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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Hot mix asphalt baghouse fines are dust particles that are captured from the exhaust gases of asphalt mixing plants. Secondary collection equipment called baghouses is commonly used to capture these very fine sized materials.
There are approximately 3,600 hot mix asphalt plants in the United States, accounting for a total annual production of 400 to 450 million metric tons (450 to 500 million tons) of asphalt paving material. About 2,300 of these hot mix plants are batch plants, with the remaining 1,300 being drum mix plants. Roughly 40 to 50 percent of all hot mix asphalt plants are equipped with baghouse collections systems. Baghouses consist of several rows or compartments of fabric filters that collect the dust during the operation of a hot mix asphalt plant. Most of these systems are preceded by cyclones, which are primary collection devices used to capture the coarser particles emitted from the plant’s dryer. Hot mix plants that do not have baghouse collection systems are equipped with wet scrubbers to control air emissions.
Drum mix and batch plants differ in their asphalt concrete production operations. In drum mix plants presized cold aggregates are fed into a drum, in accordance with preselected mix design proportions. The aggregates are dried and mixed with asphalt cement, which is introduced at the end of the drum (coating zone), in a continuous process. In batch plants unsorted aggregates are introduced into a dryer and subsequently screened into different size fractions, stored (in hot bins), and fed, by batch weight, into a separate pugmill mixer where the hot aggregates are mixed with asphalt cement.
Simplified line diagrams of batch and drum mix plant operations are presented in Figure 2-1. Baghouse fines collected in the baghouse may be routed directly to the asphalt production facility or stored in a silo for subsequent use as a mineral filler additive in the mix.
It is estimated that approximately 5.4 to 7.2 million metric tons (6 to 8 million tons) of baghouse fines are generated annually by the U.S. asphalt production industry.(1)
CURRENT MANAGEMENT OPTIONS
Most asphalt producers whose plants are equipped with baghouses try to recycle as much of the dust back into their own paving mixes as possible. Although precise figures are not available, it is estimated that as much as 80 to 90 percent of baghouse fines are currently being recycled into hot mix asphalt.
Figure 2-1. Batch plant and drum mix plant operations.
Additional information on the use of baghouse fines can be obtained from:
National Asphalt Paving Association (NAPA)
5100 Forbes Boulevard
Lanham, Maryland 20706
Although most of the baghouse fines are returned to the asphalt mixing plant, some producers (probably less than 10 percent) with excess dust dispose of the dust by sluicing it to a settling pond or returning it to the quarry.(2) Where wet scrubbers are employed for dust control instead of baghouses, the washed fines are generally discarded.
Baghouse fines are almost exclusively recycled within the asphalt production facility.
The properties of baghouse fines that might be obtained from an asphalt production facility are influenced by the sizing and moisture content of the cold feed aggregates, the type of aggregate feed material(s), the type of asphalt plant (batch or drum mix), and the design of the dust collection and handling system.
Although baghouse dusts are frequently referred to as baghouse fines, there can be a considerable variation in the fineness of baghouse dusts from one plant to another. This variability is related mainly to the efficiency of the primary collection (i.e., cyclone, if available) system and the nature of the cold feed aggregate. The percentage of material in the cold feed that passes the 0.6 mm (No. 30) sieve is important, since aggregate particles as coarse as the 0.6 mm (No. 30) sieve can be entrained in the exhaust gas.(2)
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Concrete Mineral Filler
The only established use for baghouse fines or dust is the return of the dust into an asphalt paving mixture as a portion of or, in some cases, all of the mineral filler. Mineral fillers can constitute up to 5 percent of some asphalt pavements.
Although baghouse fines are usually very fine-grained, plants without a primary collector or cyclone have retained from 0.8 to 5.8 percent of the dust on a 0.6 mm (No. 30) sieve. This is about the expected maximum particle size of baghouse fines.(2)
The size distribution of baghouse fines consists of a coarse fraction and a fine fraction, with the dividing size being the 0.075 mm (No. 200) sieve. There can be a considerable range in the percentage of dust particles passing the 0.075 mm (No. 200) sieve. Plants without a primary collection system often collect dust with less than 50 percent of the material collected passing the 0.075 mm (No. 200) sieve. On the other hand, more than half of the plants with a primary collection system collect dust with 90 to 100 percent of the particles finer than the 0.075 mm (No. 200) sieve.(2)
Other relevant physical properties of baghouse fines are specific gravity, specific surface, hygroscopic moisture, and Atterberg limits. Table 2-1 summarizes the observed range of values for the physical properties of baghouse fines. With few exceptions, baghouse fines normally absorb less than 2 percent moisture at 50 percent relative humidity. Baghouse fines contain little or no clay and will generally have little or no trouble meeting the plasticity requirement for mineral filler, which limits the plasticity index value to 4.0.
With few exceptions, the pH of baghouse fines is alkaline, with values ordinarily ranging from 7.2 to 10.8 for dusts from gravel, granite, or traprock aggregates and values ranging from 11.0 to 12.4 for dusts from limestone and dolomite aggregates. The chemical properties of baghouse dust can be expected to reflect the properties of the feed aggregate.
Table 2-1. Typical range of physical properties of baghouse dusts.