US20080136069A1 - Air plasma induced low metal loss - Google Patents

Air plasma induced low metal loss Download PDF

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Publication number
US20080136069A1
US20080136069A1 US11/870,591 US87059107A US2008136069A1 US 20080136069 A1 US20080136069 A1 US 20080136069A1 US 87059107 A US87059107 A US 87059107A US 2008136069 A1 US2008136069 A1 US 2008136069A1
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plasma
furnace
metal
dross
melting
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US11/870,591
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Ganta S. Reddy
Jainagesh A. Sekhar
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Micropyretics Heaters International Inc
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Micropyretics Heaters International Inc
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B4/00—Electrothermal treatment of ores or metallurgical products for obtaining metals or alloys
    • C22B4/005—Electrothermal treatment of ores or metallurgical products for obtaining metals or alloys using plasma jets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B21/00—Obtaining aluminium
    • C22B21/0084—Obtaining aluminium melting and handling molten aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
    • C22B9/006—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals with use of an inert protective material including the use of an inert gas

Definitions

  • 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.
  • any electrically operated system for the melting of aluminum impacts favorably on dross formation as electrically heated systems minimize convection.
  • 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.
  • 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.
  • Furnace temperature ° C. Furnace current, Observations: Time (B-type sensors) Amps Airtorch, ° 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.
  • Furnace temperature ° C. Furnace current, Observations: Time (B-type sensors) Amps Airtorch, ° 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.
  • 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:
  • a Plasma AirtorchTM 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.
  • the melting or holding environment comprises of the total atmosphere in the melting or holding device.
  • 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.
  • 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.
  • FIG. 2 A typical device in which the method of air plasma melting can be done is shown in FIG. 2 .
  • 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).
  • 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.
  • 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.
  • 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.
  • 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 .
  • molten metal 1 . 2 and any separated impurity (for example a entrained sprue filter) 1 . 13 are collected as shown.
  • three plasma generators 1 . 7 are shown.
  • the charge can include ingots, cut pieces of metal, metal chips, or metal waste, or mixed debris and metal.
  • Dross Oxide and complex oxide scale(s) formed on molten aluminum or other metals which can additionally contain trapped metal as well as fluxes.
  • 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.

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.

Description

    PATENTS CITED IN TEXT
  • U.S. Pat. No. 5,963,709 and pending patent application Ser. No. 10/725,6161
    • Other US Patents
  • U.S. Pat. No. 3,648,015.
  • U.S. Pat. No. 5,403,453.
  • U.S. Pat. No. 5,387,842.
  • U.S. Pat. No. 5,414,324.
  • U.S. Pat. No. 5,456,972.
  • U.S. Pat. No. 5,669,583.
  • U.S. Pat. No. 5,938,854.
  • U.S. Pat. No. 6,146,724.
  • U.S. Pat. No. 6,245,132.
    • Application:
  • 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:
      • 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.
      • 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.
      • 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 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
  • 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
    Figure US20080136069A1-20080612-P00999
  • 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.
  • 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
  • Equipment: (H23) Total 23 kW
    Heating Elements: Molybdenum disilicide
    Plsama Airtorch ™ #: BR
    Power Rating: 10.0 KW
    Inlet input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit dia of Plasma Airtorch ™: ¾″ diameter × 0.5″ length
    exit nozzle
    Target: Furnace pouring spout/launder
    Material of the Charge: 356 Aluminum alloy ingot
    {Tliquidus = 615 C., Tsolidus = 555 C.}
    Weight of Charge: 33 + 6 + 12.5 = 51.5 pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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
  • Equipment: (H23) Total 23 kW
    Heating Elements: Molybdenum disilicide
    Plasma Airtorch ™ #: BR Power rating: 10.0 KW
    Inlet Input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit: ¾″ diameter × 0.5″
    Target: Furnace pouring spout/launder
    Material of the Charge: Aluminum alloy ingot
    Weight of Charge: 35 pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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
  • Equipment: (H23) Total 23 kW
    Heating Elements Molybdenum diSilicide
    Plasma Airtorch ™ #: BR Power rating: 10.0 KW
    Inlet Input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit: ¾″ diameter × 0.5″
    Target: Furnace pouring spout/launder
    Material of the Charge: Aluminum alloy ingot
    Weight of Charge: 17 + 17.5 = 34.5 pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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
  • Equipment: (H23) Total 23 kW
    Plasma Airtorch ™ #: BR Power rating: 10.0 KW
    Inlet Input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit: ¾″ diameter × 0.5″
    Target: Furnace pouring spout/launder
    Material of the Charge: 356 Aluminum alloy ingot
    {Tliquidus = 615 C., Tsolidus = 555 C.}
    Weight of Charge: 33.5 Pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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
  • Equipment: H23) Total 23 kW
    Plasma Airtorch ™ #: BR Power rating: 10.0 KW
    Inlet Input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit: ¾″ diameter × 0.5″
    Target: Furnace pouring spout/launder
    Material of the Charge: 380 Aluminum alloy ingot.
    Weight of Charge: 27.5 pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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
  • Equipment: (H23) Total 23 kW
    Plasma Airtorch ™ #: BR Power rating: 10.0 KW
    Inlet Input to Plasma Airtorch ™: Compressed air, ~3-4 CFM.
    Exit: ¾″ diameter × 0.5″
    Target: Furnace pouring spout/launder
    Material of the Charge: Copper
    Weight of Charge: 1.35 pounds
  • Furnace temperature,
    ° C. Furnace current, Observations: Time
    (B-type sensors) Amps Airtorch, ° 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:
  • 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.
  • 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.
  • 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™ 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.
  • 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 ++2e −=2N+E (approximately 1480 kJ/mole)

  • 2N=N 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 ˜1 CFM of air plasma contains in excess of 1023 atoms and one percent ionization leads to nearly 1022 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
  • A Plasma Airtorch™ with a ¾″ 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.
  • 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.
  • 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
  • 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™. 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.
  • 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).
  • 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.
  • 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
  • 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.
  • 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
  • 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.
  • Melting: All processes involving partial or fully molten metal whether in containment, direct melting or transfer configurations.
  • Dross: Oxide and complex oxide scale(s) formed on molten aluminum or other metals which can additionally contain trapped metal as well as fluxes.
  • 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.

Claims (16)

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.
US11/870,591 2004-10-21 2007-10-11 Air plasma induced low metal loss Abandoned US20080136069A1 (en)

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