J. Cent. South Univ. (2014) 21: 1741−1746 DOI: 10.1007/s11771­014­2118­x Removal of tin and extraction of indium from acid­dissolved solution of waste indium­tin targets LI Rui­di(李瑞迪), YUAN Tie­chui(袁铁锤), FAN Wen­bo(范文博), ZHOU Li­bo(周立波), WU Hao­bo(吴浩波), LI Jian(李健) State Key Laboratory of Powder Metallurgy (Central South University), Changsha 410083, China © Central South University Press and Springer­Verlag Berlin Heidelberg 2014 Abstract: The recovery of indium from waste indium tin oxide (ITO) target has great significance for the economy and environment. Based on our previous study on the optimization of acid leaching technique, the present study focuses on tin removal via zinc substitution and indium recovery from a tin­free leach solution. The results show that when the amount of added zinc powder and reaction time increase, the tin removal effect can be improved. The optimal conditions obtained are as follows: additional content of zinc powder from 20 g/L to 25 g/L, reaction temperature of 60 °C, and reaction time from 3 h to 4 h. Under this condition, the tin removal rate exceeds 98%, and the tin content in the tin removal solution is lower than 0.05 g/L. After tin removal, the substitution time could be reduced from 3−5 d to 1−2 d by neutralizing the residual acid by using alkaline residue and maintaining the pH value less than 2. The indium recovery rate is also improved when this condition is used. The indium content in the tin residue is reduced to lower than 0.1% and the acid­insoluble β­SnO2 could be obtained via the strong nitric acid leaching of the indium­containing tin residue. Indium could be recovered from ITO with a high purity of 99.995% via electrorefining. Key words: tin removal; leaching; ITO waste targets; recovery rate 1 Introduction Transparent conductive oxides are widely used in the opto­electronic field because of their special visible light transmission and good electrical conductivity. Among these oxides, indium tin oxide (ITO) has the best combined properties, thus it has many applications. The ITO target is prepared by mixing indium oxide and tin oxide powders and then by molding, sintering, mechanical milling and joining to an oxygen­free copper backboard. However, 10%−30% of ITO target is left as the residue during the shaping process. The ITO film is usually prepared by magnetron sputtering, in which the atom can be sputtered by bombarding high­energy particles and deposited on the substrate to form ITO films. During the sputtering process, 40%−60% of the ITO target is left as the residue. The main compositions are indium oxide and tin oxide with an indium content of 72%−75% and a tin content of 6%−8%. Thus, waste ITO target has a significant recyclable value [1−3]. TOMII and TSUCHIDA [4] proposed an indium recovery method by leaching ITO. KRAJEWSKI and HANUSH [5] demonstrated a method of indium solvent extraction from leached acidic or alkaline solution. KANG et al [6−7] improved the solvent extraction process by changing the solvent, and PC88A was selected as the most suitable solvent. HSIEH et al [8] studied the recovery of indium by hydrometallurgy and hot immersion to hydrolyze tin, but the spin­off has a high indium content. HASEGAWA et al [9] introduced a new technique for the treatment of waste ITO glass by using aminopolycarboxylate chelants in combination with a mechanochemical treatment process. A route for indium recovery from spent commercial PtSnIn/Al2O3 catalysts was introduced by using strong basic mesoporous and macroporous anion exchange resins [10]. LI et al [11] proposed a process involving leaching­ neutralization­precipitation­zinc cementation to recover indium. However, the indium was easily absorbed into hydrous tin oxide, which resulted in a decrease in the recovery rate by 78%−81%. LI et al [12] studied a recovery method of indium from ITO target, which includes acid leaching, tin removal from the leach solution via sulfide precipitation, and deposition of sponge indium. MA et al [13] developed an efficient rough vacuum­chlorinated separation method for the recovery of indium by using NH4Cl as a chlorinating agent. The results indicated that the rough vacuum atmosphere can increase the recovery ratio of indium and simultaneously reduce the influence of tin [13]. KATO et al [14] proposed a process that involved homogeneous liquid–liquid extraction separation and concentration of indium. The leaching of indium from ITO by using an Foundation item: Project(2012BAE06B01) supported by the National High Technology Research and Development Program of China Received date: 2013−11−01; Accepted date: 2014−01−15 Corresponding author: YUAN Tie­chui, Professor, PhD; Tel: +86−731−88836476; E­mail: [email protected]
1742 acid was considered as an effective and universal method, which has been used in practical applications. Tin can also be leached and coexist with indium ion in a leach solution. The study of tin removal from an acid leach solution is necessary. However, only a few studies have systematically addressed this issue. Several methods are available for the removal of tin from the leach solution, such as hydrolysis, sulfide precipitation, sponge indium replacement, and tin powder replacement. In the hydrolysis method, the as­produced Sn(OH)4 is used as a cementing material, which is difficult for solid−liquid separation. The sulfide precipitation method can effectively remove tin, but the tin slag is always accompanied with a high indium content. Tin can be substituted by indium, but the replacement rate is so low and needs a long time. Compared with indium, zinc ion can effectively replace tin ion because it has a lower potential than indium ion. In our previous study, the parameter optimization of acid leaching of an ITO target was investigated. Based on our previous study, the present study focuses on tin removal by zinc substitution and indium recovery from a tin­free leach solution. 2 Experimental Figure 1 shows the flow chart of tin removal and indium recovery from the leaching solution of waste ITO target. Tin was substituted by zinc powder and a tin J. Cent. South Univ. (2014) 21: 1741−1746 removal solution formed which contained a large amount of indium ion. The experimental process of replacing indium by a zinc plate was as follows: alkaline residue was added into the tin removal solution by stirring the solution and carefully controlling the addition rate to ensure that the pH value was below 2. Zinc plate was added into the solution for 1 d, and the sponge indium was removed until the indium content was below 0.01 g/L. As­substituted sponge indium was then washed and briquetted. The relative density of waste ITO target was 98%, and the chemical compositions are listed in Table 1. The waste ITO target for acid leaching was pre­processed by caustic washing, drying, and smashing. This experiment was performed using a water bath heater with a temperature error of ±1 °C. Motor agitation and vacuum pump filtration were performed for liquid−solid separation. The material was exsiccated using an electric heating oven with a temperature error of ±1 °C. 3 Results and discussion 3.1 Tin removal by replacement reaction 3.1.1 Theory base The tin ion in the leaching solution can be replaced by zinc powder. The indium concentration in the leaching solution is two orders higher than the tin concentration. The added zinc powder can also replace the indium ion, and the as­replaced indium can then replace the tin ion. The main chemical reactions are as Fig. 1 Flow chart showing tin removal and indium recovery from acid leaching solution
J. Cent. South Univ. (2014) 21: 1741−1746 1743 Table 1 Experimental results of tin removal by zinc replacing Ion concentration before Ion concentration after Experiment Zinc addition tin removal (g/L) tin removal/(g∙L −1 ) −1 No. content/(g∙L ) Sn In Sn In Compositions of precipitated sediment/% w(Sn) w(In) 1# 2.35 110.52 10 0.58 108.71 88.25 2# 2.35 110.52 10 0.61 108.88 3# 2.35 110.52 15 0.11 107.49 4# 2.35 110.52 15 0.15 5# 1.97 108.79 20 6# 1.97 108.79 7# 2.01 8# Tin removal rate/% 5.27 75.32 85.39 5.42 74.04 75.11 20.42 95.32 108.53 74.62 21.88 93.62 0.035 106.11 68.57 25.44 98.22 20 0.038 105.47 69.21 25.07 98.07 110.34 25 0.034 105.77 60.05 31.42 98.31 2.01 110.34 25 0.025 106.04 60.31 32.37 98.76 9# 2.47 115.07 30 0.011 111.49 54.13 40.74 99.56 10# 2.47 115.07 30 0.027 110.77 53.89 40.35 98.91 follows: 3Zn+2InCl3=2In+3ZnCl2 Zn+SnCl2=Sn+ZnCl2 2Zn+SnCl4=Sn+2ZnCl2 2In+3Sn 2+ →2In 3+ +3Sn 4In+3Sn 4+ →4In 3+ +3Sn (1) (2) (3) (4) (5) The tin removal effect depends on the potential differences among Zn, In and Sn, as shown in Eqs. (6)− 0 (8). The standard potentials are e Zn = −0.763 V,
2 + /Zn 0 e In =
3+ /In −0.34 V,
0 e Sn 2+ /Sn = −0.14 V, and
0 e Zn 4 + /Zn 2 +
=
0.15 V. Figure 2 shows the relationship between oxidation potential and ion activity. Sn 2+ /Sn has the highest oxidation potential; thus, tin can be substituted by zinc. 0 Zn →Zn 2+ +2e, e Zn 2 + /Zn = e Zn 2 +
/Zn 0 In→In 3+ +3e, e In 3 + /In = e In 3 +
/In 0 . 059 æç 1 ö÷
lg 2 + (6) ça ÷
2 è Zn ø
0 . 059 æç 1 lg 3 +
ça
3 è In Fig. 2 Effect of ion activity on oxidation potential ö
÷
÷
ø
(7) 0 Sn →Sn 2+ +2e, e Sn 2 + /Sn = e Sn 2 +
/Sn 0 . 059 æç 1 ö÷
lg 2 +
ça ÷
2 è Sn ø
(8) The electromotive force of Eq. (4) can be described as
0 0 e = e Sn - e In 2 + 3 +
/Sn /In 0 . 059 3 + 0 . 059 2 +
lg a In +
lg a Sn 3 2 (9) Assuming that the reaction reaches equilibrium, ε= 2 +
3 +
0, the relationship a Sn = 10 -14 . 5 a In can be obtained, which shows that the activity of Sn 2+ is much lower than that of In 3+ . Thus, the Sn 2+ ion concentration can be decreased to an extremely low extent. In the same approach, based on reaction (5), Sn 4+ concentration can also be decreased to a significantly small value. Given that 0 0 e Zn is much higher than e In , the replacement 2+
3+
/Zn /In reaction between Zn and Sn 2+ , Sn 4+ is easier than replacing Sn 2+ , Sn 4+ with In. Based on this phenomenon, zinc powder was used to replace Sn ion in this experiment. However, both zinc addition content and reaction time have great effect on tin removal. A low zinc powder content could not replace the tin ion completely, whereas a high zinc powder content could replace indium ion although the tin ion could be replaced entirely. In the same approach, a short reaction time could not yield a complete tin removal, but a long reaction time will influence the production efficiency. Therefore, the effect of additional content of zinc powder and reaction time on tin removal was studied as follows. 3.1.2 Effect of additional content of zinc powder on tin removal The replacement experiment was performed using different zinc powder contents at 60 °C for a reaction time of 4 h, which are fixed as constants. The detailed experimental results are listed in Table 1. The increase in
1744 additional content of zinc powder improved the tin removal effect. The Sn ion concentration before tin removal was about 2 g/L to 3 g/L. When a zinc powder content of 10 g/L was applied, the tin removal rate was merely 74.68%. The indium content in the tin sediment was only 5%, and the tin concentration after tin removal was above 0.5 g/L. When a relatively higher zinc powder content of 15 g/L was added, the tin removal rate increased to 94.47%, whereas the indium content in the tin sediment increased to 20%−22%. When the zinc powder content was increased to 20%, the tin concentration in the solution can be decreased from 2%− 3% to 0.05%, with a tin removal rate of 98% and an indium content in the tin sediment of about 25%. Further increasing the zinc powder content could not significantly remove the tin in the solution, but the indium content in the tin sediment markedly increased. When the zinc powder content reached to 30%, the indium content in the tin sediment increased to 40%, which remarkably reduced the direct indium recovery rate. The relationship between zinc powder content and tin removal rate is shown in Fig. 3. The tin removal rate became flat and exceeded 98% when the zinc powder content was more than 20%. Considering the indium direct recovery rate and cost, the optimal additional content of zinc powder should be 20 g/L to 25 g/L. Fig. 3 Effect of additional content of zinc powder on tin removal rate 3.1.3 Effect of reaction time on tin removal The replacement experiment was performed in different replacement reaction times at 60 °C and a zinc powder content of 20 g/L, which are fixed as constants. The relationship between tin removal rate and reaction time is shown in Fig. 4. A tin removal rate of 90% can be obtained at a reaction time of 1 h. When reaction time was prolonged to 3 h, the as­received tin removal exceeded 98% and then plateaued, and the tin concentration in the solution was lower than 0.05 g/L. J. Cent. South Univ. (2014) 21: 1741−1746 Fig. 4 Effect of zinc replacement reaction time on tin removal rate Considering the aforementioned factors, the optimal zinc replacement time was 3 to 4 h. 3.2 Sponge indium preparation by zinc replacement reaction Sponge indium was produced via the zinc plate replacement reaction. After the replacement reaction, a large amount of residual hydrochloric acid was observed (about 50 g/L). Therefore, large amounts of hydrogen may be formed via replacement reaction, which is dangerous and consumes a lot of zinc plate. The alkaline residue, which is formed in the sponge indium cast, contains large amounts of alkali, hydrosoluble Na3InO3, acid­soluble indium, and indium oxide. The main chemical reactions are as follows: NaOH+HCl=NaCl+H2O (10) 2In+6HCl=3InCl3+2H2↑ (11) In2O3+6HCl=2InCl3+3H2O (12) NaInO2+4HCl=NaCl+InCl3+2H2O (13) The alkaline residue can be used to neutralize the residual acid of tin removed from the solution to achieve the following objectives: 1) when the alkaline residue is used to neutralize the acid, the dosage of zinc plate used for the replacement reaction can be minimized; 2) the risk is decreased because the amount of hydrogen, which may be formed during the replacement process, can be greatly reduced; 3) the reaction velocity can be accelerated such that the reaction time can be decreased from 3 d to 5 d without alkaline neutralization for 1 d to 2 d with alkaline neutralization; 4) the metal indium, indium oxide, and Na3InO3 can be dissolved in the tin removal solution and can be used to replace sponge indium, thereby improving the indium recovery rate, especially the direct recovery rate. Notably, the increase in speed should be carefully controlled to guarantee that
J. Cent. South Univ. (2014) 21: 1741−1746 1745 the pH value is less than 2 in the tin­removal solution, which can inhibit In(OH)3 precipitation. In+HNO3→In(NO3)3+H2↑+NO↑+NO2↑ (16) Sn+HNO3→SnO2+H2O+NO↑+NO2 ↑ (17) 3.3 Briquetting and casting After briquetting, the results show that the as­substituted sponge indium contained 5% to 10% water. The as­received sponge indium contained impurities of zinc and tin. When casting, the impurities of zinc and tin could be removed by adding caustic soda at 300−450 °C. Zinc and tin can react with caustic soda via the following chemical reactions: Zn+HNO3→Zn(NO3)2+H2↑+NO↑+NO2 ↑ (18) Zn+2NaOH=Na2ZnO2+H2O (14) Sn+2NaOH=Na2SnO2+H2O (15) The as­formed Na2ZnO2 and Na2SnO2 tend to enter the alkaline residue to separate these residues from indium. After casting, the indium production rate was above 90% with chemical compositions of w(In)>99% and w(Sn)<0.2%, which reached the standard of indium anode for electrorefining. After electrorefining, indium with a high purity of 99.995% was obtained, which can be used to prepare the ITO target. In summary, the direct recovery rate of indium exceeded 95%, and the total recovery rate of indium exceeded 98%. 3.4 Processing for tin residue Although the main chemical composition in tin residue is tin, it also contains a large amount of indium and a small amount of zinc. The detailed composition is as follows: 70%−80% Sn, 20%−30% In, and <1% Zn. Therefore, processing the tin residue is necessary to recycle the indium after indium−tin separation. The leached residue of ITO target also has a small amount of indium (<1%), which can also be recycled simultaneously. 3.4.1 Strong nitric acid leaching When strong nitric is used to leach the tin residue, the leaching velocity is very high. Thus, the leaching reaction can be finished in 1 h with adequate stirring and acid content via the following reactions: Table 2 lists the detailed experimental results. After nitric acid leaching, the residue compositions contained <0.1% indium and >78% tin with a theoretical content of 78.77% SnO2 (mass fraction). Thus, after nitric acid leaching, the residue was pure SnO2, which can be used to recycle tin via reduction smelting. The indium content in the leaching residue of the ITO target was low, and the nitric acid was in excess after leaching of the tin residue. As such, it can be reused to process the leaching residue and the tin residue. It can not only improve the In 3+ concentration of the leaching solution but also decrease the usage of ammonia. 3.4.2 Ammonia neutralization and indium precipitation After strong acid leaching of the tin residue, the as­formed In(NO3)3 solution contained large amounts of HNO3, which is a strong oxidative agent, and even diluted nitric acid. However, the presence of In(NO3)3 solution is not suitable for zinc plate substitution because it could not yield porous sponge indium but could form large amounts of water during briquetting and consequently result in a low indium production rate during casting. When ammonia is used for the neutralization, In 3+ is deposited as In(OH)3 if the pH value exceeds 4.6, and the as­formed NH4NO3 is left as residue in the solution. By washing and drying, the precipitated In(OH)3 can be transformed to InCl3 solution by dissolving hydrochloric acid. When zinc plate is used in the substitution of InCl3, porous sponge indium can be obtained. The detailed experimental results are illustrated in Table 3. The indium precipitation rate exceeds 99.5%, and the indium content in the indium residue exceeds 68.5%; therefore, the precipitate is pure In(OH)3. Finally, the In(OH)3 precipitate could be dissolved using hydrochloric acid and substituted using zinc plate to produce sponge indium, which is similar to the previously introduced procedures. Table 2 Nitric acid leaching of tin residue results Composition of Composition of Composition of leached Leaching condition tin residue/% leached residue/% solution/(g∙L −1 ) Experiment Kind No. Liquid­to­solid Temperature/ Reaction Sn In Zn w(In) w(Sn) In Sn ratio °C time/h Tin 1 79.33 18.78 0.75 3:1 70 1 0.08 78.11 59.69 0.041 residue Tin 2 69.48 28.00 0.66 3:1 70 1 0.10 78.05 87.48 0.033 residue Leached 3 74.37 1.56 — 3:1 70 1 0.05 78.66 4.88 0.01 residue Leached 4 76.22 0.77 — 3:1 70 1 0.05 78.45 2.41 0.01
residue 1746 J. Cent. South Univ. (2014) 21: 1741−1746 Table 3 Ammonia neutralization and indium precipitation results Experiment Concentration of indium before Concentration of indium after Concentration of indium in Indium precipitation No. precipitation/(g∙L −1 ) precipitation/(g∙L −1 ) precipitation residue/% rate/% 1 59.69 <0.01 68.54 99.51 2 59.69 <0.01 68.51 99.77 3 87.48 <0.01 69.01 99.99 4 87.48 <0.01 68.78 99.99 4 Conclusions 1) When the added zinc powder content and reaction time increase, the tin removal effect could be improved. The following optimal conditions are obtained: additional content of zinc powder of 20−25 g/L, reaction temperature of 60 °C, and reaction time of 3−4 h. In this condition, the tin removal rate exceeds 98%, and the tin composition in tin removal solution decreases to less than 0.05 g/L. 2) Before the zinc plate substitutes the indium ion and when the alkaline residue is used to neutralize and maintain the pH value lower than 2, the substitution time can be reduced from 3−5 d without alkaline neutralization to 1−2 d with alkaline neutralization. Thus, the indium recovery rate can be improved. 3) The indium­contained tin residue can be leached using a strong nitric acid, which enables the indium content in the tin residue to be reduced to less than 0.1% and improves the indium recovery rate. Moreover, the acid­insoluble β­SnO2 can be obtained. 4) Indium and tin can be recovered from ITO with a high purity of 99.995% of metal indium, which can be used for the preparation of the ITO target. [4] [5] [6] [7] [8] [9] [10] [11] [12] References [13] [1] [2] [3] ALFANTAZI A M, MOSKALYK R R. Processing of indium: A review [J]. Minerals Engineering, 2003, 16(8): 687−694. SINCLAIR W D, KOOIMAN G J A, MARTIN D A, KJARSGAARD I M. Geology, geochemistry and mineralogy of indium resources at Mount Pleasant, New Brunswick, Canada [J]. Ore Geology Reviews, 2003, 28(1): 123−145. PARK K S, SATO W, GRAUSE G, KAMEDA T, YOSHIOKA T. Recovery of indium from In2O3 and liquid crystal display powder via [14] a chloride volatilization process using polyvinyl chloride [J]. Thermochimica Acta, 2009, 493(1/2): 105−108. TOMII K, TSUCHIDA H. Solvent extraction recovery process for indium: USA, US 4292284 [P]. 1981−09−29. KRAJEWSKI W, HANUSH K. Process for fluid­fluid extraction of gallium, germanium or indium from liquid solution: USA, US 4666686 [P]. 1987−05−19. KANG H N, KIM K Y, KIM J Y. Recovery and purification of indium from waste sputtering target by selective solvent extraction of Sn [J]. Green Chemistry, 2013, 15: 2200−2207. KANG H N, LEE J Y, KIM J Y. Recovery of indium from etching waste by solvent extraction and electrolytic refining [J]. Hydrometallurgy, 2011, 110: 120−127. HSIEH S J, CHEN C C, SAY W C. Process for recovery of indium from ITO scraps and metallurgic microstructures [J]. Materials Science and Engineering B, 2009, 158(1/2/3): 82−87. HASEGAWA H, RAHMAN I M M, EGAWA Y, SAWAI H, BEGUM Z A, MAKI T, MIZUTANI S. Recovery of indium from end­of­life liquid­crystal display panels using aminopolycarboxylate chelants with the aid of mechanochemical treatment [J]. Microchemical Journal, 2013, 106: 289−294. MARINHO R S, da SILVA C N, AFONSO J C, da CUNHA J W S D. Recovery of platinum, tin and indium from spent catalysts in chloride medium using strong basic anion exchange resins [J]. Journal of Hazardous Materials, 2011, 192: 1155−1160. LI Yan­hui, ZHANG Hui, YANG Yong­feng, ZHANG Jin­tao, WANG Zhi­ping. Recovery of indium from waste ITO target [J]. Journal of the Chinese Rare Earth Society, 2002, 20: s256−s258. (in Chinese) LI Y H, LIU Z H, LI Q H, LIU Z Y, ZENG L. Recovery of indium from used indium­tin oxide (ITO) targets [J]. Hydrometallurgy, 2011, 105(3/4): 207−212. MA En, LU Ri­xin, XU Zhen­ming. An efficient rough vacuum­chlorinated separation method for the recovery of indium from waste liquid crystal display panels [J]. Green Chemistry, 2012, 14: 3395−3401. KATO T, IGARASHI S, ISHIWATARI Y, FURUKAWA M, YAMAGUCHI H. Separation and concentration of indium from a liquid crystal display via homogeneous liquid–liquid extraction [J]. Hydrometallurgy, 2013, 137: 148−155. (Edited by FANG Jing­hua)