Phosphate ores are one of the most important secondary sources of rare earth elements (REEs). The Esfordi phosphate concentrate (Yazd Province, Iran) contains 13.1% phosphate, 34.6% calcium, and 1.09% REEs. After selective leaching of the concentrate with 35% nitric acid at 60°C for 70 min, a pregnant leach (nitrophosphate) solution containing 143.6 g/L calcium, 53.1 g/L phosphorus, and other impurities was obtained. Sulfuric acid was used to remove calcium from the pregnant leach solution in the form of gypsum, and the effect of the stoichiometric ratio of sulfate to calcium was investigated. The results showed that at a sulfate-to-calcium molar ratio of 1, 99.5% of calcium, 95.2% of cerium, 92.8% of La, and 96.5% of Nd were precipitated from the solution. Phosphate co-precipitation with gypsum was < 10%. As the stoichiometric ratio of sulfate to calcium increased, the co-precipitation rate of phosphate decreased. The results of the present study showed that in the two-stage leaching method of REE production from phosphate ores, calcium precipitation in the form of gypsum is an effective method for purifying the solution from calcium.

中文翻译：

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钙沉淀法提纯硝酸磷酸盐溶液为石膏

磷酸盐矿石是稀土元素 (REE) 最重要的次要来源之一。Esfordi 磷酸盐浓缩物（伊朗亚兹德省）含有 13.1% 的磷酸盐、34.6% 的钙和 1.09% 的 REE。在 60°C 下用 35% 硝酸选择性浸出浓缩液 70 分钟后，得到含有 143.6 g/L 钙、53.1 g/L 磷和其他杂质的富浸液（硝基磷酸盐）。使用硫酸以石膏的形式从富集浸出液中去除钙，并研究了硫酸盐与钙的化学计量比的影响。结果表明，在硫酸盐与钙的摩尔比为 1 时，99.5% 的钙、95.2% 的铈、92.8% 的 La 和 96.5% 的 Nd 从溶液中沉淀出来。磷酸盐与石膏的共沉淀<10%。随着硫酸盐与钙的化学计量比增加，磷酸盐的共沉淀率降低。本研究结果表明，在磷矿生产稀土元素的两段浸出法中，以石膏形式沉淀钙是从钙中提纯溶液的有效方法。