Sieving: Working, Construction, Types, Standard, Conversion and Application

The sieving technique is used widely to separate fine powder from coarse powder by utilizing sieves of specific sizes. By passing the powdered material through a sieve, we can determine the level of separation. Sieves are numbered for easy distinction between them.

Working & Construction

Size separation of powder occurs as the powdered material passes through a series of sieves. These sieves are arranged in descending order, with the larger ones positioned at the top and the smallest at the bottom. The receiving opening port is attached to the bottom sieve. Initially, the material is loaded onto the upper sieve. Mechanical sieve shakers or electromagnetic devices are employed to agitate the sieves, facilitating the passage of particles through them. In Pharmaceuticals Sives are made of stainless steel, (Grade SS 316). Choosing a welding type is also important in pharmaceuticals because it can lead to contamination and the chances of adding traces in materials becomes high. So to avoid this Argon welding is preferred to attach joints in sieves.

In the pharmaceutical industry, generally, one sieve is used at a time to separate coarse powder. Sieves are selected based on their numbers to obtain the desired powder size.

What is Sieve Number?

The sieve Number/mesh size denotes the number of holes present per linear inch inside a sieve. for example, A sieve is called 12 Number, if 12 holes/ meshes are present per linear inch.

Learn: How to check Sieve mesh size?

Types of Sieving:

In Pharmaceuticals Generally, two types of sieve are used:

1. Safety Screening: These types of sieves are used to remove the contamination, or any foreign particle present in the powder, contamination may be from the worker wearing PPE equipment. It overall helps to get good quality products.

2. Grading: Grading is the system used to separate the powder as per its required size. These are used to separate only oversized and undersized powder particles.

Other Sieve commonly used in pharmaceutical, and other industries based on their MOC and processing are:

1. Woven wire Sieves
2. Bolting cloth Sieves
3. Bar Screen
4. Punched plates

1. Woven Wire Sieves: These are general purpose sieves and are widely used in pharmaceutical industries. It is of two types i) Plain weave and ii) Twilled weave.

2. Bolting cloth Sieves: These are made of silk, Nylon, and cotton cloth. Nylon cloths generally have micrometer openings used for the separation of fine powders.

3. Bar Screen: It is used in handling large and heavy materials. These bars are fixed in parallel positions and held by crossbars and spacers.

4. Punched Plates: These types of sieves is used for Coarse sizing, Screens are prepared by using sheet metal of various thickness with perforated holes. The hole may be round, oval, square, or rectangular.

Different Sieving Techniques/ Sieving Mechanism

Manual: Manual method is employed by using hands to get desired powder size.

Ultrasonic Deblinding System: While a standard sieve can quickly and accurately screen most powders, certain pharmaceutical powders may pose challenges due to stickiness or irregular-shaped particles. Such issues can lead to mesh-blinding problems. To solve this ultrasonic vibration of the stainless steel mesh wires in a powder screening machine is used. High-frequency, low-amplitude vibrations effectively remove the risk of apertures getting blocked.

As this technique helps to reduce the surface tension, Rendering the stainless steel wires friction-free. As a result, particles, whether slightly larger or smaller than the mesh, are prevented from causing blinding or obstruction.

Standards used for Sieve:

In Pharmaceuticals mainly two standards are used for example:

• U.S Standard
• Tyler Standard

U.S standard: it is based on the opening of 1mm. It means, Sieve mesh is called 20 mesh if per 1mm area total count hole present is 20.

In Indian Pharmaceuticals Tyler standard is used. Tyler standard is the ratio of width opening in a successive sieve is √2. The Tyler standard is based on the size of the opening in wire cloth having 80 openings per linear inch is called 80 mesh size.

Conversion Table from Mesh to Micron:

The conversion table for Mesh to Micron, inch is as Follows:

Mesh Size	Approximate Opening Size (inches)	Opening Size (microns)
3	0.265	6730
4	0.187	4760
5	0.157	4000
6	0.132	3360
7	0.111	2830
8	0.0937	2380
10	0.0787	2000
12	0.0661	1680
14	0.0555	1410
16	0.0469	1190
18	0.0394	1000
20	0.0331	841
25	0.0278	707
30	0.0234	595
35	0.0197	500
40	0.0165	400
45	0.0138	354
50	0.0117	297
60	0.0098	250
70	0.0083	210
80	0.007	177
100	0.0059	149
120	0.0049	125
140	0.0041	105
170	0.0035	88
200	0.0029	74
230	0.0024	63
270	0.0021	53
325	0.0017	44
400	0.0015	37

Conversion Table for Mesh to opening in mm:

Mesh size	Opening size in (mm)
3	6.730
4	4.750
5	4.000
6	3.350
7	2.830
8	2.360
10	2.000
12	1.700
14	1.400
16	1.180
18	1.000
20	0.850
25	0.710
30	0.600
35	0.500
40	0.425
45	0.355
50	0.300
60	0.250
70	0.212
80	0.180
100	0.150
120	0.125
140	0.106
170	0.090
200	0.075
230	0.063
270	0.053
325	0.045
400	0.038

Troubleshooting in Sieving Process:

In the granulation section of the pharmaceutical industry, During the sieving process Sometimes, powders create a static charge, probably while in vibration mode. This static charge can be dangerous because it can cause the powder to burn. The burning of the powder can be a serious problem as it can lead to fires or explosions. Therefore, it is important to take precautions and control static charge during the sieving process to ensure safety in the pharmaceutical industry.

Advantages:

• Cost-effective
• User-friendly

Disadvantages:

• Possibility of sieve clogging if the powder is inadequately dried
• Attrition may occur during the shaking process.

FAQs on Sieving