An aggregate operation is one big, unbroken chain with screening being the link to high-capacity, multiple-product output. Its function in the processing flow is twofold: to size and separate material ahead of crushing or washing circuits; and to size and separate material in preparation for final product stockpiling. Bottom line, crushers produce the material; screens separate the material; and screening efficiency affects the efficiency of the crushers.
Most producers will agree that vibrating screens must be properly selected and designed, or they will be the biggest bottleneck within an operation. That’s important to note — as today’s trend is toward larger screens to increase capacity within larger plants. “Everyone wants more tons per hour across the screen. However, the key to optimum screening is maximizing capacity without losing efficiency. That may involve a good amount of trial and error, as there are a lot of operating parameters to consider,” said Joe Schlabach, vice president of marketing and sales for Deister Machine Co. Inc.
Schlabach stresses that screening is both art and science. The art of screening lies in the meticulous fine tuning, tweaking and synchronizing of screen setups within a limitless number of applications. Its science is described as that of stratification. In other words, the vibration of the screen deck agitates the material causing it to stratify, allowing the larger particles to remain on the top deck, and the smaller particles to fall through the openings of the screening surface. Screening efficiency is calculated as the percentage of the undersize passing through the openings divided by the percentage of undersize in the feed. For example, if a screen is only 75-percent efficient, then 25 percent of the material within the desired product range is being rejected with the oversize material.
Operating parameters and proper bed depth
Schlabach explains that maximum screening efficiency results from proper adjustments in speed, stroke, rotation (or throw) direction and angle of inclination. Each of these parameters affects one of the most important facets in screening — proper depth of bed.
As feed material is a mixture of varying sizes, oversize material will restrict the passage of undersize material, which results in a build-up or bed depth of material on the surface of the screen. Bed depth diminishes as the undersize material passes through the screen openings. For efficient screening, the material bed should not reach a depth that prevents undersize from stratifying before it is discharged. The industry rule of thumb is this: Depth of bed (in dry screening) should not exceed four times the opening size at the discharge end of the screen. Consequently, with a ½-in. opening, the depth of bed at the discharge end should not exceed 2-in., for example.
Schlabach says that loading screens too heavily is a common practice, and one which leads to a carryover problem and less screening efficiency. He shares some basic issues regarding operating parameters and the need to experiment to find the optimal balance.
- Increasing speed has its trade-offs. Speed may decrease depth of bed, but also increases the G-force, which decreases bearing life. Using the proper opening size for the desired particle separation, along with increased speed will leave a minimal percentage of desired product size in the oversize. Alternatively, combining increased speed with a slightly larger opening size may allow a percentage of oversize in the desired product specification.
- Increasing stroke delivers a higher carrying capacity and travel rate, while reducing plugging, blinding and enhancing stratification; however, it can also create some inefficiency when lightly loaded decks lead to material bouncing. Generally, coarse separation requires increased stroke and less speed, while fines separation needs less stroke and higher speed.
- Rotation (or throw) direction can dramatically impact incline screen performance. Running counter flow or uphill, increases material retention time and action on the screen, potentially giving the particles more opportunity to find an opening — and ultimately increasing efficiency. Direction of rotation has little effect on a linear-type horizontal screen.
- Increasing the angle of inclination causes faster material travel, which can be advantageous in certain dry screening applications. Although, there may be a point where too much incline may hinder efficiency as fines may roll right over the media rather than passing through. Consider adjusting both linear and triple-shaft horizontal screens for inclination as well. One can realize some gain in capacity, rate of travel and productivity by adding some incline to the horizontal screen.
Incline vs. horizontal screens
Schlabach says there are a limited number of applications where a horizontal screen is more suitable than an incline screen. These may include portable applications or plants where proper clearance for an incline is not available; or applications with heavy water use, such as a dredge-fed screen. Overall, he says, most producers realize than an incline screen is the optimal choice.
Aside from the fact than an incline screen is lower in initial cost than a horizontal unit, Schlabach stresses than an incline model is less prone to plugging and uses gravity to reduce its energy and horsepower requirements. He explains differences in rate of travel between the two units. At 45- to 50 ft. per min. (and at a specific tonnage), a horizontal screen will experience diminished capacity due to a greater depth of bed. Alternatively, on a 20-degree incline, and at 70- to 75-ft.-per-min. travel rate, an incline screen will deliver up to 25-percent more capacity than a linear-stroke horizontal machine. Unlike the latter, the circular motion of an incline screen results in less stress to the vibrating frame.
Proper screen specification
Specifying the right screen involves making sure that the manufacturer understands your production goals and is supplied with complete application data, which includes information such as tons per hour, material type, feed gradation and top particle size, particle shape, application type (wet or dry?), type of screen media and deck opening, and the method of material feed. Armed with accurate information, the manufacturer can customize the screen setup for maximum performance. For example, with a known feed gradation, the manufacturer can analyze the loading on each deck. If a deck has a heavier depth-of-bed ratio relative to the opening, that deck may be specified at a steeper angle than an accompanying deck. Therefore, one might have an incline screen at 20 degrees on the top deck, and up to 24 degrees on the bottom deck where it’s more heavily loaded.
Application problems and solutions
The main obstacles to efficient screening are plugging, blinding and carryover. Each can be minimized with a variety of solutions.
- Plugging happens when near-size particles become lodged, blocking the openings. Solutions may include increasing stroke, changing media wire diameter or opening shape, using urethane or rubber media, and adjusting crusher settings.
- Blinding is caused by fine particles that stick to the surface media due to moisture, and gradually cover over the openings. In this case changing stroke and increasing speed may help. Also, if changing the screen media does not improve the situation, one may consider ball trays or heated decks. Ball trays incorporate rubber balls into pockets beneath the screen cloth. As the machine vibrates, the balls strike the media to free collected material. Heated decks have an electric current in the wire that heats and dries material, so that it easily knocks itself loose as the screen vibrates.
- Carryover is when excessive undersize particles fail to pass through the openings. Solutions may involve changing stroke, speed, or reversing screen rotation; changing wire diameter or the shape of the opening to increase open area; changing the angle of inclination; changing feed tonnage; controlling feed segregation; and centering feed on the screen.
Proper screening delivers big benefits. It prevents undersize material from passing into the crusher, minimizing wear and tear on the crushing plant. It also produces accurately sized products. Above all, mastering the art and science of screening improves efficiency while also increasing capacity, reliability and profitability.
Screen media
Every producer wants higher capacity, lower costs per ton, and less downtime. With that said, choosing the proper screen media for a given application is the key to delivering sizing accuracy and maximum throughput within the screening circuit.
Basically, screen media can be described as the mesh or screening surface applied to each deck of the vibrating screen. Typically the screen box contains two or three decks, with each deck having a different sized mesh for the separation of various particle fractions. The screen surface or media is made of one or more removable screen panels.
Media specification
There are many screen media options as to material type; opening size and type; and fastening style. It’s important to work closely with screen media and screen box manufacturers to design the screens that best suit given applications.
“Consider all the factors that affect an operation when selecting screen media. The lowest initial price may be the highest overall cost. So the primary consideration should be cost per ton,” said Jon Anderson, applications engineer at Durex Products Inc. (now Weir Minerals).
Anderson stresses that producers should supply screen media manufacturers with complete, accurate application data during the specification process. Factors that affect media choice are many, but largely concern the given screen box parameters, material characteristics, and production goals. For example, the material specification that must be produced will determine the type of screen opening chosen.
During the selection process, producers should address numerous questions such as: Is the material wet or dry? Will blinding or plugging be a problem? How abrasive is the material? Will there be much impact on the screening surface? What is the top size and the bottom size fed to the screen deck? How much screening area do I have?
Do I need to wash the material? Do I need to be concerned about noise? What is the cost and life expectancy of the media?
Importantly, producers should also examine the issue of maximum open area versus maximum wear life. Decide if more open area (more screen throughput) or longer wear life (available with urethane and rubber screens) is important — as one cannot have the maximum in both. With wire cloth, some wear life will be sacrificed, and with synthetic screens, some open area will be sacrificed.
Media material
Screen media material choices include wire cloth; perforated and flame-cut plate; polyurethane; rubber; and hybrid media, which combines wire screens with polymer strips.
Originally, wire cloth or perforated steel plate were the only choices, until more wear-resistant materials such as urethane and rubber came on the scene. “If an operation is changing out wire cloth screens more than five times per year — due to wear — then rubber or urethane is highly recommended,” says Anderson.
Recent developments in media include improved wire cloth media with higher tensile strength for longer wear life; and also a greater demand for the hybrid screen which delivers longer wear life and increased throughput.
- Wire cloth: Wire cloth is the best option if an operation has frequent media changeouts due to varying product specifications. Wire-cloth media can comprise high carbon wire, tempered high carbon wire or stainless steel wire. Carbon wires are best in high-impact applications, while stainless steel reduces blinding and is better against corrosion. Surface openings may be square or may include rectangular slots, long slots or accuslots. Each has advantages and disadvantages. Square openings offer the most accurate sizing, longest wear life, but the lowest open area. Rectangular slots increase throughput, reduce plugging and blinding, but compromise sizing accuracy. Long slots offer high open area and production, less plugging and blinding, but also offer less screening accuracy and the passing of flats and slivers. Accuslots deliver all the advantages of the long slots, but also prevent the passing of flats and slivers. As such, the use of accuslots is growing in popularity.
- Perforated and flame cut plate: A good alternative for secondary screening, plate screens are available in various steel types and hardness. To provide greater abrasion resistance and longer wear life, steel plates have seen recent improvements in quality with options available all the way up to the 400- to 500-Brinell range (a measurement of the hardness of the steel plate), a range suitable for top or middle decks with moderate to large, highly abrasive feed.
- Polyurethane: Available in different durometers, urethane is generally applied in wet applications, or where water is added or is in slurry form. Urethane is also the best choice for dewatering. Urethane is not well suited to dry applications. Abrasion-resistant and able to absorb moderate to high impact, urethane media is offered with square, slotted, zipper or polyaccuslot openings. The latter delivers high open area and prevents plugging and blinding. New chemistries and formulations have further improved wear life in urethane media.
- Rubber: Rubber media is ideal in dry, high-impact applications. It may also be recommended in a wet-screening application such as where a plant is processing only natural sand and/or gravel. Self-cleaning rubber screens are often used in fine, sticky, or near-size material applications. Openings are square with a molded taper relief — to prevent blinding from fines buildup, and to gain greater sizing accuracy.
Hybrid screens
Hybrid screens combine wire screens with rubber or urethane strips, depending upon the manufacturer. This screen type is a highly popular option in dry screening applications where materials contain high-moisture content and a high proportion of fines. The accuslot-type openings (zigzagging wire) minimize the opportunity for chips and slivers to pass. The screen’s high-strength wires are held in place by molded rubber (or urethane) strips, allowing the optimal flexing action required to keep screen openings clear. While rubber molded strips provide superior wear life in all applications, they are particularly ideal in hot and humid environments where thermoplastic polyurethane is not as effective.
Wear life
Basically, wear life for any type of media is largely determined by its mass. What is the diameter of the wire or the thickness of the urethane? Bottom line, is the media heavy enough to handle a given top size material and peak feed rate? Synthetic screens (rubber or urethane) will wear far longer (often more than 10 times longer) than wire or plate screens.
Modular media systems
Modular-paneled synthetic screens allow changeouts of only the worn areas, or the rotation of panels to extend wear life. Ongoing improvements in modular systems have further extended potential wear life, with an increase in open area as well. Newly designed modular systems offer greater ease of installation (without additional parts), and are better engineered for retrofitting applications.
As an example, the criteria for optimum screen media at a new Tennessee-based sand and gravel plant is increased wear life and uptime, greater open area, and minimized blinding problems. Modular-screen systems are installed in each of the triple-deck screens. The producer finds that going with urethane screen media versus wire is one of the major ways to get more production from the new plant. The urethane is a higher capital investment, but they have increased uptime significantly. The operation was getting anywhere from 4- to 6-weeks maximum with wire. With urethane, they are getting at least 18 months. Minimizing screen changeouts is also an advantage, especially since the dual 8-ft x 20-ft. screens operate atop a 66-ft. tower. They also like the convenience of the modular systems as they can quickly change a couple of 1-ft x 4-ft. urethane panels rather than replacing the entire screen. And, since they have wear on only a portion of their screen — the smaller the panel, the less they have to replace.
Screen media installation
Screen media is attached to the frame in any number of ways. Proper installation, which includes tightening or tensioning the screen surface against the supporting frame, is integral in prolonging the life of the screen. Also, clamp rails must fit properly and there must be good bucker bar rubber underneath the screen. “The biggest cause of premature screen failure is improper screen installation and a lack of maintenance. Install them right and keep them tight,” says Anderson, who adds that after initial installation, run the screen for five to ten minutes and then repeat the tightening process. Recheck the tension at the end of each shift.
Maximizing efficiency
Screening experts agree that it is impossible to be 100-percent efficient; however, with the right equipment, the right operating parameters, and the right installation and maintenance — operations will certainly maximize efficient, high-capacity screening.
FIXING THROUGHPUT PROBLEMS
When flex-mat-type self-cleaning screens were introduced to North America in 1996, they were the first entry in a new screen media category. The term “flex-mat-type” came from the trademarked name of the first product on the market. Up to that point there had been screens on the market that were able to claim some self-cleaning properties — most were made of slotted woven wire or “piano” wire (also known in the U.S. as harp wire). But even when these screens did clean themselves of material build-up, they often slowed production in many applications. Flex-mat-type screens, however, became a real solution to eliminate serious blinding (clogging) and pegging problems on one or more screen decks, while significantly increasing material production.
Blinding or clogging is the build-up of fine material on a screen deck — a common problem with humid or moist material. Blinding creates production problems because it often greatly reduces material throughput on the screen. If the screen box’s shaking action is not strong enough to clear the deck of fine material, the screen’s openings become clogged. Eventually the entire screen can be covered with material that won’t fall through the openings — defeating its purpose of sizing the material into different specifications. Screens can also experience problems with pegging. Pegging typically occurs on the top and middle decks of screens, or in washing applications, when stones that are near to the correct size fall into the screen openings, but cannot completely fall through — effectively plugging the openings. Once they’re pegged, screens often become blinded by fine material, as well. If a screen is blinded/clogged or pegged badly enough, it will cause production rates for the entire plant to fall — because the screens either continue to run at reduced capacity, or the operation must shut down regularly to clean its screens.
Design is key
Flex-mat-type screens are made of separate unwoven, crimped wires, running from hook-to-hook on the screen and bonded in place with polyurethane or rubber strips that run across the wires and center on each of the screen box’s crown bar supports. When they are installed, the screens are stretched “tight as a drum.” The screen’s design, with proper tensioning at installation, causes the wires to vibrate independently at different frequencies with the shaking of the screen box, preventing material from accumulating between the wires — while still allowing only the proper spec material to fall through.
Flex-mat-type screens typically are offered in diamond, triangular and wave patterns, with different size openings for different applications. The diamond configuration is used for precise material sizing. The triangular screen pattern can either have heavy-gauge wire with large openings for high-impact applications, or smaller-gauge wire and small openings for fine screening. The wave configuration is used to remove fines, and also for materials that are difficult to screen — such as compost or other wet materials.
Flex-mat-type screens work very well to eliminate blinding on bottom decks. To solve severe blinding problems, installation of flex-mat-type on both the bottom and middle decks might be needed. With the correct opening sizes, these screens will also virtually eliminate top and middle deck pegging.
Because they are specialized, premium products, flex-mat-type screens have a higher initial cost. But the return on investment (ROI) quickly exceeds the cost in most cases because the screens can help increase a plant’s production by up to 40 percent. With this increased throughput, many producers can avoid the cost of installing additional high-frequency screen boxes. And with the vibrating action of the wires causing fines to reverberate off the rock, flex-mat-type screens also have allowed some producers to eliminate the need for washing equipment to get a clean product.
Measurable benefits
- Eliminates blinding (clogging) and pegging on all three screen decks.
- Ends the need to stop production to clean screens.
- Non-woven configuration creates up to 30-percent more open area than woven wire screens.
- Produces a cleaner product.
- Lasts up to three times longer than woven wire.
- Increases production by up to 40 percent.
Continuous evolution
During the decade after its introduction, several screen manufacturers began to introduce their own versions of flex-mat-type screens — effectively creating a new “flex-mat” category of screen media.
Significant differences exist from manufacturer to manufacturer. For example, some flex-mat-type screens are manufactured with heavy, thick polyurethane strips — even for screens with small openings — or with cross wires embedded in the polyurethane or rubber.
And flex-mat-type screens continue to evolve. By 2006, through continuous R&D, its original manufacturer introduced screens with greater wire diameters and opening sizes — from 30 mesh to 4 in. — to handle a larger number of screening applications. Because of this, flex-mat-type screens became a solution for production problems on all three screen decks.
In 2007, a double-wire version of flex-mat-type screens was introduced. A double-wire flex-mat-type screen is lighter overall, and produces better vibration than heavy-gauge wire in high-impact applications. Ongoing research continues to explore adaptations of flex-mat-type technology to address additional screening challenges and opportunities.
The need for expertise
Because flex-mat-type screens are unlike woven wire, polyurethane or rubber screens in the way they are manufactured and the way they perform, it takes skill and experience to specify exactly what type and size of flex-mat-type screen is right for each application. Experienced equipment dealers or manufacturers must take the time to visit an operation, analyze its screening challenges and offer specific, cost-effective solutions.
An experienced representative will also know how to install the flex-mat-type screens correctly. For example, flex-mat-type screens must be stretched “tight as a drum” to ensure proper vibration. Loose flex-mat-type screens can’t vibrate — so they can’t effectively solve blinding/clogging or pegging problems. Improper installation also creates out-of-spec openings (and, therefore out-of-spec material), and can cause premature wear and breakage.
Sources/Authors: Deister Machine Co., Weir Minerals and Gary Pederson