Mineralogy and distribution of the secondary and trace bauxite elements through the Bayer process and its by-products

This study was designed to elucidate the mineralogy and distribution of minor and trace elements (especially the rare earth elements — REEs, scandium — Sc, gallium — Ga and vanadium — V) during the production of alumina from bauxite ore in the Bayer process. The investigation was set up as a case study in the Aluminium of Greece refinery (Mytilineos S.A.). Bauxites can contain up to 50 chemical elements in concentrations above 1 mg/kg, most of which can be considered as trace elements. Of those trace elements, at least five metals or groups of metals (light REEs, heavy REEs, Sc, Ga and V) occur in elevated concentrations in Greek karstic bauxites and at the same time have been listed as critical raw materials by the European Commission. Bayer process accumulates those trace elements either to process liquor or to the main by-product known as the bauxite residue. Given the enhanced concentrations of certain trace elements, there exists an interest to extract some of those metals as valuable by-products of the Bayer process. However, the mineralogy of those critical metals in Bayer process materials as well as their distribution patterns during alumina production are so far poorly constrained. Therefore, the aim of this dissertation is to (1) explain the distribution characteristics of trace elements (especially those of the REEs, Sc, Ga, V, Th, As and U), and (2) elucidate the mineralogical characteristics of trace elements (especially the REEs and Sc) in the Bayer process materials including bauxite and the residues derived from it. Representative solid and liquid samples were collected from the complete flowsheet of the Bayer process, including bauxites from six different sources as well as derived residues. Additionally, minor by-products such as scales from plant piping and solids from different filters were collected and examined. The trace element composition of the samples was mainly determined by inductively coupled plasma mass spectrometry (ICP-MS) and optical emission spectrometry (ICP-OES) as well as with instrumental neutron activation analysis (INAA). Trace element concentrations were used in combination with the plant mass flow data to compile mass balance models of the trace elements to explain the distribution patterns of the trace elements in the Bayer process. Mineralogical characteristics of the trace elements were studied with a combination of microanalytical techniques. General microscale characteristics were described by using scanning electron microscopy (SEM), nanoscale investigation was performed by high resolution transmission electron microscopy (HRTEM). Specific microscale analyses and in situ trace element quantification were performed by electron microprobe coupled with wavelength dispersive spectrometers (EPMA-WDS). In situ trace element analyses were also carried out by laser ablation (LA) ICP-MS and in situ phase identification by Raman microscopy. The assessment of trace element distribution revealed that the elements are mainly divided into two categories: (1) those that dissolve and accumulate into the Bayer process liquor and achieve different saturation levels (Ga, V, Cr, As, U), and (2) those that pass through the Bayer circuit in the composition of solid mineral matrix and do not occur in dissolved forms (Sc, REEs, Th). From the elements in the first category, only Ga and U have the property to enter to the composition of the aluminium hydroxide produced in the Bayer process. Namely, about 70% of the initial quantity of Ga and about 10% of U can be found in the aluminium hydroxide product, while the rest of their mass is separated with bauxite residue. Almost the complete mass (mostly >98%) of the other investigated trace elements (REEs, Sc, Th, V, Cr, As) ends up in the bauxite residue. It was found that even though the REEs pass through the Bayer process in the forms of solid phases, their mineralogical properties and chemical composition of minerals are changed during the bauxite digestion. The precursor REE phases in bauxites are mainly REE fluorocarbonates of the bastnäsite group and cerium oxide in the form of cerianite (CeO2). During the bauxite processing, they interact with the caustic process liquor as well as with the dissolved Ca, Ti and Fe ions and the resulting phases found in bauxite residue are REE ferrotitanates. They eventually form a solid solution series between the ideal end-members (Ca,Na)(Ti,Fe)O3 and (REE,Ca,Na)(Ti,Fe)O3, depending on the extent of transformation that has taken place. It is characteristic to the REE phases in bauxite residue that they are surrounded by calcium titanate shells, that might inhibit the leaching efficiency of the REEs from bauxite residue. The modes of occurrences of Sc, however, remain practically the same in the bauxite and in the derived residue. Most of the Sc is associated with hematite, followed by goethite. Smaller amount of Sc is found in the composition of aluminium oxyhydroxide phases (boehmite and diaspore) and zircon. In bauxite residue, hematite, goethite and zircon host respectively 55±20 %, 25±20 % and 10±5 % of the total Sc. Since most of boehmite/diaspore is digested in the Bayer process, the proportion of Sc associated with those phases is assumed to be released during the digestion and thereafter probably precipitated on the mineral surfaces of bauxite residue’s particulates. The results obtained in this work provide an insight of the fate of selected trace elements in the Bayer process in terms of their distribution and mineralogy. The obtained knowledge can be utilised to improve and plan the extraction of trace elements that are also considered as critical raw materials. The compiled distribution models can also be used as one of the sources in global or local critical metals resource estimations. In addition, the elucidated mineralogical characteristics of especially the REEs and Sc provide explanations of why very high recovery rates of those elements are difficult to be achieved when leaching experiments of bauxite residue are performed