Study of the Flowability Properties, Morphology and Microstructure of Hazelnut (Corylus avellana L.) Shell Waste Particles Obtained by Milling

Mechanical milling is a relevant preliminary processing operation that is widely used for the reuse of various types of agro-industrial waste. The objective of this study was to conduct milling experiments of hazelnut (Corylus avellana L.) shell waste at different times (0.5, 1 and 1.5 min) and subsequently evaluate the particle size distribution (PSD) of the powders obtained by sieving methodology. In addition, flowability parameters were determined for the particles retained on the sieves, and their morphology and microstructure were examined using several microscopy techniques. The results demonstrated that the hazelnut shells were successfully fractionated under the milling conditions investigated (short milling times ≤ 1.5 min), and the histograms of the PSD exhibited a wide dispersion of sizes (≤1.7 mm). The particles retained from sieve100 to residue exhibited poor or no flow, attributable to the high degree of cohesion between them. Morphological analysis based on optical microscopy and image analysis revealed that there was an increase in the aspect ratio parameter when the particle size decreased, meaning that the particles had elongated shapes. Microscopic analysis (SEM, AFM and CLSM) showed that the particles exhibited complex shapes and a comparable microstructure, comprising tightly packed clusters of sclerenchyma cells. From the microscopy images obtained (SEM and AFM), it was inferred that the cracks generated during blade impacts propagate along the middle lamella of the cells, allowing the cluster-like arrangement to be preserved. The CLSM results demonstrated that as the size of hazelnut shell particles decreases, the exposure of lignin on its surface is favored. The findings of this study demonstrate that hazelnut shell waste can be readily pre-processed using a blade grinder, thereby facilitating its reuse in applications that demand fine particle sizes (e.g., bioadsorption of pollutants and the production of biocomposite materials). Likewise, the results concerning the flowability parameters, microstructural arrangement, and morphological features of the different particle fractions obtained are crucial variables that must be considered. These variables significantly influence the possible applications for the revalorization of this type of agro-industrial waste.