Purpose: Currently, almost all cyanide-free gold leaching processes are still in the development stage. Proactively investigating their environmental impacts prior to commercialization is of utmost importance. In this study, a detailed refractory gold concentrate process simulation with mass and energy balance was built for state-of-the-art technology with (i) pressure oxidation followed by cyanidation and, compared to alternative cyanide-free technology, with (ii) pressure oxidation followed by halogen leaching. Subsequently, the simulated mass balance was used as life cycle inventory data in order to evaluate the environmental impacts of the predominant cyanidation process and a cyanide-free alternative. Methods: The environmental indicators for each scenario are based on the mass balance produced with HSC Sim steady-state simulation. The simulated mass balances were evaluated to identify the challenges in used technologies. The HSC Sim software is compatible with the GaBi LCA software, where LCI data from HSC-Sim is directly exported to. The simulation produces a consistent life cycle inventory (LCI). In GaBi LCA software, the environmental indicators of global warming potential (GWP), acidification potential (AP), terrestrial eutrophication potential (EP), and water depletion (Water) are estimated. Results and discussion: The life cycle assessment revealed that the GWP for cyanidation was 10.1 t CO₂-e/kg Au, whereas the halogen process indicated a slightly higher GWP of 12.6 t CO₂-e/kg Au. The difference is partially explained by the fact that the footprint is calculated against produced units of Au; total recovery by the halogen leaching route for gold was only 87.3%, whereas the cyanidation route could extract as much as 98.5% of gold. The addition of a second gold recovery unit to extract gold also from the washing water in the halogen process increased gold recovery up to 98.5%, decreasing the GWP of the halogen process to 11.5 t CO₂-e/kg Au. However, both evaluated halogen processing scenarios indicated a slightly higher global warming potential when compared to the dominating cyanidation technology. Conclusions: The estimated environmental impacts predict that the development-stage cyanide-free process still has some challenges compared to cyanidation; as in the investigated scenarios, the environmental impacts were generally higher for halogen leaching. Further process improvements, for example in the form of decreased moisture in the feed for halide leaching, and the adaptation of in situ gold recovery practices in chloride leaching may give the cyanide-free processing options a competitive edge.

Iron control in the atmospheric acid leaching (AL) of nickel laterite was evaluated in this study. The aim was to decrease acid consumption and iron dissolution by iron precipitation during nickel leaching. The combined acid leaching and iron precipitation process involves direct acid leaching of the limonite type of laterite followed by a simultaneous iron precipitation and nickel leaching step. Iron precipitation as jarosite is carried out by using nickel containing silicate laterite for neutralization. Acid is generated in the jarosite precipitation reaction, and it dissolves nickel and other metals like magnesium from the silicate laterite. Leaching tests were carried out using three laterite samples from the Agios Ioannis, Evia Island, and Kastoria mines in Greece. Relatively low acid consumption was achieved during the combined precipitation and acid leaching tests. The acid consumption was approximately 0.4 kg acid per kg laterite, whereas the acid consumption in direct acid leaching of the same laterite samples was approximately 0.6–0.8 kg acid per kg laterite. Iron dissolution was only 1.5–3% during the combined precipitation and acid leaching tests, whereas in direct acid leaching it was 15–30% with the Agios Ioannis and Evia Island samples and 80% with the Kastoria sample.

Pyrometallurgical metal production results in side streams, such as dusts and slags, which are carriers of metals, though commonly containing lower metal concentrations compared to the main process stream. In order to improve the circular economy of metals, selective leaching of copper from an intermediate raw material originating from primary base metal production plant was investigated. The raw material investigated was rich in Cu (12.5%), Ni (2.6%), Zn (1.6%), and Fe (23.6%) with the particle size D₈₀ of 124 µm. The main compounds present were nickel ferrite (NiFe₂O₄), fayalite (Fe₂SiO₄), cuprite (Cu₂O), and metallic copper. Leaching was studied in 16 different solutions. The results revealed that copper phases could be dissolved with high yield (>90%) and selectivity towards nickel (Cu/Ni > 7) already at room temperature with the following solutions: 0.5 M HCl, 1.5 M HCl, 4 M NaOH, and 2 M HNO₃. A concentration of 4 M NaOH provided a superior selectivity between Cu/Ni (340) and Cu/Zn (51). In addition, 1–2 M HNO₃ and 0.5 M HCl solutions were shown to result in high Pb dissolution (>98%). Consequently, 0.5 M HCl leaching is suggested to provide a low temperature, low chemical consumption method for selective copper removal from the investigated side stream, resulting in PLS (pregnant leach solution) which is a rich in Cu and lead free residue, also rich in Ni and Fe.

Natural resources and raw materials such as metals and minerals are often taken for granted in today’s society. Without them, the offerings of an enormous variety of modern conveniences, including computers and mobile phones, would not be possible. The production of these everyday items depends on a secure, sustainable, and reliable supply of critical raw materials. In addition, product development also requires new hybrid materials when targeting lightweight structures, etc. However, the mandatory recyclability of new products or materials is not obligatory or even prevailing practice in present manufacturing business. Therefore, the main research question in this article is: “how to solve the challenge of recycling in industrial system?” In this article, a comprehensive approach to recycling based on the findings of research projects is presented. Simplified, this involves moving the challenge from the end of the product’s life cycle to the beginning, to the design or even to material development phases. Life cycle and system thinking and material know-how in the design phase are found to be essential elements of a new approach to recycling. This approach stems from the material development and market economy perspectives. Furthermore, the vulnerability of the industrial system to create uncertainty to recycling is also demonstrated.

This paper presents a method for externalising and formalising knowledge involving the selection of hydrometallurgical process flowsheets for gold extraction from ores. A case-based reasoning (CBR) system was built using an open source software myCBR 3.0. The aim of the systems is to recommend flowsheet alternatives for processing a potential gold ore deposit. Nine attributes: Ore type, Gold ore grade, Gold distribution, Gold grain size, Sulfide present, Arsenic sulfide, Copper sulfide, Iron sulfide and Clay present were modelled and several literature sources of actual gold mines and processes were used for acquiring cases for the system. After preliminary testing, functional evaluation of the built CBR system was carried out by using five real mining projects as test cases. Additionally, human experts in the field of gold hydrometallurgy were interviewed to demonstrate the benefits of the CBR system as it holds no human biases towards any processing techniques. It was found that the suggestions of the CBR system provided useful information and direction for further process design and performed well compared to the interviewed human experts, thus confirming that the system is of practical relevance to the process engineer designing an industrial gold processing plant. The current model was found to be a functioning basis for further development through additional attributes, adjusted attribute weighting and increased number of cases.Peer reviewe

In 2018, global crude steel production was approx. 1.8 gigatonnes while its direct CO₂ emissions stood at 2 giga tonnes (2017), representing 23.5% of all direct industrial emissions. Globally iron and steel industry (ISI sector) is the second largest emitter after the cement industry while in Finland and Sweden the ISI sector is the largest industrial source of fossil CO₂. Biocarbon received from forest sector residues, such as bark, sawdust and lignin, provides an opportunity to a rapid reduction in fossil CO₂ emissions in the industry. Use of biocarbon for reduction purposes bestows an asset for both the forest and metals production and processing industry in Finland as well as in the Scandinavian countries. In this study, the focus was on the techno-economic evaluation of biocarbon production (1) from bark-based black pellets produced in a stand-alone plant close to a steel mill and (2) from bark produced in a pulp mill integrated plant. The economics of biocarbon production costs from bark appear promising, providing also an ample source, at an annual potential of up to 95,000 tonnes of biocarbon from a single pulp mill. GRAPHICAL ABSTRACT