Patent Application
20170211243
The present invention relates to a procedure for the production of tempered bituminous mixtures to form the upper layers of the roadbed of highways and other road infrastructure utilising material recovered from deteriorated and/or aged layers of bituminous mixtures, rendering possible the utilisation at a rate of up to 100% of recovered material. The invention furthermore covers a production plant for the execution of said procedure.
In general terms a bituminous mixture may be defined as a combination of bitumen (also called asphalt) and stony mineral aggregates in preset proportions determining the physical properties thereof and, finally, the performance thereof for a given use.
The conservation of the highway assets entails planned operations of renovation and conservation to maintain the adequate structural durability and functional response thereof. In the most widespread pavement typology, that of bituminous roadbeds, one of the most habitual and frequent operations is that of milling and removal of deteriorated and/or aged layers and the substitution (replacement) thereof by new layers utilising newly produced bituminous mixtures.
The product generated by milling deteriorated and/or aged bituminous mixtures is habitually denominated RAP (reclaimed asphalt pavement), or MRMB (Spanish acronym for material recovered from bituminous mixtures); the former term shall be utilised hereinafter.
RAP is constituted by materials of high technical and economic value, for which reason the reutilisation or recycling thereof is desirable.
Hot procedures for the production of replacement bituminous mixtures are known which may utilise a certain percentage of RAP (up to approximately 50%), however not 100% by virtue of the fact that the high temperatures attained during the processes of hot production (approximately between 160° C. and 180° C.) deteriorate (burn′) the bitumen present in the RAP, damaging the properties thereof as a consequence.
Cold procedures for the production of bituminous mixtures with bituminous emulsion permitting the utilisation of 100% RAP are known. However, these mixtures present significant disadvantages, such as the requirement for a curing period (normally of one year as a minimum) and the weakness of the mechanical properties thereof in the initial moments subsequent to the laying thereof on site. For these reasons, when these procedures are used it is essential to add an additional wearing course on top of the layer wherein RAP is being employed.
Moreover, in the known procedures of production of tempered bituminous mixtures, such as that described in patent ES 2 368 980 A1, the utilisation of RAP is not included.
Consequently, there currently exists a great surplus of RAP, it being treated as waste and having the economic and environmental disadvantages implied thereby.
The present invention is orientated towards the resolution of this problem.
An object of the present invention is to provide procedures for the production of bituminous mixtures having a high level of mechanical and functional properties and permitting a percentage of RAP recycling of up to 100%.
Another object of the present invention is to provide procedures for the production of bituminous mixtures permitting reducing the cost thereof with respect to the known procedures.
A further object of the present invention is to provide production plants for the execution of the aforementioned procedures.
In a first aspect, these and other objects are achieved through a procedure of continuous production of bituminous mixtures from reclaimed asphalt pavement (RAP) and from a bituminous emulsion comprising the following steps:
a) Provision of RAP having a granulometry smaller than a maximum diameter D3 determined by the sieves of the shredder to be employed;
b) Processing said RAP to obtain at least two RAP fractions, the maximum sizes whereof do not exceed D1 mm and D2 mm;
c) Subjection of predetermined quantities of said RAP fractions to a process of mixing and heating by hot gas flow such that the heating times of each fraction are adapted to the granulometry thereof, being shorter and having less intensity of exposure for a fraction of smaller size (by virtue of disposing of a greater quantity of binder adhered to the aggregate, this latter having a greater specific surface than the aggregate of greater size, in this manner preventing the deterioration of the binder through the exposure to hot gases) than for another of greater size (disposing of a lesser quantity of binder, being enabled to be exposed for a greater time to the hot gases without the binder deteriorating), the temperature of the mixture at the end of the step being comprised between 90° C. and 120° C.;
d) Blending the mixture obtained in the foregoing step with a predetermined quantity of bituminous emulsion supplied at a temperature comprised between 60° C. and 80° C. approximately. The final mixture subsequent to the step d) will attain a temperature of between 90 and 110° C.
Advantageously, the granulometry of the first fraction is comprised between 0 mm and 7 mm (and more advantageously between 0 mm and 5 mm) and the granulometry of the second fraction is comprised between 4 mm and 30 mm (and more advantageously between 5 mm and 25 mm).
The procedure admits the incorporation of virgin aggregate into the mixture in order to improve the properties thereof and also, should it be so desired, to make tempered mixtures having a rate of recovery of RAP of less than 100%.
In the cases wherein 100% of the starting material is RAP, the quantity of bituminous emulsion is comprised between 2.5% and 3% by weight of the total weight of the bituminous mixture, contributing a much lower percentage of bitumen than that required when recycled materials are not utilised.
In a second aspect, the aforementioned objects are achieved by means of a plant for the production of bituminous mixtures from reclaimed asphalt pavement (RAP) and from a bituminous emulsion comprising:
a) a unit for pretreatment of the material proceeding from old roadbeds, whether by virtue of milling or through the demolition of a bituminous pavement, wherein the material having more than a maximum diameter D3 mm is rejected. The fraction comprised approximately between a diameter of D2 mm and D3 mm is fed to a shredder. The fraction smaller than D2 mm is taken directly to the output screen together with the product from the shredder. This shredder separates, with the minimum fragmentation possible of the stony matrix, the fractions bound by the binder, that is to say that what it does in essence is ‘unstick’ the same. This pretreatment unit may incorporate a magnetic separator preventing the input of pickaxes or metal remains present in the material to be fed. The product from this stage obtained at the output screen is two or more RAP fractions at ambient temperature.
b) a first hopper for a first RAP fraction having a granulometry the maximum size whereof does not exceed D1 mm, a second hopper for a second RAP fraction having a granulometry the maximum size whereof does not exceed D2 mm and a tank of bituminous emulsion, they being provided with dosing devices of the material contained therein;
c) a drum dryer having an entry-end burner for the heating and mixing of materials configured as a parallel flow drum, wherein the material to be heated and the hot gases proceeding from the combustion chamber thereof circulate in the same direction, provided with two inlets of material at differing distances from the combustion chamber thereof;
d) a first transport device (for example, a conveyor belt) disposed for transporting preset quantities of the first RAP fraction to the inlet of the drum dryer most distant from the combustion chamber thereof, and a second transport device (for example, a conveyor belt) disposed for transporting preset quantities of the second RAP fraction to the inlet of the drum dryer most proximate to the combustion chamber;
e) a blender, to blend the mixture realised in the drum dryer with a preset quantity of bituminous emulsion, proceeding from the tank of bituminous emulsion, disposed in such manner that it receives the mixture realised in the drum dryer by gravity.
Advantageously, the drum dryer comprises a system of controlled recirculation of a portion of the effluent fumes and is provided with a bag filter for the treatment of the unrecycled fumes.
Advantageously, the blender comprises a system of measurement of the energy consumption thereof permitting estimation of the humidity of the final bituminous mixture.
The plant may also comprise a device for raising the mixed product to a holding hopper, in this manner adapting the continuous flow of the installation to the discontinuous flow of the lorries. The raising device may be, for example, a scraper conveyor. For long holding times, instead of a hopper, a silo of greater size, lagged and heated at the base thereof, may be used.
The plant may also comprise a third and a fourth hopper for virgin aggregate connected to a second transport device in order that this latter may also transport preset quantities of virgin aggregate to the drum dryer. The granulometry of the virgin aggregate will be that required to obtain the final granulometry of the desired mixture, having the percentages determined in the working formula obtained in the laboratory.
The third and fourth hoppers may also be utilised in an alternative manner for a third and a fourth fraction of RAP, should it be desired that more than two fractions of recycled material be employed in the production procedure.
Other characteristics and advantages of the present invention will be appreciated from the detailed description hereinafter of illustrative embodiments of the object thereof in relation to the figures appended.
The continuous procedure for the production of tempered bituminous mixtures with RAP
The steps of the procedure of the invention will be described in a detailed manner hereinafter in relation to
a) The first step consists in the provision of RAP having a granulometry smaller than D3 mm, RAP of greater than this diameter consequently being rejected. Preferably, the granulometry will be comprised between 0 mm and a value of D3 of less than 60 mm.
b) In the second step the RAP is processed to obtain at least two fractions f1 and f2, the maximum sizes whereof do not exceed D1 mm and D2 mm, disposed in stockpiles A1 and A2. In a preferable manner, the value of D1 will be less than 7 mm and the value of D2 will be less than 30 mm.
This treatment may be realised by means of a first operation of shredding the RAP comprised between D2 and D3 in, for example, a shredder, and a second operation of screening the shredded material and the remainder of the RAP (smaller than D2) to obtain said two RAP fractions f1 and f2. It is important to note that the shredder separates the RAP with the minimum fragmentation of the stony matrix, that is to say that what it does in essence is ‘unstick’ the aggregate bound by the bitumen.
The treatment of this step may also include the separation of metal materials present in the RAP by means of magnetic separators.
Preferably, the first fraction f1 has a granulometry comprised between 0 and 7 mm (and more preferably between 0 mm and 5 mm) and the second fraction f2 has a granulometry comprised between 4 mm and 30 mm (and more preferably between 5 mm and 25 mm).
At the end of the step there are thus obtained two (or more) RAP fractions at ambient temperature having two (or more) clearly differentiated granulometries, wherein the humidity has been significantly reduced. In this manner the quantity of bitumen present in each RAP fraction may be calculated with greater exactitude, and with this information the working formula of the final bituminous mixture and the quantity of bituminous emulsion to be added in the fourth step may be optimised.
c) In the third step predetermined quantities of RAP fractions f1, f2 (and, as appropriate of other fractions), obtained in the foregoing step are subjected to a process of mixing and heating by hot gas flow such that the heating times are shorter for the RAP fractions of smaller size and that the temperature of the mixture is comprised between 90° C. and 120° C. at the end of this step c).
In an embodiment of the procedure, the execution of this step is carried out by firstly feeding hoppers T1, T2, destined for RAP fractions f1, f2 and provided with dosing devices D1, D2, from the stockpiles A1 and A2; secondly, supplying preset quantities of said fractions f1, f2 to transport devices, for example, conveyor belts B1, B2, carrying them to two differentiated inlets of a drum dryer TS wherein said fractions are mixed and heated differentially, permitting in this manner a rate of recycling of RAP of 100%, by virtue of the reasons stated hereinbefore.
Consequently, in the procedure of the present invention, the RAP is heated directly, however being executed in such a manner as not to affect the bitumen present therein.
The heating of the two fractions f1, f2 is carried out by contact with a hot gas flow ac (avoiding contact with the flame as occurs in the counter-current drum dryers utilised in bituminous plants such as that described in patent ES 2 368 980 A1) and the heating time differs for the two fractions f1 and f2, they being made to enter the drum dryer TS at different points in order to accommodate the differing granulometry thereof and, furthermore, the different quantities of binder (bitumen) in order not to affect the properties of the latter.
The fraction f2, having the coarser RAP, is introduced through the inlet most proximate to the source of hot gases ac (at the commencement of the drum dryer TS), more time being required to transmit the quantity of heat required to the coarser fraction. The fraction f1, having the finest aggregate, is introduced through the inlet most distant from the source of hot gases ac, situated at an appropriate point of the drum dryer TS such that the fraction f1 attains the temperature required at the outlet from the drum dryer TS, comprised between 90 and 120° C.
In addition to the RAP fractions f1 and f2, this step may include an additional fraction of virgin aggregate to improve the characteristics of the bituminous mixture. In this case, this material would be supplied to the drum dryer TS by means of the feeder belt B2 utilised for RAP fraction f2. The granulometry of the virgin aggregate will be that required to obtain the desired final granulometry of the mixture having the percentages determined in the working formula obtained in the laboratory. In an alternative manner, instead of employing virgin aggregate, this step may include a third and a fourth fraction f3 and f4 of RAP, should it be desired that more than two fractions of recycled material be employed in the production procedure. In a preferred manner, the granulometries of the third and a fourth fraction f3 and f4 will be comprised between D1 mm and D2 mm. As appropriate, this material would also be fed to the drum dryer TS by means of the feeder conveyor B2 utilised for RAP fraction f2.
d) In the fourth step, the mixture obtained in the foregoing step is mixed with a predetermined quantity of bituminous emulsion supplied at a temperature comprised between 60° C. and 80° C.
The mixture f1+f2 obtained in the foregoing step is discharged into a bitumen mixer or blender AM, wherein it is mixed together with a preset quantity of bituminous emulsion eb received at 60 to 80° C. from a tank T3 provided with a dosing device D3. In the present procedure, when 100% RAP is utilised as starting material, the quantity of bituminous emulsion eb is comprised between 2.5 and 3% by weight.
The bituminous mixture mb obtained subsequent to the fourth step may be supplied directly to a means of transport to carry it to the destination thereof or be stored by means of a raising device in a holding hopper or silo, which may be lagged and heated.
The plant for the production of tempered bituminous mixtures with RAP
An embodiment of the plant for the production of bituminous mixtures according to the invention will be described in a detailed manner hereinafter utilising two RAP fractions and, as appropriate, virgin aggregate, in relation to
The fundamental components thereof are the following:
In a unit for the pretreatment of the material proceeding from the milling or demolition of a bituminous pavement (proceeding from aged roadbeds) the material exceeding D3 mm is rejected. The fraction comprised approximately between D2 mm and D3 mm is fed to a shredder, whilst the fraction smaller than D2 mm is taken directly to the output screen together with the product from the shredder. This shredder separates the RAP with the minimum fragmentation of the stony matrix, that is a say what it does in essence is ‘unstick’ the fractions bound by the binder. This pretreatment unit may incorporate a magnetic separator preventing the input of pickaxes or metal remains present in the material to be fed. The product of this stage obtained at the output screen is two or more RAP fractions at ambient temperature. The values of D3 and D2 will depend on the size of the installation and on the final material to be obtained, habitual values being 60 mm for D3 and 30 mm for D2.
Hoppers 1, 2, 3, 4
These are hoppers having control of the level and weight contained in each thereof and independent weigh feeders for dosing the content thereof. The hoppers 1 and 2 are destined for the storage of, respectively, the fine fraction and the coarse fraction of the RAP produced in a unit for the pretreatment of the RAP. The hoppers 3 and 4 are destined to store virgin aggregate of differing granulometry which may also be utilised as starting material for the production of bituminous mixtures (either to improve the mixture with 100% RAP or for mixtures having rates of recycled material of less than 100%). In an alternative manner, the third and fourth hoppers may be also utilised for a third and a fourth fraction of the RAP, should it be desired that more than two fractions of recycled material be employed in the production procedure.
Drum Dryer 5
In a preferential embodiment this is a parallel flow (co-current) drum, driven by electric motors having a fuel oil burner (although any other fuel may be employed such as, for example, gas). The combustion chamber 6 thereof is disposed in an entry-end position and it has two inlets or rings in different positions. That most proximate to the combustion chamber 6 is destined for the coarsest RAP fraction(s) and, as appropriate, the virgin aggregate, and the most distant for the finest fraction of the RAP. This latter must be located at a point of the drum dryer 5 as distant as possible from the burner (in order to damage the bitumen as little as possible), however at a point such as to permit that said fraction attains the required temperature at the outlet from the drum dryer, comprised between 90° C. and 120° C.
The configuration thereof with entry-end flame and the great length thereof permit increasing the dwell time of the RAP fractions in order to achieve the required transfer of heat whilst, however, preventing direct contact of the RAP with the flame. It being a matter of a parallel flow drum moderates the extreme contacts of temperature with the materials fed to the same. The materials and the hot air circulate in the same direction, differing from that occurring in the drum dryers of many bituminous plants and, in particular, that described in ES 2 368 980 A1.
It is provided with frequency variators permitting modification of the speed of rotation.
It is connected to a bag filter 13 having an extractor fan for the purpose of reducing harmful emissions whilst promoting the combustion of the flame.
On being a matter of a parallel flow drum, less thermally efficient than a counter-current drum, it includes a system of recirculation of the effluent fumes. That is to say, a given percentage of the exhaust fumes from the drum dryer 5 is not sent to the bag filter 13 but is recirculated within the drum dryer 5 by means of a system of recirculation ducts and electrovalves.
The combustion gases may be mixed at the commencement of the cycle thereof: they may be combined with a portion of the recirculated gases and another of fresh air by means of a tangential fan creating the turbulence required for homogeneous heating (the quantity of fresh air may be adjusted). By means of this system of partial recirculation of gases the efficiency of the drum in co-current or in parallel is improved.
The combination of the entry-end burner (and, in consequence, the absence of direct contact with the flame) and appropriate control of the temperature by means of the partial recirculation of the fumes leads to the treatment of the bitumen present in the RAP being very judicious, not damaging it and rendering possible the reutilisation thereof.
The burner of the drum dryer 5 is fed by fuel oil (although any other fuel may be employed such as, for example, gas) by means of a pump from a tank at approximately 40° C. and it is filtered by means of in line filters.
Transport devices 7, 8 from the hoppers 1, 2, 3, 4 to the drum dryer 5. The quantities of the RAP fine and coarse fractions required from time to time are delivered, respectively, to transport devices 7, 8 (in the example shown in
Tank 10 of Bituminous Emulsion.
It is a heated and lagged cistern having sufficient capacity to store the bituminous emulsion required for the process. It is connected to a frequency variator driven pump, governed by means of a control system (PID) based on the readings of a flowmeter, to supply the bituminous emulsion to the blender 9.
Blender 9.
The blender 9 receives the flow of material egressing from the drum dryer 5 and the bituminous emulsion from the tank 10 at a temperature comprised between 60 and 80° C.
The blender 9 is disposed beneath the drum dryer 5 in such manner that it receives the mixture of the RAP fractions (and, as appropriate, of virgin aggregate) by gravity, the final mixture attaining a temperature comprised between 90 and 110° C. It is consequently a matter of a disposition differing from that of the plants of tempered bituminous mixtures such as that described in ES 2 368 980 A1.
The blender 9 includes a system of measurement of the energy consumption in order to estimate, utilising appropriate correlation tables, the humidity of the final mixture. This parameter is fundamental because, in order to ensure adequate compaction when laying on site it is necessary that the humidity of the mixture ranges between 0.5 and 1%.
The installation provides for the possibility of adding water to the blender in order to achieve the desired humidity in the final mixture.
Devices 11, 12 of Evacuation of the Bituminous Mixture.
The bituminous mixture egressing from the blender 9 is carried to a holding hopper 12 by means of a raising device, such as, for example, a slat conveyor 11.
Although the present invention has been described in connection with various embodiments, it may be appreciated from the description that various combinations of elements, variations or improvements may be made thereto and falling within the scope of the invention defined in the claims attached.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201431103 | Jul 2014 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2015/000054 | 4/17/2015 | WO | 00 |
