Citation: Gontijo, V.L.; Teixeira, L.A.V.; Ciminelli, V.S.T. The Effect of Iron- and Calcium-Rich Waste Rock’s Acid Baking Conditions on the Rare-Earth Extraction. Minerals 2023, 13, 217. https://doi.org/ 10.3390/min13020217 Academic Editors: Kenneth N. Han and Kwadwo Osseo-Asare Received: 5 January 2023 Revised: 30 January 2023 Accepted: 31 January 2023 Published: 2 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article The Effect of Iron- and Calcium-Rich Waste Rock’s Acid Baking Conditions on the Rare-Earth Extraction Vitor L. Gontijo 1 , Leandro Augusto Viana Teixeira 2 and Virgínia Sampaio Teixeira Ciminelli 1,3, * 1 Acqua Institute, Belo Horizonte 31270-901, MG, Brazil 2 Vale S.A, Innovation Strategy and Ecosystem, Nova Lima 34006-270, MG, Brazil 3 School of Engineering, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, MG, Brazil * Correspondence: ciminelli@demet.ufmg.br Highlights: • Effect of low-temperature sulfation, H 2 SO 4 to ore ratio and time on REE extraction. • Differences in REE host minerals and their impact on these elements extraction. • Increasing sulfuric acid availability reduces iron dissolution from Fe-rich sample. • Inhibitory effect on the crystallization of Ca-sulfate by REE uptake during leaching. Abstract: The work investigates the effect of sulfuric acid baking on rare earth element (REE) extraction from two waste rock samples from a phosphate mine. The role of different mineralogical assemblages and the degree of alteration (i.e., weathering), and the behavior of the main impurities– iron, phosphorus, and calcium–on REE extraction are emphasized. For both samples, the sulfuric acid baking at 25 ◦ C, during 15 min, H 2 SO 4 :sample (w/w) mass ratio of 0.45:1 is the best condition for achieving the selective leaching of REE. For the iron rich-sample, the increase in temperature reduces REE extraction and increases iron dissolution. The corresponding sulfuric acid consumed by goethite (α-FeOOH) from 25 ◦ C to 160 ◦ C acid baking is ten times higher than that required for the monazite (REEPO 4 ) reaction. Conversely, higher REE and lower iron extraction are observed by increasing (sulfuric acid/sample) mass ratio (0.95:1). Due to the high sulfuric concentration during dissolution, a local saturation zone close to the dissolution front caused the precipitation of iron oxyhydroxides. The calcium-rich sample shows lower REE extraction by leaching (63% maximum) mainly due to the entrapment of REE-bearing minerals by a gypsum layer, and lanthanide’s uptake by calcium sulfate compounds formed during leaching. The results were discussed with the help of a detailed characterization of the residues. Keywords: sulfuric acid-baking; rare earth element; calcium sulfate; by-product; monazite; apatite 1. Introduction The distinguishing role of rare earth elements (REE) in the clean energy industry makes them a key point for the recent global movement towards electrification. The major contribution of these elements is in the production of catalysts, permanent magnets, phosphors, metal alloys for rechargeable batteries, and ceramic materials [1–3]. Therefore, several studies addressed the recovery of these elements from secondary sources or as a by-product [4–7], from tailings [8–10], and recycling end-of-life products [11–13]. The main reserves of REE in Brazil are located in alkaline carbonatite, granitic for- mations, and marine placers. The placer-type deposits are found across all the Brazilian coastline, and the most important reserves are in the Southeast region (i.e., Espírito Santo and north of Rio de Janeiro). Carbonatites and alkaline rocks are found mainly in the south- east, central-west and north regions (i.e., Minas Gerais, Goiás, São Paulo and Amazonas). The REE-carrying minerals in the different provinces of Brazil are primarily monazite and secondly bastnasite. The content of rare earth oxides (REO) varies from 1.0% to 8.0%, Minerals 2023, 13, 217. https://doi.org/10.3390/min13020217 https://www.mdpi.com/journal/minerals