WO2010011171A1

WO2010011171A1 - Method for preparing an asphalt compound

Info

Publication number: WO2010011171A1
Application number: PCT/SE2009/050860
Authority: WO
Inventors: Roger Lundberg
Original assignee: Ncc Roads Ab
Priority date: 2008-07-21
Filing date: 2009-07-02
Publication date: 2010-01-28
Also published as: EP2303966A4; EP2303966A1; CN102066495B; SE532658C2; SE0801719L; CN102066495A; RU2011102024A; RU2502760C2

Abstract

The invention relates to a method for preparing an asphalt mixture in a mixer (15). According to the invention the method comprises the steps of, loading a dry aggregate comprising stone material principally free from filler into the mixer (15), thereafter loading a foamed binder mixture into the mixer (15) that contains said dry aggregate, which binder mixture is foamed when it leaves a prefoaming chamber (12) and is loaded into the mixer (15), whereupon mixing of said aggregate and said foamed binder mixture takes place, and thereafter loading a filler (6) into the mixer (15), whereupon final mixing of the asphalt mixture takes place.

Description

METHOD FOR PREPARING AN ASPHALT MIXTURE Technical field of the Invention

The present invention relates generally to a method and a mixer for preparing an asphalt mixture, for subsequent in- termediate storage and future spreading on a road or the like. The present invention relates specifically to a method and a mixer for preparing a so called hot asphalt mixture.

Background of the Invention and prior art Asphalt is basically a mixture of stone material/aggregate and a binder, and is used as a surface course, binder course and/or base course on for example roads, airports, parking sites, etc. Conventional preparing of so called hot asphalt mixture embrace warming the aggregate up to approxi- mately 16O⁰C and that all the aggregate is placed in a mixer whereupon heated binder is provided in liquid form. Thereafter, the asphalt mixture is kneaded, at a process temperature of approximately 160⁰C, to its final consistency, for subsequent intermediate storage and future spreading. Hot asphalt mixture has by tradition in general better characteristics than previous attempts with preparing warm asphalt mixture (temperature of preparing approximately 70-120⁰C) and so called cold asphalt mixture (temperature of preparing below 70⁰C) , however, the preparing of hot asphalt mixture is very energy consuming, and also large amounts of nitric oxides and carbon dioxide emanates during the preparing and spreading of hot asphalt mixture, which is harmful to the environment and provide a worse working environment for the workers spreading the asphalt. Thus, for a time back the aim has been to lower the preparing temperature, near to the lower limit for hot asphalt mixture, without compromising on the quality of the spread asphalt, and thus to prepare hot asphalt mixture having similar characteristics as conventional hot asphalt mix- tare prepared ac approximately 160⁰C. EP 1,398,351 Bl disclose a method directed towards preparing warm and hot asphalt mixture, in which method all the aggregate is loaded into a mixer, a foaming agent in the shape of a polymerized fatty acid is added to a heated bitu- men, whereupon the resulting binder mixture is added to the aggregate as a foam. US 2005/0018530 Al discloses a method directed to the preparing of asphalt mixtures in the lower area of hot asphalt mixture, in which method the aggregate is added to the asphalt mixture in two portions. More pre- cisely, a first amount of dried aggregate principally free from fine material is loaded into the mixer, thereafter heated and liquid bitumen is added, thereafter wet aggregate comprising water, sand and fine material is loaded into the mixer, whereupon the bitumen is made to self expand. How- ever, the resulting asphalt mixtures according to these techniques are impaired by negative characteristics. The largest negative characteristics is that the filler in the EP-document and the fine material in the US-document absorb a very large amount of the bitumen/bitumen mixture since the filler is present in the asphalt mixture when the bitumen/bitumen mixture is foamed/self expanded. When such a asphalt mixture is spread onto the roads a proportionately small amount of the bitumen/bitumen mixture will be bonded to the stones in the aggregate and a proportionately large amount of bitumen/bitumen mixture will be bonded to the fine material or filler. When the spread asphalt is used the bitumen/bitumen mixture which is bonded to the fine material or filler will run the risk of being rapidly ripped off from the asphalt. Thus, according to these methods there is not provided a hot asphalt mixture showing similar characteristics as conventional hot asphalt mixture, regarding for instance durability, adhesiveness, etc. Summary of the objects and characteristics of the Invention

The present invention aims at obviating the aforementioned disadvantages and failings of previously known methods for preparing asphalt mixtures, and at providing an improved method. A primary object of the present invention is to provide an improved method for preparing an asphalt mixture of the initially defined type, which provides an asphalt mixture, prepared near to the lower limit of hot asphalt mixture, having similar or better characteristics than conven- tional hot asphalt mixture, for instance regarding durability, adhesiveness, the presence of cavities, stiffness/toughness, etc., of the spread asphalt.

It is another object of the present invention to provide a method, which emanates less smoke and smell by reduced dust emission.

It is another object of the present invention to provide a method, which is better for both the environment and the working environment by means of lower energy consumption and also by means of lower emission of carbon dioxide and nitric oxides.

It is yet another object of the present invention to provide a method, which provide an asphalt mixture which is more easy to work with, regarding preparing as well as spreading. According to the invention at least the primary object is attained by means of the initially defined method, which is characterized by the steps of, loading a dried aggregate comprising stone material principally free from filler into the mixer, thereafter loading a foamed binder mixture into the mixer that contains said dried aggregate, the binder mixture is foamed when it leaves a prefoaming chamber and is loaded into the mixer, whereupon said aggregate and said foamed binder mixture is mixed, and thereafter a filler is loaded into the mixer, whereupon final mixing of the asphalt mixture takes place. Thus, the present invention is based on the insight that it is crucial for the asphalt which is made from the asphalt mixture that the binder mixture is allowed to bond to the stones of the aggregate in thick layers/coatings before the heavily absorbing filler is added to the asphalt mixture.

Preferred embodiments of the present invention are further defined in the dependent claims.

Preferably the binder mixture comprises bitumen, an ad^¬ hesion agent in the form of a fatty acid and a foaming agent in the form of water, whereupon a strong and long lived foam is provided, which contributes to a asphalt mixture which is easy to work and which bonds to the stones of the aggregate in thick layers before the filler is added.

Further advantages and features of the invention will be apparent from the other dependent claims as well as from the following detailed description of preferred embodiments.

Brief description of the drawings

A more complete understanding of the abovementioned and other features and advantages of the present invention will be apparent from the other dependent claims as well as from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

Fig. 1 is a schematically shown side view of an inventive mixer, and also chosen parts of an asphalt mixing plant, and Fig. 2 is an enlarged schematic view from above of the mixer according to figure 1.

Detailed description of preferred embodiments

Reference is by way of introduction primarily made to figure 1.

Asphalt mixture consist to the largest amount of an ag- gregate i, or stone material, generally amounting to at least 80 percentage by weight, preferably approximately 90 percentage by weight, of the total weight of the asphalt mixture .

The aggregate is further divided into different frac- tions, or size classes, partly in fractions comprising filler, sand and stone, for instance 0-2, 0-4, 0-8, 0-11, 0- 16 and 0-22, partly in fractions that principally merely comprises sand and stone, for instance 4-8, 8-11, 11-16 and 16-22. Where the numbers represent the size of the holes in millimeter of, a net that lets the aggregate past and a net that will not let the aggregate past, respectively. The amount of aggregate from the respective fraction is primarily determined by the final application for the specific asphalt mixture, i.e. where and for what purpose the final as- phalt will be used. In an ordinary occurring asphalt mixture about half of the aggregate originate from fractions comprising filler, like 0-4 and 0-8, and the rest from fractions principally free from filler, like 4-8, 8-11 and 11- 16. Each fraction of the aggregate 1 is stored in a specific raw material silo 2, two of which are shown. When the preparing of a new batch of asphalt mixture is started a predetermined amount of aggregate 1 from the respective raw material silo 2 is loaded onto a conveyer belt or the like, whereupon the aggregate is directed towards a drier 3. In the shown embodiment the drier 3 is constituted by a coun- terflow rotary drier, which has a slight inclination downwards and also rotates during operation. Aggregate 1 is feed into a first upper end of the drier 3 and is discharged from a second lower end, as a consequence of the rotary motion of the counterflow rotary drier during operation. In the drier 3 the aggregate 1 is exposed to heat and is heated to a temperature between 100 and 13O⁰C, preferably approximately 120⁰C, whereupon the aggregate 1 is dried. In the drier 3, among other things, the aggregate 1 is "thrown" in the form of a throwing curtain through the flow of heat that is generated by a heat source in the drier 3. In the throwing curtain tne filler, moisture and flue gas are separated from the aggregate 1 and leaves the drier 3 via a chimney flue 4, arranged in said first upper end of the drier 3. Along the chimney flue 4 there is arranged a separator 5 comprising a filter (not shown) . In the separator 5 the filler 6 is collected and directed towards a filler silo 7, and moisture and flue gases is filtrated and discharged via a chimney 8. Furthermore, the aggregate which principally is free from filler is heated further in order to reach the above mentioned requested temperature.

The heated aggregate consisting of stone material principally free from filler is thereafter transported by an elevator 9, or the like, to an aggregate silo 10. Said ag- gregate silo 10 may be arranged in such a way that the heated aggregate once again is sorted into the different fractions, i.e. in separate sections of the aggregate silo 10, as is shown in figure 1, or in such a way that it only comprises one section. The aggregate silo 10 is arranged m such a way that the aggregate is allowed to fall through a number of nets having gradually decreasing hole diameter, wherein the aggregate of a certain size pass one net but not the subsequent and is then directed towards the corresponding size or fraction section in the aggregate silo 10. Thus, the smallest fraction is accumulated furthest to the left of figure 1, m the aggregate silo 10, and the largest fraction is accumulated furthest to the right in figure 1. The filler 6 is generally representing particles smaller than 0,063 millimeters, however the filler 6 may be allowed to be some- what bigger, for instance smaller than 0,08 millimeter, without affecting the inventive method. Furthermore, sand and stone is thus that part of the aggregate that has sizes greater than the filler, and generally less tnan 22 millimeters, and is there above referred to as stones. It shall be pointed out tnat IT: IS central for tne _τ/eation tiat tne filler 6 preferably comprises the dust that is separated from the aggregate 1 during the heating/drying.

The second essential element in an asphalt mixture is a binder mixture, which has an effect of holding the aggregate together in the finally obtained asphalt. The amount of binder mixture in the asphalt mixture is like the amount of aggregate 1 dependent on the purpose and application of the finished asphalt, and amounts to between 3 and 10 percentage per weight, preferably approximately 5-7 percentage per weight, of the total weight of the asphalt mixture.

The binder mixture comprises according to a preferred embodiment of the invention a binder, an adhesion agent and a foaming agent.

The preparing of said binder mixture comprises the steps of, conducting the binder via a pipe 11 towards a pre- foaming chamber 12. Thereafter the adhesion agent is added to the binder via a pipe 13 arranged connected to said binder pipe 11, whereupon the mixture of binder and adhesion agent is loaded into the prefoaming chamber 12. Thereafter the foaming agent is loaded directly into the prefoaming chamber 12 via a pipe 14, in order to be mixed with the binder and the adhesion agent. The binder is heated to a temperature between 130 and 200⁰C, preferably 180⁰C, before it is mixed with the adhesion agent in the pipe 11. The ad- hesion agent and the foaming agent may preferably have the same temperature as the surroundings, which will facilitate the handling of them.

The binder is preferably constituted by bitumen, which for instance may be extracted from raw oil or from naturally occurring bitumen, or be prepared according to other know methods within the technical field, or is constituted by mixtures thereof. Preferably so called penetration bitumen is used, instead of emulsion binder and viscosity binder that conventionally is used in cold and warm asphalt, re- spectively. One measure to classify bitumen of different bitumen sort, or bitumen compounds, is so called penetration. Bitumen from different classes is used for different purposes and applications, in combination with different mixtures of aggregate as mentioned above. The penetration number for a specific type of bitumen is measured by pressing a needle into a test piece of bitumen at a specific force and during a specific time period and at a specific temperature, and the depth the needle has reached is measured in tens of mil- limeters, dmm. Generally, bitumen having a penetration number between 50 and 430 dmm is used. A frequently used bitumen sort has a penetration number of 160-220 dmm.

The adhesion agent that is added to the binder mixture constitutes at least 0,6 percentage per weight of the total weight of the binder mixture, and comprises a fatty acid, such as pine oil fatty acid. The foaming agent that is added to the binder mixture constitutes between 1 and 10 percentage per weight, preferably 4-5 percentage per weight, of the total weight of the binder mixture. Reference is now also made, in connection with the description of the inventive method for preparing an asphalt mixture, to figure 2.

The inventive method comprises the steps of loading aggregate comprising stone material principally free from filler into the mixer 15. Said aggregate is for instance constituted by predetermined amounts of each aggregate fraction that are intermediately stored in the aggregate silo 10, whereupon said aggregate is dry mixed in order to obtain a homogenous aggregate mixture. According to one embodiment the dry mixing is carried out for three seconds . Thereafter a foamed binder mixture is loaded into the mixer 15, whereupon said aggregate and said foamed binder mixture are mixed in order to obtain thick binder mixture layers on each stone/particle of the aggregate. According to one embodiment the loading of the foamed binder mixture as well as the mixing of it together with the aggregate are carried out during four seconds. Thereafter a filler 6 is loaded into the mixer, whereupon final wet mixing of the asphalt mixture is carried out in order to obtain a homogenous asphalt mixture. According to one embodiment the loading of the filler 6 is carried out for 2 seconds and the final wet mixing is carried out for sixteen seconds. Abovementioned time periods are examples for a batch of asphalt mixture which is prepared in the mixer 15 designed for 2,5-5 tons of asphalt mixture. The process temperature in the mixer is preferably between 100 and 120⁰C during the preparation, more preferably approximately HO⁰C. It shall be pointed out that said process provide hot asphalt mixture, even thus applicable temperatures are in the border area of the crossing from warm to hot asphalt mixture, according to today's defini- tions .

In figure 1 an aggregate scale 10a is arranged below the aggregate silo 10, in order to determine how large amounts of the respective aggregate fraction that shall be loaded into the mixer 15. The binder mixture is in a liquid form when it is in the prefoaming chamber 12. When the binder mixture is loaded into the mixer 15 it is sprayed via a nozzle 16 under height pressure into the mixer 15, for instance at a pressure of 3- 6 Bar. When the binder mixture enters atmospheric pressure, which generally exists in the asphalt mixture mixer, the binder mixture expands up to approximately 20 times and consequently a foam is created, which by time will settle as the foaming agent is vaporized. The binder mixture shall preferably remain, at least partly, in the foamed state dur- ing the major part of the remaining mixing time.

Foamed binder mixture is more advantageous than liquid binder mixtures, as the temporarily increased volume also provide a lowered viscosity, thereby the binder mixture is more easily mixed with the aggregate and is distributed more evenly among the aggregate. Thus, a better coating of tne binder mixture on the aggregate is ootained at lower energy consumption. More exactly, according to the inventive method the binder is made to foam by adding a foaming agent. The foaming agent may be any material, or mixtures of materials, that will make foam of the heated binder. The ability of the foaming agent to expand when it is heated above its boiling point is crucial, and is preferably constituted by water. The content of foaming agent decreases/vaporizes in the binder mixture during the entire preparing and spreading time period, and when the spreading is finished a passable road that does not need any further hardening is directly obtained. It shall however be pointed out that the finished asphalt may comprise and is allowed to comprise a certain amount of foaming agent.

The object of the adhesion agent is, above all, to re- ject the foaming agent from the stones of the aggregate in order to obtain a better bond/adhesion between the binder and the aggregate.

Thereto, it has also been shown that the adhesion agent also entail that the binder mixture foam will have a longer survival period before it settles. Furthermore, the adhesion agent has a lubricating effect in the asphalt mixture when it is prepared in the mixer 15 as well as when the asphalt mixture is spread. Thus, the asphalt mixture, thanks to the adhesion agent and the foaming agent, gets a good workabil- ity, i.e. is easy to pack when spread, despite a spreading temperature below 100⁰C, preferably between 80 and 100⁰C.

The filler 6 that is added to the asphalt mixture after the adding of the foamed binder mixture, is preferably constituted by between 3 and 8 percentage per weight of the to- tal weight of the asphalt mixture. The filler 6 that is loaded into the mixer 15 under the preparing consists of at least partly of the same material as the aggregate, usually granite or the like, but may also comprise other material. For instance, so called stone rich aggregate, from which a small amount of filler may be separated in the heating process seep, may require that a certain amount of limestone filler is added to the filler 6 that is loaded into the mixer 15. The limestone filler may conventionally represent maximum 50% of the filler 6 that is loaded into the mixer 15. Other additive to the filler 6 that is loaded into the mixer 15 may be cellulose filler, which preferably constitute maximum 1 percentage per weight of the total weight of the asphalt mixture.

The advantage of adding the filler 6 after the foamed binder mixture and the aggregate principally free from filler are mixed, is that thick binder coatings/layers are obtained on each stone of the aggregate, thereafter the filler 6 enters these coatings and absorb that part of the binder mixture that has not attached to the stones of the aggregate. This results in a dense asphalt in which the rain water does not leak through and runs the risk of breaking the asphalt or wash the binder off. By means of the inventive method an asphalt mixture is obtained which provide a packed asphalt having a cavity extent of approximately 3%.

One other advantage of the present invention is that it has been shown that the process temperature in the mixer 15 does not require to be raised as usual when recycled asphalt is added to the asphalt mixture during the preparing.

In the inventive mixer 15 there is arranged at least one mixer shaft 17 supporting a number of vanes or mixer arms 18. Preferably the inventive mixer 15 comprises two parallel cooperating mixer shafts 17, which are individually rotary supported in a receptacle 19, comprised in the mixer 15 and thereto are arranged to rotate in opposite directions. More precisely the mixer shafts 17 rotates in such a way that that the asphalt mixture is moved from outside/below and inwards/upwards. Each mixer shaft 17 comprises preferably eight vanes 18 arranged in pairs, and the vanes 18 of each pair have mutually diametrical placing in relation to the mixer shaft 17. Each mixer shaft 17 is ar- ranged on such a level in the receptacle 19 that a bigger part volume of the asphalt mixture tha_ is prepared is situ- ated above the mixer shaft 17 than below. Thereto, the vanes 18 present an active carrier surface 20 having an axial length, across the mixer shaft 17, that is 1,5 - 4 times larger than the width thereof. Each carrier surface 20 ex- tends preferably from a free outer end in the radial direction to said mixer shaft 17, and presents preferably an uniform cross section along the length thereof. The carrier surface present in said cross section preferably a part that is plane and principally in parallel with the mixer shaft 17, and a part that is curved in the opposite direction to the direction of rotation R of the mixer shaft 17, as may be seen in figure 2. Thereto, each vane 18 is bend in the free outer end like a shovel. The inventive mixer "lifts and brings around" the asphalt mixture, unlike prior art mixers which merely "knead" the asphalt mixture.

Feasible modifications of the invention

The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is intended to cover all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined by the wording of the appended claims and the equivalents thereof. Thus, the equipment may be modified in all kinds of ways within the scope of the appended claims.

For instance the binder mixture may comprise different additives besides the ones mentioned in the description above, for instance wax in order to obtain a more rigid as- phalt mixture.

It shall also be pointed out that all information about/concerning terms such as upper, lower, etc., shall be interpreted/read having the equipment oriented according to the figures, having the drawings oriented such that the ref- erences can be properly read. Thus, such terms only indicates mutual relations in the shown embodiments, which relations may be changed if the inventive equipment is provided with another structure/design.

It shall also be pointed out that even thus it is not explicitly stated that features from a specific embodiment may be combined with features from another embodiment, the combination shall be considered obvious, if the combination is possible.

Claims

1. A method for preparing an asphalt mixture in a mixer (15), which method is characterized by the steps of:

- loading a dry aggregate comprising stone material princi- pally free from filler into the mixer (15),

- thereafter loading a foamed binder mixture into the mixer (15) that contains said dry aggregate, which binder mixture is foamed when it leaves a prefoaming chamber (12) and is loaded into the mixer (15), whereupon mixing of said aggre- gate and said foamed binder mixture takes place, and

- thereafter loading a filler (6) into the mixer (15), whereupon final mixing of the asphalt mixture takes place.

2. The method according to claim 1, characterized in that the process temperature in the mixer (15) is between 100⁰C and 120⁰C.

3. The method according to claim 1 or 2, characterized in that said aggregate, during the process step preceding the loading of the aggregate in the mixer, is heated up to between 100⁰C and 13O⁰C and is thereby dried.

4. The method according to claim 3, characterized in that at least parts of said filler (6) is separated from said aggre- gate during said drying.

5. The method according to any preceding claim, characterized in that the asphalt mixture comprises at least 80 percentages per weight of said aggregate, based on the total weight of the asphalt mixture.

6. The method according to any preceding claim, characterized in that the asphalt mixture comprises between 3 and 10 percentages per weight of the binder mixture, based on the total weight of the asphalt mixture.

7. The method according to any preceding claim, characterized in that the binder mixture comprises a binder, an adhesion agent and a foaming agent.

8. The method according to claim 7, characterized in that the preparing of the binder mixture comprises the steps of:

- first conveying the binder via a pipe (11) towards the prefoaming chamber (12), - thereafter loading the adhesion agent into said pipe (11), whereupon the mixture of bitumen and adhesion agent is loaded into the prefoaming chamber (12), and

- thereafter loading the foaming agent into the prefoaming chamber (12) .

9. The method according to claim 8, characterized in that the binder has a temperature is between 13O⁰C and 200°C, before it is conveyed via the pipe (11) towards the prefoaming chamber .

10. The method according to claim 8 or 9, characterized in that the binder mixture will foam when it is loaded from the prefoaming chamber (12) into the mixer (15) .

11. The method according to any of claims 8-10, characterized in that the binder is constituted by bitumen.

12. The method according to any of claims 8-11, characterized in that the binder mixture comprises at least 0,6 per- centage per weight of adhesion agent, based on the total weight of the binder mixture.

13. The method according to any of claims 8-12, characterized in that the adhesion agent comprises a fatty acid, such as pine oil fatty acid.

14. The method according to any of claims 8-13, characterized in that the binder mixture comprises between 1 and 10 percentages per weight of foaming agent, based on the total weight of the binder mixture.

15. The method according to any of claims 8-14, characterized in that the foaming agent is constituted by water.

16. The method according to any preceding claim, character- ized in that the asphalt mixture comprises between 3 and 8 percentages per weight of said filler, based on the total weight of the asphalt mixture.

17. The method according to any preceding claim, character- ized in that said filler comprises particles of a size smaller than 0,063 mm.

18. The method according to any preceding claim, characterized in that at least parts of said filler is constituted by the same material as said aggregate.

19. The method according to any preceding claim, characterized in that said filler comprises maximum 50 percentages per weight of limestone filler, based on the total weight of the filler.

20. The method according to any preceding claim, characterized in that said filler comprises maximum 1 percentage per weight of cellulose fiber, based on the total weight of the asphalt mixture.