Citation: Kaljuvee, T.; Tõnsuaadu, K.; Einard, M.; Mikli, V.; Kivimäe, E.-K.; Kallaste, T.; Trikkel, A. Thermal Behavior of Estonian Graptolite–Argillite from Different Deposits. Processes 2022, 10, 1986. https://doi.org/10.3390/pr10101986 Academic Editor: Aneta Magdziarz Received: 24 August 2022 Accepted: 24 September 2022 Published: 1 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). processes Article Thermal Behavior of Estonian Graptolite–Argillite from Different Deposits Tiit Kaljuvee 1, *, Kaia Tõnsuaadu 1 , Marve Einard 1 , Valdek Mikli 1 , Eliise-Koidula Kivimäe 1 , Toivo Kallaste 2 and Andres Trikkel 1 1 Departmentof Materials and Environmental Technology, TallinnUniversity of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia 2 Department of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia * Correspondence: tiit.kaljuvee@taltech.ee; Tel.: +372-6202812 Abstract: Graptolite–argillites (black shales) are studied as potential source of different metals. In the processing technologies of graptolite–argillites, a preceding thermal treatment is often applied. In this study, the thermal behavior of Estonian graptolite–argillite (GA) samples from Toolse, Sillamäe and Pakri areas were studied using a Setaram Labsys Evo 1600 thermoanalyzer coupled with the Pfeiffer OmniStar Mass Spectrometer. The products of thermal treatment were studied by XRD, FTIR, and SEM analytical methods. The experiments were carried out under non-isothermal conditions of up to 1200 ◦ C at different heating rates in the atmosphere containing 79% Ar and 21% O 2 . The differential isoconversional Friedman method was applied for calculating the kinetic parameters. All studied GA samples are characterized with high content of orthoclase (between 38.0 and 57.3%) and quartz (between 23.8 and 35.5%), and with lower content of muscovite, jarosite, pyrite, etc. The content of organic carbon in GA samples studied varied between 7.3 and 14.2%. The results indicated that, up to 200 ◦ C, the emission of hygroscopic and physically bound water takes place. Between 200 ◦ C and 500–550 ◦ C, this is followed by thermo-oxidative decomposition of organic matter. The first step of thermo-oxidation of pyrite with the emission of water, carbon and sulphur dioxide, nitrogen oxides, and different hydrocarbon fragments indicated the complicated composition of organic matter. At higher temperatures, between 550 ◦ C and 900 ◦ C, the transformations continued by dehydroxylation processes in clay minerals, and the decomposition of jarosite and carbonates took place. At tempera- tures above 1000–1050 ◦ C, a slow increase in the emission of sulphur dioxide followed, indicating the beginning of the second step of thermo-oxidative decomposition of pyrite, which was not completed for temperatures of up to 1000 ◦ C. Kinetic calculations prove the complicated mechanism of thermal decomposition of GA samples: for Pakri GA samples, it occurs in two steps, and for Silllamäe and Toolse GA samples, it occurs in three steps. Preliminary tests for the estimation of the influence of pre-roasting of GA samples on the solubility of different elements contained in GA at the following leaching in sulphuric acid is based on Toolse GA sample. Keywords: graptolite–argillite; IR-spectroscopy; kinetics; SEM; solubility; TG-DTA-MS; XRD 1. Introduction Graptolite–argillite (black shale) originally formed in the shallow sea areas of the Baltica paleocontinent and in the deeper part of oceans where the various sediments transported by rivers have been deposited. Over the hundred million years that followed, these sediments were partially moved inland by different tectonic and glacial forces. Throughout the Earth’s history, especially during the Phanerozoic eon, the oceanic anoxic (reduced level of oxygen) and euxinic environments (increase in the content of sulphides, especially hydrogen sulphide) in the deeper layers of ocean have played crucial roles in the formation of GA [1–7]. The major forcing function behind oceanic anoxic events (OAE) was an abrupt rise in temperature caused by rapid increase in carbon Processes 2022, 10, 1986. https://doi.org/10.3390/pr10101986 https://www.mdpi.com/journal/processes