The invention relates to electrolysis apparatus for the reduction of solid feedstock and methods of using the apparatus.

Background

The present invention concerns apparatus and methods for the reduction of a solid feedstock comprising a metal compound or compounds, such as a metal oxide, to form reduced products. As is known from the prior art, such processes may be used, for example, to reduce metal compounds or semi- metal compounds to metals, semi-metafs, or partially-reduced compounds, or to reduce mixtures of metal compounds to form alloys. In order to avoid repetition, the term metal will be used in this document to encompass all such products, such as metals, semi-metals, alloys, intermetallics, and partially- reduced products.

In recent years there has been great interest in the direct production of metal by reduction of a solid feedstock, for example, a solid metal-oxide feedstock. One such direct reduction process is the Cambridge FFC electro- decomposition process (as described in WO 99/64638). In the FFC process a solid compound, for example a solid metal oxide, is arranged in contact with a cathode in an electrolysis cell comprising a fused salt. A potential is applied between the cathode and an anode of the cell such that the compound is reduced. In the FFC process, the potential that produces the solid compound is lower than a deposition potential for a cation from the fused salt. For example, if the fused salt is calcium chloride, then the cathode potential at which the solid compound is reduced is lower than a deposition potential for depositing metallic calcium from the salt. Other reduction processes for reducing feedstock in the form of a cathodically- connected solid metal compound have been proposed, such as the polar process described in WO 03/076690 and the process described in

WO 03/048399. Conventional implementations of the FFC process and other electrolytic reduction processes typically involve the production of a feedstock in the form of a preform or precursor, fabricated from a powder of the solid compound to be reduced. This preform is then painstakingly coupled to a cathode to enable the reduction to take place. Once a number of preforms have been coupled to the cathode, then the cathode can be lowered into the molten salt and the preforms can be reduced. It can be highly labour intensive to produce the preforms and then attach them to the cathode. Although this methodology works well on a laboratory scale, it does not lend itself to the mass productions of metal on an industrial scale.

It is an aim of the invention to provide an electrolysis apparatus, and a method of using an electrolysis apparatus more suitable for the reduction of a solid feedstock on an industrial scale.

Summary of the Invention

The invention provides an electrolysis apparatus for the reduction of a solid feedstock and a method of reducing a solid feedstock as defined in the appended independent claims to which reference should now be made.

Preferred or advantageous features of the invention are set out in various dependent sub-claims.

Thus, in a first aspect the invention may provide an electrolysis apparatus for reduction of a solid feedstock comprising, a first electrode having an upper surface, the first electrode being located within an electrolysis chamber of the electrolysis apparatus, a second electrode having a lower surface, the second eiectrode being disposed above the upper surface of the first electrode within the electrolysis chamber, and a retaining barrier for retaining the solid feedstock within an area of the upper surface of the first electrode bounded by the retaining barrier, the retaining barrier being formed from an electrically insulating material.

It may be advantageous for the retaining barrier to be formed from a material that reduces during the reduction of the feedstock, for example the retaining barrier may be made from the same material as the feedstock. It may be preferred that the retaining barrier does not reduce during

electrolysis. In this event it is preferable that the barrier is made from a material that does not reduce under the reduction conditions for a particular feedstock. In a second aspect, the invention may provide a method of reducing a solid feedstock in an electrolysis apparatus comprising a first electrode having an upper surface, and a second electrode having a lower surface that is spaced above the first electrode, comprising the steps of, retaining a volume of solid feedstock on the upper surface of the first electrode, the feedstock being retained by an electrically-insulating retaining barrier, and operating the apparatus such that current flows between the first electrode and the second electrode, and the solid feedstock is reduced.

Preferably the method of the second aspect of the invention is performed using the apparatus of the first aspect of the invention.

In a third aspect, the invention may provide an electrolysis apparatus for reduction of a solid feedstock comprising, a first electrode having an upper surface for supporting the solid feedstock, a lower surface and a peripheral surface substantially perpendicular to the upper surface and the lower surface, and a second electrode disposed above the first electrode, in which the peripheral surface is shielded by a first insulator formed from an electrically- insulating material such that, when the apparatus is in use, current flows into or out of the first electrode through its upper surface but not through its peripheral surface.

Preferably the insulator is a suitable insulating ceramic material, for example a ceramic material that is substantially inert under cell operating conditions for example a material comprising yttrium oxide, aluminium oxide, titanium oxide, tantalum oxide, or silicon nitride.

In a fourth aspect, the invention may provide a method of reducing a solid feedstock in an electrolysis apparatus comprising a first electrode having an upper surface, a lower surface and a peripheral surface, the peripheral surface being electrically insulated, and a second electrode disposed above the first electrode, the method comprising the steps of, retaining a volume of solid feedstock on the upper surface of the first electrode, and operating the apparatus to pass current between the first electrode and the second electrode such that the solid feedstock is reduced, the current flowing through the upper surface of the first electrode but not through the peripheral surface of the first electrode.

The methods and apparatus of the various aspects and embodiments of the invention described above are particularly suitable for the production of metal by the reduction of a solid feedstock comprising a solid metal oxide. Pure metals may be formed by reducing a pure metal oxide and alloys and intermetallics may be formed by reducing feedstocks comprising mixed metal oxides or mixtures of pure metal oxides. Some reduction processes may only operate when the molten salt or electrolyte used in the process comprises a metallic species (a reactive metal) that forms a more stable oxide than the metallic oxide or compound being reduced. Such information is readily available in the form of thermodynamic data, specifically Gibbs free energy data, and may be conveniently determined from a standard Ellingham diagram or predominance diagram or Gibbs free energy diagram. Thermodynamic data on oxide stability and Ellingham diagrams are available to, and understood by, electrochemists and extractive metallurgists (the skilled person in this case would be well aware of such data and information).

Thus, a preferred electrolyte for a reduction process may comprise a calcium salt. Calcium forms a more stable oxide than most other metals and may therefore act to facilitate reduction of any metal oxide that is less stable than calcium oxide. In other cases, salts containing other reactive metals may be used. For example, a reduction process according to any aspect of the invention described herein may be performed using a salt comprising lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium, barium, or yttrium. Chlorides or other salts may be used, including mixture of chlorides or other salts. By selecting an appropriate electrolyte, almost any metal oxide may be capable of reduction using the methods and apparatuses described herein. In particular, oxides of beryllium, boron, magnesium, aluminium, silicon, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, germanium, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, and the lanthanides including lanthanum, cerium, praseodymium, neodymium, samarium, and the actinides including actinium, thorium, protactinium, uranium, neptunium and plutonium may be reduced, preferably using a molten salt comprising calcium chloride.

The skilled person would be capable of selecting an appropriate electrolyte in which to reduce a particular metal oxide, and in the majority of cases an electrolyte comprising calcium chloride will be suitable. Specific embodiments of the invention

Specific embodiments of the invention will now be described with reference to the drawings, in which;

Figure 1 is a schematic diagram illustrating an electrolysis apparatus according to an embodiment of the invention;

Figure 2 is a plan view of a cathode for the electrolysis apparatus of figure 1;

Figure 3 is a schematic diagram illustrating an electrolysis apparatus according to an embodiment of the invention;

Figure 4 is a schematic diagram illustrating an electrolysis apparatus according to an embodiment of the invention; Figure 5 is a schematic diagram illustrating an electrolysis apparatus according to an embodiment of the invention.

Figure 1 illustrates an electrolysis apparatus 10 embodying the invention. The apparatus comprises a stainless steel cathode 20 and a carbon anode 30 situated within a housing 40. The anode 30 is disposed above, and spatially separated from, the cathode 20. The housing 40 contains a calcium chloride based molten salt electrolyte 50, and both the anode 30 and the cathode 20 are in contact with the molten salt 50. Both the anode 30 and the cathode 40 are coupled to a power supply 60 so that a potential can be applied between the cathode and the anode.

Both the cathode 20 and the anode 30 are formed as plates, and the plates are substantially horizontally oriented, with an upper face 20a of the cathode 20 facing towards a lower face 30a of the anode 30.

The apparatus 10 further comprises a retaining barrier 70 formed from titanium oxide. The retaining barrier is seated on the upper surface 20a of the cathode 20 and forms a boundary surrounding an area of the upper surface 20a of the cathode 20.

Figure 2 shows a plan view of the cathode 20. The cathode can be seen to be a substantially circular plate. An area of the upper surface 20a of the cathode 20 is visible in the central portion of the plate. A peripheral region of the upper surface is obscured in this plan-view by the substantially annual retaining barrier 70 that is seated on the upper surface 20a of the cathode 20.

In use, a solid feedstock, for example a feedstock comprising a plurality of titanium oxide preforms, or tantalum oxide preforms, is retained in contact with the upper surface 20a of the cathode 20 by the retaining barrier 70. The retaining barrier allows a volume of feedstock to be retained in contact with the cathode without the feedstock falling off the upper surface 20a. A potential is applied between the anode and the cathode sufficient for the solid feedstock to be reduced. The apparatus 10 may be used to reduce any solid feedstock comprising a metal oxide. The apparatus may be of particular advantage when used in an FFC electro-deoxidation process, for example as described in WO 99/64638.

Figure 3 illustrates an electrolysis apparatus 310 embodying the invention. The apparatus has a cathode 320, an anode, and a molten salt 350, substantially as described above in relation to the apparatus of figure 1.

Furthermore, the apparatus comprises a retaining barrier 370 for retaining feedstock in contact with an upper surface of the cathode 320. The apparatus additionally comprises a bipolar electrode situated between the anode 330 and the cathode 320. The bipolar electrode 380 is associated with a second retaining barrier 371 for retaining a volume of feedstock in contact with its upper surface.

In use, a solid feedstock is loaded onto an upper surface of both the cathode 320 and the bipolar electrode 380. A potential is applied between the cathode 320 and the anode 330 causing the upper surface of the bipolar electrode 380 to become cathodic. The potential at the surface of both the cathode 320 and the bipolar electrode 380 is sufficient to cause reduction of the feedstock. Figure 4 illustrates a further electrolysis apparatus 410 embodying the invention. The apparatus 410 is substantially the same as the apparatus 10 described above in relation to figure 1 , and has a cathode 420 and an anode 430 situated in contact with a molten salt 450 within a housing 440. The apparatus also has a retaining barrier for retaining feedstock in contact with an upper surface of the cathode 420. The electrolysis apparatus 4 0 of figure 4 differs from that described above in relation to figure 1 in that the retaining barrier 470 is not seated on the upper surface of the cathode 420, but is situated adjacent to the cathode 420. The retaining barrier is sufficiently close to the cathode 420 to act as a barrier for feedstock retained on the surface of the cathode 420.

Figure 5 illustrates a further electrolysis apparatus embodying the invention. The apparatus 510 comprises a cathode 520, an anode 530, and a bipolar electrode 580 all disposed in contact with a molten salt 550 in a housing 540. The anode 530 and the cathode 520 are electrically coupled to a power supply.

The cathode has a plate structure comprising an upper surface 521, a lower surface 522 and a peripheral surface 523. Likewise, the bipolar electrode has a plate structure comprising an upper surface 581, a lower surface 582 and a peripheral surface 583. The peripheral surface 523 of the cathode is shielded by an insulator 590 formed from an electrically insulating material, for example yttria or boron nitride. The peripheral surface 583 of the bipolar electrode is also shielded by an insulator 591.

The insulators 590, 591 act to prevent the flow of electric current into or out of the peripheral surfaces of the cathode 520 or the bipolar electrode 580.