Linking Automated Scanning Electron Microscope Based Investigations to Chemical Analysis for an Improved Understanding of Ash Characteristics

The movements and efforts of a circular economy, aiming to tap into the resource potential of ash, require an intimate knowledge of the material; often, target elements within this material are part of complex ash phases. This work shows how automated SEM investigations measure up to other laboratory techniques for the analysis of elemental composition and particle size. Three sewage sludge ash (SSA) samples have been studied in this comparison, showing material variation for SSA and highlighting the strengths and shortcomings of the methods chosen. Inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence (XRF) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX) show relevant phosphate phases, but also a number of other elements. The extent of the accompanying elements, most likely hindering efficient phosphorus (P) recovery, varies. Propensities for detection in fine-grained and largely amorphous material such as ash vary, as is explored in this thorough comparison. ICP-OES data suffers from incomplete sample mobilization, and XRF-derived values suffer from matrix effects. Both are the only techniques studied which show trace elements, such as potentially toxic elements. SEM-EDX automated mineralogy delivers more reliable data for main elements while not reporting traces. By showing SEM-EDX automated mineralogy particle size distributions, alongside laser diffraction derived particle size distributions, the extent of the strain ash puts on traditional techniques is visible. Ashes tend to agglomerate, and the porous nature of particles hinders accurate detection. This work highlights where SSA recycling needs to be careful and hints at the extent of discrepancies between different methods. When understanding ash as a potential resource and designing efficient extraction strategies, this knowledge is crucial.

Automated summary

This study investigates the application of automated SEM investigations for elemental composition and particle size analysis, comparing ICP-OES, XRF, and SEM-EDX techniques in the study of sewage sludge ash (SSA) samples, highlighting the strengths and weaknesses of each method and the variation in material characteristics. The results show discrepancies in the detection of phosphate phases and accompanying elements using different techniques, indicating challenges in traditional analysis methods for fine-grained and amorphous materials like ash.