Hydrometallurgy is one of the main routes for obtaining metals that are needed for society development and for our everyday life. Chapter One presents the basics of hydrometallurgy, namely its main stages leaching, purification and/or concentration of pregnant leach solutions (PLSs), and metals recovery. Chapter Two focuses on the gold extraction processes that involve the use or addition of industrial grade oxygen to optimise the processes. In particular, it looks at how oxygen can be used to increase the throughput and/or gold recovery and make the processes more flexible. Chapter Three gives an overview of the microbially-mediated metal transformations in which iron oxides potentially provide an applicable biotechnological method for efficient removal of pollutants from ground waters and wastewaters. Chapter Four assesses the hydrometallurgical process based on leaching, deionisation, and purification of bis(trifluoromethylsulfonyl)amide salt including RE components.