Slag-What is it Good for?

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Slag. Depending on where you’re from, it may be an insult, a term meaning trash, or, in our case, the waste left over from metal smelting or refining. Outside of the construction industry, it might seem that uses for slag are considered limited.

But some recent research here at USGS might change slag’s poor public image. It turns out that, although slag is most known for being what’s left when metals have been removed, slag itself might be good at removing some negative chemicals from the environment.

Image shows industrial piles of gray rock
Pile of steelmaking slag at the ArcelorMittal Indiana Harbor steelmaking facility, Indiana. Photograph by Nadine Piatak, USGS.
(Public domain.)

Environmental Antacids

Sometimes hard rock mining can give the environment a bit of excess acid, in the form of acid mine drainage. Acid mine drainage can happen when air and water mingle with various minerals such as iron sulfide (also known as pyrite or Fool’s Gold), creating sulfuric acid. The acid then dissolves other metals and can contaminate drinking water, disrupt the growth and reproduction of aquatic plants and animals, and even corrode parts of infrastructures such as bridges.

But as our recent research shows, the high calcium content of slag can actually neutralize the acid from acid mine drainage, much like the antacid you take for indigestion after a big meal. Not only that, but it can even reduce acids that have built up in soils.

We looked specifically at ferrous slag, the leftover material from the smelting of iron and steel, in the Chicago-Gary area of Illinois and Indiana. Ferrous slag is currently underutilized. Although the construction industry does use some slag as an aggregate, most is simply discarded. However, slag could be used to treat acid soils or acid mine drainage. Doing so would both offset the cost of restoring abandoned mine areas, as well as decrease steel manufacturers’ current waste footprint.

Image shows orange, discolored water in a small canyon
Orange, iron-rich precipitate (ochre) from outflow of Lead Queen mine tunnel, after late September 2014 monsoon storm. Photo by Glen E. "Gooch" Goodwin, Photographer - used with permission.
(Copyright Glen E. "Gooch" Goodwin, Used with Permission)

Too Much of a Good Thing

Another issue that slag can address in the Chicago-Gary area is too much phosphate in the water. Phosphate is an important nutrient for plants and is a key ingredient in most fertilizers. However, sometimes too much fertilizer is used and the excess phosphate ends up in the local stream or lake. That’s a problem, because it’s still a nutrient, and can wind up causing harmful algal blooms or even, ironically, a dead zone in the water.

So how can slag help? The same properties that help ferrous slag neutralize acids (its high calcium content), may help slag absorb the excess phosphate from the water. With excess phosphate in water being a significant issue in the Chicago-Gary area, this benefit from slag could be another use for the material and could decrease the need to mine new natural materials for water treatment applications.

Start with Science

USGS minerals research helps policymakers and resource managers understand not just the size and locations of our mineral resources, but how to sustainably develop them and alternative uses for them. Learn more about this project here.