Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WEAR-RESISTANT, SINGLE PENETRATION STAVE COOLERS
FIELD OF INVENTION
The present invention relates to stave coolers for circular
furnaces with steel containment shells, and more particularly to
cast-iron and cast-copper stave coolers with a single
penetration required of a steel containment shell to accommodate
a steel collar that entirely support the weight of the stave
cooler inside a smelting furnaces, and that passes all the
piping inlets and outlets through in one group for liquid
cooling. The object of constructing the steel collars this way
being to provide a match of the coefficients of expansion in the
one penetration by using similar alloys to minimize stresses and
avoid bonding and embrittlement issues with the connecting welds
to the containment shells
BACKGROUND
Steel and non-ferrous metals are being smelted throughout
the world in circular furnaces with steel containment shells.
Some of these employ panel type stave coolers that completely
line the interior walls to cool refractory bricks mounted to
their hot faces. Their individual cooling actions are delivered
by liquid coolants that circulate inside each stave cooler with
piping that passes through penetrations of the steel containment
shells to access an external heat exchanger. Each penetration of
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the steel containment shell requires reliable welds and seals to
keep the hazardous process gases both inside the furnace and
away from its operating personnel.
Production rates exceeding three tons of hot metal per
cubic meter of working volume per day are now being reached with
modern blast furnaces. This was made possible by using improved
burden materials, better burden distribution techniques, tighter
process controls, very high hot-blast temperatures, oxygen
enrichment technology, pulverized-coal injection, and natural
gas fuel enrichment. All of which result in much higher average
heat loads and fluctuations that land on the stave coolers
mounted inside the steel containment shells of up-to-date blast
furnaces.
Integrated steelworks use blast furnaces to supply
themselves the pig iron they use to make steel. The large gains
being made in furnace-productivity have also placed overwhelming
demands on cooling system capacities. The liquid-cooled stave
coolers in blast furnaces first developed in the late 1960's
became inadequate. High conductivity copper stave coolers have
been needed since the late 1970's because these are better able
to deal with the intense process heats now being generated in
state-of-the-art, high stress furnaces. Copper stave coolers
have also proved themselves capable of delivering furnace
campaign lives that now exceed fifteen years.
The average thermal load levels a stave cooler will be
subjected to depends on where it will be positioned within a
blast furnace and how the furnace is operated. See Fig. 1. Cast-
iron staves can still be successfully used in the less demanding
middle and upper stack areas of blast furnaces, but the much
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higher average heat loads below in the lower stack, Belly, Bosh,
Tuyere Level, and Hearth all require the use of higher
performing, but more costly copper staves.
Cast iron staves are less efficient at cooling than are
copper staves because the cast iron metal is relatively much
lower in thermal conductivity. Their inherent thermal resistance
allows heat to pile up too high if too much loading is
presented. Poor internal bonding can add unnecessarily to the
overall thermal resistance. Otherwise, cracks develop in the
cast iron and the cracking can propagate into the steel pipes
themselves. Cast iron staves have a de-bonding layer that adds
to a thermal barrier between coolants circulating in its
internal water-cooling tubes and the hot faces of the cast iron
stave body. Both such effects conspire in reducing the overall
heat transfer abilities of cast iron staves.
Such inefficiencies in cast iron stave heat transfer
performance can overstress cast iron staves when hot face
temperatures drive up over 700 C. Thermal deformations are hard
to avoid. Cast iron stave bodies can also suffer phase-volume
transformations when operated at very elevated temperatures.
Fatigue cracking, stave body material spalling, and cooling
pipes exposed directly to the furnace heat are common failures.
Stave coolers can also be used in reduction vessels for the
production of direct reduced iron (DRI).
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A stave cooler is described by Todd Smith in United States
Published Patent Application US-2015-0377554-A1, published
12/31/2015. The Abstract reads,
A stave comprising an outer housing, an inner
pipe circuit comprising individual pipes housed
within the outer housing, wherein the individual
pipes each has an inlet end and an outlet end and
wherein each pipe may or may not be mechanically
connected to another pipe, and a manifold,
integral with or disposed on or in the housing;
wherein the inlet and/or outlet ends of each
individual pipe is disposed in or housed by the
manifold. The manifold may be made of carbon
steel while the housing may be made of copper.
Todd Smith further adds, "Each of the inlet and outlet ends of
each individual pipe may be surrounded in part by cast copper
within a housing of the manifold."
When liquid-cooled stave coolers are disposed inside the
steel containment shells of smelting furnaces, each conventional
coolant connection must have a corresponding penetration or
access window in the shell in order to complete the hose
connections outside. And, conventionally, each stave cooler must
be bolted to or otherwise mechanically attached to the steel
containment shell to provide vertical support to itself and the
refractory brick lining it supports and cools on its hot face.
The hot smelting inside the furnaces produces very hot,
toxic, and often flammable process gases that will find escape
paths between the refractory bricks, and between the stave
coolers and out through any openings in the containment shell.
So these penetration points must have good gas seals. One
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penetration is easier to seal and keep sealed than several.
While two or more fixed points will lead to thermally induced
mechanical stresses.
But because the stave coolers, containment shells, and
refractory brick are all subject to thermal expansion forces,
the gas seals can be compromised over the campaign years by
constantly being worked back and forth. Stave coolers like those
described by Todd Smith, have many independent circuits of
coolant piping inside, and each produces pairs of coolant
connection ends that must be passed out back and through the
containment shell.
Todd Smith describes a "manifold" that can be made of
carbon steel on the back of a housing that may be made of
copper. He points out that his stave 100 provides for ease of
installation since it reduces the number of access holes or
apertures required in the furnace shell 51 necessary for the
inlet/outlet piping 108 to and from 100 through furnace shell
51. And he says, at paragraph [0094], that stave 100 is of very
strong construction to provide much of the support necessary for
installation of the stave 100 on furnace shell 51. The effects
of stave expansion/contraction due to temperature changes in the
furnace are minimized since individual pipe connections to
furnace shell have been eliminated. And, stave 100 reduces weld
breaches in pipe connections with furnace shell 51 since such
connections have been eliminated. Todd Smith says further that
his stave 100 reduces the importance/criticality of any support
bolts needed to help support stave 100 on furnace shell 51 since
such bolts are no longer relied upon to independently support
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stave 100 since manifold 106 carries much of the load required
to support stave 100 on furnace shell 51.
A stave cooler that has one-only through-bulkhead neck that
is always collared in an appropriate steel is needed in the
industry to control process gas sealing and containment. All of
the coolant piping from all the coolant circuits within a single
rectangular copper body must pass through in a single tight
group to then connect externally outside the steel containment
shell. This minimizes the adverse effects of thermal expansion
and contraction to manageable levels. Tightly grouping the
individual pipe connections through the furnace shell limits the
deteriorating forces at work.
Towards these ends, stave coolers must depend entirely for
their vertical mechanical support by a single hanging of the
through-bulkhead in a single corresponding penetration of the
containment shell. Carrying only "much of the load" leaves the
door open to more than one penetration of the steel containment
shell per stave cooler. The two jobs of supporting the stave
cooler's weight, and connecting all the coolant piping, must
always be shared in a single through-bulkhead neck.
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SUMMARY
Briefly, cast-iron and cast-copper stave cooler embodiments
of the present invention have all of the stave cooler's weight
supported inside a furnace containment shell by a single gas-
tight steel collar on the backside. All the coolant piping in
each cooler has every external connection collected and routed
together through the one steel collar. A wear protection barrier
is disposed on the hot face. Such is limited to include at least
one of horizontal rows of ribs and channels that retain metal
inserts or refractory bricks, or pockets that assist in the
retention of castable cement and/or accretions frozen in place
from a melt, or an application of an area of hardfacing that is
welded on in bead, crosshatch, or weave patterns.
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SUMMARY OF THE DRAWINGS
Fig. 1 is a cross sectional view diagram of a vertically
orientated metal smelting or converting furnace embodiment of
the present invention with a steel containment shell that has
only one penetration per stave cooler for liquid coolant
circulation;
Fig. 2 is a cross sectional view diagram of a middle
section of a furnace like that of Fig. 1, and represents the way
stave cooler embodiments of the present invention seal out the
escape of process gases with steel-to-steel welds around steel
collars on the protruding necks, and have castable refractory
cement packed in behind them. Bricks are inserted into tapered
grooves. When pockets are provided instead, the pockets are
filled with refractory castable or ram. The steel containment
shell is penetrated only once per stave cooler, and all piping
for liquid coolant circulation is gathered together in a single
group to pass through the protruding necks inside their
respective steel collars;
Figs. 3A-3C are cold face, side, and bottom edge view
diagrams of a stave cooler embodiment of the present invention;
Fig. 4 is a cross sectional diagram of a copper casting
mold useful in making the stave coolers of Figs. 1, 2, 3A, 3B,
and 3C;
Fig. 5 is a perspective view diagram of a stave cooler
embodiment of the present invention like that of Figs. 1, 2, and
3A-3C;
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Fig. 6 is a perspective view and cutaway diagram of a stave
cooler embodiment of the present invention like that of Figs. 1,
2, and 3A-3C mounted and welded inside a steel containment
shell;
Fig. 7 is a functional block diagram in a schematic type
view of a cooling system embodiment of the present invention
that is intrinsically safe from boiling liquid expanding vapor
explosion (BLEVE) should any of its liquid, water-based coolant
escape or leak into a pyrometallurgical furnace;
Fig. 8 is a cross sectional view diagram of a stave cooler
embodiment of the present invention hanging inside a steel
containment shell. This view details the location of a
"specialty weld" that joins carbon steel and stainless steel (or
nickel alloy) parts of a steel collar embodiment of the present
.. invention;
Fig. 9A is a plan view diagram of a hot face of a stave
cooler fitted with pockets and hardfacing welding overlays; and
Fig. 9B is a cross-sectional view of one pocket of Fig. 9A
taken along line 9B-9B.
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DETAILED DESCRIPTION OF THE EMBODIMENTS
Iron smelting furnaces operate in highly reducing
environments and produce dangerous levels of toxic and highly
flammable carbon monoxide (CO) gas. Carbon monoxide is a
colorless, odorless, and tasteless gas that is slightly less
dense than air. It is toxic to hemoglobic animals when
encountered in concentrations above about 35-ppm. Carbon
monoxide is produced from the partial oxidation of carbon-
containing compounds. It forms when there is not enough oxygen
to produce carbon dioxide (CO2), such as when smelting iron. In
the presence of atmospheric concentrations of oxygen, carbon
monoxide burns with an invisible blue flame, producing carbon
dioxide.
It is therefore very important to control and stop errant
carbon monoxide process gases that pass through gaps between
stave coolers, cracks in the castable refractory cement, and
seals welded into the steel containment shells at the coolant
connections and stave support fasteners.
Copper is highly preferred over cast iron for stave coolers
because the thermal conductivity of copper is so much better
than cast iron. But copper is relatively soft and easily
abraded, compared to cast iron. The churning and roiling of the
"coke" inside a furnace is highly abrasive to the walls,
especially in the upper reaches. Copper stave coolers must
therefore have some sort of abrasion resistant facing
incorporated into their hot faces if they are to survive in a
campaign that extends ten years or more.
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Fig. 1 represents a typical blast furnace 100 in which
various stave cooler embodiments of the present invention have
been installed inside a steel containment shell. Fig. 6 shows
the novel way these mount and assemble in detail.
In reduction smelting, the ore is reduced by carbon in the
presence of flux to yield molten metal and slag. Coal is used
instead of coke in reduction vessels that produce DRI. The
typical blast furnace 100 includes a steel containment shell 102
with several essential zones of operation inside: a stack 104, a
belly 106, a Bosch 108, a Tuyere level 110, and a hearth 112.
The average operating temperatures are much more severe in the
lower elevated stack 104 and below, and therefore heat loading
is more demanding on its stave coolers. Compared to those in the
middle stack 104 and above.
A liquid-cooled, cast iron type stave cooler embodiment of
the present invention is therefore used in the middle stack 104
and above. Such cast iron stave coolers are referred to herein
by the general reference numeral 114. Cast iron material offers
superior abrasion resistance, but is not as thermally conductive
as copper. Its inherent thermal resistance is problematic and
iron stave are prone to cracking.
A cast copper type stave cooler embodiment of the present
invention is therefore used in the lower stack 104 and below.
Such cast copper stave coolers are referred to herein by the
general reference numeral 116. High quality copper material
offers superior thermal conductivity, but is easily abraded by
the agitation and churning of the materials inside the furnace,
and therefore must include an abrasion resistant facing
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incorporated into the entire outside surface area of the hot
faces of each cast copper stave cooler.
Fig. 2 represents a section of an iron-smelting furnace 200
in an embodiment of the present invention that uses either cast
iron stave coolers 114 (Fig. 1), or cast copper stave coolers
116. In this example, the insides of an external steel
containment shell 202 are lined with copper stave coolers 204.
These each have a single protrusion 206, and each such
protrusion 206 is jacketed in a steel-to-steel welding collar
208.
A completed annular steel-to-steel weld 210 secures the
mounting of each copper stave coolers 204 and prevents the
uncontrolled escape of process gases 212. A castable refractory
cement 214 is packed in behind each copper stave cooler 204, in
front of the inside walls of the steel containment shell 202, to
further prevent any uncontrolled escape of process gases 212.
Cast copper stave coolers require an abrasion resistant
facing or layer incorporated into their hot faces if their
campaign lives are to exceed ten years. Cast iron stave coolers
do not because the cast iron itself is very wear resistant.
The hot faces of the copper stave coolers 204 can therefore
be finished in a number of different ways to accommodate
materials to limit erosion caused by roiling abrasion inside a
typical smelting furnace coke 218.
A conventional technique has been to horizontally groove
the hot faces to retain rows of refractory brick, castable
refractory cement, or even cast iron metal inserts. In
alternative embodiments, the hot faces include a weld overlay or
spray coating of abrasion resistant metal or ceramic. For
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example, nickel and chromium for the weld overlay and metal
spray coatings. Silicon dioxide is useful for the ceramic spray
coatings.
A further option that will increase abrasion resistance
involves machining vertical or horizontal grooves into the hot
faces for the later insertion of matching metal inserts during
installation.
Fig. 2 simplified a range of possible abrasion resistant
facing types by its showing rows of refractory bricks 216
inserted into horizontal grooves on the hot faces. Such bricks
would ordinarily continue over to cover the copper lips of the
grooves. Alternatively, the entirety of the hot faces of the
stave coolers can be deeply dimpled or pocketed to better retain
castable refractory cement, instead of grooving or slotting.
Smelting furnace coke 218 will helpfully form a layer of
accretion 220 as it chills on the hot faces of the copper stave
coolers 204. Such accretion includes condensed gases, slag, and
metal. An internal arrangement of liquid coolant piping 222
inside the copper stave coolers 204 are all routed in a single
group for external connection with hoses 224 outside the steel
containment shell 202. They must all pass through the one,
single protrusion 206 of their respective stave cooler 204.
Conventional drilled billet block type of stave cooler
fabrication is not a practical alternative embodiment of the
present invention because too much drilling and plugging is
required to get all the internal coolant passageways to begin
and end in a single group within the single protrusion 206
(inside steel-to-steel welding collar 208).
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Iron-smelting furnaces that use liquid-cooled copper stave
coolers inside their steel containment shells can leak carbon
monoxide (CO) gas through any of the many penetrations in the
containment shell provided for the liquid coolant connections.
These penetrations all need to be sealed, and the seals must
stay tight over the campaign life of the furnace. Carbon
monoxide gas is very toxic, odorless, colorless, and can burn
very hot in ordinary air with an invisible flame. These are why
it's so hazardous. Embodiments that require welding a steel
collar to a drilled billet are not preferred due to an inherent
high probability of weld failure.
In one liquid-cooled stave cooler embodiment of the present
invention for smelting furnaces with steel containment shells, a
solid copper stave body is cast in a flattened and rectangular
shape. They may also be curved slightly to fit better in
upright, cylindrical and round furnaces. These stave coolers are
typically about 2.5 meters tall, 1.0 meter wide, and 120 mm
thick. So in general, embodiments like liquid-cooled stave
coolers 114, 116, and 204 are substantially taller than they are
wide, and are substantially wider than they are thick.
Figs. 3A-3C represent a cast copper cooler stave 300 in a
typical embodiment of the present invention. All corners and
edges are finished eased and rounded. (Sharp edges adversely
concentrate mechanical stresses in the castable refractory
cement.) A copper body 302 is cast over preformed and pre-shaped
independent circuits of coolant piping 304 and 306. A single,
protruding neck 308 is collared completely by a steel collar
310.
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Steel collar 310, copper neck protrusion 308, and copper
body 302 will not bond together very well in a steel-to-copper
weld. A much more secure and gas tight attachment is needed. So
steel collar 310 is preferably embedded into the copper of neck
308 and body 302 during casting. See Fig. 4. For casting
purposes, steel collar 310 may be fabricated in two parts. A
first part, e.g., of stainless steel, cast into the copper
stave, and then the second part, e.g., carbon steel, only
attached to the first part by specialty welding after such
casting is completed.
The entire weight of these copper stave coolers bear
entirely on their steel collars 310, and so the two must never
separate even with this burden. The embedded end of steel collar
310 can be advantageously fabricated to have its edges turned
out in a flare to mechanically "lock" into the copper casting.
Anchors 813 (Fig. 8) could also be added to the steel collars to
increase mechanical locking with the copper.
Turning now to the problem of sealing necks 308 to their
corresponding penetrations in the steel containment shells,
neither cast iron nor cast copper stave coolers would weld very
well directly, without steel collar 310, because of their
respective metal dissimilarities, e.g., cast iron to steel, or
cast copper to steel. But, good gas tight welds outside the
containment shell are mandatory to stop the escape of errant
process gases and to mechanically support and secure the stave
cooler to the containment shell.
And so any part of the stave coolers that passes through
steel containment shells 102, 202 must be "adapted" to be able
to have that part welded to the steel of the containment shell.
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The copper in neck 308 is a continuous part of the copper
casting of body 302. Such copper casting in neck 308 may not
completely fill the spaces inside the distal end of steel collar
310. And so those spaces left can be stuffed with a packing
material to impede any wayward process gases that get as far as
inside neck 308.
Figs. 3A-3C are intended to illustrate that all the
independent circuits of coolant piping in a stave cooler must be
grouped together and terminate only within neck 308. These
independent circuits are then externally connectable, e.g., with
flexible coolant hoses 224 (Fig. 2,) outside steel containment
shell 202.
The placement and orientation of neck 308 on the cold face
of body 302 is critical. This one point provides all the
vertical support of the entire weight of stave cooler 300 on the
inside of containment shell 102, 202. Stave cooler 300 should
hang straight on its own like a picture frame does on a single
hook on a wall, as in Fig. 3A. However, with respect to Fig. 3B,
it may be necessary for the bottom to tilt in or out toward the
inside of containment shell 102, 202, relative to the top, in
order to follow the inside profile and contours of the furnace.
A number of bolts or struts may be disposed on the cold
face for attachment to or standoff from the steel containment
shell. These can help set any top or bottom forward tilt of the
liquid-cooled stave cooler needed to push away from its
otherwise hanging straight and vertical with respective to Fig.
3B.
The stave cooler 300, as seen in Fig. 3A, will hang the
straightest if neck 308 is disposed close to the top edge and
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straddles an imaginary lateral middle line. If the construction
of stave cooler 300 is symmetrical about this imaginary lateral
middle line, its center of gravity (COG) will be bisected.
Neck 308 and steel collar 310 are shown in Figs. 3A-3C as
nearly square with rounded corners. But they can also be
configured in the shape of a cylindrical "can". The
corresponding penetrations provided in the steel containment
shells 102, 202, would of course have to be round or oval.
Special casting and fabrication methods may be needed to
construct copper cast stave coolers 300.
Fig. 4 represent a method 400 for casting and fabricating,
for example, copper cast stave coolers 300. Copper casting
methods are both ancient and well known. Therefore many of the
conventional details of copper casting need not be described
here.
A mold 402 is split open to receive a network 404 of pre-
shaped and pre-formed pipes and fittings. A steel-to-steel
welding collar 406 is prepositioned inside of the top of mold
402, and enclosing the coupling ends of pipe network 404.
Mold 402 is positioned flat and level with steel-to-steel
welding collar 406 pointing up and proud of the mold. A molten
liquid flow of copper 408 is desired to come up and rise gently
and evenly from under the center. Feeding from the edges would
promote one sided shrinkage. The pour rises up inside and around
to embed the steel-to-steel welding collar 406 and completely
immerse and bond with pipe network 404. The pour is continued up
to a particular level 410, and then the whole allowed to cool
slowly and solidify.
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A pure crystalline formation of the copper during casting
is not preferred because such copper castings will not bond well
with the coolant piping. A small grain copper is best, but not
at the expense of electrical conductivity quality control
measures that fall below a minimum of 80% of International
Annealed Copper Standard (IACS). (Thermal conductivity tracks
electrical conductivity, and electrical conductivity is simple
and easy to measure in manufacturing.)
The best performance under high average heat loads in stave
cooler use in smelting furnaces requires a balance of factors
like molten metal heat, cooling rate after the pour, alloys
added to improve strength and control grain sizes, deoxidants,
optimized pipe bonding with the casting, and not falling below
an electrical conductivity of 80% IACS so the thermal
conductivity will be relatively free of the thermal resistance
and gradients that plague cast iron.
An open space 412 may be deliberately left inside steel-to-
steel welding collar 406.
The steel-to-steel welding collars here should have a tight
seal with the protruding necks. (To prevent errant escaping
process gases.) A practical way to construct these steel-to-
steel welding collars is to use a length of structural steel
tubing with rounded corners and no seams or welds. Large
diameter round pipe is also possible. Preferably, the steel used
in the structural steel tubing comprises a type of steel that
has a thermal coefficient of expansion that matches the thermal
coefficient of expansion of the steel of which the steel
containment shell is comprised.
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The casting of copper inside a steel-to-steel welding
collars of carbon steel may not result in a clean joint between
the two. It may be better to use a stainless steel or nickel
alloy here for the collar if that is a problem. The level of
liquid molten copper that is flooded into the steel-to-steel
welding collar from below during casting can be limited to
filling the bottom half only. The inside of the top half can be
stuffed later with some suitable packing to prevent errant
escaping process gases.
Each liquid-cooled stave cooler embodiment includes at
least two independent circuits of coolant piping all of which
are disposed as flat loops in a single common layer. One loop
can often be laid inside another loop. All such independent
circuits of coolant piping are arranged inside the solid copper
stave bodies to be uniform, parallel, and proximate to the
insides of the hot faces.
Each end of each independent circuit of coolant piping are
all turned up together in a single group inside and through both
the protruding neck and inside the steel-to-steel welding
collar. Anchors 813 (Fig. 8) added to the steel collars would
help to increase any mechanical locking with the cast copper.
This requirement will frustrate drilling in billet methods
because too many plugs become necessary to be practical.
In general, a liquid-cooled stave cooler for smelting
furnaces with steel containment shells comprises a single,
copper casting of a stave body that is rectangular in shape with
a top edge, a bottom edge, left and right side edges, a hot
face, and a cold face. Each such stave body is substantially
taller than it is wide, and that is substantially wider than it
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is thick. Each stave may be straight or curved in plan, or
straight, bent, or curved when viewed from the sides. The staves
are configured to be cemented to the inside of a steel
containment shell of a smelting furnace, e.g., to seal the
escape of process gases.
There are at least two independent circuits of coolant
piping all of which are cast into the stave body as flat loops
in a single layer and arranged to be uniform, parallel, and
proximate to the inside of the hot face.
An abrasion resistant facing is often incorporated into the
entire outside surface area of the hot face of copper stave
coolers. A shield material with a higher abrasion resistance
than copper to the churning and roiling of material inside a
furnace is needed. It is placed to environmentally protect the
copper casting of the stave body. If a copper stave coolers is
not protected with an abrasion resistant facing, then the copper
stave cooler must be sufficiently liquid-cooled to always chill
and maintain for itself a protective layer of frozen accretion
on its hot face.
Copper stave cooler embodiments of the present invention
will therefore invariably have a single, protruding elongated
neck of the single copper casting is disposed proximate to the
middle of the top edge and on the cold face of the stave body.
It is configured to vertically support the entire weight of the
liquid-cooled stave cooler within the steel containment shell
from a single penetration. A steel-to-steel welding collar
completely jackets the end of the protruding elongated neck.
Such preferably comprises a prefabricated material similar to
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structural steel tubing having rounded corners and no seams or
welds.
Every stave cooler embodiment of the present invention will
therefore always have a steel-to-steel welding collar made of a
type of steel with a thermal coefficient of expansion that
substantially matches the thermal coefficient of expansion of
the type of steel of which a steel containment shell is
comprised. Each end of each independent circuit of coolant
piping are all turned up together in a single group inside and
through the protruding elongated neck jacketed by the steel-to-
steel welding collar.
Some stave cooler embodiments of the present invention will
include an abrasion resistant facing incorporated into the
entire surface area of the hot face can include a number of
horizontal and parallel grooves cast into the solid copper stave
body to retain one of refractory brick, castable refractory
cement, and metal inserts.
These abrasion resistant facings may alternatively include
a grid pattern of deep rectangular surface pockets or dimples
cast into the solid copper stave body to retain castable
refractory cement.
Any abrasion resistant facing incorporated into the entire
surface area of a hot face may further alternatively include a
deposited layer of weld metal on copper material.
The correct tilting and angular set of heavy stave coolers
inside the containment seals into wet castable refractory cement
during construction can be assisted by placing a number of
struts or bolts on their backsides as spacers to the steel
containment shell. Castable refractory material is placed after
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the stave coolers are installed, and these devices can maintain
a standoff and tilt of the liquid-cooled stave cooler it would
otherwise not assume.
In every embodiment, an annular steel-to-steel weld of
matching types of steel is required between the outside of the
steel-to-steel welding collar and the inside of a corresponding
penetration of the steel containment shell. The critical
advantage of making a good gas seal during construction and then
maintaining later over the campaign life is process gases are
prevented from escaping from the inside of the steel containment
shell and injuring personnel or damaging equipment. Limiting to
one penetration, and avoiding metal stress concentrations from
material mismatches are avoided. Such reasons have been the
cause failures of conventional seals, especially over long time
periods of use.
Invariably, the independent circuits of coolant piping used
in copper stave cooler embodiments comprise pipes of flexible
tubing cast in liquid molten copper inside a mold which was
flooded from the bottom. The liquid molten copper is allowed to
slowly rise up and slowly cool inside the steel-to-steel welding
collar.
As is conventional, a number of rows of parallel and
horizontal grooves may be alternatively disposed on the entirety
of the hot face. These assist in an attachment of refractory
bricks or castable refractory cement.
Generally, all the outside corners and edges of stave
cooler embodiments of the present invention are finished to be
eased and rounded. Such assures that fewer thermal stresses will
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be imposed on any castable refractory cement in contact with
such points.
Fig. 5 represents a stave cooler 500 in an embodiment of
the present invention. Such is illustrated as a flat panel, but
it may be advantageous to work in some convex or concave
curvature. Here, stave cooler 500 comprises a flat panel body
502 of either cast iron or cast copper. If cast copper, a hot
face 504 can include horizontal grooving 506 to lock in and hold
conventional refractory bricks (not shown). Cast copper wears
and abrades more easily than cast iron, so cast copper stave
coolers need the protection afforded by conventional refractory
bricks and other abrasion resistive materials.
Cast copper embodiments of stave cooler 500 comprise a
small grain copper with a balance of factors like molten metal
heat, cooling rate after the pour, alloys added to improve
strength and control grain sizes, deoxidants, optimized pipe
bonding with the casting, and not falling below an electrical
conductivity of 80% IACS so its thermal conductivity will be
relatively free of thermal resistance and gradients.
Stave cooler 500 further comprises a numbers of liquid
coolant pipe loops or tubing embedded with the flat panel body
502 just inside hot face 504. These circulate liquid coolant
that is pumped in and pulled out through a single external
piping connection group 510 which is all collected together
through a single steel collar 512. The single steel collar 512
is embedded into the flat panel body 502 during iron or copper
casting and includes an annular flare 514, anchors, or other
device to mechanically lock the pieces together, since simple
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bonding between dissimilar metals can be inadequate in these
severe applications.
The operational safety of stave cooler embodiments of the
present invention can be improved by circulating liquid coolants
within them that are water-based but nevertheless intrinsically
safe from boiling liquid expanding vapor explosion (BLEVE).
Essentially, no more than 50% water is blended in with a single
phase glycol alcohol like methanol ethylene glycol (MEG). The
MEG operates as a desiccant and binds the water in a physical
absorption. The present inventor, Allan MacRae, has disclosed
the particulars of this in United States Patent Application
15/968,272, filed 5/1/2018, and titled, WATER-BASED HEAT
TRANSFER FLUID COOLING SYSTEMS INTRINSICALLY SAFE FROM BOILING
LIQUID EXPANDING VAPOR EXPLOSION (BLEVE) IN VARIOUS PYRO-
METALLURGICAL FURNACE APPLICATIONS.
Every corner and edge of stave cooler 500 is eased and
blunted to reduce cracking and separation of castable cement
that is typically packed around and behind stave coolers to
prevent outflows of hazardous process gases past them.
Fig. 6 represents the advantageous and novel way that stave
cooler 500 mounts inside a circular furnace 600 with a steel
containment shell 602. Only one penetration hole 604 is provided
in steel containment shell 602 for each stave cooler 500. Steel
collar 512 passes through and is continuously, and gas-tight
welded all around with a steel-steel-steel weld 606. Such weld
606 must provide a long life, high reliability gas seal to keep
internal hazardous process gases, like carbon monoxide (CO),
from escaping. The full weight of stave cooler 500 is borne by
the simple hanging of steel collar 512 inside the one
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penetration hole 604. Weld 606 and castable cement all around
stave cooler 500 keep it from slipping off inside.
Water makes an excellent choice as a coolant because its
low viscosity makes it easy to pump and its high specific heat
means that coolant pumping volumes and speeds can be kept as low
as is possible. A balanced combination of these considerations
means the pumps in water-based cooling systems can be
economized. But introducing water-based coolants into high heat
ferrous and non-ferrous pyrometallurgical furnaces runs a risk
of boiling liquid expanding vapor explosion (BLEVE).
Fig. 7 represents a water-based cooling system 700 in an
embodiment of the present invention that is intrinsically safe
from BLEVE. A heat transfer fluid mixture 702 comprises water,
glycol alcohol, and corrosion inhibitors in a homogeneous
solution that are circulated around in a closed loop by a liquid
pump 704. The percentage of water used in the heat transfer
fluid mixture 702 has both high and low limits. In general,
water can in this use can range from 10% to 50%.
The minimum percentage of water that can be used is limited
by the adverse impacts of increasing viscosity and reduced
specific heat that bear on the acquisition and operating costs
of liquid pump 704. As viscosity increases, it requires a
greater pumping effort and a stronger liquid pump 704 to
maintain a minimum coolant velocity 706. And as the specific
heat of heat transfer fluid mixture 702 is decreased by diluting
the water, the greater will be the pumping effort required of a
larger capacity liquid pump 704 to maintain a higher, minimum
level coolant velocity 706 that will compensate for the
inefficiency.
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In practice, the heat transfer fluid mixture must have a
room-temperature viscosity of less than 20 mPa.s. And the heat
transfer fluid mixture 702 must have a specific heat greater
than 2.3 kJ/kg.K. Otherwise, the requirements for a suitable
pump 704 become unreasonable and/or unmanageable.
The maximum percentage of water that can be used safely is
limited by the risks of BLEVE. Short of that threshold, the
mixed coolant blend 702 will burn, and not BLEVE, if it escapes
from a cooler 708 with a steel collar 709 into a high heat
ferrous or non-ferrous pyrometallurgical furnace 710. All the
coolant circulation for each stave cooler 708 passes through in
a single grouping within its respective steel collar 709. Stave
cooler 708 is essentially the same as stave coolers 114, 116,
206, 300, and 500 of Figs. 1, 2, 3A-3C, and 5.
Intermolecular bond types determine whether any two
chemicals are miscible, that is, whether they can be mixed
together to form a homogeneous solution. Here, the water and
glycol in the heat transfer fluid mixture 702 easily join
together in a homogeneous solution. When two chemicals like
water and glycol mix, the bonds holding the molecules of each
chemical together must break, and new bonds must form between
the two different kinds of molecules. For this to happen, the
two must have compatible intermolecular bond types. Water and
MEG glycol do. The more nearly equal in strength the two
intermolecular bond types are, the greater will be the
miscibility of the two chemicals. Usually there is a limit to
how much of one chemical can be mixed with another, but in some
cases, such as with CH3OH (MEG) and H20 (water), there is no
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limit and any amount of one is miscible in any amount of the
other.
As a consequence, the percentage of water in the heat
transfer fluid mixture 702 will have a practical range between
10% and 50%. The optimum percentage of water plus corrosion
inhibitors in the heat transfer fluid mixture 702 is generally
about 25%. No excess water is left unabsorbed to support a
BLEVE.
The heat transfer fluid mixture 702 is circulated in a
closed system and pressurized by a pressurization system 712.
Typical pressures run 2-7 bar. Raising the pressure inside the
closed system raises the boiling point of the heat transfer
fluid mixture 702. The minimum boiling point of the heat
transfer fluid mixture 702 under pressure should be no less than
175 C.
A particulate filter 714 is used to remove rust particles,
exfoliated mineral scale, and other solid contaminants from the
heat transfer fluid mixture 702 as it circulates.
A chiller or heat exchanger 720 is used to remove and
dispose of the heat gained by the heat transfer fluid mixture
702 in circulation, e.g., a cooler 708 inside furnace 710. Such
chillers and heat exchangers are conventional.
Although Fig. 7 shows only a stave cooler 708, such could
just as well be a panel cooler, or a cooling jacket for a top
submerged lance (TSL), torch, or Tuyere to receive the benefits
of intrinsically safe operation from BLEVE. Conventional
applications dangerously bring water-based liquid coolants into
close proximity with pyrometallurgical furnaces.
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Fig. 8 concerns itself with the characteristics of various
metals to alloy or not alloy with other metals. Associated with
that is how well metals will physically bond with other metals.
A stave cooler installation 800 in an embodiment of the
present invention mounts a cast-iron or cast-copper stave cooler
802 inside a carbon-steel containment shell 804. A single steel
collar 806 embedded at one end into stave cooler 802 provides
the entire support of the weight by hanging from a single
penetration 808 in containment shell 804. A carbon-steel-to-
carbon-steel weld 810 stoppers process gas inside from passing
through penetration 808.
Carbon steel does not bond well with copper, and the two
often produce a "dirty" interface between them that causes
gassing and porosity 812 during fabrication. Anchors 813 can be
added to the steel collar 806 to improve its mechanical lock
with the stave body casting.
Embodiments of the present invention join together a
carbon-steel collar part 814 to a stainless-steel or nickel
alloy collar part 816 with a "specialty weld" 818 that together
serve as steel collar 806.
Collar part 816 typically comprises either a 300-series
austenitic stainless steel or a nickel alloy. Type-304 and type-
316 are both acceptable, as are type-309 and type-310. Referring
to these as "300-series austenitic stainless" is a bit clearer
to most. The 400-series martensitic stainless steels have a
coefficient of thermal expansion close to the low carbon steel
used in steel shell plate, but such can easily suffer from
embrittlement during the casting process. Duplex grades, those
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half way between the 300-grades and 400 grades of stainless
steel, could also be used effectively for collar part 816.
A dirty interface and porosity 812 will be avoided with the
use of collar part 816 because the copper contacts only the
stainless steel or a nickel alloy. However, the bonding of
stainless steel or nickel alloy with copper, is no better than
for carbon steel.
Welding austenitic stainless steels (collar part 816) to
carbon and low alloy steels (collar part 814) are conventional
in the process and construction industries. The British
Stainless Steel Association (Sheffield, UK) says dissimilar
metal welds involving stainless steels can be done using most
full fusion weld methods, including tungsten inert gas (TIG) and
metal inert gas (MIG). Welds using consumable fillers allow for
better control of joint corrosion resistance and mechanical
properties.
When deciding which weld filler to use, the joint (at weld
818) is considered to be stainless, rather than the carbon
steel. Over-alloyed fillers, e.g., with increased nickel
content, can avoid dilution of the alloying elements in the
fusion zone of the parent stainless steel.
Common combinations of dissimilar steels involving
stainless steel include plain carbon or low alloy structural
grades and austenitic stainless steel grades such as 1.4301
(304) or 1.4401 (316). Carbon and alloy steels less than 0.20% C
do not normally need a preheat when being welded to austenitic
stainless steels. Carbon and alloy steels with carbon levels
over 0.20% may require a preheat. High restraint joints, where
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the material thickness is over thirty millimeters, should also
be preheated. Temperatures of 150 C are usually adequate.
Carbon steels are more prone to hydrogen associated defects
than are austenitic stainless steels, and so the welding
consumables must be dry. Standard 308 type filler can be used
for joining a stainless steel to carbon steel, and the more
highly alloyed fillers, such as the 309 type (23 12L to BS EN
12072) are preferred. Cracking in the weld dilution zone can be
a problem if a 308 type (19 9L to BS EN 12072) filler is used,
because there can be too little ferrite, and martensite may form
on cooling.
In higher temperature service, the differences in thermal
expansion rates of the steels and filler can lead to thermal
fatigue cracking. Long exposure times at these temperatures to
welds with enhanced ferrite levels can result in embrittlement
due to sigma phase formation. Nickel based fillers, such as
Inconel, can produce better welds with lower thermal expansion
rates than do the stainless steel fillers.
"Specialty weld" 818 thus cannot be done effectively
outside the shop. But weld 810 can always be done on site.
Cracking 820 inside the body of stave cooler 802 can lead
to cracking of internal piping 822 and a loss of its circulating
liquid coolant 824. Coolants 824 comprised of water can be the
cause of BLEVE and serious explosions and loss of life. So in
the case of cast iron used in the body of stave cooler 802, a
de-bonding paint 826 is applied to internal piping 822 during
casting to prevent crack propagation.
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Crack propagation into internal piping 822 is not a problem
when copper casting is used for the body of stave cooler 802,
and so de-bonding paint 826 is not necessary.
A hard facing 830 of abrasion resistant material can be
applied as a thin layer on the hot face of stave cooler 802 to
protect the stave cooler from wear and increase its campaign
life. Depending on the exact materials used in hard facing 830,
an intermediate layer 832 may be needed to improve bonding and
durability.
The materials needed to intermediate between the materials
of a more outer coating and a copper base or cast iron base are
generally understood by artisans. However, which materials and
what deposition processes are needed to apply such hard faces to
our stave cooler base substrates of copper or cast iron are
limited to those that through empirical experience produce the
longest campaign lives.
Hard facing 830 here comprises an alloy of nickel and
chromium, and/or molybdenum, and/or niobium.
Sandmeyer Steel Company (Philadelphia, PA) says its Alloy
625 is an austenitic type of crystalline structured nickel-
chromium-molybdenum-niobium alloy with outstanding corrosion
resistance and high strength over a wide range of temperatures
from cryogenic to 1800 F (982 C)
The strength of Alloy 625 derives from a solid-solution
hardening of the nickel-chromium matrix in the presence of
molybdenum and niobium. Precipitation-hardening treatments are
not required.
Alloy 625 is outstanding in a variety of severe operating
environments in its resistance to pitting, crevice corrosion,
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impingement corrosion, intergranular attack, oxidation and
carburization in high temperature service, and is practically
immune to cracking caused by chloride stress corrosion.
Alloy 625 can be easily welded to copper and processed by
standard shop fabrication practices.
Coolers principally cast from pure copper and that
circulate water inside provide the best in high performance and
are able to work in the severe environments of modern copper and
iron furnaces. However, the relatively soft copper needs
protection from wear, and the water in the coolants needs to be
kept from BLEVE.
Wear in these furnaces is a combination of abrasion,
impacts, metallic, corrosion, heat and other effects.
Castable cement slathered on the hot face surfaces of
copper stave coolers can protect the copper from wear during
use. The relatively cool surfaces precipitate and freeze jackets
of accretion from the melt, and these form a principal wear
barrier.
Other nickel-chrome alloys suited for abrasion resistance
include Alloy-122, Alloy-622, Alloy-82, and Alloy-686. Some
nickel-chrome alloys particularly suited for corrosion
resistance include Alloy-122, Alloy-622, Alloy-686, and NC
80/20. In each case, minimum nickel content should be 55%,
minimum chrome content 18%, and maximum iron content should be
6%.
But sometimes the frozen accretions will crack, scale,
separate, and sluff off to expose the bare copper surface. New
patches will freeze in place immediately, but the process and
brief exposures can cause significant wear over the campaign
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life. Grooves, texturing, and pockets embedded as contour
features in the hot face surfaces help to retain both castable
cement and frozen accretions.
Metal and refractory brick inserts are also conventional
ways that copper stave coolers have been shielded from wear. But
the machining needed to finish off the grooves, ribs, and
channels needed to retain the metal and refractory brick inserts
is expensive. It is also very challenging to keep the inserts in
tight firm contact with the stave cooler. Any looseness in the
fit will allow the inserts to get too hot and that will
accelerate wear. A stave cooler that would suffer this
particular kind fate would be the types described by Todd Smith
in US Patent Application Publication US 2015/0377554, published
12/31/2015.
The refractory bricks illustrated in Todd Smith's Fig. 3,
do not keep tight hold of the ribs and channels embedded in the
stave cooler hot faces (as illustrated in Todd Smith's Fig. 4
and 5). These refractory bricks do appear to have an advantage
of being directly insertable, rather than needing to be slid in
from the stave coolers' sides. Sliding in may not always be
possible, especially in vertically oriented cylindrical
furnaces.
Figs. 9A and 9B represent applications in which copper
stave coolers 900 and their hot faces 902 especially cannot be
protected with refractory brick or metal inserts for practical
or economic reasons. A number of pockets 904 are distributed on
hot face 902. A hard facing weld overlay 906 is applied in bead,
crosshatch, or weave patterns on the more exposed raised
perimeters of hot face 902 surrounding each pocket 904.
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Fig. 8, represents a hard facing 830 that is applied over a
buffer or intermediate layer 832. Depending on the materials
used in the hard facing 830, it may not be necessary to include
any buffer or intermediate layer 832.
Various welding techniques can be used to fuse both similar
and dissimilar materials to the copper metal surface of stave
coolers 802 and 900. The hard facing 830 can be applied by
welding beads 906 in groups in those portions of the hot face
surface more subject to wear than others. In some cases, that
will mean the entire surface will require a weld overlay, e.g.,
no pockets.
An improved copper stave cooler embodiment of the present
invention has increased wear resistance to at least one of
abrasion, impact, metal-to-metal contact, heat, and corrosion on
an included hot face surface. A hardfacing comprising at least
one alloy of nickel and chromium is fused on by welding.
Sometimes to less than the entire surface, and only on those
portions of the hot face surface predetermined to be more
exposed during use to wear than are any other portions. The
hardfacing is typically applied as a weld overlay of molten
metal in an inert shield gas.
In Figs. 9A and 9B, these copper stave coolers 900 can be
further improved by including a plurality of castable cement
retention pockets 904 disposed across the surface of the hot
face 902. Each such pocket 904 includes inwardly tilting,
shallow walls and footings 908 that operate to better retain a
castable cement filling when in use. A perimeter of raised and
more exposed copper base material surrounds each of the
plurality of pockets. So, the application of such hardfacing is
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economized by placing it in bead patterns 906 on only the raised
and more exposed copper base material of the perimeter.
Preferably, the copper base material to receive welding
overlays is the equivalent of UNS C12000 if wrought or UNS
C81100 if cast, which includes deoxidants and low residual
phosphorous that promote good welds, reduced copper grain size,
an electrical conductivity of at least 80% IACS, and improved
embrittlement resistance during welding.
Although particular embodiments of the present invention
have been described and illustrated, such is not intended to
limit the invention. Modifications and changes will no doubt
become apparent to those skilled in the art, and it is intended
that the invention only be limited by the scope of the appended
claims.
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