HK1107374A - Electrolytic reduction of metal oxides such as titanium dioxide and process applications - Google Patents

Description

Electrolytic reduction of metal oxides, such as titanium dioxide, and process applications thereof

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This application is a divisional application of application 01805455.2 filed on 8/12/2002.

The present invention relates to improvements in the electrolytic reduction of metal compounds, particularly in the reduction of titania to produce titanium metal.

International patent specification PCT/GB99/01781 describes a method of removing oxygen from metals and metal oxides by electrolytic reduction. Therefore, it is referred to as "electrolytic reduction method" in this document. The method involves electrolysis of an oxide in a molten salt and wherein the electrolysis is carried out under conditions such that a reaction of oxygen occurs at the electrode surface rather than a cation deposition reaction of the salt and such that oxygen dissolves in the electrolyte. The metal oxide or semimetal oxide to be reduced is in the form of a solid sintered cathode.

The present inventors have developed improvements to this approach that greatly improve the effectiveness and utility of the general technique.

The general technique is described below: by passing through M₂Electrolysis of solid metals, metal compounds or semimetals M in molten salts of Y or mixtures of salts₁A method for removing oxygen from O, which comprises conducting electrolysis under conditions such that a reaction of oxygen other than M occurs at the surface of an electrode₂And oxygen is dissolved in the electrolyte M₂And Y is as defined above.

M₁Selected from Ti, Zr, Hf, Al, Mg, U, Nd, Mo, Cr, Nb, Ge, P, As, Si, Sb, Sm or any alloy thereof. M₂Any of Ca, Ba, Li, Cs and Sr may be used. Y is Cl.

Powder production by sintering metal oxide pellets

The present invention has determined that sintered pellets or powders of metal oxides, particularly titanium dioxide or semimetal oxides, can be used as electrolytic raw materials for use in the above-described processes, provided that suitable conditions exist. This has the following advantages: which can produce very efficiently titanium metal powder, which is currently very expensive, directly. In this way, the powdered titanium dioxide in pellet or powder form preferably has a diameter of 10 microns to 500 microns; more preferably 200 micron in diameter.

Semimetals are elements that have certain characteristics associated with metals, one example being boron, other semimetals being apparent to those skilled in the art.

In one embodiment of the invention, the titanium dioxide pellets comprising the cathode are placed in a basket below a carbon anode, the anode being located in a crucible containing molten salt therein. When the oxide pellets or powder particles are reduced to metal, they are prevented from sintering by keeping the particles in motion by any suitable means, such as in a fluidized bed apparatus. Agitation is provided by mechanical vibration or by injecting gas under the basket. The mechanical vibration may be in the form of, for example, an ultrasonic transducer mounted outside the crucible or on a control rod. The key variables to adjust are the vibration frequency and amplitude to achieve a certain average particle contact time that is long enough to produce reduction, but short enough to prevent the particles from binding into a solid mass. A similar principle applies to agitation by gas, but here the flow rate of the gas and the size of the bubbles are variables that control the particle contact time. An additional advantage of using this technique is that the powder batch is reduced uniformly and rapidly due to the small particle size. Agitation of the electrolyte also helps to increase the reaction rate.

In the above embodiment, titanium is obtained from titanium dioxide by the method. However, the method can be applied to most metal oxides for producing metal powders.

Production of powders by deposition of Ti on a cathode

The inventors have determined that if titanium is deposited on the cathode (based on the electrolytic process described above) from another titanium source of higher positive potential, the titanium deposited thereon is dendritic in structure. This form of titanium is easily broken into powder because the individual particles of titanium are only bonded together through small regions.

This effect can be used to produce titanium from titanium dioxide. During this refinement of the above method, a second cathode is provided, which is maintained at a more negative potential than the first cathode. When the deposition of titanium on the first cathode has proceeded sufficiently, the second electrode is switched on, causing titanium to dissolve from the first cathode and deposit onto the second cathode in dendrites.

The advantage of this method is that the dendritically deposited titanium is easily converted into powder. The process also adds an additional refining step during the reduction of the titanium dioxide, which should result in a higher product purity.

Use of continuous powder feedstock

One improvement of the electrolytic process that the present inventors have developed is to have a continuously fed metal oxide or semi-metal oxide in powder and pellet form. This allows for a constant current and higher reaction speed. Carbon electrodes are preferred for this. In addition, cheaper raw materials can be used, since the sintering and/or forming stages can be omitted. The oxide powder or pellet feed falls to the bottom of the crucible and is gradually reduced by the electrolytic process to a semi-solid mass of metal, semi-metal or alloy.

In this method, a conductive crucible is made with a cathode filled with molten salt, in which an anode is inserted. The titanium dioxide powder or pellets are fed into a crucible where they undergo reduction at the bottom of the crucible. The thick arrows indicate the increase in thickness of the reduced feedstock.

Improved feedstock for electrolytic reduction of metal oxides

The problem with the process described in WO99/64638 is that: to obtain reduction of the oxide, the electrical contact must be maintained for a period of time at a temperature at which oxygen readily diffuses. Under these conditions, titanium will diffusion bond with itself, producing a coherent mass of material rather than a free-flowing powder.

The present inventors have determined that the diffusion bonding problem can be mitigated when subjecting a sintered mass of a metal oxide mixture consisting essentially of particles generally greater than 20 microns in size and finer particles less than 7 microns to electrolysis.

Preferably, the finer particles comprise 5-70% by weight of the agglomerates. More preferably, the finer particles comprise 10-55% by weight of the agglomerates.

High density agglomerates of powder of about the desired size are made and then mixed with appropriate proportions of very fine, unsintered titanium dioxide, binder and water and formed into the desired raw material shape. This material is then sintered to achieve the required strength for the reduction process. The resulting feedstock after sintering but before reduction consists of high density aggregates in a low density (porous) matrix.

The use of such a bimodal distribution of the powder in the raw material for the sintering stage is advantageous because it reduces the shrinkage of the shaped raw material during sintering. This in turn reduces the chance of cracking and disintegration of the formed feedstock, resulting in a reduction in the number of off-spec products prior to electrolysis. The strength required or available for the sintering feedstock for the reduction process is such that the sintered feedstock has the strength required for operation. When a bimodal distribution is used in the raw materials, cracking and disintegration of the raw materials from sintering are reduced, and therefore, the proportion of the raw materials for sintering having a required strength is increased.

The raw material in the form of a briquette can be reduced using conventional methods and the result is a friable briquette that is easily broken into powder. The reason for this is that during the reduction the matrix shrinks significantly, resulting in a sponge-like structure. However, the granules shrink to form a more or less solid structure. The matrix can conduct electricity to the pellet, but is easily broken after reduction.

Titania feedstocks are produced from raw ore (sandy ilmenite) by the sulfate process, rutile or anatase involving a number of steps.

In one of these steps, titanium dioxide in the form of an amorphous slurry is calcined. The present inventors have determined that titania amorphous slurry can be used as a main raw material for producing titanium by an electrolytic reduction method and has an advantage of lower production cost than crystalline calcined titania. The electrolytic process requires sintering of the oxide powder feedstock into a solid cathode. However, it has been found that amorphous titania does not sinter well; even if mixed in advance with an organic binder, it tends to crack and disintegrate. This occurs because the fine particle size amorphous material prevents the powder from being tightly packed prior to sintering. The result is a large shrinkage during sintering, thus producing a brittle sintered product. However, it has been determined that satisfactory results after sintering can be obtained if small amounts of more expensive calcined materials are mixed with the amorphous material and the organic binder. This amount should be at least 5% of the calcined material.

Examples

1kg of rutile ore (titanium dioxide content 95%) from Richard Bay Minerals, SouthAfrica, having an average particle size of 100 microns was mixed with 10% by weight of the rutile calciner discharge from Tioxide, manufactured by the sulphate process, which had been ground in a mortar and pestle to ensure fine particle agglomerate size. An additional 2% by weight of binder (methyl cellulose) was added and the whole mix was shaken with a mechanical shaker for 30 minutes to ensure that a homogeneous raw material was produced. The resulting material was then mixed with distilled water until the consistency of the paste was close to that of putty. This material was then laid flat by hand on an aluminium foil approximately 5 mm thick and then scribed to a square 30 mm on a side by a scalpel. The material was then dried overnight in a drying oven at 70 ℃. Upon removal from the dry box, the rutile may then be peeled from the aluminum foil and cut into squares as marked with a scalpel. The binder provides significant strength to the stock and so a 5 mm diameter hole can be drilled in the centre of each square for mounting on the electrode at a later stage. Since no shrinkage is expected during the sintering stage, it is not necessary to calculate the shrinkage margin when calculating the size of the pores.

Approximately 50 squares of rutile were charged into a room temperature air furnace, and the furnace was then turned on and heated to 1300 c (about 30 minutes for heating time) at its natural rate. After 2 hours of incubation at this temperature, the furnace was turned off and allowed to cool naturally (about 20 ℃ per minute at the beginning). When the rutile is below 100 ℃, it is removed from the furnace and stacked on a M5 threaded stainless steel rod used as a current carrier. The total rutile loading was 387 grams. The bulk density of this form of the feedstock was measured to be 2.33 ± 0.07kg/l (i.e. 55% compactness) and its strength for handling was found to be sufficient.

The feedstock was then electrolyzed at an electrolyte temperature of 1000 ℃ for 51 hours at up to 3V using the method described in the above-mentioned patent application. The material obtained after washing and removal of the electrode rod weighed 214 grams. Oxygen and nitrogen analysis showed these interstitials to be present in an amount of 800ppm and 5ppm, respectively. The product had a form very similar to the form of the starting material, but the color changed and there was a slight shrinkage. Due to the method used to manufacture the raw material, the product is brittle and can be broken into very fine powder with fingers and pliers. Some of the particles are larger, so the material is passed through a 250 micron sieve. After using this simple crushing technique, about 65% by weight of the material can pass through a 250 micron sieve.

The resulting powder was washed in hot water to remove salt and very fine particles. Then washed in glacial acetic acid to remove CaO and then finally washed again in water to remove the acid. The powder was then dried in a drying oven at 70 ℃ overnight.

The results are expressed as the concentration of the calciner discharge required to obtain a practical post-sinter feedstock strength. About 10% at 1300 ℃, about 25% at 1200 ℃ and at least 50% at 1000 ℃ are required, but still produce very weakly bonded raw materials.

The discharging of the calcining furnace can use cheaper amorphous TiO₂Instead. The key requirement for such a "matrix" material is that it is easy to sinter and has significant shrinkage during sintering. Any oxide or mixture of oxides meeting these criteria is practical. For TiO₂This means that the particle size must be less than about 1 micron. It is estimated that at least 5% of the calcined material should be present in order to provide any significant strength to the sintered product.

The raw material aggregates do not have to be rutile sand but can be produced by sintering and crushing processes, and in principle there is no reason to suggest that alloy powders cannot be produced by this method. It is contemplated that other metal powders may be made by this method.

Production of metal foams

The inventors have determined that metal or semi-metal foams can be produced by electrolysis using the above described method. Initially, a foam-like metal oxide or semimetal oxide preform is prepared, then passed through a molten salt M₂Electrolytically removing oxygen from said foam-structured metal oxide preform in a mixture of Y or a salt, which comprises carrying out the electrolysis under conditions such that a reaction of oxygen rather than M occurs on the surface of the electrode₂Deposited and oxygen dissolved in the electrolyte M₂And Y is as defined above.

Titanium foams are attractive for many applications such as filters, medical implants and structural fillers. However, to date, no reliable method has been found for producing titanium foams. Partially sintered alloy powders are similar to foams, but the production costs are high due to the high cost of titanium alloy powders, and the achievable porosity is limited to about 40%.

The present inventors have determined that if a foamed sintered titania preform is produced, it can be reduced to a solid metal foam by using the above-described electrolytic process. Various established methods can be used to produce foamed titanium dioxide materials from titanium dioxide powders. It is required that the foam preform must have open air cells, i.e., air cells that are interconnected and communicate with the outside.

In a preferred embodiment, a natural or synthetic polymer foam is wetted with a metal (e.g., titanium) or semi-metal oxide slurry, dried and fired to remove the organic foam, leaving an open "foam" that is an inverted version of the original organic foam. The sintered preform is electrolytically reduced to convert it into a titanium or titanium alloy foam. And then washed or vacuum distilled to remove salts.

In an alternative method, metal oxide or semimetal oxide powders are mixed with an organic blowing agent. These materials are typically two liquids which, when mixed, react to release a blowing gas and then cure to give a cured foam having an open or closed cell structure. The metal or semi-metal powder is mixed with one or both of the precursor liquids prior to generating the foam. The foam is then fired to remove organics, leaving a ceramic foam. This ceramic foam is then electrolytically reduced to obtain a metal, semi-metal or alloy foam.

Production of alloyed Metal Matrix Composites (MMC)

Metallic, semi-metallic or alloy MMCs reinforced with ceramic fibers or particles such as borides, carbides and nitrides are known to be difficult and expensive to produce. For SiC fiber reinforced titanium alloy MMC, existing methods all use solid state diffusion bonding to produce a 100% dense composite and differ only in the way the metal and fiber are mixed prior to hot pressing. Current methods introduce the metal in the form of a foil, wire or powder, or by plasma spray deposition onto an arrangement of fibers, or by coating individual fibers with a metal, semi-metal or alloy vapor.

For particle enhanced titanium alloy MMC, the preferred conventional production method is by powder mixing and hot pressing. Liquid phase processes are generally not preferred because of the size and distribution of the phase formed by the liquid phase. However, it is also difficult to obtain a uniform distribution of the ceramic particles by mixing the metal and ceramic powders, especially when the powders have different size ranges, which is always the case with titanium powders. In the proposed method, fine ceramic particles, such as titanium diboride powder, are mixed with titanium dioxide powder to obtain a homogeneous mixture prior to sintering and electrolytic reduction. After reduction, the product was washed and vacuum annealed to remove salts, and then hot pressed to obtain a 100% dense composite. Depending on the reaction chemistry, the ceramic particles either remain unchanged by electrolysis and hot pressing or are converted into another ceramic material which then becomes a reinforcing agent. For example, in the case of titanium diboride, the ceramic reacts with the titanium to form titanium diboride. In a variation of the new process, instead of ceramic reinforcing powders, fine metal powders are mixed with titanium dioxide powders, intended to form a finely dispersed hard ceramic or intermetallic phase by reaction with titanium or another alloying element or elements. For example, boron powder may be added and reacted in a titanium alloy to form titanium boride particles.

The present inventors have determined that for the production of fiber reinforced MMCs, individual SiC fibers may be coated with an oxide/binder slurry (or mixed oxide slurry for alloys) of appropriate thickness, or the fibers may be mixed with an oxide slurry or slurry, resulting in a preformed sheet consisting of parallel fibers in an oxide and binder matrix, or may be cast or pressed from an oxide slurry or slurry into a complex three-dimensional shape containing silicon fibers in place. The coated fibers, preformed sheets or three-dimensional shapes can then be fabricated into the cathode of an electrolytic cell (with or without a pre-sintering step) and the titanium dioxide reduced by electrolysis to the metal or alloy coated on the fibers. The product was then washed and vacuum annealed to remove salts and then hot isostatically pressed to produce a 100% dense fiber reinforced composite.

Production of metal, semimetal or alloy parts

The inventors have determined that metal or semi-metal or alloy components can be manufactured by electrolysis using the above described method.

Near net-size titanium or titanium alloy parts may be prepared by electrolytically reducing a ceramic replica of the part made from a mixture of titanium dioxide or a mixture of titanium dioxide and a suitable oxide of an alloying element. Ceramic replicas can be produced using any of the well-known ceramic article production methods, including pressing, injection molding, extrusion and slip casting, followed by firing (sintering), as previously described. Fully dense metal parts can be obtained by sintering with or without pressure in an electrolytic cell or in a subsequent operating step. Shrinkage of the part during conversion to metal or alloy should be taken into account so that the ceramic replica is proportionally larger than the desired part.

This method has the advantage of producing metal or alloy parts that approach the net shape of the final desired shape. And the costs associated with other forming methods such as machining or forging can be avoided. The method is particularly suitable for small complex-shaped parts.

Claims (22)

1. A method of producing a metal matrix composite comprising:

(a) mixing a particulate reinforcing agent with a metal oxide or semi-metal oxide powder to provide a mixture;

(b) sintering the mixture; and

(c) by using a molten salt M₂Electrolytically removing oxygen from the sintered mixture in the mixture of Y or a salt, comprising performing the electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂And Y is as defined above.

2. A method of producing a fiber reinforced metal matrix composite, comprising:

(a) coating the reinforcing fibers with a metal oxide or semimetal oxide/binder slurry to produce a preform; and

(b) by using a molten salt M₂Electrolytically removing oxygen from the sintered mixture in the mixture of Y or a salt, comprising performing the electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂And Y is as defined above.

3. A method of producing a metal or semi-metal or alloy component comprising:

(a) providing a ceramic replica of the component from a metal oxide or semi-metal oxide or an oxide mixture of suitable alloying elements; and

(b) by using a molten salt M₂Electrolytically removing oxygen from the sintered mixture in the mixture of Y or a salt, comprising performing the electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂And Y is as defined above.

4. At M₂Y molten salts or mixtures of salts from solid metals, metal compounds or semimetals M by electrolysis₁A method for removing oxygen from O comprising conducting electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂Y, wherein the metal or semi-metal oxide is in the form of a pellet or powder.

5. The method of claim 5, wherein the granules or powder are stirred.

6. At M₂Y molten salts or mixtures of salts from solid metals, metal compounds or semimetals M by electrolysis₁A method for removing oxygen from O comprising conducting electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂Y, wherein the metal or semi-metal oxide is in the form of a powder or sintered pellets, which is continuously fed into the molten salt.

7. A method for producing a metal or semi-metal foam, comprising the steps of: preparing a foam-like metal oxide or semimetal oxide preform, in M₂Removing oxygen from the foam-structured metal oxide preform by electrolysis in a molten salt or mixture of salts of Y, comprising performing electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂And Y is as defined above.

8. The method of claim 7, wherein the metal oxide or semi-metal oxide preform is produced by infiltrating a polymer foam with a metal oxide or semi-metal oxide slurry, followed by drying and firing.

9. The method of claim 8, wherein the metal oxide or semi-metal oxide preform is produced by:

(a) mixing said metal oxide or semimetal oxide powder with an organic blowing agent to release a blowing gas;

(b) curing to obtain a cured foam; and

(c) firing the foam to remove the organic matter.

10. The method of claim 8, wherein the metal oxide or semi-metal oxide preform is sintered metal oxide or semi-metal oxide particles.

11. At M₂From solid by electrolysis in molten salts or mixtures of salts of YBulk metal, metal compound or semimetal M₁A method for removing oxygen from O comprising conducting electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂Y, wherein the electrolysis is performed on a sintered body of a metal oxide mixture comprising predominantly particles having a size greater than 20 microns and finer particles less than 7 microns.

12. The method for electrolytic reduction of metal oxides of claim 11, wherein the sintered body is additionally formed by mixing a binder and water.

13. The method of any one of claims 11 or 12, wherein the finer particles comprise 5-70 wt% of the sintered body.

14. The method of any one of claims 11-13, wherein the finer particles comprise 10-55 wt% of the sintered body.

15. A feedstock for the electrolytic reduction of metal oxides, the feedstock comprising a sintered body of a mixture of metal oxide particles having a size greater than 20 microns and finer particles less than 7 microns.

16. A feedstock as claimed in claim 15, in which the finer particles comprise 5 to 70% by weight of the sintered body.

17. A feedstock as claimed in claim 16, in which the finer particles comprise 10 to 55% by weight of the sintered body.

18. A method according to any one of the preceding claims wherein M is₁Selected from Ti, Zr, Hf, Al, Mg, U, Nd, Mo, Cr, Nb, Ge, P, A s, Si, Sb, Sm or any alloy thereof.

19. A method according to any one of the preceding claims wherein M is₂Ca, Ba, Li, Cs and Sr.

20. A process according to any one of the preceding claims wherein Y is Cl.

21. At M₂A process for removing oxygen from titanium dioxide by electrolysis in a molten salt or mixture of salts of Y, comprising carrying out the electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂Y, and the titania starting material is in the form of a sintered amorphous slurry containing 5 to 95% calcined titania.

22. A method of producing titanium powder from titanium dioxide comprising the steps of:

(a) providing titanium oxide as a first electrode;

(b) at M₂A process for removing oxygen from titanium dioxide by electrolysis in a molten salt or mixture of salts of Y, comprising carrying out the electrolysis under the following conditions: i.e. so that the reaction of oxygen rather than M takes place on the electrode surface₂Deposited and oxygen dissolved in the electrolyte M₂In (1).