Modern Quarrying Q2 2019

SCREENING – PART 1

Screening efficiency is how precise a screen is when separating a feed material of different sized particles into different size groups.

stockpiles afterwards, if 85 t of your undersize material is stockpiled, and an additional 5 t of undersize material is in your ‘oversize stockpile’, your screening efficiency would be calculated as (85÷90) x 100 = 94,45%,” explains Abelho. Abelho adds that, as a general rule, no screen is 100% efficient, and for all intents and purposes, a screen operating at 95% efficiency is often considered ‘perfect’. Most commercial specs will specify a range of over/ undersized material that is allowed in the final product. “Typically, these specifications offer up to 5%-15% deviation on the sizing. This means your screening can in some cases be as low as 85% efficient, and still be commercially acceptable. Note that your product may be in specification, but this inefficiency may be costing you money due to production shortfalls. Understanding the end-product requirements is therefore important,” adds Abelho. Key factors Lots of factors should be considered in order to achieve screening efficiency. According to Kukard, thickness of the bed depth of material running over the screen deck (overloading the deck) is one of the crucial factors, as well as screening media, moisture, angle of the screen deck, speed of the screen deck, amplitude of the screen and the wear of the screening media. Sukdhoe says there are several factors that affect screening efficiency and these include: • Material Travel Rate – the speed at which the material travels down the screen; • Stroke Characteristics – the amplitude, direction of rotation, type of motion and the frequency of the vibrat- ing screen;

the comparison of the undersize material carried over the screening media and discharged with the oversize product in contrast to the input amount of undersize that was initially fed into the screen. “Screening efficiency is the degree of accuracy at which undersize material is passed through an open- ing. It is measured by the quality and grading of the screened material. The screen length will determine the efficiency of the screen,” explains Sukdhoe. According to Kukard, screening efficiency is the clas- sification of different products according to a grading curve of the specific application. Sampling of the differ- ent stockpiles will be taken and put through a sieving process to determine if there is undersize material in the oversize. Abelho says, in simple terms, screening efficiency is how precise a screen is when separating a feed material of different sized particles into different size groups. Material that is smaller than the given aperture is denoted as under-size, anything larger than the aperture is considered over-size. “Often the size of the material that passes the aper- ture is referred to as the cut size. Part of the reasoning behind this is that the screen angle, material depth and other factors will influence the material that passes the aperture. To give an idea, a screen at an 18-degree angle with an aperture of 8 mm may produce a cut-size of 6 mm (i.e. 95% of the material is -6 mm). Abelho adds that screening efficiency is expressed as a percentage and denotes the percentage of undersize in the original feed that is separated by the screen at the given aperture. “A straightforward illustration would be taking a feed of 100 tph and separating at a given aperture size. When doing a sizing analysis of your

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MODERN QUARRYING QUARTER 2 - 2019