Effect of Grinding Approaches on the Preparation of Hematite Concentrate

This study investigates the influence of two distinct grinding approaches on the preparation and optimization of hematite concentrate intended for pelletizing, with emphasis on particle size distribution, Blaine number, and morphological evolution. A total of ۶۰۰ kg of hematite concentrate from the Jalal-Abad deposit (Kerman, Iran) was homogenized and processed using either multi-stage high-pressure grinding rolls (HPGR) or a combined ball mill (BM)-HPGR approach, each applied to approximately ۳۰۰ kg of material. In the first approach, the concentrate underwent five HPGR passes under controlled moisture and pressure conditions, whereas in the second approach, particles larger than ۱۰۶ μm were selectively removed, ground in a BM until complete passage below ۱۰۶ μm was achieved, recombined with the fine fraction, and finally subjected to a single HPGR pass. Particle size distribution was assessed through wet sieve analysis, while specific surface area was evaluated using Blaine air permeability. The results reveal clear distinctions between the two grinding approaches. Multi-stage HPGR grinding generated a broader particle size distribution with P۸۰ and P۶۰ values of ۹۶ and ۵۸ μm, respectively. Conversely, the BM+HPGR approach produced considerably finer and more uniform particles, reflected by significantly lower P۸۰ and P۶۰ values of ۵۷ and ۳۲ μm. Despite the higher Blaine number obtained through HPGR-only grinding (increasing from ۷۴۱ cm²/g in the feed to ۱۸۴۶ cm²/g), the second approach achieved a Blaine value of ۱۶۷۰ cm²/g with a more desirable particle size profile. These findings demonstrate that although Blaine number is often regarded as a key indicator of pelletizing performance, it cannot independently predict grinding efficiency or particle size distribution suitability [۱]. A sample with a slightly lower Blaine value may still exhibit superior P۸۰ and P۶۰ indices, resulting in improved packing behavior, more efficient balling, and enhanced pellet microstructure. Detailed morphological assessment using FESEM images along with geometric measurements of particles in ImageJ (Fig. ۱) provides further insight into the relationship between grinding mechanism and particle characteristics. The initial concentrate contains predominantly spherical particles with smooth surfaces, indicative of minimal mechanical stress during formation. After five HPGR passes, particle morphology shifts dramatically, displaying elongated shapes with sharp edges, cracked surfaces, and serrated fracture features [۲, ۳]. These characteristics align with the pressure-induced breakage mechanism of HPGR, which promotes micro-crack propagation and intergranular splitting [۴]. In contrast, the BM+HPGR sample exhibits finer, non-spherical particles formed through a combination of impact-dominated and compression-driven fracture events [۵, ۶]. This mixed morphology results in more effective breakdown of coarse particles and increased structural uniformity, consistent with the narrower particle size distribution obtained through wet sieving. Overall, the results confirm that while HPGR grinding is highly effective in generating ultrafine particles and elevating Blaine number, it has a limited role in reducing the coarser fractions that strongly influence P۸۰ and P۶۰. This explains the modest improvement in particle size indices despite substantial increases in Blaine number. On the other hand, the combined BM+HPGR method significantly enhances particle size uniformity, reduces mean particle size, and promotes a balanced morphology composed of both fractured and elongated particles. These combined effects enhance the formation and quality of green pellets and potentially improve the mechanical properties of indurated pellets in subsequent pelletizing stages. The findings emphasize that optimizing grinding processes for pellet feed preparation requires simultaneous evaluation of particle size distribution, Blaine number, and morphology rather than relying on any single parameter. In particular, incorporating a pre-grinding step to reduce coarse particles can substantially improve the final quality of hematite concentrate, even when the overall Blaine number is lower than that obtained through intensive HPGR treatment alone.

کلیدواژه ها:

Grinding ، Particle size distribution ، Blaine number ، Morphology