Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/403276
Crushing For Cubicity OPERAT M ost crushed stone and sand is produced for use in concrete and as- phalt, and there is gen- eral agreement that a cubical stone is ideal for use in these products. " e more elongated and fl aky the material is, the more water, cement or binder you require," says George Fensome, area manager, Americas, for Sandvik Construction's crusher product line. "A cubical aggregate allows you to reduce the amount of cement and water, while still produc- ing a strong concrete product, so it is be er. e benefi t is similar for asphalt," he adds. According to Bradly Estes, plants manager, Western Region, for Gran- ite Construction Co., natural sand is cubical in nature. W hen it comes to crushing aggregate to form a cubical particle, the geologic makeup, crush- ing stage, and type of crusher all play a part in producing the desired end product, he says. For instance, in a sedimentary de- posit such as limestone, the strength of some rock is lower in one direc- tion than in another due to mineral cleavage and other weak layers that occur in the parallel planes. Rocks with these characteristics tend to become fl at and elongated when crushed — especially with compres- sion crushing. "Impact crushing, more specifi cally with rock-on-rock action, helps such stone to break along its natural lines, rounding off weak edges and sharp angles so that the particle more closely resembles natural sand," notes Estes. e goal with primary crushing is to reduce the particle size. Some re- fi ning of shape may take place in the secondary stage, but most particle refi nement for shape takes place in the tertiary and even the quaternary stages. "Many operators will try to crush with only two stages, but you usually need at least three stages," Fensome says, explaining that feed size plays into this. A primary jaw or impact crusher may take a feed with a top size of 39 inches. e second- ary crusher could take an 8-inch to 10-inch feed. e top size feed for the tertiary crusher may be 4 inches, but typically it is 2 inches. "If you have a jaw, and you're going for a 2:1 or 3:1 ratio, a 36-inch rock suddenly goes to a minimum of 12 inches," Fensome continues. " at means you're feeding a 12-inch rock at the secondary stage to a crusher that may accept a top size of only 8 inches because you're trying to reduce it too quickly. "People o en try to save a penny at the start of the process, and it ends up costing them a dollar more in the end. You can't cut corners," he adds. AGGREGATES MANAGER Typically, when crushing aggregates for particle shape, the desired output is a cubical product. In concrete production, a cubical ag- gregate product requires less water and cement than a fl at and elongated product. Similarly, with asphalt production, cubical aggre- gate requires less asphalt binder. One difference is production of ag- gregate for Superpave, which does specify a more angular product. It has been argued that rock-on-rock crushing produces the most cubical product. The graph shows the ideal crusher type for aggregate refi nement, based on hardness and abrasion. HSIs and shoe-and-anvil vertical shaft impact crushers (VSIs) are preferred for softer, less abrasive stone. Autogenous VSIs and cone crushers are better with more abrasive, harder stone. Ultimately, for product shape, an autogenous VSI will produce the best quality product. 1 The quest for cubicity 4 More about crusher choices