Continuous process

Abstract

A process for producing a metal from the corresponding metal halide by reaction of the metal halide at elevated temperature in a reactor comprising a fluidised bed, the fluidised bed comprising seed particles of an inert material that has a melting point higher than that of the metal.

Description

The present invention relates to a process for producing a metal by the reduction of the corresponding metal halide. More specifically, the present invention relates to a process for producing a metal in a fluidised bed reactor that offers operational advantages when compared with conventional techniques.

It is known to produce metals by reduction of metal halides. Thus, titanium tetrachloride can be reduced by reaction with a reductant such as magnesium, sodium or hydrogen under suitable reaction conditions. One way in which this reaction is typically carried out involves a fluidised bed of seed particles of the metal it is desired to produce. In this case the reactants are delivered into the fluidised bed with the intention that metal formed by the reduction reaction is deposited on the surface of the seed particles. In turn this results in growth/coarsening of the seed particles. Seed particles that have grown to a suitable extent may be removed continuously and replenished with fresh seed particles.

In this type of method it is important to ensure that the seed particles do not sinter and agglomerate in the bed since this can defluidise the bed and disrupt continuous operation thereof. Interestingly, sintering and agglomeration of the seed particles can take place at temperatures below the recognised melting point of the constituent metal under the prevailing pressure conditions encountered in the bed. For example, for titanium seed particles agitated vigorously by an inert gas, it has been observed that sintering may actually commence prior to introduction of reactants at temperatures as low as 600-700° C. This is surprising given that the melting point of titanium is 1670° C.

It would be desirable to avoid the sintering and agglomeration phenomena, especially during start-up of the process when the fluidised bed is agitated and brought up to a suitably elevated temperature.

Accordingly, the present invention provides a process for producing a metal from the corresponding metal halide by reaction of the metal halide at elevated temperature in a reactor comprising a fluidised bed, the fluidised bed comprising seed particles of an inert material that has a melting point higher than that of the metal. As will be explained, use of seed particles formed from the inert material allows the process to be initiated (on start-up) and thereafter run continuously without sintering and agglomeration.

It is essential that at least a portion of the seed particles used on start-up of the fluidised bed are formed from the inert material as described. Herein the term “inert” means that the material does not undergo any chemical reaction with any species present in the reactor under the prevailing conditions that are encountered. The inert material is selected so that the seed particles remain solid and as discrete particles under the prevailing conditions that will be encountered in the reactor. It is also a requirement that the inert material has a melting point that is higher than the metal it is desired to produce. Typically, the melting point of the inert material is at least 30° C. higher than the melting point of the metal to be produced. The maximum temperature encountered in the fluidised bed during operation of the process of the invention will be below the melting point of the inert material.

In one embodiment it has been found that satisfactory start-up of the fluidised bed may be achieved without sintering and agglomeration by use of a mixture of seed particles of the inert material and of the metal to be produced. Typically, the weight ratio of inert material to metal will be from 20:80 to 80:20, and one skilled in the art would have no difficulty in manipulating the relative proportions of the seed particles to achieve the desired technical effect in accordance with the present invention.

In a preferred embodiment however on start-up of the process, i.e. before introduction of reactants, the fluidised bed consists solely of seed particles of the inert material. During start-up the fluidised bed is agitated and brought up to a suitably elevated temperature. This may be achieved by injection into the fluidised bed of a heated inert gas such as argon.

The inert material that is used in practice of the invention may vary between reaction systems and operating conditions. Taking into account the requirements already mentioned, the inert material may be selected from metallic or non-metallic materials. Examples of such include metal oxides, nitrides and silicates, metals or metal alloys. Particles of coke may also be used. Preferably, the inert material is silica. This is a cheap and abundant material. Mixtures of different types of inert materials may make up the fluidised bed.

The particle size of the inert seed particles is usually within the range 500-1000 μm, for example from 500-750 μm.

After the fluidised bed has been brought up to the desired temperature, the reactants are introduced. Surprisingly, it has been found that metal may be deposited on the surface of the inert seed particles. As the reaction proceeds the inert seed particles grow as fresh metal is deposited on the outer surface of the particles. It has also been found that such “composite” particles are much less prone to sintering and agglomeration when compared with particles formed from the metal only. The exact reason for this is not clearly understood, although it is believed to be associated with the structure of the metal formed at the surface of the inert seed particles.

As metal deposition continues composite particles comprising a core of inert material and a coating of deposited metal will be continuously discharged from the reactor. Uncoated seed particles may also be discharged. Sampling of the discharged particles may be used to determine when no further seed particles and composite particles are present in the fluidised bed. At that point it can be assumed that particles discharged subsequently will be of the metal per se. These particles may then be isolated after discharge from the bed for subsequent processing as necessary.

It has also been observed that fresh metal particles may be produced in addition to coarsening of the inert seed particles. The exact mechanism for this is not understood in detail. However, it is believed that fragments of metal detached from composite (inert materials/metal) particles may themselves function as seed particles for metal deposition.

The sintering and agglomeration problem may be most prevalent on start up of the fluidised bed. The use of seed particles formed from an inert high melting point material allows start up to be effected with minimal or no sintering and agglomeration. Thereafter the fluidised bed may be run continuously. As noted there may be a period of time before all of the inert seed particles are discharged from the fluidised bed (as composite particles). Subsequently the fluidised bed may be replenished with seed particles formed from the metal it is desired to produce. In this respect the fluidised bed would be operated in conventional manner. If during operation of the fluidised bed sintering and agglomeration are observed, it may be appropriate to introduce into the bed seed particles of the inert material to overcome the problem. In this case there will also be a period of time before the inert seed particles are discharged again.

In another embodiment the process is operated so that inert seed particles are always present in the bed. This will involve continuous addition of fresh inert seed particles as coated seed particles are discharged from the bed. In this case it will be necessary to separate composite particles from pure metal particles that have been discharged. This may be done on a density basis.

In one embodiment of the invention the fluidised bed (and reactor) are operated such that the fluidised particles are at a higher temperature than the reactor walls. This may help to reduce incidence of sintering when the particles come into contact with the reactor walls. The same effect may be achieved by varying the material from which the walls of the reactor (and other parts of the rector that come into contact with fluidised particles) are constructed.

Other factors that may be manipulated in order to avoid sintering and agglomeration during start-up and/or subsequent operation of the fluidised bed reactor include the reactor geometry, the temperature profile in the reactor, seed particle size, the flow conditions and contact pattern of reactants in the fluidised bed, the points and manner in which the reactants are injected into the fluidised bed. One or more of these may be adjusted in order to optimise the effect of the present invention.

The method of the invention may be applied in the manufacture of a variety of metals as might usually be produced by the kind of reduction reaction described herein. Examples include titanium, hafnium, zirconium, vanadium and aluminium. The metal halide is typically the chloride. Suitable reductants include magnesium, zinc, hydrogen and sodium.

The operating conditions necessary to effect the desired reduction reaction will vary between reaction systems and one skilled in the art would be familiar with the kind of conditions to be used.

The bed of seed particles is usually fluidised using an inert gas such as argon. The rate at which the gas is injected into the bed may vary depending upon such things as particle size and density, volume occupied by the particles, and reactor design.

The following examples illustrate embodiments of the present invention.

EXAMPLE 1

A cylindrical reaction vessel with a conical base having an internal diameter of 200 mm and an aspect ratio of 10 was charged with 40 grams of 500-1000 μm titanium sponge particles. High purity argon was then passed through the bed of starter particles at a rate of 65 standard litres of argon per minute to promote vigorous fluidisation. Once the gas temperature measured 50 mm above the upper surface of the bed reached around 850° C., the bed sintered into a solid mass before the reactants could be admitted. This solid mass is shown in FIG. 1.

EXAMPLE 2

Example 1 was repeated except the starter bed of 500-1000 μm titanium particles was replaced with sand particles having a particle size between 500 and 600 μm. To achieve vigorous fluidisation, the high purity argon flowrate was increased to 83 standard litres per minute. Once the temperature measured 50 mm above the upper bed surface reached 900° C. the two reactant feeds were applied. Titanium tetrachloride was supplied at a rate of 80 millilitres per hour as a vapour at a temperature of around 700° C. In this example, the reductant phase was magnesium metal, which was supplied at a rate of 36 grams per hour as a vapour in conjunction with a low volume argon gas carrier stream at a temperature of around 700° 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. Over the 148 minutes of continuous feeding, the level of external heating was increased such that the temperature of the gas leaving the bed increased to 1037° C. Even at this elevated temperature the bed remained fluidised with indications that sinter free operation at even higher temperatures was possible. Particles removed from the reactor were found to consist of the original sand particle totally encapsulated in a 10 μm shell of titanium. This is shown in FIG. 2, with the cross-section of the particle being shown in FIG. 3.

EXAMPLE 3

Example 1 was repeated except the starter bed of 600-500 μm sand particles was replaced with low volatile content coke particles having a particle size between 600 and 500 μm. High purity argon was then passed through the bed of starter particles at a rate of 42.4 standard litres of argon per minute to promote fluidisation. Once the temperature measured 5 cm above the upper bed surface reached 990° C. the two reactant feeds were applied. Titanium tetrachloride was supplied at a rate of 86.35 millilitres per hour in a vapour form at a temperature of around 700° C. As with Example 2, the reductant phase was magnesium metal, which was supplied at a rate of 40.16 grams per hour as a vapour in conjunction with a low volume argon gas carrier stream at a temperature of around 700° C. Upon addition to the fluidised bed, the temperature of the gas leaving the bed increased by around 18° C. due to the exothermic nature of the reaction. Over the 345 minutes of continuous feeding, the level of external heating was increased such that the temperature of the gas leaving the bed increased to 1010° C. Particles removed from the reactor were found to consist of the original coke particle completely encapsulated in a titanium based shell.

EXAMPLE 4

A cylindrical reaction vessel with a conical base having an internal diameter of 200 mm and an aspect ratio of 10 was charged with 200 grams of graded (600-500 μm) sand particles. High purity argon was then passed through the bed of starter particles at a rate of 23 standard litres of Argon per minute to promote fluidisation. Once the temperature measured 5 cm above the upper bed surface reached 970° C. the two reactant feeds were applied. Titanium tetrachloride was supplied at a rate of 220 millilitres per hour in a vapour form at a temperature of around 700° C. In this example, the reductant phase was zinc metal, which was supplied at a rate of 262 grams per hour as a vapour in conjunction with a low volume argon gas carrier stream at a temperature of around 700° 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 decreased by around 12° C. consistent with thermodynamic expectations. Over the 60 minutes of continuous feeding, the level of external heating was maintained such that the temperature of the gas leaving the bed increased was around 960° C. Even at this elevated temperature, where titanium has been observed to sinter, the bed remained fluidised with indications that higher temperatures were possible. Particles removed from the reactor were found to consist of the original sand particle encapsulated in a 10 μm shell of titanium.

Claims

1. A process for producing a metal from the corresponding metal halide by reaction of the metal halide at elevated temperature in a reactor comprising a fluidised bed, the fluidised bed comprising seed particles of an inert material that has a melting point higher than that of the metal.

2. A process according to claim 1, wherein the melting point of the inert material is at least 30° C. higher than the melting point of the metal to be produced.

3. A process according to claim 1, wherein the inert material is selected from the group consisting of metal oxides, metal nitrides and metal silicates, metals, metal alloys and coke.

4. A process according to claim 3, wherein the inert material is silica.

5. A process according to claim 1, wherein the particle size of the seed particles is from 500 to 1000 μm.

6. A process according to claim 1, wherein on start-up the fluidised bed comprises a mixture of seed particles of the inert material and of the metal to be produced.

7. A process according to claim 6, wherein the weight ratio of seed particles of inert material to seed particles of metal is from 20:80 to 80:20.

8. A process according to claim 1, wherein on start-up the fluidised bed consists solely of seed particles of the inert material.

9. A process according to claim 1, which comprises continuously discharging particles from the reactor and sampling the particles to determine when seed particles and composite particles comprising the seed particles are no longer present in the fluidised bed and then isolating particles discharged subsequently from the reactor.

10. A process according to claim 9, wherein the fluidised bed is replenished with seed particles formed from the metal.