Flotation of coarse coal particles in a fluidized bed: The effect of clusters

This paper is concerned with the separation of coarse particles from siliceous gangue, in a fluidized bed froth flotation column, the NovaCell. Coarse particles are allowed to settle in the base of a column, and are fluidized by an upward-flowing stream of liquid. Air bubbles are introduced with the stream, which collide with hydrophobic particles in the bed, carrying them to the top of the vessel as flotation product. The paper describes the use of bubble clusters as a means of recovering particles in a flotation machine. Clusters are buoyant aggregates of bubbles and particles. Copious quantities were observed to form in the fluidized bed, rising to the top of the flotation column. They were present in such volumes that the froth was unable to absorb them all, and a thick cluster layer developed beneath the froth. This layer was drawn off and separated on a screen as a secondary flotation product, in addition to the conventional froth product. Here we describe the flotation of coal particles from different sources with a top size of 2 mm, in a NovaCell fluidized bed contactor. High combustibles recoveries were achieved, at acceptable ash contents, over the whole size range. The froth phase contributed approximately 65% of the total, the balance appearing on the screen. Particles of all sizes from 0 to 2 mm were represented in the froth discharge. The NovaCell gave two flotation products and two tailings streams. It was possible to control the tails from the fluidized bed so that the solids fraction was as high as 60-70%, in the size range 300 mu m-2 mm. The results highlight the value of coarse particle froth flotation from a product de-watering perspective, when compared with flotation in conventional cells that are restricted to a top size of 500 mu m. The ultrafine fraction less than 74 mu m in diameter in the product was reduced by a third when the top size was increased to 2 mm from the normal top size of 500 mu m for coal. The fact that low-buoyancy clusters can form beneath the froth, provides an alternative explanation for the phenomenon known as froth dropback.