Size Separation: Large pieces of material are usually estimated visually but difficulties arise only when powders are to be estimated. The aim of any estimation, including particle size separation, is either analytical or preparative. Size separation is a unit operation that involves the separation of a mixture of various size particles into two or more portions using sieving surfaces. Size separation is performed by processes known as sieving, sifting, segregation, etc. Size separation is a method of classifying particles into fractions based on size. Granular mixtures separate according to particle size when shaken, with large particles rising; a phenomenon termed the 'Brazil-nut effect'. It is a well-known fact that differences in particle density affect size separation in mixtures of granular particles.
Particles of many kinds and various sizes have played
an important role in the development and production of dosage forms. If
particles are spherical or cubical, it would be easy to characterize them.
Unfortunately, most of the particles or granules in pharmaceuticals are of
irregular size and shape. Therefore, various methods and techniques have been
developed to characterize such particles. Particle size analysis is important
in studying particle behavior in a medium during industrial applications.
Particle size analysis in various processes involves many concepts and
techniques; however, this chapter is focused on the methods of particle size
separation utilizing mechanical techniques.
Objectives of Size Separation:
Particles of many kinds and various sizes play an important role in performing a specific function in the development and use of dosage forms. If particles are spherical or cubical, it is easy to characterize them. Unfortunately, most of the particles employed in pharmaceuticals are of irregular size and shape. Therefore, it is desirable to develop methods and techniques to characterize particles of irregular size and shape, and this is the main objective of particle size separation. Apart from this other objectives of size separation are:
- Size separation is useful in grading powders or granules. Any solid materials after size reduction never give particles of the same size but contain particles of varying sizes. The size-reduced particles are then passed through sieves to get fractions of the narrow size range.
- To control size variations in the materials. During tablet granulation the granules should be within a narrow size range, otherwise, weight variation will take place during tablet punching.
- To classify materials into different sizes for the desired purpose.
- To enhance performance (topical powders), efficacy (powders for inhalation), and stability of dosage forms (suspensions).
- To judge uniformity in mixing materials.
- To avoid variations in the bulk properties of materials.
- To specify the quality parameter of intermediate and finished dosage forms.
Applications of Size Separation:
Particle size separation is important in studying particle behavior in a medium as in many analytical sciences and industrial applications. Size separation has the following major applications in pharmacy:
- Size separation has significance in formulating uniform dosage forms concerning drug content.
- It can be used to obtain granules of the required size.
- It helps to ensure good flowability.
- It can also be useful in separating undesirable size solids from desired ones.
- It has applications in the determination of particle size and their distribution in the given samples.
- It can also be used to investigate the efficiency and validate size reduction equipment.
- It is used to obtain monosized powders/particles which undergo the least size segregation.
Official Standards For Powder Size:
Standards for powders used in pharmaceuticals are
reported in the British Pharmacopoeia (B.P.) which states that “the degree of
coarseness or fineness of a powder is differentiated and expressed by the size
of the mesh of the sieve through which the powder can pass”. The B.P. specifies
five grades of powder and the number of the sieve through which all the
particles must pass, Table.1.
Table.1: Standards for
Powders given B.P.
Grade
of powder |
Sieve number through which all
particles must pass |
Coarse |
10 |
Moderately coarse |
22 |
Moderately fine |
44 |
Fine |
85 |
Very fine |
120 |
The B.P. specifies the use of a smaller size of sieve for the coarser powders but states that not more than 40 % shall pass through. The relevant grades of powder and sieve number are presented in Table.2. Thus, the coarse powder is that wherein all the particles pass through a No. 10 sieve and not more than 40 % through a No. 44 sieve. This is usually referred to as a 10/44 powder.
The B.P. states that when a powder is described by a
number, all particles must pass through the specified sieve and when a
vegetable drug is being ground and sifted, none must be rejected. This
classification is a must if the character of a vegetable drug is compared with
a chemical substance.
Table.2: Classification of Powders as per B.P.
Grade
of powder |
Sieve number through which all
particles must pass |
Sieve number through which not more
than 40 % of particles pass |
Coarse |
10 |
44 |
Moderately coarse |
22 |
60 |
Moderately fine |
44 |
85 |
Fine |
85 |
Not specified |
Very fine |
120 |
Not specified |
The chemical materials are generally homogeneous. If a certain quantity of powder is required, an excess may be ground to get a sufficient amount of the desired size range by sieving, and the oversize particles may be discarded. As we know vegetable drugs consist of a variety of tissues of different degrees of hardness that softer tissues are ground first and oversize mass is obtained by sifting contain a higher proportion of the harder tissues. In many cases, constituents are not distributed uniformly through vegetable tissues. For example, digitalis contains glycosides concentrated in the mid-rib and veins of the plant. Hence, if tailings are discarded while grinding and sifting the drug, it is likely that a high proportion of the active constituents may be lost.
The British Pharmaceutical Codex has given a further
grade of powder known as ultrafine powder. For these powders, it is required
that the maximum dimension of at least 90 % of the particles must be not
greater than 5 µm and none must be greater than 50 µm. Determination of
particle size for this grade is carried out by a microscopic method.
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