WO2013127869A1

WO2013127869A1 - A method of improving slag skimming performance in a hot metal desulphurisation process

Info

Publication number: WO2013127869A1
Application number: PCT/EP2013/053952
Authority: WO
Inventors: Zushu LI; Andy Ferguson; John Smith
Original assignee: Tata Steel Uk Limited
Priority date: 2012-03-01
Filing date: 2013-02-27
Publication date: 2013-09-06

Abstract

This invention relates to a method for the desulphurisation of hot metal, comprising intermixing the hot metal to be desulphurised with a slag containing as the principal ingredients lime,magnesium and a slag modifier, wherein the slag modifier is glass, and to the said slag modifier.

Description

METHOD OF IMPROVING SLAG SKIMMING PERFORMANCE IN A HOT METAL DESULPHURISATION

PROCESS

The invention relates to a method for the desulphurisation of hot metal and to a slag modifier for the modification of desulphurisation slag.

Historically, soda ash was used in conjunction with magnesium deep injection (known as "soda ash-Mg process") in the hot metal desulphurisation process in BOF steelmaking plants. In this process, a certain amount of soda ash is added into the hot metal stream as the metal is being poured from a torpedo into a transfer ladle. The soda ash acted as a slag modifying agent to aid slag skimming as well as a desulphurising agent. The term "hot metal" is used to indicate the molten iron product from the blast furnaces which is normally transported to the steel works in a so-called torpedo ladle.

In order to improve the hot metal desulphurisation process efficiency, which is more and more demanded by the increasing tonnage of steel produced and the increasing amount of low sulphur products required, the soda ash-Mg process is often upgraded to a lime/magnesium co-injection hot metal desulphurisation process. This cuts the desulphurisation time by up to 50%, or sometimes even more. However, the use of lime and magnesium has increased the slag bulk, altered the slag consistency and deteriorated slag conditions. Efficient slag skimming was sometimes not possible without slag modification for the heats with low hot metal temperature (< 1300 °C) and high CaO/Mg injection for producing ultra low sulphur steel grades. This also increased the steel yield loss as the total iron (TFe) content in the slag was sometimes found to be 80 wt% in the form of entrapped iron droplets and direct skimming iron loss was increased due to extended skimming operation.

The slag on top of the hot metal before desulphurisation is mainly composed of blast furnace slag and the remaining slag from previous heats. The other sources include spalling from the iron runner, heat insulation and a fraction of oxides formed by reaction between Fe, Mn, Si, V and Ti in the hot iron with oxygen in the air. The desulphurisation by CaO-Mg co-injection brings the CaO and MgO into the slag. The formation of MgO from the injected Mg and the injected CaO increase the slag basicity (%CaO/%Si0₂) and MgO content to up to %CaO/%SiO₂=5.0 and MgO=25% respectively which results in dramatic increase in the melting temperature and viscosity of the post-desulphurisation slag. Therefore, the post-desulphurisation slag, which can be considered mainly as CaO-Si0₂-MgO-AI₂0₃ system, has a high viscosity and melting temperature.

In order to generate a more fluid slag and to reduce the amount of hot metal entrapped within the slag, slag modifying agents are often added with the desulphurising agent or directly into the hot metal ladle before or after hot metal pouring to decrease the viscosity and melting temperature of the post- desulphurisation slag. Various materials have been used as slag modifying agents such as fluorspar, dolomitic lime, silica, sodium carbonate, sodium chloride, potassium chloride, potash, cryolite, potassium cryolite, colemanite, calcium chloride, calcium aluminate, sodium fluoride, anhydrous borax, nepheline syenite and soda ash.

The object of this invention is to provide a method of improving slag skimming performance in hot metal desulphurisation process in BOF steelmaking.

It is also an object of this invention to provide a method of reducing the yield loss by reducing the total iron content in the slag.

It is also an object of this invention to provide a method of reducing the yield loss by reducing direct skimming iron loss.

One or more of these objects can be reached by a method for the desulphurisation of hot metal, comprising intermixing the hot metal to be desulphurised with a slag containing as the principal ingredients lime, magnesium and a slag modifier, wherein the slag modifier is glass comprising at most 5% of Al₂0₃, preferably comprising at most 2.6% of Al₂0₃.

The inventors found that by adding glass as a slag modifier that the slag is made more fluid and becomes easier to skim off. The skimming time is reduced significantly. Therefore, direct skimming iron loss was much reduced. Also, the iron content in the slag is reduced considerably from about 80% in the slag without glass addition to the hot metal to a value of between 20 to 40% in the slag with addition of glass as a slag modifier to the hot metal. The build up on the skimming block and charging lip of the charging ladles was observed to be minimal, much lower than during standard operations. When glass was used as a slag modifier the build up on skim blades was considered better as less jack hammering was required and easier removal of skulls from the blade was reported. No effect was noticed on the build up on the lances.

It is important that the slag modifier has a low alumina content or below 5% and preferably of at most 2.6%. It is preferable that no additional soda ash and/or no silica sand and/or no Fe₃0₄ and/or no exothermic agent of metallic aluminium (for heating the slag) is added to or present in the glass to be used as a slag modifier.

No negative environmental issues have been identified.

The composition of the slag changes when glass is added. Without glass, the typical iron content of the slag is 80% and the non-metallic slag fraction is typically 43% Fe, 24% CaO, 10% Si0₂ and <0.08% Na₂0. With a glass addition there is typically 20-40% Fe and the non-metallic slag fraction is typically 25- 35% Fe, 15-30% CaO, 25-35% Si0₂ and 2-6% Na₂0. The phase diagram of the CaO-MgO-Si0₂-AI₂0₃ system shows that the melting point of the slag is over 1500°C and increases as the amount of lime is added, i.e. the slag will be more solid for lower steel sulphur aims. Typical iron temperatures arriving at a BOS plant are 1250-1400°C and therefore the slag is likely to be mainly solid at these temperatures. By adding glass the amount of Si0₂ is increased and consequently the melting temperature of the slag is reduced towards the iron temperatures seen in production. This is observed as a more fluid slag and consequently the slag appears easier to skim off.

The reduction of iron in the slag, less direct skimming iron loss, and subsequent skim iron recovery will result in increased iron remaining in the charging ladle, giving a resultant increase in yield. Assuming the liquid cast weight does not alter, less scrap will be required.

The inventors found that it is possible to use recycled glass as a slag modifier. This reuse of old glass is attractive from an energy conservation and carbon footprint point of view.

In an embodiment of the invention, the glass or the recycled glass, or mixtures thereof, comprise:

• At least 50% Si0₂ and/or at most 80% Si0₂

• At most 20% CaO

• At least 5% Na₂0 and/or at most 12% Na₂0,

• At most 0.2% Pb

· At most 0.1% of any one of Sr, BaO and Zr0₂

• At most 0.5% of a fluoride component

• No Na₂C0₃ and K₂C0₃ except as an unavoidable impurity

When the glass satisfies these compositional requirements, then the skimming performance is excellent, whereas the pick-up of undesirable elements from the glass by the hot metal is minimised. For that reason, a preferable embodiment of the inventions is that the slag modifier comprises no Pb, Sr, BaO, Zr0₂ or fluoride component except as an unavoidable impurity. These elements are considered particularly harmful for the resulting steel and their presence should be avoided. Also when glass contains elements such as BaO and Pb these also transfer across to the slag; this may limit the uses of the material from the skim pit and therefore it is recommended to avoid glass with these contaminants. Preferably the Na₂0 content of the slag modifier is at most 10%, more preferably at most 9%.

Fluorine-containing modifiers are detrimental to both environment and hot metal ladle refractories; and Na₂C0₃ and K₂C0₃ additions (used as the raw materials of slag modifiers) is also not good for environment, ladle lining life and waste gas cleaning system. Also, because of the complex preparation process and its cost of raw materials, these slag modifying agents can be much more expensive than the glass, recycled or otherwise.

In an embodiment of the invention at least 1% CaO is present in the slag modifier.

In an embodiment of the invention the glass is added in the form of powder or cullet or granules. The inventors found that the form in which the glass is added is important. Wet glass should be avoided. Also, the size of the glass particles influences the occurrence of fumes. Moist particles and very small sized particles are more likely to cause fume. Also, the contaminants like Pb are believed to be the cause for fumes. Although the skimming performance is not affected by the occurrence of fumes, it is preferable that the glass is added in the form of granules having a maximum size of about 50 mm, preferably having a maximum size of about 25 mm, and having a minimum size of about 2 mm.

In a preferable embodiment glass cullet of 5-10 mm in size is used. No fumes were observed during hot metal pouring onto the glass cullet and slag skimming when using material of this size. Using cullet with a maximum size of between 10 to 50 or 10 to 25 mm showed a similar performance to 5-10 mm cullet, but these larger sizes are currently more costly than the 5-10 mm size fraction. Therefore, although from a technical performance the glass works just as well as the 5 to 10 mm cullet, it is currently less cost efficient to use the 10-25mm size.

In an embodiment of the invention at least 0.3 kg/tonne hot metal was used. A suitable upper value is 11 kg/tonne hot metal. This upper and lower boundary provided excellent results. According to a second aspect the invention is also embodied in glass or recycled glass for use as slag modifier for the modification of desulphurisation slag in the desulphurisation of hot metal, said glass comprising at most 5% of Al₂0₃, and preferably at most 2.6% of Al₂0₃.

An embodiment of the slag modifier for the modification of desulphurisation slag comprises:

• At least 50% Si0₂ and/or at most 80% Si0₂

• At most 20% CaO

• At least 5% Na₂0 and/or at most 12% Na₂0,

· At most 0.2% Pb

• At most 0.1% of any one of Sr, BaO and Zr0₂

• At most 0.5% of a fluoride component

• No Na₂C0₃ and K₂C0₃ except as an unavoidable impurity

In a preferred embodiment of the slag modifier for the modification of desulphurisation slag the slag modifier comprises no Pb, Sr, BaO, Zr0₂ or fluoride component except as an unavoidable impurity, and/or the slag modifier comprises at least 1% CaO.

In another embodiment the slag modifier has:

• at least 70% Si0₂, and/or

· 8 to 12% CaO, and/or

• 7 to 10% Na₂0, and/or

• 0.1% maximum Pb.

The invention will now be further explained by means of non-limitative examples. Table 1 provided a typical slag chemistry with and without glass addition

Table 1

Overall Slag Chemistry / %

Iron / % CaO Si0₂ Na₂0

Standard operation 80 24 10 0.08

With 1000kg glass addition 20-40 15-30 25-35 2-6 Table 2 provides an analysis of the glass used as slag modifiers.

Table 2

Using the slag modifiers the slag composition prior to skimming was determined. The results of that analysis are provided in table 3.

Cr₂0₃ 0.02 0.02 - 0.02

BaO - 2 - -

Zr0₂ - 0.43 - -

V₂0₅ - 0.05 - -

Sr - 1.4 - -

C 4.5 - - -

S 2.79 1.58 - -

These results clearly show that the addition of glass as a slag modifier is very effective in reducing the iron content of the slag. Meanwhile the reduction in both skimming time and direct skimming iron loss was also confirmed for the heats with the addition of glass as a slag modifier. The trials also showed that the use of a very fine glass powder leads to a good result in terms of slag modification, skimming time reduction, reduction of iron in the slag and increased iron yield, but it also leads to the formation of fumes, and this is not preferred as it may interfere with the proper operation of the filtering systems. The results for Batch 3 are comparable to Batch 2, and results for Batch 5 are comparable to Batch 4.

Claims

1. A method for the desulphurisation of hot metal, comprising intermixing the hot metal to be desulphurised with a slag containing as the principal ingredients lime, magnesium and a slag modifier, wherein the slag modifier is glass comprising at most 5% of Al₂0₃.

2. A method according to claim 1 wherein the slag modifier is glass comprising at most 2.6% of Al₂0₃.

3. Method according to claim 1 or 2 wherein the slag modifier is recycled glass.

4. Method according to any one of claims 1 to 3 wherein the glass or a recycled glass for use as a slag modifier comprises:

• At least 50% Si0₂ and/or at most 80% Si0₂

• At most 20% CaO

• At least 5% Na₂0 and/or at most 12% Na₂0,

• At most 0.2% Pb

• At most 0.1% of any one of Sr, BaO and Zr0₂

• At most 0.5% of a fluoride component

• No Na₂C0₃ and K₂C0₃ except as an unavoidable impurity

5. Method according to claim 4 wherein the slag modifier comprises no Pb, Sr, BaO, Zr0₂ or fluoride component except as an unavoidable impurity.

6. Method according to claim 4 wherein the slag modifier comprises at least 1% CaO and/or at most 10% Na₂0, preferably at most 9% Na₂0.

7. Method according to any one of claims 1 to 6 wherein the glass is added to the hot metal in the form of powder or cullet or granules.

8. Method according to any one of claims 1 to 7 wherein the glass is added to the hot metal in the form of granules having a maximum size of about 50 mm, preferably having a maximum size of about 25 mm.

9. Method according to any one of claims 1 to 8 wherein at least 0.3 kg of glass per tonne of hot metal is added to the hot metal, preferably at least 1 kg/tonne of hot metal.

10. Method according to any one of claims 1 to 9 wherein at most 11 kg of glass per tonne of hot metal is added to the hot metal, preferably at most 5 kg/tonne of hot metal, more preferably at most 3 kg/tonne of hot metal.

11. Method according to any one of claims 1 to 10 wherein the glass is added :

• directly onto the bottom of a ladle prior to pouring the hot metal into the ladle, and/or

• onto the hot metal during or after pouring the hot metal into the ladle, and/or

• onto the hot metal desulphurisation slag after the desulphurisation, and/or

• by injecting it into the hot metal during the desulphurisation, preferably along with the lime and/or Mg.

12. Glass or recycled glass for use as slag modifier for the modification of desulphurisation slag in the desulphurisation of hot metal, said glass comprising at most 5% of Al₂0₃, and preferably at most 2.6% of Al₂0₃.

13. Slag modifier for the modification of desulphurisation slag according to claim 12 comprising :

• At least 50% Si0₂ and/or at most 80% Si0₂

• At most 20% CaO

• At least 5% Na₂0 and/or at most 12% Na₂0,

• At most 0.2% Pb

• At most 0.1% of any one of Sr, BaO and Zr0₂

• At most 0.5% of a fluoride component

• No Na₂C0₃ and K₂C0₃ except as an unavoidable impurity

14. Slag modifier according to claim 13 wherein the slag modifier comprises no Pb, Sr, BaO, Zr0₂ or fluoride component except as an unavoidable impurity, and/or wherein the slag modifier comprises at least 1% CaO.

15. Slag modifier according to claim 13 or 14 having :

• at least 70% Si0₂, and/or

• 8 to 12% CaO, and/or

• 7 to 10% Na₂0, and/or

• 0.1% maximum Pb.