Recycling is becoming part of the production process in many industries and the asphalt sector is no exception. The advantages of reprocessing reclaimed asphalt are not just economic in an increasingly price sensitive market, the process also preserves a valuable natural resource.
Prior to the asphalt planings being used in the recycling process, they have to be broken down to an acceptable size without reducing the original stone content. Benninghoven has developed recycling granulator to break asphalt blocks and plannings to a recyclable size without reducing the original stone content thereby creating minimum fines, all at the most economic production level. The planings are fed into a single shaft granulator where they are broken down and fed up to a single deck screen.
Materials go into the feed hopper and are forced in the single shaft granulator by oscillating hydraulic rams. The granulated material is then conveyed to the single deck screens after passing a magnetic belt detector, which removes any metal in the feed. The single deck screen separates the ready to size material for stockpiling, the oversize material is passed through the secondary double shafted granulator. This double shaft granulator is equipped with an adjustable setting to enable final product grading. The final material is produced to meet acceptable gradings that enables further processing to give maximum economical results.
Reclaimed asphalt planings can be added in quantities of up to 25% direct into the asphalt plant paddle mixer, where it is mixed homogenously with virgin aggregates. This method is in common use throughout the UK and Ireland for processing small percentages of RAP.
A further innovative add on system, which can result in up to 35% asphalt planings being processed, is by using the existing plant dryer. Here the planings are introduced into the discharge end of the dryer via a slinger conveyor to mix with new material in the drum. Heat transfer continues during the materials movement to the plant mixing section, where it bypasses the screen and is introduced to the mixer through the weigh section.
Although hot asphalt recycling has become an accepted practice throughout Europe, it is only now that operators are striving to keep down costs, maximising their operating efficiency together with projecting a more sustainable image, that they have started to embrace this new technology. Where larger quantities of reclaimed asphalt planings need to be processed - above 35% and up to 100% - then hot recycling is the only option. For many years Benninghoven have been developing new and improved methods of processing these waste asphalt materials. Their designs have made possible this high percentage of RAP recovery and provided operators with equipment, which helps preserving valuable natural resources.
A feature of the hot recycling plant is that the drum is mounted at high level to facilitate the movement of reclaimed materials into the existing asphalt plant process. At the heart of the system is a separate 11 m long large volume uniflow drum, which both dries and heats the recycled asphalt. The design of the drum internals ensures the burner flame makes no contact with the moving reclaimed material inside the drum. In the heat exchange zone, material travels smoothly through the cross section of the drum, which results in optimum heating combined with economic use of the burner and low exhaust gas temperatures.
In the next hot gas zone the materials are agitated, so protecting the drum skin from thermal overheating. The combustion zone of the hot RAP drum is fitted with special finger lifters, which vibrate during the production process to prevent clogging of the material during the heating process. The drum is fully lagged to minimise radiation loss and insulated with special zinc coated steel. Drive is by four 22 kW geared motors with friction drive.
A special compact, multi-fuel burner, which can burn a variety of fuels, including oil, natural gas, liquefied gas and coal, is used and is fitted with its own internal axial fan which provides the total air required for the combustion process. Frequency inverter control of the main blower fan provides the benefits of a perfect air/fuel ratio throughout the burner's operating range, thereby obtaining optimum running efficiency.
Hot recycled materials are discharged from the drum through an enclosed conveyor then by chute into a holding buffer silo. This is fully insulated and clad to maintain the temperature of the materials. From here it is batch weighed to the plant mixer as required. A feature of hot recycling systems is that the drum is mounted at a high level to facilitate the movement of the reclaimed materials into an existing plant process.
In recent months, Benninghoven have secured a major order for a state-of-the-art, high level, hot recycling plant for a site at West Drayton, with installation to start in May 2008. This will be the first plant of its type in the UK.