Minergy’s LWA production facility was constructed as a means to beneficially reuse coal combustion fly ash from a power plant located in Oak Creek, Wis. LWA was commercially produced between 1994 and 2000, at which time the properties of the fly ash changed as a result of a new source of coal, thus allowing the fly ash to be marketed directly into the concrete industry without additional processing. This plant combined the fly ash, paper mill sludge from a coated paper mill in Wisconsin and municipal sludge from the Milwaukee Metropolitan Sewerage District to form a LWA product, meeting ASTM C330 and C331 LWA standards. The LWA was used in the local Milwaukee and Chicago construction markets.
The process receives ash and sludge via trucks, although a continuous feed of sludge or ash could be incorporated, depending on distances and building interferences. Each stream is proportioned and fed to an enclosed, high-intensity mixer. The resulting homogeneous mixture is fed to a pelletizing system that creates aggregate units according to the sizing required for the particular market. The pellets are fed to a traveling grate dryer to remove moisture and then to a rotary kiln for manufacturing into lightweight aggregate. Waste heat from the product cooler and kiln exhaust gases is recovered and used for the drying process.
In the first half of the rotary kiln, the organic fractions of the ash and sludge are combusted, releasing significant heat, and forming the cellular structure required for the low density lightweight aggregate. Nearly 85 percent of the thermal energy required for the process is provided by the organic fraction of the ash and sludge. In the second half of the kiln, the pellets are heat hardened by exposure to high-temperature gases (1800 to 2000 degrees Fahrenheit) from the upstream combustion. This atmosphere creates the conditions necessary for the pellet sintering, creating the high-strength properties necessary for construction applications.
The lightweight aggregate discharges from the rotary kiln into a cooling device, which recovers heat from the pellets and directs it to the traveling grate for drying the incoming moist pellets. Crushing and screening of the product can then be done in accordance with the requirements of the customer markets.
The lightweight aggregate technology uses the best available control technologies to clean exit gases. Dust and particulate emissions are captured in a high-efficiency bag house, while dust and mechanical collector residuals are recycled back into the process. The gases are then cleaned in scrubbing and particulate control devices before being emitted.
With the lightweight aggregate technology, the ultimate disposition of the sludge occurs when it enters the process and is transformed into the aggregate product. An important feature of the LWA technology is that the environmental liabilities associated with sludge and ash are essentially eliminated. Organic compounds, pathogens and odors are completely destroyed in the process. Trace metals are permanently stabilized in the ceramic-like aggregate product. Water leach test results of the LWA product are within primary and secondary drinking water standards. As a result, the material has received exemptions from solid waste regulations.
The facility design had the capacity to process 60,000 wet tons of sludge and 90,000 tons of fly ash into 100,000 tons of lightweight aggregate.
Minergy LWA Properties and Use
The stone-like end product, Minergy LWA™, was suitable for use in a broad range of concrete products and geotechnical applications. Minergy LWA was sold to concrete producers throughout the Midwest. It is used to reduce dead loads and improve fire ratings of concrete in numerous construction projects. For example, past projects include office buildings in the Chicago area; a student commons facility at St. Olaf College in Minnesota; a hospital in Ft. Wayne, Ind.; Michigan Tech University’s Environmental Sciences Building; and in Wisconsin, VA Hospital in Madison, 8th Street Bridge in Sheboygan, and Miller Park, Midwest Express Center and the Hilton Hotel in Milwaukee.
Minergy LWA was also used to produce lightweight concrete masonry. Compared to conventional masonry units made with sand and stone, lightweight concrete masonry has higher fire ratings and higher R-values. Since lightweight concrete masonry is about 20 percent lighter than conventional masonry, it is easier to install and transport. Many users report fewer back injuries and fewer complaints due to repetitive motion ailments. Lightweight masonry units are slightly more expensive than conventional concrete masonry, but the cost of a typical wall made of lightweight masonry is generally less, due to increased productivity and lower transportation costs. The life-cycle costs are less due to energy savings.
Minergy LWA was also used for geotechnical applications. Lightweight backfill reduces the loads on poor subsoils, thus minimizing settlement. Lightweight backfill behind retaining walls exerts lateral pressures that are three to four times less than conventional sand, stone or clay backfill. Typical backfill projects include highway embankments, bridge abutments, fill for road construction, segmental retaining walls and lightweight fill for building structures.
Minergy LWA was also used to produce lightweight soils for rooftop gardens and parks such as McArthur Square in Milwaukee, Wis., and Monona Terrace in Madison, Wis. The thermal resistivity of Minergy LWA was about four times higher than sand or stone and is often used to backfill water mains that do not have adequate cover or to insulate other frost-susceptible structures. Lightweight aggregate can be used for perimeter insulation for slabs on grade and building foundations.