In past ramblings on particle size analysis we have touched on shakers, ASTM standards, sieve checking, separating small particles, inhibitors like static charges and how to get sieve tests done. A couple of things that we rarely addressed.
* Why the heck do I need to do particle size analysis anyway?
* If I need to check particle size, what is the best way?
Where Does Particle Size Matter? (Lots of Places)
When you deal in individual day to day sieve testing issues, you don’t always see the big picture of where and why we need to test for patrice size distribution.
For example when we look at a very course and rough concrete sidewalk, do we often ask, “How Come”? Part of the reason was probably a bad distribution of gravel pieces in the concrete mix.
Here are few more particle size related issues:- Why does the soap powder flow easily out of the box? This time the particle sizes were right.
- How smooth is the instrument case, that was powder painted?
- How long does the Kool Aid Mix stay in suspension?
- How efficient is the asthma inhaler?
- How long does it take for the tablet to dissolve?
Just a small sample of every day experiences that are influenced by particle size. I’m sure you can add many more to this list of applications.
Analysis by Mechanical Methods (Sieving)
To get product design details and quality control right, for these types of product, we need to measure particle size. How do we determine this and figure out the distribution of different particles, i.e. big, medium, small
One way to start thinking about this is to visualize the design and construction of a rock garden. Part of the task is to design for a mix of different sized stones. In selecting the stones at the RO
CK store, you can organize them by size and size categories. How many size categories do you have? How many stones in each? What happens to the look of your garden if you add a lot more large stones? Or if you add a lot of small stones? You probably cut and try and change change the mix of size categories until you have the ideal look. For this single project you can count and categorize the stones by hand.
If have large amount of this stuff you need a better way to count.
Suppose you get a load of mixed size stones. Sieves could be good way separate the stones into size categories.
This is how the separation would work. You’ve’ve decided that you want to separate the stones into four categories. To
do this you set-up a stack of four sieves and a collection pan. Starting at the top, each sieve down has smaller openings than the one above.
After loading a sample of stones into the top sieve, we shake the stack.The shaking should continue until the smallest sized stones have all fallen into the collection pan.
When the shaking is finished, you count the stones left in each sieve. This gives you the number of stones that are larger than the holes in the top sieve, like wise what’s left in the second sieve are the larger than the holes in the second sieve but smaller than the holes in the first sieve. This continues for each sieve until you count the number of particle in the collection pan - Smaller than the fourth sieve.
Limits of Mechanical Analysis (How Small Can You Go)
For the terrarium example you keep changing the mix until you get the distribution that looks best. You set this based on the sieve tests you did.
For convenience of measuring, most sieve analysis formulas use the weight retained in each sieve to represent the proportion of the total for each particle size category.
For many materials, getting a good representative result for sieve analysis is easy .
However, when a sample has particles that stick together (agglomerate), or are statically charged, special shaking techniques, such as extra whacking, wet sieving or ultrasonic nudging are needed.
Getting good particle size distribution for very small particles (sizes smaller than around 20 to 100 Microns) may require more than a shaker with a strong tapping motion. To get good particle separation, at these small sizes, vacuum or ultrasonic techniques may work. Depending on the material, the use of electro formed sieve media and ultra sonic enhanced instruments can work down to the 2 micron level.
The Low End (Unimaginable Small Sizes)
Several other methods are commonly used to measure really fine stuff: Optical Imaging, Laser Light Diffraction, Gravitational Sedimentation, electron microscopy and Xray Analysis are some of these techniques.
Computer enhanced image analysis can get down to the one micron (1µ) range. It will also help with particle counting and evaluating particle shape.
Nanometers, so small it’s hard to visualize. Get these into suspension and shine a laser through it. The laser will diffract, or bounce off the particles and give a size distribution pattern. Laser Diffraction can work down to the 100 nanometer range.
For a range of these ultra small particle to around around 20 nanometers, a technique called Sedimentation can be used. This measures the time for different particles to descend through a solution.
Getting down to the unbelievable size of 1 nanometer meter, detection moves into highly complex (and costly) processes such as electron microscopy and Xray analysis
In a Nutshell (Particle Size is a Big Deal)
Particle size distribution affects the efficacy of many, many products. It covers a range in size from five Inch stones to sizes of less than a nanometer. The instrumentation to do these measurements is a rich assortment of techniques from simple to exotic.
The focus of our work is in the mechanical techniques of particle distribution analysis (about 20µ to 125 millimeters). To keep a perspective of the broad scope of particle size analysis It is good for us to sometimes visit the work being done to analyze particle size distribution in the sub micron arena.
Hope this ramble is useful. Please share it with colleagues who you think will have an interest.
Art
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P.P.S. Here is a great video about the world of quiet sieve shakers.
