US2559683A - Electric enamel furnace - Google Patents

Description

y 1951 R. E. SKINNER ETAL ELECTRIC ENAMEL FURNACE 4 Sheets-Sheet l Original Filed March 15, 1949 INVENTOR$ P085327 8'. Skid/IVER By GLENN any? WEE A I w ATTOENEV.

y 10, 1951 R. E. SKINNER ET AL 2,559,683

' ELECTRIC ENAMEL FURNACE Original Filed March 15, 1949 4 Sheets-Sheet 2 mmvrom 202cm E5K/1V/VEE, By GLEN/V mmmrma y 10, 1951 R. E. SKINNER ET AL 2,559,683

ELECTRIC ENAMEL FURNACE Original Filed March 15, 1949 4 Sheets-Sheet 3 INVENTORS ROBERT E. SKIN/YER,

BY GLENN H. Ml V TYRE muwm OTTO ENE Y6 y 1951 R. E. SKINNER ETAL ELECTRIC ENAMEL FURNACE 4 Sheets-Sheet 4 Original Filed March 15, 1949 Patented July 10, 1951 ELECTRIC ENAMEL FURNACE Robert E. Skinner, Lakewood, and Glenn H. McIntyre, Cleveland Heights, Ohio, assignors to Ferro Enamel Corporation, Cleveland, Ohio,

a corporation of Ohio Original application March 15, 1949, Serial No. 81,478. Divided and this application May 4, 1949, Serial No. 91,394

1 Claim. 1

This application is a division of our co-pending application Serial No. 81,478 filed March 15, 1949.

This invention relating as indicated, to method and apparatus-for making vitreous enamel, has specific reference to method and apparatus for continuously electrically melting such enamels.

As is well known to those familiar with the art of glass or enamel manufacture, vitreous enamel is a complex vitreous glass made opaque by the addition of opacifying ingredients. The formulation of vitreous enamels are complex because of necessity. Among the features that must be considered in the manufacture of such an enamel are, a vitreous enamel must be fusible on a metal surface, have a high degree of opacity, the proper coefficient of expansion and workability.

Among the ingredients employed in the manufacture of vitreous enamels the following usually occur: feldspar, quartz, borax, soda ash, zinc oxide, sodium nitrate, bone ash, fluorspar, cryolite, sodium silico fluoride, zirconium oxide, and titanium oxide. As is very apparent when a pile of material containing the above ingredients is subjected to a hgh temperature, some such materials namely borax, soda ash, sodium nitrate, and others will flux out at low temperatures before the other ingredients are melted. Consequently some of the materials, will separate and flow away from the massflefore others, resulting in inconsistency in the quality of the frit produced.

Other difliculties encountered in the conventional methods of manufacture of vitreous enamel have been dusting loss on feeding and the loss of expensive volatile ingredients, particularly the fluorides. As the raw materials become heated in a fuel fired smelter the fluorides begin to volatilize before the rest of the materials become molten. Thus the beneficial properties of the fluorides are in part lost before they are able to react with the other materials in the formation of a vitreous enamel.

The method or methods heretofore employed for melting the materials used in the manufacture of vitreous enamels has been to dump a considerable quantity onto a sloping floor of a ,-fuel fired smelter and to then melt down such mass and to permit the enamel as it melts, to run into a fining chamber or to immediately discharge the same into a water bath.

Fuel fired smelters because of necessity areso constructed that the raw material in the melting chamber is heated virtually only on the surface and depends on infrared radiation to heat the material throughout its entire mass.

It is therefore a principal object of our invention to provide a method of and apparatus for manufacture of vitreous enamel or the like which will result in a product of consistent quality.

It is a further object of our invention to provide a method of and apparatus for the manufacture of vitreous enamel whereby theloss of fluorine during manufacture is at an absolute minimum.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

We have discovered a new method and apparatus for the smelting of porcelain enamel. The apparatus operates electrically and takes several forms.

In said annexed drawings:

Fig. 1 is a horizontal sectional view through a smelter constructed in accordance with the principle of our invention and adapted to carry out the process comprising our invention;

Fig. 2 is a transverse sectional view of the apparatus illustrated in Fig. 1 taken on a plane substantially indicated by the line'2-2;

Fig. 3 is a transverse sectional view of the electrode controlling apparatus;

Fig. 4 is a horizontal sectional view of a second form of smelter constructed in accordance with the principle of our invention and adapted to carry out the process comprising our invention;

Fig. 5 is a transverse sectional view of the apparatus illustrated in Fig. 4 taken on a plane substantially indicated by the line 3-3 Fig. 6 is a horizontal sectional view through a third form constructed in accordance with the principle of our invention and adapted to carry out the process comprising our invention;

Fig. 7 is a transverse sectional view of the apparatus illustrated in Fig. 6 taken on a plane substantially indicated by the line 4-4.

Referring now more specifically to the drawings and more especially to Figs. 1 and 2, the smelter here illustrated in horizontal cross section and transverse section respectively as one embodiment of the apparatus comprising my invention may be round or square in form and constructed of refractory materials. The hearth is built in the conventional manner with firebrick for the body of the smelter I and a suitable super duty" refractory for the curbwall 2 and bottom for better resistance to attack by the molten glass. The body of the smelter above the bath line, or curbwall, need not be as thick or of as high quality refractory as in fuel smelters due to absence of hot combustion gases. Also due to the absence of hot combustion gases the volume of smelter above the bath line is at a minimum.

A projection 3 is provided in the smaller body to permit a bridge wall 4 to be suspended. An overflow spout 5 is located beyond the bridge wall, the elevation of the spout determining the bath line 6. The bridge wall 4 is suspended in such a way as to prevent unmelted material from flowing out of the smelter. An opening 1 is provided for draining the smelter at the completion of operation.

Three electrodes 8 are introduced through the cover to project below the normal glass level. Electric current from a 3-phase delta 230 volt source is conducted through the electrodes and then through the molten glass 9. Heating is accompl shed by the Joule effect of the molten glass. The specific resistance of the glass is 0.5 to 2.0 ohms per cubic inch in smelting temperature.

Raw ingredients to be melted are introduced by means of a continuous feeder I!) through an opening l l in the smelter. The elevation of the feeder is such that there is always a pile of raw material floating on top of the bath and above the level of the feeder to minimize dust loss. The raw materials cover the entire bath and melt from the bottom. The heat loss is thus greatly reduced and the volatile ingredients retained more completely.

The power consumption is 250 kw. hr., with frit production of 700 lbs./hour. About 357 watts per pound of frit are consumed.

Since this type of smelter operates at 230 line voltage, the current used is dependent on the depth of the electrodes in the bath. The deeper the electrodes, the greater is the contact area and the total resistance path between the electrodes is correspondingly lower. The electrodes may be raised and lowered to maintain the desired line amperage, either by hand cranking the electrode winches orby push button operation of reversible motors to operate the winches.

Automatic control also is accomplished by means of current relays which operate each electrode motor to maintain desired line amperage. Referring now to Fig. 8, the secondary of the current transformer 30 is connected in series through in the raw material depth. Furthermore, the graphite electrodes are slowly eaten away under the bath line so that continuous electrode feed is necessary. Since continual adjustment of the electrode depth is required to maintain uniform conditions, the automatic controls are desirable.

Referring now more especially to Figs. 4 and 5. this smelter has been provided with a fining zone l3 which is heated independently of the meltin is supplied by single phase transformers which the ammeter and two current relays 32 and 33.

Each relay can be adjusted to make or break contact at any value between 1 and 5 amperes. The contacts of relay 32 are connected to close when the current falls below its rating. Relay 33 makes contact when the circuit exceeds its rating. When the contacts are made according to the value of the secondary current the reversing contactor 31 is energized to operate the electrode motor 38 in a direction to either raise or lower the electrodes until the current in the lines and the secondary circuit has returned to normal.

If a current transformer of 1000 to 5 ratio is used and it is desired to operate the smelter at 750 amperes on each line, relay 32 will be adjusted to make contact at about 3.5 amperes. The contact will be held until the current has reached 3.75 amperes where it will open. This corresponds to 750 amperes in the primary lines. When the current falls to 700 amperes (3.5 amperes secondary) contact is again made until the current returns to 750 amperes. Likewise relay 33 is adjusted to make contact when the secondary current reaches 40 amperes (800 amperes primary). The electrode motor then raises the electrode until the current has dropped to 3.75 amperes.

The current flow is not steady due to the vigorous action around the electrode and to variation may have variable voltage taps to control the fining power. The resistance of the molten porcelain enamel in the fining zone is much more uniform than in the melting zone where raw material tends to cause bath resistance fluctuations. Therefore, automatic control over the power input to the fining zone is not necessarily required. The voltage range is to volts. Only 20-25% of the power input of the melting zone is required in the fining zone since this heat is only required to maintain the molten porcelain enamel at the proper temperature. The fining zone is of such length and volume that when raw bath ingredients are fed to the melting zone the porcelain enamel discharged at 5 by overflowing will be smelted to the correct degree.

Total power consumption of this smelter is 500 kw. hr., with frit production of 1200 lbs/hour.

About 417 watts per pound of porcelain enamel are consumed.

Openings IS in the melting and fining zone are connected to a stack to carry off fumes from the smelter. Since the surface of the glass in the melting zone is almost completely covered with raw material there is very little heat loss in the stack gases.

The smelter is started by filling the melting zone l2 with raw material, with the bridge wall 4 lowered to act as a gate. A piece of graphite is laid in the raw material directly under two electrodes so that when lowered their pointed ends will touch the graphite. By using a current limiting reactor in the line an arc can be placed between each electrode and the piece of graphite. After a short time a puddle is formed into which the electrodes can be lowered, the piece of graphite removed, and heating continued by direct resistance. As the molten portion enlarges the third electrode is inserted and the bridge wall is raised allowing the molten porcelain ena'mel to flow into the finding zone.

While the aforementioned vertical electrode smelters are constructed in accordance with the principle of our invention and are adapted to carry out the process comprising our invention they have certain disadvantages namely: relatively high electrode consumption per pound of frit, excessive overheating around the electrodes which causes vigorous boiling of the bath which in certain types of enamels is harmful, uncertain control of power, and excessive wear of the smelter bottom. Therefore in the preferred embodiments of our invention we use a smelter as illustrated in Figs. 6 and '7 which overcomes these previously enumerated difficulties. 7

Referring more specifically to Figs. 6 and 7, the smelter here illustrated makes use of side wall electrodes having an area of contact with the molten porcelain much larger than that with the use of vertical electrodes. The resultant lower current density eliminates overheating adjacent each electrode and produces a more uniform temperature condition in the entire bath.

The melting zone 20 and fining zone 2| are separated by a refractory wall 22 having a throat 23 for passage of molten porcelain enamel from the former zone to the latter. This solid wall with the submerged throat is more satisfactory than the suspended bridge wall since it has longer life. A flue opening 28 is located in the roof of the melting chamber 20. Two passageways 29 are provided through the wall 22 just below the roof of the fining chamber, for the passage of hot gases. In this manner the hot gases flow from the fining chamber into the melting chamber counter currentto the flow of molten'material.

One of the major problems in developing this smelter concerned the electrodes. If graphite were used it would be necessary to provide a means of replacement since molten porcelain enamel will erode graphite even under the best of conditions. A study of electrode materials revealed that pure molybdenum metal was highly resistant to the action of molten porcelain enamel. Tungsten might also be used but would be much more costly. Nickel was found to be useful only with certain types of poreclain enamels. Although the molybdenum metal withstands the corrosive action of the molten porcelain enamel it will burn readily if exposed to air at elevated temperatures. For this reason, the entire electrode must be kept below the bath level at all times. When the smelter is drained at the end of a run some enamel will adhere to the face of the electrode thus protecting some portions from oxidation. However, the edges and corners will not remain sufliciently covered. The use of an inert gas such as carbon dioxide to flood the smelter while draining will prevent serious oxidation. Thus proper care during draining will insure long life of molybdenum electrodes.

The rectangular molybdenum plates 24 are located at the sidewalls so that they are completely immersed in the molten porcelain enamel. Current is conducted to them by water cooled pipes 25 inserted through the sidewalls. A layer of chilled porcelain enamel surrounds the water cooled pipes at the point where they are inserted through the wall of the smelter, thus preventing the molten enamel from escaping.

The area of these electrodes is larger than that obtainable by using vertical electrodes as in the previously discussed smelters. This reduces the current density surrounding the electrode and eliminates localized overheating. The vigorous boiling around the electrodes is thus eliminated and better contact is made between the molten porcelain enamel and the electrode. With the side wall electrodes there is no contact resistance as in the vertical electrode smelters. Thus the only resistance is that of the bath itself which is governed by the spacing between the electrodes, bath depth, and width.

The total resistance then is less than in the vertical electrode type wherein the total resistance is the contact resistance plus the bath resistance. Therefore, a lower voltage is required on the larger electrodes than is used on the vertical electrodes. Since the bath resistance hinges in proportion to the bath temperature it is also necessary to have variable voltage control to compensate for temperature variation.

This smelter makes use of variable voltage control through saturable core reactors. A power transformer is used to supply power in the desired voltage range dictated by the dimensions of the hearth. The saturable reactor consists of a series of A. C. windings on a core along with D. C. windings. By allowing a very small direct current to flow in the D. C. windings the A. C. voltage is regulated. If the direct current is increased the A. C. voltage to the load increases and vice versa. Thus the voltage to the electrodes is varied in a stepless manner in any desired range. The output voltage may be controlled bythe bath temperature or by power input, as indicated by a, controlling pyrometer.

Both the melting zone and the fining zone are controlled by separate reactors and transformers. In this way the power and temperature in each zone can be controlled independently to suit the desired conditions.

The raw material is introduced by two screw feeders 26 so located to uniformly distribute the material over the molten bath. The elevation of the feeders is such that there is always a pile of 30 raw material ahead of the feeder discharge. The

raw material is pushed into the smelter rather.

than dropped, thus reducing dust loss through the stack. The molten porcelain enamel is discharged at spout 21 and the rate is governed by the rate of feed of raw material, although the power input must be regulated according to the rate of feed.

Broadly stated this invention comprises the process and apparatus for smelting vitreous enamels by passing an electric current through a mass of enamel forming constituents and heating such mass to a fluid state by its own internal electrical resistance.

This process has the advantage of being able 0 to heat internally substantially uniformly the entire mass. As previously stated the fuel fired smelter causes segregation of the enamel batch due to the surface heating and subsequent separating out of some of the ingredients at lower temperatures.

Prior art practices have shown the use of electric smelters for the manufacture of clear glass, however, these prior art electric smelters ar not applicable to the manufacture of porcelain en- 55 amel. In the manufacture of clear glass transparency is of the utmost importance and it is therefore most essential that the molten materials from which the glass is made is absolutely free of undissolved or suspended material known to those versed in the art as seeds.

Therefore in the fining of clear glass all smelters must be so constructed that the discharged molten material is 100% fined.

The design and function of our electric smelter for porcelain enamel must be different from that used in the manufacture of clear glass. Porcelain enamel may be generally defined as a composition which includes usually a sodium-borosilicate glass matrix in which are held in uniformly distributed suspension the opacifying compounds which are usually undissolved. The function of the smelter for the manufacture of porcelain enamel is therefore, not to separate out the crystalline or undissolved components such as the opacifying agents but to insure that the same will be evenly distributed throughout the entire bath so that the resultant product may have uniform characteristics such as opacity, etc.

Furthermore in the manufacture of clear glass, the glass batch contains little or no fluoride compounds. As is well known the fluoride compounds are extremely corrosive and cause rapid destruction of electrodes and refractories unless the smelter is constructed so as to minimize this destruction.

It can now be readily seen that an electric smelter for the manufacture of porcelain enamel must be able to first overcome the problem of segregation during the melting process, secondly the smelter must discharge a molten mass that is not 100% flned, and thirdly the smelter must be constructed so as to minimize the destructive influence of fluorides.

Electric smelters usually generate higher temperatures than fuel fired smelters and one would naturally expect that the result of usin an electric smelter would be to increase fluorine loss. However, we have provided a method and apparatus in which the opposite is true.

The use of our electric smelter for producing vitreous enamels has greatly minimized the loss of fluorides during the smelting process. Due to the fluorine retention during electric smelting it is possible to produce a vitreous enamel from a raw batch containing 4% less fluorine than ordinarily would be used in producing the same enamel on a fuel flred smelter.

The fluorine retention during electric smelting applys to both classes of enamels known to the trade as Cover Coats and Ground Coats.

So that the foregoing statements are more clearly understood the following examples are iven:

Example I The following formula is atypical zirconium bearing white cover coat enamel The above raw batch when smelted on a fuel fired smelter produced a frit having an average of 7.77% fluorine. This constitutes a loss of 4.61% fluorine during the smelting process.

The same formulation as above when smelted in an electric smelter produced a frit containing 10.45% flourine. This constitutes a loss of only 1.93% fluorine during the smelting process.

From the foregoing Example I it now becomes apparent that the electric smelter produces a vitreous enamel with less loss of fluorine.

It will be observed that when employing the process and apparatus comprising my invention, the raw materials from which the finished porcelain enamel is to be produced, are fed into the melting chamber and are quickly and efliciently reduced to a molten state for discharge to the fining chamber. The raw materials are heated internally substantially uniformly and the possibility of overburning the ingredients has been brought to an absolute minimum.

Our invention has the further advantage of keeping the fluorine loss at an absolute minimum thus making it possible to produce porcelain enamel from a raw batch which contains less fluorine than heretofore has been used.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in the following claim, or the equivalent of such, be employed.

We therefore particularly point out and distinctly claim as our invention:

A smelter for the continuous production of porcelain enamel comprising the combination of a hearth comprising melting and fining zones, a confining structure comprising upstanding walls and a roof, arranged about and over said hearth and forming therewith substantially closed adjacent melting and fining chambers, said melting chamber provided with a charge opening for raw material and a flue opening for discharging hot gases and said fining chamber provided with a discharge opening for melted material, the material supporting surface of said hearth being substantially a straight line inclined towards said discharge opening, separate passages respectively for hot gases and melted material connecting said chambers, said passage for hot gases disposed substantially adjacent the roof of said fining chamber wherein the hot gases flow countercurrent to the flow of the molten material and said passage for melted material being restricted and submerged below the normal bath level, four horizontal electrodes disposed below the normal bath level and arranged in pairs opposite each other in the side walls of said melting zone and two horizontal electrodes disposed below the normal bath level and arranged opposite each other in the side walls of said fining zone, said electrodes being of the same polarity on one side and of opposite polarity on the opposite side, a power circuit suflicient to supply an electric current through the electrodes in said melting chamber to effect a heating current flow through a mass of porcelain enamel forming materials, and a separate power circuit, in said fining chamber sufficient to supply an electric current to maintain the said melted material in fining chamber in a molten state.

ROBERT E. SKINNER. GLENN H. MCINTYRE.

REFERENCES CITED The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number Name Date 1,267,317 Erskine May 21, 1918 1,552,555 Grauel Sept. 8, 1925 1,610,377 Hitner Dec. 14, 1926 1,799,980 Hartford Apr. 7, 1931 1,820,248 Raeder Aug. 25, 1931 1,827,471 Hitner Oct. 13, 1931 1,880,541 Wadman Oct. 4, 1932 1,897,973 Wadman Feb. 14, 1933 1,905,534 Wadman Apr. 25, 1933 1,928,289 Henry et a1. Sept. 26, 1933 1,944,855 Wadman Jan. 23, 1934 2,089,690 Cornelius Aug. 10, 1937 2,122,469 Hitner July 5, 1938 2,159,361 Atkinson et al. May 23, 1939 2,283,188 Cornelius May 19, 1942 2,314,956 Slayter et al Mar. 30, 1943 2,413,037 De Voe Dec. 24, 1946 2,471,531 McIntyre et a1. May 31, 1949

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