Apparatus for electrolysis of molten oxides

Abstract

The invention provides improved electrodes for electrolytic cells operating with molten salt electrolytes. Nonconsumable iridium-based anodes of the invention facilitate the release of gaseous oxygen from oxide-containing melts, for example in the electro-chemical production of liquid or gaseous reactive metals from oxides. Cathode substrates of the invention are constructed of a tungsten-based alloy and enable deposition of an overlying liquid-metal cathode. Incorporation of the anode and cathode substrate of the invention into molten-oxide cells establishes a novel method for electrolytic extraction of titanium and other reactive metals.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]This invention relates to electrolytic cells for the production of metals from molten electrolytes. In particular, this invention provides novel anodes and cathode substrates for use with oxide melts and liquid or gaseous metal products. More particularly, an electrochemical technique for deposition of liquid titanium from titanium dioxide is described.[0003]2. Background Information[0004]Electrolytic processes acting on molten salt electrolytes have been used to produce several important metals, examples of which are given in U.S. Pat. No. 5,185,068, the entire disclosure of which is incorporated by reference.[0005]Electrolytic reduction in extractive metallurgy has seen its greatest commercial success in the production of aluminum. The invention of the Hall-Héroult process in the 1880s gave birth to a new industry and transformed aluminum from a precious metal into a ubiquitous material, commonplace in construction, t...

AI Technical Summary

Benefits of technology

[0025]Another object of the invention is metal smelting with significantly reduced emission of greenhouse gases.

[0041]Incorporation of the anode and cathode substrate of the invention into an electrolysis cell containing molten-oxide electrolyte establishes a novel method for electrolytic extraction of reactive metals. In a preferred embodiment, the iridium-based anode and tungsten-based cathode substrate co-operate in a cell for optimum production of liquid titanium metal directly from an oxide feedstock. Titanium production according to the invention proceeds without release of greenhouse gases: Because the metal is reduced directly from oxide, preparatory processing is much cleaner and simpler. Since the invention deposits titanium in liquid form, subsequent treatment is far easier than in current methods.

Problems solved by technology

However, significant problems also attend the use of carbon anodes, related to their preparation and to their consumption during aluminum smelting.

Arranging these materials for treatment exposes workers to injurious carbon dust.

Consequently, anode fabricators must undertake expensive filtering, collecting and treatment operations.

Furthermore, anode outgassing generally persists throughout the active lifetime of the electrode.

Moreover, interactions between the anode and the molten electrolyte during cell operation consume the anode.

Anode consumption is problematic for several reasons.

First, it makes cell operation more difficult.

It is difficult to maintain uniform anode current loading during operation due to the continuously changing topology of the electrolyte-anode interface.

Anode changes are labor intensive and disturb the thermal balance and electrical current distribution in the cell.

Despite substantial effort, no fully satisfactory inert anode has been identified.

While an inert anode has been viewed as a highly desirable target, its discovery would provide an enhancement to an already-workable system—not a pre-requisite for a viable process.

The titanium sponge product is contaminated with excess magnesium and magnesium chloride.

Although the FFC process moves in the right direction by obviating chlorination of titanium dioxide it is nonetheless environmentally suspect due to the halide electrolyte.

Also, the FFC process has not become economically viable owing to the long times required to remove all the oxygen from the cathode, solid-state diffusion being extremely slow.

However, such high temperatures and highly corrosive conditions constitute a set of design constraints even more stringent than those for an inert anode for the comparatively benign physicochemical climate of the Hall-Héroult cell.

Even if the standard noble metals—gold, silver and platinum—were not conventionally regarded as prohibitively expensive for large-scale industrial applications, they would not be suitable candidates for use in a liquid titanium cell owing to either their relatively low melting temperatures (silver and gold) as compared with that of titanium or, in the case of platinum, to its lack of structural integrity.

Beyond designating an inert anode, the design of apparatus for electrolytic extraction from oxide media poses many challenges not faced in the Hall-Héroult cell.

Considering titanium again as an illustrative example, materials that may be serviceable at aluminum's relatively low melting temperature may well lack structural integrity at the higher temperatures—exceeding 1700° C.—required for deposition of titanium metal in liquid form.

The carbon substrate of the Hall-Héroult cell is not compatible with reactive metals such as titanium in this instance because these metals react, to an undesirable extent, with carbon to form carbides.

If the electronic conductance of this element is too low, metal deposition at a commercially acceptable rate will be achieved only by application of a large voltage, which in turn will translate into unacceptably high electric power cost.

However, RHMs have other properties that detract from their suitability for cathode support in electrolytic cells: RHMs are expensive and also mechanically brittle and therefore difficult to shape.

To date, no material has been shown to meet the performance and practical requirements for a cathode substrate in a cell containing an oxide melt at temperatures exceeding 1700° C.

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