How Asphalt is Produced
The ingredients used to make asphalt are aggregates, sand and liquid asphalt cement, a petroleum product.
Asphalt plants can be either batch or drum plants. Drum plants produce material on an almost-constant basis, which can be stored for several days in heated silos. Batch plants make a measured amount of material which is hauled to job sites for immediate use or can be stored. Since asphalt is no longer workable if it cools, it must be either stored at a constant temperature or transported for immediate use to a job site.
Both types of plant include cold-feed bins where the aggregates are stored before use, an asphalt cement storage system which is strictly regulated and controlled for safety reasons and an emission control system.
In a batch plant, aggregates are stockpiled by fraction size in a stockpile area and loaded in separate 'cold feed' bins by size. The plant operator sets the feed rate of each cold feed bin depending on the mix that is being produced. Aggregates are taken from the cold feed bins and fed into a drying machine which tumbles the material through hot air to remove moisture and bring the aggregate to the desired temperature. The aggregate is then discharged into a bucket elevator that takes it to the top of the plant where it is discharged through a chute onto a screen deck. The screen deck sorts the aggregates into different sizes and drops them by size into separate hot bins. They are then weighed by fraction size and dropped into a pugmill mixer. The weighing process ensures that the precise amount of material is fed into the mix, depending upon the 'mix design'. The liquid asphalt cement is also weighed and injected or gravity fed into the mixer with RAP (Recycled Asphalt Pavement), if it is being used. The mixture is mixed in the pugmill until all aggregate particles are coated with the correct amount of liquid asphalt cement depending on the mix being produced.
This process is controlled through plant automation. After mixing, the finished product is discharged directly into the customers' trucks for transportation to a job site or can be sent up a slat conveyor and transfered in storage silos.
Each plant includes an emission control system to remove dust and prevent it from escaping into the air. Plant control systems and scales are also checked and verified frequently by state regulators.
In a drum plant, cold feed bins also control the amount of aggregate and RAP being taken into the mix. Aggregates and RAP are tumbled in a drum of hot air, either parallel-flow or counter-flow. As the aggregate moves through the drum it is heated to remove any moisture. Towards the rear of the drum, liquid asphalt cement is introduced to the mix to produce the finished asphalt product which is transferred to storage silos.
Drum plants are fitted with a bag house which extracts hot air from the drum and feeds it into a bag house, literally a piece of equipment filled with hanging filter bags, which extract any dust from the hot air and return it to the mix.
Each plant is equipped with a control house where an operator can control the quantities of cold feed, RAP and asphalt cement used in each mix. The plant operator is also able to monitor the silos and load trucks, print tickets and take orders.
The asphalt industry is continually making advances and striving to minimize the environmental impact of asphalt production. All plants are strictly monitored by environmental agencies and operate tight controls to minimize their impact on the local environments in which they operate.
In addition, did you know that asphalt is one of the most recycled materials in everyday use today? Through the use of RAP - Recycled Asphalt Pavement, which is removed from road surfaces, ground and introduced to the asphalt production process, asphalt has been successfully reused for many years now.
Another well-known advance in this area is the development of Warm Mix Asphalt, which is ever more widely available across the US. Warm Mix It is produced at lower temperatures than Hot Mix Asphalt. There are various technologies available to produce it, but typically either a small amount of water is added to the aggregates/cement mix or a small amount of wax or foam can be added. These additions alter the viscosity of the mix and allow asphalt to be produced temperatures 50 to 100 degrees Fahrenheit lower than for Hot Mix Asphalt, thus generating considerable energy savings. Several Tilcon plants now produce Warm Mix Asphalt.
Another example is the fact that, these days, burners used to heat the aggregate mix in asphalt plants are capable of using Recycled Fuel Oil and Natural Gas both of which have less impact than traditional fuels.