This also means that millions of tons of asphalt are being removed – a job primarily performed by cold milling machines. They are the first link in the process chain, which continues with crushing and screening plants as well as asphalt mixing plants in the next step and ends with the reclaimed asphalt being repaved by pavers as part of the new mix. Machines designed to work together, innovative processes, and state-of-the-art technologies all have a major impact on the efficiency of this process chain. Smooth processes make it possible to increase the cost-effectiveness and sustainability of asphalt recycling.
The Challenge: Increasing the feed volume of RAP
Recycling old asphalt is an economic imperative in order to conserve natural resources. As a result, the industry is constantly looking for solutions to optimize the processes within the asphalt mixing plant. One of the main approaches is to increase the volume of reclaimed asphalt pavement used in all of the different recipes produced using “hot and cold” processing technologies. Using the maximum amount of old asphalt not only protects the environment, it also positively affects asphalt mix prices.
In the traditional parallel flow recycling method, the temperature is limited to 130 °C due to the exhaust gas emissions, but the exhaust gas temperatures are physically higher, which leads to increased energy consumption and the need for additional dedusting. In order to achieve a mix temperature of 160 °C, white mineral must be overheated in this case.
The Solution: The counterflow method – recycling rate of 90 + X%
Unlike conventional parallel recycling drums, in a recycling drum with hot gas generator the recycled material is heated indirectly using the counterflow method, meaning that the material in the drum flows towards the heat source. This makes it possible to achieve higher material temperatures while simultaneously reducing the exhaust gas temperature. The outflow temperature of 160 °C corresponds to the temperature of subsequent processing, while the exhaust gas temperature lies above the dew point at approx. 100 °C.
And the technology becomes even greener when the upstream process steps – processing the particle-size distribution curve (PSD curve) and milling the asphalt – are implemented using state-of-the-art processes and technologies.
Screening plants: Ideal particle-size distribution curve increases amount of RAP added
In the case of complete post-processing, i.e. also of the remaining oversize grain, impact crushers equipped with a secondary screening unit would also be used.
Intelligent milling for cost-effective and sustainable recycling
First, however, the RAP must be recovered by removing individual layers of the road structure. By removing the asphalt in layers using cold milling machines, the milled material can be fed into the material cycle for reuse separately for each type of mix, depending on its structural suitability and its qualitative properties. The selective recovery of the surface course, binder course, and base course alone offers significant environmental and financial advantages.
The formula applicable to the drying process required to produce the new asphalt in the asphalt mixing plant is simple: Material that is 1 % drier saves 1 liter of heating oil per ton of finished asphalt during subsequent processing into recycled mix – as well reduced CO2 emissions due to the process requiring less energy.