隨著稀土元素的需求大幅增加，尋找新的次級來源和發展有效且環保的萃取技術是全球重要的議題。磷石膏作為肥料工業中的主要副產物，雖然僅擁有微量的稀土元素，然而每年可觀的生產量可以讓磷石膏成為回收稀土元素的重要供給來源。因此，從磷石膏中回收稀土元素不但有機會降低全球供應風險，還能減少長期存放磷石膏所造成的環境問題。
本研究探討次臨界水萃取技術應用於自磷石膏中回收稀土元素，其中我們探討參數包括酸種類、酸濃度、固液比、反應時間和反應時間對於磷石膏中釔、鑭、鈰和釹的萃取影響，並與傳統萃取方法比較萃取效率和能源消耗。實驗結果顯示，萃取效率隨著濃度增加和固液比減少而增加，反應溫度和反應時間增加會造成石膏轉換為硬石膏並降低萃取效率。使用0.1 N 鹽酸、固液比10 g/L、100°C、反應5分鐘時，自磷石膏中萃取釔、鑭、鈰和釹的效率最高，分別為100.36%, 97.50%, 100.06% 和96.58%，每公斤總消耗能量為4.2×106千焦耳。相較於傳統萃取，次臨界水萃取同時擁有低耗能和高效率的優點，是一種極具競爭力的綠色技術。

The significant increase demand for rare earth elements (REEs), making the search of new secondary sources and the development of effective and environmentally friendly extraction methods be considered as important global issue. As the main by-product in the production of fertilizer, although phosphogypsum (PG) only has trace amounts of REEs, the considerable annual production makes PG an important source for REEs recovery. Therefore, recover REEs from PG not only reduce the global supply risk, but also reduce the environmental problems caused by long-term storage.
This study put the emphasis on the leaching efficiency of REEs, including yttrium (Y), lanthanum (La), cerium (Ce) and neodymium (Nd), from PG utilizing subcritical water extraction (SWE). The effects of acid type, acid concentration, solid to liquid ratio, reaction temperature, and reaction time on REEs from PG were investigated. The leaching efficiency increased with increasing acid concentration and decreasing S/L. The reaction temperature and reaction time could cause the transformation of gypsum to anhydrite, and affect the leaching efficiency of REEs. The experimental results of SWE, conducted at 0.1 N HCl, 10 g/L, 100°C, and reaction for 5 min, has the best leaching efficiency, 100.36% of Y, 97.50% of La, 100.06% of Ce and 96.58% of Nd, respectively. The leaching efficiency and energy consumption were compared with conventional extraction. And the total energy consumption in SWE is 4.2×106 kJ/kg. From the comparison, SWE could be a highly competitive green technology which is more effective and energy-efficient than conventional extraction.

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