Wall structure for carbon baking furnace

A carbon baking furnace having spaced-apart, hollow flue walls defining a soaking pit therebetween. Each of the flue walls is formed of refractory bricks and has a pit face facing the pit and a flue face facing an inner flue gas passage. A coating is provided on the pit face of the flue walls. The coating increases the emissivity value of the pit face, wherein the emissivity value of the pit face is greater than the emissivity value of the flue face.

FIELD OF THE INVENTION

The present invention relates generally to the formation of carbon anodes for smelting of aluminum, and more particularly to a flue wall structure for a carbon baking furnace.

BACKGROUND OF THE INVENTION

One step in the production of aluminum is the smelting of alumina into aluminum metal. The smelting takes place in large, steel, carbon-lined furnaces known as reduction cells. The carbon lining is called a cathode. Alumina is fed into the cells where it is dissolved into molten cryolite (a liquid that can dissolve alumina and conduct electricity at about 970° C.). Carbon block anodes are electrically conductive and are used to introduce electricity into each cell.

The carbon anodes are made in a three-step process. First, petroleum coke and recycled carbon from used anodes are mixed with liquid pitch. This mixture is heated to form a hot paste. The paste is then cooled, and hydraulically pressed or vibrated into a mold to form an anode block. In the second step of the process, the carbon anodes are then “baked” in a carbon baking furnace. This “baking” process helps rid the anodes of impurities and improves their strength and electrical conductivity. Lastly, the carbon anode is then bonded to a metal rod using molten cast iron. This rod allows the anode to be suspended from the reduction cell's super structure during the smelting process.

Perhaps the most important step in forming the carbon anode is the baking process. Precise, uniform heating is necessary to produce a uniform chemical conversion of the raw material to the finished anode block with the desired electrical and physical properties that are required for aluminum smelting. In this respect, the center temperature of the anode is critical and it is important that such temperature be maintained during the heating portion of the “baking” process.

The present invention provides an improved flue wall structure for use in baking carbon anodes in a carbon baking furnace.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, there is provided a carbon baking furnace having spaced-apart, hollow flue walls defining a soaking pit therebetween. Each of the flue walls is formed of refractory bricks and has a pit face facing the pit and a flue face facing an inner flue gas passage. A coating is provided on the pit face of the flue walls. The coating increases the emissivity value of the pit face, wherein the emissivity value of the pit face is greater than the emissivity value of the flue face.

In accordance with another aspect of the present invention, there is provided a flue wall in a carbon baking furnace having a soaking pit for soaking carbon blocks. The flue wall is formed of refractory brick and has a flue face and a pit face. The flue face is in communication with hot combustion gases for heating the flue wall and the pit face is in communication with the soaking pit for conveying heat from the combustion gases to the soaking pit. The pit face of the flue wall is coated with a material that increases the emissive properties of said pit face at elevated temperatures.

In accordance with another aspect of the present invention, there is provided a flue wall having an inner surface defining an inner chamber to be heated by a burner and an outer surface for heating an area adjacent the outer surface. An outer surface coating is provided on the outer surface of the flue wall. The outer surface coating increases the emissivity of the outer surface. An inner surface coating is provided on portions of the inner surface. The inner surface coating increases the emissivity of the portions of the inner surface.

In accordance with yet another aspect of the present invention, there is provided a heat exchanger, comprised of an inner surface to be heated by radiative heat and convective heat, and an outer surface for heating an area adjacent the outer surface. An outer surface coating is provided on the outer surface. The outer surface coating increases the emissivity of the outer surface. An inner surface coating is provided on portions of the inner surface that are primarily heated by radiative heat. The inner surface coating increases the emissivity of the coated portions of the inner surface.

An advantage of the present invention is a flue wall in a carbon baking furnace that provides more efficient heating of anode blocks within the carbon baking furnace.

Another advantage of the present invention is a flue wall as described above that reduces the likelihood of significant heat variations along the surface of the flue wall.

Another advantage of the present invention is a heat exchanger that provides more efficient heat transfer from a burner on one side of the heat exchanger to the other side of the heat exchanger.

These and other advantages will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings and the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting same,FIG. 1is a perspective view of a portion of a typical carbon baking furnace10. Carbon baking furnace10includes a plurality of narrow, rectangular pits12, where a plurality of carbon blocks, shown in phantom lines in the drawings and designated14, are stacked one upon another. Each pit12is basically defined by two end walls22,24and a bottom wall26and two spaced-apart flue walls32. End walls22,24, bottom wall26and flue walls32are formed of refractory bricks and refractory shapes, as pictorially illustrated in the drawings.

Each flue wall32is essentially a hollow structure defining an inner space or cavity34(best seen inFIG. 2). Baffles36are disposed within cavity34in flue wall32at locations to define a serpentine path or passageway through flue wall32, as illustrated inFIG. 2. A flame42and hot combustion gases44are directed into flue wall32from a burner46, as is conventionally known. In the embodiment shown, a burner46directs a flame42and hot combustion gases44flow into each flue wall32through the upper end of flue wall32, as best seen inFIG. 2. Hot combustion gas44follows the serpentine path through flue wall32to an exit port48formed in end wall24.

In one embodiment of the present invention, a coating62is applied to pit face54of each flue wall section32A,32B. Coating62is a high emissivity coating that increases the emissivity of pit face54at elevated temperatures, e.g., in the range of 800° C. to 1200° C. In this embodiment, flue face52is not coated with a high emissivity coating62. As a result, the emissivity value of a pit face54of a flue wall section32A,32B is higher than the emissivity value of flue face52, at the baking temperature of carbon baking furnace10.

Coating62may be comprised of any commercially available high emissivity coatings that will increase the emissivity of pit face54at the operating temperatures of baking furnace10. By way of example, and not limitation, coating62may be comprised of one of several types of high emissivity coatings sold by Wessex Incorporated of Blacksburg, Va., under the registered trademark EMISSHIELD®.

Coating62may be applied to the surface of individual refractory bricks that form pit face54of flue wall32. Preferably, coating62is applied per manufacturer's instructions, on pit face54or flue wall32between baking operations.

Referring now to the operation and use of the present invention, carbon anodes14are stacked within pit12of furnace10. Anodes14are stacked one upon another to generally form a wall of anode blocks in the center of pit12, as generally illustrated inFIG. 3. A space exists on both sides of the anode block wall between the surface of the anode blocks and the facing pit surfaces of the opposing flue walls32. This space or gap is filled with loose carbon material, designated72in the drawings.

In a conventionally known manner, hot combustion gases44are forced into cavity or space34within flue wall32. As illustrated inFIG. 2, combustion gases44flow in a serpentine path around baffles36within cavity34and exit the flue wall through exit port48. Combustion gases44within flue wall32heat the refractory bricks forming flue wall sections32A,32B. As pictorially illustrated inFIGS. 3–5, the heat is conducted through the refractory brick of flue wall sections32A,32B into pit12. More specifically, the heat radiates from pit face54into pit12. Carbon powder72within pit12helps conduct the heat of flue wall32to carbon anodes14. Coating62on pit face54facilitates the emission of heat from pit face54into pit12. In this respect, all surfaces emit thermal radiation. However, at a given temperature and wavelength, there is a maximum amount of radiation that any surface can emit. Surfaces with high emissivity values can emit thermal radiation more rapidly than surfaces with low emissivity values. By coating pit face54with a coating62having a high emissivity value at the operating temperature of furnace10, the ability of pit face54of flue wall32to radiate heat into pit12is increased. Moreover, the ability to radiate heat more rapidly from the surface of flue wall32provides a more uniform heating surface along pit face54. For example, the temperature of combustion gases44may vary along the serpentine path through flue walls32. Moreover, corners of cavity34may have temperatures lower than other areas within cavity34. By increasing the emissivity of pit face54, variations in temperature across pit face54can be reduced. The present invention thus provides a flue wall structure having more efficient heat transfer to pit12in a carbon baking furnace10.

It is also believed that emissive coatings, such as the aforementioned EMISSHIELD® coating, may improve the alkali resistance of flue wall32of carbon baking furnace10, thereby prolonging the useful life of furnace10by preventing penetration of alkali, as well as other impurities given off by the anodes during the baking process, into the refractory brick forming flue wall32. In addition, it is further believed that coating62on pit face54will reduce the adherence of carbon powder72onto pit face54during each soaking cycle.

In another embodiment of the present invention, in addition to applying coating62to pit face54, coating62is applied to select area(s)82of flue face52. Specifically, coating62is applied to area(s)82of flue face52where radiative heating is the primary mechanism (mode) for heating flue face52. Coating62is not applied to areas of flue face52where convective heating is the primary mechanism for heating flue face52.

More specifically, burner46produces flame42within cavity34of flue wall32. Flame42will extend from burner46into a cavity of a certain length. Area(s)82of flue face52around or near flame42, the primary mechanism of heat transfer is radiative heating. Radiative heating is the result of electromagnetic radiation, i.e., light waves (photons) hitting flue face52of flue wall32. It is believed that having high emissivity coating62on area(s)82of flue face52where radiative heating is the principal mechanism of heating will cause flue wall32to absorb heat more rapidly and to heat up faster, since absorbability and emissivity are the same thing.

Further along the serpentine passage through flue wall32, radiation heating is not the primary mode for heating flue wall32. In this area, convection heating heats flue wall32. Convection heating is the result of the transfer of energy by molecular interaction between the molecules of the heated gases44within flue wall32interacting with molecules along flue face52. In these areas, high-emissivity coating62would not be applied to flue face52because coating62would cause flue wall32to heat more slowly because flue surface52would radiate away, i.e., into cavity34, from heat absorbed by flue wall32through convection.

By providing coating62only on those area(s)82of flue face52where radiative heating is the primary mechanism for heating, flue wall32is heated more rapidly. The thermal energy absorbed by flue wall32is then radiated into pit12by pit surface54.

In summary, by providing coating62along pit surface54and along those area(s)82of flue surface52where radiative heating is the primary mechanism for heating flue wall32, a more efficient structure for radiating thermal energy from cavity34within flue wall32to pit12is provided.

The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purposes of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. For example, the present invention finds advantageous application in any flue wall having an inner surface defining an inner chamber to be heated by a burner and an outer surface for heating an area adjacent the outer surface. In this respect, the outer surface would be coated with a material increasing the emissivity of the outer surface at elevated temperatures. Portions of the inner surface of the flue wall would be coated with a material increasing the emissivity of those portions of the inner surface at elevated temperatures. The portion(s) of the inner surface of the flue wall to be coated with the material are those areas that are primarily heated by radiation heating of the burner or source of combustion. Similarly, the present invention includes a heat exchanger having an inner surface to be heated by radiative heat and convective heat, and an outer surface for heating an area adjacent the outer surface. An outer surface coating would be applied to the outer surface of the heat exchanger to increase the emissivity of the outer surface at elevated temperatures. An inner surface coating would be applied on those portions of the inner surface of the heat exchanger that are heated primarily by radiative heat. The inner surface coating would increase the emissivity of those portions at elevated temperatures. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.