Leftover Mining Debris Has Potential for Pavement
It looks like the dusty, sandy, gritty substance left by plows, winter salt, and grime. It’s like the debris that street sweepers brush up and wash away once the snow has melted. But it’s so much more than basic dirt. It’s what researchers, engineers, and mining employees call taconite tailings. It’s the substance left over when the taconite ore is extracted from the quarried rock. In the past, storing the tailings was simply part of the job, but there were environmental concerns about where to take the piles of mining debris; worries about local water safety, land and soil absorption, and community health.
Larry Zanko, a researcher from UMD’s Natural Resources Research Institute (NRRI), has led the feasibility and evaluation study for Minnesota Department of Transportation (MnDOT) on use of taconite tailings to replace mineral aggregates in asphalt mixtures. The previous studies by Larry and MnDOT formed the basis for current study, where the main focus is on developing fine-graded asphalt mix, using significant quantities of taconite tailings, that are well suited for local agencies (cities, townships and counties). UMD Assistant Professor Eshan Dave approached the city of Duluth with the idea of developing a mix that is rich in taconite tailings and that can be used for the city’s roadway improvement projects. With UMD’s help, the city of Duluth applied and received a research grant from MnDOT’s OPERA (Local Operational Assistance Research) program.
This past spring and summer, UMD civil engineering students in the first master’s cohort have been advised by Dave, who teaches several courses in civil engineering, to research the potential use of taconite tailings on city roads. Several students have diligently begun and continue the research. “I found out about the opportunity through word of mouth,” said masters student Waylon Munch. “Professor Dave needed the help, and I thought it sounded interesting. It has definitely proven to be exactly that—I wasn’t aware of the intricacies of the materials that went into our roads. It has been a great eye-opening experience.”
Since last spring, Munch has been working on the research necessary for the city of Duluth to determine the most ideal mixture for testing in summer 2012 on portions of Yosemite Avenue in Duluth. The test pavement will consist of three, 1,000-foot long trail sections representing different types of asphalt mixtures.
“It isn’t purely taconite tailings,” said Munch. “It’s a mixture of several substances, and our main job is to research the varying substances, record our findings, and report them to our professor. He then sends the information to the city of Duluth and they study the results of our taconite tests.”
Taconite itself is a type of iron-bearing rock that contains 15 to 30% iron content. Taconite tailings are the leftovers once the iron ore has been sifted, and in the past, they were stored in landfills along the north shore. According to Larry Zanko, “the leftover byproduct rock, in this case fine aggregate (tailings), from Minnesota’s taconite mines, can be a very attractive material for designing asphalt pavements. Among other things, it is exceptionally durable and is harder than most, if not all, conventional aggregate materials, two properties that can significantly improve safety by providing excellent skid resistance for pavement surfaces."
According to the UMD student researchers, the process is fairly basic when creating the taconite mixture that they study. “The tailings are mixed with oil,” said masters student Justin Baker. He points to the gyratory compactor that looks like an industrial mixer with an attitude. “You put the black mixture that looks like asphalt into the container and turn on the compactor. The pressure mimics the weight and push of a steamroller. This process creates the “puck” that we later freeze, thaw, and test for mechanical properties. The results are recorded and later studied by the city engineers.”
The process of mixing the tailings with the oil, also known in engineering circles as the “binder”, is the main component that the students focus on. What kind of oil was used in the sample? How much of the tailings? What was the volume once the puck was weighed before and after submersion in 77-degree water?
“This project has really helped me decide what I want to do with my future,” said Baker. “It has helped me gain knowledge about transportation engineering.”
Although the tests in the lab offer very basic conclusive evidence as far as an ideal mixture for northern roads, the real test will be in the spring when the mixture is taken to the road and given a chance to hold up against traffic, weathering, and the northern climate.
“This is really exciting for the Civil Engineering department,” said Munch. “We haven’t even graduated students from the first entry, and we have a contract with the city. The city of Duluth trusts us, and they’re utilizing our work. A grad student couldn’t ask for a better opportunity.”
Written by Christiana Kapsner, February 2012
Photography and Design by Christiana Kapsner
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