EP2726236A1

EP2726236A1 - Continuous process for the production of titanium alloy powders

Info

Publication number: EP2726236A1
Application number: EP12730738.7A
Authority: EP
Inventors: James Manio Silva
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2011-07-01
Filing date: 2012-06-18
Publication date: 2014-05-07
Also published as: JP2014518334A; WO2013006256A1; CN103635274A; CA2839873A1

Abstract

There is provided a process for the continuous production of titanium powder from the corresponding metal chloride using a reductant metal in a molten electrolyte medium. By operating under dilute conditions, the temperature of the process remains below about 150°C above the molten electrolyte feed temperature.

Description

CONTINUOUS PROCESS FOR THE PRODUCTION OF

TITANIUM ALLOY POWDERS FIELD

[0001] The systems and processes described herein relate to a process for the continuous production of titanium alloy powders, as well as titanium alloy powders made by the process.

BACKGROUND

[0002] Elemental metals may be produced by reduction of the corresponding metal chloride. For example, titanium is produced by reduction of titanium tetrachloride, which in turn can be made by chlorinating relatively high-grade titanium dioxide ore.

[0003] The reduction of titanium tetrachloride to titanium metal has been attempted using a number of reducing agents including hydrogen, carbon, sodium, calcium, aluminum, and magnesium. The magnesium reduction of titanium tetrachloride has proved to be a viable commercial method for producing titanium metal. However, when performed as a batch process (such as the Kroll process), this requires significant material handling resulting in opportunities for contamination and quality variation from batch to batch. Numerous continuous processes have been disclosed, but a key problem with titanium production by metal reduction of titanium tetrachloride is that the reaction is both fast and highly exothermic. The Armstrong process, for example, is continuous, but uses molten sodium as the reaction medium, which poses safety risks. Continuous processes that utilize a molten electrolyte as the reaction medium have been proposed, such as variants on the Sumitomo process, but the heat removal method taught in these processes involves cooling coils placed inside the reactor, which limits the practical scale of operation. Therefore, in order to realize increased reactor productivity, it is desirable to use a continuous process that can also provide consistency in the final product. SUMMARY

[0004] The methods described herein provide a continuous process for the production of a titanium alloy powder. The process comprises contacting a metal chloride and a reductant metal within a molten electrolyte medium. Conditions are controlled in the reactor so that the temperature within the contacting vessel does not reach more than 150°C above the molten electrolyte feed temperature.

DESCRIPTION OF THE DRAWINGS

[0005] These and other features, aspects and advantages of the methods and systems presented herein may be further understood and/or illustrated when the following detailed description is considered along with the attached drawings.

[0006] FIG. 1 is an illustration of a process according to one embodiment; and

[0007] FIG. 2 is a graphical depiction of the molten electrolyte recirculation rate required to achieve reactor exit temp of 800°C as a function of electrolyte feed temperature.

DETAILED DESCRIPTION

[0008] The present specification provides certain definitions and methods to better define the described techniques and systems and to guide those of ordinary skill in the art in their practice. Provision, or lack of the provision, of a definition for a particular term or phrase is not meant to imply any particular importance, or lack thereof; rather, and unless otherwise noted, terms are to be understood according to conventional usage by those of ordinary skill in the relevant art.

[0009] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the relevant art. The terms "first", "second", and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms "front", "back", "bottom", and/or "top", unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of "up to about 25 wt.%, or, more specifically, about 5 wt.% to about 20 wt.%," is inclusive of the endpoints and all intermediate values of the ranges of "about 5 wt.% to about 25 wt.%," etc.).

[0010] The modifier "about" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). Reference throughout the specification to "one embodiment", "another embodiment", "an embodiment", and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described features may be combined in any suitable manner in the various embodiments.

[0011] One embodiment of the described technique provides a method for continuous synthesis of titanium alloy powder from the corresponding metal chlorides (e.g., vanadium chloride, aluminum chloride, boron chloride, etc.) using a reductant metal (e.g. potassium) in a molten electrolyte medium (e.g., potassium chloride). Because alloy formation reactions occur at elevated temperatures (e.g. 800°C) and are highly exothermic, conventional single pot batch reactions are limited to very small scales (e.g. 1 lb Ti/day). The present process enables large-scale operation by conducting the reaction in a manner such that the temperature within the reactor does not rise to a level more than about 150°C above the molten electrolyte feed temperature. In particular embodiments, the temperature may be held to below about 100°C about the molten electrolyte feed temperature. [0012] Such an approach allows the entire process (from raw materials to final part) to be conducted without approaching the melting point of titanium. This represents a dramatic process simplification compared with melt processing to make titanium alloys. This process thus results in significantly lower production costs than melt processes. Further, this process enables synthesis of alloys that are not attainable by processes that require melting of titanium and alloying metals.

[0013] To maintain the desired lower temperature within the reactor, it is necessary to provide a way to manage the heat produced by the reaction. In one embodiment, this is done by feeding a precooled, dilute solution of a reductant metal in molten electrolyte and a mixed metal chloride stream into the reactor. In some embodiments, the reactor may be a substantially adiabatic tubular reactor. In others, a substantially adiabatic continuous stirred tank reactor may be used.

[0014] In particular embodiments, the metal chloride stream may comprise titanium chloride (TiCl₄), vanadium chloride (VC1₄), aluminum chloride (A1C1₃), boron chloride (BCI3) or a combination of any number of these. The metal chloride stream is fed into the desired reactor at a temperature and pressure suitable to maintain the mixture as a molten liquid, e.g., at a temperature of from about 0°C to about 100°C and 1 atmosphere pressure for TiCl₄. If a combination of metal chlorides is utilized, they may be fed to the desired reactor separately, or in any combination.

[0015] The temperature increase within the reactor is limited by controlling the concentration of reductant metal in the electrolyte to be from about 0.5 wt% to about 2 wt% (wt reductant vs. wt reductant plus electrolyte). Prior to entering the reactor, the electrolyte stream is sent to the cathode chamber of an electrochemical cell. Reductant metal (e.g. K) is produced at the cathode by reducing electrolyte cations (e.g. K⁺). Chloride ions migrate across an ion-permeable barrier to the anode compartment, where they are oxidized to chlorine, which exits the cell for further processing. The cathode compartment is agitated to ensure a uniform solution of reductant metal and electrolyte. [0016] Temperature control of the process is further facilitated by sending molten electrolyte from the cathode compartment to a molten salt-cooled heat exchanger to precool the electrolyte prior to feeding it to the reactor. A slight excess of the reductant metal (e.g. less than or equal to about 1%) above the stoichiometric requirement is fed to the reactor to ensure that all reactive metal chloride (e.g. TiCl₄) is consumed. This helps to eliminate all reactive metal chlorides or subchlorides (e.g. TiCl₂) from the product stream leaving the reactor.

[0017] The product stream is sent to a settling tank, where alloy particles fall by gravity, and may be periodically removed from the settling tank.

[0018] In some embodiments, the reactor may comprise a substantially adiabatic tubular reactor or a substantially adiabatic continuous stirred tank reactor (CSTR). The reactor product stream feeds a settling tank. Alternatively, other embodiments may use a substantially adiabatic CSTR and settling tank combined into a single vessel, where a baffle separates the agitated reaction zone from the settling zone.

[0019] Figure 1 is a block flow diagram of the process according to one embodiment. In process 100, a metal chloride stream 102 (e.g. TiCl₄, A1C1₃, VC1₄, BCI₃) and a pre- cooled solution 106 of reductant metal (e.g., potassium) in molten electrolyte (e.g., potassium chloride) are fed to a substantially adiabatic tubular reactor 104. The reactor is operated under dilute conditions (e.g. a mass ratio of electrolyte to Ti product of about 100). This allows the temperature within reactor 104 to be limited to no more than about 150°C above the temperature of the molten electrolyte feed 106. In some embodiments, temperature may be limited to no more than about 100°C, above the molten electrolyte feed temperature by proper control of the mass ratio.

[0020] In the embodiment shown in Figure 1, heat is removed from process 100 by passing electrolyte through external heat exchanger 114. Molten salts, for example a eutectic of potassium nitrate and potassium nitrite, which are often utilized for high- temperature heat transfer, may be used on the cold side of heat exchanger 114. Because heat is removed from the molten electrolyte (to precool the molten electrolyte) under non-reactive conditions, the reaction between the reductant metal (e.g., K) and metal chloride stream (which in some embodiments, may be TiCU/MCl_x, wherein M comprises aluminum or vanadium and x is an integer from 1 to 4) may be conducted under substantially adiabatic conditions in a reactor that offers controlled, highly turbulent conditions that facilitate fast mass transfer.

[0021] Electrolyte is fed from the settler vessel 108 to cathode compartment 116 of electrochemical cell 120 to regenerate reductant metal (in cathode compartment 116) and chlorine (in anode compartment 118), which is removed. As shown in Figure 1, cathode compartment 116 may be provided with stirring mechanism 122 to agitate the contents thereof in order to convect reduced metal from the cathode and facilitate dissolving the reduced metal into the electrolyte. A solution of reductant metal in electrolyte is removed from the cathode compartment and is cooled in heat exchanger 114. This precooled stream is recycled to reactor 104, which in some embodiments, may be an adiabatic tubular reactor that is a motionless mixer or static mixer.

[0022] Reactor product 112 feeds into settler vessel 108 to allow the metal or alloy powder to settle to the bottom of settler 108. In some embodiments, the product stream may be cooled, e.g., to a temperature of from about 550°C to about 850°C. The lower temperature range is accessible when eutectic mixtures of electrolytes such as KCl/LiCl are used. For example, the melting point of a 41 mole% LiCl/59 mole% KC1 eutectic mixture is 357 degrees Centigrade. Alternatively, pure electrolytes may be used such as KC1, which has a melting point of 771 degrees Centigrade.

[0023] The wet alloy powder is periodically discharged from settler 108 for final purification, e.g., salt removal. The generation of solid alloy particulates is not expected to cause plugging problems since the large excess of electrolyte (required to limit the temperature increase across the reactor) will flush precipitated alloy out of reactor 104. [0024] The electrolyte and alloy fines (particles that did not settle in the settling tank) are then returned to cathode compartment 116 of electrochemical cell 120, where they pick up reductant metal for another cycle through reactor 104. Inerts are vented from the settling tank through a fines filter to the atmosphere.

[0025] The examples presented below are intended to be merely illustrative, and should not be construed to be any sort of limitation on the scope of the claims.

Example 1

[0026] Material and energy balances are conducted using Aspen Plus™, a steady-state process simulator, in order to establish how much electrolyte (per unit Ti production) is required to absorb the heat of reaction for a given precooled electrolyte temperature and a specified reactor product temperature.

[0027] Figure 2 shows the ratio of the mass flow rate of eutectic salt (41 mole% LiCl/59 mole% KCl) to titanium produced as a function of the electrolyte feed temperature. For each point on the curve, the molar ratio of K to TiCl₄ is 4/1 , the reactor exit temperature is 800°C, and all reductant metal is consumed. The TiCl₄ feed is at 25 °C and 1 atm. This figure is consistent with the KCl electrolyte example shown in Example 2. Thus, the LiCl/KCl electrolyte needs to be cooled to about 600°C at a mass flow rate ratio of about 75/1 (electrolyte flow to Ti production) in order to achieve a reactor exit temperature of 800°C.

Example 2

[0028] This example illustrates the heat management problem associated with the production of titanium metal from TiCl₄.

[0029] 1 kg of KCl is added to a vessel and heated to 800°C, and then 32.7 gm of potassium are generated at 800°C and added to this mixture. Then 39.61 gm TiCl₄(g) (initially at 25°C) is added to this vessel. After all the potassium is consumed, 10.0 gm Ti(s) is produced, and the temperature of the mixture reaches 931°C (assuming no heat loss). Thus, even with a 100/1 KCl/Ti mass ratio, the temperature of the system increases by nearly 130°C due to the dissolution and reaction alone. Thus, the heat management issue is extremely important..

[0030] Besides the heat of reaction, electrical energy must be supplied to conduct the following electrochemical reactions.

K⁺ + e^"→ K -2.924 V

2Cr→Cl₂ + 2e^~ -1.3583 V net voltage -4.28 V

[0031] The net voltage (-4.28 V) is used to calculate electrical power requirements, and does not contribute to the heat balance. The overpotential (to drive the electrochemical reactions) results in Ohmic loss, which contributes to the heat balance. Table I shows the contributions to the heat balance for lab-scale (1 lb/day Ti), development-scale (50 lb Ti/day), and pilot-scale (500 lb Ti/day) operation. The heat required to evaporate and heat TiCl₄ from 25°C to 800°C represents about 13% of the total heat released (Reaction heat+Ohmic loss); the heat of vaporization of chlorine is zero for chlorine made at temperatures above about 160°C.

Table I. Reaction Heat Management

aKA: 1000 amps; assumed current efficiency: 50%>

bassumed overpotential: 0.5 V

cReaction heat + Ohmic loss - T1CI₄ latent and sensible heat [0032] Analysis of Table I shows that, at production rates of about 50 lb Ti per day and higher, active cooling of the system will be required. This heat must be removed without freezing the electrolyte. Thus, heat exchange against a high-temperature cooling medium, e.g. a molten salt bath, can be preferred in some embodiments.

[0033] The various embodiments described above thus provide a way to achieve production without exceeding the melting temperature of the desired metal. These techniques and systems also allow production to be continuous. Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0034] Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. For example, the use of active cooling elements as described with respect to one embodiment can be adapted for use with a variety of different alloys or combinations of electrolytes. The various features described, as well as other known equivalents for each feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure.

[0035] Although the systems herein have been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the techniques and systems can extend beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the systems and techniques herein and obvious modifications and equivalents thereof. Thus, it is intended that the scope of this disclosure should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.

Claims

WHAT IS CLAIMED IS:

1. A process for the production of an alloy powder comprising: providing a metal chloride to a reactor; supplying a reductant metal within a molten electrolyte medium to the reactor; contacting the metal chloride with the molten electrolyte medium; and controlling conditions within the reactor so that the temperature within the reactor remains below about 150°C above the temperature of the molten electrolyte feed.

2. The process of claim 1, wherein the metal chloride comprises titanium chloride, aluminum chloride, vanadium chloride, boron chloride, or combinations of these.

3. The process of claim 1, wherein the reductant metal comprises a Group I or Group II element.

4. The process of claim 3, wherein the reductant metal comprises sodium, potassium, magnesium, or calcium.

5. The process of claim 4, wherein the reductant metal comprises potassium.

6. The process of claim 1 , wherein the molten electrolyte medium comprises sodium chloride, potassium chloride, lithium chloride or combinations of these.

7. The process of claim 6, wherein the molten electrolyte medium comprises potassium chloride.

8. The process of claim 1, wherein the reactor is a substantially adiabatic tubular reactor or continuous stirred tank reactor.

9. The process of claim 8, wherein the reactor is a substantially adiabatic reactor.

10. The process of claim 1, wherein the temperature within the reactor remains below about 100°C above the temperature of the molten electrolyte feed.

11. A titanium alloy powder made according to the process of claim 1.