How do you choose?
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 or batch production? What are the advantages that each of these two methods offer?
The answer is that there is no ideal option. For every single site and for every single 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. The role that Marini plays is to accompany the client and guide him towards his best personal choice.
This 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: 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 quality.
In continuous plants 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.
In both types of plant, 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 the 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 ending up by mistake in the sand hopper, or 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, batch type plants are a compulsory requirement.
In batch type asphalt plants the 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 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 batch plants, 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 plants the metering system is normally volumetric, using variable-speed feed screws which have replaced the previous pneumatic metering system.
In batch plants, the mixer with double arms and paddles means the mixing quality is undoubtedly better when compared with continuous plants because it is forced. This feature is particularly important when dealing with ‘special products’ (porous asphalt, splittmastik, 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.
The final decision
Naturally, this is left to the client to decide, but in general terms a solution can be found as follows:
Batch Plants – where flexibility, work for various clients and special formulas are required.
Continuous Plants – where low operational costs and continual production are essential, of course if you have constant quality in the aggregates..