Improving Coal Flotation Recovery Using Cavitation Concepts

Froth flotation is the most widely used solid-solid separation process for coal and minerals beneficiation. However, its high process efficiency is limited to a narrow particle size range, which is usually 10-100 ~lm beyond which its efficiency decreases sharply, especially for oxidized coal that is more difficult to float due to weak hydrophobicity The particle-bubble collision, attachment and detachment are the most critical steps in the flotation process. The low flotation recovery of fine particles is mainly due to the low probability of bubble-particle collision while the main reason for poor flotation recovery of coarse particles is the high probability of detachment of particles from bubble surface. Fundamental analysis indicated that use of smaller bubbles is the most effective approach to increase the probability of collision and reduce the probability of detachment. The main goal of the proposed project is to enhance recovery of difficult-to-float coal particles (-0.15 mm or -100 mesh), particularly ultrafine «44 or -325 mesh ~m) particles using a flotation column and a mechanical flotation cell featured with a hydrodynamic picobubble generator or an air eductor for feed preconditioning. Preliminary results indicate that the flotation rate of -100 mesh Coalberg coal can be increased by at least 200% by using an air eductor to pretreat the feed with submicron bubbles. Picobubbles (usually less than 1 ~m) selectively attach to the hydrophobic coal particles. They are characterized by an inherently high probability of collision with particles and high probability of attachment and low probability of detachment due to their tiny size, low ascending velocity and rebound velocity from the surface, and high surface free energy to be satisfied, and therefore are very effective for enhancing flotation recovery of fine and coarse particles. Other major advantages of the proposed technique include lower collector dosage and air consumption since picobubbles are produced from air naturally dissolved in water and they act as the secondary collector on particle surfaces thereby resulting in considerably lower operating costs.