Re-use of agro-industrial wastes by hydrometallurgical applications

This paper represents a preliminary step to determine the possibility for valorisation of residue organic substances, coming from agro-industrial activities, in a hydrometallurgical circuit. The idea started to develop a production process for manganiferous products of industrial interest, such as MnO2, by using an innovative chemical leaching, at low environmental impact. The experimental activity was conducted at laboratory scale, utilising manganiferous ores. Sulphuric acid was used as leaching agent, while sugar was added as carbohydrate, to simulate the presence of molasses from agro-industrial activities. The main goal of the experimental activity was the electrolytic manganese dioxide recovery, at high degree of purity, from leaching solutions containing Mn, by electrowinning process, also considering the effect of the presence of impurities, such as organic substances and iron. Electrowinning lab-scale operations were conducted to determine the process parameters and to evaluate the quality of the deposit. The purity of the MnO2 residues was determined by diffractometric analysis (RXD). The results show that, also in presence of iron and sugar in leaching solution, it is possible to achieve high MnO2 recovery after electrowinning process (95%). The energetic consumption increases from about 1 kWh/kg (pure solution containing only Mn) to about 2 kWh/kg, in the case of contemporaneous presence of Mn, Fe3+ and sugar in solution. The further work consists in the scale up of the process in continue system, utilising molasses from agro-industrial activities to perform the leaching tests.