US20050262967A1

US20050262967A1 - Method of recycling brazing sheet

Info

Publication number: US20050262967A1
Application number: US10/855,758
Authority: US
Inventors: David Leon
Original assignee: Alcoa Inc
Current assignee: Howmet Aerospace Inc
Priority date: 2004-05-27
Filing date: 2004-05-27
Publication date: 2005-12-01
Also published as: WO2005118893A3; WO2005118893A2

Abstract

The present invention relates to a method of recycling aluminum clad materials. In particular, the method relates to recycling clad materials such as brazing sheet comprised of two or more aluminum alloys, by selective melting of one of the alloys. The feedstock is agitated during heating to allow separation of the molten alloy from the unmelted core, and the molten alloy is removed, such as by draining off during the process.

Description

FIELD OF THE INVENTION

The present invention relates to a method of separating and recycling metallic clad materials. In particular, the method relates to recycling clad materials such as brazing sheet comprised of two or more aluminum alloys, by selective melting of one of the alloys.

BACKGROUND INFORMATION

Aluminum and aluminum alloys can be joined by a wide variety of brazing and soldering processes. Brazing, by definition, employs a filler material having a liquidus above 840° F. and below the solidus of the base metal. Brazing is distinguished from soldering by the melting point of the filler metal: solders melt below 840° F.
Brazing differs from welding in that no substantial amount of the base metal is melted during brazing. Thus, the temperatures for brazing aluminum are intermediate between those for welding and soldering.
Brazing sheet typically consists of a low melting brazable aluminum alloy roll bonded or clad with one or more aluminum brazing alloys, and possibly a sacrificial alloy. Brazing sheet, which is available with one or both sides clad, provides a more convenient method of supplying the filler metal than wire, shims, or powder and is particularly convenient for mass-produced complex assemblies. Commercial filler metals for brazing aluminum are typically aluminum-silicon alloys containing 7-12% Si. The filler metal is clad to an aluminum alloy core having a relatively low Si content, on one or both sides, by means of roll bonding or other fabrication methods. During the production of brazing sheet, significant amounts of scrap can be produced. Brazing sheet scrap is also produced during the forming and assembly process. Because the scrap contains both aluminum alloys with a high Si content and aluminum alloys with a low Si content, simple melting of the scrap is inadequate for recycling the metal materials, because this would result in an combined aluminum alloy having a raised Si content, too high to be used for producing similar type metallic core alloys, unless diluted with substantial amounts of pure (prime) aluminum or alloys having a very low Si content.
Various attempts have been made to separate the clad alloy from the metallic core in the scrap, and various methods are known to separate metals of different melting temperatures. For example, Intemational Publication WO03/024665 describes a method of decoating metallic coated scrap pieces by heating the scrap to a temperature above the solidus temperature of the metallic coating and below the liquidus temperature of the metallic core and agitating the scrap to effect separation. Abrading particles are added to the scrap to further facilitate separation of the alloys. The particles are characterized as being brought into fluidization during the agitation of the metallic coated scrap pieces, thereby forming a fluidized bed.
International Publication WO02/101102 describes a method of separating alloys using a rotary melting furnace.
U.S. Pat. No. 3,556,500 describes a method of separating various metals using heat and vibration.
In all of the above methods, the focus is on recovery of the core alloy only, and the methods do not attempt to minimize contamination and/or oxidation of the clad alloy. There remains a need to develop a method which can recover both the clad and core alloys in a useful form, so that they can be reused without the need for significant additional processing.

SUMMARY OF THE INVENTION

The present invention solves the above need by providing a method of recycling metallic clad materials made of two or more different metals or metal alloys. The method comprises providing a feedstock of the clad materials having at least two metals or metal alloys having different melting temperatures from each other. The feedstock is heated in a furnace to effect melting of the metal having the lowest melting temperature, producing a first molten metal and a second unmelted metal. The scrap feedstock is agitated while heating to separate the molten metal from the unmelted metal, and the molten metal is removed from the furnace. Using the method of the present invention, both clad and core metals or metal alloys can be recovered, without contamination due to the addition of particles such as abrading particles employed in prior art practices, during the recycling process.
It is an object of the present invention, therefore, to provide a method of separating and recycling metallic clad materials such as brazing sheet, based on the temperature differences in the metals or metal alloys used in the clad materials.
It is a further object of the present invention to provide a method of separating clad and core aluminum alloys in brazing sheet for further use.
These and other objects of the present invention will become more readily apparent from the following figures, detailed description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by the following drawings in which:
FIG. 1 is a schematic diagram of the steps carried out in an embodiment of the method of the present invention.
FIG. 2 is an exemplary chart of solidification ranges for an embodiment of the present invention.
FIG. 3 is a schematic side view of an embodiment of a suitable apparatus for carrying out the method of the present invention in batch mode.
FIG. 4 is a schematic front view of the embodiment shown in FIG. 3.
FIG. 5 is schematic view of an additional embodiment of a suitable apparatus for carrying out the method of the present invention in a continuous operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a method of separating and recycling metallic clad materials comprising providing a feedstock of metallic clad materials having at least a first and a second metal or metal alloy, the first and second metals or metal alloys having different melting temperatures; heating the feedstock in a furnace to effect melting of the metal having the lowest melting temperature, to produce at least a first molten metal and a second unmelted metal; agitating the feedstock while heating to separate the first molten metal from the second unmelted metal; and removing the first molten metal from the furnace. As used herein, the terms “metal” and “metal alloy” will be used interchangeably to mean any metal whether in pure or alloyed form.
The method of the present invention can be used to separate any metallic clad materials having different melting temperatures. Suitable clad materials include, but are not limited to, clad aluminum alloys such as brazing sheet, aluminum-steel composite sheets (aluminum alloy clad on a steel core) and galvanized steel (zinc coating on a steel sheet). The cladding can be on one or both sides of the core material. Preferably, the first and second metals are each aluminum alloys. Most preferably, the clad material is brazing sheet
Referring now to FIG. 1, a flow diagram of an embodiment of the method of the present invention is shown. Scrap clad material is loaded 1 into the furnace, either in batch mode or in a continuous process. The furnace is preheated 3 to an intermediate temperature ranging from about 900° F. to 1135° F., preferably to about 1000° F. In this embodiment, the clad material is brazing sheet. For other types of metallic clad materials, one skilled in the art can determine whether there is enough difference between the melting temperatures of the two or more metals to allow efficient separation by the method of the present invention. Typically, a difference in solidus temperature of about 20° F. is sufficient for using the methods of the present invention, as most furnaces permit control of temperature to within about ±10°.
During preheating, no agitation is necessary. Once the material reaches the intermediate temperature agitation of the furnace is begun 5. The furnace and material are then further heated to the melting temperature of the alloy having the lower melting temperature. For brazing sheet, this temperature range is about 1145°-1180° F., preferably 1150°-1160° F. The temperature is held at the target set point and agitation continued for a predetermined time interval, preferably from about 0 to about 15 minutes, preferably about 5 minutes. Agitation must be of a sufficient frequency and amplitude to effect complete separation of the clad from the core. One skilled in the art can easily determine the appropriate degree of agitation to achieve separation.
During the period of combined heating and agitation the clad material selectively melts and is shaken off the core metal due to the agitation and scrap-to-scrap contact. At 7 is shown a decision point, in which the core can be left to cool down and collected into scrap bins 15 as solid material, or can be transported to a secondary melter 9 for recycling. At 11 is shown a decision point regarding the clad, in which the clad can be left to solidify and collected as cast pig in a pig mold 17 or collected as molten material and transported to a secondary furnace 13 for recycling. If cast, the clad is further classified 19 as RSI (recycled scrap ingot).
Preferably, the metallic clad materials are brazing sheet scrap, which is typically comprised of two or more of 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX or 7XXX series aluminum alloys. Typically, for brazing sheet the clad layers will be about 3% to 25% by weight of the total material, while the core will be about 50% to 97% by weight of the total material.
The aluminum clad brazing sheet scrap comes from a variety of sources. Most scrap is from hot mill edge trim, and the pieces range in size from about 8 in.×70 in.×12 in. to 0.10 in.×2 in.×8 in. in size. Other suitable types of scrap include slitter scrap off the cold mill. This type of scrap ranges in thickness from about 0.25 inches down to 0.004 inches. Scrap from the manufacture of components using brazing sheet, as well as post-consumer scrap such as scrap radiators and the like can also be recycled using the methods of the present invention. The process does not require that the pieces be of any particular thickness, although preferably the thickness is greater than about 0.045 inches. Materials thinner than this may not be stiff enough at the treatment temperature to survive the agitation, and may shatter and enter the clad stream, thus contaminating it.
No pretreatment of the scrap prior to loading in the furnace is necessary. There may be some trace amounts of rolling lubricant on the pieces, but this will be removed during heat-up.
Typically, scrap will be segregated at the source based on the core alloy series, to be recycled back into new sheet ingot based on the original core alloy used, with minor adjustments to the alloy composition. Clad alloys can be melted and mixed together, in the case of brazing sheet, because the clad is always a 4XXX series alloy and can be recycled as 4045 or 4343. Core alloys have more value when segregated, although in certain circumstances some alloys can be mixed with others, because cross contamination would not be too detrimental. For example, 1XXX series, which are 99% aluminum, can be mixed with any of the others; 2XXX series, which have Cu as the main ingredient can be absorbed into 7XXX and to a lesser degree 3XXX, and 3XXX series can be mixed into 5XXX series alloys (e.g., used beverage can (UBC) 5182 lids and 3004 bodies are melted together to go into the 5XXX series).
The separation methods of the present invention allow recovery of different alloys having compositions that can be effectively remelted using traditional methods, known to those in the industry, into original alloy families, without the need for substantial adjustment to the alloy composition. As used herein, the term “effective remelting” is defined as separation of the two components such that at least about 80% of each separated component can be used in the formulation of separate molten metal quantities suitable for casting a target composition. Also as used herein, the phrase “without the need for substantial adjustment” means that the amount of pure pig or other pure metal units needed to adjust the alloy composition to a suitable target composition will be about 75 wt. % or less, more preferably about 50 wt. % or less, based on the weight of the recovered alloy. The temperature selected for heating will depend on the particular metals or alloys being separated and the liquidus and solidus temperatures of each metal or alloy. Referring now to FIG. 2, solidification ranges are presented for brazing sheet with specific alloys in the 3000 and 4000 series. In this preferred embodiment, there is a 55° F. gap (and opportunity for selective melting) between the core solidus temperature and the clad liquidus temperature, indicated in FIG. 2 as the cross-hatched area. As described above, since most furnaces can control temperature to within a 20 degree band (±10°), a suitable operating temperature would be about 1145°-1180° F. Preferably, the range is about 1150° to 1160° F. This lower range ensures that oxidation of the melt is minimized, since recovery of both alloys is desired.
Any suitable furnace can be used, so long as there is provided a means for removing the molten alloy during the separation process. An exemplary heating and shaking assembly for operation in batch mode is shown in FIGS. 3 and 4. In this embodiment, a source of heated gas 4 is supplied to a heating chamber 8 and monitored by a thermocouple 6. Scrap is loaded into the chamber through a door 10. A perforated plate 12 is held in place at the bottom of the chamber by retaining nuts 24. A first 14 and second 15 bracket is used to hold the heating and shaking assembly in place, and a first 16 and second 17 curved flex joint provides flexion and insulation. Molten metal flows through the perforated plate 12 through a first trough 18 and a second 20 trough into the molten metal collection pan 22 inside the molten metal collection chamber 34. The molten metal can be removed from the collection chamber 34 through the access door 38. A first 26 and second 28 vibrator on a first 36 and second 37 support structure provides the agitation means.
In one embodiment, heating is effected through the use of a heated stream of gas. A commercially available combustion system can be used to heat up a stream of gas in a commercially available heat exchanger. Heating can also be accomplished by other methods including, but not limited to, electrical heating, radiant or solar heating, or the like. The stream of gas is a mixture of air and nitrogen which controls the total amount of oxygen available in the heating chamber. A low oxygen content is important to limit oxidation of the molten metal, thus increasing recovery. Those skilled in the art will be able to adjust the oxygen content to control oxidation, and understand that oxygen content can also be controlled through the introduction of the products of combustion (POC) or other inert or low-oxygen content gasses.
In an additional embodiment a continuous process is used, as exemplified by the schematic diagram shown in FIG. 5. Scrap material is loaded onto a continuous belt 60 which moves through a preheat furnace 64 having a heating system 66 and an exhaust stack 62. Heating of the preheat furnace can be accomplished by any suitable type of heating, including combustion, electrical, radiant and the like. The scrap is preheated in the preheat furnace 64 and then conveyed out of the preheat furnace 64 onto a vibrator feed 68 having one or more vibration means 69 provide agitation while conveying the scrap, and controlling the rate of feed, to the main heating chamber 80. The main heating chamber 80 is a standard heating chamber having flow control dampers 70, including a flow control damper 82 at the exhaust outlet 84, a combustion air source 72, a fuel source 74 and an ignition chamber 76 for ignition of the fuel, using an ignition device 78. A hot gas circulation fan 71 recirculates the hot gas used for heating through the hot gas conduit 100. As with the preheat furnace, any method of heating can be used to heat the scrap material.
The scrap material is conveyed through the main heating chamber 80 by means of a vibrator feeder having a perforated plate 88 which allows the melted clad material to collect under the plate in a trough 90. The trough 90 is set at angle sufficient to allow runoff of the molten clad into a collection crucible 92. The solid core scrap material is conveyed out of the main heating chamber 80 on a secondary conveyor belt 93 where it is further conveyed to a core scrap bin 95.

EXAMPLES

The following examples are intended to illustrate the invention and should not be construed as limiting the invention in any way.
Dual-sided clad brazing sheet hot mill edge trim pieces of varying clad compositions and different clad thickness were loaded in a batch lab-scale unit that provided hot gas (air plus nitrogen) to heat the scrap to the target set point. When the scrap material achieved the preheat set point, the shaking mechanism (vibration) was started to separate the clad from the core. A perforated plate in the heating chamber allowed the melted clad to drain from the heating chamber into a collection pot, where it was allowed to cool. The efficiency of the separation was measured in terms of weight distribution and composition analysis of the core and clad fractions.

Example #1

Hot mill trim scrap was preheated, using the method of the present invention, to 1000° F. and agitation started. When the scrap achieved the target set point of 1160° F. a five-minute countdown was started. An analysis of the weight distribution and composition of separated core and clad materials is shown in Tables 1 and 2.

TABLE 1

EXAMPLE #1 WEIGHT SEPARATION

Recovered Recovered CORE CLAD

Percent Percent Actual Actual Separation Separation

CORE CLAD % CORE % CLAD Efficiency Efficiency

86.8 13.2 83.9 16.1 103.5 82.0

Lab-scale tests recovered about 86.4% of the metal load in the core “bin”, with a range between about 84.7 and 88.8%, indicating that clad separation was about 103% efficient when compared to the actual value. (Some of the core alloy dissolved into the clad upon melting). The amount of clad segregated was about 82% (100−(16.1−13.2)/16.1×100). There are some “system losses” due to a) oxidation of the melted clad and b) material hang-up.

TABLE 2


EXAMPLE #1 COMPOSITION ANALYSIS

	Composition of hot
	mill edge scrap if	Recovered	Recovered
Element	melted completely	3XXX Core	4XXX Clad

Si	˜1.38	˜0.34	˜7.60
Fe	˜0.54	˜0.56	˜0.64
Cu	˜0.04	˜0.06	˜0.00
Mn	˜0.96	˜1.09	˜0.23
Mg	˜0.01	˜0.01	˜0.00
Cr	˜0.00	˜0.00	˜0.01
Ni	˜0.01	˜0.01	˜0.01
Zn	˜0.14	˜0.03	˜0.79
Ti	˜0.01	˜0.01	˜0.00
Be	˜0.0000	˜0.0000	˜0.0000
Pb	˜0.00	˜0.00	˜0.00
Sr	˜0.000	˜0.000	˜0.000

Example #2

Hot mill trim scrap was preheated, using the method of the present invention, to 1000° F. and agitation started. When the scrap achieved the target set point of 1150° F. a five-minute countdown was started. An analysis of the weight distribution and composition of separated core and clad materials is shown in Tables 3 and 4.

TABLE 3

EXAMPLE #2 WEIGHT SEPARATION

CORE CLAD

Percent Percent Actual Actual Separation Separation

CORE CLAD % CORE % CLAD Efficiency Efficiency

77.2 22.8 79.6 20.4 97.0 111.8

Lab-scale tests recovered about 77.2% of the metal load in the core “bin”, with a range between about 76.9 and 77.4%, indicating that clad separation was about 97% efficient when compared to the actual value. (Some of the clad alloy remained adhered to the clad upon melting). The amount of clad segregated was about 111% indicating that clad material hung-up in the system from previous trials was released into this sample.

TABLE 4


EXAMPLE #2 COMPOSITION ANALYSIS

Si	˜2.70	˜0.21	˜11.1
Fe	˜0.21	˜0.22	˜0.30
Cu	˜0.23	˜0.28	˜0.03
Mn	˜0.79	˜0.99	˜0.15
Mg	˜0.25	˜0.25	˜0.21
Cr	˜0.00	˜0.00	˜0.00
Ni	˜0.00	˜0.00	˜0.01
Zn	˜0.01	˜0.01	˜0.01
Ti	˜0.02	˜0.02	˜0.02
Be	˜0.0000	˜0.0000	˜0.0000
Pb	˜0.00	˜0.00	˜0.00
Sr	˜0.00	˜0.00	˜0.00

As can be seen in Tables 2 and 4, when brazing sheet scrap is melted, the composition of the resulting alloy is too high in Si to be recycled into anything but lower valued casting alloys like A380 and A319. By comparison, selective melting of the clad and separation from the core results in a core composition having minimal Si contamination, thereby increasing its value.
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims

1. A method of separating and recycling metallic clad materials comprising:

providing a feedstock of metallic clad materials having at least a first and a second metal, said first and second metals having different melting temperatures;

heating the feedstock in a furnace to effect melting of the metal having the lowest melting temperature, to produce at least a first molten metal and a second unmelted metal;

agitating the feedstock while heating to separate the first molten metal from the second unmelted metal; and

removing said first molten metal from the furnace.

2. The method of claim 1, wherein said first and second metals are each an aluminum alloy.

3. The method of claim 2, wherein said metallic clad material is a brazing sheet.

4. The method of claim 3, wherein said brazing sheet is comprised of two or more alloys selected from the group consisting of 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX and 7XXX series aluminum alloys.

5. The method of claim 4, wherein said brazing sheet is comprised of two or more alloys selected from the group consisting of 3XXX and 4XXX series aluminum alloys.

6. The method of claim 5, wherein said melting is effected at a temperature of about 1145°-1180° F.

7. The method of claim 6, wherein said melting is effected at a temperature of about 1150°-1160° F.

8. The method of claim 1, wherein said agitating is effected for up to about 15 minutes.

9. The method of claim 1, wherein said method is substantially a continuous process.

10. The method of claim 1, wherein said method is carried out without the requirement of additional materials to improve separation.

11. The method of claims 2 through 10, wherein said separation results in molten and unmelted alloys having compositions that can be remelted into original alloy families without the need for substantial adjustment to the alloy composition.

12. The method of claim 1, wherein said first metal is an aluminum alloy and said second metal is steel.

13. The method of claim 1, wherein said first metal is zinc and said second metal is steel.