Lixiviation and biolixiviation of zero-valent copper

Abstract

Lixiviation and biolixiviation of zero-valent copper

Fransen William

Faculty of Applied Sciences, Master in mining and geological engineering (academic year 2016-2017) Supervised by Prof. S. Gaydardzhiev.

Since its discovery, production of copper has never ceased to increase. Its mains source has always be mining of ores. Currently, around 75% of the world copper production relates to copper-from-ore, also called primary copper. Because reserves are finite and recycling of this metal have been highly investigated these last decades. Most of the Cu-bearing scraps, considered as the secondary copper, are recycled through pyrometallurgical routes. These processes are perfectly optimised and generally widely inspired from mining processes, in the cases where Cu-bearing scraps are not already treated in primary copper production circuits. This study focuses on hydrometallurgy. In the case of copper recycling, such techniques are implemented only on E-waste, also called WEEE (Wastes from Electrical and Electronic Equipments). In 2014 around 41.8 million tons of E-waste were produced and thus amount never ceases to increase. Their average composition is 50% of iron, 21% of plastics, 13% of non ferrous metals and 16% of other materials including rubber, ceramics or concrete. Their are considered as very heterogeneous however, most of them carry a common device: a Printed Circuit Board (PCB). It is characterised on average by 28% of metals, 23% of plastics and the rest consists of ceramics. They maximum copper grade is around 20%, together with notions of resource efficiency, energy savings and environmental concerns, it explains why these last decades so many researches have been focused on copper recycling from PCB. Hydrometallurgy recycling of metals from PCBs is already implemented at industrial scale (e.g. Umicore). Biohydrometallurgical routes are currently limited to lab scale experiments and many of them are related in the literature. For copper recycling, most of them focuses on using iron-oxidising and/or sulphur-oxidising micro-organisms. They act as catalysts in oxidation of ferrous iron and/or reduced sulphur respectively. Advantages of using micro-organisms are recognised, as well as the disadvantages. One of the main limiting parameter of using micro-organisms to enhance leaching of copper from PCBs is related ot the PCB themselves. Indeed, they are recognised as toxic for micro-organisms and above a certain pulp density, which is already too low to enable profitable bioleaching, PCBs inhibit cell growth. This limitation partially prevents the fully understanding of interaction taking place during bioleaching. Through different experiments, this work aims to investigate abiotic leaching and bioleaching of metallic copper rods. Because everything is about chemical reactions, reaction kinetics are studied. Because micro-organisms catalyse ferrous iron oxidation, the kinetics of this reaction is studied in abiotic and biotic conditions. It reveals very low abiotic oxidation rates at 30 and 40° C. However three of the four studied consortia, working at 30 and 42° C, show ferrous iron biooxidation rates 2 to 20 times higher than the abiotic ones, confirming these consortia carry iron-oxidising micro-organisms. Bioleaching experiments point out the fact, under the chosen operational conditions, use of micro-organisms does not help to increase really the recovery of copper. This fact could be explain by the design of the bioleaching experiments. These lasts are characterised by too long incubation time led to passivation of metallic copper rods by the precipitation of jarosite on their surface. Therefore, it is concluded the use of the present consortia is above all profitable for biooxidation purposes. A theoretical process is proposed in order to illustrate the application of bioleaching at larger scale and it is based on results of the present experiments. To avoid passivation issues, copper digestion and ferric iron regeneration are separated in two reactors: an abiotic leaching tank and a biooxidation tank. This process works in batch mode and in theory, it would enable to treat 880tons of shredded copper cables per year and recovery, yearly, 44 tons of copper. Whatever it be, this study, like many other, reminds the interest of investigating more on biohydrometallurgy applied to copper recycling.