Measurement of gas dispersion parameters in a reflux flotation cell

Abstract

Performance of mineral flotation process in terms of grade and recovery is critically dependent on achieving the optimal gas dispersion in a flotation cell which involves three parameters − superficial gas velocity (j_G), gas holdup (${\overline{\alpha }}_G$), and bubble size (d_sea). In mechanical flotation cells, these parameters have been extensively studied, however current understanding of these parameters in the Reflux Flotation Cell (RFC™) remains significantly limited. This study aims to experimentally measure these parameters in a pilot-scale RFC™ system using high-speed imaging and differential pressure measurement technique by varying four operating fluxes which include − gas (j_G = 0.42 to 2.09 cm/s), feed (F = 1.46 to 1.88 cm/s), wash water (W = 0.21 to 0.63 cm/s), and underflow (U = 1.46 to 1.88 cm/s). It was noted that increasing F and W fluxes resulted in smaller d_sea due to increased shear rate and corresponding bubble breakups. The relationship between d_sea and F was linear, while it plateaued with W. With F and W fluxes, the d_sea decreased from a maximum of 1.37 and 1.11 mm to a minimum of 0.44 and 0.62 mm, respectively. Conversely, U flux increased d_sea at low j_G but decreased it at higher j_G, with values ranging from ∼ 0.48 to 0.97 mm. Gas holdup exhibited a linear increase with j_G but decreased with higher F and W fluxes or reduced U flux. The ${\overline{\alpha }}_G$ ranged between ∼ 0.37 and 0.72 for F and ∼ 0.46 and 0.65 for W, while U varied from ∼ 0.30 to 0.64. Feed flux had a stronger impact on d_sea, whereas U flux was noted to affect ${\overline{\alpha }}_G$ more. Suitable empirical correlations were also developed to predict d_sea and ${\overline{\alpha }}_G$ which agreed well with the experimental data. The overall hydrodynamics was characterised by two distinct flow regimes − bubbly flow, with minimal bubble coalescence at low j_G, and churn-turbulent flow, characterized by frequent coalescence at high j_G. It was noted that wash water flux destabilises the bubbly flow regime into churn turbulent flow with transition occurring at ${\overline{\alpha }}_G$ ≥ 0.6 and j_G = 1.46 cm/s.