Drum mixer and method

The productive capacity of a drum heater and mixer for asphalt paving composition intended for processing recycled materials can be increased substantially by spraying a curtain of water vertically into the pathway of the raw material as it falls into the drum intake and before any substantial contacts with heated gases or flame and by spraying a curtain of water into the pathway of the heated gases and flame for controlling the temperature profile across the interior of the drum. The latter curtain of water is disposed at about thirty-five degrees to a plane perpendicular to the drum axis. Additional sprays of water downstream from the angular curtain of water may be supplied if necessary.

BACKGROUND OF THE INVENTION 
This invention relates to drum mixers and the like for aggregate and 
asphaltic materials such as may be used in connection with road surfacing, 
more particularly to such mixers for processing re-cycled road surfacing 
materials. This invention also relates to the method of processing of 
re-cycled road surfacing materials to the end of minimizing, if not 
eliminating, atmospheric pollution both of a particulate nature and of 
noxious fumes. 
Drum mixers and/or dryers are well known including ones for processing or 
re-cycling used bituminous or asphaltic materials from existing roads and 
highways. U.S. Pat. No. 4,039,171, Aug. 2, 1977 and U.S. Pat. No. 
4,143,972, Mar. 13, 1979 may be referred to in this connection. 
Such prior drum mixers include an elongated drum that is adapted to rotate 
and, when in use, is mounted at a downward angle from the intake to 
discharge so that the processed material will move therethrough under the 
force of gravity. The drum is mounted on a framework supported by wheels 
at one end and is adapted at its other end to be attached to a tractor for 
towing the apparatus to the point of use. 
At the intake end of the drum there is a source of high temperature gases 
which in the burning condition are blown into the drum and at this same 
end there is a conveyor arrangement for carrying the material to be heated 
into the drum. Essentially, at the intake end of the drum the raw 
materials which may be broken up pieces of old road surfacing, for 
example, come into contact with the flame and/or hot gases of the 
combustion process, are melted and carried through the drum to the 
discharge end for re-application to the road surface. Also at the intake 
area of the drum there are spirally arranged flights on the interior 
surface (at an angle to the drum axis) for conveying the raw material 
downstream. At the termination of the spiral, or angular, flights there 
are further mixing flights arranged parallel to the axis of the drum and 
cause the material the drum to tumble for mixing purposes. In this process 
additional asphalt, for example, may be added in order to make the desired 
end composition of the prescribed constituency. 
In the prior devices liquid asphalt has been provided at the intake end of 
the drum in order to mix with the fine materials entering thereat for 
preventing the fines from entering into the current of heated gases and 
being carried through the drum to the discharge end and thus avoiding 
particulate contamination of the atmosphere. However, this would 
ordinarily cause burning of the asphalt or other similar material at the 
intake and thus cause noxious fumes to be emitted at the discharge end. To 
avoid this the prior art devices have used various schemes for reducing 
the temperature gradient of the hot gases and flame across the interior of 
the drum. These schemes have included heat dispersion screens, or grids, 
as in U.S. Pat. No. 4,039,171 and a completely enclosed combustion chamber 
having air intake holes disposed along its length and a truncated cone 
exit having a series of holes through which the heated gases flowed as in 
U.S. Pat. No. 4,143,972. 
While these schemes may have solved the problems of controlling 
particulates and fumes into the atmosphere they, nevertheless, have the 
serious disadvantage of relatively low throughput or productive capacity. 
Various governmental agencies, such as the Boards of Health and the 
Environmental Protective Agency, determine the standards of operation of 
drum mixers of the nature involved in this application such that the 
amount of the particulate matter as well as noxious fumes introduced into 
the atmosphere is severely limited. 
The temperature gradient controlling schemes of the prior art, not only 
result in a relatively reduced productive capacity, but they are 
relatively difficult to maintain and are expensive in the first instance. 
Injecting water spray into the cone of flame in a combustion chamber also 
is known as in U.S. Pat. No. 3,748,080, July 24, 1973. Such arrangements 
have been complicated and have not been suggested for drum mixers in 
connection with the re-cycling of bituminous road paving materials. 
Accordingly, it is an object of the invention to provide an improved drum 
mixer of the nature discussed wherein re-cycled bituminous paving material 
may be processed at an increased productive capacity while at the same 
time minimizing the emission of particulate material and noxious fumes. 
It is a further object of the invention to provide an improved drum mixer 
of the character described which is simple in construction, efficient in 
operation and simple to maintain. 
Further advantages and objects of the invention will become apparent as the 
description proceeds. 
SUMMARY OF THE INVENTION 
In carrying out the invention according to one form there is provided in a 
bituminous paving composition mixer that includes a rotatable drum having 
a discharge end and an intake end, means for the introduction of raw 
materials into the intake end, gaseous heating apparatus disposed adjacent 
the intake end including means for blowing heated gases and flame into the 
intake end of the drum for melting the bituminous material, and means for 
controlling the temperature distribution of the flame and gases across the 
interior of the entrance of the drum during operation, the last named 
means comprising first apparatus for generating a series of water sprays 
at the entrance to the drum directed into the path of the flame and heated 
gases. Preferably, the spray consists of a series of relatively closely 
spaced streams of water directed into the pathway of the flame and gases 
from each side thereof and at an angle of about thirty-five degrees to a 
plane at right angles to the drum axis. The combination of sprays 
comprises essentially a water curtain. In the event that additional 
moisture is needed, one or more additional vertical pipes are disposed 
inside of the drum and include a series of horizontally directed holes 
through which water streams may flow into the pathway of the flame and 
gases. The development of undesired amounts of atmospheric contaminating 
noxious fumes is, accordingly, avoided. Additionally, for controlling the 
particulate material in the raw stock a futther conduit is disposed 
horizontally across the entrance of the drum and ahead of the water 
curtain and other sprays described so as to dampen and coagulate the 
particulate matter before it comes into contact with the flame and gases. 
These particulates, or fines, are, therefore, not caught up into the 
gaseous current and carried through the drum mixer. Particulate 
contamination of the atmosphere is, accordingly, avoided.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, there is shown a drum mixer 10 comprising a drum 
11 mounted on a frame 12 which is adapted to be supported by a series of 
wheels 13 at one end and may be adapted to be supported at its other end 
14 by the fifth wheel of a wheeled tractor for towing the drum to any 
point of use. 
The intake end 15 of the drum includes a fire wall 16 with an opening 
therethrough through which the flame and hot gases may enter the drum. The 
hot gases and flame are created by a burner 17 and blower 18 as is well 
understood in this art. The ignited fuel and air mixture from the burner 
17 pass through the ignition port 19 and thus into the drum interior. 
Disposed underneath the burner 17 is a conveyor belt 21 that carries the 
raw material such as gravel aggregate, or broken up parts of asphalt 
pavement which are to be re-cycled, into the intake of the drum, the 
aggregate or re-cycled asphalt pavement being designated by the reference 
character 22. The raw material 22 may be supplied to the input of the 
apparatus by a further supply conveyor shown by the reference character 
23. 
Adjacent the forward end of the drum 11 are a series of blades, or flights, 
24 that are in effect spirally formed on the interior of the drum surface. 
This is shown by the fact that the flights 24 are disposed at an angle as 
may be seen in FIG. 1. Because of the angular disposition of the flights 
24, the material 22 conveyed into the interior of the drum and falling on 
to its interior surface is picked up by the flights 24 and is moved 
through the interior of the drum to the point at which further mixing 
flights 25 are disposed. 
The exterior surface of the drum includes a pair of tires 27 and 28 that 
surround the circumference of the drum. The tires 27 and 28 rest on 
rollers 29 of which only one is shown. The roller 29 may be driven by a 
suitable motor 31 and gear box 32 so that the drum 11 may be rotated 
during the mixing process. 
For controlling the amount of particulate matter introduced into the heated 
gas current and for controlling the temperature across the interior 
surface of the drum to prevent combustion of the bituminous or asphaltic 
material, the water spray system of the invention is introduced into the 
input area of the drum. 
Vertically extending pipes 33 and 34 are disposed at the respective sides 
of the drum interior and are connected together by a common pipe 35 and 
are supplied from a supply pipe or conduit 36. Small holes 37 are formed 
in each of the vertical pipes 33 and 34 and are relatively closely spaced 
together. The axes of the holes 37 are disposed at an angle of about 
thirty-five degrees to a plane perpendicular to the axis of the drum. 
Thus, when water squirts out of holes 37 a curtain of water in effect, is 
formed whose sides are at thirty-five degrees as indicated. The sides of 
the water curtain are shown by the dotted lines 38. The combined streams 
of water 38 form, in effect, a water shield protecting the environment 
exterior thereto from the intense heat of the flame and heated gases 
formed by the burner. 
If additional water shielding is necessary, a further pair of water pipes 
39 and 41 are disposed in line with each other and further downstream with 
respect to the vertical pipe 34. The pipes 39 and 41 are disposed at only 
one side of the drum as may be seen in FIGS. 3 and 4. Each of the vertical 
pipes 39 and 41 include a series of small holes 42 from which water may 
squirt directly across the drum 11, that is to say in a direction 
perpendicular to the axis of the drum. The additional streams of water or 
water shields may be designated by the reference characters 43 and 44. 
Disposed between the conduit or pipe 35 and the intake of the drum is a 
further pipe or conduit 45 extending transversely of the drum and 
underneath the area where the material being conveyed into the drum by the 
conveyor 21 falls into the drum. The pipe 45 has a series of small holes 
46 disposed therein which holes are disposed with their axes vertically, 
that is to say that a stream of water coming from them will project 
vertically into the pathway of the falling raw material. In this manner 
any fine particles which may be in the form of dust, for example, come 
into contact with the water spray and are settled down into the interior 
of the drum for exposure to the temperature inside of the drum to be 
appropriately melted rather than to be picked up and swept along by the 
current of heated gases moving through the drum. Particulate contamination 
of the atmosphere is thus avoided. The pipe 45 is connected to a supply 
pipe 47 through which water is supplied. Appropriate valves, shown 
diagrammatically, are provided for all of the supply pipes as will be 
understood. 
It has been found that for best results the holes 37, 42 and 46 should be 
about one-sixteenth of an inch in diameter and should be supplied with 
water at about one hundred fifty pounds per square inch. If too much water 
is delivered, the capacity of the unit to deliver properly mixed materials 
is apt to be reduced. Water supplied at a rate of about thirty to fifty 
gallons per minute will enable the apparatus to deliver about three 
hundred to three hundred fifty tons of mixed aggregate per hour. 
Referring to FIG. 2, there is shown diagrammatically a temperature profile 
of the flame and heated gases in the interior of a conventional drum 
relative to the distance of the intake end of the drum. The temperature at 
the peak 48 which may exist at about the end of the flights 24, in the 
absence of the water curtain of the invention, can reach the vicinity of 
twenty-two hundred degrees Fahrenheit. Flame and gases at this 
temperature, of course, will burn small particles of asphalt or the like 
and cause heavy dense smoke to come out of the exhaust 49 of the drum. The 
emission of such heavy smoke is, of course, objectionable under 
governmental and other regulations and has to be prevented. When water is 
supplied under the pressure and volume as indicated above through the 
conduits 33, 34 and 35 and, if necessary, 39 and 41 the maximum 
temperature in the profile of temperature is much reduced. This is shown 
in FIG. 7 where the maximum temperature would be the greatest extent of 
the curve 51 which can be seen is of much lesser magnitude than the peak 
48 shown in FIG. 6. The maximum temperature with the water curtain 
apparatus of the invention functioning would be about 1200.degree. F. 
Under these conditions the particles of asphalt or the like would not burn 
and dense smoke would not emit from the exhaust 49. An experienced 
observer can, by observing the density of the exhaust at 49, conclude 
whether or not more or less water should be applied to the various pipes 
of the system as disclosed. 
It has also been observed that supplying too much water can reduce the 
capacity of the unit to produce appropriately mixed aggregate and asphalt. 
Having the water spray vertically through the openings 46 from pipe 45 into 
the pathway of the particles of asphalt or the like falling from the 
conveyor 21 into the interior of the drum 11, dampens these particles and 
causes them to collect in the interior of the drum as already described. 
There is no tendency for these fine particles or fines as they are known 
in the trade to accumulate on the conveyor belt, for example, which could 
necessitate frequent cleaning thereof as has been the case with some known 
systems. 
The piping system of the present invention can be easily retrofitted into 
existing drum mixer units. It is likewise less expensive than other known 
drum mixer systems in the first instance. 
The invention also enables drum mixer units to re-cycle one hundred percent 
of used material without having to add some additional new or virgin 
material at the very beginning. In those instances where additional 
asphalt or the like needs to be added, it may be added near the middle or 
toward the end of the drum mixer as through a conduit or pipe 51. 
Along with these advantages the production rate of units is dramatically 
increased from known capacities of about one hundred seventy-five tons per 
hour to three hundred to three hundred fifty tons per hour. 
While one form of the invention has been disclosed it will be understood 
that other modifications may be made that come within the spirit and scope 
of the disclosure.