Asphalt production in Wisconsin

The asphalt pavement production sector is the process of taking sand and aggregate and mixing it with asphalt or bitumen (the thick black material left from distilling crude oil) to form hot bituminous concrete mixture that can be laid down as a smooth road surface.

First, sand, gravel or rocks are sorted into piles according to their size. Usually this involves a crushing operation to gradually reduce the solid rock to a usable size for the mix formula. Sometimes the mix material comes from solid rock deposits, such as limestone, that are loosened by blasting and then crushed in a series of crushing and grading equipment. Some material is found in glacial deposits of sand mixed with all kinds of rocks carried along by the glacier. Some operations include washing the sand with water to remove the very small soil particles.

Although asphalt plants are often located in quarries, the quarry operation is not actually part of the asphalt production sector and the quarries may be under different owners and management. DNR staff have to review each situation to be sure permits and inspections of all the regulated activities are directed to the management that is responsible for that operation.

Mixing begins by pulling a certain percent of material from each of several bins with different size sand and aggregate particles. An end loader fills bins from the top and a computer controlled belt unloading system keeps the material flowing evenly from the bottom of the bin. In the first section the material is thoroughly dried. The fuel for this is frequently waste automobile crankcase oil with fuel oil as a backup. For permanent plants natural gas may be economically available and results in reduced air emissions.

After drying and prior to adding asphalt the recycled asphalt pavement (RAP) is added from a separate bin and conveyer in whatever proportion specified in the mix formula. The material will now be at least 300 degrees F and the asphalt is added at that temperature. The asphalt is kept in a liquid state after being trucked into the site at 300 degrees in insulated tankers and stored in heated tanks. After a short mixing period to coat all the particles with asphalt the bituminous concrete mixture is conveyed to the load out storage tank.

The hauling trucks of the customers, some hauling for the mixing company and some hauling for independent paving companies and government units, drive unto a scale under the load out tank. The correct tonnage is dropped into the truck and as the truck pulls forward to cover the load a computer printed invoice is handed to the driver.

Environmental Issues

1. Use of Recycled Material - The asphalt sector has a long history of burning waste motor vehicle crankcase oil for the beneficial recovery of heat. Burning used oil does increase air emissions in comparison to natural gas or virgin fuel oil but with proper air emission controls permit limitations can be achieved. The mixing of recycled asphalt pavement into new bituminous concrete is also standard practice in this industry. Whenever an asphalt road is torn up, all the old asphalt is either hauled to an asphalt plant or recycled on site as roadbed for the new pavement. Chunks of asphalt hauled off the road site are ground into the desired particle size and stock piled until needed in a mix formula. The sector has achieved 100% recycle of used asphalt, which makes it the most recycled material in Wisconsin. Their has been some demonstration projects to evaluate the beneficial use of waste foundry sand to be mixed in small percentages into road base material under the pavement but it is currently not being used regularly.
2. Air Emissions - The mixing and drying of asphalt material produces dust and all plants are required to have emission controls to remove particulate. The state of the art control equipment is cyclones and baghouses to remove particulate and coincidentally some other pollutants. Generally the particulate collected is recycled directly back to the asphalt mixing drum but it can be stored when necessary. Other pollutants from combustion are controlled by careful adjustment of burners and monitoring impurities in the fuel. Each of the 114 asphalt facilities with air permits report the discharge of 6 pollutants. These are: total particulate, particulate under 10 microns, sulfur dioxide, nitrogen oxides, reactive organic gas (volatile organic chemicals), and carbon monoxide.
3. Wastewater and Stormwater - The washing of sand takes a large volume of water. However with a properly designed settling basin all of this water can be recycled. The fines that are washed out of the sand settle to the bottom and can eventually be recovered for reuse. Each facility has a storm water pollution prevention plan that assures that all stormwater that might pick up pollutants from the site is directed to appropriate treatment, which is generally a settling pond, so that only clean water is discharged off site. During the operating season most stormwater is reused as dust control on plant roads and material handling conveyers. Stormwater at each site is regulated by general WPDES permit number WI-S067857.