Traditionally the sand industry has not considered magnetic separation as a process alternative except in a few special applications.

The sand deposits exploited to supply the glass and ceramics industries have until recently been of low enough iron content to enable production with little or no processing of the sand. Whenever processing was required screening and attrition scrubbing techniques have been employed together with a certain amount of the froth flotation.

Now that high quality reserves are diminishing and quality specifications are becoming more strict, more processing is required to maintain quality standards. Additional environmental pressures have also reduced the attractiveness and cost effectiveness of the chemical based froth flotation process. As a result there has been a resurgence of interest in the purely physical processes of gravity and magnetic purification.

Factors Effecting Physical Processing

Of course not all sand deposits respond to physical treatment, methods and the response is dictated by the form in which the iron appears.

Generally where the iron is in the form of a discrete mineral such hematite, magnetite ilmenite chromite and biotite micas, physical separation processes work very well as the iron is in a liberated form.

Often this is not the case, with iron bearing minerals being included within the silicon particles. Much higher magnetic fields are required to remove such grains as their effective magnetic susceptibility is reduced by the extra silica mass. This type of particle is very difficult to separate using gravity means as the particle S.G. is only slightly increased. The S.G. difference between pure silica and included silica grains is therefore very small and if any separation by S.G. does occur it results in the appearance of such grains in the middlings fraction.

Similarly, with the 2 types of iron coating which appear on the surface of grains. Iron bearing clay minerals can be physically bonded to the silica surface but these bonds can easily be broken with attrition techniques.

However, silica grains with iron chemically bonded to the surface cannot be treated by physical means as such grains tend to be effectively non-magnetic or have a 2.0 S.G. difference chemical processing is the only way to purify such grains.

Magnetic Separation Techniques

Normally silica sand producers prefer to process material in the wet state to obviate the need for costly drying, although some producers process after any drying stage to achieve maximum benefit.