Nearly any organization shredding and separating E-scrap will have a screening step in their process for removal of certain size fractions. Often not the star of the show, the screener plays a key role in efficient recovery.
Types of screens:
There are two main types of screens found in most plants including rigid screens and flexible screens. Each has a benefit and a weakness. Rigid screens are lower in cost, the most common, and can accommodate tight footprints. A rigid screen moves as one with the vibratory feeder pan it is attached to. It needs to be thick enough, have bracing underneath, or be under tension to ensure it does not vibrate independently of the mass it is attached to or be subject to early failure and poor performance. These screens are also more feasible to use when a very wide vibratory feeder is needed to present material to a process such as a sensor sorter. There are variations on the types of rigid media including perforated plates, finger screens, geometrical opening variation, and 3D screens. For e-scrap, perforated plates (sheet of steel or stainless with matrix of simple round holes of a certain size) and finger screens (overlapping wedge shaped tabs of steel cascading) are the most commonly used and available. The rigid screen has one advantage the flexible screens do not which is choice of vibratory motion. For most types of vibratory feeders and pans, there are single mass and dual mass. The economical single mass is a brute force simple approach with a single metal pan sitting on springs driven by two vibratory eccentric motors (or electromagnetic resonator for smaller models) bolted directly to it spinning in opposite directions from themselves. Dual mass is more of a tuned approach with a second mass of steel and only one eccentric motor attached to the main feeder pan by springs. An oscillation occurs based on the amount of springs, vibratory force, and dampeners to create vibratory motion. This action also provides some amount of counterbalance and they transmit less vibration to the structures they sit on which is important if foundations are thin or machine platforms are prone to vibrate. Dual mass is also the only drive option that will be discussed in more detail for the flexible screens. These dual mass pans overall can have higher amplitude and are more tunable, thus making them adaptable depending on application. Why does this matter? Higher amplitude throws the material larger vertical amounts which promotes better screening. Dual mass also use about1/2 the amount of drive motor KW and are more efficient with their energy. Rigid pans do not need significant tuning and have few ways to fail. If transmitting vibration is of concern, many now offer isolator frames which is a secondary frame sitting on rubber springs that dampens the vibration transmitted.
Flexible screens work on dual mass similar platforms but the vibratory motors or eccentric weights sit on the main larger mass. The flexible screen mats sit on a suspended smaller mass with special rubber springs on both the top and bottom to promote a horizontal motion to move materials as the whole pan vibrates vertically to provide agitation. Some models have two eccentric weight motors while others use a energy efficient pedestal mounted motor drive type that transmits rotation to an eccentric weighted shaft through a u-joint enabled driveshaft. The screen media is unique and typically made of polyurethane. Screens are comprised of parallel “mats” which can be convenient when only needing to replace several worn areas instead of the whole piece of media as in the case of rigid screens. This flexible media comes in many variations of thickness and geometric holes. For e-scrap, rounded slots and thinner thicknesses perform well. Flexible screens have a higher acquisition cost, but they screen much more efficiently per foot of linear travel and are resistant to copper wire fouling the media which is not the case on the rigid counterparts. For dust control, rigid screens usually come with a top cover making them easy to connect to plant ventilation systems. A big weakness will be their large cost as their width increases making them a non-starter past 6 feet in width typically. Remember, for all types of screens driven by eccentric weight motors, the amount of weight spun by the motor is proportional to the ability of the screener to carry the material load.
Screen locations in the line:
One purpose for screening is to recover finer particles after shredding that will often contain copper wire and fractured pieces of circuit boards and surface mount components rich in precious metals. Another reason is to enable better dust control and isolate out extremely small particles that can later become airborne. To serve these purposes, placing a screener directly after the shredder can be a valuable choice. Use care to make sure the screener is rated for the shredder maximum throughput and perhaps allow a small safety factor. When screens are over-run, they remove less fines per linear foot and the fines will enter the next downstream process with is often magnetic separation for ferrous metals recovery. The magnet can remove some fines which will then be entering the steel stream and lower the plants overall recovery of precious metals. For maximum performance, select a flexible screen for this step if budget allows to avoid copper wire fouling and enjoy high screening efficiency. For rigid screens, a 6mm round hole can be effective since it is less prone to metal screws becoming trapped at that size. For flexible screens, a 6x10mm slot can produce a decent fines stream for the smelter.
In the metals yard auto shredding segment, many rounds of sizing occur prior to eddy current separation for maximum non-ferrous recovery. Due to less volume and available capital, it would be rare for an e-scrap line to have a second screener before the eddy current, but if affordable including a second eddy current, the separation and recovery can be impressive. I have built one plant with such a setup and was very pleased with the non-ferrous result.
The next most likely place to add a screener is prior to any sensor sorting equipment which in the case of an e-scrap plant would likely be an inductive sorter (metal sensor). These sensor airjet sorters have improved in resolution, but 15-17mm is a common size to screen. Why is screening required even when the sensors are rated for smaller particles? Two reasons. First, a vibratory feeder the same width of the sensor’s high speed belt or slide is needed to properly present a mono-layer to the machine. Secondly, the burden on the airjets is important to monitor. If the airjets are firing too fast, errors and air turbulence can occur resulting in non-target particles such as plastic entering the metals sort. Air supply can become strained and the machine can risk a shutdown if minimum pressure is not maintained. More fines entering increase the total number of particles per square foot on the high speed belt and lead to airjet over firing. Indeed, the fines contain recoverable metals and need a further step depending on business model. One work around scenario can be to run the fines over the same sensor sorter after shift. A small batch feeder can meter feed the fines onto a special tangential side exiting feeder and slide down onto the metals sorter bypassing the screener feeder and allow separation.
Besides keeping adequate spares including screen media, consider springs, dampeners, and most importantly the motors. These motors are not always easy to find and often have a lead time from the manufacturer or motor rewind shop. Monitoring the stroke of the screen is a quick indicator to check and is important to know. A higher stroke can mean a spring is broken while a low stroke can lead to poor performance. For the case of rubber springs, they can collapse over time and consider measuring them or checking for any that are lower than the others. For rigid screens, cleaning the media from wire tangles will likely be frequent. One tool that can sometimes help is a pneumatic de-scaler. Lifting the motors, covers, or rigid media can be awkward especially when maintenance platforms are not present or the layout is close to a wall. Invest in the right lift equipment and crew. A telestacker, forklift attachment, or small crane may be needed. Plan ahead and do a risk assessment on the job ahead of time with your plant safety team. Happy screening!
I hope you enjoyed this short article and find it helpful to your business. If you would like to learn more about our company please follow us on LinkedIn and see our website. We are experienced recycling plant designers and develop plants with an operator’s point of view. The systems we deliver perform well long term, use leading technologies, and are tailored to exactly fit our client’s application.
These opinions are not an endorsement but what worked well for me on previous e-scrap plants built over 18 years. There is not one correct formula for screen selections or sizes and it is very much a decision based on the business goals and capital budget.