Patent Application
20140110269
The present invention relates generally to the electrolytic production of nitrogen trifluoride and fluorine, and in particular, to the use of anodes made from parallel ordered anisotropic carbon, including needle coke and mesophase carbon, that exhibit certain physical properties 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. Traditional anodes are made from isotropic coke and exhibit high macroporosity as a result of the large particle size used, often greater than 100 microns. 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 NF3 or 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 NF3 or F2 and thereby produce higher purity NF3 or F2, anodes which have better dimensional integrity, and anodes that minimize or reduce polarization.
The present invention provides for the production of nitrogen trifluoride, fluorine, or both using an electrolytic cell where the anode is made from parallel ordered anisotropic carbon or coke, including needle coke, lenticular coke, mesophase carbon, and incipient mesophase carbon, including mesocarbon microbeads, all as defined by J. Speight, Handbook of Petroleum Product Analysis, John Wiley & Sons, 2002. Carbons of this type exhibit substantially parallel ordered or layered domains of various sizes, in contrast to randomly layered or concentrically layered (onion-like) carbons, or amorphous or glass-like carbons. It has been discovered that by using such an anode comprised of parallel ordered anisotropic carbon, lower cell voltage and higher current density are achievable than with conventional anodes made from sponge, shot, concentrically layered, amorphous, or any isotropic cokes. Furthermore, it has been discovered that such anodes exhibiting certain physical characteristics offer reduced and minimized production of by-products, such as CF4; thus, greatly improving the purity of the nitrogen trifluoride or fluorine produced. The physical properties offering this advantage include small particle size, low open porosity, high density, and/or a reduced quantity of microporous carbon such as that arising from the carbonization of oxygen-containing carbon precursors or the use of porous cokes like sponge coke. 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.
Carbon articles of the type described in this document are typically characterized using apparent density, among other physical properties. Because these materials consist of aggregated and bound or sintered particles, they are porous. The extent of this porosity, measured as a percentage of total article volume, as well as features such as average pore diameter, pore size distribution, and whether the pores are interconnected (open) or isolated (closed), are all functions of the processing conditions and techniques. Pure, crystalline graphite represents the highest density packing for sp2-hybridized carbon, such as that found in the carbon articles described herein. The presence of pores reduces the apparent density from this theoretical maximum of about 2.23 g/cm3. Most carbon and graphite articles, including conventional and the disclosed carbon anodes for the production of NF3 and F2, have an apparent density of about 1.5 g/cm3 to 1.9 g/cm3.
Although no ASTM standard test for porosity of carbon materials currently exists, several techniques known in the art are applied routinely. For example, mercury porosimetry and gas adsorption data can be analyzed using the Washburn equation and Brunauer-Emmett-Teller theory, respectively. Most synthetic carbon and graphite articles made via bound or sintered carbon powders exhibit open porosity of about 8% to about 20% using mercury porosimetry. It has been reported in the literature that higher density for a given carbon material, which necessarily indicates reduced total porosity, also corresponds to lower open porosity and fewer small pores (Properties and Characteristics of Graphite, R. G. Sheppard, Dwayne Morgan, D. M. Mathes, D. J. Bray, eds., POCO Graphite, Inc., 2002).
Without being bound by a particular theory, reduced porosity is believed to lead to lower surface area that is accessible by the liquid electrolyte. The formation of CF4 is believed to be minimized by reducing this accessible surface area and eliminating the trapping of electrolyte in small pores.
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 parallel ordered anisotropic carbon (e.g., mesophase carbon, such as mesocarbon microbeads), to obtain nitrogen trifluoride or fluorine. The anode may have an active geometric surface area up to about 70,000 cm2 or more, for example. In the case of mesocarbon microbeads, 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, with porosity less than about 20%, or more preferably less than about 15%. Also, the mesocarbon microbeads may have an average particle size ranging from about 1-5 microns in diameter.
In another 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 needle coke. The needle coke may be bound together with a suitable binder that may contain low amounts of oxygen in the precursor, such as highly aromatic pitch binder or mesophase-forming pitch. Furthermore, the anode may be comprised of particles less than 50 microns, or more preferably less than 20 microns, may have a density greater than 1.6 g/cm3, or more preferably greater than 1.7 g/cm3 and most preferably greater than 1.8 g/cm3. The anode is preferably baked to a temperature not higher than about 1600° C. The needle coke in this embodiment could also be replaced by other parallel ordered cokes, such as lenticular coke. In a further embodiment, the coke or binder could be replaced by mesophase, incipient mesophase coke or pitch, or an amorphous but mesophase-forming pitch.
By using carbon conforming to the invention 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 350 mA/cm2.
In another embodiment, the present invention provides an electrolytic cell for producing nitrogen trifluoride or fluorine comprising an anode comprising parallel ordered anisotropic carbon, 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. In another exemplary embodiment, the anode consists of needle coke bound with a highly aromatic binder and exhibiting low porosity (e.g. less than 20% porosity) and density above 1.7 g/cm3, formed by isostatic molding.
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:
The present invention provides for the production of high purity nitrogen trifluoride and fluorine using anodes comprising parallel ordered anisotropic carbon. In particular, a process of producing nitrogen trifluoride or fluorine includes performing electrolysis of an electrolyte by using an electrolytic anode comprising mesophase carbon, mesocarbon microbeads, needle coke, or other parallel ordered anisotropic carbon 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.
Parallel Ordered Anisotropic Carbon
The present invention provides for the production of high purity nitrogen trifluoride and fluorine using anodes comprising parallel ordered anisotropic carbon or coke. The anode for producing nitrogen trifluoride or fluorine is comprised of parallel ordered anisotropic carbon, such as mesocarbon microbeads (or MCMBs) or needle coke. As used herein, “parallel ordered anisotropic carbon” or “parallel ordered anisotropic coke” is intended to encompass a class of carbon, which exhibits substantially parallel ordered or layered domains, in contrast to randomly layered carbons, concentrically layered (onion-like) carbons, amorphous carbons, or disordered glass-like carbons. Parallel ordered anisotropic carbon or coke may include needle coke, lenticular coke, mesophase carbon, incipient mesophase carbon, and mesocarbon microbeads, for example, as defined by J. Speight, Handbook of Petroleum Product Analysis, John Wiley & Sons, 2002.
As used herein, “mesophase carbon” or “mesocarbon” is the optically anisotropic, graphitizable carbon phase derived from fusible organic compounds. Needle coke and related parallel ordered anisotropic carbons are sometimes considered to be a “mesophase” of carbon, though often the two terms refer to different physical forms of carbon that exhibit similar microstructural properties. 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).
As used herein, “needle coke” is the optically anisotropic, acicular coke comprising ordered, parallel layers or carbon, or any carbon conforming to the definition of needle coke proposed in “Recommended Terminology for the Description of Carbon as a Solid,” IUPAC, Pure & Appl. Chem., Vol. 67, No. 3, pp. 473-506, 1995. It is understood that physical alteration of needle coke by grinding or size reduction does not remove it from this definition, even to a particle size near 1 micron.
In an exemplary embodiment, the anode is primarily comprised of parallel ordered anisotropic carbon. 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 parallel ordered anisotropic carbon. In other words, the parallel ordered anisotropic 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% parallel ordered anisotropic carbon. In an exemplary embodiment, the anode is a substantially pure parallel ordered anisotropic carbon. In other words, the anode is mostly or substantially all parallel ordered anisotropic carbon, as opposed to coke or carbonized pitch or anodes that by chance contain some small amount of parallel ordered anisotropic carbon. In addition, the anode is mostly parallel ordered anisotropic carbon, as opposed to binders, fillers, or other aids known in the art.
In one embodiment, the anode is primarily comprised of mesophase carbon, such as mesocarbon microbeads. 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.
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. Similarly, in another embodiment the anode for producing nitrogen trifluoride or fluorine is comprised of needle coke and a binder with a maximum particle size of 20 microns, or more preferably a particle size less than 10 microns, a density greater than 1.6 g/cm3 or more preferably greater than 1.7 g/cm3 and less than 15% porosity, or more preferably less than 10% porosity. The needle coke and binder may be molded into the desired form.
In another embodiment, the anode is primarily composed of parallel ordered anisotropic coke, such as needle coke, bound with a suitable binding agent that results in a minimal amount of porosity in the final article. Aromatic pitch, mesophase pitch, coal tar pitch, and the like are preferred binders, while oxygen-containing binders such as polyfurfural alcohol or phenolic resin are less preferred. In all cases, the anode is not graphitized.
Parallel ordered anisotropic cokes, including needle coke and 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, decant oil, pyrolysis residue, 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. Parallel ordered anisotropic cokes and 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 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.
Needle coke, mesophase carbon, mesocarbon microbeads, or other forms of parallel layered anisotropic coke 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, Asbury Carbons, Inc. with offices in Asbury, N.J., 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 anisotropic carbon.
The parallel ordered anisotropic carbon is used to create the anode. For example, the anode may be molded from a blend of the parallel ordered anisotropic coke and suitable pitch binder or from mesocarbon microbeads. The anode may be molded using any suitable molds and molding techniques known in the art. In one embodiment, the coke/pitch blends or 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 pitch or 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 form is sintered. If a binder such as pitch is used, the binder will soften and then melt, filling the intersticies between coke particles before carbonizing to hold the finished body together. In contrast, 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 parallel ordered anisotropic carbon or mesocarbons 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 parallel ordered anisotropic carbon 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 parallel ordered anisotropic carbon 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.
In a preferred embodiment, the binder, pore filler, or sintering aids comprise an aromatic pitch, aromatic synthetic pitch, or other carbon precursors known to produce graphitizing carbons when heated.
The formed 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 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 500-1500° 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 parallel ordered anisotropic carbon (e.g., mesocarbon microbeads) preferably has a high density of about 1.7 g/cm3 or higher. It may also be preferred that the anodes comprising parallel ordered anisotropic carbon have a low porosity (e.g., less than 20% porosity, preferably, less than 15% porosity, and more preferably, less than 10% 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 anodes of the present invention are preferably not graphitized. Graphitized carbon materials, including needle coke, 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).
As compared to conventional extruded anodes, the anodes of the present invention including parallel ordered anisotropic cokes, including needle coke and 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 parallel ordered anisotropic carbon 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), and 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.
Carbon articles made from parallel ordered anisotropic cokes, including needle coke and 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”).
Anodes of the present invention 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 parallel ordered anisotropic 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 wetability. 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 Mg F2 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 parallel ordered anisotropic carbon, such as mesocarbon microbeads, which form the 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 parallel ordered anisotropic carbon material is advantageous as an anode material. The parallel ordered anisotropic carbon may exhibit a low porosity, for example, less than 15% or less than 10% porosity. Furthermore, the parallel ordered anisotropic carbon 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 parallel ordered anisotropic carbon material of the present invention contains no definite diffraction peaks other than the broad peak between 25-30° indicating that the parallel ordered anisotropic carbon does not contain any well-formed graphite or other crystal structures.
In one embodiment, the present invention provides an electrolytic cell for producing nitrogen trifluoride or fluorine comprising an anode comprising parallel ordered anisotropic carbon (e.g., 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.
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., NF3 or 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.
The cell 25 shown in
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 parallel ordered anisotropic carbon-based anode 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.
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.
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.
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%.
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%.
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.
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.
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.
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.
The electrolysis described in Example 1 was repeated using an anode comprised primarily of needle coke with a pitch-based binder. The anode had an apparent density of 1.75 g/cm3 and total porosity of 15%. The anode was not graphitized. The cell was operated at a current density of 70 mA/cm2. The cell temperature was 130° C. The anode potential during the test was 5.15 V vs. Cu/CuF2 reference, the selectivity to NF3 was 88%, and the CF4 content of the NF3 product was 30 ppm.
The electrolysis described in Example 4 was repeated at a current density of 178 mA/cm2. The cell temperature was 140° C. The anode potential during the test was 5.47 V vs. Cu/CuF2 reference, the selectivity to NF3 was 88%, and the CF4 content of the NF3 product was 20 ppm.
These results are summarized in the Tables below where IP=isostatically pressed, MCMB=mesocarbon microbeads, LD=low density, and HD=high density.
The electrolysis of Example 4B was repeated using an anode of the same composition as in that example, but having been graphitized by heating to temperatures above 2000° C. The selectivity and CF4 levels were identical, but the anode operated unstably with an anode potential varying between 6 and 7 V.
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.
The electrolysis described in Example 5 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.
The electrolysis described in Example 5 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.
The electrolysis described in Example 6 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.
This application is a continuation-in-part of U.S. patent application Ser. No. 13/859,263, filed on Apr. 9, 2013, which 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 Oct. 19, 2012, and U.S. patent application Ser. No. 61/790,810, filed on Mar. 15, 2013. The content of each priority application is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61716259 | Oct 2012 | US | |
| 61790810 | Mar 2013 | US |
