Modern Quarrying Q1 2019

It is crucial to design and manufacture crushing plants with appropriately sized and staged equipment.

to 10:1. They are more acceptable to larger feed sizes than cone crushers, but still giving an output size similar to cone crushers. As far as the vertical shaft impactor (VSI) crusher is concerned, Scherf argues that this is the most misunderstood of all crushers and often called a ‘shaping’ crusher. He reasons that the VSI is an impact crusher that uses a rotor to accelerate material and sling it against a rock wall built around the rotor. The VSI is a critical part in obtaining high specification road stone, or as a pre-milling crusher to increase mill throughput in mining applications. It is often used in sand making applications, especially in applications such as silica sand and other abrasive feeds where the operating cost of a HSI would be too high. Geology matters Is geology a factor that determines the type of crusher to be deployed in an application? Singleton says crush- ers will work in most applications, and it only depends on how long it will last. “By doing proper applications and process engineering when you select the crusher and consider the duty, as well as the ore characteristics like solid specific gravity, abrasion index, bond work index, among other parameters, one will be able to achieve the required output no matter the geology,” he says. Dickson says the end result the client is looking for would determine

what crusher gets deployed for the job. “Ideally you would not want to necessarily deploy an HSI into a high silica application, but if the client is looking tomaximise the fines generation and understands the wear costs or rand per ton cost, this could be an option,” says Dickson, adding that capex vs tonnage can be a major factor when deciding what crusher to deploy. Ravenscroft is of the view that knowing the geology of the material is very important as materials react differently when under pressure or impact in the crushing chambers. “Knowing the geology of the material assists in specifying the correct crusher for the specific rock application. Further to this, knowing the product size distribution of the ROM allows the crusher to be spec’d correctly. Should there be a high fraction of fines in the feed ROM, pre- screens can be fitted to the jaw, cone and impact crushers,” he says. According to Scherf, in order to operate economically and safely, having the correct crusher for feed material is critical. For instance, in a coal application, using a cone crusher carries a very high risk of igniting the coal, due to the compression of the material in a small chamber. He is of the view that using an HSI on a very hard granite or basalt can result in the hammers breaking at worst, or very high operating costs at best. “Knowing the silica content or abrasion index of the feed material is critical in selecting not only the

hard or abrasive feeds,” says Scherf. Ravenscroft says a cone crusher is classified as a secondary and/or tertiary crusher. “It further reduces the size of the rock at a 4/5:1 ratio and supplying desired end product sizes. Cone crushers can also supply a quality shaped product by using the crusher within its desired parameters and crushing ratios,” he says. When it comes to horizontal shaft impactors (HSI), Dickson says as long as the client understands the wear costs and ultimately the rand per ton cost to produce, there is a place for this type of crusher in most applications, especially demolition, recycling of asphalt and applications where the client needs high reduction of material. Scherf says while it can be used as a primary crusher in certain applications, a horizontal shaft impactor comes to the fore as a secondary or tertiary crusher in softer, non-abrasive rocks such as limestone. “As the name implies, it crushes using the impact of the feed material against metal hammers. Perhaps the most versatile of all the crusher types, offering the best in terms of reduction ratio and producing a cubicle product, the operating cost in abrasive or hard feed materials limits its practical applications,” says Scherf. Ravenscroft says impact crushers can be classified as primary/ secondary crushers or secondary/ tertiary crushers. He adds that they have a higher crushing ratio of up

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QUARTER 1 - 2019 MODERN QUARRYING