AMERICAN INSTITUTE O F MINING AND METALLURGICAL ENGINEERS

Technical Publication No. 986 IC~nas C. Iaon AND ST~EL DIVIBION. NO. 212) . ~ ~ -. - ----- -. ----

DISCUSSION OF THIS PAPER IS INVITED. ~iscussiod in &tins (2 copies) ma be sent to the Secretary, American Institute of Minin,g and Met?llurgical En 'neers. 29 West 39th street, New York. N. Y: Unless specla1 arrangement 18 made, dl?cusaon of tEs paper mll close Aprll 1, 1939. Any discuss~on offered thereafter should preferably be In the form of a new paper.

Induction Furnaces for Rotating Liquid Crucibles

BY E. P. BARRETT;* W. F. HOLBROOK,~ AND C. E. WOOD,% MEMBER A.I.M.E.

(Detroit Meeting. October, 1938)

THE high-frequency laboratory induction furnace with a rotating liquid crucible enables research workers to conduct certain investigations heretofore very difficult or impossible to realize because vessels are not obtainable that are capable of resisting the physical conditions or the chemical actions to which they are subjected. Applications for patents covering commercial uses of centrifugal liquid crucibles were filed in the United States in 1919,' in France in 1925, in the United States and Germany in 1926,2 and in France in 1927.3 In 1928, Schuette and Maier,4 of the Bureau of Mines, reported the construction of a whirling table beneath a high-frequency induction furnace.

When a vessel containing a liquid is rotated, the inner surface of the liquid takes the shape of a paraboloid under the simultaneous action of centrifugal force and giavity. The internal dimensions of the paraboloid rotating liquid crucible. are ' governed by the diameter of the container and i t s speed of rotation. Melted Wood's metal was poured into cylinders 2% in. in diameter, which were rotated at 180, 300, and 468 r.p.m. When the metal was solid, a slurry of plaster of Paris was added, and rotation continued until the plaster had set. Fig. 1, a photograph of sections of the cylinders, shows the effect of speed of rotation of the containers upon the shape of the inner surface of the rotating liquid crucibles.

Three types of high-frequency induction laboratory furnaces using liquid-metal crucibles were- 'designed and constructed by the Blast

Manuscript received at the office of the Institute July 7, 1938. Published by permission of the Director, U. S. Bureau of Mines.

* Metallurgist, Blast Furnace Studies Section, Metallurgical Division, U. S. Bureau of Mines, Minneapolis, Minn.

t Assistant Chemist, Blast Furnace Studies Section, Metallurgical Division, U. S. Bureau of Mines.

$ Acting Supervising Engineer, Blast. Furnace . Studies Section, . Metallurgical . .

Division, U. S. Bureau of Mines. References are at the end of the paper. Copyright, 1938,by the Amin<an 1nstituG of Mining and Metallurgical Engineers, Inc. METAL^ TECHNOLO~Y. December, 1938. Printed in U. 8. A.

2 INDUCTION FURNACES FOR ROTATING LIQUID CRUCIBLES

Furnace Studies Section of the Metallurgical Division of the Bureau of Mines :

1. A furnace in which a graphite crucible packed in lampblack in a refractory tube was rotated concentrically within the coil.

2. A furnace in which a super-refractory crucible containing molten metal was rotated within a graphite-lined stationary cylindrical furnace.

3. A cylindrical furnace that was rotated as a unit.

FIG. 1.-EFFECT O F SPEED OF ROTATION ON SHAPE AND SIZE OF ROTATING LIQUID CRUCIBLES.

Type 1 Fz~rnace

The inductor coil (5% in. outside diameter, 4% in. inside diameter and 6% in. high), consisting of 35 turns of flattened copper tubing, was mounted in a transite-board box. A $f6-in. layer of sheet mica was placed inside the coil for electrical insulation. The die in which the %-in. copper tubing was flattened and the mandrel on which it was wound are shown in Fig. 2. The coil and the transite-board box with coil are shown in Fig. 3.

A table 12 in. square by 18 in. high, for mounting the coil, was con- structed from 155 by %-in. angle iron, with a transite-board top % in. thick having a central opening 8 in. in diameter. A vertical shaft %-in. dia. by 15 in, long, was installed a t the center of the table. A disk 7% in. in diameter and four rings, each 7%-in. outside diameter by 4x411. inside diameter, of %-in. transite board, were bolted to a circular steel plate attached to the upper end of this shaft. The weight of the rotating unit was supported by a ball thrust-bearing. A refractory tube, 4%-in. outside diameter and 12 in. long, was centered in the opening in the transite-board rings, and the space between the tube and the rings was filled with a slurry of RA-162 Alundum cement.

The most serviceable refractory tube (4%-in. outside diameter, 335-in. inside diameter and 12 in. long) was made in the laboratory from a mixture of equal parts of Thermolith and RA-162 Alundum cements

E. P. BARRETT, W. I?. HOLBROOK AND C. E. WOOD 3

moistened with a 10 per cent solutioll of sodium silicate. After air-drying for 24 hr. the tube was fired to 1250" to 1300" C. in about 2 hr. and cooled

FIG. 2.-I?ORMING AND WINDING FLATTENED COPPER TUBING ON A 4%-IXCH hIANDREL.

in the furnace. The refractory tube, mounted in the rotating mechanism, and the completed furnace are shown in Fig. 4.

4 INDUCTION FURNACES FOR ROTATING LIQUID CRUCIBLES

A jig was used to center a graphite sleeve in the refractory tube with respect to the axis of rotation, and lampblack was packed between the sleeve and the tube for heat insulation. The containers for the rotating liquid crucibles were machined from graphite electrodes, the outside diameter of the container being in. less than the inside diameter of the graphite sleeve. A section of the furnace is shown in Fig. 5 and the relative position of the liquid crucibles and slags in Fig. 6.

.a :dd b FIG. 4.-HIGH-FREQUENCY ROTATING INDUCTION FURNACE, TYPE 1.1 a, refractory tube and rotating mechanism; b, completed furnace.

Type 2 Furnace

Type 2 furnace was designed to eliminate the use of graphite con- tainers for rotating liquid crucibles. The mounted inductor coiI used in type 1 furnace was lined with bonded mica sheet H6 in. thick. A graphite cylinder 7% in. long, 234 in. inside diameter and 3% in. outside diameter, was centered within the coil and lampblack packed between the graphite and the mica insulat?[on. The graphite cylinder or heating unit did not rotate in this furnace.

The refractory tube and transite-board rings attached to the rotating shaft used with type 1 furnace were replaced by a refractory ring 734 in. in diameter by 2 in. thick with a 3-in. opening a t the center. A graphite spindle of 255-in. dia. and 6 in. long was centered in the opening in the refractory ring and held in place with Alundum cement. The rotating mechanism, refractory crucible, and completed furnace are shown in Fig. 7, and a section of the furnace in Fig. 8.

E. P. BARRETT, W. F. HOLBROOK AND C. E. WOOD 5

Controlled Atmosphere.-The upper end of the rotating shaft and the lurnace were enclosed in a transite-board box having a detachable cover.

TYPE 1,

FIG. 6.-RELATIVE POSITION O F LIQUID CRUCIBLES AND SLAGS COOLED WHILE ROTATING. a, section of ferrous sulphide and slag in a graphite crucible; b, inverted section of

iron and slag; c, top view of iron and slag in a refractory crucible.

To prevent damage should there be an explosion, a 3-in. dia. opening in the lower portion of one side of the box was covered with a piece of

6 INDUCTION FURNACES FOR ROTATING LIQUID CRUCIBLES

Jis-in. asbestos paper painted with sodium silicate solutioil. An atmosphere of nitrogen under slight pressure was maintained in the box to prevent the entrance of air and subsequent formation of carbon monoxide due to contact with incandescent carbon. A 1-in. pipe attached to the cover was fitted with a window through which the charge was observed, and the temperature was determined by sighting with an optical pyrometcr. Briquets of slag or other material were introduced into the rotating liquid crucible through a tee and plug without changing the atmosphere within the furnace.

Rejractory Crucibles in W h i c h Rotating Liquid Crucibles Were Formed.-- The experiments for which type 2 furnace was designed could not be

---a -r -

a b FIG. 7.-HIGH-FREQUENCY INDUCTION LABORATORY FURNACE, T Y P E 2 , USING ROTATING

CRUCIBLES. a, rotating mechanism, spindle and crucible; b, completed furnace.

performed in liquid crucibles in graphite containers. The low-carbon iron that formed the rotating liquid crucibles was held in fused-magnesia or fused-alumina crucibles, 236 in. high by 2%-in. dia., formed by the vibrator method developed by Barrett and H o l b r ~ o k . ~ To facilitate centering on the spindle, the refractory crucibles were formed with a projection a t the center of the bottom, which fitted into a hole a t the center of the top of the graphite spindle. The diameter of the hole in the spindle was in. greater than that of the projection on the bottom of the refractory crucible.

T y p e 3 Furnace

This furnace was designed to eliminate graphite parts, lampblack heat insulation, and preformed refractory crucibles. The heat was generated

E. P. BARRETT, W. F. HOLBROOK AND C. E. WOOD

FIG. 8.-SECTION OF HIGH-FREQUENCY INDUCTION LABORATORY FURNACE, TYPE 2, USING ROTATING LIQUID CRUCLBLES.

8 INDUCTION F U R ~ A C E S FOR ROTATING LIQUID CRUCIBLES

in the charge by direct induction. An inductor coil identical with that used in types 1 and 2 was mounted in a transite-board frame attached to a steel plate welded to the upper end of a vertical hollow shaft. The shaft was made from a piece of 1-in. seamless tubing 25 in. long, turned to 1%2-in. outside diameter and mounted in the center of an angle-iron frame 18 in. square by 28 in. high, constructed in a manner similar to that used on the shaft for types 1 and 2. The weight of the rotating parts restecl on a ball thrust-bearing. The coil in the transite-board

frame is shown in Fig. 9. W a t e r Connections.-Inasmuch

as the coil rotated in type 3 fur- nace, i t was necessary that both inlet and outlet connections for the cooling water be installed in the hollow shaft. The details of con- struction of the hollow shaft are shown in Fig. 10. The coil was insulated electrically from the shaft by two pieces (each about 40 in. long) of 96-in. reinforced hose be- tween the ends of the coil and the copper tubes extending from the

hollow shaft, as shown

ings and Brushes.-The electrical energy was transmitted to the inductor coil through two

rbon brushes, each pair in parallel in contact

with slip rings 894 in. outside di- ameter made from 1 by %-in. brass and installed concentrically a t the

coil. One of each pair FIG. 9.-COIL AND TRANSITE-BOARD FRAME of brushes and the slip rings are FOR ROTATING INDUCTION FURNACE, TYPE 3. pictured in Fig. 11.

lMethod of Lining.-A layer of bonded mica sheet H6 in. thick was placed inside of the coil. By the use of a jig a piece of graphite electrode 83$ in. long, 2%-in. dia. a t the bottom and 2lXs-in. diameter a t the top was centered in the coil. Dry fused magnesia, of mixed grain sizes to provide a dense mix, was rammed between the graphite form and the mica to within 3 in. of the top. A dry mixture of fused magnesia and 3 per cent boric acid was used to form the next 1% in. of lining. The space remaining was filled with a stiff mix of RA-162 Alundum cement and water. When the cement had set, a piece of firebrick was placed on

E. P. BARRETT, W. I?. HOLBROOK AND C. E. WOOD

10 INDUCTION FURNACES FOR ROTATING LIQUID CRUCIBLES

top of the graphite form and the graphite heated to about 1550" C., a t which temperature a thin layer of the fused magnesia next to the graphite was sintered to form a crucible in the furnace. The graphite form was removed immediately after the power was turned off.

FIG. 11. F I G . 12. FIG. 11.-ROTATING INDUCTION FURNACE, TYPE 3, WITH FRONT OF BOX REMOVED. FIG. 12.-ROTATING INDUCTION FURNACE, TYPE 3, I N POURING POSITION.

To facilitate removal of the graphite form, a hole 1% in. in diameter by about 2 in. deep w m turned in the upper end. A groove about

in, square was turned a t the bottom end of the 1% by 2-in. hole to provide a grip for the tongs used to remove the graphite form.

The top of the furnace was covered with a refractory lid made from a mixture of equal parts of RA-162 Alundum cement and Carbofrax cement No. 3, moistened with a 10 per cent solution of sodium silicate.

E. P. BARRETT, W. F. HOLBROOK AND C. E. WOOD 11

Reinforcements of Nichrome wire were placed inside the lid. After air- drying overnight i t was fired to 1250° C.

The transite-board box was fitted with a removable cover to which was attached a piece of 135-in. pipe with a glass window through which the charge was observed and the temperature determined by sighting with an optical pyrometer. Briquets of slag or other material were intro- duced into the rotating liquid crucible through a tee and plug without removing the top of the furnace.

To prevent oxidation of the molten metal, an atmosphere of nitrogen under slight pressure was maintained in the box. The nitrogen was introduced into the upper portion of the crucible through a piece of flattened %-in. Allegheny 55 seamless tubing inside of the 135-in. pipe and extending through the refractory cover. The observation tube, nitrogen inlet and refractory cover are shown in Fig. 10.

About 435 lb. of molten Armco iron, when rotated a t 280 r.p.m., formed a liquid crucible approximately 2% in. in internal diameter at the top by 2% in. deep.

To preserve the lining it was necessary to remove the molten metal a t the end of each melt. This was readily accomplished by attaching a refractory-lined spout and by tilting the furnace so that the metal was poured into a split-type mold of 2%-in. diameter. The small ingots were remelted in the furnace. Fig. 12 shows the furnace in the tilted position.

1. High-frequency induction furnaces using rotating liquid metal crucibles are especially adapted to a study of slag-metal reactions.

2. Rotating liquid metal crucibles are slag proof and eliminate contamination of the slag by the refractory.

3. Type 2 induction furnace for rotating crucibles was the simplest to construct and most flexible to operate, and provided the smallest temperature gradient in the hot zone.

4. Rotating induction furnace, type 3, was the most difficult to con- struct. Graphite parts and preformed refractory crucibles were elimi- nated. The iron used for the rotating liquid crucibles was heated direct by i~duction from electrical energy transmitted through slip rings.

5. The high-frequency induction laboratory furnace using rotating liquid-metal crucibles is a useful tool for research workers.

12 INDUCTION FURNACES FOR ROTATING LIQUID CRUCIBLES

1. E. F. Northrup: Method and Apparatus for Melting Oxides, etc., without Con- tamination. U. S. Patent 1378189 (May 17, 1921).

2. J. Maximoff and M. Steela de Costa (n& Vincent): Vorrichtung zur Ausfuhrung phyahlischer oder chemischer Vorglinge. German Patent 470748 (Jan. 10, 1929); Centrifugal Liquid Cmcible. U. 8. Patent 1684800 (Sept. 18, 1928).

3. M. Ribaud: Proc6d6 et dispositifa pour effectuer des traitements thermiques de mati&res, plus particulierement au four electrique. French Patent 632343 (Jan. 7, 1928).

4. C. N. Schuette and C. G. Maier: High-frequency Induction Furnace for Chemical Preparations above 1000' C. Arner. Electrochem. Soc. Preprint No. 18 (Sept. 1928).

5. E. P. Barrett and W. F. Holbrook: An Improved Method for Forming Fused Magnesia Crucibles. Ind. and Eng. Chem. (1938) 10, 91-93.