Properly sizing a screen requires consideration of key material and machine characteristics, along with desired product specifications. 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.
Processors screen for different reasons, among them are producing a product with a desired particle distribution or characteristics, splitting a feed stream to separate processing lines, removing fines or removing finished product prior to an additional reduction step.
In the aggregate and mining industries, products typically have a required size distribution whether they are for an asphalt mix, coal to feed a boiler or a metallurgical process. Screens serve as a process control to protect process equipment, possibly a boiler for power generation or to maintain the finished characteristics of a product such as asphalt or concrete. A screening operation can also control the shape of the product whether 2D, 3D or elongated. Selecting the proper machine can help eliminate problematic particle types.
In recycling and waste stream applications, screening is often used as a means to split the feed stream between separate processing lines. Because of limitations in downstream sorting capacity, whether manual (picking stations) or mechanical (sensor sorters or magnetic separators), multiple processing lines are often required to handle the volume of the feed stream. With bulky materials, large pieces such as OCC (card board), wood or plastic film, will hide smaller items on a belt from a manual pickers view, reducing recovery. In a mechanical sortation process, sorters function best when they can process a narrow feed size range of particles. By tuning the process line for a narrow size range of particles, recovery of valuable commodities can be maximized. With current technology, there are often limitations in the ability to detect and eject small particles. If fines in a process cannot be effectively sorted, remove them from the process creating increased capacity and improve plant function by eliminating the problems associated with fines.
Screening also performs as a multiplier for plant maintenance and cleanliness. Fine material can be removed from the process and treated separately, greatly reducing maintenance concerns throughout the plant. Fines collect in other machinery, creating health and explosion hazards and ferrous fines create problems in magnetic sortation processes.
There is often a fraction of raw material that is already properly sized. In a size reduction process such as crushing, grinding, shredding or shearing, feeding finished product unnecessarily into a reducer creates avoidable wear and power consumption. Size reduction machines are often the most expensive wear related operating cost in a plant as well as the largest consumers of power. Pre-Screening the material or scalping, reduces unnecessary operating cost and increases plant capacity. If secondary size reduction is required, removing properly sized material performs the same function. If the size reduction step in a process is the bottle neck, the addition of efficient screening is often the most cost effective way of increasing the capacity of a size reduction circuit.
What is screening and how does it work?
Screening is the probability of a particle of a given size passing through an opening of a given size. As the size of the particle approaches the size of the opening, the probability of passing that particle through the screen decreases. Conversely, as the particle size decreases and the screen opening increases, the probability of passing the particle increases. Each opening in a screen presents an additional chance for a given particle to pass through. The more openings a particle is presented with, the higher the probability of passing that particle.
In real world screening, this is witnessed through the particle distribution of the fines product and the coarse product. 100% efficiency is the unicorn of screening. It happens in theory but not in practice. Screening efficiency is defined as the ratio of the material that should pass through a given opening and the material that actually passed through that opening. In practice, the product distributions will be shaped like a curve. Fine particles pass the quickest with the highest efficiency while near size particles are the hardest to pass and pass with reduced efficiency.
When feeding a given distribution of material to a screening machine, there are three main steps to screening performed on a single screen deck. In the first area of screening, bed stratification takes place. Bed stratification involves the activation and agitation of the material through the screen motion or the movement of the material. Smaller particles travel downward to contact the screen mesh while larger particles remain at the surface. Stratification is an important step in screening as only particles that are in contact with the screen apertures may pass through them. In cases of an overloaded screening machine or wet, matted recycled materials, stratification is greatly reduced and the efficiency of the machine is compromised. In the bed stratification section, fine particles begin to pass through the screen but in limited quantities. Only the small particles that are initially on the bottom of the feed begin to pass. The second area of the screen deck is called crowded screening. In this section the smaller particles are in contact with the screen media and begin to pass in volume. If you were to sample fines material from just this section of screening, you would find a product much finer than your screen opening. Because of the volume of the material required to pass the deck, particles are fighting each other to pass through. This crowding of multiple small particles trying to pass the deck decreases the effective sizing of the screen through this section. In this section the majority of the undersize material pass through the deck. The last section of screening is called separated screening. In this section, the majority of the undersize material is already removed and the near size material has the opportunity to pass. Because the majority of the fines are removed, crowding of particles is no longer a factor. By increasing the screen length, you can increase screen efficiency by providing a longer time and probability for near size material to pass through the screen media.
Every application and material presents a unique challenge for a particular screen. Even the same material at a different location will not have exactly the same characteristics.
The particle distribution describes the sizes of the discrete particles in any feed stream. The percentage of material at each size will determine the capacity of a given machine. Key factors include the percentage passing the chosen sizing point; the more material required to pass through a screen, the larger the required screen area for the duty. The top-size of the material will also have a direct impact on the material’s screening. If a large fraction of large particles are present, scalping may be required to reduce the impact on the chosen machine and improve screen-ability. Assuming a small sizing is required with a feed stream with a high percentage of large pieces, performing a scalping cut at a coarser intermediate size will increase the capacity of the smaller cut point. By removing a significant percentage of material to the scalping deck, the load on the secondary deck is reduced, reducing material crowding and increasing efficiency. The half size of the material describes how fine the material stream is. The half size is the percentage of material that passes a size half the size of the desired screen opening. The larger this percentage, the finer the material and the quicker it will pass through a given screen surface. Understanding the feed material gradation is essential to efficient screening.
Moisture greatly effects the screening characteristics of any material. There are two types of moisture; surface and inherent. Inherent moisture is the moisture present inside of a material. Surface moisture is the moisture that is coating the exterior of a particle. Surface moisture has the greatest impact on the screening performance of a material. Surface moisture causes the adherence of fines particles to the coarser particles, limiting the ability to remove them through screening. Typically, the smaller the desired screen opening the larger the impact of additional moisture will have on the screen efficiency. In addition to the fines particles sticking to coarse particles and being carried over, fines particles will stick to and blind the screen openings. Blinded screen openings reduce the effective open area of a given machine, reducing efficiency and capacity.
The shape of particles in both the feed stream and the desired products affects the screen selection for the application, and understanding the type of particle is critical. The main types of particles include three dimensional, approximated by a cube or a sphere; two dimensional, approximated by a sheet of paper or a business card; and elongated particles approximated by a rod or a pencil. Understanding whether these particles are desired in the product or to be eliminated changes the approach to screening. Two dimensional particles may present a problem where riding or surfing takes place; fine particles ride on top of the larger flat pieces and bypass the screen openings. In this case, some sort of turning or agitation is required to prevent the fines from riding on the larger pieces. Eliminating elongated pieces from the undersize requires care to ensure particles are laying down during conveyance across the screen surface. The screen openings and materials can be optimized to reduce the passage of undesired shapes.
Density of the material helps to describe the load and bulk the screening machine must convey. A bulky material might be low tonnage and have a small proportion of material passing the screen openings but because of its volume it requires a large machine to convey the material. Conveying volume and maintaining effective bed depth are key to handling low density materials.
When choosing a machine for a screening application, there are also important operating characteristics that must be considered. These may be present in an existing system or can be incorporated into the design of a system.
Regular, consistent feed is critical to the proper operation of a screen. Irregular feed will result in the passage of oversize material when the screen load is running at its minimum or result in carryover of fines when the load is at its maximum. Typically, the carryover of fines is more detrimental to the overall process, so a screen should be sized to handle the maximum spot load it will see. Typically load to a screen is discussed as tons per hour. Tons per hour is an average measure, so with irregular feed, the actual load to a screen can be many times this stated figure. In the case of operating online with a shredder in a recycling operation, the operator of the shredder can determine the feed of the system. Because shredders are commonly grapple-fed and rarely consistently fed, they produce surges and ebbs in the feed stream. If irregular feed is identified, a surge hopper can be added to regulate the feed which often will allow a smaller, properly sized screen to be used.
Similar to the impacts of a changing feed rate, a changing material stream will greatly affect the efficiency of the screen. An increase of fines in a material stream will result in a greater screen area needed to pass those fines. A decrease in density of the stream will result in a greater need for conveyance and adjustments to the screen motions. Understanding the variability of your feed stream allows you to size for your worst case situation. In cases where the feed stream changes in such a way that the screen becomes oversized, actions can be taken by operators to reduce the screen area by blanking off or bypassing areas of the screen to reduce detrimental effects of running under capacity. Few adjustments can be made during operation to compensate for under sizing of a machine without having additional effects on the operation and products. It’s better to oversize than undersize.
In a size reduction operation, whether shredding or crushing, the size reducer does not produce a uniformly sized product. Typically the output from a reducer represents a distribution of particle sizes with some particles much larger than the desired size and some particles much smaller. Often a screen is used in a quality control function to eliminate the oversize or unreduced material passing through the reducer and recirculate the oversize back to the same reducer. This scenario is called a recirculation load. Every reduction machine has an efficiency, where the efficiency is represented by the ratio between the oversize in the feed and the oversize in the discharge. Understanding the efficiency of the reducer in a recirculation scenario or reduction circuit is critical to sizing the screen. Each time oversize is returned to the reducer, the load to the screen will increase until it reaches a stable point. For example, with a reducer operating at 50% efficiency and a feed rate to the system of X, the actual load to the screen will be 2X and the rate of material passing through the screen and system will be X. For a reducer operating at 80% efficiency and a feed of X, the actual load to the screen would be 1.25X. For a reducer operating at 20% efficiency, the load to the screen would be 5X. With an undersized screen operating inefficiently in a circuit, excess material will be returned to the reducer, further lowering the reducer efficiency and compounding the load within the system. Properly sizing the screen in a circuit is critical to the function of the system.
The depth of the material on the screen affects the ability of the screen to stratify the material and allow the fines to pass through the deck; this is called bed depth. If the bed depth is too deep, the material will not stratify and fines will be carried over the screen without encountering a hole, resulting in inefficient screening. If bed depth is too low, the agitation of the material from the motion of the screen will result in material bouncing and spending less time in contact with the screen deck. When the particles are not in contact with the screen media, there is less chance of them passing through the deck resulting in reduced screen efficiency. With low bed depth, elongated particles get a chance to stand on end and pass through the screen. Maintaining proper bed depth keeps them lying flat and limits their passage. A good rule of thumb for bed depth is that the height of the material depth above the screening surface at the discharge end, should be no deeper than 4 times the opening sizing on the screen. Following this rule helps maintain screening efficiency.
Bed depth should be uniform across the width of the screen. Although material may have a uniform bed depth at the discharge end of the screen, the feed end is critical and where stratification takes place. If the depth is not even, it will be excessively deep in the middle of the screen, resulting in poor stratification and carryover of fines. It will also be too light on the sides of the screen resulting in passing of elongated oversize pieces. Care should be taken to spread the feed across the width of the screen and not to feed in the middle without proper feed distribution. To help mitigate the effect of bed depth, factors such as the angle of the screen, the width of the screen and the length of the screen should be adjusted.
Increasing the angle or adjusting the stroke can help to control the bed depth. Increasing the angle of the screen for an inclined screen increases the travel speed of the material, helping to reduce the height of the material at the feed end. The stroke and movement of other types of machines will also help the material travel more quickly. Although this will help you reduce the bed depth, increasing travel speed will result in the material being in contact with the screen surface for a reduced amount of time, having less chance of passing through the screen and resulting in reduced screen efficiency. Increasing the length of the screen can help compensate for this travel speed by providing additional dwell time on the deck, increasing efficiency. As a rule of thumb, screen length is for efficiency and screen width is for capacity. Increasing the width of the screen allows the same load to be spread over a wider area at start of the screen, reducing the bed depth and providing a chance for proper stratification of the material. Understanding how these characteristics of your screen interact with the required load of the screen help ensure proper bed depth on the screening surface for maximum screening efficiency.
When choosing the opening of a screening surface, there is the nominal size of the screening mesh and the effective size of the mesh. Because of crowding of particles on the screening surface there is a reduced probability of near size particles passing through the mesh. The nominal size of the mesh is the actual diameter of the opening on the deck. The effective size is the size of the product that the mesh produces. In a given application, the chosen nominal mesh size will produce an effective product size smaller than the nominal size. When this concept is understood, this knowledge can be used to increase screen capacity in a given footprint. Because larger openings mean more open area in a given surface, the probability to pass material of a given size increases and the capacity increases. Using larger openings in the areas of the screen with crowded screening will increase capacity while maintaining the effective screening size. As the majority of the fines are already removed towards the discharge end of the screen, using a smaller screen opening closer to the effective size will result in less passage of oversize.
The design of the openings on the screen surface help to determine the efficiency of the machine, the capacity of the machine and the quality of the product. Open area on a screen, the ratio between the area of the holes and the screen surface, helps to determine the screen capacity. By decreasing the space between holes, webbing or wire diameter, the open area may be increased, increasing capacity. The trade-off in reducing the wire diameter or webbing between holes is reduced strength and media wear life. Adjusting the shape of the opening on a deck will also affect the open area. Round holes provide the most accurate sizing, the lowest open area but greater strength and wear life. Square openings provide better open area but slightly reduced sizing accuracy because of the possibility of particles to passing through at a diagonal. Slotted openings provide the most open area but an increased risk of elongated material passing through or material passing through the length of the slot. Slots also help reduce plugging or blinding by increasing the flexibility of the screen media. Because screening is largely a gravity driven operation, placing a slot on an incline will reduce the effective length of a slot, limiting the oversize problem and still receiving the benefit of increased open area. Choosing a proper screening opening to maximize screen efficiencies while reducing the trade-offs should not be overlooked in screen sizing.
Screening is often given a cursory glance in a process but can have great effect on the capacity of a system and the quality of your products. Often the screen is one of the least expensive pieces of your system while having an outsized impact on your profits. Efficient screening is the quickest return on investment within your plant. Although outshined by processes that physically transform a material stream such as size reduction or color sortation, proper screening optimizes all these processing steps. Because the effect of screening is less observable it is often overlooked. Do so at your own detriment.
This article is a cursory glance at the basics of screening. Please contact us with any specific questions about your process or topics that you would like to investigate further. AEI is also available for screening consultations and education classes for your operations and engineering personnel. Please feel free to contact us at sales@AEIscreens.com or (717)-656-2131. We specialize in the most demanding screening applications.
Below is a list of references used in compilation of this blog and for additional reading on the topic:
Kelley, Errol and David J. Spottiswood, Mineral Processing Short Course, Colorado School of Mines, June 2012
Kelley and Spottiswood, Indroduction to Mineral Processing, Denver: John Wiley & Sons, 1989.
Vibrating Screen Manufactures Association, Vibrating Screen Handbook, Stamford, 1980.
Korte, DJ, Dry Screening Tests Conducted with Bivi-TEC Screen, South Africa: Coal Tech, 2008.
Aggregates Equipment Inc, Bivi-TEC Operations and Maintenance Manual, Leola, PA: 2012.