Asphalt rubber blending, asphalt rubber plant
Just like in any other processes the manufacturing of asphalt rubber can be batch or semi-continuous. Since complete dispersion of the crumb rubber takes time it is not really continuous, but semi-continuous. In common practice both systems are available.
Capacity requirements vary from 8-60 ton/hour.
General components of an asphalt rubber plant consists of
• a storage and dosing system of raw materials,
• heating system to provide appropriate process temperature during the whole process,
• blending system, which consists of pre-mix and curing parts. This part of the plant makes sure the crumb rubber is fully wetted (by the bitumen) and forms a homogenous, non settling suspension with bitumen.
Scenario of a common asphalt rubber plant
A – hopper, B – heat exchanger for bitumen, C – direct fired heater, D – pre-mix tank, E – curing tank
High diversity of controls of various manufacturers can be found in practice. Some end users prefer fully automated controls, some like the cheaper manual controls, but this is a part where plant cost can be reduced.
1. Raw material storage and dosing system
Depending on the number of solid additives single or dual hopper is employed for raw material storage. The dosing system is capable of metering raw materials into the pre-mix tank accurately according to asphalt rubber binder formula.
Usually there is no storage tank for neat bitumen included in asphalt rubber blending equipment, but the bitumen transporter truck is employed for this purpose. The bitumen goes through a metering and a heat exchanger system before introducing to the pre-mix tank.
2. Heating system
Usually a direct fired heater is responsible for providing enough heat capacity for the whole system. The heating oil is heated by the direct fired heater and circulated in a liquid-liquid heat exchanger (this is optional in some plants), curing tank, heated pipes etc. The heat exchanger is employed to elevate the temperature of non modified bitumen and to make sure the blending temperature meets the requirements.
3. Blending system
The mechanical blender normally consists of two parts of pre-blending and curing.
3.1 Asphalt rubber blending (pre-mix tank)
The pre-mix tank should be designed properly as the high concentration of the crumb rubber has a significant temperature drop effect on the asphalt rubber blend right after introducing the crumb rubber into the asphalt.
The residence time is usually a few minutes, the speed of agitation is high and no mechanical high shear is applied. The working principals of the mixer are different from colloid mill provided grinding.
The rubber particles need to be fully wetted by bitumen by the end of this stage. A binder is homogenous and gets transferred to the curing tank for further agitation.
3.2 Asphalt rubber curing (curing tank)
An asphalt rubber curing tank is most commonly referred to as a reaction tank, but there is no real chemical reaction between the rubber (unless the rubber is pre-treated with certain chemical additives) and the bitumen. The curing temperature varies but it usually is around 180oC.
The curing tank usually has two continuously agitated compartments for continuous asphalt rubber binder supply. The capacity of the curing tank compartment varies, depending on the final capacity.
The final product is a homogenously dispersed crumb rubber - bitumen suspension (slurry), and is immediately mixed with aggregates at the hotmix (HMA) plant according to actual job mix formula.
Majority of the asphalt rubber plants deal with recipe based processes, where the process time is pre defined (based on laboratory test results).