Agitating – Mortar mixer type – Methods
Reexamination Certificate
1999-11-26
2001-03-06
Soohoo, Tony G. (Department: 1723)
Agitating
Mortar mixer type
Methods
C366S022000, C366S025000, C366S012000, C034S131000
Reexamination Certificate
active
06196710
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a machine for manufacturing asphalt paving compositions, and more specifically, to a dust return/distribution system for such a machine. In a preferred embodiment of the invention, a dust return/distribution system is provided for a continuous-process hot-mix asphalt plant having a rotary drum dryer and a concentric fixed shell mixer.
BACKGROUND OF THE INVENTION
Machines for the manufacture of asphalt paving compositions are well-known. Hot-mix asphalt plants, for example, include a means for heating and drying aggregate and a means for mixing the heated and dried aggregate together with asphalt cement or liquid asphalt to form a paving composition.
Hot-mix asphalt plants typically include a rotating aggregate dryer in which aggregate is heated and dried before being mixed with asphalt cement, an asphalt cement supply system and a mixing tower or chamber where the aggregate and asphalt cement are mixed together. The dryer of this apparatus is typically oriented so as to have an upper end and a lower end, so that aggregate which is introduced into the dryer at the upper end will move towards the lower end by gravity flow and by the action of a series of flights mounted on the interior surface of the rotating dryer. A burner is located at the lower end of the dryer, and the hot air and exhaust gases from the burner move toward the upper end of the dryer, against the flow of the aggregate. As the aggregate is tumbled through the exhaust gases, the material is heated and dried. The hot, dry aggregate is discharged from the dryer at the lower end and introduced to a mixing tower or chamber where it is combined with asphalt cement or another binding material to form an asphalt paving composition or asphalt concrete mixture.
As the aggregate is tumbled and dried in the dryer, a quantity of dust is typically created and carried upwardly by the hot gases of combustion. Because of particulate emission regulations, it is unacceptable to exhaust the dust to the atmosphere. Furthermore, depending on the speed of rotation and the temperature at which the dryer is operated, the quantity of dust may represent a significant percentage of the fine aggregate material needed in the particular mix. Therefore, dust collection or recovery systems such as baghouses and cyclone separators are known for removal of the dust from the gas stream before further processing of the combustion gases and/or exhaustion to the atmosphere. The dust which is recovered in the dust recovery system may then be introduced to the mixing tower or chamber for inclusion in the asphalt mix. This introduction of recovered dust has generally been made through a single inlet which deposits the dust at a single point in the mixing chamber. While this single inlet method of dust introduction accomplishes the primary task of return of the dust to the mixing chamber, we have learned that the introduction of dust at more than one point in the mixing chamber allows the dust to be more readily combined with, and more evenly distributed throughout, the asphalt mix than if the dust is introduced at a single point in the mixing chamber.
Accordingly, there is a need to provide an improved apparatus for manufacturing asphalt paving compositions which may avoid or minimize the mixing problems that result from the introduction of recovered dust at a single point in the mixing chamber.
ADVANTAGES OF THE INVENTION
Among the advantages of the present invention is its provision of an improved apparatus for manufacturing asphalt paving compositions which minimizes the problems caused by the introduction of the recovered dust into the mixing chamber at a single point. This improvement is provided by introducing and dispersing the recovered aggregate dust over a substantial portion of the area of the mixing chamber. Another advantage of the invention is that the mixing effort required to achieve a thorough blending of the dust into the mixture is reduced by dispersing the dust over a substantial portion of the area of the mixing chamber.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art to which the invention relates from an examination of the drawings and the ensuing description.
EXPLANATION OF TECHNICAL TERMS
As used herein, the terms asphalt paving composition, asphalt paving material and asphalt concrete mixture refer to mixtures of various aggregates, including crushed stone, sand, lime, dust and the like, with asphalt cement or asphalt binder, which mixtures are prepared for paving purposes. Asphalt paving materials that are prepared or manufactured in a process employing heat may be referred to as hot-mix asphalt, and the equipment that is used to make hot-mix asphalt may be referred to as a hot-mix asphalt plant.
As used herein, the terms asphalt cement and asphalt binder refer to the binding material which is added to the aggregate and other materials in the mixing chamber of a hot-mix or other asphalt plant to form asphalt paving material.
As used herein, the term mixture refers to the combination of materials, including at least some fluid or partially fluid materials, in the mixing chamber of a machine or plant for the manufactured or production of asphalt paving material.
As used herein, the term mixing chamber refers to the portion of the asphalt manufacturing plant or equipment where the dried aggregate and asphalt cement are mixed together.
As used herein, the term dust refers to the aggregate fines and particulates which are entrained in the (gases of combustion as the aggregate is dried in a hot-mix asphalt plant.
SUMMARY OF THE INVENTION
We have observed that the quality of an asphalt paving composition can be improved by lowering the concentration of recovered dust which is introduced to the mixture at any single point. While metering the flow of the recovered dust which is introduced at a single point would appear to offer a solution, the large amount of dust which is typically recovered makes this impractical. Consequently, the present invention provides an apparatus for making asphalt paving, material which includes a heating/drying chamber and a mixing chamber. The heating/drying chamber includes an inlet for aggregate material, a burner that is adapted to heat and dry aggregate material in the heating/drying chamber, a first outlet for gases of combustion and aggregate dust and a second outlet for dried and heated aggregate material.
The mixing chamber includes a first inlet for heated and dried aggregate material, a second inlet for asphalt cement, and a third inlet for aggregate dust which is adapted to distribute the aggregate dust over a substantial portion of the area of the mixing chamber. The mixing chamber also includes means for mixing heated and dried aggregate material and aggregate dust with asphalt cement within the mixing chamber and an outlet for asphalt paving material.
The apparatus also includes means for conveying heated and dried aggregate material from the outlet of the heating/drying chamber to the first inlet of the mixing chamber, means for collecting aggregate dust exiting from the first outlet of the heating/drying chamber and means for conveying this aggregate dust to the third inlet of the mixing chamber.
The invention also includes a method for making asphalt paving materials using the apparatus described above. According to this method, aggregate is introduced into the inlet of the heating/drying chamber, where it is heated and dried. The heated and dried aggregate is conveyed to the first inlet of the mixing chamber and introduced into the mixing chamber through such inlet. Asphalt cement is introduced into the mixing chamber through the second inlet. Dust generated by the motion and the drying of the aggregate is collected as it exits from the first outlet of the heating/drying chamber and conveyed to the third inlet of the mixing chamber and distributed throughout the mixing chamber. The aggregate, dust, and asphalt cement are mixed in the mixing chamber to form
Keylon Gary R.
Swanson Malcolm L.
Astec Industries Inc.
Chambliss Bahner & Stophel P.C.
Soohoo Tony G.
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