During secondary copper production both internal slags and a final copper slag is produced. In contrast to the final copper slag, which is a process by-product, internal slags are typically recycled to the preceding aggregate. Therefore also the part of copper and other valuable elements like nickel, tin, lead and zinc reporting to the final slag is kept low. On the other hand, other elements which influence the process negatively like aluminium and chromium are also recirculated and lead to an increase of density, melting temperature and therefore also viscosity of the resulting slags. This results in a more complicated slag treatment and influences aggregate capacity, i.e. (black copper smelter and the converter) and copper loss to slag, negatively.

This study is investigating a slag treatment (slag reduction) of high-copper bearing slags from secondary copper production to meet the described challenges by avoiding internal slag recirculation. But besides that, the process results to additional value in that the secondary slag produced through appropriate fluxing (i.e., through slag design) can be easily used for construction purposes. For this investigation an in-depth knowledge about not only the thermodynamics but also kinetics of this process is required.

This study is built on three pillars: Thermodynamical modelling, kinetic investigations and experimental tests in a medium scale. The thermodynamical modelling is conducted using FactSage™ with inclusion of the copper database. An open-system approach is used to model the reduction process by hydrogen. Through that several simulation steps are taken into account, where the hydrogen is added in fractions which are forming a thermochemical equilibrium with the slag and the resulting metal phase. After reaching equilibrium (during each step) the gas phase is removed and a new gas phase is formed by a new addition of hydrogen and the creation of a new thermodynamical equilibrium.

Kinetics investigations were performed by using thermogravimetric methods with the combined analysis of the off-gas stream using mass spectrometry for the achieved ratio of hydrogen to steam. This ratio can be used as a measure for the reduction progress. Firstly, the slag under investigation is mixed with hematite (Fe₂O₃) and silica (SiO₂) to achieve a secondary slag near fayalitic composition (45-50 wt.-% FeO and 35 wt.-% SiO₂). The non-fluxed slag would for be rich in alumina but contains also a non-negligible proportion of chromium(III)-oxide. Both compounds are known for increasing slag viscosity. The chemical composition of the resulting metal phase and the secondary slag is analysed using SEM-EDX.

The experimental trials in medium scale are performed with around 0.5-0.75 kg slag material, depending on the mass of needed fluxes. Here the gas is injected via a lance in the molten slag system at given temperature. Via weighing before and after the experiment the mass loss during reduction, i.e., due to zinc and lead fuming can be estimated. By means of chemical analysis of the achieved metal phase as well as of the resulting secondary slag the reduction degree in this scale can be evaluated. The main goal is to achieve a secondary slag which contains less than 1 wt.-% of copper and other valuable elements and a secondary slag with an iron oxide content and silica content of 45-50 wt.-% and 35 wt.-%, respectively.

It can be seen that the majority of slag reduction is completed within a few minutes and is therefore faster than when using carbon monoxide as a typical reducing agent, as long as diffusion can be neglected. In reality, this is not the case: due to the distance e.g., between the lance tip and the outer diameter of the reaction vessel, the necessary reduction time is extended, as the process is increasingly diffusion-controlled.

With increasing temperature, an accelerated reduction can be observed due to a reduced viscosity of the slag and an improved mobility of the hydrogen, whereby the reduction of the hematite added as an additive can be considered complete even before the reduction of the slag.

The composition of the metal melt plays an important role in the production of high-quality aluminum castings. A melt with high hydrogen content often leads to defects and macro porosity [1]. Gas purging treatment and the use of melting salts for degassing are commonly used to reduce the hydrogen content. However, a consistent solidification of the entire cast part cannot always be realized. In these cases, undesired macro porosity may occur due to hydrogen excess in comparison to the amount of porous seeds in the melt. 

In recent years, the Institute of Nonferrous Metallurgy and Purest Materials has identified two ways of positively influencing this hydrogen porosity. On the one hand, it was found out that it is possible to use a special melt additive to adjust the ratio between the hydrogen dissolved in the melt and the existing pore nuclei so that the hydrogen released during solidification is finely distributed in the casting [2]. On the other hand, it was shown that the use of reactive filter materials can positively influence the precipitation of the atomically dissolved hydrogen and thus generate denser castings [3,4]. Both processes are presented and the efficiency and influence of the respective filter materials and additives is explained. 

Increasingly, scientific and technological developments are moving to a more environmentally friendly direction [1]. Therefore, the necessary adaptation of state-of-the-art processes with modified systems to an overall cleaner and more energy efficient state is imminent and requires a lot of research work, a more detailed look at processes and new test equipment. This is particularly true for the metallurgical industry, where carbon is needed not only as an energy source but also for reduction, and where the transition to greener processes has to work in tandem with the difficulties of recycling new complex multi-metal wastes such as magnets, batteries, e-waste and complex slag systems.

The Institute of Nonferrous Metallurgy and Purest Materials (INEMET) aims to look more closely at utilizing hydrogen as a key challenge for the near future with regard to metallurgical process decarbonization [2]. This is planned as a substitute for fossil fuels, e.g., natural gas for smelting copper cathodes prior to casting; nonetheless the influence of hydrogen on the copper melt, furnace refractory, fluid-dynamics, heat transfer and process control have to be assessed. In addition, the utilization of hydrogen for the molten phase reduction, e.g., in the reduction metal oxides, e,g. SnO2 in the context of a smelting process or of iron ore in the context of fluidized bed gas-solid processes is planned. [3].

The use of atmospheric thermal plasma jets as an alternative to conventional gas burners is also being investigated. The ability to use different gas compositions enables new ways of heating and treating slags, scrap and ores and is identified as a key technology in modern metallurgy [4]. Thermal plasma can be used to form species such as atomic hydrogen or even H⁺, which can reduce any metal oxide, allowing processes that cannot be decarbonized with molecular H₂ to be carried out completely without CO₂ emissions [5]. The fuming behavior of melt components can differ in these systems hence opening further pathways for metal refining.

Various new sensors are being installed and used to improve the measurement capabilities and to combine all the sensor data to gain better process knowledge. For example, new phase-differentiating melt height measurements are being tested with radar sensors. The aim is to identify the height of the slag and metal phases in a smelting unit operation. In addition, acoustic measurements may aid to analyze process fluid-dynamics. To link the different parameters of the experiments, a digital twin (on-line process model receiving experimental data) of the Institute's TSL is being built. Developments realized within the EU-HORIZON Mine.io project will be analyzed in detail.

In order to enable new process designs for industrial use, the key expertise lies in scaling up from laboratory scale to pilot scale experimental campaigns. To this end, new experimental fields are being designed within a newly planned furnace hall.

All in all, the directions for future-oriented pyrometallurgical research have been set and will be realized and carried out hand in hand throughout the university, by undergraduate/ graduate students, technical/ academic staff and industry alike.

The metallurgical industry is continuously seeking sustainable methods for the valorization of materials, such as tin residues, which arise as a byproduct during production processes for example in soldering printed circuit boards. This study focuses on the utilization of green, non-fossil reducing agents, specifically biomasses, for the recovery of tin from industrial residues. These can contain valuable amounts of tin and other valuable metals (e.g. Ag and Cu) that can be recovered and reused, essentially making its valorization not only environmentally imperative, but also economically beneficial. When treated correctly the produced secondary slag can become a valuable base product for cement production. This study aims to prove exemplary pathways for holistic valorization of two distinct tin residues.

Biomasses, abundant and renewable, from agricultural, forestry and other organic sources, are considered carbon-neutral due to the fact that they absorb as much carbon during their “life”, as they release when utilized. This more climate friendly status holds especially true for low-grade byproducts. In pyrometallurgy, the use of biomasses as reducing agents is a rapidly growing field of research, providing greener alternatives to the traditional reducing agents such as coke [1][2][3]. This work, is also aimed to explore the effectiveness of different biomasses in the reduction of tin oxides from the residues to their metallic form [4]. 

With regard to the experimental procedure, various biomass types such as straw, wood, coconut shells etc. were used [5] and compared against traditional coke. The reduction process was carried out in crucible experiments under inert gas in completely molten systems, while optimizing the parameters of temperature, reaction time and tin residue / biomass ratio as well as fluxing, in order to minimize the concentration of impurities in the metallic phases. 

For some residues a prior leaching step is explored and compared against direct pyrometallurgical treatment. Neutral and acidic leaching was investigated with the purpose of decreasing Cl and S amounts which are potentially undesired in the following pyrometallurgical step.

In conclusion, the study demonstrates the feasibility of using biomass reducing agents as greener reducing agents for the valorization of tin residues. The approach aligns with the principles of circular economy and offers a pathway towards more sustainable metallurgical processes. The successful recovery of tin using biomasses could lead to a reduction in the industry’s carbon footprint and contribute to the conservation of natural resources. 

Zinc (Zn) is utilized in many industrial applications, such as batteries, cosmetics, pharmaceuticals, and metal production. Due to urbanization and the depletion of high-grade ore deposits, efficient resource management is required from both primary and secondary resources. In terms of the latter, the most widely used recycling method of Zn-containing scrap is the Waelz process, particularly for electric arc furnace dust (EAFD). The Waelz process is a pyrometallurgical technique where the Zn scrap is loaded into a rotary kiln with a carbon-containing reducing agent at 1200-1300 ^oC to extract Zn [1]. Zn subsequently vaporizes and oxidizes in the gas stream to form particulate ZnO, which is then collected on bag filters.

In Europe, approximately 250,000 tons/year of Zn is recovered via the Waelz process. However, the process also generates nearly 800,000 tons/year of slag. Utilization of the “Waelz Slag” is hindered due to the lack of environmental compatibility [2], mainly because of the complex chemical and mineralogical composition. As a result, Waelz slag is largely landfilled, even though the iron content exceeds that of high-grade iron ores (~25% iron). 

Numerous studies have investigated the recycling potential of Waelz slag by different methods, such as in a vertical retort [1], in a top-blown rotary converter, and as a charge to an electric arc steelmaking furnace. However, these studies were either theoretical, or in the early stages of development. Nevertheless, the main component of Waelz slag is iron (Fe), followed by Zn, manganese (Mn), and lead (Pb). Of these, Fe and Zn represent the target elements for downstream utilization. 

The project’s primary goals are to generate pig iron, slag (ideal for the building materials sector), and Zn-rich fly ash (for Zn recovery). Information from a number of analytical techniques, including X-ray fluorescence (XRF), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and mineral liberation analysis (MLA), were employed to augment parameters for simulation of one-kilogram experiments conducted in an induction furnace using FactSage 8.2.

The study employs an iterative approach where the result of each experiment serves as a guide for the subsequent experiments. The highest total iron recovery is 83.18%. A combination of the FactSage and Einstein-Roscoe viscosity models was used to determine slag viscosity, implying that viscosity depends more on composition than temperature. Addition of 16% SiO₂ and 3% Al₂O₃ shows a high slag viscosity and delayed Mn reduction, possibly due to insufficient Si dissolved in the metal phase and the system being furnace cooled, giving time for nucleation of Mn-containing phases. The calculated actual oxygen partial pressure on all experiments ranges from 10^-8 to 10^-21. XRD analysis of dust recovery filter paper confirmed the presence of Zn. The slag produced in this study has similar compositions to those studied by Grudinsky et al. that can be used as concrete material to enhance its properties [3]. Overall, the study opens the way for holistic valorization of Waelz slag, resulting in more sustainable Zn resource management.

Tin is one of the earliest metals used in human history. The amount of tin produced and consumed worldwide in the last ten years has been estimated to be between 300,000 - 400,000 tons annually [1]. Not only is tin an essential constituent of tin bronze, it is also a critical component of alloys for making solders, which are essential for the major drivers of green energy transition; electric and autonomous vehicles, solar PV, semiconductors, etc. [2]. Tin from cassiterite, SnO₂ (main source of tin), has over the years been processed via the pyrometallurgical route. Sulfurization and roasting are primary steps in the process, which are carried out to thermally enrich SnO₂ content in case of low-grade concentrates. Afterwards SnO₂ is treated in reactors, where carbon-based reducing agents are used to reduce tin to the metallic form at high temperatures [3], after which the resulting tin produced is further refined to obtain a marketable grade [4]. The carbothermic reduction of cassiterite, has however, seen several drawbacks such as the generation of environmentally harmful waste gases (e.g., CO₂), high energy and equipment costs, as well as low selectivity with regard to impurities contained in the ore which are difficult to be separated at elevated temperatures [5]. 

A hydrometallurgical extraction route is proposed as a potential alternative processing method for tin extraction from cassiterite to achieve a higher degree of sustainability. This is because it ensures the reuse of chemicals in the process loop and allows for a higher metal recovery at a significantly lower energy consumption and greenhouse emissions [6]. Three different acids, (methanesulfonic acid, sulfuric acid, and oxalic acid) were investigated for their potential to leach tin from cassiterite, and they all proved futile, which supports already existing literature regarding the high chemical stability of cassiterite. A pre-treatment step was deemed necessary to render tin water soluble for subsequent hydrometallurgical processes. 

A reduction of cassiterite in a hydrogen-controlled environment to produce SnO slag, from which tin can easily be leached in acid or alkaline media was investigated.  The formation of SnO slag can be accompanied with the production of a tin metal phase depending on the H₂/concentrate ratio used. During experimentation, a high purity tin nugget (99.5 wt.%) was produced at a reduction temperature of 1300 ⁰C at 30 g H₂/ kg concentrate. The slag formed was soluble in sulfuric acid solution, from which tin extraction is being examined. Other pre-treatment options such as soda roasting and alkaline fusion are being investigated with regard to technological, economic and environmental feasibility.

This study explores a sustainable approach to pyro-metallurgical recovery of metallic raw materials from mixed sulfidic fine-grained waste streams, named as Theisenschlamm [1, 2]. As part of the FINEST project (https://finest-project.de/), Subproject 3 "FINEST Disperse Metals," our focus is on optimizing the secure blending of fine and ultra-fine-grained material flows to recover valuable metals through a multi-stage pyro-metallurgical recycling process. Specifically, we investigate the utilization of calcium- and zinc-rich industrial residues as alternative feeds for the pyro-metallurgical metal recovery process.
Using FactSage™ 8.2 software, we simulate and evaluate the behavior of the slag systems throughout both the oxidation and reduction stages of the process. Ternary phase diagrams are constructed for the key components of the slag systems, providing insights into phase equilibria, solidification behavior, and the stability of various phases under different thermal conditions [3].
A significant aspect of this work involves calculating the viscosity of the slag during the high-temperature processing stages, as this property is critical for ensuring efficient metal separation and refining [4]. Viscosity calculations are performed using the Einstein-Roscoe model, integrated with the Quasi-chemical model from FactSage™ platform, to predict the flow behavior of the slag in relation to its composition and temperature. These findings offer a deeper understanding of the impact of alternative flux materials on slag characteristics, contributing to process optimization.
This detailed modeling-driven approach not only facilitates the refinement of metal recovery processes from complex waste streams but also promotes sustainable circular economy practices by reducing dependence on traditional flux materials and enhancing resource efficiency in pyro-metallurgical recycling [5].