November/December 2000
As the use of galvanized steel grows in industry, so does the need to remove the zinc coating from the resulting scrap. At the moment, there’s only one place in the world to do it.
By Robert L. Reid
Robert L. Reid is managing editor of Scrap.
Of the roughly 7 million tons of zinc produced worldwide each year, about half ends up on the surface of steel to protect it against rust, especially for automobile bodies and parts. This zinc-coated, or galvanized, steel has been a growing part of the scrap stream for decades—up 500 percent since 1980, according to Argonne National Laboratory, part of the U.S. Department of Energy. And the amount of galvanized scrap will likely continue to increase as galvanized steel finds its way into new markets, from highway guardrails and streetlight poles to home construction and playground equipment.
Unfortunately, zinc-coated scrap can create problems in the melts of steelmakers and foundries. Foundries using induction furnaces can’t tolerate any zinc in their charges, experts say, while integrated steelmakers using basic oxygen furnaces (BOFs) are likewise reluctant to melt galvanized scrap because the zinc can damage furnace linings and inhibit their ability to recover steel fines from zinc-laden baghouse dust.
As for minimills using electric-arc furnaces, they sometimes opt not to melt galvanized scrap at all, replacing it with scrap complements such as DRI in their charges, or more commonly they melt galvanized scrap and then landfill the baghouse dust, one veteran industry observer notes.
But such dust “is a hazardous waste that must be treated at a significant cost before it is approved for land disposal,” explains the Argonne Web site, which puts the environmental compliance costs of melting galvanized steel at roughly $150 million a year.
Moreover, various sources speculate that even though landfilling zinc dust is currently permissible, environmental regulations often change. Thus, Argonne states, “an economical process is needed to strip and recover the zinc from scrap to prepare it for steelmaking.”
To that end, Argonne helped fund a dezincing effort in the United States that so far has been successful as a pilot plant, but which has faced a slow and somewhat rocky road to full commercialization. Internationally, several other dezincing processes have been developed over the past decade, but again not on a commercial level (see “Competing Solutions” later in this story).
But in Europe, a one-time partnership between France and the Netherlands has established the world’s first commercial dezincing operation, with production of dezinced steel currently running at 50,000 mt annually.
And while Compagnie Européenne de Dézingage (CED) also experienced some rough growing pains—including numerous explosions during its electrochemical processing and the recent pullout by one of the partners—the company now says it is profitable, has plans to increase its dezincing capacity, and has even begun to think about spreading its technology to other companies and countries.
L’Attaque Chimique
CED was created in 1995 by France’s scrap metal giant Compagnie Française des Ferrailles (which recently changed its name to CFF Recycling) and the Netherlands-based steelmaker.
Koninklijke Hoogovens (which merged with British Steel in October 1999 to form the Corus Group). The patented dezincing process—which features a chemical bath, or l’attaque chimique in French—grew out of similar technology used by Hoogovens for detinning. But the steelmaker has been engaged in a restructuring effort recently, which limited its interest in the CED project, notes Robert Marechalle, CED’s plant manager. This past July, CFF bought out Corus’s shares and became sole owner of CED.
The dezincing operation is located on a CFF scrap facility in Saint Saulve, France, near the northern industrial town of Valenciennes. This gives CED easy access to prompt scrap from two relatively nearby automotive stamping plants, one in Douai and the other in Maubeuge, both owned by French carmaker Renault.
These two sources provide CED with about 4,500 mt of galvanized sheet metal scrap a month, with the incoming scrap containing an average thickness of 10 microns of zinc coating on each side, though the process has handled thicknesses of up to 40 microns, Marechalle notes. The process works with both electroplated and hot-dipped zinc coatings, as well as galvannealed. But the scrap must arrive loose—the system can’t handle baled scrap. And while the technology can theoretically be used for obsolete cars, it’s not practical or economical, Marechalle notes. So CED focuses primarily on prompt scrap from automotive stamping plants.
At present, CED’s infeed must be segregated into two big piles that are processed separately because the majority of this incoming scrap (roughly 3,000 mt a month from the Maubeuge site) is coated with zinc on only about 80 percent of its surface area, while the 1,500 mt from Douai is almost 100-percent coated (and thus takes longer to process). This ratio will change by the end of the year, though, when the Maubeuge plant starts producing almost fully coated prompt scrap, Marechalle notes.
While this will end the need to segregate its infeed material, CED must make some changes to prepare for a scrap stream with a consistently higher zinc content, Marechalle explains. As a result, additional equipment is being installed this December.
Once segregated, the galvanized scrap is loaded via a pedestal crane into CED’s 1,500-hp Lindemann shredder for the first of two shreddings. (Material that’s too small or too large to be easily shredded is simply sent to the neighboring CFF facility’s scrap inventory, with about 1 or 2 mt a month being “rejected” in this manner.) The initial shredding is designed to separate metal sheets that have stuck together and produce cracks in the scrap’s zinc coating for better penetration by the chemical bath.
The shredded material is then loaded by a Caterpillar mobile crane into a collecting hopper and carried by conveyor belt inside the main CED building, which was formerly a railroad equipment dismantling facility.
Here, the shredded galvanized scrap is immersed in a bath of sodium hydroxide solution within the plant’s dissolution reactor. Carried through the bath by conveyor, the scrap spends anywhere from 30 minutes to an hour—depending on the material’s zinc content—soaking in the bath, which consists of roughly 9 to 11 percent sodium hydroxide by weight at a temperature of 174o to 187oF. The reactor can handle about 10 mt of galvanized scrap an hour.
At the end of this process, the zinc has dissolved off the steel and into the sodium hydroxide solution. The scrap then moves out of the dissolution reactor and into a drum washer, where cascading water cleans off any remaining sodium hydroxide solution, as well as dirt and grit such as sand that may have collected on the greasy galvanized scrap. Such dirt is then collected in pits underneath the drum washer for disposal.
The scrap metal—now dezinced but thoroughly soaked with water—is then conveyed back outside, where it is sent through the shredder a second time. It emerges from this second shredding “hot, dry, and ready to go,” Marechalle says. According to information in CED’s U.S. patent, when 3,000 mt of zinc-coated sheet metal was processed this way, some 2,935 mt of zinc-free material was recovered, with a residual zinc content of less than 100 ppm.
CED ships out as many as nine trucks a day of dezinced steel, Monday through Friday (though the dezincing process continues over the weekend). Its primary customer is Française de Mécanique, a foundry jointly owned by Renault and Peugot that makes automobile motors in a town near Lille, roughly 50 miles away.
On an experimental basis, CED has also produced a dezinced product that was sent through the shredder three times. The company recently sent about 150 mt of this triple-shredded steel to a specialty BOF customer for use as cooling scrap (which helps control furnace temperatures in steelmaking). This use could represent a promising new market as nongalvanized steel scrap becomes harder and harder for BOF operators to find, Marechalle notes.
The Link to Zinc
Though CED’s process is primarily aimed at producing dezinced steel scrap, the effort also recovers the zinc that was removed from the metal. This is accomplished by electrolysis, which sends an electrical current through the zinc-laden solution from the dissolution reactor.
First, the sodium hydroxide bath is pumped from the reactor to a twin set of electrolysis lines, which consist of a series of conical electrolyte cells bearing magnesium cathodes and anodes. Each line has approximately 10 cells, holding about 1,000 gallons of solution apiece at roughly the same temperature as during the dissolution phase.
Rectifiers reduce the incoming electrical current from 400 volts AC to the 42 volts DC required to draw the zinc out of the sodium hydroxide solution and deposit it on the magnesium cathodes in the form of a fine powder. The cathodes are then vibrated, which causes the zinc powder to fall off so it can be collected at the bottom of the electrolyte cells.
A decanting process then removes any remaining liquid, leaving behind the dry zinc powder. Roughly 65 mt of zinc powder is recovered from 3,000 mt of galvanized steel, according to CED’s U.S. patent example. The zinc powder is then stored outdoors in large metal bins for approximately a week, during which it slowly oxidizes—a heat-generating process that gradually lightens the powder’s color from black to gray-white.
Currently, about one truckload of zinc oxide powder is shipped every five or so days to two primary customers—Metal-europ, near Lille, and Netherlands-based Union Minière, both of which use the material in animal feed, paint, and tires.
Originally, CED used a press under a nitrogen atmosphere to form the zinc powder into roughly 10-inc- diameter discs. But there was always the potential that the discs would break open while in transit or storage, start oxidizing, and potentially set a truck or warehouse on fire, notes Peter Klut, process engineer for Netherlands-based Triple M Engineering & Contracting, which has been troubleshooting the CED plant for the past five years. So now the zinc oxide is kept in powder form and away from all flammable materials until oxidation is complete.
Growing Pains and Plans for Growth
Today, the highly automated CED plant runs smoothly—with just three employees per shift needed for the round-the-clock, continuous operation. (Given the plant’s five-shift schedule, plus shredder operators, a maintenance crew, and assorted managers, CED employs 22 people.) But in its early days, during the mid-1990s, the project got off to a potentially disastrous start.
“Most of the problems involved the electrolyte cells,” notes Klut, who explains that the combination of electricity, oxygen, hydrogen gas from the sodium hydroxide solution, and the flammability of zinc led to a number of explosions that threatened to shut down the facility. Indeed, when Robert Marechalle joined the project in December 1996, he was given just three months to correct the problems or see the plant close.
Various troubleshooting efforts included adapting the conveyor that carries metal through the dissolution reactor, reworking the mechanisms for vibrating the zinc powder off the cathodes, and modifying the gas extraction system so it would vent the hydrogen and oxides out of the cell and into the large, open space of the plant building. But in some cases, these solutions created new problems, Klut notes. For instance, venting the electrolyte cell gases into the air created an aerosol mist that would fill the building, destroying the clothing and irritating the skin of exposed employees. So the adaptation itself needed to be adapted, with aerosol collectors added atop each cell.
Together, though, the various changes did the trick. In December 1996, CED produced less than 1,000 mt of poor-quality scrap that the company’s customers did not like, Marechalle notes. By the time the three-month deadline arrived, CED had tripled its production to 3,000 mt of good-quality material. The plant reached 4,000 mt two months later and began making a profit a month after that.
But the continuous operation requires continuous attention. Just last year, for instance, process improvements helped boost electrolysis efficiency 50 percent. And this December, CED plans to install new equipment to accommodate that anticipated increase in the zinc content of its galvanized scrap supply. The new equipment includes a second dissolution reactor and a third electrolysis line, which should boost the plant’s capacity to 60,000 mt—perhaps even 65,000 mt—of dezinced steel annually, Marechalle says. To increase capacity further, though, the facility would need to move to a larger building and install a larger shredder, he adds.
Once these upgrades are in place, CFF Recycling will have invested some 50 million francs in the project (or more than $6.5 million U.S. dollars, depending on the exchange rate), notes Didier Groult, chairman of CED and CFF Recycling’s scrap operations throughout northern France. Though CFF’s original partner in CED—Hoogovens—pulled out this past summer, the scrap conglomerate sees CED’s efforts as part of its core business of recycling and so hopes to spread the technology throughout the world.
Europe alone could probably support 10 or so dezincing facilities, Marechalle suggests, adding that such sites need to be built within about 50 miles of both suppliers and customers to keep transportation costs low. At presstime, Didier Groult was scheduled to discuss CED’s processes at a zinc conference in the United States, which would also allow him to examine the potential market conditions in North America.
Already, CED has received or sought patents for its dezincing process in every country that manufactures automobiles, from the United States and Canada to Japan and Turkey, Marechalle notes.
The company’s overall expansion plans may involve building new dezincing facilities in other locations, helping other companies set up dezincing operations in which CED retains a partial ownership, or licensing the technology outright, Marechalle speculates.
Competing Solutions
If CED does someday establish a presence in North America (where its parent company, CFF Recycling, already operates scrap plants in Texas, Oklahoma, and Mexico), the company could find itself in a kind of dezincing déjà vu, competing with a roughly similar process that also dissolves galvanized coatings in a hot sodium hydroxide solution while using electrolysis to recover the zinc.
In fact, the two approaches—CED’s and the one partially funded by Argonne National Laboratory—are so similar that CED officials wonder whether they’ll have to fight in court to maintain their U.S. patent rights.
The Argonne-funded approach, which is also patented, was developed by a company whose name has changed over the years, but which today is known as Metal Recovery Technologies Inc. (MRTI). MRTI has operated a pilot dezincing plant in East Chicago, Ind., for several years but in 1998 transferred its technology to a company now known as Meretec Inc., a joint venture between MRTI and Meyado International Ltd. (London). Meretec hopes to open a commercial dezincing operation in East Chicago next year that can handle 100,000 to 150,000 tons of galvanized scrap annually.
As in Europe, other facilities would be needed to meet all of North America’s dezincing needs. But so far the U.S. project has gotten off to a slow start: The East Chicago plant had been scheduled to open much earlier, with a much higher annual production rate. Financing and technical problems delayed its start date and scaled back the anticipated output.
MRTI’s dezincing process differs from CED’s primarily by using a rotary drum rather than a horizontal conveyor to immerse the galvanized scrap in the sodium hydroxide solution. MRTI initially used a horizontal conveyor, notes Bill Morgan, one of the patent holders and a director of Meretec, but the switch to a rotary drum, which tumbles the galvanized scrap in the solution, “improved the overall dezincing performance.” The resulting dezinced scrap was tested by a nearby foundry, which considered the material “very acceptable,” Morgan says.
Like CED, MRTI plans to focus, at least initially, on shredded galvanized scrap from automotive stamping plants as well as construction projects.
As part of an earlier effort, a predecessor of MRTI and the American Iron and Steel Institute had experimented with dezincing baled scrap.
While the process did reduce the zinc content of the bales, MRTI ultimately decided not to work with bales because they’re difficult to handle and difficult to drain after dezincing and washing. Moreover, the zinc levels still remained higher than MRTI preferred, Morgan says.
While MRTI and CED both steer clear of baled scrap, MRTI would someday like to try dezincing obsolete scrap from shredded cars, Morgan adds.
At least one industry expert, though, predicts that old cars will never be de-zinced. Richard Burlingame, who spent 25 years as director of research for the Luria Brothers scrap operations, says the galvanized scrap will be too intermingled with other material after shredding and that grease and dirt in the vehicle would also hinder the process. Moreover, Burlingame remains skeptical of the effectiveness of current dezincing technology, questioning whether residual zinc will cause problems when the dezinced steel is melted and thus restrict how much dezinced steel can be used in a charge.
Both CED and MRTI argue that tests show their dezinced steel won’t cause problems and can be used pound for pound in place of black scrap that was never galvanized.
Some industry observers also question the economics of dezincing, which obviously requires the dezincing firm to charge either a premium for the dezinced steel or a tolling fee. In response, Morgan points to test cases in which MRTI purchased galvanized busheling from an automaker for $145 a ton, dezinced the material, and then sold the dezinced scrap to a foundry for $168 a ton. And CED, which notes that it has been profitable for the past several years, adds that the sale of the recovered zinc alone is paying for its upcoming new equipment installations.
While CED and MRTI have the most fully developed dezincing processes, other approaches have been developed (though apparently not commercialized) over the past decade by other companies in North America, Japan, Spain, and Italy. As the use of galvanized steel spreads, the amount of zinc-coated metal entering the scrap stream will only rise—while black scrap becomes harder to find and more expensive to buy.
At which point, the future for dezincing could look shiny indeed. •
As the use of galvanized steel grows in industry, so does the need to remove the zinc coating from the resulting scrap. At the moment, there’s only one place in the world to do it.