GB2039536A - Desulphurising molten metals - Google Patents

Abstract

A desulphurising composition for molten ferrous metals, comprises a sintered mixture of lime in an amount exceeding 60% by weight, a total of at least 15% by weight of one or more alkali metal or alkaline earth metal fluorides and at least 5% by weight of alumina. The composition which has little tendency to absorb moisture and tends neither to agglomerate nor to introduce hydrogen into the molten metal may be injected into the melt in a ladle.

Description

SPECIFICATION Treating molten metals This invention relates to treating molten metals, particularly to desulphurising molten ferrous metals.

The harmful effects of sulphur in steel are well known and many desulphurisation processes have been proposed. Such processes include so-called external desulphurisation processes, i.e. ones conducted outside the furnace, and have been proposed both for removal of sulphur from iron in ladles and, more recently, for removal of sulphur from steel in ladles.

Methods known for external desulphurisation of iron include (1 ) stirring a desulphurisation agent such as calcium carbide into the metal, (2) plunging desulphurisation agents e.g. magnesium-impregnated coke under the surface of the metal and (3) injecting particulate desulphurisation agents e.g. lime.

In the case of steel it is known to effect ladle desulphurisation by bagged additions of lime-based materials containing fluxing agents. It has also been proposed to desulphurise steel in basic ladles by deep injection of particulate desulphurisation agents such as calcium silicide, calcium carbide or magnesium.

The known materials for external desulphurisation of iron or steel all have one or more disadvantages.

Serious disadvantages include removal of less sulphur than it would be desirable to remove, contamination of the iron or steel by matter derived from the desulphurisation agent and environmental pollution arising from the nature of the desulphurisation agent.

The present invention provides a composition which enables disadvantages of the known materials for external desulphurisation of iron or steel to be avoided or substantially reduced.

According to the invention, a composition for desulphurising molten ferrous metals comprises a sintered mixture of lime, in an amount in excess of 60% by weight, a total of at least 15% by weight of one or more alkali metal or alkaline earth metal fluorides and at least 5% by weight of alumina.

The sintered mixture may contain additional ingredients, e.g. sodium carbonate preferably in an amount not exceeding 15% by weight, more preferably 1 to 10% by weight, but it is preferred that the mixture should consist entirely or substantially entirely of the lime, the fluoride(s), the alumina and any sodium carbonate included. The amount of lime is up to 80% by weight and preferably the amount of the fluoride(s) is not more than 30% by weight.

It has been found in accordance with the invention that desulphurisation agents for the purposes in question are most effective if they are highly basic and that lime is particularly advantageous over other basic oxides as it is both cheap and highly basic, much more basic than magnesium oxide for example.

Likewise, whilst desuiphurisation agents containing relatively low proportions of lime are known, it has now been found that it is advantageous to have more than 60% by weight of this ingredient as this promotes an advantageously high basicity of the composition.

Despite the fact that high proportions of lime are desirable, lime by itself is unsatisfactory in that it does not melt at practical liquid iron and steel temperatures and thus sulphur removal only occurs by liquid-solid surface reactions and high addition rates e.g. 15 to 20 kg/tonne of iron are therefore necessary. Such addition rates create problems in disposing of the large quantities of slag produced and result in excessive chilling of the iron treated. Furthermore, at such high addition rates, the proportion of fines in powdered lime gives pollution problems.

Silica is a cheap material that is known to react with lime at iron and steel making temperatures to form a product of much lower melting point and desulphurisation agents containing some lime and as much as 20% by weight of silica are known. However, it has been appreciated in accordance with the present invention that silica is an undesirable constituent as it is acidic and therefore diminishes the basicity provided by the lime. In the compositions of the invention silica preferably is absent or does not exceed 2% by weight.

The compositions of the invention include one or more alkali metal or alkaline earth metal fluorides, preferably calcium fluoride. The inclusion of the fluoride(s) serves to render the composition meltable at relatively low temperatures to form a slag. The fluoride is not acidic and thus does not diminish the basicity provided by the lime. It has been found in accordance with the invention that basicity can be maintained at an advantageously high level and a composition of advantageously low melting point simultaneously obtained if the proportion of fluoride(s) is 15% by weight or more.Whilst smaller amounts of calcium fluoride e.g. about 11 to 12% by weight also reduce the melting point of lime-based mixtures to some extent, it has been appreciated in accordance with the invention that the lower melting points obtainable with the use of 15% by weight or more of fluoride(s) present a substantial advantage.

When a desulphurisation agent is added to a ladle before or during pouring iron or steel into the ladle the turbulence promotes contact between the agent or slag formed from it and the molten metal and this promotes desulphurisation. However, once the ladle is full there is little or no turbulence and usually substantially no further desulphurisation. As the tapping time may be as short as four minutes even for a 300 tonne heat, there is very little time for a fluid slag to be formed and to act on the metal. The compositions according to the invention have the advantage that they can form fluid slags very rapidly.The compositions may start melting at 1150"C, melting may be complete at 1 2500C and the slag may be very fluid at 1350 C. The rapid formation of a very fluid slag greatly favours rapid desulphurisation.

The compositions of the invention include alumina and it has been found in accordance with the invention that this assists rapid reaction of the components to form a very fluid slag at relatively low temperatures. The alumina has little influence on the basicity provided by the lime. Inclusion of less than 5% by weight of alumina means that the alumina then exerts little beneficial effect.

Inclusion of sodium carbonate in the compositions increases their basicity and decreases their melting point but, despite these advantageous effects, it is, as already indicated, preferred that the content of sodium carbonate should not exceed 15% by weight as it has been observed in accordance with the invention that high sodium carbonate contents tend to result in an undesirably high evolution of fume when the compositions are used.

In the compositions of the invention the ingredients mentioned above are in the form of a sintered mixture and it has been found in accordance with the invention that this yields a variety of advantages. First, the sintering means that the ingredients form a slag much more rapidly than if they were used as a simple mixture. Secondly, the sintering results in a significant reduction of the amounts of dust and fume produced in use. Thirdly, the sintering greatly reduces the tendency of the compositions to absorb moisture.This is particularly important in compositions containing such a high proportion of lime, which normally tends to absorb moisture readily from the atmosphere, and the reduced moisture content renders the compositions more free-flowing and easier to handle and facilitates injection into molten metal, the risk of blockage of the injection iance and associated supply equipment being greatly reduced. The reduced moisture content is also a substantial advantage in that is reduces the risk of the metal becoming contaminated with hydrogen to such an extent that it is embrittled.

The compositions of the invention preferably consist entirely or substantially entirely of the sintered mixture.

A method according to the invention for making a composition according to the invention comprises forming a mixture of particles of the ingredients of the sintered mixture and sintering the mixture.

The mixture is preferably sintered using techniques similar to those used in making Portland cement, a rotary cement-roasting kiln giving a temperature of about 1100 to 1 2000C being suitable. The resulting sintered material may be screened to give a powder or granular product. Preferably not more than 15% by weight of the product is of particles of size 0.075 mm or less.

According to the invention a method of desulphurising a molten ferrous metal comprises contacting the molten metal with a composition according to the invention.

The molten metal may be contacted with the composition by any of the known methods such as injection beneath the surface of the metal using an inert or a hydrocarbon carrier gas or addition to a vessel such as a ladle before or during pouring of the metal into the vessel.

The metal may be iron or steel and the composition is especially suitable for desulphurising steel.

The following Examples illustrates the invention.

Example 1 A mixture of particles of the following ingredients in the percentages given was made and the mixture sintered: lime 61 % by weight fluorspar 24% by weight alumina 10% by weight sodium carbonate 5% by weight The mixture was sintered in a 1 2m long rotary kiln rotating at 1 r.p.m. The necessary heat was supplied by a burner giving a temperature of 11 500C in the burner zone, diminishing to 250"C at the back end of the kiln.

The mixture was fed into the kiln at 200 kg/hour and the hard, sintered product obtained screened to - 12 mm.

500 kg. of the screened, sintered product was added at a rate of 5.5 kg/steel tonne to a ladle as steel at 1640"C was tapped from an electric furnace into the ladle. The addition was complete when the ladle was 1/3 full. Alloying, including recarburising, agents were also added and the temperature of the steel in the ladle was 1620 C.

The steel in the furnace had a carbon content of 0.06% and a sulphur content of 0.024%. After the treatment the carbon content was 0.12% and the sulphur content 0.012% whilst the silicon manganese and phosphorus contents were respectively 0.25%, 1.05% and 0.014%. In contrast, to achieve the same desulphurisation (50%) usng an unsintered mixture required an application rate of about 9 Kg./steel tonne and dust and more fume is associated with the unsintered mixture.

In a further trial, 500 Kg. of the screened, sintered product was put in the bottom of a ladle and steel tapped on to it from an electric furnace. The sintered product was used at a rate of 7 Kg./steel tonne.

Before tapping the steel analysis was: 0.07% C, 0.26% Si, 0.62% Mn, 0.034% P, 0.107% S, 0.019% Al and after the treatment the analysis was: 0.09% C, 0.35% Si, 0.75% Mn, 0.035% P, 0.035% S, 0.01% Al.

Thus, 67% desulphurisation was achieved at an application rate of 7 Kg./steel tonne, a substantial improvement on the unsintered mixture. Again, dust was eliminated and fume much reduced.

Example 2 A mixture of particles of the following ingredients in the weight percentages given was made and the mixture sintered: lime 70% by weight alumina 5% by weight fluorspar 25% by weight The sintered product was suitable for desulphurising molten ferrous metals and was superior in desulphurisation efficiency to the unsintered mixture and was dust-free and gave little fume.

Example 3 A mixture of particles of the following ingredients was made and the mixture sintered: lime 65% by weight alumina 10% by weight fluorspar 25% by weight The properties of the sintered product were generally similar to those of the product of Example 2.

Example 4 A mixture of particles of the following ingredients was made and the mixture sintered: lime 65% by weight alumina 25% by weight fluorspar 10% by weight The properties of the sintered product were generally similar to those ofthe product of Example 2.

Example 5 A mixture of particles of the following ingredients was made and the mixture sintered: Lime 70% by weight alumina 15%byweight fluorspar 15% by weight The properties of the sintered product were generally similar to those of the product of Example 2.

Claims (11)

1. A composition for desulphurising molten ferrous metals comprising a sintered mixture of lime, in an amount in excess of 60% by weight, a total of at least 15% by weight of one or more alkali metal or alkaline earth metal fluorides and at least 5% by weight of alumina.

2. A composition according to claim 1 in which the proportion of the fluoride(s) is not more than 30% by weight.

3. A composition according to either preceding claim in which the sintered mixture contains sodium carbonate.

4. A composiiton according to claim 3 in which the proportion of the sodium carbonate is 1 to 10% by weight.

5. A composition according to any preceding claim not containing more than 2% by weight of silica.

6. A method of making a composition according to any preceding claim comprising forming a mixture of particles of the ingredients in appropriate proportions and sintering the mixture.

7. A composition made by a method according to claim 6.

8. A method of desulphurising a molten ferrous metal comprising contacting the molten metal with a composition according to any of claims 1 to 5 and 7.

9. A method according to claim 8 in which the composition is added to a ladle before or during adding the metal to be treated to the ladle.

10. A method according to claim 9 in which the metal is added to the ladle over a period of about four minutes or less and desulphurisation occurs partly during this period and partly when all the metal has been added to the ladle.

11. A composition according to claim 1 which melts completely at a temperature of about 1 250"C or less.