WO1997038142A1 - Desulphurising calcium carbide blend - Google Patents

Abstract

A desulphurising blend for basic pig iron comprises at least 50 wt. % of pulverulent calcium carbide with a particle size of < 0.2 mm, 1-20 wt. % of pulverulent or granular magnesium with a particle size of < 0.8 mm and 5-15 wt. % of pulverulent or granular aluminium with a particle size of < 0.8 mm, and optionally other pulverulent additives such as lime, calcium carbonate, carbon products and/or grain coating agents, wherein the total amount of additives is 30 % or less, said blend reducing iron loss during desulphurisation.

Description

 Desulfurizing mixture based on calcium carbide

Field of the invention

The invention relates to a desulfurizing mixture based on calcium carbide intended to be injected using a lance into the liquid cast iron obtained at the blast furnace before passing it to the converter.

State of the art

Although the pig iron and the slag are poured separately from the blast furnace, there remains an appreciable quantity of slag in the ladle, of the order of 5 kg / t. In addition, the use of a desulfurizing product creates sulfur-rich dross which must be separated from the leaving metal for the converter. At this stage, the scouring is done mechanically, which causes metal losses by entrainment. It is known that obtaining a fluid desulphurization slag makes it possible to significantly reduce the metal losses during the cleaning of the pockets. To do this, sodium carbonate, which decomposes into sodium oxide, the melting point of which is 1132 ° C., is added to the metal ladle before descaling. The dross obtained then contains sodium harmful to the environment, hence the search for other solutions to reduce metal losses.

Furthermore, the addition to desulfurizers of reducing metals, such as aluminum or magnesium, to accelerate desulfurization by eliminating the oxygen dissolved in the cast iron, has also been known for a long time, for example by patent FR 1168646 filed by IRSID in 1957. Patent FR 2317361 by KLOCKNER-WERKE describes a desulfurizer based on calcium carbide, calcium cyanamide or lime powder to which 0.5 to 3.5% of powder is added. of aluminum or magnesium of the same particle size. Also known, from patent EP 0398674 by ELKEM METALS, the addition to calcium carbide of calcium aluminate with low melting point in order to obtain a more complete desailfuration of the baths with constant quantity of mixture injected. Subject of the invention

The Applicant has found that a desulfurizing mixture based on micronized powder of calcium carbide and containing both magnesium powder and aluminum powder, the latter in a well defined concentration range, allows a significant reduction iron losses during desulfurization. The subject of the invention is therefore a desulfurizing mixture of refining pig iron consisting of at least 50% (by weight) of powdered calcium carbide with a particle size <0.2 mm, from 1 to 20% of powdered magnesium with a particle size <0.8 mm and from 5 to 15% of aluminum powder with a particle size <0.8 mm, with optionally other powder additives such as lime, calcium carbonate, carbonaceous products and / or coating agents in a total amount of at most 30%.

Description of the invention

The mixture according to the invention is based on technical calcium carbide, which generally contains around 80% by weight of C ₂ Ca, the rest being lime, ground to a particle size less than 0.2 mm, with less than 25% of particles in the particle size range 0.1 - 0.2 mm This carbide can be added with lime in addition to that resulting from the manufacture of technical carbide, calcium carbonate, carbon products such as carbon black , graphite or anthracite, or other products intended to improve the flowability of the mixture or to release gases allowing a good distribution of the desulfurizer in the hot melt. It is also possible to add up to 2% of a binder for coating the grains, for example hydrogenated castor oil as described in patent EP 0511121 of the applicant.

The magnesium is added to the mixture in the form of a powder or granules of pure metal or of a magnesium-based alloy with a particle size <0.8 mm. The grains can also be coated, like those of calcium carbide, with an organic product, for example based on saturated aliphatic acids or their esters as described in patent EP 0485309 of the applicant.

Finally, 5 to 15% of powder or granules of aluminum or aluminum-based alloy is added to the mixture, which can also be coated with a product identical to that used for magnesium. _ „„ „„,., PCT7FR97 / 00592 O 97/38142

3

The mixing of the powders or granules is advantageously carried out in a mixer operating under neutral gas, for example nitrogen.

The mixtures according to the invention have a desulfurization efficiency at least equal to that of mixtures where aluminum is replaced in equal quantity by magnesium and therefore do not cause any overconsumption of product, which is quite unexpected since aluminum, unlike magnesium, is not by itself a desulfurizer. On the other hand, a slag is formed which is more fluid, easier to clean and whose cleaning leads to much lower iron losses. Finally, dross does not contain any products harmful to the environment such as sodium oxide.

Example

We compared the desulfurization efficiency and the iron losses of 2 desulfurizing mixtures of the prior art based on technical calcium carbide, the first containing 20% (by weight) of magnesium granules, the second 20% of powder. of lime aluminate, to a third mixture, in accordance with the invention, containing 10% of magnesium granules and 10% of aluminum granules.

The tests were carried out on pockets of 300 t of cast iron. The initial sulfur content was close to 250 ppm, while the final content was around 55 ppm. The effectiveness of a desulfurizer is defined by the ratio of the number of moles removed from the cast iron to the number of moles of desulfurizer injected, that is to say, in the present case, moles of C ₂ Ca and Mg, raluminium not being in itself a desulfurizing product. The iron loss is defined by the ratio of the mass of pig iron contained in dross to the mass of pig iron contained in the desulfurization pocket. The following results were obtained for the 3 mixtures, which are the average of 20 tests carried out under conditions as close as possible.

En comparant les mélanges 1 et 3, on constate que leur efficacité en désulfuration est très voisine, mais que l'utilisation du mélange 3 conduit à diviser la perte en fer d'un facteur 3,7.

By comparing mixtures 1 and 3, it can be seen that their desulphurization efficiency is very similar, but that the use of mixture 3 leads to dividing the iron loss by a factor of 3.7.

By comparing mixtures 2 and 3, it is surprisingly found that aluminum leads to a much more effective desulfurization and a division of the iron loss by 3, whereas one would have expected similar results, since the aluminate introduces alumina into the slag and the aluminum oxidizes in the pig iron to form alumina which dissolves in the slag.

Claims

1. Desulfurizing mixture for injection into liquid refining pig iron, consisting of at least 50% (by weight) of calcium carbide in powder with a particle size <0.2 mm, from 1 to 20% of magnesium in powder or granules with a grain size <0.8 mm and from 5 to 15% of aluminum powder or granules with a grain size <0.8 mm, with possibly other powdered additives such as lime, calcium carbonate, carbon products and / or grain coating agents, the total amount of additives being at most 30%. 2. Mixture according to claim 1 in which less than 25% of the calcium carbide grains have a particle size of between 0.1 and 0.2 mm.