微波誘發裂解回收廢液晶顯示器之銦

Abstract

銦由於存量稀少且提煉困難，而被定義為稀有金屬，其大多被製造成透明導電材料氧化銦錫(indium tin oxide, ITO)，廣泛應用於各種電子產品配備的液晶顯示器(liquid crystal display, LCD)當中，為當今社會促進科技以及生活便利性的發展過程中不可或缺的材料，然據學者專家預估原生銦礦將在2025年消耗殆盡，因此諸多研究開始朝從廢棄液晶顯示器中回收銦的方向努力。 本研究係將微波誘發裂解程序融入濕式冶金法，提升從廢棄液晶面板中回收稀有金屬銦的效率，得到含高純度、高濃度之含銦水溶液，以利最終還原回收程序的進行。本研究之金屬銦回收流程包含四個階段，分別為微波誘發裂解、酸浸溶蝕、萃取、反萃取。 在微波誘發裂解階段，以熱裂解去除液晶面板中的有機物，並使液晶面板中的層狀結構彼此脫離，提升後續酸溶程序金屬銦的溶出效率。首先先觀察液晶面板受微波加熱的情形，探討其升溫速率、最高溫度與微波功率之關係，再根據熱重分析所得到的液晶面板最佳熱裂解溫度361.2 ℃，獲知液晶面板的最適微波功率150 W，接著以此微波功率找出最佳微波誘發裂解時間50 min。 於酸浸溶蝕程序，將固相中的銦轉移至液相，提升銦的純度。本研究將熱裂解完的液晶面板浸泡在硫酸中，覆蓋加熱至90 ℃，轉速設定360 rpm，消化2 hr後關掉攪拌，靜置2 hr至室溫，取樣過濾後以ICP-OES測定。此階段在考量回收率、pH值後得出之最佳硫酸濃度為0.5 M，其硫酸溶出液中目標金屬銦的回收率為98.27 %、純度為40.07 %。 接著進入萃取程序，利用有機溶劑D2EHPA溶於煤油之萃取劑，對硫酸溶出液中的銦進行萃取，使其分離純化並濃縮。本研究以轉速800 rpm攪拌混和水相及有機相，並持續5 min，再關掉攪拌靜置5 min待溶液分層，取樣過濾後以ICP-OES測定。依據測定後之結果得到最佳的D2EHPA體積分率及O/A分別為20 %以及1：10，此條件下有機相中的銦濃度為228.23 ppm、回收率為81.7 %、純度為86.17 %。 最後以反萃取將目標金屬銦轉移回水相，並達到二次分離純化及濃縮的效果。本研究以鹽酸作為萃取劑，與有機相在轉速800 rpm的情況下攪拌混和5 min，再靜置5 min待其分層，取樣過濾後以ICP-OES測定。根據計算出來的反萃取效率、回收率、濃縮程度、純度選擇最佳的鹽酸濃度及O/A分別為6 M及10：1，此最佳參數下鹽酸溶液中的銦濃度為1892.38 ppm、回收率為68.99 %、純度為99.98 %。 本研究之成果顯示利用濕式冶金法輔以微波誘發裂解之稀有金屬銦回收流程，對於銦的回收效率優異，最終產物含銦水溶液中，銦的純度、濃度皆很高，分別為99.98 %、1892.38 ppm，利於進行後續如電解精煉、置換等還原回收程序，對於最終得到固體金屬銦有偌大幫助。

Indium is a kind of rare metal because of its scarcity in the earth’s crust and difficulty in refining. The major application of indium is indium tin oxide (ITO), a transparent current-conductive material playing a critical role in the liquid crystal display (LCD) function. With the mass production of LCD screens, indium resource was estimated to be exhausted by 2025. Therefore, the recovery of indium from waste LCD is important and urgent. The indium recovery process in this study incorporates the microwave-induced pyrolysis in the hydrometallurgy to enhance the recovery efficiency of indium from waste LCD and get high purity and concentration of indium aqueous solution. The process include four steps: microwave-induced pyrolysis, leaching, extraction, and stripping. First, the microwave-induced pyrolysis process can remove the organics and make the separation between the layers of LCD panel to enhance the leaching rate in the following process. According to the thermal gravimetric analysis (TGA) results, the maximum decay rate of waste LCD occurred at 361.2 °C. Consequently, The microwave-induced pyrolysis process was carried out at the microwave power of 150 W for the processing time of 50 min. Secondly, in the leaching process, indium can be dissolved in the acid solution. 98.27 wt.% of the indium was leached out in 0.5M sulfuric acid with 1:10 solid/liquid ratio, 2 hr, 90 ℃ and stirring at 360 rpm. The purity and concentration of indium are 40.07 % and 25.97 ppm. Thirdly, di(2-ethylhexly)phosphoric acid (D2EHPA) can extract indium from the sulfuric acid solution to separate indium from the other metals and enrich the indium concentration. In the extraction process, the best condition for indium was 20 % (v/v) D2EHPA dissolved in the kerosene at organic-to-aqueous phase ratio (O/A) of 1:10. The purity, concentration and recovery rate of indium are 86.17 %, 228.23 ppm and 81.7 wt.%. Finally, indium in the loaded organic phase can be stripped by hydrochloric acid to separate and enrich indium again. In the stripping process, 68.99 wt.% of the indium was recovered in 6 M hydrochloric acid at O/A of 10:1. The purity and concentration of indium in the final production are 99.98 % and 1892.38 ppm. In this study, the final product (hydrochloric acid solution) containing high purity and high concentration of indium is beneficial to electrolytic refining or replacement to get indium metal. The result indicates that the recovery process of indium from waste LCD by microwave-induced pyrolysis is a promising technique.