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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Separation and Concentration Techniques

The separation and concentration of the valuable mineral can take place after the ore is crushed, ground, and classified into the required particle size distribution. There a number of different techniques are employed in concentrating the valuable minerals. These techniques exploit differences in physical or chemical properties of the valuable and gangue minerals.

Basically, there are four kinds of separation and concentration techniques:

i. Sorting – based on appearance, colour, texture, optical properties and radioactivity

ii. Gravity and Dense-Medium Separation – Separation based on specific gravity of the valuable mineral relative to the gangue and the carrying medium such as water. In dense-medium separation, the a carrying medium is a mixture of water, magnetite, or ferrosilicon. The paramagnetic properties of the medium allow it to either remain in suspension at a predetermined slurry density or to be separated from water for cleaning and reuse.

iii. Magnetic Separation – separation based upon natural or induced differences in magnetic susceptibility of the minerals within the ore.

iv. Froth Flotation – separations based on the surface chemistry properties of a mineral. The natural or modified surface property of the mineral determines its ability to attach to an air bubble and float to the surface.