Metallurgical apparatus – Process
Reexamination Certificate
2000-04-12
2002-08-20
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Process
C266S087000, C266S099000, C432S024000
Reexamination Certificate
active
06436335
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for controlling a carbon baking furnace. The invention is particularly concerned with so-called ring furnaces, commonly used in baking carbon bodies to form the anodes for use in electrolytic production of aluminium.
2. Description of the Prior Art
The quality of the anodes seriously affects the cost of production of aluminium. Therefore, the baking process is, in terms of cost, one of the key concerns in aluminium production. Within the anode production process, the baking, which constitutes about 70% of the total cost, represents by far the most expensive stage. Except for capital costs, which are mainly caused by the furnace, all other costs, i.e. about 70%, are immediately affected by the firing and control system of the furnace. Depending on potline operating cost and anode cost, an economical balance has to be found between two opposing objectives.
High anode quality causes higher coke cost, shorter refractory lifetime, and higher energy cost, while low anode quality causes higher electrolysis cost. A specific combination of all parameters can be found, which leads to the lowest overall cost. This cost-minimum defines a specific anode quality. The task of the carbon plant is to produce this defined anode quality. The baking costs for coke, refractory and energy are effected by the average of anode quality. But for the electrolysis the average of the anode quality is not the only important parameter. If the portion of “bad” anodes exceeds a specific value, the whole electrolysis suffers. Therefore, the same portion of bad anodes can be achieved at a lower average of quality, i.e. at lower baking cost, if the consistency of the anode quality is increased.
The consistency is affected by the furnace design, the firing equipment, and the control philosophy. This invention is focused on an advanced control philosophy to increase the anode quality consistency and the flexibility in terms of a most efficient combustion.
The anode bake quality is defined by the heat treatment. Each anode has to be brought to a specific temperature for a specific time. More precisely, the quality of the anode is determined by the time during which the body is kept at a temperature above the sintering temperature. As a parameter to indicate the quality, a time integral of the temperature above this sintering temperature can be defined as the “baking index”.
Ring furnaces have been used for many years in the baking of carbon anodes. These furnaces (
FIG. 1
) include a cooling zone
100
having five adjacent cooling sections
101
,
102
,
103
,
104
and
105
, a firing zone
200
having four adjacent firing sections
201
,
202
,
203
and
204
, and a pre-heat zone
300
having three pre-heat sections
301
,
302
and
303
. Air at ambient temperature enters the furnace flue system through a cooling inlet, such as a manifold
1
, and flows through the flues in a tortuous path as indicated by the curved arrows. Manifold
1
is shown at the entry of cooling section
105
, but a further inlet may be provided at subsequent cooling sections in the fire direction. Firing frames
2
,
3
,
4
and
5
are provided for the infection of gaseous or liquid fuel into the firing sections. The flue gases leave the furnace through an exhaust outlet such as manifold
6
. The general direction of forward movement of the flue gases is referred to as the direction of the fire and is indicated by arrow
10
.
Green anodes initially charged into the furnace pits are progressively treated by changing the heating/cooling conditions by moving the exhaust manifold
6
, the gas (or other fuel) injection firing frames
2
,
3
,
4
,
5
, and the cooling manifold
1
progressively in the “direction of fire”
10
, as illustrated in FIG.
1
. The draft within the flues is initially provided by air which enters through the cooling manifold
1
. The cooling manifold
1
preferably injects air under positive pressure into the last cooling section
105
and the air flows forward either under its own positive pressure or because it is drawn forward by the negative pressure exerted by the exhaust manifold
6
at the other end of the active zone of the furnace. The draft through the flues is controlled so that there is negative pressure within the flues in at least the firing or baking sections
200
and in the preheat sections
300
.
The air which initially enters is cold and has at least its normal oxygen content. The temperature of the cooling air increases as it moves through the cooling sections
100
towards the firing sections
200
, due to heat transfer from the anodes in the adjacent pits, thus progressively cooling the baked anodes. The air reaching the firing sections
200
is thus elevated in temperature to such a degree that it will support the combustion of gaseous fuel injected into the firing sections
200
through the firing frames
2
,
3
,
4
,
5
connected to each such section
204
,
203
,
202
and
201
. If a liquid fuel is used, the temperature of the incoming air is such as to support both the combustion and vaporization of the fuel. The temperature of the flue walls in the latter firing sections
204
(that is the rearmost firing section in the direction of the fire) may be raised to approximately 1225° C. by combustion of the incoming fuel in the relatively high oxygen content incoming air in the second firing section
202
, the oxygen content of the forwardly moving flue gases has been reduced and the temperature within the flue will also be lower than that in the third and fourth firing sections
203
and
204
. The temperature and oxygen content of the flue gas falls further in the first firing section
201
, so that the temperature of the flue gases leaving the first firing section
201
may have fallen to about 1000° C.
In the preheat sections
300
the temperature of the unfired anodes is progressively raised by the hot flue gases which have a relatively low oxygen content after much of the oxygen has been used in the combustion process of the firing sections
200
. However, as the temperature of the unfired anodes is progressively raised, volatile materials in the pitch, which are used to bind the carbon material forming the anodes together, are released and burn in the residual oxygen of the flue gases. The temperatures of the flue walls in the first preheat section
301
, where the unfired anodes are first subjected to the heated flue gases, may be in the range 200 to 500° C. In this first preheat section
301
, all the heating of the unfired anodes takes place by extraction of the residual heat from the flue gases.
In the second preheat section
302
, the temperature may rise to between 500 and 800° C., and the anodes are heated both by the incoming flue gases and the combustion of the pitch volatiles which are driven off as the anode temperatures are raised. In the third preheat section
303
of the illustrated embodiment, the flue wall temperature may reach 800 to 1000° C. due to the combined action of the incoming flue gases and the combustion of further pitch volatiles.
The flue gases are removed via the exhaust manifold
6
after passing through the flues of the first preheat section
301
. The furnace section
401
preceding the first preheat section is packed with unfired anodes after the fired anodes from the previous pass of the fire have been unloaded from that section. The section
401
packed with unfired anodes then becomes the first preheat section when the manifolds and firing frames are next moved forward.
The condition of the flue gases in any active zone of the furnace can be controlled by adjustment of the amount of air supplied through the cooling manifold and extracted through the exhaust manifold, as well as by the amount of fuel gas injected into each firing section.
Each pit section is subjected in use to different heating conditions in order to bake green or unbaked carbon bodies into a desired anode material. The condition in each section is altered progressively between controlled li
Innovatherm Prof. Dr. Leisenberg GmbH & Co. KG
Kastler Scott
Pandiscio & Pandiscio
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