The blast furnace and its control

Chapter 3

The blast furnace and its control

The blast furnace In the majority of large steelworks the basic equipment used for reducing iron ores to elemental iron is the blast furnace. Modern furnaces are of considerable size; production of 10 0001 of iron per day can readily be achieved in some plants. Sintered iron ores, mixed with a proportion of rubble ore, metallurgical coke and fluxes, are fed into the top of the blast furnace. A i r at a temperature of about 1000 °C is blown under pressure through tuyeres placed near the lower end of the furnace. Figure 3.1 shows, in a simplified form, the British Steel Corporation's new 1 10 0001 d" blast furnace at Redcar. T h e processes of reduction from iron ore (as oxides) to elemental iron as the burden moves down through the furnace are described in many text books, a good recent text being that by Peacey and 1 Davenport . A good detailed description is also given in The Making, Shaping and 2 Treating of Steel, published by the United States Steel Corporation . H o w e v e r , it is useful to give a very brief description of the plant and the processes involved as the material moves down the furnace. This is shown diagramatically in Figure 3.2 which includes a temperature diagram of a furnace. Figure 3.2 also gives a materials balance of the blast-furnace process for a large furnace. T h e upper part of the reduction stack is a gently expanding cylinder topped by a bell, or, in some modern furnaces, a 'bell-less' top. A t Redcar the stack top (or throat) diameter is 10.8 m inside, expanding to 15.3 m. T h e total height of the furnace is 38 m. Below the widest part of the stack, the 'bosh' region thins to 14.0 m diameter. B e l o w the bosh lies a cylindrical carbon hearth. Carbon is a suitable refractory, as the liquid iron (known as 'hot-metal') is saturated with carbon in this part of the furnace. A i r heated to around 1000°C is injected via holes (tuyeres) 3 between the bosh and the hearth. In the Redcar furnace (used here as an example of the latest design of modern blast furnace) there are 36 tuyeres in all, fed by a large manifold known as the 'bustle main'. R a w materials are fed from the sinter and coke plants to a system of bunkers where sinter, pellets and coke are screened and weighed, before being charged via a belt system to the furnace top. T h e top is designed (Figure 3.3) to spread these materials (the charge) evenly across the tops of the charge already in the furnace (the burden). T h e 'bell-less' top, designed by Paul Wurth S . A . , comprises a right

36

The blast furnace and its control

Figure 3.1 Cross section of the Redcar blast furnace

and left hopper, each having an upper and lower seal valve. T h e charge from the conveyor is charged into a shuttle chute which feeds the hoppers alternately. A s one hopper is being charged, the other releases the charge into the furnace via a rotating chute which ensures even distribution of the material across the top of the burden (the stockline). T h e furnace proper is of self supporting, free standing design in which the furnace shell from the hearth base to the furnace top ring is carried on a base ring at foundation level. T h e shell is surrounded by a four-column tower which carries the furnace top platform, Paul Wurth S . A . top machinery and the upper part of the charging conveyor. T h e gas uptakes are also carried on the tower structure with the expansion joints. T h e furnace shell acts as a support for 720 internal stave coolers of Russian design which form the primary cooling circuit. This is supplemented by three additional circuits: tuyere noses; underneath cooling iron, notches and tuyere bodies; and stave shelves and stove valves. A l l the water circuits are fed with near

The blast furnace and its control Top

20966 t d "

gas Sinter 7 8 2 0

t d

_

1

— ι

37 1

I — C o k e 4500 t d "

1

Pellets 8 4 8 0 t d ~ -

Top

£

1

Limestone 4 0 0 t d ~

1

Shaft

H o t blast 14976 t d "

1

Oil Bosh

Iron 1 0 0 0 0 t d " Slag 3 0 0 0 t d "

1

1

Hearth 0

1000

2000

Centre-line temperature (°C)

Figure 3.2 Materials balance and temperature in the blast furnace

boiler-quality treated water and each is on a closed flow system coupled to a cooled heat exchanger. The cast house is arranged as two interconnected but independent areas, each of which serves two iron notches. A n 851 crane is installed in each cast house, with secondary lifting devices in each of the four taphole sectors. The furnace hearth comprises carbon side walls and a central composite plug. The latter has a base of 3 m of carbon capped with 1 m of dense fireclay. T h e hearth is under cooled with water. Both the bosh parallel and lower stack have a ceramic lining, though the stack lining is of lower grade than the bosh lining. The four blast stoves are of external-combustion chamber design. Each has a 2 heating surface of approximately 82 000 m and is fitted with ceramic burners and facilities to burn enriched blast-furnace gas sufficient to achieve a blast temperature of 1350°C at maximum blowing conditions. T h e stove installation is sufficiently versatile for either operator control in a cyclic mode or computer control in a staggered parallel mode. The furnace is designed to operate at a top pressure of 2.5 bar. This pressure is regulated by the position of the variable venturi in the gas-cleaning plant and the gas flow from the furnace. T h e air blast is generated from a 55 M W turbo blower 3 which is capable of blowing 9 3 0 0 N m of air at 5.2 bar gauge pressure. T h e gas-cleaning system comprises a dust catcher followed by a high-energy scrubber of _ 1 the variable venturi type. It is capable of cleaning down to 9 m g n m . Dirty water collected from the gas-cleaning system passes to a clarifier from which the overflow water is cooled and recirculated. Sludge from the clarifier is pressure filtered prior to removal from the blast furnace area.

38

The blast furnace and its control 70888

Figure 3.3 Bell-less top (from Steel Times, Vol. 207, No. 9, p. 170 (September 1979)) Near the stockline the material is dried, this being necessary because iron ore is stored in the open, as is coke, and takes up a variable amount of moisture from the atmosphere. T h e amount of moisture absorbed affects the thermal efficiency of the process and so is a significant parameter.

Modern blast-furnace practice T h e Redcar blast furnace described above incorporates all the features necessary for modern, high-production furnaces. Its main plant and operating statistics are as follows:

The blast furnace and its control

39 1

Design fuel rate total coke oil Hearth diameter Inner volume Blast volume Blast pressure T o p pressure Blast temperature ( m a x ) Oxygen

lOOOOtd' 60% to 40% sinter 40% to 60% pellets 1 535 kg r hot metal ( H M ) 1 450 kg r H M 1 S^kgr H M 14 m 3 4373 m 3 -1 8000 N m m i n 4.3 bar 2.5 bar 1350°C T o 3%

Materials Sinter Pellets Blast Coke Limestone Oil

7820 t d " 1 8430 t d ' 1 14976 t d ' 1 4500 t d ' 1 460 t d " 1 700 t d '

A l l for: Iron Slag T o p gas

10000 t d " 1 3000 t d " 1 20966 t d "

Output Burden

1

1

T h e production rate compares favourably with other modern furnaces (see Table 3.1). T A B L E 3.1 Blast-furnace performances

Germany Schwelgern Holland Ijmuiden France Dunkerque Italy Taranto Japan Oita Kashima Fukuyama No. 5 Mizushima No. 4 Fukuyama No. 4 Oita No. 1 Tobata No. 1

Nominal capacity 1 (td- )

Hearth diameter (m)

Useful volume

10 000

13.6

3595

68.8

2.75

6 800

13.0

3667

57.2

1.85

10 000

14.2

3850

61.5

2.60

9 600

14.0

3358

62.3

2.86

12 000 12 000 11 000 10 000 10 000 10 000 10 000

14.8 14.8 14.4 14.4 13.8 14.0 13.4

5070 5050 4617 4323 4197 4158 4140

69.7 69.7 67.5 61.4 64.8 64.9 70.9

2.37 2.38 2.38 2.31 2.38 2.41 2.42

Output 2

tm- d-

1

3

l

tm- d~

The blast furnace and its control

40

Redcar Japanese practice

33

1

2.57tm" dd useful volume 3 1 2.3-2.4 tt m m" d" total volume 4

Details of other, older furnaces are given by von Bogdandy and E n g e l l who quote 3 _1 1 the excellent figure of 2 . 5 5 t m ~ d for a Mannesman furnace making 2000 td" . Production rates of this order are not common with older furnaces - specific iron 3 _1 capacities are around 1.75tm~ d for these smaller furnaces, many of which are still in operation today. T h e main features which are necessary to achieve high production rates are: a. b. c. d. e.

High top pressure, to 2-3 bar. G o o d burden distribution (in the case of Redcar, using a Paul Wurth t o p ) . High blast temperature, up to 1300°C. Excellent burden preparation. Some oil and oxygen injection in the blast.

The blast furnace process Inputs and outputs In this thermally efficient counter-current reactor, hot air (1000-1300°C) is blown into a shaft down which a mixture of carbon, in the form of coke, and iron ore flows. T h e coke is burnt to form C O and C 0 2 and the ore, which is either haematite or occasionally magnetite ( F e 2 0 3 and F e 3 0 4 respectively), is reduced, first to Wustite ( F e O ) and then to F e , in a liquid form saturated with C ( i . e . about 4 % ) . (Note: Wustite is in fact Fe^O where χ is around 0.97.) T h e input material is usually sintered oxide, lump ore straight from the boat, or pellets, formed in a rotating kiln and bound together with bentonite. Small amounts of millscale and steelmaking slag also give an iron charge. T h e coke input, always of high metallurgical quality, is carefully made near the furnace in coke ovens, by heating mixtures of coking coal, having 20-30% volatiles, in the absence of air. T h e resulting coke must be porous to allow oxygen-carbon reactions to proceed readily, and yet strong enough to avoid being crushed under several tens of metres of burden. In this way gases can be forced by the blowers up the furnace even when the coke softens, as it does near the tuyere level. Fluxes comprising C a O and M g O are also added to the burden in order to flux the alumina and silica gangue in the burden. C a O also fluxes out sulphur into slag rather than allowing it into the hot metal ( F e + 4 % C ) . T h e fluxes are usually added as limestone and dolomite. T h e hot metal produced is, as stated above, iron saturated with carbon. It is tapped at regular intervals near the furnace hearth and usually comprises: C Si S Ρ Μη

4-5% 0.3-0.7% 0.01-0.04% 0.1% 0.1%

The blast furnace and its control

41

These are 'average' figures for high-grade, low-phosphorus ore. Some inland European ores have up to 1% phosphorus, which is not eliminated in the blast furnace, but remains a problem for the steelmaker. T h e use of such ores is diminishing. Slags, which are tapped just above the hot-metal tap-holes, are usually of the composition: 30--40% 5--15% 35--45% 5--15% 1-- 2 %

Si02 A 1 20 3 CaO MgO S

T h e slag basicity ratio ( C a O + M g O ) / ( S i 0 2 + A 1 2 0 3 ) is in the region of 1.1 or 1.2 T h e waste gases which emerge from the top of the furnace are usually in the region of: CO C02 H2 N2 H 20

22% 22% 3% 50% 3%

T h e ratio of C O / C 0 2 is an important indicator of the operation of the furnace.

Following the process in the furnace It is useful to start at the bottom of the furnace when air (at about 1000 ° C ) is blown into a mass of coke via the tuyeres. In this we follow the description given by 1 Peacey and D a v e n p o r t , although the reader could also consult von Bogdandy and 4 E n g e l l for a very full description of the process involved in the blast furnace. 5 Peacey and Davenport have in turn used the work of Cavaghan and W i l s o n and 6 L o w i n g who have in recent years thoroughly investigated this process, using gas probes into the furnace stack. W e will now look at the furnace in detail from the lower hearth areas to the top. Bosh region H e r e oxygen reacts with carbon, in the form of coke, to form carbon dioxide ( C 0 2 ) , which immediately reacts again with carbon to form carbon monoxide. C02

( g)

+ C ( s)

2 C O ( g)

( + 172000 kJ).

T h e heat of reaction is given per kg mole of C 0 2 . ( T h e convention that exothermic reactions have negative overall energy change is used; all energy changes are given in kJ/kg m o l e ) , ( g ) indicates gas and (s) indicates solid.

The blast furnace and its control

42

Reduction and fusion zones A b o v e the tuyere region lies a mass of alternate layers of coke and fused metal and slag. This layering structure, which follows the charging pattern, is important in allowing C O to flow up through this region (the fusion z o n e ) . A b o v e this zone the burden is mainly Wustite, F e O , coke and solid gangue. T h e C O formed above the tuyeres reacts as follows. T w o cyclic reactions take place: C O ( g) 4- F e O ( s) - * F e ( s) + C 0 2

( - 1 7 000 kJ)

( g)

and the coke gasification C02

( g)

+ C ( s) - > 2 C O ( g)

( + 1 7 2 OOOkJ)

These reactions follow a pattern in which: 1. C O reduces F e O to F e , yielding a gas having 70% C O and 30% C 0 2 . 2. This gas reacts with coke to form C O ( 1 0 0 % ) . 3. This then reduces F e O to F e , . . . and so on up this region of the furnace. A s C 0 2 + C —> 2 C O is a highly endothermic reaction, the temperature of the gases in this part of the stack is sharply reduced, as seen in gas temperature profiles (for 5 example, Figure 3.2) recorded by Cavaghan and W i l s o n . Thermal reserve zone A b o v e the 1200 Κ (927°C) isotherm, the coke gasification reaction, C02

( g)

4- C ( S) - > 2 C O ( g ),

markedly slows but the reduction of Wustite takes place instead: C O ( g) + F e O ( s) - > F e ( s) + C 0 2

.

( g )

A s this reaction is only slightly exothermic, the gases cool slowly above the 1200 Κ level. T h e ratio of partial pressures for C O and C 0 2 ( r C O / C 0 2 ) is 2.3 at 1200 K . A s the gasification reaction ceases, C O becomes depleted above 1200 K . A s can be seen from Figure 3.2 a constant temperature zone is reached, which is known as the 'thermal reserve' zone. In this zone haematite ( F e 2 0 3 ) and other higher oxides of iron are reduced according to: C O ( g) + 3 F e 2 0 3 ( s) - > 2 F e 3 0 4 ( s) + C 0 2 C O ( g) + F e 3 0 4

- * 3 F e O ( s) + C 0 2

( s )

( g)

( g)

( - 4 8 000 k J ) , ( + 50000 k J ) .

A s the reduction of haematite can be carried out at low C O partial pressures, unreduced haematite only occurs near the top of the burden.

The blast furnace and its control

43

Top zone In the upper quarter of the shaft the temperature of the gas quickly falls by the net endothermic nature of reducing higher oxides to Wustite. T h e gases are also cooled by contact with cold charge materials. A t 'room temperature' the over-all reaction CO + F e 20 3

2FeO + C 0 2

has an energy change of +13 000kJ per kg mole of C O .

Silicon reactions in the lower furnace T h e lower furnace, comprising the bosh, tuyere and hearth, is the region where the important reduction of silica occurs. It is true to say that it is in this region that the quality of the hot metal is determined. Other parts of the furnace will determine the efficiency in terms of fuel usage (coke rate) but the variation and mean level of silicon in the hot metal are the result of reactions in the lower furnace. T h e chemical processes in this part of the furnace are still not clearly understood, but the current state of knowledge has been well reviewed by 12 T u r k d o g e n and his co-workers. T h e high temperatures reached in this region lead to the reduction of the stable oxides M n O , S i 0 2 and the phosphates. Silicon and manganese are dissolved in the iron and slag is formed from the ore gangue. T h e reactions related to silicon will form the main interest owing to the effect silicon has on steelmaking practice. A variable Si level in the hot metal will lead to control and quality problems in the steel-shop, while a high level of Si can lead to very high slag volumes which can reduce steelmaking plant utilization in extreme cases. It is now believed that the following sequence of reactions occurs as iron metal droplets pass down through the lower furnace. 1. T h e formation of SiO and SiS in the combustion zone. 2. T h e transfer of silicon and sulphur to metal and slag droplets in the bosh. 3. T h e oxidation of silicon by iron and manganese oxides in the slag as the iron droplets pass through the slag layer. 4. T h e desulphurization of metal droplets as they pass through the slag layer. There are three reaction equilibria of primary importance: S i 0 2 + 2C = Si 4- 2 C O , M n O + C = Mn + C O , C a O + S + Ç = CaS + C O , where the underscore indicates elements dissolved in iron.

44

The blast furnace and its control

T h e processes for Si may be simplified as follows: Zone

Chemical behaviour

Si level Very low

Bosh

Fe

Tuyere

Gangue -

Slag

Slag-*—

Hot metal reservoir

Fe + 4%C

Si Si

•Metal

3-4%

-Metal

Reducing to 1-2% 1-2%

13

W e can see from this simplified version of recent papers by Decker and S c i m e r , Tsuchiyer

14

and T u r k d o g e n

12

that ( a ) the silicon level in the hot metal varies up to

3% or thereabouts in the bosh, but that ( b ) silicon is 'leached-out' as metal droplets pass down the slag. T h e scientific evidence for ( a ) and ( b ) is from Tsuckiya's paper which demonstrated that the silicon transfer to the metal is via the formation of silicon monoxide from the coke ash in the high-temperature region of the tuyere zone. It also pointed out that as the metal droplets pass through the slag layer, some of the silicon picked up earlier is oxidized by iron oxide and manganese oxide in the slag. This argument was substantiated by the silicon-concentration profile in the bosh, determined from

the analysis of samples taken

from

a quenched

experimental blast furnace. T h e silicon content of the metal droplets reaches a maximum at the tuyere level and decreases in the slag layer.

T A B L E 3.2 Summary of reactions and thermal behaviour in various regions of the iron blast 1 furnace (from Peacey and Davenport ) Zone

Chemical behaviour

Hearth

Saturation of iron with carbon and final reduction of ( C a O ) 3, P 2 0 5 , MnO and Si0 2

Tuyere raceways

Coke and hydrocarbons are oxidized to C 0 2 then CO

Large evolution of heat from combustion of coke with hot air

Bosh

Impurity oxides are reduced and iron absorbs carbon during percolation of metal and slag droplets

Transfer of heat from ascending gas to descending coke and droplets of metal and slag

Fusion zone

Formation and melting of slag, final reduction of FeO

Transfer of heat from ascending gas to melting solids

Cyclic reduction zone

Wustite reduction and coke gasification

Temperature of ascending gas falls rapidly

Middle of shaft

Reduction of F e 20 by CO over much of the zone. Little or no reaction in higher portions due to depletion of CO (chemical reserve zone)

Steady temperature (1200 K) throughout (thermal reserve zone)

Upper quarter of shaft

Reduction of F e 20 2 and F e 30 4 to Wustite

Temperature of gases decreases rapidly due to transfer of heat to cold incoming solids

Thermal behaviour

The blast furnace and its control

45

Turkdogen and his co-workers have studied the second process, ( b ) , in detail, in the laboratory. They passed liquid iron droplets through a 10 cm deep slag column and plotted silicon in metal against M n O content in the slag. T h e 4 mm diameter iron droplets contained Si in the range 0.63-1.59%. T h e slag basicity was 1.5. A s M n O increased, so the Si level in the droplets fell to below 0.2% - well below that expected from equilibrium calculations. Summary Table 3.2 sums up the reactions taking place in the various zones of the blast furnace.

The rist diagram Introduction A most useful geometrical method for rapidly understanding and predicting changes in blast furnace operation due to variations in blast and burden parameters 7 has been devised by Rist . It is well described in the text by Peacey and 1 D a v e n p o r t , whose treatment is followed here. If w e say that the furnace is running smoothly then: "Fe = " F e

nh = n°c no = n o where nx, n°x are the number of moles of each element χ entering ( i ) and leaving ( o ) the furnace, per mole of useful Fe in the hot metal. T h e ways in which iron, carbon and oxygen enter and leave the furnace are as follows: Element

Into furnace

Out of furnace

Fe C Ο N2

Fe oxides Coke Oxides, blast Blast

Hot metal (Top gas) CO, C 0 2 , C-in-Fe (Top gas) CO, C 0 2 (Top gas) N 2

This assumes that: 1. T h e Fe content of the slag is very low. 2. Oxides in flux and gangue all leave as oxides in the slag. g

If we now define ( 0 / C ) as the ratio: (moles of oxygen)/(moles of carbon) in the top gas, the molar fraction of C O and C 0 2 in the top gas will then be given by: X*co2 = ( O / Q - l and =

2-(0/C)

The blast furnace and its control

46

If w e define the molar oxygen fraction in the charge of iron oxide ( O / F e ) * as being the number of moles of Ο per mole of Fe in iron oxides, then ( O / F e ) * = 3/2 ( O / F e ) * = 4/3

for haematite ( F e 2 0 3 ) , for magnetite ( F e 3 0 4 ) . m

N o w if carbon content in the hot metal is given by: ( C / F e ) = (moles of C in H M ) / ( m o l e s of Fe in H M ) then the various metal balances are:

a.

Fe balance Since the only outlet of Fe is in the hot metal and riFe and nFe are in terms of moles of Fe in the hot metal, it follows that: = l.

n^=n°Fe

b.

C balance „°

= „ « + (C/Fe)

m

as carbon leaves ( i ) via top gases and ( i i ) in hot metal, which is essentially liquid Fe saturated with C. W h e r e n& is moles of C leaving via top gas and m ( C / F e ) is moles of C in M M , for mole of product Fe.

c.

Oxygen enters the furnace as blast and iron oxides, so

Ο balance 9

g

n°0 = n c(?/C) ,

l

and n 0 = N* +

(0/Fe)

x

The stoichiometric equation Since "b

= n°c

nh = n°0 + ( O / F e r n°0=

%

n cx

(O/C)

8

so n% + ( O / F e ) * = rit x ( 0 / C )

g

(with no moles of Ο in blasts per mole of product F e ) . A s the carbon input either ( i ) reacts with oxygen in the blast and oxides ( n c ) or ( i i ) dissolves in the hot metal («c

=

moles of active C which react with Ο in blast.) the moles of carbon in the top gas, nc = « c , and 8

Aig + ( O / F e ) * = n£ x ( O / C ) .

The blast furnace and its control

Figure 3.4 (a) Rist diagram; (b) the chemical reserve zone; (c) distribution of thermal and chemical zones in the blast furnace

47

48

The blast furnace and its control 8

So a plot of ( O / F e ) (the molar ratio of oxygen to iron in the oxides) against ( O / C ) (the molar ratio of oxygen to carbon in the top gas) will be a straight line through 0/C=

0, O/Fe =

0/C=

( 0 / C ) , O/Fe = ( O / F e ) * .

-nl

and g

This line is known as the operating line. Typical examples are given in Figures 3.4(a) and 3.5, together with the meaning of the parameters. F e 20 3

1

2

O/Fe

Increasing blast temperature

Figure 3.5 Rist diagram showing permissible operating regions

Use of the diagram It is further shown in Peacey and Davenport that the operating line must pass near the point O/C = 172000/283000 and O/Fe = ( O / F e ) * -

283W'

The blast furnace and its control

49

where S is the thermal demand of the particular charge used. A s O / C must be between l ( p u r e C O ) and 2(pure C 0 2 ) this line must lie between the two lines shown in Figure 3.5. A s the quantity S can be calculated and O / C measured by top gas analysis, the operating line can be plotted for any given set of furnace conditions. T h e effect of some changes in operating factors on the line is shown in Figure 3.5. T h e Rist diagram shows graphically the use which can be made of 'the top gas ratio' C O / C 0 2 in understanding furnace operation. (Further examples of the diagrams are given in Figures 3.6 and 3.7.) A

Figure 3.6 The operating line with points given by W is at equilibrium between Wustite and gases. Ρ given by thermal demand

Rist has stated that the 'indirect reduction' is said to take place mostly in the isothermal zone and to proceed in two stages, from haematite to Wustite and from Wustite to iron. T h e oxygen transfer slows down considerably as the equilibrium of the gas with the Wustite and iron is approached. This situation can be visualized as a 'chemical pinch point' on the oxygen exchange diagram (Figure 3.4b). It has been

The blast furnace and its control

50

accounted for theoretically and studied on a small scale model in the laboratory. Ideally, if sufficient time is allowed, equilibrium is closely approached and a chemical reserve zone of pure Wustite develops. T h e blast furnace shaft can thus be viewed as being made up of two oxygen exchangers in series, extending from the stockline to some level in the shaft where solution loss becomes so active as to prevent any C 0 2 build up in the gas. T h e resulting distribution of the various zones,

0

C 0 2 (%)

28

Figure 3.7 Blast-furnace gas composition profiles: Δ, calculated values (Ref 10)

taking into consideration both heat and oxygen transfer, is shown in Figure 3.4c. Changes in the heights of these zones and in their degree of separation or overlap are related to changes in various parameters in a way which is understood only qualitatively. A quantitative description of these relationships requires data about rates of heat transfer and of reduction which are not yet available with the required degree of accuracy. Simple and safe models are obtained by incorporating the thermal pinch point and dividing the whole process in only two steps, by a plane or surface where the temperature difference between gas and solids is a minimum. In view of the close approach to thermal equilibrium generally obtained with self-fluxing burdens, this difference is neglected altogether or given a small constant value. Other models have attempted to give a more refined description of the blast-furnace process by dividing it into more than two steps. A larger number of secondary reactions can be taken into account, each being assigned a temperature interval. In spite of the increased complexity, there is no way around some empirical formulation of the efficiency of the heat and oxygen exchange.

Methods of control T h e best and most effective method of control of the blast furnace is to ensure consistency of burden, charging quality and composition, and the other main

The blast furnace and its control

51

parameters of operation, blast volume, humidity and temperature. It is for this reason that so much care has to be given to the burden preparation by sintering and in the blending of fluxes and coke. Considerable effort must be expended to ensure consistent blast volume, humidity and temperature. Care must also be taken to make sure that the burden moves smoothly down the furnace. This can be achieved, in part, by careful spreading of the burden; the new form of top known as the 'bell-less' top or Paul Wurth top has been developed for this purpose. It is also possible to check the temperature distribution at the top of the burden, although not the exact way that the burden lies at the top of the furnace (despite many years' effort on the part of research workers, who have so far been beaten by the extremely inclement conditions above the burden, due to the large quantity of hot gas laden with very hard corrosive particles and a temperature of several hundred degrees Celsius). T h e Spirotherm equipment, described later, gives the operator a good picture of the temperature distribution over the top of the burden. Complete consistency of burden and operation is impossible to achieve. There are inevitable interruptions to the smooth chemistry of the operation caused either by periods when the furnace must be turned 'off-blast' for maintenance purposes or when the bed chemistry varies slightly, at the end or the beginning of a bedding pile, or when ore changes are dictated by availability. It is for this reason that it is desirable to introduce both feed-forward and feed-back control models to aid furnace operation. In view of the importance of blast-furnace operation, extensive research has been carried out in this area since the mid-1960s. Most of the major steelmaking countries have developed techniques for ensuring that controllable variables such as blast temperature, volume and humidity can be used to take account of unavoidable changes in burden chemistry and furnace conditioning. Because it is not possible to carry out detailed measurements of the chemical processes as they are going on inside the blast furnace, it is necessary to build models (using digital computers) which approximate in some way to the chemical reactions proceeding as the burden moves down the furnace. These models are used in a variety of ways to indicate feed-back by 'tuning' the blast and feed-forward by charging the coke/ore ratio in order to obtain consistent furnace operation. Studies of blastfurnace dynamics have shown that there are limits to the controllability of blast-furnace process by 'fine tuning' action and the response to such action is slow. For economic reasons the furnace is worked very near to maximum blast volume and temperature and so scope for variation is limited to about ± 0 . 1 5 % silicon with a corresponding small amount of action on sulphur and phosphorus. Silicon is the main controlled variable because variability in silicon directly affects the steelmaking process. Steelmaking in modern, high-speed oxygen vessels, either top or bottom blown, works most efficiently when the silicon level is low and constant. T o o much silicon in the hot metal can cause slopping and dangerous practice in the converter. Inconsistency in the silicon content will lead to need for a degree of control of the steelmaking process which is sometimes not possible to obtain, and to longer blows and re-blows. T h e use of the computer in conjunction with the blast furnace has also become essential for the control of various other activities associated with the furnace. A list of such computer applications covers a number of different areas:

52

i. ii. iii. iv. v. vi.

The blast furnace and its control

Control of the process. Thermal and sequence control of hot stoves. Control of burden weights and composition and charging sequence. Fault detection on instrumentation. Cooling control and fault detection. T h e working of so-called 'policy' models, i.e. models which predict the expected response to variations in iron ore quality, flux qualities, etc. which might be demanded by purchasing policy from time to time.

This chapter concentrates on items ( i ) and ( v i ) . T h e others, though important, are more in the province of computer software. It must clearly be stated that off-line 'policy' models are much more widely used than 'control' models. In fact, it is almost mandatory for a large blast furnace to have a set of policy model programs associated with it. O n e such, very successful, set of programs, developed and widely used by the British Steel Corporation, will be described in some detail below. This will be followed by a discussion of 'control' models developed in recent years. T h e use, in practice, of these control models is much less widespread than that of the policy, steady state, model. Blast-furnace process models T h e computer-based models built for the blast furnace are essentially of three types: a. Statistical, in which large numbers of measured inputs and resulting outputs are correlated into equations predicting the effect of any change of output parameters which will result from a change in an input variable. b. Simple thermodynamic, using heat balances for the whole furnace to give a derived parameter which can be used to control the furnace by means of variations in coke rate and blast moisture. c. Models which take into account both heat balances within various sections of the furnace and chemical reaction rates within the furnace. 9

A simple example of a statistical model is given by V o i c e and D i x o n who correlated the burden weight (less moisture) and coke consumption. A simple linear relationship exists between these parameters, whose exact values depend on the exact method of burden preparation. Many more complex models of this type have been built, but their use is limited as they can only be valid over narrow ranges of the variables. T h e most widely used examples of a simple thermodynamic model were developed by I R S I D and C R M . These involved the use of derived parameters Wu and Ec respectively (Wu is equivalent to Ec). A detailed account of the C R M model is given later in this chapter. T h e more complex models have been developed to the extent that they can be used to predict the benefits of changes in practice with great reliability. Such models are also used to give operators and managers a continuous indication of the

The blast furnace and its control

53

state of running of the furnace. This type of model will now be discussed in detail, 10 starting with a m o d e l which, although now considerably updated, is a good basis 11 for understanding modern blast-furnace m o d e l s . Λ predictive model of the

blastfurnace

Measurement and control of the furnace furnace consists of: 1. 2. 3. 4. 5.

T h e whole central 'package' of the

Inputs. Outputs. Measurements. Process models. Control actions.

Inputs are the burden quantities and analyses, the blast temperature, volume and humidity, and the amount of oil coal or other injectants at the tuyeres. T h e outputs are essentially the hot metal analysis, temperature and quantity, with an emphasis on silicon levels in the hot metal. Measurements comprise: a. Charge weights

sinter, rubble ore, carbonates, additives, scrap, coke,

and also charges per hour b. Blast

volume, temperature, moisture, oil additions, oxygen additions, pressure.

c. T o p gas

temperature, pressure, composition (percentage n2, C O , C 0 2 , h2).

T h e structure is basically a two-zone thermochemical model with the interface between the zones defined as the level at which the atomic O/Fe ratio of the iron oxides is unity. A comprehensive heat and materials balance is performed over each zone and all relevant furnace parameters are considered. This division into two zones is also used by Peacey and Davenport, who point out that such division through the chemical reserve zone has the advantages that: 1. There is no carbon gasification in or above the chemical reserve zone so that all carbon in the charge descends through the top segment into the bottom before it can take part in any chemical reaction.

54

The blast furnace and its control

2. T h e only iron-bearing material crossing the zone is Wustite. A l l higher oxides have been reduced to Wustite by the time they go down into the chemical reserve zone, and all reduction to Fe goes on below the dividing line. Figure 3.4 shows this division in relation to: 1. Furnace segments. 2. Temperature profiles. 3. O / C ratio in the gases. The model itself consists of a set of nonlinear simultaneous algebraic equations which have to be solved. These equations not only include heat and materials balances with accompanying yields but also definitions of conditions at the furnace top, the interface between the zones at the tuyere region and the furnace hearth. There are also many equations relating to furnace performance and parameters, both measured and calculated. T h e majority of equations are based on theoretical laws of conservation of mass and energy, but several are based on empirical relationships obtained by research workers in many countries. T h e model is flexible in that not only can any of the equations be modified but equations can be added or removed. Successive results from the model can be used to optimize conditions. T h e model itself finds a unique solution to the set of equations from the data presented. Operating modes The model has two basic operating modes, 'assessment' or 'predictive' which will be described in turn. The aim of assessing a practice is generally to determine the efficiency of the operation and to discover any material imbalances. There are several possible ways of assessing a practice with the model. It is usual with assessment to calculate furnace parameters which either cannot be measured or are measured with the least accuracy. A new practice can be assessed as follows: a. T h e weights and analyses of the burden, hot metal and slag are used to calculate the yields of F e , C a O , S i 0 2 , A 1 2 0 3 , M g O , Μ η , Ρ and S. b. Some of the data from ( a ) together with coke analysis, tuyere injectant details, top gas percentages of H 2 , C O and C 0 2 are used to determine the coke rate, the blast furnace requirement, the hydrogen utilization, the proportion of direct/ indirect reductions and the approach to equilibrium at the F e O level. c. T h e information from ( a ) and ( b ) together with the temperatures of top gas, blast, hot metal and slag are used to calculate the heat losses and flame temperature. d. Some of the preceding data, together with the furnace and tuyere sizes, production rate, blast and top gas pressures, are used to calculate the output and fuel rate indices, permeability tuyere gas and bosh gas velocities. T h e predictive mode is usually used to predict the blast furnace performance for some change in the operating parameters of the furnace. Normally, the process variables such as heat losses, yield factors, hydrogen utilization, and approach to

The blast furnace and its control

55

equilibrium are those calculated from an assessment of a base practice or are assumed to be reasonable values; using these, the effects upon the furnace fuel and production rates of changing certain furnace parameters are calculated. A relatively simple example of using the model in predictive mode is the effect upon the furnace performance of increasing the tuyere injection rate without compensating with blast temperature or oxygen enrichment as follows: a. Either a base practice is assessed and the calculated percentage approach to equilibrium at the F e O level, the hydrogen utilization and the heat losses per unit time obtained or reasonable values of these variables are estimated. These parameters are considered to remain constant when the only parameter being changed is the tuyere injectant rate. b. Keeping the three parameters described in ( a ) constant together with such parameters as blast temperature and oxygen constant and the O/Fe ratio of the burden material, enables the coke rate, the top gas analysis, the blast requirement and the flame temperature to be calculated for different levels of tuyere injectant. T h e effect upon the production rate of the injectant can also be estimated by comparing the bosh gas volumes with that of the base case. From information such as this, technical and economic advantages of high levels of tuyere injectant may be estimated. M o r e applications of the predictive mode of the model are listed. Applications of the model The model is capable of prediction and assessment of blast-furnace performance and is not intended for on-line control purposes. It must be emphasized that to ensure the reliability of any predictions or assessments made by the model, the data upon which these are based should be reliable and preferably be averaged over approximately one week. Some examples of the use of the model are as follows: 1. Assessment with the model T o carry out routine monitoring of furnace operation the model would be used in the assessment mode; this determines the efficiency of the operation and any data imbalances. It is not recommended that operation is assessed more frequently than once per week (unless operation is particularly steady); this allows adequate time for the material balancing process. Assessment is particularly useful during furnace trial periods and the results are immediately available for operating, technical and managerial staff as a weekly log of furnace operation. The model is sufficiently flexible to be used for assessment of different plants working under different constraints and as such the results of assessment of each plant are compared. A typical computer output from the assessment is included later. This output shows not only the data that is required from a furnace for an assessment, but also includes the information that can be calculated from this data. 2. Prediction with the model The model provides particular benefits when it is used to predict blast-furnace performance for some change in the operating characteristics. N o t least of these benefits is to use the predictions from the model to plan the programme of costly furnace trials to provide the most useful information and in some cases to eliminate the need for these trials altogether.

56

The blast furnace and its control

There are many areas of furnace operation where prediction with the model is useful and the following are some of those areas that have been examined to date. T h e model can perform burden and slag calculations including certain useful predictions about slag chemistry. It will, for example, calculate the slag weight and analysis to be expected from a particular burden, including the sulphur partition. A l s o , if the required slag composition is input, then the necessary weight and analysis of one of the burden components can be calculated, such as a flux addition to a burden consisting of acid pellets and sinter. T h e ease with which the combined oxygen can be removed from different ores, pellets and sinters can be measured by simple laboratory techniques to give a figure for the burden's reducibility. T h e model can be used to estimate the effect on operation, including most importantly the fuel consumption and production rate of changes in the burden's reducibility. T h e model has been used to assist an investigation into the replacement of coke by various tuyere injectants including oil, coal, tar and coal/water slurry. This involves prediction of the coke rate at different levels of injection while maintaining constant the other process variables. These results were compared to those reported from operating practices throughout the world and the model updated where necessary. O f particular interest is the variation in the behaviour of coke replacement by injectant with different tuyere conditions. A clearer understanding of this phenomenon allows a more reliable prediction of coke replacement to be made for different operating conditions. A n assessment of the effect of changing blast temperature, oxygen content and steam additions, often in conjunction with tuyere injection, upon furnace fuel and production rates, together with the relationship between blast pressure and top pressure and its effect on output, is aided by the model. A n investigation into the utilization of energy in the overall ironmaking process can use the model to define areas where the efficiency of energy usage may be impaired by changing the furnace operating parameters. T h e overall ironmaking process is considered to be the blast furnace linked to coke areas, burden preparation units and external desulphurization.

Typical example of output T h e values of the variables not marked must be collected, averaged for the period of operation being assessed, and input into the model. T h e following data must be input:

- 1

Weight of all input materials ( t d ) . Analysis of all input materials. - 1 Production rate of hot metal ( t d ) . Analysis of hot metal and slag. Temperature of hot metal. T o p gas analysis, temperature and pressure. Blast temperature, humidity, oxygen content and pressure. Tuyere injectant rate and analysis (no injectant used in this case).

The blast furnace and its control

57

T h e values of the important operating variables determined by the model include: Coke rate (kg/t hot metal) determined from three different sets of data 1 Heat losses ( M J r ) . 3 -1 3 _ 1 Blast volume ( N m t and N m m i n ) . 3 1 3 T o p gas volume ( N m Γ and calorific value KJ N m ) Process variables such as: C O and H 2 utilization, approach to chemical equilibrium, indirect reduction, flame temperature. Various aerodynamic functions. Yields of all input materials.

Λ dynamic simulation model of the blastfurnace The heart of any policy model of the blast furnace is a simulation model. O n e such model, which has been well described by W o o d , is the basis of a suite of widely used policy models. In this the working volume of the furnace is divided into a number of horizontal zones, about 1 m in depth. T h e burden material in each zone is specified in terms of its weight, chemical analysis, and temperature. Simulation is achieved by passing a trial sample of gas up through each of the zones in turn, calculating the reaction and heat transfer at each level, and finally updating the contents of each zone, making allowance for the downward movement of material from zone to zone. Materials leaving the bottom slice are accumulated and cast in step with the plant. Charging is simulated simply by adding material to the contents of the top zone. These calculations are carried out at each dump of the large bell and when programmed in F O R T R A N take Vis of real time to perform. O f necessity, assumptions have to be made in order to allow the model to be built. A n empirical basis is suggested by the proportions of C O and C 0 2 in the stack as a function of height above tuyere level. Consider as an example the gaseous reduction of iron ore in the stack: F e 3 0 4 + C O i ± 3FeO + C 0 2 . The experimental curves of the percentages of C O and C 0 2 are smooth and, if split into the regions discussed earlier, can be approximated by single exponential curves, and generated by a single linear differential equation. T h e concentration of iron oxides is large when compared with those of the gaseous components and so the suggested form for the differential equation is that for a pseudo first-order reaction. d

~

3 4

*^ ° at

= a (%CO -

A : % C 0 2) .

Fixed values for the parameters a and k can be found which give close agreement between calculated and experimental results. H o w e v e r , as conditions in the stack change, so these parameters (and thus the top gas analysis) will also change. T h e

The blast furnace and its control

58

equation can be extended to cover variation of the heat level in the stack and upper bosh, by introducing the Arrhenius effect and for the effects of hydrogen in the gas, giving -d

F e 3Q 4 dt

= a (1 + g % H 2 )

( % C O - k%C02)

exp ( - J E / Λ Θ ) ,

where a and k are parameters calculated from the plant top gas analysis, g, E, and R are constants, and θ is the burden temperature. Similar approaches, based on basic theory, experimental results, and physical argument, are followed for the other reaction rates and heat transfers and are discussed fully elsewhere. T o overcome uncertainties associated with burden velocity down the stack, the velocity in the lower bosh is taken to be proportional to the rate at which carbon is burnt at the tuyeres. For the upper stack this is taken from the stockrod velocity. Intermediate values are taken by interpolation. In the simulation, the first-order differential equations above are 'run' with a set of parameters corrected by experience, gained by comparing estimated and actual values of outputs (notably C O / C 0 2 and hot metal silicon levels). T h e model is timed by use of the first operating lines. Each parameter change either shifts or rotates the line. If operating lines are drawn for both plant and model, then the difference indicates which parameters may be altered to bring the two lines together. A certain amount of trial and error is unavoidable, but the lines normally converge after about eight attempts. This model can be used to run fast cast-to-cast Si predictions. The calculation involves a second version of the simulation which runs with assumed inputs, for the next six hours, using all available low-priority computer time in order to predict the future path of the plant, Si(t + τ / ί / 0 ) , where the forward time τ is from 0 to 6 h. T h e required step change in blast humidity or temperature, U, is then calculated using a simple linearized model of operation about the present working point, and minimizes the difference between the target and the estimated silicon levels, that is

0

where the silicon content Si(r + xlU) at time t + τ given the input U is equal to: Si(f + τ/υο)

+ Α(τ)(ί/ -

U0),

and h(x) is the step response from the linearized model, t is the casting start time, UQ is the assumed input level when the fast simulation is run, and Si(t + x/U0) is the silicon trajectory predicted in this run. The CRM method for blast furnace

control 15

In a classic series of thorough investigations carried out between 1960 and 1975 the Centre de Recherches Métallurgiques, L i è g e , established a method of

The blast furnace and its control

59

control of blast furnace operation based on a thermal parameter Ec using variations of humidity and temperature of the blast, the flow of fuel oil injected at the tuyeres and coke/sinter ratio as controlled parameters. These workers constructed a model which showed that parameter Ec, which is representative of the thermal content of the burden at tuyere level, can easily be related to the silicon content of the tapped metal. This correlation exhibits a time gap, the Ec variations being ahead of those in silicon tapped in hot metal. This conclusion is especially important as it means that detection is ahead of the controlled physical phenomenon. T h e investigations carried out in Belgium also gave a very precise idea of the way the blast furnace was operating. It is first also necessary to examine how the parameter Ec, the derivation of which will be given in detail below, can be worked out from knowledge of blast-furnace parameters, notably hot waste gas analysis. It is also important at the outset to discover the time constant induced by variations in the parameters mentioned above (blast, humidity, temperature, e t c ) . T h e response of silicon to variations in blast temperature and fuel oil flow is very slow. T h e relationship between blast moisture and silicon is faster but is more complex. W h e n blast humidity is increased silicon drops for about 8 h and then rises gradually above its initial level, whereas silicon variations respond to coke sinter ratios after dead time of 12 h by means of two time constants equal to 4 h 20 min which is rather faster than the air blast humidity variation. Unfortunately, the silicon content of the iron in the following cast can only be slightly influenced (because of these comparatively slow time constants) by about 0 . 1 % by action at the tuyere level, i.e. by feedback control. In order to bring corrective action in the region of ± 0 . 1 5 % Si feedback control cannot be used and feed-forward control by control from the burden chemistry and other major parameters must be resorted to as quickly as possible.

Transfer

function

Owing to the importance of a full understanding of the effects of variations in blasts and other parameters on silicon levels, workers at C R M carried out a very detailed study of how the speed with which cast silicon and the parameter Ec varied with variations of these basic parameters. These will be discussed in turn. Variation creased.

in blast moisture

T w o effects occur when blast moisture is in-

a. A rapid cooling effect at the bottom of the furnace due to a decrease in the flame temperature. b. A better preparation of the burden due to an increase in the reducing power of gases and a better gas distribution in the shaft, followed after several hours by relative heating of the output of hot metal that is partial or total cancellation of the cooling effect. T h e transfer function can be expressed as F = Fx + F 2 , where Fj corresponds to effect ( a ) and F2 corresponds to effect ( b ) above. The two transfer functions were

The blast furnace and its control

60

found by the workers at C R M to be of the form: Λ

exp(-STa)

Β exp (1 + STA) where F is the transfer coefficient:

(1 +

(-STB) STB)

change in output input change

35

, (in % S i / g H 2 0 / m :N )

where τ is a time constant, ( h ) Τ is time from input change, ( h ) l S is factor of dimension T~ . Their experimental results are shown in Table 3.3 (which also shows the relationship of changes in silicon to changes in blast temperature, volume and coke rate). It can be seen that the main characteristic time constant between blast moisture and silicon is in the order of 8h. This is illustrated in Figure 3.8 which shows variations in silicon following blast moisture disturbances. Typical values 2 2 are: Λ = - 0 . 3 7 x H T ; τΛ = 0.3; Β = 0.41 x ΙΟ" ; τΒ = 1.6; and ΤΒ = 11.3.

οCN X

E f f e c t o f F2

Effect o f F

Figure 3.8 Effects of blast moisture variation on Si

Blast temperature T h e relationship between blast temperature and silicon is slow. T i m e delays of the order of 6 h occur in the blast temperature/silicon transfer function. Fuel oil injection C R M work confirms that this is of little value in carrying out feedback control of silicon levels, because of the very long time delay between control action and effect. Ratio dry coke/sinter of the order of 4h.

A time delay of 12 h is associated with a time constant

χ{

+

^ +

^

0

0.17% Si 10 000 m 3 h"1

Volume of wind

i + .5A 7 ) i ( + S t ) A ,i

0

-0.9% Si 1 0 g H 2O

Moisture of wind

Transfer f u n c t 1 0n F S -l (

0

12

TA(h)

0.4% Si 100%

0.15% Si 0.01 coke/sinter

Λ

Temperature of wind

Coke/Sinter

Quantity

T A B L E 3.3 Transfer function, Si and furnace parameters

1

1

6

τΑ(η)

0

0

0

4V3

τΑ'(η)

-

0.03% Si 10gH 20

0

0

Β

-

Q Ö

0

0

TB(h)

-

15

0

0

τΒ(η)

-

0

0

xBi(h)

The blast furnace and its control 61

The blast furnace and its control

62 COMPUTER

INPUTS

OUTPUT

Burden Fe u n i t s

Wanted coke (dry)

Coke

0

Coke moisture

2

f o rCO reduction

0

2

forH

0

2

forC reduction

Volume Moisture 0 2, oil

Combustion C

Enthalpy

reduction

H o t metal

make

Heat o f : F u s i o n , cast F e , slag R e d u c t i o n - M n O , P 20 5, FeO Dissociation H 20

Wind

Temperature

2

Coke w t (wet)

wind coke charge

ι Reduction-Si02 Slag s u p e r h e a t

E n t r y gas Gas Pressures

Top Gas CO C02 H2 N2

Thermodynamic Parameters

Figure 3.9 Calculation of Ec from furnace parameters

Three stages of blast furnace

control 15

In a seminal series of papers Vidal, Lucas and others have set out what they consider to be the three most important stages in the control of a blast furnace. T h e purpose of these stages is as follows. First stage i. T o limit variations in the composition and size of the charge as much as possible, where size means the size of the parts of the burden which are fed in through the top of the furnace. It is necessary, therefore, to keep the total iron content and the basicity (iron/silicon ratio) of the charge as well as the degree of oxidation and the reducibility of the burden within certain limits. (Methods of measurement of reducibility will be discussed later.) Methods of ensuring that the basicity is constant have already been discussed in Chapter 2.

The blast furnace and its control 0.60

r

-40

Ii 22

63

ι

ι

ι

ι

ι

I

I

I

I

I

6

14

22

6

14

22

6

14

22

6

19.8

20.8

21.8

22.8

I

I

14 2 2

I

I

6

14

23.8

L 22

24.8

Time/Date

Figure 3.10 Average value of Ec and silicon content in pig iron

(1 +ST2)

(1

+Sr2)

Pred icted :si

(%;

y 0.4 0 34 0.3

0

. S

ο=

0.03%

• K

0.2

0.1

y

'

S

y

= 0.54% Si/100th/tf = 215 min

y

r2

= 95 min

0.34 O b s e r v e d Si (%)

Figure 3.11 Correlation between predicted and observed Si in hot metal

64

The blast furnace and its control Target silicon

Correction o f slow fluctuations Casting Target d r y

H 20

coke weight

period

coke

_ L _

Wet coke charged

Correction t o

Silicon content

target w e t weight Blast furnace Sinter

Sintering

Prediction model

Sinter Correction

quality

Continuous theoretical Moisture

Adjustment

- Target sinter quality

Blast v o l u m e

j

r~

Blast t e m p e r a t u r e — I

I I

I

Fuel v o l u m e

Volume

adjustment

Temperature

Target silicon

|

% silicon

«-

Blast m o i s t u r e — ι

silicon Tuyere

% silicon averaged o v e r η casts

Blast furnace Furnace t o p gas

Calculation of

Ec

(1 +ST,) (1 +ST 2) I

i £ c averaged o v e r η casts % predicted silicon

Figure 3.12 Method proposed by CRM for controlling blast-furnace dynamics using the parameter £"c

ii. T o measure coke moisture in order to control the shaft at the top via the coke/iron ratio. iii. T o eliminate excessive variations in the 'granularity'. Second stage cerned with: i. ii. iii. iv.

T o have reliable measuring equipment which must be con-

Conditioning of the blast in terms of flow rate, temperature and moisture. Fuel quantities injected. Composition and temperature of waste gas. A full knowledge of the charge in terms of total iron units, reproducibility, etc.

Third stage T o be able to control, at the tuyeres, the blast temperature, moisture and fuel oil injection. The feedback scheme set out by C R M depends critically on these factors.

The blast furnace and its control

65

A

Blast data

T o p gas analysis

Mathematical model (calculation o f k W )

Blast furnace

L o w - pass f i l l ter

Β last-

Pig i r o n analysis

Blast temperature

Reference a l g o r i t h m for kW

1 h Si

Si d e s i r e d ASi PI controller

Control algorithm AkW, kW

AkWf kW desired

Figure 3.13 The Hoesch control system

Blast furnace measurements can be used to find a thermal balance (cooling losses do not vary significantly). A t constant burden composition, a thermal balance shows a prediction of iron temperature - this has been found to be related to Si content. Figure 3.9 shows how C R M compute a thermal parameter Ec by difference. Ec has been well correlated with the Si content of tapped hot metal. Figure 3.10 shows such a correlation, both as a function of varying Si and (in one case) with time. T h e transfer function between Ec and Si (percentage level in hot metal) is (see Figure 3.11) Κ (1 + Sxx)(\

+ Sx2)

The blast furnace and its control

66

The complete system is shown in Figure 3.12. It is in use in a similar form at I R S I D at H o o g o v e n s and in France - see Figure

3.13.

T h e parameter Ec is the sum of the heat of reduction of Si and the superheat of Fe and slag. This can be calculated from the input parameters in Figure 3.9 and used to predict the hot metal silicon level. Some feedback control has been used, mainly in Belgium and Holland, using the blast moisture as the control parameter, with some success.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Peacey, J. G. and Davenport, W. G. The Iron Blast Furnace. Pergamon, Oxford (1979) The Making, Shaping and Treating of Steel. United States Steel Corporation, Pittsburg (1971) BSC - Teesside Division. Steel Times (Special Edition). (September 1979) von Bogdandy, L. and Engell, H.-J. The Reduction of Ores. Springer-Verlag, Berlin (1971) Cavaghan, N. J. and Wilson, A . R. The use of probes in the blast furnace. J.I.S.I. 208, 215 (1970) Lowing, J. Diagnostic approach to overcoming blast furnace operational problems. Proceedings of the American Institute of Metallurgical Engineers, 36, 212-33 (1977) Stab, C , Rist, A . and Michard, J. J.I.S.I. 207 85-95 (1967) British Steel Corporation Copyright Voice, E. W . and Dixon, K, ISISpecial Publication, No 75, page 6 Wood, Β. I., ISI Special Publication, No 152, page 146 Private communication from J. Perman Turkdogan, E. T., Physical chemistry of high temperature Technology, Academic Press (1980) Decker, A . and Scheimer, R., CRM Metal Rep, 12 37 (1967) Tsuchiya, N., Tokuda, M. and Ohtassis, M, Metall. Trans, B7, 315 (1976) Poos, A , Vidal, R. and Luchers, J.,Journes. Int. de Sidérurgie, B-7 (1965). Also with Van Langen, J. in Jour. Met. 17, 1379(1965)