Effect of calcination of Estonian black shale on the solubility of metallic elements in sulfuric acid environment

In this study, the effect of calcination, with and without additives, on the solubility of metallic elements from black shale (BS) in sulfuric acid was investigated. The experiments were carried out with Estonian BS samples from Pakri and Toolse deposits, which mineral part mainly consists of quartz, orthoclase, muscovite and pyrite. Thermal pretreatment of the BS samples with and without additives (Na2CO3, K2SO4) was carried out at 700 and 850 °C before leaching experiments, which were performed in 10% H2SO4 at 90 °C for 3 h with S/L ratio of 0.75:50. Thermal analysis revealed emission of hygroscopic and physically bound water between 50 and 110 °C, decomposition of organic matter between 200 and 550 °C and decomposition of pyrite starting at around 500 °C. Dehydroxylation of muscovite and chlorite, and decomposition of jarosite, dolomite and unreacted Na2CO3 residue, takes place between 400 and 900 °C. Analysis of leaching solutions showed an increase in V solubility as a result of calcination. Solubilities of V, Mo, Al and K can be further enhanced by using additives during calcination. Therefore, thermal pretreatment, with or without additives, could be a promising method to increase the solubility of different metallic elements in a sulfuric acid environment.