Acid decomposition using concentrated H2SO4 at elevated temperature (>200 °C) is a common process to recover rare earth elements (REE) from refractory minerals where the reactions form REE sulfates, which are dissolved during a subsequent water leach. While the decomposition of REE orthophosphates and fluorocarbonates is well-documented, investigations focusing on REE silicates are more limited. The current study focuses on allanite-(Ce), since, in addition to the REEs, the presence of other sulfate-forming cations in the crystalline structure can help better define the decomposition of complex silicates. After treatment using concentrated H2SO4 at temperatures between 175 °C and 225 °C, important passivation effects from the formation of amorphous silica layers were observed. Decomposition occurs rather during the water leach if its temperature is maintained at near-boiling conditions for 20 h, suggesting that water helps attenuate passivation. Consequently, experiments were performed where water was premixed with H2SO4 during the initial acid treatment to lower concentration down to 55 wt%. The water addition drastically enhanced decomposition rate at acid treatment temperatures down to 100–125 °C, where REE recoveries >90 wt% were achieved after a room-temperature water leach. The extent of passivation may be linked to the solubility of produced sulfates. With concentrated acid, low sulfate solubility induces local saturation at the crystal dissolution front leading to diffusion-limited cation transfer across the growing silica layer. When the water content is increased, the sulfate-forming cations can enter in solution providing efficient transport, hence minimizing passivation. This decreases both acid treatment and water leach temperatures, significantly reducing energy consumption without additional reagent costs.

中文翻译：

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最大限度地减少在硫酸介质中铝榴石 (Ce) 分解过程中钝化的影响，以回收稀土

在高温 (> 200 °C) 下使用浓 H2SO4 进行酸分解是从难熔矿物中回收稀土元素 (REE) 的常见过程，其中反应形成 REE 硫酸盐，硫酸盐在随后的水浸过程中溶解。虽然 REE 正磷酸盐和氟碳酸盐的分解有充分的记录，但针对 REE 硅酸盐的研究更为有限。目前的研究集中在allanite-(Ce)，因为除了稀土元素之外，晶体结构中其他形成硫酸盐的阳离子的存在有助于更好地定义复杂硅酸盐的分解。在 175°C 和 225°C 之间的温度下使用浓 H2SO4 进行处理后，观察到无定形二氧化硅层形成的重要钝化效应。如果将温度保持在接近沸腾的条件下 20 小时，则在水浸出过程中会发生分解，这表明水有助于减弱钝化。因此，进行了实验，其中在初始酸处理期间将水与 H2SO4 预混合，以将浓度降低至 55 wt%。在酸处理温度低至 100-125°C 时，加水显着提高了分解速率，其中在室温水浸后实现了 REE 回收率 > 90 wt%。钝化程度可能与产生的硫酸盐的溶解度有关。对于浓酸，低硫酸盐溶解度会导致晶体溶解前沿的局部饱和，导致扩散限制的阳离子转移穿过生长的二氧化硅层。当含水量增加时，形成硫酸盐的阳离子可以进入溶液，提供有效的传输，从而最大限度地减少钝化。这降低了酸处理和水浸出温度，显着降低了能源消耗，而无需额外的试剂成本。