Note: Descriptions are shown in the official language in which they were submitted.
~20~
This invention relates generally to methods of
operating soaking pits to prolong the useful period between
bed replacement.
The modern soaking pit has been developed to pro-
vide uniform heating of ingots to the desired temperature
with a minimum of over-heating of the surface. In mos-t
modern clesigns, this is accomplished with automatic con-
trols. The normal range ~or heati.ng ingots is about 2~00-
2500F. The proper temperature level varies with grades
of steel and sizes o~ ingots and characteristics of the
rolling mill. Low-speed mills with many passes require
the higher level of heating for certain grades o~ steel.
Soaking pits serve the dual function of heating and acting
as a reservoir to correct irregularities in the flow of
ingots between the steel melting shop and the primary rol-
ling mills. Brie1y, soaking pits are deep chambers, or
furnaces, o~ square, rectangular or circular shape, into
which ingots are placecl in an upright position through an
opening at the top. A removable cover closes the pit open-
ing. A serias of pits, installed usually in rows areplaced under cover of a building adjacent to the entering
side of the blooming or slabbing mill to be served. The
top of the pit is usually several feet above ground level.
The pits are spanned by one or more electrically operated
traveling cranes equipped with a traveling hoist for charg-
ing the ingots into the pits and for lifting them out as
they are needed by the mill.
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The soaking pits are usually lined with a re-
fractory material, such as alumina bench brickO The
bottom, or 100r, of the soaking pit is subjected to the
greatest wear due to the weight o~ ingots, scale buildup,
5 and localized heating from the ingots being treated, as
well as puddling from molten metal. General practice has
been to protect the soaking pit bottom with a granular
material which soaks up slag, molten scale and molten
metal and prevents -these materials from directly contacting
lO the refractory lining of the pit bottom. In "wet bottom"
practice coke-breeze, a by-product from the coke ovens
of the steel mill, has been typically used as a bedding
material, or "bottom-making" material.
Where coke breeze is used as a bot-tom-making
15 material, pit bottoms are made up with coke breeze to a
depth of approximately 12 inches to 16 inches. Ashes from
burned breeze, or breeze contaminated with scale, refrac-
tory or other material, are removed through cind0r holes,
of which there ~re usuall~ two, located in the ~ottom of
20 the pit. Bottoms for the older pit designs yenerally
are made up each dayi in modern pits they are made up only
every 5-7 days under normal ~onditions. A recent trend
is to utilize what is known as a dry-bottom practice, in
which dolomite or magnesite is used as a 2-inch to ~-inch
25 thick covering on the pit hearth.
In "dry bottom" practice, loose refractory
crushed brick, magnesite or dolomite is typically used as
the bedding material. Such materials are considerably
more expensive than coke breeze, but the operation of the
30 "dry bottom" soaking pit may continue for up to ~ive
months. When these "dry bed" soaking pits have their
bedding material changedj the unit is closed down, and
completely cooled, the total downtime being about a week.
There is a growing need to replace magnesite and dolomite
35 used in soaking pits because the materials are increasing
in cost due to shortages. The amount of bedding material
required in a typical soaking pit is about 10-20 tons,
determined by the operating axea to be covered and the
depth of the bedding rnaterial.
A Japanese olivine material has apparently
been used as a "furnace floor material" in open hearth
furnaces, electric furnaces and soaking pits in Japan.
It is not known whether this Japanese olivine material
had any advantages over other materials, and it is not
known whether it was used in combination with other mater-
ials. See: Bamba, et al, Olivine-Serpentine Resources
in Hokkaido and Their Use, Gypsum & Lime 163:33ff, 1979.
Japanese olivine referred to in the Bamba et
al article is obtained from large deposits on the island
oE Hokkaido. The analysis (e~p.ressed as oxi~es) of this
15 olivine was reported to be: SiO2 39.8~aJ0.5~; M~O ~16.1-
~7.5~6; E'eO 12.5-13.5%. The heat stability of this olivine
is limited because oE the limited stability of this mater-
ial at temperatures of about 1400E', so this olivine is
not considered useful as a refractory material.
Japanese Patent 78 123,318 dated October 27,
1978, describes the use of iron ore of 5-6 mm diameter
as a soaking pit bed material at a thickness of 300-450
mm. It was reported that bed life was about one month.
See: CA 90:108012c (1979).
The present invention is directed to an improved
method o:E operating a meta:L ingot soak:ing pit having a
horizontal bottom, sidewalls and top defining a heating
chamber, said m0thod including the step of covering the
bottom of said soaking pit with a layer o:E a particulate
olivine material having an iron o~ide content less than
about 11% measured as FeO, and having a heat fusion point
of at least about 2600F, said olivine material providing
improved handling, uniform thermal insulation and a sub~
stantially horizontal uniform supporting bed for metal
ingots being treated in said soaking pit, whereby the
ingots retain substantially vertical positions during
treatment.
~o~
This invention is directed to the use of heat
resistant olivine as a replacement for coke breeze and
other bedding materials in soaking pits for steel ingots,
and particularly, for "wet-bottom" soaking pits. It has
been found that olivine obtained from U. S. deposits has
significantly less iron oxide content, and is considerably
more heat stable than the aforesaid Japanese olivines.
Olivine obtained from U. S. deposits has a
relatively high fusion point (2600-3200F). It has a
high heat capacity, good compressive strength and low
thermal expansion. All of these proper-ties make it highly
suitable ~or use as a bedding material in soaking pits.
There are many advantages obtained by replacing coke
breeze with such olivins. The most important of these
is the substantially improved bed life. Typical bed life
in "wet bottom practice'l usin~ coke hree~e as the b~cldln~
material is 3-7 days. Ol~vine used in the same soaking
pit has typically provided a bed life of about forty days,
and in some cases up to fifty-nine days. Ano-ther important
advantage o~ using olivine is that it can be reused after
removal from the soaking pit. The recovered olivine can
be used in a blast furnace as a stabilizer additive as
described in United States Patent 4,066,443 granted
January 3, 1978.
In accordance with the invention, a preferred
aspect of an olivine containing less than about 11% iron
oxide, measured as FeO, ancl, preEerably, less than about
10%, and having a heat fusion temperature of about 2600-
3200F has been found to be useful as a bedding material
in soaking pits for steel ingots to provide protection for
the refractory liner in the bottom of the soaking pit, and
to greatly extend pit life. Olivine minerals suitable
for this purpose, preferably have the following typical
analysis (expressed as oxides): MgO 40-49% by wt; SiO2
39-45% by wt;
~ll2~ ;53
-6- '
FeO 6.0-10% by wt; A].203,0.1-.5% by wt; Cr2O3 0.3-0.9% by
wt; K2O and Na2O trace; loss on ignition <6~; real density
3.2-3.5 g/cc. The preferred particle size is from less than
about 1 inch, up to about 3/8 inch (U.S. Standard screen
scale), and the useful particle size includes a range from
less than about 2 inches to more than about 1/~ inch ~U.S.
Standard screen scale~. The olivine aggregate is tough and
durable and mechanically strong comparecl to limes-tone or
dolomite. Soaking pit beds whi,ch comprise olivine are more
stable in supporting the ingots so that they remain more
vertical, and are easier to remove. In the typical "wet-
bottom" practice, olivine replaces coke breeze, and provides
the following advantages:
a) oliv,ine particles neither fuse nor breakdown,
and olivine is a clean mineral, containing less than about
2~ free silica;
b) the dense olivine particles resist penetra-
tion of slag (molten scale) or molten metal;
c) the olivine bedding material. remains effec-
ti-~Je as a protective material for the expenfiive refractory
bottom liner of the soaking pit for a much longer time than
coke breeze;
d) by increas.ing pi.t life by about 5-14 times
over coke breeze, the use of olivine reduces soaking pit
cleanour frequency;
e) time and labor for maintenance and cleanout
reduced.
f) gas consumption associated with reheating
the soaking pit after cleanout is reduced;
7_
g) the bed is stable under load, allowing verti-
cal ingot placement and more efficient heating;
h) the recovered olivine may be used in the
blast furnace to stabilize the furnace operation.
EXAMPLE
Coarse olivine, having a particle size in the range
of 1 1~4" to 3/8" was spread in the bottom of a soaking pit
which has the Eollowing bottom dimension~: width 10 ft;
length 20 ft; and depth 15 ft. About 11 tons of olivine
was used to form an ~ven, protective layer over the soakin~
pit bottom of about 6-14 inches deep.
The ollv.ine~ should have the followincJ chemica:L ~n~
sis (expressed as oxides): MsO 40-~9~6 by wt; S~02 39~~5%
by wt; FeO 6-10% by wt; A1203 0.1-0.5% by wt; Cr203 0.3~
0.9% by wt; K2O and Na2O trace; fusion point 2800-3200F;
loss on ignition <6%; real density 3.2-3.5 g/cc; and was
obtained from olivine deposits near Addie, North Carolina.
Other olivines obtalned Erom various deposits in
North Carolina and the state of Washington, which are also
use~`ul in the practice o~ t}liS i.rlVe.nt.iOn have the fol.lowirlg
ana.l~sis range (expressed as oxides): MgO 33-50% by wt;
sio2 38-50~o by wt; FeO 4-11% by wt (expressed as FeO);
A1203 0.1-0.5~ by wt; Cr2O3 0.3~0.9% by wt; K2O and Na20
trace; fusion point 2600~-3200F. The other physical proper-
ties are comparable to the Addie, North Carolina/ sample
reported above.
Be~ore filling or "making up" the soaking pit bed,
the soaking pit is unloaded and cooled to abou~ 1000F.
~L2~653
-8--
The previously used bedding ma-terial is removed, and then
the above olivine material is loaded in the bed and spread
to a uniform level thickness therein. In a typical method
of operation, the soaking pit is then refired, and quickly
heated to a ternperature of about 1600F, and heating is
then continued in increments until the operating temperature
of about 2450F is reached. Coke oven gas, blast furnace
gas, or combinations of both, can be used as the fuel.
The soaking pit is then operated in the usual manner
to heat cold ingots to rolling temperature; equali~e temper-
atures in those ingots not totall~ so].idified; and to serve
.~!, as a storage area for hot ingo-ts untll they can be ro].led.
Residence t.ime in the soaking pit can vary considerably,
depending on temperature and pro~uction requirements.
The a~ove olivine material, used as described, has
given a very prolonged "wet bottom" soaking pit life up to
fifty-nine days in at l.east one case, and on the average
about 25-40 days when the bed was stirred regularly between
ingot loads. This prolonged life is greatly improved over
the t.ypical bed life of 3-7 days when co]ce breeze is used
as the beddin~ rnaterial in a "wet bottom" ~ractice soak.ing
pit. ~verage bed lie using the above olivine material may
vary depending on speciic conditions, but i.n all cases, it
has been typically more than three times as long as coke
breeze bedding materials, when used under the same condi-
tions.
The prolonged bed life obtained using the above
olivine material represents a substantial savings in labor,
energy and production time. When the cost of oliv.ine is
balanced against these operational economics, its use in
"wet bottom practice" soaking pits represents a substantial
improvement over the prior practice.
In addition to the above advantages, when the ingots
are removed from the soaking pit bed, after "soaking", the
olivine separates readily from the bottom of the ingots.
m is is important, because any material which adheres to
the end of an ingot can end up in the rolled product.
Furthermore, hard re-fractory lumps would be undeslrable in
contact with the rolls.
