SK283419B6 - 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

Field of the invention

The present invention relates to a calcium carbide desulfurization mixture to be injected by means of a nozzle into liquid pig iron obtained from a blast furnace prior to passing it through the converter.

BACKGROUND OF THE INVENTION

Although pig iron and slag are cast separately from the blast furnace, a considerable amount of slag remains in the casting pan, of the order of 5 kg / ton. On the other hand, the use of a desulfurizing agent generates numerous sulfur impurities that need to be separated from the metal leaving the converter. At this stage, the removal of the slag is carried out mechanically, causing loss of metal by abduction.

It is known from the prior art that the recovery of liquid slag from the desulfurization phase makes it possible to significantly reduce the metal loss during removal of the slag from the casting ladles. For this purpose, sodium carbonate, which decomposes to sodium oxide having a melting point of 1132 ° C, is added to the metal casting pan before the slag is removed. However, the resulting impurities contain sodium, which is a detrimental factor for the environment, so this lack implies the need to develop other solutions to reduce metal loss.

On the other hand, the addition of reducing metals, such as aluminum or magnesium, to desulfurizing agents has been known for a long time to accelerate desulfurization by removing oxygen dissolved in the melt, e.g. WERKE discloses a desulphurizing agent based on powdered calcium carbide, calcium cyanamide or lime, to which 0.5 to 3.5% powdered aluminum or magnesium with the same granulometry is added.

The addition of low melting calcium aluminate to calcium carbide is also known from the European patent EP 0398674 of ELKEM METALS to achieve a better bath desulfurization at a constant feed rate.

SUMMARY OF THE INVENTION

It has been found that the desulfurization composition based on micronized calcium carbide powder, which contains both powdered magnesium and powdered aluminum, wherein the aluminum allows a significant reduction in iron losses during desulfurization, at a correspondingly limited concentration interval. The present invention also provides a desulfurized melt desulfurization composition comprising at least 50% by weight of calcium carbide powder with a particle size < 0.2 mm, 1 to 20% by weight of magnesium powder with a particle size < 0.8 mm and 5 to 15% by weight of aluminum powder with a particle size < 0.8 mm, optionally with other powder additives, for example lime, calcium carbonate, carbon products and / or coating agents, the total amount of these optional additives being at most 30% by weight.

The composition of the invention is based on the technical calcium carbide which comprises generally about 80% by weight of CaCl _2, the balance being lime, ground to a particle size of less than 0.2 mm and containing particles of particle size 0.1 to 0.2 mm less than 25%. This carbide may be additivated with lime added in addition to lime derived from the production of technical carbide, calcium carbonate, carbonaceous products such as coal, graphite or anthracite, or other products that can improve the castability of the mixture or release gases that permit good distributing the desulfurizing agent in the liquid melt. It is also possible to add up to 2% of a binder to coat the grains, for example hydrogenated castor oil, as described in the European patent EP 0511121 of the same applicant as the present PV.

Magnesium is fed to the mixture in the form of a powder or granules of pure metal, or a magnesium-based alloy with a particle size <0.8 mm. The grains may also be coated, as in the case of calcium carbide, with an organic product, for example based on saturated aliphatic acids or esters thereof, as described in European patent EP 0485309 of the same applicant as the present PV.

5-15% of aluminum powder or granules, or an aluminum-based alloy, which can also be coated with the same formulation as in the case of magnesium, is then added to the mixture.

The mixture of powders or granules is preferably produced in a mixer under an inert gas atmosphere, for example under a nitrogen atmosphere.

The compositions of the invention have desulfurization efficiency at least equal to that of compositions where aluminum is replaced by the same amount of magnesium, and thus do not lead to excessive product consumption, which is unexpected because aluminum, unlike magnesium, is not itself a desulfurizing agent. On the other hand, a more liquid slag is formed, which is easier and easier to remove, the removal of which leads to significantly lower iron losses. Finally, the resulting impurities contain no environmentally harmful product, such as sodium oxide.

DETAILED DESCRIPTION OF THE INVENTION

The calcium carbide desulfurization composition will be described in more detail in the following, which is illustrative only and is not intended to limit the scope of the present invention.

Example

The desulfurization and iron loss efficiency of two prior art calcium carbide desulphurization mixtures was compared, the first containing 20% by weight magnesium granules, the second 20% calcium aluminate powder, and the third mixture, according to the invention, containing 10% magnesium granules and 10% aluminum granules.

The experiments were carried out in casting ladles with 300 tons of melt. The initial sulfur content was about 250 ppm, while the final content was about 55 ppm. The efficacy of the desulfurization agent is defined as the ratio of the number of moles of melt removed to the number of moles-injected desulfurization agent, i.e. in this case the mol CaCl ₂ and Mg, since aluminum is a desulphurizing agent alone. Iron losses are defined by the ratio of the pig iron mass contained in the impurities to the pig iron mass contained in the melting desulfurization vessel.

For these three mixtures, the following results were obtained, which is an average of 20 experiments carried out under the most similar conditions.

mixture effectiveness Loss of iron 1 22% 1.1% 2 18% 0,9% 3 23% 0.3%

Comparing mixtures 1 and 3, it can be concluded that their desulphurisation efficiency is very similar, but that the use of mixture 3 leads to an iron loss by a factor of 3.7.

Comparing mixtures 2 and 3, it can be surprisingly observed that aluminum leads to a significantly more efficient desulfurization and partitioning of iron losses by a factor of 3, although similar results could be expected because alumina introduces alumina into the slag and that aluminum oxidizes in the slag. melt to form alumina that dissolves in the slag.

Claims (2)

PATENT CLAIMS CLAIMS 1. A desulphurization mixture for the injection of purified pig iron comprising at least 50% by weight of powdered calcium carbide with a particle size <0.2 mm, 1 to 20% by weight of magnesium in the form of powder or granules with a particle size <0, 8 mm and 5 to 15% by weight of aluminum in the form of powder or granules with a particle size <0.8 mm, or other powder additives, for example lime, calcium carbonate, carbon products and / or grain coating agents, the total amount of these optional additives being at most 30% by weight. A desulfurization mixture according to claim 1, characterized in that it contains less than 25% calcium carbide grains with a particle size range of 0.1 to 0.2 mm.