Pugmill Mixing

Figure 8-14.-Typical batch plant cumulative scale settings and cycle.

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Asphalt Introduction From the weigh-hopper, the aggregates are deposited into the plant pugmill (mixing chamber) and are blended with the proper proportion of asphalt. In a typical plant system, asphalt is weighed separately in a weigh bucket before being introduced into the pugmill. When the asphalt reaches a predetermined level in the weigh bucket, a valve in the delivery line closes to prevent excess asphalt from being discharged into the bucket. The asphalt is then pumped through spray bars into the pugmill. Asphalt buckets should be checked for accuracy in the mornings. New asphalt loosens some of the old asphalt that accumulated the previous day on the sides and bottom of the bucket. Loss of this accumulated asphalt changes the tare weight of the bucket. Pugmill Mixing Asphalt and aggregates are blended in a chamber called the pugmill. The pugmill consists of a lined mixing chamber with two horizontal shafts on which several paddle shanks, each with two paddle tips, are mounted. The paddle tips are adjustable and fairly easily replaced. The paddle areas are adjusted to ensure there are no “dead areas” in the pugmill. A “dead area” is a location where aggregates can accumulate out of reach of the paddles and not be thoroughly mixed. Dead areas can be avoided by making sure the clearance between the paddle tips and the liner is less than one half of the maximum aggregate size. Nonuniform mixing can occur if the pugmill is overfilled (fig. 8-15). When the plant is operating at full production, the paddle tips should be barely visible at the surface of the material during mixing. If the material is too high, the surface aggregates will tend to “float” above the paddles and will not thoroughly mix. Conversely, in a pugmill containing too little aggregate (fig. 8-15), the tips of the paddles rake through the material without mixing it. These problems can be avoided by following the manufacturer’s pugmill batch rating recommendation. Normally, the rating is based on a percentage of the capacity of the pugmills “live zone.” This live zone (fig. 8-15) is the net volume in cubic feet below a line extending across the top are of the inside body shell radius with shafts, liners, paddles, and tips deducted. Figure 8-14 presents the mixing cycle during which the aggregates, mineral filler, and asphalt are blended in asphalt hot mix in the pugmill. The length of time Figure 8-15.-Typical pugmill. between the opening of the weigh box gate (Step 6 in the figure) and the opening of the pugmill discharge gate (Step 9) is referred to as the batch mixing time. The batch mixing time must be long enough to produce an homogeneous mixture of evenly distributed and uniformly coated aggregate particles. If the mixing time is too long, the lengthy exposure of the thin asphalt film to the high-aggregate temperature in the presence of air can affect the asphalt and reduce the durability of the mix. The speed of the mixer shafts and the arrangement and pitch of the paddles are factors governing the efficiency of the mixing. Most job specifications require the use of a timing device to monitor batch mixing time. 8-14

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