EP1408157A2

EP1408157A2 - Method and device for producing asphalt mix

Info

Publication number: EP1408157A2
Application number: EP20030023201
Authority: EP
Inventors: Karl Gunnar Ohlson
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-10-11
Filing date: 2003-10-13
Publication date: 2004-04-14
Anticipated expiration: 2023-10-13
Also published as: SE0203057L; DE60335593D1; EP1408157A3; EP1408157B1; SE525421C2; SE0203057D0

Abstract

The invention relates to a method for producing an asphalt mix that includes a coarse fraction, a fine fraction, filler and bituminous binder with the following steps in the mentioned order:

a) mixing the coarse fraction and the liquid binder in a mixer (10) and

b) discharging the filler with a controlled flow rate to powder the binder liquid which is distributed over the surface of the coarse fraction with the filler and

c) spreading the fine fraction over the layer of filler,

wherein the discharge of the filler and the fine fraction is controlled in such a way that the filler and the fine fraction becomes enclosed in a combined suspension where the filler, the fine fraction and the coarse fraction are parts of a closed liquid volume. For improving the mixing method in such way that the time intervals and flow rates can be easier optimised to develop the closed liquid volume and that the production of an asphalt mix with still further reduced tendency to crack is producible, the invention proposes that the filler and/or the fine fraction is discharged into a downwardly directed material flow in the mixer (10). Further, a device for carrying out said method is described.

Description

The invention relates to a method according to the preamble of claim 1 as known from EP 0 756 615 B1.
Reference is first made to WO 83/00700 which describes a method and apparatus for the production of asphalt concrete. According to this previous method, the aggregate is divided into a coarse fraction, a fine fraction and a filler portion. First, the coarse fraction and the bituminous liquid are mixed, then the filler fraction is added, and last, the fine fraction is added. The apparatus for carrying out such production method is described in detail. The mixing method of WO 83/00700 has been commonly known as "KGO II" mixing method. The known mixing method is characterized by force mixing. The filler is discharged at once, and then the mixing is conducted by the force of mixing paddles.
Reference is now made to the above mentioned EP 0 756 615 B1. In such patent, there is shown and described a test equipment and method for testing of asphalt specimens, called TOD test. The specimens are heated and the deformation in the specimens is determined. The deformation over temperature is plotted in a graphic. The curve of a specimen produced by the KGO II mixing method still shows sharp buckles and cracks. This shows the tendency of an asphalt to crack. In EP 0 756 615 B1, a new mixing method is given reducing such tendency to crack. The mixing method of EP 0 756 615 B1 - known as KGO III mixing method - is characterized by controlling the flow rate and time intervals for discharging the filler and the fine fraction. The filler discharge is such controlled that the filler is powdered over the liquid distributed over the surface of the coarse fraction. To understand this, one can compare the mixing methods with a preparation of a pancake dough. If the flour is discharged at once into the milk, a forced mixing is necessary. Nevertheless, the dough becomes lumpy. If, however, the flour is powdered over the milk when mixing the dough, lumps are avoided. The same technique is used in the KGO III mixing method. By such mixing, the bituminous liquid, the filler and the fine fraction material becomes enclosed in a combined suspension where the filler, the fine fraction and the coarse fraction are parts of a closed liquid volume. This condition is testable by the TOD-test. The flow rate and the time intervals for discharging the filler and the fine fraction are controlled in such a way that the closed liquid volume is established.
With the mixing method of EP 0 756 615 B1, one has changed from a force mixing to a flow mixing. Further details of the theory and the praxis of such flow mixing are given in "Method and means used in production of asphalt mix. Theory of suspension, mixing method, method of testing, method of analyses", dissertation at Lund University by Ohlson, Karl Gunnar, Institutionen för Trafikteknik, LTH, bulletin 132, available at the Lund University, Box 118, SE-22100 Lund (Sweden) under the code "CODEN LUTVDG/(TVTT-3110)/1-113/(1995)", Lund 1995. This dissertation is incorporated here by reference.
When trying to conduct the KGO III mixing method in practice, namely in some asphalt plants, especially such one as given in WO 83/00700, some problems have been raised. It was particularly difficult to find the optimised time intervals and flow rates for developing the closed liquid volume.
Starting from the state of art as given in EP 0 756 615 B1, the object of the invention is to improve an mixing method according to the preamble of claim 1 in such way that the time intervals and flow rates can be easier optimised to develop the closed liquid volume and that the production of an asphalt mix with still further reduced tendency to crack is producible.
Such object is achieved by the mixing method having the steps of enclosed claim 1.
According to the invention, the filler and/or, especially, the fine fraction is discharged into a downwardly directed material flow in the mixer.
Advantageous embodiments of the invention are given in the dependent claims. A device for carrying out the invention is given in the further independent claim.
The invention shall be used in all production by mixing methods where the mix of coarse fraction and the bitumen are added to the mixer before the filler and the fine fraction are admixed. In fact the method refers to the Swedish patent no. 506 581 from 1994-04-19, and the corresponding EP 0 756 615 B1, respectively.
The invention shows the exact position for discharge of the filler and the fine fraction to the mixer of the asphalt plant that produces suspension asphalt mixes for road paving. In suspension asphalt mixes, the bitumen, the filler, and the fine fraction particles are permanently bound to the surface area of the coarse fraction particles. Asphalt plants that lack the invention produces sedimentary asphalt mixes which segregate shown by that the fine fraction material in the paving is more or less diverted from the coarse fraction. This can be verified, for example, by the TOD test.
Suspension in asphalt mix is formed from the filler material where the main portion is smaller than, for example, 0.074 mm. The suspension also includes the fine fraction of the aggregate that normally is smaller than size, for example, 4 mm. The coarse fraction consists of particles larger than the particles of the fine fraction. The suspension is developed on the surface of the coarse fraction aggregates.
In accordance with SE patent 506 581 and EP 0 756 615 B1, respectively, the filler material is spread in the bituminous liquid by the flow in the mixer, in penetrable thickness over the films of liquid already developed on the surface area of the coarse fraction. the distribution is obtained through choice of time intervals between adding the components and through control of the flow rates when spreading the material over the flow of the coarse fraction in the mixer. The production by the KGO III (as known from EP 0 756 615 B1) is called "Flow Mixing".
When spreading the fine fraction in the bituminous liquid, this shall also be made by calibration of the intervals and flow rates in the same manner as when spreading the filler material.
One reason for the present invention is the discovery of that the filler material and the fine fraction have different requirements for being mixed by the flow. The reason for this is that the quantity of fine fraction material is a number of times larger than the quantity of the filler.
The invention is to be used in production of asphalt mix where the coarse fraction and the bitumen liquid are mixed before the addition of the filler and the fine fraction material as in the SE patent no. 506 581.
According to an advantageous embodiment using a mixer with essentially horizontal rotating mixing shafts having radially extending paddles or the like mixing tools, the fine fraction material is discharged along the mixers wall that is parallel to the shaft of the mixer in order to create an excessive pressure in the material in the mixer, and gravitation force that increases the rate of flow relative to the rate of rotation of the paddles, which lessens the work of the paddles in the materials whereby the coatings of the suspension develop the mix into a structure that remains permanent until the mix is paved.
According to the invention, the asphalt mixers downwards directed material flow is loaded with the fine fraction material in order to increase the rate of flow of the material in the mixer in relation to the rate of rotation of the paddles by the gravitation force of the fine fraction material.
Advantageously, the load on the material stream increases the rate of flow in the mixer by the equalization of high pressure in the loaded zone and the low pressure in the unloaded zone. This is possible, since the mixed materials in the mixer behave like a liquid during the flow mixing method.
A further advantage is the increase in the flow rate in the mixer in relation to the rate of rotation of the paddles and that this lessens the paddle work and shear in the mix.
According to a further advantageous embodiment, the method for production suspension asphalt mix by change from force mixing to flow mixing is characterized by admixing the bituminous liquid and the coarse fraction of mix to make the mix flow like a liquid to which flow the filler, normally particles smaller than about 0.074 mm, is flow mixed by adding at a slow flow rate whereafter the fine fraction material with particles normally smaller than about 4 mm (and greater as the filler particles) is flow mixed by spreading of the material over paddles or the like mixing tools in a zone where the flow is downwards, this creating an excessive pressure in the material by gravitation force and position energy. This increases the rate of flow relative to the speed of rotation of the paddles, and by this, lessens the shear force of the mixing tools in the mixer whereby the coatings of the suspension on the surface of the coarse fraction develops the mix into suspension.
In a still further advantageous embodiment, the downwards stream in the mixer is loaded, especially with the fine fraction material, as to produce a levelling out of the fluid mix, by this increasing the flow rate of the stream in the mixer in relation to the speed of rotation of the paddles.
It is preferred that the paddle work in the mix is reduced by spreading the fine fraction material over the mixer's paddles of the downwards flow.
An advantageous embodiment of a device for carrying out the method in accordance with the invention has a plate or a drum that guide the inflow of material to the mixers wall, parallel to the shafts of the mixer, in order to connect and to load the downwards flowing material stream for spreading to the other parts of the mixer not loaded with the fine fraction. Preferably, the paddles in the downwardly flowing stream are loaded with the inflow of material for spreading the fluid mix to the lower part of the mixer.
A preferred embodiment of the invention is described in further detail below with reference to the enclosed drawings in which

Fig. 1: is a plan view of an asphalt mixer for producing asphalt mix; and
Fig. 2: is a sectional view through the mixer of Fig. 1.

In a preferred embodiment of the invention, an asphalt mix is produced as described in EP 0 756 615 B1. Reference is also made to WO 83/00700 with further details in view of the separation of an aggregate into a coarse grained portion, a fine grained portion and a filler portion. Still further, reference is made to the dissertation of the inventor Ohlson, Karl Gunnar: "Method and means used in production of asphalt mix", Lund University, bulletin 132, 1995 (details see above).
The figures show a mixer used in the preferred embodiment. The mixer 10 has a mixer body 11, having vertical side walls 12, 13 and end walls 14, 15. There is a bottom 17, comprising two portions 18 which are arcuate in cross section and which are connected together adjacent the longitudinal centre of the mixer body 11. Means for mixing materials within the mixer body 11 comprises a pair of shafts 21 and 24 which are supported adjacent the ends by suitable gearings 25. The shafts 21, 24 are simultaneously rotated by means of an electric motor 20 and suitable gearing 26. The direction of rotation is shown by arrows 27, 28.
The shafts 21, 24 are on axes from which the radii of the bottom portions 18 extend as best shown in fig. 2, and each shaft 21, 24 is provided with a plurality of mixing tools. Here, paddles 30 are used which are connected with the shafts 21, 24 by means of arms 31.
As shown in fig. 2, the paddles 30 are moving upwards in a center portion of the mixing body 11, as shown by arrow B. Near the side walls 12, 13, which extend parallel to the shafts 21, 24, the paddles 30 are moving downwards.
The coarse fraction, the filler fraction and the fine fraction are discharged by a discharging means, generally indicated by D. Said discharging means D has an opening 42 closable by a closing means, indicated E. Further details for providing the aggregate are not shown, but known in the art, see for example WO 83/00700. The discharging means D has a guiding means C which leads the discharged materials away from the upwards directed zone B to a zone A2 or A1, alternatively, where the flow in the mixer is directed downwards. In the embodiment shown, the fine fraction guiding means C for guiding the fine fraction into zone A2 has a plate 44 which directs the inflow of material 50 to the sidewall 12.
By the SE patent 506 581 and EP 0 756 615 B1, it is made known that the flow of the filler is to be connected to the flow of the coarse fraction in the mixer through a slow discharge of these materials, whereas by the shown embodiment of the invention, the spreading of the fine fraction in the bituminous liquid is made by fast discharge to the area marked by zone A1, alternative A2 shown in figure 2.
At a fast discharge, the flow of the fine fraction material will produce an overload of the material by which the gravity of the material and the nature of the fluid to flow from high to low areas increases the flow rate of the fine fraction in to other parts of the mixer that are less loaded.
With this technique, producing of a batch of asphalt mix last only about 1 minute. To put in the coarse fraction needs about 3 sec. Thereafter, the bitumen is sprayed in within 7 - 8 sec. Then the coarse fraction and the bitumen are mixed for about 7 -10 sec. Then the filler flow is started with the optimised flow rate and time, this is typically about 17 sec. After the optimised time interval, the fine fraction is added. Although the fine fraction is typically about 25 % by weight of the total aggregate and the filler is only about 7 to 9 %, the adding of the fine fraction takes only up to 10 sec., depending on the circumstances at and above the discharge gate. Thus, the time for adding the fine fraction is, compared to the masses involved, much less as the time for adding of the filler. Thus, by choosing the optimal location for discharging the fine fraction, it is possible to reduce essentially the total time for flow mixing a batch of asphalt. The mentioned time intervals are adjustable to meet the requirements for flow mixing.
As visible from Fig. 2, the mixer 10 has three zones, one (B) in which the flow of materials is directed upwards, and two (A1 and A2) in which the flow is directed downwards. Fig. 2 shows an example of a mechanical solution by said plate 44, mounted to steer the flow of the fine fraction to the place of discharge in accordance with the invention. The plate 44 can be installed permanently. Preferably, however, the guiding means can be made movable for only directing the fine fraction (or the filler and the fine fraction) into the downwardly directed material flow.
The previously known asphalt plants were discharging filler and fine fraction material in the zone B where the material flow is upwards, between the two shafts 24. This caused the flow of the coarse fraction in the mixer to stop. When the flow of the coarse fraction has been stopped as described, the flow mixing ended; and to start the flow required force mixing by the paddles, this resulting in that only part of the coarse fraction particles become coated by suspension. Fine fraction particles not suspended in the liquid, rode on top of the mix being the result of the upwards pressure created by the paddles work. The invention overcomes that problem by directing the fine fraction, and, if wished, the filler into a downwardly directed stream.
This reduces the shear force of paddles 30 on the filler bitumen films of the coarse fraction when mixing in the fine fraction material whereby the mix develops into a suspension. The paddle work is reduced through that the fine fraction is spread, when added to the mixer, over the paddles 30 in the zone A1 or A2 where the force of gravitation and the nature of flowing mix to level out, both work in the same direction as the paddles 30 in the mixer 10.
The force of gravitation is the result of the position energy of the material in zone A1 (alternative zone A2). The levelling energy is a result of over loading, this caused by the inlet of the fine fraction in zone A1 or A2. Example of such force is a container divided in half by a wall. If filled with water on the one side of the wall and a valve is opened at the bottom of the defining wall, the water from the filled side will flow to the other side until the water is levelled in the two parts.
The invention exposes a combination of three forces for reducing the shear of the paddles, 1) the energy of the flow, 2) the force of gravitation, and 3) the position energy. As long as the mix is fluid, the materials are mixed by the flow. If the admixing of the fine fraction divert from the invention, the mixing process will change from flow mixing to force mixing and the mix consistency from fluid to clogged.
At the place of adding the fine fraction in the method of the invention, all paddles of the mixes drive the flow in the same direction. This lessens the paddle work compared to conventional asphalt mixers. The turning paddles as well as the rest of the paddles on the shaft of the invention, drive the fluid mix in a flow parallel with the mixers sides 12, 13 at the end of the shaft. No accumulation is found at the turning paddles when driving the liquid mix. EP 0 756 615 B only describes time intervals and control of flow rate as the means for mixing. The manner and place for adding the fine fraction in the mixer is not accounted for.
The invention is used on different types of asphalt plants for production of suspension asphalt. Without the invention, many plants fail to produce suspension asphalt paving.
The invention develops a liquid mixing process. There is no shear in liquids. This is one reason for that the suspension develops on the surface of the coarse fraction and remains there until the mix is paved.

Claims

Method for producing an asphalt mix that includes a coarse fraction, a fine fraction, filler and bituminous binder with the following steps in the mentioned order:

a) mixing the coarse fraction and the liquid binder in a mixer (10) and

b) discharging the filler with a controlled flow rate to powder the binder liquid which is distributed over the surface of the coarse fraction with the filler and

c) spreading the fine fraction over the layer of filler,

wherein the discharge of the filler and the fine fraction is controlled in such a way that the filler and the fine fraction becomes enclosed in a combined suspension where the filler, the fine fraction and the coarse fraction are parts of a closed liquid volume,
characterized in that
the filler and/or the fine fraction is discharged into a downwardly directed material flow in the mixer (10).
Method according to claim 1,
characterized by using a mixer (10) with a mixing shaft (21, 24) extending in a horizontally direction and having paddles (30) for driving the flow of the material to be mixed and by discharging the fine fraction material along a mixer wall (12, 13) extending essentially parallel to the shaft (21, 24) where the paddles (30) are rotated in a downward direction.
Method according to any of the preceding claims
characterized by using a mixer (10) with at least two mixing shafts (21, 24) having mixing paddles (30) wherein the shafts (21, 24) and their paddles (30) are rotating in opposite directions so that the paddles (30) on the side near to the other shaft are moving upwards and the paddles (30) located near to a wall (12, 13) of the mixer (10) which wall (12, 13) extends essentially parallel to the shafts (21, 24) are moving downwards.
Method according to any of the preceding claims,
characterized in that the flow rate for discharge the fine fraction is bigger than the flow rate for discharging the filler and/or that the fine fraction is discharged in a shorter time as the filler.
Method according to any of the preceding claims,
characterized in that the fine fraction is discharged, so that fine fraction material is aggregated above the fluid level of the downwardly directed flow in the mix.
Method according to any of the preceding claims,
characterized in that the fine fraction is discharged into the downwardly directed mix flow with a velocity of the mix flow.
Device for producing asphalt mix and for carrying out the method of any preceding claim, with a mixer (10) and discharge means (D) for discharging materials to be mixed into the mixer (10), wherein the discharging means (D) has a fine fraction separating means for separating a fine fraction from a coarse fraction and a filler;
characterized in that the discharging means (D) leads the fine fraction via a fine fraction guiding means (C) to a location (A2) of the mixer (10) where the materials to be mixed are moved downwards.