CA1262506A - Chemical vapor purification of fluorides - Google Patents

Abstract

CHEMICAL VAPOR PURIFICATION OF FLUORIDES

Abstract A chemical vapor purification process for the preparation of high purity metal fluorides using the thermodynamic separation of cations by formation of a gaseous metal-containing compound, vapor transport, and fluorination is disclosed.

Description

5~6 8a~-3-036 CN -1-CHEMICAL VAPOR PUl'cIFICATION OF FLUORIDES

This invention relates to processes for preparing metal halides. More particularly, this invention relates to preparing high purity metal fluorides.

Work on the purification of fluorides has intensified greatly because of interest in such applications as heavy metal fluoride glass optical fibers, tunable solid state lasers, and dielectric layers for complex semiconductor ; structures. Initially rapid advances were made in increasing the purity of these materials, but recent progress has slowed although impurity levels remain orders of magnitude higher than can be tolerated in some applications.
For lnstance heavy metal Eluoride glasses show potential ~or fabrication of extraordinarily low loss optical flbers operating in the 2-~ ~ m region of the infrared. One of the most serious problems is contamina-tion with certain divalent transition metals, some rareearth ions, and the hydroxyl ion. These impurities have strong absorptions in the optimum optical region. It is necessary to attain impurity levels of one part per billion for some of these contaminants to exploit the potential for these glasses.
A number of purification approaches are used at present, including wet chemical processing and vapor treatment. Wet processing is well known, and has limita-tions in the degree of purification because of recon-tamination from the background levels of contaminants ~; present in the processing chemicals. Current vapor preparation techni~ues are limited to dealing with the existing fluoride compound, and suffer from low or non-existent thermodynamic driving force for removal of the contamination. The "reactive atmosphere process"
(RAP) has been used to purify a number of starting !~

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materials, and has an important effect on the hydroxyl content, but rather little effect on the other contami-nants. Physical vapor transport (sublimation) has been used to purify ZrF4 and AlF3. BaF2 and GdF3 have been purified by subliming the transition metals out of them, showing greater than an order of magnitude improvement in Fe2+, but little effect on the other contaminant ions.
Sublimation is limited by the ratio of the vapor pressure of the contaminant species to the desired compound.
~ne of the basic limitations of such processing is that the starting materials are often of limited purity, and the process is required to provide more purification than reasonable. In addition, there are some fundamental limitations on the amount of contaminant than can be practically removed because of similar vapor pressure of the contaminant compound over its solid solution with the major compound~

Accordingly, the present invention provides a chemical vapor purification process for preparing high purity metal fluorides comprising: reac~ing a metal with a reactive transport agent in the presence of a stoichiometric excess of metal to generate a gaseous metal-containing compound; isolating the gaseous metal-containing compound from the starting materials, and reacting the isolated gaseous metal-containing compound with a fluorinating agent to form a solid metal fluoride.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing in which:
The FIGURE is a schematic diagram of a reactor suit-able for use in an embodiment of the present invention.
For a bet~er understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference i5 made to the following , ~

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disclosure and appended claims in oonnection with the above-described drawings.

The chemical vapor purification process of the described embodiment overcomes the normal limitations of treating existing compounds with an active atmosphere or agent, and replaces known processes with a new process that completely avoids the normal limitations of treating existing compounds with a reactive atmosphere or other a~ent. Processes such as RAP have limitations because they can only approach the purity asymptotically, and have limited thermodynamic potential.
In the process of the embodiment, the desired metal ion is selectively extracted because of thermody-namic partitioning between the desired metal cation and the contaminating cations. A reactive transport a~ent, such as chlorine, bromine, or iodine, is reacted with the desired metal and generates a gaseous meta]-containin~
compound. The gaseous metal-containing compound contain-ing the highly purified metal cation is then isolated from ~; the starting materials/ which include the excess unreacted metal and any contaminants. The gaseous metal-containing compound is then reacted with a fluorinating agent to form the desired solid metal fluoride in a step that further enhances the purification of the metal fluoride compound.
A fluorinating agent i5 any compound capable of reacting with the gaseous metal-containing compound to form the desired metal fluoride. Examples of ~luorinating agents include such compounds as fluorine gas, sulfur hexafluoride, hydrogen fluoride, and nitrogen trifluoride.
The process of the embodiment involves the use ; of a continuous sequential reaction using direct reaction of a metal with the reactive transport agent to ~orm a gaseous metal-containing compound. This reaction is ~; followed by fluorination of the metal cation contained in the vapor, after the gaseous metal-containing compound had :~ ; ~., :

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The reactions for zirconium and aluminum with a chlorine reactive transport agent are presented below:
Zr(s) + 2Cl~(g) ~ ZrC14(g) (la) ZrC14(g) + 4/x RFx(g) ~ ZrF4(s) -~ 4/x RClx(g) (lb) Al~s) + 3/2 C12(g) ~ AlC13~) t2a) AlC13~g) + 3/x RFx(g) ~ AlF3(s) + 3/x RClx(g) t2~, In the above equations, RFX represents the fluorinating agent wherein R is a convenient cation, such as C, S, N or H, that forms a vapor ea~ily with the anion of the reactive transport agent and does not interfere with other aspects of the reaction, and x represents the number of fluoride ions necessary to balance the charge of the ; cation. Fluorine gas (F2) may also be used as the fluorinating agent~
2irconium and aluminum are two of the most suitable candidates for the process, since they are of current interest in preparing fluoride glasses for optical fibers.
Hafnium may be substituted for zirconium in equation (1 above, and gallium or indium appear to allow similar - reactions to equation (2). Such substitutions in the glass provide a longer wavelength cutoff by replacing a lighter cation with a heavier one, decreasing the ~` fundamental vibrational frequency, thus moving the -` infrared cutoff to longer wavelengths, which is desirakle.
Examples of other metals that can be used in the present method include titanium, zinc, cadmium, and mercury.
The chemical vapor purification process of the embodiment offers the advantages of not requiring high temperatures to carry out the process and of using and producing reactants and products which are chemically compatible with available materials. For example, with a :
chlorine reactive transport agent, chlorine readily reacts ~- with aluminum and zirconium to form volatile chlorides of the metals. Carbon, in the form of gxaphite or vitreous . ~
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carbon, does not react with chlorine, bromine, or iodine;
nor is carbon likely to react with the metals at the temperature involved.
In reactions (la) and (2a) abo~e, a chlorination reaction is the first step in the overall process. As stated previously, bromine and iodine are other potential active agents suitable for use in the ~irst step since they both form volatile species with the cations of interest.
10 The purification capability oE the process is based on the existence of near equilibrium conditions in the reaction of the first step and the presence of a stoichio-metric excess of the desired metal. The presence of the excess metal avoids reaction and transport of undesirable species. The excess metal condition is easily achieved since a small amount of the reactive transport a~ent is metered into a vessel that contains an inclefinitely large quantity of the metal to be transported. The purification potential was tested in calculation by establishing reaction conditions with one mole of the desired metal;
adding either 0.01 or 0.0001 mole of metallic iron as a contaminant; and an excess of the desired metal (20 mole %
excess in the following example). Iron was chosen as a contaminant for this calculation because it is one of the critical contaminants that must be controlled. Tempera-tures of 400 to 600 K were examined for aluminum, and 650 to 900 K for zirconium.
Table I lists the species which result from the chIorination of a zirconium sample which contains an iron metal contaminant. It is important to notelthat the unreacted iron is present with the unreacted zirconium metal, and that the value of the mole fraction for iron present is 8.3 x 10 . The only vapor species present in a mole fraction ~10 that contains iron is FeC12, at an estimated partial pressure of 2.6 x 10 atm at 900 K.
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As can be seen from Table I, essentially no iron species are present as yaseous compounds that can be transported under the conditions chosen, especially at lower temperatures. Effectively all the chlorine has been used to transport the zirconium as ZrCl4, using 5/6 of the zirconium metal starting material. Less than 10 12 of the iron present in this example is transported, even at the highes-t temperature, although the vapor pressure of FeCl2 is constant over the range of impurity iron considered.
There also exists a possibility that a compound will form from all three elements present in the reaction for which no thermodynamic data exists that has a high enough vapor pressure to be transported. In addition, equilibrium conditions do not exist except in thermodynamic calculations, and some transport can certainly be antici-pated in any real reaction~ By a choice of conditions of temperature and transport rate, such carry-over can be minimi~ed.
Additional calculations have shown that essentially identical results hold down to 0.0001 mole % excess metal.
The kinetics of the reaction, especially the forma-tion of the volatile metal chloride, may be rate control-ling at the lower temperatures, establishing a practical minimum temperature. Past experience has shown this to not be a problem in the aluminum reaction at temperatures 475 K, i.e., khe reaction ~oes to completion.
Table II shows the result oE including all of the ~ problem transition metal contaminants and water vapor in ; the initial aluminum charye that is reacted. Lanthanides were not included because of the paucity of reliable data, and the yeneral knowledge tha-t their metals and chlorides have very low vapor pressureO

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TABLE II
Chlorination of Aluminum with Contami~ants Products Mole Fract:ion : Temperature (kelvin) Al(s) 2.8 x 10 2.8 x 10 2.9 x 10 AlCl 1.7 x 10 8.7 x 10 2.5 x 10 AlC12 7.9 x 10 5.3 x 10 2.0 x 10 AlC13 1.8 x 10 2 6.0 x 10 3 1.6 x 10 3 Al2C16 7.0 x 10 1 7.1 x 10 1 71 x 10 1 Al23~S) 4.7 x 10 5 4.7 x 10 5 4.8 x 10 C0(9) 1.4 x 10 4 1.4 x 1~ 4 1.4 x 10 4 Cu(s) 1.4 x 10 4 1.4 x 10 ~ 1.4 x 10 4 Fe(s) 1.4 x 10 4 1.4 x 10 4 1.4 x 10 4 HCl -11 8.5 x 10-12 5.8 x 10

2 1.4 x 10 4 1.4 x 10 1.4 x 10 4 Content of Metal Starting Material Species Moles ;~ Al 1.2 :.
; Co O.0001 "~ ' ' Cu 0. 0001 Fe 0.0001 2 o . 0001 `~``;~ 30 2 1c5 ~:: Table II again suggests that the transition metal ~: : contaminants are unlikely to present a problem. The : presence of hydrogen at a vapor pressure of ~10 4 causes less certainty in establishing a low OH content in the final fluoride product, but an excess of the fluoxinating '~

agent is expected to provide the necessary control. The products that were considered but found to be less than 10 16 mole fraction were not listed because of the very extensive list of 78 combinations, but important ones such as FeC12, CuCl~, CoC12, OH and ~2 were included on that list.
Similar calculations were performed for a variety of conditions for the fluorination reactions of equations (lb) and (2b). A variety of fluorinating agents were ; 10 examined, including ~F4, F2, SF6, SF4, and HF. All were found to be thermodynamically feasible. For instance, one of the fluorinating agents, SF6, although a very stable compound, has been shown experlmentally to react wlth ZrCl~. However, carbon tetraEluoride (CF~) has not been shown to react -to any useful degree. ~11 of the reactions examined did proceed as desired in useable temperature ranges. Water vapor was also included in some of the calculations, and a low vapor pressure was indicated. Fluorine (~2) can also be used as a fluor-;~ 20 inating agent in the present invention.
The experimental configuration selected for use in carrying out the present invention is a temperature zone tube reactor. A reactor having temperature capabilities of up to 800C is suitable for carrying out the method of the present invention. This permits the use of conven-tional tube furnaces.
Minimum and maximum temperatures for use with various metals in the method o:E the present invention are set forth below in Table III.

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5~6 TABLE III
Temperature (C) Metal Minimum Maximum Zr 330 900 Hf 320 1,000 Al 180 1,200 Ga 200 1,000 In 600 1,200 The minimum temperature is the temperature at which a particular metal chloride sublimes at one atmosphere.
The maximum temperature is the sublimation temperature for a particular metal fluoride.
A particularly inert and/or pure liner is used in the reactor in order to avoid recontaminating the puri-fied fluoride product, and to prevent attack of the liner by the fluorides. An example of a suitable liner material is vitreous carbon coated graphite since its inertness is well recognized in the presence of fluorides and the absence of oxygen and water vapor. Preferably the reactor is protected from attack by atmospheric oxygen by using a shielding tube outside of the reaction tube, with an inert gas passing in the annulus.
The schematic configuration of the reactor is shown in Figure 1. A predetermined quantity of the reactive transport agent is carried from a source of reactive transport agent tnot shown) to a first reactor 30 by a first line 11. The first reactor contains a sample of the metal to be converted into a metal fluoride. Strips of reasonably pure metal/ preferably in the form of a metal foil, are placed in the first reactor 30 in a manner such that the flow must pass over a substantial surface area of material. In this manner, the very important excess metal condition is maintained to mini-mize the transport of contaminants. For example, the metal sample can be~suspended from the upper surface of the first reactor. In the case ~of aluminum and a :
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A second carrier line 31 transports the gaseous metal-containing compound to the second reactor 50. The second carrier line 31 must also be heated to a tempera-t~re sufficiently high to avoid condensation oE the gaseous metal-containing compound~
The metal-containing vapor is then reacted in the second reackor with a suitable fluorinating agent. The fluorinating agent is introduced into the second reactor 50 via a third carrier line 41. A buffer gas can option-ally he used to prevent the flow of the fluarinating agent out of the second reactor and into the second carrier line 31 or the first reactor 30. The use of the buffer gas avoids premature reaction of the f~uorinating agent with the gaseous-metal-containing compound. The buffer gas can be any gas, which is inert to reaction with the other reactants e.g., argon. The resulting metal fluoride powder falls into container 60. Because there is a concern that the metal fluoride powder could ~;~ be recontaminated from both the reaction atmosphere and subsequent handling, the used gaseous reactants are vented by an exhaust 70 for disposal.

EXAMPLE
An amount of ZrF4 in excess of 100 grams was pre-pared during a two hour period using the chemical vapor purification process of the present invention. ~hlorine ;~ was used as the reactive transport agent and hydrogen ~ fluoride was used as the fluorinating agent. This ;~ specific run was performed using half the flow of reac-tive transport agent that the equipment is capable of providing, and half the flow of fluorinating agent required for stoichiometry~ The ZrF~ was largely ~ ' , 5~)6 collected in the appropriate container, and a powder was found on the cooler base of the reactor. Qualitative examinations suggested that the latter material was basically ZrCl4~
Spark source mass spectroscopy (SSMS) analyses of both the starting metal and two independent analyses of the produc-t are reported in Table IV. The absence of deleterious rare ear-th metals in the Eluoride is signifi-cant as trace ~uantities were found in the starting metals.

TABI,E IV
Analyses of ZrF4 and Zr Metal Parts per Million ~Weight) GTEI, GTEL Northern Mat. Eva1. Mat. Eval. ~nalytical ElementZr metal ZrF~ ZrF4 Fe 2100 2.1 0.5 Ni 41 1.2 ~ 0.03 20Mn 46 1.7 0.5 Cr 140 0.4 0.3 V 5 0.7 0.04 Co 3 6.0 nd Cu 31 0~4 0.0 Hf 72 9.0 25.0 ~; Zn 1 nd < 0.04 Sn 16 nd nd Ce 0.5 nd nd Nd 0.8 nd nd Pr 0.3 na nd Sm 0.5 nd nd While there has been shown and described what are at present considered the preferred embodiments of the inven~ion, it will be obvious to those skilled ln the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the appended claims.

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS::

1. A chemical vapor purification process for preparing high purity metal fluorides comprising:
reacting a metal with a reactive transport agent in the presence of a stoichiometric excess of metal to generate a gaseous metal-containing compound;
isolating the gaseous metal-containing compound from the starting materials; and reacting the isolated gaseous metal-containing compound with a fluorinating agent to form a solid metal fluoride.

2. A method in accordance with claim 1 wherein the reactive transport agent is selected from the group consisting of chlorine, iodine, and bromine.

3. A method in accordance with claim 1 wherein the fluorinating agent is selected from the group consisting of fluorine gas, sulfur hexafluoride, sulfur tetra-fluoride, hydrogen fluoride and nitrogen trifluoride.

4. A process in accordance with claim 1 wherein the metal is selected from the group consisting of zirconium, hafnium, aluminum, gallium, indium, titanium, zinc, cadmium, mercury and mixtures thereof.

5. A process in accordance with claim 4 wherein the metal is selected from the group consisting of zirconium, hafnium, aluminum, gallium, indium, titanium, and mixtures thereof.

6. A chemical vapor purification process for preparing high purity aluminum fluoride comprising:
reacting aluminum metal with a reactive chlorine transport agent in the presence of a stoichiometric excess of aluminum to generate a gaseous aluminum chloride compound;
isolating the gaseous aluminum chloride compound from the unreacted aluminum; and reacting the isolated gaseous aluminum chloride with a fluorinating agent selected from the group consisting of F2, SF6, SF4, HF, and NF3 to form a solid aluminum fluoride.

7. A chemical vapor purification process for preparing high purity zirconium fluoride comprising:
reacting zirconium metal with a reactive chlorine transport agent in the presence of a stoichiometric excess of zirconium to generate a gaseous zirconium chloride compound;
isolating the gaseous zirconium chloride compound from the unreacted zirconium; and reacting the gaseous zirconium chloride with hydrogen fluoride to form a solid zirconium fluoride.