Modern Asphalt Mixing Plants
At today's asphalt mixing plants, emissions are low and well controlled. Typically, it takes only three to five people to run an asphalt mixing plant. In every country, the asphalt industry must comply with stringent regulations and specifications with respect to materials used, process conditions, and pavement specifications. These regulations and specifications are designed to protect the environment as well as to ensure the quality, durability, smoothness, and safety of the roads.
Today's asphalt plants are highly mechanized and utilize state-of-the-art computerized process control technology.
There are two types of asphalt plants: batch plants and drum plants. In both, the mineral aggregates are heated and dried in a rotating drum. In batch plants, aggregates are stored in hot bins prior to mixing with bitumen in discrete batches before being stored or loaded into trucks. In drum plants, the mixing of the aggregate and the bitumen takes place in the same drum, after which it is stored in a silo before being loaded into trucks for delivery. Today the predominant plant type in the U.S. and New Zealand is the drum-mix plant. Batch plants prevail in Europe, South Africa, and Australia.
Various asphalt mix formulas are used for the various types of pavement materials. These formulas are engineered to meet the needs of the owner of the pavement. In the case of major roads, highways, and airport runways, the owners are typically governmental entities.
In the case of parking areas, low-volume roads, and other facilities, many owners are from the private commercial market, but they often use specifications from government agencies.
Bitumen is stored in heated tanks on site between 150°C (302°F) and 180°C (356°F), which enables the viscous liquid to be pumped through insulated pipes to the mixing plant. The mineral aggregates – stone, sand, and gravel – are stored in stockpiles at ambient temperature. In addition to virgin aggregates, most facilities have stockpiles of reclaimed asphalt pavement (RAP). The aggregate stockpiles are neatly sorted by type and size.
Aggregate and reclaimed materials are taken from various stockpiles and loaded into specific bins. Each size of aggregate and reclaimed asphalt material is fed onto conveyor belts in proportions specified by the job mix formula and transported to be dried in a drum.
At a batch plant, the aggregates are dried and heated in a rotating drum, where the aggregates tumble through a stream of hot air. After drying, the aggregates and any fillers are then mixed in batches with the exact proportions of bitumen and possibly RAP in a second machine called a pug mill.
In contrast, at a drum mix plant, the bitumen is added to the dried aggregates and continuously mixed in the same drum used for drying. Here, the RAP and bitumen are added to aggregate far downstream from the source of heat.
Every part of the plant has enclosures and/or control technologies. Most plants are fuelled by natural gas or fuel oil, and state-of-the-art scrubbers keep combustion related emissions very low.
Dust is controlled in the baghouse, where fines and dust are collected on the outside of filter bags, while clean air passes through the center of the bags. The fines are periodically subjected to bursts of air which force them to the floor of the baghouse, where they are collected for metering back into the paving mix. Clean air is vented out the top.
Most plants are on permanent sites, but even portable mixing plants have the advanced environmental controls that are seen on plants on permanent sites.