March 1, 2013
A three-stage blending process can yield maximum efficiency and prolong wear life.
By Florian Festge
They say good things come in threes. In the case of screening for aggregate operations, profit comes in three distinct phases: layered, basic, and sharp screening. But how good they are — whether they maximize efficiency and improve the potential for success — depends on having the right screen media on the deck for each of the three phases.
A relatively new method for achieving that goal is a blended screen media approach that incorporates the right screen media for each phase and the right mix of media for the process overall. The goal in implementing a blended solution is to find the optimal combination of open area and durability. Hit the sweet spot, and an operator can maximize product quality and profits by increasing efficiency of classification while minimizing downtime and maintenance costs.
Three distinct phases and outcomes
During the first phase of screening, the layered phase, a deep bed containing coarse and fine particles hits the screen at the feed end of the deck. In the basic phase, the particles begin to stratify as fine material settles at the bottom and larger material climbs to the top of the bed. The sharp phase occurs toward the discharge end of the deck. It is during this final phase that the near-size particles move into direct contact with the screen media and have the last opportunity to fall through the openings.
There are typically two scenarios aggregate operations want to avoid when it comes to screening.
The first situation is the result of a deck that completes screening too early. In this scenario, particles travel only about a third of the way down the deck until all the undersize material has passed through the openings. While the main mission — classifying the material — is still achieved, the entire deck isn’t being used to its full capability. The results are diminished profits from premature screen media wear.
Consider this example: An operation produces 400 tons per hour and charges $10 per ton. Due to high abrasion, the media with high open area wears out quickly and needs to be replaced after only two weeks of operation. Replacing the top deck takes two people roughly four hours. At about $4,000 an hour of product not being processed and sold, that screen change-out costs $16,000 in lost revenue alone.
The second scenario involves screening that does not finish completely. In this case, undersized particles travel over the discharge end and contaminate the material. This can double a company’s production cost if the material needs to be re-screened, not to mention the lost efficiency and time. And if the material ends up too contaminated to be sold, profitability is lost altogether. In the worst-case scenario, a company may not recognize it is producing contaminated product, which can lead to costly warranty claims and negative image.
Consider this example: The same operation produces 400 tons per hour, and it costs the company $6 per ton to produce the material. Due to insufficient open area at the discharge end of a vibrating screen, the media does not allow the near-sized material to pass. As a result, the final product is contaminated and must be reworked. This means that the company needs to rework its material for a total cost of $12 per ton. With a sales price of $10 per ton, the company is losing $800 per hour until all of the material is re-processed.
A possible third scenario, optimal screening, is the ultimate goal of every operation. The bulk of undersized material passes through the openings about two-thirds of the way down the screen deck, and the last third of the deck allows all the near-sized particles to find an opening. The final product meets specification and can be sold for top dollar. However, while a company is often satisfied with achieving optimal screening, it seldom recognizes the potential to increase profits even further.
Consider this example: The same operation produces 400 tons per hour and charges $10 per ton. Its screening is considered optimal, and it is producing high-quality product. Due to greater impact at the feed end of a vibrating screen, the screen media at that end wear out sooner than the remaining panels and need to be replaced. Replacing the first section takes two people roughly one hour. At about $4,000 an hour of product not being processed and sold, that screen change-out costs $4,000 in lost revenue.
In all three scenarios, the key to achieving the most profitable screen surface is finding the right balance between wear life and open area — the amount of open space on a screening surface. Increasing open area normally leads to a decrease in wear life, while reducing open area usually increases the wear life of the screen media.
Five steps to optimal screening
Customized solutions that blend screen media types on the same deck ensure the longest product life, limit downtime, and maximize profit potential. A professional analysis will determine the right combination of open area and wear life appropriate for each of the three phases of screening. But determining the ideal blended media solution requires much more than trial and error. It requires a trained professional to delve into the numerous intricacies that impact the bottom line.
A professional specializing in optimized screen media generally follows five integral process steps, which can be applied to deck types using tensioned screen sections or modular screen panels. These include the following:
Vibration analysis. Using a wireless tool specifically designed for vibrating screens, the professional can detect faults and conduct measurements of the machine’s dynamic operating conditions. Prior to making any changes to the screen media, the measurements are imported into a computer system that uses a tuning wizard to optimize mechanical performance through recommended machine settings.
Machine inspection. The results of the vibration analysis also build the foundation for a complete machine inspection of body components, the suspension system, and wear parts. Special attention is given to screen media supporting parts such as bar rails, rail liners, and tension rails.
Screen media audit. A complete review of the current screen media setup is conducted. Using a consultation guide, the audit will take the phases of screening into account while evaluating material characteristics and overall product quality. The professional will also examine environmental factors, screen condition, signs of premature failure, and any evidence of blinding and pegging. Blinding happens when moist granules fill the opening and clog the screen surface, while pegging refers to aggregates mechanically lodged in the opening of a screen.
Screen media recommendation. The results of the vibration analysis and the screen media audit build the framework for the screen media recommendation. Recommendations will be based on the correct balance between performance and durability, while catering to the appropriate phase of screening. The professional will recommend screen media technology based on categories such as anti-pegging and anti-blinding characteristics, open area, and wear life. All recommendations should be visualized using computer software that allows for the personalized configuration of a deck.
Screen media implementation. After all recommendations are discussed and a solution is agreed upon, the final process of implementation can begin. Implementing the blended screen media gradually, by replacing one screen section at a time, allows the professional to monitor the levels of production and effects of the adjustments. With each change, production rates and final output qualities are evaluated to ensure the screen media combinations are becoming more efficient. This process is continued until the best possible results have been achieved. It’s all about making modifications for better return on investment — using the proper media for each phase of a unique screening operation.
Screen media options
Whether feed materials are wet, dry, or exceptionally abrasive, selecting the correct screen media is the key to a professionally blended deck. Open area and wear-life requirements are different during the individual phases, requiring a unique approach to each.
The market boasts numerous screen media options, and each offers a unique set of features for individual applications. Some of the most effective products that professionals may consider during their blending approach include the following:
Polyurethane — Among the more durable materials used in screen media, polyurethane provides a long wear life with little maintenance. The flexible polymer is resistant to tears and abrasion, while premium polyurethane products also offer tapered openings that reduce blinding and pegging. This makes the material a good choice in higher wear areas in both wet and dry applications, but these positive characteristics traditionally come at the expense of open area.
Woven wire — At the other end of the spectrum is woven wire. It is commonly referred to as wire cloth or wire mesh screen, and it comes in a variety of alloys, weave types, and diameters. Woven wire offers some of the greatest open area and can be used for a wide range of applications. It also is known to make the “cleanest” cut when referring to accuracy. These strong advantages come at the expense of wear life, but fortunately not all woven wire is the same. When selecting woven wire, wire with very high-in-carbon content allows for optimum characteristics in tensile strength and ductility.
Hybrid screens — The newest screen media technology is hybrid screens. Hybrid screens combine the excellent wear-life characteristics of polyurethane with the great open area of woven wire cloth. This combination lasts up to six times longer than traditional wire cloth, but offers as much as 80 percent more open area than modular polyurethane. The hybrid’s tapered openings either accept or reject near-size material to provide a self-cleaning effect, which eliminates nearly all blinding and pegging. The result is a screen that is extremely durable and highly productive.
Self-cleaning screens — Self-cleaning screens combine individual wires using flexible polyurethane strips, allowing the wires to move at frequencies higher than the G-force generated by the vibrating screen and eliminating blinding and pegging. The unique design of flat lying wires even offers higher open area than woven wire, while outlasting it by up to 25 percent. Through careful attention to the various phases of screening, a thorough analysis of the site’s needs, and use of the right mix of media, an operation can extend screen life, reduce downtime, and improve production of high-quality products.
Ontario operator improves the bottom line
For a company in Madoc, Ontario, about three hours northeast of Toronto, Canada, implementing a customized blend of screen media positively impacted the operation immediately.
Danford Construction Ltd. has been a family business for more than 50 years. Its Aggregates Division handles a variety of materials in a granite quarry, two limestone quarries, a traprock quarry, and eight gravel pits.
In 2011, Danford called in W.S. Tyler as a consultant to help customize screening for its granite operation. Following vibration analysis, Danford customized its deck with the ideal blend of screens.
“We are getting eight times the wear life out of the new screen set-up, adding two weeks production time per year, and have reduced our screening costs by 75 percent as a result,” says Jamie Danford, owner of Danford Construction, Ltd.
Danford and other companies have learned that expert guidance and fully customized configurations can pay for themselves in the short-term and dramatically increase profitability over time.
Florian Festge is the president of W.S. Tyler Canada, where he is responsible for coordination of strategic and operational efforts within the company’s screening media and vibrating screen divisions. He is a business administration graduate of the Technical University of Dresden and has a master’s degree with a focus on innovation management and production.