Alberto Gallifuoco

Rare earth elements (REEs) are essential to many low-carbon and digital technologies, yet the primary supply is geographically concentrated; waste electrical and electronic equipment (WEEE) could act as an “urban mine”, but recovery pathways remain fragmented. We synthesize the evidence through a structured literature review of Scopus and Web of Science indexed studies focusing on WEEE-derived feedstocks for REE recovery: 148 records were screened and 51 papers met the inclusion criteria. Reporting of the search and study selection process follows PRISMA 2020. We coded each study by WEEE source/fraction, core technology family, and process configuration, target REEs, performance reporting, environmental proxies, and maturity, and discussed gaps against circularity goals. Results show an intense concentration on a few feedstocks, permanent magnets (22 studies), fluorescent lamps (16), and batteries (6), with only limited attention to multi-source streams. Hydrometallurgical routes dominate, while biometallurgical options are less explored. Recovery is more frequently reported than selectivity and environmental indicators, and most solutions remain at proof-of-concept maturity. Due to the heterogeneity of feedstocks, process configurations, and reported metrics, the findings were synthesized qualitatively (no meta-analysis). This review highlights priorities for future work: multi-source and heavy rare earth elements focused feedstocks, more selective and intensified flowsheets, harmonized performance reporting, and scale-up supported by life-cycle and cost assessments.

The paper industry has historically relied on virgin raw materials as the primary resource for production. Moreover, it requires high energy and water consumption and use of chemicals like chlorine. In response to these environmental challenges, the paper industry is progressively moving toward more sustainable practices aligned with circular economy’s principles. However, the shift to recycling introduces new challenges, particularly the generation of new solid typologies of residues, such as pulper rejects and fine screen debris. This study investigates hydrothermal carbonization (HTC) as a circular solution for transforming such two solid residue streams into an effective solid fuel. More specifically, it verifies the technical feasibility of such technology on a laboratory scale. Moreover, the energy potential and environmental impacts of the obtained fuel are evaluated. Finally, it is checked whether the obtained hydrochar is compliant with the EU directive regarding Solid Recovered Fuel (SRF). Our findings confirm that HTC is an effective solution for the transformation of investigated solid residues. Moreover, all obtained hydrochars have a higher level of high heating value when compared with the original inputs (up to 83,6%). In this respect, the operating severity (250 °C) and liquid-to-solid ratio emerged as primary levers to raise the heating value while curbing halogens, mercury, and SRF-critical inorganics. Finally, the obtained hydrochars meet EU legislation in terms of energy and environmental performance, reaching even SRF Class 1 levels. This in turn confirms that HTC process parameters and feedstock mix can be properly tuned to deliver SRF-grade solids with stable, high-quality combustion behavior.

This paper introduces fractal analysis to study the kinetics of biomass hydrothermal carbonization. The reacting water-biomass slurry is a complex system leading to hydrochar by microscale reactions constrained into fractal topological boundaries. Literature and purpose experimental data check equations adapted from the fractal-like repertoire. More general models are derived from a shortcut, stochastic-based formalism, avoiding the mathematical sophistication of fractal calculus. Fractal equations explain observed data better and with fewer parameters than traditional mass-action network models (0.91202 < R ² < 0.99998, average 0.97957) over a wide range of biomass and operational conditions. Exploratory experiments highlight the surface fractal dimension of hydrocars and their variation concerning that of parent biomasses (from 2.00 to 2.84). The confluence of fitting success and evidence of HC fractality encourages prosecuting research for making fractal kinetics a fully fledged tool of hydrothermal carbonization studies.

This paper explores the potentialities of hydrochar in protein separation and enzyme immobilization for non-energy biorefinery applications of hydrothermal carbonization. An innovative experimental procedure monitors soluble protein-hydrochar interactions and enzymatic reactions in a continuously stirred tank reactor. The hydrochar comes from hydrothermal carbonization of silver fir (200 °C, 30 min, 1/7 solid/water ratio) and standard activation (KOH, oven, 600 °C). Bovine serum albumin, a non-active, globular protein, was adsorbed at ≤3300 mg/g. Sip's isotherms fitted data well (R² = 0.99999). The immobilization used a commercial β-glucosidase, which catalyzes the hydrolysis of cellobiose to glucose, a bottleneck of the cellulose to fermentable sugar bioconversion network due to the fast enzyme deactivation. The hydrochar adsorbed ≤26 w/w% of enzyme. The heterogeneous biocatalyst operational stability was 24 times that of the soluble one. The results encourage further investigations and foreshadow process schemes coupling hydrothermal carbonization and industrial bioconversions.

An innovative process based on hydrothermal carbonization, gasification, and solid oxide fuel cells (SOFCs) technologies was developed using a commercial process simulation software called ASPEN Plus. The object of this work is to study plant efficiency under various operating conditions. The hydrothermal pre-treatment (HTC) at 200 and 250 °C was modelled as a black box based on the experimental results. The gasifier was modelled as a single reactor vessel with both the fluidized bed steam gasification of solid fuel and the hot gas cleaning system. The SOFC was modelled as a simple grey box with the ASPEN Plus blocks. The effect of HTC temperature and steam/carbon (S/C) ratio on the syngas composition and yield and plant efficiency was studied. The results show that the gasification of hydrochar obtained at 200 °C with S/C ratio of 0.6 gives the best results, namely an energy output of SOFC equal to 1.81 kW/kg_Biomass, and overall process efficiency of 36%.

This paper introduces Bayesian statistical methods for studying the kinetics of biomass hydrothermal carbonization. Two simple, specially developed computer programs implement Markov-chain Monte Carlo methods to illustrate these techniques' potential, long since established in other areas of chemical reaction engineering. A range of experimental data, both from this study and the literature, test the soundness of a Bayesian approach to modeling biomass hydrothermal carbonization kinetics. The first program carries out parameter estimations and performs better or equal than the traditional deterministic methods (R² as high as 0.9998). For three out of the 22 datasets, the program detected the global minima of the parameter space, while the deterministic least-square found local values. The second program uses Gillespie's algorithm for the statistical simulation of the reactions occurring in hydrothermal carbonization. Comparing six basic kinetic models with literature data tested the stochastic simulation as a tool for assessing biomass conversion reaction networks rapidly. Among the simple models discussed, reaction scheme 3 fitted better to the experimental data (R² > 0.999). The proposed approach is worth extending to more complex, time-consuming computer models and could support other techniques for studying hydrothermal conversions.

The kinetics of biomass hydrothermal carbonization is modeled by the MaxEnt principle, without assuming a reaction network. Modeling is in good accordance with the experimental data concerning a broad range of biomass and reaction conditions.