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Wet Sieving
While it’s recommended to sieve dry materials whenever possible, sometimes dry sieving can’t produce an acceptable degree of separation between the individual fractions.
Wet sieving is a procedure used to evaluate particle size distribution of a granular material, or aid in particle size analysis by removing fines that may impede the separation process.
The wet sieving procedure applies to solids that are practically insoluble in water, not affected by water; for example: solids that expand when wet would not work, or material that remains unchanged by heat up to 110°C.
(Wet Sieving Methods)
There are a number of methods that can be used when performing wet sieving. The procedure you decide to use is based on your end goal. (Tip: Determine total dry sample weight before beginning the process because you’ll need this weight for final calculation.)
Particle Size Analysis Method involves the operator introducing water manually or through a fixture at the top of the sieve stack using a special bottom pan fitted with a drain and tubing to carry waste water away, and both agitation and separation results from the mechanical action of a sieve shaker. (Tip: Soak the sample if time allows to make the process easier.)
Conventional ASTM or ISO Test Sieves can be used and are often fitted with Sieve Seal Gaskets to control leakage. Procedures are similar to conventional dry sieving, just messier. Use caution to prevent material loss when rinsing the individual fractions into containers for drying and subsequent weighing.
Also consider using dedicated wet sieving equipment, some with vibration and vacuum systems, as well as waste water recovery and recycling. (Tip: Add Sodium Hexametaphosphate or an anti-flocculent such as Calgon to the soaking solution to break down clumps and disperse particles.)
Wet-Wash Preparation Method involves agitating the sample as it is sprayed with water or while it is in a water suspension. The agitation is often done by hand using a Wet Wash Sieve under water running from a faucet or dispersed through a spray fixture. (Tip: Use nozzles or sprayers to disperse water and evenly wet the sample during sieving or agitation) Mesh sizes for these sieves are selected based on application requirements for particle sizes to retain. #200 (75µm) and #325 (45µm) are examples of popular opening sizes and optional backing cloth reinforces the mesh for longer service life.
A variation of this Wet Wash method is to place the entire sample in a pan, cover with water, then agitate by hand. (Tip: Use gentle, controlled water pressure to prevent accidental sample loss) The waste water is decanted over a wet wash sieve and more water is added to the pan, continuing the process until the waste water is relatively clear.
The Aggregate Washer device mechanizes this process, saving time and increasing repeatability. Runoff can be filtered with a wet-wash or standard sieve positioned near a drain or fitted with a drain pan.
An Elutriation Sieve is a hollow plastic cylinder with large ports in the sides and covered with stainless steel mesh. A sample is placed in the cylinder and water and compressed air are introduced through tubes to gently agitate the sample and wash away fines.
For rough separations or sample preparation, a Wet/Dry Sieve Vibrator can hold one or two sieves over a bucket while being washed.
Once wet sieving is complete, the remaining sample is placed in a pan and placed in to an oven for drying to the separation continues process as a dry sieving test.
It’s important to be aware that any wet sieving method has the potential for sample loss during the process. Some material may be washed away during agitation or decanting, or may be forced into crevices of the sieve and become trapped. (The percentage loss is very small and the accuracy and efficiency of wet sieving compared to dry sieving is worth the process if necessary for your application.)
For any questions concerning wet sieving samples or for any Gilson product, please contact the Gilson Technical Support team.
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