Extration of scandium from non-explored resources using hydrometallurgical route with focus on sustainable development.

Author(s)

Amilton Barbosa Botelho Junior

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2021-08-26

Examining board members

Jorge Alberto Soares Tenório; Fernando Gabriel da Silva Araujo; Juliana de Carvalho Izidoro; Marcelo Borges Mansur; Mauricio Leonardo Torem

• Advisor: Jorge Alberto Soares Tenório; Denise Crocce Romano Espinosa

Abstract

Scandium is one of the elements presented in the list of rare earths metals, being the most valuable among them. It´s widely used in lightweight aluminum alloys, electronic devices, lasers, lighting, and solid oxide fuel cells (SOFCs). Scandium and all rare earth elements are considered critical by the European Union, Brazil, and the USA due to the risk of supply chain interruption, economic importance, limited resources, low recycling rate, and practically irreplaceable in green technologies application. For this reason, it is essential the study of extractive route of scandium from new resources to design an economic and technological feasible process. Therefore, the current and growing demand of an element crucial to the development of sustainable society will be met. Bauxites are considered the main source of scandium in the world, which are the raw material for alumina production by the Bayer Process. After alumina extraction, almost all scandium went through the residue generated called bauxite residue (or red mud). Its estimated up to 4 billion tons of the residue are stored in dams worldwide containing 30-100mg/kg of scandium, which may value between US$400 4,500billion. The rare earth element would represent 95% of the economic value of the residue. Zirconium is reported as the second most valuable. The literature has shown that sources containing more than 20mg/kg deserve exploration due to their economic feasibility. Moreover, the discovery of new primary reserves is crucial for its supply. Among the extraction techniques, hydrometallurgy achieves the highest scandium obtaining rates mainly in trace concentration. On the other hand, there are two main problems: the synthesis of silica gel which reduces scandium extraction and increases the acid consumption, and the separation of scandium from the contaminants. For this reason, the goal of the thesis was the study of scandium extraction from two sources unexplored: bauxite residue from a Brazilian process and silicate-based ore from a Canadian source. The leaching process and separation by solvent extraction were studied. The materials\' characterization was carried out by X-ray diffraction, energydispersive X-ray fluorescence, particle size distribution, scanning electron microscopy coupled with energy-dispersive, loss of ignition, total organic carbon, and inductively coupled plasma atomic emission spectrometry. Leaching experiments of bauxite residue were carried out with H2SO4 and H3PO4, where the effect of solid-liquid ratio, time, temperature, H2O2 dosage, and acid concentration were evaluated. Direct leaching and dry digestion/sulfation followed by water leaching were studied for scandium extraction from silicate-based ore using H2SO4. The effect of acid dosage and the roasting temperature was also evaluated. Solvent extraction technique was studied for scandium separation using Alamine 336, D2EHPA, and Cyanex 923. The effect of pH, temperature, extractant concentration, TBP mixture, and A/O ratio were explored. Results show that scandium and zirconium content in bauxite residue was 43.5mg/kg and 1329.8mg/kg, respectively, and 36.4% of Fe2O3, 23.3% of Al2O3 21.6% of SiO2. The main mineral phases of the residue were quartz, sodalite, gibbsite, goethite, hematite, boehmite, and gypsum. Scandium and zirconium content in the silicate-based ore was 191mg/kg and 8,090mg/kg, respectively. The material had 36.3% of Fe2O3, 4.61% of Al2O3, and 39.4% of SiO2. The main mineral phases of the silicate-based ore were dickite, ferrohornblende, fayalite, hedenbergite, and albite. The extraction of scandium from bauxite residue achieved 92% using H2SO4 20%, solidliquid ratio equals 1/10 for 8h at 90°C. Almost 0% of silicon was leached. The H2O2 little contributed to silicon oxide formation during the acid leaching of bauxite residue. The extraction rates in H3PO4 leaching were similar to H2SO4 leaching, where the efficiency for scandium, aluminum, and iron achieved up to 90%, while silicon was 13%. The extraction of the valuable elements from the silicate-based ore by direct leaching increased from 40% (25°C) to 80% (90°C). The extraction of scandium from direct leaching and sulfation followed by water leaching was 13.5% and 5.6%, respectively. All zirconium was separated from the solution using Alamine 336 10% in kerosene, A/O ratio equals 1:1, at pH 1.0 for 15min at 25°C. No synergic effect was observed between amine extractant and TBP. Cyanex 923 was more selective for scandium than D2EHPA, where the separation factor for Sc/Fe was 288 and 99, and Sc/Ti was 98 and 21.5, respectively, considering 10% of organic extractant, A/O ratio equals to 1:1 at 25°C for 15min. Stripping of zirconium achieved 92% using Na2CO3 for concentration above 0.25mol/L. Scrubbing of Cyanex 923 for contaminants removal may be carried out with HCl 5mol/L with losses of 0.1% of scandium. All remained scandium may be stripped using H3PO4 5mol/L. The extraction of scandium and zirconium (as co-product) reached 92% and 25%, respectively. According to the flowchart proposed, it would be possible to obtain 4kg of scandium and 31.9kg of zirconium from 100 tons of bauxite residue. The process would generate up to US$ 46,626 of scandium oxide or US$ 1,940,980 of scandium fluoride. The process design and this thesis are strictly connected to the sustainable development goals number 7 (7.2, 7a), 8 (8.2, 8.4), 9 (9.2, 9.4, 9.5, 9b), and 12 (12.2, 12.4, 12.5, 12.6, 12a). (AU)

• FAPESP's process: 18/03483-6 - Kinect and thermodynamic studies for scandium recovery from Brazilian red mud using hydrometallurgical techniques
• Grantee: Amilton Barbosa Botelho Junior
• Support Opportunities: Scholarships in Brazil - Doctorate