VALORIZATION OF SECONDARY METAL RESOURCES THROUGH HYDROMETALLURGICAL PROCESSES

Abstract This executive doctorate project comes from the collaboration between the company Engitec Technologies S.p.A. and the Università degli Studi di Milano. The student joined the company staff, gaining access to the company laboratories and pilot plants. The project was focused on the study and optimization of hydrometallurgical processes for the recovery of metals from secondary resources. The two hydrometallurgical processes object of this work are proposed as alternatives to the state-of-the-art technologies applied worldwide for the recovery of zinc and lead. These two processes are capable to produce zinc and lead metals at high purity, avoiding purification and pyrometallurgical steps. Through a critical analysis of the available literature, research work initially focussed on the comprehension of the mechanisms and reactions at the base of the recovery processes. New alternative ways and optimizations were then put forward. In addition, thanks to a period spent working directly on a pilot plant installed in South Africa, identification of industrial-scale issues was pursued. As a result of this experimentation, many improvements were proposed and unravelled, resulting in ad-hoc equipment and concepts, conference presentations and a patent. The doctorate work brought to a series of steps forward in the comprehension and development of these processes, and to the permanent employment of the student in the company staff. Summary Reserves of natural resources are limited and nowadays it is necessary to pay more and more attention to the opportunities that a circular economy can provide. The technologies objects of this study are proposed as sustainable solutions to the shortage of raw materials from primary sources. The zinc technology is able to extract metal from zinc-bearing materials, as steel mill dusts, electrolysis wastes, mineral ores, and by-products. The Lead technology is able to extract lead metal directly from lead-acid-battery pastes; however, it can be applied in many other fields, as Cu/Au/Ag/Zn refining processes. These processes are based on hydrometallurgical approaches and avoid completely the high-temperatures treatments typical of pyrometallurgy. At the same time, the peculiar chemistry of Engitec processes (based on ammonium chloride) leads to a simpler series of treatments, as the removal of the chlorine-washing step in the standard Zinc sulphate electrolytic process. The Zero Liquid Discharge policy (ZLD) is adopted by the company for all hydrometallurgical processes, implying that no liquid effluents are released in the environment. The side-streams of the plants are designed to be solid products or by-products, decreasing the water and CO2 footprint as much as possible. The two processes for recovering zinc and lead are called EZINEX® and F.A.S.T. respectively and are based on the same concept with minor differences. The research focused primarily on the EZINEX® process due to market-related reasons. The first step is the leaching of the target metal using concentrated ammonium chloride solutions: the achievement of high extraction yields is one of the essential elements for these processes. The problem related to the presence of zinc-ferrites in steel mill dust was addressed by testing different reducing agents, although a satisfying solution is still to be found. From the experience gained on the pilot plant installed in South Africa, it was possible to notice that the presence of calcium in solution is harmful for the plant heat exchangers. It was hypothesized and verified with experimental tests the possibility to keep under control the calcium concentration through the controlled precipitation of calcium sulphate without the addition of further reagents and using the ammonium sulphate that is generated in the scrubber. During the leaching step, chemical species other than zinc are also brought into solution. Final metal at high purity can be obtained only if these species are managed and removed before electrolysis. Examples of these impurities are copper, silver, manganese, lead, and cadmium. Manganese is removed by the addition of permanganate that transforms the dissolved manganese into a filterable manganese dioxide, by a comproportion redox reaction. Other metal species (Cu, Ag, Pb, Cd) are removed exploiting a cementation reaction. The analysis of demanganization reaction was performed by kinetic studies, together with electrochemical and chimico-physical experiments. A connection between the redox potential and the Mn(II) concentration was pointed out, and a mathematical function correlating the redox set-point and the plant solution pH was derived. This equation is able to control the on-field equipment to optimize the reagent consumption. At the same time, separation of the solid manganese oxide product was improved by exploring techniques alternative to simple filtering, as: i) flocculating agents, and ii) the most promising centrifugal solid-liquid separation. Cementation of copper, silver, cadmium, and lead by addition of metallic zinc powder has a considerable impact on the cost of refined zinc. Therefore, alternative methods to perform this purification were evaluated. A first study involved the testing of a rotary reactor loaded with metallic zinc pellets. A refresh of the zinc active surface, covered by the reaction products, is achieved by mixing and friction between the zinc pellets. A consequent increase in reaction yield was proved by investigating the influence of solution flow rate, rotation speed, and Zinc load. In addition, the necessary zinc for the cementation reaction can be self-produced by simple treatments of the electrolyzed zinc metal. A second alternative to zinc metal powder is the use of an electrodepuration cell. By polarising the cathodes of the cell to a certain potential it is possible to selectively reduce various interfering cations without the use of chemical reagents. Initially the reduction potentials of the interferents were determined and then different lab-scale cell configurations were realized and tested. The potentialities of this technique were confirmed. However, optimization of the cell design is still an open question. The electrolysis cell is the final step of the process. Zinc is reduced to its metallic form using electrical current. A fundamental aspect of this process is represented by the choice of the anodic material. The advantage of using activated titanium electrodes is a reduction in power consumption of about 17%, however, the use of activated titanium requires precise considerations, such as the absence of manganese in the electrolysis solution. The effect of manganese on the stability of these catalytic electrodes was studied and it was concluded that manganese is deleterious to their durability. The metallic zinc deposit of the plant quality was analysed. Several commercial additives were applied to increase the compactness and smoothness of the deposit. The campaign of experiments led to identify as the best additive a mixture of gum arabic, aescine and agar. The development of the technology involved chemical/engineering/industrial challenges with consequent advantages and disadvantages. This doctorate, starting from Engitec previous experience, explored and solved some problems, partly detectable only in a pre-industrial plant such as the one built in South Africa.