Abstract Coal combustion ash and pre-combustion coal refuse are currently under consideration as potential sources of rare earth elements. One of the early steps in recovering REEs from coal by-products is often acid leaching, which can result in low pH leachates with complex aqueous chemistry. The aim of this work was to understand the connection between REE solubility, pH, and major elemental components of leachates for coal by-products. To accomplish this, we investigated the effects of solids concentration (i.e., pulp density) and pH adjustment on REE solubility in acid leachates of coal fly ashes from the Powder River Basin (PRB) and Appalachian Basin in the United States, and a coal processing refuse from the Southwestern U.S. For PRB ashes, the concentrations of soluble REEs generally increased with increasing pulp density; however, at pulp density values above 80–100 g/L, the soluble REE concentrations in the leachates were markedly lower. Similarly, the soluble concentrations of other major solutes (Fe, Al, Si) that leached from PRB fly ashes were also non-linear with pulp density. These major elements tended to reach maximum concentration values at 60–70 g/L pulp density. In contrast, for the Appalachian fly ashes and the coal by-product, soluble concentrations of REE and major elements in leachates increased linearly with pulp density. Chemical equilibria calculations of mineral saturation indices indicated that trends in soluble REE concentrations could be explained by saturation conditions for Fe and Al-(hydr)oxides and possibility sulfate minerals, but not lanthanide hydroxides. Furthermore, pH adjustment of the acid leachates showed that REEs and many major solutes were removed from solution at pH values above 4.5, also consistent with Fe- and Al-(hydr)oxide precipitation. These results highlight the importance of understanding the chemical composition of leachates when designing REE recovery processes for low-grade geologic feedstocks and that precipitation of hydr(oxide) or sulfate minerals of major elements rather than discreet formation of REE mineral phases could be used for process optimization.

中文翻译：

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主要元素组成控制粉煤灰和煤副产品浸出过程中稀土元素的溶解度

摘要 燃煤灰分和燃烧前的煤渣目前正在考虑作为稀土元素的潜在来源。从煤副产品中回收 REE 的早期步骤之一通常是酸浸，这会导致低 pH 浸出液具有复杂的水性化学。这项工作的目的是了解 REE 溶解度、pH 值和煤副产品渗滤液的主要元素成分之间的联系。为实现这一目标，我们研究了固体浓度（即纸浆密度）和 pH 值调节对来自美国粉河盆地 (PRB) 和阿巴拉契亚盆地的粉煤灰酸浸出液中 REE 溶解度的影响，以及煤炭加工来自美国西南部的垃圾 对于 PRB 灰烬，可溶性 REE 的浓度通常随着纸浆密度的增加而增加；然而，当纸浆密度值高于 80–100 g/L 时，渗滤液中的可溶性 REE 浓度显着降低。同样，从 PRB 飞灰中浸出的其他主要溶质（Fe、Al、Si）的可溶浓度也与矿浆密度呈非线性关系。这些主要元素往往在 60–70 g/L 纸浆密度时达到最大浓度值。相比之下，对于阿巴拉契亚飞灰和煤副产品，浸出液中 REE 和主要元素的可溶性浓度随矿浆密度线性增加。矿物饱和指数的化学平衡计算表明，可溶性 REE 浓度的趋势可以用 Fe 和 Al-（氢）氧化物以及可能的硫酸盐矿物的饱和条件来解释，但不是镧系元素氢氧化物。此外，酸浸出液的 pH 调节表明，在 pH 值高于 4.5 时，REE 和许多主要溶质从溶液中去除，这也与 Fe-和 Al-（氢）氧化物沉淀一致。这些结果强调了在设计低品位地质原料的 REE 回收过程时了解渗滤液化学成分的重要性，并且主要元素的氢氧化物（氧化物）或硫酸盐矿物的沉淀而不是 REE 矿物相的离散形成可用于过程优化。