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Tailored solutions for hot metal pretreatment facilities

Dr. Robert Robey, Chief Metallurgist, Mark Whitehead, Director – Sales and Process

Technology, SMS Mevac UK Limited

(Published in MPT International, 2013, Issue 6, pp34-38)

Abstract

The need to remove impurities from liquid iron (hot metal) has long been recognised. Sulphur removal has been the main focus for hot metal pretreatment. Due to the use of lower quality raw materials or because of lower final product phosphorus requirements, in recent years demand has increased for the removal of phosphorus from hot metal. For effective phosphorus removal, lower hot metal silicon levels are a pre-requisite. This paper presents various solutions for hot metal pretreatment facilities for desiliconisation, dephosphorisation and/or desulphurisation.

Introduction

Liquid iron, supplied direct from the blast furnace is normally delivered to the steelmaking

plant in refractory lined torpedoes or ladles. The choice of transport vessel is dependent upon

the distance travelled and the techno-economic aspects of the process. Torpedoes offer a

way to minimise the temperature losses of the liquid iron during transit since their design

incorporates a large thermal mass with a small opening at the top. The open top ladle

approach avoids the high capital cost of a torpedo fleet and the subsequent process of

pouring from the torpedo into a hot metal ladle for transfer to the primary melting furnace. The

downside is that higher heat losses from the surface of the ladle are possible.

When the pre-treatment of hot metal began, reagents were added to the blast furnace runner.

Later, reagents were added by subsurface injection into the liquid iron in the torpedo. Almost

invariably, the requirement was for sulphur removal. Reagents such as soda ash, lime or

calcium carbide were injected into the iron though a submerged, refractory coated steel tube.

Sulphur was removed but the final sulphur level that could be achieved was limited because

there was a tendency for sulphur to revert from the sulphur laden slag back into the iron. This

occurred because deslagging is difficult, so much of the slag generated was transferred into

the hot metal transfer ladle. Additionally, over many heats, some of the sulphur rich slag

would be retained in the torpedo where again the sulphur could revert back to the iron and the

effective iron carrying capacity of the torpedo was gradually reduced by the build up of slag.

Desulphurisation

Modern steel plants now desulphurise the liquid iron once the metal has been poured from the

torpedo into a hot metal transfer ladle. These ladles have a pouring spout to transfer the liquid

iron into the primary melting furnace, whether it be a BOF vessel, electric arc furnace or

CONARC® (combined converter and electric arc furnace). Opposite the pouring spout a

deslagging lip can be added to allow for easier removal of the sulphur laden slag after reagent

addition, before transfer to the primary melting furnace. The use of a hot metal transfer ladle

makes for much easier access to remove the high sulphur slag generated during the

desulphurisation process.

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There are three methods commonly in use for hot metal desulphurisation: (a) deep injection

through a refractory coated lance in the torpedo [torpedo injection]; (b) deep injection through

a refractory coated lance in the hot metal transfer ladle [ladle injection]; and (c) bulk reagent

addition by mechanical stirring [mechanical stirring]. Torpedo injection is mainly a legacy

process, installed many years ago at various plants where this was the best available

technology at the time. The choice of whether to use ladle injection or mechanical stirring is

often decided by an evaluation of the availability and cost of reagents for use with either

process, temperature losses and an analysis of the time available for the desulphurisation and

slag removal steps.

The fundamentals of desulphurisation rely on the chemical reactions between the common

reagents and sulphur. The reagents, in order of desulphurising efficiency are magnesium

(Mg), calcium carbide (CaC2) and lime (CaO). Unfortunately the most effective reagent for

sulphur removal is also the most costly per unit weight and therefore is not necessarily the

reagent of choice. The choice of reagent may also influence the number of stations required

for a given final sulphur level. A high sulphur removal requirement will result in longer

treatment times at the desulphurising station and larger amounts of slag generation. Typically

the lowest level of sulphur required leaving the unit is 0.002 wt%. However, the actual final

sulphur requirement is very dependent on the steel grades being produced. The incoming

sulphur level, the final sulphur level required, the consequent treatment time required and the

ladle handling logistics determine the number of stations required to supply the primary

melting furnaces with the volume of desulphurised liquid iron requested.

Figure 1 Desulphurisation plant with tilting ladle transfer car

During treatment, the reagent is pneumatically conveyed by nitrogen into the liquid iron.

During injection, large amounts of fume are generated and this needs to be captured by the

fume extraction system. Likewise, the fume given off during the slag removal procedure must

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also be captured. The best way to achieve this is by placing the ladle within a substantially

enclosed area. Where this is not possible, a close fitting fume hood should be used directly

above the ladle. If the ladle is placed on a moving car, one end of the moving car can be

made as a refractory coated wall which can act as one side of the enclosure as the ladle

moves under the treatment area. Alternatively once the ladle transfer car is inside the

enclosure, moving doors can be used to close the treatment area at the ladle entry end. The

tilting mechanism for the ladle is incorporated into the car, as in figure 1. This usually

comprises of an hydraulic cylinder to lift the ladle about a fulcrum or a rack and pinion type

arrangement. These systems guide the ladle lip to the right position with respect to the

deslagging machine and slag pot for collecting the slag from the deslagging operation.

When the shop crane is used to place the ladle onto stands within the fume enclosure, a

travelling hood arrangement is used to move a fume cover over the ladle. In this case, an

hydraulically operated hook mechanism may be used to tilt the ladle about its trunnions to

facilitate the deslagging operation (see figure 2).

Figure 2 Desulphurisation unit with ladle stand, tilting hook and travelling fume hood

Where a travelling fume hood design is used, the reagent injection lance can be mounted on a

swivelling carriage that moves out over the travelling cover once it is in the treatment position

as in figure 3. In most other cases the lance can be mounted on a fixed carriage located

directly above the ladle treatment position. A second, duplicate lance carriage is often

supplied so that, in the event of a powder lance blockage, a new lance in the second position

can be used to finish the treatment. This ensures high availability and minimum interruption to

the supply of desulphurised hot metal to the primary melting furnace.

The quality and economic supply of each reagent plays a part in deciding which reagent or

combination of reagents to use. Whilst mono-injection of CaO or CaC2 is sometimes selected,

the longer treatment times and larger slag bulks generated may require more treatment

stations to achieve the same plant throughput as a co-injection system. Co-injection of

reagents allows the supply of Mg and CaO or Mg and CaC2 powders down a single lance at

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the same time. In a co-injection system, two injection dispensers are used which feed into a

common injection line. At the start of the treatment the lower value reagent (CaO or CaC2) is

passed into the injection line with the nitrogen carrier gas. As the powder exits the lance, the

lance is driven into the molten metal. When the lance reaches the bottom position, the Mg

dispenser outlet is opened and Mg is injected simultaneously with the CaO or CaC2. The use

of co-injection makes efficient use of both Mg and the other reagent to give a shorter

treatment time.

Figure 3 Swivelling injection lance arrangement over travelling fume hood

A further development of the co-injection system is the Eco-injection system. This system has

three injection dispensers available, one each containing CaO, CaC2 or Mg. Using a

mathematical model, the system recommends materials, quantities and combinations that

should be injected considering the aim sulphur, the start sulphur level, the cost of the

reagents, the iron temperature and chemistry and the time available for treatment. This type

of system offers the steelmaker the most flexibility in operating the desulphurisation plant and

gives the lowest possible treatment cost

Where space for the addition of further hot metal desulphurisation stations in a particular steel

plant is limited or a shorter treatment time is required it is possible to increase the capacity of

the stations by installing additional injection dispensers and operating a Twinjection™ system [1]. For this type of system the injection time is reduced by using co-injection through two

separate channels down a single refractory coated lance. Because there are two separate co-

injection streams, the rate of reagent addition is effectively doubled and the overall treatment

time is reduced without reducing reagent efficiency

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Mechanical Stirring

In most situations the capital cost of building a mechanical stirring station for hot metal

desulphurisation is higher than for a ladle injection type desulphurisation station, however, this

can be offset by the lower operating costs associated with the use of lower cost reagents. For

the case of mechanical stirring, the reagent is added as a bulk addition onto the surface of the

liquid iron as the melt is stirred with a refractory coated impellor (figure 4.). These are

granular materials rather than the fine powders used in deep injection.

Figure 4 Mechanical stirring impellor and support structure

A strong vortex within the metal stream is generated by the rotating impellor. This disperses

the reagent particles deep into the hot metal where they flux and react with the liquid iron. For

mechanical stirring the reagent particle size can be much larger, typically 1 to 5 mm may be

used. Whilst CaC2 can be used, this is uncommon and the predominant reagent is burnt lime.

Because of the need for stiff structures to counteract the reaction forces from the rotating

impellor, the impellor is usually mounted in a fixed structure and the ladle is brought to the

mechanical stirring station on a transfer car. Alternatively, the ladle can be placed on a ladle

stand and the the impellor can be brought over the ladle by using a moving gantry type

arrangement. The reagent can be added by pneumatic conveyance, blowing the material onto

the surface of the melt or by a simple feeder and chute arrangement from the storage

hoppers.

After the stirring treatment the ladle requires deslagging, so a ladle tilting mechanism and

deslagging machine are still required. Since Mg cannot be used in mechanical stirring

stations (the Mg would simply burn on the surface of the hot metal), considerably more lime is

used than for deep injection systems. This causes higher temperature losses and generates a

higher slag bulk which must be removed. With the slag, some pure iron is also removed, so

overall metal losses are often higher for mechanical stirring stations, although iron recovery is

possible using standard slag recovery systems.

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Desiliconisation and dephosphorisation

There are two reasons why steelmakers look to hot metal pre-treatment for the removal of

phosphorus. Firstly, in Japan, the desire to produce steels with very low final phosphorus

levels has led to the practice of removing phosphorus from the hot metal ladle, or even via the

two stage converter process. Secondly, however, more and more individual steelmakers are

looking at the feasibility of using lower cost, higher phosphorous containing raw materials for

blast furnace operations. This can result in more converter reblows for high phosphorous or

the need to remove phosphorous via hot metal pre-treatment or further phosphorus removal in

the primary melting furnace. In order to remove phosphorous from liquid iron it is necessary to

have a low silicon content, less than 0.2% [2]. Therefore the pre-treatment of hot metal for

dephosphorisation is commonly preceded by desiliconisation [3].

The amount of silicon to be removed varies widely depending upon the silicon level of the hot

metal. The simplest way to remove the silicon is by the addition of iron oxides in the blast

furnace runner or into the pouring stream as the torpedo fills the hot metal transfer ladle, figure

5. The sizing of the iron oxide particles can be relatively coarse, but the particle granularity

has to consider the mechanism of addition and delivery rate that is required. The feed rate of

material into the blast furnace runner has to be matched with the flow rate of the liquid iron

and the size of the reagent storage vessel has to reflect the quantity of reagent that is likely to

be required each day. A high silica slag is generated from the desiliconisation treatment

which must be removed at some stage. When the iron oxides are added into the pouring

stream from the torpedo to the ladle, then the iron oxides can be added relatively quickly. The

energy of the pouring stream is used to vigorously mix the iron oxides throughout the ladle

and effectively remove the silicon. Deslagging is required in both cases prior to subsequent

processing.

Figure 5 Desiliconisation arrangement at the hot metal pouring station

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Alternatively, it is possible to carry out desiliconisation in the torpedo car using injection of iron

oxides. However, injection processes in the torpedo have proven troublesome in that build up

of the generated slag can occur resulting in a significant reduction in capacity of the torpedo

car. Therefore, where possible, torpedo injection is avoided.

Once the hot metal transfer ladle arrives at the pre-treatment station, the first course of action

is to skim off the silica rich slag by tilting the ladle and using a deslagging machine to remove

the slag into a slag pot. Now the station can be used for phosphorus removal.

The process for removing phosphorus requires an oxidising environment. Additionally, the

slag produced during dephosphorisation needs to be fluxed to improve its phosphorus

retention capacity. The temperature loss due to the injection of iron oxides and fluidising

agent (CaO/CaF2) is compensated for by blowing of oxygen onto the ladle surface through a

supersonic lance at the same time as the deep injection of reagents. Therefore, for

dephosphorisation, in addition to the refractory coated lance for co-injection of iron oxides and

CaO/CaF2, a second lance arrangement for oxygen blowing is required, figure 6. As well as

phosphorus, some silicon, manganese and carbon are also oxidised during oxygen blowing.

The production of CO and subsequent oxidation to CO2 requires that the fume extraction

system is designed to post combust CO and has water cooling to protect the duct and to

reduce the fume offtake temperature. The requirement for water cooling of the fume hood

favours the provision of a ladle transfer car rather than a moving fume hood arrangement for

location of the ladle car inside the fume enclosure.

Figure 6 Typical dephosphorisation layout including reagent and oxygen lances.

Once the dephosphorisation treatment is complete, the ladle must again be tilted and

deslagged to lessen the chance of reversion of the phosphorus from the slag back to the hot

metal. If the next step is desulphurisation, then a deoxidation reagent is added from a supply

hopper to reduce the oxidation state of the slag.

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Conclusion

The use of mono-injection, co-injection, Eco-injection, Twinjection™ or mechanical stirring

technologies can be used for sulphur removal at hot metal pretreatment plants. A range of

mechanical equipment, tailored to each steelmaker’s specific needs, offers a sulphur removal

solution to match the treatment time available.

In combination with sulphur removal, hot metal pretreatment plants designed for the removal

of silicon and phosphorus offer steelmakers cost effective solutions for using lower quality

blast furnace raw materials or improving final sulphur and phosphorus level in steels.

References

[1] R Robey et al. MPT International, 28 (2009), No.5, pp 24-28.

[2] B. C. Welbourn, Technical Steel Research, Report No. EUR 12007 EN.

[3] E. T. Turkdogan, “Fundamentals of Steelmaking”, (The Institute of Materials, London, 1996)