Flowability and Blender Selection
Selecting the right blender is often considered an art rather than a science. A good blender design should ensure the following:
- There are no stagnant regions in the blender.
- The blender must promote different flow velocities in different sections of the blender.
- The blender operation must not segregate, or demix, mixture ingredients.
Powder flow properties can simplify blender selection by allowing the prediction of the behavior of materials in different types of blenders. Knowledge of basic material flow properties and segregation tendencies provides guidance in selecting the right blender for the application.
Shear, convection and diffusion are the three primary mechanisms of blending of solids. The extent to which each of these mechanisms occurs depends on the flow properties of the powder to be blended and the type of the blender used.
Diffusion blending is characterized by small scale random motion of solid particles. Blender movements increase the mobility of the individual particles and thus promote diffusive blending. Tumbler blenders like the double cone blenders and v-blenders function by diffusion mixing. With the diffusion mechanism, particles migrate through a dilated or expanded bed of powder. The ability of the bed to dilate and the mobility of powder particles depend on the cohesive strength of the powder. Powders with lower cohesive strength dilate more readily. Shorter blend times are achieved if the major component of the blend is relatively free flowing. A tumbler blender works best with ingredients that have similar angles of repose and only enough cohesiveness to prevent sifting.
Convection blending is characterized by large scale random motion of solid particles. In convection blending groups of particles are rapidly moved from one position to another due to the action of a rotating agitator. Likewise, in shear blending mechanism, the splitting of the material beds takes place by the high intensity shearing action of chopper blades. The material agglomerates are disintegrated by overcoming cohesion. As a result cohesive powder may blend faster in convective blenders like Ribbon Blenders and Plow Mixers because of chaotic patterns created by the blender action.
If the different components of the blend do not adhere to one another, free flowing materials may segregate easily during post blender handling. Common particle segregation mechanisms include sifting, angle of repose, fluidization (air entrainment), and dusting (particle entrainment). A good blend can be achieved if the minor component is somewhat cohesive or has a tendency to adhere to the major component of the blend. This is referred to as adhesive blend. Likewise, a good blend can be obtained and maintained if the blend as a whole is slightly cohesive compared with a blend that is free flowing. Therefore, blending action must be compared with actions that result in segregation.