PART 3 OF 3: MICRONIZER OR AJM (AIR JET MILL) OR A FLUID ENERGY MILL:

INTRODUCTION TO PARTICLE  CHARACTERIZATION AND SIZE DISTRIBUTION ANALYSIS (PSD): 

In order to understand the theory and methods of effective Micronization, one must understand about Particles and the Particle Size Distribution in detail.

A particle is a discrete sub portion of a substance. Although, the materials which are made up of particles include powders, granules, suspensions, emulsions, slurries, aerosols and sprays, we are limiting our discussion to powders and granules only.

It is extremely important to study and carry out routine analysis of the product Particle Characterization, as it helps yield superior control over the product properties, thereby leading to superior quality production. The properties of particles undertaken for studies often includes Particle Size, Particle Shape, Surface Properties, other Mechanical Properties and Microscopic properties. Depending on the material Or product under study, the parameters Or combination of the parameters may vary. However, the most common parameters chosen for analysis are Particle Size and Shape.

(Pic ref: This is the actual report of  PSD analysis after Jet-Milling of Guanidine Nitrate done for one of my Clients. Guanidine Nitrate is from Nitroglycerin family, hence carries Explosive characteristics).

When it comes to defining the size of a particle, one cannot always imagine any particle to be a prefect sphere, hence, ideally a 3D or a Three Dimensional approach is always better or recommended. Infact, many a times a particle size is determined with the concept of Equivalent Spheres, where, the particle size (say for example the diameter) is defined by the diameter of an equivalent sphere, which posses the same properties (as that of particle under study) like mass Or volume etc. The different measurement techniques require different equivalent sphere models, hence, may yield different Or non uniform results.  

(Pic ref: www dot malvernpanalytical dot com)

 PARTICLE SIZE DISTRIBUTION OR PSD: 

Since, it is not possible for each particle of a sample to posses the same dimensions (also known as Mono Dispersion), PSD will mean the Statistical Distribution of particles of different sizes. As a common practice, the Particle Size Distribution is represented as a Frequency Distribution Curve Or a Cumulative Distribution Curve. 

(Pic ref: www dot malvernpanalytical dot com)

The 'Mean' here can be of three types viz. 

1. Number Length Mean Or Arithmatic Mean (Important with respect to the total number of particles)

2. Surface Area Mean Or Sauter Mean Diameter (Important with respect to the total surface area)

3. Volume Moment Mean Or De Brouckere Mean Diamater (Important with respect to the size of those particles which constitute the bulk of the sample volume).

As you may be aware from the basic studies of statistics that Median is the middle most number when all the sizes are arranged in an ascending order. Similarly, a Mode is the size of the highest frequency.

WHAT IS MEANT BY 'PARTICLE CHARACTERIZATION TECHNIQUES':

This concept becomes important once we have the sample in hand to be analyzed for PSD. There are a many such characterization techniques available, and one can choose the most suitable based on questions like important properties of the particles, size range, Wet Dispersion Or Dry dispersion of the sample and budget available etc.

Based on all these parameters, the most common Techniques used are as under:

1. Laser Diffraction (uses Mie theory of light scattering and assumes Volume Equivalent Sphere model): Measurement of PSD by measuring the angular variations in the intensity of the light scattered as the laser beam passes through a dispersed sample. 

2. Dynamic Light Scattering (also known as Photon Correlation Spectroscopy Or Quasi Elastic Light Scattering): Powder Particles in fluid suspension undergo Brownian motion due to the thermally induced collisions between suspended particles and solvent particles. When illuminated with laser, the intensity of the scattered light fluctuates at a rate which is dependent on the size of the particle.

3. Automated Imaging: This technique is useful for particles of size ranging for One Micron to Several Millimeters. The analysis of the individual particle images captured from dispersed samples yields particle size, shape and other physical properties.

4. Electrophoretic Light Scattering (ELS): Here the Electrophoretic Mobility of particles in dispersion is measured. Electrophoretic mobility means the velocity of mobility of the charged particles when the dispersion is introduced to a cathod and an anode. 

SAMPLE DISPERSION:

As discussed above, a Particle Characterization Technique requires a Dispersion, whereby, the particles are spatially separated. The two most common types are dispersions include a Wet Dispersion and a Dry Dispersion. 

Incase of a Wet Dispersion, a liquid is used for creating a suspension of the powder particles in the liquid by lowering the surface energy of the particles and thus reducing the forces of attraction between particles.

Incase of a Dry Dispersion, a stream of Flowing Gas Or Dry Clean Air makes the particles suspended by various mechanisms viz. Particle to Particle Collision, Particle to Wall Collision and Velocity Gradient caused by shear stress etc. Dry Dispersion is not recommended for particles of size less than One Micron. This is because there may be very high forces of attraction between the fine particles.

 SUMMARY AND THE APPLICATION OF THE DISCUSSION FOR MICRONIZATION:

The PSD analysis and Particle Characterization Principles and Techniques must be known to the Process and Quality department, as without which they cannot deduce the Extent of Micronization including the Feed Micron Size, Final Micron Size, behavior of the parties under the influence of the turbulence and other design parameters.

Note: Please do not forget to read through Part 1 and Part 2 of this series, as it is the continuation of the discussion.