Method of recovering valuable metals from izo scrap

0001The present invention relates to valuable metals from used indium-zinc oxide (IZO) sputtering targets or IZO scraps such as IZO scraps generated during production (collectively referred to as "IZO scraps" in the present specification). Regarding the collection method. The "recovery of valuable metal" described in the specification of the present application shall include compounds such as oxides and hydroxides having the valuable metal as a constituent element.

0002In recent years, indium-zinc oxide (In₂O₃-ZnO: Generally called IZO) Sputtering targets are widely used for transparent conductive thin films and gas sensors in liquid crystal display devices, but in many cases, they are placed on a substrate or the like using a thin film forming means by the sputtering method. A thin film is formed on the surface. The thin film forming means by this sputtering method is an excellent method, but when a transparent conductive thin film is formed using a sputtering target, for example, the target is not uniformly consumed. A part of the target that is heavily consumed is generally called an erosion portion, and the sputtering operation is continued until just before the consumption of the erosion portion progresses and the backing plate supporting the target is exposed. And after that, I am exchanging with a new target. Therefore, many non-erosion portions, that is, unused target portions, remain in the used sputtering target, and all of them are scrapped. Also, during the production of the IZO sputtering target, scrap (mill ends) is generated from the polishing powder and cutting powder. Generally, zinc oxide (ZnO) is contained in about 10.7 wt%, but most of them are indium oxide (In).₂O₃).

0003High-purity materials are used as IZO sputtering target materials, and indium is particularly expensive, so it is common to recover indium from such scrap materials, and at the same time, recover zinc. There is. As this indium recovery method, a method combining wet purification such as an acid dissolution method, an ion exchange method, and a solvent extraction method is conventionally used. For example, after cleaning and crushing IZO scrap, it is dissolved in hydrochloric acid, hydrogen sulfide is passed through the solution to precipitate and remove impurities such as zinc, lead, and copper as sulfides, and then ammonia is added to neutralize the IZO scrap to neutralize it with water. This is a method of recovering as indium oxide. However, the indium hydroxide obtained by this method has poor filterability and takes a long time to operate, contains many impurities such as Si and Al, and the indium hydroxide produced depends on its neutralization conditions and aging conditions. Since the particle size and particle size distribution fluctuate, there is a problem that the characteristics of the IZO target cannot be stably maintained when the IZO target is subsequently manufactured.

0004The prior art and its advantages and disadvantages are introduced below. One of them is an etching method of a transparent conductive film in which an ITO film adhered on a substrate is reduced by an electrochemical reaction in an electrolytic solution, and the reduced transparent conductive film is further dissolved in the electrolytic solution (). See Patent Document 1). However, the purpose is a method of obtaining a mask pattern with high accuracy, which is a technique different from the recovery method. As a pretreatment for recovering valuable metals from ITO, there is a technique for separating impurities contained in the In-based brazing material used for joining with the backing plate in an electrolytic solution (see Patent Document 2). However, this is not about the direct technology of recovering valuable metals from ITO.

0005When recovering indium from intermediates obtained as a by-product of the zinc refining process or ITO scrap, tin is separated as a halide sulfate, then reduced with hydrochloric acid or an aqueous nitric acid solution, and then the pH of this aqueous solution is adjusted to 2 to 2 to. Disclosed is a technique for separating the indium component in an aqueous solution by adjusting the value to 5 to reduce metal ions such as iron, zinc, copper, and tarium to a substance that does not easily precipitate (see Patent Document 3). This technique has a problem that the purification process is complicated and the purification effect cannot be expected so much. In addition, as a method for recovering high-purity indium, ITO is dissolved in hydrochloric acid, and alkali is added to the pH to adjust the pH to 0.5 to 4, tin is removed as a hydroxide, and then hydrogen sulfide gas is blown into the copper. , A technique for removing harmful substances such as lead as sulfide and then electrowinning indium metal by electrowinning using this solution is disclosed (see Patent Document 4). This technique also has a problem that the purification process is complicated.

0006The ITO indium-containing scrap is dissolved in hydrochloric acid to prepare an indium chloride solution, and an aqueous solution of sodium hydroxide is added to the solution to remove tin as tin hydroxide. , There is a method of filtering this, converting indium hydroxide after filtration into indium sulfate, and using this to obtain indium by electrolytic sampling (see Patent Document 5). This is an effective method with a large purification effect, but it has the disadvantage that the process is complicated.

0007A step of dissolving ITO indium-containing scrap with hydrochloric acid to obtain an indium chloride solution, a step of adding an aqueous sodium hydroxide solution to the indium chloride solution to remove tin contained in the scrap as tin hydroxide, the step of removing the tin hydroxide. There is a method for recovering indium, which comprises a step of replacing and recovering indium with zinc from the solution after removal (see Patent Document 6). This method is also an effective method with a large purification effect, but has a disadvantage that the process is complicated.

0008The suboxide-containing casting scrap that floats on the molten metal indium is taken out and inserted into an atmosphere furnace, and after the furnace is once evacuated, argon gas is introduced and heated to a predetermined temperature to remove the suboxide-containing casting scrap. A method for recovering metallic indium to be reduced is disclosed (see Patent Document 7). Although this is an effective method in itself, it has the disadvantage that it is not a basic method for collecting ITO scrap. From the above, a method that is efficient and has versatility in the recovery process is required.

0009<patcit num="1"><text>Japanese Patent Application Laid-Open No. 62-290900</text></patcit><patcit num="2"><text>Japanese Unexamined Patent Publication No. 8-41560</text></patcit><patcit num="3"><text>Japanese Unexamined Patent Publication No. 3-82720</text></patcit><patcit num="4"><text>Japanese Unexamined Patent Publication No. 2000-169991</text></patcit><patcit num="5"><text>Japanese Unexamined Patent Publication No. 2002-69684</text></patcit><patcit num="6"><text>JP-A-2002-69544</text></patcit><patcit num="7"><text>Japanese Patent Application Laid-Open No. 2002-241865</text></patcit>

<p num="0010"> The present invention provides a method for efficiently recovering indium and zinc from IZO scrap such as an indium-zinc oxide (IZO) sputtering target or IZO scraps generated during production in order to solve the above problems. It is in.</p>

<p num="0011"> The present invention provides a method for recovering valuable metals from IZO scrap, which recovers indium and zinc as hydroxides of indium and zinc by electrolyzing the IZO scrap in a pH-adjusted electrolytic solution. The method for recovering valuable metals from IZO scrap of the present invention is characterized in that an insoluble electrode is used for the anode or cathode and IZO scrap is used for the other cathode or anode which is the opposite electrode of each. Further, in this case, the polarities of both the anode and the cathode are periodically inverted and electrolyzed, that is, the polarities are alternately changed and electrolyzed (mutual inversion of the anode polarity cathode polarity). This makes it possible to efficiently recover indium and zinc as hydroxides. Conventionally, such a technique does not exist, and there is no literature suggesting this method. Therefore, the method for recovering valuable metals from IZO scrap according to the present invention is a basic invention.</p><p num="0012"> Since IZO scrap is an oxide-based ceramic, it cannot be expected that valuable metals will be recovered by the electrolytic method. However, although IZO itself is an oxide-based ceramic, it has conductivity. Focusing on this point, the present invention attempts to recover valuable metals (indium or zinc and compounds thereof) by electrolysis, and makes it possible. It is already known that IZO itself has conductivity. This is zinc oxide (ZnO) and indium oxide (In)₂O₃) Is considered to be due to oxygen deficiency of IZO oxide, which is a sintered body. The present invention utilizes the conductivity of the IZO itself, but the conductivity of the IZO itself can be realized without finding and judging that valuable metals can be recovered by electrolysis, and without conducting many experiments. It should be understood that it cannot be done.</p><p num="0013"> When collecting conventional IZO scrap, multiple steps such as crushing the IZO scrap, dissolving it with a strong acid, reducing, substituting, sulphurizing, precipitating, neutralizing, filtering, solvent extraction, ion exchange, casting, etc. Is manufactured through a process of appropriately combining. The problem in these steps is that impurities are mixed in the crushing process of IZO scrap, and it is necessary to further remove the impurities mixed in the crushing step in the subsequent steps, which makes the process more complicated. It means that it will be. Therefore, it can be understood that the ability to recover valuable metals directly from IZO scrap by electrolysis has a great advantage.</p><p num="0014"> In the method for recovering valuable metals from IZO scrap of the present invention, it is desirable to reverse the polarities of the anode and cathode when the voltage rises above a certain level during electrolysis. As will be described later, the conversion of the polarities of the anode and the cathode is a means for improving the recovery efficiency, and the voltage is an index thereof. Therefore, it is possible to detect the time when the voltage rises and set the polarity reversal timing accordingly. Generally, if the equipment is fixed, the optimum conditions for the reversal time can be constantly grasped, and therefore the reversal can be performed at a fixed time accordingly. Therefore, it should be easily understood that the control of the reversal timing of the anode and cathode polarities is arbitrary and not constrained by this condition. Further, in the above electrolysis, it is preferable to reverse the polarities of the anode and the cathode in a cycle of 1 minute to 10 minutes. However, the timing of polarity reversal is also a condition that can be arbitrarily changed depending on the capacity of the electrolytic cell, the amount of IZO scrap, the current density, the voltage, the current, and the type of electrolytic solution. It will be easily understood that this indicates suitable conditions and is not bound by these conditions as above.</p><p num="0015"> When recovering a valuable metal from the IZO scrap of the present invention, it can be recovered as a hydroxide of indium and zinc after the above electrolysis. It is desirable to adjust the pH of the initial electrolyte to 8-12. This is a suitable condition for efficiently recovering as a hydroxide of indium and zinc. In this case, if the pH is lower than 8 or higher than pH 12, it dissolves as an ion and is electrodeposited, resulting in a decrease in efficiency.</p><p num="0016"> As mentioned above, the choice of pH is optional. The essence of the present invention is to recover as hydroxides of indium and zinc by electrolysis that periodically reverses the polarity, and unless this exists as a known technique, the reason why it should be limited to the above pH is Absent. Further, it is clear that even if the pH is devised or improved later, it is within the scope of the above idea of the present invention and is included in the present invention. As the electrolytic solution, it is desirable to select a material that does not generate harmful gas and that is not contained as an impurity in these substances when it is recovered as a hydroxide of indium and zinc. From this, a solution of sodium sulfate, sodium chloride, sodium nitrate, ammonium sulfate, ammonium chloride, ammonium nitrate, potassium chloride, potassium nitrate, potassium sulfate and the like can be arbitrarily selected and used.</p><p num="0017"> However, it will be understood that solutions other than the above can be used as the electrolytic solution as long as the IZO scrap can be electrolyzed in consideration of production efficiency. It is clear that the selection of the electrolytic solution is merely to arbitrarily select a solution that meets the conditions under which the IZO scrap can be electrolyzed, and is not the essence of the present invention. The present invention has achieved the object by recovering the hydroxide of indium and zinc obtained by electrolysis when recovering the valuable metal from the IZO scrap, but further, the water of indium and zinc. It is also possible to roast the oxide and recover it as an oxide of indium and zinc.</p><p num="0018"> In this way, once it can be recovered from IZO scrap as hydroxides of indium and zinc, they can be further roasted to obtain a mixture of indium oxide and zinc oxide, which can be used as it is as a raw material for IZO materials. it can. Further, if necessary, indium oxide or zinc oxide can be further added to change the amount of the component thereof, or other elements can be added to sinter and regenerate the IZO target. The present invention includes all of these. Further, the hydroxide of indium and zinc obtained by the above electrowinning is acid-leached to obtain a solution of indium and zinc, the pH of this solution is adjusted to remove zinc as zinc hydroxide, and indium is electrowinned. You can also do it.</p><p num="0019"> As described above, in the recovery of valuable metals from the IZO scrap of the present invention, if the IZO scrap itself to be electrolyzed is a scrap made of a high-purity material, the purity can be maintained as it is, and high-purity indium and zinc water It can be recovered as an oxide. It goes without saying that this is a significant advantage of the present invention. It does not require a conventional complicated process and a process of removing impurities mixed in during production, has an excellent merit of increasing production efficiency and enabling recovery of high-purity valuable metal. Further, the electrolytic conditions such as the current density are not uniquely determined because they are scraps such as scraps, and the current density is appropriately selected and carried out according to the amount of the scraps and the properties of the materials. The temperature of the electrolyte solution is usually in the range of 0 to 100 ° C, but room temperature (15 to 30 ° C) is sufficient.</p>

<p num="0020"> Indium-Zinc Oxide (IZO) Hydroxylation of indium and zinc is extremely easy because IZO scraps such as sputtering targets or IZO scraps generated during manufacturing are used and this is simply electrolyzed as a cathode consisting of insoluble electrodes and scraps. It is an excellent method that can be efficiently recovered as a product or as a mixture of indium oxide and zinc oxide. Further, in the recovery of valuable metals from the IZO scrap of the present invention, if the IZO scrap itself to be electrolyzed is a scrap made of a high-purity material, the purity can be maintained as it is, and high-purity indium and zinc hydroxides can be maintained. Can be recovered as. This is a significant advantage of the present invention. It does not require the conventional complicated process and the process of removing impurities mixed in during the production, has an excellent merit that the production efficiency is increased and high-purity valuable metal can be recovered.</p>

0021According to the present invention, the indium-containing scrap of the IZO target can be easily recovered as a hydroxide of indium and zinc by electrolysis. Further, by roasting the obtained hydroxide of indium and zinc, it can be efficiently recovered as a mixture of indium oxide and zinc oxide. The roasting temperature is 100 to 1000 ° C. It is preferably 100 to 500 ° C. Moisture remains below 100 ° C, and sintering occurs above 1000 ° C, so it is desirable to keep the above range. However, the choice of roasting temperature is optional. As described above, the present invention is based on the essence of the present invention to roast the hydroxides of indium and zinc obtained by electrolysis that periodically reverses the polarity of the present invention, unless this exists as a known technique. There is no reason to be limited to the roasting temperature. Further, it is clear that even if the roasting temperature is devised or improved later, it is within the scope of the above idea of the present invention and is included in the present invention.

0022As the electrolytic solution, a solution of sodium sulfate, sodium chloride, sodium nitrate, ammonium sulfate, ammonium chloride, ammonium nitrate, potassium chloride, potassium nitrate, potassium sulfate or the like can be arbitrarily selected and used. In addition, when the anion is chlorine-based, chlorine gas is generated due to the passivation of the anode, and when the anion is nitrate-based, nitrogen oxide gas is generated due to the passivation of the anode and the nitrogen load of wastewater is increased. Since there is, it is necessary to pay attention to the processing. Sulfuric acid-based materials have almost no of these problems, so it can be said that sulfuric acid-based materials are suitable materials. However, there is no reason to prevent the use of other electrolytes as long as the above problems can be solved. In addition to this, it is also possible to use a generally known and known additive in order to increase the current efficiency. Thus, it will be understood that if indium oxide and zinc oxide can be recovered at the same time, it will be easy to produce regenerated IZO.

0023No special electrolyzer is required. For example, an insoluble electrode may be used for the anode or cathode, and IZO scrap may be used for the other cathode or anode, which is the opposite electrode, for electrolysis. Whether the initial anode or cathode is an insoluble electrode or an IZO scrap is not a particular problem. The reason is that the polarity is periodically reversed. It should be easily understood that the invention of the present application is applicable in any case. This makes it possible to avoid an increase or contamination of impurities beyond those contained in IZO scrap. As the insoluble electrode, already known ones can be used. Carbon or the like is a preferable material, but it is not necessary to limit the material to this insoluble electrode. All what are generally referred to as insoluble electrodes are applicable. It can be easily understood that the selection of the insoluble electrode does not have the essence of the present invention.

0024It is desirable to adjust the electrolysis conditions appropriately according to the type of raw material. The only factor to adjust in this case is production efficiency. In general, it can be said that the productivity is better when electrolyzing with a large current and a high voltage. However, it is not necessary to be limited to these conditions, and the choice is optional. The electrolysis temperature is not particularly limited, but it is desirable to adjust the temperature to 0 to 100 ° C for electrolysis. It can be sufficiently electrolyzed at room temperature. Scraps that have become scraps can be placed in a cathode box (basket) and electrolyzed. If the scrap itself has a predetermined size (size that can be used as an electrode), it can be used as it is as an electrode plate.

0025When the cathode made of an insoluble electrode or IZO scrap is energized and electrolysis is started, oxygen gas is generated at the insoluble anode. However, the generation of this oxygen gas is not a particular problem. At the cathode of IZO scrap, hydrogen gas is generated when energization is started, and IZO scrap is reduced to hydrogen to become indium-zinc metal (IZO + H).₂ In-Zn metal). Hydrogen is generated by electrolysis of water (H)₂O 1 / 2H₂+ OH^-). This indium-zinc metal accumulates on the surface of the cathode of IZO scrap. Part of it precipitates as a mixture of indium hydroxide and zinc hydroxide.

0026However, when the energization time becomes long, some thickness of In-Zn metal accumulates on the surface of the cathode of IZO scrap, and only sponge-like In-Zn suboxide is formed under the surface layer of In-Zn metal. And no further reduction will proceed. This is because the in-Zn metal surface layer suppresses the penetration of hydrogen, and the current flows only in the In-Zn metal surface layer, and the current flow inside the IZO scrap is suppressed, which promotes the progress of electrolysis. It is considered to be the main cause of interference. In this state, the desired electrolysis is suppressed at the cathode of the IZO scrap. here,<u style="single">Insoluble electrode</u>With the anode<u style="single">IZO scrap</u>Invert the polarity of the cathode. This is a very important process.

0027As a result, the In-Zn metal accumulated on the surface of the new anode (old cathode) is dissolved. Since the electrolytic solution is retained in the neutral region, it precipitates as a hydroxide. The precipitate thus obtained can be recovered as a hydroxide of indium and zinc. The main reaction formula is (In-Zn In)³⁺+ Zn²⁺ In (OH)₃+ Zn (OH)₂). A small amount of oxygen is generated at the new anode. On the other hand, in the new cathode (old anode), since an insoluble electrode is used, there is essentially no fluctuation and only hydrogen gas is generated.

0028The above steps promote the precipitation of hydroxides of indium and zinc. However, if this state is continued, only the surface layer of the new cathode will be in-Zn metalized again, but the inside will not be energized and electrolysis will not proceed. Before this state is reached, the polarity is changed again. By repeating this, the precipitation of hydroxides of indium and zinc can be constantly promoted. By adopting the step of periodically reversing the electrode, the generation of gas, that is, hydrogen and oxygen, generated in the electrode is significantly reduced as compared with the case where one fixed electrode of the anode or the cathode is used. This shows that the generated gas is effectively consumed for oxidation and reduction.

0029The conversion of the polarity of the anode and the cathode is a means for improving the recovery efficiency, and the voltage is an index thereof. Therefore, it is possible to detect the time when the voltage rises and set the polarity reversal timing accordingly. If the equipment is fixed, the optimum conditions for the reversal time can be grasped empirically, and the reversal can be performed at a fixed time accordingly. Further, according to the experiment, it is preferable to reverse the polarities of the anode and the cathode in a cycle of 1 minute to 10 minutes. However, the timing of polarity reversal is also a condition that can be arbitrarily changed depending on the capacity of the electrolytic cell, the amount of IZO scrap, the current density, the voltage, the current, and the type of electrolytic solution.

0030Next, an embodiment will be described. It should be noted that the present embodiment is for showing an example of the invention, and the present invention is not limited to these examples. That is, it includes other aspects and modifications included in the technical idea of the present invention.

0031(Example 1) Horizontal 20 mm x vertical 100<u style="single">mm</u>× 90 g of IZO (indium oxide-zinc oxide) plate-shaped scrap (scrap) with a thickness of 6 tons was used as a raw material. The components in this raw material are zinc oxide (ZnO) 10.7 wt% and the balance indium oxide (In).₂O₃) (The ratio of metal is In: 73.8wt%, Zn: 8.6wt%, and the rest is oxygen (O)). This raw material was used as a cathode, and carbon, which is an insoluble anode, was used as the anode. Electrolysis was performed using 1 L of an electrolytic solution containing 70 g / L of sodium sulfate at a pH of 9.0 and an electrolytic temperature of 30 ° C. Voltage is 10V (constant voltage), current is 2.95A (at the start) to 1.2A (at the end), energization time (5 minutes x 12 cycles of polarity conversion)<u style="single">A total of 60 minutes (1 hour) was carried out. Usually, the total time is 600 minutes (10 hours).</u>.. .. As a result, a mixture of indium hydroxide and zinc hydroxide was precipitated in the electrolytic cell. This causes In (OH)₃Is 10g (In grade: 69.23wt%), Zn (OH)₂Obtained 2 g (Zn grade: 7.73 wt%). This indium hydroxide and zinc hydroxide<u style="single">of</u>The purity of the mixture was comparable to that of scrap.

0032(Example 2) Further, the indium hydroxide and zinc hydroxide thus obtained<u style="single">of</u>The mixture is roasted at 150 ° C and In oxide (In₂O₃) And a mixture of Zn oxide (ZnO) could be obtained. This mixture weighed about 12 g. The ratio obtained by this method is usually In₂O₃It was in: 90wt% and ZnO: 10wt%, and could be used as a raw material for recycled IZO.

0033(Example 3) Indium hydroxide and zinc hydroxide obtained by electrolysis<u style="single">of</u>The mixture was further acid-leached with sulfuric acid to form a solution of indium and zinc, and indium was further electrolyzed at an electrolytic temperature of 30 ° C and a current density of 2 A / dm.²Electrowinning was performed under the condition. From the above, we were able to recover In's valuable metal from IZO scrap. The yield of In was 98%.

0034(Example 4) Electrolysis was performed at pH 10 using the IZO offcuts of Example 1 as the cathode, Pt as the anode, and a solution of sodium nitrate 100 g / L. As a result, indium<u style="single">And zinc</u>Hydroxide was obtained. The recovered amount and purity were the same as in Example 1.

0035(Example 5) The amount of current was kept constant at 2A, and the polarity was set to be reversed when the voltage reached 10V or higher. Others were the same as in Example 1, and the integrated current amount was also the same. As a result, the recovered amount and purity were almost the same as those in Example 1.

0036(Example 6) The cycle was set to 1 minute, and the other parts were electrolyzed under the same conditions as in Example 1. As a result, the recovered amount and purity were the same as in Example 1.

0037(Example 7) The cycle was 10 minutes, and the other parts were electrolyzed under the same conditions as in Example 1. As a result, about 9 g of In oxide (In₂O₃) And a mixture of Zn oxide (ZnO) were obtained, the purity was comparable to that of Example 1.

0038(Example 8) 10 kg of IZO offcuts was placed in a cathode box, Pt was used as an anode, and electrolysis was performed in an electrolytic solution of sodium chloride 100 g / L and pH 10.5. Polarity conversion was performed at 5-minute intervals. The integrated current of electrolysis was 10,000 AHr. As a result, In oxide (In₂O₃) And a mixture of Zn oxide (ZnO) could be obtained in an amount of about 6 kg. The purity of the obtained mixture was about the same as in Example 1.

0039(Comparative example 1) Using the same IZO scrap as in Example 1, this was used as the cathode and insoluble carbon was used as the anode. The electrolysis conditions were the same as in Example 1. As a result, indium-zinc metal was obtained for the cathode, but it was only on the surface of the cathode, and the entire cathode could not be metallized, and effective recovery was not possible.

0040In the above examples, zinc oxide (ZnO) is around 10 wt% and the balance is indium oxide (In).₂O₃) IZO (Indium Oxide-Zinc Oxide) scraps or scraps were used, but In₂O₃It goes without saying that the electrolytic conditions such as current density and pH can be arbitrarily changed according to the amount of ZnO components, and the amount of components of this raw material does not have to be particularly limited. In particular, IZO may change the zinc oxide (ZnO) content from 5 wt% to 30 wt%, and even in such a case, the present invention can be sufficiently applied. In addition, there are some IZOs to which a smaller amount of sub-ingredients are added, but it goes without saying that the present invention can be applied to these as long as IZO is basically the basic ingredient. The present invention is for the anode or cathode.<u style="single">Use an insoluble electrode and the other cathode or anode that is the opposite of each</u>It can be seen that valuable metals can be efficiently recovered from IZO scrap as a mixture of indium hydroxide and zinc hydroxide by using IZO scrap and changing the polarity.<u style="single">Ka</u>To.

0041The present invention uses indium-zinc oxide (IZO) sputtering target or IZO scrap such as IZO scraps generated during manufacturing as the cathode, and electrolyzes this as the anode and cathode, so that indium hydroxide and indium hydroxide and the cathode are extremely simple. It can be efficiently recovered as a mixture of zinc hydroxide and further as a mixture of indium oxide and zinc oxide. Further, in the recovery of valuable metals from the IZO scrap of the present invention, if the IZO scrap itself used for electrolysis is scrap made of a high-purity material, the purity can be maintained as it is, and high-purity indium hydroxide and zinc hydroxide can be maintained. Can be recovered as a mixture of indium oxide and zinc oxide. This is a significant advantage of the present invention. It does not require the conventional complicated process and the process of removing impurities mixed in during manufacturing, has the excellent merit of increasing production efficiency and enabling recovery of high-purity valuable metals, and has the excellent advantage of being able to recover valuable metals from IZO scrap. It is extremely useful as a method for recovering valuable metals.