Main results

Various pre-treatment, metal extraction, metal recovery and residue valorisation methods were developed with promising results. First, selected materials were sampled and characterised. Pre-treatment studies of some of the materials provided promising results. Roasting was an attractive method to pre-treat jarosite and landfilled Zn-rich sludge. A promising flowsheet was also developed for the ASR material based on magnetic and density separation techniques. Results of the leaching and environmental characterization of fayalitic slag showed a substantially decreased solubilisation of most elements. For some materials, physical pre-treatment did not provide any added value. Such materials have to be processed without pre-treatment, i.e. be application of downstream processing methods.

The objective of high leaching yields (>90 %) was achieved for laterites (Ni, Co, Mg: atmospheric acid leaching; Ni: solvometallurgy), jarosite (Zn, Pb, Ag: roasting and two-stage leaching; Ge: autotrophic bioleaching; Zn: DES), Cr-rich sludge (Cr and Ni: heterotrophic bioleaching) and landfilled Zn-rich sludges (Zn: heap leaching, DES and solvometallurgy). Plasma-pyro technology performed well with all studied materials. To develop economically and environmentally viable processes, selectivity and flexibility were studied reaching the METGROW+ beyond the state-of-the-art for majority of the sample materials.

Results from physico-chemical metal recovery experiments showed that the proposed processes were valid for the clean-up of the pregnant leach solution (PLS) and for the recovery of metals. The target selectivity of >85% combined with a recovery yield of >90% was achieved for Ni and Co in Greek laterites PLS, for Zn in Zn-rich sludge PLS and for Zn and Pb in jarosite PLS. Combinations of different metal recovery methods showed promising results.

The reactivity of leach residues for the residue valorisation showed clear potential for the use of certain residues as supplementary cementitious materials (SCM) and the production of alkali activated inorganic polymers (AAIP), which may be used as binders in the construction sector or construction materials with various beneficial properties. In some cases, the reactivity increased by mixing two different residues.

Validations were completed for Greek laterites using acid leaching and chemical precipitation or heap leaching, for Polish laterites using acid leaching and solvent extraction, for Cr-rich sludges using heap leaching, and for goethite and fayalitic slag using plasma-pyro technology. Zn-rich sludges were validated through modelling. All validations showed technically promising results (Table 1).

Various assessments (e.g. LCA, TEA, sensitivity, risks) were done for complete process flow sheets for all material streams with most promising technologies. Processes Building Information Modelling (BIM) and digital twin methodology were used as METGROW+ visualization tools for atmospheric leaching of Polish saprolitic laterite and for plasma-pyro treatment of Zn-rich sludge. The approach included the physical layout over the terrain and the process equipment layout including mechanical, electrical and piping connections. BIM methodology could be used as a tool to assess how to scale up the most promising processes from the pilot to the industrial scale in the bases of the TEA assessment developed in the project.

The main outcome of the project, the free public version of the METGROW+ toolbox (https://app.metgrowplus.eu), was developed. The toolbox aids in process selection decision making.

Table 1. Main validation results in METGROW+.

Greek laterites

Targeted metals Ni and Co Atmospheric leaching resulted approximately

85% Ni recovery and feasible sulphuric acid consumption. Ready process for sales.

Products:o Ni/Co containing MHPo Fe-silicate leach residue for SCM-

precursor use Heap leaching resulted to efficient Ni and Co

recovery from very poor laterite ores Progress beyond state of the art included

utilization of poor laterites, which have not previously been processed. Also, improvements in atmospheric leaching feasibility were made by the used process. Wastewater was successfully treated using biological sulfate reduction and the generated sulfides could be used to precipitate Ni and Co.

Polish laterites Targeted metals Ni and Mg Both H2SO4 and HCl leaching resulted to over

90% Ni recovery and solvent extraction to ~100% Ni recovery

Products: Ni and Mg concentrates from HCl

leaching NiSO4 solution from H2SO4 leaching MgSO4 raffinate for fertilizer use from

H2SO4 leaching Fe containing leaching residue for SCM-

filler use Progress beyond state of the art included the

utilization of poor laterites and economical recovery of targeted metals.

Cr-rich sludge Targeted metals Ni and Cr Heap leaching has resulted to leaching recovery of

88% Ni and 68% Cr Products:

Cr/Ni products Progress beyond state of the art included the

development of hydrometallurgical process to selectively extract Cr and Ni from neutralization sludge, and the granulation procedure to pretreat sludge for the heap leaching process.

Fayalitic slag and goethite Targeted metals Zn and Pb ≈100% Zn recovery into the fly ash in both

processes Marketable products:

Zn/Pb dust Metal fraction (Fe>95%) Slag (SiO2/CaO based) for the production

of alkali activated inorganic polymers (AAIPs)

Progress beyond the state of the art included the processing of this type of slags and sludges to recover metals with plasma smelting technology.