US2744944A

US2744944A - Rotating electric phosphorus furnace

Info

Publication number: US2744944A
Application number: US476096A
Authority: US
Inventors: Jr Marcus M Striplin; Stanford A Hardin; William H Bundy
Original assignee: Tennessee Valley Authority (ATV)
Current assignee: Tennessee Valley Authority (ATV)
Priority date: 1954-12-17
Filing date: 1954-12-17
Publication date: 1956-05-08
Anticipated expiration: 1973-05-08

Description

May 8, 1956 Filed Dec. 17, 1954 BURDEN M. M. STRIPLIN, JR., ET AL 2,744,944

ROTATING ELECTRIC PHOSPHORUS FURNACE I5 Sheets-Sheet 1 Mag.

IN V EN TORS'.

gW/M

y 3, 1956 M- M. STRIPLIN, JR. ETA'L 2,744,944

ROTATING ELECTRIC PHOSPHORUS FURNACE 3 Sheets-Sheet 2 Filed Dec. 17, 1954 JNVEN TORS.

y 1956 M. M. STRlPLlN, JR., ETAL 2,744,944

ROTATING ELECTRIC PHOSPHORUS FURNACE oasavHo 0 d NOi/BH 'Mx woudwnsuoo uaMod MARCUS M. STRIPLIN JR. STANFORD A. HARDIN WILLIAM H. BUNDY INVENTORS.

BYZLMZZ d 777W W United States Patent O ROTATING ELECTRIC rnosrnonus FURNACE Marcus M. Striplin, In, and Stanford A. Hardin, Florence, and William H. Bandy, Sheffield, Ala., assignors to Tennessee Valley Authority, a corporation of the United States Application December 17, 1954, Serial No. 476,096

7 Claims. (Cl. 13-10) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention herein described may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty therefor.

This invention relates to an improved design of electric furnaces for the production of elemental phosphorus by smelting charges of phosphate rock, carbonaceous material such as coal or coke, and silica.

Electric furnaces have been used for smelting phosphate rock for many years, yet the design of such furnaces remains essentially empirical and no one can predict with confidence just what changes in furnace operation characteristics will follow a change in design. Prior furnaces have had the disadvantage of being unable to utilize finely divided phosphate rock. This material usually is mined as a fine sand and must be agglomeratedeither by sintering, nodulizing, briquetting, or in some other manner-before it is suitable for a furnace charge. The process of agglomerating phosphate rock frequently adds percent or more to the total cost of producing phosphorus.

Another major cost factor in producing phosphorus is the amount of electric power consumed in the smelting operation. In the past it has been believed [Chemical and Metallurgical Engineering 45, 378 (1938)] that the power required per ton of phosphate rock smelted increases rapidly with power input per square foot of hearth area and, both for this reason and because of destruction of furnace linings by excessive heat, it has been customary to design electric phosphorus furnaces to operate at a power input below 30 kilowatts per square foot of hearth area, although the advantages to be derived from high production at high power input were understood.

It is an object of this invention to provide an improved furnace for smelting phosphate rock having high capacity and operable at high power input per square foot of hearth area.

Another object is to provide a furnace that has a very low energy requirement per ton of phosphate rock smelted.

Another object is to provide such a furnace that is long-wearing in operation at high capacity.

Still another object is to provide an improved furnace in which fine phosphate rock may be smelted.

Other objects and advantages of our invention will become apparent as this disclosure proceeds.

We have found that these objects are attained in an electric furnace which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for slowly rotating said crucible at controlled speed; a stationary cover on said crucible; a gastight seal between the crucible and the cover; three graphite electrodes located at the apices of an equilateral triangle, each having a diameter which may be calculated wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in 2 the range from 0.036 to 0.043, preferably about 0.039, the electrodes being disposed to extend substantially vertically through the cover at a center-to-center spacing which may be calculated po.so

wherein C is center-to-center electrode spacing in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026, preferably about 0.0022, each electrode being at a distance from the interior wall of the crucible which may be calculated wherein C is center-to-center electrode spacing in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026, preferably about 0.0022, each electrode being at a distance from the interior wall of the crucible which may be calculated paso necessary to maintain the peripheral ohm factor r, calculated wherein E is the electrode voltage in volts, I is the electrode current in amperes, and D is the graphite electrode diameter in inches, in the range from 0.75 to 1.25, preferably about 1.0; and means for maintaining a uniform selected burden depth in the furnace. A desired electrode voltage is predetermined by the selected transformer tap, and the current required to give a total power input in the range given above is controlled by raising or lowering the electrodes to maintain the pe ripheral ohm factor r in the stated range.

In order to obtain the combination of results comprising lower power required per ton of phosphate rock smelted, long wear of furnace, high smelting capacity, smooth downward flow of burden in the furnace, and ability for smelting phosphate fines, we have found that a combination of five factors in furnace design is necessary. These factors are (l) electrode placement as described above, which results in reducing substantially the distance between electrodes from that in other furnaces; (2) the narrow wall clearance described, which is about two-thirds of the clearance used in other furnaces; (3) the means for energizing the electrodes at the high rate of power input described, together with means for maintaining the peripheral ohm factor in the high range described; (4) means for rotating or oscillating the crucible at low speed; and (5) means for maintaining burden in the furnace at a selected depth.

In the attached drawings, Figure 2 is a plan of the top of a furnace embodying principles of our invention; Figure l is a vertical section through the furnace on the line 11 of Figure 2; Figure 3 illustrates one means for rotating the furnace slowly; Figure 4 is a graph of total power consumption in a furnace of our novel design per ton of P 05 charged; and Figure 5 is a cross section on the line 55 in Figure 1 and shows details of one preferred means for water cooling the bottom of the furnace, in smaller scale.

With reference to Figure l the numeral 5 designates a round vertical crucible. This crucible has a carbon hearth 6, a taphole 7 at the level of the hearth floor for withdrawal of molten ferrophosphorus, and a taphole 8 at a higher level for the withdrawal of slag. Additional tapholes may be provided if desired. The number of tapholes should be such as to have one of each set available in the tapping area at all times. Crucible 5 is rotatably mounted on pivot 9. Rails 11 and rollers 12 carry the weight of the crucible and permit easy rotation. A means for rotating the crucible slowly, or for oscillating it through an arc of 120 if preferred, is provided and is illustrated as motor 14, an assembly of reducing gears 15, drive pinion 16, and gear 17 in larger scale in Figure 3. A pinion and sprocket chain drive may be substituted, if desired, and a reversing switch is preferably installed when it is desired to oscillate the crucible through 120 instead of rotating it completely.

A stationary cover 18 is superimposed on crucible 5 and is supported by supporting members 19. The joint between the stationary cover and the rotatable crucible isclosed by a gas-tight seal. prises an annular trough 21 attached to the exterior of crucible 5 adjacent to the top of the crucible, a molten mixture of lead and antimony 22, preferably containing about l to 13 percent antimony, in the trough, and an annular flange 23 attached to the stationary cover 18 and projecting down below the surface of the leadantimony mixture.

Three graphite electrodes 24 project down through suitable openings in the stationary cover 18 to a level which, in operation, will be deep in the furnace burden. Graphite electrodes are used because of their greater mechanical strength. Carbon electrodes of somewhat smaller size could be substituted, but we have found that it is unwise to do so because their brittleness and fragility greatly increase electrode breakage. These electrodes are of a size which may be calculated wherein D is electrode diameter in inches, P is total power in watts per electrode, and k is any constant in the range from 0.036 to 0.043, but is preferably about 0.039. The electrodes are so disposed that their vertical central axes lie at the apices of an equilateral triangle and have a center-to-center distance which may be calculated wherein C is center-to-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026, preferably about 0.0022.

The hearth section 6 of the crucible is of such size that the clearance between the interior wall and the bottom of electrodes is about one-third less than in other phosphate reduction furnaces. This clearance may be calculated from the equation wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is any constant in the range from 0.0015to 0.0019, but is preferably about 0.0016. Each electrode is surrounded by a suitable seal 26 disposed in opening 27 through the stationary cover. This seal may be of any type that permits vertical reciprocating movement of the electrode. It is preferably water cooled.

A means for raising and lowering the electrodes vertically is provided.- This means preferably comprises a system of cables 28 and 29 operated by pulleys and winches not shown. It is essential that vertical travel of the electrodes be provided in the range necessary to maintain the peripheral ohm factor r, calculated This seal preferably comwherein E is the arc voltage in volts, I is the electrode current in amperes, and D is the electrode diameter in inches, in the range from 0.75 to 1.25, preferably about 1.0.

This high peripheral ohm factor is quite different from that previously described in the literature. It has been believed that the peripheral ohm factor for a phosphate smelting furnace should be in the range from 0.4 to 0.5, as shown in Canadian Chemical Processing 35, 62629 (1951).

A means is provided for energizing the electrodes with 3-phase alternating current with one electrode on each phase and a total furnace power input in the range from 40 to 50 kilowatts per square foot of hearth area. This means preferably comprises a suitable power line 31, and transformer 32 (of the capacity described above), bus bars 33, and electrode contact members 35. Suitable electrical connections, preferably delta connections, are provided so that one electrode is on each phase. In the interest of clarity, only one bus bar is shown in its entirety in Figure 1, and only parts of the bus system are shown in Figure 2 for the same reason.

A means for maintaining a selected burden depth in the furnace also is provided. This means preferably is a source of burden (not shown), a main feed chute 37 communicating with a large central chute 38, and a plurality of smaller peripheral feed chutes 39.

An oifgas duct 50 (shown in Figure 2) is provided for withdrawing furnace gases containing the vaporized phosphorus produced.

A means for water cooling the furnace bottom also is provided. This means preferably comprises a source of water 41, a pan 42 for holding water in contact with the bottom of furnace shell 43, and suitable drains 44. Recirculating pumps may be added if desired, or suitable spraying equipment may be substituted for the preferred pan arrangement. We have found that water cooling, rather than conventional air cooling, adds greatly to the length of life of the furnace by preventing the slow sagging or warping of supporting members, which occurs in the course of a year or two in other furnaces, without causing undue loss of heat that interferes with smelting efficiency. We prefer to introduce cooling water at the center of the bottom of the furnace by making pivot 9 hollow and placing the inlet water line 41 in this hollow space. The direction of flow of cooling water is thus radial toward all parts of the circumference of the furnace bottom and at decreasing velocity.

Figure 5 shows our preferred construction of pan 42 and auxiliary equipment for cooling the furnace bottom. This figure illustrates water inlet line 41 entering at a central location. From points near line 41 radially arranged ribs 45 extend to points near the circumference of pan 42 and divide the pan into sectors. These ribs cause uniform radial flow of cooling water from inlet 41 to drains 44, which receive overflow from pan 42 at points high enough to ensure good contact of cooling water with the bottom of the crucible. Best results are obtained when the cooling water is introduced under sufiicient pressure to cause turbulent flow at the center of pan 42. This changes to streamlined flow as the water passes through wider areas toward the circumference.

In operation, the furnace is charged with a burden of phosphate rock, coke, and silica to a depth that may be calculated H=kEp in which H is depth of burden in feet, Ep is phase-to phase voltage, and k is any constant in the range from 0.029 to 0.04, preferably about 0.031. This depth is maintained fairly uniform over the entire hearth area in introducing burden through the large central feed chute 38 and through the smaller peripheral chutes 39, which are kept filled with charge at all times. The electrodes are then adjusted by vertical movement and are energized so that the peripheral ohm factor will be in the range from 0.75 to 1.25, preferably about 1.0, at a total power input of about 30 to 50 kilowatts per square footof hearth area, or preferably from 40 to 50 kilowatts per square foot. This high power input per square foot of hearth area is quite different from that recommended by the prior art. In the past it has been believed [see Chemical and Metallurgical Engineering 45, 378 (1938)] that the power required per ton of phosphate rock smelted in an electric furnace increases rapidly with power input per square foot of hearth area. It has been customary to operate phosphate smelting furnaces at a total power input below 30 kilowatts per square foot of hearth area, although the advantages to be derived from lower labor costs and higher rates of production that would accompany higher power input were understood.

The graph shown as Figure 4 in the accompanying drawings illustrates the change in power required per ton of P205 smelted with varying power input per square foot of hearth area in a furnace of the design described above. We have found that the power required to smelt phosphate rock equivalent to a ton of P205 in a furnace having the features of electrode size and placement, wall clearance, and rotation as described above does not increase rapidly at power inputs of 30 kilowatts per square foot of hearth area and above, but decreases to a minimum near a power input of about 40 kilowatts per square foot. Thus this graph shows that we can operate at very low cost for electricity in the range from 35 to 42 kilowatts per square foot. We prefer to operate in the range from minimum consumption of electricity to the right on this curve up to about 50 kilowatts per square foot. At power inputs in the range from about 40 to 50 kilowatts per square foot, the cost of electricity rises as one proceeds to the right along this curve. The increase is not sharp, however, and is offset by lower labor costs and other economies derived from high rate of production. An uncooled stationary furnace cannot be operated at such high power input without appreciably shortening the life of the furnace lining.

As the furnace heats up, motor 14 is started and the crucible is rotated so that the electrodes pass laterally through the burden at a speed in the range from about 0.66 to 5.45 inches per hour at the center of the electrode circle. The speed selected in this range will depend upon the size composition of the burden. When the phosphate rock is entirely in the form of fines we prefer to operate in the lower part of this range, i. e., from 0.66 to about 1 inch per hour. When the phosphate rock is entirely in the form of lumps we prefer to operate in the upper part of the rangenear 5 inches per hour. When the burden contains phosphate of varying sizes, for example a mixture of 6 0 percent fines with 40 percent nodules, speeds in the middle part of the range are indicated. The limiting speed is always that which will cause lateral pressure on the electrodes sufficient to push them out of plumb, or to break them. The maximum speed at which we have been able to rotate the furnace is 5.45 inches per hour. Even with a burden containing all phosphate in lump form, we have not been able to operate beyond that speed without causing undue lateral pressure on the electrodes.

As the crucible with its burden rotates, the electrodes melt their way through the charge. Thus troublesome crusts which might retain gas under pressure are constantly melted away and troubles from pufling and localized overheating are eliminated. The burden feeds down smoothly as it melts, so that rodding and other labor frequently necessary to cause even feeding are substantially eliminated. Elimination of puffing of the furnace makes it relatively easy to maintain a slight positive pressure within the furnace to cause even flow of vaporized phosphorus from the furnace through outlet 50 without much leakage of vapor through electrode seals.

It will be seen from the above description that many changes may be made in this furnace. Various types of covers and seals may be used; other means for cooling the bottom may be substituted; different driving means for rotation or oscillation may be selected; and other electrical connections or systems can be used without departing from the spirit of this invention, which is limited only by the following claims.

We claim as our invention:

1. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gastight seal between the crucible and the cover; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter which may be calculated PO-45 D-k wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.036 to 0.043; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated wherein C is centento-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026; each electrode being at a distance from the interior wall of the crucible which may be calculated poso wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is any constant in the range from 0.0015 to 0.0019; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with 3-phase alternating current with one electrode on each phase with a total furnace power input in the range from 30 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor r, calculated wherein E is the electrode voltage in volts, I is the electrode current in amperes', and D is the electrode diameter in inches, in the range from 0.75 to 1.25; and means for maintaining a selected uniform burden depth in the furnace.

2. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter which may be calculated P015 D is wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.038 to 0.039; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated 7 wherein C is center-to-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.0021 to 0.0022; each electrode being at a distance from the interior wall of the crucible which may be calculated wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is about 0.0016; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with 3-phase alternating current with one electrode on each phase with a total furnace power input in the range from 40 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor r, calculated T 1rD wherein E is the electrode voltage in volts, I is the electrode current in amperes, and D is the electrode diam eter in inches, at about 1.00; and means for maintaining a selected uniform burden depth in the furnace.

3. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter Which may be calculated.

wherein D is electrode diameter in inches, P is total fur nace power input in watts, and k is any constant in the range from 0.036 to 0.043; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated wherein C is center-to-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026; each electrode being at a distance from the interior wall of the crucible which may be calculated.

wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and. k is any constant in the range from 0.0015 to 0.0019; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with 3-phase alternating current with one electrode on each phase with a total furnace power input in the range from 30 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor r, calculated for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter which may be calculated POAS D k wherein D is electrodediameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.036 to 0.043; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated wherein C is center-to-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.002 to 0.0026; each electrode being at a distance from the interior wall of the crucible which may be calculated wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is any constant in the range from 0.0015 to 0.0019; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with 3-phase alternating current with one electrode on each phase with a total furnace power input in the range from 30 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain'the peripheral ohm factor r, calculated wherein E is the electrode voltage in volts, I is the electrode current in amperes, and D is the electrode diameter in inches, in the range from 0.75 to 1.25; means, comprising a large central feed chute and a plurality of smaller feed chutes disposed near the circumference of the crucible, for maintaining a selected uniform burden depth in the furnace; and means for water cooling the bottom of the furnace comprising a pan beneath and near the bottom of the crucible, a centrally located water inlet in the pan, and a plurality of peripherally located drains leading from the pan at a level high enough to maintain contact of water with the crucible bottom.

5. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter which may be calculated Pens D-k wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.038 to 0.039; said electrodes being disposed to extend substantially vertically through the cover at .a center-to-center distance which may be calculated wherein C is center-to-centerelectrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.0021 to 0.0022; each electrode being at a distance from the interior wall of the crucible which may be calculated kpaso wherein Sis the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is about 0.0016; means, com prising a suitable transformer tap and electrical connections, for energizing the electrodes with 3-phase alternating current with one electrode on each phase with a total furnace power input in the range from 40 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor 1', calculated wherein E is the electrode voltage in volts, I is the elec trode current in amperes, and D is the electrode diameter in inches, at about 1.00; means, comprising a large central feed chute and a plurality of smaher feed chutes disposed near the circumference of the crucible, for maintaining a selected uniform burden depth in the furnace; and means for water cooling the bottom of the furnace comprising a pan beneath and near the bottom of the crucible, a centrally located water inlet in the pan, and a plurality of peripherally located drains leading from the pan at a level high enough to maintain contact of water with the crucible bottom.

6. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, rotatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover comprising an annular trough attached around the exterior of the crucible adjacent to the top thereof, a molten mixture of lead and antimony containing about 10 to 13 percent antimony contained in the trough, and an annular flange attached to the stationary cover and projecting down below the surface of the lead-antimony mixture in the trough; three vertical graphite electrodes disposed at the apices of an equilateral triangle, each of the electrodes having a diameter which may be calculated wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.036 to 0.043; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated wherein C is center-to-center electrode distance in feet, P is total furnace power input in Watts, and k is any constant in the range from 0.002 to 0.0026; each electrode being at a distance from the interior Wall of the crucible which may be calculated wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is any constant in the range from 0.0015 to 0.0019; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with S-phase alternating current with one electrode on each phase with a total furnace power input in the range from 30 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor r, calculated wherein E is the electrode voltage in volts, I is the electrode current in amperes, and D is the electrode diameter in inches, in the range from 0.75 to 1.25; means, comprising a large central feed chute and a plurality of smaller feed chutes disposed near the circumference of the crucible, for maintaining a selected uniform burden depth in the furnace; and means for water cooling the bottom of the furnace comprising a pan beneath and near the bottom of the crucible, a centrally located water inlet in the pan, and a plurality of peripherally located drains leading from the pan at a level high enough to maintain contact of water with the crucible bottom.

7. An electric furnace for smelting phosphate rock which comprises in combination a vertical, round, ro tatably mounted crucible having a carbon hearth; means for rotating the crucible at slow, controlled speed; a stationary cover superimposed on the crucible; a gas-tight seal between the crucible and the cover comprising an annular trough attached around the exterior of the crucible adjacent to the top thereof, a molten mixture of lead and antimony containing about 10 to 13 percent antimony contained in the trough, and an annular flange attached to the stationary cover and projecting down below the surface of the lead-antimony mixture in the trough; three vertical graphite electrodes disposed at the apices of an equilateral trian le, each of the electrodes having a diameter which may be calculated 1 0.45 D k 3 wherein D is electrode diameter in inches, P is total furnace power input in watts, and k is any constant in the range from 0.038 to 0.039; said electrodes being disposed to extend substantially vertically through the cover at a center-to-center distance which may be calculated kpaso wherein C is center-to-center electrode distance in feet, P is total furnace power input in watts, and k is any constant in the range from 0.0021 to 0.0022; each electrode being at a distance from the interior wall of the crucible which may be calculated wherein S is the distance from the electrode center to the interior wall of the crucible in feet, P is the total furnace power input in watts, and k is about 0.0016; means, comprising a suitable transformer tap and electrical connections, for energizing the electrodes with B-phase alternating current with one electrode on each phase with a total furnace power input in the range from 40 to 50 kilowatts per square foot of hearth area; means for raising and lowering the electrodes vertically through a distance necessary to maintain the peripheral ohm factor 1', calculated wherein E is the electrode voltage in volts, I is the electrode current in amperes, and D is the electrode diameter in inches, at about 1.00; means, comprising a large central feed chute and a plurality of smaller feed chutes disposed near the circumference of the crucible, for maintaining a selected uniform burden depth in the furnace; and means for water cooling the bottom of the furnace comprising a pan beneath and near the bottom of the crucible, a centrally located water inlet in the pan, and a plurality of peripherally located drains leading from the pan at a level high enough to maintain contact of water with the crucible bottom.

References Cited in the file of this patent UNITED STATES PATENTS 586,686 Heath July 20, 1897 958,855 Danne May 24, 1910 1,496,232 Klugh June 3, 1924 1,807,090 Pistor May 20, 1931 2,427,037 Winters Sept. 9, 1947

Claims

1. AN ELECTRIC FURNACE FOR SMELTING PHOSPHATE ROCK WHICH COMPRISES IN COMBINATION A VERTICAL, ROUND, ROTATABLY MOUNTED CRUCIBLE HAVING A CARBON HEARTH; MEANS FOR ROTATING THE CRUCIBLE AT SLOW, CONTROLLED SPEED, A STATIONARY COVER SUPERIMPOSED ON THE CRUCIBLE; A GASTIGHT SEAL BETWEEN THE CRUCIBLE AND THE COVER; THREE VERTICAL GRAPHITE ELECTRODES DISPOSED AT THE APICES OF AN EQUILATERAL TRIANGLE, EACH OF THE ELECTRODES HAVING A DIAMETER WHICH MAY BE CALCULATED