Lecture 10- Particle Sizing II Flashcards
Number, area & mass distributions
-distribution used needs to be specified
-type of distribution obtained depends on the sizing method used
*sieving & sedimentation= provide the mass of material in a given size band
*coulter counter= measure the number of particles in a given band
Median
Splits the distribution in two halves; 50% of the mass/particle number larger and 50% of the mass/particle number smaller
Symbol= D50
Measures of dispersion
Particles in a sample area= all same size= MONODISPERSED
Range of particle sizes= POLYDISPERSE
Normal distributions
Interest largely= arise when an item is subject to random variation
Found in natural systems such as:
-distribution of heights of people
-distribution of tablet weights in a batch
Normal distribution + log normal distribution activity
Slides 13-17
Particle size analysis
-size range analysis
-wet/dry methods
-manual/auto methods
-speed of analysis
Sieving
-stacking the sieves in order of ascending aperture size + placing the powder on the top sieve whilst agitating it; powder is classified into fractions
Reliever is placed at the bottom to collect the fines and a lid is placed on top to prevent loss of powder
Best to use when;
-separated fraction required for further study
-material must have a good flowability
-material must be handled dry
Avoid when;
-much fine powder is present
-particles are fragile= may break during the process
-material adheres to the sieve/forms clumps
Sieving errors
Sieve may be regarded as a series of gauges which reject/pass particles. Probability that a particle will present itself at an aperture depends on;
-particle size distribution of the powder
-number of the particles on the sieve
-physical properties of the particles
-method of shaking the sieve
The size distribution given by a sieving operation depends also on the following variables:
◦ 1) Duration of sieving.
◦ 2) Variation of sieve aperture.
◦ 3) Wear.
Microscopy
-used to analyse individual particle size; particle sizes are visible therefore the operator can assess easily whether or not they are properly dispersed
-particles selected = representative of the bulk
The slide is placed on a microscope stage which is capable of movement in perpendicular directions and is examined in strips. As a particle image passes over the scale it is sized and recorded
The lognormal distribution
Particles reduced by grinding follow lognormal distribution
Particles grown by crystallisation often show a lognormal distribution of size
Electrical sensing zone
Powder is mixed into an electrolyte to make a dilute suspension.
This suspension is passed through a tiny hole (aperture) in a glass tube.
Electrodes are placed on both sides of the hole.
When a particle passes through the hole, it displaces electrolyte.
This bends the current flow around the particle → increases resistance.
Resistance change is measured using Ohm’s Law (V = IR).
The amount of resistance = related to the size (volume) of the particle.
Bigger particle → more resistance change → easier to calculate particle volume/size.
