Hi-recycle asphalt batch plant

Abstract

A method for producing an asphalt concrete blend that includes the steps of drying and heating aggregate in a single and only dryer, preferably via direct heating, and without use of a pre-dryer. The aggregate is then combined with asphalt cement, preferably via indirect heating, such that the blend has an asphalt cement components (ACC) percentage of at least 25% percent to form an aggregate mix. Liquid asphalt cement may also be added to the aggregate mix. Preferably, the asphalt cement content of the final asphalt concrete blend is provided by a maximum of 70% ACC. In certain cases, the asphalt cement content of the asphalt concrete blend is comprised of ACC and liquid asphalt cement.

Claims

1. A plant configured for the batch production of an asphalt concrete blend comprising a high percentage of asphalt cement components (ACC), the plant comprising: a single and only dryer for mixing and for carrying out all drying operations required in producing the asphalt concrete blend by the plant, the single and only dryer having: a drum; a burner directing a flame in a generally axial direction into a heating chamber within the drum; a mixing chamber in selective communication with the heating chamber; means for preventing aggregate from passing from the heating chamber to the mixing chamber in a first mode of operation and for enabling aggregate to pass from the heating chamber to the mixing chamber in a second mode of operation; a plurality of mixing paddles located within the mixing chamber that are sized and configured to mix ACC located within the mixing chamber; and an ACC chute through which ACC may be passed into the mixing chamber, wherein the plant is configured to produce an asphalt concrete blend having a high percentage of ACC.

2. The plant of claim 1 further comprising a frame that is sized and configured to mount the single and only dryer at ground level and at an angled orientation in order to provide an upper end and a lower end that is vertically lower than the upper end, wherein the ACC chute is located at the lower end of the single and only dryer and the aggregate chute is located at the upper end of the single and only dryer.

3. The plant of claim 1 wherein the plant further comprises: a slide gate located on the single and only dryer that is movable between a closed position, wherein the single and only dryer operates in the first mode of operation and virgin aggregate exits the single and only dryer after being dried and heated but without being mixed in the mixing chamber, and an open position, wherein the single and only dryer operates in the second mode of operation and aggregate passes from the inner drum through open slide gate and into mixing chamber.

4. The plant of claim 1 further comprising a source of liquid asphalt cement and a secondary mixer that is external to the single and only dryer that is configured to receive aggregate and ACC from the single and only dryer, to receive liquid asphalt cement from the liquid asphalt cement source, and to mix the aggregate and ACC with liquid asphalt cement in forming the asphalt concrete blend.

5. A method for producing an asphalt concrete blend comprising a high percentage of asphalt cement components (ACC), the method comprising the steps of: providing a single and only dryer for mixing and for carrying out all drying operations required in producing the asphalt concrete blend, the single and only dryer having: a drum; a burner directing a flame into a heating chamber within the drum; and a mixing chamber in selective communication with the heating chamber; operating the dryer in a first mode of operation in order to prevent aggregate from passing from the heating chamber to the mixing chamber; operating the dryer in a second mode of operation in order to enable aggregate to pass from the heating chamber to the mixing chamber; drying and heating aggregate in the single and only dryer without use of a pre-dryer; and combining asphalt cement with the aggregate such that the asphalt concrete blend has an ACC percentage of at least 25 percent.

6. The method of claim 5 further comprising the step of adding liquid asphalt to the asphalt concrete blend, wherein all of the liquid asphalt of the asphalt concrete blend is added to the asphalt concrete blend after the asphalt concrete blend is dried and heated by the single and only dryer.

7. The method of claim 6 wherein the liquid asphalt is added in a mixer that is external to the single and only dryer.

8. The method of claim 6 wherein the liquid asphalt is added in a batch tower that is external to the single and only dryer.

9. The method of claim 5 wherein the asphalt concrete blend is comprised of ACC and liquid asphalt cement.

10. The method of claim 5 wherein the aggregate is dried and heated via direct drying and heating in a first portion of the single and only dryer and, thereafter, the aggregate is combined with ACC and heated via indirect heating in a second portion of the single and only dryer.

11. The method of claim 10 wherein the combined aggregate and ACC are combined with liquid asphalt cement.

12. A method for producing asphalt concrete blends comprising the steps of: providing a single and only dryer having: a drum; a burner directing a flame into a heating chamber within an interior of the drum; a mixing chamber that is in selective communication with the heating chamber; means for preventing aggregate from passing from the heating chamber to the mixing chamber in a first mode of operation and for enabling aggregate to pass from the heating chamber to the mixing chamber in a second mode of operation; a plurality of mixing paddles located within the mixing chamber that are sized and configured to mix aggregate located within the mixing chamber; heating and drying first aggregate within the heating chamber of the single and only dryer; and combining the first aggregate with asphalt cement to form a first asphalt concrete blend.

13. The method of claim 12 further comprising the step of providing a mixer that is external to the single and only dryer and combining the first aggregate with the asphalt cement in the mixer to form the first asphalt concrete blend.

14. The method of claim 12 wherein the first aggregate is combined with the asphalt cement in the mixing chamber to form the first asphalt concrete blend.

15. The method of claim 14, wherein the asphalt cement comprises asphalt cement components.

16. The method of claim 14 wherein the asphalt cement is liquid asphalt.

17. The method of claim 14 further comprising the step of providing a mixer that is external to the single and only dryer and combining the first asphalt concrete blend with liquid asphalt in the mixer to form a second asphalt concrete blend.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numerals represent like elements throughout the several views, and wherein:

(2) FIG. 1 is an overhead view of a conventional production plant for the batch production of asphalt concrete using ACC;

(3) FIG. 2 is a first side elevation view of the production plant depicted in FIG. 1;

(4) FIG. 3 is an opposing side elevation view of the production plant depicted in FIG. 1;

(5) FIG. 4 is an overhead diagrammatic view of a production plant for the batch production of asphalt concrete using ACC according to a first embodiment of the present invention;

(6) FIG. 5 is a sectional view of a direct dryer used in the production plant of FIG. 4;

(7) FIG. 6 is an enlarged view of the boxed portion of the production plant shown in FIG. 4 identified as “FIG. 6”;

(8) FIG. 7 is a side elevation view of a portion of the production plant shown in FIG. 4; and

(9) FIG. 8 is a side elevation view of a portion of a production plant for the batch production of asphalt concrete using ACC according to an embodiment of the present invention.

DETAILED DESCRIPTION

(10) This description of the preferred embodiments of the invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. The drawings are not necessarily to scale, and certain features of the invention may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness.

(11) Referring now to FIG. 4, there is provided a production plant 200 for the batch production of asphalt concrete blends having a high percentage of ACC according to a first embodiment of the present invention. In this particular case, plant 200 is a Hi-RAP batch plant that produces up to 400 tons of asphalt concrete per hour utilizing approximately 65% RAP (or ACC) by weight. In this particular case, plant 200 includes a single and only dryer 202, a 76,000 cubic feet per minute (cfm) baghouse 204, and batch tower 206. Dryer 202 is preferably a DOUBLE BARREL® brand Model XHR dryer, depicted in FIG. 5, which is sold by Astec, Inc. of Chattanooga, Tenn. Dryer 202 is preferably located at ground level to facilitate access for maintenance, etc. The dryer 202 includes a generally cylindrical fixed outer drum 208 mounted on an inclined frame 270 that is preferably sized and configured to enable ground access to the dryer such that repairs and maintenance activities can be carried out from the ground. The dryer 202 includes a heating chamber that is located within a generally cylindrical inner drum 210 that is adapted to rotate with respect to the outer drum. The inner drum 210 is rotatably mounted on the frame by a plurality of bearings and is driven to rotate by a suitable drive system. Preferably, this drive system comprises a variable frequency drive that is adapted to vary the rotational speed of inner drum 210 with respect to outer drum 208. The above-described frame, bearings, and drive system are conventional components known to persons of skill in the art and, therefore, are not shown or described in detail. A burner 212 located at lower end 214 of the dryer 202 (on the left side as viewed in FIG. 5) directs a flame 216 in a generally axial direction into the interior of inner drum 210. Burner 212 may be equipped with a variable frequency drive that is adapted to vary the amount of excess air in burner as well as its firing rate. Such variable frequency drives are described in U.S. Pat. No. 8,863,404 (incorporated herein by reference).

(12) ACC is delivered from one or more ACC supply bins 218 (FIG. 4) into the outer drum 208 of dryer 202 after passing through a scalping screen 220 to remove oversized material, by belt conveyor 221 or other similar conveyance means, through chute 222 into mixing chamber 224 between the outer drum 208 and inner drum 210, where it is heated. As discussed earlier, production facilities that have been conventionally used to produce asphalt concrete from aggregate materials that include a high percentage of ACC often utilize a pre-dryer for ACC, which directly exposes the ACC to the flame and burner which causes a number of safety and maintenance issues. However, in this case, since there is preferably no pre-dryer and the ACC is located only (i.e., exclusively) within the mixing chamber 224 of the dryer 202, it is never directly exposed to flame 216 or to the burner 212 itself. Instead, the ACC is only indirectly heated, which limits the risk that the liquid asphalt cement in the ACC will oxidize, reduces the risk of fire, and also limits fouling that occurs at the burner from ACC fines.

(13) Virgin aggregate is delivered from one or more aggregate supply bins 226 of a bin coldfeed system 228 into the inner drum 210 of dryer 202 after passing through a scalping screen 230, by belt conveyor 231 or other similar conveyance means, through chute 232 located at upper end 234 of the dryer, whereupon it is dried and heated. The ACC supply bins 218 and the aggregate supply bins 226 of the bin coldfeed system 228 are each preferably equipped with a variable speed feeder that can control the discharge of material from the bin onto conveyors 221, 231, respectively.

(14) The interior of the inner drum 210 is functionally separated into a combustion zone located in the vicinity of burner flame 216 and a drying zone located between the combustion zone and the upper end 234 of dryer 202. Because upper end 234 of dryer 202 is elevated above lower end 214, the virgin aggregate delivered into the interior of the inner drum 210 through chute 232 will move towards the lower end and is dried and heated as the inner drum rotates. Preferably, inner drum 210 includes a plurality of V-flights (not shown), such as are described and shown in U.S. Pat. No. 8,863,404, on its inner surface to facilitate this heating and drying process. Combustion products and exhaust gases generated during the operation of dryer 202 rise out of the inner drum 210 through exhaust gas outlet 236 and are conveyed to baghouse 204 (FIG. 4) by conduit 238. The baghouse 204 is operated in a conventional manner to process the exhaust gases from the dryer 202. Preferably, a controller (not shown, but described in U.S. Pat. No. 8,863,404, and located in control center 240) is adapted to control the temperature of the exhaust gases from dryer 202 that pass through exhaust gas outlet 236 by regulating variable frequency drive systems on inner drum 210 and burner 212.

(15) With continued reference to FIG. 5 and with further reference to FIGS. 6 and 7, a slide gate 242 that is selectively movable between a closed position and an open position is preferably located at the lower end of inner drum 210. When the slide gate 242 is in the closed position, the plant 200 operates in a first mode of operation where virgin aggregate that is delivered to the inner drum 210 via chute 232 does not enter the mixing chamber 224. Instead, the virgin aggregate passes out of the inner drum 210 of the dryer 202 through outlet 233 and then is conveyed vertically to a screen 244 located on top of the batch tower 206 (which is a 400 ton per hour high-RAP batch tower in this particular case) by way of a bucket elevator 246 or other similar conveyance means. The virgin aggregate is then held and kept hot in temperature-insulated hot bins located within the batch tower 206 until ready for introduction into the final asphalt concrete blend.

(16) On the other hand, when the slide gate 242 is in the open position, the plant 200 operates in a second mode of operation where an aggregate mix formed by combining virgin aggregate together with ACC may be produced within dryer 202. More particularly, virgin aggregate may pass from inner drum 210 through open slide gate 242 and into mixing chamber 224 to be mixed with heated ACC material located there to form an aggregate mix. Inner drum 210 supports a plurality of mixing paddles 248, which extend away from an outer surface of the inner drum into mixing chamber 224. ACC delivered into mixing chamber 224 through chute 222 is thoroughly mixed with the virgin aggregate by mixing paddles 248 to form an aggregate mix. In certain preferred embodiments, the aggregate mix is comprised of up to 70% ACC. Asphalt cement may be injected into the aggregate mix within the dryer 202. However, in certain preferred embodiments, no additional asphalt cement is injected into the aggregate mix inside of the dryer 202. Rather, as discussed below, additional asphalt cement is injected into the aggregate mix within a mixer, such as a pugmill or continuous rotary mixer 250 (shown in FIG. 4), to form the final asphalt concrete blend.

(17) In certain embodiments, the blend produced by the dryer 202 is the final asphalt concrete blend and no further processing is required. However, in other embodiments, the blend proceeds from the dryer 202 for further processing. For example, referring again to FIGS. 4 and 6, the aggregate mix may be conveyed from outlet 249 into rotary mixer 250, where it is further mixed with asphalt cement that is delivered to the mixer through supply line 252 from asphalt cement storage 254, which is comprised of two 30,000 gallon tanks in this particular case. The combination of dryer 202 and rotary mixer 250 is preferably a DOUBLE BARREL® brand Model DBXHR dryer with mixer that is sold by Astec, Inc. of Chattanooga, Tenn. In still other cases, as discussed further below, the aggregate mix is transported from dryer 202 to batch tower 206 for further processing, which may include further blending with virgin aggregate and the injection of asphalt cement. In each case, however, all drying and heating of the aggregate mix preferably occurs within the dryer 202.

(18) Referring again to FIGS. 6 and 7 and FIG. 8, when the dryer 202 is used to produce a aggregate mix of ACC and virgin aggregate, the aggregate mix may pass through the dryer 202 and then to batch tower 206. In particular, the aggregate mix exits the dryer 202 from 249 and is then conveyed vertically to a batcher 256 that is located near the top of the batch tower 206 by way of a separate bucket elevator 258 or other similar conveyance means. From there, the batcher 256 drops the aggregate mix into an insulated bin 260 in order to avoid segregation. In this particular case, the bin 260 is a 50-ton surge bin that is provided with 3-inch fiberglass insulation and an embossed aluminum skin to limit heat loss. However, the size of the bin 260 may vary according to the needs of the production plant 200. A weigh hopper 262 is positioned under the bin 260 to receive and weigh a quantity of the aggregate mix that is to be used in the next batch processed in the batch tower 206. The weighed quantity of aggregate mix is then directed into pugmill 264 within the batch tower 206 by way of gravity chute 266 (or via traverse conveyor 268, as shown in FIG. 8). Within the pugmill 264, the aggregate mix may be further combined with virgin aggregate that is stored in hot bins below screen 244, as discussed above, to form the final asphalt concrete blend. Thereafter, the aggregate mix and virgin aggregate may be further mixed with additional liquid asphalt cement that is delivered to the pugmill 264 through supply line 252 from asphalt cement storage 254 (shown in FIG. 4). In the end, the final asphalt concrete blend may be comprised of 0% ACC (i.e., 100% virgin aggregate plus asphalt cement) up to and including 100% ACC blend. Advantageously, the presently-disclosed methods and apparatus permit a Hi-RAP asphalt concrete blend to be produced using a single and only dryer 202 to carry out all heating and drying of the aggregate and ACC.

(19) Thus, it may be seen that methods, systems, assemblies, facilities, and combinations of components according to the present general inventive concept, as described above, allow for the production of asphalt concrete from aggregate materials including a high percentage of ACC while using only a single dryer or a single dryer and a single mixer for both virgin aggregate and ACC, without using a separate pre-dryer for heating and drying ACC. Additionally, embodiments of the present invention, as described above, allow for ground access to the dryer, thereby making the frequent and routine maintenance of the dryer much faster and much safer. Furthermore, various embodiments of the present invention avoid injecting liquid asphalt cement within the dryer in order to reduce the maintenance interval. Instead, liquid asphalt cement is injected within a pugmill within a batch tower or within a separate rotary mixer.

(20) Although this description contains many specifics, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments thereof, as well as the best mode contemplated by the inventor of carrying out the invention. The invention, as described and claimed herein, is susceptible to various modifications and adaptations as would be appreciated by those having ordinary skill in the art to which the invention relates.