Method for melting aluminum scraps

Economically advantageous melting of aluminum scraps is obtained by first compressing said aluminum scraps under a magnitude of pressure exceeding the yield strength of the material making up said scraps for thereby converting them into a compressed mass having an apparent specific gravity greater than the specific gravity of normal aluminum melt and subsequently introducing the compressed mass into a molten aluminum bath.

BACKGROUND OF THE INVENTION 
This invention relates to a method for advantageously melting aluminum 
scraps in a freshly molten aluminum mass (hereinafter referred to as 
"molten bath") for thepurpose of reutilization of the aluminum contained 
in the scraps. The term "aluminum scraps" as used in this specification is 
meant to embrace both scraps of aluminum and scraps of aluminum alloys. 
In the melting of aluminum scraps in the molten bath, it is extremely 
important from the economic point of view to minimize possible loss of 
aluminum due to melting. 
The melting of aluminum scraps is usually performed in the atmosphere. The 
temperature of the molten bath is very high, generally exceeding 
660.degree. C., and frequently ranging from 720.degree. C. to 780.degree. 
C. When aluminum scraps are introduced into the molten bath at such high 
temperatures, they are liable to react with oxygen and nitrogen contained 
in the ambient air and produce aluminum oxide and aluminum nitride. These 
products are called "melt slag" and they are responsible for said melting 
loss. To minimize this melting loss, it is imperative that said reaction 
should be prevented from occurring, i.e. the aluminum scraps should be 
caused to sink under the molten bath as quickly as possible. For this 
purpose, there has actually been employed the liquid-heel process, the 
liquid-flux process or the method which comprises pressing and immersing 
scraps into the molten bath. 
The liquid-heel process comprises the steps of depositing aluminum scraps 
on the surface of the molten bath and subsequently forcing these aluminum 
scraps into the interior of the bath. This forced submersion of aluminum 
scraps is effected by a manual method or a mechanical method. 
Specifically, the manual method effects the forced submersion of aluminum 
scraps by causing the scraps floating on the surface of the molten bath to 
be pushed down into the interior of molten bath by use of a rabbler which 
consists of an iron bar and a disc or square of iron plate attached at an 
angle to the forward end of said iron bar. This operation entails immense 
labor and is the most physically taxing job involved in any kind of 
melting operation. The mechanical method generally employs bladed puddlers 
in the case of iron-hot furnaces or grid puddlers in the case of hearth 
furnaces of various types. These two methods both aim to prevent aluminum 
scraps from undergoing oxidation or nitrification by minimizing the 
duration of exposure of such aluminum scraps to the ambient air while they 
are floating on the surface of molten bath. These methods, however, are 
such that they cause the aluminum scraps to undergo oxidation and 
nitrofication to a considerable extent. Thus, they suffer from the 
heaviest melting loss of all the types of melting methods. 
The liquid-flux process accomplishes desired melting of aluminum scraps by 
introducing said scraps in a mass of flux melted in advance. A typical 
flux to be used for this purpose consists of 50 to 70% by weight of NaCl, 
25 to 45% by weight of KCl, 3 to 10% by weight of CaF.sub.2, etc. for 
example. In the case of this composition, the flux is melted and kept at 
temperatures in the range of from 720.degree. C. to 780.degree. C. when 
the aluminum scraps are introduced. This method entails very little 
melting loss because the aluminum scraps are melted immediately after 
their entry into the molten flux. In this method, however, since the 
molten flux absorbs oxides and consequently becomes viscous and 
deteriorated, the melting operation requires 200 to 300 kg of molten flux 
per ton of aluminum scraps placed therein. The cost of melting by this 
method, therefore, is quite high. 
New clippings and solids of pure aluminum (as defined by the United States 
NASMI Standards for Non-ferrous Metal Scraps) have large surface areas and 
small bulk densities for the unit weight and, therefore, are quite liable 
to undergo oxidation and nitrification upon exposure to the ambient air. 
To cope with the difficulty, there has been adopted a method for 
decreasing their apparent surface areas per unit weight by pressing the 
clippings and solids generally under a pressure of 30 kg/cm.sup.2. This 
pressing indeed increases their bulk densities whose initial values are 
usually on the order of from 0.28 to 0.48. Actually, however, they are 
increased by 1.6 times at most. 
An object of the present invention is to provide a method for advantageous 
melting of aluminum scraps, which method neither requires use of any flux 
nor necessitates forced submersion of floating scraps into the interior of 
the molten bath but is applicable generally to all types of aluminum 
scraps and suffers from less melting loss than any other methods directed 
to the same purpose heretofore put to actual use. 
BRIEF SUMMARY OF THE INVENTION 
With a view to fulfulling the object described above, the inventors made a 
study in search of a method which gives aluminum scraps a property such 
that the scraps immediately submerge under the molten bath when they are 
brought into contact with the molten bath. They have, consequently, 
developed a method whereby said immediate submersion of scraps is obtained 
by compressing the scraps under a magnitude of pressure greater than the 
yield strength of the material making up the scraps for thereby increasing 
the bulk density of said scraps so much as to equal or surpass the 
specific gravity of the molten bath. The present invention has originated 
in the development of this principle. To be more specific, extraneous 
particles are first removed from the aluminum scraps either after the 
scraps are first finely shredded into particles preferably having a fixed 
particle size distribution as in the case of scraps which issue from 
plates or shaped articles or without being further reduced in size as in 
the case of new clippings or solids of sufficiently small dimensions. 
Thereafter, the finely particulate aluminum scraps of either or both of 
the above described types are compressed under a magnitude of pressure 
greater than the yield strength of the material of which they are made 
and, thus, converted into a compressed mass having an increased bulk 
density as mentioned above. Finally, the compressed mass of scraps is 
introduced into the molten bath.

DETAILED DESCRIPTION OF THE INVENTION 
The method conventionally adopted chiefly for the purpose of pressing 
aluminum scraps having a relatively large apparent surface area per unit 
weight such as new pure aluminum clippings and solids or aluminum foil 
scraps for thereby decreasing said apparent surface area of the scraps and 
then subjecting the pressed mass of scraps to melting will be described. 
In this case, the intial apparent specific gravity of the aluminum scraps 
is about 0.48 and the apparent surface area of the scraps is decreased to 
about one half in consequence of the pressing performed under a pressure 
of about 30 kg/cm.sup.2, for example. This means that the apparent 
specific gravity of the pressed mass of scraps is not more than 1.6. When 
the mass of scraps prepared by this method is placed in the molten bath to 
be melted therein, the behavior of the mass observed in the molten bath is 
as illustrated in FIGS. 1 and 2. In the drawing, 1 denotes a container, 2 
a molten bath and 3 a mass of aluminum scraps. 
The relation between temperature and specific gravity of pure aluminum is 
shown in the following table. 
Table 1 
______________________________________ 
Temperature 
.degree.C. 
25 660(S) 660(L) 
700 720 750 800 
Specific gravity 
2.698 2.55 2.368 2.357 
2.352 
2.345 
2.332 
______________________________________ 
It is seen from the table that the specific gravity of the molten bath 
generally ranges from 2.33 to 2.37, though it is variable to some extent 
with the temperature. 
When a mass of aluminum scraps is placed in the molten bath, therefore, it 
floats in the upper part of the molten bath, with a part thereof exposed 
to sight above the surface of the molten bath as illustrated in FIG. 1. 
FIG. 2 shows the subsequent behavior of the mass of aluminum scraps while 
it is in the process of melting. The mass of scraps 3 shown in FIG. 1 
produces a partially disintegrated portion 4 while it remains floating on 
the surface of the molten bath. Hence it is inevitably susceptible to 
oxidation and nitrification. This is to say that the mass of scraps still 
suffers from melting loss to a considerable extent, although the pressing 
has served the purpose of decreasing the apparent surface area of scraps 
and, consequently, repressing possible melting loss discernibly. In the 
conventional method, scraps issuing from plates or shaped articles are 
thrown directly into the molten bath either as they are or after bein 
shredded into fragments of, for example, diameters from 300 to 500 mm. In 
this case, the apparent specific gravity of the fragments of scraps is 
generally about 0.28 and the melting yield is 93.7%. Whether the aluminum 
scraps are new pure aluminum clippings and solids or aluminum foil scraps 
or those issuing from shaped articles, the method inevitably entails 
appreciable melting loss. 
In contrast, the method of the present invention effects the desired 
melting of aluminum scraps by first compressing the scraps under a 
magnitude of pressing greater than the yield strength of the material 
making up the scraps for thereby converting them into a compressed mass 
having an apparent specific gravity at least equalling and desirably 
exceeding the specific gravity of the molten bath, namely an apparent 
specific gravity of more than 2.3 and preferably more than 2.50 and 
subsequently introducing the compressed mass into the molten bath. 
Now, a typical process for working the method of this invention will be 
described with reference to the flow diagram of FIG. 3. 
FIG. 3 shows the process as being performed by using aluminum scraps 
issuing from shaped articles. In this case, the scraps are assumed to be 
shredded twice. They are subjected to the first shredding and then, after 
separation of foreign particles such as debris of paper and paints 
therefrom, are subjected to the second shredding, followed by magnetic 
separating which serves to remove iron pieces from the aluminum scraps. 
The extent of these shredding and separating operations are variable with 
the kind of aluminum scraps under treatment and the kind of extraneous 
particles contained in the scraps. At any rate, these operations are 
desired to be carried out to the extent of converting the scraps, by the 
end of the second shredding, into particles most (more than 80%) of which 
have maximum diameters of between 2 and 20 mm. The shredded particles of 
aluminum scraps obtained at the end of the second shredding are desired to 
have a particle size distribution to be described in further detail 
hereinafter. The particles of aluminum scraps thus prepared are 
subsequently compressed with a magnitude of pressure equalling or 
preferably exceeding the yield strength of the material making up the 
aluminum scraps. A double-action type hydraulic molding press or a 
withdrawal molding press, for example, can be used for the purpose of 
compressing the particles of aluminum scraps. 
The yield strength of aluminum scraps is variable with the particular 
composition they possess. The relation between composition and physical 
properties of a typical aluminum and aluminum alloy (B209-74 according to 
ASTM) is shown in Table 2 below. 
Table 2 
__________________________________________________________________________ 
Tensile 
Yield 
Composition (wt %) 
strength 
strength 
Elonga- 
(except Al) kg/cm.sup.2 
kg/cm.sup.2 
tion 
Alloy Fe 
Si Cu Mg Mn (MPa) (MPa) 
(%) 
__________________________________________________________________________ 
.BHorizBrace.* 
0.05 
* * 775.about.1092 
245 
.intg. 
1 1100-0 
1.0 0.20 
0.05 
0.05 
(76.about.107) 
(24).sup.** 
28.sup.** 
1122.about.1480 
990 
2 1100-H14 
" " " " (110.about.145) 
(97).sup.** 
1.about.10 
1551 
3 1100-H18 
" " " " (152).sup.** 
-- 1.about.4 
* * 1.0 
990.about.1337 
347 
.intg. 
4 3003-0 
0.7 
0.6 
" " 1.5 
(97.about.131) 
(34).sup.** 
14.about.23 
1408.about.1827 
1194 
5 3003-H14 
" " " " " (138.about.179) 
(117).sup.** 
1.about.10 
1898 1684 
6 3003-H18 
" " " " " (186).sup.** 
(165).sup.** 
1.about.4 
.BHorizBrace. 
* 2.2 1755.about.2184 
673 
.intg. 
* 
7 5052-0 
0.45.sup.* 
0.10 
2.8 
0.10 
(172.about.214) 
(66).sup.** 
14.about.18 
2745 2255 
8 5052-H38 
" " " " (269).sup.** 
2.about.4 
.BHorizBrace. 
3.9 
0.20 
0.40 
2255 1122 
* .intg. 
.intg. 
.intg. 
9 2014-0 
0.7 5.0 
0.8 
1.2 
(221).sup.* 
(110).sup.* 
10.about.16 
4500 4010 
10 
21014-T6 
" " " " (441).sup.** 
(393).sup.** 
6.sup.** 
__________________________________________________________________________ 
.sup.* maximum- 
.sup.** minimum- 
The foregoing process flow of the method of this invention has been 
described with reference to an operation involving the melting of aluminum 
scraps issuing from plates, shaped articles, etc. In case where the 
aluminum scraps happen to be new pure aluminum clippings and solids, 
aluminum foil scraps, etc., the object of this invention can fully be 
accomplished by separating them from extraneous particles and immediately, 
in their unshredded state, compressing the scraps under a magnitude of 
pressure greater than the yield strength of the material making up the 
scraps. 
The relation of aluminum scraps with the pressure used for the compression 
and the compressed mass of aluminum scraps as obtained in experiments will 
be described. 
Experiment 1 
Method of experiment-- Aluminum scraps were finely shredded and separated 
from extraneous particles to afford aluminum particles. The aluminum 
particles were compressed with a double-action type hydraulic molding 
press to produce a compressed mass. The relation between the kind of 
aluminum scraps, the magnitude of pressure used for the compression and 
the bulk density of the compressed mass is shown in Table 3. 
Table 3 
__________________________________________________________________________ 
Aluminum scraps as raw material 
Kind 1100 P 5052 P 6063 S 
__________________________________________________________________________ 
Composition (except Al) 
Fe+Si, Cu, Mn, Zn 
Si, Fe, Cu, Mn, Mg 
Si, Fe, Cu, Mn, Mg, Cr, Zn, Ti 
(wt %) 1.0,0.05.about.0.20,0.05,0.10 
0.08,0.10,0.10,0.10,2.5 
0.5,0.35,0.10,0.10,0.55,0.10,0.10,0.10 
1 
Distribution of particles diameter 
less than 2 mm 5 wt % 5 wt % 5 wt % 
2mm .about.20 mm 
90 wt % 90 wt % 90 wt % 
more than 20 mm 5 wt % 5 wt % 5 wt % 
Bulk density 0.82 0.817 0.914 
Yield strength 1400 1700 1500 
__________________________________________________________________________ 
After compression 
Pressure applied (kg/cm.sup.2) 
1670 1074 1240 
Bulk density 2.42 2.33 2.22 
Pressure applied (kg/cm.sup.2) 
2148 1790 2864 
Bulk density 2.61 2.56 2.34 
Pressure applied (kg/cm.sup.2) 
2864 2148 3222 
ulk density 2.66 2.61 2.44 
__________________________________________________________________________ 
It is clear from Table 3 that the specific gravity of the compressed mass 
of aluminum scraps equals or exceeds that of the molten bath when the 
pressure applied is greater than the yield strength of the aluminum scraps 
under treatment. 
The particle size distribution of the shredded particles of aluminum scraps 
also has some effect upon the force with which the compression is 
effected. To be specific, the specific gravity of the compressed mass of 
aluminum scraps can easily and quickly be increased so much as to equal or 
exceed that of the molten bath when the particle size distribution 
satisfies a prescribed requirement. In case where the particles have a 
substantially uniform coarse size, however, they are susceptible to the 
phenomenon of spring-back which renders the compression infeasible. 
Desirably the shredded particles of aluminum scraps have a particle size 
distribution such that particles of the size groups indicated below are 
contained at percentages falling in the following respective ranges. 
Particles measuring up to 2 mm: 3 to 7% by weight 
Particles measuring from 2 mm to 20 mm: 88 to 92% by weight 
Particles measuring 20 mm or over: 3 to 7% by weight 
Experiment 2 
An experiment similar to Experiment 1 was performed on new pure aluminum 
clippings and solids or aluminum foil scraps. The aluminum scraps were 
subjected, in their unshredded form, to compression immediately after 
removal of extraneous particles. The relation between the pressure used 
for the compression and the bulk density of the compressed mass is shown 
in Table 4. 
Table 4 
__________________________________________________________________________ 
Aiuminum scraps as raw material 
Kind 1060 1100 
Form foil new pure al clippings and solids 
Composition (except Al) 
Fe, Si, Cu, Mn, Zn, 
Fe+Si, Cu, Mn, Zn 
(wt %) 0.35, 0.25, 0.05, 0.03, 0.03 
1.0, 0.05.about.0.20, 0.05, 0.10 
Bulk density 0.232 0.473 
Yield strength 
960 1400 
__________________________________________________________________________ 
After compression 
Pressure applied (kg/cm.sup.2) 
1790 1790 
Bulk density 2.33 1.90 
Pressure applied (kg/cm.sup.2) 
2506 2864 
Bulk density 2.42 2.32 
Pressure applied (kg/cm.sup.2) 
2864 3222 
Bulk density 2.59 2.48 
__________________________________________________________________________ 
The data of this table indicate that in the experiment, the compression 
performed under a magnitude of pressure greater than the yield strength of 
aluminum scraps increased the bulk density to more than 2.35 times the 
original value. 
The greatest dimension of the compressed mass is desired to be not larger 
than 140 mm. If the compressed mass has a dimension greater than 140 mm, 
then the melting requires greater time. This is particularly true where 
the apparent specific gravity of compressed mass equals the specific 
gravity of the molten bath. 
The magnitude of pressure under which aluminum scraps are compressed is 
required to exceed the yield strength of the material making up the 
scraps. Especially, the pressure is desired to be 600 kg/cm.sup.2 greater 
than the yield strength. If the pressure is not greater than the yield 
strength, then the aluminum scraps fail to form creeps and does not 
acquire an increased apparent specific gravity. 
FIG. 4 and FIG. 5 show how a compressed mass of aluminum scraps prepared 
and placed in the molten bath by the method of this invention behaves 
immediately after its introduction into the molten bath and while it is in 
the process of being melted in the bath. In the drawing, 5 denotes a 
compressed mass. Normally, the compressed mass settles to the bottom of 
molten bath as illustrated, immediately after its introduction into the 
molten bath, there to be melted. Thus, the duration of the exposure of the 
aluminum scraps to the ambient air is minimized and, consequently, the 
melting loss is minimized. 
The method of the present invention may appear to be similar to the 
conventional method which effects the required melting of new clippings of 
pure aluminum by pressing the scraps. However, the method of the present 
invention compresses the aluminum scraps for the purpose of increasing the 
apparent specific gravity of the scraps, whereas the conventional method 
presses the scraps solely for the purpose of decreasing the surface area 
of scraps. Thus, the conventional method sufficiently attains its object 
by pressing the scraps with a low pressure. In contrast, the method of 
this invention is required to convert aluminum scraps into a compressed 
mass and, for this purpose, it is absolutely necesary to compress the 
scraps under a magnitude of pressure greater than the yield strength of 
the scraps. 
Specifically, the present invention has an object different from that of 
the conventional method which resides in decreasing the apparent surface 
area of scraps. Thus the two methods entirely differ from each other in 
terms of the magnitude of pressure exerted upon the aluminum scraps under 
treatment. The apparent maximum specific gravity of scraps after 
application of such pressure is only 1.6 in the case of the conventional 
method and as much as 2.35 or over in the case of the method of the 
present invention. 
Now the effect of the present invention will be described with reference to 
working examples of this invention and comparison examples. 
EXAMPLE 1 
Turnings from aluminum slabs (having a composition of 0.45% of Fe, 0.18% of 
Si and the balance of Al, yield strength of 1000 to 1400 kg/cm.sup.2 and 
apparent specific gravity of 0.35) were placed in a container 140 mm in 
diameter and compressed under a strength of 140 tons with a double-action 
type hydraulic press, to afford a compressed mass having a height of 85 
mm, a weight of 3309 g and an apparent specific gravity of 2.53. The 
compressive strength used in this case was 1,400,000 kg.div. (70.times. 
70.times. 3.14)= 91 kg/cm.sup.2. About two tons of such compressed masses 
were melted in a molten bath within a 5-ton reverberatory furnace. The 
melting yield was 98.5%. 
Comparison Example 1 
About two tons of turnings from aluminum slabs identical with those used in 
Example 1 were placed, in their unaltered form, in a molten bath within a 
5-ton reverberatory furnace to be melted therein by the liquid-heel 
process. The melting yield in this case was 94.2%. 
Comparison Example 2 
The same turnings from aluminum slabs as those used in Example 1 were 
compressed with a 60-ton press into a mass having a cross-sectional area 
of 300 mm.times. 400 mm so as to decrease the apparent surface area. The 
compressive strength was 60,000 kg.div. (300.times. 400)= 0.5 kg/mm.sup.2 
and the apparent specific gravity was 1.45. About two tons of the 
compressed mass was melted in the molten bath within a reverberatory 
furnace by following the procedure of Example 1. The melting yield in this 
case was 95.3%. 
Comparison shows that the melting yield obtained in Example 1 was higher 
than that obtained in Comparison Example 1 or Comparison Example 2. 
EXAMPLE 2 
New clippings of pure aluminum having a thickness of about 0.5 mm (having a 
composition of 0.45% by weight of Fe, 0.18% by weight of Si and the 
balance of Al, a yield strength of 1200 to 1400 kg/cm.sup.2 and an 
apparent specific gravity of 0.28 to 0.48) was compressed by a compressive 
force of 140 tons in much the same way as in Example 1, to afford a 
compressed mass having a height of 80 mm, a weight of 3139 g and an 
apparent specific gravity of 2.55. About two tons of such masses were 
melted in the molten bath within a 5-ton reverberatory furnace by 
faithfully repeating the procedure of Example 1. The melting yield in this 
case was 99.1%. 
Comparison Example 3 
About two tons of new clippings of pure aluminum identical with those used 
in Example 2 were placed in the molten bath within the same reverberatory 
furnace as used in Example 1 and melted by the liquid-heel process. The 
melting yield in this case was 95.2%. 
Comparison Example 4 
The same new clippings of pure aluminum as those of Example 2 were 
compressed with a 60-ton press into a mass having a cross-sectional area 
of 300 mm.times. 400 mm for the purpose of decreasing the surface area. 
The compressive force was 60000 kg.div. (300.times. 400)= 0.5 kg/mm.sup.2 
and the apparent specific gravity was 1.49. About two tons of such masses 
were placed within the reverberatory furnace and melted by repeating the 
procedure of Example 2. The melting yield in this case was 96.3%. 
Comparison shows that the melting yield obtained in Example 2 was higher 
than that of Comparison Example 3 or Comparison Example 4. This clearly 
indicates the effectiveness of this invention. 
EXAMPLE 3 
Aluminum scraps 1000 mm in length and 0.3 to 1.5 mm in thickness (having a 
composition of 0.45% by weight of Fe, 0.18% by weight of Si and the 
balance of Al and a yield strength of 1000 to 1400 kg/cm.sup.2) were 
subjected to the first shredding to produce compressed fragments about 300 
mm in average diameter. Then, the shredded fragments were separated of 
extraneous particles such as paper and paint debris by gravity separation. 
The fragments were thereafter subjected to the second shredding into 
particles having an average particle diameter of 15 mm (with a particle 
size distribution such that particles having diameters up to 2 mm 
accounting for 5% by weight, those having diameters from 2 to 20 mm 
accounting for 95% by weight and those having diameters of 20 mm or over 
accounting for 5% by weight respectively). The particles were freed from 
iron particles by means of magnetic separation and, thereafter, compressed 
under a magnitude of pressure of 2000 kg/cm.sup.2 with a double-action 
type press, to afford compressed masses. These compressed masses had an 
average weight of about 2 kg and a bulk density of 2.50. A total of 2300 
compressed masses weighing 4600 kg were placed in a 5-ton reverberatory 
furnace and melted. The melting yield in this case was 97.6%. 
Comparison Example 5 
The same aluminum scraps as those of Example 3 were pressed, in their 
unaltered form, with a hydraulic press. The pressed masses were treated in 
the reverberatory furnace by faithfully following the procedure of Example 
3. The melting yield in this case was 95.6%.