Pharmaceutical Mixing

Process that results in randomization of dissimilar particles within a system. Mixing is one of the most common pharmaceutical operations. It is difficult to find a pharmaceutical product in which mixing is not done at one stage or the other during its manufacturing. The terms "mixing" and "blending" are often used interchangeably, but technically they are slightly different. Blending is a process of combining materials, but blending is a relatively gentle process compared to mixing. In terms of the phase of material, blending is the process of solid-solid mixing or mixing of bulk solids with small quantity of liquid. The terminology mixing is more closely associated with liquid-liquid, gas-liquid, and viscous materials.

Importance of Mixing Technology

Mixing is a critical process because the quality of the final product and its attributes are derived by the quality of the mix. Improper mixing results in a non-homogenous product that lacks consistency with respect to desired attributes like chemical composition, color, texture, flavor, reactivity, and particle size.

The wide variety and ever increasing complexity of mixing processes encountered in industrial applications requires careful selection, design, and scale up to ensure effective and efficient mixing. Improved mixing efficiency leads to shorter batch cycle times and operational costs. Today's competitive production lines necessitate robust equipment that are capable of fast blend times, lower power consumption, equipment flexibility, ease of cleaning, and a gamut of customized features. In addition to blending components, many modern mixers are designed to combine different process steps in a single equipment, e.g. coating, granulation, heat transfer, drying, etc.

I. Liquid mixing or Wet mixing:

Wet mixing may be divided into following two subgroups:

1. Mixing of liquids and liquids

a) Mixing of two miscible liquids

b) Mixing of two immiscible liquids

2. Mixing of liquids and solids

a) Mixing of liquids and soluble solids

b) Mixing of liquids and insoluble solids

1. (a) Mixing of two miscible liquids (homogeneous mixtures e.g. solutions) – mixing of two miscible liquids is quite easy and occur by diffusion. Such type of mixing does not create any problem. Simple shaking or stirring is enough but if the liquids are not readily miscible or if they have very different viscosities then electric stirrer may be used.

1.(b) Mixing of two immiscible liquids (heterogenous mixtures e.g. emulsions) – two immiscible liquids are mixed to effect transfer of a dissolved substance from one liquid to another loquid. When two immiscible liquids are mixed together in the presence of an emulsifying agent an emulsion is produced. For the production of a stable emulsion, the mixing must be very efficient.

2. (a) Mixing of liquids and soluble solids (homogeneous mixtures e.g. solutions)- in this case soluble solids are dissolved in a suitable liquid by means of stirring. It is a physical change, a soluble solid is converted to a solution.

2.(b) Mixing of liquids and insoluble solids (heterogeneous mixtures e.g. suspensions) – when insoluble solids are mixed with a liquid a suspension is produced which is an unstable system.

II Solid-Solid Mixing or Dry Mixing:

Dry mixing is a process in which two or more than two solid substances are intermingled in a mixer by continuous movement of the particles. Mainly, the object of mixing operation is to produce a bulk mixture which when divided into different doses, every unit of dose must contain the correct proportion of each ingredient. The degree of mixing will increase with the length of time for which mixing is done.

References:

 1. Wikipedia
2. The Theory and practice of Industrial Pharmacy,

    (Leon. Lachman, H.A. Lieberman, J.L. Kanig)