The correct choice of media can be critical to the effectiveness of screening. Wire, rubber and polyurethane are all contenders, while suppliers have been focusing on making screen-panel maintenance simpler and quicker.
By Simon Walker, European Editor
|Sandvik advises its customers that it is important to review the whole process when problems occur in screening operations. Understanding media choices and performance characteristics—and having the knowledge to conduct a proper study to identify the actual problem—is key to finding an effective solution.|
Last month (May 2014, pp.66-71), E&MJ looked at some of the major suppliers of screens and screen media, with specialties ranging from dewatering in coal washeries to aggregate production, scalping and trash capture in carbon-in-leach (CIP) plants. Operators clearly have a large number of choices, with some manufacturers offering packages of both the screen hardware and the media to go with it, while others specialize in the development of media that can be fitted to other companies’ screens.
As a follow-up to that, E&MJ asked a number of industry experts for their views on developments that have taken place in screen media over the past five to 10 years. The questions covered topics such as the materials now available for different applications, the design of screen media, and problems that they commonly encounter in connection with the incorrect use of media on screening plants. They were also asked for their advice on how screening costs can be reduced, as well as their predictions about screen-media developments and usage trends in the future.
A Question of Materials
E&MJ’s first question concerned developments over the past five to 10 years in the materials used in screen media, and their durability.
Derrick Corp.’s sales manager for North America, David Perkins, told E&MJ that the company has continued its quest to produce the finest, long-lasting screen panels in the world. Derrick uses its own proprietary blends of urethane materials to achieve a panel that is robust, while maintaining the highest open area available, he said, noting that in 2013, Derrick produced its finest urethane panel yet at 45 µm (325 mesh).
According to Sandvik, over the past 10 years, it has developed a complete range of screening media, encompassing modular, reinforced and tensioned panels. In applications with high abrasion, such as hard rock and metalliferous ores, it recommends the use of a high wear-resistant rubber, with the flexibility of the rubber being chosen based on the separation and application criteria. Separation at smaller sizes requires more flexible rubber, while heavy-duty applications (coarse materials and larger separations) require stiffer media. The quality of the rubber is key, with each product needing a high-performance rubber with outstanding mechanical properties, the company said.
Jeff Easton, a principal process engineer at WesTech Engineering, told E&MJ that since the mid-2000s, the company has put significant effort into improving its specialty screening products such as its media retention screens. Adsorption carbon as well as newer synthetic resins must be retained in a leach vessel by the inter-stage screen while at the same time allowing the mineral slurry to pass through, he explained.
“Wear on the screen media can be significant, impacting not only longevity but also consistency in the retention aperture over time as well,” he said. “WesTech has combatted these challenges by utilizing a high wear-characteristic alloy and steeper wire wedge angle, allowing its screens to not only resist wear, but also maintain an appropriate aperture range ensuring less loss of near-size value-loaded carbon or resin.”
Speaking for the Canadian company, Major Wire Industries, President and CEO Jean Leblond said that high-performance, self-cleaning screen media using polyurethane strips have been achieving increasing market awareness, replacing woven wire on the one hand and polyurethane or rubber on the other. The company’s Flex-Mat 3 eliminates virtually any blinding and pegging, he said, and accelerates stratification due to its independently vibrating wires. “It lasts between three and seven times as long as woven wire, giving up to 50% more higher-quality production,” Leblond said, adding that Flex-Mat 3 produces up to twice as much as polyurethane or rubber media for users looking for more tons per hour.
Les Naday, mining industry manager for Polydeck Screen Corp., said there has been some improvement in wear resistance and durability because of new formulations that have become available for rubber and polyurethanes. These formulations have proved to be beneficial in specific cases, he said, emphasizing the way in which manufacturers can now tailor their screen media products to meet individual operations’ requirements.
|FLSmidth’s screening experts point out that the company’s use of high-quality spring-steel frames allows modular screen media to achieve a much higher ‘open area’ while reducing the installed mass.|
For FLSmidth, the company’s general manager for screen media, David Sibley, also pointed out that while investigation into improved materials for the manufacture of polyurethane screen panels is ongoing, none of the many materials tested in the last few years has demonstrated qualities over and above the material currently in use. In addition, the use of high-quality spring-steel frames has enabled modular screen media to achieve a much higher “open area,” with percentages of up to 55%, while reducing the installed mass.
The company has also introduced hybrid modular polyurethane panels using different types of screen media such as wire screens, perforated material and wedge wire to bring different characteristics to modular screening.
Screening specialist for Tenova Delkor, Rhett McElvenny, looked at materials used in linear belt screens. “The screening medium used for the linear belt screens has historically been a woven polyester mono-mono construction,” he said. “Some duties can use polypropylene cloths. However, these pose tracking problems, particularly on larger machines. Some duties may also use metal link mesh as the screening medium, but these are quite specific to a particular duty.”
Screen Media Design
Having asked the experts about screen media developments, E&MJ extended this to the design of screening media. Have there been any noticeable changes in this area, such as the wider use of modular designs?
Naday (Polydeck): Modular media has taken hold in mining applications, allowing the mixing of various duties to meet maintenance cycles. Most large mines take advantage of modular panels’ flexibility as well as their safe and easy installation and removal.
LeBlond (Major Wire): Around the world, large mining operations use polyurethane or rubber while smaller operations are largely using woven wire.
Easton (WesTech): “Specialty screening products differ significantly from traditional vibrating products in that the material being screened does not necessarily have to move across the screen surface. This opens a much wider range of possible materials and configurations that can be used for screening media.
“For example, WesTech’s linear screen utilizes a moving belt of screen fabric with flexible roving, allowing the media to wrap around return drums. The fact that the media moves to the discharge eliminates the need for the material itself to slide and wear on the media surface. This configuration also allows for easy cleaning of the media in the return cycle, eliminating pegging and plugging issues.”
Sandvik: More and more customers are realizing the benefits of modular media: easy to handle, fast installation and moulded holes with a relief angle. This prevents problems with pegging.
McElvenney (Tenova Delkor): The media have not changed greatly; the medium is a woven cloth and the aperture determines the cloth model used. “Tenova Delkor has increased the size of the actual screening device in the past five years from 25-m2 units to 32 m2 and more recently, to a 40-m2 machine,” he said.
Perkins (Derrick): The company is committed to manufacturing urethane panels for exclusive use on screening machines designed and manufactured by ourselves. There is a strong relationship between the performance of the panel and the machine it is installed on. “We believe that this is truly a situation where one-plus-one is greater than two, so to speak.”
Sibley (FLSmidth): “In most new process plants, screening machines are being sized according to the open area and use of modular screen panels. Further to this, we have developed and supplied our BPS (Bolted Pin System) polyurethane modular screen panels, where the unique design gives a superior open area and reduced installed mass of the panels on the screen decks.”
The advantage of a higher open area and lower installed mass adds to the screening efficiency and the effective consumption and use of input energy. In addition, the BPS panel-fixing method has proved to be user-friendly for plant maintenance staff, with less plant downtime for routine deck replacement, Sibley said.
|Weir Minerals’ Armor polyurethane wire screen media offers the combination of high-grade wire cloth with a molded coating of premium polyurethane.|
Common Screening Problems
E&MJ next asked each of the respondents about any specific problems they see on a regular basis that arise from operators not using appropriate screening practice.
Sandvik: It is important to take the whole process into consideration when problems occur on a screen. Sometimes there is a lack of process knowledge, and adjustments are done on the screening media instead of maybe making changes at the crusher settings, chutes or feeders.
When a screen is not performing, it is important to really understand the underlying reason for the problem. An example might be if there is a problem with through-put, where material is not being screened out and just keeps recirculating. Having the knowledge to do a proper study to identify the real problem is key here.
Even if the problem is shown to be on the screen, the actual cause might be somewhere else. Just making changes to the screening media when it comes to hole sizes and thickness could lead to an endless road of changes that don’t really solve the problem. The cause of the poor screening might instead be non-optimal screen settings, where changes to the speed or stroke could provide the solution.
Another common problem is uneven loading over the width of the screen, or feed segregation, in which case changes to the feeding arrangement is the solution. It is also important to remember that making adjustments to the crusher may also help. In reality, there is often very limited benefit in changing the properties of the screening media before the feeding arrangements and screen settings have been optimized. Our teams often run into “screening problems” that have other causes than the screening media.
Perkins: “Performance problems arise from improper installation of the panels on the machines. Derrick is developing mechanisms that mitigate this problem. However, with such a large installed base worldwide of older technology, coupled with the long life of the machines, this is an ongoing training hurdle for the natural turn-over experienced by most operators.”
McElvenney: “The most common problem that we see is screens being fed with too much coarse material as a result of mill discharge cyclones being overloaded and ‘roping.’ This results in the screen receiving slugs of coarse material and subsequently flooding.”
Leblond: “A large percentage of the woven wire media in use break because of poor installation that is itself a result of a lack of know-how on proper practice. Since 2008, Major Wire and its authorized dealers have given a simplified screen maintenance seminar to more than 6,000 operators to teach best practice in the installation of tensioned screen media on shaker screens.”
|Polydeck says its modular panels are easy to place and remove, but they—like all modular products—must be installed correctly for maximum effectiveness.|
Naday: While modular panels are simple and easy to install and remove, difficulties caused by untrained contractors doing maintenance are seen. Sometimes panels are left in operation after they have failed, causing costly repairs, and sometimes the installation is done incorrectly.
Easton: The greatest difficulties arise when particular screening technologies are misapplied. For example, using a high-energy vibratory screen on material that is very friable can cause significant attrition and particle-size degradation, leading to losses as well as plugging. Often in mineral applications, the need to classify material by particle size is superseded by the need to classify on particle density. If size classification techniques are attempted when both size and density classification are required, the results can be disastrous.
Sibley: “Over a long period, we have observed that screening machines are not correctly maintained or setup for the screening duty they are being utilized for. We frequently find that the amplitude and speed are not appropriate to the cut size, and the ‘g’-forces are incorrect for effective stratification and screening efficiency.
“This can result in shorter screen life from the abrasion caused by deeper bed depths. In addition, poor screening efficiency can lead to higher recirculating loads. This in turn can have the effect of increasing the cost per ton in downstream equipment, such as higher wear rates of crusher manganese and the over-generation of fines in the product.
“As far as spillage is concerned, poor housekeeping and inappropriate maintenance are health and safety risks. They also push up the cost per ton, but can be corrected with proper plant management. The old saying of ‘penny wise, pound foolish’ springs to mind, with the main emphasis being placed on initial cost rather than the cost per ton. Disregarding the overall importance of any screen media used in a process also has a negative impact on achieving the lowest cost per ton.”
|Major Wire developed its line of crown bar adaptors, pictured here, to help producers who had flat screen decks that were limited to only using modular type screen media but wanted to use Major Wire’s Flex-Mat 3 tensioned high-performance screen media.|
Cutting Screening Costs
Here, the respondents gave their views on the measures that operators can take to reduce screening costs within the overall cost of mineral processing.
Sandvik: Make a close study of the process, ensuring to really get what is being aimed for. It might be better overall economy to produce a smaller tonnage of higher-quality material than a higher total tonnage at lower quality. It is most important to fully know and understand the process. Study the screens very closely; in general, there is a lot of focus on crushers, but screens are allowed to run with very little attention—unless problems occur. Having large amounts of unscreened final product will only cause high circulating loads in the process, so causing unnecessary wear on all the other equipment involved.
Naday: Screens do not get the respect given to more costly equipment, even though screening is a critical part of the process that can cause exorbitant costs in other equipment. A screening expert can determine and recommend the optimal parameters for an operator’s specific conditions. Questions to ask include: Is the screen running at the correct speed? Is the media performing its metallurgical duty? Is the media maintenance fine-tuned to the plant maintenance cycle? Are fines adequately removed from the circuit?
McElvenney: As a belt linear screen receives and cleans a considerable slurry flow, it is imperative that it is regularly maintained to ensure free roller movement, which in turn reduces stresses applied to the screening cloths. Flood detection sensors also reduce flood events.
Sibley: As noted above, the cost per ton can be improved with appropriate management. To assist this process, plant surveys will identify problem areas where the plant is not performing adequately or according to its original design. Based on the survey, corrective action can be planned with the incorporation of the latest technologies where appropriate.
Easton: A firm understanding of the classification requirements in a particular application are the best starting tools an engineer or operator can use. Recognizing what the process requirements and constraints are, and reconciling those with equipment solutions, is critical. Having the right screening and classifying equipment to achieve the desired process at the best combination of capex and opex will ensure the highest return on investment.
Leblond: The answer to this question varies depending on whether tensioned media or modular panels are being used on shaker screens. Looking specifically at tensioned-wire media, either woven or Flex-Mat 3, the media should wear and not break. Operators should know how to install screen media so as to eliminate breakage.
Perkins: “The single largest contributor currently to reducing operating costs within mineral processing is the application of the Derrick Stack Sizer. Derrick’s application engineers are enabling concentrators to drastically reduce grinding costs through the use of fine screening to replace more traditional separation devices like hydrocyclones.”
|Derrick Corp. uses its proprietary blends of urethane materials to achieve a panel that is robust, while maintaining the highest open area available. In 2013, Derrick produced its finest urethane panel yet at 45 μm (325 mesh).|
Finally, E&MJ asked the experts how they see screening practice developing, especially in terms of screen media de- sign and properties.
Easton: “Continued development of materials of construction as well as configurations will inevitably improve valuable material recovery. Thinking outside of the box is key to finding better ways of classifying and screening. Just because this is how it always has been done is no longer a valid selection criteria. Undoubtedly there will be those who stick with ‘if it ain’t broke, don’t fix it,’ but rest assured soon they may be left in the dust by those interested in finding a better way.”
Sandvik: More advanced products are to come, both regarding material and design. Customers are requesting longer lifetimes combined with high efficiency. That will not be solved just by using thicker media; it has to be something else.
Leblond: “I see Flex-Mat 3 replacing 75% of the woven-wire screen media and 25% of the polyurethane or rubber media, around the world.”
Naday: More and more focus on the balance between screens and the surrounding equipment is being seen. Attention is needed to producing screen media with large open areas, to material blends, and especially to consistently reproduce modular screen media for maintenance planning and consistent availability.
Sibley: The industry is constantly on the lookout for methods to improve existing processes and to develop new processes for products. It follows that as these innovations are implemented, new screening technologies may be needed, so existing screen media will need to be examined and modified to complement any innovations.
Weir Minerals’ global product manager for screen and screen media products, Kurt O’Bryan, sees a potential trend developing with some customers, where wire screen media could be an option. “Customers are starting to question what kind of media they are using. Some are considering switching from urethane back to wire, increasing efficiency in spite of higher maintenance requirements,” he said.
“Adding value to plant design and operation will become recognized as a new area of development in the near future. Special knowledge and experience with regard to screen media and its application should be considered in the initial stages of plant design, and acted on positively to ensure that when installed, all the criteria affecting the screen media are taken into account so as to maximize the process efficiency.”