METHOD FOR RECOVERING CASTING REFRACTORY COMPOSITIONS FROM INVESTMENT CASTING SLURRIES AND FROM CORES

Abstract: A slurry containing Y₂O₃ and a binder is gelled and is then ignited to remove residual organic compounds. The resulting product is powdered and then calcined to crystallize the silica binder. The calcined mixture is treated with a heated acid to dissolve Y₂O₃ and undissolved binder residue is then removed by filtration. The solution is diluted and treated with oxalic acid to precipitate yttrium oxalate which is then separated and calcined. Cores are processed in a similar manner, except that gelling and ignition are unnecessary.

Description: The present invention generally relates to the chemical treatment of casting slurries and cores, and more particularly to the recovery of Y₂O₃ from investment casting slurries and investment casting cores. Yttrium oxide (Y₂O₃) is an important and useful metal casting refractory. It is thermodynamically stable in the presence of most reactive engineering metals including titanium, titanium alloys, and columbium (niobium). General information regarding Y₂O₃ metal casting processes is presented in Feagin, R.C., "Casting of Reactive Metals into Ceramic Molds", Report of Sixth World Conference on Investment Casting, pp. 4:01 - 4:13, 1984; and Calvert, E.D., "An Investment Mold for Titanium Casting", United States Department of the Interior (Bureau of Mines), Report of Investigations No 8541, 1981. The use of Y₂O₃ as a refractory material in both investment and core casting processes normally involves the production of a slurry containing both Y₂O₃ and a hydrolyzed binder. An example of such a slurry is described in U.S. Patent No. 4,703,806 of Lassow, et al. Most often, silica binders are used, such as tetraethyl ortho silicate. Once the casting process is completed, the slurry is usually discarded. However, this undesirable for both financial and environmental reasons. Recycling of the slurry to reclaim Y₂O₃ would offer a significant cost savings, and eliminate disposal problems. The present invention involves a method whereby Y₂O₃ can be reclaimed from used casting slurries and cores which contain Y₂O₃ in combination with a binder. The method is highly efficient, and results in the recovery of up to 90% or more of the Y₂O₃ originally contained in the slurry. Also, the recovered Y₂O₃ has a purity level of nearly 100%. This is accomplished using a minimal number of process steps, as described in detail below. Summary of the Invention It is an object of the present invention to provide an efficient method for recovering Y₂O₃ from investment casting slurries and cores. It is another object of the invention to provide a method for recovering Y₂O₃ from investment casting slurries and cores capable of producing a high percentage yield of Y₂O₃. It is another object of the invention to provide a method for recovering Y₂O₃ from investment casting slurries and cores which yields a Y₂O₃ product of nearly 100% purity. It is another object of the invention to provide a method for recovering Y₂O₃ from investment casting slurries and cores which uses a minimal number of process steps. It is a further object of the invention to provide a method for recovering Y₂O₃ from investment casting slurries and cores which enables a nearly complete separation of binder materials from Y₂O₃. It is a still further object of the invention to provide a method for recovering Y₂O₃ from investment casting slurries and cores which is especially well suited to large scale casting operations. In accordance with the foregoing objects, a highly efficient method for recovering Y₂O₃ from casting slurries containing Y₂O₃ in combination with a silica binder is disclosed. The used slurry is initially treated with a gelling agent which causes the slurry to gel. The gelled slurry is then ignited to remove any residual organic compounds. After ignition, the product is crushed and pulverized to form a powder. Thereafter, the powder is calcined at a temperature sufficient to crystallize the silica binder in the powder. The calcined product is subsequently dissolved in heated acid for a selected time period. The resulting solution contains dissolved Y₂O₃ and undissolved silica binder, which is filtered and removed. The Y₂O₃ in the solution is then precipitated, preferably using oxalic acid to produce yttrium oxalate [Y₂(C₂O₄)3·9H₂O]. The Y₂(C₂O₄)₃·9H₂O is ultimately removed from the solution and calcined to produce high purity Y₂O₃. A similar process is disclosed for recovering Y₂O₃ from casting cores. These and other objects, features and advantages of the invention shall be described in greater detail below in the following detailed description of a preferred embodiment, example, and drawing. Brief Description of the Drawings Fig 1. is a schematic representation of the process steps used in conjunction with the present invention to recover Y₂O₃ from an investment casting slurry. Detailed Description of a Preferred Embodiment The present invention includes a highly effective and efficient method for recovering Y₂O₃ from investment casting slurries containg both Y₂O₃ and a binder. The binders typically used in such slurries are those known in the art, including hydrolyzed ethyl silica or colloidal silica binders, and particularly tetraethyl ortho silicate. The latter material is commonly sold under the trade name Ethyl Silicate 40, which contains 40% SiO₂ and is available from the Stauffer Chemical Company of Stamford, Connecticut. Other exemplary binders include colloidal alumina and colloidal zirconia. It is desirable from both a cost and environmental standpoint to reclaim the Y₂O₃ in the slurries for re-use at a later date. To accomplish this, a slurry is first treated to form a solid mass. With reference to Fig. 1, this is preferably accomplished by initially reacting the slurry with a gelling agent 10. The gelling agent 10 raises the slurry pH, causing the binder to gel. Compounds suitable for this purpose include ammonia, dibutylamine, and morpholine (tetrahydro-p-oxazine). Since the amount of gelling agent 10 depends on various factors including slurry composition, quantity and age, visual inspection of the slurry during addition of the gelling agent will be most effective in determining when gelation is occurring, and how much gelling agent to use. However, a sufficient amount of gelling agent should be added in order to cause gelation within about 15 minutes to 1 hour. Upon mixing the gelling agent 10 into the slurry, the gelation reaction occurs spontaneously, and a visual inspection of the slurry will indicate when gelation is completed. As shown in Fig. 1, the gelling agent 10 and slurry are combined in a gelation pan 12 which typically consists of a conventional stainless steel tray or similar apparatus. After the completion of gelation, the gelled slurry is ignited in a furnace or vented ignition chamber 14. The gel will burn because it contains various amounts of volatile organic compounds, including ethanol which is a by-product produced during hydrolysis of the binder. It may be necessary to mix, stir, or puncture the gelled slurry mass to ensure complete ignition. Combustion removes nearly all residual organic materials in the gelled slurry. For example, silica binder materials burn, leaving a silica-containing residue. The gel is allowed to continue burning until all of the combustible organic compounds are gone. At that point, the gel will stop burning. Next, the burned materials is introduced into a jaw crusher 16, followed by the optional use of a hammer mill 20. Use of the hammer mill 20 depends on the volume of material to be treated along with other production parameters. The hammer mill 20 is most likely to be used in large scale processing operations involving substantial quantities of the burned material. Thereafter, the product is pulverized in a disk pulverizer 22. As a result, a powder is produced having a particle size not exceeding about 100 mesh. The process works with coarser particles although it takes longer. The powdered product is then calcined in a calciner 30 at a temperature of between 815.6 and 1093.3°C (1,500 and 2,000°F) for a time period sufficient to crystallize binder residue in the powdered product. Typically not less than about 2.0 hours is required to crystallize a silica binder. Crystallization facilitates subsequent filtration and removal of the binder residue, as described below. Calcination traditionally involves the heating of a material at a high temperature without fusion in order to effect physical and chemical changes in the material. In the present invention, the calciner 30 typically consists of a ceramic container otherwise known as a "sagger" box which is filled with the powder and placed in a kiln for firing. After treatment in the calciner 30, the calcined product is introduced into a dissolver unit 32. A typical dissolver unit 32 would include a jacketed tank in association with an agitator system. Added to the product within the dissolver unit 32 is at least one acid 34 heated to a temperature of about (65.6 - 98.9°C) 150 - 210°F. The acid is selected for its ability to dissolve Y₂O₃ without significantly dissolving binder residue. Exemplary acids, suitable for use with silica binders, include nitric or hydrochloric acid solutions (with nitric being preferred) having a normality ranging from about 6.0 to 15.0. The time required for dissolving the calcined product in the dissolver unit 32 is approximately 4 - 6 hours. Between about 0.227 and 0.680 kg (0.5 and 1.5 pounds) of calcined product are preferably used per 4,55 ℓ (gallon) of acid. In a preferred form of the invention, acidic vapors are routed from the dissolver unit 32 through a fume scrubber 36 of conventional design prior to release. At this stage in the process, a solution is produced which contains dissolved Y₂O₃ and undissolved binder residue. The solution is then cooled to room temperature (approximately 21.1°C (70°F)), followed by removal of the undissolved binder residue which may be accomplished in two ways. Using procedure "A" as shown in Fig. 1, the solution is first introduced into a filter 40 preferably of the standard vacuum pan variety. The collected binder residue is then treated with a basic material 43 (e.g. NaOH, KOH, or Na₂CO₃) in a neutralizer tank 42. The resulting non-toxic, neutralized compound may then be suitably disposed of in a landfill or other waste storage area. In procedure "B", the solution containing undissolved binder residue is introduced into a conventional centrifuge 50, with the collected silica binder being passed into a neutralizer tank 52 of the same type as neutralizer tank 42 wherein binder residue is treated with a basic material 53 comparable to those listed above. The binder-free solution from subprocess "A" or "B" is then passed into at least one polishing filter 60 which consists of any conventional filter unit or filter media capable of removing particulates as small as 0.2 µm. After filtering, the solution is diluted and precipitated in a precipitator tank 62. In the precipitator tank 62, a temperature of about 21.1 - 65.6°C (70°F - 150°F) (preferred = about 46.1°C (115°F)) is maintained simultaneously with the addition of a precipitating agent 64. The precipitating agent 64 most suitable for use in the invention consists of oxalic acid. The oxalic acid may be added in either crystal form (dihydrate) or as a solution. For example, a 9% solution of oxalic acid in water will effectively work in the precipitator tank 62. However, prior to the addition of the precipitating agent 64, deionized water 68 is added to the solution. The deionized water 68 is designed to dilute the solution in the precipitator tank 62. Dilution enables the amount of precipitating agent 64 needed for Y₂O₃ recovery to be decreased. The deionized water should be added to the solution in a water : solution volume ratio ranging from about 1 : 1 to 8 : 1 depending on the concentration of the acid in the solution. For example, 13,6 ℓ (3 gallons) of deionized water would preferably be added to 4,55 ℓ (1 gallon) of solution containing 0,45 kg (1 pound) of Y₂O₃. When oxalic acid is used in the precipitator tank 62, yttrium oxalate [Y₂(C₂O₄)₃·9H₂O] is formed as a solid precipitate. This precipitate is then introduced into a filter 70, preferably of the vacuum pan type as described above, to separate the precipitate from remaining liquid. After filtration, deionized water 74 is passed through the filter cake in an amount sufficient to wash away any soluble impurities in the Y₂(C₂O₄)₃·9H₂O. The collected Y₂(C₂O₄)₃·9H₂O is then dried in a dryer 78 preferably consisting of a conventional convection oven at a temperature of about 65.6°C (150°F), or whatever is necessary to remove any remaining moisture. After drying, the Y₂(C₂O₄)₃·9H₂O is calcined in a calciner 80 of the same type as calciner 30 at a temperature of at least 815.6°C (1500°F) for about 2 hours to remove all of the waters of hydration associated with Y₂(C₂O₄)₃·9H₂O. The resulting product consists of highly pure Y₂O₃ which may be used again in the preparation of investment casting slurries. Use of the foregoing process is beneficial from both a cost and environmental standpoint. It enables complete removal of the binder in order to attain a recovered Y₂O₃ product having nearly 100% (99.9%) purity. In addition, recovery yields of up to 90% Y₂O₃ or more with respect to the amount of Y₂O₃ in the initial slurry are obtainable. EXAMPLE In accordance with the present invention, a slurry sample was prepared, used as a casting refractory, and treated to recover Y₂O₃. The slurry contained the following ingredients: Component Weight Per Cent of Composition Y₂O₃ flour 88.0 silica-containing binder (hydrolyzed ethyl silicate containing 12% SiO₂) 12.0 100.0% The process is not limited to the recovery of Y₂O₃ from this specific type of slurry. To recover Y₂O₃, the slurry was first gelled for about 15 minutes in a stainless steel gelation pan using a solution having a 1 : 1 volume ratio of ethanol and dibutylamine. Approximately 3 ml of this solution was used Per 0.907 kg (2 pounds) of slurry. After gelation, the slurry was ignited in an oven and allowed to burn until the residual organic materials therein were gone (about 4 hours). Next, the solid product was crushed and pulverized to a powder having an approximate particle size of 100 mesh. This product was then calcined at a temperature of 1800°F for two hours in a kiln. 22,7 kg (Fifty pounds) of the calcined product were subsequently mixed with 227,3 ℓ (fifty gallons) of 9N nitric acid at a temperature of 76.7°C (170°F) for about 6 hours to form a solution. The resulting solution was then cooled and vacuum filtered to remove the residual silica binder. After filtration, the solution was diluted with approximately 681.9 L (150 gallons) of deionized water, followed by precipitation of the Y₂O₃ with 45.4 kg (100 pounds) of oxalic acid crystals. The precipitated Y₂(C₂O₄)₃·9H₂O was removed from the solution, dried, and calcined in a kiln at 1,000°C for 2 hours. The resulting Y₂O₃ had a purity of 99.9%. In addition, the percentage yield of Y₂O₃ was approximately 91 - 94%. The foregoing method may also be used to recover Y₂O₃ from solid Y₂O₃ preformed cores produced through injection molding, transfer molding and other conventional techniques. A typical Y₂O₃ core would be formed using the Y₂O₃ flour described above (see Example) in combination with an aqueous or non-aqueous binder known in the art (e.g. tetraethyl ortho silicate) to produce a mixture suitable for molding. The mixture would preferably consist of about 60-90% Y₂O₃ flour and about 10-20% binder. To recover Y₂O₃ from a solid Y₂O₃ core, the core is first removed from the casting by conventional mechanical and/or chemical means. Preferably, water blasting may be used, followed by chemical leaching of the casting materials with an acid such as HNO₃. Removal of the core results in the production of large pieces or chunks or core which are dried in an oven and pulverized using conventional pulverizing machinery to form a powder having a particle size not exceeding 100 mesh. Thereafter, the powder is treated in the same manner as described above with respect to the slurry materials. The key differences in treating used Y₂O₃ core materials as compared with the treatment of Y₂O₃ slurries involves elimination of the gelation and ignition steps. Gelation is not required because the Y₂O₃ core materials are already in solid form. Ignition is not required because firing of the core will have eliminated all residual organic materials which may have been present in the core. In addition, calcination of the powdered product may not be necessary if it is determined that residual organic materials are not present therein. The foregoing procedure may also be used in recovering Y₂O₃ from cores which have not been used in casting processes but are defective for one reason or another and must be destroyed. The process steps are the same as described above for used Y₂O₃ core materials, except that ignition of the core materials prior to pulverization would be done in order to remove residual organic materials if such materials were determined to be present. Finally, if acid leaching is used in the initial stages of core removal, the used acid solution will retain some residual Y₂O₃. To recover the Y₂O₃, the acid solution is passed through the polishing filter 60 in Fig. 1, and added to the solution already in precipitator 62 followed by the treatment procedure described above. The features disclosed in the foregoing description and in the following claims may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.

Claims: A process for the recovery of Y₂O₃ from a used investment casting slurry having Y₂O₃ in combination with a binder, said process comprising: forming a solid mass from said slurry; pulverizing said mass to form a powder; forming a solution from said powder, said solution having yttrium dissolved therein, with said binder remaining undissolved, separating said binder from said solution; and removing yttrium from said solution after said separating of said binder. The process of claim 1 wherein said forming of said solid mass from said slurry comprises the steps of: gelling said slurry through the addition of at least one gelling agent; and igniting said slurry after said gelling. A process for the recovery of Y₂O₃ from solid yttria investment casting core materials having Y₂O₃ in combination with a binder, said process comprising: pulverizing said core materials to form a powder; forming a solution from said powder, said solution having yttrium dissolved therein, with said binder remaining undissolved; separating said binder from said solution; and removing yttrium from said solution after said separating of said binder. The process of any preceding claim wherein said powder is calcined in order to crystallize said binder therein. The process of any preceding claim wherein said solution is formed from said powder by contacting said powder with at least one acid. A process according to claim 2 or according to either claim 4 or claim 5, as appendant to claim 2, wherein the gelling agent comprises at least one of ammonia, dibutylamine and morpholine. A process according to any preceding claim wherein the pulverizing produces a powder having a particle size not exceeding approximately 0.25 mm. A process according to claim 4 or according to anyone of claims 5 to 7, as appendant to claim 4, wherein said powder is calcined at a temperature within the range of approximately 800 to 1100°C. A process according to claim 5 or according to anyone of claims 6 to 8, as appendant to claim 5, wherein the acid includes at least one of nitric acid and hydrochloric acid. A process according to claim 5 or according to anyone of claims 6 to 9, as appendant to claim 5, wherein the acid has a normality within the range of approximately 6 to approximately 15. A process according to claim 5 or according to anyone of claims 6 to 10, as appendant to claim 5, wherein the contacting engulfs the use of 0.06 to 0.18 kg of said powder per liter of said acid. A process according to claim 5 or according to anyone of claims 6 to 11, as appendant to claim 5, wherein the acid is heated to a temperature within the range of approximately 65°C to approximately 100°C. A process according to any preceding claim wherein the Y₂O₃ is dissolved from said powder over a time period of approximately four to six hours. A process according to claim 4 or according to anyone of claims 5 to 13, as appendant to claim 4, wherein the separating of the crystallized binder residue from the solution comprises the steps of cooling the solution and crystallized binder residue, followed by filtration to isolate the crystallized binder residue from the solution. A process according to any preceding claim wherein the step of removing yttrium comprises adding a precipitating agent to the solution in order to form an yttrium precipitate, removing the yttrium precipitate from remaining liquid and calcining the removed yttrium precipitate in order to produce a high purity Y₂O₃ product. A process according to claim 15 wherein the solution is diluted with water prior to adding of the precipitating agent. A process according to claim 15 or claim 16 wherein the precipitating agent comprises oxalic acid. A process according to any preceding claim wherein the binder is a silicate binder.

Inventors: KEMP, WILLIAM W., STURGIS, DAVID H.
Applicants: PRECISION CASTPARTS CORPORATION (AN OREGON CORP.)
Application year: 1989
Main classification: B22C1
IPC codes: B22C1, C22B7, C22B59, C01F17