Desempenho do amido de sorgo como depressor na flotação catiônica reversa de minério de ferro

Abstract

In the iron ore flotation process, the dependence on corn-based depressants, subject to competition with the food industry, creates uncertainties in costs. Because of this, various alternative botanical sources of starch have been studied to assess their applicability as depressants in the ore flotation process. In this study, the use of sorghum-derived depressants in the reverse cationic flotation of iron ore was investigated in bench-scale. In the ore characterization, it was found that about 64% of the iron was concentrated in particles smaller than 38 μm. The predominance of hematite and quartz minerals was identified through various techniques such as X-ray diffraction and scanning electron microscopy. Other mineral species, including micas, gibbsite, clay minerals, titanium oxides, manganese oxides, barite, carbonates, apatite, and secondary aluminum phosphates, were identified through MLA modal analysis. Approximately 87.3% of hematite particles were fully liberated above 150 μm, increasing to 98.5% in fractions below 25 μm, indicating a high degree of hematite liberation in the ore. Regarding depressant characterization, corn grits had a lower oil content (1.253 ± 0.113%) compared to sorghum flour (2.780 ± 0.057%), due to the degermination process included in grits production. Additionally, the gelatinization of sorghum starch at different starch:NaOH ratios was investigated. Gelatinization was most effective at starch:NaOH ratios below 7.5:1. The flotation test planning was divided into five stages. The optimal flotation time obtained in the first stage was 4 minutes, which was chosen as ideal for conducting the subsequent phases of the flotation tests. In the second stage, it was observed that replacing corn grits with sorghum flour had no significant impact on the flotation results. Both depressants produced concentrates with iron content around 63% and iron recoveries of approximately 83%. This performance equivalence can be explained by similarities in properties between sorghum and corn. Sorghum starch showed even more favorable performance in flotation compared to its corresponding flour, producing concentrates with iron content around 65% and iron recoveries exceeding 90%. In the third stage, it was found that both the variation in sorghum- based depressant dosage and flotation pH were significant in the process performance. Flour achieved better results with a dosage of 1700 g/tFe at pH 10.0, reaching iron content above 64% and Fe recoveries in the range of 93%. In the fourth stage, which evaluated the effect of the starch:NaOH ratio on flotation results, it was evident that excess NaOH negatively affected the process due to potential flocculation and clathrate formation effects. The most satisfactory results were obtained with a ratio between 5:1 and 10:1, suggesting the possibility of reducing the amount of NaOH in the process without compromising concentrate quality. The cleaner flotation results (final stage) indicated that gelatinized sorghum flour at the optimized ratio of 7.5:1 performed satisfactorily, with concentrates containing iron content above 67%, and iron recovery around 90%. Furthermore, comparative cleaner tests between corn grits and sorghum flour under standard conditions showed no significant differences. Both depressants generated concentrates with iron content around 67% and iron recoveries between 88% and 90%. Therefore, the results obtained throughout the study highlight that sorghum flour can be a technically effective alternative as a depressant in the reverse cationic flotation of iron ore.