Mineral classification

1. Introduction

After initial liberation of a mineral constituent from its ore by crushing, grinding and screening, separation of minerals by size is normally attempted by a classifying process. In mineral processing operations, classification and separation of mixtures of fine and coarse particles and also of lighter and heavier particles may be performed in a wet or dry state. The majority of separations are carried out in a liquid environment because of an increased efficiency. The basic technique employed is to allow particles to settle under gravity in a liquid medium (usually water). The higher terminal velocity of irregular shaped, coarser, heavier particles allows these particles to reach the bottom of the vessel at a faster rate compared to particles that are smaller and lighter. Removing the settled particles while the others are still settling offers a simple means of a separation. For very small particles, such as clay or silt, whose size approaches colloidal dimensions, long times are required to settle and the small difference in settling rates of these fine particles leads to low separation efficiency. To accelerate the settling rate of these fine particles, centrifugal forces are employed such as in cyclones or hydrocyclones.
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What is hydrocyclone?

Hydrocyclones have become one of the most important and widely used classifiers in the mineral processing industry. They are also used for de-sliming, de-watering, de-gritting and thickening processes. They are most commonly employed in closed circuit within grinding circuits and are used to return coarse material back to the ball or rod mill for further grinding. The main advantages of cyclones are that they have large capacities relative to their size and can separate at finer sizes than most other screening and classification equipment.

The separation mechanism in hydrocyclones relies on centrifugal force to accelerate the settling of particles. The slurry enters the cylindrical section tangentially above a conical section. The velocity of the slurry increases as it follows a downward helical path from the inlet area to the smaller diameter underflow end. As the slurry flows along this path, centrifugal forces cause the larger and denser particles to migrate to the fluid layer nearest the wall of the cone. Meanwhile, the finer or lower specific gravity particles remain in, migrate to, or are displaced toward the center axis of the cone. As the swirling slurry approaches the underflow tip, smaller and lighter material closer to the center reverses its axial direction and follows a smaller diameter rotating path back toward the top overflow discharge pipe.