Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/403276
OPERATIONS ILLUSTRATED AGGREGATES MANAGER Voices of Experience Bradly Estes "T ypically, a cubical aggregate product is what you want in the end," says Bradly Estes, plants manager, Western Region, for Granite Construction Co. "But sometimes certain specs, such as Washington DOT Superpave, require a minimum angularity for your fine aggregate that pushes you away from cubical." at said, Estes says that the geologic characteristics of a deposit will dictate what effort is required to make a cubical or angular product. "You look at the finished product you want to make and the aggregate material that is available, and select your crusher for each stage based on those parameters," he says. If a deposit tends to produce an end material that is flat and elon- gated, compression crushing may exacerbate the problem by creating a more angular particle. A higher reduction ratio also will produce a more angular aggregate product. "Some form of rock-on-rock crushing is usually required to make a cubical product," Estes notes. "With rock-on-rock, the aggregate will break along the natural lines and round off the weak edges for a smoother particle that is similar to natural sand." Impact crushers are the first crusher types that come to mind for rock-on-rock crushing, whether they are horizontal sha impact crushers (HSIs) or vertical sha impact crushers (VSIs). Modern, high-speed cone crushers using a fine cone liner configuration also will provide some rock-on-rock crushing , as long as they are kept choke-fed. "Most producers are familiar with a jaw-cone-cone configuration," Estes says. "And most times, a cone is the cheapest way to go over the long haul. Wear costs are low over time with the modern cones, and you do get a good particle shape. But if you're looking purely at particle shape, an autogenous (rock-on-rock) VSI is best. e modern cones still cannot produce a particle shape that is as good as what you can get from an autogenous VSI." Coming from the West Coast, Estes says that many alluvial sand and gravel deposits in his region yield pea gravel that is difficult to crush with a cone. "In order to create the high-fracture aggregate we need for asphalt, we oen need a VSI that will nick and fracture the gravel, versus a cone that has a harder time breaking it," he says. Estes offers some additional tips for achieving a desired particle shape in crushing. • A lower reduction ratio will yield a beer product shape than a larger reduction ratio in one stage. • A closed circuit allows the machine to be consistently choke fed. • Choke feeding a cone increases the rock-on-rock crushing action. • Slowing a cone crusher allows more throughput and less reduction, but produces a coarser product. • Slowing a VSI reduces the fracture count for beer product shape, while producing fewer minus-200 fines. G eological characteristics will dictate many of the con- siderations for crusher type. According to George Fensome, area manager, Americas, for Sandvik Construction's Crusher Product Line, a soer rock that has low abrasion characteristics, such as limestone, may call for a horizontal sha impact (HSI) crusher at the primary and secondary stages. "For basalt and granite, a jaw and cone may be the best choice for the primary and secondary stages," he says. When you get to the tertiary stage, especially with a harder and more abrasive material, if you want a cubical fine aggregate product, your best choice is an autogenous VSI (rock-on-rock vertical sha impact crusher). What Mother Nature achieves over millions of years to make cubical sand – through weathering, erosion, glacial deposits, and so on — a VSI crusher can ac- complish in seconds." Fensome explains that producers and manufacturers are more familiar and more comfortable with cone crushers than VSIs. VSI crushers gained some popularity in the 1990s with the advancement of asphalt pavement technology, but they also have a reputation for being high-maintenance. "It's true that a VSI will take more looking aer than a cone, but the product you get out of a VSI is beer quality," says Fensome. "As the mantle and bowl liners wear on a cone, and you continue to tighten it down, eventually you are going to get ma- terial that is out of spec. With a VSI, even when your wear parts become more worn, the product gradation never changes," he adds. Feed size and reduction ratio are critical at any stage, says Fensome. For a tertiary VSI, the maximum feed will be approx- imately 2 inches. For this reason, a VSI will never be suitable as a primary crusher. A primary jaw or HSI must reduce material quickly. e secondary crusher reduces the material even more. At the tertiary stage, refinement for shape begins to take place. "Every crusher has its ideal application dictated by physics. When you try to use a crusher outside of the boundaries it's capable of, you're going to have problems," Fensome says. He adds that every crushing chamber is designed to operate optimally at a certain speed, and VSIs are no different. "e faster your speed with a VSI, the lower your throughput and the lower your product quality," Fensome notes. "It is the rock- on-rock action that gives you the correct fracturing to produce a cubical product from rock all the way down to sand, and 51m to 55m per second is ideal for that. At 65m per second, the crusher becomes inefficient. By slowing it down and introduc- ing the proper feed, you will hit that sweet spot where the material impacts correctly." George Fensome ▼