US2473311A - Method for producing metal alloys in a submerged resistor type induction furnace - Google Patents

Description

June 14, 1949. T ETAL' METHOD FOR PRODUCING METAL ALLOYS IN A SUBMERGE RESISTOR TYPE INDUCTION FURNACE 1 Filed July 3, 1945 3 Sheets-Sheet l 1 mlllllllllllfllllhIIIIIIIIIII%M 117; m, r r

INVENTOR MANUEL TAMAMARIO TAMA JLLOYD HOFF ATTORNEY June 14, 1949. TAMA r 2,473,311

METHOD FOR PRODUCING METAL ALLOYS IN A SUBMERGED RESISTOR TYPE INDUCTION FURNACE Filed July 3, 1945 3 Sheets-Sheet 2 DEG. F. 4

' TEMPERATURES AT COLD F'ACE 0F HUSH NG FOR. CORRESPONDING WALL :r-ubrw ta:

C 1 2.0 :0 40 so so 400 h-BTU/'F/FT'/Hr INVENTOR. MANUEL TAMA,MAR|0 TAMALLLOYD HOFF BY fi firww ATTORNEY June 14, 1949. M. TAMA ETAL 2,473,311

' METHOD FOR PRODUCING METAL ALLOYS IN A SUBMERGED RESISTOR TYPE INDUCTION FURNACE filed July 3, 1945 3 Sheets-Sheet 3 INVENTOR v MANUEL um, mmo TAMAJJLLOYD mrr ATTORNEY Patented June 14, 1949 METHOD FOR PRODUCING METAL ALLOYS IN A SUBMERGED RESISTOR TYPE IN- DUCTION FURNACE Manuel Tania and Mario Tama, Morrlsville, Pa,,

and James Lloyd signors to Ajax ton, N. 1.

H05, Pennington, N. 1., as- Engineering Corporation, Tren- Applicatlon July 3, 1945, Serial No. 603,026 4 Claimn (Cl. 13-34) The invention is a continuation in part of our copending patent application Serial No. 597,282, filed June 2, 1945, which has become abandoned, and it relates to a method for producing metal alloys in an induction furnace of the submerged resistor type for the melting of alloys and particularly lead copper alloys which have a tendency to segregate constituents of a low melting point. The denomination as copper lead alloys includes alloys in which copper and lead are the chief ingredients, but in which other elements, for instance tin, may also be present. Such alloys are used chiefly as hearing metals and the lead content may vary from to 40%.

Induction furnaces of the submerged resistor type have been in successful operation for more than twenty-five years for the melting of many non-ferrous metals and their alloys including the leaded brasses, containing 60-62% Cu, 1 /-3% Pb, balance zinc; however, all previous attempts to adapt thistype of furnace to the melting of lead rich copper alloys containing more than 5 per cent of lead have been unsuccessful. This important field of application has, therefore, been closed to the submerged resistor type induction furnace, a fact which has been mentioned in several publications, for instance in the book entitled Electric Brass Furnace Practice by H. W. Gillet and E. L. Mack, Bulletin 202, of the Department of the Interior. Washington, D. C., 1922, pages 265 and 272, and also in the book entitled Electric Melting Practice, by A. G. Robiette, London, 1938. This latter book contains on page 138 the following passage in ref erence to melting copper lead alloys in furnaces of the submerged resistor type:

For copper zinc alloys in which lead does not exceed 3 per cent and base refractory has been found eminently suitable, there being no appreciable attack on the lining. Successful lining cements in use also contain a mixture of chrome ore and refractory clay. When high-lead alloys are melted, the lead rapidly penetrates the lining, short-circuiting the primary to earth, and causing the overload coil to trip. When leaded mixtures are melted in the rocking arc furnace, appreciable penetration of lead into the pores of the refractory is evidenced by the high lead loss for initial charge with a new lining. Lead also forms a very fluid silica with the free silica in a clay base refractory. Copper oxide is equally severe on ,this lining material, both slagging and penetration of oxide taking place, with the result that copper alloys containing in the region of 85 to 100 per cent copcopper 85 per cent, a clay less. 1

2 per greatly reduce the life of of lining described below."

Moreover, it is the general belief that if at least about 5% was also believed that lead oxides and copper oxides were the offenders, that they would inevitably attack the linings and that nothing it otl er than using eletion of such oxides.

This invention, however, is physical laws, as will become specification proceeds.

based chiefly on apparent as this resulted in the findings described below. These alloys, and particularly the tin containexperiments and investigations upon which this invention is based. The lead and tin contents of the segregations are very high and much higher than that of the metal charged into the furnace. The chemical composition is erratic and in no way related to the analysis of the charge. In ex periments made in a water cooled furnace with an alloy containing 25% Pb, 5% Sn, balance copper and impurities exudations resulted having 93% Pb, 1.5% Cu, plus impurities. The meltin point is sometimes as low as about 600 F. and

that the coils of the primary circuit are often attacked and destroyed. The exudations of low melting temperature penetrating through the refractory walls of the secondary block tend in the rammed type many cases also to form a second ring of metal around the primary. As soon as a new secondary is formed in this manner, more electric current is absorbed and the process of deterioration is accelerated.

Numerous attempts to remedy this situation by improving or changing the refractories have failed entirely. thus demonstrating that the choice of a good refractory does not lead to success. As pointed out by Robiette, lead oxide and copper oxide are the most active compounds to attack the linings and especially those containing silica. It has, therefore, been the belief that silica should be entirely eliminated from the refractory walls of induction furnaces to be used for melting copper lead alloys In contradistinction to this assumption it was found, as will be described later, that silica containing refractories can be used for melting these alloys if the physical rules are observed which govern this invention..

A way to cope with the problem might have been to considerably increase the thickness of that portion of the refractory block which extends between the secondary slot and the coolin duct, which for brevitys sake will be termed in the following: the thickness of the secondary bloc However, in the designing of induction furnaces of the submerged resistor type, there has always been a trend to reduce the wall thickness of the secondary block as much as possible in order to preserve a high power factor. The greater the distance between the primary and secondary winding, the larger the magnetic leakage will be and the lower the power factor, see Electric Furnaces in the Iron and Steel Industry," Rodenhauser, Schoenawa and von Baur, New York, 1920, page 187. Decrease of power factor is not the only inconvenience caused by the increase of the distance between primary and secondary. The power input of a furnace of given dimensions will also be decreased and the amount of refractory material needed for preparing the lining will be larger. In consequence of this recognition, the thickness of the secondary block has been reduced to about 2 /z"-2%" in moderen furnaces, which has proven to be most economical.

In contradistinction to this generally accepted rule it was found that in melting lead rich alloys an increase of the thickness of the secondary block to about 4" was most decisive in improving the performance of the furnace, notwithstanding the decrease in power factor and other disadvantages which ensued from this measure.

However, the increase of this thickness of the secondary block alone did not solve the problem. Even if the number of break-throughs could be reduced, they were by no means eliminated.

Therefore, the necessity resulted of effectin a larger temperature drop from the hot face of the secondary block which is filled with molten metal to its cold face adjacent to the cooling duct. This drop had to be considerable in order to establish at the cold face of the block temperatures which will lie below the melting point of an lead and tin rich segregation having, as previously mentioned, melting points of as low as 600 F. and less.

A satisfactory answer to the problem of eliminating break-throughs of the refractory linings due to the attack by these low melting segregations was finally found in a combination of the above mentioned increase of the wall thickness and an increase ofthe surface coefficient of heat lining during transmission at the cold face of the secondary block.

It is, therefore, the primary object of this invention to construct a submerged resistor type induction furnace for the melting of lead rich alloys and particularly tin containing alloys of this kind, where the penetration of the secondary block by low melting segregations and/or exudations formed from the molten charge is prevented.

It is a further object of the invention to obtain, without a substantial increase of the thickness of the secondary block lining, a temperature drop from the hot face of the secondary block towards its cold face which will result in establishing at the cold face temperatures below the melting point of any of the segregations which can be formed from the metal bath.

It is also an object of the invention to attain the above stated aims without substantially changing the furnace construction.

With these and other objects in view which will become apparent as this specification proceeds, the invention comprises in its broad aspect the maintenance in a submerged resistor type induction furnace of a relationship between the thickness of the secondary block and its surface coefficient of heat transmission which will establish at the cold face of the block a temperature below the melting point of any segregation or exudation apt to be formed from the molten alloy. In the case of the copper lead alloys in most cases, a. temperature or about 600 F. or less will satisfactorily serve the aims of the invention and will inhibit any break-throughs of molten metal through the refractory block.

As a further means to assure a satisfactory operation of the furnaces in the melting and particularly the continuous melting of high lead alloys in conformity with this invention, the maintenance of an uninterrupted motion of the charge and a corresponding temperature control was found to be of particular importance.

Temperature control is usually effected in furnaces of the present type by disconnecting the power when a certain high temperature is reached and connecting again when a lower limit of temperature is attained.

In the instant case the furnaces are operated on the so-called high-low power control. In this control method the furnace is connected to a high voltage supply, which gives high power until the maximum desirable temperature is reached; then connection is made with lower voltage.

By using this latter method, the metal contained in the furnace and in particular in the melting channels is in continuous complete motion. If the charge were allowed to become at rest, the formation of segregation is accordingly enhanced; this results in a seepage of the lead to the bottom of the furnace with incumbent attack of the lining.

The invention may be advantageously applied to the furnaces described and illustrated below and also to the water-cooled furnaces which form the subject matter of U. S. patent application Serial Number 550,442 of the applicants, Manuel Tama and Mario Tama, now Patent Number 2,389,218.

In all cases, the refractory lining of the furnace should be rammed in very densely with compressed air ramming tools, to prevent mechanical erosion. Special care should be taken to prevent cracks from developing in the the ramming, drying or preheating.

. face of the secondary block The copper lead rich alloys will penetrate through small pores or cavities of the refractory lining, even if no cracks are present, unless the temperature gradients described above are maintained.

In order to obtain the required temperature drop. the thickness of the secondary block should be about 4" or slightly more, and the surface coefllcient of heat transmission in its cold face should be at least 30 B. t. u. per degree F., sq. ft., hr. These figures refer to the shortest distance between the portion of the secondary block which is filled with molten metal and the cooling duct'and which will usually coincide with the center of the cooling duct. The thickness of 4" and the coefficient of heat transmission of 30 B. t. u. per degree F. sq. ft. hr. as enumerated above refer to furnaces which have been lined with refractories of the mullite and high alumina typ a The increase of the surface coefficient of heat transmission can be obtained byincreasing the velocity of the cooling medium. The air is blown through the coil duct 9 by a motor driven blower l8; by varying the rotational speed of the blower the velocity of the cooling air can be increased and decreased.

The relationship between velocity and heat transfer coefficient for air cooling in ducts is given in the following equation from Industrial Heat Transfer by Alfred Schack, New York, 1933, page 114.

In this equation V0 is the air velocity referred to standard conditions, 32 F. and 29.92 in. mercury, ft./sec.'; and D is the inside diameter of the duct in ft.

The air cooling ducts of induction furnaces of the instant type are not simple in shape like ordinary tubularv ducts. The passage of air is a small cavity located between the outside surface of the refractory lining and the transformer. The latter consists of two parts, namely, the core and the coil, and air will flow also through the passages formed between them. In the equation mentioned above, there will be no precise equivalent for the diameter D in the cooling ducts of submerged resistor type induction furnaces.

Applicants have found, however, that said formula can be used with a certain degree of accuracy in th following manner.

Here V0 is the air velocity, as above, and C is a constant which can be determined by measurements on existing units. If the amount of air flowing through the ducts, the air temperatures at inlet and outlet, the surface temperature and the surface of the duct are measured, C can be determined. After the value of C has been determined for a certain velocity, other values of It can be calculated for other velocities.

In the case of liquid cooled furnaces, similar laws will apply. a

A further means of accomplishing the objects of this invention consists in applying to the cold a good heat-conducting material having a very smooth surface exposed to the cooling air stream, customarily applied to the of the instant type; a slit copper bushing should be preferably used for this purpose.

abacking or cover of primary in i'umaces 6 As an additional important means to control the temperature at the cold face of the secondary block, in accordance with the principle of this invention the use of a multi-layer coil and the thereby greatly reduced heat formation has been found to be of particularly great help.

A furnace to be used for melting lead alloys in conformity with the teachings of this invention is illustrated by way of example in the accompanying drawings, in which Fig. 1 is a vertical sectional elevation,

Fig. 2 is a vertical sectional elevation on line 2-2 of Fig. 1, g

Fig. 3 is a perspective view of the copper bushing used between the secondary block and the air conduit, and

Fig. 4 is a graph showing the relation between the thickness of the secondary block, the surface coefficient of heat transmission and. the cold face temperature of the secondary block,

Fig. 5 is a part vertical sectional elevation similar to the one of Fig. 1 of a furnace provided with a multilayer coil. 1

In the drawings, i designates a housing containing a hearth adaptedto hold the molten copper lead alloy and provided in the usual manner with a refractory lining 3. Suitable insulating material may be inserted between lining 3 and the housing i. The charge is heated by means of currents induced in the secondary slot 5 formed as a. loop in the secondary block 6. The loop is threaded by a primary circuit composed of iron core I0 and surrounded by primary coil 1. The primary coil which has given the most satisfactory results, is made of flat wound copper wire of rectangular cross section insulated with fiber glass and wound in two layers. A flat wound multilayer coil develops less heat than an edge wound single layer coil occupying the same place and, therefore, helps in reducing the temperature prevailingat the cold face of the refractory block.

I The thickness of the secondary block which extends between the hot face 8 of the block and the cooling duct 9 is designated with 4., This is the critical wall thickness, the dimension of which is tight against the copper bushing ii. is provided with a longitudinal slit I2 to prevent short circuiting. It may be stated here that the use of slit copper bushings as a protective means for refractory linings is not novel per se and is claimed only in combination with the increase of the thickness of th secondary block and proper cooling conditions.

Fig. 3 shows further details of the copper bushing in perspective. It consists of a cylinder l5 made of soft copper sheet rolled to size. transversal ribs i 3 and I4 likewise of copper are brazed to cylinder l5 and are used for centering and supporting the bushing within thefurnace casing. Suitable electric insulation, for instance, mica is used between ribs i3, i4 and the furnace case. The longitudinal slot l2 of trapezoidal shape is filled-with a spacer iii of asbestos cement or any other suitable insulator.

In Fig. 5 a furnace of the present type is shown which is provided with a double layer coil I having air ducts l9 between the coil layers; the application of multilayer coils, which as such are known, offers a more intense cooling capacity which was found to be of particular usefulness in furnaces of the present type.

The graph illustrated in Fig. 4 shows a num- The latter her of curved lines marked with a thickness of the secondary block ranging from 3 inch to 6 inch, which range would appropriately cover the block thicknesses to be considered in the performance of this invention. On the ordinate the temperature ranging from 1200 F. to F. and onthe abscissae the surface coefficient in h in B. t. u., F., ft. /hr. from 0'' to 100 are marked whereby the abbreviated language used for this formula means "British thermal units per degree Fahrenheit of surface temperature per square foot per hour, assuming an approximate constancy of environment or room temperature. By means of this graph the temperature at the cold face of the secondary block or at the copper bushing ll may be determined for different values of the surface coeflcient It.

.If it is intended to maintain a temperature of 600 F. at the cold face of the secondary block 0, is is apparent from the graph that this may be achieved with a block thickness of 3 inch. and a surface coefllcient of 40 or with a 4 inch thickness and a surface coefficient of 29.

The following are details of the construction of a specific furnace of 150 kw. capacity made in accordance with the teachings of the present invention. The values contained in the graph shown in Fig. 4 correspond to the details of the particular furnace described in the following.

' The copper bushing is made of copper sheet rolled to 11" outside diameter. The secondary slot has a cross section of i" x 3" with rounded edges and the radius of the slot was of The thickness of the secondary block which, as described above, is a critical dimension. in the furnace design, was equal to 4 A copper bushing of these dimensions has enough rigidity to insure proper contact with the rammed lining and, consequently, proper heat dissipation; the latter is also increased by the smooth inner surface of the bushing. Under the conditions existing in this furnace, the temperature on both sides of the bushing, in a transversal direction, will be the same. The coil is made up of two wires 0.150" x 0.600" connected in parallel and wound in two layers flatwise. Fiberglass insulation is used between layers. Number of turns is 44 and length of the coil A multi-layer flat wound coil of such dimensions has very small losses and contributes thereby to reduce the temperature of the lining, which is the principal requirement of the instant method. Two blowers are used to force the cooling air through the duct of this furnace supplying together a volume of air of 1,090 cu. ft. per minute. The temperature maintained regularly in the hearth and the melting channels of this furnace is 2200 F. The lining was of the mullite type.

In operating furnaces of this type it is important to know at all times the temperature of the lining at the cold face in order to be sure that the temperatures and temperature gradients called for in the present invention are maintained. A thermocouple located in the refractory lining at the critical point and touching the surface of the copper bushing ll, Fig. 1, serves these purposes well. The thermocouple should be conveniently connected to a sensitive recording instrument, which will not only record at all times the lining temperature but also warn the operators in case metal begins to seep through the refractory, a condition which will be indicated by a sharp rise of temperature.

The adaptation of a submerged resistor induction furnace to the melting of lead rich copper and also lead rich tin containing copper alloys signifies a distinct progress in the art, as it provides a means to obtain these alloys in a perfectly uniform homogeneous distribution of their original components.

We claim:

1. A method of producing metal alloys adapted to form low melting point segregations and particularly lead-rich copper alloys in a submerged resistor type induction furnace having in a refractory-iined casing an upper melting hearth, a refractory block underneath the hearth, a secondary melting loop opening into the hearth and an inner cooling duct located in this block in concentric relationship with said loop, an inductor unit in said cooling duct, said p and said cooling duct enclosing therebetween an annular refractory body having its outer face adj acent to said melting loop and its inner face adjacent to said cooling duct, said method comprising maintaining a relationship between the thickness of said annular refractory body of between four to six inches and the surface coefllcient of heat transmission at its cold face of between 28.0 to 18.0 B. t. u./F./ft.=/ho. to create at said cold face a temperature of about 700 F;

2. A method for producing metal alloys adapted to form low melting point segregations and particularly lead-rich copper alloys in a submerged resistor type induction furnace having in a refractory-lined casing an upper melting hearth, a refractory block underneath the hearth, a secondary melting loop opening into the hearth and an inner cooling duct located in this block in concentric relationship with said loop, an inductor unit in said cooling duct, said loop and said cooling duct enclosing therebetween an annular refractory body having its outer face adjacent to said melting loop and its inner face adjacent to said cooling duct, said method comprising maintaining a relationship between the thickness of said annular refractory body of between four to six inches and the surface coefficient of heat transmission at its cold face of between 32.5 to 23 B. t. u./F./ft.=/ho. to create at said cold face a temperature of about 600 F.

3. A method for producing metal alloys adapted to form low melting point segregations and particularly lead-rich coppe'r alloys in a. submerged resistor type induction furnace having in a refractory-lined casing an upper melting hearth, a refractory block underneath the hearth, a secondary melting loop opening into the hearth and an inner cooling duct located in this block in concentric relationship with said loop, an inductor unit in said cooling duct, said 100p and said cooling duct enclosing therebetween an annular refractory body having its outer face adjacent to said melting loop and its inner face adjacent to said cooling duct, said method comprising maintaining a relationship between the thickness of said annular refractory body of between four to six inches and the surface coefficient of heat transmission at its cold face 01 between 39.0 to 32 B. t. u./F./ft./ho. to create at said cold face a temperature of about 500 F.

4. A method for producing metal alloys adapted to form low melting point segregations and particularly lead-rich copper alloys in a submerged resistor type induction furnace having in a refractory-lined casing an upper melting hearth, a refractory block underneath the hearth, a secondary melting loop opening into the hearth and inner cooling duct located in this block in concentric relationship with said loop, an induc- REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Tama Feb. 13, 1945 Crafts Aug. 12, 1913 Brayton, Jr. Jan. 1, 1924 Foley Aug. 30, 1927 Foley Mar. 6, 1928 Summey Nov. 10, 1936 Tama et a1. June 9, 1942 Tama Jan. 25, 1944 Tama Feb. 22, 1944 Tama et al. Jan. 30, 1945 Tama et al May 1, 1945, Tama et a1. Aug. 7, 1945 Tama et a1. Nov. 20, 1945

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