US2890879A - Apparatus for melting and handling molten metals - Google Patents

Description

June 16, 1959 F. L. HOWARD 7 APPARATUS FOR MELTING AND HANDLING MOLTEN METALS Filed Sept. 9, 1954 4 Sheets-Sheet l INVENTOR FRANK L. HOWARD ATTORNEY June 16, 1959 F. L. HOWARD 2,390,879

APPARATUS FOR MELTING AND HANDLING MOLTEN METALS Filed Sept. 9, 1954 4 Sheets-Sheet 2 INVENTOR FRANK L HOWARD ATTORNEY F. L. HOWARD June 16, 1959 APPARATUS FOR MELTING AND HANDLING MOLTEN METALS Filed Sept. 9, 1954 4 Sheets-Sheet 3 N E L O M MOLTEN lNV EN TOR FRANK L. HOWARD ATTORNEY ATT NEY 4 Shets-Sheet 4 F. 1.. HOWARD June 16, 1959 APPARATUS FOR MELTING AND HANDLING MOLIEN METALS Filed Sept. 9, 1954 APPARATUS FOR MELTING AND HANDLTNG MOLTEN NIETALS Frank L. Howard, Trentwood, Wash, assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Application September 9, 1954, Serial No. 455,028 2 Claims. (Cl. 266-43) This invention relates to the handling of molten metals. More particularly, this invention relates to a method and apparatus for handling molten metals, e.g., aluminum and aluminum alloys, wherein loss of metal due to penetration into the refractory lining of the containing receptacle, chemical and physical attack of the refractory lining by the melt, contamination of subsequent melts and problems involving cleaning of the receptacle lining have been eliminated or substantially reduced.

In the casting ofmetals such as aluminum and aluminum alloys, e.g., continuous or direct chill casting or other casting procedures, the metal is generally melted in open hearth or reverberatory furnaces which may be heated by means of oil, gas, coal or coke. The open hearth furnace usually comprises a melting hearth and a holding hearth lined with suitable refractory material and being in metal flow relationship. The charge of aluminum and any desired alloying constituents are preferably first added to the melting hearth to be melted and thereafter the molten metal is transferred to the holding hearth where it is subjected to cleaning treat ments and where control is had of the composition and temperature of the molten bath. The treated. molten metal may then be transferred from the holding hearth to the casting mold by means of a refractory lined transfer trough or poured directly from the furnace into the mold which may contain a baffle or metal distributor made of refractoryrnaterial. Alternatively, the molten metal may be tapped from the holding hearth into a heated holding ladle, also lined with a suitable refractory material, and the ladle moved to the casting station where the molten metal is, poured into a suitable transfer trough or directly into a mold. Although the open hearth or reverberatory furnace is conventionally used in melting aluminum and aluminum alloys, other types such as electric resistance heated or induction heated furnaces are sometimes utilized and, like the reverberatory furnace, are lined with a suitable refractory material. I I

The expression continuous casting referred to hereinabove pertains to a process wherein molten metal is fed into an open-ended mold which is continuously cooled and the casting is continuously Withdrawn by any suitable means fromthe opposite end while at the same time a supply of cooling fluid continuously contacts the embryo casting as it emerges from the mold. Such a casting process may be of a strictly continuous nature (in which the casting is cut to length without interruption of the casting procedure) or where the casting is of a semi-continuous nature, i.e., a casting of desired length may be cast, the flow of metal stopped, the casting removed and the procedure commenced anew.

The handling of aluminum and aluminum alloys in the manner above described has presented many problems in the past with regard tolmolten metal-refractory contact. Molten aluminum attacks most refractories by both chemical and physical action. Aluminum acts as ited States Patent a powerful reducing agent and is capable of reducing compounds such as silica and iron oxide to silicon and iron, respectively. Molten aluminum is also capable of penetrating into the pores of refractory material (bricks, plastic refractories, mortars, castables, ramming mixes, etc.) to a high degree in certain instances, resulting in a considerable increase in surface area exposed to chemical action. Such penetration may also give rise,- in the case of intermittently heated furnaces, to spalling or rupture when metal, which has solidified within the pores, is remelted, as the coeflicient of expansion of the metal is considerably greater than that of the refractory. The attack and penetration of refractories by molten aluminum, besides the disadvantages flowing therefrom of loss of metal and decreased refractory life, present a serious problem of contamination of the melt by element pick-up from the refractory and contamination of subsequent melts of different composition due to .prior melt metal contained within the pores of the refractory.

A further problem present in the melting of aluminum and its alloys is one of cleaning the refractory to free it of metal and skim or dross. During use of the furnace, metal and skim tend to adhere to and build up on the refractory walls and this coating gradually becomes quite dense and hard. The coating is generally removed periodically by chipping and, as a result, the refractory may be damaged.

It has been found, according to the present invention, that chemical and physical attack of refractories by molten metal such as aluminum and aluminum alloys, loss of metal due to penetration into the refractories, contamination of melts, and problems involving cleanability of refractories can be eliminated or substantially reduced by providing at least. that portion of the refractory to be contacted by the molten metal with a protective coating of glaze comprised essentially of boric oxide. Such a coating serves as an excellent barrier against contact and attack of the refractory by the molten metal. Additionally, such coatings are very adherent to the surface of the refractory with the result that there is little tendency for loss of coating due to flaking, peel ing, etc. Moreover, theprotective coating is of low cost and can be readily and simply applied. 7

Accordingly, it is a'primary purpose and object of this invention to provide a novel method and means for handling molten metals which eliminates or substantially reduces the problems attendant in metal handling as heretofore known. t

Another object of this invention is to provide a novel method of handling molten metal, e.g., aluminum and aluminum alloys, wherein loss of metal by penetration into the refractory containing surface and contamination of the molten metal is eliminated or substantially reduced.

A' further object of this invention is to provide an improved receptacle or furnace structure for the melting and handling of molten metals wherein physical .and chemical attack of the refractory lining bythe melt is eliminated or substantially reduced and the cleanability of the refractory is greatly improved, thereby increasing the useful life of the refractory lining.

Other objects and advantages of the invention will .be readily apparent from the following detailed description thereof.

As set forth hereinabove, the present invention generally comprises providing the containing (refractory) surface or surfaces for the molten metal with a protective coating of glaze comprised essentially of boric oxide. Although there are various methods of applying the glaze, the preferred method is to first form a solution of boricacid in a suitable liquid vehicle such as water. The term solution, as used herein, is meant to include Patented June 16, 1959 3 unsaturated or saturated solutions. or dispersions of the material in the vehicle. The refractory lining of the furnace, or at least that portion which will be in contact with the molten metal is thereafter given a coating of the solution. The coating step can be accomplished in various manner, e .g., spraying, brushing or swabbing the solution upon the refractory surface.

After the refractory surface has been'coated with the solution'ofboric acid the coating is dried. Although the coating may be dried at room temperature, it has been found that for practical commercial operations, from the standpoint of time, it is preferred to dry the coating by first subjecting it to air at room temperature and thereafter to. heat the coating to a temperature of from about 60 C. to, 160 C. for a predetermined time. Care should be exercised in not using a drying temperature so high that the volatile constituents of the coating volatilize at such a rapid rate as to cause a portion of the boric acid to. be removed from the surface of the refractory material. After the coating is dried it is then subjected to firing at a temperature of from about 500 C. to about 1200 C., the preferred temperature-being in a range of from about 800 C. to 1000 C. The heating of thecoating to the firing temperature can be accomplished over a wide range of heating rates, heatingrates as low as 11 C./hr. and as high as 200 C./hr. and over having been used. For practical purposes the rate of heating should preferably be as high as possible. However, there are certain instances where a low heating rateis desirable as where some of the refractory material is in the form of a castable refractory and still contains some water which must be removed. The coated refractory is held at the selected firing temperature for a time sufficient to transform the boric acid into boric oxide and to cause the melting or softening of the resultant boric oxide such that sufficient flow ofthe materialwill occur to produce a continuous coating of boric oxide glaze.

It is to be understood that the firing temperature of the boric acid coating could be less than 500 C. or over 1200* C., but that for practical purposes the broad range is limited to from about 500 C. to 1200 C. Boric oxide, being a glassy material, melts or softens over a considerably wide range of temperature. If a temperature less than about 500 C. is used as the firing temperature, the

viscosity is so great that the time period necessary to provide a satisfactory continuous coating would be greatly increased. On the other hand, if the firing temperature used is over about 1200 C. there tends to occur a slow vaporization of the boric oxide.

Refractory lined receptacles or furnaces treated according to the instant invention as above described are provided with a continuous, adherent boric oxide glaze which eliminates or substantially reduces chemical and physical attack of the refractory by molten aluminum and its alloys contained therein, loss of metal due to penetration into the refractory and contamination of subsequent melts of different composition. Moreover, the glaze makes it possible to remove the solidified metal and skim or dross from the furnace lining with ease. Such coating also possesses the property of having good resistance to peeling', flaking or chipping during furnace operation.

The thickness or weight of the boric oxide coating can be readily varied, asdesired. Preferably, it is desired to use a coating Weight single application) of from about 0.10 to 0.40 pound of boric acid per square foot of refractory surface. One method of increasing the thickness of the protective coating is by originally coating the refractory with thej boric acid solution, drying the coating, and thereafter repeating the operation one or more times untilft-he desired thickness of boric acid coating has been applied. Thereafter, the coating can be fired to produce a relatively thick boric oxide glaze on. the refractory. Where a relatively thin glaze is desired, this may be accomplished merely by varying the concentration of boric acid in the original solution, which, as, mentioned hereinbefore, is meant to include any suitable proportions of boric acid and vehicle. Where a slurry is used, it has been found desirable to periodically agitate it to ensure proper distribution of the dispersed material. Additionally, it may be desirable in certain instances, depending on factors such as boric acid particle size, percent of vehicle used, and time the slurry is contained in a receptacle, to add thereto a small but effective amount of a conventional dispersing agent, e.g. Bentonite or methylcellulose, for purposes of stabilizing the dispersion and hence retard settling out of the particles. The amount of dispersing agent to be added, if desired, can be determined with a nominal amount of trial and error experimentation.

Although the above-described embodiment of the invention pertains to the use of boric acid as the starting material, it is to be understood that other materials can be used, the primary requisite being that the resultant fired coating consist essentially or substantially of boric oxide. Boric oxidemay be used as the starting material rather than boric acid. In this case, however, it is preferable to use a suitable non-aqueous vehicle such as normal butanol. Additionally, mixtures of boric acid and boric oxide in a suitable vehicle can be satisfactorily used.

When using an aqueous solutionof boric acid the vehicle content may vary widely to suit the particular application. The vehicle, e.g. water, can be present in from small but effective amounts up to as much as 70% by weight or over, the only restriction being that the solution should be one which can be applied to the refractory material with ease and with substantial uniformity. Where the vehicle content is toov high, the resultant coating will necessarily be very thin and, as such, may necessitate repeated coating operations. On the other hand, where the vehicle content is very low, the coating solution has a tendency to be more or less stiff and difficult to apply to the refractory material in'an even manner. For practical purposes it has been found preferable to use a vehicle content of from about 40% to 70% by weight.

It is contemplated, within the scope of the invention, that various constituents may be added to the coating solution to facilitate ease of applying the wet coating to the refractory material and of drying and firing the coating. A small amount of binder may be added to the solution to aid in holding together the boric acid or boric oxide particles during and after drying in preparation for the step of firing the coating. Materials such as gum arabic, etc., can be suitably used in small amounts as binder. For purposes of promoting the spreading and wetting of the coating on the refractory material, small but effective amounts of materials such as Nonic 2'18 tion composition. Such materials, as hereinabove enumerated for use in facilitating ease of applying and drying the Wet coating are generally volatilized during the step of firing and hence are not present in the final fired coating.

Although the principal objects of the invention are realized by the provision of the coating of boric oxide glaze on the refractory materiahit is further contemplated within the scope of the invention that certain constituents may be added to the coating in 'minor amounts to provide additional qualities to the glaze. For example, carbides may be added to increase the abrasion resistance of the coating. Compounds such as alumina can be used to control the viscosity of the glaze at its ultimate service temperature. It is to be understood that the particular compounds or materials and the amounts thereof which are added will necessarily be dictated to a certain extent byfactors such as refractory material being used, thickness of" glaze desired, composition of molten metal being handled, and possible mechanical wear the glaze will be subjected to in use.

The improved results obtained by the invention are more fully illustrated with reference to the various photographs (actual size) shown in the accompanying drawings, Figures 1 to 8, which represent the results of laboratory tests performed on refractory materials. These test results are intended as specific illustrations of the invention and are not to be regarded as a limitation thereof. I

The refractory materials utilized were Korundal fire brick having an approximate chemical composition of 91% alumina, 8% silica and 1% (titanium oxide, iron oxide, calcium oxide, magnesium oxide and alkalies) and Walsh XX high heat duty fire brick having an approximate chemical composition of 24-43% alumina, 52-60% silca, 22.4% titanium oxide and 36% (iron oxide, calcium oxide, magnesium oxide and alkalies). For each test one refractory brick was unprotected while another brick of the same refractory was provided with a coating solution of the invention comprising, approximately, by weight, 35.4% boric acid and 64.6% water together with about 0.01% by weight Nonic 218 Wetting agent. The coating was applied to the surface or surfaces to be in contact with molten aluminum alloy by brushing or spraying and the coating weights ranged from 0.18 to.0.23 pounds of boric acid per square foot of brick surface (equivalent to about 0.10 to 0.13 pounds of boric oxide per square foot of brick surface). The coatings were subjected to room temperature atmosphere for 18 hours, heated to 110 C. for 24 hours, and fired at 870 C. for about 30 minutes to produce the glaze. The metal uti lized in the tests was 758 aluminum alloy, the chemical composition of which was approximately 1.6% copper, 0.30% iron, 0.19% silicon, 0.10% manganese, 2.5% magnesium, 5.8% zinc, 0.20% chromium, 0.03% titanium, balance aluminum.

Figures 1 and 2 illustrate cross-sections of Walsh XX bricks glazed and unprotected, respectively, after having been immersed for 96 hours in molten 75S alloy maintained at a temperature of about 760 C. No metal penetration could be detected in the glazed brick of Figure 1, but the unprotected brick shown in Figure 2 was penetrated by metal to a depth of about onehalf inch. It is to be noted with further reference to Figure 2, that the central portion of the cross-section appears dark. This dis-coloration is believed due to the partial reduction of oxide impurities in the brick.

Figures 3 and 4 illustrate cross-sections of Korundal brick glazed and unprotected, respectively, after having been immersed for 269 hours in molten 7 5S alloy maintained at a temperature of about 760 C. No metal penetration could be detected in the glazed brick of Figure 3, but the unprotected brick shown in Figure 4 was completely penetrated by the molten metal.

Figures 5 and 6 illustrate cross-sections of Walsh XX bricks which were each provided with a cavity containing molten 75S alloy at 760 C. for 48 hours. Figure 5 illustrates the brick wherein the cavity was provided with the glaze prior to receiving the molten metal whereas Figure 6 represents the brick wherein the cavity was unprotected. No metal penetration could be detected in the glazed brick of Figure 5, but considerable penetration had taken place in the case of the unprotected brick shown in Figure 6. Also, it was found that this metal penetration caused ultimate rupture of the brick.

Figures 7 and 8 illustrate the external surface appearance of Korundal" bricks glazed and unprotected, respectively, which were partially immersed for 269 hours, in molten 75S alloy maintained at about 760 C. It is to be noted that in the case of the unprotected brick molten metal penetration extended about 1% inches above the maximum metal level, possibly due to capillary efiects.

For purposes of determining the extent of boron pickup by the molten metal from the protective coating, ad-

ditional tests were conducted wherein glazed Korundal bricks were immersed for 260 hours in molten 99.85% aluminum maintained at about 760 C. Chemical analysis of the metal showed 0.000% boron. Analyses were also made of the aluminum alloy used in the tests hereinbefore discussed, utilizing the protective coating, with the result that no boron was found in the metal.- Analyses were also made of the metal contained in the cavity bricks shown in Figures 5 and 6 to determine the extent of silicon pick-up by the metal and it was found that the metal in contact with the unprotected Walsh XX brick picked up about 0.40 pounds of silicon per square foot of exposed brick surface, where as the metal in contact with the coated brick showed no silicon pick-up.

Figure 9 is a diagrammatic illustration of one form ofapparatus embodying the principle of the invention and wherein there is shown a melting hearth or furnace 1, a holding hearth or furnace 2, transfer means 3 for facilitating passage of molten metal from the melting hearth to the holding hearth, and a boric oxide coating 4 provided on that portion of the hearths adapted to be in contact with the molten metal.

The protective coating of the instant invention markedly improved the cleanability of the refractory. The adherent film of metal and skim or dross on the surface of the coated bricks was readily removed thereby appreciably reducing the possibility of contamination of subsequent melts of different composition resulting in off-analysis of the melt, whereas that on the surface of the unprotected bricks was difiicult to remove and in some instances impossible to remove without damaging the refractory.

Due to the ease of cleaning off the metal and skim or dross and the absence of metal penetration, it is now possible, in the operation of reverberatory furnaces, to form an aluminum melt of one composition, remove same from the furnace, and provide an aluminum melt of different composition in the same furnace ready for casting without the necessity of an intermediate wash cast to remove contaminants as was heretofore used. By the expression wash cast is meant that after a first aluminum melt of a given composition is removed from the furnace and cast or other disposition made thereof, another body of molten metal is provided in the furnace and preferably agitated for the purpose of dissolving or otherwise removing residue of the previous molten metal composition adhering to the surface of the refractory brick. The composition of the was metal used may be identical to that which will subsequently be charged to the furnace or it may be a composition such as aluminum of commercial purity or other composition having the same or less alloying constituents as that to be subsequently charged to the furnace and containing such alloying constituents in smaller amounts. After a predetermined period of time the wash metal is withdrawn from the furnace and the desired production metal charge added thereto. In certain instances it has been found heretofore that more than one wash cast was necessary in order to avoid undesirable contamination of the subsequent melt.

It will thus be seen from the hereinabove description that by practice of the invention the treatment and handling of molten metals, e.g., aluminum and aluminum alloys, in refractory lined apparatus such as ladles, furnaces, transfer troughs, crucibles, or other form of receptacle, can be accomplished with elimination or substantial reduction of prior known disadvantages such as chemical and physical attack of the refractory material, loss of metal due to penetration into the refractory material, contamination of the molten metal by reduction of refractory constituents and/or by the presence of metal penetration from prior melts of dissimilar composition, and difficulty in removing the adherent film of metal and skim or dross from the refractory surface.

Also the refractory life has been greatly increased. Additionally, the invention can be utilized with relatively small cost inasmuch as the protective coating material is of low cost and can be readily and simply applied. Furthermore, the invention makes it possible to utilize cheaper refractory material for purposes of receptacle lining than has heretofore been satisfactorily used since the tendency for physical and chemical attack of the refractory and contamination of the melt thereby is substantially reduced.

Although the invention has been specifically described, by way of examples, with reference to refractory material of the alumina-silica series, it is to be understood that it has application to other refractory materials which are compatible with boric oxide, that is, no undesirable reactions occur therebetween at the firing temperature used to produce the desired glaze. Also, although the invention has been specifically illustrated with regard to the handling of molten aluminum and its alloys, the invention has application to the handling of other metals wherein similar problems exist.

It will be understood that various changes, modifications and alterations may be made in the instant invention without departing from the spirit and scope thereof and, as such, the invention is not to be taken as limited except by the appended claims, wherein I claim:

1. A receptacle for use in the melting of metals, said receptacle being provided with a lining of refractory material and at least that portion of said refractory lining adapted to be contacted by molten metal being provided with an adherent and continuous coating on the surface thereof, said coating consisting essentially of boric oxide.

2. A reverberatory furnace for use in the melting and holding of metals comprising a melting hearth and a holding hearth, said hearths being in molten metal flow relationship and being provided with an adherent and continuous refractory lining, and at least that portion of said lining adapted to be contacted by molten metal being provided with a coating consisting essentially of boric oxide.

References Cited in the file of this patent UNITED STATES PATENTS 82,313 Heatley Sept. 22, 1868 137,554 Lansdowne Apr. 18, 1873 285,462 Canavan Sept. 25, 1883 861,487 Veitch July 30, 1907 1,815,605 Allen July 21, 1931 1,835,113 Iredell Dec. 8, 1931 1,852,162 Harris et al. Apr. 5, 1932 2,027,065 Sadtler Ian. 7, 1936 2,301,027 Ennor Nov. 3, 1942 2,376,518 Spence May 22, 1945 Poland June 6, 1950

Claims (1)

1. 2. A REVERBRATORY FURNACE FOR USE IN THE MELTING AND HOLDING OF METALS COMPRISING A MELTING HEARTH AND A HOLDING HEARTH, SAID HEARTS BEING IN MOLTEN METAL FLOW RELATIONSHIP AND BEING PROVIDED WITH AN ADHERENT AND CONTINUOUS REFRACTORY LINING, AND AT LEAST THAT PORTION OF

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