Patent Application
20090120239
The present invention provides a process for the manufacture of a metal or alloy by reduction of the corresponding metal halide. The invention also relates to an apparatus suitable for carrying out the process.
The present invention will be described with particular reference to the manufacture of titanium by the reduction of titanium tetrachloride with magnesium. However, it is to be appreciated that the principles underlying the present invention are more generally applicable so that the invention may be employed in order to manufacture other metals by similar kinds of reduction reaction.
The Kroll process 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:
TiCl4(g)+2Mg(l)=Ti(s)+2MgCl2(l)
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 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 are melting to produce titanium of acceptable quality.
A variety of other processes for titanium production have also been proposed. However, these also have disadvantages associated with them. Thus, U.S. Pat. No. 2,827,371 describes a process for manufacturing titanium in which titanium tetrachloride and sodium are reacted in an inert atmosphere in a bed of sodium chloride particles. However, the resultant titanium powder has to be post-treated by heating to 850° C. in an inert atmosphere in order to render it less reactive.
U.S. Pat. No. 4,877,445 discloses a fluidised bed process in which titanium tetrachloride vapour is reacted with magnesium vapour at a temperature of 1000 to 1200° C. The reaction is carried at low absolute pressure (in the range 20 to 50 Torr) in order to prevent condensation of unreacted magnesium and of the magnesium chloride reaction product. Condensation of these species can result in product contamination and/or reactor fouling. In practice, on an industrial scale it is extremely difficult to maintain the low operating pressure required in this type of process.
JP 3-150326 teaches an alternative approach that may be carried out at atmospheric pressure. Here gaseous titanium tetrachloride and gaseous magnesium are blown into a fluidised bed of titanium seed particles with a stream of heated inert gas. The amount of inert gas delivered to the bed is controlled with the intention of ensuring that in the reactor the partial pressures of unreacted magnesium and magnesium chloride remain below their respective equilibrium vapour pressures at the temperature in the reactor. Use of the inert gas in this way enables the reaction to be carried out at atmospheric pressure whilst avoiding condensation of unreacted magnesium and/or magnesium chloride. In the process described the reactants and heated inert gas are fed from below into the fluidised bed using a dispersal plate. It is the intention that titanium produced during the reaction will form as a result of heterogenous phase reaction at the surface of the seed particles, and that the particles will be removed from the reactor when they have grown to a particular size.
Although not explicit from the disclosure of JP 3-150326, it is believed that significant gas phase reactions will take place in the reactor due to uncontrolled mixing of the reactants on introduction into the fluidised bed. Such homogeneous (gas) phase reactions often result in the formation of fine, sub-micron size titanium particles rather than growth of the seed particles. This can itself present problems since the fine titanium particles produced are difficult to handle and are more prone to sintering and oxidation than larger sized particles. Furthermore, the fine particles can cause oxidation combustion during post-treatment processing.
These problems associated with the process disclosed in JP 3-150326 can also perhaps be inferred from JP 3-150327, by the same applicant. The process of JP 3-150327 uses a main fluidised bed reactor as described in JP 3-150326 and a second fluidised bed for treatment of titanium particles continuously removed from the main reactor. One function of this second fluidised bed is to separate titanium particles of the desired size from fine titanium particles that have been produced in the main reactor. It is the intention to return the fine particles to the reactor for growth by titanium deposition. It is believed that one skilled in the art would appreciate that it is accepted in JP 3-150327 that a significant proportion of fine titanium particles will be produced in the main reactor as a consequence of the way in which the process is carried out.
The present invention seeks to overcome the disadvantages associated with these known processes. Thus, the invention seeks to provide a process for the production of a metal by reduction of the corresponding metal halide that is capable of producing the metal in high yield and with good purity. The invention also seeks to provide a process for metal production that can be operated at atmospheric pressure and that does not result in the production of significant amounts of fine metal particles. In one preferred embodiment, the process of the present invention may be operated continuously.
Accordingly, the present invention provides a process of producing a metal from the corresponding metal halide by reaction of the metal halide with a reducing agent to form the metal and a halide of the reducing agent, the reaction taking place at elevated temperature and at atmospheric pressure in a reactor comprising a fluidised bed of seed particles of the metal, which process comprises:
injecting into the reactor an inert gas at a rate and in an amount effective to form a fluidised bed of the seed particles and to ensure that unreacted reductant and the halide of the reducing agent do not condense in the reactor under the prevailing conditions of temperature and pressure;
maintaining the maximum temperature in the reactor below the melting point of the metal; and
delivering the metal halide and reducing agent into the fluidised bed in a manner that favours formation of the metal on the seed particles over formation of the metal by homogeneous gas phase reaction between the metal halide and reducing agent.
In the context of the present invention the metal produced may be a single metal or an alloy of two or more metals. Unless otherwise stated, the term “metal” is used to denote single metals as well as alloys.
The crux of the present invention resides in taking deliberate steps to promote contact of the reactants on the surface of the seed particles making up the fluidised bed, thereby ensuring heterogeneous phase reaction and deposition of metal on the surface of the seed particles. It will also be appreciated that the effect of this is to reduce, or eliminate altogether, interaction between the reactants in the gas phase that would lead to formation of fine metal particles. There is not believed to be any disclosure in the prior art of controlling the way in which the reactants are delivered into the fluidised bed, and thus come into contact with each other, with the intention of promoting formation of the metal at the surface of the seed particles making up the fluidised bed. In taking specific steps to control how the reactants come into contact with each other, the process of the present invention favours coarsening of the seed particles by deposition of metal on the surface of the particles rather than the production of new, fine particles of the metal by homogeneous gas phase reaction of the reactants.
Without wishing to be bound by theory, it is believed that it is possible to promote the heterogeneous phase reaction on the surface of the seed particles making up the fluidised bed by delivering the reactants into the fluidised bed in such a way that one of the reactants is present and available for reaction at the surface of the seed particles before there is any contact of this reactant with the co-reactant. The manner in which the process of the invention is performed is therefore likely to depend upon the kind of surface interactions that exist between a given reactant and the seed particles under the conditions encountered in the fluidised bed. For example, if it is known that a gaseous reactant becomes adsorbed onto, or impregnates, the surface of the seed particles on contact with them, it is necessary in accordance with the invention to ensure that this adsorption or impregnation takes place before this reactant contacts the co-reactant. An understanding of these surface interactions is important for operation of the invention and apparatus design.
Successful implementation of the invention will also involve an understanding and/or control of how the seed particles flow within the fluidised bed as the process of the invention is carried out. As noted, it is believed to be important that one of the reactants is “associated” with the seed particles prior to coming into contact with the co-reactant. The manner in which the seed particles flow in the region in which the relevant reactant is delivered is likely to have an impact on this association. Furthermore, after this reactant has become suitably associated with the seed particles, movement of the particles within the fluidised bed is usually relied upon for contact of the reactants. By adopting this approach it is possible to introduce the reactants into the fluidised bed in such a way that avoids uncontrolled contact, and thus homogeneous gas phase reaction, of them. For the desired growth of the seed particles by metal deposition, it is necessary for repeated reactions to take place on the surface of the seed particles. It is therefore also important that, after having been coated with metal, the coated seed particles are recirculated in order to become associated once more with the relevant reactant. After this association the seed particles are “primed” for reaction with the co-reactant, and flow into a region within the bed where the co-reactant is present. In this way the seed particles are constantly cycled through the respective regions occupied by reactants with formation of metal on the surface of the particles each cycle.
In practice, a variety of factors are likely to influence the way in which the seed particles flow within the fluidised bed. Such factors may include the shape of the reactor, the initial size of the seed particles, how the inert gas is delivered and/or the temperature conditions and variations thereof in the reactor. It may be necessary to manipulate some or all of these variables in order to optimise performance of the present invention. Additionally, the way in which the reactant is delivered into the fluidised bed and, possibly, the residence time required for the necessary association of reactant and seed particles are likely to be relevant factors for successful operation of the invention. Association of the reactant and the seed particles is likely to be instantaneous on delivery of the reactant into the fluidised bed but, if not, this must be taken into account. This may be a consideration if it is intended that the reactant will undergo some phase change before the required association with the seed particles occurs. The effect of any and all of these factors may be assessed by computer modelling and/or by experiment, thereby enabling a suitable reactor and accompanying operating conditions to be designed.
In an embodiment of the invention, both reactants are delivered into the fluidised bed in gaseous/vapour form and one of the reactants is absorbed by or impregnates the seed particles prior to contact with the co-reactant. Typically, in this embodiment, it is the metal halide which becomes associated with the seed particles.
This embodiment may be put into practice by delivering the reactants into separate regions of the fluidised bed. In this case the points of delivery of the reactants and the flow of the seed particles are such that the seed particles come into contact with one of the reactants (the first reactant), thereby becoming associated with it, with subsequent flow of the particles into a region where the other (second) reactant is present. Reaction between the reactants then takes place at the surface of the seed particles with metal being deposited on the surface of the particles. After having been the site for this reaction, the flow of the seed particles within the bed is such that the particles are then circulated within the bed so that they come into contact again with the first reactant.
This embodiment may be put into practice with individual injection nozzles for each reactant. These may be spaced and positioned within the reactor as necessary given the recognised flow patterns of the seed particles within the fluidised bed. Alternatively, it may be possible to achieve the desired effect by use of a single injection nozzle delivering independent streams of reactants. Here the invention may be put into practice using an injection nozzle having concentric outlets for the reactants. For example, the nozzle may comprise a central conduit for one reactant with a surrounding annular conduit for the other reactant. In this case it is usual that the flow of seed particles within the fluidised bed would be across a stream of reactant delivered through the annular conduit and into the stream of the reactant delivered through the central conduit, with subsequent re-circulation of the particles, and so on. In this case it will be appreciated that the flow of particles will suit delivery of the first reactant (to be associated with the seed particles) through the annular conduit with the second reactant being delivered through the central conduit.
It may be possible to prevent premature contact of the reactants by injecting inert gas between adjacent streams of reactants. The intention here is that the inert gas will prevent reaction of the reactants until such time as one of them becomes associated with the seed particles. However, in this case, the inert gas should not interfere with the desired flow of seed particles within the reactor. In the concentric nozzle design mentioned, the inert gas may be delivered through an annular conduit provided between the central and annular conduits through which the reactants are delivered.
In another embodiment, one of the reactants is present in the fluidised bed as a liquid and this reactant wets or coats the surface of the seed particles prior to contacting the co-reactant as a result of particulate flow. Here the same general principles apply as described above for gaseous reactants, although when using a reactant, in liquid form other delivery mechanisms, such as gravity feeds, may be employed.
In accordance with the invention an inert gas is used to manipulate the partial pressures for unreacted reductant and the halide of the reductant that is formed after the reduction reaction. The amount of inert gas that is delivered to the reactor may be varied appropriately according to the required partial pressures in order to prevent condensation of these species at the intended operating temperatures of the reactor. How this works may be illustrated with reference to the titanium tetrachloride/magnesium reaction system. Here it is desired to prevent condensation of the magnesium chloride produced, and of any unreacted magnesium. As the following table illustrates the vaporisation temperature of magnesium chloride at atmospheric pressure (i.e. when the partial pressure of the magnesium chloride is 1 atmosphere) is about 1418° C., and that of magnesium is about 1100° C. At reduced absolute pressure, the temperature at which the partial pressure of these species corresponds to the absolute pressure becomes lower and thus the vaporisation temperatures are lower.
To enable the production process to be run at reduced temperature whilst avoiding condensation of magnesium chloride (magnesium has a lower boiling point at the same pressure), it is conventional to operate the process under low pressure conditions. Thus, if the process is operated at an absolute reactor pressure of 50 Torr (as per U.S. Pat. No. 4,877,445) it is necessary for the reactor temperature to be in excess of about 1068° C. to avoid condensation of magnesium chloride. However, in accordance with the present invention, the same effect can be achieved by reducing the partial pressure of the magnesium chloride rather than by reducing the absolute pressure in the reactor. This can be done by injecting into the reactor an appropriate amount of inert gas. This allows the process of the invention to be operated at atmospheric pressure but below the recognised boiling temperature of magnesium chloride. Thus, if it is desired to operate the process of the invention at 1320° C. and at atmospheric pressure, it is necessary to use an amount of inert gas to reduce the magnesium chloride partial pressure to 0.5 atm. The boiling point of magnesium chloride is then 1311° C., i.e. below the operating temperature chosen.
The use of greater amounts of inert gas will allow the process to be operated at lower temperatures (at atmospheric pressure) whilst avoiding magnesium chloride condensation. However, in practice, it is generally not convenient and/or economic to use very high inert gas dilution to achieve low operating temperature. Injection of large amounts of inert gas into the fluidised bed may also cause unwanted turbulence and make it difficult to achieve the desired particulate flow patterns within the fluidised bed.
One skilled in the art would have no difficulty in utilising inert gas in order to manipulate the partial pressures in this way, particularly in the light of the teachings of JP 3-150326 and JP 3-150327. Usually argon or helium is employed as the inert gas.
The seed particles used in practice of the invention are formed of the same metal it is desired to produce by the reduction reaction. Depending upon the mechanism involved, it may be possible to enhance the intended association of one of the reactants with the seed particles by varying the surface characteristics and/or initial size of the seed particles. Usually, the seed particles have an initial diameter of from 200 to 500 microns but not necessarily limited to this range. It is generally desired to remove the coarsened particles from the reactor. If the process of the invention is operated in a batch-wise fashion, the coarsened particles are removed and the reactor re-stocked with suitably sized fresh seed particles.
This said, in accordance with a preferred embodiment of the invention, the process of the invention is operated continuously. To do this it is necessary to remove seed particles that have been coated with metal and grown to a predetermined size and replenish the reactor with small seed particles. These may be self-seeded metal particles produced in the bed from attrition of the metal coating on existing seed particles and/or fresh seed particles added via an inlet into the bed for on-going metal deposition. Particles of desired size may be withdrawn from the reactor through a suitable outlet. Essentially, the coarsened particles are removed from the reactor through a self-regulating process based on the particle size and fluidisation conditions. The removed particles are subsequently cooled. It is important that the removal of coated particles, production of self-seeded particles in the bed and introduction of fresh seed particles do not have an adverse effect on the desired flow patterns of particles making up the fluidised bed.
Typically, the reduction reaction is exothermic. In this case, and according to a preferred embodiment of the invention, at least one of the reactants is delivered into the fluidised bed as a solid or liquid with subsequent phase change of the at least one reactant as a result of the temperature in the bed. The phase change is endothermic and this may help to moderate the temperature within the reactor as the reduction reaction proceeds. If a change of phase of one or both of the reactants is to be relied upon, it is still necessary to ensure that the general principles of the present invention are applied and preserved.
Herein the region in the fluidised bed where reaction between the reactants takes place is termed the “reaction zone”. In a preferred embodiment, after deposition of the metal on the surface of the seed particles, the freshly coated particles move rapidly from the reaction zone (by virtue of flow patterns established in the fluidised bed) and are rapidly quenched in lower temperature regions outside the reaction zone. As the reduction reaction is typically exothermic, the temperature of the reaction zone is higher than other parts of the fluidised bed where the reduction reaction is not on-going.
It is desirable to control the temperature within the fluidised bed in order to ensure that any fine metal particles that may have been formed will sinter, without any sintering of larger metal particles. If the reaction is exothermic it may in fact be necessary to take specific steps to reduce the average temperature within the fluidised bed. Attempts to moderate the temperature within the reactor should however be considered in the light of the general principles applicable in the present invention. The temperature may be manipulated by altering the temperature of the reactants and/or by the amount and temperature of the inert gas delivered and/or by relying on reactant phase change, as mentioned above. Energy released as a result of the reaction may be used to convert the reactants to gases by suitable heat exchange systems. If insufficient energy is available from the reactor itself, additional energy may be derived from burning a fuel, such as methane, in air.
The present invention is preferably employed for producing titanium by reduction of titanium tetrachloride with a suitable reductant, such as magnesium or zinc. This said, the present invention may be used to produce alloys and other metals by analogous reduction reactions using suitable reactants. Thus, the invention may be used to produce zirconium, silicon, tantalum or niobium from the corresponding chlorides. One skilled in the art would be familiar with suitable reductants to be used in each case.
With respect to the titanium tetrachloride/magnesium reaction system, in one embodiment, both reactants are delivered into the fluidised bed as gases. In gaseous form the titanium tetrachloride may be adsorbed onto or impregnate the surface of the titanium seed particles making up the fluidised bed. The titanium tetrachloride is therefore contacted with the seed particles to ensure that this takes place before contact with the magnesium.
As an alternative, the magnesium may provided in the fluidised bed in the form of a molten liquid. Magnesium may wet/coat the surface of the seed particles and it may therefore be contacted with the seed particles in order to form the necessary association of reactant/seed particles. The flow pattern of seed particles within the bed and the delivery of the gaseous titanium tetrachloride are such that the magnesium wets/coats the seed particles prior to contacting the titanium tetrachloride. In a preferred embodiment, the magnesium is delivered into the reactor as a solid with the temperature being sufficient to melt the magnesium immediately. This phase change can be used to moderate the reactor temperature-along the lines described already.
Embodiments of the present invention are illustrated in the accompanying non-limiting Figures is which:
In the following the Figures are discussed with reference to the titanium tetrachloride/magnesium reaction system. However, this is for illustrative purposes only and should not be taken as restricting the embodiments shown in the Figures to this reaction system.
In the embodiment illustrated in
With suitable manipulation of operating parameters, as described above, it is believed that the flow pattern of seed particles local to each concentric nozzle will be as shown in
After being coated with Ti in the reaction zone the coated seed particles move out of the reaction zone and into cooler parts of the fluidised bed where they are rapidly quenched after coming into contact with other particles in the bed.
Typically, for the reduction of titanium tetrachloride with magnesium, the fluidised bed is operated at a temperature of 1200° C. at atmospheric pressure, the temperature in the reaction zone corresponding to about 1880° C.
The MgCl2 produced by the reduction reaction remains as a gas at the conditions prevailing in the reactor. At lower temperatures of operation it would be necessary to use larger quantities of argon, to give a lower partial pressure Of MgCl2 vapour, thereby avoiding condensation of MgCl2 as per the principles discussed above
MgCl2 gas and argon exit the top of the fluidised bed reactor and enter a quenching device which could be a circulating fluidised bed quencher made up of particulate MgCl2. Here the argon is cooled rapidly and the MgCl2 condenses on the surface of the MgCl2 particles in the bed. Various alternative quencher designs could be used. The MgCl2 particles can be used to generate magnesium in an electrolytic cell by conventional process.
Titanium particles of appropriate size may be withdrawn from the fluidised bed reactor and fresh seed particles supplied. In this manner the process of the invention may be run continuously.
An alternative embodiment for operating the process of the invention is shown in
The flow pattern of the coated seed particles is such that they subsequently flow into a region of the fluidised bed where TiCl4 is present. The reduction reaction then takes place on the surface of the Mg coated seed particles resulting in MgCl2 (which is a gas under the prevailing conditions) and a fresh coating of Ti on the seed particles. As with the other embodiment described above, the flow pattern of particles in the bed is such that the titanium coated particles are circulated through a cooler region of the reactor system where quenching takes place. This may take place in the same or different vessel. The quenched particles my then be available for recirculation in proximity to the Mg inlet where they would be coated with Mg for subsequent reaction, or discharged from the reactor with suitable replenishment of said particles. The MgCl2 may be processed as described above in relation to
A high heat transfer rate is possible because of the large amount of transfer surface area available per unit volume of the fluidised bed. This permits rapid levelling of any temperature surges either from incoming reactants and/or inert gas, or from reactions within the fluidised bed.
In both these embodiments it is important that the seed particles are suitably associated with one of the reactant prior to contacting the co-reactant. This may be achieved by variation of process parameters as described.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004904305 | Jul 2004 | AU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AU2005/001126 | 7/29/2005 | WO | 00 | 1/22/2009 |
