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Particle size distribution (Granulometry)

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Dry Sieve Analysis of Granular Products

Introduction

The granule size of a granular formulation is required to be determined for regulatory purposes. The granule size is expressed as a range of two sizes of sieve (x1 and x2) where residue levels (ie proportion not passing the sieve) of ≥ 90% and ≤ 10% are attained.

Method

The procedure detailed in CIPAC MT 170 was followed, using the following range of sieve screen sizes:

·       2800 µm

·       2000 µm

·       1000 µm

·       500 µm

·       250 µm

·       125 µm

·        75 µm

 

 


At the beginning of the method it states that the required test substance amount is calculated as follows:

m=nx D x 20 g

 

Where:

m= mass required (g)

n= number of sieves (1 in this case as each sieve was assessed separately)

D = tap density (g/mL)

 

This gave a value for test substance amount of 7.43 g.

 

However the last section of the method states that 10 g of test substance should be used.

 

Due to the two different values being given for required test substance amount in the CIPAC method and in order to ensure completeness the analysis was carried out twice. Once with approximately 10 g of test substance being used for each sieve and once with approximately 7.43 g of test substance being used for each sieve.

 

The first testing carried out using 10g used all the sieve sizes quoted. However when the test was repeated with the 7.43 g only the sieves from the smallest that allowed all the test substance to pass through it (500 µm) and downwards in size were used.

 

Starting with the largest screen sieve first the sieve was placed onto a receiver pan of known weight. The required amount of test substance of specified weight (m) was accurately weighed out and transferred onto the sieve. A lid was fitted to the sieve and the sieve assembly was then lifted and dropped a distance of 1 cm onto a rubber mat a total of 25 times using the assembly described in CIPAC method MT 170. The weight of the test substance that had collected in the receiver pan (dx) was then determined and recorded. This procedure was then repeated with each of the other sieves.

The residue remaining on each sieve was then calculated as follows:

Where: m

dx= weight of test substance passing through sieve into receiver pan (g)
m= weight of test substance originally taken (g)

The range of granule size was then determined from the first two sieve sizes where residue was ≥90% and ≤10%.