Batch plants obtain their name because during operation the asphalt mixture is produced in batches. The size of batch varies according to the capacity of the plant pugmill (the mixing chamber where aggregate and binder are blended together). A typical batch is approximately 6000 lb.
At a batch plant, aggregates are blended, heated and dried, proportioned, and mixed with binder to produce asphalt mixture. When recycled materials are used, they are proportioned and mixed with the heated aggregates prior to mixing with the binder. A plant may be small or large, depending on the type and quantity of asphalt mixture being produced, and also may be stationary or portable. Certain basic operations are common to all batch plants: 1) Aggregate storage and cold feeding 2) Aggregate drying and heating 3) Screening and storage of hot aggregates 4) Storage and heating of binder 5) Measuring and mixing of binder and aggregate and recycled materials (when used) 6) Loading of finished asphalt mixture Aggregates are removed from storage or stockpiles in controlled amounts and passed through a dryer to be dried and heated. The aggregates then pass over a screening unit that separates the material into different sized fractions and deposits the aggregates for hot storage. The aggregates and mineral filler (when used) are then withdrawn in controlled amounts, combined with binder, and thoroughly mixed in a batch. The asphalt mixture is loaded directly into trucks or placed in a surge bin, and hauled to the paving site. Figure 3-9 illustrates the major components of a typical batch plant. Each component or group of related components is discussed in detail in sections that follow; however, an overview of the processes required in plant operations helps the Technician to understand the functions and relationships of the various plant components. 3-15 Cold (unheated) aggregates stored in the cold bins (1) are proportioned by cold-feed gates (2) on to a belt conveyor or bucket elevator (3), which delivers the aggregates to the dryer (4), the aggregate is dried and heated. Dust collectors (5) remove undesirable amounts of dust from the dryer exhaust. Remaining exhaust gases are eliminated through the plant exhaust stack (6). The dried and heated aggregates are delivered by hot elevator (7) to the screening unit (8), which separates the material into different sized fractions and deposits the aggregates into separate hot bins (9) for temporary storage. When needed, the heated aggregates are measured in controlled amounts in to the weigh box (10). The aggregates are then dumped into the mixing chamber or pugmill (11), along with the proper amount of mineral filler, if needed, from the mineral filler storage (12). Heated binder from the hot binder storage tank (13) is pumped into the binder weigh bucket (14) which weighs the binder prior to delivery into the mixing chamber or pugmill where the binder is combined thoroughly with the aggregates. From the mixing chamber, the asphalt mixture is deposited into a waiting truck or delivered by conveyor into a surge bin.