Winter has settled over the crushing plant at Rekefjord Stone outside Egersund. The operators have weekly maintenance stoppages, where four hours are set aside to clean the sieving sheets in a single sifting machine.
Inside an aiming machine, an operator lies on his back working with the impact chisel and compressed air. The frost smoke lingers on his breath as he works to remove the frozen debris that has clogged the stitches in the sieve. On such cold days, the work stands still even longer than usual.
After half an hour, he crawls out of the narrow aiming as he breathes heavily.
“Switch, now we’re switching,” he says, wiping sweat from his forehead.
At the opening of the aiming machine, Olav Gilje, technician at Nordic Bulk, looks for signs that cause the sight cloths to clog.
“I suggest running a test with four different tablecloths,” Gilje concludes.
Up to four hours a week with compressed air and knocking of compact soil was the solution before the analysis was performed.
New aiming cloth yielded results
They started a test phase with four different sight mats. Hole size, material, supplier and hardness varied on all the fabrics. Finally, three of the tablecloths were weeded out. One tablecloth emerged as the clear winner, tested under several different conditions.
A modular soft polyutherane sieve cover (PU) from German supplier Kûper was optimal for this specific case. The soft sieve mat gives a trampoline effect, which means that it does not overgrow with the sticky masses.
“PU sieves are alive, so there is no doubt that they work and sift the mass in a completely different way than steel cloth, which is completely dead,” explains Georg Herveland, operator at Rekefjord Stone.
“Visibility has never been better than now. I’ve been working here for 40 years and we’ve had a lot of work on that term, but now it’s great,” he adds.
The before/after image shows the huge difference the right canvas can make to the aiming process.
Testing is crucial
“You would think that what works in one place should work another, but unfortunately that’s not the case,” says Gilje from Nordic Bulk.
The consistency and stickiness of the material, humidity, type of sieve, number of strokes and speed of the sieving machine are all factors that affect the properties of the sieve cloth.
Gilje uses sieve cloth analysis to find the right fabric for the conditions, which will provide optimal sifting of the masses. Then he tests up to seven different tablecloths at a time.
“Choosing the optimal fabric without testing is almost impossible. There are simply too many different factors at play, says Gilje, who makes no secret of the fact that he recommends testing to customers who wear tight masks in their sights.
Olav Gilje believes that sieve cloth analysis can be a useful tool that many quarries in Scandinavia can benefit greatly from.
Steel mesh is not the first choice
Many crushed stone plants in Scandinavia choose to use traditional steel mesh. Gilje believes that PU is a far better alternative because it often aims better and has a longer lifespan if you choose the right fabric.
Herveland from Rekefjord Stone supports the claim about sieve cloth made of steel, and believes that polyurethane can be a better alternative, as long as the sieve mat is of high quality.
We don’t have to screw on a single tablecloth, and if the module is worn we just move them around. It’s just like building blocks
Georg Herveland, operator at Rekefjord Stone
“With PU, it’s all the more important to have high-quality tablecloths. You can buy cheaper, but when it breaks, the mass goes into the wrong place and often pollutes the entire next silo, which is expensive,” says the experienced operator.
He believes that the German suppliers Isenmann and Kûper are of top quality.
“They’re so durable, after three years, not a single mask has broken with us,” he said.
Consistency and stickiness of material, type of term, number of strokes and speed are all factors affecting the sieve fit. Olav Gilje believes that testing is necessary to find the solution that provides the best economy.
Easier maintenance increased well-being
Besides the financial gain, Rekefjord Stone is satisfied with the HSE aspect, as the operators avoid the heavy work with little space available.
“You can feel the steel cloth on your knees and elbows. Now we can just knock on the sheet and the debris bounces off, maintenance goes much faster,” says Herveland.
In addition to being difficult to work with, the traditional steel fabric took up much of the capacity of the operators during each maintenance stop.
“We don’t have to screw on a single tablecloth, and if the module is worn out, we just move them around. It’s just like building blocks,” he concludes.
Three years after the new tablecloths were installed, it looks like this. Neither cleaning nor cloth replacement has been necessary during this period.
Negative ripple effects throughout the term
The sieve at Rekefjord Stone has three layers of sieve mats that stand horizontally, where ever finer soil is separated out as the mass falls downwards.
“When the top layer is tight, none of the sheets do their job,” explains Gilje, who often recommends PU or rubber sieve mats.
The extra weight on the top cover means that the sight has a skewed weight distribution in relation to the vibration settings. Some sieves have elliptical vibration, others circular, others linear. Common to all of them is that if they are top-heavy, it will cause extra stress and wear.
“There could be several tons clogging the top layer. The springs, gears and motor get extra load that makes the vibrations in the entire visibility too slow,” adds Gilje.
Read also: 8 common mistakes that kill your carrier
Traditional steel cloth was blamed
“Steel sheets are considered a good alternative in the sifting process at many crushing plants in Norway, but PU is often a better alternative,” Gilje points out.
The problem with steel cloth is they often have little flexibility. In addition, they rust when exposed to soft material, which causes loose matter to stick.
“There are an incredible number of sight mats in different thicknesses and materials. You have modular cloths, Iseflex, finger sights, side-tensioned and long-tensioned tablecloths, rubber, PU or steel to name a few. If you have the wrong tablecloth, it provides more maintenance and lower quality of the end product, says Gilje and pauses before adding,
“I think a lot of crushers would have been surprised if they tested different tablecloths.
