Whether driven by feelings of environmental stewardship or ever-tightening environmental standards, the end result is the same steel industry views and practices for handling electric-arc furnace (EF) dust are changing.
Zinc isn't the problem in the dust generated by the steel industry's EF shops; other elements in the melt shop's by-product have prompted the U.S. Environmental Protection Agency (EPA) to label it a hazardous waste named K061.
In the past, mini-mills and some other EF shops looked at it as simply a waste product to be hauled away. Some sent it to processors who, if they didn't extract much metal from the dust, turned it into a solid that could be disposed of. That practice has changed for many EF steelmakers. The material is now turned over to one of a handful of companies that recover the zinc and iron. When the zinc is removed, what is left is an iron-rich material. Unfortunately, it also contains elements like chromium, lead and mercury that are deemed too toxic for landfills. Should that material or those compounds containing the elements deteriorate, they could leach into the local water table and contaminate it. That's why the EPA has restricted its use. Much of the iron-rich material is used as aggregate in asphalt and concrete, some of which is rendered inert and some recaptured as iron.
Chromium, mercury and a whole list of other materials have been designated as hazardous by the EPA, according to Elaine Laps, a spokeswoman for Timken Co. But the Canton, Ohio-based company has made sure that it is handled properly. Timken has a sophisticated system that collects all of the melt shop's baghouse dust and ensures that none escapes into the atmosphere. "We went back to recycling because it was the right thing to do. It is more expensive to do it this way, but it is what the company wanted to do," she said.
Timken's decision to install an elaborate collection system with piping and vacuum lines wasn't financial-based; it was environmental, Laps said. "They take absolutely every step necessary to make sure that dust does not get into the air."
Timken's system includes sealed pipes and a vacuum system, so the dust is always contained. The vacuum system pulls the dust through pipes into a silo, where it is stored until it's collected by the contractor. Even while the dust is poured into the truck, another vacuum system is running to capture any fugitive dust.
Steelmakers are uncertain of the percentages of various elements fumed off from the furnaces. Melters focus on the residuals left in the heat and ensuring they don't exceed specifications. Few seem to know whether the proportion of zinc collected in the baghouses has risen in recent years as more and more vehicles made with electrogalvanized sheet steel reach the end of their useful lives and are shredded.
One scrap buyer at an EF mill said he assumes the volume has grown, but added that the melters haven't complained. "Zinc isn't the bad actor as far as they are concerned," he said, because it is blown off into the baghouse. "The real problem is the copper. If it's too high, we've blown that heat."
Most mini-mills and EF melt shops have a fourth hole on the furnace that collects much of the dust and delivers it to the baghouse and silos, said Barry Smith, environmental engineer at Steel Dynamics Inc. (SDI), Butler, Ind. The other three holes in the roof of the furnace are for the electrodes. Some, like SDI, also have hoods over their furnaces to capture so-called fugitive emissions from the furnaces. These also help to keep dust off crane rails and provide a better working environment inside the mill.
Some zinc and other metals might still be trapped in the melt, but the melters keep track of the chemistry and make sure they don't exceed what customers will accept, Smith said. However, "most is fumed off and condenses into the dust."
There are alternatives to shipping the dust to companies that recapture the zinc at refineries. A few companies treat the dust on-site at some mills to render it inert and suitable for disposal in a Class D non-hazardous landfill. Others want the iron-rich material as well as the zinc. Baghouse dust can contain as much as 60-percent iron and perhaps 25-percent zinc.
Not all of the dust processors are interested in iron, but there are a few, Smith said. One is British-based ZincOx Resources Plc, which operates the Big River Zinc smelter in St. Louis and is building an EF zinc dust recovery plant in York, Ohio. ZincOx will recapture the zinc then take the iron-rich material and further refine that to produce high-grade iron like pig iron.
Still, it doesn't matter if the material collected in the silo is as safe as mom's apple pie, Smith said, noting that the EPA has ruled that it is a hazardous waste and it must be handled as such. "We have to abide by all the rules, training, documentation, disposal and transportation associated with a hazardous waste, and it is quite a burden," he added. "Everybody is under very tight standards not to let this dust waft out of the building and openings. The whole steel industry has a lot of mechanisms available to them that they have employed to keep a clean shop."