US3025155A - Chloridizing aluminum - Google Patents

Description

March 13, 1962 M. F. LEE ET AL CHLORIDIZING ALUMINUM Filed NOV. 21, 1958 MAURICE F. LEE KENNETH J. BRONDYKE 4 PAUL D. HESS INVENTORS 7g;

ATTORNEY 3,025,155 Patented Mar. 13, 1962 3,025,155 CHLURHDIZHNG ALUMliNUM Maurice F. Lee, Allegheny Township, Westrnoreland County, Kenneth J. Erondyhe, Oakrnont, and Paul D. Hess, Arnold, Pa, assignors to Aluminum Company of America, Pittsburgh, Pa, n corporation of hennsylvauia Filed Nov. 21, 1958, Ser. No. 775,626

- 9 Claims. (Cl. 75-68) This invention relates to the removal of certain metallic impurities from molten aluminum, and, more specifically, to a method for removing the metallic impurities by reaction with chlorine.

The term aluminum as used herein refers to aluminum and to alloys containing at least 50 percent by weight of aluminum.

Certain metallic impurities are frequently encountered in commercial aluminum, including magnesium, sodium and calcium. These may be introduced through remelting of scrap metal or, in the case of sodium it may come from the refining process. Although the presence of these elements is often desirable in some alloys, in other instances these metals are detrimental to the properties desired in the final product.

Generally, the sodium impurity content does not exceed about 0.2 percent by weight and the calcium content is less than about 0.5 percent. However, magnesium may range as high as 5.0 percent.

It has been proposed to remove magnesium and sodium from molten aluminum by reaction with gaseous chlorine or a reactive vaporous chloride, such as aluminum chloride and organic chlorides. The magnesium and sodium, being more electropositive than aluminum, preferentially react with the chlorine or chloride to form the chloride salt, which rises .to the surface of the melt and may be skimmed off. This process has been referred to as chloridizing aluminum.

Heretofore, this treatment of the aluminum has been accomplished by bubbling the chlorine gas or vaporous chlorides into the molten metal while it was in the melting or holding furnace. Although the process has proven generally beneficial, large volumes of chlorine have been required for substantial removal of the metallic impurities. As a result, appreciable amounts of the aluminum have also been attacked by the chlorine (except when aluminum chloride was employed as the reagent). Furthermore, the presence of chlorine vapors in a melting room have caused severe corrosion problems.

Moreover, because of the quantities of chlorine and the time consumed in the chloridizing treatment, such practice has been restricted to furnaces or ladles and could not be adapted to use in the metal transfer line. Also, considerable amounts of the finely divided solid chlorides as well as other solid non-metallic impurities were entrapped in the molten metal and passed into the cast articles because of the lack of suitable filtration.

The term reactive chlorine-containing vapor, as used herein, includes chlorine gas and vaporous chlorides which are reactive with metals more electropositive than aluminum. The vaporous chlorides, however, should not introduce any contaminants into the molten metal; among those suitable are aluminum chloride and chlorinated hydrocarbons.

It is an object of this invention to provide an improved method for chloridizing aluminum.

Another object is to provide a method for chloridizing the metal in a molten metal transfer line.

It is also an object to provide a method for chloridizing and filtering the molten metal in a single operation.

A specific object is to provide a method wherein sodium,

magnesium and calcium impurities may be removed from aluminum by treatment with reactive chlorine-containing vapor.

Other objects and advantages will become apparent from the following detailed description and attached drawing wherein:

FIG. 1 is a longitudinal section of apparatus suitable for practice of the present invention as located in a molten metal transfer line;

FIG. 2 is a top plan view of the apparatus in FIG. 1 Without showing the molten metal and refractory granules.

It has now been discovered that metallic impurities more electropositive than aluminum, such as sodium, magnesium and calcium, may be subtantially eliminated from molten aluminum by a method in which the molten aluminurn containing the metallic impurities is passed through a bed of refractory bodies while in contact with reactive chlorine-containing vapor. The impurities, being more electropositive than aluminum, preferentially react with the reactive chlorine-containing vapor to form chlorides, and these may thereafter be separated from the purified aluminum.

The chlorides of the impurities formed may be in the molten or solid condition, depending upon the temperature of the molten metal. Magnesium chloride which has a melting point of 1306 F. (708 C.) is conveniently removed as a molten salt by maintaining the unit at a temperature above about 1350 P. so that the salt rises to the surface of the treating vessel and is periodically skimmed olf. Sodium chloride and calcium chloride, on the other hand, which have melting points respectively of 1474 F. (801 C.) and 1421" F. (772 C.) are conveniently removed by filtering as solids at a temperature of about 1350 to 1400" F., although they too may be removed as molten salts by increasing the operating temperature.

The amount of reactive chlorine-containing vapor utilized in the present invention is dependent upon the quantity of metallic impurity present and the desired purity of the final product. For example, to reduce the magnesium content from about 3.5 percent by weight to less than 0.2 percent, a substantially stoichiometric amount of chlorine gas may be employed. However, to reduce the amount of magnesium below 0.1 percent, increasingly greater proportions of chlorine are necessary, generally about 2 to 3 times the stoichiometric proportion. This amount, however, is still far less than the quantity employed in the conventional purification treatments.

Similarly, the sodium content may be reduced to below about 0.005 percent by using substantially stoichiometric quantities of reactive chlorine-containing vapor, but further reductions may be effected by employing up to about five times the amount theoretically required.

For the purposes of this invention, excessive quantities of reactive chlorine-containing vapor are not desired. Whereas the conventional chloridizing techniques have generally used 30 or more times the theoretical amount, this has been at the expense of aluminum and of equipment which is attacked by the chlorine. In this invention, the amount of reactive chlorine-containing vapor utiiized is between 0.5 and 5.0 times that stoichiometrically required depending upon the original impurity content and the desired final value.

It has further been found that by passing the metal through a bed of refractory granules 3 to 14 mesh in size while in contact with the reactive chlorine-containing gas not only are solid chloridized impurities, i.e. sodium and calcium chlorides, and other solid nonmetallic impurities filtered out in the treating bed but the effectiveness of the gas in removing impurities is increased. In this method, the bed should be at least six inches in depth, and preferably eight inches or more, and the reactive chlorine-containing vapor and molten metal should be in contact through at least two inches of bed.

When the chloridized impurities are to be skimmed off the top of the metal in the treating vessel, larger size granules may be employed, on the order of A to 1 inch. However, a greater depth of bed will generally be required, usually 12 inches or more. In practice, a 12 inch bed of inch alumina balls has proven quite satisfactory.

The refractory utilized for the bed must be inert to aluminum. It must also have a melting point higher than the temperature of treatment, possess high hardness and be of sufficient density to gravitationally remain in place during the operation. Chromite, corundum, forsterite, magnesia spinel, periclase, silicon carbide and zircon are suitable refractory materials. Of these, tabular alumina is preferred. All of these materials with the exception of forsterite and zircon are free from silica; but in the case of these two, the silica is chemically combined in such a manner that it is not attacked by the molten aluminum. For this reason, these materials are regarded as being inert towards molten aluminum.

As is well known, chlorine gas and the vaporous chlorides are highly reactive and tend to attack even the above refractories. However, it has been found that considerable service can be obtained before the attack becomes detrimental. Because of the possibility of attack, corundum or alumina granules are preferred since the product of reaction with chlorine is aluminum chloride, which readily escapes from the molten metal and does not become contaminant.

In the embodiment of this invention wherein a bed of fine refractory granules is utilized to filter out the chlo ridized impurities, it has been found desirable to prepare the bed by adding preheated refractory granules to an initial body of molten aluminum in the treatment vessel or container, and to allow the granules to settle gravitationally therein. The bed thus formed has shown improved filtering efiiciency and faster metal flow rates, especially when a reverse or U-shaped metal flow through the treatment container is utilized. Further details on this method may be found in the copending application of K. J. Brondyke and P. T. Stroup, Serial No. 655,746, filed April 29, 1957, now Patent No. 2,863,558. However, the refractory bed may also be prepared dry, i.e., by adding the refractory to the container in the absence of an initial body of molten aluminum. In either instance, the refractory and container should be preheated to a temperature sufiicient to avoid chilling of the molten aluminum, generally about 1200 to 1800 F., and preferably above 1400 F.

Referring now to FIGS. 1 and 2, wherein apparatus is illustrated which is suitable for the practice of the present invention and in which the metal is simultaneously filtered to remove solid impurities. The refractory chamber or crucible 2 is partitioned by the transverse bafile 4 to provide a chloridizing section 6 in which there is situate a gas device 8, which may be a porous or perforated manifold, and which is connected to the gas supply by the entry tube 10. Alternatively, several porous or perforated tubes may be employed. As illustrated, a layer of large refractory particles 12 has been placed at the bottom of the chamber 2, and the fine ceramic bodies 14 constituting the refractory filter medium have been deposited thereon. If the simultaneous filtering action is not desired, the fine ceramic bodies 14 may be eliminated and the layer of large refractory bodies 12 continued to a greater height, preferably at least 12 inches.

In the operation of the apparatus, molten aluminum 16 passes from the inlet trough 18 through the refractory bed of fine ceramic bodies 14 wherein it is in counter-current contact with reactive chlorine-containing vapor rising from the gas device 8. Solid chlorides, as well as any other solid impurities, are removed by the filter bed. The metal 16 then flows under the baffle 4 and upward in the discharge section 20 from which it is discharged into the outlet trough 22.

The surface of the chloridizing section is periodically skimmed to remove molten chloridized impurities, while the solid impurities are removed by the filter bed.

Indicative of the efficacy of the present invention are the following examples wherein aluminum containing impurities was chloridized in apparatus of the type illustrated in the attached drawing.

Example 1 A 1 /2 inch layer of 4 inch alumina balls was deposited in the base of the refractory chamber and the unit was preheated to a temperature of about 1400 F. A porous graphite diffuser was placed upon the balls and then more alumina balls which had been preheated to 1400 F. were superposed to provide .a bed depth of 10 inches. Chlorine gas and aluminum metal fiow were started as quickly as possible to prevent burning of the graphite diffuser. The metal flow rate through the apparatus was 600 pounds per hour and chlorine gas was introduced at the rate of 65 cubic feet per hour (70 F. and 760 mm. pressure). The temperature of operation was 1400 F. and the magnesium chloride formed rose to the surface as a molten salt from which it was periodically skimmed.

The magnesium content of the aluminum was reduced from 0.46 to 0.06 percent by weight, a reduction of 87.5 percent.

Example 2 A porous graphite diffuser was placed in an 8 inch bed of alumina balls and an 8 inch filter bed of 3 to 6 mesh tabular alumina was deposited thereon. The crucible, balls and tabular alumina had been preheated to a temperature of 1400 F. The metal flow rate was 3000 pounds per hour and the chlorine flow rate was 1 /2 cubic feet per hour (70 F. and 760 mm. pressure) and the temperature of the unit was maintained at about 1400 F. The sodium chloride formed was removed by the filter bed .as were other solid impurities.

The sodium content of the aluminum was reduced from 0.0051 to 0.0003 percent by weight, or a reduction of 94 percent.

Example 3 A porous graphite diffuser was placed on a 6 inch bed of /8 to inch alumina balls and another 8 inches of alumina balls were added to the crucible. The alumina and crucible had been preheated to a temperature of 1400 F. Molten aluminum was passed through the unit at a rate of 3000 pounds per hour and chlorine was fed into the bed at the rate of 3 cubic feet per hour (70 F. and 760 mm. pressure). The temperature of the unit was maintained at about 1400 F. In this unit, a slight skim formed on the surface of the metal in the chloridizing section.

Analyses of the metal indicated that the treatment had reduced the sodium content from 0.0024 to 0.0006 percent by weight, for a reduction efficiency of 79.3 percent.

As can be readily observed from the foregoing examples, the method of the present invention is highly effective in the chloridizing of aluminum. The invention is conveniently practiced in metal transfer lines at high flow rates, thus eliminating the long periods of treatment in holding furnaces conventionally employed. Since the efficiency of the system is high, there is little problem of corrosion as opposed to the present practices wherein excesses of chlorine are necessary.

Having thus described the invention, we claim:

1. The method of chloridizing aluminum containing metallic impurities more electropositive than aluminum comprising: providing a bed of refractory bodies inert to molten aluminum in a container in a metal transfer line having means for introducing reactive chlorine-containing vapor into said bed; preheating said refractory to a temperature of 1200 to 1800 F.; completely covering said bed with molten metal and passing molten aluminum containing said metallic impurities through said refractory bed; simultaneously introducing reactive chlorinecontaining vapor into said refractory bed and molten aluminum to react with said metallic impurities to form chlorides the amount of chlorine-containing vapor employed being relatively small and related to the stoichiometric proportions required to form chlorides with the metallic impurities; and thereafter separating said chlorides.

2. The method in accordance with claim 1 wherein said metallic impurities are at least one metal selected from the group consisting of sodium, calcium and magnesium.

3. The method in accordance with claim 1 wherein the amount of reactive chlorine-containing vapor is 0.5 to 5.0 times that stoichiometrically required to react with said metallic impurities.

4. The method of chloridizing aluminum containing metallic impurities more electropositive than aluminum comprising: providing a bed of refractory bodies in a container in a metal transfer line having means for introducing reactive chlorine-containing gas into said bed, said refractory bodies being inert towards molten aluminum and 3 to 14 mesh in size; preheating said refractory to a temperature of 1200 to 1800 .1 completely covering said refractory bed with molten metal and passing molten aluminum containing said metallic impurities downwardly through said bed; simultaneously introducing reactive chlorine-containing vapors within the refractory bed and above the bottom thereof to react with the metallic impurities to form chlorides, the amount of chlorine-containing vapor employed being relatively small and related to the stoichiometric proportions required to form said chlorides, the said molten aluminum and chlorinecontaining vapor passing in counter-current relationship to each other and retaining any solid chlorides in said bed as the metal passes therethrough.

5. The method in accordance with claim 4 wherein said refractory bed is at least six inches in depth.

6. The method according to claim 4 wherein the refractory bed is maintained at a temperature between 1350 and 1400 F.

7. The method in accordance with claim 6 wherein the refractory bed is at least 12 inches in depth.

8. The method of chloridizing aluminum containing metallic impurities more electropositive than aluminum comprising: providing a bed of refractory bodies in a container in a metal transfer line having means for introducing reactive chlorine-containing gas in said bed, said refractory bodies being inert towards molten aluminum and A to 1 inch in size, preheating said refractory bodies to a temperature of 1200 to 1800 F.; completely covering said refractory bed with molten metal and passing molten aluminum containing said metallic impurities downwardly through said refractory bed while maintaining the bed and molten aluminum at a temperature of 1350 to 1600 F.; simultaneously introducing reactive chlorine-containing vapor within the refractory bed and above the bottom thereof to react with the metallic impurities to form chlorides, the amount of chlorine-containing vapor employed being relatively small and related to the stoichiometric proportions required to form said chlorides, the said vapor being in counter-current contact with said downwardly flowing aluminum, any liquid chlorides rising to the surface of the molten aluminum in said container and any solid chlorides being retained in said refractory bed; and thereafter separating the said liquid chlorides from the top surface of the molten aluminum.

9. The method in accordance with claim 8 wherein said metallic impurity is magnesium.

References Cited in the file of this patent UNITED STATES PATENTS 449,815 Cragg Apr. 7, 1891 1,820,141 Jessup Aug. 25, 1931 1,972,432 Girsewald Sept. 4, 1934 1,994,358 Holstein Mar. 12, 1935 2,821,472 Peterson Ian. 28, 1958 2,863,558 Brondyke Dec. 9, 1958 FOREIGN PATENTS 297,086 Great Britain Sept. 11, 1928 684.048 Great Britain Dec. 10, 1952

Claims (1)

1. THE METHOD OF CHLORIDIZING ALUMINUM CONTAINING METALLIC IMPURITIES MORE ELECTROPOSITIVE THAN ALUMINUM COMPRISING: PROVIDING A BED OF REFACTORY BODIES INERT TO MOLTEN ALUMINUM IN A CONTAINER IN A METAL TRANSFER LINE HAVING MEANS FOR INTRODUCING REACTIVE CHLORINE-CONTAINING VAPOR INTO SAID BED; PREHEATING SAID REFACTORYTO A TEMPERATURE OF 1200 TO 1800*F.; COMPLETELY COVERING SAID BED WITH MOLTEN METAL AND PASSING MOLTEN ALUMINUM CONTAINING SAID METALLIC IMPURITIES THROUGH SAID REFRACTORY BED; SIMULTANEOUSLY INTRODUCING REACTIVE CHLORINE CONTAINING VAPOR INTO SAID REFACTORY BED AND MOLTEN ALUMINUM TO REACT WITH SAID METALLIC IMPURITIES TO FORM CHLORIDES TTHE AMOUNT OF CHLORINE-CONTAINING VAPOR EMPLOYED BEING RELATIVELY SMALL AND RELATED TO THE STOICHIOMETRIC PROPORTIONS REQUIRED TO FORM CHLORIDES WITH THE METALLIC IMPURITIES; AND THEREAFTER SEPARATING SAID CHLORIDES.

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