Sulfuric acid baking of monazite ((Ce,La,Th)PO) bearing ores is one of the major processes used in commercial production of rare earth elements. In hydrothermal vein type rare earth deposits, apatite is often found together with monazite. Rare earth enriched fluorapatite ores may also contain significant amounts of rare earths hosted in monazite. An understanding of the effect of apatite on the sulfuric acid baking of monazite is therefore important for the development of effective sulfuric acid based treatment methods for such ores. In this work, the addition of a natural hydroxyapatite (Ca(PO)(OH)) sample to a monazite acid bake followed by acid leach was investigated using a combination of chemical analyses, SEM-EDS, XRD and TG-DSC. Hydroxyapatite addition significantly decreased the dissolution of rare earth elements from monazite in the leach after baking at temperatures above 300 °C. For a bake temperature of 500 °C, the rare earth dissolution in the leach dropped from 80% for monazite alone, to 30% for a 1:1 (/) addition of hydroxyapatite. This decrease in rare earth leaching was attributed to the formation of an insoluble thorium and rare earth bearing polyphosphate. The rare earth elements were incorporated into this polyphosphate phase in preference to calcium. At 800 °C, monazite was re-formed, causing a further reduction in rare earth extraction, while simultaneous formation of calcium pyrophosphate (CaPO) led to an increase in calcium and phosphorus dissolution. The detrimental effect of apatite could be partially overcome by the addition of goethite. Addition of goethite to the acid bake of a monazite/apatite mixture at 500 °C improved the total rare earth dissolution from 29% to 85% in the subsequent leach. Results also demonstrated that the order of reactivity, in terms of formation of polyphosphates is Fe > REE > Ca.

中文翻译：

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磷灰石对独居石精矿硫酸焙烧的影响及针铁矿添加对稀土浸出的益处


含独居石（(Ce,La,Th)PO）矿石的硫酸焙烧是稀土元素商业化生产的主要工艺之一。在热液脉型稀土矿床中，磷灰石常与独居石一起发现。富含稀土的氟磷灰石矿石还可能含有大量以独居石形式存在的稀土。因此，了解磷灰石对独居石的硫酸烘烤的影响对于开发针对此类矿石的有效的基于硫酸的处理方法非常重要。在这项工作中，结合化学分析、SEM-EDS、XRD 和 TG-DSC，研究了将天然羟基磷灰石 (Ca(PO)(OH)) 样品添加到独居石酸烘烤中，然后进行酸浸。在 300 °C 以上的温度下烘烤后，添加羟基磷灰石可显着降低浸出液中独居石中稀土元素的溶解。对于 500 °C 的烘烤温度，浸出物中的稀土溶解度从单独独居石的 80% 下降到按 1:1 (/) 添加羟基磷灰石的 30%。稀土浸出的减少归因于不溶性钍和含稀土多磷酸盐的形成。稀土元素优先于钙并入该多磷酸盐相中。在800°C时，独居石重新形成，导致稀土提取进一步减少，同时焦磷酸钙（CaPO）的形成导致钙和磷溶解增加。通过添加针铁矿可以部分克服磷灰石的不利影响。在 500°C 下将针铁矿添加到独居石/磷灰石混合物的酸烘烤中，可以将随后浸出中的总稀土溶解度从 29% 提高到 85%。 结果还表明，就多磷酸盐的形成而言，反应性顺序为 Fe > REE > Ca。