Whitepapers

Solid-solid product formulations are mixed to meet a variety of objectives. Theseinclude blending of ingredients (such as in the preparation of animal feeds, fertilizer, glass batches, food products, etc), heating/cooling/drying (like in the processing ofmetal powders and plastics) and coating of solid particles with small amounts of liquid (similar to the manufacture of pigments, pharmaceuticals and chemical blends).

Three basic blenders can achieve the above objectives. These are small-scale random motion (diffusion), large-scale random motion (convection) and shear. Diffusion occurs readily for free-flowing powders. But for most rapid mixing, in addition to diffusive movement, large groups of particles should be able to intermix as well.

Among solid-solid blending systems, the three most common types are the horizontal ribbon blender, the cone screw vertical blender and the tumble blender. Of these three, the ribbon blender particularly demonstrates convective high speed mixing.

A ribbon blender consists of a U-shaped horizontal trough and an agitator made up of inner and outer helical ribbons that move material in opposite directions. The ribbons rotate up to approximately 300 fpm, and move materials both radially and laterally. The rotating element can also be replaced with a paddle design agitator.

The blending action of the vertical blender's slow turning screw is far gentler than that of a horizontal blender. The blending screw orbits a conical vessel wall while it turnsand gently lifts material upward. The materials are then thrust at the upper most batch level towards the center of the vessel, and then move slowly back down the center, while mixing with materials being moved upward by the orbiting screw.

Products that require low-impact blending are indeed best handled in a vertical blender or a tumble blender. The latter is a rotating blender that comes in interchangeable double-cone and vee-shaped configurations. Asymmetric vessels designed to reduce blend times and improve uniformity are also available. Generally, tumble blenders operate ata speed of 5 to 25 revolutions per minute. With this type of blender, diffusion is the main mechanism for mixing.