Trends and perspectives in the use of organic acids for critical metal recycling from hard-metal scraps

Hard-metal sector, strategic for the industrial economies, is suffering from the reduced availability and price volatility of its main feedstock: critical W and Co. In 2021, a 73.5 kt W and 9.2 kt Co demand for hard-metal production (65% and 5.3% of global demand, respectively), was recorded. Hard-metal scrap recycling is hence desirable for both environmental and economic reasons. A significant recovery of W and Co from manufacturing by-products and scraps is already good practice in the hard-metal industry (42% for W and 22% for Co). However, there is still a lot to do to meet the technical-economic-environmental sustainability in materials and energy enhancement for pursuing a green economy model. Indeed, Chemical Modification and Direct Recycling, which are the most widely employed industrial approaches, typically involve energy and/or harsh chemicals-intensive treatments which require expensive equipment and skilled workers. In the last decade, research efforts have been spent on implementing alternative materials reclamation processes from hard-metal scraps based on the use of bio-based organic acids with the view to increase the rate and quality of the recycled materials exploiting their peculiar metal complexing action as well as to preserve natural resources and prevent the disposal of potentially toxic/polluting substances. Despite the preliminary stage of the research, organic acids were demonstrated to be powerful but gentle agents for the selective leaching of cobalt from WC-Co-based materials as well as promising agents for WO3 dissolution. Indeed, thanks to their acid and complexing properties, they can stabilize metals in their oxidized form giving soluble products and preventing passivation phenomena. Furthermore, organic acids can be obtained by renewable biomass transformation, limiting the request for high-impact industrial chemicals. Hence they points out key features making them promising for the design of eco-friendly recovery processes. In this context, the different industrial approaches to the recovery and recycling of Hard-metal wastes, with specific reference to the role of bio-derived organic acids in hydro- and solvo-metallurgical processes, will be critically reviewed with the view of opening a discussion on the perspectives of their use in designing circular economy models in HM manufacturing as economically, technically and environmentally sustainable as possible

Valorisation of organic residues through hydrothermal carbonization

Hydrothermal carbonization (HTC) is a thermochemical process which can directly convert wet organic materials producing a carbon-rich solid (hydrochar, HC) and a liquid phase (process water, PW). Hydrochar has chemical and physical properties that make it similar to natural peats and coals. Depending on the process conditions, mostly temperature (180-250°C) and residence time (few hours), this material can be enriched in its carbon content, modifying its structure and providing it interesting characteristics that make it possible to be used for several applications, like energy production, as a soil ameliorant, adsorbent material for different pollutants, and some others reported in literature. In this work, the HTC process was applied to different organic materials such as hemp, grape marc, spent coffee grounds, AD (anaerobic digestion) digestate from hemp, from cow manure, and from agricultural residues, cigarette butts, surgical masks and gloves. Feedstocks and resulting products were physically and chemically characterised to be tested for different applications (e.g., energy production by combustion or AD, soil ameliorant, peat substitute in growing media or fertigation, or to be biologically treated in aerobic wastewater treatment plant). Compared to the feedstocks, hydrochars showed for all the tested materials strong physical and chemical modification (e.g., carbon content increased, and ash reduced). Due to the increased HHV, hydrochar may be used as solid fuel, which entails a higher energy recovery compared to the feedstock. Following HTC, all the tested materials showed a reduced volume (over 70%, in some cases) and an increased density. The possibility to use HC as peat substitute in growing media was tested for cow manure digestate, spent coffee grounds, and grape marc, showing that low amount of HC can partially substitute peat with similar or even better results. However, the addition of higher amount showed inhibition. Pre- and post-treatments tested (extraction, drying, washing) seem to have positive effect on seed germination and plant growth, removing some phytotoxic compounds. However, since these steps can be expensive, further tests should be carried out to understand which compounds cause inhibition, how to remove them or whether it is possible to destroy or avoid producing them by adequately setting the HTC process parameters. Process water (PW) should be properly treated since it contains many different compounds which can become an environmental problem. In this work, the PW valorisation was tested through characterising the liquid. The high amount of nutrient in PW may suggest the use in fertigation; however, germination tests showed inhibition effect. The aerobic treatability was tested through the bioassay on nitrifying bacteria. The high amount of VFAs may make PW a suitable substrate for AD to produce biogas. This work demonstrated that hydrothermal carbonization is a suitable process for the treatment of a wide range of organic materials. Depending on the feedstock, the resulting products may have different properties and characteristics which make them feasible for diverse applications. In general, all the feedstock tested may be used as solid fuel with several advantages, such as higher energy content, reduced volume, possible reduced emission during combustion, ease of dewatering, etc. The use on soil is promising when low amount of HC is used. However, pre- or post-treatments may increase the concentrations. Process water may be biologically treated: anaerobically, to produce biogas and remove some organic compounds and aerobically, but an acclimatation phase may be necessary. HTC proved to be a promising process in the biorefinery concept, especially when integrated with other processes (AD, aerobic degradation, etc.). Materials and energy can be recovered by HTC and re-introduced in the productive cycle and in the market, promoting the circular economy and the zero-waste concept

Benefits and Limitations of Using Hydrochars from Organic Residues as Replacement for Peat on Growing Media

New technologies for the production of peat-substitutes are required to meet the rising demand for growing media in horticulture and the need to preserve natural peatlands. Hydrothermal conversion of organic residues into char materials, hydrochars, with peat-like properties may produce such substitutes, reducing environmental impacts and CO2 emissions from improper management. To assess their potential as a component in growing media, cress seed germination tests are used to assess hydrochars from digestate (D), spent coffee grounds (SCG), and grape marc (GM). Pre and post-treatments (extraction, washing, and drying) are applied to remove phytotoxic compounds associated with process waters retained on the hydrochars, and a nitrification bioassay with process water is used to predict their toxicity. All hydrochars achieve similar or better germination results compared to their feedstock, showing a potential to replace at least 5% of peat in growing media. SCG and GM hydrochars show inhibition above 5%, while all post-treated D-hydrochar mixtures produce >3 times longer roots than the control. The nitrification test shows a high sensitivity and good agreement with the high inhibition trends found in the germination tests with process water. Such tests can be a good way to optimize process combinations for the hydrothermal production of peat replacements

Valorization of Face Masks Produced during COVID-19 Pandemic through Hydrothermal Carbonization (HTC): A Preliminary Study

The COVID-19 pandemic has led to the increased use of disposable face masks worldwide, resulting in a surge of potentially infectious waste. This waste must be safely managed and disposed of to prevent the spread of the virus. To address this issue, a preliminary study explored the use of hydrothermal carbonization (HTC) as a potential method for converting surgical mask waste into value-added carbonaceous materials. The HTC treatments were conducted at 220 °C for 3 h with or without the addition of acetic acid. The resulting hydrochar was characterized using several techniques, including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and N2-physisorption analyzers. The study found that the masks formed a melt with reduced mass (−15%) and volume (up to −75%) under the applied conditions. The carbon content and higher heating value (HHV) of the produced hydrochars were higher than those of the original masks (+5%). Furthermore, when acetic acid was added during the HTC experiment, a new crystal phase, terephthalic acid, was produced. This acid is a precursor in surgical mask production. The study suggests that hydrothermal carbonization could potentially achieve sanitization and volume reduction in non-renewable and non-biodegradable surgical masks while also producing a solid fuel or a raw material for terephthalic acid production. This approach offers an innovative and sustainable solution to manage the waste generated by the increased use of disposable face masks during the pandemic

Combined biohydrogen and polyhydroxyalkanoates production from sheep cheese whey by a mixed microbial culture

The present study investigates the combined production of biohydrogen and polyhydroxyalkanoates (PHA) from sheep cheese whey through a 3-stage bioprocess, i.e. dark fermentation, selection of PHA storing microorganisms, and PHA accumulation. Batch dark fermentation tests (Stage I) were performed on raw cheese whey under different pH operating conditions, avoiding either the addition of inoculum or substrate pre-treatment to support the economic and technical feasibility of the proposed process. The performance of the fermentative stage was assessed in terms of biohydrogen and soluble metabolites production yields. The dark fermentation effluent was used as organic acid-rich feedstock either for selecting and harvesting PHA storing microorganisms from a mixed microbial culture without the addition of external nutrient sources (Stage II) or for the PHA accumulation by the selected biomass (Stage III). The results of the study support the possibility of achieving combined recovery yields of 5.3 L biohydrogen and 7.6 g PHA per litre of fed sheep cheese whey in the case of optimal dark fermentation pH setting (pH = 6). Such outcomes underline the untapped potential of sheep cheese whey for the recovery of high-added value bioproducts

Combined valorization of grape marc

The implementation of the concepts of the circular economy raises the qualitative and quantitative level of residue valorisation, as a higher integration of waste-derived resources in the market logics and regulations is required. In this framework, grape pomace is one of the most promising agro-industrial side-streams as it contains organic acids, sugars, and lignocellulosic materials suitable to be converted into biofuels or soil improvers, but also compounds of great interest for economic sectors such as medicine, nutraceuticals and cosmetics. The full exploitation of grape pomace, therefore, implies the adoption of a sequence of processes that allows both the recovery of intact compounds with high market value and the valorization of the exhausted biomass. The present study focused on the recovery of polyphenols from grape pomace, combined with the anaerobic digestion, composting or hydrothermal carbonization (HTC) of the exhausted biomass to convert it into value-added materials and energy. The polyphenols extraction was based on mild solvent characteristics and operating process conditions; the effects of pomace preliminary grinding and different values of the liquid-to-solid ratio were explored. The extraction yields, the concentration of total polyphenols and single compounds in the extract, and the antioxidant properties of the extracts were determined. Finally, biochemical methane potential, composting and HTC tests were performed on the exhausted grape pomace and, for comparison, also on the raw grape pomace. The results show that 76.5 g of extract per kg of dry grape pomace can be recovered, which contains 2.3% w/w of original total phenols (roughly 1.8 g total phenols per kg of dry grape pomace). Preliminary grinding had beneficial effects on the phenols extraction yield and total phenols content in the extract, and the antioxidant activity of the extracts proved to be proportional to the total phenols content. A theoretical value of biomethane recovery of about 140 NLCH4/kg volatile solids was derived from the BMP tests conducted on the exhausted grape pomace; although this figure refers to optimal process conditions, it is reasonable to assume that the production of biogas achievable on the real scale would be such as to cover at least the energy needs of the polyphenols extraction process. The composting process proved to be able to deal with the phenol presence without negative effects, being instead more influenced by the particle size of the substrate. Indeed, the total phenols content decreased during the composting treatment, probably because the polyphenols were incorporated into biosynthetic pathways leading to the formation of humic molecules. The size reduction carried out upstream of the phenol extraction resulted in faster and more complete composting. The hydrochars obtained through the thermochemical conversion of grape pomace showed a carbon content increase (+26%) and improved quality as a solid fuel (+40%), suggesting their use as soil amendment or for energy production. Further energy recovery can be achieved through biomethane production from the HTC process water (obtaining up to 137 mL of biomethane per gram of fed COD)

Processes, applications and legislative framework for carbonized anaerobic digestate: Opportunities and bottlenecks. A critical review

Char is a valuable product obtained from thermochemical conversion processes of different biomass feedstocks, mainly pyrolysis and hydrothermal carbonization (HTC). In this work, anaerobic digestion (AD) integration with pyrolysis/HTC is critically reviewed, considering anaerobic digestates as feedstocks for char production. This virtuous interconnection can boost sustainable digestate valorization in the circular economy framework. Different substrates for AD are investigated, including sewage sludge, food waste, agricultural residues, and animal manure. The available thermochemical technologies, including pyrolysis, HTC and other processes are considered, analyzing the effects of substrate characteristics and process parameters on char quality. The possible fields of char application are successively presented, including agricultural application, energy recovery, pollutants adsorption, catalysts production, and electrochemical technologies; the advantages and drawbacks of each application are highlighted. Limitations still preventing the full-scale application of digestate-derived char production and utilization include the variability in substrate characteristics and the presence of undesired pollutants (especially in sewage sludge digestate), full-scale development of thermochemical plants, lacking legislative frameworks, uncertain economic sustainability, limited eco-toxicological studies, and stakeholders’ acceptance. Future research needed on the topic is finally depicted, with the aim of widening digestate reuse applications, as thermochemical processes may prevent safety concerns linked to direct agricultural reuse, leading to sustainable biorefinery platforms