The Open Circuit and Closed Circuit Grinding Process

In many machines, the feed material is reduced to a satisfactory size by passing it once through the machine. If the material is passed only once through the machine (crushing or grinding), and no attempt is made to return the oversized material to it for further reduction, the process is known as open-circuit grinding.

If the partially ground material from the machine is sent to a size separation unit, from where the undersize is withdrawn as the product and the oversize material is returned to the machine for reground, the process is known as closed-circuit grinding.

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In the case of coarse particles, the size separation unit is a screen or grizzly while it is some form of classifier in the case of fine powers. Closed-circuit grinding through useful for any crusher, it is commonly employed for machine-yielding fine products. 

The size separation unit is sometimes incorporated inside a machine (ultrafine grinder) but it is common practice to have a such unit outside the size reduction equipment. A flow sheet for typical closed-circuit grinding consisting of a set of size reduction machines and separators is shown in the diagram.

The product from a crusher is screened into three fractions 

Oversize, intermediate, and fines. The oversize is returned back to the crusher. The intermediate particles are fed to a rod mill where they are reduced in size and are then fed to a ball mill along with the fines from a size separation unit.

The ball mill shown in the upper diagram runs wet i.e. the stream of water is introduced in the to mill with the feed material to carry the product to a Dorr classifier, which furnisher a simple method for separating oversize and undersize. The oversize obtained as sludge from the classifier is repulped with more water and returned back to the ball mill. 

Closed-circuit grinding requires less energy as compared to open-circuit grinding and is of the most value in reduction to fine and ultrafine sizes.

The equipment which is used in industry are 

1. Jaw crusher used in the Cement industry.

2. Ball Mill used in the Paint industry.

3. Ultrafine grinders in used Cosmetic and Pharmaceutical industries.

4. Cutters in the used leather Tanning industry.

5. Hammer Mill in the use Food industry.

Size reduction operation is carried out in coal washeries, ore processing, the cement industry, the paint industry, the chemical industry, and the food processing industry.

All Size Reduction Equipment feed size to product size 

1. Blake Jaw Crushers in feed size is 1500-40mm and product size is 50-5mm.

2. Gyratory crusher in feed size is 1500 to 40mm and product size is 50 to 5 mm.

3. Crushing rolls in feed size 50 to 5mm and product size 5 to 0.1 mm (200 mesh).

4. Hammer mill in feed size 50 to 5 mm and product size 5 to 0.1 mm.

5. Ball Mill in feed size 5 to 2 mm and product size 0.1 mm (about 200 mesh size) 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane