Abstract:
Proposed is a method for collecting valuable metal from an ITO scrap in which a mixture of indium hydroxide and tin hydroxide or metastannic acid is collected by subjecting the ITO scrap to electrolysis in pH-adjusted electrolyte, and roasting this mixture as needed to collect the result as a mixture of indium oxide and tin oxide. This method enables the efficient collection of indium hydroxide and tin hydroxide or metastannic acid, or indium oxide and tin oxide from an ITO scrap of an indium-tin oxide (ITO) sputtering target or an ITO scrap such as ITO mill ends arisen during the manufacture of such ITO sputtering target.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for collecting valuable metal from an ITO scrap such as a spent indium-tin oxide (ITO) sputtering target or ITO mill ends arisen during the manufacture of the ITO sputtering target (hereinafter collectively referred to as an “ITO scrap” in this specification). 
     In recent years, an indium-tin oxide (In 2 O 3 —SnO 2 : generally known as ITO) sputtering target is being widely used for a transparent conductive thin film of an LCD device or a gas sensor. In many cases, however, a thin film is formed on a substrate or the like by employing the sputtering method as the thin-film forming means. 
     Although the sputtering method as the thin-film forming means is a superior method, if a sputtering target is used to form a transparent conductive thin film, the target will not be consumed uniformly. A portion of the target with severe consumption is generally referred to as an eroded portion, and the sputtering operation is continued until immediately before a backing plate supporting the target becomes exposed due to the further consumption of the eroded portion. This target is subsequently replaced with a new target. 
     Accordingly, a spent sputtering target will have numerous non-eroded portions; that is, unused portions of the target, and all of these portions become scrap. Moreover, even during the manufacture of ITO sputtering targets, scraps (mill ends) will arise from grinding dust and cutting wastage. Generally speaking, the scrap contains roughly 9.7 wt % of tin oxide (SnO 2 ), but the scrap is mostly indium oxide (In 2 O 3 ). 
     Because high-purity materials are used as the ITO sputtering target material and because indium is particularly expensive, indium and tin are simultaneously collected from the foregoing scrap materials. As this kind of indium collection method, conventionally, a method that combines wet refining such as the acid solution process, ion exchange method, and solvent extraction method is used. 
     For instance, there is a method of subjecting an ITO scrap to cleansing and pulverization, dissolving the ITO scrap in nitric acid, precipitating and eliminating impurities, such as zinc, tin, lead and copper, as sulfide by passing hydrogen sulfide through the solution, thereafter adding ammonia to neutralize the solution, and collecting the resulting indium hydroxide. 
     Nevertheless, the indium hydroxide obtained by the foregoing method has inferior filtration property, requires long operational process, and contains large amounts of impurities such as Si and Al. In addition, with the created indium hydroxide, since the grain size and grain size distribution will vary depending on the neutralization condition, maturization condition and other conditions, there is a problem in that the characteristics of the ITO target cannot be stably maintained upon subsequently manufacturing such ITO target. 
     Conventional technology and its advantages and disadvantages are described below. 
     As one example of such conventional technology, there is an etching method of a transparent conductive film including the steps of reducing an ITO film deposited on a substrate by an electrochemical reaction in the electrolyte, and dissolving the reduced transparent conductive film in the electrolyte (refer to Patent Document 1). However, the object of this method is to obtain a mask pattern with high precision, and relates to technology that is different from the collection method. 
     For pretreatment to collect valuable metal from ITO, there is also technology of separating, in the electrolyte, the impurities contained in an In-based brazing filler material used in the bonding with the backing plate (refer to Patent Document 2). Nevertheless, this method does not relate to technology of directly collecting valuable metal from ITO. 
     Moreover, upon collecting indium from an intermediate obtained as a by-product of the zinc refining process or an ITO scrap, disclosed is technology of separating tin as halogenated stannate, performing reduction treatment with hydrochloric acid or nitric acid solution, subsequently adjusting the pH of this aqueous solution to 2 to 5, reducing metallic ions of iron, zinc, copper, thallium and the like in order to make the metallic ions into a substance that will not precipitate easily, and separating the indium component in the aqueous solution (refer to Patent Document 3). With this technology, however, there is a problem in that the refining process is complicated, and a superior refining effect cannot be expected. 
     Further, as a method for collecting high-purity indium, disclosed is technology of dissolving ITO in hydrochloric acid, adding alkali thereto to make the pH 0.5 to 4, eliminating tin as hydroxide, subsequently blowing hydrogen sulfide gas in order to eliminate hazardous materials such as copper and lead as sulfide, and electrowinning indium metal by performing electrolysis using the obtained solution (refer to Patent Document 4). There is a problem with this technology in that the refining process is complicated. 
     In addition, proposed is a method of dissolving an ITO indium-containing scrap in hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to this solution to eliminate tin as tin hydroxide, additionally adding a sodium hydroxide solution after the elimination to obtain indium hydroxide, filtering the obtained indium hydroxide to obtain indium sulfate from the filtered indium hydroxide, and obtaining indium by electrowinning the indium sulfate (refer to Patent Document 5). Although this is an effective method with a significant refining effect, there is a drawback in that the process is complicated. 
     Also proposed is a method for collecting indium including the steps of dissolving an ITO indium-containing scrap in hydrochloric acid to obtain an indium chloride solution, adding a sodium hydroxide solution to the indium chloride solution to eliminate tin contained in the scrap as tin hydroxide, substituting indium with zinc from the solution after eliminating the tin hydroxide, and thereafter collecting indium (refer to Patent Document 6). Although this is also an effective method with a significant refining effect, there is a drawback in that the process is complicated. 
     Additionally disclosed is a method for collecting metallic indium including the steps of extracting suboxide-containing cast scrap floating on molten metallic indium, introducing this into an atmosphere furnace, introducing argon gas into the furnace after vacuating the furnace once, heating the furnace to a prescribed temperature, and reducing the suboxide-containing cast scrap (refer to Patent Document 7). 
     Although this is in itself an effective method, there is a drawback in that this is not a fundamental collection method of ITO scrap. 
     In light of the above, a method that is efficient and with a versatile collection process is being sought.
     [Patent Document 1] Japanese Patent Laid-Open Publication No. S62-290900   [Patent Document 2] Japanese Patent Laid-Open Publication No. H8-41560   [Patent Document 3] Japanese Patent Laid-Open Publication No. H3-82720   [Patent Document 4] Japanese Patent Laid-Open Publication No. 2000-169991   [Patent Document 5] Japanese Patent Laid-Open Publication No. 2002-69684   [Patent Document 6] Japanese Patent Laid-Open Publication No. 2002-69544   [Patent Document 7] Japanese Patent Laid-Open Publication No. 2002-241865   

     SUMMARY OF THE INVENTION 
     In order to overcome the foregoing problems, the present invention provides a method for efficiently collecting indium and tin from an ITO scrap of an indium-tin oxide (ITO) sputtering target or an ITO scrap such as ITO mill ends arising during the manufacture of such ITO sputtering target. 
     The present invention provides a method for collecting valuable metal from an ITO scrap including the steps of obtaining a mixture of indium hydroxide and tin hydroxide or metastannic acid by subjecting the ITO scrap to electrolysis as an anode in pH-adjusted electrolyte, and collecting indium and tin as a mixture of indium hydroxide and tin hydroxide or metastannic acid. 
     As the electrolyte upon collecting valuable metal from the ITO scrap, an acid solution of sulfuric acid, hydrochloric acid, nitric acid or the like, or an alkali solution of sodium hydroxide, potassium hydroxide, ammonium hydroxide or the like is primarily used. 
     The foregoing electrolyte solutions are preferable examples. There is no particular limitation on the electrolyte solution that may be used, and conditions for efficiently collecting valuable metal may be arbitrarily selected. Desirably, pH of the electrolyte solution is adjusted to 3 to 11. In particular, pH of 5 to 9 is a preferable condition for efficiently collecting a mixture of indium hydroxide and tin hydroxide. 
     The conditions including the current density for performing electrolysis to the scrap such as mill ends are not uniformly defined, and the current density is appropriately selected according to the amount of such mill ends or the property of the material. The liquid temperature of the electrolyte solution is generally in the range of 0 to 100° C., and preferably in the range of 20 to 50° C. 
     The method for collecting indium and tin as a mixture of indium hydroxide and tin hydroxide or metastannic acid by electrolysis according to the present invention is an extremely simple method since the ITO scrap is simply subject to electrolysis as an anode. Nevertheless, no conventional technology has previously adopted this kind of method. 
     The present invention is also able to collect indium and tin as a mixture of indium oxide and tin oxide by roasting the mixture of indium hydroxide and tin hydroxide or metastannic acid obtained by the foregoing electrolysis. 
     Once a mixture of indium hydroxide and tin hydroxide or metastannic acid is collected from the ITO, this may be further roasted to obtain a mixture of indium oxide and tin oxide, and it can be used as is as the raw material of an ITO material. Moreover, as needed, it is possible to recycle the ITO target with ease by sintering the ITO material after adding indium oxide or tin oxide and changing the component amount, or after adding other elements. The present invention covers all of the foregoing aspects. 
     The provided method for efficiently collecting a mixture of indium hydroxide and tin hydroxide or metastannic acid, and even a mixture of indium oxide and tin oxide is a superior method in which indium oxide and tin oxide can be collected extremely easily; that is, an ITO scrap of an indium-tin oxide (ITO) sputtering target or an ITO scrap such as ITO mill ends arising during the manufacture of such ITO sputtering target is simply subject to electrolysis as an anode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method for collecting a mixture of indium hydroxide and tin hydroxide or metastannic acid simply by subjecting an indium-containing scrap of an ITO target to electrolysis. In addition, by roasting the mixture of indium hydroxide and tin hydroxide or metastannic acid, a mixture of indium oxide and tin oxide can be efficiently collected. The roasting temperature is set in a range of 100 to 1000° C., and preferably in a range of 100 to 500° C. Moisture content will remain if the roasting temperature is less than 100° C., and sintering will occur if it exceeds 1000° C. Thus, the roasting temperature is set to be within the foregoing range. 
     As the electrolyte, an acid solution of sulfuric acid, hydrochloric acid, nitric acid or the like, or an alkali solution of sodium hydroxide, potassium hydroxide, ammonium hydroxide or the like is primarily used. Moreover, in order to improve the current efficiency, well-known additive agents may also be used. As described above, if indium oxide and tin oxide can be simultaneously collected, it should be understood that a recycled ITO can also be manufactured easily. 
     A special electrolytic apparatus is not required. For instance, the ITO to be subject to electrolysis is used as the anode, and a corrosion-resisting electrode such as carbon is used as the cathode mother plate when performing electrolysis. It is thereby possible to avoid the increase or inclusion of impurities in the anode. 
     It is desirable to adjust the current density as needed based on the type of raw material. The factor to be adjusted in this case is only the production efficiency. Although there is no particular limitation on the electrolysis temperature, desirably electrolysis is performed by adjusting the temperature to be in a range of 0 to 100° C. Since the current efficiency will deteriorate if the electrolysis temperature is less than 0° C. and the evaporation of the electrolyte will increase if the electrolysis temperature exceeds 100° C., a more preferable range of the electrolysis temperature is 20 to 50° C. 
     EXAMPLES 
     The present invention is now explained in detail with reference to the Examples. These Examples are merely illustrative, and the present invention shall in no way be limited thereby. In other words, various modifications and other embodiments based on the technical spirit claimed in the claims shall be included in the present invention as a matter of course. 
     Example 1 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). The raw material was used as an anode, and electrolytic refining was performed in a mixed solution of sulfuric acid and sodium sulfate. The electrolytic condition was pH of 4 and an electrolysis temperature of 50° C. 
     Consequently, In hydroxide and Sn hydroxide precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of In hydroxide and Sn hydroxide was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of In hydroxide and Sn hydroxide obtained as described above was additionally roasted at 150° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Example 2 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). The raw material was used as an anode, and electrolytic refining was performed in a nitric acid and ammonium nitrate solution. The electrolytic condition was pH of 6 and an electrolysis temperature of 50° C. 
     Consequently, indium hydroxide and metastannic acid precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of indium hydroxide and metastannic acid was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of indium hydroxide and metastannic acid obtained as described above was additionally roasted at 150° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Example 3 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). 
     The raw material was used as an anode, and electrolytic refining was performed in a hydrochloric acid solution. The electrolytic condition was pH of 5 and an electrolysis temperature of 30° C. Consequently, In hydroxide and Sn hydroxide precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of In hydroxide and Sn hydroxide was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of In hydroxide and Sn hydroxide obtained as described above was additionally roasted at 200° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Example 4 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). The raw material was used as an anode, and electrolytic refining was performed in an ammonium chloride solution. The electrolytic condition was pH of 8 and an electrolysis temperature of 50° C. 
     Consequently, In hydroxide and Sn hydroxide precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of In hydroxide and Sn hydroxide was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of In hydroxide and Sn hydroxide obtained as described above was additionally roasted at 150° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Example 5 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). The raw material was used as an anode, and electrolytic refining was performed in a mixed solution of sodium hydroxide and sodium nitrate. The electrolytic condition was pH of 9 and an electrolysis temperature of 50° C. 
     Consequently, indium hydroxide and metastannic acid precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of In hydroxide and metastannic acid was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of indium hydroxide and metastannic acid obtained as described above was additionally roasted at 150° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Example 6 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). The raw material was used as an anode, and electrolytic refining was performed in a sodium hydroxide solution. The electrolytic condition was pH of 10 and an electrolysis temperature of 30° C. 
     Consequently, indium hydroxide and metastannic acid precipitated, but were not electrodeposited on the cathode side. According to the foregoing process, a mixture of In hydroxide and metastannic acid was collected from the ITO (indium oxide-tin oxide) mill ends. 
     The mixture of indium hydroxide and metastannic acid obtained as described above was additionally roasted at 400° C. to obtain a mixture of In oxide (In 2 O 3 ) and Sn oxide (SnO 2 ). This mixture was approximately 1.8 kg. The ratio obtained by this method was generally 72 to 89 wt % of In 2 O 3  and 28 to 11 wt % of SnO 2 , and this mixture was usable as the raw material of recycled ITO. 
     Comparative Example 1 
     2 kg of ITO (indium oxide-tin oxide) mill ends were used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). 
     The raw material was used as an anode, and electrolytic refining was performed in a sulfuric acid solution. The electrolytic condition was pH of 2 and an electrolysis temperature of 50° C. Consequently, Sn hydroxide precipitated, but indium electrodeposited on the cathode side. According to the foregoing process, it was not possible to simultaneously collect a mixture of Sn oxide from the ITO (indium oxide-tin oxide) mill ends. This is considered to be because of the strong acidity of pH2. By roasting Sn hydroxide, approximately 180 g of tin oxide was obtained. 
     Comparative Example 2 
     2 kg of ITO (indium oxide-tin oxide) scrap was used as the raw material. The components in this raw material were 9.7 wt % of tin oxide (SnO 2 ) and the remainder indium oxide (In 2 O 3 ). 
     The raw material was used as an anode, and electrolytic refining was performed in a sodium hydroxide (NaOH) solution at pH12. The electrolytic condition was as follows. Consequently, indium hydroxide precipitated partially, but 
     Sn hydroxide did not precipitate. The metals of indium and tin electrodeposited on the cathode side. According to the foregoing process, it was not possible to simultaneously collect a mixture of In hydroxide and Sn hydroxide from the ITO (indium oxide-tin oxide) scrap. 
     This is considered to be because of the strong alkali of pH12. By roasting indium hydroxide, approximately 1600 g of indium oxide was obtained, but the yield was inferior. In addition, it was not possible to simultaneously collect a mixture of In hydroxide and Sn hydroxide or in oxide and Sn oxide. 
     Although in each of the foregoing Examples, ITO (indium oxide-tin oxide) mill ends or ITO scraps containing 9.7 wt % of tin oxide (SnO 2 ) and remainder indium oxide (In 2 O 3 ) were used, it is possible to arbitrarily change the electrolytic conditions such as the current density and pH according to the component amount of In 2 O 3  and SnO 2 , and it goes without saying that there is no particular limitation on the component amount of the raw material. In particular, although the content of tin oxide (SnO 2 ) in the ITO is changed between 5 wt % to 30 wt %, the present invention can still be applied to this case. 
     In addition, although there are cases where small amounts of accessory components are added to the ITO, so as long as ITO is the basic constituent, it goes without saying that the present invention can also be applied to this case. 
     The present invention yields a significant industrial advantage since provided is a superior method in which indium oxide and tin oxide can be collected extremely easily and efficiently; that is, an ITO scrap of an indium-tin oxide (ITO) sputtering target or an ITO scrap such as ITO mill ends arising during the manufacture of such ITO sputtering target is simply subject to electrolysis as an anode.