WO1986007097A1

WO1986007097A1 - Metal halide reduction with molten sodium/potassium alloy

Info

Publication number: WO1986007097A1
Application number: PCT/AU1985/000113
Authority: WO
Inventors: William Reginald Bulmer Martin
Original assignee: The University Of Melbourne
Priority date: 1985-05-27
Filing date: 1985-05-27
Publication date: 1986-12-04
Also published as: GB2185493A; GB8701633D0; JPS63500389A; JPH06104869B2; DE3590793T1; GB2185493B

Abstract

A method of obtaining a desired metal, selected from the group consisting of titanium, aluminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten, which comprises reacting a halide of the desired metal with an alkali metal reducing agent at a temperature at which the reducing agent is molten, in order to produce the desired metal and halide of the metal reducing agent.

Description

METAL HALIDE REDUCTION WITH MOLTEN SODIUM/POTASSIUM ALLOY This invention relates to chemical processes. In another aspect this invention relates to chemical processes invol ing reactive metal(s) in the liquid state at temperatures and pressures such that the other reactants, generally covalent halides, are present in compact phase, i.e. not in the gaseous phase. I have found that in chemical processes wherein one of the reactants is a suitable metal or is a suitable metal mixture, in the liquid state, substantial and unexpected advantages accrue from employment of the liquid metal in considerable stoichiometric excess. Inter al ia, the invention takes advantage of the extrao dinarily high capability to transfer heat which is exhibited by metals in the liquid state. The excess liquid metal also functions as a materials transfer agent. The present invention provides a method of obtaining a desired metal selected from the group consisting of metals capable of existing in the form of a compound capable of being reduced with a liquid metal reducing agent which comprises a reaction comprising contacting such a compound of said desired metal in substantially compact form with a liquid metal reducing agent whereby to obtain said desired metal. Preferably said compound is a covalent halide. Preferably said desired metal is selected from the group consisting of titanium, aluminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten. In a particularly preferred aspect the present invention provides a method of obtaining a desired metal selected from the group consisting of titanium, aluminium, iron, manganese hafnium, zirconium, tantalum, vanadium, uranium and tungsten which comprises a reaction comprising contacting a halide of said desired metal in substantially compact form with a liquid metal reducing agent whereby to obtain said desired metal. Particular advantage is derived from application of the invention to recovery of titanium from its te t achl o i de which is directly derivable from ores such as rutile. High purity titanium metal is today a strategic material, for example in space research and for high speed aircraft. It is expensive to produce by conventional means; firstly because it is so reactive at high temperatures, especial ly temperatures greater than 500°C and, it is particulary reactive with respect to oxygen. A development of the invention envisages the preparation of al loys which are required to be free to impurities, particularly oxygen, such as iron titanium and titanium iron manganese alloys. Alloys such as these can be used to store hydrogen in the form of hydrides and the amount of hydrogen which can be stored is inversely dependant on the amount of oxygen conta ination. The alloys referred to above as hydrogen stores may provide a satisfactory source of hydrogen for use as a fuel for internal combustion engines and for storage of energy via the fully reversible heat of 'reaction. Oxygen is ubiquitously present in air and water and only a few parts per mil lion are required to adversely affect the properties of high purity titanium metal and the types of alloys mentioned herein. Consequently very special techniques are needed to produce titanium metal in oxygen- free condition. It is recognized that "commercial pure" titanium capitalises upon the presence of impurity oxygen which is controlled, like carbon in steel, to increase the stiffness in ambient temperature conditions for corrosion resistant duty as in chemical plants. Because of the special nature of the covalent titanium halide compounds, particularly the chlorides, it is possible to purify them entirely from oxygen reasonbly easily and in a continuous plant. However, existing processes for recovery of titanium metal from the hal ide invariably result in surface contamination from air and/or water arising from recovery of the "sponge" from solidif ed melts. Only coarsely crystalline material with low surface/volume ratio can be used for high purity metal. Fine titanium adsorbs so much water and/or air on its surface that subsequent melting under argon at minimum pressure to maintain an electric arc yields a metal ingot unacceptably high in oxygen for high purity metal. Furthermore, fine titanium powder is generally pyrophoric and must be handled in an inert atmosphere. The invention is also applicable to the obtaining of other metals as noted above and of these another particularly economical ly significant metal is aluminium. However, care needs to be applied in obtaining an anhydrous halide and it is desirable that the halide be in l iquid phase. Many halides may be put in l iquid phase, if that is not their normal condition, by heat and/or pressure so as to liquefy them but it is also possible to dissolve halides in inert solvents such as a paraffin or suspend it in a liquid or even use reactive solvents such as fe ic chloride and titanium tetrachl or i de.. However, experimental work does show that even if the halide is in solid form reaction does occur. Preferably said liquid metal reducing agent comprises a metal selected from the group consisting of Group I metals or a mixture containing a Group I metal. Preferably said liquid metal reducing agent is a mixture of sodium and potassium alloy. Said liquid metal reducing agent may contain at least one of calcium and magnesium. It is desirable that said reaction is conducted at a temperature not greater than the boi l ing point or subl imation point of the halide under the pressure pertaining. It is desirable that said reaction is conducted at a temperature such that solid by-product halides of said liquid metal reducing agent are formed. The reaction is greatly exothermic. When using NaK alloy in the presence of approximately equivalent amounts of reactants without cool ing, the evoked heat caused the reaction to "run away" with a resultant explosion. This is avoided, in accordance with the present invention, by employment of the liquid metal reducing agent in considerable excess to transfer the heat to an external sink. The excess liquid metal reducing agent not only displaces the equilibrium in favour of the reaction; while part of the liquid metal reducing agent reacts and ceases to be metal lic, the excess continues to act as a potent reductant but also acts as a highly efficient heat transfer medium, in situ, at the actual reactive zone where the heat is produced. A high yield of fully reduced desired metal is obtained by operating under these conditions, at a suitable reactor temperature. Partially this is because the strongly electro positive liquid metal reducing agent not only acts as a carrier and heat transfer medium but also as an unrestricted source of electrons by electronic conduction, once the covalent bonds of the halide of the desired metal are split. Substantial ly full reduct on to desired metal occurs. There was no evidence that lesser reduction to ^*■ undesired metal cations occurs. Working with excess liq.uid metal reducing agent in intimate contact wtth a compact phase in contrast to a disperse vapour, ^'is an important advance over prior art processes which at low temperatures have produced desired metal in low yields, due to partial reduction to lower halides. If desired, heat might be removed from the reaction by circulating the liquid metal reducing agent to a cooling station but in general the conductivity of the liquid metal reducing agent wil l be effective in carrying heat to the wall of a reaction vessel from which heat may be removed. In one instance at least portion of said liquid metal reducing agent is circulated between the reaction vessel and reaction product removal station whereby to convey reaction products away from said reaction vessel. Preferably said liquid metal reducing agent is present in stoichiometric excess. Preferably said desired metal that is produced is removed from said reaction with the liquid metal reducing agent and is thereafter separated from by-product halides of said liquid metal reducing agent and from the liquid metal reducing agent. The operating pressure of the process should preferably be maintained by rare gas, e.g. argon, the current economic choice. The process according to the invention may be carried out continuously and in such case excess liquid metal may be employed as a carrier to remove the reaction products at low temperature from the reaction vessel to suitable filters, screens, decantation vessels and/or centri fuges or vacuum distil lation stages. A liquid slurry of the l iquid metal reducing agent and reaction products wil l flow and can be pumped by known means. The product salts (sodium chloride and/or potassium chlori de) may be separated from the much heavier desired metal powder in a centrifuge, and the excess sodium, potassium or NaK alloy may then be centrifuged or filtered from these separately. Finally the residual NaK metal may be evaporated under high vacuum from the titanium powder after particle modification if desired, in a higher temperature loop. In general, complete removal of liquid metal reducing agent halides from liquid metal reducing agent which is to be reused is not considered essential as such halides probably act as seeds for reaction initiation. Anhydrous ammonia wi ll be found useful in removing traces of liquid metal reducing agent from desired metal. Preferably said reaction is initiated by l iquefying said halide and, if necessary, a precursor material in solid form of said liquid metal reducing agent. It i s poss ib le to pass l iqui d sodi um meta l co un te re urre nt to by-product halides of said l iquid metal reducing agent wherbey to regenerate potassi um metal from potassium chloride. For high purity compact product metal, powder produced by the process of the present invention is preferably directly melted by the electron beam technique, which avoids the contamination experienced in the use of electric arcs on water washed metal powder or that which has been exposed to the air. An electric arc must have a mini um amount of gas present to ionise, and wi ll not operate in an ultra high vacuum that would strip all gases off hot surfaces. In conventional arc processes argon or helium atmospheres are employed but the absolute pressure necessary prevents good stri pping of air and water vapour form the washed metal powder. Electron beam melti ng is general ly becomi ng the preferred compacting means in rare and contami natable metal technology, for example that of titanium, hafni um, zirconium, tantalum and tunsten. Thus in a preferred embodiment of the invention, metal powder from which the residual NaK al loy has been evaporated, is fed directly into the melting electron beam without ever having been exposed to air or water. In these circumstances small particle si ze is advantageous. Also electroslag melting may be applicable. ■ In another embodiment powder suitably conditioned., in a, hot l oop may be re leased to ambi ent and hand led conventionally for use in powder metallurgy or for hydrogen storage as hydride. Treatment with an alcohol or ketone has been found to reduce and in some cases eliminate pyrophoric problems. Because liquid sodium reduces potassium from its molten salts (cf vice versa in aqueous media), no di fficulty in keeping potassium captive in the system is anticipated. In one proposed embodiment of the invention, for continuous - production of titanium, liquid Na metal from an electrolytic reduction cel l would enter the process counter current to outgoing mi xed product NaCl and KC1 stream. This counter current contacting, with sufficient heat and mass transfer stages, would both retain the K in the system and conserve the heat. Then overall the feed to the process would be, TiCl^ and Na metal and the products, be Ti metal and NaCl which latter could be recycled directly to an electrolytic cel l from which in turn C 1 would be available either to an integrated TiCl^ production plant or for sale according to economics of procurement of Ti C 14 and titanium source materials, e.g. ruti le plus carbon, at the plant site. For economic reasons site melted Na brought into the site, may be better than recycle to an integrated electrolytic cell , without affecting the generality of the principle of captive K, which essentially removes its cost from the economics of production, this being essentially tied to T i C 1 and Na in and Ti + NaCl out of the essential process. Si i l ar considerations apply to other desired metals. EXAMPLE 1 Titani um metal was recovered from T i C 1 by reaction with a large excess of NaK alloy. The apparatus incorporated means for evacuating the system to below 10 microns of mercury; a supply of inert gas; the means for external heating and cooling a reactor, with a heat transfer medium inert to NaK. The reactor was made of pyrex glass so that the reaction mixture was clearly visible. A stirrer totally isolated from ambient was built into the reactor, and means for sampling ^'whi le stir ing while under inert gas or vacuum were provided. Safety of operation was a paramount consideration. The stirrer was adjustable and made of nickel tubing, as it is known that nickel is an inert reaction vessel material for the preparation of titanium from its chlorides. As autocatalysis may be important in the mechanism, faci lities were provided for both nickel and titanium propellers on the stirrer. The reactor was made of pyrex glass, surrounded by a pyrex glass jacket through which high flash point, low viscosity oi l was pumped to either heat or cool the reactants. The jacket, in particular the base, was designed to maintain high heat transfer rates at the vessel wal ls. Inside the reactor, the stirred liquid NaK al loy itself constituted an excellent heat transfer medium. On one side of the vessel was provided a connection to a burette containing Ti C 1 , and on the other side a vent to a mercury lute. A sampling probe was also provided. A thermocouple was connected into the cooling systm at the point where the oil leaves the glass jacket surrounding the reactor. The lute provided was adapted to vent to atmosphere any pressure surge which may occur in the system, yet al low a high vacuum to be applied to the system. Argon gas was used as the protective gas, as is customary the preparation of titanium, but facil ities fo using nitrogen were also bui lt i nto the system. The argon used was a commercially pure grade, and was purified of any traces of water vapour and oxygen before admission following evacuation. The titanium tetrachlori de used was laboratory reagent grade, which was disti lled in an atmosphere of nitrogen before use, the boi ling range 133°C to 136°C being col lected. 60/40 mole % NaK sodium- potassium alloy was prpared. 30 g (1020 mi ll imoles) of the al loy was charged into the reactor via a No. 3 porous filter disc, u-nder the cover of argon gas, and the tet ra ch 1 o ri de was placed in a burette which had a fitting to connect it to the reactor. The stirrer was set running and 2.00 m l. ( 18 mi llimoles) of T i C 14 was added to the al loy. The reaction mixture was stirred vigorously. The top layer of al loy became darker and went through a wide range of colours; gold, blue, pink and green all being noticeable. Then another 1.20 ml (11 millimoles) of TiCl4 was added and once agai n many colours were visible, then 70 seconds after the addition, there was a flash of light, and a recorder showed a rapid 0.8°C increase in the temperature of cooling fluid from about 25°C. A dark grey substance was now seen inside the apparatus intermixed with the liquid metal. There was evidence of a considerable increase in the viscosity of the excess liquid metal. The whole system was evacuated to remove any remaining T i C 1 (none was found in subsequent tests on the cold trap washings). To facilitate analysis of the products, two lots of 25 ml. of ethanol (436 mi llimoles each) were added to destroy the alloy. When all the alloy was destroyed, suction was appl ied to a sample probe inserted into the reactor and as much material as possible was withdrawn into the fi lter. The solid's were filtered and washed with ethanol and then dried under vacuum. The reaction vessel also contained a quantity of rather fi ner sol ids and these were also col lected and washed with ethanol, but kept separate from the coarser samples. The two samples were extracted with hot water, and after drying, the mass of watei — insoluble material was 1.53 g, 1.12 g from the coarse sample and 0.41 g from the fine samp! e. Metallurgical examination indicated that both samples after the above treatment comprised non-pyrophori c titanium. For purpose of examination, they were dissolved in hot dilute sulphuric acid, precipitated with ammonia and ignited to Tiθ - The yield was close to theoretical. EXAMPLE II The- procedure of Example I was repeated excepting that AICI3 was used in lieu of Ti Cl 4. Aluminium metal was produced although yields were low and reaction times long probably due to the fact that the A 1 C 13 remained in sol id state throughout the process; the apparatus used being incapable of holding the pressure necessary to liquefy A 1 C 13 under the temperatures that were practical. EXAMPLE III While the process exemplified in Example II proved the process of this invention as applicable to aluminium, further experiment was made to improve yields by conducting the process inside a sealed bomb so as to ensure liquefaction of A 1 C 13. 30g of a 60/40 mole % NaK sodium - potassium alloy was charged into a bomb under inert atmosphere conditions and lOgm of dry freshly prepared AT CT 3 was also charged into the bomb under inert atmosphere conditions. The bomb was sealed and heated to 300°C and maintained at that temperature for 15m. After cool ing, the bomb was unsealed under an inert atmosphere and the NaK alloy was destroyed and product metal was extracted similarly as in Example I. A yield of aluminium of 80% of theoretical was obtained. This example indicates the desirability of the halide being in liquid phase. By the use of the exemplary processes we can provide a pure metal such as titanium, aluminium or an al loy such as Ti Fe suitable fo the new hyd ide storage technology or other metals or mixtures of metals for which truly anhydrous halides are preferred.

Claims

CLAIMS 1. A method of obtaining a desired metal selected from the group consisting of metals capable of existing in the form of a compound capable of being reduced with a l iquid metal reducing agent which comprises a reaction comprising contacting such a compound of said desired metal i n substantially compact form with a liquid metal reducing agent whereby to obtain said desired metal. 2. A method as clai ed in claim 1, wherein said compound is a covalent halide. 3. A method as claimed in claim 1, wherein said desired metal is selected from the group consisting of titani um, al uminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten. 4. A method of obtaining a desired metal selected from the group consisti ng of titanium, aluminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten which comprises a reaction comprising contacting a halide of said desired metal in substantially compact form with a liquid metal reducing agent whereby to obtain said desired metal. 5. A method as claimed in claim 4, wherein said desired metal is titanium. 6. A method as claimed in clai 4, wherein said desired metal is aluminium. 7. A method as claimed in claim 4, wherein the halide is an anhydrous halide. 8. A method as claimed in claim 4 wherein the halide is in liquid phase. 9. A method as claimed in claim 8, wherein the halide is in solution. 10. A method as claimed in claim 8, wherein the halide is in suspension. 11. A method as claimed in claim 8, wherein the halide is in liquefied form. 12. A method as claimed in claim 4, wherein the halide is in solid form. 13. A method as claimed in any preceding claim wherein said liquid metal reducing agent comprises a metal selected from the group consisting of Group I metals or a mixture containing a Group I metal. 14. A method as claimed in claim 13, wherein said liquid metal reducing agent is a mixture of sodium and potassium al loy. 15. A method as claimed in claim 13, wherein said liquid metal reducing agent addition alloy includes at least one of calcium and magnesium. 16. A method as claimed i n any preceding claim wherein said reaction is conducted under substantial ly anhydrous conditi ons. 17. A method as claimed in claim 4, wherein said reaction is conducted at a temperature and pressure such that the halide and said liquid metal reducing agent are maintained in liquefied form. 18. A method as claimed in claim 17, wherein said reaction is conducted at a temperature not greater than the boil ing - μoint or sublimation point of the halide under the pressure pertaining. 19. A method as claimed in claim 17, wherein said reaction is conducted at a temperature such that solid by-product halides of said liquid metal reducing agent are formed. 20. A method as claimed in any preceding claim, wherein said reaction is conducted under substantially oxygen free conditions. 21. A method as claimed in any preceding claim which is conducted in a reaction vessel and wherein at least a portion of said l iquid metal reducing agent is circulated between the reaction vessel and a reaction product removal station whereby to convey reaction products away from said reaction vessel. 22. A method as claimed in any preceding claim, wherein said l iq ui d meta l reduci ng agent is present i n stoichiometric excess. 23. A method as claimed in any preceding claim wherein said reaction is effected under an inert atmosphere. 24. A method as claimed in claim 4, wherein said desired 1 metal that is produced is removed from said reaction with

2 the liquid metal reducing agent and is thereafter separated

3 from by-product halides of said liquid metal reducing agent

4 and from the liquid metal reducing agent.

5 25. A method as claimed in any preceding claim wherein said

6 reaction is initiated by liquefying said halide and, if

7 necessary, a precursor material in solid form of said liquid

8 metal reducing agent.

9 26. A method as claimed in claim 14, comprising passing

10 liquid sodium metal countercurrent to by-product halides of

11 said liquid metal reducing agent whereby to regenerate

12 potassium metal from potassium chloride.

13 27. A method as claimed in any preceding claim, including

14 applying an alcohol or a ketone to the desired metal that is

15 produced prior to exposing it to ambient atmosphere.

16 28. A method of producing a metal substantial ly as

17 hereinbefore described with reference to any one of the

18 Examples.

19.. 29. The articles, things, parts, elements, steps, features,

20 methods, processes, compounds and compositions referred to

21 or indicated in the specification and/or claims of the

22 appl ication individually or col lectively, and any and al l

23 combinations of any two or more of such.