EP0485309B1 - Product based on coated magnesium for the desulphurizing of cast iron or liquid steel - Google Patents

Abstract

Product for desulphurising cast irons into liquid steels, based on magnesium particles coated with the aid of an organic product based on saturated aliphatic acid or their solid esters and a refractory powder.

Description

TECHNICAL AREA

The invention relates to a product for desulfurization of cast irons or liquid steels, based on coated magnesium particles.

STATE OF THE ART

The use of magnesium powder is well known for the desulfurization of cast iron and steel. However, due to its high vapor pressure at the temperature of liquid iron or steel baths, magnesium presents difficulties of use which we try to overcome by coating the particles of magnesium, the diameter of which is generally less than 2 mm, a layer of fine powder mineral product consisting of grains whose diameter is generally less than 150 microns.

Various means have been proposed for adhering the grains of the mineral product to the magnesium particles. For example, the adhesion can be carried out using organic binders which has certain advantages.

Thus in patent FR 2 331 618 (AIKOH) the magnesium is first wetted with an organic binder, the mineral powder is then added to finally obtain Mg grains surrounded by an organic film itself covered with a layer of mineral product which therefore adheres to magnesium via said organic film.

The organic products more particularly cited in this patent are:
. starch, dextrin, molasses, which are sugars, carboxymethylcellulose, phenolic resins, urea resins, melamine resins, furan resins, epoxy resins, polyester resins, pitch and tar.

Similarly, application EP 292 205 describes a similar coating where the magnesium is first covered with a first layer of a hydrophobic product and then with a second layer of a particulate refractory product. These hydrophobic products which are also organic products, are selected from aliphatic or aromatic liquid oils with a low viscosity at 25 ° C., or solid at room temperature but liquid at low temperatures (for example waxes or paraffin).

The refractory mineral product used is generally constituted by metal oxides, mixtures or complexes of oxides (for example: MgO, Al₂O₃, SiO₂, TiO₂, vermicullite, ...), by fluorides, by carbon. .

However, according to US Pat. No. 4,533,572, the magnesium particles can also be coated only with a varnish or a layer of carbon, by prior coating of said particles with the aid of polymerizable oils which are then heat treated and decomposed more. or less completely to finally obtain said varnish or said carbon layer; the oils used can be animal fats, petroleum oils with high boiling point, fatty acids, etc.

Desulfurizing products, based on coated magnesium, obtained using refractory products and organic binders, however, have drawbacks.
First of all, they are not completely impermeable to humidity, which gives off hydrogen in the event of contact with water (liquid or vapor); this release of hydrogen obviously presents a risk which must be avoided.
In addition, a slight rise in temperature can cause softening of the organic matter and thus cause deterioration of the refractory coating of the magnesium grains; this results in a risk of sticking and agglomeration of the grains between them, causing difficulties and irregularities in processing during the introduction of the desulfurizing agent into the liquid metal. These harmful temperature rises generally occur during the storage of products in exposed places, for example near ovens, sunshine ...

Given these drawbacks, the applicant has sought an organic product which is sufficiently resistant to temperature, while being easy to use and guaranteeing perfect protection against humidity.

DESCRIPTION OF THE INVENTION

The invention is a product for desulfurization of cast irons or liquid steels based on magnesium grains coated with an organic product which is solid at room temperature and a refractory powder, characterized in that the organic product is based on saturated aliphatic acids or their esters.

Solid acids or esters are usually used at low temperature, preferably below 60 ° C or better of 80 ° C, the melting point not generally exceeding 110 ° C, with a clear melting, without going through a softening phase.

The esters usually used are the glycerine triesters or their mixtures, in particular the saturated triesters obtained by hydrogenation of castor oil, which essentially leads to glycerol trihydroxystearate. Hydrogenated sunflower oil, glycerol stristearate and sometimes stearic acid can also be used.

These products have a high molecular weight, with a number of C atoms generally equal to or greater than 16 for acids, to 51 for their triesters.

It can be seen that the solid coating products of the invention make it possible on the one hand to make a coating of Mg which is stable at high temperature, without it being necessary to carry out chemical transformations of starting materials in order to obtain it. On the other hand, it may be recalled that with this stable coating, an Mg powder can withstand severe temperature storage conditions (proximity of ovens, sun, etc.) without its flowability being deteriorated.

The esters of the invention generally constitute most or preferably all of the organic product used for coating the magnesium. Thus said organic product may contain other organic products than the esters of the invention, in particular unprocessed residues of the products used to obtain said esters.
They are particularly suitable for coating magnesium on which they adhere remarkably thanks to their non-zero but low acidity which makes it possible to react very superficially with the Mg particles. For example hydrogenated castor oil has an acid number of 4 (which represents the weight of KOH in mg to neutralize 1 g of product) and melts at 86 ° C.

As a refractory coating product, any infusible metal oxides or other powdered refractories known from the prior art can be used, but also desulfurizing agents such as C₂Ca, CO₃Ca, CaO ... or other minerals such as carbons ... or their mixtures.

In the case where the refractory product is sensitive to humidity, such as for example C₂Ca, it is advantageous to coat it beforehand with an organic product, preferably those already mentioned, before coating it with Mg according to the invention.

CaO

54 à 60%

MgO

3 à 7%

SiO₂

24 à 30%

Al₂O₃

9 à 13%

However, the Applicant has found that it was advantageous to use as a refractory product, thus serving to coat the grains of magnesium agents with the aid of the esters according to the invention, a slag in the form of grains having the following composition (in % weight):

CaO

54 to 60%

MgO

3 to 7%

If

24 to 30%

Al₂O₃

9 to 13%

• . à la température des bains de fonte ou d'acier en fusion, il est à peu près en équilibre thermodynamique avec le magnésium avec lequel il ne réagit donc pratiquement pas; on évite ainsi une consommation parasite de magnésium;
• . si on le fabrique par fusion, il donne lieu, après coulée, à une transformation allotropique qui, au cours du refroidissement, le transforme naturellement en une poudre fine constituée de grains de diamètre moyen 40 µm environ.
• . il a une conductibilité thermique d'environ 0,5 Wm⁻¹K⁻¹ significativement plus faible que celle des oxydes habituels dans les mêmes conditions et donc favorise un effet retard dans l'action du magnésium dont les réactions sont jugées trop rapides et trop violentes.

Such powdered slag has the following advantages:

• . at the temperature of baths of cast iron or molten steel, it is almost in thermodynamic equilibrium with magnesium with which it therefore practically does not react; this avoids parasitic consumption of magnesium;
• . if it is produced by fusion, it gives rise, after casting, to an allotropic transformation which, during cooling, naturally transforms it into a fine powder made up of grains with an average diameter of about 40 μm.
• . it has a thermal conductivity of about 0.5 Wm⁻¹K⁻¹ significantly lower than that of the usual oxides under the same conditions and therefore promotes a delay effect in the action of magnesium whose reactions are considered too rapid and too violent .

The magnesium particles are usually less than 2 mm in size, preferably between approximately 300 and 800 μm and the refractory product powder has a grain size between approximately 0 and 80 μm.

In general, 0.5 to 1.5 parts by weight of ester according to the invention are used (for example hydrogenated castor oil) per 5 parts by weight of refractory protection product, most often about 1 part of ester for 5 parts of protective slag. The proportion of coating product relative to the desulphurizing agent can range from 2% to 25% (weight).

The coated magnesium desulfurizing product according to the invention can be mixed with other desulfurizing agents, in particular with calcium carbide, or with other mixtures based on calcium carbide (for example mixture of the carbonate type of calcium + calcium carbide + carbon), with or without lime. It can also be mixed with inert products.

To obtain the desulfurizing product according to the invention, it is possible to proceed as follows:
the magnesium particles and the ester according to the invention in the solid state are introduced simultaneously into a heated mixer so as to initially produce particles uniformly covered with ester; the refractory product powder is then introduced and particles coated with slag are then obtained which are removed from the mixer and then cooled.

Thus, the invention makes it possible to obtain desulfurizing products which are inert to water and humidity, giving no release of hydrogen and not even agglomerating, under severe conditions of storage or use. These products have improved safety in use.

EXAMPLE 1

Various products for desulfurization based on magnesium, hydrogenated castor oil and slag powder of between 0 and 80 μm were also prepared according to the process described above, also described above.

These products were injected at a dose of 0.1% into a liquid iron to be desulfurized at 1350 ° C containing 0.05% sulfur. The characteristics and the results are given in the following table:

All these products are inert to water, that is to say that when they are immersed in water at pH 7, no hydrogen is observed, result confirmed after 2 hours of immersion.
It is noted that the delay effect increases with the quantity of refractory product and tends to improve the quality of the cast iron obtained, up to a certain limit.

In comparison, the same immersion test for 2 h was carried out with magnesium particles coated according to the prior art.
Thus, coated magnesium was prepared according to Example 1 of EP 292 205; 25 g were immersed in water at pH 7 and 19 ° C, and a regular evolution of 50 cm³ of hydrogen was observed per hour.

A similar evolution of hydrogen was observed under the same conditions with magnesium coated with a phenolic resin and with magnesia according to the teaching of patent FR 2331618.

EXAMPLE 2

A desulfurizing product was prepared containing 20% of product No. 1 and 80% of a pulverulent mixture of composition (% by weight):

This product was injected into the same liquid iron as in Example 1, at a dose of 0.25%. The final sulfur content obtained was 0.012%.

EXAMPLE 3

Mg (300-800 µm)

80%

Huile de ricin hydrogénée

3%

C₂Ca (inférieur à 100 µm)

17%

A mixture based on magnesium, hydrogenated castor oil and micronized calcium carbide, of composition:

Mg (300-800 µm)

80%

Hydrogenated castor oil

3%

C₂Ca (less than 100 µm)

17%

Such a product according to the invention is particularly suitable for use in admixture with a powder also of calcium carbide. It has great storage security in hot environments, on the other hand, it is sensitive to water due to the presence of calcium carbide on the surface.
This product was injected in the same liquid iron as in Example 1 with the same proportion of 0.1%. The final sulfur content obtained is 0.005%.

The above drawback was avoided, in a complementary test, without altering the safety of storage in a hot environment, by coating C₂Ca with 1% (relative to C₂Ca) of hydrogenated castor oil before l '' introduce into the mixer containing Mg coated.

Claims (10)

1. A product for the desulphurisation of molten pig iron or steel based on magnesium grains coated using a solid organic product at ambient temperature and using a refractory powder, characterised in that the organic product is based on saturated aliphatic acids or their esters, with precise melting in the absence of a softening phase.
2. A product according to Claim 1, characterised in that the organic product based on saturated aliphatic acids or their esters has a melting point above 60°C.
3. A product according to any one of Claims 1 or 2, characterised in that the organic product based on esters is selected from the triesters of glycerine or mixtures thereof.
4. A product according to any one of Claims 1 to 2, characterised in that the organic product based on esters is selected from hydrogenated castor oil, hydrogenated sunflower oil, glycerol trihydroxystearate, glycerol tristearate or stearic acid.
5. A product according to any one of Claims 1 to 4, characterised in that the refractory product is a powder of a desulphurising agent such as C₂Ca, CO₃Ca, CaO, or of another mineral such as carbons or of mixtures thereof.
6. A product according to any one of Claims 1 to 4, characterised in that the refractory powder is a powder of electric-furnace slag of the following weight composition:

   CaO   54 to 60%

   MgO   3 to 7%

   SiO₂   24 to 30%

   Al₂O₃   9 to 13%
7. A product according to any one of Claims 5 or 6, characterised in that the refractory product is coated with an organic product beforehand.
8. A product according to any one of Claims 1 to 7, characterised in that the magnesium is less than 2 mm in size, and the refractory powder is less than 80 µm in size.
9. A product according to any one of Claims 1 to 8, characterised in that 0.5 to 1.5 parts (by weight) of said organic product based on ester is used per 1 to 5 parts of powder of refractory product, and that this coating product combination represents 2 to 25% of the weight of magnesium.
10. A process for obtaining the product of any one of Claims 1 to 9, characterised in that spherical particles of deslphurising agent and of said organic product based on aliphatic acids or their esters are first of all produced, and that they are then covered with the powder of refractory product.