The basic principle of this separation method was already defined in the middle of the nineteenth century. However, a change over for the treatment of minerals has followed from the construction of proper machines i.e. airjigs and airtables as late as in the twenties and thirties of this century. The main application of this technology was subsequently found in coal processing and reached quite a great dimension in the two following decades. The number of applications of this method in mineral dressing plants decreased rapidly and has almost vanished in the fifties in favour of wet concentration procedures. It has been for a long period as long as up to the eighties that no more, consideration was taken for applying dry gravity separation in the field of mineral processing also due to the successful introduction of flotation. Due to the fact that labour cost were increasing remarkably within the past period after the end of worldwar II also the economic conditions in the agricultural sector became more though. As a result there came up a great demand for harvesting machines. Since agricultural products can only be processed by dry methods in a prevailing number, highly developed machines were introduced for the dry processing of food and feed stuff.

To derive benefit from this development, some of the machine manufacturers tried to make use of this knowledge also in the field of mineral dressing. But, limited application followed because mineral dressing experts did not take to much notice of dry gravity separation for they still claimed that this processing method is of very limited accuracy which is no longer the truth.

The future necessity of introducing dry gravity separation methods in mining and in the treatment of solid waste is evident for several reasons:

1. Dry processing technology has developed within the past few decades rapidly and as a result, separation efficiency could be improved remarkably.
2. Cost for investment and operation is comparatively lower than for wet dressing plants.
3. Due to environmental regulations which have become more rigorous this days, wet processing linked with tailing pulp to be carried off will cause increasing costs for environmental protection installations. Cost for dedusting in dry processing operations are much lower and even more important, no chemical reagents are involved.
4. Air is available at any place in the world, while water can be rare in some areas.

In order to meet the a.m. considerations, FREN has been engaged in the field of dry gravity separation for almost a decade now.

When we started test series in 1985 the applicability of dry gravity separation was limited to a grain size of some 0.3 mm. Comprehensive development work in DGS technology was being carried out by FREN to overcome this limit. Both the test operational part and the design work for new mechanical concepts lead to the construction of a modified air table working in the mode of a two way separator which can properly separate mixtures of mineral components sizing as small as in the ten micron range.

Mineral and solid waste have been successfully treated at granulometries between some 10 microns and 50 mm so far. The grades of recovery of executed tests were ranging between 85 % and 99% depending on the kind of feed material and grain size fraction.

Principle of the Advanced Two-Way DGS

The material to be separated is fed on to a vibrating air permeable deck. The deck has a slope against the vibrating transport direction. An air upstream is generated by a fan below the deck.

The air upstream causes the floating of the light components downwards the slope while the heavy particles are sinking on to the deck and are transported upwards by friction on the vibrating deck.

Conditions for a Successful Separation

As a basic condition feed material has to be as dry as it can be classified into size fractions by dry screening.

The frequency of the vibration has great influence on the efficiency of DGS.

Throughput capacity is a strict function of the machine parameters and is also dependent on the size fraction of the feed.

The grade of recovery is a function of the grade of liberation of course. The size range for the feed will be specified by the density difference of the components.

Another influence of the separation results i given by the shape of the component’s grains.

Case Histories

On reclamation of chromite from foundry sands quartz/feldspar separation and concentrating gold bearing pyrite will be given at the paper.

Prospects

FREN is being involved permanently in the further development of DGS technology with regard to a variety of minerals and solid industrial wastes. This will keep FREN a step ahead of the competitors.

Hence, we make continuous effort to extend the field of application of DGS.

A remarkable reduction of cost for waste disposal and an alliviation of the waste deposit problems is the goal of our activity in DGS.