One of the most important jobs in a road project is the asphalt mixture. It involves economic and environmental considerations. At the moment, scientists are researching about two main concepts:
- The way of saving raw materials by recycling old pavements or using rubber powder from waste tyres.
- Decrease the temperature of the mixture to avoid toxic emissions from the hot bitumen and, on the other hand, save fuel in the dryer drum.
Anyway, the main equipment to do all kind of asphalt mixtures is the Hot/Cold Mix Plant. In this post we are going to analyse what you can find in the global market of Hot Mix Plants (HMP).
The scheme we are going to follow is the next:
- DISCONTINUOUS AND CONTINUOUS HOT MIX PLANTS
- KNOWING THE PLANTS
- COLD BINS
- DRYER DRUM
- SCREENING AND HOT BINS
- MIXER, DOSAGE AND HOT STORAGE
- BAG FILTER
- BITUMEN TANKS
1 DISCONTINUOUS AND CONTINUOUS HOT MIX PLANTS
The first classification we can do among the HMP is attending to the work cycle. As a first approach, while in continuous plants the aggregates flow from the cold bins to the truck in a continuous way and the mixing is continuous too, in a discontinuous plant, the aggregates stop in the hot bins and the mixing is made by batches. Move on to the point
a ) Discontinuous Plants. We can see the next diagram :
Firstly, you introduce the classified aggregates into the cold bins. Usually, each bin makes a volumetric dosage and feed the material to the collecting weighing conveyor belt. Usually the sand bins have a device to dosage by weight.
The aggregates go to the dryer where the moisture is evaporated and aggregates are heated until the mixture temperature. At the same time, thanks to the fan, the filler goes to the bag filter, where it will be stocked to add to the mixer when necessary.
The hot aggregates go to the screen box and after screening they are sending to several hot bins by size.
Once the recipe is introduced in the computer, the system dosage the exact weight of aggregates, bitumen and filler for make one batch. Then it puts them together into the mixer, and after of the mixing period, the system feeds the mixture into the hot storage silo or to the truck. After that, the process is repeated again and again until complete the required production.
The important aspects of this kind of plants are:
- The hot screening: permits a best classification because there is no filler adhered to the aggregates.
- The dosage of aggregates, filler and bitumen in batches, by weighing: it has a higher control over the dosage.
There are several statements that people say regarding the discontinuous plants and they have nothing to do with the kind of plant. I´m going to say a couple of examples:
- It doesn´t matter the quality of the classification of the aggregates because we are going to screen the aggregates. It´s not entirely true. If the aggregates are bad classified, then the hot bins fill in a different way as the recipe requires and provoke the overflowing of one of the bins. You have to extract this material and transport to the stockpile.
b ) Continuous plants. We can see below a similar diagram than the other above
The process until the end of the dryer is similar than the above mentioned (if we have an external mixer instead of dryer drum mixer). If we have dryer drum mixer, the mixture is made inside of this drum. In both cases the dosing of the filler and the bitumen is in a continuous way.
Once the mixture is ready (in the external mixer or in the dryer drum mixer) it goes to the hot storage silo or to the truck.
The important aspects of continuous plants are:
- Very simplified and for this reason, we will have a reduction of the costs of maintenance.
- If you are going to use RAP for recycling, the process is easier than in a discontinuous one.
- The process take place in a continuous way, for this reason the starts and stops don´t benefit to the quality of the mixture.
There are several statements that people say regarding the continuous plants and these characteristics have nothing to do with the kind of plant. I´m going to say a couple of examples:
- The dosage of bitumen is in a volumetric way. It´s not entirely true. There are continuous mass counter for the bitumen.
- De dosage of filler is in a volumetric way. It´s not entirely true. There are continuous weight systems for the filler.
- The mixer is a part of the drum dryer. It´s not entirely true. There are plants with an external mixer in continuous way
- The dosage of aggregates is in a volumetric way. It´s not entirely true. There are continuous weight systems especially for the sand, which is more sensitive to the moisture.
Summarizing, the question that all customers ask is: “ok, all is clear but, which is the best option?” There is no philosopher´s stone. Both of them have advantages and disadvantages. It depends on the use in each project, but as the life of the plant is longer than one project , the user have to evaluate the kind of job he has to do in his portfolio and choose the proper plant.
2 KNOWING THE PLANTS
In this epigraph we go to show the main components of the plants, especially those that have more variability around the world. In my view, the best way to see it is to follow the aggregates flow from the stockpile to the truck.
a ) COLD BINS:
The aggregates are classified according to grain size and type and stocked in the stockpiles, close to the cold bins. Usually we need one hopper for each kind of aggregate and size. The standard recommendation is from 4 to 6 bins (two of them for sand). It is highly recommendable to have moisture measurement devices to know continuously the moisture of the sand.
Every hopper has a system for dosage. The possibilities are:
- Volumetric dosage frequency controlled and a collecting weighing conveyor belt.
- By using dynamic weighing with load cells in each hopper.
b ) DRIER DRUM:
This is one of the most important parts of the plant. Furthermore, there are a lot of researches and developments about its type and its place.
The main function of the dryer is to dry the moisture of the aggregates and rise their temperature by exposing to the flame. Finally you obtain in one side hot aggregates and on the other hand the gases and the filler going to the bag filter.
There are several kinds and sizes of burners but the manufacturers of the plants know how to choose the best for each plant. There are burners consuming fluid gas, natural gas, fuel oil, fat oil, carbon dust or diesel, so you can use the more efficient in each case.
The first classification is:
- Parallel flow vs counter flow system:
- In the parallel flow system the flow of aggregates and the flame way are the same.
In this case the temperature of the gases outlet is approximately 150ºC while the temperature of the aggregates outlet is approximately 122ºC.
- In the counter flow system, the feeding of aggregates to the tromel is opposite to the burner and for this reason the flow of the aggregates and the gases are in the opposite direction.
In this case the temperature of the gases outlet is approximately 60 ºC while the temperature of the aggregates outlet is approximately 122ºC.
Summarizing, we can observe than the counter flow is more thermally efficient than parallel system, because the heat is gradually applied to the aggregates and, at the end, waste less energy by the exhaust.
- Dryer without mixer vs drum mix :
- Dryer drum without mixer:
The drum is only used to elevate the temperature. This option is used in discontinuous plants and it is used in plants with external mixer too.
- Drum mix dryer: in this case the mix is made inside of the drum. The traditional units operate with a parallel flow.
In other cases, the mix area is physically separated from the burner to avoid overheat the bitumen and operate in counter flow way. They take advantage of the length of the lighter to create a mix zone without flame. Furthermore, if you want to recycle with RAP, the old material is not exposed to the flame in this area , as the environmental laws say.
The brand who has the majority of developments of this kind of dryers is Astec, and his cutting edge product is the double barrel dryer. It takes advantage of the outer drum for mixing the aggregates with the bitumen and filler.
In my view, this is the system which permits the highest recycling rates.
- Dryer at ground level vs elevated drum:
This classification has sense if we are talking about discontinuous plants.
- Dryer at ground level are all of the ones that we have been talking about so far.
- Elevated drum: the drum is positioned on the top of the plant and we upload the cold aggregates to this position by using an inclined conveyor belt.
This is completed with a special system of screening drum that we will talk about in the next chapter.
As far as I know, the two manufacturers that include this dryer are Lintec and Linnhoff. The advantages of this position, according to the manufacturers, are Fuel efficiency, no hot elevator required, noise reduction and easy maintenance.
c ) SCREENING AND HOT BINS:
This chapter has sense when we talk about discontinuous plants. Once the aggregates are heated, they are classified and put in hot bins.
The classification we are going to do is:
- Vibrating screens vs screening drum
- Vibrating screening: The system used is a vibrating box with flat sieves.
Depending on the manufacturer, we would have from three to four hot bins; furthermore you should consider the oversize aggregates. Take into account that the more hot bins you have, the easier is to make the recipe of the mix.
- Screening drum: we have talked about this in the previous chapter. It is a set with heater and screen drum with several sieves. The sieves are placed in the outer drum.
In both cases, as I told at the very beginning of the post, if we don´t classify properly in the cold bins, we will have a problem in the hot bins because the level inside won´t be suitable for the mix.
d ) MIXER, DOSAGE AND STORAGE
- We are going to consider the external mixer because we have already talked about the drum mixer. If we consider a continuous plant, we usually will have a continuous mixer
On the other hand, if we consider Batch plants, we will use a plug-mill mixer with twin shafts. This mixer has mixing arms and mixing blades. The movement by counter rotating shafts is used to improve the mixing .There are single shaft mixer, but I consider that is not the best option. The mixer is heated by thermal oil to avoid cooling the mixture. The mixer is covered internally by a special wear parts to protect the piece.
- Inside the mixer we prepare the recipe and we need a correct dosage of the products. The main products we use are: aggregates, bitumen, filler, polymers, additives, etc.… Each of them comes from a different part of the plant and has a different way of measure. In principle, the best way of dosage is the static weight or dynamic weight, but in several cases it doesn´t matter the small difference with other measurement systems.
- Aggregates: It is used dynamic weight in continuous plants. We spoke about this in the cold bins section. In Batching plants we should use static weight from the hot aggregates.
In continuous plants there are several ways:
- By volume: it´s not the best way to weight. The temperature affects the density of the bitumen and hence the discrepancies respect to the weight can be very important. Take into account that the dosage by ton is around 5% and it is the more expensive raw material.
- Continuous mass counter: is the more accurate way for these plants. I have worked with several systems and, in my view, the most accurate is the one who use the Coriolis acceleration principle.
In discontinuous plants the best way is by static weight by cells.
In continuous plants:
- By volume through a rotary valve
- By weighing with screw conveyor or a small weighing hopper in continuous way.
In Batching Plants:
- By weighing in the weigh hopper
- Other additives:
As the asphalt mixtures are developing to be more efficient and more environmental friendly, the use of additives to achieve it is higher. Depending on the state (liquid or solid) and the shape, we feed the additive in one place or another.
One of the most important additive is the polymer to modify the bitumen. The best manufacturers of plants have equipment for bitumen modification with polymers, such as
- Once we have the mixture ready, we can unload directly the mix over the truck or we can stock the mix in a storage silo. There are two kinds of storage silos:
- Short term storage: it is only to permit to have a little flexibility in the production. The range of capacity goes from 5 t to 40 t. You can suppose you are manufacturing bitumen mixtures and the next truck is not arriving. If you don´t have silo you should stop the plant and start again after a while.
This is always necessary if we have a continuous plant where the stopping suppose to break the operational status (it is very critical in continuous plant)
- Medium term storage: it is used when you need manufacture several recipes of mixture and you should dispatch them throughout the day. The capacity and the number of compartments are variables. They are usually provided by heater system to avoid the cooling of the mixture
e ) BAG FILTER
The bag filter has two main functions: on the one hand, it recovers the filler of the aggregates picked in the drum up to add to the mixture measured if it were necessary and, on the other hand, purify the exhaust gases in order to produce less environmental impact. The filter usually has a primary section where the gases impact and change of speed. This makes to fall the coarse particles to the screw conveyor for the process, the fine filler goes to the bag camera.
Attending to the cleaning system and to the internal section there are:
- Air pulse with continuous chamber: is the last image
- Reverse pulse /modular: it´s divided in sections and the cleaning is by reverse pulses with a rotary valve
Attending to the bag material:
- Filter of polyaramide bag: the most famous brand is NOMEX by DUPONT. We talk about the density and the resistance to the temperature as the main characteristics.
- Filter of laminated polyester. It permits to obtain more filtrating surface thanks to the folding of the surface. One problem is that this material resists less temperature than polyaramide bag and could cause inconveniences in the range of high temperatures.
f ) BITUMEN TANKS
It contains the core raw material of the process. An insulating material covers their entire external surface. They need a heater system to maintain the bitumen in the proper state to introduce it in the mixture (160ºC-º180ºC).
The tanks may be provided with: a recirculating system by pumps, fill pumps, heated and insulated pipelines, agitator systems for modified asphalts, polymers, etc.…
Attending to the position we can classify: horizontal and vertical. Normally mobile plants use horizontal tanks
Attending to the heater system there are:
- Electricity heating system: great developments are obtained in this aspect, encouraging big savings of energy.
- Thermal oil heating system: is the most traditional system. It´s a reliable system but, over time, a solidified film cover the heating pipes and make the process less effective.
- Direct fire: this system is nowadays all fashioned.
After such a bunch of sheets talking about asphalt plants we have seen that the manufacturers have technology and variety of possibilities to make a customized plant. Is the buyer who has to know what his company needs in the next years to perform his business.
Continuous vs batch plants, vibrating screens vs rotary screens, etc. are decisions that should be studied by experts at sight of the company. It´s recommendable to talk to several suppliers and strong suppliers of each kind of plant, thus we will know the strengths and weaknesses of each plant for our company.
In my view, this links should be clicked before the purchase of a Hot Mix Plant:
Depending on the geographical market, we have other manufacturers:
Other webs I have consulted in this post: