Abstract The present work describes a process to recover rare earth elements (REEs) by selective precipitation with ammonia from phosphogypsum (PG) waste, a by-product of the phosphate industry. A pretreatment step was carried out with a solution of sodium chloride and sodium carbonate (60 g/L) in order to assure, respectively, a better solubilization of impurities and a conversion of PG to a calcite matrix easily attacked in an acid medium. Enriching a sulfuric acid solution (15%) with REEs was performed by double contacting the same acid solution with appropriate amounts of the pretreated PG by maintaining the operating conditions (L/S = 3, 100 °C and 2 h) each time. The leaching method produced a sulfuric leach liquor rich with rare earths with a final concentration of 4308 mg/L as well as two anhydrite solids. The latter, having the same level of purity, could be used safely in many industrial applications. Afterward, the progressive addition of ammonia (10%) to the REEs-rich liquor led to form three precipitates at different pH values: 1.1; 2.3 and 6. The ICP/MS analysis of the precipitation liquors showed that the optimum pH for the precipitation of REEs is 1.1 allowing their recovery with an efficiency of 98.5%; while precipitations at higher pH of 2.3 and 6 had a lower recovery yield of respectively 42.4% and 39.3%. The X-ray diffraction analysis of the precipitate obtained at pH 1.1 showed that it corresponds to an ammonium sulfate compound. Analyzes with photoluminescence technique showed the characteristic luminescence bands of Ce3+, Gd3+, Dy3+, Pr3+ and Sm3+(after excitations at λExc = 260 nm and λExc = 440 nm) confirming the presence of REEs in the precipitate.

中文翻译：

---

从磷石膏加工后的硫酸浸出液中分步沉淀回收稀土

摘要 目前的工作描述了一种通过从磷石膏 (PG) 废物中选择性沉淀氨来回收稀土元素 (REE) 的工艺，磷石膏 (PG) 废物是磷酸盐工业的副产品。预处理步骤是用氯化钠和碳酸钠 (60 g/L) 的溶液进行的，以分别确保更好地溶解杂质和将 PG 转化为容易在酸性介质中侵蚀的方解石基质。通过每次保持操作条件（L/S = 3、100°C 和 2 小时），使相同的酸溶液与适量的预处理 PG 进行双重接触，从而用 REE 富集硫酸溶液（15%）。浸出方法产生富含稀土的硫酸浸出液，最终浓度为 4308 毫克/升，以及两种硬石膏固体。后者，具有相同的纯度水平，可以在许多工业应用中安全使用。之后，向富含 REE 的液体中逐步添加氨 (10%) 导致在不同 pH 值下形成三种沉淀物：1.1；2.3 和 6. 沉淀液的 ICP/MS 分析表明 REE 沉淀的最佳 pH 值为 1.1，使其回收效率为 98.5%；而在 2.3 和 6 的较高 pH 值下沉淀的回收率较低，分别为 42.4% 和 39.3%。在 pH 1.1 下获得的沉淀物的 X 射线衍射分析表明它对应于硫酸铵化合物。光致发光技术分析显示了Ce3+、Gd3+、Dy3+、