Coal-based direct reduction and magnetic separation of lump hematite ore

Minerals & Metallurgical Processing , 2014, Vol. 31, No. 3, pp. 150-161

Kou, J.; Sun, T.; Tao, D.; Cao, Y.; Xu, C.

The reduction behavior of low-grade lump hematite ore (100% 20-50 mm) has been evaluated by performing
a series of coal-based direct reduction roasting tests followed by magnetic separation under different process
conditions such as roasting time, temperature, reductant type and dosage, and grinding stage. To investigate
mineralogical transformation and reduction characteristics of lump hematite ore, roasted ores and magnetic
concentrates were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy
dispersive spectrometer (EDS) and polarizing microscope. The results have shown that a magnetic concentrate
with a total iron (TFe) grade of 90.7% was produced at an iron recovery of 83.9% when lump hematite ore was
reduced at 1,200° C for five hours using 35% lignite as a reductant followed by two stages of grinding, for 20 min
and 30 min each, and low-intensity magnetic separation. The degree of reduction is closely related to the change
in the internal structure of the roasted ore. Lignite with a higher fixed carbon content and lower ash generated
roasted ore with a more highly porous structure, which provides better permeability of reducing gas. Magnetite,
fayalite and wustite were the dominant intermediates of coal-based direct reduction of lump hematite ore. Parts
of wustite reacted with SiO2 or Al2O3 in gangue and generated fayalite or hercynite. FeO in fayalite was replaced
by CaO to produce wustite, which was finally reduced to metallic iron. However, Fe in fayalite and hercynite
produced with low contents of CaO cannot be reduced into metallic iron.