Shoe manufacturing wastes: Characterisation of properties and recovery options

Using the renowned shoe manufacturing sector in Marche Region (Italy) as a case-study, this paper deals with the characterisation of (1) the chemical and physical properties of representative types of shoe manufacturing wastes, (2) the quality profile of a particular material recovery finalised to a soil destination, and (3) the flue gas emissions from burning tests in view of the potential for energy recovery. The property characterisation has primarily revealed the following aspects: a general condition of resulting higher ash contents in shoe waste materials having a mineral or synthetic processing origin; and relatively high energy contents, ranging on the whole expressed as LHV (lower heating value) from 15,710 kJ kg-1 (for vegetable-tanned leather dust) to 42,439 kJ kg-1 (for natural rubber). The characterisation of a fertiliser that can be generated from the particular waste type of vegetable-tanned leather dust has determined an interesting quality profile classifiable as organic-nitrogen fertiliser with acceptable values of the humification parameters and heavy metal contents significantly below the assumed quality limits. Finally, simplified pilot-scale burning tests have provided the following findings: higher NOx emissions produced from leather-based materials as compared with the cellulose-based material, which are likely attributable to the substantial “fuel nitrogen” content of the original animal skins; higher HCl emissions produced from the leather-based materials in comparison with the cellulose-based material; and a more even combustion process occurred for the leather dust than for the other shoe materials.