Anodes for the Electrolytic Production of Nitrogen Trifluoride and Fluorine

A process and an anode for the production of nitrogen trifluoride or fluorine where the anode in the electrolytic cell is made primarily from mesocarbon microbeads. The mesocarbon microbead anodes minimize the production of CF4 and improve the purity of the nitrogen trifluoride or fluorine gas produced. Additionally, the anodes may be molded, instead of extruded or machined, providing for improved dimensional and mechanical integrity of the anode.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e) to earlier filed U.S. patent application Ser. No. 61/716,259, filed on 19 Oct. 2012 and U.S. patent application Ser. No. 61/790,810, filed on 15 Mar. 2013 and incorporating both applications in their entirety by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to the electrolytic production of nitrogen trifluoride and fluorine, and in particular, to the use of anodes made from mesophase carbon, including mesocarbon microbeads for generating nitrogen trifluoride and fluorine.

Nitrogen trifluoride (NF3) is a stable gas that has little reactivity at room temperature. Fluorine (F2), on the other hand, is a reactive gas with most materials at ambient conditions. Both NF3 and F2 have found growing use in semiconductor manufacturing. For example, NF3 is typically used as an etchant for silicon or silicon oxide layers on a semiconductor substrate or as a CVD chamber cleaning gas where it is activated in situ.

On an industrial scale, NF3 may be manufactured by a fluorination process. There are two principle methods for fluorination: direct fluorination (DF) and electrochemical fluorination (ECF). In electrochemical fluorination, an electrolyte may be electrolyzed in an electrolytic cell to produce the NF3. F2 is produced in an electrochemical process that resembles the ECF process for producing NF3. Traditional electrolytic cells use a carbon steel cathode and extruded, carbonized anodes made from carbon coke particles and a carbon pitch binder, for example. The traditional extruded carbon anodes are carbonized at temperatures below 1000° C. and are typically not graphitized, which requires temperatures in excess of 1500° C. There are a number of drawbacks associated with traditional extruded carbon anodes, however, as described in the literature, for example in the reference Ellis, J. F. and G. F. May, “Modern Fluorine Generation” in Fluorine, the First Hundred Years, R. E. Banks, D. W. A. Sharp, and J. C. Tatlow, eds., Elsevier Sequoia, 1986.

One problem associated with electrochemical fluorination is contamination of the electrolytically-generated NF3or F2 with CF4 (tetrafluoromethane or carbon tetrafluoride). Contamination of any sort is a concern because a high purity NF3 or F2 is desired and is required in many industries, such as the semiconductor industry. CF4 is practically impossible to separate from NF3. J. Massonne, CHEMIE INGENIEUR TECHNIK, v. 41, N 12, p. 695 (1969). Thus, any CF4 contamination reduces the purity of the resulting NF3 and cannot be readily removed. With respect to F2, although CF4 may be formed, it can be separated and removed, this requires additional and expensive process steps to purify and recover purified F2, however.

Another problem associated with traditional carbon anodes, such as those with carbon coke with pitch binder, is that the anodes need to be extruded, machined, or both to be formed into the anode shape. The anodes, however, may not be of precise shape and design to properly function and may not be reproducible. This results in poor dimensional and mechanical integrity of the anodes.

Another problem is polarization of the anode when the anode becomes passivated and stops functioning. This state is indicated by higher than normal cell voltage and is referred to as “polarization.” When carbon-type anodes are used to manufacture F2 or NF3, polarization is a major cause of cell failure. Extreme situations are sometimes referred to as the “anode effect.” M. Jaccaud, R. Faron, D. Devilliers, and R. Romano, “Fluorine” in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2000. By preventing polarization, cells are allowed to be run longer between rebuilds, thereby cutting the cost of production.

Thus, there remains a need for anodes which form fewer by-products in the electrolytic production of NF3or F2 and thereby produce higher purity NF3or F2, anodes which have better dimensional integrity, and anodes that minimize or reduce polarization.

BRIEF SUMMARY OF THE INVENTION

The present invention provides for the production of nitrogen trifluoride, fluorine, or both using an electrolytic cell where the anode is made from mesophase carbon, such as mesocarbon microbeads. It has been discovered that by using such an anode comprised of mesophase carbon or mesocarbon microbeads the production of by-products, such as CF4, is greatly reduced and minimized; thus, greatly improving the purity of the nitrogen trifluoride or fluorine produced. Additionally, the anodes may be molded, instead of extruded or machined, providing for improved dimensional and mechanical integrity of the anode. In other words, unlike traditional anodes, requiring extruding and/or machining, the shape of the molded anode is set by the mold making it more precise and reproducible providing for a higher quality and better functioning anode. This also allows the geometry of the cell to be more consistent which allows electrolyte circulation and gas bubble disengagement to be more reproducible. The anodes of the present invention also exhibit improved resistance to polarization along with other benefits as described herein.

In one embodiment, the present invention provides a process of producing nitrogen trifluoride or fluorine comprising performing electrolysis of an electrolyte by using an electrolytic anode comprising mesophase carbon, such as mesocarbon microbeads, to obtain nitrogen trifluoride or fluorine. The anode may have an active area up to about 70,000 cm2 or more, for example. The mesocarbon microbeads may be isostatically pressed mesocarbon microbeads. In one embodiment, the anode consists only of molded and self-sintered mesocarbon microbeads and includes no binders or other additives to mold or sinter the anode. The mesocarbon microbeads are also preferably not graphitized. In one embodiment, the anode made from molded mesocarbon microbeads is of high density, e.g., a density of 1.7 g/cm3 or higher. Also, the mesocarbon microbeads may have an average particle size ranging from about 1-5 microns in diameter.

By using mesophase carbon, such as mesocarbon microbeads, as the anode material, it is possible to produce nitrogen trifluoride and fluorine at higher purities, with little or substantially less CF4 compared to traditional extruded carbon anodes. For example, the process may produce less than 100 ppm of CF4, preferably less than 75 ppm of CF4, even more preferably less than 50 ppm of CF4 in pure nitrogen trifluoride or fluorine product gasses. The selectivity in the electrolytic process for producing the nitrogen trifluoride or the fluorine may be 70% or greater, preferably 80% or greater.

Suitable electrolytes may be selected by one of ordinary skill in the art. To produce nitrogen trifluoride, the electrolyte may be a binary electrolyte or a ternary electrolyte, for example. The binary electrolyte may include HF and NH4F or other suitable binary electrolytes known in the art. The ternary electrolyte may include HF, NH4F, and one of KF, LiF, CsF, or the like, or other suitable ternary electrolytes known in the art. For example, a ternary electrolyte composition may comprise about 35-45 wt % HF, about 15-25 wt % NH4F, and about 40-45 wt % KF. To produce fluorine, the electrolyte may be a binary electrolyte, for example, including HF and KF.

The nitrogen trifluoride process and the fluorine process may be conducted under appropriate conditions and operating process parameters, including temperatures and current densities, selected by one of ordinary skill in the art. For example, nitrogen trifluoride may be produced at temperatures of about 120-140° C. and current densities up to about 250 mA/cm2. Fluorine may be produced at temperatures of about 80-90° C. and current densities up to about 250 mA/cm2.

In another embodiment, the present invention provides an electrolytic cell for producing nitrogen trifluoride or fluorine comprising an anode comprising mesophase carbon or mesocarbon microbeads, a cathode, and an electrolyte composition comprising HF, optionally KF, and optionally NH4F. The electrolytic cell is operated to produce nitrogen trifluoride or fluorine at high purities with little or no contamination of CF4. In an exemplary embodiment, the anode consists of self-sintered isostatically pressed mesocarbon microbeads. In another exemplary embodiment, the anode consists of isostatically molded mesocarbon microbeads optionally with a phenolic resin sintering aid.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:

FIG. 1 shows two views of (a) a mesophase carbon sphere and (b) a section through the mesophase sphere shown in (a);

FIG. 2 is a cross-sectional view of one embodiment of an electrolytic cell useful in this invention;

FIG. 3 is a cross-sectional view of another embodiment an electrolytic cell useful in this invention; and

FIG. 4 is an X-ray diffraction pattern for mesophase carbon, which may be suitable to form the anodes in accordance with the present invention.

 DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for the production of high purity nitrogen trifluoride and fluorine using anodes comprising mesophase carbon or mesocarbon microbeads. In particular, a process of producing nitrogen trifluoride or fluorine includes performing electrolysis of an electrolyte by using an electrolytic anode comprising mesophase carbon or mesocarbon microbeads to obtain nitrogen trifluoride or fluorine with high selectivity and with reduced or minimal amounts of CF4.

As used herein, “anode” means the electrochemically-active portion of the electrode where the nitrogen trifluoride or the fluorine is generated in the cell when current is applied to the cell.

As used herein and in the claims, the terms “comprising” and “including” are inclusive or open-ended and do not exclude additional unrecited elements, compositional components, or method steps. Accordingly, the terms “comprising” and “including” encompass the more restrictive terms “consisting essentially of” and “consisting of.” Unless specified otherwise, all values provided herein include up to and including the endpoints given, and the values of the constituents or components of the compositions are expressed in weight percent or % by weight of each ingredient in the composition.

Mesophase Carbon

The anode for producing nitrogen trifluoride or fluorine is comprised of mesophase carbon, such as mesocarbon microbeads (or MCMBs). As used herein, mesophase carbon is the optically anisotropic, graphitizable carbon phase derived from fusible organic compounds. Mesophase carbon can be separated from optically isotropic material in the form of small particles, often referred to as mesocarbon microbeads. Thus, mesophase carbon is intended to encompass carbon having an optically anisotropic phase. In other words, the carbon exhibits optical anisotropy when observed under a polarizing microscope (e.g., an optical microscope with polarized light). FIG. 4 depicts an X-ray diffraction (XRD) pattern for a suitable type of mesophase carbon. The sharp peaks with indication marks at the top, are from ZnO, which is added as an internal calibration standard during the analysis and are not part of the carbon anode. As is evident, there are no graphite peaks in the XRD indicating that well-crystallized graphite is not present. The XRD shows only one broad peak between 25-30° indicating poorly-registered graphene-type planes of carbon. There are no peaks at lower angles indicating that that the mesophase carbon does not contain any well-crystallized or well-formed graphite or other crystal structures.

In an exemplary embodiment, the anode is primarily comprised of mesophase carbon or mesocarbon microbeads. As used herein, “primarily” indicates that that component is present in a greater amount than any other component of the relevant composition, for example, the anode is largely or exclusively mesophase carbon. In other words, the mesophase carbon makes up the majority of the anode more so than any other component. In particular, the anode may comprise at least 40%, at least 50%, at least 60%, at least 75%, at least 90%, at least 95%, or at least 99% mesophase carbon. In an exemplary embodiment, the anode is a substantially pure mesophase carbon. In other words, the anode is mostly or substantially all mesocarbon, as opposed to coke or carbonized pitch or anodes that by chance contain some small amount of mesocarbon. In addition, the anode is mostly mesocarbon, as opposed to binders, fillers, or other aids known in the art.

In one embodiment, the anode for producing nitrogen trifluoride or fluorine is comprised of mesocarbon microbeads. FIG. 1 depicts an example of a mesocarbon microbead including a depiction of (a) a mesophase sphere 100 and (b) a section through the mesophase sphere 100a. The mesophase sphere 100 may include two poles 110, a trace of lamellae direction 120, and an edge of disk 130 of the mesophase sphere 100. Although the mesocarbon microbead shown in FIG. 1 has a lamellar structure, mesocarbon microbeads made by other routes may have other forms. The mesocarbon microbeads may be spherical in shape, or may have elongated or irregular shapes, for example.

The microbeads may have a bead diameter up to about 100 μm (e.g., about 1-100 μm in diameter). In an exemplary embodiment, the mesocarbon microbeads may have an average particle size ranging from about 1-5 microns in diameter. The mesocarbon microbeads may have a high specific surface area ranging from 1,000 to 4,000 m2/g, for example.

Mesophase carbon or mesocarbon microbeads may be produced, for example, by heating a bituminous precursor, such as coal tar, coal tar pitch, a petroleum heavy oil, petroleum pitch, emulsion-polymerized plastics, synthetic pitch, surfactants, or small molecules, to cause the low-molecular material to be converted into a high-molecular material through repeated polycondensation, thus producing small optically anisotropic particles commonly referred to as mesocarbon microbeads. Mesophase carbon can also be produced synthetically from aromatic molecules, such as napthalene. For example, the precursor material may be heated at 200-600° C. (depending on the precursor) to generate a “mesophase” green carbon particle or greenbody. Isotropic, non-mesophase carbon may be optionally removed by solvent extraction to generate a pure mesophase carbon. The green carbon particles are then molded or pressed into the desired shape and then may be baked to sinter and remove volatiles. Various methods of producing mesophase carbon are known, such as those taught and described in world patent WO 2006/109497 and Korean patent 10-2006-0138731.

The mesophase carbon or mesocarbon microbeads may be produced or obtained from any suitable supplier or distributor, such as CR Tech with offices in Korea, MWI with offices in Rochester, N.Y., Graftech International with offices in Parma, Ohio, Y-Carbon with offices in Bristol, Pa., Timcal Graphite and Carbon, Ltd. with offices in Bodio, Switzerland, Qinhuangdao Huarui Coal Chemicals Co., Ltd. with offices in Tangshen, China, MTI Corporation with offices in Richmond, Calif., Linyi Gelon New Battery Materials Co., Ltd with offices in Shandong, China, Osaka Gas Chemicals Co., Ltd with offices in Osaka, Japan, SGL Carbon SE with offices in Wiesbaden, Germany, China Steel Chemical Corporation with offices in Kaohsiung, Taiwan, ROC, SEC Carbon, Ltd. with offices in Amagasaki, Japan, or other suppliers producing mesocarbon microbeads.

The mesocarbon microbeads are used to create the anode. For example, the anode may be molded from the mesocarbon microbeads. The anode may be molded using any suitable molds and molding techniques known in the art. In one embodiment, the mesocarbon microbeads are isostatically pressed (e.g., cold isostatically pressed) to form the anode. Cold isostatic pressing (CIP) includes applying pressure to a mold at substantially room temperature (e.g., using a fluid as a means of applying pressure to the mold at a temperature of about 20-25° C.). The part optionally may be heated in the mold or while under applied pressure to soften the mesocarbon during forming. The part may or may not undergo heating or sintering after being released from the mold. In an exemplary embodiment, the molded mesocarbon microbeads are sintered. Due to the nature of the mesocarbon microbeads, the mesocarbon microbeads may self-sinter at low temperatures (e.g., about 400-600° C.). Self-sintering means the microbeads are pressed and fused together and sintered or heated, but require no binders, resins, fillers, or the like in order to mold and sinter the anode part. In one embodiment, the anode consists only of molded and self-sintered mesocarbon microbeads. The mesocarbon may also be formed using other techniques known in the art, including, but not limited to, isostatic pressing, uniaxial pressing or extrusion.

In another embodiment, at least one stabilization aid or sintering aid, such as phenolic resin, may be added for the purpose of stabilizing the formed mesocarbon without or in addition to oxidation. The stabilization aid may contain oxygen or sulfur. For example, small amounts of stabilization or sintering aids, such as phenolic resins, may be provided to introduce oxygen that serves to cross-link the mesocarbon and imparts resistance to deformation when the form is heated during the carbonization process. The anode may comprise 10% or less, 8% or less, 5% or less, 3% or less, or 1% or less of the stabilization or sintering aid.

The formed mesocarbon greenbody may optionally be oxidized, for example, by exposure to air at elevated temperature, to stabilize the physical form and reduce or eliminate deformation during subsequent heating. In other words, the mesocarbon greenbody may be oxidized by heating in air or an oxygen-containing gas. Suitable conditions for oxidizing mesocarbon forms are known in the art. Various methods to oxidatively stabilize mesophase carbon articles have been described in the literature, such as by F. Fanjul, M. Granda, R. Santamaria, and R. Menendez, “On the chemistry of the oxidative stabilization and carbonization of carbonaceous mesophase.” Fuel. 2002 November; 81(16):2061-70.

The sintering treatment may also be following by a densifying heat treatment, for example, at temperatures of about 500-1300° C. The density or apparent density of the molded anode may be a low density ranging from about 1.60-1.65 g/cm3. In one embodiment, the anode formed from mesocarbon microbeads preferably has a high density of about 1.7 g/cm3 or higher. It may also be preferred that the anodes comprising the mesocarbon microbeads have a low porosity.

The anode may be of any suitable size and shape as would be ordinarily used in electrolytic cells known in the art. For example, the anode blade may range from about 1.5 to 2.5 feet long, about 6-10 inches wide, and about 1-3 inch in thickness. The anode blade may be flat and/or may comprise other surface features, including grooves, ridges, indentations, pyramids, and the like in order to improve the surface area or other features known in the industry for producing anode blades having an extended geometric surface area, for example, as described in U.S. Pat. Nos. 5,290,413 and 4,511,440. The anode may have any suitable active surface area. The shape and physical features of the anode may be formed during the molding process, or they may be machined at any time after forming the greenbody using conventional fabrication techniques.

The mesocarbon microbeads are preferably not graphitized. Graphitized carbon materials, including mesocarbon microbeads and conventional extruded carbon anodes, are typically baked after being molded, extruded, or otherwise formed to remove volatile materials and sinter or consolidate the bulk carbonaceous material. This baking can occur at temperatures up to about 1300° C. At higher temperatures, typically greater than 1500° C. but depending on the type of carbon material, the carbon begins to form larger graphitic domains, and the electrical resistance decreases. This is called graphitization. Many types of carbon articles are partly or fully graphitized. Anodes for NF3 and F2 production are preferably not graphitized because this can lead to poor performance in an electrolytic cell (e.g., graphite is attacked by the fluorine product and also disintegrates due to intercalation by various components in the electrolyte). Accordingly, in another embodiment, an anode is provided comprising mesocarbon microbeads that are ungraphitized (i.e., they have not been graphitized). Such anode is free of graphitized mesocarbon microbeads and may also comprise an anode current connection.

As compared to conventional extruded anodes, the anodes of the present invention including mesocarbon microbeads provide for a number of benefits such as: (1) reduced formation of CF4, thus, providing higher purity NF3 and F2; (2) shorter manufacturing times (for example, without the need for additional separation steps in F2 production); (3) less machining required to create finished anode; (4) lower manufacturing costs for the anode, which translates into lower cost of production for NF3 and F2; (5) improved resistance to polarization; (6) the ability to operate at higher current density; and (7) reduced operating cell voltage.

By using mesocarbon microbeads to form the anode, it is possible to produce nitrogen NF3 and F2 at higher purities, with little or substantially less CF4 compared to traditional extruded anodes. For example, the process may produce less than 100 ppm (by volume), preferably less than 75 ppm (by volume), even more preferably less than 50 ppm (by volume), most preferably less than 25 ppm (by volume) of CF4 in pure NF3 or F2. Thus, 25 ppm is equal to 25 molecules of CF4 per million molecules of NF3 or 25 mL of CF4 per million mL of NF3. The selectivity in the process for producing the NF3 or F2 is also preferably high and may be on the order of 70% or greater, preferably 80% or greater, even more preferably 85% or greater, or even 90% or greater selectivity for NF3 or F2.

Mesocarbon microbead anodes also may exhibit improved wetting characteristics without incorporating a traditional wetting aid. Traditional carbon anodes often develop a low-energy surface as a result of their interactions with the electrolyte and electrochemically-produced species, such as fluorine, during the production of NF3 or F2. The wetting of the anode surface by the electrolyte becomes poor, resulting in a higher cell operating voltage and an increased tendency to polarize. For example, an anode of known geometric surface area can be used to produce NF3 via electrolysis of a ternary KF—HF—NH4F molten salt electrolyte, with composition of 40 wt % HF, 18 wt % NH4F, and 42 wt % KF, at or near 130° C. for at least 150 hours at a current density of 70 mA/cm2, or more preferably 180 mA/cm2. Upon removal from the melt and cooling to about room temperature, the anode can be washed with water without physically abrading the active surface and dried. The surface energy of the active surface can then be determined using known surface tension inks or markers (“dyne pens”).

Mesocarbon anodes were found to show a high surface energy of 65 dyne/cm or greater after more than 150 hours of operation at 70 mA/cm2 across at least 30% of the active surface. In other words, the anodes of the present invention comprising mesophase carbon exhibit retained wettability over an extended duration without needing a wetting agent. In contrast, conventional extruded carbon anodes display surface energies below this value, for example, often below 55 dyne/cm and require a wetting agent to improve the wettability. For example, P. Hough, “Fluorine Production and Use—An Overview” in Electrochemistry in the Preparation of Fluorine and its Compounds, W. Childs and T. Fuchigami, eds., The Electrochemical Society, 1997 suggests incorporating additives into the carbon to enhance wetting. Similarly, U.S. Pat. No. 7,608,235 uses the addition of MgF2 or AlF3 to traditional carbon anodes to improve wetting. Additives such as these, however, add cost to the anodes and may contaminate the electrolyte and the anode manufacturing facility.

It has been surprisingly found, unlike other carbon forms, the substantially pure mesophase carbon or mesocarbon microbead anodes resist the formation of a low-energy surface without the need for such wetting additives, resulting in lower cell operating voltages and the ability to operate at higher current density than conventional carbon anodes. Lower cell voltage results in reduced power consumption during the manufacture of NF3 or F2, while increased current density permits more NF3 or F2 to be produced from a given cell. Moreover, removal of traditional wetting aids from the anode composition reduces cost and contaminations in the manufacturing process.

Other attempts have been made to improve upon the conventional extruded carbon anode material. U.S. Publication No. 2010/0193371 describes the use of a conductive diamond film on glassy carbon. U.S. Publication No. 2010/0252425 suggests the use of mesophase carbon as a filler or binding agent for a more desired carbonaceous component with a specified X-ray diffraction pattern. It also suggests that a high degree of porosity is desirable, and that the specified carbonaceous material needs a conductive diamond coating to perform well as an anode. The present invention does not require expensive diamond films. Instead, the present invention establishes that a properly prepared mesocarbon material is advantageous as an anode material. Furthermore, the mesocarbon materials of the present invention do not exhibit the diffraction pattern specified in U.S. Publication No. 2010/0252425 requiring crystallized, well-formed graphite or other crystal phases. As discussed above, the mesophase material of the present invention contains no definite diffraction peaks other than the broad peak between 25-30° indicating that the mesophase carbon does not contain any well-formed graphite or other crystal structures.

Electrolytic Cell

In one embodiment, the present invention provides an electrolytic cell for producing nitrogen trifluoride or fluorine comprising an anode comprising mesocarbon microbeads, a cathode, and an electrolyte composition. The electrolytic cell is operated to produce nitrogen trifluoride or fluorine. The process of forming nitrogen trifluoride or fluorine includes electrolyzing an electrolyte, for example, using an electrolytic cell. Any suitable electrolytic cell known in the art may be selected by one of ordinary skill in the art.

For example, the electrolytic cell may include a container or housing comprised of walls inert to and for containing the electrolyte. The anode and cathode may be connected to a source of direct current. For example, the electrode may be positioned in the container for immersion into the electrolyte, such that when current is applied the electrodes are made electrochemically anodic and cathodic. A partition wall or separation skirt may be disposed for preventing fluorine or nitrogen trifluoride from being mixed with hydrogen during the electrolysis when the fluorine or nitrogen trifluoride is generated at the anode and hydrogen is generated at the cathode. In general, the partition wall may be disposed vertically.

Any materials may be used to construct the components of the cell so long as the materials are durable when exposed to the corrosive conditions of the cell. Useful materials for the cell body and separation skirt are iron, stainless steel, carbon steel, nickel or a nickel alloy such as MONEL®, TEFLON®, and the like, as known to a person of skill in the art. The material(s) of construction for the cathode is not specifically limited so long as the cathode is made of a material which is useful for that purpose as known to a person of skill in the art, such as nickel, carbon steel, and iron.

FIG. 2 shows a schematic representation of one example of an electrolytic cell apparatus, which may be suitable for the production of nitrogen trifluoride or fluorine in accordance with the present invention. The electrolytic cell apparatus may include an electrolytic cell 25 having an electrolyzer body 26, side faces 51, 52, and an upper lid or covering 28. The cell 25 is partitioned into anode chambers 17 and cathode chambers 18 by vertically disposed gas separation skirt 19 and diaphragm 22. Anodes 20 are disposed in the anode chambers 17, and cathodes 21 are disposed in the cathode chambers 18. The electrolyte 23 is disposed in the electrolytic cell 25 and the level 27 of the electrolyte 23 is the height of the electrolyte 23 above the bottom surface 53 of the electrolytic cell 25. The level of the electrolyte 23 may be determined by a level indicator 31, and the level 27 may be controlled between a high level set point 32 and a low level set point 33, for example. Additionally, the composition of the electrolyte 23 may be sampled by an electrolyte sample port 41.

The electrolytic cell 25 may include feed tubes 12 and 16 for feeding raw materials or the components that make up the electrolyte 23. In general, the feed tubes 12 and 16 are provided in the cathode chamber 18. The anode chamber 17 may have an anode product outlet pipe 11 for withdrawing the product gas mixture (e.g., NF3or F2) from the electrolytic cell 25. The cathode chamber 18 may have a cathode product outlet pipe 13 for withdrawing gas from the electrolytic cell 25. The electrolytic cell 25 can include a temperature detector 30, temperature adjusting means 29, and the like to control the appropriate process parameters during electrolysis.

If desired, the electrolytic apparatus of the present invention may further comprise additional components, such as purge gas pipe connections in the anode and cathode chambers 17, 18. A purge gas source, such as nitrogen for example, may be connected to the anode chamber 17 and/or the cathode chamber 18 (not shown) of the electrolytic cell 25 to provide for a purge of the electrolytic cell 25 for safety reasons, to provide a blow-out means for clogged pipes, or to otherwise provide for the proper functioning of the inlet and outlet tubes and pipes and other instrumentation.

When the cell 25 is operated, the nitrogen trifluoride or fluorine containing gas is generated at the anode 20 and hydrogen is generated at the cathode 21. When used to produce nitrogen trifluoride, the gases generated in the anode chamber 17 may comprise nitrogen trifluoride (NF3), nitrogen (N2) and fluorine (F2), for example. When used to produce fluorine, the gases generated in the anode chamber 17 may comprise fluorine (F2), for example. In addition, HF may optionally be present in the gas leaving both the anode chamber 17 and cathode chamber 18.

FIG. 3 shows a cross sectional view of an electrolytic cell 25 similar to the one shown in FIG. 2 except that the cell 25 shown in FIG. 3 comprises only one anode chamber 17 and one cathode chamber 18. The anode chamber 17 has one anode 20 and the cathode chamber 18 has one cathode 21. Like components in FIGS. 2 and 3 are numbered the same.

The cell 25 shown in FIG. 3 comprises a current controller 39 that supplies current to the anode 20 through anode current connection 14 and to the cathode 21 through cathode current connection 15 at a level that can be increased or decreased within a target range specified by the operator or the control process for the electrolytic cell 25.

Although specific electrolytic cells 25 are described and shown herein, the cell 25 could include any known or hereafter developed cell design. For example, the cell type may include the ICI fluorine cell design described in Fluorine, The First Hundred Years, R. E. Banks, D. W. A. Sharp, and J. C. Tatlows, eds. Elsevier Sequoia, Netherlands, 1986.

The electrolytic cell may be capable of producing NF3, F2, or both, and the processes are substantially similar. The minor differences between the production of NF3 or F2 include the use of different electrolyte solutions and different operating conditions. Otherwise, the two processes are substantially identical. The cells are almost interchangeable, and the anodes used in both are the same mesocarbon material as described herein. As noted above, the undesired by-product CF4 is made in both processes. The only difference is that CF4 and F2 can be separated by distillation, whereas CF4 and NF3 cannot practically be separated. In either case, it is preferable to not produce CF4 because then the separation requires an additional process step.

(a) Production of NF3

Nitrogen trifluoride may be produced by using the electrolytic apparatus of the present invention along with an electrolyte comprised of any known electrolyte that is useful in making nitrogen trifluoride. For example, suitable electrolytes may include ternary electrolytes (e.g., an HF-containing molten salt of ammonium fluoride (NH4F), potassium fluoride (KF), and hydrogen fluoride (HF). In addition, the molten salt electrolyte may also contain other additives such as cesium fluoride, lithium fluoride, and the like. In an exemplary embodiment, the ternary electrolyte composition may comprise about 35-45 wt % HF, about 15-25 wt % NH4F, and about 40-45 wt % KF. The concentrations may be expressed in terms of mol % NF4F and HF ratio. The HF ratio is defined by the equation below:


HF Ratio=(moles of HF titratable to neutral pH)/(NH4F(moles)+KF(moles)).

The HF ratio represents the ratio of the solvent to salt in the electrolyte. In some embodiments with the ternary electrolyte, it may be preferable to operate the electrolytic cell with the NH4F concentration in the range of 14 wt % and 24 wt %, more preferably between 16 wt % and 21 wt %, most preferably between 17.5 wt % and 19.5 wt %; with the HF ratio preferably between 1.3 and 1.7, more preferably between 1.45 and 1.6, most preferably between 1.5 and 1.55. In other embodiments, the preferred concentration range may vary depending on the operating conditions such as applied current and electrolyte temperature. It is desirable to choose the concentration range based on a balance between high efficiency of the electrolytic cell and safe operation.

This invention is not limited to any specific electrolyte composition, and any description herein referring to, for example, the ternary electrolyte is for convenience only. It is understood that any electrolyte useful for making NF3 can be substituted into the description and is included in the invention.

The nitrogen trifluoride electrolytic process may be conducted under appropriate conditions known in the art, including temperatures and current densities. For example, nitrogen trifluoride may be produced at temperatures of about 100-140° C., preferably about 120-130° C. and current densities up to 250 mA/cm2.

(b) Production of F2

In the case of fluorine, a fluorine-producing electrolyte may include a binary electrolyte. For example, the binary electrolyte may include a hydrogen fluoride (HF)-containing molten salt of HF and KF. In addition, the HF-containing molten salt electrolyte may also contain other additives such as ammonium fluoride, cesium fluoride, lithium fluoride, and the like.

The HF ratio may be similar to those described above in order to achieve a balance between high efficiency of the electrolytic cell and safe operation and may be defined as:


HF Ratio=(moles of HF titratable to neutral pH)/(KF(moles)).

This invention is not limited to any specific electrolyte composition, and any description herein referring to, for example, the binary electrolyte is for convenience only. It is understood that any electrolyte useful for making F2 can be substituted into the description and is included in the invention.

The fluorine electrolytic process may be conducted under appropriate conditions known in the art, including temperatures and current densities. For example, fluorine may be produced at temperatures of about 80-90° C. and at a current density up to 250 mA/cm2.

EXAMPLES Example 1 Production of NF3 with Mesocarbon Anode

A ternary electrolyte with composition 40 wt % HF, 19.5 wt % NH4F, and 40.5 w % KF was electrolyzed in a 250 mL laboratory cell to generate NF3. The cathode was carbon steel, and the cell was equipped with a Cu/CuF2 reference electrode. The anode was isostatically-pressed mesocarbon microbeads with an active area of 2.25 cm2. The anode and cathode product gases were kept separated by means of a TEFLON® skirt that extended below the liquid line. The cell was operated at 130° C. The electrolysis was conducted in galvanostatic mode with an applied current density of 70 mA/cm2. The anode gas was analyzed after allowing the cell to reach steady state by means of gas chromatography.

The anode gas contained 71 ppm CF4 on a pure NF3 basis. The selectivity to NF3 was 70.7% defined as:


NF3 Selectivity=(moles of NF3 produced)/(moles of NF3 produced+moles of N2 produced).

After more than 150 hours of operation as an anode at 70 mA/cm2, the anode was removed from the electrolyte, cooled to room temperature, washed with water without abrading the active surface, and was well wet by a 70 dyne/cm ink (a high surface energy ink) over about 50% of the surface, indicating a surface energy above this value. The remaining surface was wet by a 58 dyne/cm ink.

Comparative Example 1 Traditional Anode

The electrolysis described in Example 1 was repeated except that the anode was replaced with a conventional extruded carbon anode. The active anode area was kept at 2.25 cm2. The anode gas contained 341 ppm CF4 (on a pure NF3 basis) and the selectivity to NF3 was 89.9%. The anode was removed after more than 150 hours of operation and subjected to the same test as Example 1, during which the anode was wet by only a 50 dyne/cm ink but not by any higher surface energy inks.

Example 2 Production of NF3 with Mesocarbon Anode

A ternary electrolyte with composition 37.5 wt % HF, 18.3 wt % NH4F, and 44.2 wt % KF was electrolyzed in a 250 mL laboratory cell to generate NF3. The cathode was carbon steel, and the cell was equipped with a Cu/CuF2 reference electrode. The anode was isostatically-pressed mesocarbon microbeads with an active area of 2.25 cm2. The anode and cathode product gases were kept separated by means of a Teflon skirt that extended below the liquid line. The cell was operated at 139° C. The electrolysis was conducted in galvanostatic mode with an applied current density of 100 mA/cm2. The anode gas was analyzed after allowing the cell to reach steady state by means of gas chromatography. The anode gas contained 20 ppm CF4 on a pure NF3 basis. The selectivity to NF3 was 77.6%.

Comparative Example 2 Traditional Anode

The electrolysis described in Example 2 was repeated except that the anode was replaced with a conventional extruded carbon anode. The active anode area was kept at 2.25 cm2. The anode gas contained 70 ppm CF4 (on a pure NF3 basis) and the selectivity to NF3 was 87.0%.

Example 3A Production of NF3 with Low Density Mesocarbon Anode

A ternary electrolyte with HF, NH4F, and KF was electrolyzed in a 250 mL laboratory cell to generate NF3. The cathode was carbon steel, and the cell was equipped with a Cu/CuF2 reference electrode. The anode was low density (1.60 g/cm3) isostatically-pressed mesocarbon microbeads with an active area of 2.25 cm2. The anode and cathode product gases were kept separated by means of a Teflon skirt that extended below the liquid line. The cell was operated at 130° C. The electrolysis was conducted in galvanostatic mode with an applied current density of 70 mA/cm2. The anode gas was analyzed after allowing the cell to reach steady state by means of gas chromatography. The anode gas contained 61 ppm CF4 on a pure NF3 basis. The selectivity to NF3 was 82.3%. The anode was evaluated for surface energy as described in Example 1, and was wet by a 70 dyne/cm ink over at about 40% of the surface, and the remaining areas were wet by a 58 dyne/cm ink.

Example 3B Production of NF3 with High Density Mesocarbon Anode

The electrolysis described in Example 3A was repeated except that the anode was replaced with high density (≧1.70 g/cm3) isostatically-pressed mesocarbon microbeads. The anode gas contained <25 ppm CF4 (on a pure NF3 basis) and the selectivity to NF3 was 84.7%. The anode was evaluated for surface energy as described in Example 1, and was wet by a 70 dyne/cm ink over at about 40% of the surface, and the remaining areas were wet by a 58 dyne/cm ink.

Comparative Example 3A Traditional Anode

The electrolysis described in Example 3A was repeated except that the anode was replaced with a conventional extruded carbon anode. The anode gas contained 341 ppm CF4 (on a pure NF3 basis) and the selectivity to NF3 was 89.9%. The anode was evaluated for surface energy as described in Example 1, and was wet by a 50 dyne/cm ink but not by any higher surface energy inks.

Comparative Example 3B Isostatically Pressed Conventional Anode

The electrolysis described in Example 3A was repeated except that the anode was replaced with an isostatically pressed non-mesocarbon anode. The composition of this anode resembles the conventional extruded carbon (i.e., based on carbonized coke and pitch rather than mesocarbon). The anode gas contained 212 ppm CF4 (on a pure NF3 basis) and the selectivity to NF3 was 88.3%. The anode was evaluated for surface energy as described in Example 1, and was wet by a 48 dyne/cm ink but not by any higher surface energy inks.

These results are summarized in the Tables below where IP=isostatically pressed, MCMB=mesocarbon microbeads, LD=low density, and HD=high density.

TABLE 1 Electrolyte: Ternary (NH4F—KF—HF) Electrolyte Temperature 130° C. Current Density: 70 mA/cm2 Anode Material CF4 Level NF3 Selectivity Traditional Extruded 341 ppm 89.9% Anodes (Comparative Example 1) IP MCMB (Example 1)  71 ppm 70.7%

TABLE 2 Electrolyte: Ternary (NH4F—KF—HF) Electrolyte Temperature 139° C. Current Density: 100 mA/cm2 Anode Material CF4 Level NF3 Selectivity Traditional Extruded 70 ppm 87.0% Anodes (Comparative Example 2) IP MCMB (Example 2) 20 ppm 77.6%

TABLE 3 Electrolyte: Ternary (NH4F—KF—HF) Electrolyte Temperature 130° C. Current Density: 70 mA/cm2 Anode Material CF4 Level NF3 Selectivity Traditional Extruded 341 ppm 89.9% Anodes (Comparative Example 3A) Non-Mesocarbon 212 ppm 88.3% Isostatically Pressed Anode (Comparative Example 3B) LD IP MCMB (Example 3A)  61 ppm 82.3% HD IP MCMB (Example 3B) <25 ppm 84.7%

Example 4 Production of F2 with a Mesocarbon Anode

A binary electrolyte with composition 40 wt % HF and 60 wt % KF was electrolyzed in a 250 mL laboratory cell to generate F2. The cathode was carbon steel, and the cell was equipped with a Cu/CuF2 reference electrode. The anode was isostatically-pressed mesocarbon microbeads with an active area of 2.25 cm2. The anode and cathode product gases were kept separated by means of a TEFLON® skirt that extended below the liquid line. The cell was operated at 88° C. The electrolysis was conducted in galvanostatic mode with an applied current density of 80 mA/cm2. The cell operated stably, discharging fluorine gas, with a cell voltage of 6.4 Volts.

Comparative Example 4 Production of F2 with a Traditional Anode

The electrolysis described in Example 4 was repeated except that the anode was replaced with a conventional extruded carbon anode. The cell operated stably, discharging fluorine gas, but with a higher cell voltage of 7.0 Volts.

Example 5 Production of F2 at High Current Density with a Mesocarbon Anode

The electrolysis described in Example 4 was repeated, except that a smaller cell holding 25 mL of electrolyte was used. The isostatically-pressed mesocarbon anode had an active area of 0.5 cm2. The cell was operated at a current density of 225 mA/cm2. The anode discharged F2 gas, with stable operation at a cell voltage of 6.8 Volts.

Comparative Example 5 Production of F at High Current Density on a Conventional Anode

The electrolysis described in Example 5 was repeated, except that the anode was a conventional extruded carbon anode. The cell was again operated at a current density of 225 mA/cm2. The cell discharged F2 gas, but with an unstable cell voltage of 7.7-8.5 Volts.

Although illustrated and described above with reference to certain specific embodiments and examples, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. It is expressly intended, for example, that all ranges broadly recited in this document include within their scope all narrower ranges which fall within the broader ranges. In addition, features of one embodiment may be incorporated into another embodiment. All publications, patents, and other documents referred to in this document are incorporated by reference herein, as though individually incorporated by reference, in their entirety for all purposes.

Claims

1. A process of producing nitrogen trifluoride or fluorine comprising:

performing electrolysis of an electrolyte by using an electrolytic anode comprising primarily mesophase carbon to obtain nitrogen trifluoride or fluorine.

2. The process according to claim 1, wherein the mesophase carbon comprises mesocarbon microbeads.

3. The process according to claim 2, wherein the mesocarbon microbeads are isostatically pressed mesocarbon microbeads.

4. The process according to claim 2, wherein the mesocarbon microbeads have an average particle size of about 1-5 microns.

5. The process according to claim 2, wherein the mesocarbon microbeads are not graphitized.

6. The process according to claim 1, wherein the anode comprises:

at least 40% mesophase carbon; and
optionally, up to 10% of a stabilizing aid.

7. The process according to claim 1, wherein the anode has a density of 1.7 g/cm3 or higher.

8. The process according to claim 1, wherein the anode consists of molded and self-sintered mesocarbon microbeads and optionally a sintering aid.

9. The process according to claim 1, wherein the anode has an active area up to about 70,000 cm2.

10. The process according to claim 1, wherein the process produces less than 100 Ppm of CF4 in pure nitrogen trifluoride or fluorine.

11. The process according to claim 1, wherein the process produces nitrogen trifluoride or fluorine with a selectivity of 70% or greater.

12. The process according to claim 1, wherein the process produces nitrogen trifluoride or fluorine with a selectivity of 80% or greater.

13. The process according to claim 1, wherein nitrogen trifluoride is produced and the electrolyte is a ternary electrolyte composition comprising HF, NH4F, and KF.

14. The process according to claim 13, wherein the ternary electrolyte composition comprises 35-45 wt % HF, 15-25 wt % NH4F, and 40-45 wt % KF.

15. The process according to claim 1, wherein fluorine is produced and the electrolyte is a binary electrolyte composition comprising HF and KF.

16. The process according to claim 1, wherein nitrogen trifluoride is produced and the electrolyte is electrolyzed at a temperature of about 120-140° C.

17. The process according to claim 1, wherein fluorine is produced and the electrolyte is electrolyzed at a temperature of about 80-90° C.

18. The process according to claim 1, wherein the process is operated at a current density of about 70-250 mA/cm2.

19. The process according to claim 1, wherein nitrogen trifluoride is produced and the process is operated at a current density of about 100-250 mA/cm2.

20. The process according to claim 1, wherein fluorine is produced and the process is operated at a current density of about 120-250 mA/cm2.

21. An electrolytic cell for producing nitrogen trifluoride or fluorine comprising:

an anode comprising mesocarbon microbeads;
a cathode; and
an electrolyte composition comprising HF, optionally KF, and optionally NH4F,
wherein the electrolytic cell is operated to produce nitrogen trifluoride or fluorine.

22. The electrolytic cell according to claim 21, wherein the anode consists of self-sintered isostatically pressed mesocarbon microbeads and optionally a sintering aid.

23. The electrolytic cell according to claim 21, where the anode exhibits retained wettability without the addition of a wetting agent.

24. The electrolytic cell according to claim 21 wherein the mesocarbon microbeads are not graphitized.

25. An anode comprising mesocarbon microbeads that are ungraphitized.

26. The anode of claim 25 further comprising an anode current connection.

Patent History
Publication number: 20140110267
Type: Application
Filed: Apr 9, 2013
Publication Date: Apr 24, 2014
Applicant: Air Products and Chemicals, Inc. (Allentown, PA)
Inventors: James Patrick Nehlsen (East Stroudsburg, PA), Kerry Renard Berger (Lehighton, PA), Reinaldo Mario Machado (Allentown, PA), Kyoung-Ho Choi (Anyang City)
Application Number: 13/859,263
Classifications
Current U.S. Class: Utilizing Fluidized Bed Or Particulate Electrode (205/348); Cells (204/242); Carbon Containing (204/294)
International Classification: C25B 11/02 (20060101); C25B 11/12 (20060101); C25B 1/24 (20060101);