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Phosphate rocks are a critical resource for the European Union, and alternative sources to assure the future production of a new generation of fertilizers are to be assessed. In this study, a statistical approach, combined with a sustainability evaluation for the recovery of materials from waste containing phosphorus (P), is presented. This work proposes a strategy to recover P and silica (SiO2) from rice husk poultry litter ash (RHPLA). The design of experiment (DoE) method was applied to maximize the P extraction using hydrochloric acid (HCl), with the aim to minimize the contamination that can occur by leachable heavy metals present in RHPLA, such as zinc (Zn). Two independent variables, the molar concentration of the acid, and the liquid-to-solid ratio (L/S) between the acid and RHPLA, were used in the experimental design to optimize the operating parameters. The statistical analysis showed that a HCl concentration of 0.34 mol/L and an L/S ratio of 50 are the best conditions to recover P with low Zn contamination. Concerning the SiO2, its content in RHPLA is too low to consider the proposed recovery process as advantageous. However, based on our analysis, this process should be sustainable to recover SiO2 when its content in the starting materials is more than 80%.

The industrial incineration of aviary manure is still far from the concept of using its ash residuals for nutrient uptake by plants and most of these materials are landfilled under futile fixed cost. Aviary manure includes poultry litter mixed with different aviary bedding materials or laying hens manure, which may be burned using different incineration technologies and conditions. This study aims to determine the comparative characterisation of P-rich ash residues sampled at Güres Energy (Turkey) fluidized bed combustion of laying hens manure and at Campoaves (Portugal) chain grate stoker combustion of rice husk poultry litter. The effect of different fuels and different combustion systems on P speciation in ash was investigated: the characterisation of global samples (bottom ash (BA), economiser fly ash (FAECO), cyclone fly ash (FACYC)) and respective size-fractions were done chemically (proximate and elemental analysis by X-ray fluorescence–XRF–and Inductively Coupled Plasma Mass Spectroscopy – ICP-MS), morphologically (detailed imaging and X-ray micro analysis by Scanning Electron Microscopy—Energy-Dispersive X-ray spectrometry–SEM−EDS) and mineralogically (X-ray diffraction–XRD). Phosphorus was detected in Güres ashes mainly as hydroxyapatite crystals alongside with CaCO 3 relics and CaO, while Campoaves ash fractions contained P also as Na–K-Mg phosphate and major amounts of rice husk relics including unburnt char and silica phases. Both Güres and Campoaves ashes are complex but the combustion of aviary manure under these setting conditions appears to be promising for P recovery owing to their high P content and limited trace elements respecting the limitations imposed by EU legislation for fertilisers applications. Graphical Abstract