JP7266939B1 - Desulfurization method for desulfurization slag in steelmaking process - Google Patents

Abstract

An object of the present invention is to reuse deoxidizing and desulfurizing slag used for ladle refining in electric furnace steelmaking. SOLUTION: The deoxidizing and desulfurizing slag remaining in the ladle at the end of casting is flowed into a slag pot, the slag is heated and oxidized by an oxygen burner, and the S content in the slag is vaporized and removed as SOx. Exhaust gas is guided to cleaning or the like and neutralized by spray cleaning with an alkaline aqueous solution. The desulfurized slag immediately flows into the original ladle and receives steel. Slag is hot recycled. Ammonium sulfate is obtained as a by-product when ammonia is used as an alkali. [Selection diagram] Fig. 1

Description

TECHNICAL FIELD The present invention relates to a method for desulfurizing and reusing deoxidizing/desulfurizing slag applied to ladle refining, which plays the latter half of refining in electric furnace steelmaking.

The current mainstream process in the electric furnace steelmaking process uses scrap iron as the main raw material, melts and oxidizes the raw material in an arc furnace, then immediately taps the steel into a ladle, and uses a so-called LF (Ladle Furnace) to arc the molten steel in the ladle. After reheating to produce slag suitable for reduction, deoxidation, and desulfurization to complete deoxidation and desulfurization, the slag is subjected to continuous casting through a two-step process of induction and adjustment to a predetermined composition and temperature.

The post-refining work goes through the following steps.
1) The molten steel in the melting furnace is tapped into a ladle. Part of the melting furnace slag flows into the ladle.
2) During tapping, a composition adjusting material, a deoxidizing agent, and a slag forming material consisting mainly of quicklime are added to the ladle.
3) Introduce the ladle truck to the arc heating device (LF) to reheat and melt the slag forming material.
4) Continue to stir the molten steel by blowing inert gas through the ventilation plug provided at the bottom of the ladle.
5) A carbonaceous material for reducing slag is charged and held to generate deoxidized/desulfurized slag.
6) Measure and sample temperature, adjust ingredients, and adjust temperature.
7) Pull out the ladle from the arc heating device, place it on the pedestal of the continuous casting device, and use it for casting.

The formation of deoxidized/desulfurized slag depends on the following reactions.
a) At the stage of 5) above, the slag composition is basic as follows.
CaO 45-55%, SiO ₂ 15-20%, Al ₂ O ₃ 10-15%, MgO 5-10%, (FeO+MnO) 5-10%.
b) Low oxides (FeO+MnO) in the slag are easily reduced by charging the carbon material. C+ _CO2 =2CO
(FeO+MnO)+2CO=Fe+Mn+ _2CO2
c) Deoxidation of the slag shifts the distribution of S from the molten steel to the slag side.
[FeS] = (FeS)
d) Part of the carbonaceous material also reduces CaO under arc to form Ca carbide.
CaO+3C= _CaC2 +CO
e) Ca carbide fixes S in the slag and desulfurization is completed.
(FeS) + ( _CaC2 ) = (CaS) + Fe + 2C
Refining reduces the concentration of lower oxides FeO + MnO in the slag to about 1.0% or less,
The CaS concentration increases from 0 to about 2-3%. In the process, the dissolved oxygen concentration drops from about 0.01% to about 0.003%, and the S drops from about 0.03% to about 0.01%.

The functions of the slag in the ladle from the start of refining to the end of casting are to stabilize the arc, prevent atmospheric contamination of molten steel, retain heat, deoxidize, and desulfurize. After casting is finished, the slag is discarded. Since the desulfurization capacity has been consumed, it cannot be reused.
After completion of casting, the slag is usually fluffed (transformed into powder) during cooling. There are many problems in disposal, such as dust disposal and difficulty in landfill disposal, and various measures have been taken, but all of them are complicated and expensive to dispose of as industrial waste.

Consider examples of treatment methods. The slag composition is adjusted so that the ladle slag solidifies without air-crushing upon cooling. Specifically, when the slag is poured from the ladle into the slag pot, a solidification agent containing borax as a main component is mixed into the slag. Although it is often practiced, there are problems such as insufficient mixability, insufficient solidification strength as a ground material, and an increase in the amount of slag.

Patent Document 1 discloses a method of holding slag in a heat-insulating mold for several tens of hours based on the new knowledge that the slag is usually air-crushed during cooling, but becomes petrified by ultra-slow cooling. there is Provides strength comparable to granite.
The principle is that the mineral compositions that follow the equilibrium phase diagram during the cooling and solidification process will appear in accordance with the progress of solidification, precipitation and transformation.
The problem is that the holding time is several tens of times longer than the tapping cycle is within 1 hour, and the processing space,
The number of molds to be processed is too large to implement.

Patent Document 2 discloses a method for greatly shortening the petrification time in the above method (slag petrification).
According to it, the petrification time can be shortened to several hours by adding waste gypsum (CaS.2H ₂ O) as a petrification accelerator in an amount of about 2% of the slag mass when deoxidizing and desulfurizing slag is produced. The method has been put into practice and the product is used for fish reef material.
Although the amount of waste gypsum added is not large, the problem is that CaS reduces the desulfurization ability of the slag, and H enters the molten steel from the hydration water, causing quality problems. .

Patent Literature 3 discloses a method for desulfurizing slag applied to desulfurization of molten pig iron instead of molten steel and a method for reusing the slag after desulfurization.
According to it, hot metal desulfurization slag is solidified without pulverizing. The solidified slag is pulverized (5 mm diameter or less), charged into a rotary kiln, burned under appropriate heating and oxidation conditions, the S content is extracted from the solid slag as SOx gas, and the residue mainly composed of lime CaO The slag is transferred to the steelmaking process and reused.
Oxidizing conditions are important for stable desulfurization from desulfurization slag. Excessive oxidation oxidizes iron particles in the slag to form FeO, which reacts with lime CaO to form Ca ferrite with a low melting point. It welds and adheres the powder particles and suppresses the desulfurization reaction. Optimal values are presented.
The method of heating and burning the sulfides in the slag to separate the S content as SOx is similar to the operation of burning (roasting) metal sulfides in non-ferrous refining.

The problems of this method are 1) that a slag pulverization process is required in advance, 2) that equipment (rotary kiln) and fuel for heating and burning the powder are required, and 3) the desulfurization rate is low. , is unattractive for application to electric furnace steelmaking. However, the gasification treatment of S in slag is very informative.

Patent Document 4 discloses a method of hot recycling the slag in the ladle after casting into the same ladle toward the next receiving steel. According to it, in special high-grade refining using neutral slag with a basicity of 1.5 or less, when the ladle is turned over horizontally for repair after casting, a kind of dam installed in the ladle prevents slag from flowing out. When the machine is rotated forward after repairing, the next steel reception is carried out while the molten slag is held at the bottom.
Not only is the slag-forming material and the industrial waste caused by the slag-forming material significantly reduced, but it is also effective in saving energy.

The problem with this method is that although neutral slag is effective for deoxidizing, it has no desulfurizing function at all. Therefore, it is necessary to limit the raw materials to high-quality materials or to perform desulfurization in advance. Not suitable for mass-produced ordinary steel.

Published Patent Publication 2015-36350 Published Patent Publication 2021-014382 Published Patent Publication 2014-201805 Published patent publication 1999-092817

In electric furnace steelmaking, the latter stage of refining involves reheating molten steel in a ladle with an arc, forming basic slag, adding carbonaceous materials at appropriate times to enhance reducibility, and deoxidizing and desulfurizing the molten steel. S in molten steel is fixed as CaS in slag. The slag that has been subjected to deoxidation and desulfurization is discarded after casting, but it is weathered and pulverized during cooling, making the disposal of industrial waste complicated and expensive.
Petrochemical treatment of slag (a type of constant temperature heat treatment) poses no problems when discarded, but the treatment efficiency is extremely low and impractical, and the addition of accelerator waste gypsum as a solution causes problems in product quality.
Desulfurization by oxidation treatment of the slag used for hot metal desulfurization makes it possible to reuse the slag, but it is not suitable for electric furnace steelmaking due to problems with equipment, energy and efficiency.
In ladle refining of high grade steel using neutral slag, it is possible to reuse slag, but because it is neutral, it has no desulfurization ability and is not suitable for mass production of ordinary steel.

The present invention is to desulfurize the deoxidized / desulfurized slag that is subjected to finish refining in the ladle of electric steelmaking to enable reuse of the slag, perform desulfurization at high speed, and perform subsequent charging while in a molten state. There are two problems to be solved: providing for refining, that is, enabling hot recycling of slag.

In the first invention, basic deoxidized/desulfurized slag that has been subjected to ladle refining in electric steelmaking is transferred to a slag pot immediately after casting is completed, and oxygen gas is blown into the deoxidized/desulfurized slag in a molten state. A desulfurization/regeneration method for ladle refining slag characterized by separating/removing S in slag as SOx gas.

The second invention has the following four conditions,
1) The method of blowing oxygen is by an oxygen gas burner shared with the oxygen gas lancer,
2) to grasp the progress of desulfurization by installing a SOx sensor in the flue of the combustion exhaust gas;
3) guiding the flue gas to a scrubbing tower and dissolving and removing SOx by showering with an alkaline aqueous solution;
4) immediately return the slag after desulfurization to the original ladle in a molten state;
The desulfurization/regeneration method for ladle refining slag according to the first invention, characterized in that one or more of the above is incorporated.

The deoxidized/desulfurized slag that has been subjected to ladle refining is oxidized by blowing oxygen gas in a molten state after casting, and S in the slag is separated and removed as SOx gas, and the slag is used as a slag forming material. It becomes possible to provide for reuse. Both the amount of slag forming material used and the amount of industrial waste disposal of waste slag are greatly reduced. Although there is some consumption of fuel, the reduction of industrial waste is more than enough.

Since the deoxidized/desulfurized slag required for ladle refining is provided in a molten state, the heat of dissolution is not required, and the efficiency of ladle refining is improved, so that power and energy can be saved.

The SOx gas generated by desulfurization of the slag is neutralized in water by a simple alkali cleaning device without being released into the atmosphere, and does not cause air pollution. The precipitated and crystallized alkali sulfate becomes, for example, ammonium sulfate and is effectively used as a fertilizer.

FIG. 2 is a conceptual diagram of performing desulfurization treatment of ladle refining slag that has been subjected to deoxidation and desulfurization. A method of working to recycle ladle smelting slag is shown.

In ladle refining, a slag-forming material mainly composed of basic quicklime is charged into an intermediate ladle. A part of the slag in the furnace is also carried over to the ladle during tapping.
The slag composition at the start of refining is CaO 45-55%, Si
15-20% O ₂ , 10-15% Al ₂ O ₃ , 5-10% MgO, and 5-15% lower oxides (FeO+MnO).
During refining, carbonaceous materials are introduced in a closed state while arc heating, and lower oxides are reduced by C and CO. After reduction, distribution of FeS shifts from molten steel to slag (desulfurization reaction).
3C+CaO= _CaC2 +CO
FeO+CO=Fe+ _CO2
MnO+CO=Mn+ _CO2
[FeS] = (FeS)
(FeS)+ _CaC2 =CaS+Fe+2C
At the stage when the ladle refining is completed, the slag composition (FeO + MnO) drops to 2.0% or less,
CaS increases from 0 to 2-4% CaS.

A method for removing S from the S-containing slag after deoxidation and desulfurization will be described with reference to FIG.
After the casting is finished, the deoxidized and desulfurized slag remaining in the ladle (not shown) is flowed into the slag pot 1 . A small amount of residual molten steel is mixed in the deoxidizing and desulfurizing slag 2 in the slag pot 1 .
Immediately after the inflow, the deoxidizing/desulfurizing slag 2 is heated by the oxygen burner 3 sharing the oxygen lance. The oxygen burner 3 is commercially available and widely used as a coherent oxygen jet burner, and can also be used as a lancer.

When the oxygen jet acts on the residual molten steel, the temperature of the molten steel rises due to oxidation, and FeO is produced, the melting point of the slag is lowered, the fluidity is increased, and the oxidation of the slag is favorably promoted.

S in the slag is combusted by oxidation and heating by an oxygen burner to become SOx gas and diffuse into the exhaust gas.
CaS+ _2O2 =CaO+ _SO3
CaS+4FeO=CaO+4Fe+ _SO3
SOx production can also be perceived from the odor of combustion gases. Combustion gases are sent to scrubber 5 via flue 4 . The SOx gas is absorbed and removed by the alkaline aqueous solution shower 6 in the washing tower 5 and released to the atmosphere. The aqueous solution at the bottom of washing tower 5 is circulated to shower 6 by a pump. A portion of the solution is phase-shifted to a cooling sedimentation tank 9 by a branch valve 8, and the sulfate 10 is transferred out of the precipitation and crystallization system. Washing water and alkali are replenished as needed.
The end point of the reaction is timely determined by continuous measurement of the SOx sensor 11 installed in the flue 4 .

A method for effectively utilizing the desulfurization treatment of the deoxidation-desulfurization slag in the current steelmaking work will be described with reference to FIG.
After casting, the ladle 21 is tilted to allow the deoxidizing and desulfurizing slag 22 to flow into the slag pot 23 . The slag pot 23 is guided to a desulfurization apparatus comprising an oxygen burner 24 and the flue gas scrubbing tower, etc., and is immediately heated, oxidized and stirred by the oxygen burner 24 . About 20 minutes of treatment vaporizes and removes most of the S in the slag.
On the other hand, the ladle 21 is overturned to repair the nozzle 26, etc., and stands upright to stand by for the next receiving steel. The desulfurized slag 25 in the slag pot 23 flows into the ladle 21 and returns.

The CaS concentration in the slag is reduced from about 2 to 4% to about 1% or less by the slag oxidation treatment.
FeO increases from about 1% to about 5%.
Consider the chemical reactions caused by slag combustion.
Three reactions A, B, C are envisioned.
(A) <CaS> + ( _O2 ) = <CaO> + SO)
ΔG°＝-37170-0.7T (298-1760K)
(B) <CaS> + 3/2 ( _O2 ) = <CaO> + ( _SO2 )
ΔG°＝-108390-16.61T (298-2000K)
(C) <CaS> + 2 ( _O2 ) = <CaO> + ( _SO3 )
ΔG°＝-130990-37.97T (298-1800K)
Comparing at 1800K (1523℃)
ΔG°(A) = -38430 cal/mol
ΔG°(B) = -138288 cal/mol
ΔG°(C) = -199336 cal/mol
ΔG°＝-RTlnK (R=1.987cal/deg・mol) to find the equilibrium constant
K(A)=a(CaO)xP(SO)/a(CaS)xP( _O2 )=46390
K(B)=a(CaO)xP( _SO2 )/a(CaS)x(P( _O2 )) ^3/2 = ^6.19x1016
K(C)=a(CaO)xP( _SO3 )/a(CaS)xP( _O2 )= ^1.615x1024
becomes. It is presumed that when solid CaS is oxidized at high temperature, sulfur trioxide (SO ₃ : sulfuric anhydride) gas is mainly produced, and reacts with moisture in the air to form sulfuric acid fog.
From the equilibrium constant values, it is likely that the reaction of C occurs preferentially even if the activity of CaS and the partial pressure of oxygen in the gas phase are considerably low.

The effect of the operation of the slag hot recycling method is that the amount of slag-forming material used is remarkably reduced. Theoretically, the basic unit approaches 0%. The effect of greatly reducing industrial waste is to reduce the associated manual and administrative work.
In the conventional method, slag-making materials are added at the time of tapping, and heating time and electric power are consumed to transform them into refined slag.

Most of the S content in the molten steel temporarily shifts to slag, but precipitates and crystallizes out as sulfate due to the desulfurization treatment of the present invention. If ammonia is used in the alkaline aqueous solution, ammonium sulfate, which can be used as a fertilizer, is obtained as a by-product.

After casting a small amount of slag produced by ladle refining and subjected to deoxidation and desulfurization reaction, approximately 1 liter (approximately 2 kg) of molten slag is sampled, held in a refractory crucible, and cut into steel while in a molten state. Heating and oxidation treatment was performed for 10 minutes with an oxygen gas torch for the above.
The S concentration in the slag before treatment was 1.2%, and after treatment it was 0.2%, confirming the desulfurization effect.
In practice, the amount of slag in one cycle is 500 to 2000 kg, and the reaction speed seems to be a problem. It is presumed that stirring becomes rate-limiting and the oxygen lance is effective.

1; slag pot 2; deoxidation and desulfurization slag 3; oxygen burner 4; flue 5; deoxidation and desulfurization slag 23; slag pot 24; oxygen burner 25; deoxidation and desulfurization slag 26; nozzle

Claims (1)

Basic deoxidized/desulfurized slag that has been subjected to ladle refining in electric steelmaking is transferred to a slag pot immediately after casting, and the deoxidized/desulfurized slag is heated in a molten state by an oxygen gas lancer and an oxygen gas burner in common.・ Ladle refining characterized by oxidizing and stirring, separating and removing S in the deoxidized and desulfurized slag as SOx gas, immediately flowing into and returning to the original ladle, and supplying it to the next receiving steel. Slag desulfurization/regeneration method.

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