When trying to maximise the efficiency and throughput of your operation’s screen media system, it begins with a ‘holistic’ view of crushing and screening,” said Craig Burke, director of engineering for Polydeck Screen Corp, based in Florida, USA, and a former plant manager for a large aggregates producer.
“Everything you tweak in a processing system affects all the other parts. To make screening more efficient, you have to understand what is going on in front and behind the screen.”
The processes that feed the screen and the processes the screen itself feeds all interact to play a role in desired throughput, Burke said.
“In some cases, increasing throughput at one point can overload another point,” he said. “Once you establish a holistic view, you can anticipate problems that may arise and plan for them.”
For example, if a producer is trying to maximise throughput on a particular product, it is important to examine the entire crushing and processing system. Burke suggests reviewing the system from finished product all the way back to the initial phase of processing.
“Good blasting technique is just as important as proper crusher operation to produce more of a desired product,” Burke said. “People want good screen efficiency, but the product may not be there to be recovered. Regardless of how well a screen is set up, if the product you want is not in the feed coming to the screen, it can’t be produced.”
Another consideration is the combination and type of screen being used for the various decks. Open area is “king” for more throughput, but sometimes must be sacrificed for longevity and limited footprint.
“You need to optimise the screen system design,” said Sam Durnavich, regional sales manager for Polydeck and a former plant manager for another aggregate producer.
A “mixed approach” – such as heavy-duty panels on the hotspots of the screen and high open area and lower-duty panels on non-heavy-duty screens – may be used to maximise throughput.
Even if the screen system is meeting throughput goals, producers need to make sure this doesn’t become all for naught in the finished product piles coming off the screens.
“If you have a dedicated finished stockpile, try to load out of it so new production falls to one side,” said Michael Vacchi, quarry manager of PJ Keating’s Cranston Quarry in Rhode Island, USA.
“If you have large stockpiles, it can contaminate the finished product. You want to manage potential contamination.”
Vacchi suggested digging out a half-moon finished stockpile, so material alwaysfalls down instead of covering the entire cone.
“That way you are only contaminating one side instead of the whole stockpile,” he said. “A lot of plants have dedicated stacking conveyors, but it may put you at risk of contaminating the entire stockpile. A radial stacker can help.”
Best screening practices
1. Screen deck set-up (Figure 1).
{{image3-a:r-w:300}}To ensure the screen deck has been properly set up, check the acceleration, displacement and all technical aspects of the screen when it is running.
If one side moves differently than the other or the G-force is set too high or too low, it can affect the performance of the screen media.
If improper set up causes these problems, a screen performance evaluation should be conducted.
Multiple readings of several places on the machine should be taken to determine if it needs to be tweaked and if it is running as efficiently as possible.
2. Screen systems customisation (Figure 2).
{{image4-a:r-w:300}}New material science enables producers to customise their screen systems to better match their production goals, while fitting various budgets.
Wire mesh has been the traditional screen media and is known for having the most open area, crucial to getting the most throughput.
However, alternate materials such as rubber and urethane – and now rubber with a metal panel in the centre – are being used to create more robust and modular screen system designs.
Lower durometer screen materials help keep the screens from plugging up while creating a consistent product.
3. A holistic view of processing (Figure 3).
{{image5-a:r-w:300}}Understand what occurs in front of and behind the screen and how interaction between these various facets affects material throughput and efficiency.
In a moderate feed, the speed and stroke play a role in efficiency.
If material is only being fed down one side of the conveyor to the screen, it will affect production and could cause unnecessary wear on the equipment.
Also, the closer in size the material is to the aperture, the harder it is to get through the screen.
A larger aperture may be necessary to sort near-size product.
4. Focus on quality control, maintenance (Figure 4).
{{image6-a:r-w:300}}Before or during each shift, the operation’s maintenance team should check screen panels. With a heavy feed rate, it’s also important to use the entire top deck of a screen system to achieve the greatest efficiency from all the decks.
As part of quality control measures, check for consistency of the crusher feed. If a crusher goes empty, material will not be screened properly because a finished product is not being sent, and the material will continue to recirculate.
Note whether crushing is being done efficiently by monitoring what is coming off the top screen and what is going back to the crusher.
A holistic view
Burke said, all too often, total plant throughput is used as a measure of efficient operation. Efficiency is more accurately thought of as: how easily you are getting product into the pile the first time you try.
“That’s when you have to look at the whole picture,” he said. “Screen operating parameters, mechanical conditions of the screen and feed distribution are just a few critical factors that can have a big effect on efficiency.”
Missing the first, or subsequent, opportunities to get product into the pile can lead to recirculating loads that bog down your plant.
{{quote-A:L-W:300-I:8-Q:"To make screening more efficient, you have to understand what is going on in front and behind the screen."-WHO:Craig Burke, Polydeck USA.}}“One key is understanding the physical characteristics of the equipment and setting up operating parameters correctly,” Burke said.
Without understanding the effect of speed and stroke on the screen media, producers may have a screen set at a speed or stroke better suited to fine material, but have a coarse material feed.
“In a case like that, you can’t get coarse material to stratify as it should so the right sizes find the apertures,” he said.
“Alternatively, some people may have a screen set at a high energy level for coarse material, and when fine material is fed to it, the material bounces too high and can’t find its way through.”
Inspecting the feed arrangement and physical condition of the screen is also important. “Is the feed stream segregated? Is the feed all to one side of the screen? Are there broken cross members or springs? Is the chute clear under the screen? Sometimes the screen is smacking up and down on packed material in the chute and shaking itself apart.”
When troubleshooting feed arrangements, check if you are feeding to the centre of the screen or to one side. Feeding from two different conveyors isn’t a “no-no”, so long as the feed box is properly mixed.
Dust suppressions, emissions
One of the biggest issues with screen throughput is managing dust suppression and emissions coming from transfer points.
“We manage this in production daily,” Vacchi said. “No matter what material you are feeding through, you are trying to manage the right amount of water to control dust and allow the material to screen out – both of which are very important.”
Quarry operators are advised to wet the material prior to hitting the screen – and not to put water over the top of the screen – so it has the best chance of being properly screened.
{{quote-A:R-W:300-I:7-Q:"If you have a dedicated finished stockpile, try to load out of it so new production falls to one side,"-WHO:Michael Vacchi, quarry manager of PJ Keating’s Cranston Quarry, USA.}}Regardless of the type of screen media used, producers often see the most success when they fabricate either a feed shoot or a diverter on the feed plate of a screen to spread out the material on the top deck.
“A lot of people don’t use the whole top deck, so it hurts screening ability through the rest of the screening,” Vacchi said. “If all of the top deck is used, you’ll get the greatest efficiency out of all the decks.”
This also makes the finished product much more consistent. If only the middle of the top deck is utilised, it’s difficult to efficiently screen material without it being spread out. It also contributes to improper wear.
“If I have a box or diverter gate dumping onto the feed plate, sometimes the metal on it will begin to wear,” Vacchi said. “Instead of taking impact on the feed plate of the screen, the first row of screen panels will take the impact. It’s only a matter of time before it will go through and cause contamination.”
Although it may not seem directly related, equipment maintenance plays a vital role in maximising throughput. “As soon as you go out of spec, things can go awry very quickly,” Vacchi said.
Right screen media
Traditionally, wire cloth has been the predominant technology because it is relatively inexpensive, fairly easy to procure and has high open area. However, it comes in large 1.2m x 2.4m sections, has a high bed depth and tends to wear out quickly.
Polyurethane and rubber media tends to have a much higher cost at the outset and may not have as much open area, but it may need to be changed only every six months instead of every 10 days, Durnavich said of conventional screen media options.
{{quote-A:R-W:300-I:9-Q:"You need to optimise the screen system design,"-WHO:Sam Durnavich, Polydeck USA.}}Now, a combination of wire and synthetic screen media is being used to give producers a screen with higher open area, combined with the advantage of lower maintenance.
A section of welded wire inside a polyurethane border in a modular format is used to customise screen system designs.
“Using this type of modular approach allows producers to choose row by row to fine-tune the screen to their needs and hit their production targets,” Durnavich said.
However, he stresses that even with the flexibility of a modular design, it’s critical for producers to work with an applications engineer and provide accurate information on production goals, current system set-up and gradation analysis.
“If one side of the screen machine is moving differently than the other side of the machine or the G-force is set too high or too low, it can affect the performance of the screen media,” he said.
Older panels tend to plug and blind. By experimenting with the material science behind the rubber and urethane used in newer modular design screens, more robust – and lighter – designs are being developed.
“Screens are being designed with lower-durometer rubbers to allow the panels to self-clean, so they don’t plug badly,” Durnavich said. “It has a trampoline-like action – a secondary motion for self-cleaning.”