Mineral raw materials from geogenic deposits end up as waste after extraction, processing and use. However, if these wastes are regarded as anthropogenic ores, the cycle can be closed if their contents are returned to the product cycle. From the perspective of a geoscientist with knowledge of the formation of deposits and the formation of mineral phases with the corresponding concentration of valuable carriers, there are opportunities to develop efficient recycling systems in adapted technological process chains.

The interplay between the findings on the formation of natural ores and the geochemical modeling of the processes, for example, with the knowledge of thermodynamic models in metallurgy, offers the option of saving even the previously unrecoverable proportion of “spice metals”. The interplay of mineralogical characterization, mechanical processing and the further processing of concentrates in metallurgical processes can achieve what is known as the “engineering of artificial minerals”.

However, mineralogical analysis not only provides the basis for understanding the structure of the material to be processed in cases where slags are specifically influenced and processed as anthropogenic magmas, but also along the entire range of waste streams from mining tailings to electronic scrap. Liberation and sorting as well as subsequent chemical processes are based on this.

The conference contribution aims to present a small kaleidoscope of experience from around 40 years of dealing with various legacies from production and consumption.

Printed circuit boards (PCBs) play a crucial role in recycling of WEEEs (waste from electronical and electronic equipment), containing valuable elements like copper, gold, nickel, palladium and many others. While e.g. copper and gold are being recycled via well-established pyrometallurgical routes (Hagelüken, 2006), others are lost in the waste stream. Additionally, hazardous contents like flame retardants and persistent organic compounds comprise further issues for recyclers. In an attempt to contribute to the improvement of recycling rates, we used a combination of different analytical tools to investigate unpopulated PCBs, such as Computer Tomography (CT), SEM-based image analysis (i.e. automated mineralogy, AM), X-ray diffraction and wet chemical methods in order to perform a case study eventually including the usage of heavy liquid separation, a new type of eddy current separator and hydrometallurgical techniques. Furthermore, we trained a particle-based separation model (PSM, Pereira et al. 2021), which has successfully been applied to primary raw materials, to predict the probabilities of individual particles reporting to different processing products. Our results indicate that PSMs are valuable tools to predict recycling efforts, we suggest to use rather 3D-methods (e.g. CT) for complex particle shapes rather than 2D-techniques (e.g. AM) to obtain particle data necessary for simulations.

Hagelüken, C. (2006). Improving metals returns and eco-efficiency in electronics recycling. In Proceedings of the 2006 IEEE international symposium on electronics and environment, San Francisco.

Pereira, L., Frenzel, M., Khodadadzadeh, M., Tolosana-Delgado, R., & Gutzmer, J. (2021). A self-adaptive particle-tracking method for minerals processing. Journal of Cleaner Production, 279, 123711.

Gypsum (CaSO₄·2H₂O) is a crucial mineral across sectors such as construction, agriculture, and biomedicine. Despite its potentially full recyclability, a shortage looms due to limited mining in Europe and decreasing production of flue gas desulfurization (FGD) gypsum, a byproduct of coal power plants. With current EU consumption at 24 MT/a (17 MT mined, 7 MT FGD), a deficit of 10-35 MT/a is projected by the 2030s as CaSO₄ becomes a critical raw material [1]. Meanwhile, substantial CaSO₄ waste is produced in various industries (e.g., phosphogypsum, red gypsum), but its recycling remains limited (10% in Germany, 5% in the EU) due to contamination and separation challenges.

This contribution introduces a sustainable, efficient wet-chemical method for converting gypsum to bassanite (CaSO₄·0.5H₂O), and thus recycling gypsum, under mild conditions (T < 100 °C) using reusable high-salinity aqueous solutions (brines with c[NaCl] > 4 M) [2]. The optimal conversion conditions (T > 80°C, c[NaCl] > 4 M) enable rapid (<5 min) and reversible transformation. Upon cooling, gypsum re-forms, offering a temperature-dependent control over phase transition. Unlike conventional thermal dehydration (150-200 °C), this approach promotes the dissolution of gypsum, allowing contaminants to be separated via selective precipitation or adsorption. Additionally, the wet-chemical process facilitates the physical removal of impurities from gypsum matrices, making it advantageous for recycling gypsum waste from sources such as demolition or urban mining, where it is often mixed with other materials.

[1] Resources, Conservation and Recycling, 2022, 182, 106328.

[2] Journal of Cleaner Production, 2024, 440, 141012.

The University of Applied Sciences Nordhausen is currently conducting research projects on the topic of resource conservation. For example the separation of different materials with the aim of recycling or utilising them as a source of raw materials / secondary raw materials. The research concentrates on the gypsum recycling and the substitution of natural gypsum with gypsum from industrial productions. This problem is a particular issue in the southern Harz region, where the interests of big mining companies and nature conservation meet. With the end of coal-fired power generation, the German market will lose around 50 % of its gypsum, as this comes from flue gas desulphurisation. Unless other alternatives are found and utilised, the resulting deficit will probably have to be covered by mining natural gypsum. The current research project ‘PhosphoGypsum’ is investigating a quantitatively promising by-product of phosphoric acid production that requires qualitative optimisation. The aim is to develop a phosphogypsum available in Europe for industrial use and to analyse the existing opportunities, risks and challenges. Various treatment methods are to be implemented in the manufacturing process of the by-product in order to optimise the quality requirements of the material.

Tungsten is considered to be inert to weathering and benign if released to the environment. The main tungsten ore minerals, scheelite and wolframite, seem to be weathering very slowly or not at all. In our research in the oxidation zone of the Ochtiná W-Mo deposit (eastern Slovakia), we observe complete weathering of the W minerals and release of W into the environment as a function of pH. The primary ores consist of quartz, pyrite, scheelite, and wolframite. The secondary minerals are iron oxides, identified as goethite by Raman spectroscopy. Some of the iron oxides have the morphology typical of schwertmannite. These Fe oxides contain 0-5 wt.% WO₃ and are products of weathering of the primary W minerals under mildly acidic conditions. Their textures change if the primary mineral was pyrite, wolframite, or scheelite. High porosity in the weathered W minerals suggest release of material, especially of W, into water percolating through the oxidation zone. X-ray absorption spectroscopy shows that W is adsorbed on the Fe oxides. In some samples, the weathering assemblage includes tungstite, hydrotungstite, W-bearing jarosite, goyazite, and baryte. Among these minerals, especially jarosite is an indicator of strongly acidic conditions. The occurrence of either Fe oxides or W oxides can be rationalized by calculation of their solubility curves as a function of pH. The solubility of Fe oxides attains its minimum at circumneutral conditions whereas that of the W oxides in strongly acidic media. This study explains the behavior of W in mining waste.