Operators of WTE plants and RDF production facilities would prefer no metal in their inbound stream, but they know how to deal with it when it’s there.
Disc screens and magnetic equipment play key roles in helping material recovery facility (MRF) operators recover the aluminum and steel cans and other metallic items brought in by curbside program collection vehicles.
For recyclers, the prices plant operators have received for their scrap metals in 2013 are down from historic highs that were reached in the boom years of 2007 and 2008. Nonetheless, metals producers or other users remain willing to pay for nearly any type of metal that can be harvested not only from MRFs, but also from refuse-derived fuel RDF and waste-to-energy (WTE) plants.
While recyclers have dollars-per-ton incentives to capture the metal that heads into their facilities, at WTE and RDF plants operators also have quality control reasons for deploying magnetic equipment to separate metals from the rest of the material stream.
Higher Priority
Producers of RDF pellets tend to know which materials are desirable as components of their fuel products, but it may not have been their first instinct to concentrate on the removal of metallic contaminants, at least in years past.
Ted Hansen, general manager of Greenwood Energy, a New York-based producer of RDF pellets, says his company continues to upgrade and give greater prominence to the role of magnets at its plant in Green Bay, Wis.
“The bottom line is you can’t have enough metal removal,” says Hansen. “The primary issue is to design your process to accommodate magnets.”
The plant’s operators have learned to pay attention to the depth of the material passing by on conveyors as well as to the speed at which the conveyor is set to make sure the magnets have an adequate opportunity to pick up metal.
“The key is to design your process for the appropriate burden depth, speed and size of the metal being removed,” notes Hansen. “Small metal items are often more challenging to remove than larger metal items [since] magnets attract larger items better than smaller items. If the burden depth is too great, an optimal magnet distance from the material cannot be achieved, or it may be difficult to pull the metal through the burden depth obstructions,” he comments.
Hansen says there can be uses for both drum magnets and overhead belt magnets in RDF applications. “The process layout and types of materials being processed will determine whether a drum magnet or overhead belt magnet is preferred. If the [belt] magnet can readily pull the metal through the materials being processed, a drum magnet may not be necessary. If not, the drum magnet will pull metals from the bottom of the raw material stream.”
Whether it is worthwhile to target nonferrous metals with additional equipment can depend on the inbound material stream with which Greenwood is working, adds Hansen.
“The source of metal is important to consider,” he remarks. “If the producer is manufacturing pellets from woody biomass, it’s probably not critical to have nonferrous separation. If it’s a waste garbage stream, nonferrous separation is very desirable.”
Operators who wish to capture every last bit of metal also can install a metal detector beyond all of the other metal removal steps, notes Hansen. “This can discard metal of any size depending on how the detector is set up.”
Manufacturers of magnetic equipment can offer an array of options to RDF and WTE plant operators, but Hansen comments that budgets for those outfitting such plants are never unlimited.
“In most cases, the capital budget dictates much of the metal removal limitations,” he remarks. “If an overhead belt magnet, eddy current separator and metal detector are affordable, that is the best way to go. If not, it becomes a series of trade-offs that are dictated by metal size and quantity, raw material sources and the process design and layout.”
Some Simple Steps
Fully recovering scrap metal from the municipal solid waste (MSW) stream before it is subject to the WTE process can involve one or more steps and a considerable level of investment in capital equipment for optimal results.
Not every WTE mass burn plant has the space available to run inbound material through a separation process. Others may simply opt not to do any presorting. Steve Hilliard of equipment distributor PennQuip Inc., Maple Glen, Pa., says there are four WTE plants in the region in which he operates and, as far as he is aware, none of them run inbound material through a magnetic presorting process.
Going Old School
As early as 1977, the authors of a paper prepared for the U.S. Environmental Protection Agency (EPA) recommended shredding the inbound stream of materials heading into waste-to-energy plants, in part to achieve increased ferrous scrap recovery.
The 1977 paper “Magnetic Separation: Recovery of Saleable Iron and Steel from Municipal Waste,” co-authored by Harvey Alter and Kenneth L. Woodruff, is still housed on the EPA’s National Service Center for Environmental Publications (NSCEP) website.
“Generally, municipal solid waste must first be shredded to produce a salable magnetic fraction,” write the authors. “In some processing schemes, the shredded waste is air classified,” the authors continue. “If magnetic equipment separation follows air classification, a cleaner and hence higher quality iron and steel can be recovered.”
While using a single overhead belt magnet may recover significant amounts of metal, “the metals recovered by this method are not always sufficiently clean to meet market specifications,” wrote the authors.
Circa 1977, Alter and Woodruff recommended in-line belt separators that use “several magnets, arranged with alternating polarities.” According to the two researchers, “The reverse magnetic fields allow the separated metals to be tumbled, causing contaminants to drop and resulting in the recovery of cleaner metals.”
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To what extent nonrecyclable MSW is prepped for its WTE process can vary depending upon the plant operator, the source material and the requirements of solid waste jurisdictions or other entities providing material.
Al Gedgaudas of magnetic equipment maker Eriez, Erie, Pa., says that in cases where WTE plant operators want to pull metals from inbound material, overhead belt magnets are more affordable upfront, but they often incur greater ongoing maintenance costs compared to a drum magnet.
If overhead belt magnets are to be deployed, Gedgaudas recommends armor-clad belting to protect against cuts, tears and gouges from sharp pieces of metal that will potentially be attracted.
Shredders, often of the low-speed, high-torque variety, can be deployed at facilities such as mixed waste processing plants or mixed construction and demolition (C&D) recycling facilities where residuals will then head to a WTE plant. In these cases, magnetic equipment is deployed in-line after the shredder to pull metals from these highly commingled streams.
As pointed out by Hansen, MSW streams are among those more likely to benefit from exposure to eddy current separators to harvest aluminum and other nonferrous metals. Operators or jurisdictions with larger budgets can consider optical units, X-ray technology, induction sorters and other innovations that allow operators to maximize the recovery and subsequent purity of the nonferrous metals contained in their commingled stream.
Whether plant operators can recover enough nonferrous metal to gain a return on such investments is tied to the volumes they handle and the value of such metals at any given time.
Cleveland-based MiniMRF LLC describes its technology as a “snap-on front-end processing system for WTE or waste conversion technologies such as gasification, pyrolysis or the production of bio-based fuels” designed to recover ferrous and nonferrous metals.
On its website, MiniMRF says its process “recovers these metals on the front end where they can be collected and sold as recyclable commodities.” The company says this not only presents an opportunity to gain revenue but it also can prevent problems at WTE facilities, where metals “can melt and plug up stoker grates in WTE systems.”
Depending on the temperature reached in the ensuing WTE process, this may be the last chance to harvest such metals, as aluminum and other nonferrous metals have a melting point well below that of iron and steel. Measured by market value, it can certainly represent an important opportunity.
After the Fire
Even after considerable efforts to harvest metal from the MSW stream before it enters the heat-intensive energy creation process, operators of traditional mass burn facilities continue to “mine” the ash that results from their process for recoverable metals.
PennQuip’s Hilliard remarks that while he has not been involved in installing magnetic equipment at the front end of WTE plants in his northeastern U.S. region, he has worked with facilities to install magnets to recover metal from the outbound ash material.
In that process, according to Hilliard, drum magnets are the preferred pieces of equipment to recover iron-bearing ash materials, with some WTE plant operators also using eddy currents and other technologies to recover nonferrous metal-bearing ash.
A diagram created by Eriez, a company that PennQuip represents, shows a drum magnet as the first piece of magnetic equipment to which such post-WTE process ash is exposed.
The ash then moves along a vibratory feeder, with one-half-inch and larger pieces of the stream passing along a stronger rare earth drum magnet, with the Eriez model known as the P-Rex.
Gedgaudas says the P-Rex models are “super” in the iron and steel recovery from ash process. “They increase ferrous recovery by approximately 50 percent,” he comments.
After the P-Rex presumably picks up the iron and steel half-inch and over pieces, remaining material moves along another vibratory feeder before being introduced to an eddy current separator, which repels (rather than attracts) pieces of material containing aluminum and other nonferrous metals.
The material harvested by the eddy current is then bounced along a vibratory conveyor to clean up the resulting nonferrous metal product.
The range of materials and metals that can enter a WTE plant can vary greatly, as can the content of the metallic ash that is recovered at the end of the process.
A 2007 study examining recovery at an incinerator in Taiwan found that extraction methods used there could recover seven common metals from mass burn WTE plant bottom ash in the following percentages:
- Iron, 41.9 percent
- Aluminum, 34.5 percent
- Zinc, 10.8 percent
- Copper, 5.7 percent
- Lead, 2.8 percent
- Chromium, 0.3 percent
- Cadmium, 0.003 percent
Magnet companies have developed increasingly powerful magnets and they also have worked together with WTE plant operators to determine the best way to prepare material and position magnets for optimal iron recovery from bottom ash.
Other technology providers are claiming progress on the recovery of nonferrous metals from fly ash or bottom ash. Inashco, based in Rotterdam, the Netherlands, says it has worked with researchers at the University of Delft, also in the Netherlands, to develop “breakthrough technology that recovers fine nonferrous metals and produces marketable construction products” from ash residue.
The company says its “Advanced Dry Recovery (ADR) technology was developed to separate the sticky, wet mineral fraction from the coarser ash fraction containing the valuable fine metal particles. This fraction can now be processed by conventional eddy current separators to liberate nonferrous metals from the mineral aggregates.”
Inashco says its process concentrates “on the ash fraction smaller than 16 millimeters that contains the highest [and] most valuable percentage of nonferrous metals. Further product upgrading is performed in the Netherlands by Inashco’s proprietary upgrading systems to ultimately create metal blends that maximize the value of recovered heavy and light nonferrous products,” the company says.
Inashco has a portable plant that has provided such recovery services to one customer in Finland in the summer of 2013. The Dutch company says the plant is scheduled to “operate for several weeks per year at one location and will be travelling throughout Scandinavia to deliver its services.”
WTE plant operators in North America may soon have access to one more option for recovering metals from the MSW stream to re-introduce them as a scrap product into the metals production loop.
The author is editor of Renewable Energy from Waste and can be contacted at [email protected].
Further Reading: Visit www.REWmag.com/rew1213-westchester.aspx to learn about how Westchester County, N.Y., has maximized metals recovery at its waste-to-energy by implementing advanced technology.