US2891297A - Method for treating materials - Google Patents

Description

W. A. KLEMM METHOD FOR TREATING MATERIALS Jupg 23, 1959 2 Sheets-Sheet 2 Filed Jan. 3, 1955 I, I l 1/! II I! II 'INVENTOR. Wl///4/fl ,4. 'K/emm.

United States Patent 2,891,297 Patented June 23, 1959 ice 2,391,297 Msrnou For TREATING William A. Klemni, Cupertino, Califl, nssignor' to Kaiser Aluminum & Chemical Corporation, Oakland, Calif This invention relates to. the production of carbon furnace linings; and more particularly it relates to an improved method for baking electrodes and to the product obtained thereby.

Various methods, have been proposed and employed in the past for baking carbon furnace linings or electrodes which are to be employed in the form of a shell or a hollow container, but these methods have exhibited the disadvantages of non-uniform heat distribution and consequent non-uniform baking, and non-uniform quality of the lining or electrode produced. Also, the prior art methods have been diflicult to control and have required constant attention by the operators during the baking process. It has been common practice, for instance, to prepare cathodes for electrolytic reduction cells by forming the cathode shape or shell with the embedded collector rods as known in this art, placing the un'baked cathode in the ppoduction line, 6. g. a line of electrolytic cells in series, connecting the anode or cathode connection to the co on source of power through suitable shunting arrangements, and thus electrically bakin g the cathode. This procedure requires use of potline power in a manner to reduce the number of pots in actual production and it also requires rather elaborate special electrical connecting lines or shunts between the operating pot connections and the pot or cathode being baked. Also, heating of the, lining is not uniform and the stresses created have often resulted in cracks and fissures in the lining. It has also been known to pro-bake sections or blocks, in a place apart from the production line of a plant, and then to assemble the pre-baked sections to form a cathode shell. This requires bonding of the blocks, to provide proper flow of electric current, after they have been assem'bled, but disadvantages attend such procedure, especially parting of the blocks at the original interfaces and heaving f the blocks, for instance due to leakage of bath material to the back of or below the blocks.

It is an object of the present invention to, provide a method for baking monolithic carbon furnace linings in the form of a shell or hollow form or shape. It is a further object of the invention to provide a device or arrangement for carrying out such baking method. It is a still further object to provide a method for uniformly baking monolithic carbon electrodes whereby the baked electrode exhibits minimal cracking tendency and an increased operating life. i

A c r ing to the pre ent, inv nt on. it ne been found thatth e above disadvantages are avoided and that a monolithic carbon furnace lining, especially of shell or hollow form, is obtained by forming the lining produeing carbonaceousmix or paste into the desired shape, suitably supported by or within a casing, and heating to cause coking, the heat being applied exteriorly to the interior or concave face of the formed mix, where the latter is in the shape of a shell or hollow form. Preferably, the carbon material to be baked is an admixture of coal, coke and a carbonaceous binder such as. tar, 0031i tar or pitch. The. prepared mix is suitably tamped, rammed, or packed to. the required thickness into a metal: casing. or desired form. In a preferred embodiment, wherein, an. electrode is prepared, for use in a reduction. cell, the required collector bars are embedded in the, mix prior to. heating After the lining or electrode has been shaped, the mass. is baked by exterior heating of the inner concave face. Heating is preferably effected: by. flame firing as will be more fully explained below. I

The operation of. this invention will be illustrated by the annexed drawings, which show. particular embodiments for carrying out the process ofthe invention. In the drawings, wherein like numerals designate like features or elements in each view:

Fig. 1 is a vertical sectional view ofa furnace and electrode assembly according to. this invention, and suitable for carrying out the. process ofthis invention;-

Fig. 2 is a plan view of a portion of the device of Fig. 1, taken on line 22 of Fig. 1;

Fig, 3.'is a vertical sectional view according to. the present invention;

Fig. 4 is a vertical sectional view of still another variant according to this invention, wherein heat is ap plied by electrical means; I i i Fig. '5 is a partial vertical sectional View of another embodiment of this invention showing the liner orfacing to support a softer cokable mass.

Referring to the drawings, Figure 1 shows a furnace for heating and coking the carbonaceous mass under an open flame, having a. steel shell 10, lined with refractory 11, and having apertures 12 for burners 13. The refractory is preferably an insulating lining, but any refractory material is useful which is capable of resistin-g flue gases at high temperatures. The furnace is open at the bottom, and when in operation is placed upon the. electrode or lining mass, orthe furnace and lining mass are brought into conjunction, as shown in the figures. The lower edge of the furnace shell and the upper edge of 'the liningor cathode structure abut at 1 5. Combustion and furnace gases go off; through stack or flue 20.

The carbonaceous shell 18, which in one embodiment is to be baked to form a cathode for an aluminum reduction cell for operation of the Hall process, is contained within steel shell 15 within which are disposed bottom lining 16 of insulating refractory material and side lining 17, also of insulating refractory material. In this embodiment, the bottom insulating material 16 and side 17 are composed of alumina ore. Alternatively, other refractory materials can be used, for example, alumina brick, diatomaceous earth, common or building brick, silica brick of various types, or other materials. A i

Carbonaceous mixture 18, which is to be baked and coked, is rammed in over the insulating materials to the desired shape and depth. During the ramming in or packing of the linings, collector rods 19, of the type well known in this reduction art, are placed in the mass and are embedded therein as ramming or packing proceeds. Two of these rods are shown but 'any' desired number can be employed. It is to be noted that Fig. 2 is a partial plan View or a plan view ofonly a portion of the length of the cathode and it illustrates a pair of burners. The cathode for a reduction cell as described is usually much longer than it is wide and it will be understood that any desired number of burners are suitably employedto provide heating over the whole length of such ane'xtensive carbonaceous mass. Fig. 2 'illiis trates a plan view of atypical portion of such heating installation and mass.

Fig. 3 represents a furnace device and a carbonaceous of another variant lining and its supporting features which are all similar to those of Fig. 1, except that the protective particulate layer is omitted and the carbonaceous lining mass is exposed directly to the source of the heat, which can be an open flame as shown, or electrical resistors as shown in Fig. 4, or any desired source of heat.

Fig. 4 shows a furnace device and the carbonaceous lining assembly which are like those of Fig. 1 with the exception of the heating means employed. In the furnace device of Fig. 4 there are disposed one or more electrical resistor units 21 connected by suitable conductors 23 to a source of electrical current (not shown) and through which current is passed to provide heat for baking the carbonaceous lining 18. Resistor units 21 are suitably supported within the furnace.

The carbonaceous lining material is prepared from any desired cokable mass or admixture. There can be employed, for example, a mixture of coke, coal and a binder such as tar or pitch; or a mixture of coke and a tar or pitch hinder, or of coal and a tar or pitch hinder; or a mixture of raw coal and coke. The mass to be coked is preferably of such consistency as will retain its shape during the coking process. Alternatively, however, there can' be employed a softer mass and during coking it is supported by a liner or inner facing of steel or other metal to prevent slumping of the mass during heating. The liner or facing is removed after coking, if desired. For instance, a suitable liner or facing 22 is of steel in the form of an. inverted trough or frustrum of a hollow, elongated pyramid, and provides a desirably shaped coked mass, as shown in Fig. 5. Where a carbonaceous binder such as tar or pitch is employed, it is preferred to employ a small or minor amount, up to about 35% thereof, and especially good results are obtained by employing in the mix from about 11% to about 18% of such binder, these percentages being based on the total weight of the carbonaceous mixture. The solid or aggregate com ponents of the mixture, such as coke or coal, are suitably crushed, milled or ground and are preferably admixed in graded particle size ranges useful to secure the best packing and densest product in the known manner. The binder is ground or fused and mixed with the solid aggregate particles and the whole then filled into the metal shell or supporting assembly and formed into the desired shape, and is then ready for firing or baking. Any desired method of mixing the aggregate and binder can be employed. It is frequently advantageous to employ aggregate in particle sizes whereof a major portion is about evenly divided between coarse particles and fine particles, and a minor portion is of intermediate particle sizes or the intermediate particle sizes are substantially eliminated. The coarse particles are preferably of a size to be substantially retained on an 8 mesh screen and the fine particles, to substantially pass through a 48 mesh screen.

The carbonaceous mix which is to be coked can be exposed directly to the action of the exteriorly applied heat. However, it is preferred to place over the carbonaceous mix a protective particulate layer, or a layer of discrete particles, for instance, as shown at 24 in Figures 1, 2, and 4, which will moderate the action of the heat. This layer preferably consists essentially of combustible carbonaceous particles, such as carbon, charcoal, coal, coke or mixtures of these materials with each other. In operation of the coking process, this combustible particle layer partially burns or oxidizes and protects the mixture to be coked, preventing burning or oxidation of the latter. Alternatively, the protective layer can be made up of noncombustible material, for example, ore particles, to retard or minimize oxidation or burning of the lining. For instance, when a cathode for analuminum reduction cell is being coked, the layer can be composed of alumina ore particles, or cryolite particles. In another embodiment, the layer can consist essentially of a mix ture of ore particles and combustible carbonaceousparticles, for example, of from 0% to %alumina ore and from 0% to 100% coke'or coal. This is sometimes advantageous in that when the coal or coke tends to consolidate during the heating step, the intervening alumina or ore particles reduce this tendency and facilitate removal of any remaining protective layer after coking of the carbonaceous lining mass has been completed. The amount of alumina ore in the mixture is suitably increased as the carbonaceous material exhibits an increased tendency to coke, in order to best ensure prevention of the formation of a hard coked covering layer and to facilitate removal of residual protective layer after coking of the lining has been completed as desired. However, from 0% to 100% of either the ore or the carbonaceous component can be employed, as stated, in the covering layer. The components of the protective layer are of particle sizes to provide a good covering bed or layer, and it will be understood that the sizes selected will be large enough so that substantially no dust entrainment occurs and that the material is not blown or carried out of the furnace by combustion gases. If desired, the bed can be replenished during the heating to coke process, for instance, if it is consumed before the heating and coking is completed. e I 1 Heat can be applied exteriorly in any desired manner, some suitable embodiments being shown in the annexed drawings. In Figs. 1, 2 and 3 the furnace shown employs combustion heating, wherein the fuel flame is directed horizontally across the heating zone. Combustion heating can be effected with any suitable fuel, e.g., natural gas, oil, powdered coal or other fuel which can be burned in burners located exteriorly of the concave shell surface. The combustion furnace gases are withdrawn through a stack disposed in the central portion of the furnace top but it will be understood that the gases can be withdrawn in any desired manner and a stack or flue can be disposed at any other part of the furnace as desired. In some instances, it is desirable to introduce the flames tangentially into the heating zone and in this instance it will be preferred to withdraw the furnace gases through a central flue. Also, radiant burners as known in the heating art can be employed, these burners being operated by burning the fuel within a ceramic sheath or cavity which becomes heated thereby and transmits the heat to the heating zone and the material to be coked. Furthermore, as shown in Fig. 4, electric resistors can be employed to heat the carbonaceous lining and cause coking thereof. For instance, the devices known as Glo-Bars, manufactured by Carborundum Company, are useful in this method, or a nichrome wire or ribbon resistor can be employed, electrical current being introduced from any desired source to the resistor employed.

The carbonaceous lining is coked by heating to the coking temperatures as known. It has been found in carrying out the method of the present invention that a solid, coked mass can be obtained at as low as 450 C., and in general, the mass is coked at a temperature of from about 700 C. to 1200 C. Heating is continued until smoking, or evolution of visible fume, has stopped, at which time the mass is rigid and maintains its shape under handling, so that it can be removed and handled, and can be installed in place of use, as desired. It will be understood that, in coking at 450 C., this temperature is that at the coolest part of the lining or electrode mass, and that the temperature within such mass is higher at points closer to the source of heat. It is convenient to measure the temperature at the bottom of the lining in order to determine when the mass will have reached the temperature at which it will have become coked, i.e. preferably 700. C. to 800 C. The temperature at the top of the lining mass is also preferably maintained at from 900 C. to 1200 C., to prevent overheating at that location. The heating of the mass to cause coking thereof is maintained at such rate that the vapors are drivel? fi t a slow, uniform, steady cell cathodes by this invention it is an advantage that no potline power is required for bakeout of replacement pots in an alumina reduction process, enabling more efficient use of potline power in the actual production line. Further advantages are that cheaper heat sources can be employed for coking such linings, cleaner pot-room conditions can be attained by way of removing the bake-out or firing furnace gases through a stack which can suitably be connected to the pot fume exhaust duct, less labor and attention are required in firing by the present invention and the bulky and cumbersome shunt connections which have heretofore been employed in baking or firing in place in the potline are hereby avoided.

In one example of operating the method of the present invention by means of a device as shown in Fig. 1, a cokable mixture is prepared by crushing, milling and grading metallurgical coke to the following desirable particle sizes:

Particle size: Amount: weight percent +.371 in. 3-10.5 .371 to +4 mesh 20.6-28.4 4 to +8 mesh 3.0- 6.1 8 to +14 mesh 1.4 2.0 -14 to +28 mesh 5.8- 9.0 -28 to +48 mesh 7.6-12.9 +48 to +100 mesh 8.1-11.9 -l00 to +200 mesh 14.2-14.9 -200 mesh 19.4-22.0

An anthracite coal is also crushed, milled and graded to particle sizes as above given for the coke component. In this example, a mix is prepared containing about 14% of pitch binder, the remaining 86% of the mix being aggregate composed of 30% of coke as described and about 70% of anthracite coal as described. This mixture is rammed into the lined steel shell as shown in Fig. 1 to a depth of about 2 to 4 inches and then collector rods 19 are set in place, after which more of the carbonaceous mix is rammed in to form the lining 18 as shown in the figure. A layer of ground coke is placed over the rammed lining, suificient to thoroughly cover it. The furnace de vice is placed over the assembly, the burners are turned on and the lining is heated. The temperature of the upper lining surface is observed at regular intervals and heating is controlled so that this temperature does not rise too high. This temperature is maintained preferably between 900 C. and 1200 C. The temperature at the lower surface of the carbonaceous lining is measured as the heating operation proceeds. When it has reached a value between 700 C. and 800 C. the carbon lining is sufliciently coked to carry electrical current uniformly when the cell is placed into operation. For a lining 14" thick (usual for aluminum reduction cells), this heating operation requires from 25 to 670 hours.

Instead of the mixture of coal, coke and pitch described in the example above, other mixes can be employed as previously stated. Also, other particle sizes can be used, and various formulae for making up cathode or carbon lining mixes, as known to the art, can be employed. If desired, the refractory linings between the metal shell and the carbonaceous mixture can be omitted. The mass is heated until coked and the end of the coking process is conveniently determined by measuring the temperature at that point of the mass which is to be coked which is farthest from the source of heat, for example, the bottom of a carbon lining or cathode being heated from above; or by observing the cessation of evolution of smoke or visible fume. The time required for such heating and coking varies, depending at least in part upon the thickness of the layer of mixture to be coked, the furnace temperature and the temperature at the top of the said mixture.

In this specification and the claims, percentages are by weight, unless otherwise indicated. The mesh sizes given herein are those of Tyler screens as defined on pages 17181719 of the Chemical Engineers Handbook, by John H. Perry, published by McGraw-Hill, second edition, 1941. The Hall process referred to herein is the method of producing aluminum metal by electrolysis of alumina dissolved in a molten bath such as molten cryolite or other molten fluoride.

Having now described the invention, what is claimed is:

1. Process for preparing a monolithic carbon furnace lining of uniform electrical conductivity for an electrolytic reduction cell which comprises ramming a cokable admixture consisting essentially of carbon particles and a carbonaceous binder into a supporting casing to form a large, thick, hollow shell lining therein, covering the interior face of said shell lining with a protective layer consisting essentially of a mixture of combustible particles and noncombustible particles, and applying heat exteriorly to said covered interior face of said shell to cause coking of said carbonaceous mixture.

2. Process as in claim 1 for preparing said lining for an aluminum electrolytic reduction cell wherein said noncombustible particles are alumina particles.

References Cited in the file of this patent UNITED STATES PATENTS 1,135,182 Hershman Apr. 13, 1915 1,714,165 Gilbert May 21, 1929 1,804,052 Haas May 5, 1931

Claims (1)

1. PROCESS FOR PREPARING A MONOLITHIC CARBON FURNACE LINING OF UNIFORM ELECTRICAL CONDUCTIVITY FOR AN ELECTROLYTIC REDUCTION CELL WHICH COMPRISES RAMMING A COKABLE ADMIXTURE CONSISTING ESSENTIALLY OF CARBON PARTICLES AND A CARBONACEOUS BINDER INTO A SUPPORTING CASING TO FORM A LARGE, THICK, HOLLW SHELL LINING THEREIN, COVERING THE INTERIOR FACE OF SAID SHELL LINING WITH A PROTECTIVE LAYER

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