All minerals are classified into polar or non-polar types according to their surface characteristics. The surfaces of non-polar minerals are characterised by relatively weak molecular bonds. The minerals are composed of covalent molecules held together by van der Waals forces, and the non-polar surfaces do not readily attach to the water dipoles, and in consequence are hydrophobic. Minerals of this type, such as graphite, sulphur, molybdenite, diamond, coal, and talc, thus have high natural floatabilities with contact angles between 60 and 90 . Although it is possible to float these minerals without the aid of chemical agents, it is universal to increase their hydrophobicity by the addition of hydrocarbon oils or frothing agents. Creosote, for example, is widely used to increase the floatability of coal. Use is made of the natural hydrophobicity of diamond in grease tabling, a classical method of diamond recovery which is still used in some plants. The pre-concentrated diamond ore slurry is passed over inclined vibrating tables, which are covered in a thick layer of petroleum grease. The diamonds become embedded in the grease because of their water-repellency, while the water-wetted gangue particles are washed off the table. The grease is skimmed off the table either periodically or continuously, and placed in perforated pots (Figure 12.3), which are immersed in boiling water. The grease melts, and runs out through the perforations, and is collected and re-used, while the pot containing the diamonds is transported to the diamond-sorting section.

Figure 12.3: Diamond...