Patent Application
20080136069
U.S. Pat. No. 5,963,709 and pending patent application Ser. No. 10/725,6161
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.
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:
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
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.
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=N2+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.
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
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.
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
A typical device in which the method of air plasma melting can be done is shown in
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.
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.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10969053 | Oct 2004 | US |
| Child | 11870591 | US |
