EP0031776B1 - Combined blowing method for refining metals in a converter - Google Patents

Description

The present invention relates to a process for introducing refining gas and brewing gas into metal refining converters, and more particularly into steelworks converters.

For refining cast iron into converters, pure oxygen blowing is generally used, which can be done in two distinct ways:

• - either from top to bottom by means of a lance cooled with water and introduced by the nozzle of the converter,
• - either from bottom to top, vertically or obliquely, by means of nozzles with two or more concentric tubes, generally arranged in the refractory bottom of the converter, and protected against excessively rapid wear by a peripheral protective agent introduced into the outer tube of each nozzle.

• (a) Une production d'abondantes fumées rousses d'oxydes de fer pendant toute la durée de la conversion, ce qui constitue une perte en fer non négligeable;
• (b) Un brassage du bain métallique dû exclusivement à la décarburation localisée dans la zone de réaction, et sur l'intensité duquel il est impossible d'agir autrement que par le biais de la vitesse de décarburation, qui est fonction du débit d'oxygène soufflé, lequel doit respecter d'autre part un certain nombre de contraintes. Ce brassage du bain par la seule décarburation limite les possibilités d'action sur le déroulement de la conversion. En particulier un déséquilibre physico-chimique, parfois important, peut se produire, à certains moments de l'opération, entre la teneur en oxyde de fer du laitier et la teneur en carbone du bain métallique, avec un certain nombre de conséquences fâcheuses, bien connues des aciéristes. De même, le manque de brassage en fin d'affinage, aux basses teneurs en carbone, empêche de maîtriser complètement la fin de l'opération, et donc de réaliser avec précision toutes les caractéristiques de l'acier recherchées au convertisseur. Il faut alors y remédier par des actions métallurgiques supplémentaires en aval du convertisseur.

The blowing of pure oxygen by lance, from top to bottom, in vertical converters, presents, among others, the following two disadvantages:

• (a) Production of abundant reddish fumes of iron oxides during the entire conversion period, which constitutes a significant loss of iron;
• (b) A stirring of the metal bath due exclusively to the decarburization located in the reaction zone, and on the intensity of which it is impossible to act other than by means of the decarburization speed, which is a function of the flow rate of blown oxygen, which must also respect a certain number of constraints. This mixing of the bath by decarburization alone limits the possibilities of action on the course of the conversion. In particular a physico-chemical imbalance, sometimes significant, can occur, at certain times of the operation, between the content of iron oxide in the slag and the carbon content of the metal bath, with a number of unfortunate consequences, although known to steelmakers. Similarly, the lack of stirring at the end of refining, at low carbon contents, prevents complete control of the end of the operation, and therefore accurately achieving all the characteristics of the steel sought in the converter. This must be remedied by additional metallurgical actions downstream of the converter.

In this type of oxygen blowing by lance, from top to bottom, no attempt has been made in practice to reduce the large quantity of red fumes emitted (which we have only captured and purified), but certain steelmakers have attempted to cause mixing of the bath and the slag by an additional means, consisting in placing in the solid bottom of the converter porous refractory plugs, through which a neutral gas, such as nitrogen or argon for example, during most of the oxygen blowing by the lance. This additional means gives interesting results, such as good mixing of the bath, but has certain disadvantages which are specific to it:

• . First of all, such porous plugs can only blow neutral gases, because oxidizing gases, and a fortiori pure oxygen, would cause much too rapid wear of the porous refractory.
• . In the choice of neutral brewing gas, we often hesitate between nitrogen, a cheap gas, but which nitrides the bath and deteriorates the calorific value of the converter gases, and argon, which does not have the first of these. disadvantages, but that is expensive.
• . Even with cheap nitrogen, the cost of consuming neutral gas is not negligible.
• . With a neutral gas, the wear of the porous refractory plugs is slow enough for this blowing mode to be usable in practice, but this wear is nevertheless still fast enough so that the life of these plugs is not always as long as that of the solid bottom and the refractory lining of the top blown converter.

In summary, this mixed blowing mode, which offers advantages, also has significant limitations.

The second type of pure oxygen blowing in a steelworks converter consists in using nozzles, vertical or oblique, arranged in the refractory bottom of the converter, blowing from bottom to top, and made up of at least two concentric tubes, the (or) inner tube blowing an oxidizing gas, which can be pure oxygen, the outer tube being traversed by a protective agent of the nozzle against its wear in service.

• . Une moindre quantité de fumées rousses d'oxydes de fer émises pendant le soufflage.
• . Un excellent brassage du bain et du laitier pendant la quasi-totalité de l'opération, à l'exception toutefois des dernières secondes, lorsque le bain atteint de basses teneurs en carbone. On y a d'ailleurs remédié par un très court brassage final par un gaz neutre remplaçant l'oxygène dans les tuyères, azote le plus souvent, argon plus rarement. Ce brassage final, très court, ne présente aucun des inconvénients cités ci-dessus pour les bouchons poreux.

This second type of blowing of pure oxygen in a steelworks converter, from bottom to top, has several advantages over blowing by lance, from top to bottom, among which it is important to cite the following two:

• . A lesser amount of red iron oxide fumes emitted during blowing.
• . Excellent mixing of the bath and the slag during almost the entire operation, with the exception, however, of the last seconds, when the bath reaches low carbon contents. This was moreover remedied by a very short final mixing with a neutral gas replacing the oxygen in the nozzles, nitrogen more often, argon more rarely. This very short final stirring has none of the drawbacks mentioned above for porous plugs.

• (a) la durée de vie des fonds munis des tuyères de soufflage, bien qu'en cours d'amélioration permanente, reste encore inférieure à la durée de vie des revêtements réfractaires latéraux du convertisseur.
• (b) Une certaine avance de la déphosphoration par rapport à la décarburation est plus difficile, et parfois impossible, à réaliser.

However, full blowing from the bottom has two disadvantages compared to blowing with a lance:

• (a) the life of the bottoms fitted with the blowing nozzles, although in the process of permanent improvement, is still less than the life of the lateral refractory linings of the converter.
• (b) A certain advance in dephosphorization compared to decarburization is more difficult, and sometimes impossible, to achieve.

Then the idea came to associate on the same converter blowing by lance, and blowing by the bottom nozzles, in order to benefit from the advantages of the two types of blowing, hoping to minimize their disadvantages. In this case, it has even been recommended to blow oxygen through the bottom nozzles under increasing pressure as the carbon content of the bath decreases.

But if the disadvantages of blowing by lance were thus reduced, if not even - for the lack of stirring for example - removed, the disadvantage of a holding of the nozzle base less good than that of the lateral refractory lining remained. This drawback was even especially marked in the case of an oxygen blowing at increasing pressure in the bottom nozzles, since the rate of wear of the nozzles increases at the low carbon contents of the bath at given oxygen flow rate, and even more with increased throughput.

The object of the present invention is to produce a mixed blowing, simultaneously from above and from below, which, combining the advantages of blowing by lance and blowing by nozzles, also makes it possible to significantly improve the life of the bottoms to nozzles thus used in such a mixed blowing.

To this end, the subject of the present invention is a process for blowing oxidizing gases, and in particular pure oxygen, for refining metals, and. more specifically for the refining of cast iron in a converter, by means of a hand from a lance blowing from top to bottom, and simultaneously on the other hand by means of protected nozzles blowing, vertically or obliquely, bottom to top, characterized both in that the quantity of oxygen blown from bottom to top by the nozzles is between 3% and 25% of the total quantity of oxygen necessary for refining the metal bath, in this that this oxygen is blown from bottom to top at practically constant flow or decreasing flow, that is to say at non-increasing flow, in that one can add to this oxygen blown from bottom to top a stirring gas, neutral or oxidant, the flow rate of which varies according to the various stages of blowing and may even be zero at certain times, so that the total amount of oxygen blown from bottom to top by the nozzles remains between 3% and 25% of the total amount of oxygen necessary for refining the metal bath, the total flow of oxygen thus blown through the nozzles remaining non-increasing, and in that the jets of oxygen blown from the bottom up, mixed or not with a stirring gas, have a diameter at most equal to 18 millimeters, and preferably at most equal to 12 millimeters, this diameter of the jets being considered at the outlet of the nozzles.

The brewing gas can be a neutral gas, such as nitrogen or argon, or a mixture of argon, that is to say, argon containing a little oxygen and not containing all other than trace gases.

This brewing gas can also be an oxidizing gas, such as carbon dioxide or water vapor, the dissociation products of which other than oxygen (carbon monoxide for the first, hydrogen for the second) cause an effect. bath brewing.

According to a particular characteristic of the invention, the flow rate of oxygen blown through the nozzles is kept constant throughout the duration of the blowing.

According to another particular characteristic of the invention, the flow of oxygen blown through the nozzles is at a decreasing flow throughout the duration of the blowing or only from a certain time. It can also be decreasing first, and constant thereafter.

According to another particular characteristic of the invention, there is added to the oxygen blown by the nozzles a stirring gas, and this for two periods: a first period of a few minutes at the critical moment of the decarburization projections which may be located in the second quarter of high phosphorus cast iron conversion, and in the third quarter into hematite cast iron; and a second period, quite short, of one to three minutes, towards the end of the blowing, at low carbon contents, period during which the instantaneous rate of wear of the nozzles increases rapidly if one blows from the pure oxygen.

In this second period, at the end of blowing, it is possible to introduce the stirring gas preferably at increasing flow rate.

If, in this final period, the flow rate of oxygen blown through the nozzles is adjusted decreasingly, it may be advisable to compensate for the decrease in the flow rate of oxygen by an equal increase in the flow rate of the mixing gas, so that the Total gas flow blown from the nozzles from bottom to top remains constant.

According to a particular characteristic of the present invention, the jets of oxygen blown from bottom to top have a diameter at most equal to 18 mm, and preferably at most equal to 12 mm, at the outlet of the nozzles. It is understood that these nozzles are protected against wear in a known manner by a protective peripheral agent, such as a hydrocarbon gas, or fuel oil, or water vapor, or car dioxide. gaseous or liquid bone, etc. By comparison, it should be recalled here that the nozzles usually used in the processes of integral blowing from the bottom have a passage diameter for oxygen generally between 28 mm and 36 mm.

The protective agent of the nozzles against wear possibly participates in the stirring of the bath but only to a small extent, because the flow rate of the protective agent is always very low compared to the flow rate of blown oxygen.

According to another particular characteristic of the invention, the total amount of oxygen blown from bottom to top by the nozzles is between 3% and 10% of the total amount of oxygen necessary for refining the metal bath.

According to another particular characteristic of the invention, the pulverulent materials, such as lime powder or limestone powder, to be introduced into the metal bath, are introduced in suspension in the oxygen of the lance, and as the latter does not represents that a fraction of the total oxygen necessary for the complete refining of the metal bath, the concentration of pulverulent matters in oxygen can be notably higher than in the processes of integral blowing by lance, which contributes, by cooling the oxygen reaction zone of the lance in the bath, to reduce the quantity of red fumes due to this oxygen.

According to another particular characteristic of the invention, the flow rate of oxygen blown from bottom to top, and, optionally, the flow rate of additional stirring gas, are regulated at each instant as a function of the oxidation state of the slag, evaluated either by an overall estimate from known elements, or by dosing of samples taken by a sublance, with measurement of the bath temperature, or by means of a device for measuring the intensity of the sound produced by the converter , in order to control at all times the difference in the balance between slag and metal bath.

• 1-Suivant la composition de la fonte, on peut accroire ou diminuer, donc moduler, à chaque instant du soufflage, la proportion d'oxygène soufflé par la lance par rapport à l'oxygène soufflé par les tuyères, ce qui introduit une très grande souplesse de réglage de l'opération.
• 2-L'effet de surchauffe locale provoquée dans le bain par l'oxygène de la lance, et qui a pour conséquence l'émission d'abondantes fumées rousses, source de notables pertes en fer, est très sensiblement réduit pour deux motifs: d'abord la quantité d'oxygène soufflée par la lance est ici inférieure à celle qu'il faut souffler dans les procédés de soufflage intégral par lance; et ensuite, chaque mètre cube d'oxygène ainsi soufflé par la lance peut contenir une concentration en poudre de chaux ou de castine notablement plus élevée que dans les procédés de soufflage intégral par lance, si bien que l'effet refroidissant de la poudre de chaux ou, au besoin, de la poudre de castine, diminue d'autant la surchauffe de la zone de réaction en face de la lance et réduit en proportion l'émission de fumées rousses, sans que le bilan thermique total de l'opération métallurgique en soit affecté pour autant. (sauf pour la castine).
• 3-Un autre avantage important de l'invention est de rendre possible à tout moment un "brassage contrôlé" du bain métallique, soit par variation du rapport des débits d'oxygène insufflés par le haut et par le bas, car chaque mètre cube d'oxyde de carbone en provenance de l'oxygène soufflé par le bas brasse plus énergiquement le bain qu'un mètre cube d'oxyde de carbone en provenance de l'oxygène soufflé par le haut, soit par réglage de la quantité optimale de gaz de brassage, neutre ou oxydant, accompagnant l'oxygène soufflé par le bas. Disposant ainsi de deux moyens de brassage dont on est maître, on rend indépendants l'intensité du brassage et la vitesse de décarburation du bain à l'instant considéré.
  
  
  Ainsi, en soufflage intégral par lance, tout retard de décarburation provoqué par des conditions de soufflage par le haut favorisant trop la déphosphoration (forte hauteur de la lance au-dessus du bain, bonne qualité de chaux réactive, scorie fluide) provoque un déséquilibre entre le carbone du bain et l'oxyde de fer de la scorie, qui se traduit, quelques instants après l'apparition de ce déséquilibre, par des réactions violentes et par de fortes projections de scories et de métal, constituant des pertes de matière métallique.
  Le brassage selon l'invention évite l'apparition de tels déséquilibres, et son réglage optimal permet d'obtenir une mattrise complète du monde opératoire. ₁
• 4.-Un autre avantage important de l'invention, résultant d'ailleurs des avantages mentionnés ci-dessus, est de permettre une "conversion rapide", par l'emploi de forts débits d'oxygène, convenablement répartis entre le haut et le bas, et évitant néanmoins les risques de projections.
• 5.-Le choix des gaz de brassage, qui ne sont d'ailleurs pas toujours indispensables, est assez vaste: si la teneur en azote de l'acier n'a aucune importance pour la qualité de l'acier à obtenir, ou bien même si on recherche une certaine teneur en azote de l'acier, pour des usages particuliers, on utilisera de l'azote, bon marché, pour le brassage.
  Au contraire, si une basse teneur en azote est recherchée dans l'acier, on utilisera de la vapeur d'eau ou du gaz carbonique si le bilan thermique de la conversion est excédentaire, ou bien de l'argon, plus cher, si l'on entend préserver le plus possible le bilan thermique de l'opération. C'est ainsi qu'on peut utiliser, parce qu'elle est moins chère que l'argon, une "mixture d'argon", à 1 % ou 2% d'oxygène, contenant à peine quelques traces d'autres gaz, et disponible dans les centrales productrices d'oxygène. De toutes manières, les consommations de gaz de brassage restent relativement faibles, quand elles ne sont pas nulles.
• 6.-Un autre avantage essentiel de l'invention est une amélioration de la durée de vie des fonds et des tuyères par rapport à la durée de vie des fonds et des tuyères de convertisseurs à soufflage intégral de bas en haut, et cela pour trois motifs: d'abord, la quantité d'oxygène à souffler par les tuyères au cours d'une conversion est comprise entre le trentième et le quart de celle qui est nécessaire en soufflage intégral' par les tuyères, c'est à dire qu'elle est notablement plus faible; ensuite, l'emploi de tuyères de plus petit diamètre (au plus 18 mm, de préférence au plus 12 mm, au lieu de 28 mm à 36 mm dans les procédés connus de soufflage intégral par le fond) assure à ces tuyères une durée de vie supérieure, et par conséquent aussi aux fonds qui les contiennent; enfin, l'addition éventuelle de gaz de brassage, en fin de soufflage, sans aucune augmentation du débit d'oxygène, et de préférence, avec une diminution de celui- ci, peut freiner l'usure des tuyères au moment où elle est maximale en soufflage intégral par le fond.

As will be understood, the type of mixed blowing according to the present invention has many advantages, some of which are common with certain other known types of mixed blowing, and of which others are specific to the present invention:

• 1-Depending on the composition of the cast iron, one can increase or decrease, therefore modulate, at each moment of blowing, the proportion of oxygen blown by the lance compared to the oxygen blown by the nozzles, which introduces a very large flexibility in adjusting the operation.
• 2-The effect of local overheating caused in the bath by the oxygen of the lance, and which results in the emission of abundant red fumes, source of significant iron losses, is very significantly reduced for two reasons: d first the quantity of oxygen blown by the lance is here less than that which must be blown in the processes of integral blowing by lance; and then, each cubic meter of oxygen thus blown by the lance can contain a concentration of powder of lime or limestone appreciably higher than in the processes of integral blowing by lance, so that the cooling effect of the powder of lime or, if necessary, castine powder, correspondingly reduces the overheating of the reaction zone in front of the lance and reduces proportionately the emission of red fumes, without the total thermal balance of the metallurgical operation in be affected. (except for the castine).
• 3-Another important advantage of the invention is to make possible at all times a "controlled stirring" of the metal bath, either by varying the ratio of the oxygen flow rates blown from above and from below, because each cubic meter d carbon monoxide from oxygen blown from below blows the bath more vigorously than a cubic meter of carbon monoxide from oxygen blown from above, either by adjusting the optimal amount of mixing, neutral or oxidizing, accompanying the oxygen blown from below. Thus having two stirring means of which one is master, one makes independent the stirring intensity and the decarburization speed of the bath at the instant considered.
  
  
  Thus, in full blowing by lance, any delay in decarburization caused by blowing conditions from the top too favoring dephosphorization (high height of the lance above the bath, good quality of reactive lime, fluid slag) causes an imbalance between the carbon of the bath and the iron oxide of the slag, which is translated, a few moments after the appearance of this imbalance, by violent reactions and by strong projections of slag and metal, constituting losses of metallic material.
  The mixing according to the invention avoids the appearance of such imbalances, and its optimal adjustment makes it possible to obtain complete control of the operating world. ₁
• 4. Another important advantage of the invention, resulting moreover from the advantages mentioned above, is to allow a "rapid conversion", by the use of high oxygen flow rates, suitably distributed between the top and the low, and nevertheless avoiding the risk of projections.
• 5.- The choice of brewing gases, which are not always essential, is quite wide: if the nitrogen content of the steel has no importance for the quality of the steel to be obtained, or even if we are looking for a certain nitrogen content in the steel, for specific uses, we will use cheap nitrogen for brewing.
  On the contrary, if a low nitrogen content is sought in the steel, water vapor or carbon dioxide will be used if the thermal balance of the conversion is excess, or argon, more expensive, if the '' we intend to preserve the as much as possible the thermal balance of the operation. This is how we can use, because it is cheaper than argon, a "mixture of argon", with 1% or 2% oxygen, barely containing some traces of other gases, and available in oxygen producing plants. In any case, the consumption of stirring gas remains relatively low, when it is not zero.
• 6. Another essential advantage of the invention is an improvement in the life of the bottoms and the nozzles compared to the life of the bottoms and the nozzles of converters with integral blowing from bottom to top, and this for three reasons: firstly, the quantity of oxygen to be blown by the nozzles during a conversion is between the thirtieth and a quarter of that which is necessary for full blowing by the nozzles, that is to say that it is notably lower; then, the use of nozzles of smaller diameter (at most 18 mm, preferably at most 12 mm, instead of 28 mm to 36 mm in the known methods of integral blowing from the bottom) ensures these nozzles a duration of superior life, and therefore also to the funds which contain them; finally, the possible addition of stirring gas, at the end of blowing, without any increase in the oxygen flow rate, and preferably, with a reduction in this flow rate, can slow down the wear of the nozzles at the time when it is maximum fully blown from the bottom.

Of course, the number and the diameter of the nozzles are calculated as a function of the flow rates envisaged for the oxygen blown from bottom to top, with or without stirring gas, and for a usual pressure for blowing from the bottom, the total passage section oxygen in all the nozzles is generally between the thirtieth and a quarter of the passage section necessary in the blowing processes integrated from bottom to top through the nozzles.

Thus, in the case where the oxygen blown through the nozzles must represent 20% of the total oxygen, or 1/5, it is possible for example to use nozzles with a diameter of 12 mm for oxygen, which, by compared to conventional nozzles with integral blowing from the bottom, 28 mm in diameter, have a section 5.45 times smaller; this ratio being close to 5, we can then keep the same number of nozzles as in full blowing from the bottom, since we must pass a volume of oxygen 5 times smaller.

One can also, still in this case where the oxygen blown by the nozzles represents 1/5 of the total oxygen, use for example nozzles with a diameter of 17 mm for oxygen, twice as small.

In order to clearly understand the invention, an embodiment of the method according to the invention for refining Thomas cast iron into steel will be described hereinafter by way of non-limiting example, in a 65-ton converter.

This Thomas cast iron at 3.6% C, 0.4% Si, 0.6 Mn, 1.8% P, requires approximately 63 Nm3 of oxygen per ton for its refining in mild steel.

In the present example, 80% of this oxygen, or 50 Nm3 per tonne of pig iron, is blown through a lance, accompanied by all of the lime necessary for refining, in the form of powder, or 135 kg per tonne of cast iron, divided into 110 Kg of lime powder and 40 Kg of castine powder (because 40 Kg of castine provide as much CaO as 25 Kg of lime).

The protected nozzles blow 20% of the total oxygen required, or 13 Nm3 per ton of pig iron.

They have a passage diameter for oxygen of 12 millimeters and are 7 in number. At 11 effective bars, their gas flow rate is 9.2 Nm3 / min per nozzle, or 64.4 Nm3 / min for the 7 nozzles . They are supplied either with oxygen alone or with a mixture of oxygen and a mixture of argon, at 1% or 2% oxygen.

• On charge dans le convertisseur 52 tonnes de fonte Thomas et 22 tonnes de ferrailles.

The metallurgical operation, to produce 65 tonnes of mild steel, by mixed blowing lasting 11 minutes and a few seconds, takes place as follows:

• 52 tonnes of Thomas cast iron and 22 tonnes of scrap are loaded into the converter.

• 240 Nm3/min en régime.

The lance will introduce into the converter: 50 Nm3 / tx52 = 2,600 Nm3 of oxygen at the rate of:

• 240 Nm3 / min under operating conditions.

• 710 Kg/min, soit une concentration moyenne de poudre dans l'oxygène de:
• 3 Kg/Nm3, dont 0,8 Kg de castine très re- froidissante par mètre cube normal d'oxygène.

This oxygen is accompanied by 5720 Kg of lime powder and 2080 Kg of limestone powder, that is 7800 Kg, in all, at an average flow rate of:

• 710 Kg / min, i.e. an average concentration of powder in oxygen of:
• 3 Kg / Nm3, of which 0.8 Kg of very refreshing castine per normal cubic meter of oxygen.

As a result, the red smoke is reduced ― but not suppressed ― due to this relatively high concentration of powder and also the use of castine powder.

• 37 Nm3 environ, soit, moins de 0,6 Nm3 par tonne d'acier.

In the 7 nozzles, for 8 minutes and a few seconds, that is to say a little before the end of decarburization, an oxygen flow of 65 Nm ³ / min is blown under a little more than 11 x 10 ¹ Pressure Pascals. Then, during the last 3 minutes, the oxygen flow is gradually reduced linearly from 65 Nm3 / min to 40 Nm3 / min, while an increasing flow of argon mixture is introduced, from 0 up to 25 Nm3 / min. The consumption of this brewing gas is thus:

• 37 Nm3 approximately, that is, less than 0.6 Nm3 per tonne of steel.

• 65x8+53x3=520+159=679 Nm3, soit 13 Nm3/t de fonte.

The total volume of oxygen blown through the nozzles is therefore slightly greater than:

• 65x8 + 53x3 = 520 + 159 = 679 Nm3, i.e. 13 Nm3 / t of pig iron.

It represents about 20% of the total oxygen blown.

After this mixed blowing of 11 minutes and a few seconds, we proceed to the deslagging of the phosphate slag, we possibly carry out a brief blowing under second slag, with the corresponding additions, and 65 tonnes of mild steel are poured into a pocket.

A second nonlimiting example will now be described below of application of the method according to the invention, in which the blowing from bottom to top is reduced to a level close to the strict minimum. In practice, in fact, there are many cases where a proportion of oxygen blown from the bottom up between 3% and 10% is already very interesting. In the example below, it is a question of refining a hematite cast iron in a 65-ton converter, but the application to a Thomas cast iron, with high phosphorus content, could be carried out in the same way way without difficulty.

• 4.5% C-0,9396 Mn-0,17096 P―0,96% Si.

This hematite cast has for analysis:

• 4.5% C-0.9396 Mn-0.17096 P ― 0.96% Si.

It requires an oxygen supply of 57 Nm3 per ton of pig iron, and an addition of 80 Kg of lime per ton of pig iron, divided into 65 Kg of lime powder and 27 Kg of castine powder (because 27 Kg of castine provide as much CaO as 15 Kg of lime).

Protected nozzles blowing 7% of total oxygen, i.e. 4 Nm3 per ton of pig iron. They have a passage diameter for oxygen of 11 millimeters and are three in number. At a pressure of 10 ⁸ effective Pascals, they are capable of a gas flow of 6.7 Nm3 / min per nozzle, or 20 Nm3 / min for the 3 nozzles.

They are supplied either with oxygen alone; either as a mixture of oxygen and argon.

• On charge dans le convertisseur 53 tonnes de fonte hématite et 21 tonnes de ferrailles.

The metallurgical operation, to produce 65 tonnes of mild steel by mixed blowing according to the invention, lasting 11 minutes and a few seconds takes place as follows:

• 53 tonnes of hematite cast iron and 21 tonnes of scrap are loaded into the converter.

• 53 Nm3/tx53=2.809 Nm3 d'oxygène, à raison de 255 Nm3/min en régime.

The lance will introduce into the converter:

• 53 Nm3 / tx53 = 2.809 Nm3 of oxygen, at the rate of 255 Nm3 / min in steady state.

This oxygen is accompanied by:

• 65 Kg / tx53 = 3.445 Kg of lime powder and:
• 27 Kg / tx53 = 1.431 Kg of castine powder, that is, in all:

4.876 Kg, at an average flow rate of:

443 Kg / min, ie an average concentration of powder in the oxygen of the lance of: 1.74 Kg / Nm3, of which 0.5 Kg of very cooling castine per normal cubic meter of oxygen.

Red fumes are reduced, but to a lesser extent than in the previous example, because, on the one hand, the proportion of oxygen blown by the lance is higher than in the previous example, and d 'on the other hand it is a hematita cast iron, which requires less lime for its refining.

In the 3 nozzles, for 10 minutes, that is to say up to a carbon content of the order of 0.150 / 0.200%, an oxygen flow of 20 Nm3 / min is blown at 10 ° effective Pascals. Then during the last minute, this flow rate is reduced to a constant level of 12 Nm3 / min and an argon flow rate of 8 Nm3 / min is added to this oxygen flow rate, which corresponds to an argon consumption of 0.12 Nm3 per tonne of steel, which contributes very little to the cost price of steel, but contributes significantly to the good performance of the nozzles and the bottom at the end of blowing.

• 20x10+12x1=212 Nm3, soit 4 Nm3 à la tonne de fonte. Il représente 7% de l'oxygène total soufflé.

In 11 minutes and a few seconds, the flow of oxygen blown through the nozzles is therefore slightly greater than:

• 20x10 + 12x1 = 212 Nm3, i.e. 4 Nm3 per ton of pig iron. It represents 7% of the total oxygen blown.

We then proceed to the cleaning; 65 tonnes of steel are poured into a ladle.

In summary, the joint effects of the small diameter of the nozzles, of the decrease in the oxygen flow rate per nozzle compared to a normal blast nozzle at the bottom, of the decrease in the oxygen flow rate at the end of blowing, and of the final addition of the stirring gas, during the period of greatest wear of the nozzles, means that the behavior of the nozzles and of the bottoms is much improved compared to that of the nozzles and of the bottoms of processes with integral blowing from the bottom. The improvement is also notable compared to known mixed processes.

A final advantage of the mixed blowing according to the invention can manifest itself in many cases, at the beginning of the blowing. We know that, quite often, priming the oxygen jet of the lance, on the surface of a more or less frozen metal bath, more or less charged with scrap, lime, etc., is difficult. On the other hand, the initiation of refining reactions by oxygen from the nozzles is always instantaneous. Also, a particular characteristic of the mixed blowing according to the invention consists in starting the blowing for a few seconds only with the oxygen from the nozzles blowing from bottom to top, regulated at a high flow rate, for example at the maximum of the oxygen pressure available , and then to introduce the oxygen from the lance, while then bringing the oxygen from the nozzles to its normal operating rate.

It is understood that it is possible, without departing from the scope of the invention, to imagine variants and refinements of details, as well as to envisage the use of equivalent means. This is how we can replace all or part of the water vapor with water spray as a possible stirring fluid.

Claims (11)

1. Process for blowing oxidising gases, especially pure oxygen, for the refining of metals, more particularly for the refining of cast steel, into a converter by means of, on the one hand, a lance blowing from the top downwards and, on the other hand, protected tuyeres blowing vertically or obliquely from the bottom upwards, characterised, at the same time, in that the quantity of oxygen blown from the bottom upwards via the tuyeres is between 3% and 25% of the total quantity of oxygen necessary for refining the metal bath, in that this oxygen is blown from the bottom upwards with a flow which does not increase, in that an inert or oxidising stirring gas can be added to this oxygen blown from the bottom upwards, at certain moments during blowing, in such a way that the total quantity of oxygen blown from the bottom upwards via the tuyeres remains between 3% and 25% of the total quantity of oxygen necessary for refining the metal bath, the total flow of oxygen blown in this way via the tuyeres remaining without an increase, and in that the jets of oxygen blown from the bottom upwards, which may or may not be mixed with a stirring gas, have a diameter at most equal to 18 mm, the diameter of the jets being considered at the outlet of the nozzles.

2. Blowing process according to Claim 1, ' characterised in that the flow of oxygen blown from the bottom upwards via the tuyeres is kept constant for the entire duration of blowing.

3. Blowing process according to Claim 1, characterised in that the flow of oxygen blown from the bottom upwards via the tuyere is adjusted in a decreasing manner over the entire duration of blowing.

4. Blowing process according to Claim 1, characterised in that the flow of oxygen blown from the bottom upwards via the tuyere is kept constant during a first part of blowing and is adjusted in a decreasing manner during the second part of blowing, or vice versa.

5. Blowing process according to any one of Claims 1 to 4, characterised in that the stirring gas is mixed with the oxygen blown from the bottom upwards via the tuyeres during two distinct periods:

-the first period at a critical moment of the decarburisation projections,

- the second period towards the end of blowing.

6. Blowing process according to Claim 5, characterised in that the flow of stirring gas increases during the second period towards the end of conversion.

7. Blowing process according to either one of Claims 3 and 4, characterised in that, towards the end of blowing, the increase in the flow of stirring gas is adjusted so as to compensate exactly the decrease in the flow of oxygen blown from the bottom upwards, so that the total flow remains constant.

8. Blowing process according to any one of Claims 1 to 7, characterised in that the jets of oxygen blown from the bottom upwards, which may or may not be mixed with a stirring gas, have a diameter at most equal to 12 mm, the diameter of the jets being considered at the outlet of the tuyeres.

9. Blowing process according to Claim 8, characterised in that the total quantity of oxygen blown from the bottom upwards via the nozzles is between 3% and 10% of the total quantity of oxygen necessary for refining the metal bath.

10. Blowing process according to any one of Claims 1 to 9, characterised in that the flow of oxygen blown from the bottom upwards and, if appropriate, the flow of additional stirring gas are adjusted at each moment as a function of the state of oxidation of the slag, so as to control at each moment the divergence from the equilibrium between the slag and the metal bath.

11. Blowing process according to any one of Claims 1 to 10, characterised in that the start of blowing is carried out for a few seconds solely with oxygen blown from the bottom upwards in a strong flow via the tuyeres, in that the oxygen from the lance is subsequently admitted, and in that the flow of oxygen blown via the tuyere is then brought back immediately to its normal value.