GB2567007A - Electroless titanium process - Google Patents

Abstract

A process is disclosed for the electroless production of titanium metal on the conductive surface of a particle suspended in a molten halide salt, wherein a reducing agent comprises lithium, sodium, potassium, magnesium or calcium and is present in the deposition bath in a concentration approaching metastable equilibrium with a source of titanium cations, wherein said concentration is lower than 2000 parts per million, ppm, of a reductant dissolved in the molten salt. The particle is preferably a particle of titanium metal and may be initially produced in the apparatus by a chemical precipitation or nucleation reaction. The source of titanium is preferably a titanium chloride, such as titanium dichloride or titanium trichloride. The process may form part of a process for forming an alloyed mixture of titanium, aluminium and vanadium for the production of the aerospace alloy Ti-6Al-4V. Also disclosed is an apparatus for carrying out the process comprising a first zone 1 for delivering a reductant, a second zone 2 for delivering a cation and a third zone 3 where the reductant and the cation are present in low concentrations.

Description

ELECTROLESS TITANIUM PROCESS

Field Of The Invention

The invention relates to electroless deposition of metallic species (M) on electrically conductive particulate substances (P) suspended in a fluid (F), and the products thereof. The invention extends to the electroless deposition of titanium for the production of titanium metal.

Background To The Invention

The inventors are aware of various method of coating metal powders.

US Patent 5,064,463 notes that coated metal powders are known in the art and have been used in conventional powder metallurgical processing.

Canadian patent CA 1287814 noted production of titanium metal as a metallic powder in a static bath provided an electric current is passed through the fluid via electrodes, as per standard electrodeposition. This patent differs from the present claim in that the present claim requires both the reducing agent (R) and the cations (M+) to be dissolved in the fluid (F), thus achieving electroless deposition.

Another known process9 is the introduction of titanium metal ions (M+) into the fluid (F) by anodic dissolution, the ions then disproportionates to coat particles with titanium metal. The substance being coated (P) then provides the reducing action.

This differs from the proposed method differs as there is the requirement for dissolved reducing substance (R).

US Patent 3,743,499 mentions a process for growing transition metal powders; however this is achieved by heat treatment and not electroless deposition.

US Patent 3,853,094 discloses a method and apparatus for electroless plating, wherein is claimed a frusto-conical apparatus with elements of bath circulation and particle suspension.

Also known is various attempts^{2,10, 13} at titanium powder production via predissolution of titanium ions (M+) in molten halides and reducing with reductants (R-) such as magnesium or sodium.

An electroless method has thus not been described in prior art to produce titanium metal or its alloys in a particulate form which is directly suitable for, and specifically focused on a product aimed at low cost powder metallurgy. Similarly the unique chemistry of titanium and its alloys require an apparatus not previously described in order to ensure particle growth in a manner as to not entrap halides and to limit production of titanium on the side walls of the apparatus.

Whereas the prior art makes use of an excess of reductant, up to 30% for magnesium in the Kroll process¹ and may even require further separation steps to remove a powder product from excess reductant9, this method utilises highly dilute concentrations of both the compound carrying metal ions (M+) and the reductant (R) which highly diluted in the suspending fluid (F) in near stoichiometric ratio, such as to facilitate electroless deposition on existing suspended particles (P) rather than nucleation or reaction on the reactor walls.

Electroless deposition:

There are essentially three methods of producing a layer of metal on a surface namely electroplating or electrodeposition method, the vapour deposition method, and the electroless deposition method.

The electrodeposition method requires special equipment, and that electric contact must be maintained to the surface being plated to ensure deposition at the correct rate and the proper potential. Vapour deposition also has some inherent disadvantages which can include the requirements of vacuum operation, or the use of compounds that are toxic, pyrophoric, or corrosive.

Electroless deposition, which is sometimes referred to as chemical reduction deposition or plating, occurs by catalytic reaction between cations (M⁺) and a reducing agent (R) dissolved in a fluid (F).

During electroless deposition, electrons (ne-) required for the reduction of the Metal cation (M⁺) are supplied by reducing agent (R) and the anodic/cathodic reactions to produce the new layer of metal (M) take place on the particle (P).

Electroless deposition is an autocatalytic process in the sense that the metal (M) deposited on the substrate or particle (P) serves as a catalyst for continuation of the deposition (also called plating) action. Once plating has been initiated on the surface of the substrate (P), it continues as long as the substrate remains in contact with fluid (F), provided there is adequate cations (M⁺) and reducing agent (R) in solution.

Because a uniform coating of any desired thickness can be applied by direct chemical reaction, without the passage of an electric current, the process was denoted electroless plating⁸.

Table 1. Properties of electroless (chemical / autocatalytic) deposition

1. Driving force: Reducing agent [R] and autocatalytic property of the deposited metal

2. Cathode reaction	M++ R —> M
3. Anodic reaction	R - ne“ -> R (Oxidized form)
4. Overall reaction	M+ + R —> M + R (Oxidized form)
5. Anodic site	Particle (P) surface
6. Cathodic site	Particle (P) surface

Titanium Production:

In the production of titanium metal the use of molten halide salts as reaction medium, as well as their presence as by-product of the reaction of titanium metal production is well known in the prior art; as is the production of titanium in metal vessels, of which steel is the predominant material of choice.

It is known in the art that in the chief industrial processes for the production of titanium metal (the so called Kroll and Hunter processes), contamination of the titanium product (known as titanium sponge) primarily occurs due to:

a) Titanium product intimately contacting the reactor sides and bottom, where reactions and product settling takes place^1,2. Contamination from reactor walls³ also limits the direct use of titanium produced by the industrially used Kroll process for titanium production in applications such as electronics. It is of interest to decrease impurity elements⁴ such as Fe, Ni and Cr from the reactor walls; for example, Fe contamination degrades film patterning and circuit element registry. As such titanium from the industrially dominant Kroll process has to be further purified post-production.

b) Entrapped halides from sintering of the mass of titanium metal (titanium sponge) at elevated temperature. Industrially produced titanium metal is thus generally not considered suitable for direct use in powder metallurgy due to chloride contamination ⁵’⁶.

c) Oxygen, which is a contaminant critical in determining the Grade of commercially pure titanium metal and its alloys. While many metals have the potential to become brittle with oxygen, titanium is particularly sensitive. Grade 3 titanium is only 0.3 percent oxygen, yet it is one-third as tough as grade 1 titanium, which is 0.1 percent oxygen.

Titanium metal’s corrosion resistance is primarily due to the stable oxide layer, however for very small particles the relative effect of the oxide layer would have the effect that products produced from such a powder would have oxygen contamination to the extent that it markedly reduces the specification or grade of the product.

It is therefore required to only use titanium particles of a size that is not disproportionately disadvantaged by their oxide layer. It was calculated⁷ that 1 micrometer particle would have an oxygen content of 0.0838% based on a monolayer of oxide, whereas a 44 micron particle would only have an oxygen content of 0.0019%. As such a process capable of increasing titanium particle size would benefit the quality and thus value, measurable in terms of the final

ASTM classification or Grade, of the product produced.

There is presently no primary process directly producing titanium metal alloy, such as Ti 6AI-4V (Grade 5) at industrial scale. Known as the “workhorse” of titanium alloys, Ti 6AI-4V is the most commonly used of all titanium alloys. Present production of this alloy is achieved by double or triple melting titanium with a master alloy of aluminium and vanadium.

Summary Of The Invention

According to a first aspect of the invention there is provided a process for electroless plating of a metal (M) onto a particle (P) suspended in a plating fluid (F), wherein is dissolved reducing agents (R) so that the metal (M) plates the particles (P), wherein the fluid (F) is a molten halide which is also the by-product of the reaction between the cation (M+) and the reducing agent (R). The reducing agent may be one or more of the substances selected from the alkali or alkali earth metals.

The invention extends to electroless plating of titanium onto the particles (P), wherein a solution of titanium cations (M+) together with a narrow class of dissolved reducing agents (R) plates the suspended particles (P) with titanium metal.

The process provides titanium metal (M) where the fluid (F) is a molten halide which is also the by-product of the reaction between the titanium cation (M+) and the reducing agent (R).

The reducing agent may be one or more of the substances selected from the alkali or alkali earth metals. The alkali metals may be selected from Lithium, Sodium and Potassium. The alkaline earth metals may be selected fromMagnesium and Calcium.

For the process of electrodeposition to occur it is required that both the cations (M+) and the reductant (R) are dissolved and simultaneously present in the fluid (F) that they may interact via the suspended electronically conductive particle (P).

An important aspect of the invention is then the composition of the electroless plating fluid (F). The plating fluid (F) may contain a source of titanium cations (M+), usually added as a titanium salt such as titanium tetrachloride. Concentrations (in terms of titanium metal cation) may vary but generally extremely low concentrations are preferred to ensure the meta-stable existence in solution of both the metal cation (M+) and the reductant (R). Metastable is herein defined as (of a substance or particle) theoretically unstable but so long-lived as to be stable for practical purposes.

The reaction conditions of the process may include a narrow alkali metal, such as sodium, concentration range where both dissolved cation (M+) and dissolved reductant (R) are present in the fluid (F) at the same time, wherein the concentration of alkali metal in solution is lower than 1 x1 O'⁴ weight percent.

It is known that metastable co-existence of titanium cation (M+) only occurs at a very low concentration of sodium in solution. In the case of reductant (R) being sodium and the cation (M+) being titanium dichloride, to have both species being dissolved in molten sodium chloride fluid (F) and the species existing in equilibrium, the concentration of sodium in solution needed for a metastable state is lower than 1 x10'⁴ weight percent.

Thus, according to the invention, a process for electroless production of titanium metal (M) on a conductive surface of a particle (P) suspended in a fluid (F) comprises the step of wetting said surface with an electroless titanium deposition bath and wherein the electroless titanium deposition bath comprises:

a. source of titanium cations (M+) present in the fluid (F) in a concentration range approaching the metastable condition for equilibrium with a chosen reductant (R), said concentration being below 1000 parts per million of the cation (M+) dissolved in the associated halide salt, however, a more preferred range for operation would be less than 500 parts per million, and a specific range between 20-500 parts per million of the dissolved cation (M+);

b. reducing agent (R) consisting of at least one compound selected from the alkali metals Lithium, Sodium, and Potassium, or the alkaline earth metals selected from Magnesium and Calcium; said reducing agent present in the fluid (F) in the concentration range approaching metastable equilibrium with the chosen source of titanium cations (M+), said concentration being below 2000 parts per million of the reductant (R) dissolved in the associated halide salt, however, a more preferred range for operation would be less than 1000 parts per million, and a specific range between 20-500 parts per million of the dissolved reductant (R);

c) fluid (F) selected from the group of halides such that the electroless deposition bath is operated with a molten halide salt as fluid (F).

The dissolved titanium cations may originate from dissolved titanium chloride species, including TiCh and TiCh, with the preferred cation coming from the presence of dissolved TiCI₂These values for concentration of titanium (M+) present in the fluid (F) is the full range of solubility of the cation (M+) in the reductant specific halide salt, e.g. TiCI₂ in NaCI for the choice of reductant (R) being Sodium. Such solubility is generally limited to a few mass percent.

According to a further aspect of the invention, there is provided an apparatus for carrying out a process as described above.

The apparatus may comprise of any arrangement of pipework or vessels essentially achieving three zones:

1. a zone for delivering the reductant (R) into the Fluid (F);

2. a zone for delivering cation (M+) into the fluid; and

3. a zone where the reductant (R) and the cation (M+) are present in concentrations low enough to exist in a meta-stable state to effect electrodeposition of metal (M) on particles (P) which are suspended in fluid (F).

The process as described, wherein M may be titanium metal and P a particle of titanium metal initially produced in the apparatus via chemical precipitation or nucleation reaction between ions carrying titanium (M+), such as a dissolved titanium subchloride, and a reducing substance (R) such as an alkali or alkali earth metal.

The process wherein M may be titanium metal and P may be a particle of titanium metal introduced into the apparatus and suspended in the fluid (F).

The process wherein M may be titanium metal and P a particle of an electronically conductive substance introduced into the apparatus and suspended in the fluid (F).

In one embodiment, P is a particle of a master alloy with composition of 60% Aluminium 40% Vanadium by weight. The particle may be coated with titanium metal via the described process of electrodeposition to produce a pre-alloyed mixture of titanium, aluminium and vanadium which may be a starting material in the production of, or directly applied as the widely used aerospace alloy Ti-6AI-4V, also known as Grade 5 titanium.

The process in which the source of titanium may consist of titanium chlorides. The process may be carried out in an apparatus for the continuous electrodeposition of titanium metal on suspended particles as described above.

Particularly desirable then is an electroless plating procedure for titanium to achieve controlled deposition of titanium metal on particles in such a way as to a) avoid trapping halide salt in the deposited metal and b) increase particle size to ensure the relative influence of the eventual protective oxide layer does not lower the grade of product produced. Further, it is desirable that such a procedure be carried out using a stable fluid, with specific focus on the molten halide fluids used in industry. Also, it is desirable that the electroless titanium plating procedure yields plating thicknesses of practical interest to improve the potential use of the product in powder metallurgy.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described, by way of non-limiting example only, with reference to the Figures herebelow. In the Figures,

Figure 1 shows shows a graphic representation of a model for autocatalytic deposition interactions vs Sodium concentration in an autocatalytic process of the invention;

Figure 2 shows an apparatus for the electroless deposition of metal on suspended particles in accordance with the invention;

Figure 3 shows particles of commercially purchased titanium powder prior to electrodeposition; and

Figure 4 shows particles with electro deposited titanium metal.

Figure 1 shows a graphic representation of a model for autocatalytic deposition interactions vs Sodium concentration from a mathematical model which was developed to estimate the number of autocatalytic interactions possible at various concentrations of dissolved reductant (R); the results shown in figure 1 are specific to the choice of the alkali metal sodium as reductant. Laboratory results and mathematical models show that the concentrations of reductant (R) at which such autocatalytic interactions are possible is low, and an increase in the concentration of the reductant (R) causes standard nucleation and similar titanium production reactions to occur, with very low possibility of autocatalytic deposition.

In order to achieve sensible growth via electrodeposition, it is required that the dissolved reductant (R) must exist in equilibrium with, and not immediately react with, the dissolved titanium dichloride cation (M⁺) to produce unwanted titanium nuclei.

The apparatus to achieve the autocatalytic deposition as discussed should include a zone dedicated to electrodeposition, being a zone continuously operates at a controlled, very low concentration of reductant (R) and cations (M⁺). Such control can be achieved by various means, including continued monitoring of pH of the reactor zones and making adjustments to reagent feed rates and other process parameters to maintain the concentration of the reductant (R) and cations within the desired range.

As illustrated in Figure 2, The apparatus can comprise of any arrangement of pipework or vessels essentially achieving three zones indicated as 1. a zone for delivering the reductant (R) into the Fluid (F), 2. a zone for delivering cation (M+) into the fluid, and 3. a zone where the reductant (R) and the cation (M+) are present in concentrations low enough to exist in a meta-stable state to effect electrodeposition of metal (M) on particles (P) which are suspended in fluid (F). The apparatus is illustrated with a preferred flow pattern; flow can be achieved in a number of standard ways such as gas-lift, mechanical agitation or fluid displacement.

In order to achieve an industrially viable production rate from such low concentrations it is required a mentionable flow of fluid (F) from various zones where cations (M⁺) and reductant (R) are pre-dissolved to low concentrations and fed to the zone of electrodeposition whilst maintaining control of the relative concentrations, as not to disturb the metastable equilibrium.

The electroless plating fluid is a molten halide, preferably the halide produced during the reaction between the metal cation (M⁺).

The present process proposes a method and apparatus in which titanium metal powder is produced which is low in contamination from entrapped halide or similar suspension fluid (F), and predominantly free of contamination from materials of construction due to contact with e.g. reactor walls and pipework. The process can be operated in batch, semi-batch or continuous manner.

In continuous operation the benefit of the fluid (F) also being the by-product of the reaction between the metal cation (M⁺) and the reductant (R) is that a continuous slurry of suspended particles (P) coated with metal (M) can overflow/be withdrawn from the apparatus. For illustration is the case of using sodium as reductant and titanium dichloride as metal cation to electrodeposit titanium metal and produce molten sodium chloride as the by-product, which is then also the fluid for further electrodeposition and particle suspension.

In an embodiment of the invention, commercially purchased titanium particles (P), shown in figure 3, were loaded into a reactor containing molten halide as fluid (F). By controlling in zone 2 of the apparatus the concentrations of metal cations (M+), present as titanium dichloride, and a reducing metal (R) in the fluid (F) such that both species were dissolved and present in a meta-stable condition, electrodeposition and particle growth of metal (M) on the particle (P) was achieved versus the expected nucleation and production of new nuclei I new small particles of metal M. Particles coated with electro deposited metal are shown in figure 4.

BELIEVED ADVANTAGES OF THE PROCESS:

Electroless metal deposition of both electrically conductive and electrically nonconductive articles is known to the prior art, however this method has not been utilised for the production of titanium powder directly usable in powder metallurgy, being:

a) The controlled growth by electrodeposition of titanium metal powder such as to increase particle size to decrease the relative contamination effect by particle protective oxide layer.

b) The production of titanium or titanium alloy powder free from halide contamination.

c) The production of a titanium or titanium alloy powder free from contamination from materials of construction due to the product being suspended in the fluid (F) vs. plating or growing on materials of construction.

d) The electrodeposition of titanium metal on particulate substances such as a master alloy, in order to achieve a pre-alloyed powder without the need for melting. Most titanium powder products undergo post production heat treatment and a powder of titanium eletrodeposited onto master alloy is expected to provide a lower cost alternative to the industrial route which requires co-melting of titanium and its alloying compounds and then atomisation to powder.

e) The option to operate this process continuously vs. the present industrial processes for titanium production which are operated batch-wise.

References:

1. Nagesh, C. R., Rao, C. S., Ballal, N. B., & Rao, P. K. (2004). Mechanism of titanium sponge formation in the kroll reduction reactor. Metallurgical and Materials Transactions B, 35(1), 65-74.

2. Withers, J. C., Laughlin, J. P., Elkadi, Y., DeSilva, J., Storm, R., Shapovalov, V., ... & Loutfy, R. O. (2009, September). Novel processing to produce Ti and Ti alloy powders on a continuous basis. In Titanium 2009 Conference, The International Titanium Association, Kona Hawaii (pp. 13-16).

3. Waseda, Y., & Isshiki, M. (Eds.). (2012). Purification process and characterization of ultra high purity metals: application of basic science to metallurgical processing. Springer Science & Business Media.

4. Rosenberg, H., Winters, N., & Xu, Y. (2001). U.S. Patent No. 6,309,595. Washington, DC: U.S. Patent and Trademark Office.

5. Low, R. J., Qian, M., & Schaffer, G. B. (1770). Chloride impurities in titanium powder metallurgy-a review. In Proc. 12th World Conf, on ‘Titanium’,(ed. L. Zhou et al.) (Vol. 1774, p. 2011).

6. Yu, C. 7., & Jones, Μ. I. (2013). Investigation of chloride impurities in hydrogenated-dehydrogenated Kroll processed titanium powders. Powder Metallurgy, 56(4), 304-309.

7. Hansen, D. A., & Gerdemann, S. J. (1998). Producing titanium powder by continuous vapor-phase reduction. JOM Journal of the Minerals, Metals and Materials Society, 50(11), 56-58.

8. Brenner, A., & Riddell, G. E. (1947). Deposition of nickel and cobalt by chemical reduction. J. Res. Nat. Bur. Stand, 39, 385-395.

9. Sytchev, J., Gondor, Z. H., & Kaptay, G. (2002). ELECTROLESS COATING

OF NON-CONDUCTING SURFACES AND PARTICLES WITH METALLIC

TITANIUM IN MOLTEN SALTS. Proc, of the Electrochemical Society, PV, 19, 803809.

10. Anderson, R. P., Donn, A., & Jacobsen, L. (2009). U.S. Patent No. 7,632,333.

Washington, DC: U.S. Patent and Trademark Office.

11. Suzuki, R. 0., Matsunaga, T., Ono, K., Harada, T. N., & Deura, T. N. (1999). Titanium powder prepared by magnesiothermic reduction of Ti 2+ in molten salt. Metallurgical and Materials Transactions B, 30(3), 403-410.

io 12. Henrie, T.A., Baker, D.H. (1960) Mechanism of Sodium Reduction of Titanium Chloride in Fused Salts. US Bureau of mines. Report of Investigations 5661, pp. 137

13. Van Vuuren, D. S., Oosthuizen, S. J., & Swanepoel, J. J. (2017). U.S. Patent No. 9,567,690. Washington, DC: U.S. Patent and Trademark Office.

Claims (10)

1. A process for electroless production of titanium metal (M) on the conductive surface of a particle (P) suspended in a fluid (F) comprising the step of wetting said surface with an electroless titanium deposition bath and characterized in that the electroless titanium deposition bath comprises:

a. source of titanium (M+) present in the fluid (F) in a concentration range approaching the metastable condition for equilibrium with a chosen reductant (R), said concentration being below 1000 parts per million of the cation (M+) dissolved in the associated halide;

b. reducing agent (R) consisting of at least one compound selected from the following alkali metals: Lithium, Sodium and Potassium, or the following alkaline earth metals: Magnesium and Calcium; said reducing agent present in the fluid (F) in the concentration range approaching metastable equilibrium with the chosen source of titanium cations (M+), wherein the metastable concentration range is lower than 2000 parts per million of the reductant (R) dissolved in the associated halide salt; and

c) fluid (F) selected from the group of halides and the electroless deposition bath is thus operated with a molten halide salt as fluid (F).

2. A process as claimed in claim 1, wherein the metastable concentration range is less than 1000 parts per million of the dissolved reductant (R).

3. A process as claimed in claim 2, wherein the metastable concentration range is between 20-500 parts per million of the dissolved reductant (R).

4. A process as claimed in any one of the preceding claims, wherein M is titanium metal and P is a particle of titanium metal initially produced in the apparatus via chemical precipitation I nucleation reaction between ions carrying titanium (M+), and a reducing substance (R) selected from an alkali or alkali earth metal.

5. A process as claimed in claim in any one of claims 1 to 3, wherein M is titanium metal and P is a particle of titanium metal introduced into the apparatus and suspended in the fluid (F).

6. A process as claimed in any one of claims 1 to 3, wherein M is titanium metal and P is a particle of an electronically conductive substance introduced into the apparatus and suspended in the fluid (F).

7. A process as claimed in claim 6, wherein P is a particle of a master alloy with composition of 60% Aluminium 40% Vanadium by weight and wherein the particle P is coated with titanium metal via the process of electrodeposition to produce a prealloyed mixture of titanium, aluminium and vanadium which can be a starting material in the production of, or directly applied as the widely used aerospace alloy Ti-6AI-4V also known as Grade 5 titanium.

8. A process as claimed in any one of claims 1 to 3 in which the source of titanium consists essentially of titanium chlorides.

9. An apparatus for carrying out a process as claimed in claim 1, which comprises of any arrangement of pipework or vessels achieving three zones, wherein the zones are defined as:

1. a zone for delivering the reductant (R) into the Fluid (F);

2. a zone for delivering cation (M+) into the fluid; and

3. a zone where the reductant (R) and the cation (M+) are present in concentrations low enough to exist in a meta-stable state to effect electrodeposition of metal (M) on particles (P) which are suspended in fluid (F).

10. An apparatus as claimed in claim 9, wherein flow between the zones is

5 achieved by gas-lift, mechanical agitation, or fluid displacement.