US3685984A - Removing metal carbides from furnace systems - Google Patents

Abstract

DEPOSITS OF METAL CARBIDES, SUCH AS ALUMINUM CARBIDE, TITANIUM CARBIDE AND SILICON CARBIDE, FREQUENTLY ACCUMULATE IN HIGH TEMPERATURE FURNACES, LADLES AND CONDUITS UTILIZED IN THE THERMAL PROCESSING OF METALS. THESE HIGHLY DURABLE DEPOSITS ARE CONTACTED WITH MOLTEN IRON, NICKEL, OR MANGANESE ALLOY TO DECOMPOSE THE DEPOSIT AND THEREBY RENDER THEM SUFFICIENTLY FRIABLE FOR CONVENIENT PHYSICAL REMOVAL.

Description

United States Patent 3,685,984 REMOVING METAL CARBIDES FROM FURNACE SYSTEMS Gilbert S. Layne and James 0. Huml, Midland, Mich,

assignors to The Dow Chemical Company, Midland,

Mich.

N0 Drawing. Filed Sept. 4, 1970, Ser. No. 69,926 Int. Cl. C22b 21/00, 21/04 US. Cl. 75-68 B 4 Claims ABSTRACT OF THE DISCLOSURE Deposits of metal carbides, such as aluminum carbide, titanium carbide and silicon carbide, frequently accumulate in high'temperature furnaces, ladles and conduits utilized in the thermal processing of metals. These highly durable deposits are contacted with molten iron, nickel, or manganese alloy to decompose the deposit and thereby render them sufiiciently friable for convenient physical removal.

In the thermal refining of aluminum, as illustrated by the teachings of US. Pats. 2,184,705; US. 2,470,305; US. 3,169,854 and US. 3,397,056, highly durable, refractory metal carbides, such as aluminum carbide, titanium carbide, and silicon carbide are likely to form an equipment surfaces. The source of the ingredients for these compounds is usually the impure metal source materials being processed and sometimes ingredients are accumulated from carbon electrodes and siliceous ingredients of refractory bricks. Eventually, the refining process must be stopped and equipment cleaned of such deposits to restore operating capacities and efficiencies. Because these metal carbides are very hard and adhere tenaciously to vessel walls, their removal presents a most difficult problem.

It is an object of the instant invention to provide an improved method for removing metal carbide deposits.

A further object is to provide an improved process for the refining of impure aluminum source materials, particularly in processes which involve reacting aluminum source materials and distilling aluminum as a subvalent aluminum halide.

A still further object is to provide a method for removing metal carbide deposits from converters utilized to contain the aforedescribed refining reaction.

Still another object is to provide a process for treating the metal carbide deposits whereby they are rendered amenable to physical removal in the form of a molten alloy and a highly friable carbon deposit, which is readily scraped from vessel walls.

In accordance with the instant invention, the foregoing objects, and other benefits as will become apparent hereinafter, are accomplished by contacting the metal carbide deposits with a molten iron, nickel or manganese scavenger metal. Examples of the metal carbide deposits are aluminum carbide, titanium carbide, silicon carbide and mixtures thereof inclusive of double carbides such as Al C -SiC. At the temperature of the molten scavenger metal, e.g. from about 1150 to about 2000 C., the metal carbides are decomposed with the metal constituent of the carbide being taken up as an alloying component of the molten metal. To the extent the molten metal is not saturated in carbon, some carbon may also dissolve. The remaining deposit thus becomes a highly friable form of solid carbon which is readily removed from the vessel wall such as by scraping, fluidization or combustion.

As used herein, scavenger metal means any metal having one or more of iron, nickel or manganese, the totalamount of such metal or metals constituting at least about 50 weight percent of the metal, so long as 3,685,984 Patented Aug. 22, 1972 the metal is characterized by a melting point less than about 2000 C. Examples of suitable metals in addition to the essentially pure elemental metals include alloys such as pig iron, carbon steel, scrap iron, silvery iron, sponge iron, iron nickel alloys, iron manganese alloys and alloys of iron with aluminum, silicon and titanium as minor constituents. It will be manifest to those skilled in the art that the particular iron, nickel or manganese containing material utilized in the instant invention should be capable of dissolving the metal constituent of the carbide and thus should not be saturated with respect to such metal. In other words, metals are operable if they already contain aluminum, titanium or silicon so long as the amounts thereof leave the metal, in its molten state, unsaturated with respect to the component to be removed.

The invention is particularly useful as applied to the removal of metal carbides from converters and conduits connected therewith employed for the refining of impure aluminum alloys by subhalide distillation processes. These processes, which are described in more detail in the above-mentioned patents, generally comprise heating a mixture of the impure aluminum source material and a halide, such as aluminum chloride or magnesium fluoride, up to a temperature where the mixture reacts. -At this temperature, an aluminum halide is formed and volatilized along with the volatile metal or volatile subhalide of the metal, e.g., AlCl. These vapors are then condensed, at least in part, to recover purified liquid aluminum and the starting metal halide as either a gaseous or liquid phase. In one embodiment, magnesium fluoride is utilized in conjunction with the impure aluminum source to produce aluminum fluoride and magnesium vapors. Condensation of these vapors, at the proper temperature, yields easily separable liquid phases of aluminum and magnesium fluoride. The process is further described by Layne and Huml in US. 3,397,056.

Often, metal carbides become partially dispersed or otherwise associated with slag or salt phases that occur in the above refining processes, producing very viscous masses. These are diflicult to drain from furnace systems. Further complication arises when these viscous masses adhere to furnace surfaces and over a period of time accumulate into persistent deposits. By the treatment of the instant invention, such masses are either rendered mobile or more easily removed.

The constituent sources and conditions which promote the formation of the metal carbides are generally well known. For instance, conditions conducive to the formation of silicon carbide are described in US. 3,284,189. Similarly, aluminum carbide deposits in converters and conduits are described in US. 3,292,914 and 3,323,909.

The contacting of the deposits with the molten scavenger metal is carried out in any convenient manner, the particular technique depending somewhat on the location of the deposit. In converters or are furnaces, the contacting is carried out simply by melting the metal in the vessel and allowing sufficient time for the reaction with the carbide to proceed. Usually no more than about 2 hours is required to effect the decomposing reaction. When the deposits are present in conduits, the molten metal may be melted in a separate vessel and poured, or otherwise caused to fiow, through the conduits to eifect the decomposition of the carbides. Preferably, but not necessarily, the molten metal is contacted with the carbide containing deposit under a protective atmosphere. Suitable for this purpose are those gases which are inert to the molten metal such as argon and helium. While the scavenger metal phase is molten, it is removed from the vessel thereby removing also the metal constituent and some of the carbon from the treated carbide deposit. Most of the carbon constituent of the metal carbide remains behind as a highly friable solid which can be readily broken or scraped as may be convenient from the vessel surfaces and physically removed or chemically reacted or dissolved from the system.

The following examples illustrate the utility of the instant invention as applied to the removal of several species of metal carbide deposits.

EXAMPLE I A graphite crucible-hearth containing about 146 grams of silicon carbide and 36 grams of carbon and approximately 75 grams of residual, magnesium fluoride-containing slag Was contacted with 437 grams of iron in the form of scrap cast iron. The crucible was heated to a temperature about 1600 C. at which point the iron scavenger metal was molten. The crucible and its contents were maintained at this temperature for 4 /2 hours. The course of the reaction was followed by periodically withdrawing liquid metal samples, which were analyzed for their silicon content. When the reaction apparently had reached steady state, it was determined, from analysis of the samples, that the molten iron contained about 18-20 percent by weight of silicon and a corresponding amount of silicon carbide had been effectively decomposed so as to produce a residual friable deposit of carbon readily amenable to physical removal from the crucible.

EXAMPLE II In another operation, conducted according to the procedure described in Example I, titanium carbide was substituted for the silicon carbide. With other conditions remaining equivalent, it was determined that the iron had absorbed about 5 weight percent of titanium thereby decomposing the corresponding amount of titanium carbide to produce friable, readily removed carbon deposit. The titanium constituent of the carbide, was simply removed as by pouring the molten iron-titanium alloy from the crucible.

EXAMPLE III A graphite crucible-hearth was charged with 454 grams of aluminum carbide and 681 grams of an iron scavenger metal in the form of scrap mild steel. The charge was heated under a protective atmosphere of argon to a temperature within the range of 17001800 C. for a twohour period. At the conclusion of the reaction period, a liquid metal sample was removed and found to contain 29.7 Weight percent aluminum. This corresponded to the decomposition of approximately 384 grams of the initially charged aluminum carbide. The alloy and residual friable carbon were easily removed simply by dumping the cooled products out of the crucible-hearth.

EXAMPLE IV Aluminum carbide was separated from an aluminum alloy, that had been prepared by the carbothermal reduction of bauxite, by slagging with aluminum sulfide (Al- S The sulfide slag contained approximately 17 weight percent of the aluminum carbide.

To illustrate the carbide deposit in the slag is similarly amenable to treatment in accordance with the invention, the slag was treated with mild steel chips in an amount of percent by weight of the total slag weight at 1500" C. for 2 hours. The reaction was conducted in a graphite crucible under a protective atmosphere of argon.

At the conclusion of the reaction, the system comprised molten metal and slag phases. Each phase was analyzed. The molten metal contained 11.5 weight percent aluminum and 3.3 percent carbon. Based on the aluminum content, it was estimated that approximately percent of the initially available aluminum carbide had been decomposed. Since the iron metal was still unsaturated in aluminum at the conclusion of the initial reaction, further amounts of slag were added to the reaction mixture and it was determined that further decomposition of the aluminum carbide occurred.

In a manner similar to the foregoing examples, substantially equivalent results are achieved by substituting other scavenger metals, such as nickel, nickel-iron alloys, manganese and manganese-iron alloys, for the scrap iron used in the above examples.

What is claimed is:

1. In a process for refining impure aluminum source materials by reacting them with a metal halide at a temperature sufiicient to volatilize the resulting aluminum monohalide and reduction product of the metal halide and condensing the vapors formed thereby to provide a pure aluminum phase and recovering the metal halide, the improvement which comprises periodically terminating the reaction and removing the molten metallic phase from the converter to thereby reveal a deposit containing one or more of aluminum carbide, titanium carbide, and silicon carbide and subsequently contacting such deposit with a molten scavenger metal.

2. A method as in claim 1 and including the additional step of removing the molten scavenger metal phase from the reaction vessel, and any residual friable carbon formed thereby, to remove the metal carbide deposits from the vessel.

3. A method as in claim 1 wherein the scavenger metal is an iron alloy.

4. A method as in claim 3 wherein the deposit contains silicon carbide.

References Cited UNITED STATES PATENTS 2,408,278 9/1946 Stroup et al. 68 R 3,290,141 12/1966 Johnson 7568 R 708,941 9/1902 Tone 7558 585,036 6/1897 Hunt 7558 2,184,705 12/ 1939 Willmore 7593 AC HENRY W. TARRING, II, Primary Examiner U.S. Cl. X.R.