Evaluation of solids suspension in a pilot-scale mechanical flotation cell: The critical impeller speed

This paper investigates solids suspension in a pilot-scale mechanical flotation cell in terms of the critical impeller speed, Njs. Understanding solids suspension has become increasingly important in recent years due to dramatic increases in flotation cell sizes but appears to be relatively poorly researched. The critical impeller speed is commonly used to indicate the effectiveness of solids suspension in stirred tanks, but has seldom been investigated in flotation cells. In this study, critical impeller speeds were visually determined and concentration profiles were measured through sample withdrawal in a 125 l Batequip (Bateman) pilot flotation cell. Two solids size fractions (75–106 and 150–250 μm) were tested, in ungassed and gassed conditions (JG = 0, 1 cm/s) and at various impeller speeds (300–900 rpm). The effectiveness of solids suspension was quantified in three ways; (i) the extent of off-bottom solids suspension, (ii) the extent of axial solids distribution (suspension height) and, (iii) the variability of axial solids distribution. Consistent trends were found when these effectiveness criteria were considered against relative impeller speed, in terms of percentage of critical impeller speed, N/Njs. As per definition, off-bottom solids suspension was found to be complete at or above 100% of Njs, with significant sedimentation occurring as the impeller speed dropped below 60% of Njs. Suspension heights consistently reached a level equivalent to 90% of the tank diameter at the critical impeller speed, and dropped off significantly as N/Njs dropped below 60%. The relative standard deviation of the vertical solids distribution consistently reached a value of around 30% at Njs. The paper concludes that the critical impeller speed, as per stirred tanks, is an appropriate measure for describing and benchmarking the effectiveness of solids suspension in a mechanical flotation cell. The authors speculate that, when solids suspension is considered as a precondition to flotation, flotation cells should not be operated at specific ‘impeller speeds’ but rather at specific ‘percentages of critical impeller speed’ analogous to the ‘critical speed’ used in the operation of grinding mills.