Chapter 14: Screening
November 1, 2007 By: P&Q Staff Pit & QuarryAn 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.
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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.
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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 1/2-in. opening, the depth of bed at the discharge end should not exceed 2-in., for example.
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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.
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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.
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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.
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