US3807988A

US3807988A - Refining hematite pig iron in a converter

Info

Publication number: US3807988A
Application number: US00224746A
Authority: US
Inventors: P Nilles
Original assignee: Metallureiques Ct Voor Res In
Current assignee: Centre de Recherches Metallurgiques CRM ASBL
Priority date: 1971-02-10
Filing date: 1972-02-09
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30
Also published as: NL7201589A; BE778603A; DE2206081C3; FR2124428A1; DE2206081B2; FR2124428B1; IT948907B; DE2206081A1; CA958896A

Abstract

A gas consisting mainly of commercial oxygen is blown in by a tuyere or tuyeres in the bottom or side wall of the converter under the surface of the molten metal. At the beginning of the refining operation, a quantity of lime is introduced through the mouth of the converter. The quantity is lower than 12 kg/kg Si in the pig iron. The total surface of the lime is at least 45 m2/t pig iron, and a part of the lime has a specific surface higher than 2.5 m2/kg.

Description

United States Patent Nilles Apr. 30, 1974 REFINING HEMATITE PIG IRON INA 2,950,186 I 8/1960 Allard 75/60 CONVERTER 2,855,293 10/1958 Savard 75/60 1,145,506 7/1915 Pasquier 75/52 Inventor: Paul Emlle Nllles, g, 2,918,365 12/1959 Kanamori 75/53 Belgium 2,864,689 12/1958 Perrin 75/59 [73] Assign e: C n r De Recherches FOREIGN PATENTS OR APPLICATIONS Metallureiques-Centrum Voor 553 120 4 1956 I 1 75 52 Research In De Metallurgie ta y Brussels Belglum Primary ExaminerL. DeWayne Rutledge [22] Filed: Feb. 9, 1972 Assistant Examiner-Peter D. Rosenberg [2]] App] No 224 746 Attorney, Agent, or FirmH0lman & Stern [30] Foreign Application Priority Data [57] ABS CT Feb. 10, 1971 Luxembourg 62570 A gas consisting mainly 0f Commercial Xyge n is Apr 7, 1971 Luxembourg 62944 blown in by a tuyere myeres in the bottom or Side wall of the converter under the surface of the molten 52 us. c1. 75/52, 75/60 rnetel- Al the beginning of the refining operation, a 51 Int. Cl. C2lc 7/00, 0210 5/34 q y of lime is introduced through the month of 58 Field of Search 75/52-60 the eenvener- The quantity is lower than 12 g Si in the pig iron. The total surface of the lime is at least 5 References Cited 45 m /t pig iron, and a part of the lime has a specific surface higher than 2.5 m /kg.

15 Claims, 1 Drawing Figure REFINING HEMATITE PIG IRON IN A CONVERTER The present invention concerns improvements in methods for refining hematite pig iron in a converter by means of an oxygen-containing gas blown in through the bottom or through the lateral wall of the converter through at least one tuyere situated under the surface of the bath of metal to be refined.

It is well known that, in refining a hematite pig iron by means of an oxidizing gas, the problem of the correct control of the development of the entire'refining operation becomes more difficult to solve satisfactorily the richer the oxidizing gas is in oxygen. One obvious reason for this is that the local speed of certain reactions and the temperatures developed at certain places in the bath are in direct proportion to this oxygen content.

It is also known that the correct development of a refining operation for hematite pig iron, particularly in refining by means of commercially pure oxygen, is bound up with a definitely pre-arranged time development of the composition of the slag. Thus, the composition of the slag, for instance, has a great effect on its participation in the decarburization reaction and consequently on the development of this reaction.

Moreover, it is known that the spattering of sparks and the overflowing of slag, in themselves quite clearly prejudicial tothe safety of the operators and to the yield of iron obtained from the refining operation, can be reduced by suitable development of the composition of the slag during the refining operation; in addition, the viscosity of the slag has an influence on the tendency of the slag to foam and overflow. Thusit is evidently important to have effective methods for controlling this development.

When one intends to carry out a refining operation of hematite pig iron by blowing oxygenthrough the bottom or through the lateral wall of the converter in the conventional manner, one must bear in mind that in practice there is only one single parameter of the operation that one can use for control. The reason for this is simple: the oxygen blown under the surface of the bath of molten metal to be refined penetrates directly into the bath before passing in part into the slag. Accordingly there is no possibility of transferring at will onto the slag a given proportion of the oxygen blown in, and of acting on its formation. The only parameter of operation that one has available is the rate of flow of oxygen blown in, and this can have only a very limited influence on the distribution of the oxygen between the metal bath and the slag. The natural consequence of this state of affairs is that one is forced to accept the development of the composition of the slag rather than be able to control it, which constitutes a considerable disadvantage. Theabsence of iron in the slag is notably at the source of difficulties in dissolution of the lime in the slag.

Various attempts have already been made toovercome this disadvantage by attempting to use another variable parameter. It has already been proposed in particular to put finely divided lime in suspension in the refining gas blown into the bath of metal through the bottom of the converter. When this was done considerable wear occurred in the tuyeres; furthermore, the transport of the powder and its distribution among the various tuyeres gave rise to serious difficulties.

The addition of lime in powder form to the top of a converter during blowing has always been considered ineffective because the powdered lime was carried away by the converter gases before reaching the bath of metal. It was not possible to achieve success without propelling the lime particles at high speed, which is what takes place in top blowing methods such as the LD-AC process.

It was also proposed to add lime in pieces to the bath of metal through the mouth of the converter (the pieces being of sufficient size to reach the bath without there being any danger of them being carried away by the converter gases); it was then found that during blowing considerable spatterings of slag and metal occurred, which prejudiced the economy of the method and extended the duration of the refining operation unacceptably.

In contrast to what current opinion has considered to be well established, the applicant has found that, in a particularly remarkable manner and quite unexpectedly, if the refining operation is effected with the blowing of commercially pure oxygen through the bottom or the lateral wall of the converter under the surface of the metallic bath, one can rapidly obtain a quite fluid and reactive slag which does not give rise to any spattering or overflowing, by introducing through the mouth of the converter before the start of the refining operation or soon after the start (preferably before the start)a quantity of lime of predetermined granulometry, provided the quantity of steel produced in the converter is taken into account.

Accordingly, the present invention provides a method of refining hematite pig iron in a converter comprising the refining operation of blowing in a gas consisting mainly of commercially pure oxygen through at least one tuyere opening out under the surface of the molten metal, the method including introducing through the mouth of the converter at the beginning of the refining operation a quantity of lime lower than 12 kg per kilogram of silicon initially contained in the pig iron, the granulometry of the quantity of lime being such that itstotal surface is at least 45 m per metric ton of pig iron, and a part of the quantity of lime being fine lime having a specific surface higher than 2.5m kg.

The best results are obtained when the lime is introduced before the start of the refining (oxygen blowing) operation, but the results are still excellent if the addition takes place just after the start of the blowing operation, when the decarburization rate has not yet reached a quarter of its maximum value.

Specific surface means the sum of the external surfaces of all the pieces contained in a kilogram of lime, all the pieces being assumed to be spherical. In practice, the lime used is assumed to be constituted of small spheres of a diameter equal to the mean granulometry excluding abrasion fines.

The sole FIGURE of the accompanying drawing is a graph of specific surface against average granulometry.

For limes of known average granulometry the specific surface can be determined for instance from the graph shown in the FIGURE, in which y is the specific surface (m /kg) and x is the average granulometry of the lime (mm). The graph plotted is a hyperbola whose equation is approximately xy 6.

The total surface per metric ton of pig iron is equal to the sum, over all the categories of lime used, of the specific surface multiplied by the corresponding amount of lime per metric ton of pig iron for each category of lime. For comparison, one can mention that in the LD process it is customary for the lime supplied to be of granulometry 20/40mm, which corresponds to a specific surface of approximately 0.2m /kg; if one introduces 60 kg of lime per metric ton of pig, iron, the total surface is equal to 0.2 X 60 l2m /t pig iron. ln the basic Bessemer or Thomas process the customary lime granulometry is /60 which corresponds to a specific surface of 0.l7m /kg.

ln proceeding in accordance with the invention, it is observed, contrary to what has already been taken as fact, that the lime with high specific surface (more than 2.5m /kg) is not carried away by the converter gases (fumes). On the contrary, under the conditions defined above, one finds that the assimilation of the very fine lime takes place before the lime has been able to be carried away by the fumes.

In accordance with an advantageous variation of the invention, the lime which constitutes the fine fraction has an average granulometry not exceeding lmm, the remainder being constituted of pieces of granulometry larger than that of the lime added at the beginning, and preferably of a granulometry of at least mm. This variation has the advantage of using a considerable part of the lime in an economical form.

In accordance with another even more advantageous variation, one uses rich lime, that is lime having a granulometry the major part of which does not exceed 0.02mm. With this variation it is possible to use a minimum quantity of lime with high specific surface.

In the case where particularly fine lime is added before the start of the blowing operation it is particularly advantageous to carry out this addition by means of lime in bags. 0

Another advantageous variation consists in effecting the deposition of the lime in powder form on the pig iron by pneumatic means. This variation is also very suitable for adding, at the beginning of oxygen blowing, lime with a granulometry which is for instance lower than lmm. The powdered lime may be fluidized with air and conducted into the mouth of the converter.

Again, it is advantageous for the quantity of lime of high specific surface (fine lime) that is lime whose specific surface is greater than 2.5m /kg, (which quantity is preferably between 0.5 and 5 kg per kg of silicon initially contained in the pig iron) to be linked to the specific volume, V, of the converter.

By specific volume, V, there should be understood the ratio between the internal volume of the converter (immediately after construction or re-lining) and the average of steel tapped into the ladle from the converter per cast (blow).

More specifically, it is recommended that for a given silicon content of the pig iron the minimum quantity Q of lime with high specific surface will be higher the lower the specific volume is.

This minimum quantity of lime of high specific surface is thus determined by an equation of the type: Q =f(i, V), in which i represents the initial silicon content of the pig iron charged into the furnace. This quantity Q, of fine lime can be greater than Q though under these conditions the operation will not be so economically bearing in mind the cost of the products.

The ratio P of the quantity Q of lime with high specific surface (fine lime) to the total quantity of lime introduced into the furnace is accordingly a function of the same factors i, V. In general, P 2 V 10 i.

In this equation, P is expressed in percent, V in M /t of steel, and i represents in weight percent the silicon content of the pig iron.

A particularly interesting example of the method of the invention is one in which: a. refining is effected by means of a gas consisting mainly of commercial oxygen, blown into the converter under the surface of the bath (for instance through the walls or the bottom) by means of at least one tuyere with two coaxial tubes, the internal tube being for the injection of the refining gas, while the peripheral tube is intended for blowing a gaseous or liquid protective fluid, i.e., a fluid with endothermic dissociation, for instance a hydrocarbon; b. at the very beginning of the refining operation, and preferably before its commencement, one introduces through the mouth of the converter a quantity of lime less than 12 kg per kg of silicon initially contained in the pig iron; the granulometry of this lime is such that the total surface (assuming the pieces to be spherical) is higher than 45m per metric ton of pig iron, and a portion of this lime (between 0.5 and 5 kg per kg of silicon initially contained in the pig iron) has a specific surface higher than 2.5m /kg of lime; c. the total quantity of pig iron, scrap, and possibly ores, charged into the converter before the beginning or during the course of the refining operation is such that the specific volume is between 0.40 and 0.90 m metric ton of steel produced; and d. the ratio P between the quantity of lime with specific surface higher than 2.5m /kg of lime and the total quantity of lime increases (for a given silicon content of the pig iron) as the specific volume of the ladle decreases.

It has for instance been found that condition ((1) can be expressed by the relationship where V is expressed in m /t of steel obtained in i represents the silicon content of the pig iron in weight percent.

It should be pointed out here that the method does not involve the injection of lime directly into or onto the bath of molten metal.

As already mentioned above, it has surprisingly and unpredictably been found that a hematite pig iron refining operation carried out as above enables a fluid and reactive slag to be obtained rapidly without spattering or overflowing, and consequently a quality steel under advantageous conditions of time and economy.

The addition of lime to the converter in the form of pieces can be effected in the conventional manner, for instance by means of a hopper, the addition of lime of granulometry below 2 mm preferably taking place either in sacks or by pneumatic means, excluding any injection of lime onto or into the metal.

The lime introduced before the blowing operation or at the beginning of the blowing operation, can be mixed with constituents such as fluidizers (spath fluor, alumina), cooling agents (ores, soda, scale), and so on, these constituents preferably having a granulometry similar to that of the lime.

One can also add compounds which melt more easily than the lime, in particular lime ferrites, to take the place of all or a part of the lime.

The granulometry of the lime ferrites will correspond to the indications given in.the above description. Example A.

A converter of 11.3 in useful volume was charged with 14.6 t (metric ton) of pig iron containing 0.55% Si, 3.2 t scrap, and 800 kg lime made up as follows:

63 percent (i.e., 504 kg lime) in powder form of average granulometry 1 mm (specific surface: 6m /kg), and

37 percent (i.e., 296 kg lime) in pieces of granulometry 20/40 mm (specific surface: approximately 0.2 m /kg).

The yield was 16.4 t good quality steel, with no losses or spattering.

Thus the quantity of lime introduced before the start or at the start of the blowing operation represents 504 kg 296 kg/l4.6 X 5.5 kg lime per kg silicon initially contained in the pig iron; the first condition is satisfied 12 kg).

The total surface of the lime is 0.2 m X 296 6 m X 504/ 14.6 or 211 m /t pig iron, satisfying the second condition 45 m P 130 150 V 10 i is also satisfied:

63 2 130 150 X l1.3/16.4 l 10 X 055 as well as the relation 0.40 V' 0.90 m lt steel produced since V 0.69.

The minimum proportion of fine lime to be added to obtain oxygen blowingwithout spattering would have been 32 percent, i. e 800 X 32/ 100= 256 kg or 3.2 kg per kg silicon initially contained in the pig iron; this minimum quantity is within the preferable range of 0.5 to 5 kg per kg silicon mentioned above.

Example B The same converter was charged with 18.5 t hematite pig iron with 0.80 percent silicon, 4.9 t scrap, and 1,400 kg lime: 700 kg in pieces (20/40 mm) and 700 kg in powder of average granulometry 2 mm (specific surface: 3 m /kg). The yield was 21.5 metric tons of steel but there 'was some spattering.

By calculating the various conditions one obtains the following results:

a. 700 700/18.5 X 8 9.5 kg of lime per kg silicon initially contained in the pig-iron (i.e. 12); ..2 m X 111 8 3 m2 X 700 e121 mute).

c. the specific surface of the fine lime is 3 m lkg (i.e., 2.5), the quantity of lime of this category being 4.7 kg/kg Si (i.e., within the preferred range);

d. the relation P( =50) 130 150 V+ 10 i is not at sfied hecayse; 1.9-. .150 ...1.1 1. /2.1; $128918? ing in a gas consisting essentially of commercially pure oxygen through at least one tuyere opening out under the surface of the molten metal, the method including depositing on the surface of the molten metal through the mouth of the converter a quantity of lime which is between 0.5 and 12 kg per kilogram of silicon initially contained in the charge of pig iron, the granulometry of the quantity of lime being such that its total surface is at least 45m per metric ton of pig iron, and a part of the quantity of lime in the amount of 0.5 to 5 kg per kilogram of silicon initially contained in the pig iron being fine lime having a specific surface greater than 2.5m /kg of lime, the quantity of lime being introduced after the molten metal has been introduced and before the decarburization rate has reached one-quarter of its maximum value.

2. A method as claimed in claim 1, in which the quantity of lime is introduced before the start of blowing.

3. A method as claimed in claim 1, in which the fine lime has an average granulometry of at most 1 mm, the remainder of the quantity of lime consisting of pieces having a larger granulometry than that of the fine lime.

4. A method as claimed in claim 3, in which the granulometry of the remainder is at least 15 mm.

5. A method as claimed in claim 1, in which the quantity of lime comprises rich lime having a granulometry the major part of which does not exceed 0.02 mm.

6. A method as claimed in claim 1, including introducing the quantity of lime in bags.

7. A method as claimed in claim 1, including introducing the quantity of lime pneumatically.

8. A method as claimed in claim 1, in which the amount of fine lime is a function of the specific volume of the converter.

9. A method as claimed in claim 8, including increasing the amount of fine lime when the specific volume is reduced, and decreasing the amount when the specific volume is increased.

10. A method as claimed in claim 1, in which the proportion of fine lime in the said quantity of lime obeys the following relationship:

where P is the proportion of fine lime in the said quantity of lime, in percent by weight, V is the specific volume of the converter in m lmetric ton of steel produced, and i is the silicon content of the pig iron charged into the converter, in percent by weight.

11. A method as claimed in claim '1, in which at least part of the lime is introduced in the form of a lime compound which melts more easily than uncombined lime.

12. A method as claimed in claim 11, in which the compound is a lime ferrite.

13. A method as claimed in claim 1, in which the tuyere comprises an inner tube and an outer tube encompassing the inner tube, the method including injecting the oxygen-containing gas through the inner tube and injecting a fluid which dissociates endothermically, through the outer tube.

14. A method as claimed in claim 1, including introducing into the converter at least one fluidizer selected from the group consisting of spath fluor and alumina.

15. A method as claimed in claim 1, including introducing into the converter at least one cooling agent selected from the group consisting of iron ore, soda and scale.

Claims

2. A method as claimed in claim 1, in which the quantity of lime is introduced before the start of blowing.
3. A method as claimed in claim 1, in which the fine lime has an average granulometry of at most 1 mm, the remainder of the quantity of lime consisting of pieces having a larger granulometry than that of the fine lime.
4. A method as claimed in claim 3, in which the granulometry of the remainder is at least 15 mm.
5. A method as claimed in claim 1, in which the quantity of lime comprises rich lime having a granulometry the major part of which does not exceed 0.02 mm.
6. A method as claimed in claim 1, including introducing the quantity of lime in bags.
7. A method as claimed in claim 1, including introducing the quantity of lime pneumatically.
8. A method as claimed in claim 1, in which the amount of fine lime is a function of the specific volume of the converter.
9. A method as claimed in claim 8, including increasing the amount of fine lime when the specific volume is reduced, and decreasing the amount when the specific volume is increased.
10. A method as claimed in claim 1, in which the proportion of fine lime in the said quantity of lime obeys the following relationship: P > or = 130 - 150 V + 10i, where P is the proportion of fine lime in the said quantity of lime, in percent by weight, V is the specific volume of the converter in m3/metric ton of steel produced, and i is the silicon content of the pig iron charged into the converter, in percent by weight.
11. A method as claimed in claim 1, in which at least part of the lime is introduced in the form of a lime compound which melts more easily than uncombined lime.
12. A method as claimed in claim 11, in which the compound is a lime ferrite.
13. A method as claimed in claim 1, in which the tuyere comprises an inner tube and an outer tube encompassing the inner tube, the method including injecting the oxygen-containing gas through the inner tube and injecting a fluid which dissociates endothermically, through the outer tube.
14. A method as claimed in claim 1, including introducing into the converter at least one fluidizer selected from the group consisting of spath fluor and alumina.
15. A method as claimed in claim 1, including introducing into the converter at least one cooling agent selected from the group consisting of iron ore, soda and scale.