The success of the concept had seen 25 successful installations in Europe, China, India and Australia, he said. And last month, the first units ordered in South Africa were due to be commissioned at BHP Billiton's Optimum colliery.

"A relatively compact processing unit weighing only 47 tonnes (empty weight), the unit has minimal space requirements and as such entails a low capital investment," Kottmann said.

The benefits were many: cost-efficient production of steam coal, savings in slurry handling, lower ash content, improved homogenisation, reduced wear and greater efficiency in handling capacity.

The process itself is simple. Coal has a lower specific gravity than rock, so in a moving fluid medium it floats on top of the denser waste product. By repeated mechanical lifting and lowering (jigging) of ROM (run-of-mine) coal crushed to a size of between 50-400mm, moving in a water bath and with subsequent free sedimentation of the loosened bulk, accurate division into layers according to density can be achieved. A weir scalper positioned ahead of the jig discharge therefore takes out virtually all waste material.

The ROMJIG has a moving frame, supported at one end, that carries a screen plate with slots. The frame is driven by a reciprocal pivoted hydraulic mechanism, which, after reaching maximum stroke displacement, drops by gravity through the water to its position. A hydraulic shock absorber takes up the downstroke energy.

Raw ROM coal passing over the jig is stratified by density due to the strokes of the screen plate. Transport of the coal is due to the inclination of the screen, and water flow through the ROMJIG.

Kottmann said unlike other processes, ROMJIGs were water and energy efficient. Only a small volume of make-up water was needed to compensate for the water drained while removing coal and discard.

In one of the first major tests of the ROMJIG, undertaken by Humboldt in conjunction with mine management and various tertiary institutions, at the Emil Mayerisch colliery at Aldenhoven in Germany, consistent 90-95% discharge of refuse was reportedly achieved.

"There was an overall reduction in the stone handled and advantages resulting from the lower percentage of refuse in the washery feed such as reduced wear on machinery and transporting equipment, less grain degradation, less dust, less slurry and reduced consumption on flocculation and flotation agents are still being calculated," Kottmann said.

That first successful introduction and practical application of a movable screen jig for primary cleaning of lumpy ROM coal had opened a wide field of future applications which was now being realised, he said.

"In fact, it wouldn't be out of line at all to say that the ROMJIG, like all the best advances in process technology, has taken a simple concept and refined it through good engineering to achieve better levels of productivity and set a new benchmark in coal beneficiation."