GB1603514A - Process for producting coated roadstone - Google Patents

Description

(54) PROCESS FOR PRODUCING COATED ROADSTONE (71) We, REDLAND AGGREGATES LIM ITED of Redland House, Reigate, Surrey and UNDERGROUND MINING MACHINERY LIM ITED, of Aycliffe Industrial Estate, Darlington, Co. Durham, England, both English Companies, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for producing coated roadstone.

In a known process for producing coated roadstone, an aggregate material, such as stone chippings, and a bituminous binder are charged into a mixer and mixed together in a current of hot air so as to cause the binder to liquefy and coat the aggregate material.

A disadvantage of the known processes of this type is that, as a rule, the aggregate material is dried thoroughly before it is mixed with the binder. As a result, the dusty constituents of the aggregate, or fines, escape in considerable quantities before the binder liquefies and binds them into the mix.

Expensive dust collecting equipment must therefore be provided.

In the process described in the complete specification of our patent number 1340762, there is described and claimed a process comprising heating and mixing an aggregate material having a water content and a solid bituminous binder in particulate or granular form proportioned to provide a composition requisite for a mix of coated roadstone to be produced, the water content being removed by evaporation at the same time as the binder is liquefied and coats the aggregate. the dusty constituents of the aggregate material being retained until such time as they are bound into the mix by the liquefied binder.

The aggregate material is cold when the binder is added to it and mixing commences, and the retention of a quantity of water in the aggregate material while the binder is being liquefied and binding the dusty constit uent into the mix, results in a process which is dustless when compared with the conventional processes in which the - aggregate is heated before its first contact with the binder.

The overall process described in detail in the specification mentioned above is carried out using two separate mixers or mixer stages.

It was later realised that the dustless process could be- simplified and expedited, and the cost of the apparatus reduced,-by carrying out a single, continuous - mixing operation in one mixer.

Accordingly, there is described and claimed in the complete specification-of our patent No. 1400721 a process for the production of coated roadstone in which an aggregate material-having a water content and a solid bituminous binder in particulate or granular form proportioned-tó provide a composition requisite for-a mix of a coated roadstone to be produced are mixed together cold, and heat is supplied as mixing continues so that the water content of the aggregate material is removed by evaporation and the binder is caused to liquefy the coat the aggregate material, the water content of the aggregate being sufficient to. retain the-dusty constituents of the aggregate taterial~until they are bound into the mix by the liquefied binder; the entire mixing and heating process being carried out within one and the same mixer, within which a current of hot gas is caused to flow to supply heat--to the mix.

It was then -realised that it is often- possible to dispense with a preliminary cold inixing operation, and to commence the hot mixing operation as soon - as practicable after the aggregate and binder has been charged into the mixer.

Accordingly, there - -is described- an'd claimed in the complete specification of our patent No. 1443424 a process in-which a aggregate material, for example, stoneohipp- ings, and a bituminous binder are charged into a mixer and mixed together to form a mix in a current of hot air so as to cause the binder to liquefy and coat the aggregate material, the aggregate material being initially cold and containing a quantity of water which is evaporated by heat supplied to the mix by the hot air but which serves to prevent the dusty constituents of the aggregate material escaping with the current of hot air until the dusty constituents are bound into the mix by the liquefied binder, heating and mixing of the materials commencing at substantially the same time.

In this process the current of hot air is supplied by an oil or gas burner and the heating and mixing takes place in a rotating drum.

To be commercially viable, the aggregates must be heated and coated in as short a time as possible, and consequently very high gas temperatures are employed, the exit temperature of the burner being as high as 170F1800-C. Unfortunately, this high gas temperature can result in the temperature of the mixing chamber becoming so great as to cause vapourising and cracking of the binder.

The present invention provides a process for producing coated roadstone in which an aggregate material and a bituminous binder are charged into a mixer and mixed together to form a mix in a current of hot air so as to cause the binder to liquefy and coat the aggregate material, the aggregate material containing a quantity of water which is evaporated by heat supplied to the mixer by the hot air and which serves to prevent the dusty constituents of the aggregate material escaping with the current of hot air until the dusty constituents are bound into the mix by the liquefied binder, and the contents of the mixer being discharged in the form of a batch after mixing, wherein, at least during the stage of the process in which the binder coats the aggregate, the temperature of the hot air is so controlled that vapourisation of the binder does not occur.

Preferably heating occurs in three stages in the first and third of which the hot air is at a first, higher temperature and in the second of which the hot air is at a second, lower temperature, moset of the water being evaporated during the first heating stage. after which the binder is added, the binder liquefying and coating the aggregate during the second heating stage, and the coated roadstone being produced at the end of the third heating stage, said second temperature being so controlled that vapourisation of the binder does not occur. This process, therefore, uses the maximum possible temperature at each stage in the cycle to allow the production of coated roadstone while limiting the temperature to prevent the production of unacceptable fumes.

Advantageously, the aggregate material is initially cold and the heating and mixing commence substantially at the same time.

Preferably, the first temperature is of the order of 17001800 C, and the second temperature is of the order of 800"C. These temperatures are burner exit temperatures, and not necessarily the actual temperatures within the mixer.

Where the hot air is supplied by an oil or gas burner, its first and second temperatures may be controlled by adjusting the flame of the burner to either high or low fire. Alternatively, the entire burner may be moved towards and away from the mixer to adjust the temperature of the hot air entering the mixer.

The invention will now be described, by way of example, with reference to the accompanying drawing, the single figure of which is a side elevation of a plant for producing coated roadstone.

Referring to the drawing, a plant for coating roadstone includes a structural framework I which permits a truck to be driven into a position for receiving hot coated roadstone discharged from a batch mixer generally designated 2 and supported on a lower platform 30 of the framework.

The batch mixer 2 comprises a barrel 3 of double frusto-conical form having a charge mouth 4 at one end. Internally, the barrel 3 is plain and devoid of lifters, but scrolls are provided in the frusto-conical parts 6 and 7.

These scrolls cause the mix to move axially during the mixing operation. The barrel 3 is mounted for rotation on its axis on support rollers in a frame 8 and is driven through gearing by a hydraulic motor generally designated at 9. The frame 8 carrying the barrel 3 is pivotably mounted between supports 10 and is connected at each side to a hydraulic ram 11 operable to tilt the barrel from a position wherein the axis is horizontal as shown at R, to a charging position wherein the axis is inclined as shown at R 1. Mounted on a platform 31 at one end of the framework 1 is an air heater 12, which is fired by an oil or gas burner 32 capable of being set to direct a flame of variable intensity into a combustion chamber through which air is forced by a blower so that heated air issues into a duct 13. The mouth of the duct 13 is so located that it is aligned with the mouth 5 of the barrel 3 when the axis of rotation of the latter is in its horizontal position. At the other end of the framework 1 an expansion chamber 17 has an intake mouth, which aligns with the mouth 4 of the barrel 3 when the axis of rotation of the latter is in its horizontal position, and a discharge mouth which discharges to a stack 19 in which is located an adjustable restrictor 119. A door is provided in the lower part of the expansion chamber 17 to allow the accumulated particles to be removed.

A conveyor (not shown) serves to convey batches of undried aggregate material into a batch check weight hopper 22 having an exit chute 221 which is closable by a helmet door (not shown) and is positioned to guide aggregate material leaving the hopper to a pivotable feed chute 222 which may be positioned to guide the material into the mouth 4 of the barrel 3.

Mounted at ground level are tanks (not shown) for molten binder and flux oil. Pipe lines connect the tanks with nozzles for discharging the respective substances into the barrel 3. The nozzle (not shown) for the binder is located, together with that for the flux oil, within the expansion chamber 17 and opens towards the intake mouth thereof, discharge of a metered quantity of binder taking place under the effect of pump or air pressure. Further nozzles (not shown) connected to a supply of water are located to spray the aggregate material before it is fed to the barrel 3.

In operation, the required grades of wet aggregate material drawn from store (which may be storage heaps exposed to the weather) are supplied to feed hoppers delivering the grades onto a conveyor belt through weighing devices such as belt weight feeders or through volumetric feeds by means of which the proportioning of the different grades forming a batch delivered to the conveyor belt can be controlled. The water content of the grades may be measured by moisture content indicators provided, for example, at the feed hoppers, to take the water content into account in weighing out the appropriate quantities of the different grades to ensure that their dry weight will comply with the desired specification of coated roadstone.The water content of the batch of aggregate material may be adjusted as necessary or desired to obtain a water content between 0.5 and fifteen per cent (preferably one to three per cent or higher) by weight by means of the spray from the water spray nozzles. One purpose of the moisture content is to hold the fines against escape in the hot air stream and accordingly the moisture is dispersed as evenly as possible throughout the aggregate material.

When the weigh hopper is full, the helmet door and chute 222 pivot into the open position and aggregate material is directed into the barrel 3, the barrel being in its inclined charging position Rl. The control system closes the door 122 and chute and signals are given to call the next batch and pivot the mixer under the action of ram 11 to the horizontal position. The motor 9 is started and the barrel rotated in the clockwise direction.

The hot mixing operation occurs in three stages and is carried out with the barrel 3 revolving at about 24 r.p.m. that is about 70% centrifuging speed. Prior to the first heating stage, the control system brings the burner 32 on to full flame. Hot air is then passed from the air heater 12, through the rotating barrel3, and is discharged to the atmosphere through the stack 19. During the first heating stage the hot air, whose temperature is of the order of 1701800"C, evaporates most of the water present in the aggregate material to form steam. At the end of this period, the binder is added to the aggregate in the barrel 3.

During the second heating stage, the hot air, whose temperature is of the order of 800"C, removes the steam and liquefies the binder which immediately coats the aggregate and fixes the fines of the aggregate material against being picked up and carried off by the current of hot air. Any coarse particles that do escape from the barrel 3 are collected in the expansion chamber 17. Prior to the fixing of the fines by the binder, they are held against escape by the water content of the aggregate. The burner 32 is brought to a low flame condition prior to the commencement of the second heating stage by the control system.

At the end of the second heating stage, the burner 32 is returned to full flame by the control system and the mixing process continues until the coated roadstone is at the required temperature. During this third heating stage, the hot air is again at a temperature of between 1700"C and 1800"C.

After mixing has been completed the mixer is tilted into position R2 and the motor 9 driven in the forward direction at half speed to discharge the hot mix into a waiting truck. The mixer is then moved slightly to bring it into the position R1, and a further cycle may be commenced. Cooling of the barrel before further aggregate material is charged into it is not necessary and indeed it has been found desirable to insulate the barrel in an attempt to conserve heat. Insulation for the duct 13 may also be provided.

The addition of the binder immediately prior to the second heating stage ensures that the binder is merely liquefied and not vapourised. Consequently, no fuming occurs and a better coating of the aggregate results than with the known process which utilises no such cooler heating stage (binder consolidation period).

The periods of the three heating stages depend upon such factors as the original water content of the aggregate material and the desired final temperature of the coated roadstone. Typically, for an original water content of 2.4% and a final temperature of 120"C, the total cycle time is 120 seconds (50 seconds for the first heating stage, 40 seconds for the second heating stage, and 30 seconds for the third heating stage).

It will be apparent that a number of modifications could be made to the process described above. In particular, instead of arranging for the burner 32 to have a full flame setting and a low flame setting, the entire heater 12 could be arranged for movement towards, and away from, the mouth 5 of the barrel 3.In this case, with the heater 12 positioned directly adjacent to the mouth 5 (as in the drawing) hot air at between 1700"C and 1800"C would be directed into the barrel 3, and with the heater 12spaced from the mouth 5, cool air is drawn into the barrel with the hot air from the burner, thereby reducing the temperature of the air entering the burner to 800 C. Thus, by moving the heater 12 away from the mouth 5 at the end of the first heating stage and back towards the mouth prior to the third heating stage, the second heating stage is again a cooler heating stage and so again a binder consolidation period results.

When it is required to use a filler for the required coated roadstone specification, this is fed by a screw conveyor, or other known means, to a separate check weigh hopper located above the main feed conveyor to the barrel 3. When the check weight is achieved, the filler is discharged onto the moist aggegate carried by the conveyor. Provision is made to mix the filler with the aggregate on the main conveyor belt, for example ploughs are provided which "fold" the aggregate over the filler. Alternatively, tines are provided to agitate the aggregate and filler so as to cause mixing. Indeed, this operation of mixing the wet aggregate with the filler has been found to be the best method of wetting the "flourlike" filler.

WHAT WE CLAIM IS: 1. A process for producing coated roadstone in which an aggregate material and a bituminous binder are charged into a mixer and mixed together to form a mix in a current of hot air so as to cause the binder to liquefy and coat the aggregate material, the aggregate material containing a quantity of water which is evaporated by heat supplied to the mixer by the hot air and which serves to prevent the dusty constituents of the aggregate material escaping with the current of hot air until the dusty constituents are bound into the mix by the liquefied binder, and the contents of the mixer being discharged in the form of a batch after mixing, wherein, at least during the stage of the process in which the binder coats the aggregate, the temperature of the hot air is so controlled that vapourisation of the binder does not occur.

2. A process as claimed in Claim 1, wherein heating occurs in three stages in the first and third of which the hot air is at a first, higher temperature and in the second of which the hot air is at a second, lower temperature, most of the water being evaporated during the first heating stage, after which the binder is added, the binder liquefying and coating the aggregate during the second heating stage, and the coated roadstone being produced at the end of the third heating stage, said second temperature being so controlled that vapourisation of the binder does not occur.

3. A process as claimed in Claim 2, wherein the first temperature is of the order of l70O-l800C and the second temperature is of the order of 800 C.

4. A process as claimed in Claim 3, wherein the hot air is supplied by an oil or gas burner.

5. A process as claimed in Claim 4, wherein said first and second temperatures are controlled by adjusting the flame of the burner to either high or low fire.

6. A process as claimed in Claim 4 wherein the entire burner is moved towards and away from the mixer to adjust the temperature of the hot air entering the mixer, whereby said first and second temperatures are controlled.

7. A process as claimed in any one of Claims 1 to 6, wherein the aggregate material is initially cold and the heating and mixing commence substantially at the same time.

8. A process for producing coated roadstone substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. It will be apparent that a number of modifications could be made to the process described above. In particular, instead of arranging for the burner 32 to have a full flame setting and a low flame setting, the entire heater 12 could be arranged for movement towards, and away from, the mouth 5 of the barrel 3.In this case, with the heater 12 positioned directly adjacent to the mouth 5 (as in the drawing) hot air at between 1700"C and 1800"C would be directed into the barrel 3, and with the heater 12spaced from the mouth 5, cool air is drawn into the barrel with the hot air from the burner, thereby reducing the temperature of the air entering the burner to 800 C. Thus, by moving the heater 12 away from the mouth 5 at the end of the first heating stage and back towards the mouth prior to the third heating stage, the second heating stage is again a cooler heating stage and so again a binder consolidation period results. When it is required to use a filler for the required coated roadstone specification, this is fed by a screw conveyor, or other known means, to a separate check weigh hopper located above the main feed conveyor to the barrel 3. When the check weight is achieved, the filler is discharged onto the moist aggegate carried by the conveyor. Provision is made to mix the filler with the aggregate on the main conveyor belt, for example ploughs are provided which "fold" the aggregate over the filler. Alternatively, tines are provided to agitate the aggregate and filler so as to cause mixing. Indeed, this operation of mixing the wet aggregate with the filler has been found to be the best method of wetting the "flourlike" filler. WHAT WE CLAIM IS:

1. A process for producing coated roadstone in which an aggregate material and a bituminous binder are charged into a mixer and mixed together to form a mix in a current of hot air so as to cause the binder to liquefy and coat the aggregate material, the aggregate material containing a quantity of water which is evaporated by heat supplied to the mixer by the hot air and which serves to prevent the dusty constituents of the aggregate material escaping with the current of hot air until the dusty constituents are bound into the mix by the liquefied binder, and the contents of the mixer being discharged in the form of a batch after mixing, wherein, at least during the stage of the process in which the binder coats the aggregate, the temperature of the hot air is so controlled that vapourisation of the binder does not occur.

2. A process as claimed in Claim 1, wherein heating occurs in three stages in the first and third of which the hot air is at a first, higher temperature and in the second of which the hot air is at a second, lower temperature, most of the water being evaporated during the first heating stage, after which the binder is added, the binder liquefying and coating the aggregate during the second heating stage, and the coated roadstone being produced at the end of the third heating stage, said second temperature being so controlled that vapourisation of the binder does not occur.

3. A process as claimed in Claim 2, wherein the first temperature is of the order of l70O-l800C and the second temperature is of the order of 800 C.

4. A process as claimed in Claim 3, wherein the hot air is supplied by an oil or gas burner.

5. A process as claimed in Claim 4, wherein said first and second temperatures are controlled by adjusting the flame of the burner to either high or low fire.

6. A process as claimed in Claim 4 wherein the entire burner is moved towards and away from the mixer to adjust the temperature of the hot air entering the mixer, whereby said first and second temperatures are controlled.

7. A process as claimed in any one of Claims 1 to 6, wherein the aggregate material is initially cold and the heating and mixing commence substantially at the same time.

8. A process for producing coated roadstone substantially as hereinbefore described with reference to the accompanying drawings.