WO2008067614A1

WO2008067614A1 - Separation method for metal recovery

Info

Publication number: WO2008067614A1
Application number: PCT/AU2007/001893
Authority: WO
Inventors: Grant Ashley Wellwood; David Hughes Jenkins; Christian Doblin; Asem Mousa; Andrew Joseph Urban
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2006-12-08
Filing date: 2007-12-07
Publication date: 2008-06-12

Abstract

A method for the recovery of a metal from composite particles comprising particles of the metal to be recovered dispersed in a matrix of another metal salt, which method comprises: a) forming a mixture by adding the composite particles to a molten salt, the temperature of the molten salt being equal to or above the melting point of the another metal salt, below the boiling point of the another metal salt and below the melting point of the metal to be recovered; b) agitating the mixture; c) allowing the particles of the metal to be recovered to consolidate; d) allowing the consolidated metal particles to settle under gravity.

Description

SEPARATION METHOD FOR METAL RECOVERY

The present invention relates to the production of titanium metal from titanium tetrachloride by reduction using magnesium (i.e. by magnesiothermic reduction). More specifically, the invention relates to the recovery of titanium produced by this reaction.

Background to invention

The Kroll process (US 2,205,854) is used the world over for production of titanium by magnesium reduction of titanium chloride. The reaction is carried out in a steel reactor where molten magnesium and gaseous titanium chloride are contacted, the titanium being produced in the form of a "sponge". Although the process has been employed for about 50 years, there is no clear understanding of the reaction mechanism involved and of sponge formation. The reaction is believed to be represented by the following equation:

TiCLKg) + 2Mg(i) = Ti(_s) + 2MgCl₂₍D

Thus, under the prevailing conditions in the reactor, the magnesium chloride by-product is produced as a liquid and this enables it to be removed periodically from the reactor.

Unfortunately, the Kroll process is a batch process with low intensity and low titanium yield due to contamination of the sponge by iron from the reactor to which the sponge adheres as it is formed. Moreover, the magnesium chloride product and any unreacted magnesium tend to remain in the interstices created in the titanium sponge and these have to be removed subsequently by a vacuum distillation step. This is also a batch operation. In view of the contamination, the sponge has to be refined through one or more stages of vacuum arc melting to produce titanium of acceptable quality. Even additional processing steps are required if the titanium is required in a powder form.

Furthermore, the process is not particularly environmentally friendly (due to waste streams and loss of batch containment), and there may also be occupational health and safety issues since the process tends to require significant manual intervention during operation. Driven by these drawbacks, efforts have been made to develop alternative processes for the continuous production of titanium. A variety of different chemical pathways have been pursued and these can broadly be classified as either "wet" or "dry" according to the physical state of the magnesium chloride by-product that is produced.

With regard to the "wet" process, some research has focussed on continuous versions of the Kroll process where titanium tetrachloride is injected into molten magnesium to produce fine titanium particles. One such approach is described by Deura et al. (1998 Met. and Matrls. Trans. 29B p 1167-1173). This involves producing titanium particles by injecting gaseous titanium tetrachloride into a molten bath of magnesium chloride that is covered with a layer of molten magnesium metal. As titanium tetrachloride bubbles through the magnesium chloride layer it reacts with the magnesium at the interface between the two liquid layers. Results from a laboratory scale system have been reported. However, the process does not appear to be practiced commercially. This is probably due to operational problems associated with the process.

In "dry" processes deliberate steps are taken to maintain the by-product magnesium chloride in a gaseous form. Thus, in their patent US 4,877,445, Okudaira et al. teach producing titanium powder in a single stage by contacting magnesium vapour and titanium tetrachloride vapour within a fluidised bed. The bed (fluidised with argon) is operated at high temperature (>l,100°C) and at low absolute pressure (50 Torr) such that the only condensed species that may be present as a result of the reaction is titanium metal. The byproduct magnesium chloride phase exists as a vapour under the prevailing conditions and is carried away from the bed with inert gas used to fluidise the bed. While the fluidised bed is conducive to hosting the process on a continuous basis, the elevated temperatures tend to cause finely divided titanium powder formed in the reactor to sinter, thereby locking the bed. There are also practical problems associated with continuous operation of the reactor at such low pressures. As a result of these issues commercial implementation of the process is untenable.

The proposals described above have in common that formation of titanium and separation of the by-product magnesium chloride from the titanium takes place in a single stage. To this end it is critical that the titanium and magnesium chloride are formed as different phases. However, regardless of whether the magnesium chloride is separated as a liquid or as a gas, conditions in the single stage operation are to a significant extent driven by the separation pathway being pursued. This can result in a compromise in terms of titanium productivity.

Applicant's published International patent application no. WO 2006/042360 describes an alternative process for producing titanium that does not suffer the disadvantages associated with the prior methods described. Thus, WO 2006/042360 describes a method for producing titanium by reaction of titanium tetrachloride with magnesium in a reactor, wherein the temperature in the reactor is above the melting point of magnesium and below the melting point of magnesium chloride. The reaction results in formation of composite particles comprising regions of titanium embedded in a matrix of magnesium chloride. The particles are then removed from the reactor and processed in order to recover the titanium.

According to the WO publication recovery of titanium may be achieved by conventional methods such as vacuum distillation or solvent leaching (using a solvent for the magnesium chloride). The solvent may be a liquid or gas. If the magnesium chloride is to be processed in order to regenerate magnesium (by electrolysis), the magnesium chloride removed from the titanium should remain anhydrous. In this case vacuum distillation (with subsequent condensation of magnesium chloride) or the use of a non-aqueous solvent should be employed

Whilst the composite particles produced by the method described in WO 2006/042360 have been found to be amenable to conventional separation technology in order to recover titanium, it would be desirable to provide an alternative approach that provides benefits over the conventional techniques. The present invention seeks to provide such an alternative approach and one that is believed to have general applicability. Summary of invention

Accordingly, the present invention provides a method for the recovery of a metal from composite particles comprising particles of the metal to be recovered dispersed in a matrix of another metal salt, which method comprises: a) forming a mixture by adding the composite particles to a molten salt, the temperature of the molten salt being equal to or above the melting point of the another metal salt, below the boiling point of the another metal salt and below the melting point of the metal to be recovered; b) agitating the mixture ; c) allowing the particles of the metal to be recovered to consolidate; d) allowing the consolidated metal particles to settle under gravity.

The present invention also provides metal particles when recovered by the method of the invention.

Detailed description of invention

In the method described, composite particles are added to a bath of molten salt. As an alternative, it is possible to heat the composite particles in order to generate molten salt.

The method of the invention may be applied to recover a variety of metals, such as titanium, aluminium, zirconium, zinc and vanadium. The invention is preferably applied to the recovery of titanium (from composite particles comprising magnesium chloride), and for the purposes of illustration the invention will be described in more detail with reference to this particular system. Application of the invention to this system is also preferred.

hi accordance with the present invention it has been found that agitation of a mixture of the composite particles in a molten salt having particular temperature characteristics leads to the formation of particles (consolidated titanium particles) that have a higher effective diameter than the titanium particles originally present in the composite particles. One important practical consequence of this is that consolidated titanium particles are more amenable to settling under gravity. Settling may also be enhanced by use of, for example, a hydrocyclone. The larger diameter of the consolidated particles may also make them more amenable to filtration or other solid/liquid separation techniques. The titanium particles present in the composite particles are generally of a size such that they would tend to remain in suspension in the kind of molten salt used in practice of the invention, despite density differential. Such titanium particles would be difficult to remove from suspension by techniques such as filtration because of filter blinding. Furthermore, the consolidated titanium particles produced in accordance with the present invention are safer to handle and/or use in conventional metallurgical techniques. They may also be less susceptible to oxidation. It is known that very fine titanium particles (around particle size 3 μm) are extremely pyrophoric.

The prevailing temperature conditions are believed to be important to implementation of the present invention. The salt (to be rendered molten) will be selected based on its temperature characteristics. Thus, the molten salt must be provided at a temperature that is equal to or above the melting point of magnesium chloride, that is below the boiling point of magnesium chloride and that is below the melting point of titanium. For the system described the temperature of the molten salt will be from 712 to 1418⁰C, typically towards the lower end of this range, for example from 712 to 850°C. The salt that is used may be a single compound or a mixture of compounds having the requisite temperature characteristics. In an embodiment of the invention the salt is a eutectic mixture. Use of a eutectic system may enable greater temperature control to be achieved and this may bring benefits in terms of the overall recovery process. In the following, unless context otherwise requires, reference to use of a salt is intended to embrace mixtures of salts also.

When recovering titanium from titanium/magnesium chloride composite particles, the salt may be magnesium chloride. Examples of eutectic mixtures of salts that may be used include the sodium chloride/magnesium chloride, potassium chloride/magnesium chloride and sodium chloride/potassium chloride/magnesium chloride systems. In this system, in principle, any salt (or mixture) may be used that is also otherwise used for electrowinning of magnesium, including but not limited to barium chloride, lithium chloride and calcium chloride and mixtures thereof. A specific example is a salt mixture having the composition 57 wt% NaCl, 20 wt% KCl, 15-20 wt% MgCl₂, 2 wt% CaCl₂ and 1 wt% CaF₂.

As will be described below, formation of the consolidated titanium particles is believed to take place by sintering of titanium particles present in the titanium/magnesium chloride composite particles. Titanium has a melting point of 1670°C although it is known that fine particulate titanium will sinter at temperatures much lower than this. The exact size and morphology of the titanium particles will determine their sintering temperature.

Advantageously, the temperature at which the method is performed, i.e. the temperature of the molten salt, is at or slightly above the sintering temperature for the titanium particles present in the composite particles. In practice the optimum temperature may be determined experimentally.

Without wishing to be bound by theory it is believed that under the prevailing temperature conditions the titanium particles are mobile within the matrix of magnesium chloride (and any other complimentary salts that may be present) in the composite particles. At elevated temperature there is a strong driving force to minimise surface area of the titanium particles and when titanium particles encounter one another a strong coherent bond may be formed. Agitation of the molten salt/composite particles mixture is believed to promote collision of the titanium particles thereby leading to formation of consolidated particles. Magnesium chloride is believed to be driven out of the composite particles and into the bulk of the molten salt. Interestingly, it has been observed that the consolidated titanium particles formed in accordance with the present invention may have essentially the same geometrical form as the composite particles and they are of similar dimension. For example, if the composite particles are spherical it has been observed that the consolidated titanium particles are hollow spherical microspheres. There may be a link between the macroscopic shape and dimensions of the composite particles and the consolidated particles that are formed in accordance with the present invention. Typically, the average particle size of the composite particles will be at least 500μm with the titanium particles in the composite particles having an average particle size of about 5μm.

Agitation may be done mechanically, for example using a bladed propeller. It has been found that the metal particles may settle rapidly when this technique is used. It should be noted however that mechanical agitation can lead to problems with materials selection and gas ingress (oxygen and nitrogen must not be allowed to contact the titanium). Alternatively or additionally, agitation may be achieved by injection of an inert gas into the mixture. Typically, the inert gas will be argon or helium, preferably argon. The degree of turbulence in the mixture caused by the agitation and the prevailing operating temperature are likely to influence the rate of formation of the consolidated titanium particles. The inert gas may be injected into the mixture at ambient temperature and it will be rapidly heated on contact with the mixture.

The difference in density between the molten salt (magnesium chloride in the particular system discussed herein) and titanium may also be a factor in leading to consolidation of the titanium particles. Solid magnesium chloride has a density of 2300 kg/m³ with this falling to 1660 kg/m³ for molten magnesium chloride. The density of titanium is higher at 4500 kg/m³.

Preferably, the magnesium chloride should also have good solubility or be readily miscible with the molten salt since it seems to be integral to the mechanism of formation of the composite particles that magnesium chloride is driven out of the composite particles during the consolidation process. As noted, the salt used is typically magnesium chloride or magnesium chloride-based, such as a salt mixture of magnesium chloride and sodium chloride and/or potassium chloride. A mixed salt bath of sodium chloride and potassium chloride may also be used.

Energetically it is preferred that the composite particles are already at elevated temperature when added to the molten salt. More specifically, the composite particles are typically at an elevated temperature that is nevertheless a temperature that is lower (e.g. > 100°C lower) that the temperature of the molten salt. This may benefit dispersion of the composite particles in the molten salt and influence the recovery process. With this in mind it may be beneficial for the method of the present invention to be applied directly downstream of the composite particle-forming reaction described in WO 2006/042360. Once formed the consolidated titanium particles will settle under gravity in the molten mixture. As noted, the settling process may be enhanced by use of, for example, a hydrocyclone. The invention may be carried out in a batch wise fashion with the consolidated particles being removed at the end of each batch operation. The method of the invention may also be implemented in a continuous fashion with addition of composite particles and removal of consolidated titanium particles. In this case it will be necessary to remove some of the molten salt as magnesium chloride will be continuously dissolving or dispersing into it.

In an embodiment of the present invention a multi-stage temperature regime is applied, in which metal particles that have been recovered from the composite are re-suspended in molten salt at an increased temperature and agitated. It has been found that this approach may give enhanced (titanium) particle agglomeration and higher salt (magnesium chloride) recovery. For example, in a two-stage process titanium/magnesium chloride composite particles may be agitated in a molten salt bath of magnesium chloride or eutectic mixture, such as sodium chloride/magnesium chloride. The first stage is performed at low temperature (typically 600⁰C to 780°C) and the separated titanium allowed to settle. The second stage involved re-suspension of the settled titanium particles with a rapid temperature increase to a higher temperature (typically 800°C to 900°C) with more vigorous agitation. This approach resulted in partial sintering and agglomeration of the titanium particles.

The consolidated titanium particles produced in accordance with the invention are typically purified, for example, by vacuum distillation or washing. The particles generally include relatively low amounts of residual magnesium chloride and the burden for removal therefore tends not to be significant. The magnesium chloride derived from the composite particles is usually filtered and then fed to an electrolytic cell for recovery of magnesium. The magnesium may be used as a reactant in the first stage reaction as described in WO 2006/042360. Vacuum distillation of the consolidated titanium particles yields relatively pure magnesium chloride and this may be delivered directly to the electrolytic cell without filtration. The method of the invention may be implemented in any suitable reactor. The materials of construction should be carefully selected based on the temperatures and materials involved.

When compared to conventional separation techniques, benefits associated with the method of the present invention include:

1. A reduction in the specific (mass) surface area of the recovered titanium (the high unit mass specific surface area of the titanium particles in the composite particles could make them strongly pyrophoric and difficult to separate once liberated from the matrix material) .

2. Particle Size Flexibility (while the small titanium particles present in the composite particle are consolidated as a result of this method, the consolidation is not very strong hence it is possible to reconstitute a titanium powder of a required particle size distribution relatively easily from the consolidated particles). 3. Exclusion of oxygen and nitrogen. During the separation stage the particles are most vulnerable to reaction with oxygen and nitrogen. However, conducting the separation/consolidation within a molten salt eliminates this possibility.

4. Simplicity (minimal sample handling and need for mechanical devices).

5. Energy Efficiency (the significant sensible heat energy of the composite particles discharged from the first stage reaction may be fully utilised).

6. Intensity (high intensity therefore small footprint plus ability to be close coupled to the first stage reaction process).

7. Industrial Appeal. The outputs from this continuous process, i.e. titanium and molten MgCl₂ are the same as those associated with the conventional Kroll process.

The method of the invention has been described herein with particular reference to recovering titanium from composite particles produced in accordance with the process described in WO 2006/042360. However, this should not be regarded as limiting and the same methodology may be used to recover any metal when present in a composite metal/salt structure, when there is a suitable density difference between the discrete (metal) phase and the material of the matrix phase and when the metal is present in a form that is too finely divided so as to be readily separated (once liberated) using gravity or centrifugal force. In this case the methodology described herein may be applied to produce consolidated metal particles that may be more readily separated by gravity.

There may be potential to apply the present invention to a composite produced according to the process described in U.S. 5,779,761 (Armstrong et. al). In that process a metal halide or mixture of metal halides is contacted with a stream of liquid alkali metal or alkaline earth metal or mixture thereof. The halide is converted to metal or alloy. The temperature is maintained below the boiling point of the alkali metal or alkaline earth metal (at atmospheric pressure) or the sintering temperature of the metal or alloy. The process is said to be suitable for the production of titanium by reaction of titanium tetrachloride with a molten stream of sodium. Particulate titanium is produced in a stream of sodium and sodium chloride.

The methodology of the present invention may be applied to the composite structure obtained on cooling the product stream produced as per the process described in US

5,779,761. hi this case after cooling the composite may be milled to form particles that may be used as feed in the methodology of the present invention. Alternatively, composite droplets may be formed by spraying and cooling the molten stream. The droplet size may be manipulated as required. The methodology of the present invention therefore offers an alternative approach to metal recovery to the conventional approaches suggested in US

5,779,761.

By way of background the process taught in WO 2006/042360 will now be described in more detail. Central to the process is the temperature in the reactor during operation of the process. Thus, it is a requirement of the invention that the temperature in the reactor be above the melting point of magnesium but below the melting point of magnesium chloride. It has been found that conversion of titanium tetrachloride to titanium at such low operating temperatures is capable of producing titanium in unexpectedly high yield and at a suitably high rate. Conventional thinking may have predicted that this would not be possible.

The reactor may be any suitably configured apparatus in which the reaction may be carried out. The reactor may be any type of gas-solid contact device. Preferably, however, the reactor comprises a fluidised bed and for convenience the invention will be described in more detail with reference to use of a fluidised bed. One skilled in the art would appreciate however that the underlying principles of the present invention may be applied in other types of reactor.

In the context of WO 2006/042360 reference to the temperature of the fluidised bed means the average or bulk temperature of the bed. There may be localised "hot spots" within the bed due to localisation of the exothermic reaction between magnesium and titanium tetrachloride. However, the temperature observed at such "hot spots" should not be taken as being representative of the bed temperature.

Disregarding localised "hot spots" within the fluidised bed, the operating requirement of the process with respect to temperature means that in the fluidised bed the magnesium reactant will be present as a molten liquid and that the magnesium chloride produced as by-product will be present as a solid. Given this requirement, the temperature of the fluidised bed will be from 650°C to less than 712°C. Usually, the bed temperature is from 650°C to 710⁰C. Selection of an operating temperature will be based on a variety of other factors, as will be explained in more detail below.

In an embodiment it is possible to introduce into the reactor elements that it is desired to alloy with the titanium being produced. In this case the temperature in the reactor must also be suitably high to render the alloying element(s) liquid. Obviously, the alloying element is selected such that magnesium will preferentially react with the titanium tetrachloride, thereby avoiding any chemical reaction involving the alloying element. The alloying elements are usually metals, such as aluminium. It is a requirement however that the temperature in the fluidised bed will remain below the melting point of magnesium chloride.

It is also possible to introduce alloying elements as halides for reduction by reaction with magnesium. In this case the alloy halides are vaporised and introduced into the reactor in combination with the titanium tetrachloride. This technique may be used to introduce aluminium and vanadium, for instance.

For convenience the process of WO 2006/042360 will be described with reference to the production of titanium, i.e. without alloying elements.

It perhaps goes without saying that at the operating temperature required titanium will be produced as a solid. It is possible for titanium particles to sinter at temperatures well below the melting point of titanium (1670°C), especially where the particles are very fine. However, at the operating temperatures employed in the process sintering is not likely to occur, even if fine particulate titanium is present in the fluidised bed.

The temperature of the fluidised bed may be determined by averaging the temperature observed at a number of locations within the bed. In this case it is desirable to measure the bed temperature at numerous locations in order to minimise the influence of "hot spots" on temperature measurement. As a preferred alternative the exit temperature of inert gas used to fluidise the bed may be taken as representative of the bed temperature. Irrespective of the method used, temperature measurement will typically involve conventional equipment such as thermocouples.

At the intended operating temperature of the process and under the prevailing conditions in the fluidised bed (including the degree of agitation of the particles making up the bed), it is important that the seed particles making up the bed do not sinter. This will have implications on selection of the seed particles for use in the process, especially on start up. In principle the seed particles may be made of any material that is capable of acting as a reaction site for the reaction between molten magnesium and titanium tetrachloride vapour. Typically, however, the seed particles will be formed of titanium or of magnesium chloride. A mixture of the two may be used. The initial particle size of the seed particles will vary depending upon the scale of operation and the desired particle size of the product particles. Broadly speaking the initial particle size is from lOμm to 2mm, more likely from 250 to 500μm.

On start-up of the process the seed particles are charged into a suitable reactor and fluidised by injection (usually from below) of an inert gas such as argon. The inert gas will be heated prior to introduction into the bed of seed particles in order to bring the bed temperature up to the desired operating temperature. As noted above, the temperature of inert gas leaving the reactor may be taken as being representative of the bed temperature. A number of parameters either manipulated in isolation or in combination can be used to control the bed temperature including the temperature of the inert gas streams being injected into the bed, heat flow across the reactor wall, reactant feed rate, reactant supply temperature (and hence phase), with the preferred strategy dependant on application specific factors like reactor configuration and scale. The rate at which the inert gas is injected into the bed can be varied to manipulate the way in which the seed particles are agitated, and the extent of agitation. With suitable selection of seed particles, and possibly particle size, sintering of particles within the bed does not become an issue. In this case the rate at which inert gas is fed into the bed of seed particles may be relatively low since it is not necessary to apply vigorous agitation in order to minimise sintering or drive the evaporation of the MgCl₂ phase by manipulation of partial pressures in the reactor.

When the seed particles have been brought up to temperature the reactants may be introduced into the bed. The titanium tetrachloride is usually supplied into the reactor in vapour form by pre-heating titanium tetrachloride from a storage reservoir. The magnesium may be supplied into the reactor as a solid, molten liquid or gas depending upon the supply technique. Normally, magnesium is supplied into the reactor as a solid or molten liquid. It may be difficult or impractical to pump molten magnesium through piping into the reactor and particulate magnesium may be more practically convenient since in this form it may be free flowing. It may therefore be preferred to use particulate magnesium as the magnesium supply to the reactor. As a guide, generally the particle size of the magnesium will be from 40 to 500μm. Having said this, any unreacted molten magnesium may be collected (drained) from the reactor and returned (recycled) to the reactor for reaction with titanium tetrachloride. This may make economic and process sense. When delivered into the reactor, whether fresh or recycled, molten magnesium may be dispersed by an in situ atomiser or similar dispersion device. The aim is to provide molten magnesium in finely divided form. Irrespective of the form in which the magnesium is supplied to the reactor, at the temperature in the reactor the magnesium will be present in molten form.

The reactants are delivered into the reactor in such a way that they will come into contact and react within the fluidised bed. The same would be true if another type of reactor was employed. In one embodiment the titanium tetrachloride is injected into the fluidised bed with the inert gas used to fluidise the bed. This will be done from below the bed through one or more suitably adapted conduits. The magnesium may be delivered through one or more inlets provided in a side wall of the reactor. In one embodiment the reactor is cylindrical and the magnesium is delivered through one or more inlets that are tangential to the side wall of the reactor. It is equally possible for the titanium tetrachloride vapour to be delivered into the reactor through one or more such inlets provided at the side wall of the reactor.

Within the fluidised bed the reactants come together and interact with solid titanium and solid magnesium chloride being formed at the surface of the seed particles. The reaction is an exothermic one and localised heating at the point of reaction will therefore take place. Without wishing to be bound by theory it is believed that this reaction takes place within the outer layer of participating particles and that the localised heating may play an important part in formation of composite particles comprising titanium and magnesium chloride. Thus, when the reaction between magnesium and titanium tetrachloride takes place titanium and magnesium chloride will be formed at the surface of the seed particles. Depending upon the temperature of the fluidised bed the heat of reaction may cause the temperature at the localised site of reaction to increase and exceed the melting point of magnesium chloride, thereby promoting correspondingly localised melting of magnesium chloride. In turn it is believed that the reactants will dissolve in or be absorbed by the molten magnesium chloride and react therein. Agitation of the fluidised bed will cause the particles that have been the site of reaction to be circulated to relatively cooler parts of the fluidised bed resulting in solidification of the magnesium chloride. This process is repeated as particles circulate in the bed.

The composite particles usually comprise regions of titanium embedded in a matrix of magnesium chloride. This is consistent with the mechanism proposed above involving localised melting of magnesium chloride and dissolution/absorption of the reactants. Typically, the composite comprises titanium and magnesium chloride at a mass ratio of about 1 : 4.

In view of the reaction mechanism that is believed to operate it may be preferable to use magnesium chloride as the seed particles making up the fluidised bed. If titanium particles are used, magnesium chloride must first be deposited on the surface thereof before being available to participate as a vehicle for the magnesium/titanium tetrachloride reaction.

Advantageously, the particles formed as a result of the reaction between the magnesium and titanium tetrachloride tend to be essentially spherical. As such they are free flowing and this is beneficial in terms of handleability.

It is preferred that the temperature of the fluidised bed is such that the exotherm resulting from the reduction reaction will have the effect of increasing the temperature (albeit in a very localised region) to a temperature equal to or above the melting point of magnesium chloride. In practice for a given reactor set-up (including rate of supply and stoichiometry of reactants, reactor design, seed particles and/or inert gas feed) it may be possible to determine the optimum bed temperature in this regard by sampling and analysis of the particles that are produced as a result of the reaction. If the particles exhibit the composite characteristics described it can be assumed that the bed temperature is set appropriately. If the composite structure is not observed the reactor set-up may be manipulated as required to achieve the desired morphology with respect to the titanium and magnesium chloride formed as a result of the reaction. As noted it is very straightforward to manipulate the bed temperature by varying the temperature of the inert gas used to fluidise the bed.

It is also important that the characteristics of the bed (including temperature and degree of agitation) and/or the rate of supply of reactants is/are such that temperature "runaway" is avoided. This is because if, as a result of the reaction, the bulk temperature of the bed increases above the melting point of magnesium chloride, sintering will start to occur. The bed temperature should be monitored and varied accordingly. This said, in a preferred aspect of the process may be run continuously and under steady state conditions without the need to actively regulate the bed temperature. In this embodiment the heat of reaction is effectively absorbed (at least due to the latent heat of fusion associated with localised melting of magnesium chloride) and distributed over the bulk of the bed. In this case the ability of the bed to act as a heat sink for thermal energy released by the magnesium/titanium tetrachloride reaction is balanced against the thermal energy that is actually released by on-going reactions within the bed based on supply of the reactants. Typically, the process is operated at or near stoichiometric ratio of the reactants based on the equation reflecting the reduction reaction. Here it may also be advantageous to feed magnesium into the bed as a solid (powder) since some thermal energy will be consumed in melting of the magnesium. In this way introduction of solid magnesium may also act as a heat sink for thermal energy generated by the reduction reaction.

It is intended that the process will be operated continuously with supply of reactants and removal of suitably sized particles. Advantageously, it has been found that it may be possible to operate the process of the invention continuously without the need to supply fresh seed particles. This is because the process may be self-seeding due to the formation of titanium and magnesium chloride as solids within the fluidised bed. In practice such collisions between particles within the bed may cause fragmentation with the resultant fragments acting as seed particles for subsequent reactions. Here it should be noted that particles are removed from the bed based on their effective aerodynamic diameter (size, density, shape) classification so that small, newly formed seed particles will be retained in the fluidised bed until they have been coarsened appropriately due to reaction between magnesium and titanium tetrachloride at the surface of the particles.

Composite particles may be removed from the bed when they have reached a suitable size. Here the coarsened particles may be removed from the reactor through a self-regulating process based on the effective aerodynamic diameter of the particles and on fluidisation conditions within the bed. In one embodiment the rate of supply of inert gas into the bed may be manipulated in order to achieve removal of suitably sized particles. In this embodiment as the rate of gas flow into the bed is reduced the ability of the gas flow to prevent particles entering the gas supply conduit will diminish until such time as particles will fall under gravity down the conduit. Varying the gas flow in this way allows the particles to be separated based on weight, heavier particles being preferentially removed over lighter ones. In this embodiment the gas supply through the conduit is used primarily for the purposes of particle separation rather than for fluidisation of the bed. Thus the reactor will therefore also be equipped with at least one further inert gas supply conduit for the purposes of fluidising the bed of particles. In one embodiment the inert gas is delivered into the bed through concentric nozzles, a central conduit of this arrangement being used for the purposes of particle separation.

After suitably sized particles, typically having a diameter of at least 500μm have been removed from the bed they are processed to recover the titanium, as per the method of the invention as described herein. During transfer of the particles from the fluidised bed and subsequent processing it is important that the particles are maintained under an inert atmosphere to prevent oxidation of titanium. The titanium present in the composite particles may be less prone to oxidation due to the magnesium chloride matrix that is present but conditions to prevent oxidation should nevertheless be employed. As noted, the composite particles formed during the process tend to be spherical and this can be an advantage in terms of particle flow during the subsequent processing stage.

Description of figures

Figure 1 is a schematic illustrating experimental apparatus used in Example 2.

Figure 2 is an Environmental Scanning Electron Microscope image showing the structure of the product obtained in Example 4 (cluster of titanium particles recovered from a eutectic salt bath).

Figure 3 is an Environmental Scanning Electron Microscope image showing the product obtained in Example 5 (cluster of titanium particles in nodule form recovered from a two- temperature salt bath).

Embodiments of the invention will now be illustrated with reference to the following non- limiting examples. Example 1 - Production of composite particles

A cylindrical reaction vessel made from stainless steel with a conical base having an internal diameter of 200mm and an aspect ratio of 4 was purged with high purity argon then heated externally to 680°C. Once the pre-heated gas temperature measured at a control point 50 mm above the upper surface normally associated with a bed reached

655°C, the system was charged with 60 grams of 500-1000μm titanium sponge particles.

Once the control point temperature had recovered to 655°C, the two reactant feeds were applied.

Titanium tetrachloride was suppled at a rate of 160 millilitres per hour as a vapour at a temperature of around 500°C. hi this example, the reductant phase was magnesium metal, which was supplied at a rate of 71 grams per hour as a finely divided powder (44-500μm) conveyed in a low volume argon gas carrier stream entering the reactor at a temperature of around 500°C. Both reactant inlets were located at the base of the fluidising zone.

Upon addition to the fluidised bed, the temperature of the gas leaving the bed increased by around 22°C consistent with the exothermic nature of the reaction. The reactor was easy to operate with the bed remaining fluidised despite its proximity to the melting point of MgCl₂ indicating sinter free operation is possible. The test produced free flowing small black spheres (0.1 to lmm diameter) which "softened" upon contact with moisture in the air, confirming that they contained anhydrous magnesium chloride (highly hygroscopic).

The rate of the reactants supplied to the reactor was intentionally increased by a factor of more than two over the duration of the experiment and no unreacted TiCl₄ was detected in the exhaust scrubber. This was another unanticipated outcome as the expectation based on conventional thinking was that the conversion of TiCl₄ to Ti would be poor at low temperatures.

In the context of a constant gas flow rate, the higher rates would have also been expected to push the bed temperature well above the melting point of magnesium chloride (712°C). In practice the control temperature, which represents the bulk bed temperature, remained below 700°C. This surprising result was later attributed to a mechanism by which the extra energy released from the reaction is absorbed by the bed in the conversion of some of the MgCl₂ on the surface of the particles from a solid to a liquid (latent heat of fusion). The process is therefore self limiting within generous limits with respect to bed temperature hence the ability to keep the bed in the apparently narrow band required (650-712°C) is significantly enhanced. The conversion of some of the surface MgCl₂ to liquid is also thought to be the mechanism by which the bed self-seeds itself; drops of liquid MgCl₂ are mechanically knocked off the particles by the action to provide new sites for reaction/deposition.

Heating of the composite particles from this run under an inert gas atmosphere produced porous titanium metal structures that assumed the shape and size of their composite particle precursors. The heating step volatilised the MgCl₂ leaving behind the titanium particles as originally envisaged.

Example 2 - Recovery of titanium

This example may be better understood with reference to Figure 1.

Consolidated clusters of fine titanium particles were recovered from a gas agitated molten salt bath (1). One hundred and fifty grams of reagent grade anhydrous magnesium chloride (MgCl₂) powder was added to a 51mm (ID) silica tube (2) terminated with a quick-fit compatible flange. Before the MgCl₂ was added, the tube was flushed with argon and from that point maintained under an argon cover to minimise hydration and oxidation reactions. The terminating flange of the silica tube was then coupled to a multi-point (x5) ground glass fitting quick-fit top (4). The silica tube containing the MgCl₂ powder was then inserted vertically into a customised furnace (5) consisting of two electrically heated sides, an insulated back and an insulated front panel equipped with a slot for visual access to the tube. The furnace (5) and therefore the tube were then heated to 800⁰C₅ which being above the melting point of MgCl₂ caused the contained powder to melt into a transparent liquid (molten salt bath). The multi-point access top had been pre-configured as follows: • Access port #1 : Fitted with a 6mm ID tube to deliver the agitating argon. The discharge point of the tube was adjusted to sit ¹A way below the liquid-gas interface.

• Access port #2: Fitted with a K-type thermocouple (6) inserted such that the sensing tip was located % of the way below the liquid-gas interface. The output from this thermocouple was connected to a display and recording unit (not shown).

• Access port #3 was fitted with a ¹A " ID nylon line (9) to exhaust gases from the tube assembly.

• Access port #4 was fitted with an inert gas flushed pre-filled delivery chamber (7) containing the Ti/MgCl₂ composite particles. The pathway between the contents of this chamber and the tube containing the molten salt was controlled by a 90° Quick Fit valve (3).

• Access port #5 was blanked off via a gas tight stopper.

Once in a molten state, a small flow of argon was bubbled (10) into the tube via the gas delivery tube (8). Exhaust gas exits the reactor through a suitably configured exhaust tube (9). The flow rate of the argon was regulated by a flowmeter which was adjusted such that the bubbling argon gently agitated the molten MgCl₂ in the tube.

Once the differential between the furnace control thermocouple located outside the tube in the furnace cavity, and the thermocouple within the molten salt phase in the tube has stabilised within 30 ⁰C, the 15 gram charge of Ti/MgCl₂ composite particles was admitted via the control valve. The composite particles were produced according to the technique described previously in WO 2006/042360. These near spherical particles, with a diameter around 500 microns, consist of a continuous MgCl₂ phase (80 mass %) containing uniformly distributed discrete titanium particles with a ds₀ of around 4 microns (20 mass%). Upon addition of the composite particles, the hitherto transparent molten salt bath became uniformly black due to obscuration of the back-lit tube. Agitation was continued for 20 minutes after which time some transparent regions began to appear. At this point the flow of agitating gas was ceased and almost immediately the solid phase responsible for the black appearance of the liquid settled to the bottom of the tube. Close observation of the settling solids indicate that they had formed some kind of a web or floe. This phenomenon produced a supernatant with a transparency comparable to the one associated with the pure molten salt.

The furnace (5) was then switched off and the tube was allowed to cool to room temperature in-situ. At this point the tube assembly, containing the now frozen salt and settled solid charge, was withdrawn from the furnace. The consolidated and settled solids were then recovered from the solidified salt plug via washing with water. Environmental Scanning Electron Microscope (ESEM) analysis of the recovered solids confirmed that the particles were composed of titanium with very little MgCl₂. The major contaminant was Si. As this contamination originated from the silica tube it is an experimental artefact that does not compromise the concept of this approach. The analysis also revealed that the mass of recovered titanium was comprised of hollow, thin walled titanium spheres lightly sintered together to form a continuous structure (floe). There was also evidence of smaller particles, perhaps broken shells or individual titanium particles washed off the composite before consolidation, embedded in the structure. Analysis of supernatant MgCl₂ samples indicates a low titanium content supporting expectations based on the clarity on the supernatant when it was in a molten state.

Example 3 {MzCh-single temperature-gas sparged)

This example used the same apparatus as used in Example 2 as illustrated in Figure 1.

A 50mm ID 285mm long round bottom transparent silica tube terminated with a glass flange was coupled to a top configured for gas-tight passage of a glass capillary for gas sparging, a K-type thermocouple and double-block solid delivery chamber similar to

Figure 1. The silica observation tube was prepared by adding a total of 150g of AR grade

(Sigma Aldrich 98% min) anhydrous MgCl₂ in a dry-nitrogen gas glove box. The prepared tube was then sealed and placed vertically in position within a 1000⁰C electrically heated box furnace controlled by a Eurotherm 3504 PID controller. The box furnace featured a vertical slot on one face in alignment with the tube to provide a full-length view of the tube in-situ. To prevent thermal shock to the tube on heat-up, the furnace was programmed to follow a defined profile:

300°C/h ambient to 12O⁰C then dwell for 2 hours

40°C/h 12O⁰C to 32O⁰C then dwell for 5 hours

300°C/h 32O⁰C to 400⁰C then dwell for 3 hours

300°C/h 400⁰C to 78O⁰C hold at 780°C

Manual rate 78O⁰C to 85O⁰C then hold for test

At the target temperature of 85O⁰C the contents of the tube were a transparent fluid, however there was a significant amount of MgO settled in the base of the tube. The molten MgCl₂ was then agitated by passing high purity argon at a rate of 80 mL/min through the sparger capillary submerged just below the surface of the molten salt. Once the system had stabilised thermally, the 16.4g charge of composite particles consisting of finely dispersed titanium particles (d₅o~4um) in a continuous anhydrous MgCl₂ phase produced in the fluid bed reactor (as per WO 2006/042360) were added. To prevent localised freezing in the vicinity of the point where the composite particles hit the molten salt interface, the addition was made slowly. Once below the surface of the molten salt the feed particles appeared to dissolve making the contents opaque. After 30 minutes of agitation the fine titanium particles that caused the opaqueness coalesced into large aggregates that then dropped out of suspension due to their higher density (s.g Ti=4.5g/cc vs s.g MgCl_2(I) 1.66g/cc). At this point the agitation ceased and all the titanium particle clusters settled to the bottom of the tube, leaving the molten salt fluid above, once again transparent. The sparging tube and the thermocouple were retracted and the furnace switched off. Once the temperature of the tube and contents had cooled naturally to near ambient (overnight), the tube was removed and the solidified solids in the bottom of the tube were washed free from the salt using water according to a standard procedure. The isolated solids were then dried and weighed. They were then submitted for elemental analysis using XRF and standard wet chemical ICPMS techniques.

The analytical results showed that solids recovered from this test consisted of 86.4wt% Ti, 2.54wt%Mg, 2.49%C1 Example 4 (MεCh/NaCl-Eutectic-single temperature-gas sparεed)

A 50mm ID 285mm long round bottom transparent silica tube terminated with a glass flange was coupled to a top configured for gas-tight passage of a glass capillary for gas sparging, a K-type thermocouple and double-block solid delivery chamber similar to Figure Ir. The silica observation tube was prepared by adding a total of 15Og consisting of AR grade anhydrous MgCl₂ together with NaCl in a dry-nitrogen gas glove box. The mole fraction of MgCl₂ in the salt mixture was 0.431 giving a eutectic melting temperature of 459⁰C. The prepared tube was then sealed and placed vertically in position within a 1000⁰C electrically heated box furnace controlled by a Eurotherm 3504 PID controller. The box furnace featured a vertical slot on one face in alignment with the tube to provide a full-length view of the tube in-situ. The box furnace was then set to 614⁰C, 155⁰C above the melting point of the eutectic salt mixture present in the tube. To avoid thermal stress to the tube a heat-up profile was used viz:

300°C/h ambient to 12O⁰C then dwell for 2 hours 40°C/h 12O⁰C to 32O⁰C then dwell for 5 hours

300°C/h 32O⁰C to 400⁰C then dwell for 3 hours

Manual ramp 400⁰C to 614⁰C then hold for test

Once at the target temperature (614⁰C) the contents of the tube, which were now a transparent fluid, were agitated by passing high purity argon through the sparger capillary submerged below the surface of the molten salt. Once the system had stabilised thermally, the 15g charge of composite particles consisting of finely dispersed titanium particles

(d₅o~4um) in a continuous anhydrous MgCl₂ phase produced in the fluid bed reactor (as per WO 2006/042360) were added. To prevent localised freezing in the vicinity of the point where the composite particles hit the molten salt interface, the addition was made slowly. Once below the surface of the molten salt the feed particles appeared to dissolve making the contents opaque. After a period of agitation the fine titanium particles that caused the opaqueness coalesced into large aggregates that then began to drop out of suspension due to their higher density. At this point the agitation ceased and all the titanium particles settled to the bottom of the tube, leaving the molten salt fluid above, once again transparent. At this point the sparging tube and the thermocouple were retracted and the furnace switched off. Once the temperature of the tube and contents had cooled to near ambient, the tube was removed and the solidified solids were washed free from the salt mixture using water according to a standard procedure. The isolated solids were then dried and weighed. They were then submitted for elemental analysis using XRP and standard wet chemical ICPMS techniques.

The analytical results showed that solids recovered from this test consisted of 87.3 wt% Ti, 3.83wt%Mg, 1.37%C1

The results showed that solids recovered from a eutectic salt mixture (as opposed to the pure MgCl₂ control) retained less salt after the standard washing profile.

Environmental Scanning Electron Microscope analysis of this solid (Figure 2) revealed a much more open structure. The reduced degree of sintering between particles provides less opportunity for salt to be captured in inaccessible voids.

Example 5 (MgCh-two stage temperature mechanically stirred)

A 50mm ID 285mm long round bottom transparent silica tube terminated with a glass flange was coupled to a top configured for gas-tight passage of a mechanical agitator (3- blade-40mm diameter impeller) shaft, a K-type thermocouple and double-block solid delivery chamber similar to Figure 1. The tube was prepared by adding a total of 15Og of AR grade anhydrous MgCl₂ (MP=712°C) in a dry-nitrogen gas glove box. The prepared tube was then sealed and placed vertically in position within a 1000⁰C electrically heated box furnace controlled by a Eurotherm 3504 PID controller. The box furnace featured a vertical slot in one face in alignment with the tube to provide a full-length view of the tube in-situ. The box furnace was then set to 742⁰C, 3O⁰C above the melting point of the MgCl₂ present in the tube. To avoid thermal stress to the observation tube a heat-up profile was used viz:

300°C/h ambient to 12O⁰C then dwell for 2 hours

40°C/h 12O⁰C to 32O⁰C then dwell for 5 hours

300°C/h 32O⁰C to 400⁰C then dwell for 3 hours

Manual ramp 400⁰C to 742⁰C then hold for test

Once at the stage 1 target temperature (742⁰C) the contents of the tube, which were now a transparent fluid, were agitated via the agitator shaft drive which was set to 61 rpm. Once the system had stabilised thermally, the 15g charge of composite particles consisting of finely dispersed titanium in a continuous anhydrous MgCl₂ phase produced in the fluid bed reactor (as per WO 2006/042360) were added quickly. The molten salt temperature dropped to 727⁰C following the addition of the feed and as a consequence there was some localised freezing of the MgCl₂ bath. Once the temperature recovered to the stage 1 target temperature of 742⁰C and the entire charge was molten again, the agitation was restored and the low-temperature plateau was held for 80 min. At this point the bath was still opaque with no visible signs of particle agglomeration. The furnace was then reset to increase the temperature of the bath to 836⁰C at an average ramp rate of 18.7°C/minute. The bath was then maintained at this second temperature plateau^" for 37 minutes. At this point the agitator and the thermocouple were retracted and the furnace switched off. Once the temperature of the tube and contents had cooled naturally to near ambient (overnight), the tube was removed and the solidified material in the bottom of the tube were washed free from the salt with water according to a standard procedure. The isolated solids were then dried and weighed. They were then submitted for elemental analysis using XRF and standard wet chemical ICPMS techniques.

The analytical results showed that solids recovered from this test consisted of 87.8wt% Ti, 1.22wt%Mg, 1.09°/oCl The results showed that solids recovered from a MgCl₂ bath held at two distinct temperatures retained less salt after the standard washing profile. Environmental Scanning Electron Microscope analysis of this solid (Figure 3) revealed a much more open nodular structure. The alternate structure of the particles provides less opportunity for salt to be captured in inaccessible voids.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method for the recovery of a metal from composite particles comprising particles of the metal to be recovered dispersed in a matrix of another metal salt, which method comprises: a) forming a mixture by adding the composite particles to a molten salt, the temperature of the molten salt being equal to or above the melting point of the another metal salt, below the boiling point of the another metal salt and below the melting point of the metal to be recovered; b) agitating the mixture; c) allowing the particles of the metal to be recovered to consolidate; d) allowing the consolidated metal particles to settle under gravity.

2. The method of claim 1, wherein the metal to be recovered is selected from titanium, aluminium, zirconium, zinc and vanadium.

3. The method of claim 1, wherein the metal to be recovered is titanium and the another metal salt comprises magnesium chloride.

4. The method of claim 1 , wherein the molten salt is a molten salt eutectic mixture.

5. The method of claim 1 , wherein agitation takes place by mechanical agitation.

6. The method of claim 1, wherein agitation takes place by injection of an inert gas into the molten salt.

7. The method of claim 6, wherein the inert gas is argon.

8. The method of claim 1 , wherein the method is performed in two stages wherein in a first stage the composite particles are agitated in the molten salt at a first temperature to produce consolidated metal particles followed by a second stage in which the consolidated metal particles are agitated in molten salt at a second temperature that is higher than the first temperature.

9. The method of claim 8, wherein agitation in the second stage is more vigorous than in the first stage.

10. Consolidated metal particles when produced by the method claimed in claim 1.