Effect of up-scaling on the quality of ashes obtained from hyperaccumulator biomass to recover Ni by agromining

Nickel (Ni) agromining aims at recovering this metal from the biomass of hyperaccumulator plants grown on serpentine soils, giving value to these low-grade agricultural soils. A process has already been designed to produce Ni salts, especially ammonium nickel sulphate hexahydrate (ANSH) from the biomass of Alyssum murale. The first stage of biomass processing consists of burning the dry plant to obtain ash, considered as a bio-ore because of its high Ni content (15-20%). This stage is perfectly controlled at the lab scale with a few grams of plant. At the pilot scale, plants are burnt in a boiler dedicated to biomass combustion; temperature cannot be precisely controlled and reaches 900 degrees C, which is likely to modify the chemical composition of ashes. The objective of this research was to compare ashes produced at lab and pilot scales and assess if the ANSH recovery from ashes obtained at higher temperature was still effective. Two batches of ashes of A. murale were prepared at the lab and pilot scales (550 and 900 degrees C) and analysed for elemental composition, mineralogy, particle size and morphology. Due to different temperatures and combustion times, the two ashes resulted to have diverse characteristics. To test process efficiency, they were used in the first process stages in which ashes are involved: potassium removal by ash washing and acid leaching to transfer Ni into solution. Extraction kinetics of potassium, the main impurity, was modelled. Results show that the discrepancy in ash composition and properties had no influence on the process. Hence, it can be up-scaled by using an industrial furnace to increase the production of nickel salts and develop a valuable method for Ni recovery by agromining. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.