Powder mixing is an operation that aims to treat two or more components, initially in an unmixed or partially mixed state, so that each unit (particle) of the components lies as nearly as possible in contact with a unit of each of the other particles in the powder bed. It is a process that results in randomization of dissimilar particles within a system.
This article will focus on the mechanisms involved in the mixing of powders.
There are three main mechanisms involved in the mixing process, related to the different kinds of particle motion:
This involves the collective transfer of large group of particles from one location to another. Convection is controlled primarily by the geometry of the system including the presence of elements such as paddles and baffles.
This type of mixing contributes mainly to the macroscopic mixing of powder mixtures, rather than microscopic mixing. The mixing rate of this process is relatively high, however, to achieve a more uniform mixing, prolonged times are required because particles within the moving group are not able to mix well.
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Shear refers to the action that one layer of powder particles moves relative to another layer of particles. Shear mixing can be caused by particles from a different powder layer, having different velocities developed in a rotary vessel or due to the compression force and extension of the powder bed. Hence, shear mixing is the movement of a layer of particles, rather than the movement of individual particles.
The rate of shear mixing is intermediate and mainly causes semi microscopic mixing.
Diffusion is known as the moment of individual particles, due to random motion of powder particles. When the powder bed is placed in a mixer, the volume of that powder bed increases, due to convection and shear forces, which drive the powder ﬂowing around the mixer. Therefore, the voids between particles increase, leading to great potential for individual particles to mix with each other. The rate of diffusive mixing is relatively low, compared with convection and shear mixing; however, it contributes to the microscopic mixing.
For most of the mixing operations, all of these mechanisms are involved, rather than only one or two of them. The mechanism that predominates and the extent to which each occurs will depend on the mixer type, mixing process conditions (mixer load, speed, etc.), and the flowability of the powder components.
Mechanisms involved in the mixing of pharmaceutical powders