Note: Descriptions are shown in the official language in which they were submitted.
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A Method for Creating a Thermally Stable Base Structure and means in
Connection with
Such a Method
The present invention concerns a method for creating a thermally stable base
structure
which may form part of furnaces such as ring section furnaces for calcining of
carbon
blocks for use in aluminium electrolysis. Moreover, the present invention
comprises
means in connection with a base structure in which the structure has proved to
remain
stable over long-term operation with high mechanical and thermal loads.
Carbon blocks such as those stated above might have a considerable weight of
several
tonnes and a length of 1.5 metres or more, depending on whether they are to be
used
as anode or cathode elements in the electrolysis cells.
The carbon blocks are loaded into the furnace in deep shafts called cassettes
or pits
with walls constructed of refractory brick work. The gap between the carbon
blocks and
the cassette walls is filled with packing material, for example coke, to
provide . good
support (stabilising of) for the carbon blocks. The packing coke serves also
to protect the
carbon blocks against air burn.
Several cassettes are built next to each other and form a section. The walls
between the
cassettes are provided with ducts for firing gases and heat is supplied to the
carbon
blocks by conducting firing gases through these ducts.
The firing gases from a section are conducted to an adjacent section in the
direction of
firing via passages arranged in or under head walls located between the
sections. In this
way, the firing gases may be drawn through several sections connected in
series in the
preheating, firing and cooling zones.
Furnaces of this type may comprise horizontal firing gas ducts in the space
below the
base of the cassettes while there is free gas conduction in the space between
the
section cover and the cassettes. The firing gas ducts in the cassette walls
connect the
space below the section cover with the spaces below the base of the cassettes.
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Moreover, a section may be divided into two parts by a barrier wall in the
space below
the cassettes. The firing gases are then conducted up through one half and
down
through the other half in the ducts of the cassette walls in the direction of
firing.
On account of the special properties of the carbon blocks, during calcining it
is
necessary to avoid large temperature gradients which may cause cracks in the
finished
product. Each section must, therefore, follow precisely the time/temperature
curve
defined for the ring section furnace.
The first phase of the heat supply to a section takes place in the preheating
zone, where
the carbon blocks reach up to approximately 600°C by means of the heat
in the firing
gases from the last part of the firing zone. Later, in the temperature
interval from 600°C
to the desired operating temperature of 1200-1300°C, heat must be
supplied by the
stated combustion of gas, oil and binding material.
!n closed ring section furnaces, the fuel can either be supplied in separate
vertical firing
shafts in the head walls or fully or partially in the space above and/or below
the
cassettes, as shown in the applicant's own patent nos. 152029 and 174364. ,
One problem related to optimal control of a ring section furnace is that it
depends on~the
condition of the brick work and firing gas ducts not being too worn so that
large leaks
occu r.
One part of the brick work which is particularly exposed is the base structure
of the
cassettes. When the carbon blocks are inserted, the base will be loaded with
several
tonnes. Moreover, during the calcining process, the temperature may exceed
1500°C in
parts of the structure. In addition to having to have high mechanical
strength, it is
important for the base structure to constitute an effective sealing surface
against firing
gas ducts installed below the base so that uncontrolled burn in the cassette
above does
not occur. Another feature of the base structure is that gas ducts from the
cassette walls
which communicate with the space below the base structure pass through it.
These
ducts should be in sealing contact with the base so that firing gas does not
leak into the
cassettes.
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One purpose of the present invention is for the above properties to be
provided even
with large thermal cycles, and the base is constructed in such a way that it
can withstand
high mechanical and thermal loads.
The present invention will be described in further detail in the following
with examples
and figures, where:
Fig. 1 shows a cross section of a section in a furnace,
Fig. 2 shows a plan view of a section in a furnace, seen from above,
Fig. 3a shows an enlarged cross section of a part of a section as shown in
Fig. 1,
Fig.3b shows further details in connection with the construction of a base
structure equivalent to the one shown in Fig. 3a,
Fig. 4 shows .an enlarged plan view of a part of a section as shown in Fig. 2,
seen from above, in which the section is taken below the base of the
cassettes,
Fig. 5 shows a longitudinal section of the section as shown in Fig. 4,
Fig. 6 shows details in connection with the construction of a base structure.
Figure 1 shows a cross section of a section in a furnace. The section 1
comprises an
outer case 2, which is lined with brick work at the sides 3, 5 and at the base
of the
furnace 4. The figure also shows cassette walls 6, 7, 8, 9, 10, 11, 12, 13,
14, which are
equipped with firing gas ducts. A number of columns 15, 16, 17 rest on the
base 4.
They support part 18 of the base structure. Accordingly, the columns 19, 20,
21 support
part 22 of the base structure. Between parts 18 and 22 there is an opening 23
through
which firing gas may pass and communicate with the firing gas ducts in
cassette wall 7.
Three layers of carbon blocks K are shown inserted in the cassette between
cassette
walls 10 and 11.
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Figure 2 shows plan view of a section in a furnace, seen from above. As the
figure
shows, the cassette walls 6, 7, 8, 9, 10, 11, 12, 13, 14 are fixed to head
walls 30, 31 at
their ends. Moreover, carbon blocks K are shown inserted in the cassette
between
cassette walls 9 and 10.
Figure 3a shows an enlarged cross section of a part of a section as shown in
Figure 1,
where an outer case 2 is lined with brick work 5 at its side and with brick
work 4 at its
base. Moreover, the figure shows three columns 15, 16, 17, which support a
part 18 of
the base structure, and columns 19, 20, 21, which support part 22 of the base
structure.
The opening 23 between the base parts 18, 22 communicates with the firing gas
ducts)
24 which runs) from the bottom to the top in the cassette wall 7. The other
cassette
walls are arranged accordingly. The cassette walls may expediently consist of
wall
structures as shown in the applicant's own Norwegian patent application no. NO
20012044, and rest against recesses 27, 28 arranged in the base parts 18, 22.
The
opening 23 shown may run as a continuous gap along the full length of the
section.
As partially shown in the figure, the base parts 18 and 22 are built up of two
layers B', C'
of refractory bricks of a relatively large area in relation to their
thickness. The bricks'
adjacent surfaces are expediently made with locking elements 25, 26 which
contribute to
the bricks in the two layers locking together. The locking elements may
consist of
interacting elevations/recesses which are adapted to each other and fit
together. Several
alternative embodiments may be used. For example, the locking elements may
consist
of longitudinal and transverse beads/cutouts or they may consist of
rotationally
symmetrical elevations/recesses equivalent to that stated in the applicant's
own patent
application no. PCT/N099/00370.
Figure 3b shows further details in connection with the construction of a base
structure
equivalent to the one shown in Figure 3a, in which four columns 15, 16, 17, 18
are
shown. A construction of a base structure with two Payers C', B' of refractory
bricks
comprises bricks with locking elements 25, 26. The figure also shows expansion
joints
60, 61 which may be arranged between the bricks. In this embodiment example,
the
refractory brick 62 is not equipped with locking elements against the
underlying bricks
64, 65 so that the layers C', B' may have the necessary mobility in relation
to the
expansion joints. Moreover, the figure shows the shape of an end brick 63,
which is
designed as the termination of the two layers against an adjacent brick in a
firing gas
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duct in a cassette wall (not shown). Accordingly, end bricks 66, 67 are shown.
They
interact with a brick 24' which constitutes part of a firing gas duct in a
cassette wall.
Figure 4 shows an enlarged plan view of a part of a section as shown in Figure
2, seen
from above, in which the section is taken below the base of the cassettes. As
in Figure
2, head walls 30, 31 are shown. They limit the length of the section.
Moreover, a number
of columns are shown. For the sake of simplicity, only columns 15, 16, 17, 19,
20, 21
and columns 33, 34, 35, 36, 37 are indicated with reference numbers. The space
below
the base structure of the cassettes is closed in terms of gas flow by means of
a tight
barrier wall 32 at the centre of the section, which causes the firing gases to
be forced up
through the cassette walls in the first half of the section and down again in
the next half.
Figure 5 shows a longitudinal section of a section as shown in Figure 4. The
figure also
shows a base structure 38 which rests on the columns. The figure shows columns
15,
33, 34, 35, 36, 37, which rest on the base of the furnace 4. Moreover, the
figure shows
the partition wall 32 and the base structure 38. At its ends 39, 40, the base
structure is
fixed to adjacent head walls (not shown) with an expansion/contraction joint.
Figure 6 shows details in connection with the construction of a base structure
where only
a section of the base is shown, seen from above. The figure shows the barrier
wall 32
and (only partially) columns 15, 17, 33, 34, 35, 36, 37 and 16. The figure is
intended to
illustrate the construction of the base structure, and rows A, B, C show the
various
stages of this construction. In row A, the installation of a layer of edge
bricks 50, 50' is
shown. The place where a gap 23 is formed between edge bricks 50' and 50"
forms the
base of the cassette walls (see Figure 3). In row B, a row of bricks 51 is
shown laid
between the rows of edge bricks 50" and 50"'. The area of the bricks is
indicated by
black lines which together form a rectangular shape. The figure shows that
each brick
may have a network of locking elements, and in the centre part of the row this
may
comprise longitudinal and transverse beads/grooves 52, 53.
In row C, the second, concluding layer is laid. In the same way as in the
previous row,
the individual bricks are shown by continuous black lines in rectangular
shapes.
Underneath this layer are interacting locking elements which lock with
complementary
elements in the layer shown in row B. Moreover, the bricks are designed so
that none of
the end edges coincides with end edges of bricks in the layer underneath.
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In this way, a stable base structure may be created in which, in principle,
every single
brick is locked permanently to one or more adjacent bricks with the exception
of the
creation of any expansion joints, where adjacent bricks must be able to move
in the
appropriate manner as described under Figure 3b.
The size of the bricks is adapted to the bearing surface constituted by the
top surface of
the columns and the joints between the bricks are laid in such a way that the
strength
properties of the floor are optimised.
