CA2843798A1 - Method and apparatus for dephosphorising liquid hot metal such as liquid blast furnace iron - Google Patents

Abstract

This invention relates to a method for dephosphorising liquid hot metal, such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, wherein a pouring stream of the hot metal is discharged from a vessel containing the hot metal into a refining unit wherein in the refining unit one or more streams of additives for forming a molten slag and one or more gaseous streams for breaking up the pouring stream of hot metal into molten metal droplets are directed into the pouring stream wherein one or more of the gaseous streams and/or one or more of the streams of additives comprises oxygen in gaseous form or in compounded form, to allow a dephosphorisation reaction between the metal droplets, the oxygen and the molten slag during the fall of the molten droplets before being collected into a receiving vessel positioned below the refining unit, and to an apparatus for performing the method.

Description

Method and apparatus for dephosphorising liquid hot metal such as liquid blast furnace iron This invention relates to a method and apparatus for dephosphorising liquid hot metal such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, hereafter referred to as hot metal.
Premium low phosphorus ores could become significantly more expensive with increasingly limited availability and under these circumstances, steel companies will need to resort to more abundant higher phosphorus ores. In order to satisfy present and future product mix requirements with regard to low phosphorus levels in the steel, higher hot metal phosphorus levels resulting from the higher phosphorus load will need to be accommodated by modification of current steelmaking operating practices.
Previous hot metal phosphorus pre-treatment production processes included silicon removal in the blast furnace runner followed by phosphorus removal in the torpedo ladle by injection of powdered and gaseous dephosphorising reagents. This procedure led to the removal of silicon from the hot metal (chemical energy) together with a significant drop in temperature in the torpedo ladle (heat energy) and consequently, a requirement for a high hot metal ratio at charge (for example >90%). For plants designed to use significant amounts of scrap in their input materials and therefore having a typical hot metal ratio at charge of between 70% to 85%, such as the majority of the European steel plants, this method is not practically viable.
Alternatively 'a spare' BOS converter can be used for hot metal phosphorus pre-treatment whereby hot metal (no scrap) is blown more or less normally in a `dephosphorising' converter, with lime or pre-fused slag for a very short treatment time (e.g. less than eight minutes). In general, this is enough time to transfer a large portion of the phosphorus load to the slag. The dephosphorised metal is then tapped conventionally into a ladle and then charged together with scrap to a `decarburising' converter. However, this procedure is not an option when there is no 'spare' BOS-converter.
Moreover when such a procedure is retrofitted to an existing steel plant it poses a logistical challenge that would require major operational changes leading to possible inflexibility in steel plant operations. On a 'Greenfield Site' a `dephosphorising' converter option will require a very high capital outlay because of the high roof height needed to accommodate the oxygen lance travel.
The object of the invention is to provide a method which allows the use of higher phosphorus ores.
Another object is to provide a method which can be readily integrated into current steelmaking operating practices.

Still another object of the invention is to provide an apparatus for carrying out the inventive method.
One or more of these objects are achieved by a method for dephosphorising liquid hot metal, such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, wherein a pouring stream of the hot metal is discharged from a vessel containing the hot metal into a refining unit (a.k.a. refining vessel) wherein in the refining unit one or more streams of additives for forming a molten slag and one or more gaseous streams for breaking up the pouring stream of hot metal into molten metal droplets are directed into the pouring stream wherein one or more of the gaseous streams and/or one or more of the streams of additives comprises oxygen in gaseous form or in compounded form, to allow a dephosphorisation reaction between the metal droplets, the oxygen and the molten slag during the fall of the molten droplets before being collected into a receiving vessel positioned below the refining unit as described in independent claim 1.
Preferred embodiments of the method according to the invention are described in the dependent claims 2 to 10.
One or more of these objects are achieved by an apparatus for carrying out the method of any one of the preceding claims wherein the apparatus comprises a vessel for containing the liquid hot metal, such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, which hot metal is preferably already desulphurised, the vessel comprising means for discharging a pouring stream of the hot metal into a refining unit, the refining unit having a single reaction chamber, wherein the refining unit is provided with at least one first injection means for injecting a gaseous stream into the pouring stream of the hot metal to break up the pouring stream into molten metal droplets, and with at least one second injection means for injecting a stream of additives into the pouring stream and/or into the molten droplets and with an outlet for waste process gas, and an outlet for allowing the molten droplets to be collected into a receiving vessel.
The pouring stream preferably exits the vessel containing the liquid hot metal in a substantially vertical direction and enters the refining unit also in a substantially vertical direction before being broken up into molten metal droplets by the gaseous stream from the at least one first injection means.
Preferred embodiments of the apparatus according to the invention are described in the dependent claims 12 to 16.
The invention is also embodied in an apparatus wherein the at least one injection means for injecting a gaseous stream and/or the at least one second injection means for injecting a stream of additives into the pouring stream are mounted in an angle a to the pouring stream of between 0 (parallel to the pouring stream) and 75 , preferably wherein the angle is at least 10 and/or at most 60 . A preferred maximum is 45 . It is the intention that the gaseous stream and the stream of additives contact the pouring stream at the angle at which the respective injection means are mounted with respect to the vertical.
The choice of a is made so as to achieve a completely broken-up pouring stream into metal droplets that have an average diameter and size range required for optimum dephosphorisation performance. The condition should be to break up the stream to the desired droplet size and to achieve the required level of oxidation and slag basicity for maximum dephosphorisation at a controlled, minimal level of decarburisation and iron yield loss. Preferably the angle of the first and/or second injection means can be changed during the dephosphorisation, preferably independently, to enable optimisation of the breaking up of the pouring stream and/or the injection of the additives into the pouring stream.
The hot metal which is discharged from the vessel into the refining unit is preferably desulphurised prior to being dephosphorised according to the invention.
The one or more gaseous streams may comprise gaseous oxygen or oxygen containing gaseous compounds. The one or more streams of additives may comprise oxygen in compounded form, e.g. in the form of an oxide or carbonate.
The hot metal dephosphorising method according to the invention is preferably positioned in the steelmaking process route between the hot metal desulphurisation plant and the BOS-converter. The vessel containing hot metal, preferably desulphurised, preferably unskimmed, provides a pouring stream of the hot metal, e.g. by bottom-pouring, into a refining unit. Although the pouring could be done otherwise, e.g. by tilting the vessel and pouring it from a taphole in the sidewall or even by pouring it like a bucket, the bottom-pouring gives the best conditions for a stable pouring stream in terms of consistency and stability, and the best conditions for shielding the pouring stream from the influences of the surrounding atmosphere. The refining unit is preferably equipped with several individual multi purpose burner modules or nozzles containing one or more injection features for injecting one or more of gaseous compounds such as oxygen, nitrogen, natural gas or other gas, or solid compounds such as lime powder, flux powder or other powder that point directly into the pouring stream either from discrete positions or from an annular ring so that the pouring stream is completely broken-up into metal droplets that have an average diameter and size range required for optimum dephosphorising performance. Instead of lime, flux or other powder, granules could be used. The nozzles provide supersonic or subsonic jets of the gaseous and/or solid compounds. The droplet size is preferably at most 20 mm, and preferably at least 1 pm.
Additional individual multi purpose burner modules or nozzles of the aforementioned type and for the aforementioned purpose may be placed at other positions in the refining unit as required. The oxygen input rate and lime powder/flux powder/other powder input rates to the refining process are matched to the hot metal pouring rate and to the required state of oxidation and chemistry of the resulting slag. Total Fe in the slag is preferably between 10% and 40% and the slag basicity (CaO/Si02) is preferably between 1.0 and 4Ø
Phosphorus is transferred from the metal droplets to the slag at a high level of efficiency of at least 50% because the design of the method ensures a very high reaction surface; a relatively low temperature (between 1200 C and 1500 C compared to 1600 C to C at tap in a conventional BOS converter process), a high state of oxidation that is at or near an optimum condition for oxidation of phosphorus; and a slag composition that is at or near optimum condition for high phosphorus capacity.
Between the vessel and the refining unit a smaller containment vessel hereafter known as a tundish may be provided to allow the dephosphorisation process in the refining unit to be not directly linked to the vessel containing the hot metal, and to continue while the vessel containing the hot metal is emptied and replaced by a full vessel. When a tundish is used, the ferrostatic head of the hot metal may be held constant by sustaining a constant height of liquid metal in the tundish. In this way, the flow condition of the pouring stream may be effectively maintained. When a tundish is used, then the stream of hot metal to be broken up in molten metal droplets is discharged into the refining unit from the tundish rather than directly from the vessel.
Within the refining unit, the dispersed metal droplets are exposed to an oxidising and basic slag environment ensuring very fast silicon and phosphorus removal together with Fe oxidation and a significant rise in temperature. At the same time, the carbon content of the dispersed hot metal droplets may also be reduced albeit by a variable amount and dependent upon their average diameter and size range. High yield losses from fume in the off gas and FeO and metallic shot in the slag may occur when the average diameter of the dispersed metal droplets is low and the size range wide.
Therefore the size of the metal droplets preferably is between 1 pm and 20 mm.
A suitable minimum droplet size is 100 pm. A suitable maximum droplet size is 3000 pm.
The metal pouring stream geometry and the number, flow conditions, nozzle geometry and relative direction of the subsonic streams or supersonic streams from the nozzles are major factors in the determination of the average droplet diameter and size range. Therefore, the number of nozzles is preferably at least one and at most eight and more preferably at least two. A suitable maximum is four nozzles. The pouring stream shape may be irregular or may be round or rectangular or a combination of both; the supersonic core length of the stream or streams can be greater than, equal to or less than the nozzle to stream distance; the relative direction of the stream of additives or gaseous streams to the pouring stream can be between 0 (parallel to the pouring stream) and 750 .

Preferably this angle (a, see Figure 2) is at least 10 and/or at most 60 . A
more preferable maximum angle is 45 . The stream of additives or gaseous streams are directed perpendicularly to the pouring stream in case of the angle a being 90 , and are tilted downwardly in case of an angle between 0 and 90 . The choice of a is made so as 5 to achieve a completely broken-up pouring stream into metal droplets that have an average diameter and size range required for optimum dephosphorisation performance.
The condition should be to break up the stream to the desired droplet size and to achieve the required level of oxidation and slag basicity for maximum dephosphorisation at a controlled, minimal level of decarburisation and iron yield loss. Preferably the angle of the first and/or second injection means can be changed during the dephosphorisation, preferably independently, to enable optimisation of the breaking up of the pouring stream and/or the injection of the additives into the droplet stream.
In an embodiment means are provided between the refining unit and the receiving vessel for collecting the dephosphorised metal which enable continuous use of the refining unit without having to stop for changing the receiving vessel when this is full.
These means may consist of a buffer vessel or buffer tundish able to collect the dephosphorised metal while the full receiving vessel is exchanged for an empty one.
The advantage of the method according to the invention is that it can be implemented with relatively low capital and running costs; minimal logistical impact; high productivity; and, the simple and effective concept of rapid refining, particularly of relatively small volumes of hot metal per unit time, via the controlled generation of hot metal droplets within a regulated oxidising and basic environment.
In an embodiment the injected additives may be recovered decarburising converter slag that has been suitably processed to injectable grade. As such, the lime flux will be pre-fused and therefore easily melted.
In an embodiment injected additives may be alloy or ferro-alloy fines. As such, the chemistry of the ambient conditions can be additionally altered. These alloy or ferro-alloy fines may be recovered alloy or ferro-alloy fines.
The invention will now be further explained by means of the following, schematic and non-limiting drawings.
Figure 1 shows a schematic drawing of an apparatus for carrying out the process according to the invention. Figure 3 shows a more realistic drawing of an apparatus for carrying out the process according to the invention.
Figure 2 shows the angle a in relation to the pouring stream (PS) and one nozzle (n).
Figure 1 shows an apparatus according to the invention with a tundish between the vessel containing the hot metal and the refining unit (indicated in figure 1 with refining vessel). The inlets indicated with oxygen injection and lime injection in figure 1 are the gaseous streams and stream of additives in the context of this invention.
Within the steelmaking process route, the apparatus is placed in-line between the hot metal desulphurising plant and the BOS-converter. Hot metal (not skimmed) and preferably desulphurised, contained in a bottom-pouring vessel is bottom-poured into a tundish and then bottom-poured from the tundish as a pouring stream into the refining unit. The flow rate of the pouring stream into the refining unit can be monitored and controlled via load cells. One or more supersonic or subsonic gas jets as outputs from oxygen/
nitrogen/
other gas/ lime powder/ flux powder/ other powder/ natural gas burner modules or nozzles which can be either combined into a single multi purpose module or nozzle or as separate injectors point directly into the pouring stream either from discrete positions or from an annular ring.
Additional individual multi purpose burner modules or nozzles containing one or more injection features for injecting supersonic or subsonic oxygen, nitrogen, other gas, lime powder, flux powder or other powder or natural gas may be placed at other positions in the top or sidewall of the refining unit as required. The oxygen input rate and lime powder/flux powder/other powder input rates to the refining process are matched to the pouring rate of the pouring stream.
Within the refining unit the metal droplets, oxygen and molten slag will chemically react during the time it takes for the metal droplets to fall under gravity into the receiving vessel. Silicon and phosphorus refining will be accompanied by iron oxidation and a rapid rise in temperature. Further refining reactions will also take place between the slag and metal in receiving ladle. This can be gas-stirred to ensure that the slag and metal approach chemical equilibrium. Some carbon oxidation is also expected and this could lead to appreciable slag foaming within the receiving ladle. Consequently, the receiving vessel will require a suitably large freeboard to accommodate this.
After completion of the batch refining process, the receiving ladle (now the transfer ladle or charging ladle), will be moved away and the slag removed with a slag skimmer unit or other slag removal device. The slag may require pre-conditioning prior to slag skimming or slag removal to reduce metallic iron yield loss. After sampling and temperature measurement the dephosphorised and partially-decarburised hot metal, which generally has a temperature of between about 1250 C and 1500 C, will then be transferred and charged to the decarburising converter that already contains its required scrap charge. During the converter decarburisation process, the oxygen flow rate can be significantly higher than current practice, and have values of up to 1500Nm3/
min.
Consequently, it is expected that the converter processing time will be substantially shorter than current practice, leading to significant gains in productivity.

The phosphorus content of the slag skimmed (i.e. removed) from the receiving ladle, which is now the charging ladle for the decarburisation process, may well be high enough to ensure that such slag could be used as a basis for a fertiliser product.
The control systems preferably include flexible and independent control of one, more or all of the following: injectant inputs, pouring stream rate, metal and slag composition sampling and control, temperature measurement and/or off-gas analysis monitoring.
The temperature affects the performance by affecting the slag capacity for phosphorus and the metal droplet size. Too high temperature may cause the dephosphorising reaction to slow, stop, or reverse. On the other hand, the reaction vessel is required to be suitably pre-heated to ensure low levels of temperature loss. Hot metal temperature changes may be influenced by a combination of chemical heat (silicon and Fe (+ carbon, manganese) oxidation), chemical heat (natural gas burner) and conductive, convective and radiative thermal losses.
Because the phosphorus loading at charging the converter is significantly lower than normal operation and hot metal silicon content will be negligible, decarburisation time in the BOS converter can be reduced by utilising a high oxygen blowing rate. This will help to increase converter productivity. Less aggressive slag could be used which will reduce the need for slag splashing and refractory maintenance and in turn will lower converter heat losses. A high flow rate oxygen lance could be employed for the converter.
The decarburisation slag could be recycled to the dephosphorising reactor where it could be injected as a pre-fused flux addition.

Claims (16)

1. Method for dephosphorising liquid hot metal, such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, wherein a pouring stream of the hot metal is discharged from a vessel containing the hot metal into a refining unit wherein in the refining unit one or more streams of additives for forming a molten slag and one or more gaseous streams for breaking up the pouring stream of hot metal into molten metal droplets are directed into the pouring stream wherein one or more of the gaseous streams and/or one or more of the streams of additives comprises oxygen in gaseous form or in compounded form, to allow a dephosphorisation reaction between the metal droplets, the oxygen and the molten slag during the fall of the molten droplets before being collected into a receiving vessel positioned below the refining unit.

2. Method according to claim 1 wherein the liquid hot metal is desulphurised prior to dephosphorising according to claim 1.

3. Method according to claim 1 or 2 wherein the one or more streams of additives and/or the one or more gaseous streams are provided by means of one or more individual multi purpose burner modules or nozzles containing one or more injection features for injecting one or more of oxygen, nitrogen, natural gas, other gas, lime powder or granules, flux powder or granules, other powder or granules.

4. Method according to any one of the preceding claims wherein the one or more gaseous streams is directed downwardly under an angle .alpha. in respect to the stream of the hot metal.

5. Method according to claim 4 wherein the angle .alpha. is between between 10° and 60°.

6. Method according to any one of the preceding claims wherein the one or more streams of additives comprises one or more of lime, prefused slag, flux, ferroalloy or alloy powder or granules.

7. Method according to any one of the preceding claims wherein the metal droplets have a diameter of between 1 µm and 20 mm and more preferably between 100 µm and 3000 µm.

8. Method according to any one of the preceding claims wherein the receiving vessel is gas-stirred.

9. Method according to any one of the preceding claims wherein the additives comprise, or consist of, recovered decarburising converter slag.

10. Method according to any one of the preceding claims wherein the hot metal in the receiving vessel is subsequently subjected to a skimming or slag removal operation to remove the slag from the hot metal prior to charging to a converter and to a decarburisation operation in the converter.

11. Apparatus for carrying out the method of any one of the preceding claims wherein the apparatus comprises a vessel for containing the liquid hot metal, such as liquid blast furnace iron or liquid metal with a composition equivalent to blast furnace iron, which hot metal is preferably already desulphurised, the vessel comprising means for discharging a pouring stream of the hot metal into a refining unit, the refining unit having a single reaction chamber, wherein the refining unit is provided with at least one first injection means for injecting a gaseous stream comprising gaseous oxygen or oxygen containing gaseous compounds into the pouring stream of the hot metal to break up the pouring stream into molten metal droplets, and with at least one second injection means for injecting a stream of additives into the pouring stream and/or into the molten droplets and with an outlet for waste process gas, and an outlet for allowing the molten droplets to be collected into a receiving vessel wherein the at least one injection means for injecting a gaseous stream and/or the at least one second injection means for injecting a stream of additives into the pouring stream are mounted in an angle .alpha. to the pouring stream of between 10° and 75°.

12. Apparatus according to claim 11 wherein the angle .alpha. of the first and/or second injection means can be changed during the dephosphorisation to enable optimisation of the breaking up of the pouring stream and/or the injection of the additives into the pouring stream.

13. Apparatus according to claim 11 or 12 wherein one, more or all of the injection means are supersonic jets.

14. Apparatus according to claim 11 to 13 wherein additional injection means are provided along the top or the sidewall of the refining unit.

15. Apparatus according to any one of claims 11 to 14 wherein the receiving vessel is gas-stirred, preferably bottom stirred.

16. Apparatus according to any one of claims 11 to 15 wherein between the vessel containing, in use, the hot metal, and the refining unit there is provided a tundish to allow continuous production and a stable and constant ferrostatic head and wherein the tundish is provided with an opening for discharging a stream of the hot metal into the refining unit.