Capital Equipment News

In Part 1 of our crushing-focused series of articles, we alluded to the substance of optimum crushing in the whole aggregate production equation. In this feature, our panel of experts further delves into product quality parameters and the significance of close cooperation between the supplier and the customer in order to come up with the best crushing solution for the job at hand, writes Munesu Shoko.

Terex addressing quality parameters in crushing

In the previous article, the major talking point was that, to execute the crushing process cost-effectively, it’s always important to deploy the correct crusher for the ideal application. This not only saves you time, but it will also save you money in the long run. The article also looked at whether geology was a major factor that determines the type of crusher to be deployed in an application. The focus now turns on how quality parameters influence the crusher to be deployed for a job at hand.

Quality parameters in aggregate production are normally based on strength, size and shape of the end product. How does the end result help determine what type of crusher is needed? Johann Pruewasser, engineer at Keestrack, says to determine the most economical crusher for the job at hand, project considerations always have to start at the end. “This means that all tooling deliberations have to be informed by the quality demand of the final product, and then go step by step from the end of the process to the primary crushing stage,” says Pruewasser.

Dewald Janse van Rensburg, MD of B&E International, says the final product requirement is the cornerstone of equipment selection and plant design, and certain combinations of jaw crushers and secondary crushers have proved to be more effective in achieving certain outcomes. “Customer requirements will also specify the grading envelope that must be achieved, which is the ratio between the coarser and finer aggregates. For any operation, the intended throughput rate is another key consideration in the choice of crushers and their sizes, and with commercial operations the market will dictate the level of demand to be satisfied, or the draw-off requirement for a specific project,” says Van Rensburg.

Tyron Ravenscroft, Finlay product manager at Bell Equipment, says the strength of the material determines the crushing ratios acceptable to that material and how many crushers will be required to crush the run of mine (ROM) to the desired product sizes. “The size of the ROM will determine the size of the primary crusher to accept the ROM. The ROM size will also determine the number of crushers required to produce the desired product sizes in the acceptable crushing ratios,” says Ravenscroft.

“The desired product shape will determine the type of crushers required. Should a high quality shape be required, an impact crusher will be necessary to get the end product into specification,” adds Ravenscroft.

More quality considerations

Sandro Scherf, CEO of Pilot Crushtec International, says one critical consideration is the end application of the aggregate produced. Some basic examples where shape and size or product size distribution (PSD) is critical to include are filter sands, road construction and asphalt topping, to mention a few. “Filter sands require very strict product curves. With too much fine material, the filter sand turns to clay, which doesn’t allow water to flow through. With too little fine material, the water passes through too easily and particles aren’t filtered out,” explains Scherf.

For road base construction, Scherf says the Colto specs on G1 and G2 base and sub-base material require at least 4-12% of 0,075 mm material, all produced from clean parent rock. “The micro fines basically act as a lubricant when compacting the road-base to ensure that you get the correct compaction levels and have a sturdy base on which to lay the road. This can generally only be achieved with the introduction 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.

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 produce 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 southern 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 better than the supplier does, through their experience from working the site. “The supplier’s main function is to make the client aware of the crushing technology available to meet the client’s production needs,” he says.

Singleton says as experts in crushing applications, flowsheet design and crusher maintenance, Weir Minerals Africa understands that uptime and maximum production are the primary drivers for an aggregate client’s profitability. “As a result, we believe that long-term partnerships with continuous support, as well as spares and service agreements, are the only way to run a sustainable business,” says Singleton.

“At Weir Minerals Africa, we understand that the more we help our customers become profitable, the more relevant we are. We do this by maintaining our machines and collaborating with clients and continuously finding innovative ways to increase their production, using their existing infrastructure,” adds Singleton.

Van Rensburg says close collaboration with the customer is always essential. “For a start, it allows the supplier to understand the application in detail and to design the right solution; but it is also important to ensure that the customer receives the support they need for smooth operations and lowest cost per ton,” he says.

Pruewasser says only if the supplier and the customer are prepared to work together closely will it be possible to determine, in partnership, the most economical crushing system which is able to fulfil the quality demand.

Ravenscroft says it’s imperative that the customer furnishes the supplier with all the necessary information on the ROM material and the desired end product requirements. “Once all this information is obtained, there are many different simulation programmes available to help determine the correct plant specifications needed to achieve the desired end product specifications. Bell and Finlay’s application specialists work closely with our customers with the initial product selection and thereafter with ongoing plant efficiencies and product support because our core belief is if we help our customers succeed so will we,” he says.

Scherf is of the view that selecting the best crushing solution is only the first step to achieving targets and ensuring a successful business. He says that a close cooperation between supplier and customer is critical for long-term success as operating conditions are continuously changing.

“Nature did not mass produce rocks to the same specifications; the deeper you quarry the more competent and hard the rock is. Crushing conditions can also change drastically in some areas due to the rainy season. Having close cooperation means that many of these challenges can be pre-emptively addressed, by ensuring that the correct spares or screening media is in stock before the rains come, for example,” says Scherf.  

Scherf says when site conditions or the contract specification change, the first person the customer should call is their trusted advisor from the supplier to ensure that fine-tuning of the plant can be done timeously to maximise productivity. “It is very hard to drop costs without compromising on quality, so improving the plant’s performance by 1% has a hugely positive financial impact over the course of a year. A simple example is a 1% improvement on production on a 100 tph plant; over a year it would make up to 2 000 tonnes difference, and that equates to 100 additional tipper trucks delivering to end-users for the same fixed operating costs,” concludes Scherf.

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