現今，銦和他的化合物有多種工業用途，並且廣泛的被使用在製造薄膜晶體管液晶顯示器(TFT-LCD)、半導體、低溫焊料和紅外探測器。但是銦是廣泛的分布於各處，並沒有單獨的儲藏區，它是鋁和鋅初級產品的副產物。生產銦錫氧化物 (ITO) 被使用於各式各樣的平板器材中，像是 TFT-LCDs，並且這會持續地成為全球銦使用的最終用途與消耗。因此，增加與多廣泛的研究有關從二次資源中回收銦。然而，傳統技術涉及顯著用量的強酸及溶劑，會環境不友善，現在一個新的技術是使用次臨界流體的水從廢棄ITO中萃取銦。此實驗使用四種有機酸，像是醋酸、葡糖酸、檸檬酸、依替膦酸，其固液比10 g/L。醋酸和葡糖酸都無法有效溶解ITO。當使用1M的檸檬酸，在100°C反應30分鐘可以溶解 35.99% 的ITO，在125°C和150°C反應30分鐘，壓力從12 kg/cm2到19kg/cm2，分別可以溶解57.28%和70.71%的ITO。 關於酸強度的影響，我們發現0.1M依替膦酸可溶解50.77% ITO，而0.5M和2.0M的依替膦酸分別可溶解96.31%和97.54%的ITO。與傳統萃取方法比較，次臨界水的萃取不只是顯著的有效率，也可以不使用強酸或其他溶劑。此外對滲濾液進行初步測試置換反應，發現滲濾液中有依替膦酸和檸檬酸配體，可以抑制銦或錫被鋁置換的機制，這些銦或錫與依替膦酸或檸檬酸配體形成的複合物跟單純的銦或錫離子相比，氧化還原電位可能會升高。

Nowadays, indium (In) and its compounds have numerous industrial applications, and it is extensively used in the manufacture of thin film transistor liquid crystal display (TFTLCD), semiconductors, low-temperature solders and infrared photodetectors. However, In is widely distributed and has no individual reserve. It is a by-product commodity from primary ones of aluminum and zinc. Production of indium tin oxide (ITO), which is used in a variety of flat panel devices such as TFT-LCDs, continues to be the major end use of In and it accounts for most global In consumption. Therefore, the recovery of In from secondary resources has received extensive study. Yet, traditional technologies involve significant amount of strong acid and solvent and may not be environmentally friendly. A novel technology using subcritical water extraction was applied to extract In from waste ITO. Four types of organic acid including acetic acid, gluconic acid, citric acid, and etidronic acid were used for this purpose at solid to liquid ratio (S/L) of 10 g/L. Neither acetic acid nor gluconic acid yielded effective dissolution of ITO. When 1 M of citric acid was used, 35.99% ITO dissolved at 100°C within 30 min, while 57.28% and 70.71% dissolved at 125°C and 150°C, respectively, as pressure varied from 12 to 19 kg/cm2. Regarding effect of acid strength, it was found that 50.77% ITO dissolved at 0.1 M of etidronic acid, and it increased to 96.31% and 97.54% at 0.5 and 2.0 M, respectively. Compared with conventional extraction methods, subcritical water extraction was not only more effective and rapid but also free from the use of strong acid or solvent. Furthermore, the preliminary test of cementation reaction of the leachate showed that indium or tin cementation by aluminum was inhibited in the presence of etidronate and citrate ligands in the leachate. The complex formation between indium or tin and etidronate or citrate ligands might increase the redox potential of the species than that of free indium or tin ions.

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