EP2238230A1

EP2238230A1 - Process

Info

Publication number: EP2238230A1
Application number: EP09706295A
Authority: EP
Inventors: Abhishek Lahiri; Xiaobing Yang; Animesh Jha
Original assignee: University of Leeds
Current assignee: University of Leeds
Priority date: 2008-01-31
Filing date: 2009-01-26
Publication date: 2010-10-13
Also published as: TW200946718A; AR070848A1; US20110083969A1; GB0807687D0; WO2009095660A1; GB0812098D0; GB0801791D0

Abstract

The present invention relates to a process for electrochemical extraction of a metal (M) from a metal (M) oxide, to a conducting electrode and to an electrolytic cell comprising the conducting electrode.

Description

Process

In recent years, the direct electrochemical reduction of TiO₂ to Ti metal in molten CaCl₂ has stimulated significant scientific and industrial interest (see for example G. Z. Chen, et al, Nature 407 (6802), 361 (2000); R. O. Suzuki and K. Ono, in Molten Salts XiH, edited by P. C. Trulove, H. C. DeLong, R. A. Mantz et al. (2002), Vol. 2002, pp. 810; T. H. Okabe et al, Journal of Alloys and Compounds 364 (1-2), 156 (2004); S. L. Wang and Y. J. Li, Journal of Electroanalytical Chemistry 571 (1), 37 (2004); and T. Nohira et al, Nature Materials 2 (6), 397 (2003)). However the formation of a stable perovskite phase in the intermediate stage of reduction hinders the diffusion of O^2" ions forming a layered structure in the pellet which slows the overall kinetics (D. T. L. Alexander et al, Acta Materialia 54 (11), 2933 (2006) and K. Jiang et al., Angewandte Chemie-International Edition 45 (3), 428 (2006)).

The conventional FFC process is of the type disclosed in WO-A-99/64638 for the formation of Ti metal from a TiO₂ pellet cathode using a carbon rod anode in a molten bath of CaCl₂ at 900⁰C at a constant voltage of 3. IV in an argon atmosphere. The FFC process involves several intermediate steps one of which includes the formation of stable perovskite phases (Alexander [supra] and C. Schwandt and D. J. Fray, Electrochimica Acta 51 (1), 66 (2005)). The formation of perovskite not only reduces the diffusion of O^2" ions but also due to larger grain size reduces the pore diffusion of CaCl₂ in the pellet. Although Jiang [supra] and R. Lilia Centeno-Sanchez et al, Journal of materials science 42, 7494 (2007) showed that an increase in porosity could be achieved by adding carbon and polyethylene precursors to the pellet or by directly starting from perovskite, the process still takes more than 24 hours to produce Ti with 3000ppm by weight of oxygen. It is evident that diffusion of O^2" ions is one of the limiting steps in the overall reduction of oxides in the FFC process. The low rate of reduction hinders the process being scaled-up and full metallisation is difficult to attain even with a small pellet. These limitations are a barrier to a continuous process and render the FFC process solely a batch process. The present invention is based on the recognition that the presence of an alkali metal oxide (or a salt from which an alkali metal oxide can be derived) serves to increase the rate of electrochemical reduction of a metal oxide in an oxygen- dissolving molten electrolyte.

Viewed from a first aspect the present invention provides a process for electrochemical extraction of a metal (M) from a metal (M) oxide comprising: applying a voltage between a cathode comprising (or consisting essentially of) or in contact with the metal (M) oxide and an anode in an oxygen-dissolving molten electrolyte in the presence of an alkali metal (M^a) oxide whereby to form an alkali metal (M^a) metallate (M) phase.

In accordance with the process of the invention, alkali metal (M^a) ions improve the diffusivity of oxygen by forming the alkali metal (M^a) metallate (M) phase. By way of example, where the alkali (M^a) metal oxide is potassium oxide and the metal (M) oxide is TiO₂, TiO₂ is reduced to nearly 100% Ti metal with 1350ppm of oxygen in less than 20 hours. The presence of a potassium titanate (K₄TiO₄) liquid phase provides an efficient O^2' ion transport medium which substantially shortens the Ti production time. This opens up the possibility of continuous Ti production at lower cost and therefore the more widespread exploitation of Ti in consumer products.

The alkali metal (M^a) oxide may be a caesium, rubidium, lithium, sodium or potassium oxide. Preferably the alkali metal (M^a) oxide is lithium, sodium or potassium oxide. Particularly preferably the alkali metal (M^a) oxide is potassium oxide.

The alkali metal (M^a) oxide may be an additive or may be formed in situ by decomposition of a decomposable alkali metal (M^a) salt into the alkali metal (M^a) oxide. Preferably the alkali metal (M^a) oxide forms the alkali metal (M^a) metallate (M) phase from a reaction of the alkali metal (M^a) oxide with a metal (M") metallate (M) phase. Particularly preferably the metal (M") metallate (M) phase is a solid phase. Particularly preferably the metal (M") metallate (M) phase is a perovskite (or perovskite-type) phase. Preferably M' ' is an alkaline earth metal, particularly preferably Ca, Sr or Ba, most preferably Ca.

Preferably the diffusivity of oxygen in the alkali metal (M^a) metallate (M) phase is higher than the diffusivity of oxygen in the metal (M") metallate (M) phase.

Preferably the alkali metal (M^a) metallate (M) phase is a liquid. Preferably the alkali metal (M^a) metallate (M) phase is a transitional phase.

In a preferred embodiment, the alkali metal (M^a) oxide is an additive. The alkali metal (M^a) oxide may be added (eg in the form of a powder) to the oxygen- dissolving molten electrolyte.

Preferably the alkali metal (M^a) oxide is in admixture with the metal (M) oxide in or in contact with the cathode. The mixture of alkali metal (M^a) oxide and metal (M) oxide may be solid or liquid (eg molten).

Preferably the process of the invention further comprises: mixing the alkali metal (M^a) oxide and the metal (M) oxide. Particularly preferably the process of the invention further comprises: forming the mixture of alkali metal (M^a) oxide and metal (M) oxide into a self-supporting mixture (eg a pellet, slab, sheet, wire, foil, basket or tube). The forming step may be pressing. The self-supporting mixture may be the cathode or may be contactable with the cathode. Preferably the self-supporting mixture is a pellet. The mixing step may be followed by heat treating the mixture.

The alkali metal (M^a) oxide may be present in the self-supporting mixture in an amount in excess of a trace amount, preferably in excess of 5wt%, particularly preferably in excess of 10wt%, more preferably in excess of 20wt%. Preferably the alkali metal (M^a) oxide is present in the self-supporting mixture in an amount in the range 10-70wt%, particularly preferably 20-50wt%.

In a preferred embodiment, the alkali metal (M^a) oxide is formed in situ by decomposition of a decomposable alkali metal (M^a) salt. The decomposable alkali metal (M^a) salt may be thermally decomposable. The decomposable alkali metal (M^a) salt may be added (eg in the form of a powder) to the oxygen-dissolving molten electrolyte

Preferably the decomposable alkali metal (M^a) salt is in admixture with the metal (M) oxide in or in contact with the cathode.

Preferably the process of the invention further comprises: mixing the decomposable alkali metal (M^a) salt and the metal (M) oxide. Particularly preferably the process of the invention further comprises: forming the mixture of decomposable alkali metal (M^a) salt and metal (M) oxide into a self-supporting mixture (eg a pellet, slab, sheet, wire, basket, foil or tube). The forming step may be pressing. The self- supporting mixture may be the cathode or may be contactable with the cathode. Preferably the self-supporting mixture is a pellet. The mixing step may be followed by heat treating the mixture.

The decomposable alkali metal (M^a) salt may be present in the self-supporting mixture in an amount in excess of a trace amount, preferably in excess of 5wt%, particularly preferably in excess of 10wt%, more preferably in excess of 20wt%. Preferably the decomposable alkali metal (M^a) salt is present in the self-supporting mixture in an amount in the range 10-70wt%, particularly preferably 20-5 Owt%.

Preferably the decomposable alkali metal (M^a) salt is decomposable into one or more gaseous species. The gaseous species may be selected from the group consisting of water and carbon dioxide. Decomposition of the alkali metal (M^a) salt into one or more gaseous species may advantageously promote electrochemical reduction by forming porosity within the cathode. Continuous formation of pores permits fast transport of molten electrolyte species (eg CaO and CaCl₂) which accelerates chemical reduction.

The decomposable alkali metal (M^a) salt may be an alkali metal (M^a) halide, carbonate, bicarbonate, hydrogen sulphide, hydrogen sulphate, nitrate, chlorate or sulphate. Preferably the decomposable alkali metal (M^a) salt is an alkali metal (M^a) bicarbonate. The decomposable alkali metal (M^a) salt may be a caesium, rubidium, lithium, sodium or potassium salt. Preferably the decomposable alkali metal (M^a) salt is a lithium, sodium or potassium salt. Particularly preferably the decomposable alkali metal (M^a) salt is a potassium salt, more preferably KCl.

The metal (M) may be a reactive metal element, semi-metal element, metal alloy or metalloid element.

In a preferred embodiment, the metal (M) forms a solid perovskite (or perovskite-type) phase in the oxygen-dissolving molten electrolyte. The solid perovskite phase may be an alkaline earth metal {eg Ca) metallate (M) phase.

The metal (M) may be one or more metals selected from the group consisting of group IIA metals, group IIIA metals, group IVA metals, group B transition metals, rare earth metals and alloys thereof. Preferably the metal (M) is one or more metals selected from the group consisting of Mg, Al, Si, Ge, group IVB transition metals, group VB transition metals, group VIB transition metals, group VIIB transition metals, group VIIIB transition metals, lanthanides, actinides and alloys thereof. Particularly preferably the metal (M) is one or more metals selected from the group consisting of group IVB transition metals, group VB transition metals, group VIB transition metals, group VIIIB transition metals, actinides and alloys thereof. Especially preferably the metal (M) is one or more metals selected from the group consisting of Ti, Nb, Ta, U, Th, Cr, Fe, steel and Zr. More especially preferred is one or more metals selected from the group consisting of Ti, Nb, Ta and Zr. Most preferred is Ti.

The alkali metal (M^a) metallate (M) phase may be M^a ₂MO₃ or M^a ₄MO₄. Preferred is M^a ₄MO₄. For example, where M is titanium, the preferred phase is M^a ₄Ti0₄.

The metal (M) oxide may be the cathode or the metal (M) oxide in admixture with either the alkali metal (M^a) oxide or the alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide may be the cathode. Preferably the metal (M) oxide in admixture with either the alkali metal (M^a) oxide or the alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide is the cathode.

Alternatively the metal (M) oxide may be in contact with a cathode. In this embodiment, the metal (M) oxide may be self-supporting (eg in the form of a pellet) and the cathode may be a bath, crucible or basket (eg a perforated basket). Alternatively the metal (M) oxide may be in admixture with the alkali metal (M^a) oxide or the alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide. The mixture may be a self-supporting mixture (eg in the form of a pellet or a perforated basket) or a molten mixture. Where the mixture is a molten mixture, the cathode is preferably a crucible. The crucible may be composed of a metal such as titanium or a titanium alloy and this embodiment advantageously prevents contamination of the oxygen-dissolving molten electrolyte.

Alternatively the metal (M) oxide may be in the oxygen-dissolving molten electrolyte in contact with the cathode. The alkali metal (M^a) oxide or the alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide may be in the oxygen- dissolving molten electrolyte in contact with the cathode. The cathode may be a metal substrate such as steel which may be in the form of a cathodic bath, crucible, basket or one or more pellets.

The oxygen-dissolving molten electrolyte may be (or contain) a compound of an alkaline earth metal (eg Ca, Sr or Ba), Li, Cs or Y (or a mixture thereof). Preferably the oxygen-dissolving molten electrolyte is a compound of Ca. The oxygen-dissolving molten electrolyte may be (or contain) a halide. Preferably the molten electrolyte contains (eg consists essentially of) CaCl₂. Particularly preferably the molten electrolyte contains CaCl₂ and an alkali metal halide (preferably a chloride). Preferred is a mixture Of CaCl₂ and KCl or of CaCl₂ and LiCl.

The anode may be carbon (eg graphite).

Typically in the process of the invention the anode is an inert anode. Preferred is an anode which is substantially unreactive with oxygen. Preferred is an anode which is substantially insoluble in the molten electrolyte.

„ ■ r fi. runner Typically the inert anode is a non-carbon anode. Preferred is an inert metal alloy anode. An inert metal alloy anode advantageously provides effective current efficiency.

Preferably the anode is composed of an Al-E-Cu based alloy comprising an intermetallic phase of formula:

Al_xEyCU_z

wherein:

E denotes one or more metallic elements; x is an integer in the range 1 to 5; y is an integer being 1 or 2; and z is an integer being 1 or 2.

The Al-E-Cu based alloy may be substantially monophasic or multiphasic. Preferably the intermetallic phase is present in the Al-E-Cu based alloy in an amount of 50wt% or more (eg in the range 50 to 99wt%). Preferably the Al-E-Cu based alloy further comprises an ordered high-temperature intermetallic phase of E with aluminium, particularly preferably Al₃E. Other intermetallic phases may be present.

In a preferred embodiment, the Al-E-Cu based alloy is substantially free of CuAl₂. This is advantageous because CuAl₂ has a tendency to melt at the elevated temperatures which are deployed typically in the process of the invention. Preferably CuAl₂ is complexed.

In a preferred embodiment, the Al-E-Cu based alloy falls other than on the E poor side of the tie line joining Al₃E and ECu₄ (eg on the E rich side of the tie line joining Al₃E and ECu₄).

In a preferred embodiment, the Al-E-Cu based alloy comprises an intermetallic phase falling on or near to the tie line joining Al₃E and ECu₄. In a preferred embodiment, the Al-E-Cu based alloy falls other than on the E poor side of the tie line joining Al₃E and AlECu₂ (eg on the E rich side of the tie line joining Al₃E and AlECu₂).

In a preferred embodiment, the Al-E-Cu based alloy comprises an intermetallic phase falling on or near to the tie line joining Al₃E and AlECu₂.

In a preferred embodiment, the Al-E-Cu based alloy falls other than on the E poor side of the ξ, Al₅E₂Cu, EAlCu₂ and P-ECu₄ phase tie line (wherein ξ is a phase falling between Al₃Ti and Al₂Ti with 3 at% or less of Cu (eg 2-3 at% Cu)).

In a preferred embodiment, the Al-E-Cu based alloy comprises an intermetallic phase falling on or near to the ξ, Al₅E₂Cu, EAlCu₂ and P-ECu₄ phase tie line.

Preferably the intermetallic phase is Al₅E₂Cu. Particularly preferably the Al-E- Cu based alloy further comprises Al₃E.

Preferably the intermetallic phase is EAlCu₂. Particularly preferably the Al-E- Cu based alloy further comprises P-ECu₄

The anode may be composed of a homogenous, partially homogenous or non- homogeneous Al-E-Cu based alloy.

Typically E has a potential in the anode which is lower than it would be in the molten electrode.

In a preferred embodiment, the anode develops a passivating layer. Preferably the passivating layer withstands oxidation in anodic conditions.

In a preferred embodiment, E is a single metallic element. The single metallic element is preferably Ti. In an alternative preferred embodiment, E is a plurality (eg two, three, four, five, six or seven) of metallic elements. In this embodiment, a first metallic element is preferably Ti. Typically the first metallic element of the plurality of metallic elements is present in a substantially higher amount than the other metallic elements of the plurality of metallic elements. Each of the other metallic elements may be present in a trace amount. Each of the other metallic elements may be a dopant. Each of the other metallic elements may substitute Al, Cu or the first metallic element. The presence of the other metallic elements may improve the high-temperature stability of the alloy (eg from 1200⁰C to 1400⁰C).

In a preferred embodiment, E is a pair of metallic elements. In this embodiment, a first metallic element is preferably Ti. Typically the first metallic element of the pair of metallic elements is present in a substantially higher amount than a second metallic element of the pair of metallic elements (eg in a weight ratio of about 9: 1). The second metallic element may be present in a trace amount. The second metallic element may be a dopant. The second metallic element may substitute Al, Cu or the first metallic element. The presence of a second metallic element may improve the high-temperature stability of the alloy (eg from 1200⁰C to 1400⁰C).

Preferably the pair of metallic elements has similar atomic radii. Preferably the atomic radius of the second metallic element is similar to the atomic radius of Cu. Preferably the atomic radius of the second metallic element is similar to the atomic radius of Al.

In a preferred embodiment, E is one or more of the group consisting of group B transition metal elements (eg first row group B transition metal elements) and lanthanide elements. Preferably E is one or more group IVB, VB, VIB, VIIB or VIIIB transition metal elements, particularly preferably one or more group IVB, VIIB or VIIIB transition metal elements.

In a preferred embodiment, E is one or more metallic elements of valency II, III, IV or V, preferably II, III or IV. In a preferred embodiment, E is one or more metallic elements selected from the group consisting of Ru, Ti, Zr, Cr, Nb, V, Co, Ta, Fe, Ni, La and Mn. In a particularly preferred embodiment, E is one or more metallic elements selected from the group consisting of Ti, Fe, Cr and Ni.

Preferably E is or includes a metallic element capable of reducing the tendency of CuAl₂ towards grain boundary segregation at an elevated temperature. In this embodiment, the metallic element capable of reducing the tendency of CuAl₂ towards grain boundary segregation at an elevated temperature may be the second metallic element of a plurality (eg a pair) of metallic elements. Particularly preferably E is or includes a metallic element capable of forming a complex with CuAl₂. Preferred metallic elements for this purpose are selected from the group consisting of Fe, Ni and Cr, particularly preferably Ni and Fe, especially preferably Ni.

Preferably E is or includes a metallic element capable of reducing the tendency of the first metallic element or Cu to dissolve in molten extractant. In this embodiment, the metallic element may be the second metallic element of a plurality (eg a pair) of metallic elements. Preferred metallic elements for this purpose are selected from the group consisting of Fe, Ni, Co, Mn and Cr, particularly preferably the group consisting of Fe and Ni (optionally together with Cr).

Preferably E is or includes a metallic element capable of promoting the passivation of the surface of the anode in the presence of a oxygen-dissolving molten electrolyte. For this purpose, the metallic element may form or stabilise an oxide film. In this embodiment, the metallic element may be the second metallic element of a plurality (eg a pair) of metallic elements. Preferred metallic elements for this purpose are selected from the group consisting of Ru, Fe, Ni and Cr. Particularly preferably E is Ti, Fe, Ni and Cr in which the formation of a combination of oxides such as iron oxides, chromium oxides, nickel oxides and alumina advantageously promotes passivation.

Preferably E is or includes a metallic element selected from the group consisting of Zr, Nb and V. Particularly preferred is V or Nb. These second metallic elements are advantageously strong intermetallic formers. In this embodiment, the metallic element is the second metallic element of a plurality (eg a pair) of metallic elements.

Preferably E is or includes a metallic element capable of forming an ordered high-temperature intermetallic phase with aluminium metal. Particularly preferably E is or includes a metallic element capable of forming Al₃E.

Preferably E is or includes Ti. A titanium containing alloy typically has electrical resistivity in the range 3 to 15 μohm cm at room temperature.

Preferably the intermetallic phase is Al₅Ti₂Cu. Particularly preferably the Al- Ti-Cu based alloy further comprises Al₃Ti.

Preferably the intermetallic phase is TiAlCu₂. Particularly preferably the Al- Ti-Cu based alloy further comprises P-TiCu₄

In a preferred embodiment, E is or includes Ti and a second metallic element selected from the group consisting of Fe, Cr, Ni, V, La, Nb and Zr, preferably the group consisting of Fe, Cr and Ni. The second metallic element advantageously serves to enhance high-temperature stability of the Al-Ti-Cu phases.

The anode may be composed of an Al-E-Cu based alloy obtainable by processing a mixture of 35 atomic % Al or more (preferably 50 atomic % Al or more), 35 atomic % E or more (wherein E is a first metallic element as hereinbefore defined) and a balance of Cu and optionally E' (wherein E' is one or more of the additional metallic elements hereinbefore defined).

In a preferred embodiment, the anode is composed of an Al-E-Cu based alloy obtainable by processing a mixture of (65+x) atomic % Al, (20+y) atomic % E (wherein E is a first metallic element as hereinbefore defined) and (15-x-y) atomic % Cu, optionally together with z atomic % of E' (wherein E' is one or more of the additional metallic elements hereinbefore defined) wherein E' substitutes Cu, Al or E. In this embodiment, the alloy may be obtainable by casting, preferably in an oxygen deficient atmosphere (eg an inert atmosphere). For example, a mixture may be melted in an argon-arc furnace under an atmosphere of argon gas and then solidified in an argon atmosphere. Alternatively in this embodiment, the alloy may be obtainable by flux-assisted melting, vacuum arc or vacuum melting using a resistance furnace. Contamination by O, C, N, S or P should be minimised.

In a preferred embodiment, the anode is at least as conducting at elevated temperature (eg at 900⁰C) as a carbon electrode. Preferably the anode is more conducting at elevated temperature (eg at 900⁰C) than a carbon electrode.

In a preferred embodiment, the decomposable alkali metal (M^a) salt may be present with an amount of endogenous hydroxide ions. A hydroxide ion decomposes at the cathode into an oxide ion (which moves to the anode) and a proton. At the cathode, this leads to the formation of occluded hydrogen in the metal (M) which may react with oxygen (for example in subsequent steps such as remelting) to advantageously lower the oxygen content of the metal (M) (eg to a level as low as HOOppm). A hydrogenated metal (M) (eg hydrogenated uranium) produced in this embodiment is useful. For example, a hydrogenated metal (M) may be a useful hydrogen storage material. The hydrogen may be removed by (for example) plasma melting.

In a preferred embodiment, the decomposable alkali metal (M^a) salt may be present with an amount of exogenous hydroxide ions. Preferably the exogenous hydroxide ions are provided by an alkaline additive. The alkaline additive may be an alkali metal hydroxide (such as lithium, sodium or potassium hydroxide), an alkali metal hydride (such as lithium, sodium or potassium hydride) or an alkaline earth metal hydroxide. The alkaline additive may be added to the oxygen-dissolving molten electrolyte.

The process of the invention may be carried out at an elevated temperature typically in the range 600-1000⁰C, preferably 850-1000⁰C (eg about 900⁰C). The process of the invention for a discrete batch of metal (M) oxide may be carried out to substantially complete conversion over a period of less than 20 hours, preferably less than 10 hours (eg 8 hours), particularly preferably less than 4 hours. This advantageously minimises energy input and therefore costs.

The voltage is typically less than the discharge potential of metals in the oxygen-dissolving molten electrolyte. For example, the voltage may be less than 3.5V (eg about 3.0V).

In a preferred embodiment, the process of the invention is carried out in an oxygen deficient atmosphere (eg an inert atmosphere such as argon).

The process of the invention typically achieves a rate of metal (M) extraction of 99% or more, preferably 99.9% or more.

The process of the invention typically produces metal (M) with an oxygen content of less than 2500ppm O₂ by weight, preferably less than 1500ppm O₂ weight.

In a preferred embodiment, the process of the invention comprises: applying a voltage between a cathode comprising TiO₂ in admixture with an alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide and an anode in an oxygen-dissolving molten CaCl₂-containing electrolyte whereby to form a liquid alkali metal (M^a) titanate phase.

In a preferred embodiment, the process of the invention further comprises: measuring the current flow between the cathode and the inert metal alloy anode over a temporal range; relating a characteristic of the current flow between the cathode and the inert metal alloy anode over the temporal range to the extent of electrochemical extraction of the metal (M) from the metal (M) oxide.

Viewed from a further aspect the present invention provides a conducting electrode comprising (or consisting essentially of) a metal (M) oxide and either an alkali metal (M^a) oxide capable of forming an alkali metal (M^a) metallate (M) phase or an alkali metal (M^a) salt decomposable into an alkali metal (M^a) oxide capable of forming an alkali metal (M^a) metallate (M) phase.

The conducting electrode may (in use) be a cathode as hereinbefore defined.

In a preferred embodiment, the conducting electrode comprises a metal (M) oxide and an alkali metal (M^a) salt decomposable into an alkali metal (M^a) oxide capable of forming an alkali metal (M^a) metallate (M) phase.

Viewed from a still further aspect the present invention provides the use of a conducting electrode or cathode as hereinbefore defined in an electrolytic cell.

Viewed from an even still yet further aspect the present invention provides an electrolytic cell comprising a cathode which comprises or is in contact with a metal (M) oxide and one or more inert anodes in contact with a fusible or fused oxygen- dissolving electrolyte in the presence of an alkali metal (M^a) oxide.

The (or each) inert anode may be as hereinbefore defined. The fused oxygen- dissolving electrolyte may be an oxygen-dissolving molten electrolyte as hereinbefore defined. The cathode may be as hereinbefore defined.

Generally the electrolytic cell is operated in an inert atmosphere (eg an argon atmosphere). Preferably the fusible or fused oxygen-dissolving electrolyte comprises CaCl₂.

In a first preferred embodiment, the electrolytic cell comprises a single inert anode. The alkali metal (M^a) oxide (eg K₂O) may be present in the fused oxygen- dissolving electrolyte. Preferably the cathode is a cathodic basket (eg a perforated basket) or crucible in which is carried the metal (M) oxide (eg in the form of a pellet).

The electrolytic cell may be a continuous cell.

In a second preferred embodiment, the cathode is a cathodic vessel which is adapted to facilitate in use continuous flow of the fused oxygen-dissolving electrolyte

14

(TV. ,^• i ' -^■. T ^ΛTiSW between a feeder end into which the fused oxygen-dissolving electrolyte is feedable and a discharge end from which the fused electrolyte is dischargeable, wherein the electrolytic cell comprises a plurality of inert anodes housed in the cathodic vessel between the feeder end and the discharge end.

Particularly preferably the electrolytic cell further comprises: a cathodic separation vessel downstream from the discharge end, wherein the cathodic separation vessel houses an inert anode.

Preferably the cathodic separation vessel houses a chlorine meter.

Preferably the cathodic separation vessel houses an oxygen meter.

Preferably the cathodic separation vessel comprises a reference electrode to assist in the measurement of current flow between the cathodic separation vessel and the inert anode. The current flow may be used to determine the extent of electrochemical reduction of the metal (M) oxide.

In the second preferred embodiment, the metal (M) oxide may be present in the fused oxygen-dissolving electrolyte (eg in the form of a suspended powder or a pellet). The alkali metal (M^a) oxide (eg K₂O) may be present in the fused oxygen- dissolving electrolyte.

In a third preferred embodiment, the electrolytic cell comprises a plurality of inert anodes housed in a vessel which contains the fused oxygen-dissolving electrolyte, wherein a mixture of the alkali metal (M^a) oxide and metal (M) oxide in contact with a cathode is present in the form of a plurality of self-supporting elements conveyable in use through the fused oxygen-dissolving electrolyte.

Each self-supporting element may be a pellet or a basket (eg a perforated basket). The self-supporting elements may be mounted on a conveyor. The self- supporting elements may be dismountably mounted on a conveyor. The self- supporting elements may be conveyed in and out of the fused oxygen-dissolving electrolyte. The self-supporting elements may be circulatory (eg recirculatory). In a fourth preferred embodiment, the electrolytic cell comprises a plurality of inert anodes housed in a vessel which contains the fused oxygen-dissolving electrolyte, wherein the alkali metal (M^a) oxide and metal (M) oxide are present in the fused oxygen-dissolving electrolyte (eg in the form a suspension) in contact with a plurality of cathodic elements conveyable in use through the fused oxygen-dissolving electrolyte.

Each cathodic element may be a pellet. The cathodic elements may be mounted on a conveyor. The cathodic elements may be dismountably mounted on a conveyor. The cathodic elements may be conveyed in and out of the fused oxygen- dissolving electrolyte. The cathodic elements may be circulatory (eg recirculatory).

In a fifth preferred embodiment, the cathode is a metal crucible containing the alkali metal (M^a) oxide and metal (M) oxide in molten admixture, wherein the metal crucible is suspended in the fused oxygen-dissolving electrolyte. The metal crucible may be composed of titanium metal or a titanium metal alloy. The fifth embodiment advantageously prevents contamination of the fused oxygen-dissolving electrolyte by the molten admixture.

The present invention will now be described in a non-limitative sense with reference to the Examples and accompanying Figures in which:

Fig 1 : K-Ti-O phase diagram at 1173K plotted using FACTSAGE thermodynamic software (C. Bale et al., FACTSAGE (Ecole Polytechnique CRCT, Montreal, Quebec

Canada));

Fig 2: Elemental map of the cross section of a partially reacted TiO₂ pellet after treatment according to an embodiment of the process of the invention;

Fig 3 : Current vs time graph for the process according to the invention at an applied voltage of 3.1V;

Fig 4a: Low magnification image of the cross section of a Ti pellet fully metallised in a LiCl-CaCl₂ molten bath;

Figure 4b: High magnification image of Ti metal obtained from the inner region of the pellet seen in figure 3; Figs 5a and 5B: XRD of a TiO₂ + KHCO₃ pellet roasted for 1 hour and electrolysed for 0.5 hours (see Fig 5a) and 1 hour (see Fig 5b) in a molten bath OfCaCl₂-LiCl showing phases of Ti (ICDD 5-682), CaTiO₃ (ICDD 42-423), CaTi₂O₄ (ICDD 1 1-29) and TiO (ICDD 8-117);

Fig 6: XRD of titanium metal formed after 20 hours of electrolysis;

Fig 7: A schematic illustration of a first embodiment of the electrolytic cell according to the invention;

Fig 8: A schematic illustration of a second embodiment of the electrolytic cell according to the invention;

Fig 9: A schematic illustration of a third embodiment of the electrolytic cell according to the invention;

Fig 10: A schematic illustration of a fourth embodiment of the electrolytic cell according to the invention; and

Fig 11 : A schematic illustration of a fifth embodiment of the electrolytic cell according to the invention

EXAMPLE 1

Method

Pellets were prepared by mixing l-2g Of TiO₂ with 0.2-0.5g of KHCO₃ at different weight ratios. In each case, the mixture was heat treated for 1 hour at 1073K and pressed in a die at a pressure of 3643atm. A hole was drilled in the pellet with a 2mm drill bit. The pellet was suspended in a steel electrode which acted as a cathode with a molybdenum wire. An Al-Ti-Cu intermetallic anode was suspended on a steel electrode with a molybdenum wire. The two electrodes were connected to a power supply which was set to a constant voltage of 3. IV.

Molten electrolytic mixtures of KCl-CaCl₂ and LiCl-CaCl₂ were prepared by taking 180gms Of CaCl₂ with 20gms of KCl and LiCl respectively. In each case, the mixture was transferred into a zircon crucible which was lowered into a furnace maintained at 32O⁰C. The mixture was heat treated for 24 hours and then transferred into an alumina crucible and heated to 800⁰C at 0.5⁰C per minute after which the temperature was raised to 92O⁰C at a rate of 2⁰C per minute. During heating, argon gas was passed into the furnace at 500ml min^" . Once the electrolyte was fully molten, the temperature of the furnace was lowered to 900⁰C. The two electrodes were lowered into the furnace and a potential of 3. IV was applied using an Agilent 665 IA DC power supply. The experiments were carried out for a period of 8-24 hours.

Pellets were removed at intervals of 30 and 60 minutes of electrolysis and washed in water for 24 hours. The pellets were finely ground using a mortar and pestle for X-ray powder diffraction analysis. The diffraction was carried out using Cu- Ka as target at a scanning rate of 0.02° sec^"1.

Results

An increase in internal porosity was achieved readily in situ by the presence of KHCO₃ in the TiO₂ pellet. As KHCO₃ decomposes, it produces potassium oxide, carbon dioxide and water. The liberated gaseous mixture of CO₂ and H₂O increases the porosity in the pellet which enhances the contact surface area between CaCl₂ and TiO₂ and facilitates rapid cathodic dissociation of TiO₂.

Besides pore formation, a much more significant reaction takes place between K₂O and CaTiO₃. K⁺ ions diffuse into the perovskite lattice which breaks the structure by forming more stable liquid potassium titanates as shown in equation [1] (ascertained from an equilibrium calculation performed using FACTSAGE see Bale [supra]). The calcium oxide formed in this reaction is dissolved in the molten salt bath until it reaches saturation:

CaTiO₃ + 2K₂O = KJiO₄ + CaO ΔG = -334349.6 J mole ¹ at T=900°C [1]

As the diffusivity of O^2' ions in the liquid phase is faster than in solid CaTiO₃, the reduction OfK₄TiO₄ to the Magneli phases through to Ti metal occurs rapidly as no major reorganisation of crystalline TiO₂ is required. The Magneli phases (Ti₄O₇, Ti₃O₅) all have a distorted rutile structure with a larger number of oxygen vacant sites. From a phase equilibrium analysis, it was established that the K₄TiO₄ liquid phase can be in equilibrium with the Magneli phase and continue to shift the equilibrium with the progression of reduction to the metallic phase (see figure 1). The formation of liquid phase increases the reaction kinetics which is evident as Ti metal was observed within the first half an hour of electrolysis. The K⁺ ions produced from the decomposition of potassium titanate reacts with the molten electrolyte and forms KCaCl₃

By controlling the volume of the liquid phase of potassium titanate, the loss of Ti in the molten salt can be prevented. If the liquid phase drains out from the solid pellet into the CaCl₂ bath, TiO₂ is then irreversibly lost into the CaCl₂ bath.

Figs 5a and 5B are the XRD pattern of the pellet at 0.5 hours (see Fig 5a) and 1 hour (see Fig 5b) of electrolysis. Phases of Ti (ICDD 5-682), CaTiO₃ (ICDD 42- 423), CaTi₂O₄ (ICDD 11-29) and TiO (ICDD 8-117) are present. A comparison of Figs 5 a and 5 b shows that the perovskite peak is suppressed as perovskite is decomposed. After 20 hours of electrolysis, the XRD pattern (Fig 6) shows that titanium metal is present.

EXAMPLE 2

A number of experiments were carried out to change the ratio of potassium bicarbonate in the pellet in the range 10-50wt%. Experiments were also conducted on the two different types of molten salt containing CaCl₂-KCl and CaCl₂-LiCl mixtures at 900⁰C with a constant voltage of 3.1V. Both processes yielded complete reduction of TiO₂ pellet to Ti metal. The residual concentration of oxygen dissolved in the Ti metal was determined by X-ray diffraction analysis (see M. Dechamps et al., Scripta Metallurgica 11 (11), 941 (1977)) and was found to be 1350ppm by weight.

A first experiment was carried out with a pellet containing 20wt% potassium bicarbonate in a CaCl₂-KCl bath for 8 hours. The elemental map in figure 2 demonstrates the formation of Ti metal layer with a thickness of 500μm beyond which there is a high concentration of calcium, titanium, potassium and chlorine. From the elemental map in figure 2, it is possible to observe discrete regions of KCl within the CaCl₂ layer which can occur via reaction [2] at 900⁰C: K₂O + CaCl₂ = CaO + KCl ΔG = -346968 J mole^"1 [2]

When the concentration of potassium bicarbonate was increased from 20wt% to 50wt% and electrolysis was performed for 20 hours, it was found that a uniform microstructure of Ti metal was formed across the cross section of the pellet with the majority of the area being metallised. Furthermore when the salt bath was replaced by LiCl-CaCl₂ and 50wt% of potassium bicarbonate was mixed with TiO₂ and electrolysed for 20hours, it also led to full metallisation. The reduction in the two molten salts proves that the formation OfK₄TiO₄ liquid phase is important for increasing reaction kinetics and is independent of the molten salt used. During electrolysis, all the experiments showed an increase in the current with the inert metallic anode which is in sharp contrast with previous observations (C. Schwandt [supra]; M. Ma et al., Journal of Alloys and Compounds 420 (1-2), 37 (2006); and R. O. Suzuki et al, Metallurgical and Materials Transactions B-Process Metallurgy and Materials Processing Science 34 (3), 287 (2003)).

Figure 3 displays the current-time plot for the reaction at 3 V. Although a smooth curve is observed, there was oscillation in the current with a variation of ± 0.1 amps during electrolysis. It can be seen from figure 3 that there is a decrease in the current for the first half hour of the process after which the current increases. Beyond two hours, there is a slow increase in current which plateaus at around 4.0amps. The large initial current is due to the use of the inert anode which has high conductivity compared to a carbon anode and therefore decreases the cell resistance. The initial decrease in current in figure 3 is due to the formation of a perovskite phase (verified from the X-ray diffraction analysis). The data for half hour electrolysis showed the presence of CaTiO₃, CaTi₂O₄, Ti₃O₅, TiO and Ti metal phases. After 4 hours, almost 95% Of TiO₂ is reduced to Ti. It is important to note that in previous experiments (Alexander [supra] and Schwandt [supra]), no titanium metal has been observed in the first 30 minutes of the process.

From the Ti-O phase diagram, it is known that Ti₃O₅ can never be in equilibrium with Ti from which it is concluded that (at an early stage) two simultaneous reactions occur. The first reaction is the formation of CaTiO₃, CaTi₂O₄ and Ti₃O₅ which dominates the phase constitution. The second reaction is the decomposition Of K₄TiO₄ to form TiO and Ti metal. Since the Magneli phases are more electrochemically conducting and the Ti metal is formed in the first hour of electrolysis, an increase in current is eminent which is what is found in figure 3. The diffraction pattern after one hour of electrolysis showed small peaks of CaTiO₃ and predominant peaks of Ti, CaTi₂O₄ and Ti₃O₅. It must be noted that the XRD does not show the presence of the potassium titanate phase because it is a transitional liquid phase during electrolysis.

The amount of Ti metal produced is not only shown by microstructural analysis but also by measuring the weight loss after electrolysis (as previously demonstrated by G. Z. Chen et al, Metallurgical and Materials Transactions B- Process Metallurgy and Materials Processing Science 35 (2), 223 (2004) in the case of electro-reduction Of Cr₂O₃ in molten CaCl₂). After electrolysis of Ig of TiO₂ pellet for 20 hours, the pellet was washed in water for 24 hours and the weight of the pellet was measured again and was found to be 0.605g. The theoretical amount of Ti produced from Ig of TiO₂ is 0.6g which is within the error of experimental observation thus verifying complete metallisation. Figure 3 shows a low magnification image of the cross section of a fully metallised TiO₂ pellet which was reduced in a CaCl₂-LiCl molten electrolyte. As evident from figure 4a, the layered structure as seen in figure 2 is absent throughout the cross section. The inset in figure 4a reveals that it has a metallic grey colour with a large number of cracks on the surface. There was 20% shrinkage in the pellet from its starting thickness of 5mm to a final thickness of 4mm. The corresponding high magnification image of the inner region (within the hole) is shown in figure 4b. The microstructure has a distinctive Ti metal morphology obtained from the electro-reduction process and compares well with literature data (Schwandt et al [supra]). The EDX from this region confirms Ti having K^α, K^p and L^α peaks. In the EDX spectrum shown in figure 4b, an oxygen peak at 1350ppm concentration in the reduced Ti metal is not anticipated. The designated oxygen peak is Ti L^α and not O K^α.

EXAMPLE 3 Figure 7 is a schematic illustration of a first embodiment of the electrolytic cell according to the invention designated generally by reference numeral 1. The electrolytic cell 1 comprises an inert alloy anode 2 and a cathodic basket 3 in a molten electrolyte 6 of CaCl₂ containing K₂O. Inside the cathodic basket 3 is a TiO₂ pellet 4 around which is formed a perovskite layer 5. The cell 1 operates at an applied voltage of about 3.1V

EXAMPLE 4

Figure 8 is a schematic illustration of a second embodiment of the electrolytic cell according to the invention designated generally by reference numeral 11. The electrolytic cell 11 is deployed for continuous metal production.

The electrolytic cell 11 comprises four inert alloy anodes 12a-d. Inert alloy anodes 12a-c are mounted in a cathodic vessel 13 containing a molten electrolyte 16 Of CaCl₂. The molten electrolyte 16 is fed continuously into the cathodic vessel 13 together with TiO₂ powder and K₂O into the feed end 20 and a controlled flow of molten electrolyte 16 from the feed end 20 to a discharge end 21 is achieved by a slope in the cathodic vessel 13.

During the continuous flow of molten electrolyte 16, TiO₂ is reduced to titanium sub-oxide. In accordance with the invention, this is only made feasible by the presence of K₂O. At the discharge end 21, there is a discharge port 22 through which titanium sub-oxide is discharged into a cathodic separation vessel 31 which houses inert alloy anode 12d and completes the reduction of titanium suboxide to titanium metal. Titanium metal is discharged from the discharge outlet 30 and the molten electrolyte is recycled to the cathodic vessel 13. To determine the end point of the process, the separation vessel 31 is fitted with a reference electrode to facilitate the measurement of a current vs time plot.

EXAMPLE 5

22 .■^•■- Λ rn>7r.-i -.τfc.τ /ri/ry.τvιτ Figure 9 is a schematic illustration of a third embodiment of the electrolytic cell according to the invention designated generally by reference numeral 21. The electrolytic cell 21 is deployed for continuous metal production.

The electrolytic cell 21 comprises three inert alloy anodes 22a-c housed in a vessel 23 containing a molten electrolyte 26 of CaCl₂. In contact with a cathode 29 is a plurality of baskets 30 each composed of a self-supporting mixture Of TiO₂ and K₂O. Each basket 30 is mounted on a conveyor which circulates the baskets 30 in and out of the molten electrolyte 26 in the direction X.

EXAMPLE 6

Figure 10 is a schematic illustration of a fourth embodiment of the electrolytic cell according to the invention designated generally by reference numeral 221. The electrolytic cell 221 is deployed for continuous metal production.

The electrolytic cell 221 comprises three inert alloy anodes 222a-c housed in a vessel 223 containing a molten electrolyte 226 of CaCl₂. TiO₂ and K₂O is added to the molten electrolyte 226 to form a suspension. A plurality of cathodic pellets 230 is mounted on a conveyor which circulates the pellets 230 in and out of the molten electrolyte 226 in the direction X.

EXAMPLE 7

Figure 7 is a schematic illustration of a fifth embodiment of the electrolytic cell according to the invention designated generally by reference numeral 1. The electrolytic cell 1 comprises an inert alloy anode 2 and a cathodic crucible 3 made of titanium or titanium alloy. The cathodic vessel 3 is suspended in a molten electrolyte 6 Of CaCl₂. Inside the cathodic crucible 3 is a molten mixture 4 of TiO₂ and K₂O. At surface A, TiO₂ is reduced to titanium metal and oxide ions are transported from surface B through the molten electrolyte 6 to the inert alloy anode 2.

Claims

1. A process for electrochemical extraction of a metal (M) from a metal (M) oxide comprising: applying a voltage between a cathode comprising or in contact with the metal (M) oxide and an anode in an oxygen-dissolving molten electrolyte in the presence of an alkali metal (M^a) oxide whereby to form an alkali metal (M^a) metallate (M) phase.

2. A process as claimed in claim 1 wherein the alkali metal (M^a) oxide is potassium oxide.

3. A process as claimed in claim 1 or 2 wherein the alkali metal (M^a) oxide forms the alkali metal (M^a) metallate (M) phase from a reaction of the alkali metal (M^a) oxide with a metal (M") metallate (M) phase.

4. A process as claimed in any preceding claim wherein the metal (M") metallate (M) phase is a perovskite (or perovskite-type) phase.

5. A process as claimed in any preceding claim wherein the diffusivity of oxygen in the alkali metal (M^a) metallate (M) phase is higher than the diffusivity of oxygen in the metal (M") metallate (M) phase.

6. A process as claimed in any preceding claim wherein the alkali metal (M^a) metallate (M) phase is a liquid.

7. A process as claimed in any preceding claim wherein the alkali metal (M^a) metallate (M) phase is M^a ₄MO₄.

8. A process as claimed in any preceding claim wherein the alkali metal (M^a) oxide is in admixture with the metal (M) oxide in or in contact with the cathode.

9. A process as claimed in any preceding claim further comprising: mixing the alkali metal (M^a) oxide and the metal (M) oxide and forming the mixture of alkali metal (M^a) oxide and metal (M) oxide into a self-supporting mixture.

10. A process as claimed in any of claims 1 to 7 wherein the alkali metal (M^a) oxide is formed in situ by decomposition of a decomposable alkali metal (M^a) salt, wherein the decomposable alkali metal (M^a) salt is in admixture with the metal (M) oxide in or in contact with the cathode.

11. A process as claimed in claim 10 further comprising: mixing the decomposable alkali metal (M^a) salt and the metal (M) oxide and forming the mixture of decomposable alkali metal (M^a) salt and metal (M) oxide into a self-supporting mixture.

12. A process as claimed in any of claims 10 to 11 wherein the decomposable alkali metal (M^a) salt is decomposable into one or more gaseous species.

13. A process as claimed in any of claims 10 to 12 wherein the decomposable alkali metal (M^a) salt is an alkali metal (M^a) bicarbonate.

14. A process as claimed in any preceding claim wherein the metal (M) forms a solid perovskite (or perovskite-type) phase in the oxygen-dissolving molten electrolyte.

15. A process as claimed in any preceding claim wherein the metal (M) is one or more metals selected from the group consisting of group HA metals, group IIIA metals, group IVA metals, group B transition metals, rare earth metals and alloys thereof.

16. A process as claimed in any preceding claim wherein the metal (M) is one or more metals selected from the group consisting of group IVB transition metals, group VB transition metals, group VIB transition metals, group VIIIB transition metals, actinides and alloys thereof.

17. A process as claimed in any preceding claim wherein the metal (M) is one or more metals selected from the group consisting of Ti, Nb, Ta, U, Th, Cr, Fe, steel and Zr.

18. A process as claimed in any preceding claim wherein the metal (M) is one or more metals selected from the group consisting of Ti, Nb, Ta and Zr.

19. A process as claimed in any preceding claim wherein the anode is composed of an Al-E-Cu based alloy comprising an intermetallic phase of formula:

Al_xEyCu_z

wherein:

20. A process as claimed in any preceding claim comprising: applying a voltage between a cathode comprising TiO₂ in admixture with an alkali metal (M^a) salt decomposable into the alkali metal (M^a) oxide and an anode in an oxygen- dissolving molten CaCl₂-containing electrolyte whereby to form a liquid alkali metal (M^a) titanate phase.

21. A conducting electrode comprising a metal (M) oxide and either an alkali metal (M^a) oxide capable of forming an alkali metal (M^a) metallate (M) phase or an alkali metal (M^a) salt decomposable into an alkali metal (M^a) oxide capable of forming an alkali metal (M^a) metallate (M) phase.

22. An electrolytic cell comprising a cathode which comprises or is in contact with a metal (M) oxide and one or more inert anodes in contact with a fusible or fused oxygen-dissolving electrolyte in the presence of an alkali metal (M^a) oxide.

23. An electrolytic cell as claimed in claim 22 comprising a single inert anode, wherein the cathode is a cathodic basket in which is carried the metal (M) oxide.

24. An electrolytic cell as claimed in claim 22 wherein the cathode is a cathodic vessel which is adapted to facilitate in use continuous flow of the fused oxygen-dissolving electrolyte between a feeder end into which the fused oxygen- dissolving electrolyte is feedable and a discharge end from which the fused electrolyte is dischargeable, wherein the electrolytic cell comprises a plurality of inert anodes housed in the cathodic vessel between the feeder end and the discharge end.

25. An electrolytic cell as claimed in claim 22 comprising a plurality of inert anodes housed in a vessel which contains the fused oxygen-dissolving electrolyte, wherein a mixture of the alkali metal (M^a) oxide and metal (M) oxide in contact with a cathode is present in the form of a plurality of self-supporting elements conveyable in use through the fused oxygen-dissolving electrolyte.

26. An electrolytic cell as claimed in claim 22 comprising a plurality of inert anodes housed in a vessel which contains the fused oxygen-dissolving electrolyte, wherein the alkali metal (M^a) oxide and metal (M) oxide are present in the oxygen-dissolving electrolyte in contact with a plurality of cathodic elements conveyable in use through the fused oxygen-dissolving electrolyte.

27. An electrolytic cell as claimed in claim 22 wherein the cathode is a metal crucible containing the alkali metal (M^a) oxide and metal (M) oxide in molten admixture, wherein the metal crucible is suspended in the fused oxygen-dissolving electrolyte.