Ajay Tripathi elaborates on the different aspects of continuous asphalt drum mix and asphalt batch mix plants and how to choose the right plant for an application.
When the moment arrives for the entrepreneur to make his decision to purchase an asphalt plant, he may leave it up to the suppliers to help decide on the best layout and configuration. The first choice that has to be made regards the two different systems: continuous asphalt drum mix/hot mix plant or asphalt batch mix plant. What are the advantages each of these two methods offer?
The answer is that there is no ideal option. For every single site and job requirement, there is a different solution and so the choice depends on careful analysis of all the 'fringe factors' and the type of output that has to be produced.
Batch mix plant is the most widespread type of asphalt plant in the world, which guarantees the highest level of flexibility in production and quality of the finished product. The batches depend on the type of production: In every 40-50 seconds, a complete batch is produced, after all the individual components have been weighed and metered separately.
This type of plant is a must for producers who work for several clients at the same time, because the specifications can be easily changed, while maintaining a high level of mix quality.
In continuous asphalt drum mix/hot mix plant, there is no interruption in the production cycle as the rhythm of production is not broken into batches. The mixing of the material takes place inside the dryer drum which is elongated, as it dries and mixes the material at the same time. Since there are no mixing tower or elevators, the system is therefore considerably simplified, with a consequent reduction in the cost of maintenance. The absence of the screen however makes it necessary to have precise controls at the beginning of the production cycle, before the aggregates are fed into the dryer and before they are consequently discharged from the dryer as asphalt mix.
In both types of plants, the production cycle begins with the cold feeders, where the aggregates are generally metered by volume; if required, the sand extractor can be fitted with a weigh-belt for metering.
Control of the total weight of the virgin aggregates, however, is effected in two different phases of the production cycle in two different plants. In the continuous type, there is a feed belt, before the moist aggregates are fed into the dryer drum, where the moisture content is set manually in order to allow for the weight of water to be subtracted. Therefore, it is extremely important for the moisture content in the aggregates, particularly the sand, to have a constant value which is continually monitored through frequent laboratory tests.
In batch type plants, the weight of the aggregates is checked after drying, before they are fed into the mixer. The weighing, therefore, in the weigh hopper is not influenced by moisture or by variable factors, such as changeable weather conditions.
Furthermore, in batch type plants, the presence of the screen means there is more accuracy in the selection of materials before mixing, therefore making the quality of the finished product more consistent. Inaccuracies are also avoided -such as large aggregates by mistake ending up in the sand hopper, inconsistencies in the supply of aggregates, or possible errors preset in the formula at the cold feeders.
For this reason, in the specifications of many countries, where quarries do not enforce adequate and strict control procedures required for the production of asphalt mix, batch type plants are a compulsory requirement.
In asphalt batch mix plant, bitumen is metered by weight through load cells in the weigh hopper. The computer system ensures that the exact quantity is fed into the mixer, calculated on the basis of the specifications and the actual weight of the aggregates. In continuous asphalt plants, the metering is generally volumetric through a litre-counter subsequent to the feed pump. Alternatively, it is possible to install a mass counter, a necessary choice if modified bitumen is used, which requires frequent cleaning operations.
In asphalt batch mix plant, the filler is metered by weight in the weigh hopper, where the recovered fines and imported fines can be separately controlled, thereby making the metering process very precise. The filler is then fed into the mixer with its own screw conveyor or by gravity.
In continuous asphalt plants, the metering system is normally volumetric, using variable-speed feed screws which have replaced the previous pneumatic metering system.
In asphalt batch mix plants, the mixer with double arms and paddles means the mixing quality is undoubtedly better when compared with continuous plants because it is forced mixed. This feature is particularly important when dealing with 'special products' (porous asphalt, split mastic, high RAP content, etc), which require a high level of quality control. In addition, with 'forced mixing' methods, the mixing time can be lengthened or shortened and thereby the mixing quality can be varied, depending on the type of material being produced. On the other hand, in continuous plants the length of the mixing action must necessarily stay constant without any change in mixing time. Naturally, this is left to the client to decide, but in general terms, a solution can be found as follows:
Advantages of batch mix plants - where there is flexibility to work with different type of material.
Uses of continuous plants - where low operational costs and continual production are essential, of course if you have consistent quality available in the aggregates.
Batch mix plant is a must for producers who work for several clients at the same time, because the specifications can be easily changed, while maintaining a high level of mix quality.
The author is Technical Director, Nilang Asphalt Equipments Pvt Ltd.