Title of Invention	METHOD OF RECOVERING RARE EARTH ELEMENT FROM COMPOSITION CONTAINING RARE EARTH FLUORIDE
Abstract	The object of the present invention is to provide a method of recovering a rare earth element from a composition containing a rare earth fluoride, involving the steps of: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an insoluble content from the solution obtained at step (2) to obtain a rare earth-containing solution; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3).

Title of Invention

METHOD OF RECOVERING RARE EARTH ELEMENT FROM COMPOSITION CONTAINING RARE EARTH FLUORIDE

Abstract

The object of the present invention is to provide a method of recovering a rare earth element from a composition containing a rare earth fluoride, involving the steps of: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an insoluble content from the solution obtained at step (2) to obtain a rare earth-containing solution; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3).

Full Text	DESCRIPTION TECHNICAL FIELD The present invention relates to a method of recovering rare earth elements from a composition containing rare earth fluorides. More specifically, the present invention relates to a method of recovering with high recovery rare earth elements from a composition containing rare earth fluorides, such as a rare earth abrasive containing a rare earth oxyfluoride or the like, that has already been used for polishing glass or the like, and also relates to a method of reproducing a rare earth abrasive. This application claims the priority of Japanese Patent Application No. 2006-067725, filed March 13, 2006, the contents of which are incorporated herein by reference. BACKGROUND ART In recent years, rare earth abrasives that contain fine powder of oxides of rare earth elements such as cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd) have been applied to various fields of technology. In particular, a rare earth abrasive disclosed in Patent Document 1 that contains rare earth fluorides is compatible to precise polishing processes, and has been often applied to polishing of glass materials or quartz crystals. For example, such a rare earth abrasive has been used for polishing, among others, glass substrates that are used for optical or magnetic disks, or used for liquid crystal displays such as a thin film transistor-type (TFT-type) liquid crystal display (LCD) and a twisted nematic-type (TN-type) liquid crystal display (LCD); a color filter for a liquid crystal television (TV); a cathode ray tube (CRT) for TV or the like; spectacle lenses; optical lenses; glass substrates for a LSI photomask; wired plate glass; substrates for a crystal oscillator. Ores that are raw materials for rare earth abrasives, for "example, include bastnaesite, monazite or the like. These ores has been mainly mined in China or in the U.S.A. In order to produce a rare earth oxide from ores such as a bastnaesite and monazite, the following treatments (1) to (9) are successively conducted in general. (1) Sulfuric acid is added little by little to the ore in a rotary kiln, followed by mixing and drying. (2) The ore is baked at 500°C to 600°C. (3) Rare earth-containing contents are leached from the baked ore into water. (4) The leaching solution is concentrated with a thickener, and impurities such as BaSO4, CaSO4, and SiO2 are removed by way of filtration whereby R2(SO4)3 is obtained. The "R" of the chemical formula refers to a rare earth element. (5) SOdium sulfate is added to the obtained R2(SO4)3, and Fe, Ca, phosphoric acid or the like are separated whereby a sulfate composite salt of the rare earth element is obtained. (6) SOdium hydroxide is added to the sulfate composite salt of the rare earth element whereby a hydroxide of the rare earth element is precipitated. (7) Hydrochloric acid is added to the hydroxide of the rare earth element, thereby producing a chloride of the rare earth element. (8) Ammonium hydrogencarbonate is added to the chloride of the rare earth element, thereby producing a rare earth carbonate. (9) The rare earth carbonate is baked, thereby producing a rare earth oxide. However, the reserves of the ores that are raw materials for producing the rare earth abrasives are limited. Furthermore, in recent years, there has been a problem in which the raw material ores are quite scarce as the demand for the rare earth abrasives increases. Therefore, various methods of recovering the rare earth elements from the waste fluid of the rare earth abrasives have been developed in order to recycle them as abrasives. For example, Patent Document 2 discloses a method of recovering rare earth elements from a rare earth abrasive that has been already used. The disclosed method includes the steps of: dissolving rare earth elements included in the used rare earth abrasive by treating the abrasive with an aqueous mineral acid solution; collecting the rare earth element-dissolving solution by separating it from the insoluble contents by way of solid-liquid separation; precipitating the rare earth elements as oxalates by adding oxalic acid and an alkali solution of oxalic acid, such that the pH of the collected solution becomes 5 or less; recovering the oxalates of the rare earth elements by way of solid-liquid separation; treating the oxalates of the rare earth elements with an aqueous alkali hydroxide solution, thereby converting them to hydroxides of the rare earth elements, and producing an alkali solution of oxalic acid; and collecting separately the hydroxides of the rare earth elements and the alkali solution of the oxalic acid by way of solid-liquid separation, wherein the collected alkali solution of oxalic acid is recycled in the step of the oxalate-salt precipitation. On the other hand, Patent Document 3 discloses a method of recovering rare earth elements as rare earth oxides. This method includes the steps of: mixing an acid into a solution containing rare earth elements, and heating the mixed solution, in order to dissolve the rare earth elements therein; removing the insoluble contents from the rare earth element-dissolving solution; adding a soluble carbonate, a soluble hydrogencarbonate, or oxalic acid thereto, thereby converting the rare earth elements present in the solution into rare earth carbonates or rare earth oxalates; separating the rare earth carbonate or the rare earth oxalate from the slurry containing them; and baking the separated rare earth carbonate or rare earth oxalate to produce rare earth oxides. These methods are suitable for recovering rare earth elements from a polishing waste fluid of abrasives containing only a rare earth oxide. However, when these methods are applied to waste fluids of the abrasives containing rare earth fluorides such as rare earth oxyfluorides, their recovering efficiency of the rare earth elements is low, and it is generally 80% or less. Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2002-224949 Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2000-87154 Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2004-175652 DISCLOSURE OF THE INVENTION An object of the present invention is to provide a method of recovering with high recovery rare earth elements from a composition containing rare earth fluorides, such as a rare earth abrasive containing a rare earth oxyfluoride or the like, that has already been used for polishing glass or the like. Furthermore, another object of the present invention is to provide a method of reproducing a rare earth abrasive. The present inventors studied intensively the above-mentioned conventional methods of recovering the rare earth elements. Accordingly, they discovered that rare earth fluorides such as rare earth oxyfluorides do not dissolve in an acid by way of the conventional methods, and they are included in an insoluble content in the form of a rare earth fluoride or the like. The insoluble content includes polishing wastes, waste particles of the polishing pad, fragments of glass or the like, and therefore, it is very cosdy to retrieve pure rare earth elements from such a mixture, and it is not practical. Therefore, the present inventors further studied a method of reducing the rare earth elements in the insoluble content by way of dissolving both a rare earth oxide and a rare earth fluoride. Consequently, it was discovered that rare earth fluorides can be completely dissolved by way of adding silicon oxide to a composition containing a rare earth fluoride, followed by heating the mixture in a hydrochloric acid aqueous solution, and that the rare earth elements were no longer included in the insoluble content. The present invention was completed based on this discovery. That is, the present invention adopts the following embodiments: [1] A method of recovering a rare earth element from a composition containing a rare earth fluoride, including: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an insoluble content from the solution obtained at step (2) to obtain a rare earth-containing solution; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3); [2] The method of recovering a rare earth element according to [1], wherein the rare earth fluoride is a rare earth oxyfluoride; [3] The method of recovering a rare earth element according to [1] or [2], wherein the composition containing a rare earth fluoride contains 4% to 10% by mass of fluorine on a solids basis; and 40% to 90% by mass of cerium oxide, 5% to 55% by mass of lanthanum oxide, and 1% to 10% by mass of praseodymium oxide on an oxide basis; [4] The method of recovering a rare earth element according to any one of [1] to [3], wherein the composition containing a rare earth fluoride contains a rare earth oxide; [5] The method of recovering a rare earth element according to any one of [1] to [4], wherein the composition containing a rare earth fluoride includes an abrasive containing a rare earth fluoride; [6] The method of recovering a rare earth element according to [5], wherein the composition containing a rare earth fluoride is a polishing waste fluid; [7] The method of recovering a rare earth element according to any one of [1] to [6], wherein the silicon oxide is at least one selected from the group consisting of colloidal silica, silica gel, fumed silica, white carbon and glass beads; [8] The method of recovering a rare earth element according to any one of [1] to [7], wherein the mixing ratio of the silicon oxide is 53 parts by mass or more with respect to 100 parts by mass of the fluorine present in the rare earth fluoride; [9] The method of recovering a rare earth element according to any one of [1] to [8], wherein the mixing ratio of the hydrochloric acid is within the range of 200 parts to 2200 parts by mass based on 35% by mass hydrochloric acid with respect to 100 parts by mass of the composition containing a rare earth fluoride on a solids basis; [10] The method of recovering a rare earth element according to any one of [1] to [9], wherein the heating temperature at step (2) is within the range of 60°C to the boiling temperature of the mixture obtained at step (1); [11] The method of recovering a rare earth element according to any one of [1] to [10], wherein the mixture is heated for 10 minutes to 10 hours at the step of (2); [12] The method of recovering a rare earth element according to any one of [1] to [11], wherein the insoluble content is removed by way of filtration at step (3); [13] The method of recovering a rare earth element according to any one of [1] to [12], wherein the insoluble content is removed at step (3) by separating the rare earth-containing solution, the silicon oxide, and the insoluble contents other than the silicon oxide; [14] The method of recovering a rare earth element according to [13], wherein the separated silicon oxide is reused at step (1); [15] The method of recovering a rare earth element according to any one of [1] to [14], wherein the rare earth element is retrieved at step (4) by obtaining a rare earth oxide from the rare earth-containing solution; [16] The method of recovering a rare earth element according to any one of [1] to [14], wherein the rare earth element is retrieved at step (4) by obtaining a rare earth fluoride from the rare earth-containing solution; [17] The method of recovering a rare earth element according to any one of [1] to [16], wherein the rare earth element is retrieved at the step of (4) as a precipitate of a rare earth carbonate or a rare earth oxalate by adding a soluble carbonate, a soluble hydrogen carbonate or oxalic acid to the rare earth-containing solution obtained at the step of (3); [18] The method of recovering a rare earth element according to [17], wherein the precipitate of the rare earth carbonate or the rare earth oxalate is further baked at step (4), thereby recovering the rare earth element as a rare earth oxide; [19] The method of recovering a rare earth element according to [17], wherein the precipitate of the rare earth carbonate or the rare earth oxalate is further reacted with hydrofluoric acid, thereby recovering the rare earth element as a rare earth fluoride; [20] The method of recovering a rare earth element according to any one of [1] to [19], further including distilling out at least a part of the hydrochloric acid between step (2) and step (4); [21] The method of recovering a rare earth element according to [20], wherein the hydrochloric acid distilled out is reused at step (1); [22] Use of the rare earth oxide obtained by the method of recovering a rare earth element according to [15] or [18] for production of a rare earth abrasive; [23] Use of the rare earth fluoride obtained by the method of recovering a rare earth element according to [16] or [19] for production of a rare earth abrasive; and [24] A method of producing a rare earth abrasive which includes the method according to any one of [1] to [21], wherein the rare earth abrasive is produced by obtaining a rare earth oxide or a rare earth fluoride from the rare earth-containing solution which is obtained by dissolving the composition containing a rare earth fluoride. In the method of producing a rare earth abrasive according to [24], in order to obtain a rare earth oxide or rare earth fluoride from the rare earth-containing- solution, a soluble carbonate or hydrogen carbonate, or oxalic acid is added to the rare earth-containing solution whereby a precipitate of a rare earth carbonate or a rare earth oxalate can be produced. If the precipitate is baked, it can be converted into a rare earth oxide. On the other hand, if the precipitate is reacted with hydrofluoric acid, it can be converted into a rare earth fluoride. The obtained rare earth oxide and the rare earth fluoride are baked, crushed and arranged into an appropriate particle size whereby a rare earth abrasive can be produced. In addition, the rare earth oxide and the rare earth fluoride can be used alone, or may be mixed at a desired ratio. According to the method of recovering a rare earth element from a composition containing a rare earth fluoride of the present invention, a rare earth element can be retrieved with high recovery and at a low cost from a composition containing a rare earth fluoride, such as a rare earth abrasive containing a rare earth oxyfluoride which has been used for polishing glass or the like. Furthermore, the retrieved rare earth element has an excellent quality, and therefore, it can be reproduced into a rare earth abrasive or the like containing a rare earth fluoride which is used for processes of fine polishing. BEST MODE FOR CARRYING OUT THE INVENTION The method of recovering a rare earth element from a composition containing a rare earth fluoride of the present invention includes: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, whereby the rare earth fluoride is dissolved therein; (3) removing an insoluble content from the solution obtained at step (2), whereby a rare earth-containing solution is obtained; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3).. The composition containing a rare earth fluoride that is applicable to the present invention includes, for example, an abrasive containing a rare earth fluoride, a polishing waste fluid produced after polishing using such an abrasive, among others. In addition, the polishing waste fluid may be concentrated or dried into a solid, or may remain as a liquid. The rare earth fluoride includes a fluorinated rare earth, rare earth oxyfluoride, or the like. Hereinafter, the present invention is described with respect to each step in the case where the composition containing a rare earth fluoride is a waste fluid of a rare earth abrasive containing a rare earth fluoride. Step (1): the composition containing a rare earth fluoride (a waste fluid of a rare earth abrasive containing a rare earth fluoride) is mixed with silicon oxide and hydrochloric acid. As described in Patent Document 1, the rare earth abrasive containing a rare earth fluoride mainly contains a rare earth oxide such as cerium oxide (for example, CeO2), lanthanum oxide (for example, La2O3), and praseodymium oxide (for example, Pr6O11); a rare earth fluoride such as cerium oxyfluoride (for example, Ce3O4F3), lanthanum oxyfluoride, and praseodymium oxyfluoride; and a composite compound thereof. Such an abrasive is generally used for polishing a glass material or quartz crystal. The used abrasive (i.e. polishing waste fluid) includes extraneous materials such as polishing wastes, wastes particles of the polishing pad, and fragments of glass. On a solids basis, such a waste fluid of an abrasive generally contains 4% to 10% by mass of fluorine, 40% to 90% by mass of cerium on a cerium oxide-basis, 5% to 55% by mass of lanthanum on a lanthanum oxide-basis, and 1% to 10% by mass of praseodymium on a praseodymium oxide-basis although the contents may vary depending on how the abrasive is used for polishing. In addition, the solids basis refers to a ratio of the element, or means that a ratio of a given element is represented by a ratio of an oxide of the element on the supposition that the mass of the solid content dried at 120°C is l00%. The waste fluid of the abrasive that has been used for polishing glass materials may include a glass content (i.e. silicon oxide). In order to recycle the abrasive for a fine polishing process, it is required to remove such a glass content. Silicon oxide is mixed into the waste fluid in the present invention although silicon oxide should be removed from the waste fluid in general. The silicon oxide present in the waste fluid that has been used for glass polishing does not improve solubility of the rare earth fluoride therein while the new silicon oxide added thereto is considered to improve the solubility of the rare earth fluoride although the reason such a difference is present is not obvious in detail. It is estimated that the surface condition of the silicon oxide previously present in the waste fluid is different from that of the newly mixed silicon oxide. Therefore, even if glass-polishing or the like results in the increase of silicon oxide in the waste fluid of the abrasive, it is required to mix fresh silicon oxide into the waste fluid in order to achieve effects of the present invention. The silicon oxide mixed into the waste fluid is not particularly limited in terms of its form. However, it is preferable that the silicon oxide be at least in a form of particles which contain silicon oxide on their surface in order to attain a larger area of the reaction interface. Such particles containing silicon oxide, for example, include colloidal silica, white carbon, fumed silica, glass beads, and silica gel. It is preferable that the particles containing silicon oxide have a narrower particle size distribution because the solid-liquid separation can be easily conducted. In addition, it is preferable that the particle size of the particle containing silicon oxide be smaller (for example, 1 nm to 100 nm) or be larger (for example, 10 µm to 5000 µm) than those of the extraneous materials such as polishing wastes, waste particles of the polishing pad, and fragments of glass because this makes it easier to remove such extraneous materials. It is preferable that the total surface area of the particles containing silicon oxide which are mixed into the waste fluid of the abrasive be broader because this can shorten the reaction time. For example, it is preferable that the total surface area be 300 cm2 or more with respect to 1 g of the fluorine present in the waste fluid, more preferably 1000 cm2 or more. 53 parts by mass or more of the silicon oxide can be preferably added with respect to 100 parts by mass of the fluorine present in the waste fluid of the abrasive, and 80 parts by mass or more thereof can be more preferably added. The maximum limit of the silicon oxide added can be determined by considering the cost, the usability or the like, and therefore, is not particularly limited. However, it may be up to about 1,000,000 parts by mass. , In addition, the material for the inner surface of a reactor which is used for the retrieving treatment may be glass. Glass is corroded by a fluorine content, and therefore, use of a glass reactor is avoided in a chemical reaction using a fluorine compound. However, the corrosion of the inner surface of the glass reactor is suppressed in a reaction that occurs when the mixture of the waste fluid of the abrasive, silicon oxide, and hydrochloric acid is heated. Accordingly, for example, a glass-lined reactor can be used in the present invention. The concentration of hydrochloric acid that is mixed with the waste fluid of the abrasive is not particularly limited. However, the concentration is generally within a rage of 15% to 45% by mass, and preferably 20% to 35% by mass, and concentrated hydrochloric acid (for example, 35% by mass) can be used. The amount of the hydrochloric acid mixed therein is preferably within a range of 200 parts to 2200 parts by mass, and more preferably within a range of 727 parts to 2200 parts by mass with respect to 100 parts by mass of the solid content present in the waste fluid of the abrasive, based on 35% by mass hydrochloric acid. Step (2): the mixture of the waste fluid, silicon oxide, and hydrochloric acid is heated, thereby dissolving the rare earth fluoride. The heating treatment is continued until the rare earth elements present in the waste fluid of the abrasive (they are generally in a form of an oxide or fluoride) are sufficiently dissolved therein. It is preferable that the heating temperature be within a range of 60°C to the boiling temperature of the mixture. The period for heating the mixture is preferably within a range of 10 minutes to 10 hours, and more preferably within a range of 0.5 hours to 6 hours. The heating treatment is conducted under reflux. Then, the reflux is terminated, the mixture is concentrated by way of heating, and a part of the hydrochloric acid can be retrieved by cooling the generated vapor. The retrieved hydrochloric acid can be reused at step (1). Thus, the mixture of the waste fluid of the abrasive, silicon oxide, and hydrochloric acid is heated whereby rare earth oxides and rare earth fluorides are dissolved therein, and an acidic aqueous solution in which the rare earth elements are dissolved can be obtained. Although the details of the mechanism are not apparent, it is considered that the rare earth elements are dissolved therein as rare earth chlorides. Glass polishing wastes present in the waste fluid of the abrasive become a silica sol, its gelation further proceeds, and they finally turn into an insoluble silica gel. The insoluble contents such as the silica gel and the waste particles of the polishing pad may be separated directly from the solution obtained at step (2) by way of filtration. When a large volume of the acidic aqueous solution of the rare earth elements is kept in insoluble contents, it is preferable mat the solution be concentrated by way of heating in order to retrieve the acidic aqueous solution of the rare earth elements at a high recovery yield. When it is concentrated by way of heating, the density (concentration) of the solution becomes high. Consequently, the silica gel becomes relatively lighter, and the silica gel floats with pad wastes or the like upon the liquid surface. This makes it easier to conduct the next step (3). In addition, hydrochloric acid is retrieved by condensing the vapor generated while concentrating the solution by way of heating. The retrieved hydrochloric acid can be reused at step (1). If the acidic aqueous solution is further heated to be concentrated, its liquid surface declines, and the silica gel and the insoluble contents floated on the liquid surface adhere to the inner wall of the reaction vessel, and they are fixed thereon. When the amount of the silica gel and the insoluble contents is too much for the area of the inner wall of the reaction vessel, a partition plate or the like may be provided inside the reaction vessel in order to increase the area of the inner wall. The volume of the silica gel and insoluble contents that are fixed on the inner wall is reduced, and the acidic aqueous solution of the rare earth elements that remain in the silica gel and the insoluble contents are released when the temperature further increases. Therefore, it is preferable that the temperature of the inner wall, the partition plate, etc. be kept at the temperature of the aqueous solution or more in order to easily cause the reduction of the volume. It is preferable that the aqueous solution be concentrated to about 25% to 75% of the initial volume before heating, more preferably about 40% to 60%. As the separation between the silica gel and the insoluble contents, and the acidic aqueous solution of the rare earth elements proceeds by way of the heat-concentration treatment, the recovery yield of the acidic aqueous solution of the rare earth elements improves at the next step (3). It is preferable that the acidic aqueous solution of the rare earth elements be concentrated while generating bubbles therein. It is more preferable that fine and uniform bubbles be generated. If a boiling chip is put into the aqueous solution, the bubbles are repeatedly generated around the boiling chip by way of heating, and the generated bubbles becomes fine and uniform. The silica gel and the insoluble contents float on the liquid surface with the bubbles. Thus, the separation between the silica gel and the insoluble contents, and the acidic aqueous solution of the rare earth elements is improved at step (3). In addition, as examples of the boiling chip, ceramics such as glass or plastics that are insoluble in the hot solution can be selected. It is preferable that the boiling chip have numerous projections on its surface from which the bubbles are generated. Step (3): obtaining a rare earth-containing solution by removing the insoluble contents from the solution obtained at step (2). After the step of (2), the acidic aqueous solution of the rare earth elements is taken out from the reaction vessel, and the insoluble contents are removed by separating them from the solution. The method of separating the insoluble contents is not particularly limited. However, solid-liquid separation procedures such as filtration and centrifugation can be applied for removing the insoluble contents. In particular, filtration is preferable because of its ease in carrying out. In this way, an acidic aqueous solution in which the insoluble contents are removed can be obtained. The removal of the insoluble contents may include steps where the solution is separated into a rare earth-containing solution, silicon oxide, and other insoluble contents other than silicon oxide. For example, such methods include a method in which unreacted silicon oxide (particles such as glass beads) is removed, and then, silica gels and other insoluble contents other than silicon oxide (for example, waste particles of the polishing pad) are removed. Also, the rare earth-containing solution and the insoluble contents may be first separated, followed by separating silicon oxide from the insoluble contents by way of filtration. The separated silicon oxide can be reused at step (1). In addition, the obtained rare earth-containing solution is diluted with water, and concentrated by way of heating whereby hydrochloric acid may be retrieved from the solution in the same manner as step (2). The retrieved hydrochloric acid can be reused at step (1). Step (4): rare earth elements are retrieved from the rare earth-containing solution obtained at step (3). The solution (acidic aqueous solution of the rare earth elements) obtained at step (3) is a solution which hardly contains impurities, and in which rare earth elements are dissolved. To reproduce an abrasive or the like from the solution, the rare earth elements can be retrieved, for example, by way of the following method. First, a soluble carbonate, soluble hydrogen carbonate or oxalic acid is added to the acidic aqueous solution of the rare earth elements, thereby precipitating the rare earth elements. The precipitates are rare earth carbonates or rare earth oxalates. The soluble carbonates or soluble hydrogen carbonates may be salts soluble in the solution (acidic solution) obtained at step (3). Alkali metal salts, alkali earth metal salts, or ammonium salts are preferably used. The alkali metals include, for example, sodium, potassium, lithium or the like. In particular, sodium and potassium are preferable. The alkali earth metals include, for example, calcium, strontium, barium or the like. With regard to carbonates and hydrogen carbonates of the alkali metals, alkali earth metals, or ammonium, sodium hydrogen carbonate, potassium hydrogen carbonate, and ammonium hydrogen carbonate are preferable, and ammonium hydrogen carbonate is more preferable. Prior to adding the carbonates, the hydrogen carbonates, or oxalic acid, the acidic aqueous solution of the rare earth elements can be diluted with water in order to control particle size where necessary. In general, the higher the dilution ratio, the larger the particle size of the obtained particles becomes. Moreover, it is preferable that the pH of the acidic aqueous solution of the rare earth elements be adjusted to 1 to 7 prior to adding the carbonates, the hydrogen carbonates, or oxalic acid thereto. It is more preferable that the pH be adjusted to 1 to 4. In addition, it is most preferable that ammonium hydrogen carbonate or oxalic acid be added thereto after the adjustment of the pH. The adjustment of the pH can be conducted by adding an alkaline content such as aqueous ammonium, sodium hydroxide, potassium hydroxide, and calcium hydroxide. However, it is preferable that aqueous ammonium be preferably used because it is easier to remove the alkaline content. For example, if aqueous ammonium is chosen, its concentration is not limited, but it may be generally selected within a range of 5% to 28% by mass. The soluble carbonate or hydrogen carbonates can be added to the acidic aqueous solution of the rare earth elements in a form of solid or aqueous solution. However, the aqueous solution is preferable. The concentration of the aqueous solution of the soluble carbonate or the soluble hydrogen carbonate, or oxalic acid solution is not particularly limited, but it can be generally selected within a range of 5% to 97% by mass. It is preferable that the amount of the soluble carbonate or the hydrogen carbonate, or oxalic acid, added thereto, be within a range of 96 parts to 180 parts by mass with respect to 100 parts by mass of the rare earth elements. The acidic aqueous solution containing rare earth elements turns into a slurry that contains rare earth carbonates or rare earth oxalates when the above-described carbonate or hydrogen carbonate, or oxalic acid is added thereto. The slurry is subjected to the solid-liquid separation, thereby collecting the precipitates. For example, the slurry including rare earth carbonates or rare earth oxalates is subjected to filtration or centrifugation whereby the precipitates of the rare earth carbonates, the rare earth oxalates or the like can be collected. The separated rare earth carbonates or the rare earth oxalates may be washed with water, and may be subjected again to the solid-liquid separation if desired. Next, the collected precipitates of the rare earth carbonates or the rare earth oxalates are baked or reacted with hydrofluoric acid. Step (4-1): if the precipitates are baked, then, they turn into rare earth oxides such as oxidized rare earth. With regard to the baking treatment, the rare earth carbonates or the rare earth oxalates are baked in the atmosphere, generally at 600°C to 1200°C, preferably at 800°C to 1100°C, and generally for about 0.5 hours to 3 hours, preferably for 0.5 hours to 2 hours. Such a baking treatment makes it possible to obtain rare earth oxides. A general baking furnace such as a box-type furnace, rotary furnace and tunnel kiln can be used as a baking device. The rare earth oxides obtained by way of baking are retrieved, crushed, and assigned into certain grain sizes (particle size and particle distribution), and the grain sizes are adjusted whereby they can be recycled as an abrasive for fine polishing. In general, they may be adjusted to the original particle size, but their particle size may be different. For example, the average particle size can be adjusted within a range of 0.1 µm to 2 µm. Step (4-2): if the precipitates are reacted with hydrofluoric acid, then, they turn into rare earth fluorides such as fluorinated rare earth. In general, the precipitates of the rare earth carbonate or the rare earth oxalates are mixed with hydrofluoric acid whereby fluorinated rare earth is obtained. The hydrofluoric acid may be added at the equivalent to the rare earth carbonates or the rare earth oxalates or more. The concentration of the hydrofluoric acid solution is not particularly limited, but a 5% to 65% solution is preferable. Step (4-3): the rare earth oxides obtained at step (4-1), and the rare earth fluorides obtained at step (4-2) are mixed at a desired ratio, and baked to obtain a mixture including the rare earth oxides and the rare earth oxyfluorides. After the mixture is crushed, and adjusted to a predetermined grain size (particle size and particle distribution), it can be reused as an abrasive for fine polishing. In general, it may be adjusted to the original particle size, but the particle size may be different. For example, the average particle size can be adjusted within a range of 0.1 µm to 2 µm. As described above, an abrasive including rare earth oxides, or an abrasive including rare earth oxides and rare earth oxyfluorides can be reproduced from the waste fluid of the abrasive that includes rare earth fluorides. In the present invention, most of the rare earth elements present in the waste fluid can be recycled. Therefore, the present invention can achieve a method of recycling that is cost-effective compared with conventional methods. Furthermore, a treatment of concentrating the solid contents may also be conducted before the above-described step (1). For example, the method of concentration such as filtration, and the sedimentation method using a flocculant can be mentioned. If a flocculant is used, for example, aluminum sulfate, polyaluminum chloride, or a polymerer flocculant can be added to the waste fluid containing the rare earth fluorides in order to allow the solid contents containing the rare earth elements to settle. Then, the sediment of the solid contents is separated and collected from the solution. This treatment is considered as a pretreatment conducted before the above-described step (1). After the pretreatment, the collected sediment is mixed with silicone oxide and hydrochloric acid. Aluminum sulfate and polyaluminum chloride can be used in a form of either solid or an aqueous solution (generally a concentration of 10% by mass or more). The polymer flocculant applied to the present invention includes, for example, commercial products such as "Kurifloc" (produced by KURITA WATER INDUTIRY LTD.), and "Orfloc" (produced by ORGANO CORPORATION). The rare earth oxides and the rare earth oxyfluorides retrieved by way of the above-described steps (1) to (4) are crushed, and adjusted into uniform grain sizes (particle size and particle distribution). Then, they are recycled as abrasives used for fine polishing. The application of the reproduced abrasives is identical to that of the original abrasives that include fine powder of oxides of the rare earth elements such as cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd), produced from the ores. Its application fields includes, for example, polishing of glass substrates that are used for optical or magnetic disks, or used for liquid crystal displays such as a thin film transistor-type (TFT-type) liquid crystal display (LCD) and a twisted nematic-type (TN-type) liquid crystal display (LCD); a color filter for a liquid crystal television (TV); a cathode ray tube (CRT) for TV or the like; spectacle lenses; optical lenses; glass substrates for a LSI photomask;,wired plate glass; substrates for a crystal oscillator. EXAMPLES Hereinafter, the present invention is described in detail with reference to Examples. However, the present invention is not limited to Examples. In addition, "part" and "%" refer to "part by mass" and "% by mass", respectively, unless specified. (Example 1) An abrasive that had a composition of rare earth oxides and rare earth oxyfluorides, shown in Table 1, and whose average particle size was 1.8 µm was used for polishing a 2.5-inch strengthened glass substrate that included mainly aluminosilicate. The polishing treatment was conducted by using a four-way-type double-side polishing machine (Type "5B" produced by Fujikoshi Machinery Corp.). The polishing conditions were as follows. The slurry concentration was 10%; the slurry-supplying rate was 500 ml/rnin; the platen-rotating speed was 90 rpm; the processing pressure was 100 g/cm2; and the polishing time was 30 minutes. The processing rate was 1 µm/min in this polishing process, and the surface roughness of the glass substrate was 0.9 nm after the polishing process. The obtained glass substrate by way of this polishing process was used for evaluating the recycled abrasives as described below. The composition of the waste fluid generated in this polishing process is shown in Table 1. The content of SiO2 increased compared with the composition of the original abrasive because of the inclusion of polishing wastes. A part of the waste fluid was concentrated by way of filtration such that its solid content becomes 30%. 100 parts (solids basis) of the waste fluid of 30% solid content, 8 parts of colloidal silica ("AEROSIL200" produced by JAPAN AEROSIL), and 1500 parts of 35% hydrochloric acid were transferred into a glass container that was equipped with a reflux device and a stirrer whereby the mixture thereof was obtained. The mixture was heated to 104°C. After ten minutes, the red slurry of the waste fluid turned into a yellow or green transparent liquid. After a while when heating was terminated, and the slurry was allowed to stand, a gel-like insoluble content remained on the bottom of the glass container. The insoluble content was removed from the solution by way of filtration. It was found that the insoluble content hardly contained rare earth elements. The solution from which the insoluble content was removed was transferred to a glass container, and diluted with pure water, thereby doubling the volume. The solution was then heated to 108°C, and the hydrochloric acid excessively present therein was retrieved by way of distillation. Heating was terminated, and the concentrated solution was cooled to room temperature, and then, pure water was added thereto such that the volume became identical to that of the solution before being concentrated. The pH of the solution was adjusted to 2 by adding 5% aqueous ammonia dropwise while stirring. 1400 parts of a 10% aqueous solution of ammonium hydrogen carbonate was added to the adjusted solution while stirring whereby a white precipitate of rare earth carbonates was obtained. The solution was filtrated, and the obtained white precipitate was washed with water. Then, this solid content was transferred to a porcelain container, and baked at 800°C for one hour whereby rare earth oxides were obtained. The product was crushed, and sorted in terms of particle size, thereby reproducing an abrasive whose average particle size was 0.5 µm. The composition of the recycled abrasive is shown in Table 1. The recovering efficiency of the rare earth elements was 96%. The above-described polished glass substrate was subjected to a finish polishing process by using the above recycled abrasive. The polishing process was conducted by using a four-way-type double-side polishing machine (Type "5B" produced by Fujikoshi Machinery Corp.) that was equipped with a suede-type polishing pad. The polishing conditions were as follows. The slurry concentration was 10%; the slurry-supplying rate was 60 ml/min; the platen-rotating speed was 90 rpm; the processing pressure was 75 g/cm2; and the polishing time was 10 minutes. The processing rate was 0.7 µm/min in this polishing process, and the surface roughness of the polished glass substrate was 0.3 nm. (Example 2) In the same manner as Example 1, a white precipitate of rare earth carbonates was obtained. 50 parts of 55% hydrofluoric acid were mixed into 100 parts of the white precipitate, and the mixture was heated at 350oC for two hours, thereby producing white rare earth fluorides. 30 parts of the rare earth fluorides, and 100 parts of the rare earth oxides obtained in Example 1 were mixed, and baked at 900°C. Then, the mixture was crushed and sorted in terms of particle size whereby an abrasive that contained fluorine and rare earth elements, and whose average particle size was 1.8 urn was obtained. The composition of the recycled abrasive is shown in Table 1. The recovering efficiency of the rare earth elements was 94%. A 2.5-inch strengthened glass substrate which mainly contained aluminosilicate was subjected to the polishing process by using the above recycled abrasive. The polishing process was conducted by using a four-way-type double-side polishing machine (Type "5B" produced by Fujikoshi Machinery Corp.) that was equipped with a urethane-type polishing pad. The polishing conditions were as follows. The slurry concentration was 10%; the slurry-supplying rate was 500 ml/min; the platen-rotating speed was 90 rpm; the processing pressure was 100 g/cm2; and the polishing time was 30 minutes. The processing rate was 1 µm/min in this polishing process, and the surface roughness of the glass substrate was 0.9 nm after the polishing process. (Example 3) A polishing waste fluid was obtained in the same manner as Example 1. The waste fluid was dried, and the remaining content was crushed, thereby obtaining a brown powder. 100 parts of the brown powder; 1500 parts of hydrochloric acid (density: 1.096 g/cm3, about 20% concentration) obtained in the same manner as Example 1; and 320 parts of glass beads having a diameter of 0.2 mm were transferred into a glass container equipped with a reflux device, and a mixture thereof was obtained. The mixture was heated to 106°C. After five hours, the red slurry of the waste fluid turned into a yellow or green transparent liquid. After a while when heating was terminated, and the slurry was allowed to stand, a gel-like product and glass beads remained in the bottom of the glass container. The container was heated again whereby 500 parts of hydrochloric acid were collected by way of distillation at 109°C. Heating was terminated, and the insoluble content remaining in the bottom of the glass container was separated and removed from the concentrated solution by way of filtration. The insoluble content was washed with water in a stainless steel sieve having a pore size of 100 µm whereby glass beads were retrieved. The solution from which the insoluble content was removed was transferred to a glass container, and diluted with water, thereby doubling its volume. A 5% sodium hydroxide aqueous solution was added dropwise to the solution while stirring whereby the pH was adjusted to 2. 1800 parts of 8% sodium hydrogen carbonate aqueous solution were added to the solution while stirring whereby a white precipitate of rare earth carbonates was obtained. The above solution was then filtered, and the remaining white precipitate was washed with water. The obtained solid content was transferred to a porcelain container, and baked at 700°C for two hours whereby rare earth oxides were obtained. Furthermore, 100 parts of the solid content obtained by washing the white precipitate with water after filtration were mixed with 50 parts of 55% hydrofluoric acid, and heated to 350°C for two hours whereby white rare earth fluorides were obtained. 30 parts of the rare earth fluorides were mixed with 100 parts of the above-obtained rare earth oxides, and 200 parts of pure water, and the mixture was water-ground such that the average particle size became 2 µm. The particles were baked at 950°C, further crushed and sorted based on particle sizes whereby abrasive which contained fluorine and rare earth elements, and whose average particle size was 1.6 µm was reproduced. The composition of the reproduced abrasive is shown in Table 1. The recovering efficiency of the rare earth elements was 96%. * The contents of Ce, La, Nd, Pr, Si, F, and CI were analyzed by way of elemental analysis. Ce, La, Nd, Pr, and Si were converted to contents by mass of CeO2, La2O3, Nd2CO3, Pr6O11, and SiO2, respectively (oxide basis), and the contents of F and CI were calculated by mass as elements, and the remainder was regarded as "Other". In addition, the mass of the solid content where the mass became constant by way of drying at 120°C was considered to be 100%. The analysis of Ce, La, Nd, Pr, and Si was conducted by using the ICP luminescence method, and the analysis of F and CI was conducted by way of ion chromatography. (Comparative Example 1) An abrasive was reproduced in the same manner as Example 1 except that no silicon oxide was added. It was evident that the insoluble content contained rare earth elements that were present as yellow rare earth fluorides or the like. The recovering efficiency of the rare earth elements was 75%. Thus, according to the present invention, the rare earth elements present in an abrasive can be retrieved at high yield, i.e. 90% or more, and an abrasive that has the same quality as that of the original abrasive can be reproduced. INDUSTRIAL APPLICABILITY According to present invention, namely the method of recovering a rare earth element from a composition that contains rare earth fluorides, rare earth elements can be retrieved at low cost and at high yield from a composition containing rare earth fluorides such as rare earth abrasives containing rare earth oxyfluorides which have been used for polishing glass or the like. Therefore, this results in saving material resources of the ores which are raw materials for rare earth abrasives. Moreover, the hydrochloric acid used in the steps of the method can be recycled, and this further results in material saving. Furthermore, rare earth abrasives reproduced from the retrieved rare earth elements can be applied to fine polishing process for glass substrates that are used for optical or magnetic disks, or used for liquid crystal displays such as a thin film transistor-type (TFT-type) liquid crystal display (LCD) and a twisted nematic-type (TN-type) liquid crystal display (LCD); a color filter for a liquid crystal television (TV); a cathode ray tube (CRT) for TV or the like; spectacle lenses; optical lenses; glass substrates for a LSI photomask; wired plate glass; substrates for a crystal oscillator. Thus, the present invention, namely the method of recovering rare earth elements from a composition containing rare earth fluorides, and the method of producing a rare earth including the steps thereof have high industrial applicability. WE CLAIM: 1. A method of recovering a rare earth element from a composition containing a rare earth fluoride, comprising: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an insoluble content from the solution obtained at step (2) to obtain a rare earth-containing solution; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3). wherein the silicon oxide is at least one selected from the group consisting of colloidal silica, silica gel, fumed silica, white carbon and glass beads. 2. The method of recovering a rare earth element as claimed in claim 1, wherein the rare earth fluoride is a rare earth oxyfluoride. 3. The method of recovering a rare earth element as claimed in claim 1, wherein the composition containing a rare earth fluoride contains 4% to 10% by mass of fluorine on a solids basis; and 40% to 90% by mass of cerium oxide, 5% to 55% by mass of lanthanum oxide, and 1% to 10% by mass of praseodymium oxide on an oxide basis. 4. The method of recovering a rare earth element as claimed in claim 1, wherein the composition containing a rare earth fluoride contains a rare earth oxide. 5. The method of recovering a rare earth element as claimed in claim 1, wherein the composition containing a rare earth fluoride includes an abrasive containing a rare earth fluoride. 6. The method of recovering a rare earth element as claimed in claim 5, the composition containing a rare earth fluoride is a polishing waste fluid. 7. The method of recovering a rare earth element as claimed in claim 1, wherein the mixing ratio of the silicon oxide is 53 parts by mass or more with respect to 100 parts by mass of the fluorine present in the rare earth fluoride. 8. The method of recovering a rare earth element as claimed in claim 1, wherein the mixing ratio of the hydrochloric acid is within the range of 200 parts to 2200 parts by mass based on a 35% by mass hydrochloric acid with respect to 100 parts by mass of the composition containing a rare earth fluoride on a solids basis. 9. The method of recovering a rare earth element as claimed in claim 1, wherein the heating temperature at step (2) is within the range of 60°C to the boiling temperature of the mixture obtained at the step (1). 10. The method of recovering a rare earth element as claimed in claim 1, wherein the mixture is heated for 10 minutes to 10 hours at step (2). 11. The method of recovering a rare earth element as claimed in claim 1, wherein the insoluble content is removed by way of filtration at step (3). 12. The method of recovering a rare earth element as claimed in claim 1, wherein the insoluble content is removed at step (3) by separating the rare earth-containing solution, the silicon oxide, and the insoluble contents other than the silicon oxide. 13. The method of recovering a rare earth element as claimed in claim 13, wherein the separated silicon oxide is reused at step (1). 14. The method of recovering a rare earth element as claimed in claim 1, wherein the rare earth element is retrieved at step (4) by obtaining a rare earth oxide from the rare earth-containing solution. 15. The method of recovering a rare earth element as claimed in claim 1, wherein the rare earth element is retrieved at step (4) by obtaining a rare earth fluoride from the rare earth-containing solution. 16. The method of recovering a rare earth element as claimed in claim 1, wherein the rare earth element is retrieved at step (4) as a precipitate of a rare earth carbonate or a rare earth oxalate by adding a soluble carbonate, a soluble hydrogen carbonate or oxalic acid to the rare earth-containing solution obtained at step (3). 17. The method of recovering a rare earth element as claimed in claim 17, wherein the precipitate of the rare earth carbonate or the rare earth oxalate is further baked at step (4), thereby recovering the rare earth element as a rare earth oxide. 18. The method of recovering a rare earth element as claimed in claim 17, wherein the precipitate of the rare earth carbonate or the rare earth oxalate is further reacted with hydrofluoric acid, thereby recovering the rare earth element as a rare earth fluoride. 19. The method of recovering a rare earth element as claimed in claim 1, comprising distilling out at least a part of the hydrochloric acid between step (2) and step (4). 20. The method of recovering a rare earth element as claimed in claim 19, wherein the hydrochloric acid distilled out is reused at step (1). ABSTRACT METHOD OF RECOVERING RARE EARTH ELEMENT FROM COMPOSITION CONTAINING RARE EARTH FLUORIDE The object of the present invention is to provide a method of recovering a rare earth element from a composition containing a rare earth fluoride, involving the steps of: (1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an insoluble content from the solution obtained at step (2) to obtain a rare earth-containing solution; and (4) recovering the rare earth element from the rare earth-containing solution obtained at step (3).

Full Text

DESCRIPTION
TECHNICAL FIELD
The present invention relates to a method of recovering rare earth elements from a
composition containing rare earth fluorides. More specifically, the present invention
relates to a method of recovering with high recovery rare earth elements from a
composition containing rare earth fluorides, such as a rare earth abrasive containing a rare
earth oxyfluoride or the like, that has already been used for polishing glass or the like, and
also relates to a method of reproducing a rare earth abrasive.
This application claims the priority of Japanese Patent Application No.
2006-067725, filed March 13, 2006, the contents of which are incorporated herein by
reference.
BACKGROUND ART
In recent years, rare earth abrasives that contain fine powder of oxides of rare earth
elements such as cerium (Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd)
have been applied to various fields of technology. In particular, a rare earth abrasive
disclosed in Patent Document 1 that contains rare earth fluorides is compatible to precise
polishing processes, and has been often applied to polishing of glass materials or quartz
crystals. For example, such a rare earth abrasive has been used for polishing, among
others, glass substrates that are used for optical or magnetic disks, or used for liquid
crystal displays such as a thin film transistor-type (TFT-type) liquid crystal display (LCD)
and a twisted nematic-type (TN-type) liquid crystal display (LCD); a color filter for a
liquid crystal television (TV); a cathode ray tube (CRT) for TV or the like; spectacle
lenses; optical lenses; glass substrates for a LSI photomask; wired plate glass; substrates
for a crystal oscillator.
Ores that are raw materials for rare earth abrasives, for "example, include
bastnaesite, monazite or the like. These ores has been mainly mined in China or in the
U.S.A.

In order to produce a rare earth oxide from ores such as a bastnaesite and
monazite, the following treatments (1) to (9) are successively conducted in general.
(1) Sulfuric acid is added little by little to the ore in a rotary kiln, followed by mixing and
drying.
(2) The ore is baked at 500°C to 600°C.
(3) Rare earth-containing contents are leached from the baked ore into water.
(4) The leaching solution is concentrated with a thickener, and impurities such as BaSO4,
CaSO4, and SiO2 are removed by way of filtration whereby R2(SO4)3 is obtained. The
"R" of the chemical formula refers to a rare earth element.
(5) SOdium sulfate is added to the obtained R2(SO4)3, and Fe, Ca, phosphoric acid or the
like are separated whereby a sulfate composite salt of the rare earth element is obtained.
(6) SOdium hydroxide is added to the sulfate composite salt of the rare earth element
whereby a hydroxide of the rare earth element is precipitated.

(7) Hydrochloric acid is added to the hydroxide of the rare earth element, thereby
producing a chloride of the rare earth element.
(8) Ammonium hydrogencarbonate is added to the chloride of the rare earth element,
thereby producing a rare earth carbonate.
(9) The rare earth carbonate is baked, thereby producing a rare earth oxide.
However, the reserves of the ores that are raw materials for producing the rare
earth abrasives are limited. Furthermore, in recent years, there has been a problem in
which the raw material ores are quite scarce as the demand for the rare earth abrasives
increases.
Therefore, various methods of recovering the rare earth elements from the waste
fluid of the rare earth abrasives have been developed in order to recycle them as
abrasives.
For example, Patent Document 2 discloses a method of recovering rare earth
elements from a rare earth abrasive that has been already used. The disclosed method

includes the steps of: dissolving rare earth elements included in the used rare earth
abrasive by treating the abrasive with an aqueous mineral acid solution; collecting the
rare earth element-dissolving solution by separating it from the insoluble contents by way
of solid-liquid separation; precipitating the rare earth elements as oxalates by adding
oxalic acid and an alkali solution of oxalic acid, such that the pH of the collected solution
becomes 5 or less; recovering the oxalates of the rare earth elements by way of
solid-liquid separation; treating the oxalates of the rare earth elements with an aqueous
alkali hydroxide solution, thereby converting them to hydroxides of the rare earth
elements, and producing an alkali solution of oxalic acid; and collecting separately the
hydroxides of the rare earth elements and the alkali solution of the oxalic acid by way of
solid-liquid separation, wherein the collected alkali solution of oxalic acid is recycled in
the step of the oxalate-salt precipitation.
On the other hand, Patent Document 3 discloses a method of recovering rare
earth elements as rare earth oxides. This method includes the steps of: mixing an acid into
a solution containing rare earth elements, and heating the mixed solution, in order to
dissolve the rare earth elements therein; removing the insoluble contents from the rare
earth element-dissolving solution; adding a soluble carbonate, a soluble
hydrogencarbonate, or oxalic acid thereto, thereby converting the rare earth elements
present in the solution into rare earth carbonates or rare earth oxalates; separating the rare
earth carbonate or the rare earth oxalate from the slurry containing them; and baking the
separated rare earth carbonate or rare earth oxalate to produce rare earth oxides.
These methods are suitable for recovering rare earth elements from a polishing
waste fluid of abrasives containing only a rare earth oxide. However, when these methods
are applied to waste fluids of the abrasives containing rare earth fluorides such as rare
earth oxyfluorides, their recovering efficiency of the rare earth elements is low, and it is
generally 80% or less.
Patent Document 1: Japanese Unexamined Patent Application, Publication No.
2002-224949
Patent Document 2: Japanese Unexamined Patent Application, Publication No.

2000-87154
Patent Document 3: Japanese Unexamined Patent Application, Publication No.
2004-175652
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a method of recovering with high
recovery rare earth elements from a composition containing rare earth fluorides, such as a
rare earth abrasive containing a rare earth oxyfluoride or the like, that has already been
used for polishing glass or the like. Furthermore, another object of the present invention is
to provide a method of reproducing a rare earth abrasive.
The present inventors studied intensively the above-mentioned conventional
methods of recovering the rare earth elements. Accordingly, they discovered that rare
earth fluorides such as rare earth oxyfluorides do not dissolve in an acid by way of the
conventional methods, and they are included in an insoluble content in the form of a rare
earth fluoride or the like. The insoluble content includes polishing wastes, waste particles
of the polishing pad, fragments of glass or the like, and therefore, it is very cosdy to
retrieve pure rare earth elements from such a mixture, and it is not practical.
Therefore, the present inventors further studied a method of reducing the rare earth
elements in the insoluble content by way of dissolving both a rare earth oxide and a rare
earth fluoride. Consequently, it was discovered that rare earth fluorides can be completely
dissolved by way of adding silicon oxide to a composition containing a rare earth fluoride,
followed by heating the mixture in a hydrochloric acid aqueous solution, and that the rare
earth elements were no longer included in the insoluble content. The present invention
was completed based on this discovery.
That is, the present invention adopts the following embodiments:
[1] A method of recovering a rare earth element from a composition containing a rare
earth fluoride, including:

(1) mixing a composition containing a rare earth fluoride, silicon oxide, and
hydrochloric acid;
(2) heating the mixture, thereby dissolving the rare earth fluoride;
(3) removing an insoluble content from the solution obtained at step (2) to obtain
a rare earth-containing solution; and
(4) recovering the rare earth element from the rare earth-containing solution
obtained at step (3);
[2] The method of recovering a rare earth element according to [1], wherein the rare
earth fluoride is a rare earth oxyfluoride;
[3] The method of recovering a rare earth element according to [1] or [2], wherein the
composition containing a rare earth fluoride contains 4% to 10% by mass of fluorine on a
solids basis; and 40% to 90% by mass of cerium oxide, 5% to 55% by mass of lanthanum
oxide, and 1% to 10% by mass of praseodymium oxide on an oxide basis;
[4] The method of recovering a rare earth element according to any one of [1] to [3],
wherein the composition containing a rare earth fluoride contains a rare earth oxide;
[5] The method of recovering a rare earth element according to any one of [1] to [4],
wherein the composition containing a rare earth fluoride includes an abrasive containing a
rare earth fluoride;
[6] The method of recovering a rare earth element according to [5], wherein the
composition containing a rare earth fluoride is a polishing waste fluid;
[7] The method of recovering a rare earth element according to any one of [1] to [6],
wherein the silicon oxide is at least one selected from the group consisting of colloidal
silica, silica gel, fumed silica, white carbon and glass beads;
[8] The method of recovering a rare earth element according to any one of [1] to [7],
wherein the mixing ratio of the silicon oxide is 53 parts by mass or more with respect to
100 parts by mass of the fluorine present in the rare earth fluoride;
[9] The method of recovering a rare earth element according to any one of [1] to [8],
wherein the mixing ratio of the hydrochloric acid is within the range of 200 parts to 2200
parts by mass based on 35% by mass hydrochloric acid with respect to 100 parts by mass
of the composition containing a rare earth fluoride on a solids basis;
[10] The method of recovering a rare earth element according to any one of [1] to [9],
wherein the heating temperature at step (2) is within the range of 60°C to the boiling

temperature of the mixture obtained at step (1);
[11] The method of recovering a rare earth element according to any one of [1] to [10],
wherein the mixture is heated for 10 minutes to 10 hours at the step of (2);
[12] The method of recovering a rare earth element according to any one of [1] to [11],
wherein the insoluble content is removed by way of filtration at step (3);
[13] The method of recovering a rare earth element according to any one of [1] to [12],
wherein the insoluble content is removed at step (3) by separating the rare
earth-containing solution, the silicon oxide, and the insoluble contents other than the
silicon oxide;
[14] The method of recovering a rare earth element according to [13], wherein the
separated silicon oxide is reused at step (1);
[15] The method of recovering a rare earth element according to any one of [1] to [14],
wherein the rare earth element is retrieved at step (4) by obtaining a rare earth oxide from
the rare earth-containing solution;
[16] The method of recovering a rare earth element according to any one of [1] to [14],
wherein the rare earth element is retrieved at step (4) by obtaining a rare earth fluoride
from the rare earth-containing solution;
[17] The method of recovering a rare earth element according to any one of [1] to [16],
wherein the rare earth element is retrieved at the step of (4) as a precipitate of a rare earth
carbonate or a rare earth oxalate by adding a soluble carbonate, a soluble hydrogen
carbonate or oxalic acid to the rare earth-containing solution obtained at the step of (3);
[18] The method of recovering a rare earth element according to [17], wherein the
precipitate of the rare earth carbonate or the rare earth oxalate is further baked at step (4),
thereby recovering the rare earth element as a rare earth oxide;
[19] The method of recovering a rare earth element according to [17], wherein the
precipitate of the rare earth carbonate or the rare earth oxalate is further reacted with
hydrofluoric acid, thereby recovering the rare earth element as a rare earth fluoride;
[20] The method of recovering a rare earth element according to any one of [1] to [19],
further including distilling out at least a part of the hydrochloric acid between step (2) and
step (4);
[21] The method of recovering a rare earth element according to [20], wherein the
hydrochloric acid distilled out is reused at step (1);

[22] Use of the rare earth oxide obtained by the method of recovering a rare earth
element according to [15] or [18] for production of a rare earth abrasive;
[23] Use of the rare earth fluoride obtained by the method of recovering a rare earth
element according to [16] or [19] for production of a rare earth abrasive; and
[24] A method of producing a rare earth abrasive which includes the method according
to any one of [1] to [21], wherein the rare earth abrasive is produced by obtaining a rare
earth oxide or a rare earth fluoride from the rare earth-containing solution which is
obtained by dissolving the composition containing a rare earth fluoride.
In the method of producing a rare earth abrasive according to [24], in order to
obtain a rare earth oxide or rare earth fluoride from the rare earth-containing- solution, a
soluble carbonate or hydrogen carbonate, or oxalic acid is added to the rare
earth-containing solution whereby a precipitate of a rare earth carbonate or a rare earth
oxalate can be produced. If the precipitate is baked, it can be converted into a rare earth
oxide. On the other hand, if the precipitate is reacted with hydrofluoric acid, it can be
converted into a rare earth fluoride. The obtained rare earth oxide and the rare earth
fluoride are baked, crushed and arranged into an appropriate particle size whereby a rare
earth abrasive can be produced. In addition, the rare earth oxide and the rare earth fluoride
can be used alone, or may be mixed at a desired ratio.
According to the method of recovering a rare earth element from a composition
containing a rare earth fluoride of the present invention, a rare earth element can be
retrieved with high recovery and at a low cost from a composition containing a rare earth
fluoride, such as a rare earth abrasive containing a rare earth oxyfluoride which has been
used for polishing glass or the like. Furthermore, the retrieved rare earth element has an
excellent quality, and therefore, it can be reproduced into a rare earth abrasive or the like
containing a rare earth fluoride which is used for processes of fine polishing.
BEST MODE FOR CARRYING OUT THE INVENTION
The method of recovering a rare earth element from a composition containing a
rare earth fluoride of the present invention includes: (1) mixing a composition containing

a rare earth fluoride, silicon oxide, and hydrochloric acid; (2) heating the mixture,
whereby the rare earth fluoride is dissolved therein; (3) removing an insoluble content
from the solution obtained at step (2), whereby a rare earth-containing solution is
obtained; and (4) recovering the rare earth element from the rare earth-containing solution
obtained at step (3)..
The composition containing a rare earth fluoride that is applicable to the present
invention includes, for example, an abrasive containing a rare earth fluoride, a polishing
waste fluid produced after polishing using such an abrasive, among others. In addition,
the polishing waste fluid may be concentrated or dried into a solid, or may remain as a
liquid. The rare earth fluoride includes a fluorinated rare earth, rare earth oxyfluoride, or
the like.
Hereinafter, the present invention is described with respect to each step in the
case where the composition containing a rare earth fluoride is a waste fluid of a rare earth
abrasive containing a rare earth fluoride.
Step (1): the composition containing a rare earth fluoride (a waste fluid of a rare
earth abrasive containing a rare earth fluoride) is mixed with silicon oxide and
hydrochloric acid.
As described in Patent Document 1, the rare earth abrasive containing a rare
earth fluoride mainly contains a rare earth oxide such as cerium oxide (for example,
CeO2), lanthanum oxide (for example, La2O3), and praseodymium oxide (for example,
Pr6O11); a rare earth fluoride such as cerium oxyfluoride (for example, Ce3O4F3),
lanthanum oxyfluoride, and praseodymium oxyfluoride; and a composite compound
thereof. Such an abrasive is generally used for polishing a glass material or quartz crystal.
The used abrasive (i.e. polishing waste fluid) includes extraneous materials such as
polishing wastes, wastes particles of the polishing pad, and fragments of glass.
On a solids basis, such a waste fluid of an abrasive generally contains 4% to 10%
by mass of fluorine, 40% to 90% by mass of cerium on a cerium oxide-basis, 5% to 55%

by mass of lanthanum on a lanthanum oxide-basis, and 1% to 10% by mass of
praseodymium on a praseodymium oxide-basis although the contents may vary
depending on how the abrasive is used for polishing. In addition, the solids basis refers to
a ratio of the element, or means that a ratio of a given element is represented by a ratio of
an oxide of the element on the supposition that the mass of the solid content dried at
120°C is l00%.
The waste fluid of the abrasive that has been used for polishing glass materials
may include a glass content (i.e. silicon oxide). In order to recycle the abrasive for a fine
polishing process, it is required to remove such a glass content. Silicon oxide is mixed
into the waste fluid in the present invention although silicon oxide should be removed
from the waste fluid in general. The silicon oxide present in the waste fluid that has been
used for glass polishing does not improve solubility of the rare earth fluoride therein
while the new silicon oxide added thereto is considered to improve the solubility of the
rare earth fluoride although the reason such a difference is present is not obvious in detail.
It is estimated that the surface condition of the silicon oxide previously present in the
waste fluid is different from that of the newly mixed silicon oxide.
Therefore, even if glass-polishing or the like results in the increase of silicon
oxide in the waste fluid of the abrasive, it is required to mix fresh silicon oxide into the
waste fluid in order to achieve effects of the present invention.
The silicon oxide mixed into the waste fluid is not particularly limited in terms of
its form. However, it is preferable that the silicon oxide be at least in a form of particles
which contain silicon oxide on their surface in order to attain a larger area of the reaction
interface. Such particles containing silicon oxide, for example, include colloidal silica,
white carbon, fumed silica, glass beads, and silica gel. It is preferable that the particles
containing silicon oxide have a narrower particle size distribution because the solid-liquid
separation can be easily conducted. In addition, it is preferable that the particle size of
the particle containing silicon oxide be smaller (for example, 1 nm to 100 nm) or be larger
(for example, 10 µm to 5000 µm) than those of the extraneous materials such as polishing
wastes, waste particles of the polishing pad, and fragments of glass because this makes it

easier to remove such extraneous materials.
It is preferable that the total surface area of the particles containing silicon oxide
which are mixed into the waste fluid of the abrasive be broader because this can shorten
the reaction time. For example, it is preferable that the total surface area be 300 cm2 or
more with respect to 1 g of the fluorine present in the waste fluid, more preferably 1000
cm2 or more.
53 parts by mass or more of the silicon oxide can be preferably added with
respect to 100 parts by mass of the fluorine present in the waste fluid of the abrasive, and
80 parts by mass or more thereof can be more preferably added. The maximum limit of
the silicon oxide added can be determined by considering the cost, the usability or the like,
and therefore, is not particularly limited. However, it may be up to about 1,000,000 parts
by mass. ,
In addition, the material for the inner surface of a reactor which is used for the
retrieving treatment may be glass. Glass is corroded by a fluorine content, and therefore,
use of a glass reactor is avoided in a chemical reaction using a fluorine compound.
However, the corrosion of the inner surface of the glass reactor is suppressed in a reaction
that occurs when the mixture of the waste fluid of the abrasive, silicon oxide, and
hydrochloric acid is heated. Accordingly, for example, a glass-lined reactor can be used in
the present invention.
The concentration of hydrochloric acid that is mixed with the waste fluid of the
abrasive is not particularly limited. However, the concentration is generally within a rage
of 15% to 45% by mass, and preferably 20% to 35% by mass, and concentrated
hydrochloric acid (for example, 35% by mass) can be used. The amount of the
hydrochloric acid mixed therein is preferably within a range of 200 parts to 2200 parts by
mass, and more preferably within a range of 727 parts to 2200 parts by mass with respect
to 100 parts by mass of the solid content present in the waste fluid of the abrasive, based
on 35% by mass hydrochloric acid.

Step (2): the mixture of the waste fluid, silicon oxide, and hydrochloric acid is
heated, thereby dissolving the rare earth fluoride.
The heating treatment is continued until the rare earth elements present in the
waste fluid of the abrasive (they are generally in a form of an oxide or fluoride) are
sufficiently dissolved therein. It is preferable that the heating temperature be within a
range of 60°C to the boiling temperature of the mixture. The period for heating the
mixture is preferably within a range of 10 minutes to 10 hours, and more preferably
within a range of 0.5 hours to 6 hours. The heating treatment is conducted under reflux.
Then, the reflux is terminated, the mixture is concentrated by way of heating, and a part of
the hydrochloric acid can be retrieved by cooling the generated vapor. The retrieved
hydrochloric acid can be reused at step (1).
Thus, the mixture of the waste fluid of the abrasive, silicon oxide, and
hydrochloric acid is heated whereby rare earth oxides and rare earth fluorides are
dissolved therein, and an acidic aqueous solution in which the rare earth elements are
dissolved can be obtained. Although the details of the mechanism are not apparent, it is
considered that the rare earth elements are dissolved therein as rare earth chlorides.
Glass polishing wastes present in the waste fluid of the abrasive become a silica sol, its
gelation further proceeds, and they finally turn into an insoluble silica gel.
The insoluble contents such as the silica gel and the waste particles of the
polishing pad may be separated directly from the solution obtained at step (2) by way of
filtration. When a large volume of the acidic aqueous solution of the rare earth elements is
kept in insoluble contents, it is preferable mat the solution be concentrated by way of
heating in order to retrieve the acidic aqueous solution of the rare earth elements at a high
recovery yield.
When it is concentrated by way of heating, the density (concentration) of the
solution becomes high. Consequently, the silica gel becomes relatively lighter, and the
silica gel floats with pad wastes or the like upon the liquid surface. This makes it easier to
conduct the next step (3). In addition, hydrochloric acid is retrieved by condensing the

vapor generated while concentrating the solution by way of heating. The retrieved
hydrochloric acid can be reused at step (1).
If the acidic aqueous solution is further heated to be concentrated, its liquid
surface declines, and the silica gel and the insoluble contents floated on the liquid surface
adhere to the inner wall of the reaction vessel, and they are fixed thereon. When the
amount of the silica gel and the insoluble contents is too much for the area of the inner
wall of the reaction vessel, a partition plate or the like may be provided inside the reaction
vessel in order to increase the area of the inner wall. The volume of the silica gel and
insoluble contents that are fixed on the inner wall is reduced, and the acidic aqueous
solution of the rare earth elements that remain in the silica gel and the insoluble contents
are released when the temperature further increases. Therefore, it is preferable that the
temperature of the inner wall, the partition plate, etc. be kept at the temperature of the
aqueous solution or more in order to easily cause the reduction of the volume. It is
preferable that the aqueous solution be concentrated to about 25% to 75% of the initial
volume before heating, more preferably about 40% to 60%. As the separation between the
silica gel and the insoluble contents, and the acidic aqueous solution of the rare earth
elements proceeds by way of the heat-concentration treatment, the recovery yield of the
acidic aqueous solution of the rare earth elements improves at the next step (3).
It is preferable that the acidic aqueous solution of the rare earth elements be
concentrated while generating bubbles therein. It is more preferable that fine and uniform
bubbles be generated. If a boiling chip is put into the aqueous solution, the bubbles are
repeatedly generated around the boiling chip by way of heating, and the generated
bubbles becomes fine and uniform. The silica gel and the insoluble contents float on the
liquid surface with the bubbles. Thus, the separation between the silica gel and the
insoluble contents, and the acidic aqueous solution of the rare earth elements is improved
at step (3).
In addition, as examples of the boiling chip, ceramics such as glass or plastics
that are insoluble in the hot solution can be selected. It is preferable that the boiling chip
have numerous projections on its surface from which the bubbles are generated.

Step (3): obtaining a rare earth-containing solution by removing the insoluble
contents from the solution obtained at step (2).
After the step of (2), the acidic aqueous solution of the rare earth elements is
taken out from the reaction vessel, and the insoluble contents are removed by separating
them from the solution. The method of separating the insoluble contents is not
particularly limited. However, solid-liquid separation procedures such as filtration and
centrifugation can be applied for removing the insoluble contents. In particular, filtration
is preferable because of its ease in carrying out. In this way, an acidic aqueous solution in
which the insoluble contents are removed can be obtained.
The removal of the insoluble contents may include steps where the solution is
separated into a rare earth-containing solution, silicon oxide, and other insoluble contents
other than silicon oxide. For example, such methods include a method in which unreacted
silicon oxide (particles such as glass beads) is removed, and then, silica gels and other
insoluble contents other than silicon oxide (for example, waste particles of the polishing
pad) are removed. Also, the rare earth-containing solution and the insoluble contents may
be first separated, followed by separating silicon oxide from the insoluble contents by
way of filtration. The separated silicon oxide can be reused at step (1).
In addition, the obtained rare earth-containing solution is diluted with water, and
concentrated by way of heating whereby hydrochloric acid may be retrieved from the
solution in the same manner as step (2). The retrieved hydrochloric acid can be reused at
step (1).
Step (4): rare earth elements are retrieved from the rare earth-containing solution
obtained at step (3).
The solution (acidic aqueous solution of the rare earth elements) obtained at step
(3) is a solution which hardly contains impurities, and in which rare earth elements are
dissolved. To reproduce an abrasive or the like from the solution, the rare earth elements

can be retrieved, for example, by way of the following method.
First, a soluble carbonate, soluble hydrogen carbonate or oxalic acid is added to
the acidic aqueous solution of the rare earth elements, thereby precipitating the rare earth
elements. The precipitates are rare earth carbonates or rare earth oxalates.
The soluble carbonates or soluble hydrogen carbonates may be salts soluble in
the solution (acidic solution) obtained at step (3). Alkali metal salts, alkali earth metal
salts, or ammonium salts are preferably used. The alkali metals include, for example,
sodium, potassium, lithium or the like. In particular, sodium and potassium are preferable.
The alkali earth metals include, for example, calcium, strontium, barium or the like. With
regard to carbonates and hydrogen carbonates of the alkali metals, alkali earth metals, or
ammonium, sodium hydrogen carbonate, potassium hydrogen carbonate, and ammonium
hydrogen carbonate are preferable, and ammonium hydrogen carbonate is more
preferable.
Prior to adding the carbonates, the hydrogen carbonates, or oxalic acid, the
acidic aqueous solution of the rare earth elements can be diluted with water in order to
control particle size where necessary. In general, the higher the dilution ratio, the larger
the particle size of the obtained particles becomes.
Moreover, it is preferable that the pH of the acidic aqueous solution of the rare
earth elements be adjusted to 1 to 7 prior to adding the carbonates, the hydrogen
carbonates, or oxalic acid thereto. It is more preferable that the pH be adjusted to 1 to 4.
In addition, it is most preferable that ammonium hydrogen carbonate or oxalic acid be
added thereto after the adjustment of the pH. The adjustment of the pH can be conducted
by adding an alkaline content such as aqueous ammonium, sodium hydroxide, potassium
hydroxide, and calcium hydroxide. However, it is preferable that aqueous ammonium be
preferably used because it is easier to remove the alkaline content. For example, if
aqueous ammonium is chosen, its concentration is not limited, but it may be generally
selected within a range of 5% to 28% by mass.

The soluble carbonate or hydrogen carbonates can be added to the acidic
aqueous solution of the rare earth elements in a form of solid or aqueous solution.
However, the aqueous solution is preferable.
The concentration of the aqueous solution of the soluble carbonate or the soluble
hydrogen carbonate, or oxalic acid solution is not particularly limited, but it can be
generally selected within a range of 5% to 97% by mass.
It is preferable that the amount of the soluble carbonate or the hydrogen
carbonate, or oxalic acid, added thereto, be within a range of 96 parts to 180 parts by mass
with respect to 100 parts by mass of the rare earth elements.
The acidic aqueous solution containing rare earth elements turns into a slurry
that contains rare earth carbonates or rare earth oxalates when the above-described
carbonate or hydrogen carbonate, or oxalic acid is added thereto.
The slurry is subjected to the solid-liquid separation, thereby collecting the
precipitates. For example, the slurry including rare earth carbonates or rare earth oxalates
is subjected to filtration or centrifugation whereby the precipitates of the rare earth
carbonates, the rare earth oxalates or the like can be collected. The separated rare earth
carbonates or the rare earth oxalates may be washed with water, and may be subjected
again to the solid-liquid separation if desired.
Next, the collected precipitates of the rare earth carbonates or the rare earth
oxalates are baked or reacted with hydrofluoric acid.
Step (4-1): if the precipitates are baked, then, they turn into rare earth oxides
such as oxidized rare earth.
With regard to the baking treatment, the rare earth carbonates or the rare earth
oxalates are baked in the atmosphere, generally at 600°C to 1200°C, preferably at
800°C to 1100°C, and generally for about 0.5 hours to 3 hours, preferably for 0.5 hours to

2 hours. Such a baking treatment makes it possible to obtain rare earth oxides. A general
baking furnace such as a box-type furnace, rotary furnace and tunnel kiln can be used as a
baking device.
The rare earth oxides obtained by way of baking are retrieved, crushed, and
assigned into certain grain sizes (particle size and particle distribution), and the grain
sizes are adjusted whereby they can be recycled as an abrasive for fine polishing. In
general, they may be adjusted to the original particle size, but their particle size may be
different. For example, the average particle size can be adjusted within a range of 0.1 µm
to 2 µm.
Step (4-2): if the precipitates are reacted with hydrofluoric acid, then, they turn
into rare earth fluorides such as fluorinated rare earth.
In general, the precipitates of the rare earth carbonate or the rare earth oxalates
are mixed with hydrofluoric acid whereby fluorinated rare earth is obtained. The
hydrofluoric acid may be added at the equivalent to the rare earth carbonates or the rare
earth oxalates or more. The concentration of the hydrofluoric acid solution is not
particularly limited, but a 5% to 65% solution is preferable.
Step (4-3): the rare earth oxides obtained at step (4-1), and the rare earth
fluorides obtained at step (4-2) are mixed at a desired ratio, and baked to obtain a mixture
including the rare earth oxides and the rare earth oxyfluorides. After the mixture is
crushed, and adjusted to a predetermined grain size (particle size and particle distribution),
it can be reused as an abrasive for fine polishing. In general, it may be adjusted to the
original particle size, but the particle size may be different. For example, the average
particle size can be adjusted within a range of 0.1 µm to 2 µm.
As described above, an abrasive including rare earth oxides, or an abrasive
including rare earth oxides and rare earth oxyfluorides can be reproduced from the waste
fluid of the abrasive that includes rare earth fluorides. In the present invention, most of the
rare earth elements present in the waste fluid can be recycled. Therefore, the present

invention can achieve a method of recycling that is cost-effective compared with
conventional methods.
Furthermore, a treatment of concentrating the solid contents may also be
conducted before the above-described step (1). For example, the method of concentration
such as filtration, and the sedimentation method using a flocculant can be mentioned. If a
flocculant is used, for example, aluminum sulfate, polyaluminum chloride, or a
polymerer flocculant can be added to the waste fluid containing the rare earth fluorides in
order to allow the solid contents containing the rare earth elements to settle. Then, the
sediment of the solid contents is separated and collected from the solution. This treatment
is considered as a pretreatment conducted before the above-described step (1). After the
pretreatment, the collected sediment is mixed with silicone oxide and hydrochloric acid.
Aluminum sulfate and polyaluminum chloride can be used in a form of either
solid or an aqueous solution (generally a concentration of 10% by mass or more). The
polymer flocculant applied to the present invention includes, for example, commercial
products such as "Kurifloc" (produced by KURITA WATER INDUTIRY LTD.), and
"Orfloc" (produced by ORGANO CORPORATION).
The rare earth oxides and the rare earth oxyfluorides retrieved by way of the
above-described steps (1) to (4) are crushed, and adjusted into uniform grain sizes
(particle size and particle distribution). Then, they are recycled as abrasives used for fine
polishing.
The application of the reproduced abrasives is identical to that of the original
abrasives that include fine powder of oxides of the rare earth elements such as cerium
(Ce), lanthanum (La), praseodymium (Pr), and neodymium (Nd), produced from the ores.
Its application fields includes, for example, polishing of glass substrates that are
used for optical or magnetic disks, or used for liquid crystal displays such as a thin film
transistor-type (TFT-type) liquid crystal display (LCD) and a twisted nematic-type
(TN-type) liquid crystal display (LCD); a color filter for a liquid crystal television (TV); a

cathode ray tube (CRT) for TV or the like; spectacle lenses; optical lenses; glass
substrates for a LSI photomask;,wired plate glass; substrates for a crystal oscillator.
EXAMPLES
Hereinafter, the present invention is described in detail with reference to
Examples. However, the present invention is not limited to Examples. In addition,
"part" and "%" refer to "part by mass" and "% by mass", respectively, unless specified.
(Example 1)
An abrasive that had a composition of rare earth oxides and rare earth
oxyfluorides, shown in Table 1, and whose average particle size was 1.8 µm was used for
polishing a 2.5-inch strengthened glass substrate that included mainly aluminosilicate.
The polishing treatment was conducted by using a four-way-type double-side polishing
machine (Type "5B" produced by Fujikoshi Machinery Corp.). The polishing conditions
were as follows. The slurry concentration was 10%; the slurry-supplying rate was 500
ml/rnin; the platen-rotating speed was 90 rpm; the processing pressure was 100 g/cm2;
and the polishing time was 30 minutes. The processing rate was 1 µm/min in this
polishing process, and the surface roughness of the glass substrate was 0.9 nm after the
polishing process. The obtained glass substrate by way of this polishing process was used
for evaluating the recycled abrasives as described below. The composition of the waste
fluid generated in this polishing process is shown in Table 1. The content of SiO2
increased compared with the composition of the original abrasive because of the inclusion
of polishing wastes. A part of the waste fluid was concentrated by way of filtration such
that its solid content becomes 30%.
100 parts (solids basis) of the waste fluid of 30% solid content, 8 parts of
colloidal silica ("AEROSIL200" produced by JAPAN AEROSIL), and 1500 parts of 35%
hydrochloric acid were transferred into a glass container that was equipped with a reflux
device and a stirrer whereby the mixture thereof was obtained. The mixture was heated to
104°C. After ten minutes, the red slurry of the waste fluid turned into a yellow or green

transparent liquid. After a while when heating was terminated, and the slurry was allowed
to stand, a gel-like insoluble content remained on the bottom of the glass container.
The insoluble content was removed from the solution by way of filtration. It was
found that the insoluble content hardly contained rare earth elements.
The solution from which the insoluble content was removed was transferred to a
glass container, and diluted with pure water, thereby doubling the volume. The solution
was then heated to 108°C, and the hydrochloric acid excessively present therein was
retrieved by way of distillation. Heating was terminated, and the concentrated solution
was cooled to room temperature, and then, pure water was added thereto such that the
volume became identical to that of the solution before being concentrated. The pH of the
solution was adjusted to 2 by adding 5% aqueous ammonia dropwise while stirring.
1400 parts of a 10% aqueous solution of ammonium hydrogen carbonate was added to the
adjusted solution while stirring whereby a white precipitate of rare earth carbonates was
obtained.
The solution was filtrated, and the obtained white precipitate was washed with
water. Then, this solid content was transferred to a porcelain container, and baked at
800°C for one hour whereby rare earth oxides were obtained. The product was crushed,
and sorted in terms of particle size, thereby reproducing an abrasive whose average
particle size was 0.5 µm. The composition of the recycled abrasive is shown in Table 1.
The recovering efficiency of the rare earth elements was 96%.
The above-described polished glass substrate was subjected to a finish polishing
process by using the above recycled abrasive. The polishing process was conducted by
using a four-way-type double-side polishing machine (Type "5B" produced by Fujikoshi
Machinery Corp.) that was equipped with a suede-type polishing pad. The polishing
conditions were as follows. The slurry concentration was 10%; the slurry-supplying rate
was 60 ml/min; the platen-rotating speed was 90 rpm; the processing pressure was 75
g/cm2; and the polishing time was 10 minutes. The processing rate was 0.7 µm/min in this
polishing process, and the surface roughness of the polished glass substrate was 0.3 nm.

(Example 2)
In the same manner as Example 1, a white precipitate of rare earth carbonates
was obtained. 50 parts of 55% hydrofluoric acid were mixed into 100 parts of the white
precipitate, and the mixture was heated at 350oC for two hours, thereby producing white
rare earth fluorides. 30 parts of the rare earth fluorides, and 100 parts of the rare earth
oxides obtained in Example 1 were mixed, and baked at 900°C. Then, the mixture was
crushed and sorted in terms of particle size whereby an abrasive that contained fluorine
and rare earth elements, and whose average particle size was 1.8 urn was obtained. The
composition of the recycled abrasive is shown in Table 1. The recovering efficiency of the
rare earth elements was 94%.
A 2.5-inch strengthened glass substrate which mainly contained aluminosilicate
was subjected to the polishing process by using the above recycled abrasive. The
polishing process was conducted by using a four-way-type double-side polishing
machine (Type "5B" produced by Fujikoshi Machinery Corp.) that was equipped with a
urethane-type polishing pad. The polishing conditions were as follows. The slurry
concentration was 10%; the slurry-supplying rate was 500 ml/min; the platen-rotating
speed was 90 rpm; the processing pressure was 100 g/cm2; and the polishing time was 30
minutes. The processing rate was 1 µm/min in this polishing process, and the surface
roughness of the glass substrate was 0.9 nm after the polishing process.
(Example 3)
A polishing waste fluid was obtained in the same manner as Example 1. The
waste fluid was dried, and the remaining content was crushed, thereby obtaining a brown
powder. 100 parts of the brown powder; 1500 parts of hydrochloric acid (density: 1.096
g/cm3, about 20% concentration) obtained in the same manner as Example 1; and 320
parts of glass beads having a diameter of 0.2 mm were transferred into a glass container
equipped with a reflux device, and a mixture thereof was obtained. The mixture was
heated to 106°C. After five hours, the red slurry of the waste fluid turned into a yellow or

green transparent liquid. After a while when heating was terminated, and the slurry was
allowed to stand, a gel-like product and glass beads remained in the bottom of the glass
container.
The container was heated again whereby 500 parts of hydrochloric acid were
collected by way of distillation at 109°C.
Heating was terminated, and the insoluble content remaining in the bottom of the
glass container was separated and removed from the concentrated solution by way of
filtration. The insoluble content was washed with water in a stainless steel sieve having a
pore size of 100 µm whereby glass beads were retrieved.
The solution from which the insoluble content was removed was transferred to a
glass container, and diluted with water, thereby doubling its volume. A 5% sodium
hydroxide aqueous solution was added dropwise to the solution while stirring whereby
the pH was adjusted to 2. 1800 parts of 8% sodium hydrogen carbonate aqueous solution
were added to the solution while stirring whereby a white precipitate of rare earth
carbonates was obtained.
The above solution was then filtered, and the remaining white precipitate was
washed with water. The obtained solid content was transferred to a porcelain container,
and baked at 700°C for two hours whereby rare earth oxides were obtained. Furthermore,
100 parts of the solid content obtained by washing the white precipitate with water after
filtration were mixed with 50 parts of 55% hydrofluoric acid, and heated to 350°C for two
hours whereby white rare earth fluorides were obtained. 30 parts of the rare earth
fluorides were mixed with 100 parts of the above-obtained rare earth oxides, and 200
parts of pure water, and the mixture was water-ground such that the average particle size
became 2 µm. The particles were baked at 950°C, further crushed and sorted based on
particle sizes whereby abrasive which contained fluorine and rare earth elements, and
whose average particle size was 1.6 µm was reproduced. The composition of the
reproduced abrasive is shown in Table 1. The recovering efficiency of the rare earth
elements was 96%.

* The contents of Ce, La, Nd, Pr, Si, F, and CI were analyzed by way of elemental analysis.
Ce, La, Nd, Pr, and Si were converted to contents by mass of CeO2, La2O3, Nd2CO3, Pr6O11,
and SiO2, respectively (oxide basis), and the contents of F and CI were calculated by mass
as elements, and the remainder was regarded as "Other". In addition, the mass of the solid
content where the mass became constant by way of drying at 120°C was considered to be
100%. The analysis of Ce, La, Nd, Pr, and Si was conducted by using the ICP
luminescence method, and the analysis of F and CI was conducted by way of ion
chromatography.
(Comparative Example 1)
An abrasive was reproduced in the same manner as Example 1 except that no
silicon oxide was added. It was evident that the insoluble content contained rare earth
elements that were present as yellow rare earth fluorides or the like. The recovering
efficiency of the rare earth elements was 75%.

Thus, according to the present invention, the rare earth elements present in an
abrasive can be retrieved at high yield, i.e. 90% or more, and an abrasive that has the same
quality as that of the original abrasive can be reproduced.
INDUSTRIAL APPLICABILITY
According to present invention, namely the method of recovering a rare earth
element from a composition that contains rare earth fluorides, rare earth elements can be
retrieved at low cost and at high yield from a composition containing rare earth fluorides
such as rare earth abrasives containing rare earth oxyfluorides which have been used for
polishing glass or the like. Therefore, this results in saving material resources of the ores
which are raw materials for rare earth abrasives. Moreover, the hydrochloric acid used in
the steps of the method can be recycled, and this further results in material saving.
Furthermore, rare earth abrasives reproduced from the retrieved rare earth elements can
be applied to fine polishing process for glass substrates that are used for optical or
magnetic disks, or used for liquid crystal displays such as a thin film transistor-type
(TFT-type) liquid crystal display (LCD) and a twisted nematic-type (TN-type) liquid
crystal display (LCD); a color filter for a liquid crystal television (TV); a cathode ray tube
(CRT) for TV or the like; spectacle lenses; optical lenses; glass substrates for a LSI
photomask; wired plate glass; substrates for a crystal oscillator.
Thus, the present invention, namely the method of recovering rare earth elements
from a composition containing rare earth fluorides, and the method of producing a rare
earth including the steps thereof have high industrial applicability.

WE CLAIM:
1. A method of recovering a rare earth element from a composition containing a
rare earth fluoride, comprising:
(1) mixing a composition containing a rare earth fluoride, silicon oxide,
and hydrochloric acid;
(2) heating the mixture, thereby dissolving the rare earth fluoride;
(3) removing an insoluble content from the solution obtained at step (2)
to obtain a rare earth-containing solution; and
(4) recovering the rare earth element from the rare earth-containing
solution obtained at step (3).
wherein the silicon oxide is at least one selected from the group consisting of
colloidal silica, silica gel, fumed silica, white carbon and glass beads.
2. The method of recovering a rare earth element as claimed in claim 1, wherein the
rare earth fluoride is a rare earth oxyfluoride.
3. The method of recovering a rare earth element as claimed in claim 1, wherein the
composition containing a rare earth fluoride contains 4% to 10% by mass of fluorine on a
solids basis; and 40% to 90% by mass of cerium oxide, 5% to 55% by mass of lanthanum
oxide, and 1% to 10% by mass of praseodymium oxide on an oxide basis.
4. The method of recovering a rare earth element as claimed in claim 1, wherein the
composition containing a rare earth fluoride contains a rare earth oxide.
5. The method of recovering a rare earth element as claimed in claim 1, wherein the
composition containing a rare earth fluoride includes an abrasive containing a rare earth
fluoride.
6. The method of recovering a rare earth element as claimed in claim 5, the
composition containing a rare earth fluoride is a polishing waste fluid.

7. The method of recovering a rare earth element as claimed in claim 1, wherein the
mixing ratio of the silicon oxide is 53 parts by mass or more with respect to 100 parts by
mass of the fluorine present in the rare earth fluoride.
8. The method of recovering a rare earth element as claimed in claim 1, wherein the
mixing ratio of the hydrochloric acid is within the range of 200 parts to 2200 parts by
mass based on a 35% by mass hydrochloric acid with respect to 100 parts by mass of the
composition containing a rare earth fluoride on a solids basis.
9. The method of recovering a rare earth element as claimed in claim 1, wherein the
heating temperature at step (2) is within the range of 60°C to the boiling temperature of
the mixture obtained at the step (1).
10. The method of recovering a rare earth element as claimed in claim 1, wherein the
mixture is heated for 10 minutes to 10 hours at step (2).
11. The method of recovering a rare earth element as claimed in claim 1, wherein the
insoluble content is removed by way of filtration at step (3).
12. The method of recovering a rare earth element as claimed in claim 1, wherein the
insoluble content is removed at step (3) by separating the rare earth-containing solution,
the silicon oxide, and the insoluble contents other than the silicon oxide.
13. The method of recovering a rare earth element as claimed in claim 13, wherein
the separated silicon oxide is reused at step (1).
14. The method of recovering a rare earth element as claimed in claim 1, wherein the
rare earth element is retrieved at step (4) by obtaining a rare earth oxide from the rare
earth-containing solution.
15. The method of recovering a rare earth element as claimed in claim 1, wherein the
rare earth element is retrieved at step (4) by obtaining a rare earth fluoride from the rare

earth-containing solution.
16. The method of recovering a rare earth element as claimed in claim 1, wherein the
rare earth element is retrieved at step (4) as a precipitate of a rare earth carbonate or a rare
earth oxalate by adding a soluble carbonate, a soluble hydrogen carbonate or oxalic acid
to the rare earth-containing solution obtained at step (3).
17. The method of recovering a rare earth element as claimed in claim 17, wherein
the precipitate of the rare earth carbonate or the rare earth oxalate is further baked at step
(4), thereby recovering the rare earth element as a rare earth oxide.
18. The method of recovering a rare earth element as claimed in claim 17, wherein
the precipitate of the rare earth carbonate or the rare earth oxalate is further reacted with
hydrofluoric acid, thereby recovering the rare earth element as a rare earth fluoride.
19. The method of recovering a rare earth element as claimed in claim 1, comprising
distilling out at least a part of the hydrochloric acid between step (2) and step (4).
20. The method of recovering a rare earth element as claimed in claim 19, wherein
the hydrochloric acid distilled out is reused at step (1).

ABSTRACT

METHOD OF RECOVERING RARE EARTH ELEMENT FROM
COMPOSITION CONTAINING RARE EARTH FLUORIDE
The object of the present invention is to provide a method of recovering a rare
earth element from a composition containing a rare earth fluoride, involving the steps of:
(1) mixing a composition containing a rare earth fluoride, silicon oxide, and hydrochloric
acid; (2) heating the mixture, thereby dissolving the rare earth fluoride; (3) removing an
insoluble content from the solution obtained at step (2) to obtain a rare earth-containing
solution; and (4) recovering the rare earth element from the rare earth-containing solution
obtained at step (3).

Documents:

3925-KOLNP-2008-(17-10-2011)-ABSTRACT.pdf

3925-KOLNP-2008-(17-10-2011)-CLAIMS.pdf

3925-KOLNP-2008-(17-10-2011)-DESCRIPTION (COMPLETE).pdf

3925-KOLNP-2008-(17-10-2011)-EXAMINATION REPORT REPLY RECIEVED.pdf

3925-KOLNP-2008-(17-10-2011)-FORM 1.pdf

3925-KOLNP-2008-(17-10-2011)-FORM 2.pdf

3925-KOLNP-2008-(17-10-2011)-FORM 3.pdf

3925-KOLNP-2008-(17-10-2011)-OTHERS.pdf

3925-KOLNP-2008-(17-10-2011)-PA.pdf

3925-KOLNP-2008-(23-01-2012)-CORRESPONDENCE.pdf

3925-kolnp-2008-abstract.pdf

3925-KOLNP-2008-ASSIGNMENT-1.1.pdf

3925-KOLNP-2008-ASSIGNMENT.pdf

3925-kolnp-2008-claims.pdf

3925-KOLNP-2008-CORRESPONDENCE-1.1.pdf

3925-KOLNP-2008-CORRESPONDENCE-1.2.pdf

3925-KOLNP-2008-CORRESPONDENCE-1.3.pdf

3925-kolnp-2008-correspondence.pdf

3925-kolnp-2008-description (complete).pdf

3925-KOLNP-2008-EXAMINATION REPORT.pdf

3925-kolnp-2008-form 1.pdf

3925-KOLNP-2008-FORM 13-1.1.pdf

3925-kolnp-2008-form 13.pdf

3925-KOLNP-2008-FORM 18-1.1.pdf

3925-kolnp-2008-form 18.pdf

3925-KOLNP-2008-FORM 3-1.1.pdf

3925-KOLNP-2008-FORM 3-1.2.pdf

3925-KOLNP-2008-FORM 3-1.3.pdf

3925-kolnp-2008-form 3.pdf

3925-KOLNP-2008-FORM 5-1.1.pdf

3925-kolnp-2008-form 5.pdf

3925-KOLNP-2008-GPA-1.1.pdf

3925-kolnp-2008-gpa.pdf

3925-KOLNP-2008-GRANTED-ABSTRACT.pdf

3925-KOLNP-2008-GRANTED-CLAIMS.pdf

3925-KOLNP-2008-GRANTED-DESCRIPTION (COMPLETE).pdf

3925-KOLNP-2008-GRANTED-FORM 1.pdf

3925-KOLNP-2008-GRANTED-FORM 2.pdf

3925-KOLNP-2008-GRANTED-SPECIFICATION.pdf

3925-kolnp-2008-international publication.pdf

3925-kolnp-2008-international search report.pdf

3925-kolnp-2008-others pct form.pdf

3925-KOLNP-2008-OTHERS-1.1.pdf

3925-kolnp-2008-others.pdf

3925-kolnp-2008-pct priority document notification.pdf

3925-KOLNP-2008-REPLY TO EXAMINATION REPORT.pdf

3925-kolnp-2008-specification.pdf

3925-KOLNP-2008-TRANSLATED COPY OF PRIORITY DOCUMENT.pdf

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Patent Number

253204

Indian Patent Application Number

3925/KOLNP/2008

PG Journal Number

27/2012

Publication Date

06-Jul-2012

Grant Date

04-Jul-2012

Date of Filing

26-Sep-2008

Name of Patentee

SHOWA DENKO K.K.

Applicant Address

13-9, SHIBADAIMON 1-CHOME, MINATO-KU, TOKYO-1058518, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	ITO, KATSURA	C/O SHOWA DENKO K.K., SHIOJIRI PLANT, 1, OAZA SOGA, SHIOJIRI-SHI, NAGANO-KEN 3996461
2	IMAI, FUMIO	C/O SHOWA DENKO K.K., SHIOJIRI PLANT, 1, OAZA SOGA, SHIOJIRI-SHI, NAGANO-KEN 3996461, JAPAN

PCT International Classification Number

C01F 17/00,C22B 3/00

PCT International Application Number

PCT/JP2007/054922

PCT International Filing date

2007-03-13

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2006-067725	2006-03-13	Japan