In general, mixing involves one or more of the following mechanisms:
Convective Mixing:
Convective mixing is a process of transferring groups of particles in bulk from
one part of the powder bed to another. It is also known as micro-mixing and is
regarded as analogous to bulk transport. Depending on the type of mixer
employed, convective mixing may occur by an inversion of the powder bed or using
blades and paddles or using a revolving screw, or by any other method of moving
a relatively large mass of material from one part of the powder bed to another.
Shear Mixing:
During this process, shear forces are created within the mass of material by
using an agitator arm or a blast of air. As a result forces within the
particulate mass slip and planes are set up. Depending on the flow
characteristics of the powder, these can occur singly or in such a way as to
give rise to laminar flow. When shear occurs between regions of different
compositions and parallel to their interface, it reduces the scale of
segregation by thinning the dissimilar layers. The shear that occurs in a
direction normal to the interface of such layers is effective as it also
reduces the scale of segregation.
Diffusive mixing:
Diffusive mixing is also known as micromixing. In diffusive mixing, the
materials are tilted to ensure that the upper layer slips and diffusion of
individual particles take place at the newly developed surfaces. This occurs
when the random motion of particles within a powder bed causes them to change
position relative to one another. Such an exchange of positions by single
particles results in a reduction of the intensity of segregation. Diffusive
mixing occurs at the interfaces of dissimilar regions that undergo shearing and
therefore it results from shear mixing. It may also be produced by any form of
agitation that results in inter-particulate motion.
Mechanisms Solid Mixing
In solid mixing, two
different dimensions in the mixing process are convective mixing and intensive
mixing. In convective mixing material in the mixer is transported from one
location to another. This leads to a less ordered state inside the mixer. The
mixing components are distributed over the other components. With time the
mixture becomes more random and after a certain time, the ultimate random state
is reached. This type of mixing is observed for free-flowing and coarse solid
materials. Physical properties of the material that affects solid mixing are
density, particle size, distribution, wettability, stickiness, and particle
shape or roughness. Usually, these factors contribute to the demixing of
macromixed solids. If solids are in fine form with cohesive nature or if it is
wet convective mixing is not enough to obtain a random state. The relatively
strong inter-particle forces form lumps. The decrease in size of the lump
requires more intensive energy which is provided either as impact force or
shear force.
Mechanisms Liquid Mixing
The liquid mixing occurs
in two stages; first, localized mixing which applies sufficient shear to the
particles of the fluid, and second, a general movement sufficient to take all
parts of the material through the shearing zone and to ensure a uniform final
product.
There are four essential
mechanisms involved in liquid mixing as follows:
1. Bulk Transport:
Movement of a relatively large portion of the material being mixed from one
location in the system to another.
2. Turbulent flow:
It is characterized by the fluid having different instantaneous velocities at
the same instant of time. The temporal and spatial velocity differences
resulting from turbulence produce randomization of fluid particles.
3. Laminar Flow:
In this mechanism, a streamlined flow is encountered in highly viscous liquids.
4. Molecular diffusion:
It is a primary mechanism responsible for mixing at the molecular level which
results from the thermal motion of molecules. It is governed by Fick's first
law of diffusion that describes concentration gradient across the system as:
`\frac{dm}{dt}=-DA\frac{dc}{dx} ...(1)`
Where,
- dm / dt = Rate of transport of mass across a surface area
- D = Diffusion co-efficient
- A = Area across which diffusion is occurring
- dc / dx = Concentration gradient
There is a decrease in
concentration gradient with time which approaches zero at completion.
Liquid mixing is a
process that can either be carried out batch to batch or can be a continuous one.
Impellers, air jets, fluid jets, and baffle mixers are the major types of
mixing equipments used for batch mixing. Impellers operate using a combination
of radial, axial and tangential flow. These might be classified into two
further types, propellers and turbines, the former being used for low viscosity
liquids while the latter for high viscosity liquids.
Mechanisms Semisolids Mixing
The mechanisms involved
in mixing semi-solids depend on the properties of the material which generally
may show considerable variations. Many semi-solids form neutral mixtures do not
tend to segregate although sedimentation may occur. The three most commonly
used semi-solid mixers are:
(i) Sigma blade mixer:
This mixer has two blades that operate in a mixing vessel that has a double
trough shape. These blades move at different speeds towards each other. It can
be used for products like granulation of wet masses and ointments.
(ii) Triple-roller mill:
The triple roller has differential speed and narrow clearance between the rollers
which develops a high shear over small volumes of semi-solid material. This
type of mill is generally used to grind semisolids to achieve complete
homogeneity in the material, for example, ointments.
(iii) Planetary mixers:
This mixer has a mixing arm rotating about its axis and also about a common
axis usually at the center of the mixing wheel. The blades attached to the arm
provide the kneading action, while the narrow passage between the blades and
the wall of the container provides shear.
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