Recovery of Critical Raw Materials from Industrial Wastes by Advanced Methods

1. Introduction and Scope

Raw materials (RMs) are crucial to the world economy. They form a strong base for industry, with a broad range of goods and applications produced for use in everyday life. Reliable and unhindered access to certain RMs is a growing concern within the EU and across the globe. To address this challenge, the European Commission has created a list of 30 critical raw materials (CRMs) for the EU, which is subject to regular reviews and updates. CRMs comprise RMs of high importance to the EU economy and those that have high levels of risk associated with their supply; moreover, they are closely linked to clean technologies.

The use of secondary RMs derived from marginal resources as industrial wastes is of strategic importance for industrial production, due to their high concentrations of valuable metals.

RMs (i.e., gold, silver, copper, zinc, manganese, nickel) and CRMs (i.e., platinum, indium, cobalt, vanadium, magnesium, antimony, niobium, and rare ores), are essential for the application of emerging modern technologies and to preserve the environment from industrial waste by avoiding the release of pollutants and their components.

Innovative processes such as bio-hydrometallurgy, electrowinning, phytoremediation, and bioprecipitation, compared with conventional processes, are characterized and advanced through the reduction in environmental impacts and energy consumption and by the degree of purity of the valuable metals obtained.

The economic value of advanced methods for the recovery of critical raw materials from industrial waste, which is closely linked to the choice and optimization of the experimental parameters of the processes, is of great importance.

The articles published in this Special Issue all contribute to the improvement of the above-mentioned methods.

I would like to thank the authors who accepted the invitation to be part of this Special Issue, helping us to produce a high-impact, high-quality Special Issue on the “Recovery of Critical Raw Materials from Industrial Wastes by Advanced Methods”.

2. Contributions

Researchers from around the globe who were investigating the recovery of critical raw materials from industrial wastes using advanced methods were invited to submit research papers in order to aid readers in recognizing common areas and connections. Of the submitted manuscripts, six articles were published in this Special Issue.

The papers are all of high scientific value, and the experimental activities that they cover fall into various disciplines, confirming the importance of studying the recovery of critical raw materials from industrial wastes using advanced methods in different scientific and technological fields. An overview of the published papers is given below.

In one study, the optimal slag conditions for a pyrometallurgical process to recover palladium (Pd) and silver (Ag) from hazardous industrial wastes, such as copper containing sludge and spent petrochemical catalysts (SPCs) at 1500 °C, are explored (contribution 1).

This study highlights that physical loss is more serious than chemical loss in metal recovery, as the latter is dependent on the thermochemical solubility of the target metals in the slag. The results emphasize the need for the precise control of slag properties to maximize metal recovery processes in conjunction with the mitigation of CO₂ emissions.

In a different study, recycling technology for degraded batteries and cathode-active materials via the thermal decomposition of polyvinylidene fluoride (PVDF) using calcination and the air-jet stripping of active materials was developed (contribution 2). The proposed air-jet erosion method for stripping calcined cathode material from Al foils allows for a flexible separation process that is damage-free for both particles and substrates, while the CaO calcination air-jet separation process application and equipment can significantly improve the PVDF decomposition and separation efficiency of the cathode materials. Low environmental impact, the high purity of the recycled material, and low costs were achieved, as compared to pyro- and hydrometallurgical methods.

In a third paper, the development of hydrometallurgical recycling processes for lithium-ion batteries (LIBs) is challenged by the heterogeneity of the electrode powders recovered from end-of-life batteries via physical methods. These electrode materials are known as black mass. The results of the hydrometallurgical treatment of mixed nickel, manganese, and cobalt (NMC) and lithium iron phosphate (LFP) black masses aimed at creating flexible recycling processes are presented in this paper (contribution 3).

The technical feasibility of LIB recycling based on re-synthesis was assessed for NMC-LFP mixed black masses, proving the possibility of both using LFP as a reducing agent for NMC leaching and the selective recovery of iron phosphate (FP) from leach liquor before precursor re-synthesis.

A different study proposed a biotechnological tool for the decontamination of soil with heavy metal(loid)s through phytostabilization or arbuscular mycorrhizal (AM)-assisted phytoextraction to prevent their entry into the food chain, followed by the subsequent recovery of critical raw materials (CRMs) (contribution 4). This biotechnology is very economical. The biomass waste is processed by hydrometallurgy, with a pure metal recovery rate of 90% (at 99% purity), which is important when calculating costs and benefits. The patent that was used covers many chemical elements, but the challenge lies in achieving the physical/chemical/biological conditions that yield the desired behavior of the chemical elements.

A novel method to recover rare earth elements (REEs) from secondary sources such as NdFeB magnets by leaching with citric acid and subsequently separating them using the solvent extraction method was studied in a fifth paper (contribution 5). The experimental investigation conducted at the laboratory scale was compared with those obtained at a pilot scale and at an industrial scale to better optimize the scaling-up of this phase of the process and to allow for the full-scale efficient recovery of REEs from NdFeB.

In a different paper, an experimental roasting study was carried out to oxidize cobalt-bearing pyrite tailings for preparing and recovering the cobalt through acid leaching (contribution 6). Cobalt is a critical metal widely distributed in nature, but cobalt ore is rarely found as an independent mineral. Cobalt-bearing pyrite tailings separated from iron ore are the primary resources used for the recovery of cobalt.

The main aim of this study was to determine and control the optimal technological process for roasting by using thermodynamic modeling for application at an industrial scale. Through oxidative roasting, the elimination of environmentally hazardous gases, such as sulfur, during the process was achieved.

3. Conclusions and Outlook

A variety of topics are included in this Special Issue, presenting recent developments regarding the recovery of critical raw materials from industrial wastes using advanced methods.