GB2506440A

GB2506440A - Apparatus for drying particulate material

Info

Publication number: GB2506440A
Application number: GB201217517A
Authority: GB
Inventors: Ian Brian Lewis
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-10-01
Filing date: 2012-10-01
Publication date: 2014-04-02
Also published as: GB201217517D0

Abstract

This invention relates to a dryer for drying particulate material, and the dryer has a rotating drying chamber 112 with a combustion zone 116 and a drying zone 118. The drying chamber has an inlet 120 for materials which are to be dried, and in use, heated and dried particulate materials pass through the drying zone and the combustion zone to an outlet 126. A burner 114 is mounted adjacent one end of the dryer and produces a flame which extends into the combustion zone. The combustion zone and the drying zone are separated by a flame shield 176 which prevents the flame from entering the drying zone but permits hot gases generated by the flame to pass around the flame shield and into the drying zone before leaving via an exhaust outlet 122. Both the combustion zone and the drying zone contain lifting elements, for example box lifters 132 and finger lifters 130, which lift and tumble the particulate material through the stream of hot gases. The arrangement may be inclined downwards towards the burner to gradually migrate the particulate material towards the combustion zone.

Description

I

APPARATUS FOR THE DRYING OF PARTICULATE MATERIAL

This invention relates to an apparatus and method for the drying of particulate material and in particular aggregates such as the aggregates used in the manufacture of asphalt.

Background of the Invention

Asphalt is the name used in the UK and Europe to denote the material, used in road building and other civil engineering applications, which comprises aggregates (e.g. crushed rock, gravel, shingle, sand and recycled broken up asphaltic road surface material) coated in bitumen. In the USA, this material is generally known as asphalt concrete.

The aggregates used in making asphalt typically contain substantial quantities of water, either because of the wet nature of the medium from which they have been extracted, or because they have been left out in the open and have therefore been exposed to atmospheric moisture. Consequently, the aggregates need to be dried before use. Moreover, in order to ensure efficient mixing of the aggregates and bitumen and maximise the binding of the bitumen to the aggregates, it is desirable that the aggregates should be heated prior to mixing with the bitumen.

A typical asphalt plant will therefore comprise a dryer for drying and heating the aggregates. A common form of dryer used in asphalt plants is a rotating drum dryer in which the heat for the drying process is provided by one or more combustion burners at one end of the drum. Air is drawn through the combustion burners and the heated gases from the burner pass along the interior of the rotating drum and out through a gas exhaust outlet at the far end of the drum. The stream of hot gases from the burner passing through the drum serves to dry the aggregates. In order to facilitate the drying and heating process, the internal side wall of the drying zone of the drum is provided with a series of scoops or blades which scoop up the aggregates from the floor of the drum, lift them to the high point of revolution of the drum and then drop them so that they fall back as a curtain of aggregates through the stream of hot gases to the floor of the drum. In most known types of drum dryer, a contra-flow arrangement is used in which the drum is inclined so that the drying aggregates gradually migrate from an inlet at the end of the drum opposite the combustion burner towards the end at which the burner is located. Once they have reached the burner end, the dried hot aggregates are either discharged directly into a mixer, where they are mixed with bitumen to form the asphalt, or they are conveyed to hot storage containers from which weighed amounts of the hot dry aggregates are discharged into the mixer.

In recent years, many asphalt plants have started to use small amounts of re-claimed asphalt (commonly known as RAP or recycled asphalt pavement) in place of a proportion of the virgin aggregates in an asphalt mix. Large amounts of potentially recyclable asphalt pavement are produced when existing asphalt road surfaces are broken up for replacement or repair. However, because of the volatile components in the bitumen, the use of recycled RAP material in asphalt manufacture presents particular problems.

The majority of plants currently processing reclaimed asphalt (commonly known as RAP or recycled asphalt pavement) fall into several categories.

Standard batching tower plants equipped with a single dryer make use of virgin aggregates (i.e. uncontaminated with recycled asphaltic materials) which are heated to temperatures to 250°C in the dryer and are then transported via an elevator, to a vibratory screen for sizing (dust, 6, 10, 14,20 & 28 mm is usual), and then to hot bin storage compartments. Required amounts of each size of aggregate are discharged from the storage bins into a weighing hopper and are subsequently discharged into a mixer where they are mixed with bitumen and reclaimed asphalt (RAP). Before entering the mixer, cold RAP is usually transported from a feed bin via an inclined conveyor which delivers it onto a stationary horizontal weighing conveyor. When the required weight of RAP is on the horizontal belt, the inclined conveyor stops. The horizontal conveyor belt then starts and discharges the RAP into the mixer when required. After the RAP has been discharged into the mixer, the horizontal conveyer belt stops and the next batch of RAP is weighed onto it, ready for the next batch of asphalt. The amount of RAP used in the asphalt mix is typically up to 25% of the total weight of the asphalt batch. The cold reclaimed asphalt will typically contain a significant quantity of moisture (up to 8-10%) and, when this contacts the heated virgin aggregates, it is expelled in the form of steam. The steam is removed from the mixer and transported to a primary exhaust system via a conduit known as the "nuisance duct".

The production of asphalt using this type of plant suffers from several drawbacks.

Firstly, during the drying of the virgin aggregates, high temperature exhaust gases of up to 170°C are discharged to the atmosphere. The process is therefore inefficient in terms of energy consumption. Secondly, using this method, only relatively small amounts (up to a maximum of 25%) of RAP can be used in the asphalt mixtures.

Plants using standard continuous mixers equipped with a single dryer operate in a similar manner to the standard batching tower plants except that there is no tower in which the heated virgin aggregates are screened and separated into size fractions.

Instead, the heated aggregates are discharged directly into the mixer and the cold RAP is also delivered into the mixer. The problems encountered in using this type of plant are similar to those encountered with the standard batching tower plants equipped with a single dryer. Thus, they have poor energy efficiency as a result of the high exhaust gas temperatures and only relatively low concentrations of RAP can be used (i.e. a maximum of 25%) in the asphalts produced by these plants.

Standard batching tower plants equipped with two dryers are a fairly recent development and are provided with a virgin aggregate dryer and a RAP dryer. The two dryers heat the virgin aggregates and the RAP separately. The virgin aggregates are then processed through a screw for sizing. The RAP is delivered to a holding hopper and then both the virgin aggregates and the RAP are weighed out accordingly for the final mix recipe and delivered directly to the mixer. Plants of this type suffer from the problems that they are very expensive to install and have high maintenance costs.

Also, because substantial bituminous deposits build up on the interior surfaces of the RAP dryers over fairly short periods of use, they require regular cleaning out using heated virgin aggregates, which is inefficient both in terms of plant operating time and also energy consumption. A further problem encountered with this type of plant is that when making asphalt mixes containing less than 50% virgin aggregates, the partially full virgin aggregate dryers do not run efficiently leading to high exhaust temperatures (and hence energy wastage) and overheating of the dryer.

A major problem with drying RAP is that if the drying temperature is too high and the RAP aggregates pass through or close to the burner flame, a large proportion of the bitumen present in the RAP is burnt off and the volatile components of the bitumen then pass into the exhaust system to the filtration system (typically a bag filter) and then out of the exhaust outlet to the atmosphere. By using separate dryers for the virgin aggregates and the RAP, the problems of overheating bitumen can be avoided but, as indicated above, such two-dryer systems tend to be inefficient.

Bernardi Impianti International of Milan, Italy, manufacture asphalt plants using so-called "RED" (RAP Ecological Dryer) dryers which are designed so that virgin aggregates and the RAP are dried in the one dryer. Dryers of this type are disclosed in EP2202473 (Bemardi Impianti International S.P.A.) The RED dryers are characterised by the presence, in the combustion zone of the dryer, of an open-ended combustor tube that is intended to receive and shield the flame and thereby prevent the flame from coming into contact with the aggregates (including the RAP) tumbling from the wall of the dryer as it rotates. By avoiding contact between the flame and the aggregate, the problem of the overheating of the RAP and the loss of volatile components of the RAP is said to be prevented 01 reduced. It is also stated that asphalts having much higher RAP contents can be produced using plants incorporating RED dryers. For example, in EP2202473, it is disclosed that it is possible to make asphalts containing 40% or more RAP.

Despite the apparent advantages suggested in EP2202473, the present applicants have found that there are substantial disadvantages associated with the RED dryers of the type disclosed in EP2202473.

A major problem is that the combustor tubes have a very short lifetime and very quickly need replacing. In addition, the hottest part of the flame typically extends out of the end of the combustor tube and is therefore in direct contact with the cascade of aggregate materials including RAP tumbling from the walls of the dryer.

A further problem is that the surface of the combustor tube is extremely hot and, as a result, RAP aggregates falling onto the tube may stick and/or burn. The burning of the bitumen produces pollutant gases which are extracted by the exhaust via the bag filter.

In EP2202473, it is suggested that the distance between the burner and the combustor tube can be set so that a venturi effect is generated in front of the burner which takes the fumes from the periphery of the combustor tube back towards the inside of the flame and back into the combustor tube. It is stated that this contributes to a reduction of pollutants. However, whilst theoretically possible, in practice the negative pressures established by the exhaust extraction fan serve to prevent burnt bituminous gases from passing back into the combustor tube. Instead, they are pulled by the extractor fan into the bag filter where the bag filter can become clogged by potentially flammable materials. Thus, in practice, contrary to what is suggested in EP2202473, a substantial amount of the bitumen present in aggregates containing a proportion of RAP is either lost or must be recovered as reclaimed dust from the bag filter. Furthermore, the exhaust gas emissions of pollutants such as carbon monoxide are above the expected levels. The applicants have tested a plant of a type disclosed in EP2022473 to make asphalt containing 20% RAP and found that the carbon monoxide emissions were over 5000ppm.

At the present time therefore, there remains a need for a process and apparatus for producing low temperature asphalt mixes that avoids the aforementioned problems.

The Invention The present invention provides a number of improvements in the drying and processing of particulate materials and in particular materials commonly referred to as aggregates and RAP (Reclaimed Asphalt Pavement) and which are used in civil engineering applications such as the manufacture of asphalt.

More particularly, the present invention sets out to provide an improved apparatus for drying aggregate materials and RAP (Reclaimed Asphalt Pavement) of the type used in asphalt manufacture and which provides substantial energy savings over the drying apparatuses used in many existing asphalt manufacturing plants.

Accordingly, in a first aspect, the invention provides a dryer for drying aggregates and reclaimed asphalt pavement (RAP) wherein: the dryer comprises a burner and a rotating drying chamber; the drying chamber has an interior comprising a drying zone and a combustion zone; the drying chamber has an aggregate inlet for receiving aggregates and RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and RAP, and an exhaust outlet; the dryer is arranged so that aggregates and RAP entering the drying chamber are advanced from the aggregate inlet through the drying zone and combustion zone to the aggregate outlet; the burner is mounted adjacent one end of the drying chamber and produces a flame which extends into the combustion zone thereby generating hot gases for drying the aggregates and RAP; means are provided for drawing a stream of the hot gases along the drying chamberfrom the combustion zone through the drying zone and out through the exhaust outlet; the drying zone has an interior surface which is configured to lift the aggregates as the drying chamber rotates and tumble the aggregates and RAP through the stream of hot gases to dry the aggregates and RAP; and the combustion zone contains a plurality of lifting elements for lifting and tumbling the aggregates and RAP; characterised in that: the combustion zone and the drying zone are separated by a flame shield which prevents the flame from entering the drying zone but permits hot gases generated by the flame to pass around the flame shield and into the drying zone; the lifting elements in the combustion zone are screened from the flame by a plurality of spaced apart shield members each of which is mounted on an internal wall of the drying chamber, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and arranged so that together they form a cascade path down which aggregates and RAP falling out of the lifting elements can tumble to a bottom of the drying chamber without contacting the flame.

The dryer comprises a burner and a rotating drying chamber. The burner, which does not generally rotate, is located at one end of the drying chamber. The drying chamber has an aggregate inlet for receiving aggregates and RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and RAP, and an exhaust outlet. There may be one or more than one aggregate inlet. The aggregates and RAP may enter the drying chamber through a common aggregate inlet or through separate aggregate inlets. Most typically, they enter the chamber through a common aggregate inlet. The aggregate inlet and the exhaust outlet are typically located at the opposite end of the drying chamber to the burner. The aggregate outlet is typically located at the same end of the drying chamber as the burner.

The rotating drying chamber component of the dryer may be referred to herein for convenience as a rotary dryer drum.

The interior of the drying chamber is divided into a number of zones and comprises a drying zone and a combustion zone. Aggregates and RAP enter the aggregate inlet and are directed into the drying zone. The wall of the drying chamber adjacent the aggregate inlet may be provided with helical blades which carry the aggregates and RAP from the aggregate inlet to the drying zone.

The drying zone has an interior surface which is configured to lift and tumble the aggregates and RAP as the drying chamber rotates. The interior surface therefore typically is provided with a plurality of protrusions which engage and hold the aggregates/RAP as the chamber rotates until an angle is reached at which the protrusions can no longer hold the aggregates/RAP and they tumble back to the floor of the chamber. The protrusions may conveniently be finger-shaped and are often referred to for this reason as finger lifters. The protrusions or fingers may be radially oriented, i.e. they may be aligned with a line extending radially inwardly to the axis of rotation of the chamber and hence are substantially perpendicular to the surface of the chamber. Alternatively, they may be arranged at an angle (e.g. about 0° to about 200, more particularly about 0° to about 10°) so that they lean in the direction of the rotation of the chamber and can therefore carry the aggregates and RAP further around the drying chamber before releasing them.

The combustion zone is also configured so that it can lift and tumble the aggregates/ RAP as the drying chamber rotates. However, rather than the more open finger lifters used in the drying zone, the lifting elements in the combustion zone typically have a more closed construction and hold the aggregates/RAP until they have passed through the high point of rotation. The lifting elements may have a scoop or box-like form with the leading face of the box being open for collecting the aggregates/RAP. Such lifting elements are conventional and are often referred to as "box lifters'.

The dryer is constructed so that aggregates/RAP entering the drying chamber are advanced from the aggregate inlet through the drying zone and combustion zone to the aggregate outlet. Typically, this is achieved by providing the inner surface of the drying chamber with a small negative angle of inclination in the direction of the aggregate outlet. Usually, the negative angle of inclination (which is typically of the order of about 3-5°) is provided by inclining the axis of rotation of the drying chamber.

Alternatively, but less preferably, the axis of rotation of the drying chamber may be substantially horizontal and the angle of inclination is provided by using a drying chamber which increases in radius towards the aggregate outlet.

The burner typically burns fuel oils, liquid petroleum gas (LPG) or natural gas. More usually, the burner will make use of fuel oils. The burner produces a flame which extends into the combustion zone of the drying chamber thereby generating hot gases for drying the aggregates. Means (such as an extractor fan) are provided for drawing a stream of the hot gases along the drying chamber from the combustion zone through the drying zone and out through the exhaust outlet.

As the drying chamber rotates, the tumbling action in the drying zone creates a curtain of aggregates/RAP falling through the stream of hot gases. Moisture present in the aggregates/RAP is heated and turned to steam and carried away by the stream of hot gases and out through the exhaust outlet. Thus, in the drying zone, the aggregates/RAP are dried and heated.

In order to prevent the flame from entering the drying zone and coming into direct contact with the aggregates and RAP, the combustion zone and the drying zone are separated by a flame shield. The flame shield prevents the flame from entering the drying zone but permits hot gases generated by the flame to pass around the flame shield and into the drying zone.

The flame shield is typically mounted coaxially with the rotational axis of the drying chamber and is typically has a conical, pyramidal or domed shape to deflect the flame radially outwardly and prevent or minimize the development of hot spots on the shield.

The peak or tip of the conical, pyramidal or domed shaped shield typically lies on the rotational axis of the drying chamber. The term "pyramidal" as used herein includes not only triangular or square based pyramids but also pyramids having a polygonal base of more than four sides. Preferably the pyramid has a polygonal base of more than six sides, for example eight to fourteen sides, and more particularly ten to twelve sides. In one embodiment, the pyramid has a dodecagonal base and the shield thus comprises twelve triangular plates or panels connected together to form the pyramid.

The flame shield is typically formed of a material which is refractory at the temperatures produced by the flame. For example, the flame shield may be formed from a refractory metallic material such as stainless steel (e.g. 5mm thick /Grade 310 stainless steel). Alternatively, the flame shield may be formed from a non-metallic refractory material.

The plates or panels making up the flame shield may be formed separately and then fixed together (e.g. by welding). Alternatively, an appropriately shaped piece of the refractory material may be folded to form the requisite number of panels and then adjoining ends connected (e.g. welded) together to form the shield. In a further alternative, two or more sub-assemblies each comprising two or more panels may be formed by folding appropriately shaped pieces of the refractory material and then connecting the sub-assemblies together! for example by welding.

The flame shield may have a plain outer edge or the outer edge may be configured to impart turbulence to the flow or hot gases around the shield. For example, the outer edge may be provided with vanes or vane-like structures to impart twist to the gas stream. In one embodiment, the flame shield has a plain outer edge.

The flame shield is typically mounted on a plurality of arms extending inwardly from the interior surface of the drying chamber. For example, there may be two to six mounting arms, and more usually thiee or four (e.g. four) mounting aims.

As the aggregates and RAP are conveyed from the drying zone into the combustion zone, they are scooped up and tumbled by the lifting elements. The lifting elements in the combustion zone are screened from the flame by the shield members and hence aggregates and RAP falling out of the lifting elements are not exposed directly to the flame. The shield members are configured and arranged so that together they form a cascade path down which aggregates/RAP falling out of the lifting elements can tumble to the bottom of the drying chamber without contacting the flame. Thus, instead of falling out of the lifting elements and down through the flame to the bottom of the drying chamber, the aggregates/RAP falling from one lifting element are directed onto the surface of the neighbouring lifting element and from there to the next neighbouring lifting element and so on until they reach the bottom of the drying chamber without coming into contact with the flame. A significant advantage of preventing the RAP from coming into contact with the flame is that loss of the volatile and combustible components of bitumen present in RAP is reduced or prevented. Consequently, less bitumen needs to be added to the asphalt mixture during the mixing process and emissions of combusted and volatilized bitumen components are reduced or prevented. Fuithermore, the problem of the deposition of bitumen iesidues on the interior of the drying chamber is substantially reduced.

The shield members form a protective screen radially inwardly of the lifting elements.

The protective screen shields the lifting elements and reduces the exposure of the lifting elements to radiant heat from the flame.

Each of the shield members is mounted on the internal wall of the drying chamber, for example by means of a mounting biacket. The shield membeis may take the form of elongate plates, for example of generally rectangular shape. Each elongate plate may extend in an axial direction along substantially the entire length of the combustion chamber. The elongate plates may be ribbed to provide greater rigidity. Typically, the plates have upturned edges to assist with retention of the aggregates.

The shield members, e.g. the elongate plates, are typically formed from a matenal which is refractory at the flame temperatures used. For example, the elongate plates can be formed from a refractory metallic material such as stainless steel (e.g. 5mm thick Grade 310 stainless steel). Alternatively, the shield members, e.g. the elongate plates, can be formed from a refractory non-metallic material.

Preferably each shield member extends continuously from one end of the combustion zone to the other. However, each shield member may alternatively be discontinuous, e.g. may comprise a plurality of individual shield members arranged end to end.

Similarly, each lifting element may extend continuously from one end of the combustion chamber to the other or may comprise a plurality of lifting elements arranged in a row.

In one embodiment, each pair of circumferentially adjacent shield members has a lifting element (or row of lifting elements) disposed therebetween. Thus, for example, the number of lifting elements (or rows of lifting elements) may match the number of shield members.

The dryer of the invention is designed to be used in an asphalt manufacturing plant. In addition to the dryer, the asphalt plant will typically comprise a mixer linked to the dryer by a duct extending from the aggregate outlet of the dryer to the mixer; an exhaust gas extraction and filtering system linked to the exhaust gas outlet of the dryer; a supply of bitumen linked to the mixer; and means for supplying aggregate (and RAP when the asphalt is to contain RAP) to the dryer.

Accordingly, in a further aspect, the invention provides an asphalt plant for manufacturing asphalt comprising a dryer as hereinbefore defined; a mixer linked to the dryer by a duct extending from the aggregate outlet of the dryer to the mixer; an exhaust gas extraction and filtering system linked to the exhaust gas outlet of the dryer; a supply of bitumen linked to the mixer; and means for supplying aggregate (and optionally RAP) to the dryer.

In the mixer, dried and heated aggregates (and RAP) are mixed with bitumen to form asphalt which is then discharged into heated and/or insulated containers for later use or is discharged directly into lorries or other vehicles to be conveyed to the point of use. In conventional asphalt plants, hot gases are vented from the mixer into the exhaust gas extraction and filtering system from which they are discharged into the atmosphere. Discharging the heated gases into the atmosphere is very wasteful of energy. In order to improve energy efficiency, the asphalt plants of the present invention recycle heated gases from the mixer back into the drying chamber. A duct is therefore typically provided to link the mixer with the drying chamber. The duct typically opens into the drying chamber at the burner end, most typically adjacent the burner.

By recycling heat from the mixer into the dryer, the heat output required from the burner to maintain the desired temperature is reduced and hence the burner temperature can be reduced thereby providing fuel savings.

In order to prevent or minimise the loss of volatile and combustible components of the bitumen in recycled asphalt, dryer temperatures are typically reduced when handling aggregates containing RAP. Whilst this saves on energy, there is a risk that the temperatures of the exhaust gases carrying the moist gases away from the dryer to the filtering system will fall below the dew point leading to condensation in the filter and consequent blockage of the filter. The asphalt plant of the present invention typically provides means for monitoring the temperature of the exhaust gases and preventing it from falling below the dew point.

In one embodiment, the asphalt plant of the invention comprises means for monitoring the temperature of the exhaust gases passing from the exhaust outlet of the dryer to the filtering system, and means for boosting the temperature of the exhaust gas if it falls below a predefined value.

The means for boosting the temperature of the exhaust gas may take the form of a by-pass duct leading from the burner end of the drying chamber to a primary exhaust duct downstream of the exhaust outlet of the dryer. The bypass duct channels heated gas from the burner end of the dryer into the primary exhaust duct. Typically one or more control valves or gates are provided in the bypass duct to control the flow of heated gas into the duct. The control valves or gates may be operatively linked to one or more sensors via a controller so that the valves or gates are automatically opened to permit heated gas to be diverted down the bypass duct if the temperature of the exhaust gas falls below a predefined value.

Thus the asphalt plant of the present invention can be operated at lower temperatures suitable for use with asphalt compositions containing larger proportions of recycled asphalt pavement (RAP). The asphalt plants of the invention can also be used for preparing the recently developed so-called "low energy asphalt (LEA) where the presence of particular additives in the compositions means that mixing temperatures as low as 95°C can be used. The construction of the dryer prevents or minimises the loss of bituminous components of the RAP passing through the dryer and the energy consumption of the dryer is reduced as a consequence of recycling heated gases from the mixer. In order to prevent the relatively lower temperature exhaust gases from falling below the dew point, means are provided for boosting the exhaust gas temperature. Thus, the asphalt plant of the invention can be used to manufacture asphalts having a much higher proportion of RAP or LEA compositions whilst reducing the energy consumption typically associated with asphalt manufacture.

Asphalt compositions containing 40% or more RAP can be prepared using the plant of the present invention. Processes for manufacturing such compositions represent a further aspect of the invention.

Accordingly, in another aspect, the invention provides a process for manufacturing an asphalt composition containing recycled asphalt pavement (RAP), which process comprises using an asphalt manufacturing plant including a dryer as hereinbefore defined; introducing into the aggregate inlet of the dryer aggregates and recycled asphalt pavement (RAP); drying and heating the aggregates and RAP in the dryer; discharging a resulting mixture of the dried and heated aggregates and RAP through the aggregate outlet of the dryer to a mixer; adding bitumen to the mixture of aggregates and RAP and mixing to form the asphalt composition.

The amount of RAP in the mixture of aggregates and RAP may be, for example from 1% to 60% by weight of the mixture, more typically from 10% to 60% by weight of the mixture.

In one embodiment of the process, the mixture of RAP and aggregates contains from 25% to 60% by weight of the RAP.

The dryer and asphalt plant of the invention can be constructed from new or they can be constructed by modifying existing dryers and plants. Thus, for example, a dryer can be retro-fitted with a flame shield to separate the combustion zone and the drying zone and with shield members for screening the lifting elements in the combustion zone from the flame.

Accordingly, in a further aspect, there is provided a method of manufacturing a dryer for drying aggregates and processing reclaimed asphalt pavement (RAP) wherein: the drying chamber has an interior comprising a drying zone and a combustion zone; the drying chamber has an aggregate inlet for receiving aggregates and the RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and the RAP, and an exhaust outlet; the drying zone has an interior surface which is configured to lift the aggregates and RAP as the drying chamber rotates and tumble the aggregates and the RAP; and the combustion zone contains a plurality of lifting elements for lifting and tumbling the aggregates and RAP; characterised in that: a flame shield is fitted in the drying chamber so that it separates the combustion zone and the drying zone; the flame shield in use serving to prevent a flame from a burner from entering the drying zone but permitting hot gases generated by the flame to pass around the flame shield and into the drying zone; and in that a plurality of spaced apart shield members are mounted in the combustion zone of the drying chamber, the spaced apart shield members in use serving to screen the lifting elements in the combustion zone from the flame, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and fitted so that together they form a cascade path down which aggregates and RAP falling out of the lifting elements can tumble to a bottom of the drying chamber without contacting the flame.

As an alternative to filling an existing dryer with the flame shield and shield members.

an existing asphalt plant can be converted to give an asphalt plant according to the invention by replacing an existing rotary dryer drum with a rotary dryer drum containing a flame shield and shield members.

Accordingly, in another aspect, the invention provides a rotary dryer drum comprising: a drying chamber having an interior comprising a drying zone and a combustion zone; the drying chamber having an aggregate inlet for receiving aggregates and RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and RAP, and an exhaust outlet; the drying zone having an interior surface which is configured to lift the aggregates and RAP as the drying chamber rotates and tumble the aggregates and RAP; and the combustion zone containing a plurality of lifting elements for lifting and tumbling the aggregates and RAP; characterised in that: the combustion zone and the drying zone are separated by a flame shield; a plurality of spaced apart shield members are mounted on an internal wall of the drying chamber in the combustion zone, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and arranged so that together they form a cascade path down which, in use, aggregates and RAP falling out of the lifting elements can tumble to a bottom of the drying chamber.

Also provided by the invention is a conversion kit for converting a dryer to give a dryer according to the invention, the kit comprising a flame shield and one or more supports for mounting the flame shield in a drying chamber of a dryer; and further comprising a plurality of shield members and mounting elements for mounting the shield members in the combustion zone of a dryer so that they form a protective screen for lifting elements in the combustion zone.

The invention further provides a flame shield configured to be fitted into a dryer of the invention, wherein the flame shield is as defined and described herein.

Still further, the invention provides a shield member configured to be fitted into a dryer of the invention, the shield member being as defined and described herein.

The invention will now be described in more detail, but not limited, by reference to the specific embodiments illustrated in the accompanying drawings Figures 1 to 9.

Brief Description of the Drawings

Figure 1 is a schematic view of the layout of a known type of asphalt manufacturing plant in current use.

Figure 2 is a schematic view of the layout of an asphalt manufacturing plant according to one embodiment of the invention.

Figure 3 is an enlarged partially sectioned side view showing the burner and drying chamber of the asphalt manufacturing plant of Figure 2.

Figure 4 is a partially sectioned side view showing the combustion zone of the dryer of Figure 3.

Figure 5 is a view from direction D in Figure 4 of the interior of the combustion zone in the dryer of Figure 3.

Figure 6 is a side view of the flame shield of the dryer of Figures 3 to 5.

Figure 7 is a view from direction D in Figure 4 of the flame shield of Figure 6.

Figure 8 is a partially sectioned side view showing the flame shield mounted in the drying chamber.

Figure BA is an end view of one of the elongate shield plates lining the combustion chamber as shown in Figures 4 and 5.

S Figure 8B is a plan view of the elongate shield plate of Figure 8A.

Figure BC is a side view of the elongate shield plate of Figure BA.

Figure 9 is a schematic side view showing the gas circulation in the apparatus of Figures 2 to 8C.

Detailed Description of the Invention

Figure lisa schematic illustration of an asphalt manufacturing plant of known type which is equipped with a single dryer and uses a standard continuous mixer.

The asphalt manufacturing plant of Figure 1 comprises a dryer 2, a mixer 4, a storage and conveying system 6 for conveying virgin aggregates to the dryer, a storage and conveyor system B for conveying RAP to the mixer and an exhaust extraction system 10 for collecting and filtering exhaust gases prior to release into the atmosphere.

The dryer 2 comprises a rotating drum 12 and a burner 14 mounted at one end of the drum. The interior of the drum 12 is divided into a "combustion zone" 16 and a "drying zone" 18. At the drying zone end of the drum are an aggregate inlet 20 and an exhaust gas outlet 22. At the combustion zone end of the drum is an aggregate discharge outlet 24 for discharging dried aggregates into a chute 26 leading to an inlet 28 of the mixer 4.

The inner surface of the drying zone 18 of the drum is lined with an array of protrusions known as linger lifters". The finger lifters, which are of conventional construction and do not need to be described in detail here, are configured to enable them to collect aggregates when the protrusions or fingers are at their lowest point of rotation of the drum, lift the aggregates as the drum rotates and then release them after the drum has moved through about 130° to 2300 so that the aggregates fall back to the lowest point of the drum.

The inner surface of the combustion zone of the drum 12 is lined with an array of box lifters 32 which take the form of elongate axially extending box section scoops having one side of the box (the side facing in the direction of rotation of the drum) open so that it can scoop up aggregates and carry them around as the drum rotates. The geometry of the box lifters is such that the aggregates do not fall out and back to the floor of the drum until after the drum has rotated through about 2400 (i.e. has reached "2 oclock").

The exhaust gas outlet 22 of the drum 12 is connected to a primary exhaust duct 34 which leads to a bag filtei 36 and then, via extiaction fan 38 and discharge chimney 40, to the atmosphere.

The burner 14 is typically set up to burn fuel oil, liquid petroleum gas or natural gas and, when in operation, directs an elongate flame into the combustion zone of the drum 12. Heated gases produced by the flame are drawn along the interior of the drum and out through the exhaust gas outlet 22 by the suction power of the fan 38.

Adjacent the aggregate inlet 20 at the drying zone end of the dium is the storage and conveying system 6 for conveying virgin aggregates to the dryer. The storage and conveying system 6 comprises storage bins or hoppers 42 which discharge aggregates onto a conveyer belt 44 and then via a transfer conveyer 46, and conveyer belt 48 to the aggregate inlet 20. Aggregates enteling the inlet 20 are tumbled by the rotating drum 12 so that a cascade or "curtain' of falling aggregates is created in the drying zone of the drum, the aggregates being dried as they fall through the stream of hot gasses produced by the burner 14. The rotational axis of the drum is inclined ala small angle (e.g. about 3-5°) downwards towards the burner end of the drum so that the drying aggregates gradually migrate along the drum towards the buinei end and the aggregate discharge outlet 24. As the aggregates pass from the drying zone 18 to the combustion zone 16, they are scooped up and carried around the drum by the box lifteis 32. Howevei, because the box lifters do not ielease the aggiegates until after they have passed through the high point of travel, i.e. until they have moved from the low point (6 oclock") through about 240° (to "2 oclock"), the greater proportion of the aggregates do not fall through the centre of the burner flame but instead fall through the outer regions of the flame oi to the side of the flame. When the aggiegates reach the burner end of the drum, they are discharged through the aggregate discharge outlet 24 and down the chute 26 leading to the inlet 28 of the mixer 4.

The storage and conveyor system 8 for conveying RAP to the mixer comprises one or more storage bins or hoppers 49 which discharge required amounts of RAP onto a conveyer belt 50 which conveys the RAP to the RAP inlet 52 in the chute 26 fiom where it falls through inlet 28 and into the mixer. Molten bitumen is introduced into the mixer via pipe 54 and motorised valve 56.

The mixer 4 comprises a mixing chamber 58 and a plurality of paddles 60 mounted on drive shafts 62 driven by motors 64. At the far end of the mixer (i.e. the end remote from the inlet 28 and the molten bitumen inlet) is an asphalt discharge outlet 66 through which hot asphalt is discharged to waiting lorries or to heated storage containers. On the upper side of the mixing chamber 58 are gas outlets 68. The gas outlets 68 lead into the secondary exhaust duct ("nuisance duct") which joins the primary exhaust duct 34. Thus, gases from the mixing chamber are extracted and directed through the bag filter 36 before venting to atmosphere through chimney 40.

The plant illustrated in Figure 1 suffers from a number of disadvantages. Firstly, such plants have very poor energy efficiency as a result of the heat lost through the exhaust gases from the dryer being vented directly to atmosphere. The extraction and venting to atmosphere of hot gases from the mixerfurther adds to the energy inefficiency.

Whilst energy could in theory be saved by reducing the temperature of the drying gases in the dryer and heating the aggregates to a lower temperature, this is made difficult because the RAP is introduced into the mixer in an undried moisture-laden and cold state. Therefore, the temperature of the aggregates emerging from the dryer must be high enough to remove water from the RAP and heat the RAP up to a temperature suitable for efficient coating with the molten bitumen.

A further disadvantage with plant of the type shown in Figure 1 is that the asphalt mixes produced by the plant can only contain relatively small proportions of RAP (up to about 25%).

Previous attempts to overcome the problems arising from the use of cold undried RAP in asphalt production have been made by using two dryers, one for the RAP and one for the virgin aggregates. However, as indicated in the introductory part of this application, such systems also suffer from energy inefficiency and, moreover, the RAP dryers require frequent cleaning to remove bituminous deposits.

It would be beneficial if the virgin aggregates and RAP would be dried and pre-mixed before entering the mixer and being mixed with bitumen. Whilst EP2202473 (Bernardi Impianti International SPA.) discloses a plant which purportedly does this, it has been found that plants of the type described in EP2202473 suffer from other disadvantages as described above, including unacceptably high carbon monoxide emissions and loss of bituminous materials through overheating in the dryer.

The asphalt manufacturing plant of the present invention overcomes or at least substantially alleviates the problems described above.

An asphalt manufacturing plant according to one embodiment of the invention is shown schematically in Figure 2.

The asphalt plant differs from the known type of plant illustrated in Figure 1 in several respects, including: (A) the arrangement of the secondary exhaust ducts removing gases from the mixer; and (B) the facility for controlled diversion of hot gas from the burner end of the drying chamber to the primary exhaust duct; (C) the manner in which the virgin aggregates and the RAP are fed into the dryer; (D) the interior layout of the dryer drum.

In Figures 2 to 9, those features which are equivalent or analogous to features found in the known apparatus illustrated in Figure 1 share the same final numeral(s). For example, the extractor fan 138 is equivalent or analogous to extractor 38 in Figure 1.

The asphalt plant of Figures 2 to 9 comprises a dryer 102, a mixer 104, a storage and conveying system 106 for conveying virgin aggregates to the dryer, a storage and conveyor system 108 for conveying RAP towards the mixer and an exhaust extraction system 110 for collecting and filtering exhaust gases prior to release into the atmosphere.

The dryer 102 comprises a rotating drum 112 and a burner 114 mounted at one end of the drum. The interior of the drum 112 is divided into a "combustion zone" 116 and a "drying zone" 118. At the drying zone end of the drum are an aggregate/RAP inlet 120 and an exhaust gas outlet 122. At the combustion zone end of the drum is an aggregate discharge outlet 124 for discharging dried aggregates and RAP into a chute 126 leading to an inlet 128 of the mixer 104.

The inner surface of the drying zone 118 of the drum is lined with an array of protrusions 130 ("finger lifters"). The finger lifters are configured to enable them to collect aggregates/RAP when the protrusions or fingers are at their lowest point of rotation of the drum, lift the aggregates/RAP as the drum rotates and then release them after the drum has moved through about 130° to 230° so that the aggregates/RAP fall back to the lowest point of the drum.

The inner surface of the combustion zone of the drum 112 is lined with an array of box lifters 132 which are constructed in an analogous manner to the box lifters described in relation to Figure 1. In addition to the box lifters, the combustion zone is provided with an array of elongate shield plates 172, the structure and manner of mounting of which can be seen in more detail in Figures 5 and BA to 8C. In the embodiment illustrated, there are sixteen shield plates but it will be appreciated that there may be fewer or more than sixteen. The shield plates 172 are spaced apart and there are narrow gaps 173 between neighbouring plates which allow circulation of hot gases.

The shield plates 172 are formed from stainless steel or other refractory materials, and are mounted on brackets 174 welded or bolted to the inner surface of the drum 112 between the box lifters 132. One shield plate 172 is mounted between each pair of circumferentially adjacent box lifters 132. The shield plates are not mounted perpendicularly to the mounting brackets 174 but instead are mounted at a slight angle, the leading edge 172A (with reference to the anticlockwise direction of rotation) being closer to the wall of the drum than the trailing edge 1 72B for reasons that will become apparent below. The edges 1 72A and 1 72B of the shield plates are turned outwardly in the direction of the wall of the drum.

At the boundary between the drying zone 118 and the combustion zone 116 is mounted a generally conical flame shield 176 (referred to hereinafter for convenience as "the flame deflector cone"). The flame deflector cone 176 is secured to the wall of the wall of the drum 112 by legs 178. The flame deflector cone is not a true cone but is actually a dodecagonal pyramid formed by triangular stainless steel panels 1 76A. The panels can be formed separately and then fixed (e.g. welded) together, or they can be formed by folding one or more appropriately shaped sheets of stainless steel and then joining (e.g. welding) the edges of the folded sheet(s) to form the pyramid. The outer edge of the flame deflector cone is reinforced by a band 176B of stainless steel and the joints at the peak of the cone are covered by a stainless steel cap 1 76C. The centre of the flame deflector cone is aligned with the rotational axis of the drum 112.

As with the apparatus of Figure 1, the exhaust gas outlet 122 of the drum 112 is connected to a primary exhaust duct 134 which leads to a bag filter 136 and then, via extraction fan 138 and discharge chimney 140, to the atmosphere.

At the burner end of the drum 112 are an aggregate discharge outlet 124, a recycled gas inlet 180 and a gas-flow bypass outlet 182. The aggregate discharge outlet 124 is linked to chute 126 which leads to the inlet 128 of the mixer 104. The recycled gas inlet is linked to secondary exhaust duct 183 which connects with the gas outlets 168 of the mixing chamber 158. In contrast to the apparatus of Figure 1, the secondary exhaust duct ("nuisance duct") 183 of the apparatus of the invention does not join the primary exhaust duct 134. Instead hot gases from the mixing chamber are extracted and fed back onto the drying chamber.

The gas-flow bypass outlet 182 is connected to the gas-flow bypass duct 184 which in turn is connected to the primary exhaust duct 134. Rotating gate valves 186 control the flow of gas along the gas-flow bypass duct.

Adjacent the aggregate inlet 120 at the drying zone end of the drum is the storage and conveying system 106 for conveying virgin aggregates to the dryer. The storage and conveying system 106 comprises storage bins or hoppers 142 which discharge aggregates onto a conveyer belt 144 and then a transfer conveyor 146, and conveyer belt 148 to the aggregate inlet 120. The storage and conveyor system 108 for conveying RAP towards the mixer comprises one or more storage bins or hoppers 148 which deposits required amounts of RAP onto the conveyer belt 150 which links with the conveyer 146 and conveyer belt 148 to carry the RAP to the aggregate inlet 120.

The mixture of aggregates and RAP entering the inlet 120 is tumbled by the rotating drum 112 so that a cascade or "curtain" of falling aggregates and RAP is created in the drying zone of the drum, the aggregates and RAP being dried as they fall through the stream of hot gasses produced by the burner 114. As with the dryer of Figure 1, the rotational axis of the drum is inclined at a small angle (e.g. about (3-5°) downwards towards the burner end of the drum so that the drying aggregates gradually migrate along the drum towards the combustion zone 116.

In general, the burner is set up so that the leading edge of the flame is just in front of the flame deflector cone 176. Hot gases produced by the flame are diverted around the edges of the deflector cone through annular gap 177. The deflector cone prevents the flame from coming into contact with the aggregates in the drying zone 118 in the event that the flame length is set incorrectly. The directions of flow of the various streams of gas in the dryer are shown in Figure 9.

In the combustion zone, the aggregates are scooped up and tumbled by the box lifters as with the apparatus of Figure 1. However, contact between the aggregates and the burner flame in the combustion zone is prevented by the shield plates 172.

The shield plates 172 perform three functions. Firstly, they shield the aggregates/RAP from contact with the bumer flame as the aggregates/RAP tumble out of the box lifters and fall back to the floor of the drum. Secondly, they prevent aggregates/RAP from falling through the flame. Thirdly they reduce overheating of the combustion zone box lifters from radiated heat. For a drum set up to rotate in an anticlockwise fashion (viewed from the burner end) as shown in FigureS, the aggregates/RAP typically begin falling out of the box lifters when they reach the 100 clock to 11 o clock position (point A in Figure 5). In the absence of the shields 172, a quantity of the aggregates/RAP would fall directly back to the bottom of the drum and, in doing so, would pass through the flame. This would result in the loss of bitumen from the RAP through combustion and evaporation, and the formation of gases containing volatile bitumen components and bitumen combustion products. With the shields 172 in place, aggregates/RAP falling out of a box lifter will fall onto the nearest shield rather than into the flame. The angle of the shield ensures that the aggregates/RAP falling onto the shield will be deflected onto the neighbouring shield and then the next neighbouring shield and so on until the aggregates/RAP reach the bottom of the drum. Thus, the shields provide a cascade for the aggregates/RAP to pass along so that they do not fall directly to the boftom of the drum. In this way, contact between the flame and the aggregates/RAP is avoided thereby minimising the loss of bitumen from the RAP and minimising the formation of pollutant gases.

The combination of the flame deflector cone and the shield plates enables the aggregates/RAP passing through the drying drum to be thoroughly dried and heated but without being exposed directly to the flame. Therefore, loss of bituminous components of the RAP is minimised or prevented. Consequently, much higher loadings of RAP asphalt can be processed than has been the case hitherto. For example, in tests carried out by the applicant, asphalt compositions containing in excess of 50% RAP have been successtully manufactured using the apparatus of Figure 2.

The arrangement of the shields in Figure 5 is set up for a drum which rotates in an anticlockwise direction (viewed from the burner end). If the drum is to be rotated in a clockwise direction, the angle of the shields is reversed so that the leading edge of the shield in the clockwise direction is closer to the wall of the drum than the trailing edge of the shield.

Once the aggregates/RAP have reached the burner end of the drum, they are discharged through the aggregate discharge outlet 124 from which they pass down the chute 126 and through inlet 128 into the mixer 104. There they are mixed with molten bitumen which is introduced into the mixer via pipe 154 and motorised valve 156. The mixer 104 comprises a mixing chamber 158 and a plurality of paddles 160 mounted on a drive shaft or shafts 162 which is driven by motor or motors 164. At the far end of the mixer (i.e. the end remote from the inlet 128 and the molten bitumen inlet) is an asphalt discharge outlet 166 through which hot asphalt is discharged to waiting lorries orto heated storage containers.

On the upper side of the mixing chamber 158 are gas outlets 168 which lead into the secondary exhaust duct ("nuisance duct") 183 which leads to the recycled gas inlet adjacent the burner. Thus, hot gases from the mixing chamber are extracted and recycled into the drying chamber, thereby reducing energy wastage, in contrast to the known apparatus of Figure 1 where the hot gases from the mixing chamber are combined with the main exhaust gas flow and are vented to atmosphere A further distinguishing feature of the apparatus of the present invention compared to the apparatus of Figure 1 is the presence of the gas-flow bypass duct 184 which links the burner end of the drum interior via gas-flow bypass outlet 182 with the primary exhaust duct 134. In general, much lower flame temperatures are used for drying RAP-rich aggregates than are conventionally used for drying virgin aggregates. Therefore the temperature of the exhaust gases entering the bag filter 136 will also be commensurately lower. One potential problem with cooler exhaust gases is that the temperature may sink below the dew point of the gas, in which case condensation of water vapour in the gas will take place in the bag filter leading to blockage of the filter.

In order to avoid this, sensors (not shown) mounted just upstream of the bag filter are used to monitor the temperature of the exhaust gas and, if it appears that the temperature is likely to sink below the dew point, the rotating gate valves 186 are actuated to divert a proportion of hot relatively dry air from the burner end of the dryer chamber along the gas-flow bypass duct and into the primary exhaust duct 134, thereby to raise the temperature of the exhaust gas and prevent it from falling below its dew point.

The apparatus of the invention has a number of advantages over known asphalt plants. Firstly, it can make be used to make asphalt compositions containing much higher RAP contents that has hitherto been possible. For example, an apparatus constructed as shown in Figures 2 to 9 has been used to manufacture asphalt containing in excess of 50% RAP. Secondly, the apparatus of the invention produces significantly reduced carbon monoxide emissions. Thirdly, the apparatus of the invention has significant advantages in terms of energy and material (e.g. bitumen and virgin aggregates) savings.

Although the dryers and apparatus of the invention are particularly advantageous when used to make asphalt compositions containing RAP, they may also be used to make asphalts containing no RAP and also so-called low energy (LEA) asphalts.

EQUivalents It will readily be apparent that numerous modifications and alteiations may be made to the specific embodiments of the invention described above without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application.

Claims

CLAIMSA dryer for drying aggregates and RAP wherein: the dryer comprises a burner and a rotating drying chamber; the drying chamber has an interior comprising a drying zone and a combustion zone; the drying chamber has an aggregate inlet for receiving aggregates and RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and RAP, and an exhaust outlet; the dryer is arranged so that aggregates/RAP entering the drying chamber are advanced from the aggregate inlet through the drying zone and combustion zone to the aggregate outlet; the burner is mounted adjacent one end of the drying chamber and produces a flame which extends into the combustion zone thereby generating hot gases for drying the aggregates/RAP; means are provided for drawing a stream of the hot gases along the drying chamber from the combustion zone through the drying zone and out through the exhaust outlet; the drying zone has an interior surface which is configured to lift the aggregates/RAP as the drying chamber rotates and tumble the aggregates/RAP through the stream of hot gases to dry the aggregates; and the combustion zone contains a plurality of lifting elements for lifting and tumbling the aggregates/RAP; characterised in that: the combustion zone and the drying zone are separated by a flame shield which prevents the flame from entering the drying zone but permits hot gases generated by the flame to pass around the flame shield and into the drying zone; the lifting elements in the combustion zone are screened from the flame by a plurality of spaced apart shield members each of which is mounted on an internal wall of the drying chamber, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and arranged so that together they form a cascade path down which aggregates and RAP falling out of the lifting elements can tumble to a bottom of the drying chamber without contacting the flame.
2. A dryer according to claim 1 wherein the flame shield is mounted coaxially with a rotational axis of the drying chamber.
3. A dryer according to claim 1 or claim 2 wherein the flame shield has a conical, pyramidal or domed shape.
4. A dryer according to claim 3 wherein the flame shield has a pyramidal shape and the pyramidal shape is a polygon-based pyramid in which the polygon has from eight to fourteen sides.
5. A dryer according to any one of the preceding claims wherein the flame shield is formed from a refractory material.
6. A dryer according to claim 5 wherein the refractory material is stainless steel.
7. A dryer according to any one of the preceding claims the shield members take the form of elongate plates, each extending in an axial direction along substantially the entire length of the combustion chamber.
8. A dryer according to any one of the preceding claims wherein each pair of adjacent shield members has a lifting element disposed therebetween.
9. A dryer according to any one of the preceding claims wherein the shield members are formed from a refractory material.
10. A dryer according to claim 9 wherein the refractory material is stainless steel.
11. A dryer substantially as described herein with reference to the accompanying drawings Figures 2 to 9.
12. An apparatus for manufacturing asphalt comprising a dryer as defined in any one of claims 1 to 11 a mixer linked to the dryer by a duct extending from the aggregate outlet of the dryer to the mixer; an exhaust gas extraction and filtering system linked to the exhaust gas outlet of the dryer; a supply of bitumen linked to the mixer; and means for supplying aggregate to the dryer.
13. An apparatus according to claim 12 wherein a duct is provided to direct hot gases from the mixer to the drying chamber; wherein the duct opens into the drying chamber at the burner end thereof.
14. An apparatus according to claim 12 or claim 13 wherein means are provided for monitoring the temperature of exhaust gas downstream of the exhaust gas outlet of the dryer and means for boosting the temperature of the exhaust gas if it falls below a predefined value.
15. An apparatus according to claim 14 wherein the means for boosting the temperature of the exhaust gas comprises a by-pass duct leading from the burner end of the drying chamber to a primary exhaust duct downstream of the exhaust outlet of the dryer, wherein the bypass duct channels heated gas from the burner end of the dryer into the primary exhaust duct.
16. An apparatus according to claim 15 wherein one or more control valves or gates are provided in the bypass duct to control the flow of heated gas into the duct, the control valves or gates being operable to permit heated gas to be diverted down the bypass duct if the temperature of the exhaust gas falls below a predefined value.
17. An apparatus for manufacturing asphalt substantially as described herein with reterence to the accompanying drawings Figures 2 to 9.
18. A process for manufacturing an asphalt composition containing recycled asphalt pavement (RAP), which process comprises using an apparatus as defined in any one of claims 12 to 17, introducing into the aggregate inlet of the dryer of the said apparatus aggregates and recycled asphalt pavement (RAP); drying and heating the aggregates and RAP in the dryer; discharging a resulting mixture of the dried and heated aggregates and RAP through the aggregate outlet of the dryer to a mixer; adding bitumen to the mixture of aggregates and RAP and mixing to form the asphalt composition.
19. A process according to claim 18 wherein the mixture of RAP and aggregates contains from 25% to 60% by weight of the RAP.
20. A method of manufacturing a dryerfor drying aggregates and processing reclaimed asphalt pavement (RAP) wherein: the drying chamber has an interior comprising a drying zone and a combustion zone; the drying chamber has an aggregate inlet for receiving aggregates and the RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and the RAP, and an exhaust outlet; the drying zone has an interior surface which is configured to lift the aggregates/RAP as the drying chamber rotates and tumble the aggregates and the RAP; and the combustion zone contains a plurality of lifting elements for lifting and tumbling the aggregates/RAP; characterised in that: a flame shield is fitted in the drying chamber so that it separates the combustion zone and the drying zone; the flame shield in use serving to prevent a flame from a burner from entering the drying zone but permitting hot gases generated by the flame to pass around the flame shield and into the drying zone; and in that a plurality of spaced apart shield members are mounted in the combustion zone of the drying chamber, the spaced apart shield members in use serving to screen the lifting elements in the combustion zone from the flame, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and fitted so that together they form a cascade path down which aggregates and RAP falling out of the lifting elements can tumble to a bottom of the drying chamber without contacting the flame.
21. A rotary dryer drum comprising: a drying chamber having an interior comprising a drying zone and a combustion zone; the drying chamber having an aggregate inlet for receiving aggregates and RAP requiring drying, an aggregate outlet for discharging dried and heated aggregates and RAP, and an exhaust outlet; the drying zone having an interior surface which is configured to lift the aggregates and RAP as the drying chamber rotates and tumble the aggregates and RAP; and the combustion zone containing a plurality of lifting elements for lifting and tumbling the aggregates and RAP; characterised in that: the combustion zone and the drying zone are separated by a flame shield; a plurality of spaced apart shield members are mounted on an internal wall of the drying chamber in the combustion zone, the shield members forming a protective screen radially inwardly of the lifting elements; and the shield members are configured and arranged so that together they form a cascade path down which, in use, aggregates and RAP tailing out of the lifting elements can tumble to a bottom of the drying chamber.
22. A conversion kit for converting a dryer to give a dryer as defined in any one of claims 1 to 11, the kit comprising a flame shield and one or more supports for mounting the flame shield in a drying chamber of the dryer; and further comprising a plurality of shield members and mounting elements for mounting the shield members in the combustion zone of the dryer so that they form a protective screen for lifting elements in the combustion zone.