In addition to the use of X-ray beams in sedimentation and centrifugal methods discussed earlier. X-ray absorption with two beams has been used as an on-line method for particle size analysis (Allen 1990). The method is based on the fact that only one of the two beams is sensitive to variation in particle size. A comparison between the two beams by a sensor can produce particle size data.
According to Allen (1990), an on-line stream X-ray fluorescence technique consists of two flow cells through which the slurry passes. The turbulent flow causes a mixing of slurry in cells. As a result of X-ray excitation, particles become fluorescent. The emission is measured and related to the particle size .
Materials that are opaque to X-ray are ideal for use in conjunction with this technique. However, such techniques are in general expensive and require special safety precautions due to radiation hazards.
3.3 Conclusion
In this chapter various methods of particle size analysis with different operating principles have been reviewed. Table 3.1 is a summary of the relative merits and disadvantages of the main techniques reviewed. Some of the techniques are not suitable for on-line size distribution measurement. Others are either expensive or require sample preparation prior to size analysis. The majority of particle counters are affected by electrical noise which is a general problem.
Sieving is a time consuming method and involves the risk of blinding and damaging of the apertures.
There are three limitations associated with microscopy; 'the resolution of image', 'the size of the field of view' and 'the height of the object under observation'. Light interaction methods on the other hand suffer from the saturation of fluid view by particles which in turn results in experimental error.
Sedimentation suffers from interference between the particles themselves. Gravitational sedimentation of fine particles less than 5 pm is subjected to convection and brownian motions and hence result in prolonged settling times.
Chapter 3 Literature survey on particle size measurement
Centrifugal methods overcome the above problem but suffer from other effects such as the 'concentration of the suspended phase, salvation, particle shape and electroviscosity.
The electrical sensing zone method is limited by bubble formation which in turn results in false signals. Also samples with wide range of size distribution must be wet sieved prior to test. This is another drawback of this method.
Elutriation is based on the analysis of proportion of particles carried off by a carrier gas at terminal velocity form a vertical column. Difficulties associated with preventing fine particles from adhering to the walls and the breaking up of the aggregated particles are the main limitations associated with this technique.
The impact sound generated following the collision of particles with a circular plate can be related to instantaneous particle size and flow rate. Data on particle size distribution on the other hand, may be obtained from the sound intensity and impact frequency. However, bouncing of particles and hence re-collision with the plate result in size measurement error.
The main drawback of adsorption methods is the necessity for cleaning up of the solid surface prior to particle size analysis. Also, there are some sites on solids known as dead-spaces where gas can not be adsorbed. This results in an error in surface estimation.
The main disadvantage of the cascade impactor is a consequence of the fact that impaction surfaces have to be layered with an adhesive frequently in order to avoid re-entrainment from one stage to the next. Field flow fractionation is based on the effect of a field perpendicular to the stream of suspension flowing through a thin open channel. The force due to the resulting velocity profile results in fractionation of particles amongst the streamlines and hence causes particle size elution. However, any changes in fluid dynamic properties requires the re-calibration of the system.
Gas cyclones are capable of producing particle size distribution data. However, they are mainly used as dust pre-collectors before filters in solid particulate processes. The use of this method as on-line particle size analyzer is inevitably limited due to pollution problems.
Lfi N)
M eth o d T ech n ica l p rin cip le S iz e
range(iam ) C o m m e n ts
Sieving Separation of^ fractions from sieving medium through apertures.
> 5
Slow technique, the easiest and cheapest method. Applicable for both wet and dry analysis. Risk o f blinding and damaging the apertures .
Microscopy : 1. Optical 2. Electron
statistical estimations
following direct observation o f a part o f the field o f view.
0 . 8 - 1 5 0 > 0.001
Very slow, limited to resolution o f the image, the size o f the field o f view and the height o f the object under observation. The most illustrative and sensitive technique used in particle size and shape studies.
light Interaction : 1. Extinction 2. Scattering
suspension o f particles in a transparent fluid with different refractive indexes and image formation.
1 -2 5 0 0 0 . 1 - 2 5
Very fast, accurate and no risk o f aggregation. Requires very small sample. Particles are highly affected by the suspension medium and the electrical noise. Error due to saturation o f field o f view by particles. Requires sample preparation prior to size analysis.
Sedimentation determination o f particle size from the rate o f settling in a fluid.
0.1 - 100
Very slow, sensitive to high concentration o f solids. Particle sizing depends on fluid physical properties. Suffers from interference between the particles themselves. Suitable for direct mass distribution analysis with small amounts o f sample.
Centrifugal Particle sizing in a centrifugal force from the rate o f settling in a fluid.
0.075 - 5
Fast settling time, overcomes Brownian motion problem during analysis o f fine particles. No risk o f disturbing the suspension. Affected by concentration o f the suspended phase, salvation and particle shape. Electrical sensing
zone
Conductivity o f suspended particles at low concentration in an electrolyte solution.
0.6 - 250
Fast technique, limited to the size o f the orifice and concentration o f suspensions. Requires sample preparation and uniform dispersion o f particles. Bubble formations may lead to false signals.
Elutriation Separation o f particles at terminal velocity from a fluid in an upward motion.
5 - 7 5
Slow technique, there are risks in adherence o f frne particles to the walls and the breaking up o f aggregated particles. The carrier fluid should be clean and dry. Water and air elutriators are both available.
Adsorption Adsorption o f gas onto the
surface o f particles. 0.001 - 0.1
Slow technique, highly affected by the process temperature and pressure. Requires careful sample preparation. Dead-spaces on solid surface result in an analytical error. Ideal for determination o f particle surface area. 1 .Cascade
Impactor 2.Gas Cyclone
separation o f high speed particles from a carrier gas when is diverging sharply.
0.4 - 15 1 -1 1
Fast technique if automated. Particle size detection depends on the design characteristics o f the apparatus and the gas velocity. Requires frequent cleaning and reconditioning.
n f
I
§1
a n>Table 3.1 A list of different commercially available methods for particle size analysis with a summary of their technical principles , merits and disadvantages.