METHOD AND APPARATUS FOR THE PRODUCTION OF TITANIUM
Field of the Invention
The present invention relates to a method and apparatus for the production of metal and, particularly, but not exclusively, to a method and apparatus for the production of titanium.
Background of the Invention Titanium is a metal with a number of highly desirable properties. It is stronger than steel, but is of lighter weight. Further, it is highly resistant to corrosion. Titanium finds extensive use in industries such as the aerospace industry and other industries were these properties are prized. A major problem, however, is that titanium is very expensive. This limits its use virtually to applications where its properties are essential so that the cost must be incurred.
The main reason for the expense of titanium is the difficulty in obtaining titanium from its ore. Present processes are very expensive. There are two existing commercial routes for production of titanium; the Kroll process and the Hunter process. In the Kroll process, titanium minerals are chlorinated in the presence of carbon to form TiCl4 that is then reduced to Ti metal using magnesium. By-products of the process such as MgCl2 is separated from the titanium sponge by washing and then the titanium sponge is crushed for further purification using vacuum arc distillation. The vacuum arc is also used to make ingot out of the crushed sponge. The Kroll process is multi-stage and expensive.
In the Hunter process sodium is used instead of magnesium as the reducing agent. This process is less widely used than the Kroll process. Sodium is a highly reactive (more so than magnesium) element and is therefore difficult and expensive to handle.
Both the Kroll process and the Hunter process are therefore, extremely expensive.
There have been a large number of attempts to replace these processes with lower cost processes . Recent efforts include a method developed by Idaho National Engineering Laboratory for reduction of TiCl using hydrogen in a plasma torch using a supersonic nozzle for quenching the plasma (US Patent No. 5749937) . For this approach, the titanium product is in a nano-powder form and requires further careful processing and handling as nano-powders are highly reactive, which may result in contamination of the titanium product .
The process therefore has to be carried out under controlled atmosphere. This process has not yet been used on an industrial scale Another recent process suggested by Chen et al (G Z Chen, D J Fray and T W Farthing, Nature 407, 361 (2000)) proposes a direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. For this process, Chen et al propose an electrolytic route for the direct reduction of titanium oxide (Ti0 ) , in which the oxygen is ionised, dissolved in a liquid calcium chloride electrolyte and discharged at the anode, leaving pure titanium at the cathode. This process is still somewhat in the development stage and has not yet been proven on an industrial scale.
It is to be understood that, if any prior art or prior art publication is referred to herein, such reference does not constitute an admission that the publication or prior art forms part of the common general knowledge in the art, in Australia or any other country. There is a need for a method and apparatus for producing titanium metal, which is less expensive than presently known processes.
Summary of the Invention
The present invention preferably enables the production of titanium by way of a direct extraction process, preferably in the absence of any reducing agent. Removal of a requirement for an agent simplifies the reaction, results in less likelihood of impurity and, preferably, lowers the expense of production. Preferably, the titanium is extracted from titanium tetrachloride. In accordance with a first aspect of the present invention, there is provided a method for the production of titanium from a compound containing titanium, comprising the steps of implementing a direct reduction reaction of the compound in the absence of any reducing agent, and driving the reduction reaction away from equilibrium whereby-to facilitate the precipitation of titanium metal.
Preferably, the reduction reaction is driven away from equilibrium rapidly.
The compound is preferably a titanium halide, and is preferably titanium chloride (TiCl4) .
Driving the reduction reaction away from equilibrium is preferably achieved by a process of rapid cooling of the reaction constituents, to rapidly drive the reduction reaction away from equilibrium.
Preferably, the reduction reaction is implemented by way of a plasma discharge to produce the reaction constituents within the plasma. Preferably, the rapid cooling is implemented by rapidly quenching the plasma. The rapid cooling is preferably implemented by driving the plasma into contact with relatively strongly cooled surfaces . The rapid cooling may also be implemented by mixing the plasma with other gases (as an alternative to driving into contact with strongly cooled surfaces or in addition to) .
Preferably, the titanium species condense at the cooled surface (where a cooled surface is used) in the form of powders that can be continuously removed from the surface.
Alternatively, the powder may be exposed to heat from the plasma for a short period so that it forms large crystals, and then the crystals are subsequently removed from the cooling surface.
In accordance with a second aspect of the present invention, there is provided an apparatus for the production of titanium from a compound containing titanium, the apparatus comprising a reaction vessel including a reaction means for implementing a reduction reaction of the compound, and means for driving the reduction reaction away from equilibrium.
The reaction implementation means preferably comprises -means for creating a plasma arc
The means for driving a reaction away from equilibrium may comprise means for rapidly cooling the reaction components .
This may comprise means for presenting a cooling surface within the reaction chamber, or means for introducing a gas to mix the plasma and therefore cause the plasma to cool, or both. In accordance with a third aspect of the present invention, there is provided titanium metal produced by a method in accordance with the first aspect of the present invention .
In a variation on the process discussed in relation to the first aspect of the present invention, reacting gases may be introduced to facilitate titanium production or to produce titanium compounds .
In accordance with a fourth aspect of the present invention, there is provided a method for the production of titanium from a compound containing titanium, comprising the steps of implementing a reduction reaction, and rapidly driving the reduction reaction away from equilibrium whereby to facilitate the precipitation of titanium metal . Other gases may be introduced to either increase the yield of titanium (reactive gas such as hydrogen may do this) or to produce compounds such as titanium nitride and titanium oxide (nitrogen and oxygen may be introduced to do this) .
Where hydrogen gas is introduced it may increase the plasma enthalpy, resulting in increasing energy dissipation in the plasma. This increases the rate of dissociation of the titanium compound (which is preferably a titanium halide, preferably titanium tetrachloride) . With this process the hydrogen does not have a reducing role. The reaction may be driven away from equilibrium by utilising any of the techniques discussed above in relation to the first aspect of the present invention.
A similar reaction as described above in relation to the first aspect of the present invention may be used to prepare other metals .
In accordance with a fifth aspect of the present invention, there is provided a method of producing a metal from a compound containing the metal, comprising the steps of implementing a reduction reaction of the compound, and then rapidly driving the reaction away from equilibrium whereby to facilitate the precipitation of the metal.
Preferably, the reduction reaction is implemented in the absence of a reducing agent and is a direct reduction. The compound may be a metal halide. The reduction reaction may be driven away from equilibrium by any of the means discussed above in relation to the first aspect of the present invention. In accordance with a sixth aspect of the present invention, there, is provided a method for the production of titanium from a compound containing titanium, comprising the steps of implementing a direct extraction of titanium from the titanium compound utilising a plasma arc process.
Preferably, the titanium compound is a titanium halide, and is preferably titanium tetrachloride.
The plasma arc process operates to dissociate the titanium atoms from the chlorine atoms directly. The reaction is preferably driven rapidly away from equilibrium to enable titanium to be obtained.
Brief Description of the Drawings
Features and advantages of the present invention will become apparent from the following description of an embodiment thereof, by way of an example only, with reference to the accompanying drawings, in which; Figure 1 is a schematic diagram of an apparatus in accordance with an embodiment of the present invention for the production of titanium;
Figure 2 is a photograph illustrating titanium produced by the apparatus of figure 1, and Figure 3 is an EDX trace of the titanium material produced by the apparatus of figure 1.
Description of Preferred Embodiment
Figure 1 schematically shows a reaction apparatus which can be used to prepare titanium metal, according to an embodiment of the present invention. A direct reduction process is employed, in the absence of any reducing agent. Attempting such a process is counterintuitive. Under equilibrium conditions, the direct reduction is thermodynamically unfeasible and one would expect the components of the reduction merely to recombine due to the reactivity, so that no or little titanium would be obtained. The present invention avoids this recombination by rapidly driving the reaction away from equilibrium.
In the method of this embodiment the reduction reaction is implemented by a plasma arc . Driving the reaction away from equilibrium is achieved by quenching the reactive' plasma by bringing the plasma into contact with strongly cooled surfaces and also by first mixing with inert gases . As the plasma is driven into contact with the cooled surfaces, the temperature of its atomic species (Ti and Cl) is sharply reduced from the temperature of the plasma at more than 5000K to the cooled surface temperature of less than 1000K. The effects of the gas flow is somewhat similar to that of the cooled surface in that the gas flow causes a sharp decrease in the plasma temperature, resulting in a low reaction rate between the Ti species and the chlorine species in the plasma. At low temperatures, the reaction rate between Ti and Cl is lower than between Ti and Cl in the gas phase of the plasma. Ti vapour condenses at the cool surface in the form of powders that can be continually removed from the surface so as to avoid increasing the temperatures of the collecting surfaces, hence avoiding a higher reaction rate between the collected Ti metal and the remaining chlorine gas in the plasma. Also, the temperature of the cooled surface can be regulated so as to avoid condensation of titanium chloride species such as TiCl2, TiCl3 and TiCl . Alternatively, the powder may be exposed to heat from the plasma for a short period so that it forms large crystals, and then the crystals are subsequently removed from the cooling surfaces . Formation of large crystals may be favoured over powders as it lowers contamination of the metal upon exposure to air.
In this embodiment, as discussed above, inert gases and strongly cooled surfaces are used together. It may be possible to use merely a strongly cooled surface or alternatively mixing with inert gases, each alone.
Referring to the drawings, figure 1 is a schematic diagram of an apparatus for carrying out the proςess. A plasma is created by operating an electric discharge 1 between a first electrode 2 (which in this embodiment is a cathode) and a second electrode 3 (which in this embodiment is an anode) . Titanium chloride is introduced into the arc through nozzles 5 distributed in an annular fashion about the arc, and a plasma 1 is formed.
The main body of the arc 1 is surrounded by a cooling cylinder 4. The cooling cylinder 4 is a water-cooled metal cylindrical tube (the water cooling arrangement is not shown in the diagram but it will be appreciated by a skilled person how this is achieved) . The cooling tube 4 may also serve as an auxiliary anode and then the arc may be operated between the cathode 2 and the auxiliary anode 4.
Other inert or reactive gases may be introduced into the arc together with the titanium chloride or independently through the gas nozzle 6 surrounding the cathode 2. In this embodiment, inert gases (such as noble gases) are introduced in order to facilitate driving the reduction reaction occurring in the arc away from equilibrium.
Gases from the plasma can exit the arc region through the gaps between the gas nozzle 6 and the titanium chloride nozzle 5 or alternatively through the region between the cooling tube and electrode 3. The length of these gaps may be adjusted by adjusting the position of the cooling tube 4 and the cathode 2, relative to the anode 2, so as to force the gas through either of the 2 exit regions. For example, the gas nozzle 6 together with the cathode 2 may be driven downwards so as to close the gap between the gas nozzle 6 and the titanium chloride nozzle 5 and hence prevent gas from exiting the plasma arc 1 region through the upper part of that gap.
In operation, the TiCl4 is driven through the arc discharge 1, dissociated in the plasma due to atomic and radiative plasma processes, and the Ti species condenses on the surface of the cooling tube 4 and elsewhere in the vessel 7, including the surface of the anode 3. The cooling tube 4 should be long enough to allow complete dissociation of the TiCl4.
Titanium metal collected on the surfaces may be continuously removed so as to optimise titanium metal production.
The apparatus of figure 1 is enclosed in the vessel 7. Working gases in the vessel, including argon, chlorine and gaseous titanium chloride are continuously removed from the vessel using a pumping system 8 connected to the vessel. The pressure in the vessel is kept below one atmosphere and the process of Ti production is preferably carried out at pressure below 300 torrs .
The gas flow through the nozzle 6 has a large influence on arc properties and hence on the titanium production process and yield. For example, for a low argon gas flow of a few litres per minute injected through the gas nozzle 6, using the configuration of figure 1 but with a closed gap between gas nozzle' 6 and titanium chloride nozzle 5, most of the titanium found is produced on the internal surfaces of the cooling tube. For the same configuration but with a gas flow of 40 litres per minute, black titanium powder is collected from the external surface of the cooling tube and the internal walls of the vessel.
Example The following are examples of actual experimental parameters :
Arc current 275A.
Argon gas flow 5 litres per minute. Closed gap between gas nozzle 6 and titanium chloride nozzle 5.
Cathode of tungsten 6.4 millimetres diameter.
Titanium chloride nozzle 6 is of ceramic and. has outlet diameter of 20 millimetres.
All other electrodes are water-cooled copper. This results in a titanium product in the form of a tubeless cylinder with large crystals on its surface.
Figure 2 shows an example of the product collected from the inner walls of the cooling tube, for the example parameters given above. The material is in a tubular form with the length of the tubular part around 5 centimetres . The materials shown in this figure have been washed in alcohol to remove residual powder that may exist in the collected materials.
Figure 3 shows a trace for an energy dispersive X-ray analysis of the crystals of figure 2. Impurities in the crystals are below the detection limits of the" analysis system and purity of the crystals exceeds 99.6%. Impurities due to corrosion of the internal part of the system due to reaction with chlorine are minimal. This high purity illustrates the advantages of the present process starting from titanium chloride, over reduction processes starting from titanium minerals. For the present process, impurities in the titanium products are minimal as purification occurs at the chlorination stage leading to the formation of very pure titanium chloride .
Although titanium chloride is used in the process of this embodiment, it will be appreciated that other starting compounds may be used. In particular, other titanium halides may be utilised or a mixture of titanium halides.
This process may also be used for the production of other metals, in some cases.
The apparatus illustrated in figure 1 and described above is only exemplary. Other configurations based on the same physical principle may also be used. For example, the inner surface of the cooling tube may be non- cylindrical and also the anode may not be a non-disc shape so as to maximise the contact area between the cooling surfaces in the plasma. Titanium powder may also be formed on the surface of the anode and the internal walls of the chamber. Furthermore, the system may be surrounded by other cooling surfaces so as to increase contact with the escaping plasma gas and increase the yield of titanium metal production. Reactive gases such as hydrogen, nitrogen and oxygen may also be injected into the vessel or into the arc in order to increase the yield of titanium production or to produce titanium compounds such as titanium nitride and titanium oxide. One advantage of using hydrogen is that it increases the plasma enthalpy, resulting in increasing energy dissipation in the plasma. This increases the rate of dissociation of the titanium tetrachloride (where titanium tetrachloride is the compound which is used as the starting material) . Hydrogen has no reducing role in this case.
Titanium crystals may be produced by exposing deposited titanium to heat from the plasma for a short period. This can be done in any way, but in one example the flow of titanium tetrachloride may be stopped and the plasma gas becomes only argon. Increasing the arc power to increase heat and radiation from the arc causes the titanium to heat up and crystallise.
The process of the present invention may be implemented on an industrial scale for the industrial production of titanium or selected titanium compounds. A series of reaction vessels may be utilised in parallel similar to the reaction vessels described in figure 1.
Large scale reaction vessels may be built.
A further preferred step in the method of the present invention would be to recirculate undissociated titanium tetrachloride (or whatever metal halide is being used as the starting product) back through the arc to increase yield.
Titanium or other metal which is produced may be continuously removed from the cooled surface. There are a number of possibilities for achieving this including mechanically scrubbing and also shaking of the collection surface using an ultrasound transducer and other possibilities.
The above example and description relates to the production of titanium from titanium tetrachloride. The present invention could be applied to produce titanium from other halides or compounds, and even to produce other metals from metal halides or other compounds and is not limited to the production of titanium.
It will be appreciated by persons skilled in the art that numerous variations and/or modi ications may be made to the invention as shown in the specific embodiments without broadly departing from the spirit or scope of the invention as described. The present embodiments, therefore, are to be considered in all respects as illustrative and not restrictive.