In the mineral processing plant, it is very important to control the grain size in the stage of crushing and grinding. Especially in metal mines such as iron ore processing line and gold ore processing, grain size control plays a decisive role in the subsequent stage of selection. 

A few typical granularity control schemes and their characteristics are described below.

1.Crushing circuits - open screening

• Screening ahead of a crusher avoids packing

• Less wear in the crusher

• Higher total capacity

• The screening media is “controlling” the product in two dimensions. No “flaky shortcuts”.

Fig1. crushing-open screening

2.Crushing circuits-closed screening

• The screens are lowering the capacity

• Calibration of the product is improved

• Better cubical shape

• Higher reduction ratio

Fig2.crushing-closed screening

3.Grinding circuits-screening

• Used for “trapping critical sizes” in AG - SAG circuits (1)

• Used for taking out size fractions from AG circuits for pebble grinding (1)

• Used in circuits with heavy minerals – avoiding over grinding (fine screening) (2)

• Screens being static (fixed cut point) are not too tolerant to changes in product size, causing variations in circulating loads.

• Mechanical damage or clogging of screening media can disturb operation.

Fig3. grinding circuits-screening

4.Grinding circuits-classification

• Classifiers being dynamic (floating cut point) are more tolerant to changes in product size as the cut point is moving with the changes.

• Cyclones, being most common, are effective as classifiers at cut points below 200 microns (1)

• Spiral classifiers are effective as classifiers at cut points up to 800 microns. For the coarse fraction solids up to 50mm (2”) can be removed by the spiral.

• Spiral classifiers and cyclones can be used complementary if cut point is coarser than 200 microns. (2)

Fig4. Grinding circuits-classification

5.Dry classifier system

A typical dry classifier system is shown below.

Fig5. Dry classifier system