The Investigation of Critical Impeller Speed in a Laboratory Mechanical Flotation Cell

One of the important steps in flotation is particle collection that occurs after the successful particle-bubble interaction. Effective solids suspension is a necessary preconditioning for particle collection and flotation. The critical impeller speed, Njs, is commonly used to indicate the effectiveness of solids suspension. The critical impeller speed refers to the minimum impeller speed where all the solids are just suspended off the bottom of the cell. In this paper, two correlations for predicting the critical impeller speed in terms of particles size (dp), relative solids density (ρs-ρl), solids concentration (X), liquid viscosity (vL) and superficial gas rate (Jg) in two and three phases conditions have been developed. The study was conducted in a laboratory flotation cell using four size fractions of quartz, barite, and galena. The results indicated that ρs-ρl, dp, X and vL are the most important variables, respectively. The critical impeller speed increased linearly with increasing Jg. Investigation of concentration profiles versus the relative height of the cell showed that suspension heights consistently reached a level equivalent to 0.80-0.85 of the cell height at the critical impeller speed. Homogeneous suspension condition will only be approached as the impeller speed was increased to approximately 120–150% of the critical impeller speed. This range can be used to predict the optimum impeller speed in order to increase the flotation efficiency.