EP0360223A2 - Agent and method for desulfurizing molten ferrous metal and a process for the production of this agent - Google Patents

Abstract

The invention relates to agents for the desulfurization of iron, containing calcium carbide and a metallic component, calcium carbide and the metallic component being in such a form that their bulk density and their grain size are in the same range. The invention further relates to a process for the preparation of the composition and a process for the desulfurization of molten iron using the composition according to the invention.

Description

The invention relates to an agent for the desulfurization of molten iron, a process for the production thereof and a process for the desulfurization of molten iron. The agent contains calcium carbide and a metallic component.

The desulfurization of pig iron outside the blast furnace by means of injection metallurgical processes is a firmly established process in the production of steel. In practice, two methods have proven particularly useful for reducing the sulfur content in pig iron, namely desulphurization in the torpedo pan and treatment of the pig iron in the charging pan in the steel mill. Both methods are used to desulfurize by injection metallurgy, i.e. the desulfurization mixture is blown into the pig iron melt by means of an immersion lance using an inert gas stream.

In practice, desulfurization mixtures of calcium carbide and magnesium with possibly further additives such as gas-releasing coal, alkaline earth carbonates, calcium oxide and calcium fluoride or calcium cyanamide have proven to be advantageous for practicing this process. Some publications in which such mixtures are described may be mentioned as examples: DE-OS 25 31 047, DE-OS 26 50 113, DE-OS 27 08 424, DE-OS 27 41 588, DE-OS 35 44 562, and Stahl und Eisen 105 (1985), No. 11, pp. 627 to 630.

A disadvantage of some of these processes is the use of a mixture of substances which is not stable to mixing. Magnesium, which enables rapid and targeted desulphurization of the pig iron, can therefore not be dosed with the necessary accuracy.

If the above-mentioned methods provide for a separate addition of the magnesium to the desulfurization mixture in the form of co-injection, a relatively large amount of equipment is required in order to be able to meter the components precisely. In order to be able to dose precisely during co-injection, magnesium is often used in a mixture with slag, with aluminum dross or other oxidic compounds. These agents also separate and therefore do not solve the problem.

For the aforementioned reasons, attempts have not been lacking to produce magnesium-containing desulfurization agents in the form of pellets, cored wires or by coating, which should give these agents simplified handling and increased process efficiency. Such a desulfurization agent is disclosed, for example, in DE-AS 12 99 670. It has a multi-layer structure and, in addition to magnesium and calcium carbide, can contain other components that have a desulfurizing effect. However, the dimensions and the composition of the compacts disclosed in this document are unsuitable for use in the co-injection process.

Furthermore, from DE-OS 24 22 072 a desulfurizing agent based on calcium carbide is known, which is coated with magnesium metal. The coating is carried out according to the method of this document by vapor deposition of the calcium carbide grain with magnesium vapor. From today's economic point of view, the process is too complex and costly intensive to use this product for co-injection, even if it is technically suitable. Desulphurization by means of a calcium carbide-magnesium metal mixture in the form of a cored wire or in the form of rods, as can be found in DE-OS 27 38 379, is out of the question of the co-injection process for understandable reasons. Finally, U.S. Patent 4,541,867 teaches the manufacture of a carbon coated granular agent which can be used as an additive to steel baths and for desulfurization thereof. The agent can consist of magnesium and calcium carbide. The coating is made by mixing the components of the agent with a polymerizable oil, its thermal polymerization and its subsequent partial thermal decomposition. This manufacturing process also requires a relatively large amount of equipment and high energy input.

In general, it should also be noted that due to the high magnesium vapor pressure at the pig iron temperatures, the magnesium introduction into the melt poses problems. Good meterability of the magnesium component is therefore essential to the process, but is not satisfactorily achieved by the desulfurization agents known today. Especially when magnesium is blown in without the co-injection partner calcium carbide, iron ejection and lance blockage occur. A disadvantage of the general type of known mixtures is that they have a filler content of 20 to 34% by weight, which is not involved in the desulfurization. Another disadvantage of known mixtures of magnesium with fillers such as aluminum, aluminum oxide or ball mill dust is that they separate relatively quickly, so that the magnesium content is initially high and then decreases. Because of this inhomogeneity, a higher use of material with a poorer effect is required.

The object was therefore to provide a free-flowing, low-slag and inexpensive agent which contains calcium carbide and a metallic component which is suitable for injection for the desulfurization of pig iron melts and does not have the disadvantages mentioned.

This object is achieved by an agent for the desulfurization of iron, containing calcium carbide and a metallic component, calcium carbide and the metallic component being in such a form that their bulk density and their grain size are in the same range.

According to the invention, there is provided an agent for the desulfurization of molten iron, which consists of two components which do not separate even when standing for a long time, during transport, during long storage in the silo or during pneumatic conveying and which can therefore be metered very well. This allows the rate of injection of the metallic component to be controlled very well, which is of fundamental importance for a targeted desulfurization with the result of an optimal cost-benefit effect. The agent according to the invention can be used alone for desulfurization. However, it is also suitable for co-injection in conjunction with other desulfurization agents, as described, for example, in EP 0 226 994 A1.

The agent according to the invention essentially consists of two components, namely calcium carbide and a metallic component. The metallic component can be calcium, magnesium or alloys thereof. Magnesium metal is preferably used for the desulfurization agent. The respective proportion of the two components in the mixture is not critical per se and can vary within wide limits. The agent according to the invention usually contains 10 to 90% by weight of calcium carbide, preferably 20 to 80% by weight, and 90 to 10% by weight, preferably 80 to 20% by weight of metallic component.

The two essential components according to the invention are used in such a way that their bulk density and their grain size are in the same range. As a rule, the bulk density of the metallic component is adjusted to the bulk weight of calcium carbide, so the bulk density is preferably set to a range from 0.7 to 1.0 g / cm³, particularly preferably 0.8 to 0.9 g / cm³ . The production of the two components with the corresponding bulk density takes place according to methods known per se.

The grain size of the two components is also set in the same range by appropriate grinding. The grain size is preferably in the range from 0.1 to 3 mm, particularly preferably 0.3 to 1 mm.

For use in the desulfurization of molten iron, the two components are mixed together, fluidized and then blown in via a lance. By using particles of the same size and the same bulk density, a very homogeneous mixture can be produced that does not separate even over a long period.

In a preferred embodiment of the method according to the invention, the particles of the two components are coated. An adhesive is applied to the particles for coating and then a finely divided dust. The coating is 1 to 10 wt .-% based on the weight of the entire grain. An oily liquid that adheres to the particles is used as the adhesive. Vegetable oils, silicone oils and / or mineral oils are suitable for this. In a second step, a fine-particle dust is applied. Silicate dusts or oxidic dusts, such as those produced in the aluminum industry, are suitable for the coating. Examples are finely divided silica, bentonite and / or furnace filter dust from the production of calcium silicon and / or ferro silicon and / or other ferro-alloys as well as other oxidic compounds such as calcium aluminates.

Due to the properties of this preferred agent, the segregation is even less and the dosing is thus possible even more precisely. A typical and particularly preferred composition contains 45 wt .-% magnesium, 45 wt .-% technical calcium carbide, the content of CaC₂ is usually 65 to 80 wt .-%, 0.5 wt .-% of an oil and 9.5 % By weight of a laminate.

The coating of the agent according to the invention is particularly advantageous. This measure produces the surfaces of the same constituents of the two components calcium carbide and metallic component. A surface of the same material, for example in the form of a coating containing silica, not only gives the agent excellent flow properties, but in particular counteracts segregation of the constituents, so that the homogeneity of the agent is fully retained during transport, handling and silo storage.

Highly viscous oils, in particular those of vegetable origin, but also silicone oils and / or mineral oils are used as adhesion promoters. In order to avoid decomposition of the carbide, preference is given to anhydrous or low-water oils, the weight fraction of which is 0.1 to 1% by weight. They form the basis for a firmly adhering, gapless coating that gives the agent the desired properties. Another advantage of this preferred embodiment is that the oil also binds fine fractions of carbide and magnesium and in this way makes the desulfurization agent dust-free. In addition, the sensitivity of the carbide component to moisture is reduced.

A layer of fine dust is applied to the adhesive layer. For this, dusts with a grain size below 10 µm are used. The proportion of finely divided dust in the desulfurization agent is 2 to 10% by weight.

The agent according to the invention is produced by simply mixing technical calcium carbide and the metallic component in the desired grain size under an inert gas atmosphere. If coated particles are used for the agent according to the invention, then after grinding the two components to the desired particle size, the particle surface is wetted with the oil and the finely divided dust is then applied. The process can be carried out batchwise in drum, trough or truncated cone mixers, as well as continuously, for example in screw mixers. Although the agent consisting of coated particles is less flammable than finely divided magnesium, the mixing process is also expediently carried out under a dry inert gas atmosphere in order to exclude moisture during production, in which the magnesium and calcium carbide are free, and at the same time the risk of a dust explosion due to possible exclude existing fine-particle magnesium.

The agent according to the invention can be blown into the molten metal without any additives or diluents using argon or nitrogen as the carrier medium. It can also be used as a co-injection partner with other desulfurization agents. Without fear of iron ejection, the agent as such or together with another desulfurization mixture can be easily blown in at a rate of 10 to 100 kg / min, the preferred blowing rate being 20 to 40 kg / min. The high blowing speed enables the blowing times to be shortened considerably and, furthermore, the filling level of the pig iron pans can be increased through the calm blowing behavior of the agent.

The productivity of the desulfurization process is considerably improved by using the agent according to the invention. In order to avoid the violent magnesium evaporation reaction in the hot pig iron melt, the agent essentially contains actively desulfurizing calcium carbide and only small amounts of inactive components.

According to the invention, a means is thus made available which permits an extremely flexible desulfurization method for technical, metallurgical and economic reasons.

• Figur 1 zeigt ein Diagramm, in dem die Veränderung der Zusammen­setzung eines hinsichtlich Korngröße und spez. Gewicht angeglichenen Gemisches aus 55 Gew.-% Calciumcarbid und 45 Gew.-% nach der Auflockerung in einem Silo dargestellt wird (+). Bei der Entnahme des Gemisches werden Magnesiumgehalte zwischen 46.4 und 42.3 Gew.-% gefunden, was einen Streubereich von 4.1 Gew.-% Magnesium bedeutet.
  Ein gleichartiges, jedoch beschichtetes Gemisch aus 45 Gew.-% Calciumcarbid, 45 Gew.-% Magnesium und 10 Gew.-% Beschichtung (□) zeigte nach Auflockerung und Entnahme Magnesiumgehalte zwischen 46.2 und 44.6 Gew.-% Mg, was einem Streubereich von 1.6 Gew.-% entsprach.
• Figur 2 In diesem Fall bestand das Gemisch aus 20 Gew.-% Magnesium der Körnung 0.3 bis 1 mm und fein aufgemahlenem Calciumcarbid der Körnung < 0.1 mm (vgl. Beispiel 8). Das Diagramm zeigt die Entmischung des Magnesiums bei der Auflockerung und anschließender Entnahme, wobei Magnesium-Gehalte zwischen 24.2 und 17.0 Gew.-% Mg erhalten wurden, was einer Streubreite von 7.2 Gew.-% Mg entsprach.
• Figur 3 zeigt ein Diagramm, in dem die Veränderungen des Entschwefelungsgrades für ein Gemisch nach dem Stand der Technik (durchgezogene Kurve) dargestellt ist; ferner wird die Streubreite des Endschwefelgehaltes S_E · 1/1000 % für ein erfindungsgemäßes Gemisch (schraffierter Bereich) angegeben.
  In jedem Fall wird die gleiche Gemischzusammensetzung und die gleiche Menge an Gemisch in die Eisenschmelze eingeblasen.
  Mit dem Gemisch nach dem Stand der Technik war der Entschwefelungseffekt am Beginn der Behandlung größer als am Ende, nachdem das Gemisch dann infolge des Sicht­effektes im Silo weniger Magnesium enthielt. Wurde das erfindungsgemäße Gemisch verwendet, blieb der Entschwefelungseffekt innerhalb eines relativ engen Streubereichs konstant.

The invention is illustrated by the following figures and examples.

• Figure 1 shows a diagram in which the change in the composition of a grain size and spec. Weight adjusted mixture of 55 wt .-% calcium carbide and 45 wt .-% after loosening is shown in a silo (+). When the mixture is removed, magnesium contents of between 46.4 and 42.3% by weight are found, which means a range of 4.1% by weight of magnesium.
  A similar but coated mixture of 45% by weight calcium carbide, 45% by weight magnesium and 10% by weight coating (□) showed, after loosening and removal, magnesium contents between 46.2 and 44.6% by weight Mg, which corresponds to a scattering range of 1.6 wt .-% corresponded.
• FIG. 2 In this case, the mixture consisted of 20% by weight of magnesium with a grain size of 0.3 to 1 mm and finely ground calcium carbide with a grain size of <0.1 mm (cf. Example 8). The diagram shows the segregation of the magnesium during loosening and subsequent removal, magnesium contents between 24.2 and 17.0% by weight of Mg being obtained, which corresponded to a scattering range of 7.2% by weight of Mg.
• FIG. 3 shows a diagram which shows the changes in the degree of desulfurization for a mixture according to the prior art (solid curve); Furthermore, the spread of the final sulfur content S _E · 1/1000% for a mixture according to the invention (hatched area) is given.
  In any case, the same mixture composition and the same amount of mixture are blown into the molten iron.
  With the mixture according to the prior art, the desulfurization effect was greater at the beginning of the treatment than at the end after the mixture then contained less magnesium due to the visual effect in the silo. If the mixture according to the invention was used, the desulfurization effect remained constant within a relatively narrow scattering range.

Examples

The hot metal was desulfurized (RE) in a charging pan containing 230 t of iron at a temperature of 1350 ° C. The mixtures used for desulfurization were blown in pneumatically by means of an immersion lance using argon.

Example No. 1 is a comparative example with a commercially available mixture for desulfurization (Mg 50 = 50 wt .-% magnesium metal + 50 wt .-% ball mill dust (Al₂O₃)).

Example Nos. 2 and 3 were performed with the agent of the invention CaM 45 of the preferred composition. CaM 45 consists of 45% by weight of technical calcium carbide, 45% by weight of magnesium metal, 9.5% by weight of furnace filter dust from FeSi production and 0.5% by weight of silicone oil. The co-injection experiments No. 4 and No. 5 were carried out with CaM 45 together with CaD C5 (95% by weight technical calcium carbide + 5% by weight flame carbon).

Example No. 6 illustrates the co-injection of an agent CaM 25 of the composition: 25% by weight magnesium metal, 65% by weight technical calcium carbide, 9.5% by weight furnace filter dust from the CaSi production and 0.5% by weight Silicone oil, together with CaD 45.

Abbreviations:

Beispiel Nr. Gemisch zur Entschwefelung Mg 50 kg Mittel kg % Mg im Gemisch zur Entschwefelung Einblasgeschwindigkeit Behandlungszeit S_A, S_E E % 1 Mg 50 435 - 50 15 kg/min. 29 min. 40 5 87,5 2 CaM 45 - 375 45 25 kg/min. 15 min. 50 5 90,0 3 CaM 45 - 408 45 24 kg/min. 17 min. 42 2 95,2 4 CaM 45 - 180 9,5 15 kg/min. 12 min. 46 2 95,6 CaD C5 - 672 42 kg/min. 16 min. 5 CaM 45 - 98 9,4 14 kg/min. 7 min. 44 5 88,6 CaD C5 - 369 41 kg/min. 9 min. 6 CaM 25 - 330 9,7 30 kg/min. 11 min. 47 3 93,6 CaD C5 520 40 kg/min. 13 min. E = degree of desulfurization of the RE
S _A = sulfur content (wt .-% · 10⁻³) in RE before treatment
S _E = sulfur content (wt .-% · 10⁻³) in RE before treatment
S = S _A - S _E

Example No. Desulphurization mixture Mg 50 kg Average kg % Mg in the mixture for desulfurization Blowing speed Treatment time S _A , S _E E% 1 Mg 50 435 - 50 15 kg / min. 29 min. 40 5 87.5 2nd CaM 45 - 375 45 25 kg / min. 15 minutes. 50 5 90.0 3rd CaM 45 - 408 45 24 kg / min. 17 min. 42 2nd 95.2 4th CaM 45 - 180 9.5 15 kg / min. 12 min. 46 2nd 95.6 CaD C5 - 672 42 kg / min. 16 min. 5 CaM 45 - 98 9.4 14 kg / min. 7 min. 44 5 88.6 CaD C5 - 369 41 kg / min. 9 min. 6 CaM 25 - 330 9.7 30 kg / min. 11 min. 47 3rd 93.6 CaD C5 520 40 kg / min. 13 min.

Example 7

A silo with 25 - 30 t of mixture 1, 2 and 3 was in each case subjected to a total of 10 m 3 gas / minute for 10 x 3 minutes. There was a 5-minute pause between the individual loosening steps so that the mixture could settle again. The entire loosening up treatment lasted 80 minutes. The mixture was then unloaded and sampled. The discharge time lasted approx. 60 minutes for 25 t, with a 6 kg sample (10 to 12 samples) being taken every 5 minutes. The removal time for the 6 kg material was approx. 20 seconds. These 6 kg were reduced to a sufficient amount for the analysis of the magnesium content using a standardized sample divider. TABLE 1 Desulfurization mixture Mg content% by weight CaC₂ content% by weight Grain size (mm) 1 CaM 45 coated 45 50 0.3-1 2 CaM 45 uncoated 45 55 0.3-1 3 CaM 20 uncoated 20th 75 Mg 0.3-1 CaC₂ <0.1 (CaM 45 = mixture with 45% magnesium content, CaM 20 = mixture with 20% magnesium content)

The results are shown in FIG. 1.

The coated mixture (CaM 45 coated) practically does not separate, the uncoated mixture (CaM 45 uncoated) undergoes only a slight separation, the size of which has not previously been achieved.

Example 8 (comparison)

A desulfurization agent according to the prior art was examined in which the grain size of the magnesium was between 0.3-1 mm and that of the calcium carbide was <0.1 mm. The loosening of this mixture caused a clear visual effect, the magnesium and calcium carbide components separating from one another, which can be seen clearly in FIG. 2. The magnesium content is initially approximately 24% by weight and ends at approximately 17% by weight. These differences in the mixture lead to problems in the desulfurization treatment, since the mixture composition and thus its effectiveness change in the course of the treatment.

Desulphurization with a mixture of different magnesium and calcium carbide grains containing 20% by weight of magnesium (mixture 3, according to table 1)

This mixture 3 was used for the desulfurization and FIG. 3 shows the different end sulfur values (S _E values) which arise during the desulfurization treatment within a 25 t mixture delivery. It can be seen that, in accordance with the visual effect shown in FIG. 3, at the beginning of the removal of the mixture from the silo, significantly lower S _E values are achieved than with the remaining amount of the mixture still present in the silo.

When the mixture according to the invention is used which is stable to mixing, the S _E values deviate only insignificantly from the desired value, which is indicated by the hatched area.

Claims (13)

1. Agent for the desulfurization of iron, containing calcium carbide and a metallic component, calcium carbide and the metallic component being in such a form that their bulk density and their grain size are in the same range. 2. Composition according to claim 1,
characterized,
that the metallic component is magnesium. 3. Composition according to claim 1 or 2,
characterized,
that it contains 10 to 90% by weight of magnesium and 90 to 10% by weight of calcium carbide. 4. Agent according to one of the preceding claims,
characterized,
that the bulk density of the components used is in the range of 0.7 to 1.0 g / cm³. 5. Agent according to one of the preceding claims,
characterized,
that the grain size of the two components is in the range of 0.1 to 3 mm. 6. Composition according to claim 5,
characterized,
that the grain size is in the range of 0.3 to 1 mm. 7. Composition according to one of the preceding claims,
characterized,
that the particles of the two components are coated with an adhesive and a finely divided dust. 8. Composition according to claim 7,
characterized,
that the coating is 1 to 10 wt .-% based on the coated particles. 9. Composition according to claim 7 or 8,
characterized,
that the adhesive is an oily liquid, preferably a vegetable oil and / or a silicone oil and / or a mineral oil. 10. Agent according to one of claims 7 to 9,
characterized,
that the coating contains finely divided silica, bentonite and / or furnace filter dust from the production of calcium silicon and / or ferro silicon and / or other ferro alloys and / or other oxidic compounds. 11. A process for the preparation of the agent according to any one of claims 1 to 6,
characterized,
that calcium carbide and the metallic component are ground to the required particle size and then mixed under an inert gas atmosphere. 12. A process for the preparation of the agent according to any one of claims 7 to 10,
characterized,
that calcium carbide and the metallic component are ground to the required size, mixed under an inert gas atmosphere and then mixed with an oily adhesive and coated with the addition of a finely divided substance. 13. Process for desulfurization of iron,
characterized,
that an agent according to any one of claims 1 to 10 is introduced into the molten iron at a rate of 10 to 100 kg / min either alone or in a co-injection process together with other desulfurizing agents.

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