Public defence in Processing of Materials, M.Sc. (Tech.) Anssi Karppinen

Title of the thesis: Leaching of gold and battery metals from industrial tailings

Thesis defender: Anssi Karppinen
Opponent: Prof. Kostas Komnitsas, School of Mineral Resources Engineering, Technical University of Crete, Greece
Custos: Prof. Mari Lundström, Aalto University School of Chemical Engineering

The modern world requires multiple metals due to the phase-out of fossil fuels and increasing electrification. At the same time, the ore grades of critical metals are declining and there is a lot of room for improvement regarding recycling efficiency. In addition to primary ores and recycled metals, metallurgical processes produce diverse side streams such as tailings, slags, dusts, residues, and scrap which contain low grades of valuable metals but are not often recovered. As ore grades decline, these side streams have more comparable metal grades to the ores.

Tailings are leftover material from primary production where the less valuable fraction of the raw material is separated from the more valuable fraction. Typically, tailings are produced in the froth flotation process where metal grades of the ore are enriched in the concentrate and tailings are generated in the underflow. In addition, other types of wastes have been classified as tailings, such as leach residue from the cyanide leaching of gold. Tailings are stored in ponds or dried and stacked near the mine site. Cumulative storage of tailings requires a great deal of space and is an environmental concern due to acid mine drainage and potential breaking of tailings dam. 

This thesis investigates leaching as treatment for tailings originating from sulfide ores. The aim was to extract valuable metals—gold, cobalt, nickel, and copper—by acidic leaching in chloride and sulfate media. Additionally, tailings were utilized as a novel reductant for battery cathode material leaching to facilitate the simultaneous extraction of battery metals from both investigated materials.

The results showed that while full valorization of valuable metals from tailings is challenging, considerable concentrations of metals can be extracted from already mined and processed materials. Chloride leaching can bring additional value to the conventional cyanide leaching process with improved capabilities on extraction of gold from refractory tailings. Common low-value minerals in tailings—pyrrhotite and pyrite—can be utilized as reductant in hydrometallurgical recycling of battery waste. Simultaneously, additional battery metals i.e., cobalt, nickel, and copper can be extracted from tailings. Hence, it is technically possible to integrate battery recycling processes with mine sites that already extract and enrich valuable battery metals from sulfide ores.

Keywords: lithium-ion battery, pyrite, pyrrhotite, oxidant, reductant

Thesis available for public display 7 days prior to the defence at Aaltodoc. 

Contact information: 
anssi.karppinen@aalto.fi