Abstract
This study investigates the removal of aluminum and iron from rare earth element (REE) containing solutions by solvent extraction with saponified naphthenic acid and by hydrolysis-precipitation. The results emphasize both, the preferential application as well as limitations of every method. We find that emulsification occurring during the solvent extraction of aluminum is caused by its slow extraction rate in comparison to the neutralization reaction and by the proximity of the pH value required for aluminum extraction and the pH value at which hydrolysis of aluminum occurs. However, by choosing a long shaking time of at least 4 h, the emulsion recedes. The formation of emulsion can be avoided by strict control of pH value during the extraction. Moreover, the loading capacity of the organic phase with aluminum is limited due to the strong increase in viscosity of the organic phase with increasing aluminum concentration and due to the gel formation. Regarding the extraction of iron, the amount of extracted ions is limited due to the overlap of the pH range required for the extraction with pH range in which sparingly soluble iron oxides/hydroxides are formed. In summary, aluminum and iron can be simultaneously removed from REE-sulfate solution by solvent extraction with saponified naphthenic acid in one extraction stage only from diluted solutions. However, in comparison to the hydrolysis-precipitation method, a higher purity of the solution is achieved. A complete removal of aluminum and iron from concentrated solutions can be achieved in two stages. First, the content of aluminum and iron should be reduced by hydrolysis-precipitation. After that, a high-purity solution can be obtained by subsequent solvent extraction by saponified naphthenic acid.
Highlights
Due to their unique properties, rare earth elements (REEs) find a broad application in different areas such as catalysts, alloys, glass polishing, ceramics and permanent magnets [1]
This paper investigates the following issues: (i) solvent extraction of aluminum, iron and lanthanum by saponified naphthenic acid; (ii) separation of aluminum and iron from lanthanum from binary-component solutions by solvent extraction with saponified naphthenic acid; (iii) separation of aluminum and iron from lanthanum from ternary-component solutions by solvent extraction with saponified naphthenic acid and by hydrolysis-precipitation
The results show that the aluminum and iron removal by the solvent extraction with saponified naphthenic acid has limitations
Summary
Due to their unique properties, rare earth elements (REEs) find a broad application in different areas such as catalysts, alloys, glass polishing, ceramics and permanent magnets [1]. The whole extraction process comprises the REE extraction from raw materials and a subsequent separation of individual REEs. the Chinese export restrictions, very few effective substitutes for certain REEs, low recycling rates, growing demand and negative effects of REE extraction on the environment impede the secure supply, especially of countries without their own Minerals 2020, 10, 556; doi:10.3390/min10060556 www.mdpi.com/journal/minerals. Alternative REE supply sources must be found [5], recycling rate has to be improved [6] as well as economic and efficient extraction processes have to be developed for the new raw materials. The extraction process depends strongly on the REE mineralogy and on the ore/concentrate grade resulting in two routes. It is possible to produce a bastnäsite concentrate containing
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