Producing titanium particulates from in situ titanium-zinc intermetallic

A process for producing salt free titanium powder by reacting zinc and a titanium halide in the presence of a reducing agent to form a solid zinc titanium product. Titanium halide vapor is introduced into a liquid alloy of zinc and the reducing agent at a temperature between 650.degree.-907.degree. C. The titanium halide is introduced beyond the titanium solubility limit in zinc to precipitate a zinc titanium intermetallic compound and also produce a liquid halide salt. The intermetallic compound forms and accumulates at an interface between the salt and liquid alloy. The compound is periodically removed from the interface, crushed into a powder, and the zinc is evaporatively separated from the titanium to produce pure titanium powder. The process preferably occurs above the peritectic decomposition temperature of Zn.sub.3 Ti, and most preferably above the peritectic decomposition temperature of Zn.sub.2 Ti, to maximize the titanium content of the resulting product.

FIELD OF THE INVENTION 
This invention concerns a process for the preparation of titanium from 
titanium halides, such as titanium tetrachloride. This invention further 
relates to production of finely divided particulate titanium and titanium 
alloys from titanium tetrachloride. 
BACKGROUND OF THE INVENTION 
Many diversified applications have been found for titanium and its alloys. 
Titanium metal has been essential to the aerospace industry since the 
early 1950's because it combines a high-strength to weight ratio with the 
ability to perform at much higher temperatures than aluminum or magnesium. 
It has therefore been used in compressor blades, turbine disks, and many 
other forged parts of jet engines and aircraft frames. It is also widely 
employed in the chemical processing industry because of its excellent 
resistance to chloride corrosion. Because of its scarcity and high cost, 
titanium has frequently been used in the form of a titanium powder to 
produce articles which are too expensive or difficult to produce by 
machining or forging from massive metal shapes. More efficient processes 
for the production of titanium powder have therefore been sought. 
A majority of the world's titanium is made by the Kroll process, which 
produces titanium "sponge" in the form of a metallic powder. The titanium 
sponge is produced by reducing titanium tetrachloride (TiCl.sub.4) with 
magnesium or sodium in a heated steel vessel. After cooling, an intimate 
mixture of titanium sponge and frozen chloride salt forms. The sponge and 
salt are separated by crushing and water leaching the products to dissolve 
the salt and produce a purer titanium product. The titanium sponge is then 
compressed into an electrode bar and vacuum arc remelted (VAR) to 
consolidate the metallic sponge. The expensive VAR process must be 
repeated once and sometimes twice to remove residual chloride salt and 
produce a clean consolidated bar of titanium. Alloying agents may be 
introduced during resulting if special purpose titanium alloys are 
desired. 
The most important consideration for any process of making titanium is to 
prevent contamination with either metallic or non-metallic impurities, 
because even small amounts of some impurities can make the product brittle 
and unworkable. This is an especially serious problem for aerospace and 
other critical applications where such impurities can lead to defects in 
the final product manufactured from titanium. It is crucial, for example, 
that titanium components of jet engines or guided missiles maintain their 
structural integrity at all times in stressful environments. To help 
preserve this integrity, many processes have been developed for producing 
titanium powder free of contaminants which impair the structural integrity 
of the end product. 
U.S. Pat. No. 4,602,947, for example, discloses a method of producing 
titanium sponge or titanium alloy powder by reducing gaseous titanium 
tetrachloride with magnesium. This method, which is schematically 
summarized in FIG. 2, produces titanium metal in the form of finely 
divided particles by first forming a liquid mixture of titanium and zinc, 
then solidifying the liquid mixture to produce finely divided alloy 
particles, and finally evaporatively separating zinc from the particles to 
produce pure titanium powder. In particularly disclosed embodiments, 
titanium chloride vapor is injected into a molten zinc-magnesium bath. 
Titanium replaces magnesium in the liquid alloy such that liquid zinc 
titanium and liquid magnesium chloride are produced. The less dense liquid 
magnesium chloride, which is completely immiscible with the liquid zinc 
titanium alloy, floats to the top of the reactor where it is removed. The 
resulting liquid zinc titanium mixture is recovered, solidified, and 
passed to a zinc evaporation zone where the zinc is sublimed to produce 
sponge titanium. 
Although the process disclosed in U.S. Pat. No. 4,602,947 produces a 
relatively pure titanium sponge product, it suffers from the expensive 
drawback of requiring large amounts of zinc. Titanium has a very low 
solubility in zinc at temperatures up to the normal boiling point of zinc 
(907.degree. C.). As a practical matter, the titanium solubility in liquid 
zinc is limited to about five weight percent. This is shown by the zinc 
rich end of the zinc titanium binary phase diagram reproduced in FIG. 1. 
This low solubility is significant because the solubility limit cannot be 
exceeded if a liquid mixture of titanium and zinc is desired. Such a 
liquid mixture is required in the '947 patent, and because of the limited 
titanium solubility, approximately 20 lbs. of zinc must be consumed for 
each pound of titanium produced. A substantial amount of zinc is also lost 
through evaporation at the elevated temperatures preferred in that prior 
process. Although a cover of molten salt theoretically prevents zinc 
evaporation up to its boiling point at the gas over-pressure (usually one 
atmosphere or less), as a practical matter it is usually necessary to 
operate at temperatures over 907.degree. C. to increase the solubility of 
titanium in zinc. The zinc evaporates at this temperature and is lost from 
the reaction. 
Other United States patents disclose methods for producing titanium sponge 
by reducing titanium chloride salts with aluminum. See, for example, U.S. 
Pat. Nos. 4,359,449; U.S. Pat. No. 4,390,365; and U.S. Pat. No. 4,468,248. 
None of these patents disclose reduction of gaseous titanium chloride by 
magnesium in a liquid zinc alloy. Other U.S. patents teach producing 
titanium powder and titanium alloy powder from binary and more complex 
zinc-titanium alloys by removing the zinc through sublimation. Such 
patents include U.S. Pat. No. 4,470,847; U.S. Pat. No. 4,595,413; and U.S. 
Pat. No. 4,655,825. Removal of zinc from zinc titanium alloys is also 
taught in U.S. Pat. No. 4,602,947. 
SUMMARY OF THE INVENTION 
The present invention overcomes the drawback of U.S. Pat. No. 4,602,947 by 
contradicting the teaching of that patent that a liquid mixture of 
titanium and zinc is desired in producing titanium powder. In the present 
invention, zinc and a titanium halide are reacted in the presence of a 
reducing agent to form a solid zinc titanium product. The solid product is 
obtained by introducing titanium halide vapor into a liquid alloy of zinc 
and a reducing metal in amounts beyond the solubility limit of titanium 
metal in zinc to precipitate zinc titanium intermetallic compounds. The 
reaction also produces a lower density salt comprised of the reducing 
metal and halide, which is immiscible with the liquid alloy and floats to 
the top of the reaction mixture. The zinc titanium intermetallic compounds 
form and accumulate at the interface between the salt and liquid alloy 
layers. The zinc titanium compounds are removed from the interface, 
crushed, and the zinc evaporatively separated to produce pure titanium 
sponge. 
In more specific embodiments, titanium tetrachloride vapor is injected into 
a liquid alloy of zinc and magnesium at temperatures above 650.degree. C. 
but below the zinc boiling temperature of 907.degree. C. Titanium 
tetrachloride injection is continued well beyond the titanium solubility 
limit to precipitate a zinc titanium product which includes intermetallic 
compounds. Zinc rich intermetallic compounds such as Zn.sub.3 Ti or 
Zn.sub.4 Ti are unstable above 650.degree. C. and decompose peritectically 
to solid Zn.sub.2 Ti and Zn.sub.2 Ti. Even Zn.sub.2 Ti is unstable above 
the peritectic decomposition temperature of Zn.sub.2 Ti (about 750.degree. 
C.), and ZnTi will be the sole product of the reaction above this 
temperature. The titanium content in ZnTi is above 40 weight percent, 
while the titanium content in Zn.sub.2 Ti is about 27 weight percent. In 
either case, the titanium content is much greater than the liquid 
solubility limit of about 10-15 atomic percent, and a process producing 
either of these intermetallic compounds is much more efficient and 
economic in its use of zinc than previous processes for titanium sponge 
production. The low reaction temperatures also diminish the amount of zinc 
lost through evaporation. 
In prior art processes, such as that disclosed in U.S. Pat. No. 4,602,947, 
approximately 20 pounds of zinc are consumed for each pound of titanium 
produced. The process of the present invention, however, requires only 
1.37 pounds of zinc per pound of titanium when the process is performed 
above the peritectic decomposition temperature of Zn.sub.2 Ti (about 
750.degree. C.) to produce ZnTi. When the process is carried out between 
the peritectic temperature of decomposition of Zn.sub.3 Ti (650.degree. 
C.) and Zn.sub.2 Ti (about 750.degree. C.), about 2.73 pounds of zinc 
would be required for each pound of titanium produced. The differing 
requirements for zinc reflect the changing atomic percent of zinc in the 
final product. In a commercial process, additional amounts of liquid zinc 
(saturated in titanium) would be attached to the intermetallic compound 
dross removed from the furnace, and more zinc would be required than the 
theoretical amounts given above. The amount of zinc required in the 
present invention, however, is much less than the amount of zinc needed to 
produce a liquid alloy that is thereafter frozen and vacuum sublimed as in 
the prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A cylindrical carbon crucible or reactor 10 is shown in FIG. 3 into which 
titanium tetrachloride vapor is introduced through a gas conduit 11. The 
conduit 11 enters reactor 10 through an open top 13 and extends along a 
sidewall 14 until conduit 11 terminates adjacent bottom 15. Before the 
reaction begins, reactor 10 contains a liquid alloy of zinc and a reducing 
metal in layer 16. In the disclosed embodiment, the reducing metal is 
magnesium, but it can also be sodium, potassium, lithium, calcium, or 
mixtures thereof. Magnesium is the preferred reducing metal. The disclosed 
process operates at one atmosphere pressure, until the zinc vapor pressure 
increases at higher temperatures. 
As TiCl.sub.4 is introduced into layer 16 of liquid zinc magnesium alloy, 
TiCl.sub.4 bubbles 17 are formed in the mixture. As the amount of 
TiCl.sub.4 introduced into and reduced to titanium in layer 16 exceeds the 
solubility limit of titanium in zinc, a zinc-titanium intermetallic dross 
18 (FIG. 4) is formed. The dross is the product of a reaction in which 
titanium displaces magnesium from the zinc to precipitate intermetallic 
compounds which then concentrate in situ beyond the solubility limit of 
titanium in zinc. The intermetallic compounds include ZnTi and Zn.sub.2 
Ti, and in preferred embodiments are limited to ZnTi and Zn.sub.2 Ti, 
rather than compounds having a higher Zn content. The magnesium liberated 
from the ZnMg alloy reacts with chlorine liberated from the TiCl.sub.4 to 
produce magnesium chloride (MgCl.sub.3) liquid which is immiscible with, 
and has a lower density than, the layer 16 or dross 18. The magnesium 
chloride, therefore, forms a liquid layer 20 at the top of the reaction 
vessel. Dross 18 accumulates at interface 22 between layers 16 and 20. 
Dross 18 is periodically removed from the crucible, for example, by an 
inert alloy sieve basket described in connection with FIG. 4 below. The 
dross is removed from reactor 10 as a solid ZnTi compound, which is 
crushed to form a powdered ZnTi product that is then heated to remove zinc 
by sublimation and yield a pure titanium powder. Examples of methods for 
subliming zinc from binary and more complex zinc titanium alloys are 
disclosed in U.S. Pat. No. 4,470,847; U.S. Pat. No. 4,595,413; and U.S. 
Pat. No. 4,655,825. 
Zinc and magnesium are continuously or intermittently replenished through 
an airlock into reactor 10. Accumulating magnesium chloride is removed 
from layer 20 by a cup, siphon pipette, or overflow weir (not shown) 
through an airlock. Thus, the process can be quasi-continuous if desired, 
rather than a batch process. Other methods of replenishing reactants and 
removing solid and liquid products of the reduction reaction are possible 
and within the scope of this invention. It would be possible, for example, 
to continuously add titanium scrap to liquid zinc and allow the titanium 
to dissolve to its solubility limit, then precipitate out the 
titanium-zinc intermetallic compound as a dross for removal. 
The titanium halide reduction reaction has fast chemical kinetics and is 
essentially stoichiometric. The reaction between magnesium and titanium 
tetrachloride produces two moles of magnesium chloride for each mole of 
titanium tetrachloride injected: 
EQU TiCl.sub.4 (g)+2 Mg.fwdarw.Ti+2 MgCl.sub.2 
A second embodiment of the invention is shown in FIG. 5 wherein a 
cylindrical crucible or reactor 30 is contained within and surrounded by a 
cylindrical furnace 32. Reactor 30 and furnace 32 for controlling the 
reaction temperature are both enclosed in a furnace chamber 34 which is 
filled with an inert gas such as argon or helium to provide an inert 
atmosphere for the reaction. Argon (at one atmosphere) is the preferred 
inert gas because of its low cost compared to helium. An inert atmosphere 
is desireable to prevent introducing impurities such as oxygen or nitrogen 
into the titanium which weaken the product and can make it brittle. An 
airlock chamber 36 communicates with furnace chamber 34 but is separated 
from it by a vacuum valve 38. A second vacuum valve 40 is interposed 
between airlock chamber 36 and the outside atmosphere. 
As shown in FIG. 5, reactor 30 contains a lower layer 42 of zinc magnesium 
liquid alloy, and an upper layer 44 of liquid magnesium chloride which is 
produced as a by-product of the reaction in reactor 30. Titanium chloride 
vapor is introduced through conduit 48 into layer 42 to form a solid 
intermetallic compound which accumulates as dross 50 at the interface 52 
of layers 42 and 44. A sieve basket 54 is suspended in reactor 30 to 
retrieve dross 50 periodically from the reactor. Basket 54 includes a 
perforated plate 56, imperforate cylindrical sidewall 58, and suspension 
hanger 60 for suspending basket 54 in the reactor. Arms 62 of hanger 60 
are connected to the top of sidewall 58 by hinges 63 at several positions 
circumferentially around the top of the sidewall. Hanger 60 is connected 
to a conventional device (not shown) for raising or lowering sieve basket 
54. 
In operation, basket 54 is suspended in reactor 30 below the surface of 
layer 42 before TiCl.sub.4 is introduced into the zinc magnesium liquid 
alloy. As TiCl.sub.4 is introduced through conduit 48 into layer 42, 
titanium displaces magnesium from the zinc and the zinc titanium dross 50 
forms at interface 52. After a predetermined period of time, or after a 
predetermined amount of dross 50 has accumulated, hanger 60 exerts an 
upward force on basket 54 to elevate the basket and move plate 56 
upwardly. The liquids of layers 42 and 44 drain through perforated plate 
56, while solid dross 50 is retained in basket 54 and removed from reactor 
30. The ZnTi dross 50 is removed from the protective inert atmosphere of 
furnace chamber 34, and into airlock chamber 36 by opening vacuum valve 
38, which allows basket 54 to enter airlock chamber 36. Vacuum valve 36 is 
then closed once again to protect the inert atmosphere in furnace chamber 
34. Valve 40 is then opened to allow dross 50 to be removed from chamber 
36 without contaminating the inert atmosphere of chamber 34. 
Zinc and magnesium are replenished by introducing them through airlock 
chamber 36 into reactor 30. Accumulating magnesium chloride is also 
removed periodically from layer 44 through airlock chamber 36, either by a 
cup, siphon pipette, or overflow weir (not shown). Alternative methods for 
removing the dross (such as slurry pumping) would also be acceptable if 
oxidation of the product was prevented or diminished. The process is, 
therefore, quasi-continuous and efficient. 
An advantage of the present invention is that it produces a zinc-titanium 
intermetallic compound having a high titanium content. The principle which 
permits the process to operate efficiently is illustrated in FIG. 1, which 
is a zinc-titanium phase diagram at one atmosphere. As the temperature 
rises upon heating, zinc melts at 419.5.degree. C. and begins to dissolve 
titanium. The curve in FIG. 1 is the liquidus composition, which is the 
composition of zinc liquid saturated with dissolved titanium at the 
corresponding temperature, e.g., point 2 at about 830.degree. C. At 
equilibrium point 2, zinc liquid is saturated with dissolved titanium. As 
further titanium is added, the excess dissolved titanium solute reacts 
with the zinc solvent to precipitate ZnTi crystals, with composition at 
point 3, from the melt in a liquid metal crystallization process. In the 
example shown in FIG. 1, an aggregate initial composition of about 13 
atomic percent titanium, point 2 will yield equilibrium products that are 
solid TiZn and saturated liquid. The relative amounts are 90 percent 
liquid and 10 percent TiZn. 
Above 650.degree. C., the peritectic decomposition temperature of Zn.sub.3 
Ti is exceeded, and a mixture of only Zn.sub.2 Ti and ZnTi are produced 
from the saturated liquid. The peritectic decomposition temperature of 
Zn.sub.2 Ti is exceeded at about 750.degree. C., and a pure ZnTi product 
is obtained at or above this temperature. The high vapor pressure of zinc 
renders difficult a precise determination of the peritectic decomposition 
temperature for Zn.sub.2 Ti. The present inventors have determined, 
however, that Zn.sub.2 Ti will peritectically decompose to liquid ZnTi at 
a temperature below 800.degree. C. and near 750.degree. C. 
A clear advantage of this invention is that Zn.sub.2 Ti decomposes 
peritectically at a temperature at which the zinc vapor pressure is not 
excessively high. Moreover, when operating above the Zn.sub.2 Ti 
peritectic temperature, the solubility of titanium in liquid zinc is very 
low (less than 10 atomic percent). In addition, the precipitation product 
ZnTi is very high in titanium (50 atomic percent, or about 42 weight 
percent). It is possible to continually introduce titanium into solution, 
letting it react with zinc to precipitate solid ZnTi, which can then be 
harvested as a dross. The process is performed above 650.degree. C., which 
is the decomposition temperature of the peritectically decomposing 
Zn.sub.3 Ti compound. Addition of excess titanium to a melt above this 
temperature will precipitate only Zn.sub.2 Ti or ZnTi, because higher zinc 
intermetallic compounds such as Zn.sub.3 Ti and Zn.sub.4 Ti are unstable, 
will not form, and if present by addition would decompose peritectically 
to Zn.sub.2 Ti or ZnTi and liquid. If the temperature of the melt is 
maintained above the peritectic decomposition temperature of Zn.sub.2 Ti 
(about 750.degree. ), addition of excess titanium will precipitate only 
ZnTi because compounds containing higher atomic percents of Zn are 
unstable and will spontaneously decompose to ZnTi. Although operating 
temperatures above the peritectic decomposition temperature of Zn.sub.2 Ti 
may cause operational difficulties, it does produce a product having a 
greater atomic percent of titanium. However, even the Zn.sub.2 Ti product 
produced between 650.degree. C. and about 750.degree. C. has a much 
greater atomic percent of titanium than the liquid solutions of titanium 
produced by prior art processes. 
Having illustrated and described the principles of the invention in two 
preferred embodiments, it should be apparent to those skilled in the art 
that the invention can be modified in arrangement and detail without 
departing from such principles.