WO2009108922A1

WO2009108922A1 - Methods for removing precious metal-containing coatings and recovery of precious metals therefrom

Info

Publication number: WO2009108922A1
Application number: PCT/US2009/035691
Authority: WO
Inventors: David C. Fairbourn
Original assignee: Aeromet Technologies Inc
Current assignee: Aeromet Technologies Inc
Priority date: 2008-02-28
Filing date: 2009-03-02
Publication date: 2009-09-03
Anticipated expiration: 2010-08-28

Abstract

A method for recovering a precious metal from a precious metal-containing coating (10) on a metallic substrate (12). The method comprises associating the precious metal-containing coating (10) with an electrolytic bath (22) that contains water, a ligand capable of reacting with the precious metal, and a chelating agent; dc coupling the substrate to a counter electrode (23) fluidly coupled with the electrolytic bath (22), and applying an operating voltage between the precious metal-containing coating (10) and the counter electrode (23) such that the precious metal in the precious metal-containing coating (10) reacts with the ligand and chelating agent. The substrate (12) may be a turbine engine blade or turbine vane segment. An electrolyte may be added to the electrolytic bath (22) to provide the ligand. The electrolyte may be an ionic salt. The chelating agent may be provided by addition of dimethylglyoxime and/or 8-hydroxyquinoline.

Description

METHODS FOR REMOVING PRECIOUS METAL-CONTAINING COATINGS AND RECOVERY OF PRECIOUS METALS THEREFROM

Cross Reference To Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 61/032,102, filed February 28, 2008, which is hereby incorporated by reference herein in its entirety.

Background

[0002] The present invention relates to stripping precious metal-containing coatings from substrates and recovering precious metals from the stripped material and, more particularly, to methods for removing platinum-containing coatings from metallic substrates and recovering the platinum such that it may be recycled.

[0003] Components of turbine engines are often manufactured from nickel-, cobalt-, or iron- based superalloy materials, which are recognized as providing greater shape and strength retention over a wider range of operating temperatures than other candidate materials for these applications. Although superalloy materials exhibit improved mechanical properties at high operating temperatures, they are nonetheless susceptible to high temperature oxidation and hot corrosion. This presents a problem for use of these materials because the efficiency of a turbine engine generally increases with increasing operating temperature. Thus, turbine engine manufacturers continue to demand materials that are capable of withstanding oxidation and corrosion at temperatures experienced due to combustion of fossil fuels. [0004] Generally, gas turbine engines include a compressor for compressing incoming air; a combustor for mixing the compressed air with fuel, such as jet fuel or natural gas and igniting the mixture; and a turbine blade assembly for producing power. In particular, gas turbine engines operate by drawing air into the front of the engine. The air is then compressed, mixed with fuel, and combusted. Hot combustion gases from the combusted mixture pass through a turbine, which causes the turbine to spin and thereby power the compressor. [0005] External surfaces of superalloy turbine engine components, which may experience direct contact with the hot combustion gases, are susceptible to high temperature oxidation and hot corrosion. Consequently, these external surfaces are frequently provided with an overlayer or diffusion coating that protects the underlying superalloy material against high temperature oxidation and hot corrosion. The coatings contain both refractory and precious metals, like platinum, that resist oxidation and hot corrosion due to aggressive gaseous species generated in the combustion of fossil fuels.

[0006] Upon reaching a point, either one that is predetermined according to a set maintenance schedule or one that is due to some unexpected occurrence, the engine is disassembled and the turbine blades and turbine vanes, for example, are removed from the engine for servicing. In a typical process of servicing the turbine blade or vane, the coating is removed or stripped from the underlying superalloy, the stripped blade is inspected, and the blade may be repaired in some fashion, Before the turbine blade or vane is placed back into the engine, a new coating is formed on it.

[0007] As the precious metal coating contains valuable precious metals, it is desirable to reclaim as much of the precious metal as cost-effectively as possible. The reclaimed metal may then be recycled, which helps to lower the maintenance costs of the turbine engine. In addition, it is desirable to recover all of the metals from the waste stream for environmental protection. However, currently available techniques, such as hydroxide precipitation, for removing the refractory metal coatings in a manner to facilitate reclaiming the precious metal therefrom, particularly platinum, are expensive and often do not yield predictable, cost-effective results. One example of an expansive, time consuming, and unpredictable method includes hand wiping with brushes or other tools.

[0008] There is a need, therefore, for methods for removing precious metal-containing coatings such that the precious metals from the coating may be economically recovered and recycled.

Summary of the Invention

[0009] In one embodiment of the present invention, a method for recovering a precious metal from a precious metal-containing coating on a metallic substrate is provided. The method comprises associating the precious metal-containing coating with an electrolytic bath that contains water, a ligand capable of reacting with the precious metal, and a chelating agent; dc coupling the substrate to a counter electrode fluidly coupled with the electrolytic bath; and applying an operating voltage between the precious metal-containing coating and the counter

-?- electrode such that the precious metal in the precious metal-containing coating reacts with the ligand and the chelating agent to form a precious metal-containing complex. In one embodiment, the metallic substrate is a turbine engine blade or turbine vane segment. [0010] In one embodiment, an electrolyte is added to the electrolytic bath to provide the ligand. The electrolyte may be an ionic salt and may further be selected from a group consisting of sodium chloride, ammonium chloride, sodium nitrate, sodium bromide, sodium phosphate, or combinations thereof. However, in another embodiment, the electrolyte may be a mixture of sodium bromide and sodium phosphate. By way of example, and not limitation, the chelating agent may be provided by addition of an organic compound, such as dimethylglyoxime. In addition, 8-hydroxyquinoline may also be added to provide another chelating agent. [0011] In another embodiment, the precious metal-containing coating is a platinum-containing coating that does not contain substantial amounts of aluminum. For these coatings, the ligand may be a chloride ion (Cl^") and the chelating agent may be supplied by an addition of dimethylglyoxime to the electrolytic bath.

[0012] In another embodiment, the method further comprises separating a portion of the precious metal-containing complex from the electrolytic bath such that the concentration of the precious metal-containing complex in the portion is greater than a concentration of the precious metal-containing complex in the electrolytic bath.

[0013] It will be appreciated that in the formation of platinum- aluminide coatings, aluminization occurs over the top of a separate platinum diffusion operation. Where there are defects associated with the aluminide diffusion operation, the aluminum-containing coating must be removed, as welding, brazing, and other repairs are difficult, if not impossible, where significant amounts of aluminum exists at the surface of the substrate. Furthermore, with regard to coatings containing high percentages of platinum, process disruptions can negatively impact the platinum coating requiring that the platinum coating be removed. However, prior art processes cannot remove a coating containing a high percentage of platinum without further damaging the substrate. Ordinarily to remove the platinum-containing coating of only platinum, the platinum- containing coating must be aluminized. Prior art processes may then be utilized to remove the platinum-aluminide coating so that the platinum coating may be reapplied. In either case, aluminiding followed by stripping significantly reduces the useable life of a turbine blade since the allowable tolerances are small, which permits only a limited number of stripping operations before the turbine blade is out of dimensional specification. Once the blade is out of specification, it is considered scrap. According to embodiments of the method of the present invention, the platinum-containing coating following platinum diffusion may be removed without aluminiding. Advantageously, removing all or a substantial portion of the platinum- containing coating before aluminization conserves the metal of the substrate and does not adversely affect the usable life of the turbine blade to the same degree as prior art processes.

Brief Description of the Drawings

[0014] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the brief description given above and a detailed description of the embodiments given below, serve to explain the principles of the embodiments of the invention.

[0015] FIG. 1 is a cross-sectional view of a substrate and a precious metal-containing coating thereon.

[0016] FIG. 2 is a schematic of a representative embodiment of a stripping system for removing a precious metal-containing coating from a substrate.

Detailed Description

[0017] With reference to FIG. 1, in one embodiment, the present invention provides a method for removing or stripping at least a portion of a precious metal-containing coating 10 from a substrate 12 without substantially damaging the substrate 12. Once stripped, an economically effective amount of the precious metal, like platinum (Pt), from the coating can be recovered and recycled. The substrate 12 may then be repaired, recoated, and placed back into service. Alternatively, the substrate 12 may be recycled as revert-quality scrap metal. Therefore, embodiments of the method of the present invention are environmentally friendly while also being cost effective.

[0018] The precious metal-containing coating 10 contains platinum or one or more of the platinum group metals (i.e., ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), or iridium (Ir)). In addition to the platinum group metals, the precious metal-containing coating 10 may contain other metals. By way of example and not limitation, where the precious metal- containing coating 10 is a platinum-containing coating 14, it may contain substantial amounts of platinum, e.g., pure platinum at the surface of the substrate 12, or a platinum alloy, such as platinum alloyed with between about 5 wt. % and about 9 wt. % aluminum, for example, about 7 wt. % aluminum, with the balance being platinum. These platinum-containing coatings are often referred to as Low Cost Bond Coatings (LBCBs) and may be used in propulsion applications. By way of further example, the platinum-containing coating 14 may contain up to about 30 wt.% aluminum, for example between about 18 wt.% and about 30 wt. % aluminum. Platinum content may range from 18 wt. % to 45 wt. % or higher. While platinum and platinum- containing coatings are referenced specifically herein, one skilled in the art will appreciate that a coating containing other platinum group metals may be removed and recycled according to the methods described herein.

[0019] Depending on the technique used to form the coating 10, the coating 10 may also contain other metals though the other metals may not be intentionally added to the precious metal- containing coating 10. These metals may include, for example, metals that are constituents of the substrate 12, such as nickel, chromium, cobalt, molybdenum, tantalum, titanium, iron, manganese, rhenium, aluminum, yttrium, cerium, tungsten, hafnium, zirconium, and niobium. The coating may also contain non-metals including, but not limited to, silicon, carbon, and/or boron.

[0020] As shown in FIG. 1, the platinum-containing coating 14 may be characterized as a layer on the substrate 12. The coating 14 may not, however, have defined boundaries, although well- defined boundaries are shown in FIG. 1. The platinum-containing coating 14 is usually defined by the technique used to form the coating 14 and may also be the result of the conditions under which the coating 14 is exposed during use of the substrate 12. As is known in the art, the platinum-containing coating 14 may be formed by diffusion of platinum into the surface of the substrate 12. Thus, the platinum-containing coating 14 may be a region of the outer surface of the substrate 12 that is modified by in-diffusion of platinum. Accordingly, the coating 14 may have high concentrations of platinum at, or near, the surface of the platinum-containing coating 14 with a gradually decreasing concentration of platinum at an increasing distance from the surface of the coating 14 to a distance where the concentration of platinum is zero. Aluminum in the platinum-containing coating 14 may be added by diffusing aluminum over top of the platinum-containing coating 14, such that the distribution of aluminum may be proportional to the distribution of platinum. [0021] The substrate 12 may be any metallic substrate capable of supporting a platinum- containing coating thereon, In some cases, the substrate 12 may be composed of a nickel-based superalloy, a cobalt-based superalloy, or an iron-based superalloy. In one embodiment, the substrate 12 may be configured as a turbine engine blade, as illustrated in FIG. 2, or a vane segment for use in power generation or in propulsion engines, particularly those used in aerospace applications. Specifically, where the substrate 12 is a turbine engine blade or vane segment, the substrate 12 may be formed from a single-crystal of one of the superalloys, set forth above, that contains rhenium.

[0022] With reference to FIG. 2 in which like reference numerals refer to like features in FIG. 1, in one representative embodiment, a stripping system 18 is utilized to strip the platinum- containing coating 14 from the substrate 12. While it is contemplated that residual platinum may remain in a portion of the substrate 12, in one embodiment, the stripping process substantially removes the platinum leaving a surface having a composition that is representative of the substrate 12. Preferably, the substrate 12 is undamaged by the stripping process. [0023] The stripping system 18 includes a processing tank 20 that contains an electrolytic bath 22, as described below. The system 18 also includes a counter electrode 23 and an external power source 30. The size of the processing tank 20 may be determined by the size of the substrate 12 and the number of substrates being stripped. However, in one embodiment, the processing tank 20 may be sized to contain about 12 gallons of the electrolytic bath 22. For example, the processing tank 20 may measure about 32 inches long by 15 inches high by 6 inches deep. In addition, a reserve tank (not shown) holding additional electrolytic bath solution may be in fluid communication with the processing tank 20. The stripping system 18 may also include a filter 24; a pump section 26 to circulate the electrolytic bath 22, particularly through the filter 24; a sensor 32; a heating element 34; and a thermocouple 36. [0024] With continued reference to FIG. 2, according to one embodiment, the platinum- containing coating 14 is associated or otherwise brought into contact with the electrolytic bath 22. While only a portion of the platinum-containing coating 14 may be wetted by the electrolytic bath 22, such as an airfoil portion as shown, so that the corresponding portion of the platinum-containing coating 14 is removed, it will be appreciated that all of the platinum- containing coating 14 found on the substrate 12 may be brought into contact with the electrolytic bath 22 during any single stripping process. In this case, the substrate 12 may be wholly or partially immersed in the electrolytic bath 22. Where the substrate 12 is immersed, in order to prevent damage to a portion of the substrate or prevent contact with the electrolytic bath 22 so that only selected portions of the coating 14 are removed, a masking agent may be applied to portions of the substrate 12. By way of example and not limitation, masking agents may include Chrome-Peel sold by Masktec, Inc. Company, or mask off tapes manufactured by Sequoia Manufacturing Corporation, Las Vegas, Nevada.

[0025] The electrolytic bath 22 contains water, a ligand, and a chelating agent. While the platinum-containing coating 14 is associated with the electrolytic bath and an operating voltage is applied via the external power source 30, the platinum from the platinum-containing coating 14 reacts with the ligand to form a platinum-containing compound. Although not wishing to be bound by theory, the chelating agent is believed to bind the platinum-containing compound, as is described in more detail below. Consequently, the platinum-containing coating 14 is gradually removed or stripped from the substrate 12. The time to remove the coating 14 may be as high as 20 hours or so depending on the amount of platinum in the coating 14 and other process factors, like temperature. Stripping is continued until the coating 14 is at least partially removed from substrate 12. However, the coating 14 may be substantially completely removed to expose a surface containing a composition that is substantially similar to the bulk composition of the substrate 12.

[0026] In preparing the electrolytic bath 22, the water is of sufficient purity. For instance, the water may be deionized or demineralized to remove impurities such that the water has a sufficiently high resistance. By way of example, the demineralized or deionized water may have an initial resistance of at least about 50 kΩ-cm, and in a further example, the water may have a resistance of about 18 MΩ-cm. One skilled in the art will appreciate that the constituents of the water will change over time due to addition of other materials, from contact with the air above the electrolytic bath 22, and/or from contact with the processing tank 20. Consequently, the exemplary resistances provided represent initial resistance prior to addition of or contact with any other materials.

[0027] An electrolyte, when added to the water, provides at least one ligand that reacts with platinum or one or more of the other metals described above. A ligand, as that term is used herein, is an atom, ion, or molecule that at least bonds with the precious metal. In one embodiment, the electrolyte is an ionic salt or combination of salts. The ionic salt is a specific ionic solid having a significant solubility in the water. The ionic salt dissociates substantially completely when added to the water and provides the ligand to the electrolytic bath 22. At least one of the ligands from the salt forms a platinum-containing compound or a precious metal- containing compound with the precious metal from the coating 10. In one embodiment, the ionic salt is one or more halide salts, which provides a halide ligand, e.g., fluoride (F ), chloride (Cl^"), bromide (Br^"), iodide (I^") and astatine (At^"). The platinum in the platinum-containing coating 14 reacts with the halide ligand to form the platinum-containing compound. Halide salts that provide a chlorine ligand (Cl^") to the electrolytic bath 22 are advantageously used for stripping LCBCs, i.e., those that do not contain appreciable amounts of aluminum. Thus, electrolytic baths 22 that contain Cl^" may be used to strip coatings that do not contain substantial amounts of aluminum, which is advantageous because aluminization of the platinum coatings is not necessary in order to remove the platinum coating. The chlorine reacts with the platinum from the coating 14 to form a platinum chloride compound (e.g., PtCl₂ or PtCl₄). It will be appreciated that the ligand may be provided by an acid having the ligand as its conjugate base. For example, where the ligand is the chloride ion (Cl^"), it may be provided by hydrochloric acid (HCl). However, the pH of the electrolytic bath 22 is normally limited to pHs that do not substantially damage the substrate 12 or breakdown the chelating agent. A pH of about 7 or above are sufficient to permit use of some chelating agents. In another embodiment, the ionic salt is substantially free of chlorides. In other words, ionic salts that contain chlorine are not intentionally added to the water. It is advantageous to use salts that are substantially free of chlorine in the removal of platinum-aluminide coatings set forth above as it is known that chloride ions contribute to pitting of the substrate.

[0028] In another embodiment, the ionic salt is selected from a group consisting of sodium chloride (NaCl), ammonium chloride (NH₃Cl), sodium nitrate (NaNO₃), sodium bromide (NaBr), a sodium salt of phosphate (NaH₂PO₄, Na₂HPO₄, and Na₃PO₄), and combinations thereof. It will be appreciated that potassium salts or other similar salts that provide the ligand may be used in place of, or in combination with other salts.

[0029] The chelating agent may be provided by an organic compound that is added to the water. As set forth above, the chelating agent reacts with the platinum-containing compound or other precious metal-containing compound and forms the precious metal-containing complex. For example, in the case of platinum, the chelating agent binds a platinum-containing compound and forms a platinum-containing chelate. The precious metal-containing complex is insoluble in the electrolytic bath and therefore precipitates making it separable from the electrolytic bath 22, as described below. In this respect, the chelating agent may accelerate the removal of platinum from the electrolytic bath 22. Consequently, removal of the platinum-containing coating 14 from the substrate 12 is accelerated by removing the platinum-containing compound from the electrolytic bath 22. One skilled in the art will observe that removal of the platinum-containing compound from the electrolytic bath 22, by binding it with the chelating agent, shifts the reaction of the ligand with the platinum in the coating 14 (the reactants) towards formation of the platinum-containing compound (i.e., one of the reaction products). Thus accelerating platinum removal from the coating 14.

[0030] One particularly advantageous organic compound is dimethylglyoxime ((CH₃)₂C₂(NOH)₂), the conjugate base of which is the chelating agent. Dimethylglyoxime is a chelant for nickel, platinum, and palladium. Dimethylglyoxime is available commercially from Fisher Scientific and Alfa Aesar. By way of example, when dimethylglyoxime and a halide salt, like NaCl, are added to the water to make the electrolytic bath 22, during a stripping operation the platinum-containing complex formed by reaction of the conjugate base of dimethylglyoxime and the platinum-containing compound is PtCl_x(dimethylglyoximate)_y. PtCl_x (dimethylglyoximate)_y precipitates and may be observed as a red-colored compound during stripping of the platinum-containing coating 14. If the electrolytic bath 22 is not perturbed, the PtCl_x (dimethylglyoximate)_y may settle to the bottom of the processing tank 20, which facilitates its removal.

[0031] By way of additional example, and not limitation, the electrolytic bath 22 may be a mixture of water, dimethylglyoxime, sodium bromide (NaBr), and a sodium salt of phosphate (NaH₂PO₄, Na₂HPO₄, and Na₃PO₄). The electrolytic bath 22 having these constituents may appear yellow. The electrolyte may be mixed from neat materials such that in 1 liter of the electrolytic bath 22, the dimethylglyoxime concentration is about 0.3 moles/liter or less, the NaBr concentration is about 0.3 moles/liter or less, and the sodium phosphate is about 0.3 moles/liter or less. As is described below, the operating temperature of this particular electrolytic bath 22 may be 12O⁰F or less, the voltage may be about 2 volts or less, though the external power source 30 may also provide constant current during the stripping operation, and the electrolytic bath 22 should preferably be agitated, such as by pumping the electrolytic bath 22 through a filter and sparger.

[0032] In one embodiment, the electrolytic bath 22 contains another organic compound that provides another chelating agent. Additional chelating agents may be added to precipitate other metals from the precious metal-containing coating 10. In a representative example, the additional organic compound may be 8-hydroxyquinoline (CgHgNOH), which provides a chelating agent that forms a chelate of aluminum. Thus, when stripping a platinum-containing coating that contains aluminum, adding 8-hydroxyquinoline facilitates separation of platinum- containing complex from other metal complexes, such as an aluminum-containing chelate. The additional organic compounds may be utilized to improve the purity of the platinum-containing complex.

[0033] In yet another embodiment, the electrolytic bath 22 may consist essentially of water, the ionic salt, and the chelating agent. In this respect, "consisting essentially of means that no other compounds are intentionally added to the electrolytic bath 22 during operation. However, impurity content of other compounds from the water, the ionic salt, the chelating agent, the processing tank 20, the substrate 12, or coating 10 may be contemplated. [0034] The electrolytic bath 22 should be stirred or otherwise agitated to provide relative motion between the platinum-containing coating 14 and the electrolytic bath 22. The relative motion may enhance the removal rate of the coating 14. For example, an ultrasonic probe (not shown) may be inserted into the electrolytic bath 22 to produce shock waves that agitate the electrolytic bath 22 at or near the surface of the platinum-containing coating 14. However, ultrasonic frequencies may not be required and may damage the substrate 12. Lower frequencies, for example, frequencies within the audible range of the human ear, may produce sufficient agitation of the electrolytic bath 22 though these frequencies may not damage the substrate 12. Another example of an agitating mechanism is a mechanical agitator, such as, an impeller; a carefully designed sparger; or the pump section 26 that continuously moves the electrolytic bath 22 through the processing and/or reserve tank or other stirring or agitating mechanisms that are known in the art with an exception that compressed air agitation may have adverse consequences.

[0035] Alternatively, or in addition to agitating the electrolytic bath 22, relative motion between the electrolytic bath 22 and the platinum-containing coating 14 may be generated by rotating the substrate 12. In one embodiment, the substrate 12 is rotated between about 4 rpm and about 6 rpm while being at least partially immersed in the electrolytic bath 22. It will be appreciated that the direction of rotation may discourage the electrolytic bath 22 from entering any holes in the substrate 12, such as cooling holes in the case of a turbine engine blade. However, if cooling holes are present, they may be temporarily sealed to prevent the electrolytic bath 22 from entering or contacting portions of the substrate 12 that do not contain a precious metal. [0036] With continued reference to FIG. 2, the counter electrode 23 is associated with the electrolytic bath 22 in the position of an cathode. The electrode 23 may be immersed in the processing tank 20 with the substrate 12 or be formed as a liner (not shown) along an inside surface of the processing tank 20. The counter electrode 23 may be formed from graphite, platinum clad niobium, a niobium alloy, or a platinized titanium anode having a mesh or solid strip configuration. The counter electrode 23 and the substrate 12 are DC coupled by an electrical path 28, which may be a conductive wire, to establish a closed electrical circuit. [0037] The external power source 30 is placed in the closed electrical circuit. The external power source 30 is capable of providing an operating voltage of, for example, at least 1 volt DC between the counter electrode 23 and the substrate 12, where the substrate 12 is in the position of an anode. Thus this arrangement may be referred to as a reverse plating operation. In an alternative embodiment, the external power source 30 is operated at a voltage of between about 2 volts and about 4 volts DC over top, in a further example, the voltage may be around 3.0 volts. However, the external power source 30 may be operated to provide a constant current during at least a portion of the stripping operation. The amperage for a normal sized turbine blade may be between 0.5 A and 0.6 A, and a normal sized vane segments may require more amperage. [0038] The sensor 32 may be used to monitor one or more constituents of the electrolytic bath 22 while stripping the platinum-containing coating 14 from the substrate 12. By way of example, the sensor 32 may be an ion selective electrode (available, for example, from Fisher Scientific) for monitoring nitrate, chloride, or phosphate ion concentrations or another suitable sensor to monitor concentration of the one or more constituents of the electrolytic bath 22. In addition, the sensor 32 may generate an output signal that is related to the concentration of the monitored constituent, which may provide an indication of the completeness of the removal of the platinum-containing coating 14 from the substrate 12 during the stripping process. By way of example only, the sensor 32 may include a pH sensor. In one embodiment, the nominal pH of the electrolytic bath 22 is around 4.0. The pH of the electrolytic bath 22, however, may be adjusted to optimize removal of the coating 14. The output signal may also be utilized to control the addition of one or more of water, electrolyte, or organic compound to the electrolytic bath 22 to maintain the respective concentration of each within a predetermined range. [0039] The thermocouple 36 may also be associated with the electrolytic bath 22 to provide a measurement of the temperature of the electrolytic bath 22. The thermocouple 36 may be one sensor in electrical communication with a temperature control system (not shown) that includes the heating element 34. The control system allows the temperature of the electrolytic bath 22 to be monitored and adjusted or controlled by providing power from a power source (not shown) to the heating element 34. In one embodiment, the temperature of the electrolytic bath 22 is elevated to above room temperature. By way of example, the temperature of the electrolytic bath 22 may be between about 115°F and about 145⁰F (about 46°C to about 63°C), and more specifically, the electrolytic bath 22 may be maintained at about 12O⁰F (about 49⁰C) with a variation of plus or minus 5 ⁰F. However, the electrolytic bath 22 may be warmed to higher temperatures, if desired, to accelerate the stripping process. Or, in one embodiment, the electrolytic bath 22 is at about room temperature and may be uncontrolled such that the temperature may vary according to the ambient environment.

[0040] In one specific embodiment, the platinum-containing coating 14 may be pretreated or prepared prior to being associated with the electrolytic bath 22. Pretreatment may include roughening the exposed surface of the platinum-containing coating 14 by grit blasting, such as, blasting the surface of the platinum-containing coating 14 with an abrasive material (e.g., 220 grit aluminum oxide). The substrate 12 may also be post treated by grit blasting after stripping the platinum-containing coating 14 from the substrate 12. It will be appreciated that treatments via grit blasting or abrading the surface of the platinum-containing coating 14 with a tool, for instance, a stainless steel brush, may be alternated between multiple stripping processes. For example, the platinum-containing coating 14 may be stripped, blasted, stripped, etc. to completely remove the platinum-containing coating 14.

[0041] According to the illustrative system described above, once the platinum-containing coating 14 is associated with the electrolytic bath 22, the external power source 30 is controlled to provide an external voltage sufficient to facilitate removal the platinum-containing coating 14 in the presence of the ligand. When the voltage is applied between the electrode 23 and the substrate 12, platinum from the platinum-containing coating 14 reacts with the ligand in the electrolytic bath 22. Subsequent formation of the platinum-containing complex facilitates the removal of the coating 14.

[0042] In one embodiment, the platinum-containing complex in the electrolytic bath 22 is recovered by separating the platinum-containing complex from the electrolytic bath 22. Recovery may occur subsequent or contemporaneous with removing the platinum-containing coating 14 from the substrate 12. For example, additional electrolytic bath solution may be continuously added to the electrolytic bath 22 which causes overflow of the processing tank 20. The overflow solution may be further processed as described below.

[0043] Separation methods may include filtering the platinum-containing complexes from the electrolytic bath 22 by pumping the electrolytic bath 22 through the filter 24. In this case, the platinum-containing complex is concentrated in the filter 24. Specifically, the electrolytic bath 22 may be pumped through a Whatman 40 or Whatman 25 medium fast paper filter to capture the platinum-containing complex in the filter 24. A vacuum filtration unit, as is known in the art, may also be used to enhance filtration of the electrolytic bath 22 from the platinum- containing complex. However, other filtration or separation methods are known in the art, and may include decanting or selectively extracting platinum-containing complexes where they tend to pool or concentrate. This may include extraction from either the bottom of the processing tank 20, if the platinum-containing complex settles, or the top of the processing tank 20, if the platinum-containing complex floats. Selective extraction of portions of the bath may further facilitate filtration or may be the sole means for separation of the platinum-containing complexes from the electrolytic bath 22.

[0044] Following separation of the platinum-containing complex from the electrolytic bath 22, the platinum-containing complex may be further processed. In one embodiment the platinum- containing complex may be heated in the presence of oxygen to evolve any organic portion of the platinum-containing complex as carbon dioxide or other carbon-containing or nitrogen- containing gases to leave a platinum-containing residue. By way of example, if a filter is used to capture the platinum-containing complex, the filter may be placed in a furnace or burned in such a manner to remove the filter and any organics. In particular, the filter may be placed in an alumina crucible and then placed in an enclosed retort furnace. The furnace may be heated to a temperature of about 1300°F and held at that temperature for 30 minutes. The crucible may then contain the platinum-containing residue. The platinum-containing residue may then be further concentrated, a described below, or recycled.

[0045] In an alternative embodiment, a chemical separation technique may be used to separate the platinum-containing complex from the electrolytic bath 22. For example, chloroform may be added to the electrolytic bath 22. Where the platinum-containing complex has a preference for chloroform over the environment in the electrolytic bath 22, the platinum-containing complex may preferentially segregate into the chloroform. The platinum-containing complex and the chloroform may be removed from the electrolyte bath 22 by decanting one or more portions containing the chloroform. Once the solution of the chloroform and platinum-containing complex is separated from the electrolytic bath 22, the solution may be heated to evaporate the chloroform, Additional heating may be used to remove any organics and to form the platinum- containing residue, as described above. While chloroform is provided in an exemplary chemical separation embodiment, other ion chromatography processes may be utilized depending on the platinum-containing complexes formed.

[0046] Whether obtained through physical or chemical separation, the platinum-containing residue may be further concentrated or purified. For example, the platinum-containing residue may be further purified by mixing the residue with nitric acid while heating. In a representative embodiment, the platinum-containing residue is mixed with a 10% nitric acid solution by volume, The mixture is heated to a temperature of about 140°F and held at that temperature for 30 minutes with constant agitation, Nitric acid dissolves any nickel-containing residues mixed with the platinum-containing residue. Nitric acid, however, does not dissolve platinum. As is known in the art, turbine engine blades may contain nickel such that during the stripping process, as described above, nickel-containing complexes are formed, which contaminate the platinum- containing complex. Thus, addition of nitric acid dissolves much of the nickel contamination. To neutralize any residual acid, ammonia may be added while heating. Subsequent filtration of the platinum-containing residue from the dissolved nickel may be more effective. The platinum- containing residue is collected, dried, and the dried and filtered residue may be recycled. [0047] While the present invention has been illustrated by the description of an embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described, Accordingly, departures may be made from such details without departing from the scope or spirit of applicant's general inventive concept.

Claims

What is claimed is:

1. A method for recovering a precious metal from a precious metal-containing coating on a metallic substrate, the method comprising: associating the precious metal-containing coating with an electrolytic bath that comprises water, a ligand capable of reacting with the precious metal, and a chelating agent: dc coupling the substrate to a counter electrode fluidly coupled with the electrolytic bath; and applying an operating voltage between the precious metal-containing coating and the counter electrode such that the precious metal in the precious metal-containing coating reacts with the ligand and the chelating agent to form a precious metal-containing complex.

2. The method of claim 1 wherein the precious metal-containing coating is a platinum- containing coating that does not contain substantial amounts of aluminum.

3. The method of claim 2 wherein the ligand is a chloride ion and the chelating agent is a conjugate base of dimethylglyoxime.

4. The method of claim 1 further comprising: adding an electrolyte to the electrolytic bath to provide the ligand.

5. The method of claim 4 wherein the electrolyte is an ionic salt.

6. The method of claim 5 wherein the ionic salt is selected from a group consisting of sodium chloride, ammonium chloride, sodium nitrate, sodium bromide, sodium phosphate, and combinations thereof.

7. The method of claim 4 wherein the electrolyte is a mixture of sodium bromide and sodium phosphate.

8. The method of claim 1 further comprising: adding dimethylglyoxime to the electrolytic bath to provide the chelating agent.

9. The method of claim 1 further comprising: adding 8-hydroxyquinoline to the electrolytic bath to provide a second chelating agent that reacts with a metal other than the precious metal.

10. The method of claim 1 wherein applying the operating voltage includes applying a voltage of at least 1.0 volt between the counter electrode and the substrate.

11. The method of claim 1 wherein the substrate is a turbine blade or a turbine vane segment.

12. The method of claim 1 further comprising: separating a portion of the precious metal-containing complex from the electrolytic bath such that the concentration of the precious metal-containing complex in the portion is greater than a concentration of the precious metal-containing complex in the electrolytic bath.

13. The method of claim 12 wherein separating includes filtering the electrolytic bath to remove the precious metal-containing complex from the electrolytic bath.

14. The method of claim 12 wherein separating includes: adding chloroform to the electrolytic bath such that the precious metal-containing complex concentrates in the chloroform; separating a solution containing the chloroform and the precious metal-containing coating from the electrolytic bath; and heating the solution containing the chloroform and the precious metal-containing complex to volatilize the chloroform.

15. The method of claim 12 further comprising: heating the precious metal-containing complex to volatilize a portion of the precious metal-containing complex and to form a precious metal-containing residue.

16. The method of claim 15 further comprising: mixing an acid with the precious metal-containing residue.

17. The method of claim 16 further comprising: mixing ammonia to the mixture of the acid and the precious metal-containing residue to neutralize any residual acid; filtering the neutralized mixture; and drying the filtered, neutralized mixture.

18. A composition for an electrolytic bath used to remove at least a portion of a precious metal-containing coating from a metallic substrate, the composition comprising: water; a ligand capable of reacting with a precious metal in the precious metal-containing coating to form a precious metal-containing compound, the ligand provided by addition of an ionic salt to the water; and a chelating agent capable of binding the precious metal-containing compound, the chelating agent provided by addition of an organic compound to the water.

19. The composition of claim 18 wherein the ligand is selected from the group consisting of fluoride, chloride, bromide, iodide, astatine, or combinations thereof.

20. The composition of claim 18 wherein the organic compound is dimethlyglyoxime.