WO2004044268A2

WO2004044268A2 - The production of hematite-containing material

Info

Publication number: WO2004044268A2
Application number: PCT/IB2003/005292
Authority: WO
Inventors: Eric Appourchaux; Thinh T. Nguyen; Vittorio De Nora
Original assignee: Moltech Invent S.A.
Priority date: 2002-11-14
Filing date: 2003-11-14
Publication date: 2004-05-27
Also published as: AU2003280106A1; WO2004044268A3; AU2003280106A8

Abstract

A method of forming a hematite-containing material for use in a high temperature oxidising and/or corrosive environment, comprises sintering by heat treatment a particle mixture comprising predominantly sinterable hematite particles and substantially non-sinterable hematite particles, with a weight ratio sinterable hematite/substantially non-sinterable hematite in the range of 0.2 to 3. The sinterable hematite particles are sintered together and the substantially non-sinterable hematite particles inhibit aggregation of voids produced by the sintering. The particle mixture may comprise additives such as titanium, zinc, copper and/or other metals or oxides thereof to improve the electrical properties of the hematite-containing material, as well as non-sinterable non-oxide ceramic particles, iron metal particles and/or ferrous oxide particles or other additives such as sodium silicate. The material can be used for the manufacture of aluminium electrowinning cell components, in particular anodes.

Description

THE PRODUCTION OF HEMATITE-CONTAINING MATERIAL

Field of the Invention

This invention relates to a method of manufacturing a hematite-containing material which can be used in high temperature oxidising and/or corrosive environments, for instance as a non-carbon anodic material of an aluminium electrowinning cell.

Background Art

Using non-carbon anodes - i.e. anodes which are not made of carbon as such, e.g. graphite, coke, etc., but possibly contain carbon in a compound - for the electrowinning of aluminium should drastically improve the aluminium production process by reducing pollution and the cost of aluminium production. Many attempts have been made to use oxide anodes, cermet anodes and metal- based anodes for aluminium production, however they were never adopted by the aluminium industry.

For the dissolution of the raw material, usually alumina, a highly aggressive fluoride-based electrolyte, such as cryolite, is reguired. Materials for protecting aluminium electrowinning components have been disclosed in US Patents 5,310,476, 5,340,448, 5,364,513, 5,527,442, 5,651,874, 6,001,236, 6,287,447 and in PCT publication WO01/42531 (all assigned to MOLTECH) . Such materials are made predominantly (more than 50%) of non-oxide ceramic materials, e.g. borides, carbides or nitrides, for exposure to molten aluminium and to a molten fluoride-based electrolyte and have successfully been used in cathode applications. However, these non-oxide ceramic-based materials do not resist immediate exposure to anodically produced nascent oxygen.

The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of non-conductive or poorly conductive oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.

Several patents disclose the use of an electrically conductive metal anode core with an oxide-based active outer part, in particular US patents 4,956,069, 4,960,494, 5,069,771 (all Nguyen/Lazouni/Doan) , 6,077,415

(Duruz/de Nora), 6,103,090 (de Nora), 6,113,758 (de

Nora/Duruz) and 6,248,227 (de Nora/Duruz) , as well as PCT publications WO00/06803 (Duruz/de Nora/Crottaz) ,

WO00/06804 (Crottaz/Duruz) , WO00/40783 (de Nora/Duruz), WO01/42534 (de Nora/Duruz) and WO01/42536 (Nguyen/Duruz/ de Nora) .

US patents 4,039,401 and 4,173,518 (both Yamada/ Hashimoto/Horinouchi) disclose multiple oxides for use as electrochemically active anode material for aluminium electrowinning. The multiple oxides include inter-alia oxides of iron, nickel, titanium and yttrium, such as NiFe₂0₄ or TiFe₂0₄, in US 4,039,401, and oxides of yttrium, iron, titanium and tantalum, such as Fe₂0₃.Ta₂0₅, in US 4,173,518. The multiple oxides are produced by sintering their constitutive single oxides and then they are crushed and applied onto a metal substrate (titanium, nickel or copper) by spraying or dipping. Alternatively, the multiple oxides can be produced by electroplating onto the metal substrate the constitutive metals of the multiple oxides followed by an oxidation treatment.

Likewise US patents 4,374,050 and 4,374,761 (both Ray) disclose non-stoichiometric multiple oxides for use as electrochemically active anode material for aluminium electrowinning. The multiple oxides include inter-alia oxides of nickel, titanium, tantalum, yttrium and iron, in particular nickel-iron oxides. The multiple oxides are produced by sintering their constitutive single oxides.

US patent 6,372,119 and WO01/31091 (both Ray/Liu/

Weirauch) disclose a cermet made from sintered particles of nickel, iron and cobalt oxides and of metallic copper and silver possibly alloyed with cobalt, nickel, iron, aluminium, tin, niobium, tantalum, chromium molybdenum or tungsten. The particles can be applied as a coating onto an anode substrate and sintered thereon to form an anode for the electrowinning of aluminium.

W099/36591 (de Nora), W099/36593 and W099/36594 (both Duruz/de Nora) disclose sintered multiple oxide coatings applied onto a metal substrate from a slurry containing particulate of the multiple oxides in a colloidal and/or inorganic polymeric binder, in particular colloidal or polymeric alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide or zinc oxide. The multiple oxides include ferrites of cobalt, copper, chromium, manganese, nickel and zinc. It is mentioned that the coating can be obtained by reacting precursors among themselves or with constituents of the substrate.

These non-carbon anodes have not as yet been commercially and industrially applied and there is still a need for a metal-based anodic material for aluminium production.

Summary of the Invention

The present invention relates primarily to a method of forming a hematite-containing material suitable for use in a high temperature oxidising and/or corrosive environment. The method comprises sintering by heat treatment a particle mixture made predominantly of sinterable hematite particles and substantially non- sinterable hematite particles, with a weight ratio sinterable hematite/substantially non-sinterable hematite in the range of 0.2 to 3, in particular 0.2 to 1.4. During the heat treatment the sinterable hematite particles sinter together and the substantially non- sinterable hematite particles inhibit aggregation of voids produced by the sintering.

When a mass of sinterable hematite particles is consolidated by sintering, the mass undergoes a volume contraction between 30 to 50% which results in the formation of cracks. However, it has been observed that the addition to the sinterable hematite particles of substantially non- sinterable hematite particles inhibits the formation of such cracks during sintering. The effect of the substantially - non-sinterable hematite particles can be reinforced by further adding: iron metal and/or ferrous oxide particles, and/or substantially non-sinterable non- oxide ceramic particles, or other appropriate additives, such as sodium silicate, as explained below. The substantially non-sinterable hematite and non- oxide ceramic particles are chemically and physically substantially inert during the sintering process. However, their presence physically inhibits aggregation of the voids formed by the sintering contraction of the hematite-based material. Thus, instead of forming compact portions of hematite separated by cracks formed by aggregation of voids, the sintering process with the substantially non-sinterable particles produces a continuous crack-free hematite-based material having throughout a generally homogeneous microporosity. This microporosity results from the local sintering contraction of the hematite which forms micropores that are inhibited from significantly migrating in the hematite-based material by the presence of the substantially non-sinterable particles that act as barriers against significant pore migration.

When the particle mixture also contains iron metal and/or ferrous oxide and is sintered in an oxidising atmosphere, the iron metal when present is oxidised into ferrous oxide and the ferrous oxide is oxidised into hematite. The formation of hematite from the ferrous oxide results in a volume expansion such that it fills pores of the hematite-based material and contributes to inhibit formation of cracks. Brief Description of Drawings

The invention will be further described by way of example with reference to the accompanying schematic drawings, in which: - Figure 1 schematically shows the structure of sinterable hematite particles;

- Figures 2 schematically shows the structure of a sintered body made solely from sinterable hematite particles;

- Figure 3 schematically shows the structure of a substantially non-sinterable hematite particle;

- Figure 4 schematically shows the cracked structure of a comparative sintered body made solely from sinterable hematite particles; and

- Figure 5 schematically shows the non-cracked structure of a sintered body made from a mixture of sinterable and non-sinterable hematite particles according to the invention. Detailed Description

Figure 1 schematically shows three sinterable hematite particle 10. Each particle 10 is in the form of a single grain of one or more hematite crystals that are schematically indicated by rectangles 11. The free surface of such a particle 10 is high compared to its volume. This permits a significant diffusion of material at the particle's surface during sintering and a good bonding between neighbouring particles 10. It is possible to use sinterable hematite particles containing more than one grain. However, the larger the particle the lesser the (relative) material diffusion and the weaker the bonding between neighbouring particles.

The sinterable hematite particles 10 can have an average size in the range from 0.07 to 1.4 micron, typically below 0.4 micron. The sinterable hematite particles may have an apparent density in the range of

5.1 to 5.7 g/cm³, in particular from 5.2 to 5.4 g/cm³.

Figure 2 illustrates the structure of a sintered body 20 made solely from sinterable hematite particles 10 shown in Figure 1. The sintered body 20 is made of an aggregation of groups 15 of sintered single hematite grains 10. During sintering, the hematite crystals 11' in particles 10 tend to combine and increase in size.

Particles 10 of groups 15 are bonded to one another by large interdiffusion of material over a significant part 10' of their surfaces. Groups 15 are connected together by sintering at locations 15' that are spaced apart over the groups' outer surfaces. Obviously, the bonding between groups 15 is much weaker than the bonding between particles 10 inside a group 15. Pores 25 are formed between groups 15. It is common that groups 15 aggregate in such a way that pores 25 form larger cracks 30 between series of groups 15 (see Figure 4) . Smaller pores 26 are also formed in groups 15 between particles 10. Overall, the sintering of a body of sinterable hematite particles 10 is accompanied by a volume contraction between 30 to 50%.

Figure 3 schematically shows a substantially non- sinterable hematite particle 10a which is produced by grinding the sintered body 20 of Figure 2. Particle 10a corresponds to a group 15 made from sinterable particles 10 and can be been obtained by breaking the (weaker) bonding at locations 15' of a group 15 of body 20 during grinding. As can be seen in Figure 3, particle 10a which is made of an agglomeration of hematite grains 10 has an irregular outer surface. The size and the shape of particle 10a are very unfavourable for using particle 10a for sintering. Such particles do not sinter properly.

The substantially non-sinterable hematite particles 10a can have an average size in the range of 0.2 to 3.3 micron, typically greater than 0.5 micron. Substantially non-sinterable hematite particles 10a can have an apparent density in the range of 4 to 4.9 g/cm³, in particular from 4.4 to 4.8 g/cm³. This reduced density (compared to the sinterable hematite particles 10) is due to the porosity between the hematite grains 10 of the non-sinterable hematite particles 10a.

Figure 4 discloses a comparative sintered structure

20 made solely from sinterable hematite particles 10.

Like in Figure 2, the sintered body 20 is made of an aggregation of groups 15 of sintered single hematite grains 10. The sintering contraction leads to an arrangement of groups 15 such that pores 25 form larger cracks 30 between series of groups 15. Adjacent crack 30, groups 15 have irregularities at locations 15' at their surfaces caused by the formation of crack 30 which breaks the bonding between facing groups 15 across crack 30.

Figure 5 discloses a sintered structure 20' according to the invention made from a mixture of sinterable hematite particles 10 and substantially non- sinterable hematite particles 10a. The sinterable hematite particles 10 are distributed between the substantially non-sinterable hematite particles 10a. As discussed above, the substantially non-sinterable hematite particles 10a form only weak connections between themselves. However, on the one hand they do not participate in the volume contraction during sintering and on the other hand they inhibit large aggregation of particles 10 and 10a. The presence of the substantially non-sinterable particles ensures that no cracks are formed in structure 20' during sintering.

Typically, pores 26 in groups 15 have a size of the order of 10 to 20 nanometer. Pores 25 between groups 15 can have a maximum size from 0.1 to 1 micron. Conversely, cracks 30, as shown in Figure 4, can have a size of 0.1 to 1 mm.

The particle mixture of the invention may further comprise particles of one or more metallic and/or oxide additives and/or precursors thereof, in particular an additive selected from Ti, Y, Yb, Ta, Mn, Zn, Zr, Ce, Ni, Fe, Cu, Ag, Pd, Pt, Co, Cr, Al, Ga, Ge, Hf, In, Ir, Mo, Mg, Nb, Re, Rh, Ru, Se, Si, Sn, Ti, V, W, Li, Ca, Ce and Nb as metal (s) and/or oxide (s).

In particular, the electrical/electrochemical properties of the hematite-containing material can be improved by selecting at least one of the further constituents from oxides of Ti, Y, Yb, Ta, Mn, Mg, Zn, Zr, Ce, Cu and Ni and/or a heat-convertible precursor. Such selected further constituents can be present in the hematite-containing material in a total amount of 1 to 15 weight%. Usually, it is sufficient for these selected further constituents to be present in a catalytic amount to achieve the electrical/electrochemical effect, in particular in a total amount of up to 5 or 12 weight%.

Alternatively or cumulatively, the hematite- containing material can comprise at least one constituent selected from Cu, Ag, Pd, Pt, Co, Cr, Al, Ga, Ge, Hf, In, Ir, Mo, Mn, Mg, Nb, Re, Rh, Ru, Se, Si, Sn, Ti, V, W, Li, Ca, Ce and Nb as metal (s) and/or oxide (s), which can be added to the particle mixture as such or as a precursor, in the form of particles or in solution, for example a salt such as a chloride, sulfate, nitrate, chlorate or perchlorate, or a metal organic compound such as an alkoxide, formate or acetate. These selected constituents can be present in the resulting hematite-containing material in a total amount of 0.5 to 15 weight%, preferably from 0.5 to 5 weight%, in particular from 1 to 3 weight%.

Small amounts of copper or copper oxides, i.e. less than 12 or 15 weight%, increase the electrical conductivity of the hematite-containing material and diffusion of iron oxide (and possibly other oxides) during the sintering of the particle mixture. This leads to the production of more conductive and denser hematite- containing material than without the use of copper metal and/or oxides.

As explained above, in addition to the sinterable and substantially non-sinterable hematite particles, the particle mixture can contain iron metal (Fe) and/or ferrous oxide (FeO) to inhibit the formation of cracks. Typically, the particle mixture has a weight ratio iron metal/sinterable hematite below 2, in particular in the range from 0.1 to 1, or a weight ratio ferrous oxide/sinterable hematite below 2.5, in particular in the range from 0.12 to 1.3. The particle mixture can contain both iron metal and ferrous oxide, with weight ratios iron metal/sinterable hematite and ferrous oxide/sinterable hematite that are in pro rata with the above ratios.

For example, the particle mixture consists of: 65 to 100 weight% of substantially non-sinterable and sintering-active hematite particles with a weight ratio sinterable hematite/substantially non-sinterable hematite in the range of 0.2 to 1; and 0 to 35 weight% of particles of one or more metallic and/or oxide additives and/or precursors thereof.

The particle mixture may also comprise substantially non-sinterable non-oxide ceramic particles, in particular ceramic particles selected from particles of nitrides and carbides, such as boron nitride, aluminium nitride, silicon nitride, silicon carbide, tungsten carbide or zirconium carbide. Non-oxide ceramics being usually less resistant to oxidation than hematite, the amount of non- oxide ceramic particles in the particle mixture is preferably maintained at a low value, e.g. below 20 or even below 15 weight%. However, when the hematite- containing material is exposed to oxidation conditions that are less severe than when it is used as an anodic material for aluminium production, the hematite- containing material can contain up to 25 weight% non- oxide ceramic particles.

When substantially non-sinterable non-oxide ceramic particles are used as a barrier against void aggregation during sintering in the particle mixture, the ratio of sinterable hematite particles/non-sinterable hematite particles can be high.

For example, the particle mixture consists of: 60 to 95 weight% of substantially non-sinterable and sintering- active hematite particles with a weight ratio sinterable hematite/substantially non-sinterable hematite in the range of 0.6 to 1.4; 5 to 25 weight% of substantially non-sinterable non-oxide ceramic particles, in particular 10 to 20 weight%; and 0 to 35 weight% of particles of one or more metallic and/or oxide additives.

Limiting the amount of constituents other than hematite also reduces the risk that during use such constituents contaminate the environment, e.g. an electrolyte of a metal electrowinning cell.

The particle mixture can be provided in a slurry that can either be applied to a substrate, or formed as a body prior to sintering. Such a slurry may comprise an organic binder which is at least partly volatilised during sintering, in particular a binder selected from polyvinyl alcohol, polyvinyl acetate, polyacrylic acid, hydroxy propyl methyl cellulose, polyethylene glycol, ethylene glycol, hexanol, butyl benzyl phthalate and ammonium polymethacrylate. The slurry may also comprise an inorganic binder, in particular a colloid, such as a colloid selected from lithia, sodium oxide, beryllium oxide, magnesia, alumina, silica, titania, vanadium oxide, chromium oxide, manganese oxide, iron oxide, gallium oxide, yttria, zirconia, niobium oxide, molybdenum oxide, ruthenia, indium oxide, tin oxide, tantalum oxide, tungsten oxide, thallium oxide, ceria, hafnia and thoria, and precursors thereof such as hydroxides, nitrates, acetates and formates thereof, all in the form of colloids; and/or an inorganic polymer, such as a polymer selected from lithia, beryllium oxide, alumina, silica, sodium silicate, titania, chromium oxide, iron oxide, nickel oxide, gallium oxide, zirconia, niobium oxide, ruthenia, indium oxide, tin oxide, hafnia, tantalum oxide, ceria and thoria, and precursors thereof such as hydroxides, nitrates, acetates and formates thereof, all in the form of inorganic polymers. When sodium silicate is used, for example in the form of an inorganic polymer as discussed above or a particulate or a precursor solution, it contributes to inhibit the formation of cracks during sintering of the sinterable hematite powder. Typically, when the hematite particles are mixed with 0.5 to 5 weight% sodium silicate, such as 1 to 3 weight%, the weight ratio sinterable hematite/substantially non-sinterable hematite can be in the range from 1 to 3, in particular from about 1.5 to about 2.5. Typically, the particle mixture is sintered by heat treatment at a temperature in the range from 800° to 1400°C, in particular from 850° to 1150°C. The particle mixture can be sintered by heat treatment for 1 to 48 hours, in particular for 5 to 24 hours. Usually, the particle mixture is consolidated by heat treatment in an atmosphere containing 10 to 100 mol% 0₂. To produce a cermet, i.e. a sintered body containing sintered hematite particles and metallic particles, the heat treatment is preferably carried out in an inert atmosphere.

One embodiment of the invention comprises providing a metal-based substrate, applying one or more layers of the particle mixture thereto and sintering the particle mixture on the substrate to form a protective coating of the hematite-containing material thereon.

The substrate can be metallic, a ceramic or a cermet or metallic optionally with an oxide layer onto which the particle mixture is applied. For example the oxide layer is integral with the substrate and is obtained by surface oxidation of the substrate.

Usually, the substrate comprises at least one metal selected from chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, silicon, tin, tantalum, titanium, tungsten, vanadium, yttrium and zirconium or an oxide thereof. For instance, the substrate comprises an alloy of iron, in particular an iron-nickel alloy optionally containing at least one further element selected from cobalt, copper, aluminium, yttrium, manganese, silicon and carbon.

The application of inorganic colloidal and/or polymeric slurries on metal substrates are disclosed in US Patents 6,361,681 (de Nora/Duruz) and 6,365,018 (de Nora) .

Advantageously, the method of the invention comprises oxidising the surface of a metallic substrate to form thereon an integral anchorage layer to which the protective coating is bonded by sintering during heat treatment. Such an integral layer may in particular contain an oxide of iron and/or another metal, such as nickel, that is sintered during the heat treatment with iron oxide from the particle mixture. Further details on such an anchoring of the protective coating are disclosed in WO03/087435 (Nguyen/de Nora) .

When used for aluminium electrowinning, the protected metal-based substrate preferably contains at least one metal selected from nickel, iron, cobalt, copper, aluminium and yttrium. Suitable alloys for such a metal-based substrate are disclosed in US Patent 6,372,099 (Duruz/de Nora), and WO00/06803 (Duruz/ de Nora/Crottaz) , WO00/06804 (Crottaz/Duruz) , WO01/42534 (de Nora/Duruz) , WO01/42536 (Duruz/Nguyen/de Nora) , WO02/08399 (Nguyen/de Nora) , WO02/097168 (Nguyen/ Duruz/de Nora) and WO03/078695 (Nguyen/de Nora) .

Another embodiment of the method of the invention comprises sintering a shaped bulk of the mixture to form a self-sustaining body. Such body can have any appropriate shape according to its end use.

Typically, hematite-containing material can be used to manufacture a component of a cell for the electrowinning of a metal such as aluminium, in particular a current carrying anodic component such as an active anode structure or an anode stem, or another cell component exposed to molten electrolyte and/or cell fumes, such as a cell cover or a powder feeder. Examples of such cell components are disclosed in WO00/40781 and

WO00/40782 (both de Nora), WO00/63464 (de Nora/Berclaz) ,

WO01/31088 (de Nora), WO02/070784 and US2003/0102228

(both de Nora/Berclaz) . The hematite-containing material of such cell components can be sintered before use by heat treating the components.

Advantageously, the particle mixture can be sintered by heat treating the cell component over the cell. By carrying out the sintering heat-treatment immediately before use, thermal shocks and stress caused by cooling and re-heating of the component between consolidation and use can be avoided.

Another aspect of the invention relates to a method of electrowinning a metal such as aluminium. The method comprises manufacturing as described above a current- carrying anodic component, installing the anodic component in a molten electrolyte containing a dissolved compound of the metal to electrowin, such as alumina, and passing an electrolysis current from the anodic component to a facing cathode in the molten electrolyte to evolve oxygen anodically and produce the metal cathodically. The electrolyte can be a fluoride-based molten electrolyte, in particular containing fluorides of aluminium and sodium. Further details of suitable electrolyte compositions are for example disclosed in WO02/097167 (Nguyen/de Nora) and PCT/IB03/04649 (de Nora/ Nguyen/Duruz) .

The cell can be operated with an electrolyte maintained at a temperature in the range from 800° to 960°C, in particular from 880° to 940°C. Preferably, to reduce the solubility of metal-based cell components, an alumina concentration which is at or close to saturation is maintained in the electrolyte, particularly adjacent the anodic component.

An amount of iron species can also be maintained in the electrolyte to inhibit dissolution of the hematite- containing material of the anodic component. Further details on such a cell operation are disclosed in the above mentioned US Patent 6,372,099.

The invention relates also to method of electrowinning a metal such as aluminium, that comprises manufacturing as disclosed above a cover, placing the cover over a metal production cell trough containing a molten electrolyte in which a compound of the metal to electrowin is dissolved, passing an electrolysis current in the molten electrolyte to evolve oxygen anodically and metal cathodically, and confining electrolyte vapours and evolved oxygen within the cell trough by means of the cover.

Further features of cell covers are disclosed in US Patents 6,402,928 (de Nora/Sekhar) , WO02/070784 and US2003/0102228 (both de Nora/Berclaz) .

A further aspect of the invention relates to a hematite-containing material suitable for use in a high temperature oxidising and/or corrosive environment, producible by the above method. The material is microporous and at least substantially crack-free. Yet a further aspect of the invention concerns a cell for the electrowinning of a metal such as aluminium, comprising at least one component having an outer part made of the above hematite-containing material.

Examples of starting compositions of particle mixtures for producing hematite-containing materials, in particular protective coatings, according to the invention are given in Table 1, which shows the weight percentages of the indicated constituents for each specimen Al-Nl.

TABLE 1

In Table 1, the sinterable hematite is designated by "S-Fe₂0₃". The substantially non-sinterable hematite is designated by "NS-Fe₂0₃". Examples of alloy compositions of substrates for application of protective coatings according to the invention are given in Table 2, which shows the weight percentages of the indicated metals for each specimen A2- U2.

TABLE 2

The "other" elements refer to minor elements such as manganese, silicon and yttrium which may be present in individual amounts of 0.2 to 1.5 weight%. Usual small impurities, such as carbon, have not been taken into account in the composition samples listed in Table 2.

Comparative Example 1

A non-coated anode was manufactured from an anode rod of diameter 20 mm and total length 20 mm made of a cast alloy having the composition of sample A2 of Table 2. The anode rod was supported by a stem made of an alloy containing nickel, chromium and iron, such as Inconel, protected with an alumina sleeve. The anode was suspended for 16 hours over a molten cryolite-based electrolyte at 925°C whereby its surface was oxidised.

Electrolysis was carried out by fully immersing the anode rod in the molten electrolyte. The electrolyte contained 18 weight% aluminium fluoride (A1F₃) , 6.5 weight% alumina (A1₂0₃) and 4 weight% calcium fluoride

(CaF₂), the balance being cryolite (Na₃AlF₆) .

The current density was about 0.8 A/cm² and the cell voltage was at 3.5-3.8 volt throughout the test. The concentration of dissolved alumina in the electrolyte was maintained during the entire electrolysis by periodically feeding fresh alumina into the cell.

After 50 hours electrolysis was interrupted and the anode extracted. Upon cooling the anode was examined externally and in cross-section. The anode's outer dimensions had remained substantially unchanged. The anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 500 micron.

Samples of the used electrolyte and the product aluminium were also analysed. It was found that the electrolyte contained 150-280 ppm nickel and the product aluminium contained roughly 1000 ppm nickel.

Comparative Example 2

Another comparative aluminium electrowinning anode was prepared as follows: A slurry for coating an anode substrate was prepared by suspending in 32.5 g of an aqueous solution containing

5 weight% polyvinyl alcohol (PVA) 67.5 g of a nitride/carbide-free particle mixture made of 86 weight% commercially available Fe₂0₃ particles (e.g. produced by

Alfa Aesar™) , 10 weight% Ti0₂ particles and 2 weight% CuO particles (with particle sizes of -325 mesh, i.e. smaller than 44 micron) . Hence, this slurry contained sinterable hematite particles but was free of substantially non-sinterable hematite particles.

An anode substrate having the composition and the dimensions of the cast anode rod of Comparative Example 1 was covered with six layers of this slurry that were applied with a brush. The applied layers were dried for 10 hours at 140 °C in air and then consolidated at 950 °C for 16 hours to form a hematite-based coating which had a thickness of 0.24 to 0.26 mm.

During consolidation, all the Fe₂0₃ particles (as depicted in Figure 1) were sintered together into a matrix (as depicted in Figure 2) with a volume contraction. Pores formed by this contraction had agglomerated to form small cracks (as depicted in Figure 4) that had a length of the order of 1.5 mm and a width of up to 20 micron. The Ti0₂ particles and CuO particles were dissolved in the sintered Fe₂0₃.

Example 1

Pqwder__Prep_aration_:

Substantially non-sinterable hematite particles for use in the particle mixture of the invention were prepared as follows:

A commercial hematite (Fe₂0₃) particulate -325 mesh (i.e. smaller than 44 micron) produced by Alfa Aesar™ having a measured average particle size of about 0.2 micron (as depicted in Figure 1) and made essentially of single grain particles having an apparent density of about 5.3 g/cm³ was shaped into a body and then heat treated for 15 minutes at 1000 °C. At this temperature the hematite particulate self-sintered to consolidate the hematite body (as depicted in Figure 2) . During the sintering, the consolidating hematite body underwent a volume contraction of about 40%.

After cooling, the sintered hematite body was ground to form substantially non-sinterable hematite particles (as depicted in Figure 3) having an average size of about 0.6 micron. The particles consisted of an agglomeration of a plurality of sintered hematite grains having an apparent density of about 4.6 g/cm³. When such a substantially non-sinterable hematite particulate is subjected to the same heat treatment, i.e. 15 minutes at 1000°C, substantially no volume contraction occurs, i.e. typically less than 1% contraction, indicating that the particulate remains substantially non-sintered. Also, unlike a sintered body made from sinterable hematite particles, such a heat treated body made from substantially non-sinterable hematite particles is very brittle and can be easily destroyed by hand because of the very poor sintering of the particles. Example 2

An aluminium electrowinning anode with a boron nitride-containing hematite coating was prepared according to the invention as follows: A slurry for coating an anode substrate was prepared by suspending in 32.5 g of an aqueous solution containing 5 weight% polyvinyl alcohol (PVA) 67.5 g of a particle mixture made of sinterable hematite (Fe₂0₃) particles

(namely in this Example, the commercial sinterable hematite particles used as starting material in Example

1) , substantially non-sinterable hematite (Fe₂0₃) particles (namely in this Example, the substantially non- sinterable hematite particles as prepared in Example 1), substantially non-sinterable boron nitride (BN) particles, zinc oxide (ZnO) particles and copper oxide (CuO) particles (with a particle size of -325 mesh, i.e. smaller than 44 micron) in a weight ratio corresponding to sample Dl of Table 1. An anode substrate having the composition and the dimensions of the cast anode rod of Comparative Example 1 was covered with ten layers of this slurry that were applied with a brush. The applied layers were dried for 10 hours at 140 °C in air and then consolidated at 950 °C for 15 minutes to form a protective hematite-based coating which had a thickness of about 0.5 to 0.6 mm.

During consolidation, the sinterable Fe₂0₃ particles were sintered together into a rαicroporous matrix with a volume contraction. The ZnO particles and CuO particles were dissolved in the sintered Fe₂0₃. The boron nitride particles and the substantially non-sinterable Fe₂0₃ particles remained substantially inert during the sintering but prevented migration and agglomeration of the micropores into cracks. Hence, as opposed to Comparative Example 2, the hematite-containing protective layer was crack-free even though it was thicker, and thus this boron nitride-containing hematite layer was able better to inhibit diffusion from and to the metal-based substrate.

Underneath the coating, an integral oxide scale mainly of iron oxide had grown from the substrate during the heat treatment and combined with iron oxide and zinc oxide from the coating to firmly anchor the coating to the substrate. The integral oxide scale contained zinc oxide in an amount of about 10 metal weight%. Minor amounts of copper, aluminium and nickel were also found in the oxide scale (less that 5 metal weight% in total) .

Example 3 An de__TestjLncr

An anode was prepared as in Example 2 by covering an iron-alloy substrate with layers of a slurry containing a particle mixture of sinterable Fe₂0₃, substantially non- sinterable Fe₂0₃, BN, ZnO and CuO.

The applied layers were dried for 5 hours at 200 °C and then consolidated by suspending the anode for 16 hours over a cryolite-based electrolyte at about 925 °C. The electrolyte contained 18 weight% aluminium fluoride (A1F₃) , 6.5 weight% alumina (A1₂0₃) and 4 weight% calcium fluoride (CaF₂) , the balance being cryolite (Na₃AlF₆) .

Upon consolidation of the layers, the anode was immersed in the molten electrolyte and an electrolysis current was passed from the anode to a facing cathode through the alumina-containing electrolyte to evolve oxygen anodically and produce aluminium cathodically. A high oxygen evolution was observed during the test. The current density was about 0.8 A/cm² and the cell voltage was stable at 3.1-3.2 volt throughout the test.

Compared to an uncoated anode, i.e. the anode of comparative Example 1, the coating of an alloy-anode with an oxide protective layer according to the invention led to an improvement of the anode performance such that the cell voltage was stabilised and also reduced by 0.4 to 0.6 volt, which corresponds to about 10 to 20%, thus permitting tremendous energy savings.

After 50 hours, the anode was extracted from the electrolyte and underwent cross-sectional examination. The dimension of the coating had remained substantially unchanged. However, ZnO had selectively been dissolved in the electrolyte from the protective coating. The integral oxide layer of the anode substrate had grown to a thickness of 200 micron, i.e. at a much slower rate than the oxide layer of the uncoated anode of Comparative Example 1.

Samples of the used electrolyte and the product aluminium were also analysed. It was found that the electrolyte contained less that 70 ppm nickel and the produced aluminium contained less than 300 ppm nickel which is significantly lower than with the uncoated anode of Comparative Example 1. This demonstrated that the protective coating of the invention constituted an efficient barrier reducing nickel dissolution from the anode's alloy, inhibiting contamination of the product aluminium by nickel. Example 4

Another aluminium electrowinning anode with a boron nitride-free hematite coating was prepared according to the invention as follows:

A slurry for coating an anode substrate was prepared by suspending in 32.5 g of an aqueous solution containing 5 weight% polyvinyl alcohol (PVA) 67.5 g of a particle mixture made of sinterable hematite (Fe₂0₃) particles, substantially non-sinterable hematite (Fe₂0₃) particles, zinc oxide (ZnO) particles and copper oxide (CuO) particles (with a particle size of -325 mesh, i.e. smaller than 44 micron) in a weight ratio corresponding to sample Jl of Table 1.

An anode substrate having the composition and the dimensions of the cast anode rod of Comparative Example 1 was covered with ten layers of this slurry and then heat treated like in Example 2 to form a protective hematite- based coating which had a thickness of about 0.5 to 0.6 mm. During consolidation, the sinterable Fe₂0₃ particles were sintered together into a microporous matrix with a volume contraction. The ZnO particles and CuO particles were dissolved in the sintered Fe₂0₃. The substantially non-sinterable Fe₂0₃ particles remained substantially inert during the sintering, preventing migration and agglomeration of the micropores into cracks like in Example 2.

Such an anode can be tested in an aluminium production electrolyte like in Example 3, which leads to similar results.

Example 5

An electrowinning aluminium anode with a hematite coating was prepared by using a sodium silicate- containing slurry according to the invention:

The slurry was prepared by suspending in 32.5 g of an aqueous solution containing 5 weight% sodium silicate 67.5 g of a particle mixture corresponding to sample Ml of Table 1, i.e. made of sinterable hematite (Fe₂0₃) particles, substantially non-sinterable hematite (Fe₂0₃) particles, boron nitride (BN) particles, zinc oxide (ZnO) particles and copper oxide (CuO) particles (with a particle size of -325 mesh, i.e. smaller than 44 micron).

An anode substrate having the composition and the dimensions of the cast anode rod of Comparative Example 1 was covered with ten layers of this slurry and then heat treated like in Example 2 to form a protective hematite- based coating.

During consolidation, the sinterable Fe₂0₃ particles were sintered together into a microporous matrix with a volume contraction. The ZnO particles and CuO particles were dissolved in the sintered Fe₂0₃. The boron nitride particles and the substantially non-sinterable Fe0₃ particles remained substantially inert during the sintering, preventing migration and agglomeration of the micropores into cracks like in Example 2. Such an anode can be tested in an aluminium production electrolyte like in Example 3, which leads to similar results.

Example 6

Examples 2 to 4 can be repeated using different combinations of coating compositions (Al-Nl) selected from Table 1 and metal alloy compositions (A2-U2) selected from Table 2.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that alternatives, modifications, and variations will be apparent to those skilled in the art.

Claims

1. A method of forming a hematite-containing material in particular for use in a high temperature oxidising and/or corrosive environment, comprising: sintering by heat treatment a particle mixture comprising predominantly:

- sinterable hematite particles; and

- substantially non-sinterable hematite particles, with a weight ratio sinterable hematite/substantially non- sinterable hematite in the range of 0.2 to 3, in particular from 0.2 to 1.4, whereby the sinterable hematite particles sinter together and the substantially non-sinterable hematite particles inhibit aggregation of voids produced by the sintering.

2. The method of claim 1, wherein the sinterable hematite particles comprise single hematite grain particles .

3. The method of claim 1 or 2, wherein the sinterable hematite particles have an apparent density in the range of 5.1 to 5.7 g/cm³, in particular from 5.2 to 5.4 g/cm³.

4. The method of any preceding claim, wherein the substantially non-sinterable hematite particles comprise particles of agglomerated hematite grains producible by sintering single hematite grain particles and grinding.

5. The method of any preceding claim, wherein the substantially non-sinterable hematite particles have an apparent density in the range of 4 to 4.9 g/cm³, in particular from 4.4 to 4.8 g/cm³.

6. The method of any preceding claim, wherein the particle mixture further comprises particles of one or more metallic and/or oxide additives and/or precursors thereof, in particular an additive selected from Ti, Y, Yb, Ta, Mn, Zn, Zr, Ce, Ni, Fe, Cu, Ag, Pd, Pt, Co, Cr, Al, Ga, Ge, Hf, In, Ir, Mo, Mg, Nb, Re, Rh, Ru, Se, Si, Sn, Ti, V, W, Li, Ca, Ce and Nb as metals and/or oxides.

7. The method of claim 6, wherein the particle mixture comprises iron metal particles and/or ferrous oxide particles .

8. The method of claim 6 or 7 , wherein the particle mixture consists of: at least 65 weight% of said substantially non- sinterable and sinterable hematite particles with a weight ratio sinterable hematite/substantially non- sinterable hematite in the range of 0.2 to 1; and - up to 35 weight% of said particles of one or more metallic and/or oxide additives and/or precursors thereof.

9. The method of any one of claims 1 to 7, wherein the particle mixture further comprises substantially non- sinterable non-oxide ceramic particles, in particular particles of nitrides and/or carbides.

10. The method of claim 9, wherein the particle mixture consists of:

60 to 95 weight% of said substantially non-sinterable and sinterable hematite particles with a weight ratio sinterable hematite/substantially non-sinterable hematite in the range of 0.6 to 1.4;

5 to 25 weight% of said substantially non-sinterable non-oxide ceramic particles, in particular 10 to 20 weight%; and

0 to 35 weight% of particles of one or more metallic and/or oxide additives.

11. The method of any preceding claim, comprising providing the particle mixture in a slurry prior to sintering.

12. The method of claim 11, wherein the slurry comprises an organic binder, in particular selected from polyvinyl alcohol, polyvinyl acetate, polyacrylic acid, hydroxy propyl methyl cellulose, polyethylene glycol, ethylene glycol, hexanol, butyl benzyl phthalate and ammonium polymethacrylate .

13. The method of claim 11 or 12, wherein the slurry comprises an inorganic binder, in particular a colloid, such as a colloid selected from lithia, sodium oxide, beryllium oxide, magnesia, alumina, silica, titania, vanadium oxide, chromium oxide, manganese oxide, iron oxide, gallium oxide, yttria, zirconia, niobium oxide, molybdenum oxide, ruthenia, indium oxide, tin oxide, tantalum oxide, tungsten oxide, thallium oxide, ceria, hafnia and thoria, and precursors thereof such as hydroxides, nitrates, acetates and formates thereof, all in the form of colloids; and/or an inorganic polymer, such as a polymer selected from lithia, beryllium oxide, alumina, silica, sodium silicate, titania, chromium oxide, iron oxide, nickel oxide, gallium oxide, zirconia, niobium oxide, ruthenia, indium oxide, tin oxide, hafnia, tantalum oxide, ceria and thoria, and precursors thereof such as hydroxides, nitrates, acetates and formates thereof, all in the form of inorganic polymers.

14. The method of any preceding claim, wherein the sinterable hematite particles and substantially non- sinterable hematite particles are mixed with sodium silicate.

15. The method of claim 14, wherein the hematite particles are mixed with 0.5 to 5 wt% sodium silicate, such as 1 to 3 weight%, the weight ratio of sinterable hematite/substantially non-sinterable hematite being in the range from 1 to 3, in particular from 1.5 to 2.5.

16. The method of any preceding claim, wherein the particle mixture is sintered by heat treatment at a temperature in the range from 800° to 1400 °C, in particular from 850° to 1150°C.

17. The method of any preceding claim, wherein the particle mixture is sintered by heat treatment for 1 to 48 hours, in particular for 5 to 24 hours.

18. The method of any preceding claim, wherein the particle mixture is sintered by heat treatment in an atmosphere containing 10 to 100 mol% 0₂.

19. The method of any preceding claim, comprising providing a substrate, applying one or more layers of the particle mixture thereto and sintering the particle - 27 -

mixture to form a protective coating of the hematite- containing material on the substrate.

20. The method of claim 19, wherein the substrate is metallic, a ceramic or a cermet optionally with an oxide layer onto which the particle mixture is applied.

21. The method of claim 18, wherein the substrate comprises at least one metal selected from chromium, cobalt, hafnium, iron, molybdenum, nickel, copper, niobium, platinum, silicon, tantalum, titanium, tungsten, vanadium, yttrium and zirconium.

22. The method of claim 21, wherein the substrate comprises an alloy of iron containing at least one further element in particular selected from nickel, cobalt, copper, aluminium, yttrium, manganese, silicon and carbon.

23. The method of any one of claims 20 to 22, comprising oxidising the surface of a metallic or cermet substrate to form thereon an integral anchorage layer to which the protective coating is bonded by heat treatment.

24. The method of any one of claims 1 to 18, comprising sintering a bulk of the particle mixture into a self- sustaining body.

25. The method of any preceding claim for manufacturing a component of a metal electrowinning cell, in particular an aluminium electrowinning cell, which during use is exposed to molten electrolyte and/or cell fumes.

26. The method of claim 25 for manufacturing a current carrying anodic component .

27. The method of claim 25 for manufacturing a cover.

28. The method of any one of claims 25 to 27, comprising sintering the particle mixture by heat treating the cell component over the cell.

29. A method of electrowinning a metal such as aluminium, comprising manufacturing a current-carrying anodic component by the method of claim 26, installing the anodic component in a molten electrolyte containing a dissolved compound of the metal to electrowin such as alumina, and passing an electrolysis current from the anodic component to a facing cathode in the molten electrolyte to evolve oxygen anodically and produce the metal cathodically.

30. The method of claim 29, wherein the electrolyte is a fluoride-based molten electrolyte.

31. The method of claim 29 or 30, comprising maintaining the electrolyte at a temperature in the range from 800° to 960°C, in particular from 880° to 940°C.

32. The method of any one of claims 29 to 31, comprising maintaining in the electrolyte an alumina concentration which is at or close to saturation.

33. The method of any one of claims 29 to 32, comprising maintaining in the electrolyte an amount of iron species to inhibit dissolution of the anodic component.

34. A method of electrowinning a metal such as aluminium, comprising manufacturing a cover by the method of claim 27, placing the cover over a metal electrowinning cell trough containing a molten electrolyte in which a compound of the metal to electrowin is dissolved, passing an electrolysis current in the molten electrolyte to evolve oxygen anodically and the metal cathodically, and confining electrolyte vapours and evolved oxygen within the cell trough by means of the cover.

35. A sintered hematite-containing material suitable for use in a high temperature oxidising and/or corrosive environment, which is producible by the method of any one of claims 1 to 28 and which is microporous and substantially crack-free.

36. A cell for the electrowinning of a metal such as aluminium, having at least one component made at least in part of the material defined in claim 35.