The present invention relates to a method of producing a metal by reaction of a metal halide with a reductant. In particular the invention relates to the production of titanium by reduction of titanium tetrachloride with magnesium using a slurry feed of the reactants.

BACKGROUND TO INVENTION

Applicant's own International patent publication WO 2006/042360 describes a method for producing titanium by reaction of titanium tetrachloride with magnesium in a fluidised bed reactor. It is fundamental to the method disclosed that the temperature in the reactor is above the melting point of magnesium and below the melting point of magnesium chloride. The titanium tetrachloride is usually supplied into the reactor in vapour or liquid form. The magnesium may be supplied into the reactor as a solid, liquid or gas. The titanium tetrachloride and magnesium feeds are separate and contact of the reactants takes place in the reactor where reaction takes place. Titanium-containing particles are produced and these are removed for subsequent purification.

The method described in WO 2006/042360 is capable of producing titanium in unexpectedly high yield. However, the form of magnesium feed into the reactor has practical constraints and implications and these must be considered carefully when implementing the process.

When used as a solid the magnesium must be delivered in fine particulate form to ensure suitable reactivity and distribution. However, in this form there can be a significant dust explosion potential and significant precautions are required to ensure safe operation.

A further practical constraint associated with the use of particulate magnesium is that it can introduce significant amounts of oxygen (as passivating magnesium oxide coating on the particles) into the reactor, and as a result into the titanium that is produced. Some applications of titanium however require very low oxygen content. Due to surface - ? -

area/volume effects larger magnesium particles would introduce relatively less oxygen when compared with smaller particles. However, there is then a trade-off in particle reactivity and distribution.

Introducing magnesium into the reactor in liquid form avoids these problems but poses significant challenges with respect to equipment design, metering and handling. The same is true when magnesium is used in gaseous form with the added issue of higher energy input.

It would be desirable to provide a method of producing titanium by the type of methodology described in WO 2006/042360 in which the reactants are supplied into the reactor in a different form from that described.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of producing titanium by reaction of titanium tetrachloride and magnesium in a reactor, wherein the titanium tetrachloride and magnesium are delivered into the reactor in the form of a slurry formed by milling magnesium particles in titanium tetrachloride. The present invention also provides this form of slurry for use, and when used, in a reactor for production of titanium.

The invention further provides a slurry comprising magnesium and titanium tetrachloride, wherein the slurry has been produced by milling magnesium particles in titanium tetrachloride.

The invention yet further provides titanium when produced in accordance with the invention.

DETAILED DISCUSSION OF INVENTION

In accordance with the present invention the dust explosion potential associated with fine particulate magnesium is avoided by dispersing the magnesium particles in titanium tetrachloride. In fact the magnesium particles are milled in titanium tetrachloride with milling leading to a reduction in the average particle size of the magnesium that is present. As milling results in a reduction in the average particle size of the magnesium, the magnesium used as starting material may be of a particle size for which there is reduced risk or no risk of dust explosion. Thus, the average particle size for the magnesium prior to milling may vary depending on the comminution technique employed and the commercially available form of the magnesium. Typically, however, the average particle size is up to about 2000 μm, for example from 1000 to 2000 μm. Magnesium powder in this size range is commercially available. Preferably, the average particle size for delivery into the reactor will be 50 to 100 μm.

In addition to mitigating the dust explosion potential, milling has a number of other practical advantages. Thus, use as starting material for milling of relatively large size magnesium particles is also beneficial with respect to the amount of oxygen that will be introduced into the reactor. As noted above, due to surface area/volume effects, relatively large particles of magnesium will include relatively less magnesium oxide (based on the weight of magnesium) when compared to particles of smaller size. The starting magnesium particles will include a particular amount of oxygen but milling in titanium tetrachloride causes a reduction in the average particle size whilst minimising or avoiding altogether oxidation of the smaller magnesium particles that are produced (fresh magnesium surfaces will be created during milling). Thus, milling produces magnesium particles that have a reduced oxygen content, as magnesium oxide, when compared with particles of the same size that have otherwise been exposed to oxidative conditions. Typically, it is desirable for the oxygen content of magnesium particles delivered to the reactor to be less than 2500 ppm, preferably less than 1000 ppm, based on the weight of magnesium particles.

Milling may be carried out in any suitable conventional apparatus such as a ball mill or impact mill. Materials selection may dictate mill design and operation. It is desirable to conduct milling in an inert atmosphere so that oxygen is not available for reaction. Thus, milling may be conducted under argon or other inert gas. It may be possible to conduct milling without an inert atmosphere provided the amount of oxygen introduced does not exceed the intended maximum levels. Any significant degree of oxidation of milled particles is to be avoided. Another advantage of milling in TiCl ₄ is that TiCl ₄ has an extreme affinity for oxygen. Thus, the TiCl ₄ effectively scavenges any tramp oxygen before it can react with fresh magnesium surfaces created as a result of milling. The titanium oxides thus produced will not be reduced during the reaction step and will therefore not contribute to the solution oxygen level in the titanium metal products. Milling ■ F^ may take place at room temperature or at elevated temperatures provided that in either case reaction of magnesium and titanium tetrachloride is not initiated or is not initiated to any appreciable extent. Reaction between these components is intended to take place in a separate reactor and not in the equipment used for milling.

With respect to the relative proportions of magnesium and titanium tetrachloride that are subjected to milling, this will generally be determined by the stoichiometry required in the reactor for these components. Generally, the magnesium and titanium tetrachloride will be milled at a ratio of 1 :3.9 w/w. However, titanium tetrachloride can be added after milling if necessary to achieve the desired ratio.

Favourably, the specific gravity of magnesium and titanium tetrachloride is generally similar, and the result of this is that once the magnesium slurry is formed little settling and ' ^" ' separation of magnesium takes place. It is preferred that the slurry has a uniform distribution of magnesium in titanium tetrachloride when delivered into the reactor. The slurry may be agitated prior to delivery into the reactor if settling of magnesium has occurred to any appreciable extent. The slurry is storage stable and can be made in advance and stored prior to use. Alternatively, the slurry may be produced on a continuous basis in the context of a continuous production of titanium. In this case the milling apparatus used to produce the slurry may form part of an overall production system including an appropriate reactor. The mill would be supplied with magnesium particles (prill) and titanium tetrachloride as required with slurry being tapped from the mill when the magnesium has reached a desired particle size. Use of the slurry as feed for the kind of method described in WO 2006/042360 for producing titanium also provides advantages in terms of reaction control. Since the slurry is of known composition in terms of reactants, it can be metered into the reactor with relative ease by conventional pumping and flow control equipment, Indeed, with knowledge of the slurry composition the weight of reactants being delivered into the reactor can be determined based on a volume flow rate of slurry into the reactor.

The reaction itself between magnesium and titanium tetrachloride in the reactor is conducted at elevated temperature (minimum about 600° C) but is exothermic in nature.

Temperature control within the reactor is important since it is desired to maintain magnesium chloride by-product in solid form. The temperature in the reactor is therefore usually maintained in the range 650° C to 710° C. Varying the rate at which the magnesium/titanium tetrachloride slurry is delivered into the reactor can be a convenient way of controlling reactor temperature. For example, if a particular feed rate of slurry results in a temperature increase that is unsatisfactory, the feed can be paused or a lower feed rate adopted until the temperature drops to a suitable level. In practice the feed rate of slurry into the reactor may be adjusted on a continuous basis in response to fluctuations in reactor temperature due to on-going reaction of magnesium and titanium tetrachloride in the reactor.

In accordance with the invention any alloying elements that it is desired to introduce to the titanium product may be added to the slurry and, if necessary, milled with the magnesium. This may be especially inserted where any alloying elements are oxygen sensitive and/or need to be finely divided to react or added in small quantities. Using this approach the required alloy ratios can be finely adjusted and verified ahead of the reaction stage.

It has been also found that use of a slurry as described herein leads to a titanium product having favourable geometry and that is friable. This provides benefits in terms of handling and/or separation downstream, such as by vacuum distillation. The reaction step produced titanium particles in the range of 4 micron which is too fine for many powder applications and also highly pyrophoric. Some form of consolidation is needed to make a powder metallurgy ready feed and avoid post production handling and oxygen damage issues. Titanium metal recovered from the vacuum distillation separation step of the composite from the process taught in WO 2006/042360 is in the form of a friable sinter that satisfies this criteria. The sinter is safe to handle in air because of its reduced surface area per unit mass however the nature of the sinter facilitates comminution under mild conditions such that a particle size distribution of the type required for powder metallurgy can be reconstituted as required. It has been found that titanium produced from a slurry feed is particularly amenable to this approach due to the favourable size and shape of the composite particles (Ti in MgCl ₂ ) produced.

Notwithstanding that WO 2006/042360 does not describe use of magnesium and titanium tetrachloride feeds combined as a slurry, the basic reactor configuration and manner of operation, including downstream processing of produced titanium, are relevant and applicable to the present invention. The disclosure of WO 2006/042360 is incorporated herein by reference.

Advantages of the slurry approach described herein include the following:

• inherently safe since avoids use as starting material of particulate magnesium with high dust explosion potential;

• low oxygen content even though slurry includes finely divided magnesium;

• enhanced efficiency and process (temperature) control;

• accurate metering of reagents possible (assuming fixed ratio of magnesium and titanium tetrachloride); • ease of introduction of alloying elements that are oxygen sensitive and/or need to be finely divided to react;

• enhanced reactivity due to use of magnesium in finely divided form;

• more favourable product geometry and enhanced performance of separation step downstream. BRIEF DESCRIPTION OF FIGURES

Embodiments of the present invention are illustrated with reference to the accompanying non-limiting drawings in which:

Figure 1 is a graph depicting oxygen content of (commercial purity) titanium particles based on the particle size magnesium reductant used;

Figure 2 is schematic illustrating a system useful for implementing the method of the present invention as claimed; and

Figure 3 shows a temperature profile in an experimental reactor in which magnesium is reacted with titanium tetrachloride.

Figure 1 shows that when magnesium particles are used in a conventional system as reductant for titanium chloride the concentration of oxygen in the produced titanium is relatively high. This is because small magnesium particles have a relatively high surface area to volume ratio and thus a relatively large amount of magnesium oxide as particle coating. As the size of the magnesium particle that is used increases the relative amount of oxygen introduced into the reaction system is relatively less for the same reason. However, whilst the use of large magnesium particles is preferred from the viewpoint of limiting the introduction of oxygen into the titanium product, large particles are of reduced reactivity when compared with smaller particles. For reactivity the magnesium particle size is desirably 50 to 100 μm. Figure 1 shows to generate titanium with less than 1000 ppm oxygen would preclude the use in a conventional reactor system of magnesium particles having a particle size below about 580 μm. For conventional magnesium particles tested by the inventors it has been found that the average amount of oxygen present is 0.166g O/m ² surface area of the particles. The present invention allows the production of magnesium particles with a lower average amount of oxygen per square metre surface area. The formation of a slurry of reactants in accordance with the present invention may be integrated as part of an overall production process. An exemplary production process is illustrated in Figure 2. This shows a production system in which a magnesium "prill" feeder 1 (capable of accepting commonly available magnesium forms, such as raspings (turnings), granules and coarse powders, the various forms having an average particle size >500 microns) delivers coarse magnesium particles 2 to a slurry unit (mill) 3. This unit 3 receives liquid titanium tetrachloride 4, metered using a pump 5 based on the weight of coarse magnesium particles 2. The slurry unit 3 mills (mulls, shears or grinds) the coarse magnesium particles in titanium tetrachloride thereby producing a slurry 6 having a reduced average magnesium particle size. Typically, milling takes place under argon to avoid any adverse effects associated with the presence of oxygen. When the average magnesium particle size is suitably low the slurry 6 may be pumped using a suitable pump 7 (a modified mono pump may be used) to an intermediate store 8 prior to delivery into a reactor 9. Additional titanium tetrachloride can be added between the store 8 and reactor 9 if the ratio of magnesium and titanium tetrachloride requires modification. The use of an intermediate store 8 is not mandatory and the slurry 6 could be delivered directly to the reactor 9 if necessary. The delivery of slurry 6 into the reactor 9 can be metered based on the prevailing temperature in the reactor 9 so that operation of the reactor 9 and production of titanium is optimised. Titanium particles produced in the reactor 9 are removed for subsequent purification and refinement (not shown).

Figure 3 shows a temperature profile in an experimental reactor in which magnesium is reacted with titanium tetrachloride. These reactants are introduced into the reactor as a slurry as per the present invention. The plot may be understood as follows. The reactor includes a fluidised bed and is brought up to a suitable operating temperature. At that point slurry is fed into the reactor at a flow rate of lOml/min (A). This causes a temperature increase up to about 685° C. At that point it is desired to reduce the temperature since it is approaching the temperature at which by-product magnesium chloride becomes a liquid. This temperature reduction is achieved by a reduced slurry flow rate of 5 ml/min (B). The temperature then drops and may be increased again by increasing the slurry flow rate to 7.5 ml/min (C). Another temperature spike follows and this can be accommodated by reducing the slurry flow rate once more to 5 ml/min (D). Subsequently, the slurry feed is stopped (E) and the temperature drops off and stabilises again. The reactor is then shut down. This figure therefore demonstrates the ease with which reactor temperature may be controlled by adjustment of slurry input. Such responsiveness would be a valuable asset on scale-up.

The present invention is described herein with reference to the magnesium/titanium tetrachloride reaction system. However, the principles underlying the present invention may be applied to other combinations of reductant/metal halide, including aluminium and titanium tetrachloride The metal halide must of course be a liquid under the conditions at which milling takes place. Thus, the invention may also be implemented using SiCl ₄ and VCl ₄ , and possibly TiBr ₄ (which is a liquid at a temperature slightly above room temperature).

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 in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.