US3232858A

US3232858A - Manufacture of magnetite

Info

Publication number: US3232858A
Application number: US261942A
Authority: US
Inventors: Alvin W Holmes; Jr John Varga
Original assignee: General Cable Corp
Current assignee: General Cable Corp
Priority date: 1963-02-25
Filing date: 1963-02-25
Publication date: 1966-02-01
Anticipated expiration: 1983-02-01

Description

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Feb. 1, 1966 A. w. HOLMES ETAL MANUFACTURE OF MAGNETITE Filed Feb. 25, 1963 BY GA,JR.

x9 ,Wiaa/ United States Patent This application is a continuation-in-part of our United States patent application Serial No. 742,470, filed June 17, 1958, now abandoned.

This invention relates to the manufacture of magnetite, particularly to a method of manufacturing magnetite suitable for castings for use as anodes and the like, and a method of manufacturing such magnetite castings.

During the last years of the nineteenth century, magnetite anodes were used almost exclusively in the electrolysis of caustic solutions to manufacture chlorine gas and sodium hydroxide. Around the turn of the century, graphite anodes began to replace magnetite anodes, because the magnetite anodes had not performed consistent ly. It is now apparent that in the manufacture of magnetite, the compositions varied widely, and no means had been developed to control adequately the composition of the magnetite.

In later years, magnetite anodes have been used to a limited extent, particularly in Europe, but the same difficulty of inadequate control over the magnetite composition has been encountered. The magnetite has been manufactured from mill scale, iron ores, or both, by melting in furnaces permitting little or no circulation of air. Because of the excessive-1y high resistivity of much of the magnetite made in this manner, it is common for the anodes made therefrom to be cast in hollow shapes and for the inner surfaces to be plated with copper to increase the conductivity.

Theoretically, magnetite (Fe O is one molecule of ferrous oxide (FeO) plus one molecule of ferric oxide .(Fe O However, most of the materials referred to as magnetite contain varying amounts of Fe O sometimes only enough to make the materials magnetic or able to conduct electricity, most of the balance being ferrous oxide. Magnetite is present naturally in some iron oxides and in mill scale, along with varying amounts of ferrous oxide. Magnetite electrodes containing too much ferrous oxide have low electrical conductivity'and are easily attacked in electrolytic baths. Thus they are inferior in performance and short-lived. Much of the failure of magnetic anodes before the present invention apparently is attributable to the presence of excessive amounts of ferrous oxide. In thepast it was observed that oxygen was lost during the melting of iron oxide materials. Thus, when magnetite (Fe O was melted, some of it was reduced to form ferrous oxide (FeO). United States Patent 931,513, Specketer, describes an attempt to counteract this by addingto the melt (after it has been removed from the furnace but while it is still molten) finely pulverized, unmelted, ferric oxide (Fe O Specketer wanted to add enough ferric oxide (Fe O to combine with all of the ferrous oxide (FeO) in the melt and provide all magnetite (Fe O But he could not be sure how much was needed, and preferred to play safe by adding what he thought would be an excess of ferric oxide, with the hope that it would dissolve uniformly and not be harmful. Specketers scheme was unsuccessful, however, as were all other attempts to overcome the problem of uncontrolled loss of oxygen content, prior to the present inven- .tion.

This invention provides magnetite of composition suitable for use in anodes and the like (such as cathodes and other electrodes) for electrolytic processes. It provides improved control over the magnetite composition, such that anodes of low resistivity can be obtained consistently, and the anodes do not need any plated surfaces. In general, the optimum magnetite content for anodes is in the range of 60 to percent, and should be at least about 50 percent.

A method of manufacturing magnetite suitable for castings for use as anodes and the like according to the present invention comprises melting preselected iron oxide materials by electric are between the materials and at least one electrode, maintaining each electrode at a controlled spacing from the materials after the initial contact made to establish the electric arc, and providing a substantial supply of free-oxygen-containing gas such as air to the surface of the molten material. The iron oxide materials preferably comprise mill scale substantially free from oil, grease, and other foreign matter, and may include other iron and iron oxide materials also. The starting materials preferably should have at least about 40 percent theoretical magnetite content, and for optimum results should have about 50 to 70 percent theoretical magnetite content.

Mill scale comprises various iron oxides. The full analysis of most mill scales would be 40 to 70 percent 1e 0,, (magnetite), 59 to 29 percent FeO (ferrous oxide), with approximately 1 percent of impurities consisting of silica, manganese oxide, etc., which were in the steel on which the mill scale was formed. The exact composition of the mill scale is not important. The essential requirement is that the theoretical magnetite content of the mill scale, plus any other iron and iron oxide materials in the melt if other starting materials are included, be at least about 40 percent, and for optimum results it should be about 50 to 70 percent. To make satisfactory anodes from such starting materials the magnetite content must not be diminished and preferably should be increased. As used herein, percent theoretical magnetite content means the number of weight units (grams, pounds, etc.) of Fe O per hundred weight units of material (Fe O plus FeO plus impurities). However, in the literature, by common usage, and in the United States patents of Blackman, 568,299, and Specketer, 800,181, and 931,513, for example, the terms magnetite, magnetic oxide, etc., are also used to mean a mixture of Fe O with varying amounts of FeO, and impurities, but with sufficient Fe O to make the material magnetic and to conduct electricity.

A method of manufacturing magnetite castings suitable for use as anodes and the like according to the present invention comprises, in addition to the above steps, pouring the molten material into molds having nonreacting cast ing surfaces, at mold temperatures of at least about 250 F., opening each mold and removing the cast material after at least the shell thereof has solidified and while the material is still red hot, placing the cast material, preferably within 30 seconds after opening the mold, in a furnace having a temperature of from about 1400 to 1900 F., and gradually reducing the temperature in the furnace, cooling the cast material to about F. gradually over a period of at least about 16 hours. Conditions should be kept within these ranges in order to provide consistently good results and avoid a high percentage of unsatisfactory cast ings. The above annealing steps are the result of many experiments, some of which were quite discouraging because of the many broken magnetite castings that resulted before the proper processing conditions were determined.

It has been found in the present invention that maintaining each electrode at a controlled spacing from the materials during melting and providing a substantial supply of free oxygen-containing gas to the surface of the molten material provide control over the magnetite composition vastly superior to that obtained by prior methods. As is brought out in more detail below, this may be done in a container having exposed to air an open top area of at least about 80 square inches per inch of depth of the material, the surface of the molten material being maintained in contact with the air, by breaking any solidified skin tending to form on the surface, for a time of about 5 to 30 minutes, thereby increasing the magnetite content of the material by about to percent to a final magnetite content of at least about 50 percent and preferably at least about 60 percent. These features of the invention also make it possible to obtain high magnetite contents consistently where desired, which could not be done by prior methods. The other features of the invention have also been found to be essential to consistently obtain magnetite anodes suitable for use in present-day electrolytic processes.

Before the discovery of the present method, repeated attempts under various degrees of direction or supervision by one of the present inventors to make satisfactory anodes by prior methods and variations thereof were unsatisfactory. Losses in magnetite content of as much as 19 percent Were common. Moreover, the losses varied uppredictably and unaccountably, and thus could not be controlled. With the method of the present invention, the magnetite content of the starting material can be increased conveniently from small amounts to about 15 percent higher magnetite content in the final product, as desired. The results are predictable and readily controlled, after routine preliminary testing in a given installation. Starting materials having relatively low contents of magnetite may be used in the present method, as they take up oxygen faster at first than do materials already high in magnetite content.

The following example sets forth the best mode presently contemplated of carrying out the invention.

A specially designed shell assembly was constructed for containing the magnetite during melting and pouring. The shell 10 for the skull-melting unit was fabricated from a standard 36-inch-diameter, %-inch-thick dished steel head. The sides of the dish were extended to 14 inches in height with %-inch plate. The open top of the dish exposed to air thus was about 80 square inches or more per inch of depth of the material that could be conveniently held therein for melting below the spout. This is a very shallow, open-top furnace, to allow access of air to the molten material, whereas regular furnaces are built either deep and narrow or with covers to reduce access of air to the molten charge.

A cooling coil 11 was brazed on the underside of the bottom and around the sides of the shell to increase the cooling during the preparation of the skull and to prevent hot spots during normal melting operations. The apron 12, spout or pouring lip 13, and bracket 14 for the electrode mast 15 were fabricated from /z-inch steel plate. To provide improved pouring control, the unit was constructed to pivot, as indicated at 16, at the pouring lip 13. A steel supporting framework 17 completed the assembly.

A rapid-acting hoist was used to tilt the furnace during casting. The hoist had a lifting speed of 30 feet per minute. The hoist also had a very rapid response when it was desired to lower the furnace. This was necessary to stop the pouring stream instantaneously.

Quick-opening molds 18-18 were designed and made of cast iron, using a cavity design such that each mold produced a /2-inch by 5-inch by 10-inch anode with an attached tapered riser. Quick-opening action was ob tained by using hinges 19-19 on one vertical edge of the mold, and providing a cam mechanism 20 at the opposite edge. Six molds 18-18 were attached to a turntable 21 with each mold 18 fixed at the same distance from the pouring lip 13 of the furnace in a position that permitted rapid filling of the molds 18-18.

Broken anodes were used as the starting material to establish the skull of magnetite in the furnace shell 10. Large pieces of anodes were placed on the bottom and sides of the bare furnace shell. No refractory was used inside the furnace shell. Small pieces of anodes 22 (approximately walnut size) were then charged to a depth of approximately three inches. The arcs to the electrodes 23-23 were then struck at 55 volts (phase to ground) and 500 amperes per phase, from a three-phase electrical source. As the initial charge melted, a small pool of molten magnetite formed under each furnace electrode 23, and the arcs became steady. The current input at this time was increased to 1000 amperes per phase. Molten magnetite flowed in between the larger pieces of broken anodes and formed a skull of solidified magnetite against the furnace shell. Additional charges of broken anodes were then added to build up the thickness of the skull. With an increase in the thickness of the skull, the current input could then be further raised to 1500 amperes per phase. The higher current input produced a larger pool and an increased melting rate.

When the desired thickness of skull height was reached, melting could be continued for at least 30 minutes without charging additional material. This 30-minute melting .period without the addition of new charging material is recommended with a new skull which may contain unfilled voids. The extended melting time enlarges the pool and enables the molten magnetite to fill the voids.

After the skull was built up to the desired thickness, the molten pool was tapped, and melting of the prepared charge of mill scale or blend of iron oxides was started.

When the melting operation was started on a solidified skull, the arcs were struck at 55 volts (phase to ground) and 500 amperes per phase. (Striking of the arcs at higher amperages on the solidified magnetite caused spattering of molten magnetite and erratic operation of the arc.) As soon as the molten pool was formed, the current was increased to 1500 amperes, and prepared charge material was shoveled in beneath the electrodes. A longhandled scoop was used to charge the mill scale or blend. After the first charge was melted, additional material was charged as rapidly as possible. A dry wood pole was used to push the mill scale or blend, that had fallen on the side, back beneath the arcs, and also to keep the pool open by breaking the solidified magnetite skin that forms on the top of the pool. After the last material was charged to produce a pool of the desired size, the power input was maintained for the desired period of time, then turned oif, and the anodes cast.

A small amount of molten magnetite was left in the skull after each cast, and was used to strike the arcs for the next heat. The arcs were struck on the molten material at 1000 amperes per phase, the current was then increased to 1500 amperes, and charging and melting were resumed.

The quick-opening molds were preheated by gas burners before casting any anodes. From numerous tests, it was determined that the mold temperature should be at least about 250 F.

The material must be left in the mold until at least the shell of the material has solidified, approximately 20 seconds at a mold temperature of 250 F., approximately 60 seconds at a mold temperature of 1000 F., and longer for higher mold temperatures. The casting must be removed while it is still red hot, however, to avoid cracking. Upon opening the mold, the casting should be removed immediately and placed in the annealing furnace as quickly as possible. The furnace should be located adjacent to the mold, so that each casting can be placed in the furnace within thirty seconds after opening the mold, preferably within a much shorter time.

The casting and handling of the anodes were carried out in accordance with the above requirements.

The best annealing cycle involved charging of the hot anodes into an electric furnace held at a furnace temperature of 1600 to 1700 F. The anodes were cooled to room temperature over a period of hours. The

extended annealing time was necessary to minimize cracking of the anodes during cooling. The anodes have a very high solidification shrinkage and are very brittle. During cooling, the difference between the temperatures of the outside surface and of the inside of the anode must be very small, to avoid ditferences in rate of contraction that would cause the anode to crack. Initial furnace temperatures of about 1400 to 1900 F. can be used. The furnace temperature should be reduced gradually to cool the cast material to about 100 F. or lower over a period of at least sixteen hours, and preferably over a much longer time. Anodes that are removed from the furnace at temperatures higher than about 300 F. usually crack.

The simplest way to supply oxygen-containing gas to the surface of the molten material is to expose the surface to the air. The amount of oxygen picked up by the material during melting and the resulting magnetite content of the melt depend upon the length of time that the molten bath is exposed to the air. Thus, the final magnetite content depends upon the melting time. With the furnace shown and described herein, for a melting time of five minutes, the magnetite content of the anodes is approximately ten percent higher than the magnetite content of the starting material. For a melting time of 30 minutes, the increase in magnetite content is approximately percent. Installations may differ in the amounts by which the magnetite increases for various melting times and exposed surface areas. Routine preliminary testing readily provides the necessary data for any new installation.

During that part of the melting period between the time charging was completed and the time that the power was cut off, the surface of the molten magnetite solidified into a thin skin and bridged over, except for the area directly under each electrode. The solidified skin was broken each time it formed. The formation of the solid skin on the surface of the pool reduced the area of the molten pool in direct contact with air. For optimum control of the supply of oxygen to the surface of the molten material, the exposed area should be kept approximately constant by breaking any solidified skin that forms. For a new installation, preliminary testing readily determines the amounts by which the magnetite content increases for various melting times and exposed surface areas.

The resistivity of the anode decreases for increasing magnetite content. Anodes containing approximately 51 percent magnetite cast in molds having temperatures of approximately 1000 F. have resistivities in the neighborhood of 0.034 ohm-centimeter. Anodes containing approximately 73 percent magnetite, similarly cast, have resistivities of about 0.017 ohm-centimeter. In the range of 50 to 75 percent magnetite content, the resistivity decreases approximately linearly with increasing magnetite content. Higher mold temperatures generally provide slightly lower resistivity for a given magnetite content.

To obtain optimum control over the magnetite content, to obtain high magnetite contents consistently where desired, and to avoid reduction of the iron oxides, it is essential to maintain the arc of each electrode at a controlled spacing from the materials during melting. Equipment for providing such control automatically is available in electric arc melting equipment. Any suitable control may be used. The primary requirement is that the electrodes avoid contact with the materials. The automatic control of course also maintains the arc spacing for the desired current.

The spacing of the electrodes from the materials in the arc furnace is not critical in the sense that it must be kept at any specific distance, but only to the extent that each electrode must be kept away from contact with the materials. If an electrode were to contact the molten materials during the melting process the hot carbon of the electrode would reduce some of the iron oxides of the molten material, which would decrease the proportion of magnetite in the final product by unpredictable amounts and thus would cause appreciable variation in composition and in electrical properties. Of course, the spacing must be close enough to maintain the electric arc. In the examples herein, the spacing is given in terms of voltage and current across the are, which comprise a measure of the spacing of the electrode from the molten bath. It would not be practical, because of the intense heat of the arc, to try to measure the spacing by physical means, especially since the level of the molten material fluctuates because of arc agitation, solid pieces falling into molten pool, etc.

Prior to this invention, the manufacture of magnetite anodes has been carried out by arc melting in furnaces either having covers or shaped so deep and narrow that little, if any, oxygen from the air could reach the molten material. Carbon from the electrodes reduced the proportion of magnetite in the anodes by unpredictable amounts, causing appreciable variation in composition and in electrical properties. The magnetite content of the anodes manufactured in this way was less than the magnetite content of the starting materials. The resistivity of the anodes was excessively high because of the low magnetite content usually obtained. Other factors may have been involved also. At any rate, the present invention overcomes the problems of wide variation in composition and high resistivity encountered in the manufacture of magnetite anodes by prior methods.

It is realized that various modifications of the invention may be made without departing from the spirit and scope thereof and without the exercise of further invention. No attempt is here made to exhaust all such possibilities. It will be understood that the words used herein are words of description rather than of limitation, and that various changes may be made without departing from the spirit or scope of the invention herein disclosed.

What is claimed is:

1. A method of manufacturing magnetite suitable for castings for use as anodes and the like comprising: melting preselected iron oxide materials comprising mill scale substantially free from oil, grease, and other foreign matter, having at least about 40 percent theoretical magnetite content, in a container having exposed to air an open top area of at least about square inches per inch of depth of said material, by electric are between the materials and at least one electrode, maintaining each electrode spaced from said materials after the initial contact made to establish the electric arc; and maintaining the surface of the molten material in contact with air for a time of about 5 to 30 minutes, thereby increasing the magnetite content of said material by about 10 to 15 percent to a final magnetite content of at least about 50 percent.

2. A method of manufacturing magnetite suitable for castings for use as anodes and the like comprising: melting preselected iron oxide materials comprising mill scale substantially free from oil, grease, and other foreign matter, having about 50 to 70 percent theoretical magnetite content, in a container having exposed to air an open top area of at least about 80 square inches per inch of depth of said material, by electric arc between the materials and at least one electrode; maintaining each electrode spaced from said materials after the initial contact made to establish the electric arc; and maintaining the surface of the molten material in contact with air, by breaking any solidified skin tending to form on said surface for a time of about 5 to 30 minutes, thereby increasing the magnetite content of said material by about 10 to 15 percent to a final magnetite content of at least about 60 percent.

3. A method of manufacturing magnetite suitable for castings for use as anodes and the like comprising: melting preselected iron oxide materials comprising mill scale substantially free from oil, grease, and other foreign matter, and other iron and iron oxide materials to provide 7 8 about 50 to 70 percent theoretical magnetite content, in of said material by about 10 to 15 percent to a final maga container having exposed to air an open top area of at netite content of at least about 60 percent.

least about 80 square inches per inch of depth of said material, by electric are between the materials and at least one electrode; maintaining each electrode spaced from References Cited by the Examiner UNITED STATES PATENTS said materials after the initial contact made to establish 563,229 9/ 1896 an 20429l the electric arc; and maintaining the surface of the molten 8001181 9/1905 Specketel' 2O4291 material in contact With air, by breaking any solidified 9 13 8/1909 Specketer 204291 skin tending to form on said surface, for a time of about 10 290O236 8/1959 Speed et a1 23 200 5 to 30 minutes, thereby increasing the magnetite content JOHN H. MACK, Primary Examiner.

Claims

1. A METHOD OF MANUFACTURING MAGNETITE SUITABLE FOR CASTINGS FOR USE AS ANODES AND THE LIKE COMPRISING; MELTING PRESELECTED IRON OXIDE MATERIALS COMPRISING MILL SCALE SUBSTANTIALLY FREE FROM OIL, GREASE, AND OTHER FOREIGN MATTER, HAVING AT LEAST ABOUT 40 PERCENT THEORETICAL MAGNETITE CONTENT, IN A CONTAINER HAVING EXPOSED TO AIR AN OPEN TOP AREA OF AT LEAST ABOUT 80 SQUARE INCHES PER INCH OF DEPTH OF SAID MATERIAL, BY ELECTRIC ARC BETWEEN THE MATERIALS AND AT LEAST ONE ELECTRODE, MAINTAINING EACH ELECTRODE