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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

October 2000

Industry's hidden assets

The by-products of the processes used to generate electricity, make paper and produce metal castings are really resources in disguise.

Bruce W. Ramme

Everyone understands the convenience of flipping a switch for electricity, or the ease of purchasing everyday paper products like tissues and plates from a grocery store shelf. Some might be aware of the cast metal pieces used in the motors, appliances and plumbing that make modern life comfortable. But it's the rare person who considers how these conveniences are produced by industries – and what industry does with what's left behind after energy or goods are created.

Wisconsin industries annually use about 25 million tons of coal, more than one million tons of special silica sands, over two million cords of pulp and more than 2.5 million tons of recycled paper feedstock to crank out the watts, cast the U-joints and roll out the paper toweling for consumers.

These industrial processes yield by-products: fly ash, bottom ash, boiler slag, flue gas desulfurization materials, spent foundry sand, wood ash and pulp/paper mill sludge. Each year, more than 1.5 million tons of coal ash, 1.2 million tons of used foundry sand and slag, and 1.6 million tons of pulp and paper mill residues from wastewater treatment are created – more than enough to fill a football field 2,000 feet in height every year.

State industries, in conjunction with Wisconsin universities, state government agencies and others, have developed beneficial uses for many of these materials. Here's a short list:

  • fly ash is used as a supplementary cementing material in concrete and concrete products for highways, building, bridges, driveways, sidewalks, masonry products, swimming pools, slabjacking and flowable backfill.
  • bottom ash, foundry sand and slag can replace crushed stone, sand and gravel.
  • boiler slag is added to roofing shingles, asphalt surfacing and sand blasting grit.
  • coal ash, wood ash and foundry sand can be used as structural fill material in constructing highway embankments, road bases, and "controlled low- strength backfill" for easy excavation.
  • paper mill by-products can be land-spread on farm fields and forests as a soil conditioner, used to carry fertilizer, reclaimed for energy, used to reinforce concrete, or to produce lignin, which has a variety of uses.
  • fly ash, wood ash and bottom ash are used as soil conditioners.

Most of these by-products also have been used in landfills for daily cover, granular drainage layers, and to solidify liquid wastes.

It makes good environmental and economic sense to use rather than discard industrial by-products. Reuse reduces the need for landfills, quarries, gravel pits and Portland cement kilns. It saves fuel. It lowers costs for building roads. And it means we won't deplete stocks of valuable natural resources as quickly for future generations.

The Wisconsin Department of Natural Resources has established rules to ensure by-products are used in ways that won't harm the environment or human health. The rules include chemical testing, permissible applications, location guidelines and public involvement on large projects.

Contributions from the coal bin

Over half of the electricity produced in the United States is generated from coal, the country's most abundant fuel. In a typical pulverized coal-fired power plant, the coal is milled, then transported with air into the furnace or boiler, where it is burned at temperatures exceeding 2000F. The residues are filtered and collected so they don't become pollutants. Bottom ash and boiler slag are collected from the bottom of the furnace. Electrostatic precipitators or "bag houses" before the chimney collect fly ash. Flue gas desulfurization materials can be collected in a fine dry form or a lumpy wet form, depending on the type of system employed. Lime or limestone-based materials are typically used to "scrub" sulfur from flue gases in these systems to reduce emissions that might otherwise contribute to acid rain..

By-products from coal burning are similar to volcanic ash, wood ash and soils.

Bottom ash and boiler slag residues range in size from fine silt to gravel-sized particles. Fly ash resembles a fine powder. Flue gas desulfurization materials are typically rich in calcium and sulfur compounds, because limestone is often used to remove sulfur from flue gases.

Approximately 1.5 million tons of coal combustion by-products are produced in Wisconsin annually. Wisconsin's utilities continue to be leaders in beneficial reuse, recycling more than 60 percent of our coal burning by-products into concrete products, road bases, gypsum board and assorted fills – well beyond twice the national average. Like other commercial materials, combustion by-products are tested to ensure that safety and quality requirements are met before the by-products are reused in construction or as fill.

Reuse provides a wealth of benefits. For every ton of Portland cement manufactured in a typical kiln, one ton of CO2 (a greenhouse gas) is emitted. By using coal combustion by-products to replace some of the ingredients in cement, we need fewer kilns, and we reduce greenhouse gas emissions. When by-products are reused, we consume fewer raw materials, and don't need to build as many new landfills, gravel pits and quarries. To replace freshly mined gravel and stone, Wisconsin Public Service Corp. uses coal ash as structural fill for highway embankments in the Fox River Valley. The freeway spur to Milwaukee Mitchell International Airport Freeway sits on an embankment of coal ash, and the freeway is paved with concrete containing fly ash.

New life for old sand

When compressed under pressure with a small quantity of a binder, ordinary silica sand is used to make molds into which molten metal can be poured, or cast. Metal castings are used in countless ways – in machines of all kinds, in engines, pipefittings and more.

Sand molds can be reused many times before they must be replaced. When the old sand is disposed of, it is treated as a solid waste – a "high-volume industrial waste" because there's so much of it – nearly 1.2 million tons each year.

Spent foundry sand serves as a gritty substitute for native gravel and stone. It's used in structural fill, embankments, granular backfill, roadway base or sub-base materials, and landfill liners or daily cover. It's also found in bricks, asphalt, and low-strength cement.

Waupaca Foundry in Waupaca and Marinette, a pioneer in finding uses for its spent sand, has participated in several state highway projects with the Department of Transportation. The foundry's sand was used in place of virgin soil to create road embankments, and crushed slag was used as roadway sub-base.

The Falk Corporation and Briggs & Stratton foundries in Milwaukee participated in asphalt pilot projects undertaken by a major highway-paving firm. The study, supervised by the University of Wisconsin Asphalt Institute, documented the suitability of foundry sands as an ingredient in asphalt and established specifications for mix ratios.

Spent sand from Grede Foundries in Milwaukee and Reedsburg is an ingredient in a cement mixture known as "flowable fill." This low-strength material makes an ideal backfill for areas such as trenches that later may be re-excavated. The Kohler Company in Kohler also has extensive experience using flowable fill in bridge abutments.

Power from paper production

Paper mill by-products include coal and wood ash, along with "fines" or fibers that are too short to be made into paper products, and residue from operating wastewater treatment plants.

Pulp and paper mills have used these materials for many years landspreading wastewater residues as a soil amendment on farm and forest lands, drying and pelletizing residuals to carry fertilizers, burning the remains for energy recovery, using boiler ash as a fill in highway projects, and incorporating boiler slag in road surfacing. Two projects illustrate how these efforts directly benefit Wisconsin residents.

Tomahawk area residents have been using boiler slag – cinders – from Packaging Corporation of America (PCA) as cover for unpaved roads. Over 5,000 tons of this material was distributed in 1999. In the future, the cinders will be incorporated in asphalt road paving.

Air travelers to the Central Wisconsin Airport have been getting a bird's-eye view of a cooperative beneficial use project between Wausau-Mosinee Paper Corporation's Mosinee mill and Wisconsin Public Service Corporation (WPSC). The airport has been engaged in a multi-year project to construct a runway overrun area to improve safety. Fill for this project has been provided by ash from the Mosinee mill and WPSC, avoiding the need to landfill the ash and providing enhanced safety for air travelers.

Other cooperative projects are underway. One is Minergy Corporation's unique cyclone boiler facility that uses a patented technology to convert approximately 400,000 tons per year of paper mill wastewater treatment residuals into 80,000 tons per year of glass aggregate. The plant also provides 5 megawatts of electric power and generates steam for P.H. Glatfelter Company's Neenah paper mill.

Another project is underway in Green Bay where WPSC is working with Green Bay Packaging Corporation to reburn industry ash and recover additional energy. The residue is then combined with utility ash and used as cement replacement or in highway embankment construction.

Why reuse is the right thing to do

Even in a state like Wisconsin, with relatively low-cost native stones and gravels, it's becoming more and more difficult to site new quarries and pits. Urbanization, non-metallic mining regulations, and a higher level of sensitivity about the environment all contribute to a new "go slow" attitude.

With over 4 million tons of industrial by-products available on a yearly basis in our state, it's also becoming harder to justify not reclaiming and reusing these valuable resources. This is especially true if the alternative is filling up landfills faster. Everyone – generators, users and society – wins when we find beneficial ways to reuse by-products.

Wisconsin Electric Power Company in Milwaukee now uses high-carbon coal ash by-products as supplementary fuel. Foundry sands are already being reclaimed and reused many times internally by foundries.

Research continues to discover new uses for by-products. Fly ash is being used on an experimental basis to produce lighter and stronger aluminum castings for the automotive industry. And coal fly ash is an ingredient in an ultra-high strength concrete that is almost as strong as steel now being developed at the University of Wisconsin-Milwaukee. Studies have shown that 10 percent bottom ash added to topsoil mixtures improves porosity, increases seed germination rates, and improves plant yield for a variety of crops. The future will see by-products added to paints, epoxies, ceramics, metals and more.

We can even reclaim by-products that were previously regarded as waste. In a pilot project at Wisconsin Electric's Pleasant Prairie power plant, landfilled coal ash is being recovered to supplement the supply of sand and gravel.

We're limited only by our own imagination and creativity. With so many environmentally sound and economically beneficial uses for by-products, the real waste would be to ignore the value inherent in these materials.

Bruce W. Ramme is Principal Engineer of Combustion Products Utilization for Wisconsin Electric Power Company. Contributing writers included Brian Mitchell representing the Wisconsin Cast Metals Association, Edward Wilusz representing the Wisconsin Paper Council, and Tom Bennwitz and Jon Brand of the DNR.

Learn more

"Innovative Applications of Coal Combustion Products," American Coal Ash Association, 1998. 99 pages.

"Fly Ash Facts for Highway Engineers," FHWA-SA-94-081, 1995. 70 pages.


American Coal Ash Association

American Foundry Society

University of Wisconsin-Milwaukee Center for By-Products Utilization

WE Energies

Beneficial uses for coal combustion products

Fly ash
  • Concrete & concrete products
  • Cement manufacturing
  • Lightweight aggregate
  • Highway embankments
  • Soil amendments
  • Flowable backfill
  • Slabjacking material
  • Liquid waste stabilization
  • Fillers for composite materials
  • Fillers for paints & plastics
Bottom ash & boiler slag
  • Sand, gravel & crushed stone
  • Aggregate for concrete products
  • Soil amendments
  • Structural fills
  • Base & sub-base for pavements
  • Anti-skid material for roads
  • Roofing shingle granules
  • Chip Seal road topping
  • Blasting grit
Flue gas desulpherization materials
  • Gypsum board
  • Soil amendments
  • Liquid waste stabilization
Beneficial uses for foundry by-products

  • Paved roadway & lot base, sub-base,
  • Subgrade fill, embankments
  • Controlled low-strength concrete (CLSM) or "flowable fill" for utility backfills or slabjacking
  • Daily cover for landfills
  • Structural fill
  • Natural aggregate substitute in manufactured concrete products or asphalt
  • Cold weather road abrasive
  • Decorative stone
  • Roadway sub-base
  • Substitute for crushed stone
Beneficial uses for pulp and paper mill byproducts
  • Landspread soil amendment on agricultural and forest land
  • Daily cover or hydraulic barriers at landfills
  • Cement manufacturing
  • Glass aggregate
  • Light weight aggregate
  • Fertilizer carrier
  • Energy recovery
  • Animal bedding
  • Industrial absorbents