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D7.4 – Recovery of elements via leaching: experiments and simulations - part 1

Year: 2026
Type: article
Abstract: This report presents the results of the first part of the research work performed to explore the recovery of valuable elements from solid gasification residues (char, ashes) via leaching. This research was implemented as a part of the work in WP7 of the BeBOP project (WP7, Enabling technologies at lab-scale for SOEC characterisation and resource recovery, T7.2, Recovery elements from solid gasification residues via leaching). Depending on the type and origin of the biomass, char will contain a certain fraction of valuable inorganic elements, including nutrients and (heavy) metals. Some of the elements found in relevant biomass types are included in the 5th list of Critical Raw Materials for the EU, e.g., phosphorus, magnesium, aluminium. Although their shares in the gasification residues are relatively low, from ppm up to precent levels, taking into account the long-term operation of the gasification plant these quantities accumulate and become substantial. In addition to value extraction, the recovery of elements from the gasification residues, prevents hazardous waste accumulation, thereby improving the overall sustainability and circularity of the bio-/e-methanol production. The goal of this research work was to study the separation (recovery) of the inorganic constituents of biomass char and ash via the leaching process while applying different process conditions. The base case experiments were batch experiments performed in vitro. The char samples from four different biomass types (clean wood, demolition wood, Miscanthus and hay) were obtained in the laboratory, while the ash sample was obtained during the gasification of Miscanthus. The leaching agents used involved water, citric acid, oxalic acid, hydrochloric acid, sodium hydroxide, ammonium acetate, ethylenediaminetetraacetic acid (EDTA) and water peroxide. Different leaching sequences were investigated, starting from a multi-step water leaching with intermediate char drying, through the leaching with the above-mentioned leaching agents after an initial water leaching step, and finally a 3-step process involving water, ammonium acetate and hydrochloric acid. In the base case experiments different leaching temperatures (25°C and 60°C) and leaching times (1, 5 and 60 minutes) were applied. In the first series (Miscanthus ash leaching) also the leaching time of 24 hours was used, and an experiment with ultrasound assisted leaching was done. In total 61 multi-step batch experiments were done. Furthermore, also different process devices were tested. A short series of experiments was carried out using a pressurized batch reactor in order to investigate the effect of elevated temperature, pressure and different gaseous atmospheres above the leachate, as well as to identify any gases possibly being released during leaching. Next, the Accelerated Solvent Extraction (ASE) device was used to explore the water leaching process limits, with high pressure, different temperatures and short residence times.After the experiments the chemical composition of the leachate was analysed, which allowed the calculation of the recovery fraction (RF) of each considered element. All leachate samples were analysed using the cuvette tests for a narrow range of elements. In addition, selected leachate samples and selected samples of char exposed to leaching were analysed using ICP-OES.In order to obtain a better understanding of the leaching process and to be able to select the process conditions in a more targeted way, a chemical equilibrium-based model of the leaching process using the FactSage software was implemented and a series of simulations was done. The analysis of the experimental results showed that selective separation of different elements is possible. Starting with water leaching allows to recover water-soluble elements (K, P, S, Na), although the recovered fractions differ among these elements. The effect of the leaching time is less pronounced than the effect of the temperature. It was also revealed that an important gain in leaching performance is achieved by the intermediate drying of the substrate and repeating the leaching step. This allowed to achieve high recovery fractions with only short exposure times (order of 1 minute) of the substrate to the leaching agent. Heavy metals were barely recovered with water, which is good from the separation point of view. Here an inverse effect of the leaching temperature was observed on lead: its amount in the leachate dropped with the increasing leaching temperature, which can later be used as a parameter to control the separation of the elements. Summarising the results of the first set of the FactSage simulations and their validation against the experimental results it can be concluded that the simulations based on chemical equilibrium and Gibbs’ free energy minimization can provide useful insight during the development of the leaching process and the recovery of valuable elements from biomass char and ash. The obtained set of results gave multiple suggestions for the required focal points of the research to be implemented during the next stages of the project, namely to gain the information about the specific compounds present in the char or ash matrix, getting additional information about the ash chemistry and phase diagrams of the species of interest. In general, it can be concluded that each leaching sequence will start with water leaching, likely followed by the second water of ammonium acetate step to remove all potassium, chlorine and sulphur. Then, preferably other nutrient elements (phosphorus, magnesium, calcium) should be recovered, finally followed by other (heavy) metals. At this stage it is not yet possible to draw conclusions about the procedures for other important elements present in the char and ash, namely silicon and aluminium. Their mobilisation into the leachate as its justification is yet to be investigated. The exact process equipment (open system or pressurized reactor) is still to be defined. The open system performed relatively well considering its simplicity, but high temperature (200°C) and pressurized conditions had a significant positive effect on the recovery fraction of potassium and sulphur. Likely solely the higher RFs of these two elements will not justify the engagement of such more complex equipment, but the final choice will depend on the overall recovery scheme and any possible process integrations (e.g., thermal integration) with the gasification or the entire bio/e-methanol production facility. The results obtained in this part of the project will be used for the further investigation of the recovery of the elements via leaching, namely in WP8 of the project (WP8, Technology validation: SOEC and gasification residues recovery, T8.2, Leaching conditions validation).
DOI: 10.5281/zenodo.20730956
Language: English
Open Access status: green
OpenAlex ID: W7165023307
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