  Citation: Ristovi´ c, I.; Štyriaková, D.; Štyriaková, I.; Šuba, J.; Širadovi´ c, E. Bioleaching Process for Copper Extraction from Waste in Alkaline and Acid Medium. Minerals 2022, 12, 100. https://doi.org/10.3390/ min12010100 Academic Editors: Carlos Hoffmann Sampaio, Weslei Monteiro Ambros and Bogdan Grigore Cazacliu Received: 27 December 2021 Accepted: 12 January 2022 Published: 15 January 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/). minerals Article Bioleaching Process for Copper Extraction from Waste in Alkaline and Acid Medium Ivica Ristovi´ c 1, *, Darina Štyriaková 2,3 , Iveta Štyriaková 2 , Jaroslav Šuba 2 and Emilija Širadovi´ c 1 1 Faculty of Mining and Geology, University of Belgrade, Djusina 7, 11000 Belgrade, Serbia; emilija.siradovic@rgf.rs 2 ekolive s.r.o., Americká trieda 3, 04013 Kosice, Slovakia; darina.styriakova@ekolive.eu (D.Š.); iveta.styriakova@ekolive.eu (I.Š.); jaroslav.suba@ekolive.eu (J.Š.) 3 Slovenian National Building and Civil Engineering Institute, Dimiˇ ceva ulica 12, 1000 Ljubljana, Slovenia * Correspondence: ivica.ristovic@rgf.bg.ac.rs Abstract: Flotation wastes are becoming a valuable secondary raw material and source of many metals and semimetals worldwide with the possibilities of industrial recycling. The flotation tailings contain oxide and sulfide minerals that have not been sufficiently stabilized and form acidic mine waters, which in turn contaminate groundwater, rivers, and reservoi6sediments. An effective way to recycle these mine wastes is to recover the metals through leaching. While the focus is on acid bioleaching by iron- and sulfur-oxidizing bacteria, alkaline leaching, and the removal of iron- containing surface coatings on sulfide minerals contribute significantly to the overall environmental efficiency of leaching. For this study, static and percolate bioleaching of copper from flotation waste at the Bor copper mine in Serbia was investigated in alkaline and then acidic environments. The aim of the study was to verify the effect of alkaline pH and nutrient stimulation on the bioleaching process and element extraction. A sample was taken from a mine waste site, which was characterized by XRF analyses. The concentration of leached copper was increased when copper oxide minerals dissolved during alkaline bioleaching. The highest copper yield during alkaline bioleaching was achieved after 9 days and reached 67%. The addition of nutrients in acidic medium enhanced the degradation of sulfide minerals and increased Cu recovery to 74%, while Fe and Ag recoveries were not significantly affected. Combined bioleaching with alkaline media and iron- and sulfur-oxidizing bacteria in acidic media should be a good reference for ecological Cu recovery from copper oxide and sulfide wastes. Keywords: copper; alkaline; acidic bioleaching; secondary raw materials 1. Introduction The copper and precious metal deposits in the territory of the town of Bor in the Republic of Serbia were discovered at the beginning of the 20th century. Since then, there have been two world wars, changes in ore processing technologies, and the Bor mine has been closed and reopened several times. Due to its importance, the entire radius around the town of Bor was the focus of investors for the exploitation of copper and gold. Since its establishment, the local company RTB Bor (Bor Mining and Metallurgical Basin) was state-owned. Constant mismanagement led to the sale of part of the company (63%) to the Chinese partner Serbia Zijin Copper DOO Bor. The main activity of the company is mining and processing of copper ore, and the production of cathode copper, copper wire, gold and silver, sulfuric acid, copper sulfate, selenium, platinum, palladium, brass, bronze, and precious metal salts. In addition to primary reserves, Bor mines have large quantities of secondary raw materials derived from centuries of mining and processing copper ores. The total amount of flotation waste in Bor is 588 million tonnes (Table 1). The old flotation waste (Figure 1) contains the most valuable source of copper for the industry. The Minerals 2022, 12, 100. https://doi.org/10.3390/min12010100 https://www.mdpi.com/journal/minerals