Optimized acid-leaching recovery of Iron from pyrrhotite waste and hydrothermal crystallization of high-purity α-Fe2O3 nanoparticles for magnetic applications

In this study, iron(III) oxide (α-Fe 2 O 3 ) was recovered from pyrrhotite ash (PA) using an optimized acid leaching process followed by hydrothermal crystallization. A one-factor-at-a-time (OFAT) approach was employed to investigate the effects of acid concentration, leaching time, temperature, and liquid–solid ratio on iron extraction efficiency. Under optimal conditions (6 M HCl, 90 °C, 120–180 min, and a liquid-to-solid ratio of 10:1 mL/g), a maximum iron recovery of 82.6% was achieved. Kinetic analysis based on the shrinking-core model demonstrated that iron dissolution is governed by a surface chemical-reaction mechanism, exhibiting the highest linearity (R 2 = 0.97) among the tested models. Temperature-dependent rate constants followed the Arrhenius relationship, yielding an activation energy of 46.66 Kj/mol. X-ray fluorescence (XRF) analysis confirmed that the synthesized α-Fe 2 O 3 possesses high purity, containing 98.76 wt% iron(III) oxide. X-ray diffraction (XRD) confirmed the phase purity and crystallinity of the extracted α-Fe 2 O 3 , while Fourier-transform infrared (FTIR) and Raman spectroscopy revealed characteristic Fe O vibrations consistent with the hematite structure. Scanning Transmission Electron Microscopy (STEM) analysis showed agglomerated particles with nearly hexagonal morphology and sizes ranging from 50 to 138 nm. Magnetic measurements at 5 K and 300 K indicated weak-ferromagnetic behavior. The recovered α-Fe 2 O 3 nanoparticles were successfully used as precursors for synthesizing cobalt and nickel ferrites using solid-state reaction. This work represents the first reported utilization of PA by-products as a source of α-Fe 2 O 3 for producing ferrimagnetic powders, highlighting a sustainable and value-added route for industrial waste valorization.