Quarry Southern Africa May 2018 - Page 15

BUSINESS Dimension stone cuts can be done in one of two ways, the first is to have a standard and the second is to optimise the size of blocks recovered. The advantage of the standard is that blocks of uniform size will be produced, but significantly more waste material is produced with a lower block recovery / yield. The optimised grid cut will yield larger blocks and minimise waste but require secondary sizing of the blocks if demanded by the market or due to slabbing equipment limitations. The dimensions of a bench are determined by physical factors and equipment limitations. The length is restricted by the limits of accurate drilling for the intersecting holes of the diamond wire cuts. The width of the bench is determined by the limits of accurate drilling and blasting, and the height of the bench is determined by the block handling equipment. The choice of mining method in a dimension stone quarry is completely dependent on the geology of the deposit. Boulder formations will largely be quarried by means of splitting methods, most commonly using specialised blasting techniques. Solid formations may require a combination of different methodologies. Marbles, slates, sandstones and quartzites are quarried by non-explosive splitting and cutting techniques. Granite and dolerites on the other hand require blasting techniques. The main consideration in the type of blasting technique is the prominence of cleavage/ weakness planes in the rock along with minimising the amount of blast induced cracking that will reduce the quality of the saleable blocks. Conventional mining of solid formations of marble and granite make use of diamond wire sawing to loosen large benches, which are then split into successively smaller blocks until these are of a size that can be handled by the loader or crane. The blocks are removed from the face and trimmed if necessary to square it and remove any remaining defects. By optimising the mining layout and extractions methods, you can better manage the challenges imposed by the dimension stone market, such as a ‘fashionable’ product versus the commodity in the ground, the customer derived demand, price elasticity, industry structure and competing products. ■ CREDIT Bowline Professional Services: www.bowlineps.co.za. “The choice of mining method in a dimension stone quarry is completely dependent on the geology of the deposit.” QUARRY SA | MAY/JUNE 2018 _ 13