Fundamentos Fusion Reductora

8/20/2019 Fundamentos Fusion Reductora

1/13

I Background and aim of the smelting reduction process

The blast furnace as the major method to produce hot metal has been used for several centuries.

Although it is still an attractive device from the view point of economical operation, energy

conversion efficiency and high productivity, its drawbacks have appeared more and more

indisputable, such as:

(1) High capital requirement for the highly integrated process which includes blast furnace, coke

oven and iron ore agglomeration facilities.

(2) Requirement of high quality coking coal.

(3)

Large scale requirement for economical operation.

(4) Environmental problems mainly caused by coking and sintering plants.

To overcome those weak points, metallurgists have been trying to develop primary ironmaking

processes along two different limes since the 1960’s or even earlier.

The main stream of the work has concentrated on the improvement of the blast furnace process and

the major effort has been on economical operation and the reduction of coke consumption 1 the

blast furnace. As a result, huge blast furnaces as big as 5000 m

3

have been put into operationtogether with many advanced facilities and technologies, the blast furnace has been improved

signfficantly [1,2]. Among those technological developments in the blast furnace, oxygen and coal

blowing are the most important and are still actively studied. The requirement of coke is the most

obvious weak point of the blast furnace because of the shortage of the resource, and the high cost of

the coking oven building. In addition, it is the main pollution source of the metallurgical

manufacturer. Oxygen and coal blowing aim to reduce the coke consumption and increase the

productivity of the blast furnace as well. Over the years, pulverized coal injection (PCI) has been

highly developed and widely used. The blast furnace is operated with coal injection rates above 170

kg/tHM and coke rates below 300 kg/tHM in some Western European countries[3]. In China, great

efforts have also been made in this technology and a high record of coal injection (279 kg/tHM) was

set together with an oxygen enrichment blast (24.2% O2 ) and high blast temperature (1 150°C)[4].

Along with the development of PCI technology full oxygen blast furnace processes were proposed[5].

From the analysis above, it can be seen that the blast furnace with PCI and oxygen blowing has been

one of the most active research fields for a long time and many improvements have been made.


2/13

Although those developments can reduce the coke consumption, the total elimination of coke is still

impossible. Also, some weak points of the blast furnace mentioned earlier still remain.

To completely eliminate coke and other drawbacks of the blast furnace, a new concept, Smelting

Reduction Process, has interested more and more metallurgists around the world. This new process

is believed to be an alternative in the next generation of ironmaking and its aims could be briefly

described as follows:

(1) Shortening of the present ironmaking process route and the elimination of coke making and

ore agglomeration.

(2)

Reducing and eventuality eliminating the dependence on coking coal,

(3) A totally continuous and environmentally harmless ironmaking process.

II Development of smelting reduction processes

II.1 The main differences of smelting reduction from BF

Since the 1 970s many different smelting reduction processes have been proposed and studied on a

laboratory scale or pilot plant scale. For better understanding of those processes or betterunderstanding of the basic ideas of smelting reduction, it is necessary to first analyze the operation of

the blast furnace briefly. Fig.1-1 shows the classical description of the blast furnace.


3/13

The existence of those areas shown in Fig.1-1 have determined both of the advantages and

disadvantages of the blast furnace. As to the disadvantages, the thermal zone and chemical zone

decide that the blast furnace has to be high enough to reach high reaction efficiency. The reducing

gas produced at the low part of the furnace and counter current gas-solid reaction have determined

that lump raw materials have to be used. The height of blast furnace and melting zone determine that

the requirement of high quality coke in the blast furnace can not be avoided. To solve those

problems, the tasks of the smelting reduction process are very clear; first, to divide the one reactor

into several reactors which include pre-reduction, final reduction and melting respectively and

secondly, to remove melting zone from the reactors. Fig.1-2 shows this basic idea of smelting

reduction.

The advantages of this separation are quite obvious. First of ah, it is possible to avoid the use of coke

which is used to support the burden and guarantee the gas flow; then it is not even necessary to use

lurnp raw materials and finally, it can make the process easily controllable and more flexible and

operate economically on a small scale.

II.2

The principies of the smelting reduction process proposais.

The development of the smelting reduction process has concentrated on the following four aspects:

(1) Pre-reduction.

(2) Final reduction and rnelting.

(3) Energy and reductant supply.

(4) Connection of each sub-process.

For the pre-reduction part, there are two types of reactors proposed, shaft furnace and the fluidized

bed. The advantage of the fluidized bed is that the kinetic condition of reaction in some cases is

better and it can operate with fine ore concentrates without any agglomeration. Meanwhile, thedisadvantage of the fluidized bed mainly comes from the lack of large scale operation experiences.

Also sticking problems iii the fluidizing operation seems to be obvious with a higher metallization

ratio in pre-reductiont6. The shaft furnace operation, on the other hand, is simple and many

experiences could be directly adopted.

For the final reduction, there are mainly two alternatives, the coke bed type and the iron bath type.

In the coke bed reactor, the final reduction and melting of the iron ores are comparable with that in a


4/13

blast furnace. The reactions proceed smoothly and are easy to control with coke bed type reactor,

but the efficiency is lower and, at least in some concepts, a small amount of coke is still needed7’8.

The second alternative, an iron bath type reactor to produce hot metal or even steel directly is a

revolutionary idea. In this type of reactor, the pre-reduced iron ore will be finally reduced and melted

either in the liquid iron bath or in the slag layer above the liquid iron bath and consequently, this

direct contact of solid and liquid supplies very good kinetic conditions for the reaction. More over,

since the principle of the iron bath reactor carne from the oxygen converter process, many highly

developed converter technologies can be adopted in this process.

Fig.1-3 shows the connections of each operating unit inside the smelting reduction process.

Fig.1-3 The connections inside a smelting reduction process

The energy and reductant adopted in the different proposals of the smelting reduction process

include pulverized coal, lump coal, electricity and low grade coke. In fact, the technical

improvements of energy and reductant supply, as well as the connections of each reactor, play the

most important role during the development of the whole smelting reduction process.

On the basis of the connections shown in Fig.1-3 concerning the reducing gas supply from the finalreduction and the requirement in pre-reduction, the possible link points between final reduction part

and pre-reduction part are schematically shown in Fig.1-4.


5/13

Fig.1-4 Possible link points between final reduction and pre-reduction in smelting reduction process

The line A-B in the figure shows the supply of reducing gas from the final reduction along with the

pre-reduced iron oxide reduced by the coal. In other words, the line A-B is based on the chemical

equilibrium and mass balance of the final reduction of the smelting reduction process. The supply of

the reducing gas could be represented by PCO, PH2 in the gas and the amount of gas from the final

reduction. Obviously, based on the chemical equilibrium and mass balance, the less final reduction

proceeds (high pre-reduction ratio), the less supply of the reducing gas will be from the final

reduction. The line C-D shows the reducing gas requirement in the pre-reduction part of the

process. Of course, it increases with the increase of the pre-reduction ratio. Obviously, point P in the

figure is the optimum operation state from the point of view of chemical equilibrium. In addition to

the chemical equilibrium, the thermal balance in the final reduction unit must be taken into account

which makes the operation state in the final reduction always away from the line A-B. In the view

point of connection between the final reduction and the pre-reduction, the area APC in Fig.1-4

represents the strong post combustion area which shows the heat demand of final reduction unit

could be met by lowering the reduction potential of gases (post combustion). The area below CPB is

also the strong post combustion area but gas re-treatment is needed for meeting the requirement ofpre-reduction. The area DPB represents the extra gas supply and it or the outlet gas circulation

should be used for the pre-reduction. The top area ( APD ) shows that besides the final reduction and

reducing gas supply to the pre-reduction, the extra energy supply in the final reduction is required to

meet the heat balance in final reduction unit by burning more fuel, small amount of post combustion

as well as electricity in some case. As the process works in this area, it usually produces high

combustion value outlet gases.


6/13

Following the analysis above, the proposals of the smelting reduction could be evaluated according

to the type of the reactor, connections of each reaction unit, energy and reductant supply as well as

the operation state.

II.3 Examples of smelting reduction concepts

a)

COIN Process

The COIN process which was developed by the Krupp Group in the early 1980s was originally

conceived as a process for melting scrap or other metallic feed stocks using coal and oxygen injected

into a BOF type converter vessel. The concept was subsequently extended to a combined reduction-

melting configuration in which melter off-gases, after cooling to the temperature required forreduction, were used directly in the reduction unit. Within the concept, both shaft and fluidized bed

reduction units were considered as alternatives to the pre-reduction reactor[9,10,11]. Fig.1-5 shows the

concept of the process. The pulverized coal injected from the bottom of the vessel was used as the

energy source and reductant. The link point between pre-reduction and final reduction is

approximately at point 3 in Fig.1-4.

Fig.1-5 Concept of COIN process


7/13


8/13

processes at present and construction was started for a pilot plant in Australia in 1991. The process is

equipped with a fluidized bed type prereduction unit and horizontal iron bath smelter. Fig.1-7 shows

the process graphically. The Hlsmelt process uses ore fines and pulverized coal as its raw materials to

produce liquid metal and the carbon content of the product can be varied from blast furnace quality

hot metal down to semi-steel compositions. Unlike most other smelting reduction processes, it does

not require an oxygen lance but uses a high velocity jet of pre-heated air which can be oxygen

enriched if necessary. The fine ore is first pre-heated and pre-reduced in a circulating fluidized bed

and then injected into the melter. With a less amount of slag, as well as a high velocity air jet rather

tan oxygen, sufficient mixing of the hot combustion gases above the bat with liquid metal droplets is

an essential part of the process to permit a high efficiency heat transfer into the iron bath by way of

radiation and convection[15,16,17]. The link point of the process is located approximately at point 1 in

FigJ-4, which indicates a significant post combustion.

Fig.17 Hlsmelt process


9/13

d) AISI Process

The project “AISI Steel Initiative” was started in 1984 by The American iron and Steel Institute. The

process development aims to smelt pre-reduced iron ores using coal, oxygen and flux in a foaming

slag above a molten metal bat at a high production rate. The process can be used to produce normal

hot metal or directly produce steel and to melt scrap as well [18,19]. Fig.1-8 shows the concept of the

AISI process. Coal is fed from the top into the slag where it chars, producing CO and consequently

causes slag foaming. Iron oxides are top fed as well, in the form of wustite pellets produced in a

separate shaft furnace. Wustite pellets will melt and be reduced in the foaming slag to form molten

iron. About 40% of the remaining CO in the produced gas is post combusted above the slag using

an oxygen lance to provide the heat to sustain the reduction. The remaining exhaust gases will be fedto a shaft furnace to pre-heat and pre-reduce the incoming pellets. The link point of the process is

located roughly at point 1 in Fig.1-4.

Fig.1-8 The concept of the AISI process


10/13

e) DIOS Process

The DIOS (Direct Iron Oxide Smeltmg) process was initiated in 1987. The process is sponsored by

the Japanese government and its technique base is the process developed by NKK in 1984. At

present, a 150,000 t/y pilot plant is under construction at NKKs Keihin Works. Fig.1-9 shows the

idea of the process. The main equipment used in the DIOS process includes the fluidized bed for

pre-reduction, a gas reforming furnace to mix coal powder into the gas and the conventional

elements of a large steelmaking converter. With some additional engineering arrangements, the

process permits the operation at high pressure. A thick slag layer above the metal bath, which is

different from the Hlsmelt process, is expected and within the slag layer, a high post combustion

ratio and high heat transfer takes place. The post combusted off gases, which are cooled somewhat,dedusted and reformed, pass into the fluidized bed where the iron ore fines are pre-heated and pre-

reduced up to a 20- 30%. Both coal and hot pre-reduced ore are top charged to the smelting

reduction furnace. A portion of the coal will be charged in the hot gas reformer before being injected

into the bath[7,20,21]. The link point of the process is located around point 2 in Fig.1-4. DIOS can also

be defined as a circulating system capable of reusing its own reaction energy.

Fg.1-9 DIOS process


11/13

f) COREX Process

The COREX process, formerly the Korf KR process, was a joint development between Korf

Engineering and Voest Alpine. The process was conceived in the late 1970s as an alternative to gas

based direct reduction and later on, it was modified to produce hot metal. A 300,000t/y plant is

currently in operation at ISCOR Pretoria Works in South Africa using local lump ore and non coking

coal[21,23,24]. From the practical view point, COREX is the most successful smelting reduction process

up to now. Fig.1-10 schematically shows the process. The process includes two main process units, a

shaft furnace for prereduction and a high pressure coal gasifier-melting vessel. The combined unit

operates at a pressure of 4 bar. Lump iron ore is first reduced to sponge iron (metallization ratio is

above 90%) in the shaft furnace by the reducing gases produced in the coal gasifier and then thesponge iron passes into the gasifier-melter where it is melted and periodically tapped. The link point

is located m the area 4 in Fig.1-4. However, there is development work at Voest-Alpine

Industrieanlagenbau (VAl) for the utilization of fine concentrates in the future[24].

Figi-l-10 COREX process


12/13

g) KAWASAKIProcess

The process was developed by Kawasaki Steel Corporation in the early 1980s. Fig.l-11 shows the

concept of the process. This process is a typical blast furnace based process which includes a coke

bed gasifier and melter coupled with a fluidized bed pre-reduction unit [7,25]. The link point between

pre-reduction and final reduction is located in the area 4 in Fig.1-4.

Fig.1-11 Kawasaki process

h) PLASMASMELT Process

The Plasmasmelt process was developed by SKF Steel in Sweden. Fig.l-12 shows the process. The

process is an integrated smelting reduction process based on a fluidized bed reduction unit and a

coke bed melting unit. Hot DRI and reducing gases are injected into the melting unit with the

reducing gases being generated by reforming coal or possibly other hydrocarbon fuels with recycled

melter off-gas in electrical plasma torches[8,26]. The link point is located in the area 4 in Fig.1-4. The

process development was stopped in 1989 for economic reasons. Even so, the Plasmasmelt process

is a fully new way of using electric power in an ironmaking process. In the future, if the price of

electricity goes down, this process concept could still have very good development potential.

III Concluding remarks

From the analysis of different proposals, the outline of an ideal smelting reduction process, for the

next generation method of ironmaking could be figured out clearly. From the view point of raw


13/13

material, energy source and reductant, coke should be totally avoided and in the present situation,

coal should be used directly as an energy source and reductant, and electricity should be avoided for

economic reasons. As for the input form of coal and iron ore, there are stifl two options, lump and

powder. The advantage of powder materials in the smelting reduction process is obvious, less cost

and high reaction efficiency but it has as a drawback the lack of large scale operation experience. As

for using lump materials, especially lumpy iron ores, many highly developed technologies could be

used and a stable operation could be guaranteed which is very important for the beginning of the

new process development. Therefore, COREX and AISI processes are using pellet or lumpy iron ore

at the first step of development. Concerning the final reduction and melting reactors, there are two

types of processes. The first one uses a shaft furnace with high pre-reduced materials characteristic

to the COREX and Plasmasmelt processes. This type of process operates at the upper right comer in

Fig.1-4 which means no post combustion and a large amount of surplus gas output. Since most iron

oxides reduction takes place in the gas-solid reaction, the operation in final reduction unit is relatively

easy to control and especially suitable for small and middle scale facilities. Clearly, the low efficiency

of the chemical reaction and heat transfer are the weak points of this type of process. The iron bath

type process is highly attractive process for large scale production since it adopts the concept of the

oxygen blowing converter. A strong bath stirring and the reaction of solid-liquid, liquid-liquid will

lead to production efficiency and the adoption of post combustion technology will make for less

energy consumptions.

Date post:	07-Aug-2018
Category:	Documents
Upload:	casimirito
View:	223 times
Download:	0 times

Fundamentos Fusion Reductora

Documents