U.S. patent application number 11/870591 was filed with the patent office on 2008-06-12 for air plasma induced low metal loss.
This patent application is currently assigned to Micropyretics Heaters International, Inc.. Invention is credited to Ganta S. Reddy, Jainagesh A. Sekhar.
Application Number | 20080136069 11/870591 |
Document ID | / |
Family ID | 39497028 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080136069 |
Kind Code |
A1 |
Reddy; Ganta S. ; et
al. |
June 12, 2008 |
AIR PLASMA INDUCED LOW METAL LOSS
Abstract
Even a very small amount of air plasma can reduce the dross
during melting. A method and device is shown, whereby substantial
saving in the cost of melting aluminum and the energy to melt
aluminum is possible by the technique of introducing a small amount
of air plasma in the melting environment. In this manner even
though the air contains oxygen, and the common practice is
presently directed at air being eliminated from the melting
environment, an air plasma is able to very effectively be
utilized.
Inventors: |
Reddy; Ganta S.;
(Cincinnati, OH) ; Sekhar; Jainagesh A.;
(Cincinnati, OH) |
Correspondence
Address: |
MICROPYRETICS HEATERS INTERNATIONAL, INC.
750 REDNA TERRACE
CINCINNATI
OH
45215
US
|
Assignee: |
Micropyretics Heaters
International, Inc.
Cincinnati
OH
|
Family ID: |
39497028 |
Appl. No.: |
11/870591 |
Filed: |
October 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10969053 |
Oct 21, 2004 |
|
|
|
11870591 |
|
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Current U.S.
Class: |
266/207 ;
266/165 |
Current CPC
Class: |
C22B 4/005 20130101;
C22B 9/006 20130101; C22B 21/0084 20130101 |
Class at
Publication: |
266/207 ;
266/165 |
International
Class: |
C22B 9/00 20060101
C22B009/00; C22B 21/00 20060101 C22B021/00 |
Claims
1-10. (canceled)
11. An apparatus for processing a metal comprising: a receptacle
for containing the metal; and at least one plasma arrangement
configured to provide a combination of a heated gas and an ionized
gas over a free surface of the metal, wherein, when the plasma
arrangement provides the combination over the free surface, a dross
that is formed when an entire charge of the metal is melted
comprises less than about 3% by weight of the entire charge.
12. The apparatus of claim 11, wherein the combination provides at
least about five percent of the total heat used for melting of the
entire charge.
13. The apparatus of claim 11, wherein the receptacle comprises a
heating arrangement configured to provide heat to the entire
charge.
14. The apparatus of claim 13, wherein the heating arrangement
comprises a plurality of resistance heating elements.
15. The apparatus of claim 14, wherein the resistance heating
elements comprise molybdenum disilicide.
16. The apparatus of claim 11, wherein the heated gas is air.
17. The apparatus of claim 11, wherein the heated gas comprises
oxygen.
18. The apparatus of claim 11, wherein the combination comprises a
small amount of the ionized gases.
19. The apparatus of claim 11, wherein the combination comprises
less than about 1% of the ionized gases.
20. The apparatus of claim 11, wherein the combination comprises
between about 0.5% and about 1% of the ionized gases.
21. The apparatus of claim 11, wherein the metal comprises
aluminum.
22. The apparatus of claim 11, wherein the receptacle comprises at
least one of a launder system or a molten metal transportation
vessel.
23. The apparatus of claim 11, wherein the at least one plasma
arrangement comprises a plurality of plasma arrangements.
24. The apparatus of claim 11, further comprising an enclosure
configured to provide an enclosed region above the receptacle,
wherein the combination is provided into the enclosed region.
25. The apparatus of claim 11, wherein the receptacle comprises at
least one of a furnace, a crucible, a holding furnace, a melting
furnace, or a casting furnace.
Description
PATENTS CITED IN TEXT
[0001] U.S. Pat. No. 5,963,709 and pending patent application Ser.
No. 10/725,6161
Other US Patents
[0002] U.S. Pat. No. 3,648,015.
[0003] U.S. Pat. No. 5,403,453.
[0004] U.S. Pat. No. 5,387,842.
[0005] U.S. Pat. No. 5,414,324.
[0006] U.S. Pat. No. 5,456,972.
[0007] U.S. Pat. No. 5,669,583.
[0008] U.S. Pat. No. 5,938,854.
[0009] U.S. Pat. No. 6,146,724.
[0010] U.S. Pat. No. 6,245,132.
Application:
[0011] The general physical and chemical characteristics of molten
aluminum include: Aluminum melts combine with oxygen, moisture, or
other oxidizing materials to form dross, and the tendency and ease
with which this dross can be entrained in the melt affects the
casting made from the melt. Other factors which affect the casting
made from aluminum and its alloys are, the readiness with which the
melt will absorb nascent hydrogen, and the evolution of hydrogen
during solidification of the casting to form porosity (the
principal source of hydrogen is moisture from the products of
gas/oil combustion); the 3.5 to 8.5% contraction in volume which
occurs when the melt solidifies and the low density of molten
aluminum which results in low hydrostatic pressure in the mold.
Good founding practice begins with good melting practice which is
almost always dependent on the type of melt casting furnace used.
As will be noted in the sections below the use of any electrically
operated system for the melting of aluminum impacts favorably on
dross formation as electrically heated systems minimize convection.
When the aluminum melts react with the atmosphere or moisture, a
dross of aluminum oxide and nitride is formed, which contains some
mechanically entrained gas and metal. Since the dross is wetted by
the aluminum melt and has about the same density, it often becomes
entrained in the melt during melting, handling, or casting, and
does not readily separate at the surface of the melt. It is
commonly believed that the quantity of dross formed during melting
increases with 1) the use of fine or badly weathered or corroded
scrap; 2) the presence of magnesium in the alloys in the charge; 3)
the increase in turbulence (such as from induction melting) which
breaks the protective oxide surface of the melt in the furnace; and
4) the increase in the temperature of gases specially air and
oxygen in contact with the surface. The oxide on the melt surface
contains a considerable amount of liquid metal, causing the dross
layer to be "wet." Common experience has it therefore that high
temperatures cause more dross and that wet dross is increased also
by a higher temperature of melting. The melting/casting furnaces
presently used in the aluminum industry can typically be classified
into three types depending on the source to power the same. These
are resistance-heated furnaces, induction-heated furnaces, and gas-
or oil-fired furnaces. The common types and their advantages are
tested in Table 1. Although each type of the existing melt furnaces
have some advantages, they all suffer from several general
drawbacks, namely high energy cost, high dross, harmful gas
generation, low quality aluminum, and high operational noise, as
individually discussed below: [0012] Resistance-heated furnace--The
resistor elements are inserted in protection tubes or otherwise
suspended and installed in the furnace lining with heat transfer to
the metal by radiation. The general temperature of operation of
these furnaces is between 700 and 1000 C. The heating elements used
are generally made of metallic wires (max temperature that these
can normally reach is about 1050 C) or silicon carbide (maximum
temperature they can reach is about 1500 C). Nevertheless the
normal use of electric furnace to melt or contain aluminum is about
700 to 800 C. The costs for investment, maintenance, and operation
of this type of furnace are high because of the cost of electricity
and when silicon carbide elements are used which frequently are
imbalanced because of aging. [0013] Induction-heated furnaces--In
addition to energy inefficiency, induction furnace are generally
characterized by high maintenance and labor costs and therefore,
the use of this kind of furnace is usually limited only to some
very special applications. Induction furnaces also cause churning
of the liquid leading to oxide (dross) inclusions. There is also a
growing concern about electromagnetic fields (EMF) in the
workplace. However, this issue remains controversial. Again when
using induction furnaces the melt is kept at about 700 to 800 C.
[0014] Gas- or oil-fired furnaces--This type of furnace is more
energy inefficient than the other two types of furnaces because of
uncontrolled combustion flames inside the furnace. All gas- or
oil-fired furnaces suffer from high noise due to the burning
explosive process and serious environmental problem due to the
release of harmful combustion product gases such as PAH, soot,
sulfur dioxide, NOx, and CO. In addition, this type of furnace
usually has low recovery rated because air is allowed into the
furnace for the operation of the gas/oil burners which results in
severe melt loss due to oxidation. Moisture in the gas often leads
to hydrogen pick up in the melt.
TABLE-US-00001 [0014] TABLE 1 Typical operating parameters of
common aluminum furnaces. Dross Energy Efficiency Capital Cost
Indirect Fixed Crucible 5-15% 7-17% low Electric Induction 5-10%
low Very high Direct Flame 5-15% Very low Very low Electric radiant
2-6% 70% Medium Sloping Dry Hearth 5-15% 18% Medium
[0015] It is clearly noted from the chart above that conventional
radiant electric heating is the most efficient and clean method of
heating. The total metal melt loss in dross could be as high as 80%
of the dross weight. In radiant rod furnaces, electric currents of
up to 4000 to 5000 amperes are commonly used to heat silicon
carbide resistance elements which radiate to the furnace load and
walls (note however as described above such elements are not the
most optimal). These furnaces are made to oscillate, thereby
facilitating conduction to the melt from the furnace walls. Radiant
rod furnaces require relatively low investment cost, but are
primarily being used as holding furnace. Operating costs are
impacted by dross formation and energy usage. Typical dross
loss
[0016] The result of reduced dross is significant from our
experiments. We find that even a small amount of air plasma in an
aluminum heating furnace can substantially reduce dross.
[0017] It is common knowledge that nitrogen gas is used as a cover
to reduce the oxidation (dross formation). There are several
technologies which are also used to recover aluminum from dross by
re-melting and cleaning means. Our invention will make possible
substantial savings in melting costs because Nitrogen a gas often
used during melting or holding aluminum to melt aluminum can be
eliminated. The dross is often reclaimed by re-melting thus
incurring energy and productivity penalties. Thus by using our
invention the energy costs are reduced for aluminum processing and
the productivity of aluminum melting can be enhanced. We anticipate
that the product of the invention can be used to separate debris
from aluminum where the debris can be sprues or dross or other
contaminants.
EXPERIMENT # 1
TABLE-US-00002 [0018] Equipment: (H23) Total 23 kW Heating
Elements: Molybdenum disilicide Plsama Airtorch .TM. #: BR Power
Rating: 10.0 KW Inlet input to Plasma Airtorch .TM.: Compressed
air, ~3-4 CFM. Exit dia of Plasma Airtorch .TM.: 3/4'' diameter
.times. 0.5'' length exit nozzle Target: Furnace pouring
spout/launder Material of the Charge: 356 Aluminum alloy ingot
{T.sub.liquidus = 615 C., T.sub.solidus = 555 C.} Weight of Charge:
33 + 6 + 12.5 = 51.5 pounds
TABLE-US-00003 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 3:08 RT RT 0 0 RT 3:09 RT RT 15.5 48.9 RT 3:45
RT RT 23 75 RT Start-up door opened 3:50 37 33 73 RT 3:55 94 37 84
RT 13.3 V, 71 A; Increased from 45 to 50% power 4:00 198 36 84 RT
4:05 203 21 62 4:10 199 20 54 RT 4:15 201 18 51 RT 4:20 228 19 63
RT 60% power 4:25 254 22.9 76 RT 4:30 324 30 94 RT Red glow
started; 65% power 4:35 370 31 96 RT 4:40 400 31 97 RT 4:45 400 39
93 120 4:50 400 37 86 342 4:55 430 37 117 527 80% power; SV = 1500
C., door closed 5:00 541 49 151 682 100% power, Pri = 188 V, Output
dropped to 95% 5:05 642 47 46 728 5:10 696 47 148 700 5:15 753 48
151 679 5:20 794 781 49 153 674 5:25 836 823 50 155 672 5:30 875
864 50 157 659 Soft metal 5:35 902 892 51 159 661 Soft; Gap bottom
closing 5:40 918 990 51 160 1019 Little quantity dripped into
crucible. Shiny liquid 5:42 923 918 52 161 1081 Metal started
pouring semi- continuously (shiny). 5:45 930 926 52 162 1123 Metal
pouring droplets; 5:47 940 935 50 155 1157 More close to continuous
pouring; 92% power 5:48 944 941 50 154 1173 Continuous pouring 5:50
954 951 50 156 1181 Continuous pouring 5:52 980 977 51 159 1188
Continuous pouring 5:54 1079 1080 50 156 1201 Stopped pouring 5:57
1142 1139 50 155 802 Door opened 5:59 1006 993 New ingot of 12.5
pounds charged 6:00 994 987 51 156 650 One more ingot of 6 pounds
charged 6:00:30 Door closed 6:02 952 962 50 155 820 Heating 6:05
952 967 50 155 996 Heating; Click noise twice inside furnace 6:10
980 993 51 158 1075 Tr. Hot; Two droplets fell. 6:12 996 1010 52
161 1097 Metal pouring semi- continuously 6:15 1013 1027 52 160
1106 Metal pouring more continuously 6:17 1029 1039 51 160 1114
Metal pouring more continuously 6:18 1040 1047 51 159 1117
Continuous almost 6:20 1052 1059 51 158 1119 Continuous almost 6:25
1091 1111 50 156 1125 Continuous almost 6:27 1101 1099 Door opened;
Furnace shutdown Results: Weight of dross = 200 grams. Metal is
shiny. % Dross = 0.855
EXPERIMENT # 2
TABLE-US-00004 [0019] Equipment: (H23) Total 23 kW Heating
Elements: Molybdenum disilicide Plasma Airtorch .TM. #: BR Power
rating: 10.0 KW Inlet Input to Plasma Airtorch .TM.: Compressed
air, ~3-4 CFM. Exit: 3/4'' diameter .times. 0.5'' Target: Furnace
pouring spout/launder Material of the Charge: Aluminum alloy ingot
Weight of Charge: 35 pounds
TABLE-US-00005 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 1:24 1500 Ingot charged into furnace; ~1 minute
for loading 1:25 1200 1:30 1034 1:35 1007 Beginning to sag 1:40
1026 Began to flow; Slow dripping Furnace opened brifely 1:45 1016
-900 Steady flow just starting 1:48 1022 Good flow rate; Steady
stream 1:50 1059 Flow slowing - ready to stop flowing Weight of
dross = 134 grams. Metal is shiny. % Dross = 0.84 Cumulative weight
of dross of 2 melts (33 + 6 + 12.5 + 35 pounds) = 334 grams.
Cumulative % dross = 0.850
EXPERIMENT # 3
TABLE-US-00006 [0020] Equipment: (H23) Total 23 kW Heating Elements
Molybdenum diSilicide Plasma Airtorch .TM. #: BR Power rating: 10.0
KW Inlet Input to Plasma Airtorch .TM.: Compressed air, ~3-4 CFM.
Exit: 3/4'' diameter .times. 0.5'' Target: Furnace pouring
spout/launder Material of the Charge: Aluminum alloy ingot Weight
of Charge: 17 + 17.5 = 34.5 pounds
TABLE-US-00007 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 3:15 590 Furnace started & flashing slowly
taken to 1500 C. 3:30 1500 766 46.5 415 Ingot charged 3:32 1400 709
52 475 Some smoking 3:45 750 674 51.9 650 Controller confg changed
from K to B type 3:50 808 729 53.3 Smoking door area 3:55 893 803
54.6 4:00 935 841 56.4 669 Melting 4:04 955 857 56.6 658 Dripping
to flow 4:10 996 900 57 Starting to pour; Good flow. 4:15 1074 981
55.3 Flow stopped Aim: Take melter to 1500 C. and charge new ingot;
4:58 1500 1411 47.8 772 17.5 pound Ingot charged 5:00 1320 1187
51.8 775 5:08 1234 1085 53.1 First drip 5:10 1228 1079 52.9 Flowing
steady 5:13 1279 1129 52.0 Flow slowing 5:14 1330 1200 50.9 Flow
stopped Weight of dross = 180 grams. Metal is shiny. % Dross =
1.149%
EXPERIMENT #4
TABLE-US-00008 [0021] Equipment: (H23) Total 23 kW Plasma Airtorch
.TM. #: BR Power rating: 10.0 KW Inlet Input to Plasma Airtorch
.TM.: Compressed air, ~3-4 CFM. Exit: 3/4'' diameter .times. 0.5''
Target: Furnace pouring spout/launder Material of the Charge: 356
Aluminum alloy ingot {T.sub.liquidus = 615 C., T.sub.solidus = 555
C.} Weight of Charge: 33.5 Pounds
TABLE-US-00009 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 10:25 274 Start up 10:35 444 31.0 131 784 10:48
743 749 421 123 799 10:55 920 924 45.8 122 814 11:10 1100 1103 39.7
115 821 Hold 11:25 1103 1103 33.1 90.7 822 11:55 1150 1150 31.2 146
822 12:05 1280 1254 44 111 822 12:10 1331 150 1:03 1065 1081 Ingot
charged 1:14 1088 1100 Steady flow 1:20 1077 1093 Steady flow 1:25
1079 Stopped flow Weight of dross = 400 grams (for expt # 4 &
5: 34 + 17.5 + 35.5 = 87 pounds total charge) % Dross = 1.013.
Metal is shiny.
EXPERIMENT # 5
TABLE-US-00010 [0022] Equipment: H23) Total 23 kW Plasma Airtorch
.TM. #: BR Power rating: 10.0 KW Inlet Input to Plasma Airtorch
.TM.: Compressed air, ~3-4 CFM. Exit: 3/4'' diameter .times. 0.5''
Target: Furnace pouring spout/launder Material of the Charge: 380
Aluminum alloy ingot. Weight of Charge: 27.5 pounds
TABLE-US-00011 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 11:25 690 40 118 669 11:30 830 40 124 11:36 915
53 152 719 11:40 1000 55 159 719 11:45 1020 42 125 719 11:50 1045
42 121 722 11:55 1085 43 123 722 12:15 1125 35 102 722 12:30 1200
38 105 725 12:35 1265 46 133 724 12:40 1331 47 133 728 1:05 1480 47
134 645 1:10 1500 47 130 742 1:20 1550 1:25 1060 44 124 796 27.5
pounds ingot charged 1:30 1053 43 125 First drips 1:35 1061 43 125
Steady drips 1:37 1065 43 125 787 Steady stream 1:39 1070 43 125
787 Stopped; Shutdown Weight of dross = 125 grams. Metal is shiny.
% Dross = 1.001%
EXPERIMENT # 6
TABLE-US-00012 [0023] Equipment: (H23) Total 23 kW Plasma Airtorch
.TM. #: BR Power rating: 10.0 KW Inlet Input to Plasma Airtorch
.TM.: Compressed air, ~3-4 CFM. Exit: 3/4'' diameter .times. 0.5''
Target: Furnace pouring spout/launder Material of the Charge:
Copper Weight of Charge: 1.35 pounds
TABLE-US-00013 Furnace temperature, .degree. C. Furnace current,
Observations: Time (B-type sensors) Amps Airtorch, .degree. C. of
start and finish Time Process Over temp. Primary Secondary
(K-sensor) pouring. 9:22 RT 41 1600 C./1.5 h 9:25 70 36 9:30 164 38
9:35 265 32 9:40 339 28 9:46 464 39 1600 C./1 hr 9:50 600 46 9:55
741 48 9:57 822 Shutdown 10:15 Re-started 10:20 432 26 10:25 504 27
10:30 563 608 35 10:36 650 698 33 10:40 701 750 39 10:45 768 821 34
10:50 836 893 39 10:55 897 959 39 11:00 971 1034 38 11:09 1092 1157
40 11:15 1176 1243 50 11:20 1237 1307 49.5 11:25 1286 1356 49 11:27
Furnace shutdown due to OVT SP was at 1350; reset to 1720 C. 11:31
1099 1172 23 11:35 1158 1229 32 11:45 1297 1365 41 11:50 1361 1432
47 11:56 1419 1487 46 12:01 1452 525 46 12:05 1474 1548 45.1 12:10
1501 1577 45.1 12:29 1571 1656 44 12:35 1581 1673 44 12:40 1594
1684 44 12:43 1600 1689 40 12:47:00 1600 1690 40 1.35 pound ingot
charged 12:48:22 Started pouring/leaking 12:48 1507 1611 12:49:00
Completed pouring 12:52 1569 1661 Shutdown 1:53 1395 1475 47
Re-started for next melt 2:02 1453 1538 46 2:15 1507 1600 45 2:28
1542 1635 44 2:40 1565 1667 44 2:45 1577 1677 44 3:01:00 to 3:01:30
1600 1701 43 5.0 pounds Cu ingot charged in 30 seconds 3;04:00 1459
3:03:30 Pouring began (Molten metal leaked from bottom hearth)
Summary of Rapid Copper Melting:
TABLE-US-00014 [0024] Time for Furnace temp, melting Time for
Furnace temp. after charging Time for (beginning of complete at the
time of & closing Run # Charge, lbs charging, M:S pouring), M:S
pouring, M:S charging, C. door, C. 1 1:35 0:20 1:22 0.38 1600 1507
2 5:00 0:30 2:00* -- 1600 1459 *Molten metal leaked through the
hearth. Dross Very Low. Shiny.
[0025] From the results and the table 2 we note that in addition to
dross reduction the energy and time required to melt aluminum is
also low when even a small amount of air plasma is present. The
heat transfer coefficient may have been increased because of the
presence of even small amounts of plasma. In our experiments we
estimate that that at least 5% of the total heat came from the
plasma generator.
[0026] Most importantly the dross content is reduced substantially
which is an unusual result and totally unexpected from common
wisdom which is that as the temperature is higher then the dross
increases especially in the presence of hot air. The reason for the
low dross, we suspect possibly comes from the air nitrogen becoming
partially ionized. However, this reasoning is only a speculation at
this stage. Normally it would be expected that an Airtorch.TM.
enhanced melting which uses hot air (i.e. hot oxygen) would show
high dross but the experiments all appear to indicate that the
dross in reality reduced substantially. As discussed below this is
thought to occur because of the plasma content in the air, albeit
small.
[0027] The surface of a metallic part especially if the surface is
electrically conducting, i.e. where electrons are available in
abundance, may give up electrons to the air plasma and also produce
heat according to the reaction:
2N.sup.++2e.sup.-=2N+E (approximately 1480 kJ/mole)
2N=N.sub.2+E
This is a manner in which nitrogen and heat automatically could be
thought to deposit on the surface of aluminum thus increasing the
energy transfer rate substantially as well as providing a cover of
nitrogen gas which prevents oxidation. Typically .about.1 CFM of
air plasma contains in excess of 10.sup.23 atoms and one percent
ionization leads to nearly 10.sup.22 ions which can easily produce
a layers of inert (non oxygen containing) atoms after absorbing
electrons from the solid or liquid metal surface. The air plasma is
expected to be mostly nitrogen plasma although the presence of
oxygen plasma may not be ruled out because the first ionization
energies of nitrogen and oxygen are very similar.
EXPERIMENT #7
[0028] A Plasma Airtorch.TM. with a 3/4'' diameter nozzle system
was used for melting small pieces of aluminum with the sample in
proximity with the hot air plasma atmosphere generated by the
Airtorch. During solidification and cooling the plasma Airtorch was
powered down slowly. The melted and solidified product looked
clean. The clean melt and resultant clean surface solid is
presumably because of the ionized plasma which protected the
aluminum from large oxidation even though the atmosphere contained
mostly air. This is an example which shows that a air plasma can be
used by itself providing all the heat required to melt
aluminum.
[0029] The melting or holding environment comprises of the total
atmosphere in the melting or holding device. When the plasma
generator is a device of the type displayed in FIG. 2 (products of
U.S. Pat. No. 5,963,709 and pending patent application Ser. No.
10/725,6161) the device can be retrofitted to any metal processing
system such as a launder or flowing metal channels or molten metal
pumps.
[0030] The typical devices which may used with the element to melt
or contain liquid metal are furnaces (batch, continuous, holding,
melting), crucibles, laddles, launder systems (channels for moving
liquid metals), holding furnaces, melting furnaces, casting
furnaces, transportation vessels for molten metals and other
similar equipment.
EXPERIMENT #8
[0031] Several small batches about 50 gms of Aluminum alloy 356
were melted in different configurations for a comparative study of
the melting surface on resolidification. One batch was heated with
a Plasma Airtorch.TM.. The result is shown as (A) in FIG. 1 (a
composite photograph). A similar melt was made by heating a small
quantity of aluminum in a regular metallic wire furnace with an air
environment. Yet another sample was heated in a propane torch gas
heating environment atmosphere. All the melts were made in a
crucible and after the melts solidified, the aluminum was removed
from the crucible by tilting the crucible and allowing the solid to
fall out. Notice how much more shiny (A) is compared to the other
two clearly indicating that the molybdenum disilicide melted
material had low dross and was clean. The metal in (A) slid out
much more easily from the crucible. The data conclusively indicates
that the use of an Air Plasma in the heater configuration reduces
the dross content and the metal loss in the dross. Air contains
both nitrogen and oxygen as the predominant gasses. An air plasma
is one that contains a nitrogen ions and electrons. Our experiments
indicate that even a very small amount of air plasma in the air can
substantially reduce the dross.
[0032] A typical device in which the method of air plasma melting
can be done is shown in FIG. 2. In this device aluminum charge is
introduced from one end into a chamber which has heating elements
and a plasma Airtorch is placed on top. The chamber now has the
environment of an air plasma. Clean liquid metal is discharged from
the other end (i.e. a liquid with low dross content).
[0033] An air plasma can be created by the products of U.S. Pat.
No. 5,963,709 and pending patent application Ser. No. 10/725,6161
(herein incorporated fully). Small amounts of thermal plasma may
also be created in very high temperature environments. Very small
amounts of thermal ionization are possible by high temperature
heating elements such as molybdenum, tungsten and molybdenum
disilicide materials. The type of useful plasma for the invention
is one which can be employed at normal or high pressure as opposed
to very low pressure plasma. Plasma can also created by RF means
U.S. Pat. Nos. 3,648,015, 5,403,453, 5,387,842, 5,414,324,
5,456,972, 5,669,583, 5,938,854, 6,146,724, 6,245,132 all
incorporated herein. Not all techniques can produce Air Plasma at
normal pressures and not all techniques except for U.S. Pat. No.
5,963,709 and Ser. No. 10/725,6161 can be considered to produce
substantial heat. Unless an air plasma is used, the cost benefits
to melting aluminum from using air instead of a gas like nitrogen,
helium or argon are difficult to realize. Of course gas plasmas may
also be employed and their use is anticipated.
[0034] The best mode appears to be the use of even a small amount
even as low as 0.5-1% (of the total environment) of air plasma in
any existing or specially constructed device which holds or melts
molten aluminum. In this manner even though the air contains oxygen
and common practice would involve hot oxygen being removed from the
environment, an air plasma is able to very effectively utilize hot
air and yet provide beneficial melting. The environment also
protects against oxidation in the solid cool down or solid heat up
stage.
DETAILED DESCRIPTION OF DRAWINGS
[0035] FIG. 1. This is a color photograph (composite photograph).
Samples A, B and C represent solidified melts of aluminum alloy 356
after melting and solidification. Note sample A is much more shiny
than sample B or sample C. Sample A was melted using a Plasma
Airtorch placed above a crucible containing the initial solid
sample. Sample B was melted by placing in crucible and melting
carried out conventionally with a wire wound electric heater
furnace (max temperature 1000 C) and sample C was melted with a gas
torch heater. Note, the shiny surface of sample A indicates less of
an oxidized surface i.e. the clear difference in the oxide/dross
levels. The samples B and C have clear wrinkled and non-shiny
oxidized surfaces.
[0036] FIG. 2 shows a aluminum melting device consisting of a
furnace box with stand 1.1, for melting and collecting liquid metal
1.2 in a crucible 1.3, through a pouring spout 1.4. The furnace
environment is contained in the refractories (insulation) 1.5.
Molybdenum disilicide heating elements 1.6 and the plasma
Airtorches (plasma generator) 1.7 provide heat to the charge
introduced from the port 1.10. The plasma and hot air from the
plasma generator 1.7 arrive at the charge through a port 1.9. The
Plasma generators 1.7 are held to the main furnace body by clamps
1.8. The metal charge 1.11 is introduced through the port 1.10
which can rotate (with the help of a motor 1.12). The molten metal
1.2 and any separated impurity (for example a entrained sprue
filter) 1.13 are collected as shown. In this embodiment of the
invention three plasma generators 1.7 are shown.
GLOSSARY
[0037] Charge: The ingot, or other parts made of metal which are
melted or heated. The charge can include ingots, cut pieces of
metal, metal chips, or metal waste, or mixed debris and metal.
[0038] Melting: All processes involving partial or fully molten
metal whether in containment, direct melting or transfer
configurations.
[0039] Dross: Oxide and complex oxide scale(s) formed on molten
aluminum or other metals which can additionally contain trapped
metal as well as fluxes.
[0040] Air-Plasma: The plasma obtained from the ionization of air.
The air plasma may contain substantially hot air and a percentage
of ionized air gases.
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