Mixing or blending has been defined as an operation “in which two or more ingredients in separate or roughly mixed condition are treated or blended so that each particle of any one ingredient is as nearly as possible adjacent to a particle of each of the other ingredients’’. Mixing is a basic step in most process sequences, and it is normally carried out -
1. To secure uniformity of composition, so that small samples withdrawn from a bulk material represent the overall composition of the mixture and
2. To promote physical or chemical reactions, such as dissolution, in which natural diffusion is supplemented by agitation.
Types of Mixing in Pharmaceutical Engineering
Danckwerts (Dankwerts, 1953) classified mixing as follows:
Positive mixing: which applies to systems that, given time, would spontaneously and completely mix. Examples are provided by two gases or two miscible liquids, and mixing apparatus is used on such systems to accelerate mixing.
Negative mixing: which is demonstrated by suspensions of solids in liquids. Any two-phase system, in which the phases differ in density, will separate unless continuously agitated.
Neutral mixing: which occurs when neither mixing nor de-mixing takes place unless the system is acted upon by a system of forces. Examples are found in the mixing of solids and of solids with liquids when the concentration of the former is high.
The Degree of Mixing
A quantitative expression that defines the state of a mix is necessary if a rational answer to the question “Is this material well enough mixed?” is to be made. Such an expression would also allow the course of mixing to be followed and the performance of different mixers to be compared.
The most useful method of expressing the degree of mixing is by measuring the statistical variation in composition of a number of samples drawn from the mix. The scale of scrutiny determines the size of the samples, and their number depends on the accuracy required of the assessment.
SD & RSD calculation formula for degree of mixing measurement:
Mechanism of Mixing and De-mixing
1. Convective mixing: The transfer of groups of adjacent particles from one location in the mass to another.
2. Diffusive mixing: The distribution of particles over a freshly developing surface.
3. Shear mixing: The setting up of slip planes within the mass.
The Rate of Mixing
The rate of mixing for any mixing mechanism can be represented by the following expression:
Different Types of Mixers in Pharmaceutical Industry
1. Ribbon Mixers
2. Impeller Mixer
3. Tumbler Mixers
4. Paddle Mixers
5. Propeller Mixers
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