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Modern Quarrying QUARTER 2 2019 1On the cover: Having operated a downtime-ridden plant beleaguered by excessive breakdowns, extreme maintenance glitches and low throughput setbacks, the quest to increase production efficiencies and reduce operating costs saw CNC Crushers purchase a full process-focused solution from Weir Minerals Africa. Four years on, the decision is paying big dividends.

For 14 years, CNC Crushers has been producing aggregates out of two old gold mines located at Buffelsfontein gold mine’s Shaft 4 and 5 in the North West Province of South Africa. The quality of the dolerite rock has made CNC Crushers a supplier of choice for a wide range of road building and construction material specifications. From an operational cost point of view, rehabilitating the waste materials of old gold mines has provided CNC Crushers with an edge, especially considering that there is no need for costly operational processes such as drilling and blasting.

However, these operational gains were initially outweighed by inefficient processing plants plagued by high wear on crusher liners, excessive maintenance problems and unplanned downtime due to constant breakdowns, resulting in low throughput and high operating costs. In 2015, Weir Minerals Africa presented a full process-focused solution to CNC Crushers, initially on a trial basis, and the results were beyond expectations.

Four years later, the operational challenges are a thing of the past and engaging the services of Weir Minerals Africa has turned out to be a game changer for CNC Crushers. “We have been in this business for 14 years, but we have only ‘opened our eyes’ in the past four years following our association with Weir Minerals Africa,” says Willie Crous, operations manager at CNC Crushers.

Speaking about the solution, Vernon Steenbok, General Manager of Comminution at Weir Minerals Africa, explains that Weir Minerals approached CNC Crushers’ operational setbacks with an integrated solutions approach. “We always approach every plant upgrade with a specific formula aimed at improving efficiency. For us, availability x utilisation x quality = efficiency. We are confident that the quality of our product can influence plant utilisation and availability. The equipment must always be available in order to get the best utilisation, that’s the starting point,” says Steenbok.

Michael Lapidoth, Product Support Manager for Trio® and Enduron® at Weir Minerals Africa, adds that utilisation and availability are two key parameters in aggregate production, especially considering that quarries are volume-driven operations selling low-margin product. For them to operate profitably, quarries need to keep volumes moving. “Availability is the proportion of time the equipment is available to be used for its intended purpose, while utilisation is the proportion of the time that the equipment is used for its intended purpose,” says Lapidoth. “In any mining environment, poor equipment availability and utilisation can endanger the success of the operation.”

Quality parameters in crushing

Quality parameters in aggregate production are normally based on strength, size and shape of the end product. How does the end result influence the type of crusher to be deployed?

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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.

Achieving screening efficiency

The last important production step in aggregates production is screening (classifying), which has to ensure high quality screening curves of end-products in accordance with standards. It is imperative that the process is executed in the most efficient manner. What is screening efficiency and what are some of the crucial factors that affect it?

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In the aggregates world, where selling prices are often fixed and very competitive, efficient screening is, more often than not, the cheapest method of dropping the overall cost per tonne on a plant. This is according to Fernando Abelho, head of sales for Africa at Pilot Crushtec International, who adds that, sadly, it is also generally the most neglected facet by plant operators.

“Many operators will spend time and money in the acquisition of best crushing equipment, then look for cheap options for their screens – be it the screen itself or the screening media. A simple example is picturing a 100 tph plant, where the correct screen or screening media could make a simple 2% difference,” says Abelho.

“In a 5-day, 7-hour operation, that’s an additional 70 t (3,5 tipper trucks) of production per week. That’s an additional +/-14 extra loads a month for the same overheads and fixed costs! This would apply to the type and quality of media, with cheaper meshes requiring more change outs, meaning more downtime and loss of production,” adds Abelho.

Speaking about the significance of efficient screening, Waylon Kukard, sales manager at Wirtgen South Africa, the supplier of the Kleemann range locally, says screening is critical to the overall aggregates production process. “Without proper screening efficiency, your final products can be contaminated and out of the grading curve. This also impacts your production rates as undersize material will go back to the crusher if you are operating in a closed circuit,” says Kukard.

The same view is shared by Rasheel Sukdhoe, product manager of the McCloskey Crushing, Screening, Stacking and Washing range at HPE Africa, who says that the screening process is the most important part of the aggregate production process. “Screens are a way to control quality and gradation of aggregate product. Prior to screening, the product might not have as much of a value, however after screening the value of the product will increase,” he says.

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Crown Publications, one of South Africa’s largest business-to-business publishing houses, came into existence in 1986. Since then, the company has grown from producing a single magazine, Electricity SA (renamed Electricity+Control), to publishing six monthly magazines, three quarterlies, and a number of engineering handbooks.