Valorization of waste Fe-rich sludge as erdite/KFeS rods under atmosphere condition and evaluation of their mutual transformation.

Pilot scale production of one-dimensional (FeS) rods was performed by using an automatic 20 L vessel at 80 °C under atmosphere condition with the resource utilization of Fe-rich sludge. The sludge was simulated at lab-scale with chemical pure of ferric trichloride. After the sludge treatment, the corresponding rods were not formed at room temperature. But by heating at 80 °C, erdite rod was well-crystallized after 0.5 h by only adding Na halite, and KFeS rod was crystallized weakly after 2 h and highly at 10 h with the addition of K halite. After 48 h heating, the rods grow radially to 300 nm for erdite, but to 5 μm for KFeS. However, at room temperature, erdite rod was converted to high crystallized KFeS in KOH water or ethanol solution, whilst the conversion of KFeS rod to erdite also occurred in NaOH water solution, but terminated in NaOH ethanol solution, without any morphology change. It is also noted that with the presence of both Na and K halite, the rod was an intermediate of erdite to KFeS with 1 μm length after heating at 100 °C but converted to 10-μm-length KFeS crystal at the temperature of > 120 °C. The thermodynamic results confirmed that during the rod polymerization, the Fe(OH)HS formation was the sole rate-limiting step and showed a positive Gibbs value of 6.45 kJ/mol at room temperature and negative values at the temperature of > 48 °C. In summary, this method not only enabled the vaporization of waste Fe-rich sludge as value-added rods without generating any secondary waste but also showed a new route for the in situ conversion of erdite/KFeS rods at room temperature.