Maximizing Metal Recovery
Covanta Energy’s Steve Bossotti has worked in the energy-from-waste (EfW) industry since its inception and has performed virtually every role a plant requires. He was in the business when most plants were built in the 1980s and ‘90s, he says, and over the years has witnessed drastic innovation.
One year ago, he became vice president of Covanta’s Organic Growth and Innovation Group, a company segment that focuses on fostering new ideas and embracing new market technologies. Two of the group’s main initiatives are to improve and maximize metal recovery efforts, as well as engage in beneficial ash reuse projects, Bossotti says, and those are things he’d like to see happen before he hangs up his hardhat. “In Europe it [ash] is widely used in roadbeds and construction materials, but in the states we haven’t been successful doing that,” he says. “There was some experimentation back in the 1980s, but it hasn’t come to fruition.”
Ash reuse is a longer-term goal than metal recovery, Bossotti says, but the latter needs to be done in order for the former to happen. That means removing the tiny bits of ferrous and nonferrous metal from the ash, a pursuit that may sound insignificant but can make a big difference at an EfW plant. From Bossotti’s perspective, it’s “all about sizing and separation.”
Metal Recovery ABCs
When municipal solid waste (MSW) is hauled into a Covanta EfW plant, waste is dumped on the floor and bulky items are removed. “You might see a refrigerator, but things are a lot different today than they were 20 years ago,” Bossotti says. “In the old days you could pay a garbage guy $10 and he’d take your refrigerator, but you can’t do that anymore, though we still get our share of bulky items.”
With metals separation, there are two things a plant wishes to accomplish: remove as much clean metal on the front end as possible, and remove bulky items that won’t burn. Once that’s done, the remaining MSW is sent through the boilers. “It goes into a pit, we fluff it and feed it into the unit and create energy,” Bossotti explains. “It then comes out the back end of the boiler onto a conveyor, and at that point it’s 23 percent by weight of what it was when it went in, and one-tenth of the volume of waste.”
The vibrating conveyor, or grizzly feeder, levels out the waste stream making metal extraction easier. The material is screened to remove bigger pieces, and what’s left goes through a second screen before exposure to a magnet that captures all of the ferrous materials—such as steel and iron—left in the ash. “The magnet size depends on the plant size; some are six-foot diameter drums,” Bossotti says. These drums roll above the conveyors and capture metals that jump to it and stick. Everything that does not adhere to the ferrous magnet continues to another conveyor, where it’s traditionally screened to three-eighths of an inch and greater and passed over an Eddy Current Separator, which utilizes rare earth magnets. “That’s where we recover what we call the large nonferrous,” Bossotti says. Nonferrous metals consist of precious metals such as gold, silver and copper, as well as aluminum, magnesium and lead.
Right now, roughly 30 percent of Covanta’s EfW plants have nonferrous metal removal systems that recover materials larger than three-eighths of an inch. In an effort to improve upon that, the company has partnered with Steinert U.S. Inc., to upgrade its technology to pick up even smaller materials. “The technology difference is that it uses double the poll changes,” Bossotti says. “The ionic charge on the particle gets confused and jumps away from the machine, or is repelled.”
Covanta’s Fairfax, Va., plant has installed the technology and is now recovering twice the amount of nonferrous metals. By adding an additional step that sends materials that fall through the last screen through another specially-designed Eddy Current Separator, very small, nonferrous particles are captured. “Materials pass over a rotating drum in a casing,” Bossotti explains. “By changing the charge on the particles several times it confuses them so they repel away, shooting into an area that we hold them in.”
The rejects or ash left over is mixed with fly ash on the back end of the boiler, and is then sent out as a combined product and disposed of in monofills. Nonferrous metals are stored in a clean area away from ash, and are shipped for sale roughly once a week. Ferrous metals are hauled out nearly six days a week. To put that into perspective, Bossotti says that some plants produce about 30,000 tons per year of ferrous metals, and one container can hold 20-30 tons maximum. “That’s a lot of trucks,” he says. “All of our plants together process up to 400,000 tons of metal per year.”
Covanta Energy isn’t the only EfW giant working to improve its metal recovery efforts, however. Waste Management-owned Wheelabrator Technologies Inc. has a plan to increase its recovered scrap metal volumes over the next five years through better existing metal recovery system performance and the deployment of new recovery technologies.
A Second Perspective
“Recovering scrap metals for recycling has always been part of our standard operating procedure at Wheelabrator waste-to-energy plants,” says Mark Lyons, senior manager of business development at Wheelabrator. “We began recovering scrap ferrous metals—iron and steel—from ash residues at our first waste-to-energy plant in Saugus, Mass., when it started up in 1975. Our original metal recovery systems were designed with screens and magnets to recover only ferrous metals, but our current metal recovery efforts include recovering both ferrous metals and nonferrous metals from the ash our facilities produce.”
A metals recovery system doesn’t have a significant impact on waste-to-energy plant operational performance, Lyons says, rather, the most important performance issue is that a metals recovery system needs to be properly maintained and operated so that it can continuously process the ash produced by the facility during operations and recover scrap metals. “Most waste-to-energy plant metal recovery systems are installed with some level of bypass functionality,” he explains. “This means that if a particular component of the metal recovery system is down for repair or maintenance, one or more elements of the system can be bypassed so that the ash being generated by a facility can still be moved through the system to the ash load-out building for disposal. However, the operational goal is to minimize metals recovery system bypass time in order to maximize the amount of scrap metal that is recovered and recycled.”
On recent technological improvements, Lyons says the biggest has been in the efficiency and power of Eddy Current Separators. Wheelabrator is currently designing several metal recovery systems that will take advantage of that. Aside from technology, he notes the importance of metal recovery systems on the economics and sustainability of EfW plants. “When most of our facilities originally came online, scrap metal prices were much lower than they are today,” he says. “Since then, metal prices have generally trended higher, while electric power rates and disposal fees have trended lower. Today, scrap metal sales represent a much more significant fraction of a facility’s total revenue.”
Bossotti agrees. “It’s no mystery that the metals market has been performing very well, and recovery technologies have advanced from when most plants were built,” he says, adding that what ends up in the trash is quite surprising. “We have one plant that pulls a large number of coins out of the ash each year; it’s astounding.”
Not all plant owner/operator models that extract ferrous and nonferrous metals allocate revenue the same way. “The receipt of revenues from the sale of recovered scrap metals at the publicly-owned plants that we operate varies from plant to plant,” Lyons says. “At some publicly-owned plants, Wheelabrator and the owner share the revenues. At other publicly-owned plants, the owner keeps all the revenues.”
Bossotti says that many client-based plants have sharing arrangements with Covanta. One main factor in such an arrangement is who is paying to get rid of the ash, because the operator is saving the client ash disposal costs for each ton of metal removed. “If there’s metal in there at the end destination, they don’t know the difference, but our goal is to remove as much as we can,” Bossotti says.
In an initiative to improve its ferrous metal products, Covanta is working to recover more of it on the front end, as they are cleaner and more valuable at that point. “At one particular plant we’re taking front-end ferrous and running it through a silo through which we blow air,” Bossotti says. “That separates out paper and plastics that might be in the waste and creates quite a clean product, and returns that waste to where it started.”
Some of the company’s plants are much smaller than others—950,000 tons of waste per year versus 200,000—so another area of focus is regional processing facilities. At one location, Covanta is building a metal recovery system at the monofill in which three of its plants in the region dispose ash. “We still extract ash before it goes into the monofill, but instead of building three systems at each plant, we will do one,” Bossotti says.
Covanta has also teamed up with Germany-based Tartech for the recovery and recycling of metals from EfW ash monofills. Covanta Tartech will utilize a proprietary and highly specialized technology provided by Tartech for recovering both ferrous and nonferrous metals remaining in ash that has already been deposited in monofills. The joint venture will develop projects at Covanta ash monofills and will look to partner with various municipal and commercial ash monofill owners.
Covanta will have 10 or more metal recovery projects completed in 2013. Not all plants will be getting the same upgrades, Bossotti says. Some plants never had non-ferrous systems and will eventually be installing new systems, while others will be getting ferrous or non-ferrous system upgrades. “Each plant is looking at its options and making the best decisions upon where to improve.”
Author: Anna Simet
Contributions Editor, Biomass Magazine