Modern Quarrying Q2 2019

CRUSHING

Should a high quality shape be required, an impact crusher will be necessary to get the end product into specification.

Scherf says this can depend on the rock characteristics and the plant layout. However, as a general rule, the HSI and VSI crushers offer the best shape. “Often a three-stage plant consisting of 1 x jaw crusher and 2 x cone crushers will also pro- duce a cubicle product. The type of cone crusher will have a significant effect on the results. A Metso HP crusher, for example, can produce a very cubicle product when utilised correctly, which a Symons type cone crusher, for example, cannot offer,” says Scherf. Due to the hardness and abrasive nature of most of the rock in south- ern Africa, Scherf says the VSI is the preferred option when it comes to a cubicle product, and would be used as a tertiary or quaternary crusher. “The main advantage the VSI has over the HSI crusher in this application, operating costs aside, is the consistency in the end product. As soon as you start feeding an HSI crusher you start having wear in the chamber, which means you require constant adjustment to achieve the same results,” says Scherf, adding that on the VSI, the wear on the rotor has a minimal effect on the product PSD, meaning you have a consistent product. Close partnership To come up with the best crushing solution for the application at hand, a closely knit cooperation between the supplier and the customer is of utmost significance. Dickson says it’s an absolute necessity, the client understands their challenges far bet- ter than the supplier does, through

which to lay the road. This can generally only be achieved with the introduc- tion of a vertical shaft impact (VSI) crusher as a tertiary or quaternary stage in the process,” explains Scherf. For asphalt topping (and high MPa concrete), Scherf reasons that it’s critical to have a cubicle shape to the stone rather than a flaky product. Flaky material is weak and can be broken by the pressure of large vehicles, while an exposed hole will fill with water and quickly form a pothole. “Cubicle material fits closer together and more compactly, ensuring that less asphalt (or cement) is required in production, thereby reducing costs significantly,” he says. Scherf adds that since flaky material doesn’t compact as tightly as cubicle aggregate does, one is left with voids or spaces between the material, which is then filled with tar from the asphalt. In hot climates the tar will soften, and this can cause deformation of the road surface when exposed to heavy traffic. “Cubicle product adds value to your end product, however, many construction projects don’t require high specification aggregate, so the added expense of shaping the end-product does not make financial sense,” argues Scherf. Shape in question If the shape of the stone is a major spec requirement, what crusher achieves the best product shape? JD Singleton, process director at Weir Minerals Africa, says if optimum shaped product is an obligation, the VSI crusher is the optimum shape-producing crusher. The same view is shared by Heath Dickson, national mining sales manager at ELB Equipment, who says, without a doubt, a VSI, and in some cases a horizontal shaft impact (HSI) crusher, will help shape up the product. Ravenscroft is of the view that an impact crusher will give the best quality shape to a stone. “An impact crusher reduces the amount of flaky material from the feed ROM and or flakiness produced from the jaw and cone crushers. It removes the brittle pieces from the stones, creating a more high quality cubic shape,” says Ravenscroft. Van Rensburg says it’s not easy to control the shape of the aggregate; different host rocks will break differently. “If for instance the material is flaky, then you can introduce specialised equipment like a VSI to break down the edges of the stone and make them more cubical,” he says. While most mining operations require ore to be crushed as fine as possible, Van Rensburg says this is not always the case. “Where fines must be kept to a minimum, there is need for less aggressive crushing, or the ore must be screened or scalped before crushing takes place to minimise further fines generation,” adds Van Rensburg. Pruewasser says every stone has a crystalline shape which is normally hexagonal. He says impact crushing systems crush the material along this crystalline structure because it is the easiest part of the rock to crush. “This always gives a better shape than crushing with shearing or pressing forces offered by cone and jaw crushers,” says Pruewasser.

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MODERN QUARRYING QUARTER 2 - 2019