[0002]The electro-decomposition or electro-reduction of solid compounds is described in, for example, International Patent Application PCT / GB99 / 01781 and “Extraction of titanium from solid titanium dioxide in molten salts” by D. J. Fray and G. Z. Chen, TMS (The Minerals, Metals and Materials Society) 2004, pp. 9-17, which are incorporated herein by reference in their entirety. In this process, also known as the FFC process, a solid compound between a metal (which term is used in this document to include both metals and semi-metals) and another substance (for example, oxygen) is electrolytically reduced. A cathode comprising or contacting the solid compound is immersed in or contacted with a fused salt and a voltage is applied between the cathode and an anode, which also contacts the salt, such that the subst...

AI Technical Summary

Benefits of technology

[0009]Preferably the melt further comprises an oxide of the alkali metal and this oxide is particularly preferably soluble in the hydroxide. A melt comprising an alkali metal oxide, which is dissolved in an alkali metal hydroxide, is a melt system that may advantageously support oxygen ion conductivity. Good oxygen ion conductivity may be advantageous in efficiently transferring oxygen from the solid compound at the cathode through the melt to be discharged at the anode.

[0012]Although the solid compound may be any solid compound comprising a metal and oxygen that is less stable than an oxide of the alkali metal, the invention may be particularly advantageous when used to reduce oxides of low stability such as an iron oxide or an oxide of cobalt, nickel, copper, zinc or lead, or when used to remove oxygen from metals or alloys comprising such metals.

[0016]Advantageously, the melting point of sodium hydroxide may be decreased with the addition of a small amount of sodium iodide or sodium bromide. This may allow electro-decomposition to proceed at even lower temperatures and so may reduce the energy needed to maintain the melt at an operating temperature and may reduce problems of corrosion in the electro-decomposition apparatus.

[0022]As inert anodes do not react during electro-decomposition, they are not consumed by the process or are consumed at an advantageously slow rate. This may allow for longer running times for a cell implementing the process, simpler cell design and lower overall anode costs. In addition, the melt and the product metal may not be contaminated by material from the anode, which may increase the working life of the melt and may reduce the number of post-processing steps required for the product.

[0024]If the alkali metal were to be produced in metallic form at the cathode and / or to dissolve in the melt, electronic conductivity of the melt may increase. This may disadvantageously reduce the electrical efficiency of the process. The process is advantageously run under conditions such that alkali metal from the hydroxide does not continuously deposit as a metal at the cathode. The melt may contain more than one species of alkali metal in which case the potential applied between the anode and the cathode is preferably not sufficient for any alkali metal that is present in the melt to deposit continuously as a metal at the cathode. An absence of dissolved alkali metal in the melt is likely to advantageously reduce or substantially eliminate electronic conductivity of the melt.

[0025]During electro-decomposition in a hydroxide melt, it is thought that hydrogen may potentially evolve on the cathode at a lower cathode potential than may be required for the reduction of alkali metal ions present in the melt to alkali metal. Any such alkali metal produced at the cathode could potentially dissolve in the melt and may disadvantageously change the properties of the melt, for example by increasing its electronic conductivity. The preferential evolution of hydrogen may advantageously prevent alkali metal ion reduction.

Problems solved by technology

A problem arises, however, in providing an anode material which is sufficiently inert in a CaCl2-based melt.

At present, there is no suitable material that can be used as an inert anode in chloride melts such as calcium chloride and, therefore, a carbon anode is generally employed.

Furthermore, the carbon anode is consumed and the product of the electro-decomposition may become contaminated with carbon.

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