Lanthanides as impurities in the Bayer production cycle of the aluminum hydroxide from Sierra Leone bauxite

Abstract

This paper is describing a careful study of the content and distribution of rare elements in the fluid and solid phases involved in dry and classified aluminum hydroxide production through the Bayer process at Alum SA, Tulcea, Romania. The source of rare elements in the Bayer process is the bauxite from Sierra Leone, a particular type of aluminous goethite-lateritic bauxite, not fully studied yet. Rare earth elements are fairly abundant in nature, but their distribution is very large, encompassing hundreds of types of minerals where the rare element appears as minor crystalline and amorphous compounds, solid solutions, or as ions adsorbed on the surface of common natural rocks. This study data show that Sierra Leone bauxite has only a small content in rare elements. Mainly, only the scandium and cerium concentrations (44.84 mg/kg and 11.49 mg/kg in bauxite residue) may reach the expected values required for eventual valorization. On the Bayer cycle, the rare metals enter with bauxite and concentrate in bauxite residues. Solubility of the rare element compounds in the Bayer process fluid phases is close to zero. In the final product, the aluminum hydroxide dried, milled and classified grades, the rare metals appear only as occlusion contaminants.