What exactly is the batch-mix technique?
-Firstly, there is the quality of the aggregates. Certain countries, such as France, favour upstream inspections and are also fortunate to have abundant stocks of exceptional aggregates. Hot screening is thus either virtually redundant or sporadic. Others suffer from a scarcity of supplies and must settle for “difficult” products whose quality is improved by screening. The batch-mix solution offers an appropriate response.
– Next, there is variable production output according to numerous different formulae. Commonly used on urban sites, this method requires instantaneous access to significant amounts of resources and needs increased storage capacity for “hot stocks”, and for the storage of finished products. The batch-mix technique also offers interesting solutions and adds value.
– The number of screen sizes is directly proportional to the complexity of the road building technology and old screens with 4 sizes are being abandoned in favour of equipment featuring 5 or 6 sizes.
– Finally, we have low-energy asphalts, along with high modulus asphalts, light-coloured asphalts, and sinthetic binders; in short, the whole range of special asphalts is certainly better suited to batch-mix solutions.
This brief overview is certainly not exhaustive and users will have different views according to their working conditions.
The batch-mix method may not be the oldest but it is the most widely used. It consists of dividing up hot mix asphalt production into quantities of between 1,000 kg and 5,000 kg. Each of these parts is generally called a batch. The granular components, along with the fillers, bitumen and any additives are weighed separately before being sent to the mixer. One of the characteristics of the batch-mix method is that the aggregates are metered separately when cold before being dried all together and re-screened when dry. Making sure that the screen sizes conform to the formulation is a key point to which engineers pay particular attention. Figure 1 of the UltiMAP explains the different functions of a traditional fixed batch-mix plant.
Cold feed bins (1)
The UltiMAP accepts up to 14 metering devices. This sub- assembly is a completely separate part of the machine and allows for the efficient management of the different formulae. The special RAP feed bins (8) complete the equipment.
The counterflow dryer mixer (2)
Traditionally, this includes different sections for heating, drying, reheating, the introduction of the recycled materials and the mixture. It is possible to adapt different types of dryer mixer to the UltiMAP according to the type of formula.
The traditional fuel oil and/or heavy fuel oil burner is replaced by gas burners
which are reputed to produce less pollution. Currently enjoying a new resurgence in popularity, coal burners are highly polluting and emit more CO2. The recycling ring remains the most efficient method and, above all, it is the least expensive solution for traditional recycling.
The bag filter (3)
Figure 2 shows the recovery of fillers and their reintroduction into the operating cycle according to their particle size – fines via the filler conveyor and coarse particles with the hot aggregates. The products can also be mixed for the re-screening of the entire mixture.
The foreign fines silo (4)
This frequently takes the form of a filler tower close to the main tower, as in the diagram in Figure 3. The surplus recovered fines are recycled and stored in a dedicated silo.
The tower (in the strict sense of the term)(5)
A growing trend is for specific customer formulae to require personalised plants to accommodate variations in cycle times, high temperatures, different bitumens, special products and additives. UltiMAP series allows for 32 basic variations based on 5 structural chassis. The metering of granular components is sequential and gradual. The accuracy and reliability of the weighing devices must be checked. A major market trend has been towards the gravity feeding of the bitumen and fillers into the mixer. The traditional mixing cycle is 45 seconds, resulting in an output of 80 batches per hour.
One of the advantages of the batch–mix method is that it allows users to change the filling coefficient of the mixers and/or to modify the production cycle. These solutions are very appealing for products containing high proportions of sand or for sequential coatings.
Finally, modern plants often have several binder supply pumps, one of which is dedicated to supplying binders, one for traditional bitumen and another for special binders.
Storage of hot-mix asphalts (6)
This is a decisive factor for an efficient operation, allowing for the delivery of different formulae to several clients at the same time. A well designed combination of under-screen storage and hot-mix storage can be used to achieve customer delivery capacities that are well above the production capacity for quite a long period.
As for continuous plants, the development of long-term storage is also an important factor.
Storage of bituminous binders
An increasing number of different binders calls for voluminous storage capacities. The trend is towards vertical storage and electric heating.
The coexistence of batch-mix and continuous plants is an established fact. Continuous plants easily dominate in America and batch-mix plants are more common in Europe, with several notable exceptions such as France, North Africa and Norway.
A well-balanced fleet should include different types of equipment. However, it is likely that certain qualitative problems concerning aggregates and the production of specific formulae shall work in favour of the batch-mix method. The appearance of mobile batch-mix plants has been an interesting development on the market in recent times.