Multiple drum mixing system

Details Multiple drum mixing system Multiple drum mixing system

1999-11-10

2002-04-02

Wilson, Pamela (Department: 3749)

Drying and gas or vapor contact with solids

Apparatus

Rotary drums or receptacles

C034S218000, C034S231000, C110S226000, C366S024000, C366S054000, C432S106000

Reexamination Certificate

active

06363625

ABSTRACT:

This application is a national stage application, according to Chapter I of the Patent Cooperation Treaty. This application claims the priority date of Mar. 10, 1998 of Canadian Patent Application No. 2,231,693 and Jul. 10, 1998 of Canadian Patent Application No. 2,243,132.
This invention deals with the field of mixing plants and in particular such a plant that employs an inner and an outer drum rotating together.
BACKGROUND TO THE INVENTION
Various situations arise in industry where a material is dried and then mixed with another material. One example is where aggregate is dried and then mixed with cement for bagging as concrete mix. Another is where aggregate is dried and then mixed with liquid asphalt to produce an asphalt mixture for paving roads.
An asphalt plant is used to prepare the asphalt mix. Conventionally such plants comprise a is rotating drum set up at an angle to the horizontal. A burner and fan are located adjacent to the high end directing a flow of hot air into the high end of the drum. Aggregate is conveyed into the high end, and is carried towards the low end by gravity and the rotation of the drum which has impellers on its inner surface. The aggregate is tumbled in the hot air flow by the rotation of the drum and the impellers lifting the aggregate and dropping it into the air stream, essentially drying and heating the aggregate.
Approximately two thirds of the way down the drum, a flow of hot liquid asphalt is introduced to the aggregate. The impellers on the drum carry the aggregate and liquid asphalt towards the top of the drum, assisting the mixing process. At the end of the drum, the finished mix is discharged onto a conveyor by paddles. The hot air is exhausted from the lower end of the drum into a filter to remove the considerable amount of dust that is carried in the exhaust air.
A significant problem with such systems is that the fine material carried out with the air not only causes pollution, but the fines are also lost from the mix. These fines are material that has been conveyed into the mixer and dried. Any such material that does not go into the mix is a loss, and must be replaced with further aggregate material, at added cost.
A second significant problem with conventional drum mixers is that the liquid asphalt is exposed to very high temperatures in the drum which causes burning of the asphalt and degrades the quality of the finished product.
Prior U.S. Pat. No. 4,600,379 to Elliot is directed to a mixer with concentric dual drums. In the Elliot device, the dual drums are further contained in a shroud to capture unburned vapours from the exhaust air for re-burning. The Elliot device provides some mechanism for removing the dust from the exhaust air for retention in the mix and removes the flow of asphalt from the direct heat of the burner. In the Elliot device the hot air is removed directly from the inner drum, requiring a further housing around the outside drum to contain the air stream and attempt to expand it to precipitate the fine material.
Since the aggregate must move in both directions in the Elliot device the drums must be kept horizontal, and that device provides only impellers to actually move the aggregate through the device, requiring aggressive impellers leading to excessive wear and possible plugging. Similar limitations apply to the device of U.S. Pat. No. 4,262,429 to Avril. It would be desirable to provide a mixer which would allow gravity to aid in moving the material through the device.
Some studies show that mixing the hot aggregate with a small amount of water before the liquid asphalt is added will also improve the quality of an asphalt mix. After heating, the surface of the aggregate rock is very dry and the addition of a small amount of water to the surface of the rock helps the liquid asphalt to adhere to and penetrate the rock.
Usually an asphalt plant is set up near where the road paving is needed, reducing the distance that the finished product must be hauled. As the asphalt must be laid on the road at a certain minimum temperature, trucking distances from the plant must be kept to a minimum. It is desirable to have a mixing plant that lends itself readily to mounting on a trailer for easy movement.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a drying and mixing plant that captures the fine aggregate material usually carried out of the plant by the exhaust air, for use and retention in the material mix, thereby also reducing the filtration capacity needed to remove these fines from the air.
It is a further object of the present invention to provide such a drying and mixing plant that provides an improved mix for certain materials, such as an asphalt/aggregate mix, by moving the second material input “around the corner” from the burner, reducing burning of the second material.
It is a further object of the present invention to provide such a drying and mixing plant that provides an improved mix for certain materials, such as an asphalt/aggregate mix, by wetting the aggregate before it is mixed with the asphalt.
It is a further object of the present invention to provide such a drying and mixing plant that provides for separate introduction of a third material into the drying and mixing plant.
It is a further object of the present invention to provide an asphalt mixing plant that provides an improved asphalt mix by moving the asphalt injection nozzle “around the corner” from the burner, reducing burning of the asphalt or by wetting the aggregate before it is mixed with the asphalt or by a combination of both.
The invention accomplishes these objects providing in one aspect a drying and mixing plant for mixing material with an additive comprising an inner drum having an input end for receiving material and an output end for discharging material, said inner drum being tapered such that the diameter of said input end is smaller than the diameter of said output end; an outer drum concentric to said inner drum and attached to said inner drum so as to rotate in concert with said inner drum, said attachment such that there is an open chamber between said inner and outer drums, the resulting inner and outer drum assembly rotatably mounted on a frame so that the axis of said drums is approximately horizontal; a drive mechanism adapted to rotate said drum assembly; said outer drum having an input end for receiving material from said output end of the inner drum, and an output end at the opposite end thereof for discharging said material, said outer drum being tapered such that the diameter of said input end is smaller than the diameter of said output end; a first shroud enclosing said input end of the outer drum; a second shroud operatively connected to said chamber at the output end of the outer drum, said second shroud directing exhaust air to an exhaust air duct, and said second shroud further directing the mixed material to a material outlet: a burner and blower directing a stream of hot air into said input end of the inner drum, said hot air thereby moving through said inner drum; thence through said chamber and thence through said exhaust air duct: an additive material input for introducing an additive material into said material after said material leaves said output end of the inner drum; impellers attached to the inner surfaces of said inner and outer drums for lifting said material towards the lop of said drums and dropping same into said air stream, and for moving said material through said drums.
For fluid additive materials, a fluid additive material input could include a plurality of fluid additive nozzles located in proximity to the entrance to the chamber, the nozzles oriented to direct a spray of fluid additive across a majority of the cross-section of the chamber.
To improve adhesion and penetration of certain additives into the material, for example asphalt into aggregate, a water nozzle could be incorporated into the mixing plant for spraying water onto the material prior to introducing the asphalt. Such spraying with water would also help to precipitate the fines fro

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