The pre-treatment work started with a detailed characterization of the four model streams (carbon steel slags, LC and HC ferrochrome slags, stainless steel slags). This work revealed the need for a multi-analytical approach to gain insight in Cr distribution, and highlighted the importance of choosing an appropriate analytical technique for reliable Cr quantification (Horckmans et al., 2019).
Magnetic separation proved to be the best method for up-concentration, resulting in a pure metallic phase with potential for reuse, and enriched fractions for leaching (Kukurugya et al., 2020). The use of microwave or electrodynamic pulse fragmentation to improve comminution efficiency were unsuccessful due to the small particle size of the target minerals and metals.
The best leaching results were achieved with (conventional and microwave) NaOH roasting and water leaching, reaching a selective Cr recovery > 95% with < 1% matrix dissolution. Heap leaching resulted in lower recovery (21 %) but could be a lower cost alternative to recover the easily leachable Cr.
The combination of multiple precipitation steps and nanofiltration allowed to reuse up to 83% NaOH and produce very pure Cr(OH)3 (90%+). Layered double hydroxides (LDH) with a very high selectivity for V over Cr and a high sorption capacity (up to 65 mg Cr/g) were engineered to withstand the alkaline milieu in the leachates. Anion exchanging extractants were used in solvent extraction to up-concentrate the valuable elements and separate them from Al. Optimized electrocoagulation removed Cr fast and completely from the solution. Overall, the developed recovery schemes recover Cr from the leachates, but with a considerable technical effort. Efficiency of the recovery could be improved by increasing the input Cr concentration.
Reusability of the matrix material after leaching is critical for the implementation of these new flow sheets. Repeated washing, in combination with carbonation was shown to reduce Cr leaching by 90%. Addition of FeS as reducing agent further reduces Cr(VI) leaching, but as yet not sufficiently to meet the stringent environmental quality criteria. Further research is needed to optimize both the leaching and subsequent washing and carbonation processes.
Upscaling trials on selected flowsheets showed slightly lower Cr recovery efficiency than laboratory trials, and highlighted necessary improvements for implementation, such as the use of a rotary kiln.
Theoretical calculations for three pilot plants at TRL 6-7 (slag input 50 kg/h, 80 t/y) showed that the proposed processes for Cr recovery are technically feasible but complex. Potential revenues through Cr and residual matrix reuse are negligible comparing to the annual costs of the pilots (<5%).
In the integrated assessment, the economic feasibility and environmental impact of the flow sheets was assessed at industrial scale (100 kt/y). Based on average costs for Europe, the cost of the CHROMIC processes is higher than landfilling or replacement of aggregates, but the overall weighted impact score is higher for landfilling. The heap leaching flow sheet for SS slag yields the best result among the CHROMIC technologies studied, mainly due to lower energy requirements. Valorization of the residual matrix after recovery of the valuable metals has a larger positive impact on the economic and environmental feasibility of the processes than the recovered metals themselves. Controlling Cr-leaching from the carbonated residual matrix material is thus essential.
The CHROMIC project produced regular newsletters and podcasts, and the “Getting to Know” series targeting the general public. The project was presented at 38 events, and resulted in 3 peer-reviewed scientific publications (7 in preparation) plus over 15 conference proceedings and other non-peer reviewed publications so far. Three waves of community involvement events targeted at lay people, professional experts, and students, respectively, were organized in four countries (Belgium, Germany, France, Italy). CHROMIC clustered with other H2020 projects and EIT Raw Materials, recently focusing on the topics Sustainability Assessment and Societal acceptance.
Exploitation of the project results is foreseen through further research projects, licensing and sales of the developed microwave equipment. So far, the project has resulted in 1 patent application (1 pending), and a follow-up research project at TUK (funded by Ministry of Education, Science, Research and sport of the Slovak Republic)
