Abstract:
The present invention relates to a process for recovering metals from indium tin oxide (ITO) scrap. It allows the selective recovery of indium and tin from waste ITO by means of a simple and environmentally benign dissolution-deposition method, with no requirement of using strong corrosive acid/alkaline chemicals (e.g. hydrochloric acid, nitric acid, sulfuric acid and sodium hydroxide) for dissolution and complicated procedures/operation. The dissolution baths can be reused without observable recovery deterioration. It significantly reduces the cost requirement in the recovery process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of U.S. provisional patent application Ser. No. 61/966,180 filed Feb. 18, 2014, and the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates a method of selective recovery of indium and tin from ITO waste through sequential dissolution and deposition, involving the method and bath formulations to selectively dissolve indium and tin from ITO waste and how to collect indium and tin from the solution. 
       BACKGROUND OF THE INVENTION 
       [0003]    Indium is an important element in electronic and energy-related industries. Its compound indium oxide (In 2 O 3 ) plays an important role in transparent conductive oxide (TCO), which is an essential material for flat panel display, solar cell and other applications. Due to its rising demand and difficulties in extraction, a shortage of indium and rising prices are expected in the future. The establishment of a safe and cost-effective method for recovering indium from indium-containing waste is, therefore, of great importance. 
         [0004]    There are many research groups working on ITO waste recovery and none of them uses the current invention to selectively recover indium and tin from the ITO waste. Park et al. (Silla University) reported a method to recover indium metal from ITO scrap with purity up to 99%. However, the process needed to use highly corrosive medium (i.e. 50% sodium hydroxide) in an indium precipitation bath at 140° C. for 4 hours, which are generally not recommended in large scale production purpose ( Bull. Korean Chem. Soc.,  2011, 32, 3796). Li et al. (Central South University) proposed a method to leach ITO from the waste using H 2 SO 4  and HCl, and remove tin from the leach solution by sulphidation. However, hydrogen sulfur (H 2 S) gas was used as the sulphidation agent which is highly toxic and combustible gas ( Hydrometallurgy,  2011, 105, 207). Benedetto et al. (Cidade Universitaria) proposed the use of organic phosphoric acid-based extractants (e.g D2EHPA) to selective extract indium from ITO-containing leach solution. However, the materials cost of these proprietary extractants is relatively high ( Minerals Eng.,  1998, 11, 447). In the above reports, they all carried out their studies using either highly corrosive chemicals or proprietary extractants, and used to produce a lot of waste in the processes. 
         [0005]    Therefore, low-cost, less toxic and reusable formulations for recovering indium and tin from ITO-containing waste are also need in the industry. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention allows selective recovery of indium and tin from ITO waste through a simple and environmentally benign process, with no requirement of using corrosive acid and alkaline (e.g. hydrochloric acid, nitric acid, sulfuric acid and sodium hydroxide) and complicated procedures/operation. Hence, the present invention significantly reduces the cost and energy requirement in the recovery process. As the present method uses a greener recycling solution, it also reduces the impact on the environment. 
         [0007]    Accordingly, the first aspect of the present invention relates to a dissolution-deposition process for selectively recovering indium and tin from ITO waste. To recover indium, the process of the present invention comprises the following steps: 
         [0008]    a) reducing the size of ITO-containing materials by shredding and crushing so as to form finely-divided particles; 
         [0009]    b) subjecting the finely-divided particles to chemical and physical cleaning for pre-treatment to substantially avoid the interference in dissolution step; 
         [0010]    c) transferring the pre-treated particles to a first dissolution bath comprising a first bath formulation at a bath temperature ranging from 60° C. to 120° C. with continuous stirring for 30-180 minutes to dissolve the pre-treated particles; 
         [0011]    d) adding 50-300% by volume of water into the solution containing the dissolved particles from step (c) to form a first mixture and filtering the first mixture so as to collect an indium-rich filtrate and a tin-rich filtrand; 
         [0012]    e) putting an indium plate in the indium-rich filtrate for 1-10 hours to remove the tin residue by a displacement reaction; 
         [0013]    f) recovering indium with purity not less than 99.9% from the indium-rich filtrate by a first deposition process; 
         [0014]    g) reusing the dissolution bath in step (c) for the next recovery cycle after water evaporation. 
         [0015]    To recover tin, the process of the present invention further comprises the following steps: 
         [0016]    h) transferring the tin-rich filtrand obtained from step (d) to a second dissolution bath comprising a second bath formulation at a bath temperature ranging from 60° C. to 120° C. with continuous stirring for 30-180 minutes to dissolve the tin-rich filtrand; 
         [0017]    i) adding 50-300% by volume of water into the solution containing the dissolved tin-rich filtrand to form a second mixture and filtering the second mixture so as to collect tin-rich filtrate and non-dissolved substrates; 
         [0018]    j) recovering tin from the tin-rich filtrate obtained in step (i) by a second deposition process; 
         [0019]    k) reusing the second dissolution bath in step (h) for the next recovery cycle after water evaporation. 
         [0020]    The second aspect of the present invention relates to bath formulations for recovering indium and tin respectively following the dissolution-deposition process of the present invention. Both the first and second bath formulations for dissolving indium and tin respectively are comprised of low-cost and less corrosive compounds, as opposed to conventional bath formulations which contain highly corrosive sodium hydroxide, hydrochloric acid and sulfuric acid. In one embodiment, the first bath formulation used in the dissolution-deposition process of the present invention comprises a mixture of organic halide and dicarboxylic acid. In another embodiment, the second bath formulation used in the dissolution-deposition process of the present invention comprises a mixture of organic halide and carboxylic acid. 
         [0021]    As compared to conventional process of recovering indium and tin from ITO-containing materials, the whole process of the present invention is done at relatively low temperature, which does not require large amount of energy. The ionic solvents in the bath formulations of the present invention can also be reused after simple water evaporation without observable recovery deterioration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a flow chart depicting the dissolution-deposition process of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Reference will now be made in detail to the presently preferred embodiment of the present invention, serve to explain the principles of the invention. These embodiments or examples are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that changes may be made without departing from the spirit of the present invention. 
       Example 1 
     Composition of Bath Formulations For Indium/Tin Recovery 
       [0024]    The first bath formulation for recovering indium from pre-treated ITO-containing particles mainly comprises the following two components: 
         [0025]    a) One or more than one kind of organic halide salts: The cation of these organic halide salts can be but not limited to tetraalkylammonium, (di-, tri- and tetra alkyl)imidazolium, alkylpyridinium, dialkylpyrrolidinum, dialkylpiperidinium, tetraalkylphosphonium, tetralkylsulfonium, dialkylpyrazolium, and N-alkylthiazolium. In this example, 2-hydroxy-N,N,N-trimethylethanaminium chloride (choline chloride) is used as the organic halide salts in the first bath formulation; 
         [0026]    b) 20-80 mol % of dicarboxylic acid: The dicarboxylic acid can be but not limited to oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid. In one embodiment, the molar ratio between the organic halide salts and dicarboxylic acid in the first bath formulation is about in 1:1. 
         [0027]    The second bath formulation for recovering tin from a tin-rich material (e.g. the tin-rich filtrand obtained from the ITO-containing scrap according to the dissolution-deposition process of the present invention) comprises the following two components: 
         [0028]    a) One or more than one kind of organic halide salts: The cation of these organic halide salts can be but not limited to tetraalkylammonium, (di-, tri- and tetra alkyl)imidazolium, alkylpyridinium, dialkylpyrrolidinum, dialkylpiperidinium, tetraalkylphosphonium, tetralkylsulfonium, dialkylpyrazolium, and N-alkylthiazolium. In this example, 2-hydroxy-N,N,N-trimethylethanaminium chloride (choline chloride) is used as the organic halide salts in the second bath formulation; 
         [0029]    b) 20-80 mol % of carboxylic acid: The carboxylic acid can be but not limited to trifluoroacetic acid, trichloroacetic acid, dichloroacetic acid, chloroacetic acid, propanoic acid, butyric acid, and valeric acid. 
         [0000]    In one embodiment, the molar ratio between the organic halide salts and carboxylic acid in the second bath formulation is about in 1:2. 
       Example 2 
     Electrodeposition Conditions for Collecting Indium/Tin from Bath Formulations with Dissolved Indium and/or Tin 
       [0030]    In Example 1, after dissolving the pre-treated particles of ITO-containing waste into the first bath formulation and being added with water by volume of 50-300%, the solution is filtered and the indium-rich filtrate is separated from the tin-rich filtrand. To collect indium from the indium-rich filtrate, an indium plate is put into the filtrate to remove any residual tin in the filtrate followed by the first electrodeposition process under the following conditions: pH not higher than 1.5; current density from 0.6 to 4 mA/cm 2  or voltage from 2V to 4V, more preferably the voltage is at about 2.6V; electrodeposition time: 30-90 minutes; temperature from 60° C. to 120° C. Indium metal is deposited electrochemically on a substrate from the tin-rich filtrate in an electrochemical cell. The substrate can be but not limited to titanium, stainless steel and graphite. The counter electrode can be but not limited to titanium, platinum and graphite. 
         [0031]    The tin-rich filtrand obtained from filtration of the first mixture is then dissolved in the second bath formulation before being added with water by volume of 50-300% to form a second mixture. The second mixture is filtered to obtain a tin-rich filtrate and a filtrand containing non-dissolved substrates. The tin-rich filtrate is then subjected to the second electrodeposition process under the following conditions: pH not higher than 1; current density from 0.8 to 5 mA/cm 2  or voltage from 2.2V to 4.5V, more preferably the voltage is at about 3.2V; electrodeposition time: 30-90 minutes; temperature from 60° C. to 120° C. Tin metal is deposited electrochemically on a substrate from the tin-rich filtrate in an electrochemical cell. The substrate can be but not limited to titanium, stainless steel and graphite. The counter electrode can be but not limited to titanium, platinum and graphite. 
       Example 3 
     Recovery of Indium and Tin from ITO-containing Scarp Using Bath Formulations and Electrodeposition Process of The Present Invention 
       [0032]    In  FIG. 1 , the recovery route of indium and tin is composed of three main steps. First, the collected and manufacturing ITO-containing scarp are crushed to reduce the waste size, and chemically washed to eliminate those organic residues (e.g. liquid crystals (LCs) in liquid crystal displays (LCDs)). Afterwards, the pre-treated powder is transferred into the dissolution bath containing the respective ionic solvent, such as the first bath formulation for dissolving indium in Example 1. In a first dissolution bath, indium and tin are dissolved from ITO (Eqn. 1 and 2), and stabilized as In(X) 2   −  and Sn(X) 2  respectively (Eqn. 3 and 4). 
         [0000]      In 2 O 3 +6H + =2In 3+ +3H 2 O  (Eqn. 1)
 
         [0000]      SnO 2 +4H + =Sn 4+ +2H 2 O  (Eqn. 2)
 
         [0000]      In 3+ +2 X   2− =In( X ) 2   −   (Eqn. 3)
 
         [0000]      Sn 4+ +2 X   2− =Sn( X ) 2   (Eqn. 4)
 
         [0000]    X: dicarboxylic acid 
         [0033]    As the ionic solvent in the first dissolution bath exhibits dissolution selectivity for In(X) 2   −  and precipitation ability for Sn(X) 2 , tin-rich filtrand can be separated from the mixture through filtration by any suitable filtering means, e.g., glass microfiber and membrane filter. Indium metal can then be collected from the bath through the electrodeposition process under certain conditions such as the respective conditions for indium recovery in Example 2. 
         [0034]    As it is evident from inductively coupled plasma atomic emission spectroscopy (ICP-OES) (Table 1), contents of indium and tin in raw ITO scrap is about 85:15 ratio. By mixing with the first bath formulation in Example 1, for example, a mixture of 2-hydroxy-N,N,N-trimethylethanaminium chloride (organic halide salt) with oxalic acid (dicarboxylic acid) in 1:1 molar ratio, it exhibits dissolution selectivity for indium over tin. In the dissolution process, indium is well-dissolved in the reaction medium and most tin oxides are precipitated. After filtration, the indium to tin ratio in the filtrate increases up to 96:4. If the molar ratio of 2-hydroxy-N,N,N-trimethylethanaminium chloride and oxalic acid is changed to 2:1 or 1:2, there is no significant change in the indium to tin ratio in the resulting filtrand; however, it takes higher temperature (about 120° C.) and longer time (about 180 minutes) to complete the dissolution process. By replacing the dicarboxylic acid with carboxylic acid (e.g. trichloroacetic acid) in the second bath formulation as in Example 1, both indium and tin are well-dissolved in the reaction medium. It exhibits no dissolution selectivity and hence contents of indium and tin after dissolution process is still about 85:15 ratio. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 ICP samples 
                 In:Sn ratio 
                 Purity, % 
               
               
                   
                   
               
             
             
               
                   
                 Raw ITO scrap 
                  85:15 
                 85% 
               
               
                   
                 After dissolution process 
                 96:4 
                 96% 
               
               
                   
                 After displacement reaction 
                 99.95:0.05 
                 99.95%   
               
               
                   
                   
               
             
          
         
       
     
         [0035]    To further reduce the tin content in the filtrate, a galvanic displacement process by indium plate is used to remove the tin impurity. In comparison, indium metal is more reductive. It can take the place of tin in the ionic solvents and promote tin deposition. After the displacement reaction, indium metal is electrochemically deposited on a substrate (e.g. titanium, stainless steel and graphite) and the indium purity can further increase to &gt;99.9%. 
         [0036]    To recover metal tin from the tin-rich filtrand, the tin-rich filtrand is re-dissolved into a second dissolution bath comprising a second bath formulation, such as the second bath formulation in Example 1 for dissolving tin. As the second bath formulation for tin recovery shows good solubility for the tin precipitate, the residues (e.g. glass and plastic) can be easily separated from the mixture by any suitable filtering means, e.g. glass microfiber filter and membrane filter. Tin metal can then be collected from the bath through the electrodeposition process under certain conditions such as the respective conditions for tin recovery in Example 2. 
         [0037]    Since the chemical composition of the bath formulations after each of the dissolution steps is basically unchanged, they can be reused without observable recovery deterioration but simply evaporating the excess water. 
       INDUSTRIAL APPLICABILITY 
       [0038]    The presently claimed method and formulations for dissolving and recovering indium and tin respectively from ITO-containing waste are useful in both waste management and treatment plant for ITO-containing waste such as display panel. The present formulations for dissolving indium and tin comprised of less corrosive and less toxic substances than the conventional formulations are reusable without observable recovery deterioration while the composition of the formulations are substantially unchanged after water evaporation, thereby becoming a better alternative to the conventional formulations in terms of cost and pollution to our environment. 
         [0039]    It is understood that the method/device/system described herein may be performed in different order, concurrently and/or together with other steps not mentioned herein but readily appreciated by one skilled in the art to obtain the method/device/system of the present invention. Without further elaboration, it is believed that one skilled in the art can, based on the description herein, modify the present invention without departing the spirit of the present invention and utilize the present invention to its fullest extend. All publication recited herein are hereby incorporated by reference in their entirety.