Positioned at, or close to, the front of the line, screens play a vital if somewhat unglamorous role in the sorting process. One particular type of screen, the disc screen, is particularly useful for separating and sizing the 2D and 3D fractions.
Tried and tested rotating drums with varying size sieve like holes in the wall, trommels, are a common sight at material recycling facilities. Raised at one end, gravity pulls the input material through them as they rotate. On its way through different size holes allow the material to be graded by fraction size. The basic design works, but there are some drawbacks. They tend to be fairly large pieces of equipment, can require frequent cleaning, and more than one machine may be required to meet the throughput of a large MRF.
Depending on the quantity and composition of the material being processed, in some instances an option to perform the same function is to use a disc screen. With a number of powered rotating shafts fitted with inter-spinning discs, these machines can grade various fraction sizes of waste while separating out the 2D fraction.
Speaking with WMW at the recent RWM exhibition in Birmingham, UK Trevor Smart, country manager at recycling equipment manufacturer Bollegraaf explains that because they are powered, one disc screen can replace two trommels. Even then each screen would have a smaller footprint than a single trommel.
Discussing the company’s StarScreen technology he says “We’ve got one of these machines running on MSW. One machine, 2.5m wide and 9m long that screens the 80mm and finer fraction at 60 tonnes per hour. If you went to a MRF with trommel screens installed at 60 tonnes per hour you’d have to have two in-feeds with two, if not four trommel screens. Because it’s powered you can get a huge amount of throughput. The screening is about the same. It’s done by modules so you can change the screen size as you go down the modules.”
Another advantage Smart highlights is power consumption. At 7.5 kW per 3m section he says that the StarScreen uses around one quarter of the energy which would be required by trommels of a similar capacity.
Keep it Clean
One issue that has been a thorn in the side for disk screens has been that of clogging. Plastic bags and films have a tendency to get caught on the shafts, wrapping around them and becoming a sticking point for yet more debris to become attached. Eventually the operator will need to stop the line and clean the machine. To tackle this issue Lubo, a subsidiary of Bollegraaf, has developed its Anti Wrapping StarScreen, which Smart says combines the advantages of a star screen and a drum separator in one machine while eliminating the disadvantages of both.
“The thing about trommels is that they’re tried and tested and they’ve been around forever,” he explains. “The wrapping issue with driven screens has always been an issue. What you don’t want to do is go in and clean every couple of hours. With a trommel once per shift you open up the side panel, clean it down and you’re done with it. In reality that’s what the StarScreen has to achieve. You don’t want to be doing it more than once a shift because you’re there to process waste, not to clean machines.”
Lubo has been testing its latest Anti Wrapping StarScreen (AWS) in France for around nine months where Smart says the system is proving its value.
The new version of the AWS can also be installed at an angle, which the company says also gives it the advantages of a ballistic separator for separating fraction size and 2D and 3D materials. In addition both the rotation speed of its stars can be adjusted. Combined this enables the machine to be tuned to a specific requirement.
“Any time you go to a MRF to see a client you want to see what’s coming in and you want to know what they want out of it. Once we’ve got those two pieces of information that drives our design process. The main driver is that it has to be cost effective for the client,” explains Smart.
Tailor Made
Another company which has developed an anti-clogging system is Italian disc screen manufacturer Ecostar. To deal with the problem it has employed a series of hexagonal or octagonal discs with a flat profile. The anti-clogging system is the company’s second international patent and works by implementing a loose sleeve on the exposed parts of the shaft between the discs which prevents film material for getting stuck.
Speaking with WMW, Fabio Cappozzo, the company’s owner reinforces Smart’s point with regard to customisation.
“We normally tailor make the machines because every customer is different. We have many different types of discs and use many different spaces between the discs depending on the application,” he explains.
The screen, which the company claims to be able to process up to 200 tonnes per hour, is produced in several versions including stationary, hooklift and mobile and comes in various sizes. It also allows operators to differentiate the preset screen width (± 20%) and has the option varying the speed of the shafts through the use of inverters.
To tailor make the Ecostar for a given application, material type, moisture content, feed gradation, product specifications and capacity should all be considered.
Canadian recycling equipment manufacturer, Machinex, also likes to talk about bespoke options when it comes to specifying its screening equipment.
Explaining the company’s approach to single stream recycling, Ghislain Thivierge explains that at the beginning of the process an OCC screen would generally be deployed to remove the cardboard.
“On the first deck it would probably have a small opening to remove the fines,” he tells WMW. “After that you would have larger openings. You can either have one screen that’s doing all the sorting or you can stack them and remove different grades of material. The first thing we need to know is what the material breakdown is and the second is the throughput. It depends on the percentage of OCC, fines and glass.”
Maintaining your investment
According to David Stairs, from US screening equipment specialist, AEI Screening Solutions, maintenance is another area where disc screens such as the Ecostar win out over trommels.
“Maintaining trommels revolves around cleaning of the drum and maintenance of the hardware required to support this large machine. Replacing everything from trunion wheels to cleaning brushes, a trommel requires significant annual expenditures to keep your plant operating,” he explains.
“The very simple design of the Ecostar screen greatly reduces your down time. The anti-clogging system cuts cleaning times on your maintenance schedule. The screen consists of a series of shafts with discs mounted in pillow block bearings. Regular maintenance consists of greasing a central lubrication point,” continues Stairs. “When it is time to change shaft, a simple four bolt connection provides for easy change over and replacement with minimal downtime.”
Future Developments
Moving forward, Thivierge says that purchasing decision for sorting equipment will be influenced by what happens to the prices for recycled materials.. He explains that Machinex has one customer who is a big player in the paper industry, so they wanted to have really high quality paper.
They were going to use multiple screens and an optical sorter to make sure they got a good purity, but since the markets went down for recovered the optical sorting waste dropped.
He also says that he is seeing increasing interest in yet another type of screening machine, the ballistic separator.
For Smart, the future will see increased integration of capabilities, with single machines offering the functionality which would have previously required multiple pieces of equipment: “The way I see it now is that you’ve got all these individual components and you’ve got to start incorporating them together.”
“For example, in the future a drum screen might incorporate a magnet and eddy current separator,” he says. “It’s bit like the analogy of a mobile phone, a digital camera and a satnav. Five years ago you had all three. Equipment now will need to become multifunctional, and not just cobbled together multifunctional, but designed multifunctional – it all has to be integrated.”
Conclusions
Screens of various types play a crucial role in the recycling process. For many applications disc screens offer a robust, versatile solution with a high throughput, but there are many options available. Selecting the right machine requires careful examination of the task at hand.
“As properly sizing a screen requires consideration of key material and machine characteristics, along with desired product specifications,” explains Stairs. “The interplay of all these factors will determine the proper machine for the application, the screen media and the efficiency of the machine. Screening is much more complicated than taking a single feed stream and making two piles.”
