Enmeshed in the World – An Overview of Sieving
Sieving is an analytical technique that has withstood the test of time due to its excellent ability to quickly provide the particle size distribution over a wide analytical range. The technique is relatively simple and cost-effective. Additionally, the sieving process inherently involves separating a body of sample into different size fractions, which may also be useful for further studies or applications. In this blog, we explore different sieve analysis options offered at Particle Technology Labs.
What is a sieve?
You may not know it but you likely use a sieve in your everyday life. When you are straining the water from pasta or simply using a salt shaker to season your food, you are utilizing a rudimentary sieve. A sieve is a mesh or lattice, typically made of a metal wire, used to separate larger granules from smaller particulates. The mesh can come in a variety of different sizes depending on the size range of the material. The sizes are called the mesh number; the larger the mesh number, the smaller the opening between the mesh. The mesh number corresponds to the number of openings within 1 linear inch on a sieve. A 100 mesh sieve would have 100 openings per inch, where each of those openings are 150 microns (more formally, micrometers or µm) in size. At Particle Technology Labs, we have sieves with openings that range from 2 inches all the way down to 20 microns.
Different sieve analysis methods: How they work and when to use them?
While sifting powdered sugar onto your cake is often done by hand, methods of agitation with higher precision are often required for analytical sieving. At Particle Technology Labs, we offer a few different methods for sieving, each utilizing a different method to move the sample through the sieve(s) in a controlled manner.
Mechanical agitation is typically applied to a stack of sieves nested together, arranging the sieve with largest openings at the top to the smallest at the bottom, with a pan underneath to catch anything finer than the smallest sieve. The sample with known weight is poured onto the topmost sieve and put onto the mechanical sieve shaker. An oscillating electromagnet generates a shaking motion to help guide the sample through the stack. Each sieve is carefully weighed with the sample retained on it. The % by mass for each size fraction is then calculated relative to the starting sample mass and reported as particle size distribution.
Mechanical agitation is best used when analyzing a larger quantity of sample with a relatively broad size distribution, such as corn meal or wood chips It is not recommended for finer particulates, due to electrostatic buildup and sample cohesion. PTL’s range of sieves available for this technique go from 2 inches all the way down to 25 µm (500 mesh).
In sonic sifting, an oscillating column of air is shot up through a stack of sieves at regular time intervals to pass the sample through the stack.
Sonic sifting is used when there are limited quantities of sample available. It is also typically used for finer sieves than mechanical agitation, as the approach is a bit more aggressive. This helps with finer particulates, as they tend to be more cohesive. The sieves available at PTL for this technique range from 2,360 µm (8 mesh) down to 20 µm (635 mesh).
Air entrainment or air jet sieving utilizes a rotating wand of air that aids in dispersing the sample through one sieve at a time. The weighed sample is poured onto the sieve and covered with a lid. The rotating column of air disperses/fluidizes the sample and allows it to fall through the sieve while a vacuum pulls down on the passed sample at an adjustable pressure. This is commonly the go-to analysis type for most pharmaceutical products that require sieving.
Air entrainment or air jet sieving is used when only the portion of sample greater than or less than one sieve is required. It is the recommended sieving technique for fine particulates due to the ability of the air flow to disperse them. PTL has sieves varying from 1,400 µm (14 mesh) down to 20 µm (635 mesh) for this technique.
Wet sieving employs the use of water to disperse the sample and pass through a sieve. Water is passed through a single or nested stack of sieves into a catch pan. The contents remaining after washing are then dried before weighing to determine a particle size distribution.
Wet sieving is typically used for samples that are already dispersed in a liquid, such as a slurry, or samples with low density that might otherwise have difficulty passing through a sieve without additional aid, such as colloidal silicon dioxide.
How long should the analysis be conducted?
The process to determine the length of time required to ensure all the sample that can pass through the sieve(s) does pass through is called endpoint determination. This is done by repeatedly sieving a sample at regular intervals until the percent mass changed on any sieve used does not change by more than a set value. The time intervals are added up to the total duration required to complete the sieve analysis.
Endpoint determination is particularly important when establishing a new sieve analysis procedure for a sample. For some materials, prolonged agitation may cause unwanted changes over time, such as a milling effect which causes the particle sizes to decrease, or moisture gain in hygroscopic materials. If these effects are observed in your materials, you may have to simply select an analysis duration for consistency, or explore other particle sizing techniques altogether.
What are some typical obstacles in sieve analysis and how to overcome them?
For samples that are very cohesive, tend to agglomerate, or have a strong electrostatic charge, a flow agent can be used to help alleviate these effects. A flow agent, such as colloidal silicon dioxide or aluminum oxide, is mixed in with the sample and will coat the particulates. As the flow aids used are typically on the nano-scale in terms of size, their presence should not impact the sizes of particulate retained on the sieve mesh. However, their mass should be accounted for in subsequent calculations.
While this simple yet powerful technique can offer many benefits, there are also limitations. For example, all approaches detailed above may not be enough to overcome cohesive forces in very small particulates. As such, alternate methods for determining particle size often need to be explored for samples smaller than 75 µm. Additionally, if the samples are elongated in shape, their orientation may affect the results. In such cases, an image analysis technique might be more appropriate for determining the particle size.
Still not sure which sieving method is right for your material? Experts at Particle Technology Labs can help you choose or guide you towards alternate particle sizing techniques. Contact us today for consultation!