Impact of high-sulfur boron-containing iron powder on pelletizing process and metallurgical properties

In response to the issues caused by reduced supply of high sulfur low silicon ore powder on the Shougang pellet production line consisting of a grate-rotary kiln-annular cooler, including insufficient sulfur source for the acid production system, decreased magnesium to aluminum ratio in the blast furnace, and deteriorated slag fluidity, a systematic study was conducted on the feasibility of using highsulfur boron-containing iron powder (HSBC, containing B₂O₃ 5%, MgO 10.18%, S 1.0% by mass) to replace Peruvian fines in the production of acid pellets. The phase composition microstructure and thermal decomposition characteristics of HSBC were characterized using a combination of X ray diffraction reference intensity ratio (XRD-RIR), scanning electron microscope energy dispersive spectroscopy (SEM-EDS), and thermogravimetric analysis differential scanning calorimetry (TG-DSC) method. Five gradient tests with HSBC mass ratios of 0, 2%, 4%, 6%, and 8% were designed and implemented on an industrial 10 kg scale disc pelletizer grate machine rotary kiln and annular cooler line for green pellet preparation, basket roasting and sampling detection. The results show that HSBC particles have a rough surface and a fibrous structure. Their particle size is relatively coarse with 72.6% below 74 μm complementing the extremely fine Macheng powder in particle size distribution. This complementarity increased the drop strength of green pellets from 6.2 times to 8.5 times. TG-DSC analysis revealed an exothermic peak at 379 ℃ corresponding to the conversion of Fe₃O₄ to Fe₂O₃ and an endothermic peak between 661.7 and 916 ℃ associated with sulfide oxidation dolomite decomposition and ludwigite lattice reconstruction with a total weight gain of 2.9%. The roasted pellets exhibited a core shell structure characterized by a liquid phase shell and a porous core with porosity increasing from 17.7% to 25.9%. As the HSBC mass ratio increased from 0 to 8%, pellet reducibility decreased from 58.50% to 50.32% while reduction swelling increased from 10.98% to 22.34%. The compressive strength after one hour of reduction improved by 169 N and the softening temperature interval Δt widened from 91 ℃ to 115 ℃. Considering metallurgical performance indicators and blast furnace adaptability, the optimal HSBC addition ratio is determined to be 5%. At this ratio, the compressive strength of the finished pellets reached 3 173 N, the desulfurization rate is 91.5% and the sulfur input is 1 117 mg/m³, which is close to the baseline level. Additionally the pellets demonstrated good reducibility at 55.02% and a controllable reduction swelling rate of 16.36%. The research findings provide a theoretical basis and technical support for the large scale application of high-sulfur boron-containing iron powder in pellet production.