CA1075471A

CA1075471A - Method of treating molten slags in steel-making process

Info

Publication number: CA1075471A
Application number: CA250,592A
Authority: CA
Inventors: Yoshio Miyashita; Kenzo Yamada; Akira Masui; Kenji Takahashi
Original assignee: Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1975-04-16
Filing date: 1976-04-15
Publication date: 1980-04-15
Also published as: JPS5523781B2; FR2307875B1; DE2616784A1; JPS51121030A; FR2307875A1

Abstract

ABSTRACT OF THE DISCLOSURE
The invention relates to a method of regenerating molten slags in a steel-making process. In accordance with the invention, oxygen is blown at the molten slags after solid carbon and fluxes are newly added thereto. The composition of the slags is varied to correspond to the intended uses thereof.
The varying of the composition of the slags and the regenerating of the slags is accomplished in a converter or any other vessel having the same facilities as those of a converter.

Description

1~75~

This invention relates to an improved method of regene-rating molten slag~ in a steel-making proces~, and more parti-cularly, to contributing to the saving oE resources and the re-moving of environmental pollution by varying the composition of slag, It is well-known that large amounts of molten slag, reaching about l~/o of the output of crude steel in the case of a steel-making process, e.g. by Basic Oxygen Furnace, is pro-duced. Such a steel-making slag consists generally of free lime (CaO), dicalcium silicata (2CaO~SiO2), and at the same time, about 15% Fe and about 1% P. In such composition, CaO reacts chemically through contact with water or carbon dioxide gas and the 2Ca0.Si02 varies its own cubic volume by phase transformation, bringing about a breakdown of the slag. When steel-making slags are employed for such services as road bed materials and aggre-gates which require granular shape, they are unsuitable because of the tendency for the above-mentioned brea~down~ When the slags are employed for such services as cement materials where there is no hindrance even if broken down, they are undesirable with res-pect to their composition, i.e., the amount of Fe and so forth in the composition. Also, in such uses as a source of lime for blast furnace operation, it is known that there is some limit on their employmen~ because of the high concentration of P in the slags. Thus it will be apparent that there are many difficulties to be overcome in using such slag~. Proposals for overcoming the above-mentioned difficulties have been suggested, including treat-ment to regenerate the slags by varying their composition. The art disclosed in Japanese Patenk Application Showa 49-83693 (laid open for public inspection) is a representative example, According to the application mentioned abo~e, the art comprises the following ~teps:
1. Molten slags in steel-making process are transferred L ~
.

~.~7~9~7~L

into an electrical smelting furnace where sufficient hot metal has been already charged.

2. The slags are heated throuyh the above hot metal to which elec-tric -thermal energy is given.

3. The composition of the slags is varied by chemical reaction betweell the slags and the hot me-tal or other flux, and the slags are subjected to the final regeneration.
Such regenerated slags may be utilized as one of the materials for making cement. However, many difficulties may be pointed out in the above-mentioned process. For instance, there is first heat loss brought about by transferring the slags into the smelting furnace from the steel-making furnace. The second weakness is that a large amount of hot metal is required as a heating medium. In such a case the required amount of heat to the slags is given by conduction of heat based on relatively expensive electrical energy (e.g. 560KW~I/slag t). A third weakness is that the blowing of a gaseous body (e.g. N2) or aqueous vapor from the bottom of the smelting furnace, or the forced agitation by rotating or stirring of said furnace or the mechanical agitation by other means are unavoidable, for which ;~
some additional equipment is required. The fourth weakness is ` that the control of the hot metal composition is required to adjust the reaction because of the regeneration of the slags through the hot metal. Thus, it should be pointed out that there are many disadvantages and difficulties to be overcome in the prior art.
This is the existing state.
This invention has been developed to overcome the above-mentioned existing state. The main features lie in the blowing of oxygen into molten slags after the addition of solid carbon and flux.

An object of this invention is to provide a regenerating J ~? - 2 -,, ~375~7~

method of molten 31ag in qteel~making process ~ithout any additional equipment.
Another object of this invention is to provide a re-yenerating method of molten slags in a steel-ma~ing process, which is most suitable for such uses as a source of lime, a raw material of cement, a material of road bed and an ag~regate of cement, etc.
In accordance with the invention, there is provided a method of regenerating molten slags in a steel-making process, in other words a method in wnich the composition of the molten slag is changed so that the composition of the slag may be varied according to the intended use thereof.
The varying of the composition of the slags and the regenexating of the slags may be accomplished in a conv~rter.
Alternatively, the molten slags may be transferred to any other vessel having the same facilities as those of a converter and then treated therein.
According to the invention there is provided a method of changing the composition of molten slag of a steel making process, comprising the steps of adding solid carbon and ~ux to molten slag remaining in a converter or furnace and blowing oxygen into said molten slag.
In a particular embodiment of the invention there is provided a proce~s of changing the composition of molten slag, comprising the steps of (A) tapping molten steel from a con-verter to separate said molten steel from molten slag t~ereby substantially leaving molten slag remaining therein, said slag comprising CaO, SiO2, MgO, MnO, FeO and P205, (B) adding solid carbon and flux to said molten slag remaining in said converter; (C) blowing oxygen onto said carbon and flux on the surface of said molten slag to generate exothermic reaction and compensate for heat loss caused by said addition of carbon - 3 ~

~6~7S~7~L

and flux without addition of any external heat, whereby said molten slag is regenerated to increase the amount of said CaO, SiO2, MgO and to decrease the amol~nt of MnO, Fe and P205, and (D) removing said regenerated slag from said converter and thereby to prepare sald converter for another steel making charge.
The carbon is added in an amount effective to reduce oxides in the slag, and the flux is added in an amount effective to maintain the fluidity of the slag.

Other objects and advantages will be apparent from the following description.
The development of this invention star-ted from thorough investigations of the possibility of obtaining regenerated slags corresponding in composition to the intended uses, in the steel-making furnace, e.g. specifically in the converter itself, based on the consideration that the above-mentioned prior art suffers in many respects, and above all the process is expensive.
The reason for utilization of the converter itself in accordance with the invention is that there is a time to spare from a tapping ; 20 operation to a following operation, and this time can be used to - regenerate the slags. In addition, the transferring of molten slags to another vessel is avoidable. ~eedless to say, slag in the converter ray be treated as it is and this is highly ,~

'` :

75~7~

Idvantageous. The reason resides in possibility of e~lploying the exis-ting converter facilities as they stand. When such a method is realized, there would be no need for additional and auxiliary facilities. Briefly speaking, it may be noted that this is a regeneratirlg method for steel-making slags which does not affect the productivity of the conver-ter in i-ts furnace operations and it does not introduce factors for increasing costs in the furnace facilities. The invention resides in providing such a method.
Data from the above-mentioned investigation reveals the following:
When the temperature of the steel-making slags is more than 1500C and a sufficient amount of (C) is present, (FeO), (P205) and (MnO), among the oxides of the slags, are reduced to (Fe), (P) and (Mn) by the following endothermic formula (1) and, at the same time, an exothermic reaction as per formula (2):
(FeO, P205, MnO) + C = Fe, P, Mn + co¦ ............. (1) 2C + 2 = 2CO ..,,.. ,,... ,.,,.. (2) 2CO + O - 2CO
The measures, according to the invention, to promote the above-mentioned reaction are as follows:
First, solid carbon and flux for raising the fluidityof the slags are added to the slags in the furnace, secondly, oxygen is blown at the slags for raising its temperature to more than 1500C and for speeding the reaction by agitating the slags.
In this case, the addition of the carbon and flux and also the blowing of oxygen are accvmplished by the existing facilities as they stand. Accordingly there is no necessity for providing facilities.
The heat loss of the endothermic reaction is com-pensated for by the combustion of either the added carbon (C)or (CO) produced by -formula (1) with the blown oxygen as shown in formula (2). Thus, as to the solid carbon playing an

- 4 -.

~547~

important part il~ this invention, ordinary coke can be employed as well. EJowever, some consideration for the size of the coke is required. That is, the size o-f coke should be prepared in less than 30 m/m, preferably about less than 10 m/m, corresponding to the capacity of an employed furnace. The above blowing of oxygen is carried out through an ordinary lance. The reasons for the blowing of oxygen are to have the molten slag come into contact with the added solid carbon by the agitation, and to bring about the combustion as shown in the formula (2). By these measures, the reactions indicated by formulas (1) and (2) are easily and stably advanced. Consequently, the melting point of the slag itself rises as the slag becomes reduced and its fluidity becomes lowered. This conversely brings about a check of the reaction itself. Flux is added in this invention to lessen the above lowering of the fluidity. Any of the known fluxes, e.g. alumina, cryolite, fluorite and others can be employed and selected in consideration of intended uses after regeneration~ A controlling agent of slag composition is further added as required. This is to vary the composition corresponding to future intended uses, not only simply regeneration of the slags.
An actual example as carried out in a pilot plant of a converter is as follows:
Elirst, after the ordinary oxygen steel-making by a 40 Kg converter was finished and the oxygen lance was raised, - the molten steel was tapped and molten slags of about 5 Kg remained as it was. Then, solid carbon of less than 2 m/m was added in the furnace successively, and the lance was pulled down to blow the oxygen. The molten slags were thereby agitated by the momentum of the blown oxygen and the reaction of the above formula (1) was advanced, and, at the same time, the combustion as per the above formula (2) was advanced. The ra-te of advance of both reactions was controlled by the nozzle shape, the number

- 5 -1~75~71 ~f nozzle holes and the height of the lance and -the amount of oxygen blown. Flux selected in accordance with future intended use was added correspondingly to lower the fluidity of the slags with the advancement of the reducing reaction. The (CO) gas genera-ted by reaction as per formula (1) was well fired at the upper portlon of the furnace so that the decrease in slag tcmperature after regeneration was only about 20C from the tapping temperature.
Table I shows the testing requirements and Table II
and III are results.

Table I Test Re uirements q Initial molten slag about 5000g Solid carbon 500g Flux cryolite 250g Amount of oxygen 100 Nl/min Regenerating time 5 min ,, Table II Variation in the Composition and Weight of the Slag along the Reaction Time CaO, SiO2 Al2O3 MgO
) _ ( % ) ( / ) ( % ) initial slag 52.20 15.31 0u89 2.85 after 2 min. 59.96 18.37 2.48 5.25 after 4 min. 63.49 19.32 3.03 5.58 after 5 min. 65.36 18.71 2.68 5.59 regenerated slag 65.41 18.75 2.70 5.61 MnO T.Fe P205 S weight temperature (%) (%) (%) (%) (g) (C) 3.85 15.302.18 0.11 about 1680 0.13 4.85 0.75 0.49 0 2.33 0.27 0.17 0 2.43 0.26 0~20 1660 0 2.38 0.19 0.21 4080 ~ J~ - 6 . .

~1)75~7~
Table III Weight and composition of recovered Fe weigh-t C Si Mn P S
(g) (%) (%) (%) (%) (%) 9~0 ~.95 0.16 13.0 2.57 0.003 -According to the above-mentioned Tables, it will be seen that the oxides of the slags are well reduced. That is, (FeO) is reduced by about 90%. This reduced (Fe) is precipitated and collected at the bottom of slags. The reducing of (MnO) reaches 100% and the reduced (Mn) transfers into the above molten (Fe). (P205) is reduced by about 90%. About 60% of the reduced (P) transfers into the above molten (Fe) and about 40% is vaporized.
Such a reducing rate may be easily obtained by controlling the oxygen blowing method, reaction temperature and time. In other words, the controlling of the above-mentioned requirements i5 varied and selected corresponding to the intended use of the regenerated slags. For example, in the case where the slag is to be employed as a source of lime for a blast furnace, the (P) concentration shown in the above Table II is further lowered below 0.1% and the decrease is easily achieved. In the case where the slag is to be employed as material for cement, a controlling agent of the slag composition is further added. Also, in cases of other use such as slag wool, aggregate or fillers in civil engineering and the construction field, the most suitable composition may be obtained by the above-mentioned control method, without any difficulty and weakness.
It is needless to say that the regeneration of slags is achieved easily in not only the converter itself but also any other vessel having the same facilities as those of the converter.
According to the above mentioned experiments, it can be seen that the regeneration after tapping can be cornple-ted ~754~3L

~i-thin about 5 minutes. Thus, the existing time margin between tapping and the subsequent steel-making operations may be fully utilized for this. The existing time margin is available in the refining process since the casting process is rate con--trolling, not only in the production controlling of steel-making but also ln the conventional refining - casting process. It is a valuable advantage to be able to carry out a treatment while the slags remain in the converter after the tapping of the molten steel, since the conventional furnace facilities can then lQ be used as they are. It will be appreciated that the treating process, according to the present invention, is not limited to use of the furnace and facilities per se. For example, the molten slag may be moved, depending on the conditions, into a container which is provided with facilities equivalent to those of the converter for performing the regeneration of the above mentioned molten slag, and further the regeneration of the slag and the varying of the composition thereof as shown in the experiments, is easily accomplished.
According to the invention, the regeneration and varying ~ -are not only performed easily, but also yield the by-product Fe in considerable amounts in the initial slag in the converter so as to reduce the requirement of obtaining Fe from other sources.
In addition, less waste materials have to be dumped reducing `~ dumping space requirements. Thus, the present invention has many industrially useful advantages.
.

, - 8 -. . .

Claims

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

1. A method of changing the composition of molten slag of a steel making process, comprising the steps of adding solid carbon and flux to molten slag remaining in a converter or furnace, said carbon being added in an amount effective to reduce oxides in the slag and said flux being added in an amount effective to maintain the fluidity of the slag, and blowing oxygen into said molten slag.

2. A method according to claim 1, wherein after said blowing, the molten slag is heated to a temperature of more than 1500°C and the slag is stirred.

3. The method of claim 1 or 2, wherein said steps are carried out in said converter or furnace between the time molten steel is tapped and the next steel charge is made.

4. The method of claim 1, wherein said solid carbon is coke and is of a size less than 30 m/m.

5. The method of claim 2, wherein said solid carbon is coke and is of a size less than 30 m/m.

6. The method of claim 1 or 2, wherein said solid carbon is coke and is of a size less than 10 m/m.

7. The method of claim 1, 2 or 4, wherein said molten slag is transferred to another vessel prior to adding of said solid carbon.

8. The method of claim 1, 2 or 5, wherein said flux is alumina.

9. The method of claim 1, 2 or 5, wherein said flux is cryolite.

10. The method of claim 1, 2 or 5, wherein said flux is fluorite.

11. A process of changing the composition of molten slag, comprising the steps of (A) tapping molten steel from a converter to separate said molten steel from molten slag thereby sub-stantially leaving molten slag remaining therein, said slag comprising CaO, SiO2, MgO, MnO, FeO and P2O5;
(B) adding solid carbon and flux to said molten slag remaining in said convertor, said carbon being added in an amount effective to reduce oxides in the slag and said flux being added in an amount effective to maintain the fluidity of the slay;
(C) blowing oxygen onto said carbon and flux on the surface of said molten slag to generate exothermic reaction and compensate for heat loss caused by said addition of carbon and flux without addition of any external heat, whereby said molten slag is regenerated to increase the amount of said CaO, SiO2, MgO and to decrease the amount of MnO, Fe and P2O5; and (D) removing said regenerated slag from said con-verter and thereby to prepare said converter for another steel making charge.

12. The method of claim 11, wherein said molten slag has added thereto a controlling agent comprising Al2O3 or SiO2 after said regeneration.

13. The method of claim 11, wherein said flux is selected from the group consisting of alumina, cryolite and fluorite.

14. The method of claim 11, 12 or 13, wherein said solid carbon is coke and is of a size less than 30 mm.

15. The method of claim 11, 12 or 13, wherein said solid carbon is coke and is of a size less than 10 mm.