CA1204653A - Olivine bedding material for soaking pits - Google Patents

Abstract

ABSTRACT
A method of operating soaking pits to prolong useful period between bed replacement. Olivine is used to replace coke breeze or crushed refractory material in the bottom of the soaking pit to protect its refrac-tory lining.
Operating periods from 40-60 days are possible when olivine is used in the soaking pit. The olivine has excellent temperature stability, also, so it is well suited for this application, in which temperatures range from about 2400-2650°F.

Description

~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.

~2(~653 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 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.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved method of operating a metal in-got soaking pit having a horizontal bottom, sidewalls and top defining a heating chamber, said method including the step of covering the bottom of said soaking pit with a layer of a particulate olivine material having an iron oxide content less than about 11% measured as FeO, and having a heat fusion point of at least about 2600°F, said olivine material providing improved handling, uniform thermal insulation and a substantially horizontal uniform supporting bed for metal ingots being treated in said soak-ing pit, whereby the ingots retain substantially vertical positions during treatment.

2. The method of claim 1, in which the soaking pit is a "dry bottom" practice soaking pit, and the olivine material has a particle size in the range of about 1/4 inch to 1 inch in diameter, and said olivine material is dis-posed in a layer from 2-4 inches in thickness over the bot-tom of the soaking pit.

3. The method of claim 1, wherein the soaking pit is a "wet bottom" practice soaking pit, and the olivine material has an average particle size of less than about 2 inches, and said olivine material covers the bottom of said pit to a depth of about 6-14 inches.

4. The method of claim 1, 2 or 3 in which the olivine material has the following analysis: MgO 38-50%
by wt; SiO2 38-50% by wt; FeO 4-11% by wt (expressed as FeO); Al2O3 0.1-0.5% by wt; Cr2O3 0.3-0.9% by wt; K2O and Na2O, trace; fusion point 2600-3200 F; loss on ignition <6%; real density 3.2-3.5 g/cc.

5. The method of claim 1, 2 or 3 in which the olivine material has the following analysis (expressed as oxides): MgO 40-49% by wt; SiO2 38-45% by wt; FeO 6-10%
by wt; Al2O3 0.1-0.5% 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.

6. An improved method of operating a "wet bottom"
practice soaking pit having a horizontal bottom, sidewalls and top defining a heating chamber used to heat treat metal ingots in a continuous batch sequence without cooling said soaking pit between batches, said method including the steps of filling the bottom of said soaking pit to a level of about 6-14 inches with a particulate olivine material having a heat fusion point of at least about 2600°F, said olivine material being capable of extending the operational life of said soaking pit to at least twenty days continuous operation without replacing said olivine material.

7. The method of claim 6, in which said olivine material is stirred and releveled between loadings with ingots to be treated.

8. The method of claim 6 or 7, in which the olivine material has the following analysis: MgO 38-50%
by wt; SiO2 38-50% by wt; FeO 4-11% by wt (expressed as FeO); Al2O3 0.1-0.5% by wt; Cr2O3 0.3-0.9% by wt; K2O
and Na2O, trace; fusion point 2600-3200°F; loss on ignition < 6%; real density 3.2-3.5% g/cc.

9. The method of claim 6, in which the olivine material has the following analysis (expressed as oxides):
MgO 40-49% by wt; SiO2 39-45% by wt; FeO 6-10% by wt;
Al2O3 0.1-0.5% 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.

10. The method of claim 9, in which the average particle size of the olivine material is less than about two inches.