The current study investigated selective recovery of rare earth elements (REEs) from spent nickel metal hydride (NiMH) batteries using two-stage acid leaching followed by precipitation at acidic pH. The first-stage conventional leaching using 2.5 M of waste phosphoric acid (H₃PO₄) at 25°C for 60 min removed 60 to 100% heavy metals, such as Ni, Co, Zn and Cd, and converted more than 90% REEs into insoluble phosphate precipitates, REE(PO₄). Subcritical water extraction (SWE) using mild concentration of sulfuric acid (H₂SO₄) in the second stage leached out all REEs. The leaching efficiency of REEs increased by increasing acid concentration, and 100% leaching efficiency of REEs, Ni, Co and Mn were achieved using 1 M of H₂SO₄, solid concentration of 20 g/L, at 125°C for 30 min. As predicted by species distribution of metals at equilibrium, REEs in the leaching solution were selectively precipitated as NaREE(SO₄)₂·H₂O at low pH (0.5 to 2) by the addition of NaOH solution, and separated from other metals (Ni, Co, Mn). Precipitation at pH 1 resulted in 82.59% of La, 90.75% of Ce, and 85.97% of Nd recovered as hexagonal rod crystals. This study provides an alternative process to selectively recover REEs from electronic waste (e-waste).

目前的研究调查了使用两阶段酸浸然后在酸性 pH 下沉淀从废镍金属氢化物 (NiMH) 电池中选择性回收稀土元素 (REE)。第一阶段使用 2.5  M 废磷酸 (H ₃ PO ₄ ) 在 25°C 下进行 60 分钟常规浸出，去除 60% 至 100% 的重金属，如 Ni、Co、Zn 和 Cd，转化率超过 90% REE 转化为不溶性磷酸盐沉淀物 REE(PO ₄ )。使用温和浓度的硫酸 (H ₂ SO ₄) 在第二阶段浸出所有 REE。稀土元素的浸出效率随着酸浓度的增加而提高，使用 1  M H ₂ SO ₄，固体浓度为 20  g/L，在 125°C 下浸出30 分钟，稀土元素、Ni、Co 和 Mn 的浸出效率达到 100% . 根据平衡态金属的物种分布预测，浸出液中的稀土元素以 NaREE(SO ₄ ) ₂ ·H _{2 的形式}选择性沉淀O 在低 pH 值（0.5 至 2）下通过加入 NaOH 溶液，并与其他金属（Ni、Co、Mn）分离。pH 值为 1 的沉淀导致 82.59% 的 La、90.75% 的 Ce 和 85.97% 的 Nd 以六方棒状晶体的形式回收。该研究提供了一种从电子废物（e-waste）中选择性回收 REE 的替代工艺。