Furnaces – Process
Reexamination Certificate
2003-10-24
2004-12-21
Rinehart, Kenneth (Department: 3749)
Furnaces
Process
C110S208000, C110S225000, C110S228000, C075S483000
Reexamination Certificate
active
06832564
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of operating a multiple hearth furnace.
BACKGROUND OF THE INVENTION
A multiple hearth furnace comprises an upright cylindrical furnace housing that is divided by a plurality of vertically spaced hearth floors in vertically aligned hearth chambers. A vertical shaft extends axially though the cylindrical furnace housing, passing centrally through each hearth floor. In each hearth chamber at least one rabble arms is secured to the vertical shaft and extends radially outside therefrom over the hearth floor. These rabble arms are provided with rabble teeth, which extend down into the material being processed on the respective hearth floor. As the vertical shaft rotates, the rabble arms move over the material on their respective hearth floor, wherein their rabble teeth plough through the material. The orientation of the rabble teeth of a rabble arm is such that they confer to the material a circumferential and a radial motion component, wherein the radial motion component is either centripetal (i.e. the material will be moved radially inwardly towards the vertical shaft) or centrifugal (i.e. the material will be moved radially outwardly towards the outer shell of the furnace). Drop holes are provided in each hearth floor, alternately in the inner zone of the hearth floor (i.e. centrally around the vertical shaft) or in the outer zone of the hearth floor (i.e. peripherally around the outer shell of the furnace). On hearth floors with a central drop hole, the rabble arms urge the material from the outer periphery of the hearth floor radially inwardly. On hearth floors with a peripheral drop hole, the rabble arms urge the material from the inner periphery of the hearth floor radially outwardly.
Operation of such a multiple hearth furnace takes place in the following manner. Solid material to be processed is supplied continuously via a material feed inlet into the uppermost hearth chamber, where it falls for example upon the outer periphery of the uppermost furnace floor. As the vertical shaft rotates, the rabble arms in the uppermost hearth chamber gradually urge the material in a kind of spiral movement over the hearth floor towards a central drop hole surrounding the vertical shaft. Through this central drop hole the material drops down onto the second hearth floor in the second hearth chamber, where the rabble arms of this chamber gradually work the material toward the outer periphery of the second hearth floor. Here the material drops through the peripheral drop holes of this second hearth floor onto the third hearth floor in the third hearth chamber. The material is then worked in the same way through successive hearth chambers, before it ultimately leaves the furnace via a material outlet in the hearth floor of the lowermost hearth chamber. Process gases move in an ascending counter-flow through the multiple hearth furnace. As the material travels downwards from hearth floor to hearth floor, it is thoroughly stirred and exposed to the hot process gases.
To optimise the process in the multiple hearth furnace, it is often of interest to feed additional material, e.g. a reducing agent as coal, on a lower hearth floor. This additional material is usually discharged by a conveyor through the outer shell of the furnace on a peripheral area of a hearth floor with a central drop hole (i.e. the rabble arms are consequently designed to urge the solid material radially inwardly, and the hearth floor immediately above has consequently peripheral drop holes). The rotating rabble arms urge the material falling through the peripheral drop holes of the next higher hearth floor and the additional material discharged by the conveyor through the outer shell of the furnace together to the central drop hole. Due to the ploughing action of the rabble teeth, both materials are thoroughly mixed before they fall through the central drop hole on the next lower hearth floor.
In many cases it would be of interest—at least from the point of view of process optimisation—to thermally precondition the additional material before adding it to the material already processed on upper hearth floors. Such a thermal preconditioning can for example comprise a preheating of the additional material to avoid an inhomogeneous temperature profile in the material bed, a preheating to dry the additional material or to evaporate other volatile components. However, in practice such a thermal preconditioning is generally not carried out, because it is considered to be too expensive in comparison to its benefits.
OBJECT AND SUMMARY OF THE INVENTION
A problem addressed by the present invention is to provide a simple and inexpensive method for thermally preconditioning a additional solid material prior to adding it to a material already processed on upper hearth floors of a multiple hearth furnace.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method of operating multiple hearth furnace with a plurality of vertically aligned hearth floors, comprises in particular following steps. A first material is fed onto the uppermost hearth floor and moved over this uppermost hearth floor before it falls through a drop hole onto the next lower hearth floor. This first material is processed in this way from hearth floor to hearth floor down to the lowermost hearth floor. A second material is fed onto one of the hearth floors to be mixed into the first material. In accordance with an important aspect of the present invention, the second material is moved separately from the first material in a separate annular zone of the hearth floor onto which it is fed before it is mixed into the first material. It will be appreciated that this method allows to provide an efficient thermal preconditioning of the second material prior to mixing it into the first material without requiring any supplementary equipment therefore.
In a generally preferred implementation of the method, the second material is fed onto an outer annular zone of a hearth floor, and the first material is dropped from a higher hearth floor onto an inner annular zone of this hearth floor. The first material is then moved in the inner annular zone of the hearth floor, and the second material is moved in the outer annular zone surrounding the first material in the inner annular zone. It will be appreciated that this way of proceeding allows to easily feed the second material through a lateral outer wall of the furnace onto the respective hearth floor.
For process reasons it may be of interest to keep the first and second material separate until they are dropped onto the next hearth floor. If this is the case, the second material is e.g. advantageously fed onto the outer periphery of the outer annular zone and moved inwardly towards the inner annular zone; whereas the first material is dropped onto the inner periphery of the inner annular zone and moved outwardly towards the outer annular zone. The first material and the second material can then be dropped through at least one common drop hole located in a fringe range between the inner and outer annular zones.
The first material and the second material may be dropped through the at least one common drop hole either onto an inner zone or onto an outer zone of a lower hearth floor. Here they are mixed by moving them from the inner zone to the outer zone, respectively from the outer zone to the inner zone, e.g. by means of rotating rabble arms as commonly used in multiple hearth furnaces.
The same rabble arms may be used for moving the first material in the inner zone and the second material in the outer zone. In the inner zone, the rabble teeth are then arranged so as to move the first material outwardly. In the outer zone, the rabble teeth are then arranged so as to move the second material inwardly.
In an alternative implementation of the method in accordance with the present invention, the second material is fed onto an inner annular zone of a hearth floor and moved herein, and the first material is dropped onto an outer annular zone of the hearth floo
Arendt Steve
Ferretti Pascal
Hutmacher Patrick
Lonardi Emile
McCormick Paulding & Huber LLP
Paul Wurth S.A.
Rinehart Kenneth
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