Application of Chemical Additives in Minerals Beneficiation – Implications on Grinding and Flotation Performance

Abstract: The application of chemical additives, known as grinding aids (GA), dates to 1930 in the cement industry. As opposed to the cement industry, where the use of GAs is on the final processing step, it could be one of the first process steps in ore beneficiation. Further to grinding performance, the successful application of GAs requires understanding the effect on ground products and possible interaction of the GA in view of downstream processes. Understanding and controlling any GA-separation reagent interactions is critical to ensure that the required downstream process efficiency and integrity of the whole value chain are maintained. This thesis work first investigates the effect of selected chemical additives on dry grinding performance and product properties. Second, the effect of the additives on surface properties and pulp chemistry, together with the resulting behavior in subsequent froth flotation separation, is investigated. Within mineral processing, the use of environmentally benign and sustainable alternatives to conventional surfactants has been growing. To this end, a natural polysaccharide-based grinding aid (PGA) (natural polymer), together with a polyacrylic acid-based grinding aid (AAG) (synthetic polymer) were used as grinding aids. The effect of PGA and AAG at varying concentrations was investigated with regard to energy consumption, particle size distribution, BET surface area, roughness, and rheology. The resulting grinding parameters are correlated with the measured rheology indices from the automated FT4 powder Rheometer. Moreover, the effect of the GAs on the flotation of quartz from magnetite was investigated using a mixture as an artificial  ore. Zeta potentials, stability measurement, adsorption test, and FTIR analyses were performed to better understand the adsorption and surface interaction mechanisms.The grinding results indicated that the application of GAs reduced energy consumption (Ec) and gave a finer-uniform product size with roughened surfaces compared to grinding without. PGA reduced the work index by 31.1%, from 18.0 to 12.4 kWh/t. The PSD became narrower and finer (P80 decreasing from 181 to 142 µm), and the proportion of the particles (38–150 µm) increased from 52.5 to 58.3%. The results generally reveal that sufficient GA dosages reduce the particle size, increase the specific surface area, and narrow the particle size distribution. Further studies on powder rheology indicated that the applied GAs resulted in improved material flowability compared to grinding without additives (in the examined dosage range). PGA showed the best performance with a 38.8% reduction of basic flow energy, 20.4 % reduction of specific energy, a 24.6% reduction of aerated basic flow energy, and a 38.3% reduction of aerated energy. There was a strong correlation (r > 0.93) between the grinding and flow parameters. These results confirmed the effect of GA on ground particles' flowability. Flotation tests on pure samples illustrated that PGA has beneficial effects on magnetite depression (with negligible impact on quartz floatability) through reverse flotation separation. Flotation of the artificial mixture ground sample in the presence of PGA confirmed the benefits, giving a maximum Fe recovery and grade of 84.4 and 62.5 %, respectively. In the absence of starch (depressant), PGA resulted in a separation efficiency of 56.1 % compared to 43.7 % without PGA. The PGA adsorption mechanism was mainly via physical interaction based on UV-Vis spectra, zeta potential tests, Fourier transform infrared spectroscopy (FT-IR), and stability analyses. Further, single-mineral flotation tests indicated that AAG enhanced the collection of quartz with minimal effect on magnetite. Mixed mineral flotation revealed that by using AAG, Fe recovery of 92.1% and 64.5 % Fe grade could be achieved with at a lower collector dosage than the reference. Zeta potentials and stability measurements showed that AAG shifts the potential, thus improving the stability and dispersion of the suspension. Adsorption tests revealed that AAG adsorbed on both quartz and magnetite, the former having a higher capacity. Fourier transform infrared spectroscopy showed that the interaction between AAG and the minerals occurs via physical interaction. The findings illustrate that GAs improved grinding efficacy at an optimum dosage and enhanced product properties. Further, the predominant mechanism of GAs is based on the alteration of rheological properties. Importantly, the feasibility of using GAs to improve grinding performance with secondary beneficial effects on flotation has been demonstrated. Using PGA, a natural green polymer, generally benefited both grinding and reverse flotation separation performance. On the other hand, applying AAG not only improved grinding efficiency but could potentially decrease the amount of collector required to achieve comparable metallurgical performance.