MIXING
Mixing may be defined as the process in which two or more ingredients are treated so that every particle of any one ingredient lies as near as possible to the particle of the other ingredient.
There are several factors which affect the process of mixing, like density of the material, particle size, shape, and proportion of the constituents; but the chance of achieving such a true homogeneous mixture is very rare. The extent to which the mixtures can be mixed depends on the rationale for which it is being carried out, e.g., formulation of a mixture containing different constituents for increasing the rate of reaction, for dispersion, or for any kind of physical change.
OBJECTIVES OF MIXING
- Chemical Reaction Enhancement: In chemical industries, mixing is used to continue a reaction appropriately which require a close contact among reacting substances.
- Simple Physical Mixtures: This is a mixture of two or more miscible liquids, in which homogeneously divided solids, etc. can be obtained by mixing.
- Physical Changes: These changes occur by mixing, e.g., supersaturated solution form crystals.
- Achieving Dispersion: This is also an objective of mixing in which two or more immiscible liquids and one or more liquids with finely divided solids are mixed to produce a quasi-homogenous substance.
CLASSIFICATION OF MIXING
1. Solid-Solid Mixing
In this process, two or more solid ingredients are blended together in a mixer by constant particle movement. This type of mixing is commonly used in pharmaceutical industries for the formulation of various types of products such as tablets, powders, and capsules.
2. Liquid-Liquid Mixing
Mostly liquid-liquid and sometimes liquid-solid mixing processes are carried out using a propeller or a turbine in a tank. In liquid-liquid mixing, the liquid system contains any one of the following liquids:
- Liquids with or without non-viscous solids, e.g., light oils.
- Liquids with or without viscous and pourable solids, e.g., heavy oils and paints.
- Liquids with solids forming stiff pastes, e.g., oil-bound distempers.
3. Semisolid Mixing
When semisolid dosage forms (such as jellies, ointments, creams, pastes, etc.) are to be mixed, the material must be brought to the agitator or is moved throughout the mixer. Low speed shear, wiping, smearing, stretching, folding, and compressing altogether are involved in a process of mixing.
TYPES OF MIXTURES
1. Positive Mixtures
These mixtures are spontaneous and irreversible, i.e., they do not require energy for mixing. But they require energy for separation. For example, mixing of two or more gases or miscible liquids such as water and milk.
2. Negative Mixtures
These mixtures require a high degree of mixing along with expenditure of energy for mixing. They can be separated without the use of energy. For example, making a suspension of solids in liquids and emulsion of two immiscible liquids such as oil and water.
3. Neutral Mixtures
These mixtures are static in nature and require an expenditure of energy for mixing as well as separation. For example, physical mixing of pastes, ointment and powders.
MECHANISMS OF MIXING
| Mechanism | Description |
|---|---|
| Convective Mixing | Mixing brought about by the bulk movement of material, frequently aided by outside factors like stirring or agitation, and transfers mass and momentum. |
| Diffusive Mixing | Mixing that happens over time as a result of the particles’ random migration caused by concentration gradients. It is also known as micromixing. |
| Shear Mixing | Mixing brought about by the application of shear forces, usually produced by revolving blades or impellers in a mixing vessel to improve mixing and create flow. |
APPLICATIONS OF MIXING
- During tablet and capsule manufacturing, wet mixing is done in the granulation stage.
- For easy compression of tablets, various components are mixed by dry mixing.
- In the manufacturing of capsules, compound powders, and dry syrups, dry blending of powder is done.
- For capsule manufacturing, pellets are formed.
FACTORS AFFECTING MIXING
- Surface Nature: The entry of active ingredients into the pores of other elements obstructs a suitable mixing, due to the rough surface of that particular element.
- Particle Density: Sudden halt of the mixing process or presence of small particles with high density accelerate the demixing process as the material with high density travels downward and residue remains at the bottom.
- Particle Size: The two powders which are of almost same particle size are easy to mix. Small particles go to the bottom by passing the gaps between the bigger particles and get separated due to the difference in particle size.
- Particle Shape: For uniform mixing the ideal particle shape is spherical. The particles of irregular shape are not separated easily if mixed together due to interlocking between the particles.
- Particle Charge: Separation or aggregation occurs as some particles employ attractive forces which are generated because of their electrostatic charges.
- Material Proportion: By mixing two powders in an equal ratio by weight and volume, finest outcomes can be obtained. Mixing occurs in the ascending order of the weights of two powders if their ratio varies to a greater extent.
EQUIPMENT FOR MIXING
For mixing and homogenisation process such mixer should be used which facilitates random mixing, and also prevents the conditions which may result in segregation. The most commonly used equipment are discussed below:
Double Cone Blender
Principle
Double cone blender involves axial mixing as the powder moves in different sections. This blender provides thorough mixing depending on its speed of rotation. It produces a homogeneous solid-solid mixture.
Construction
The body of double cone blender consists of two cone-shaped sections joined to a central cylindrical section at their bases. The rotational axis is perpendicular to the cone axis and passes through the cylindrical section. The blender body is held by two lateral supports, one of which is fitted with the driving motor.
Working
The autoclaved double cone blender’s angle is adjusted with the help of a moving wheel so that the material can be conveniently loaded. After loading, the lid of the blender is closed and secured. After blending, the safety guard is removed, the blender is adjusted to an angle for convenient unloading, and the lid is opened.
Applications
- Employed for preparation of pharmaceuticals, food, chemical, cosmetic products.
- Used for uniform mixing of granules or dry powders.
- Suitable for homogeneous mixing of small amount of powders.
- Used for providing heating and cooling effect with jacketed construction.
Turbine Mixer
Principle
Turbines rotate at a lower speed than propellers, and the ratio of vessel diameter to impeller diameter is low. Turbines are used for mixing high viscosity liquids because of greater shear forces. They are especially used for preparation of emulsions.
Construction
Turbines consist of a circular disc having short blades attached to it. Their diameter is 30-50% of the vessel diameter. They rotate at 50-200 rpm speed. Different turbines have blades of different shapes, like straight, curved, pitched, or vertical.
Applications
- Effective for high viscous solutions, e.g., syrups, glycerine, etc. due to high shear forces.
- Suitable for liquids of large volume and high viscosity.
Triple Roller Mill
Principle
The material experiences a high shear developed by the differential speed and the narrow space between the rollers. Under the influence of this shear, aggregates and particles get crushed, and the drug gets distributed uniformly throughout the semi-solid base.
Construction
A triple roller mill consists of three rollers of equal diameters made of hard abrasion-resistant material (stainless steel). These rollers are arranged parallelly and horizontally fixed to a rigid frame. A hopper is fixed between the first two rollers, and the last roller has a scrapper attached to it.
Applications
It is used for mixing solid powders in an ointment base.
Silverson Mixer – Homogeniser
Principle
Silverson emulsifier works on the principle of shearing forces and turbulence produced by high-speed rotors. The fluid passes through fine spaces formed by closely placed perforated metal sheets under the influence of turbulence.
Construction
Silverson emulsifier consists of long supporting columns connected to a motor providing support to the head. It has a centrally located shaft with one end connected to the motor and the other end connected to the head. Turbine blades are present in the head, surrounded by a mesh enclosed by a cover having openings.
Applications
It is used for mixing creams, ointments, sauces, flavouring emulsions, and pharmaceutical suspensions of globule or droplet size ranging from 2-5 µm.
