GB2148778A - Manufacturing mastic asphalt blocks - Google Patents

Abstract

In an installation for manufacturing mastic asphalt hot bitumen mixed with hot limestone filler and unheated grit is a mixer from which the asphalt is discharged into a plurality of moulds 52 in order to mould blocks. The filled moulds 52 are conveyed by a conveyer 54 through a cooling tower 50 in which they pass through an array 56 of spray nozzles arranged to spray water onto the moulds 52 so that at least some of the water impinging on the moulds 52 initially forms steam and thereby cools the asphalt. Inside the cooling tower 50 the moulds 52 are conveyed along a vertical zig-zag path so as to fit a relatively long cooling path into a relatively small ground area. The mastic asphalt blocks produced have at least one groove formed in their upper surface. Heat is extracted from the asphalt in the mixer and returned to the means for heating the limestone filler. A microprocessor controls the filling of the moulds 52 in order to render the weight of the moulded blocks substantially consistent. <IMAGE>

Description

1 GB 2 148 778 A 1

SPECIFICATION

Manufacturing mastic asphalt The present invention relates to a method and to an installation for producing mastic asphalt and to an improved mastic asphalt block.

The invention is defined in the claims appended hereto to which reference should now be made.

Mastic asphalt has been manufactured previ- ously by methods in which the main constituents of the mastic asphalt, which generally include bitu men, limestone grit and a powdered limestone filler, are mixed and then heated together for fur ther mixing. The resulting molten mixture is then poured into moulds and allowed to cool and solid ify into blocks which are convenient for transport ing to the sites on which the asphalt is to be used.

A considerable amount of labour is involved in these existing methods and they are wasteful in terms of energy consumption. Consequently the asphalt produced using these methods is relatively expensive.

Some attempts have already been made to over come the disadvantages of these existing methods.

For example, in order to six bitumen with other materials it is necessary to keep it relatively warm so that it remains in a relatively fluid state other wise it would simply be too viscous to handle effi ciently at all.

When the warm bitumen is added to the other cold constituent materials, its temperature drops and it must be re-heated before mixing is effected.

This cooling and reheating of the bitumen used in manufacturing the asphalt increases the heat en ergy used in the manufacturing process.

It has, therefore, been proposed that the energy consumed in the manufacture of mastic asphalt could be reduced by heating the grit before mixing it with the bitumen and grit so that the hot constit uent materials can be mixed without supplying fur ther heat to the mixer. This avoids reheating the already hot bitumen and saves considerable en ergy.

Even this improved method, however, consumes 110 a great deal of heat energy, and quite a large pro portion of this energy is, in practice, simply lost into the atmosphere. The heat loss into the atmos phere is not only wasteful and expensive but also makes the the working area around the mixers very hot which is unpleasant for those persons working there.

We have appreciated that it would be highly ad vantageous to recover at least some of the heat which has heretofore been dissipated in the atmos phere and to re-use it for heating the unmixed con stituent materials. W$ have, therefore, proposed that heat should be recovered during the mixing of the constituent materials and used in heating the filler for asphalt mixed subsequently.

In addition to considerably reducing the heat in put required, this arrangement has a further ad vantage. Although mixing of the constituent materials is best carried out at a temperature of around 1800C, the asphalt is best poured into the 130 moulds at a lower temperature, from around 120'C, the optimum temperature being about 1500C. Thus, if the heat exchange takes place in the latter stages of mixing, the asphalt can be cooled to a temperature ideal for moulding by the time the mixing is complete and the delays which have been necessary in the past while the asphalt is allowed to cool prior to moulding are avoided.

Previously, once the hot asphalt has been poured into the moulds, it has been allowed to cool and solidify naturally. However, the asphalt blocks may take as long as four hours to solidify completely and this not only introduces a great deal of delay into the processing of the asphalt but also necessitates the provision of large areas of space in which the moulds can be stored during cooling.

We have, therefore, proposed that the entire cooling process should be effected by passing the filled moulds through an array of water spray nozzles in a vertical zig- zag path. In addition to enabling the moulds to travel along a relatively long path in quite a small ground area, this arrangement has the advantage that the use of water sprays in removing heat from the moulds is more efficient than immersing the moulds in water because water sprayed onto the surfaces of the moulds in the initial stages of cooling tends to form steam and so large quantities of heat are removed from the moulds as the latent heat needed to turn water into steam is much greater than the heat required to merely heat the water.

As mentioned above, mastic asphalt is usually supplied to the contractors who use it in the form of solid blocks. These blocks generally have a nominal weight of 25kg as this amount can easily be melted in an ordinary bucket. In practice, the blocks vary considerably in weight, size and shape and are, therefore, difficult to stack and store.

We propose to overcome this problem by using a monitoring arrangement incorporating a control system which checks the weights of the filled moulds and adjusts the time that the discharge gate through which the asphalt is poured into the moulds is opened so as to render the blocks uni form in weight and size. This has the advantage that the resulting blocks can be formed into a self supporting stack which is simply strapped for transporting and does not require a pallet or any other form of packing. This is only possible because the blocks are of uniform size.

On site, it is often desirable to break up the blocks of asphalt to enable them to be melted more easily. Existing blocks are usually broken up by striking them with a hammer and it is quite common for the blocks to break into uneven-sized pieces or into a number of very small fragments which cannot be easily melted. It is, therefore, desirable to split the block evenly into a number of suitably sized pieces and we have found that this can be achieved by forming the blocks with one or more grooves or indentations. When the block is gently tapped it will break, in a controlled manner, along the line of the groove or indentation.

Thus the method of the invention in its various 2 GB 2 148 778 A 2 aspects enables an improved mastic asphalt block to be produced more quickly and cheaply than hitherto.

An embodiment of the invention will now be de scribed in detail with reference to the drawings, in 70 which; Figure 1 is a schematic block diagram showing a mastic asphalt manufacturing plant in accordance with the invention; Figure 2 is a front elevational view of the mixing 75 arrangement of the plant of Fig. 1; Figure 3 is a side elevational view of the second ary mixer of the plant of Fig. 1; Figure 4 is a simplified plan view of the mixer of Fig. 3; Figure 5 a front elevational view of the cooling tower of the plant of Fig. 1; Figure 6 is a side elevational view of the cooling tower of Fig. 5; Figure 7 is a schematic block diagram of the mould filling station of the plant of Fig. 1; Figure 8 is a perspective view of a self-supporting stack of asphalt blocks; and Figure 9 is a perspective view of a single asphalt block.

The manufacturing plant shown in Figs. 1 and 2 enables mastic asphalt to be produced from the raw materials bitumen, limestone grit and powdered limestone filler by an almost completely au- tomatic process.

Bulk supplies of bitumen, grit and filler are held in separate hoppers 10, 12 and 14, respectively and are supplied to a primary mixer 16 in the proportions necessary to produce the grade of asphalt re- quired. Bitumen is delivered hot to the the hopper 10 and is kept warm at a temperature between 17WC and 2000C to render it sufficiently liquid to flow reasonably easily and is then carried directly to the primary mixer 16 through a pipeline 11.

Grit and filler from the hoppers 12 and 14 respectively are drawn upwards by means of elevators 13 and 15.

The filler is fed from the hopper 14 into two heaters 18 where it is heated to a temperature of about 20WC and then is carried downwards by gravity to the primary mixer 16, either directly, as shown in Fig. 2 or via a weighing hopper 20 as shown in Fig. 1. The grit is not heated prior to mix ing but is added to the other materials in the pri mary mixer 16, again, either directly from the 115 hopper 12 or via the weighing hopper 20.

The heat retained in the bitumen and the filler is sufficient to permit the asphalt to be mixed without further heat input into the mixer 16. The mixer 16 may be of the conventional "vertical" mixer type having a mixing vessel of, typically, 6 tonnes capacity with a central vertical rotating shaft carrying one or more sets of angled mixing blades or paddies which effect the mixing of the raw materials to form mastic asphalt. The mixer 16 is set to operate on a thirty-minute mixing cycle and once the mixing period is over, a gate is opened at the base of the mixer 16 to allow its contents to be discharged by gravity through a steeply angled shute 22 into a secondary mixer 24.

The secondary mixer 24, which is shown in detail in Figs. 3 and 4, serves two purposes. Firstly, it subjects the asphalt to further mixing, so as to ensure that the raw materials are properly blended and the resulting asphalt is as homogeneous as possible, and, secondly, it acts to cool the asphalt, which not only permits heat to be recovered and re-used but also ensures that the asphalt is at the optimum temperature for moulding.

The mixer 24 is, again of the vertical type consisting of a mixing tank 26 enclosing a central rotary spindle 28 which carries two sets of angled mixing paddles 30. Hot asphalt from the primary mixer 16 enters the mixing tank 26 through an opening 32 at the top of the tank 26 and the cooled asphalt is discharged through an outlet valve (trea cle valve) or opening at its base which is closed during mixing by a pneumatically-operated gate 34.

The secondary mixer 24 is also provided with an outer jacket 36 of insulating material which en closes a number of vertical passages 38 for the flow of a heat exchange medium. The passages 38 are joined at their upper and lower ends by annu- lar passages 40 and 42 respectively to form a network through which the heat exchange medium, which may, for example, be thermal oil, circulates. As it circulates, the cold thermal oil entering the network of passages contacts the hot asphalt through the wall of the mixing tank 26 and heat is transferred from the asphalt to the oil. The hot oil then leaves the secondary mixer 24 and passes through a heat exchanger 44 where the heat is removed and returned to the heaters 18 for use in heating filler for the next batch of asphalt.

Combining the secondary mixer 24 with a heat exchange arrangement in this way enables considerable savings in terms of heat energy to be made and, as mentioned above, has the advantage that once the secondary mixing cycle is complete, the asphalt has been cooled to a temperature of around 150'C, which is ideal for moulding. Furthermore, if the secondary mixer 24 is made sufficiently large for example, of 13 tonnes capacity, the filling of the moulds can be entirely separated from the mixing process with the secondary mixer 24 acting as a buffer between the primary mixer 16 and the mould-filling equipment. It is also possible, if desired, to provide further primary mixers which discharge into the same secondary mixer.

Once the asphalt has been poured into the moulds, which is preferably carried out as de scribed in detail below, the moulds must be cooled so that the asphalt solidifies into fairly rigid blocks.

Rapid cooling of the filled moulds is achieved by drawing the moulds through an array of water sprays. The filled moulds are carried from the fill ing point at the outlet of the secondary mixer 24 by a suitable form of conveyor into a cooling tower 50, which is shown in Figs. 5 and 6.

Inside the cooling tower 50 the moulds 52 are coupled to a chain conveyor 54. The conveyor 54 runs in a zig-zag path up-and-down along the length of the tower 50 so as to fit as long a path as possible into a relatively small ground area. Each 3 GB 2 148 778 A 3 vertical span of the chain conveyor, may, for example, be about 30 feet (9 metres approximately) in length.

The chain conveyor 54 is surrounded along its sinuous zig-zag path by an array 56 of nozzles which spray water onto the moulds 52. The water which initially contacts the moulds 52 and their contents is turned into steam, thus removing a relatively large quantity of heat from the asphalt in the form of the latent heat needed to turn the water into steam. Thereafter, heat is removed by merely heating the spray water. Typically, the water temperature rises from about 1WC to about 160C.

Consequently, the moulds 52 and their contents are cooled rapidly in a relatively small space.

At the end of the chain conveyor 54 remote from the mixers, the moulds 52 are uncoupled from the conveyor 54 and carried away for the asphalt blocks to be unmoulded and packaged prior to being transported to the sites where the asphalt is to be used.

As mentioned above, mastic asphalt blocks produced in existing processes tend to vary a great deal in size and shape and are, therefore, difficult to package, because they cannot be stacked, and to transport. Frequently they are also difficult to break up into even-sized pieces. These disadvantages may be overcome in the following manner.

In the installation of the invention, the mou)ds 52 in which the asphalt blocks are formed, are conveyed continuously around a closed-loop path which passes through a mould-filling station 53 and an unmoulding station 55.

The filling of the moulds 52 is regulated by a control system incorporating a micro-processor 60 as shown in Fig. 7.

Each mould 52 is carried to the mould-filling station 53 which is directly below the discharge gate 34 of the secondary mixer 24. As mentioned above, the discharge gate 34 is pneumatically-actuated and can be moved to open and close the discharge outlet at the base of the mixer 24. When the gate 34 is open, asphalt fails vertically under gravity from the outlet into a mould 52 which is positioned directly below it. The use of a vertical outlet arrangement avoids the problems due to drag and a build-up of asphalt in the shute which arise when an inclined outlet shute is used.

The pneumatic actuator 35 of the gate 34 is controlled by the microprocessor 60. As each mould 52 reaches the mould-filling station 53, the microprocessor 60 causes the actuator 35 to open the gate 34. The gate 34 remains open for the period required for a sufficient amount of asphalt to be discharged into the mould 52 to fill it. The gate 34 is then closed.

The filled mould 52 is then conveyed through a weighing station 64 and a signal representing the weight of the filled mould input into the microprocessor 60. The microprocessor 60 then compares this signal to a stored value, which can either be a desired nominal weight or an average value or the weight of the next preceding mould. The micropro- cessor 60 then adjusts the time for which the dis- charge gate 34 is opened to fill subsequent moulds so that the amount of asphalt discharged and, hence the size and weight of the blocks are as con sistent as possible.

Alternatively, where single moulds are used, each mould may in addition be weighed prior to being filled. The signal representing the weight of the empty mould can then be used by the micro processor to provide a signal representative of the actual weight of asphalt in the mould which is then compared to a desired value.

The moulds 52 in which the blocks are formed are in the form of a generally rectangular tray consisting of four compartments, each rectangular, having a single integral side wall and a separate spring-biased loose bottom. The side wall is not quite perpendicular to the bottom but tapers slightly towards it, typically at an angle of about 8' to the perpendicular, so that the asphalt blocks can more easily be removed from the mould. The use of a loose bottom allows the finished block to be simply pushed out of the mould by suitable unmoulding equipment at the unmoulding station 55. Prior to being filled, the inside of each mould is dusted with a little of the powdered limestone filler material to prevent the asphalt sticking to the mould. As a result, the finished blocks tend to have a surface layer of limestone filler which helps to prevent them sticking together. The empty moulds 52 are returned by means of a conveyor 57 to the mould-filling station 53 for re-use.

Because the blocks produced by the installation of the invention are of a uniform size, they can easily be formed into a self-supporting stack 70, as shown in Fig. 8 of the drawings. For this reason, it is preferred to make the blocks rectangular in shape, as rectangular blocks can be stacked more easily using conventional stacking machinery than blocks of other shapes.

Each stack 70 is formed by a conventional stack ing machine 59 located downstream of the un moulding station 55 and consists of, in this case, eight layers of blocks arranged so that the blocks of each layer straddle the gaps between the blocks of the layer below. The stack 70 is held together by means of four straps 72 which pass tightly around it to prevent the blocks moving apart. At the top and bottom of the stack 70, spaces are left between adjacent blocks to form channels 74 into which the forks of a fork-lift truck can be inserted to nove the stack. A stack formed in this way is entirely self-supporting and does not require a pallet or any other form of packing.

Where such stacks are stored for some time be- fore being transported, changes in ambient temperature may cause the blocks to contract and the straps 72 to loosen. It is therefore, preferred that each strap 72 should be left with an end free when initally placed on the stack 70 so that it can be pulled up and re-tightened immediately before moving the stack 70.

The loose bottom plate of each compartment of the moulds 52 in which the blocks are formed is formed with two equally-spaced transverse ribs of triangular cross-section which produce in the fin- 4 GB 2 148 778 A 4 ished asphalt block two correspondingly spaced, triangular cross-section grooves or indentations 80, as shown in Fig. 9. The ribs may either be formed integrally with the bottom plate of the mould or by positioning two pieces of angle iron on the base of 70 the mould.

The grooves 80 enable the block to be broken easily into three equal sized parts by tapping the block on a suitable surface.

We have found that, for a 20kg block, the best results are obtained with grooves whose cross-section takes the form of a right-angled isosceles triangle, the sides of which are at least one inch (approximately 2.5cm) in length. Although it is possible to use, for example, a similarly-sized groove of equilateral triangle cross-section, blocks having such grooves are more difficult to release from their moulds than those having grooves of right-angled triangle cross-section.

As will be seen from the above description, the method of the invention enables an improved mas tic asphalt block to be manufactured more cheaply by making savings in both the energy consumed and the manufacturing time. Furthermore, the plant described above is almost completely auto matic and can be operated by as few as two men who may be housed in a small air-conditioned control cabin 90, shown in Fig. 2, thus avoiding the need for personnel to work in the unpleasantly hot areas around the heaters and mixers. In addition, the plant is very compact and, consequently, occu pies relatively little space and it is envisaged that it could be erected on site in instances where large quantities of mastic asphalt will be needed.

Claims (42)

1. A method of cooling mastic asphalt, the method comprising discharging the asphalt into a plurality of moulds, and conveying the moulds through an array of spray nozzles arranged to spray water onto the moulds so that at least some of the water impinging on the moulds forms steam and thereby cools the asphalt; at least a portion of the path along which the moulds are conveyed ex tending between levels which are spaced from one another in a vertical direction.

2. A method according to claim 1 in which each mould is conveyed along a sinuous or zig-zag path through the array of spray nozzles.

3. A method according to claim 2 in which the sinuous or zig-zag path lies in a substantially verti cal plane.

4. A method according to claim 2 or 3 in which the moulds are conveyed by means of a continu- 120 ous conveyor.

5. A method according to any of claims 1 to 4 substantially as hereinbefore described with refer ence to the drawings.

6. A method of manufacturing mastic asphalt, the method comprising the steps of (a) heating at least some of the constituent mate rials which are to be combined to form the as phalt; (b) mixing the constituent materials; and 130 (c) cooling the asphalt to a temperature suitable for discharging into a mould; at least some of the heat removed from the as phalt 14 during cooling being recovered and re used in heating the constituent materials of asphalt manufactured subsequently.

7. A method according to claim 6 in which mix ing is effected without further heating of the con stituent materials.

8. A method according to claim 6 or 7 in which the cooling is effected during the latter stages of mixing.

9. A method according to any of claims 6 to 8 in which cooling is effected by bringing the asphalt into thermal contact with a heat exchange medium.

10. A method according to claim 9 in which the heat exchange medium is brought into thermal contact with the asphalt during mixing of the constituent materials.

11. A method according to any of claims 6 to 10 substantially as hereinbefore described with reference to the drawings.

12. A method according to any preceding claim in which the cooled asphalt is discharged into a plurality of moulds through a discharge outlet which can be selectively opened and closed, the weight of each successive mould being measured after filling and the length of time for which the discharge outlet is opened to discharge asphalt into each mould being varied in response to the measured weight of preceding filled moulds so as to render the weight of asphalt discharged into successive moulds substantially uniform.

13. A method of discharging mastic asphalt into a plurality of moulds through a discharge outlet which can be selectively opened and closed, the method comprising measuring the weight of each mould after filling and varying the time for which the discharge outlet is opened to discharge asphalt into each mould in response to the measured weight of preceding filled moulds so as to render the weight of asphalt discharged into successive moulds substantially uniform.

14. A method according to claim 12 or 13 in which the said measured weight is compared to a stored value, the time for which the discharge outlet is opened being varied in dependence on the difference between the said measured weight and the stored value.

15. A method according to claim 14 in which the stored value is the measured weight of a preceding mould.

16. A method according to any of claims 13 to 15 substantially as hereinbefore described with reference to the drawings.

17. A method according to any preceding claim in which the asphalt is cooled subsequently to being discharged into a plurality of moulds; cooling of the asphalt being effected by spraying water onto the moulds so that at least some of the water impinging on the mould forms steam and thereby cools the asphalt.

18. A method of manufacturing mastic asphalt substantially as hereinbefore described with refer- GB 2 148 778 A 5 ence to the drawings.

19. Apparatus for the manufacture of mastic as phalt comprising means for heating at least some of the constituent materials from which the asphalt is formed prior to mixing, means for mixing the constituent materials, means for cooling the as phalt to a temperature suitable for discharging into a mould and a heat exchanger arrangment associ ated with the cooling means for recovering heat from the asphalt and returning it to the heating means for heating the constituent materials for as phalt to be produced subsequently.

20. Apparatus according to claim 19 in which the means for mixing includes a primary mixer ar ranged to discharge its contents into a secondary 80 mixer which forms the cooling means.

21. Apparatus according to claim 20 in which two or more primary mixers are arranged to dis charge their contents into a single secondary mixer.

22. Apparatus according to claim 20 or 21 in which the walls of the secondary mixer are formed with at least one passage for the circulation of a heat exchange medium.

23. Apparatus according to claim 22 in which 90 the heat exchange medium is oil.

24. Apparatus according to any of claims 19 to 23 substantially as hereinbefore described with ref erence to the drawings.

25. Apparatus according to any of claims 19 to 24 including means for discharging asphalt into a plurality of moulds through a discharge outlet provided in the cooling means, the discharging means including means for weighing each mould after fill- ing and for providing a signal indicative of the weight of each mould and control apparatus coupled to receive the said signal and to vary the time for which the discharge outlet is opened to discharge asphalt into each mould in dependence on the said signal so as to render the weight of asphalt discharged into successive moulds substantially uniform.

26. Apparatus for use in the method of claim 13 comprising means for weighing each mould after filling and for producing a signal indicative of the weight of each mould and control apparatus coupled to receive the said signal and to vary the time for which the discharge outlet is opened to discharge material into each mould in dependence on the said signal so as to render the weight of material discharged into successive moulds substantially uniform.

27. Appparatus according to claim 25 or 26 in which the control apparatus includes a micropro- cessor.

28. Apparatus according to any of claims 25 to 27 in which the discharge outlet can be selectively opened and closed by means of a pneumaticallyactivated movable closure.

29. Apparatus according to any of claims 25 to 28 substantially as hereinbefore described with reference to the drawings.

30. Apparatus according to any of claims 19 to 29 including means for cooling the asphalt subse- quently to its having been discharged into a plural- ity of moulds, the means for cooling comprising an array of spray nozzles and a conveyor for conveying the moulds through the said array; the spray nozzles being arranged to spray water onto the moulds carried by the conveyor so that at least some of the water impinging on the moulds forms steam and thereby cools the asphalt and the conveyor being such that at least a portion of the path along which the moulds are conveyed extends be- tween levels which are spaced from one another in a vertical direction.

31. Apparatus for use in the method of claim 1 comprising means for discharging the asphalt into a plurality of moulds, an array of spray nozzles and a conveyor for conveying the moulds through the said array; the spray nozzles being arranged to spray water onto moulds carried by the conveyor so that at least some of the water impinging on the moulds forms steam and thereby cools the $aterial and the conveyor being such that at lest a portion of the path along which the moulds are conveyed extends between levels which are spaced from one another in a vertical direction.

32. Apparatus according to claim 30 or 31 in which the conveyor is arranged to convey the moulds along a sinuous or zig-zag path through the array of spray nozzles.

33. Apparatus according to claim 32 in which the sinuous or zig-zag path lies in a substantially vertical plane.

34. Apparatus according to any of claims 30 to 33 including means for removing the asphalt from the moulds subsequently to being conveyed through the array of spray nozzles.

35. Apparatus according to any of claims 30 to 34 substantially as hereinbefore described with ref erence to the drawings.

36. Apparatus for use in the manufacture of mastic asphalt, the apparatus being substantially as hereinbefore described with reference to the drawings.

37. A mastic asphalt block having formed in a surface thereof at least one groove for facilitating breaking of the block.

38. A block according to claim 37 in which the groove extends from one surface of the block to a surface opposite the said one surface.

39. A block according to claim 37 or 38 in which the groove is of triangular cross-section.

40. A block according to claim 39 in which the groove is of right-angled isosceles triangle crosssection.

41. A block according to claim 39 or 40 in which the length of each side of the triangular cross-section is not less than one inch.

42. A mastic asphalt block substantially as hereinbefore described with reference to Fig. 9 of the drawings.

Printed in the UK for HMSO, D8818935, 4/85, 7102. Published by The Patent Office, 25 Southampton Buildings. London, WC2A lAY, from which copies may be obtained.