GB2212411A

GB2212411A - Mixing process and apparatus for bituminous materials

Info

Publication number: GB2212411A
Application number: GB8826669A
Authority: GB
Inventors: George Griffin Mcintosh
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-11-16
Filing date: 1988-11-15
Publication date: 1989-07-26
Anticipated expiration: 2008-11-15
Also published as: GB8826669D0; GB2212411B

Abstract

A process for the production on site of a batch of hot macadam, asphalt, hot mastic, sealant or like material of predetermined composition comprises heating a batch of prepacked, premixed graded aggregate or other particulate filler in a heater (1), melting a prepacked batch of bitumen or other asphalt binder in a melter (2) and mixing said heated aggregate with said molten asphalt binder in a mixer 3. The heater (1) includes a rotary drum with an internal helical flight, and the mixture 3 includes a static drum which is heated by burners and which is provided with mixing blades on a rotary shaft. <IMAGE>

Description

"Mixing process and apparatus" The present invention relates to a mixing process and apparatus and more particularly relates to a process for mixing hot macadam, asphalts, mastics and similar coated materials and to transportable apparatus for performing such mixing on site. Macadams, asphalts, mastics and similar materials are mixtures of bitumens, tars, asphalt and like binding agents with aggregate and filler.

It is the current practice to manufacture coated materials in large mixing plants and then transport them hot to the site of use. This procedure is highly unsatisfactory when relatively small amounts of material are needed, e.g. for small to medium reinstatements of road surfacing by Local Authorities and contractors.

The normal practice for carrying out such repairs is as follows.

The contractor will require a suitable truck to send to the nearest static mixing plant to collect a minimum of 2-3 tonnes of patching material, whether he requires 100 kg or 3 tonnes. Such visits will encounter inevitable delays caused e.g. by breakdown of the plant or runs of materials for large contracts, which will take precedence over the contractor's requirement.

Having collected the material, the contractor has to protect it as best he can on his return to the site area. As a result of cooling of the materials on the return journey, there is normally a definite crusting on the top and sides of the load. The temperature of crusted material may be outside specified limits and thus produces potential wastage. It will be appreciated that the material cannot be supplied in a super-heated state to allow for cooling on the return journey because'the bitumen would become too fluid and would tend to separate from the aggregate.

On most small to medium reinstatements, the hot material will have to be handled several times before it reaches the pot-hole or other reinstatement site and will suffer cooling all the time it is handled. As a result, it is highly likely that the material will have cooled below the temperature at which proper compaction can take place. Thus the material will not be properly compacted and will not adhere well to the walls of the reinstatement site. The repair will probably fail after relatively short usage and the reinstatement process will have to be repeated. At best, a sub-standard road surface will result from slow compaction of the reinstatement by vehicles.

Commonly, a considerable proportion of the original load of material will be impossible to use and will have to be discarded. If adverse weather conditions such as rain, frost or snow prevail, the likelihood is that virtually all of the collected material will be discarded, or perhaps some may be used to reinstate on a temporary basis.

In an effort to avoid these problems, it is known to "cut back material with suitable solvents to produce a product which can be used in the cold state and which hardens by evaporation of the solvents.

Such cut back material is difficult to use, does not compact sufficiently, and frequently fails.

The solvents take too long to evaporate, so that the reinstatement suffers traffic damage before it has adequately hardened. It is normally impracticable to divert traffic from the reinstated area for a long period in order to allow proper hardening to take place.

There is, therefore, a need for a process and transportable apparatus able to produce small quantities of quality materials on site. Apparatus has been proposed to recycle old materials, adding bitumens at a haphazard rate to rejuvenate them.

In US-A-4,072,435 and US-A-3,820,914 there is disclosed portable self-propelled apparatus for mixing hot materials on site. A plurality of grades of aggregate are stored in separate compartments; bitumen is stored in a liquid condition in another compartment. When hot material is required, suitably metered supplies of aggregate are withdrawn from the bulk storage compartments and after heating are mixed with liquid bitumen. This process is replete with opportunities for operator's errors, whereby the type or proportion of one or more of the aggregate components is improperly selected, e.g. where a substitution is made to replace a size fraction which has become exhausted on site.

The alternative use of premixed aggregate is also unsatisfactory as a result of the tendency of mixed aggregate to segregate under movement and vibration, causing the bulk supply to become inhomogeneous.

Moreover the transport of large quantities of liquid bitumen is clearly hazardous. Still further, the apparatus is bulky, very heavy, and requires some time to reach operating temperatures; it is not suitable for producing small batches of accurately mixed hot material on demand. The shut-down procedure for the liquid bitumen tank, pump and pipe-work is also complex and time-consuming.

There is, therefore, still a need for a process and apparatus capable of producing rapidly, on site small amounts of hot material of predetermined and accurately controlled composition.

Thus in one aspect, my invention provides a process for the production on site of a batch of hot macadam, asphalt, hot mastic, sealant or like material of predetermined composition which comprises heating a batch of prepacked, premixed graded aggregate or other particulate filler, melting a prepacked batch of bitumen or other asphalt binder, and mixing said heated aggregate with said molten asphalt binder.

The prepacked aggregate generally comprises at least two grades mixed in a desired ratio, and preferably has a low moisture content e.g. below 0.5% by weight and more preferably below 0.2%.

It may be packed into sacks or the like, suitably in quantities convenient for manual handling. The pre-packed aggregate may be packed together with the correct quantity of bitumen or other asphalt binder necessary to produce the macadam, asphalt, mastic or sealant material of desired composition.

A block of bitumen may e.g. be cast in a cardboard carton or tub e.g. of aluminium foil, or may be wrapped in a suitable manner to control unwanted movement and adhesion prior to use. If desired, the prepacked bitumen may be physically associated with the corresponding amount of graded aggregate, e.g. in a combined pack.

In a further aspect, my invention provides portable apparatus for mixing hot macadam, asphalt, hot mastic, sealant or like materials, comprising heating means for heating a batch of prepacked, premixed graded aggregate or other particulate filler; means for melting a prepacked batch of bitumen or other asphalt binder; and mixing means for intimately mixing said melted bitumen with said heated aggregate According to a preferred aspect of my invention, the aggregate is heated to a suitable temperature by passing through a drum which is externally heated.

The passage of the aggregate through the drum may be assisted by an Archimedean screw which either rotates inside the drum or, more preferably, is attached internally to the drum as one or more inwardly-projecting helical flights which, upon rotation of the drum, cause the aggregate to move longitudinally of the drum.

I have found that when the drum is heated internally by a gas or oil burner, as in known portable macadam-making apparatus, a large quantity of noxious dust is entrained in the flue gases.

The dust is in part a fine fraction of the aggregate and in part is generated by self-abrasion engendered by the rotation of the drum. This dust causes environmental pollution and can only be removed by a bulky cyclone separator, or the like. However when the drum is heated externally, the fine fraction in the aggregate composition remains part of the coated product and no hazard or inconvenience is caused. Moreover, for a given size, the externally heated drum has a far higher output of heated aggregate. Such heater is applicable in other technical fields and forms a further aspect of my invention.

The aggregate heater must be capable of producing controlled heating of various sizes of single and mixed aggregates, sands, fillers, etc, e.g. the prepacked, premixed graded aggregate described above. Moreover it must be able to heat the aggregate to a desired temperature without separating the various particle sizes. The preferred aggregate heater described above is able to meet these criteria, while being physically compact and non-pollutinq.

The aggregate heater may be powered by drive means, e.g. by an electrical or hydraulic drive, which may be adjusted, either manually or by automatic means, to achieve a desired exit temperature for the aggregate. In order to keep the apparatus as compact and light as possible, it is preferred that the aggregate heater discharges directly into the mixing means, with6ut any intermediate storage area for the heated aggregate.

The means for melting bitumen comprises means for rapidly melting and discharging in molten or partly molten form an amount of bitumen or other asphalt binder sufficient to bind the batch capacity of the mixing means. It is thus unnecessary to store molten bitumen, with all the problems which this engenders, particularly on shut-down. Instead, a batch of bitumen is prepared and used immediately, in the requisite quantity to provide the desired batch composition.

I have found that a block of bitumen can be melted rapidly by allowing said block to slide down a heated convergent chute, slipway or nozzle.

Preferably at least one of the surfaces of said chute, slipway or nozzle contacting said block is corrugated, grooved or provided with fins or other projections, so as to maximise the area of heated surface in contact with the block and assist in drainage of the liquid bitumen. It is preferred to provide at least one, e.g. two, vertical chutes communicating directly with the mixing means.

Not only is the need for pumps and pipework completely avoided but the chutes can be so disposed that blocks of bitumen therein are moved by the blades of the mixing means, thus assisting the melting and drainage processes. The exit from the convergent chute, slipway or nozzle is so dimensioned that a small piece of partly-melted bitumen is finally discharged, which on contact with the heated aggregate is rapidly melted and homogenised therewith.

The mixing means is a heated drum provided with mixing blades mounted on a shaft extending longitudinally of the drum. A discharge gate in the base of the drum permits the completed batch to be discharged downwardly onto the ground or into a receiving vessel. The bitumen melting means may be heated with waste heat from the burners heating the mixing drum, e.g. by suitable ducting of hot gases.

The utility of the apparatus will be evident from the foregoing description. The apparatus may be mounted on the back of a truck or on a trailer and is fully transportable. It thus enables materials of accurate composition to be mixed on site and used immediately. It is even possible to discharge the material from the mixer straight into the reinstatement site. The temperature of the material can be accurately controlled, and hence a reinstatement of optimum quality can be effected. The material can be discharged at a relatively high temperature in order to ensure that it adheres and compacts well, without any risk of bitumen draining from the interstices of the aggregate prior to use.

If a number of different types of reinstatement have to be undertaken, a suitable mix for each purpose can be prepared as necessary by the one apparatus. For such purposes, the heated aggregate and bitumen may be supplied in turn to two or more mixers so that two or more different mixes may be prepared simultaneously. Moreover, mastics of excellent quality may be prepared on site, which has never before been possible.

Finally, and most importantly, only the exact quantity of material necessary for the purpose in hand need be prepared. As little as 100 kg may be satisfactorily mixed. There is no waste and hence the economics of the process are enormously improved compared to conventional practice.

Although the invention has been described primarily in relation to the preparation of macadam and asphalt for highway reinstatement and the like, it will be appreciated that the apparatus is equally applicable to the preparation of other hot mastic or sealant materials for more- specialised purposes.

For instance, one may compound a bridge or road deck jointing material including a propation of rubber crumb. Such compositions have previously often required several hours of mixing to incorporate the rubber crumb satisfactorily, but the apparatus of the invention has been found to produce a good quality mixture in less than 20 minutes. The crumb should, of course, be added directly to the mixing drum without being pre-heated.

One embodiment of my invention will now be described by way of example only with reference to the accompanying drawings, wherein: Fig. 1 is a schematic plan view of the apparatus of the invention; Fig 2. is side elevational view of the apparatus of Fig. 1; Fig. 3 is an internal view, partially in section, of the aggregate heater; Fig. 4 is an internal view, partially in section, of the bitumen melting means and the mixer means; and Fig. 5 is a cross-section along the line A -A in Fig. 4.

Referring now to Figs. 1 and 2, the apparatus comprises three portions: the aggregate heater 1; the bitumen melters 2; and the mixer 3. The apparatus is mounted on a wheeled chassis, shown schematically at 28 in Fig. 2.

The aggregate heater comprises a rotating drum 4 (Fig. 3) having an internal helical flight 5. The flight 5 comprises a portion of coarser pitch at the entry end 6, forming approximately two turns, to ensure a brisk intake of aggregate into the durm. The entry end rotates in a housing 29 to constrain aggregate to enter the drum. The drum is supported on rdllers (not shown) bearing on the collar 7, and is driven through shaft 8 by the hydraulic motor 29 (Fig. 1).

Aggregate to be heated is tipped into the hopper 9 and covered by a lid (not shown). The aggregate immediately enters the drum 4 which is heated by the burner assembly 10 which consumes LPG or furnace oil.

The helical flight 5, on rotation of the drum, causes the aggregate to be urged towards the exit end 11 and eventually to be discharged through the aperture between the drum and the baffle plate 12. The heated aggregate falls directly into the mixer 3, described below. The temperature of the drum 4 is monitored by an attached thermocouple to provide automatic regulation of the burner 10.

The bitumen-melting portion of the apparatus comprises two convergent throat members 13 of rectangular cross-section, having flared mouths 14 dimensioned to receive a prepacked block of bitumen.

The block is dimensioned so as to produce maximum surface area in order to minimise time required for melting. In use a block of bitumen is allowed to rest in each mouth 14 and to enter the throat 13, which is heated by hot gases in the space 15 surrounding each throat member. The hot gases are the exhaust from burners 18, which are constrained by a casing 19 surrounding the mixer and rise up the spaces 15 and exit to the atmosphere at 16.

The blocks of bitumen melt rapidly, and slide down the convergent throats 13 until they contact the corrugated portion 17. The increased surface area of this portion completes the melting process, which typically takes less than 5 minutes.

The mixer 3 comprises a static drum 20, heated by burners 18 and housing a longitudinally extending shaft 21 on which are mounted a plurality of cruciform mixing blades 22. The blades carry angled paddles 23, which are alternately angled in opposite senses (not shown in Fig. 4). This combined arrangement of paddles 23 and drum 20 greatly increase the intensity of the mixing action. Shaft 21 is driven by the hydraulic motor 24 (Fig. 1).

Discharge gate 25 extends along the length of the mixing drum 20 and is of generally cylindrical form having a wide central aperture 26 through which (as shown in Fig. 5) the completed material composition may be discharged. Rotation of the gate assembly through 900 by handle 27 will interrupt the discharge. Webs are provided at spaced intervals across the aperture 26 to prevent undue flexing of the assembly under mechanical stress.

In use of the apparatus, two bags of prepacked mixed aggregate are tipped into the hopper 9, and two blocks of bitumen are placed in the mouths 14. The aggregate is drawn through the drum 4 and discharged, heated, into the mixing drum 20.

At the same time, the blocks of bitumen melt and begin to protrude into the drum 20, where they are repeatedly moved by the paddles 23 into the heated corrugations 17. After a few minutes of mixing the macadam or asphalt composition of excellent quality is ready to be discharged, often directly onto the site of reinstatement, or closely adjacent thereto. Moreover the aggregate heating and mixing processes are carried out in the enclosed drums 4 and 20, and a minimal amount of dust is liberated into the environment, making the apparatus suitable for use on urban sites.

Claims

Claims:

1. A process for the production on site of a batch of hot macadam, asphalt, hot mastic, sealant or like material of predetermined composition which comprises heating a batch of prepacked, premixed graded aggregate or other particulate filler, melting a prepacked batch of bitumen or other asphalt binder, and mixing said heated aggregate with said molten asphalt binder.

2. A process according to claim 1, wherein said aggregate has a moisture content below 0.5% by weight.

3. A process according to claim 1 or 2 wherein said bitumen is in the form of a block which has been cast in a cardboard carton or in a tub, or wrapped in a manner to control unwanted movement and adhesion prior to use.

4. A process according to any of the preceding claims wherein the prepacked bitumen is physically associated with the corresponding amount of graded aggregate in a combined tack.

5. A process according to any of the preceding claims wherein the resulting batch of hot macadam is utilised on site for highway reinstatement.

6. A process according to claim 1, substantially as described herein.

7. A portable apparatus for mixing hot macadam, asphalt, hot mastic, sealant or like materials, comprising heating means for heating a batch of prepacked, premixed graded aggregate or other particulate filler; means for melting a prepacked batch of bitumen or other asphalt binder; and mixing means for intimately mixing said melted bitumen with said heated aggregate.

8. Apparatus according to claim 7 wherein said heating means comprises an externally heated drum, passage of the aggregate or other particulate filler through said drum being assisted by an Archimedian screw.

9. Apparatus according to claim 8 wherein said Archimedian screw is attached internally to the drum as one or more inwardly-projecting helical flights which, upon rotation of the drum, cause the aggregate to move longitudinally of the drum.

10. Apparatus according to claim 8 or claim 9 and wherein said drum is powered by drive means which may be adjusted to achieve a desired exit temperature for the aggregate or other particulate filler.

11. Apparatus according to any of claims 7 - 10, wherein said means for melting bitumen comprises a heated convergent chute, slipway or nozzle down which a block of bitumen may be slid.

12. Apparatus according to claim 11, wherein at least one of the surfaces of said chute, slipway or nozzle contacting said block is corrugated, grooved or provided with fins or other projections.

13. Apparatus according to claim 11 or 12, wherein at least one vertical chute is provided, said chute communicating directly with the mixing means.

14. Apparatus according to any of claims 7-13, wherein said mixing means comprises a heated drum provided with mixing blades mounted on a shaft extending longitudinally of the drum.

15. Apparatus according to claim 14, wherein said heated drum is provided with a discharge gate in the base of the drum for discharge of the completed batch.

16. Apparatus according to claim 14 or 15, wherein said longitudinally extending shaft carries a plurality of cruciform mixing blades which carry angled paddles, said paddles being alternately angled in opposite senses.

17. Apparatus according to any of claims 7 - 16, wherein said bitumen melting means is heated with waste heat from burners heating said mixing means.

18. Apparatus according to any of claims 7 - 17, mounted on the back of a truck or on a trailer.

19. Apparatus according to claim 7, substantially as described herein.

20. Apparatus according to claim 7, substantially as illustrated in the accompanying drawings.

21. A method for reinstating a highway which comprises mixing a batch of hot macadam on site in apparatus according to any of claims 7 - 20 and discharging a batch of hot macadam onto said reinstatement site or closely adjacent thereto.