PRODUCTION OF LIQUID HOT METAL
Amit Chatterjee Adviser to the MD Tata Steel
IIT, Kharagpur, 26th October, 2010
PRODUCTION OF LIQUID HOT METAL
Amit Chatterjee Adviser to the MD Tata Steel
IIT, Kharagpur, 26th October, 2010
PART - I IRONMAKING IN BLAST FURNACES Present Scenario and General Features
IRON ORE TO LIQUID STEEL -
30 - Fe2O3
Iron ore
Direct molten steelmaking
%
n e 20 g y x O
10 -
Direct reduction
Blast furnace and Smelting Reduction Liquid steel
Remelting DRI, HBI
Scrap
0 Scrap
Refining
%
n5 o br a C -
| 0
Hot metal
| 200
| 400
| 600
| 800
| 1000
Temperature OC
| 1200
| 1400
| 1600
BLAST FURNACE IRONMAKING Amongst all the ironmaking processes, the blast furnace still holds the dominant position. The blast furnace has remained up-to-date competitive with the new technologies.
and
Hot metal production rates of 8000-10,000 tpd, fuel rates of around 450-470 kg/thm (270-275 kg coke plus 175-225 kg coal), furnace availability ranging between 95-98% and campaign life of 15-20 years are benchmarks today.
SIZE AND NUMBER OF BLAST FURNACES IN THE WORLD IN 2007 140 s 120 F B 100 f o 80 r e b 60 m 40 u N 20
130
80 59
63 38
35
34
21 8
9
7
4
0 0 0 0 0 5 0 0 0 0 0 5 0 0 0 0 0 5 0 0 0 0 0 5 0 0 0 0 0 5 0 0 0 0 5 0 1 0 6 1 2 2 3 3 4 4 5 5 1 0 0 1 0 1 0 1 0 1 0 1 0 1 5 0 1 0 1 0 1 0 1 5 5 0 1 0 5 0 5 0 5 0 5 0 1 2 2 3 3 4 4 5
BF inner volume, m 3
In 2007, there were around 490 BFs varying in size from 500 to
CHANGE IN SIZE OF BLAST FURNACES AT POSCO, S.KOREA
BLAST FURNACES IN INDIA BF size, m3 (inner vol.)
No. of Furnaces
Combined inner vol., ‘000 m3
Production, Mpta
> 3000
2
6.4
4.0
2000-3000
8
17.2
8.5
1500-1999
8
14.1
6.5
1000-1499
15
16.2
7.5
500-999
8
3.9
3.0
< 500
~ 30
~ 5.5
~ 2.5
Total
68
61
32
INCREASING BLAST FURNACE SIZE IN INDIA Steelworks Big BFs
Big BFs Under project planning stage
Inner vol., m3
Capacity, Mtpa
JWS
4019
3.0
Tata Steel - ‘G’ (after upgrdation)
2308
1.8
Tata Steel - ‘H’
3800
2.5
Tata Steel - ‘I’
3800
2.5
Vizag # 3
~ 4000
3.0
Bhilai # 0
~ 3800
2.7
IISCO
~ 3200
2.2
Bokaro 2 – rebuild
~ 2600
1.8
~ 4000 x 2
6.4
1680 x 2
3.0
Tata Steel – KPO Medium and small BFs under construction
JSW, JSPL, Bhusan
MACRO-FEATURES OF A BLAST FURNACE The furnace is a refractory lined steel shell filled with material viz. coke, iron ore, sinter, pellets, flux, etc. from the stockline down to the bottom. The process goes on continuously for several years till the furnace is shut down for repairs and modification. The inputs and outputs are represented per metric ton (i.e. tonne) of hot metal .
Preheated air at 1000-1250 O C is blown through tuyeres into the furnace. It may be enriched with some pure oxygen, moisture. Most modern furnaces also inject pulverised coal. Exothermic combustion of coke and coal by oxygen of air gasifies carbon into CO and also provides heat. The highest temperature zone of the furnace (1900-2000OC) is at the level of tuyeres – the raceway.
BLAST FURNACE PLANT
BLAST FURNACE REFRACTORY LINING For containing heat, lining is important. It is subjected to: • Carbon monoxide attack. • Action of alkali and other vapours high temperature. • Abrasion by moving solid charges. • Attack by molten slag and metal. • Effect of furnace design and operation. Alumino-silicates and carbon are refractory materials most commonly employed for BF lining. Ordinary fireclay bricks containing 40-45% Al 2O3 are used in the upper stack. 60% Al 2O3 (known as high duty fireclay) is employed for lower stack, belly and bosh. There have been attempts to use silicon carbide bricks in the bosh region as inner refractory lining. Carbon is the popular refractory in the hearth. It has very high
HOT BLAST STOVES
HOT BLAST STOVES 25-40% of the total BF gas generated is consumed in pre-heating the blast in hot blast stoves. Each furnace has at least three stoves. The stove is a tall cylindrical (height 20-36 m, diameter 6-8 m) steel shell lined with insulating bricks inside. The interior of a stove has a combustion chamber, and a heat re-generator unit, which consist of refractory bricks arranged as a checker work. As gases flows through the checker work, heat is exchanged with checker bricks. The stoves operate in cycles. During heating cycle, the blast furnace gas is burnt with air in the combustion chamber. The hot flue gas heats up the bricks. This requires 2-4 hours. Then the combustion is stopped and air at room temperature is blown through the stove in the reverse direction. The air, blown by turbo-blowers, gets heated following contact with hot checker bricks. Then flows into the blast furnace through tuyeres. This is the cooling cycle of the stoves. Lasts 1-2 hr. Since cooling is faster than heating, a minimum of 3 stoves are required – one on cooling and two on heating.
DEVELOPMENTS IN BF IRONMAKING
TECHNOLOGICAL IMPROVEMENTS IN BFs IN GERMANY AND EFFECT ON COKE RATE
IMPROVEMENTS IN BLAST FURNACES Maximum size of blast furnaces stabilised at about 15 m hearth diameter; inner volume of 5000-6000 m 3. Maximum productivity achieved 2.8-2.9 t/m 3 /day using conventional raw materials. Maximum output is 12000 tpd; equivalent to 4 Mtpa. Coke consumption (without coal injection or other fuel) is at best about 450 kg/thm, i.e 3.15 Gcal or 12.5 GJ, with recoverable excess BF gas of energy value 3-4 GJ. Iron ore beneficiation becoming mandatory for reduction of slag volume from 300-350 kg/thm to 200 kg or even 100-150 kg/thm using high grade pellets (66-68%).
OPERATIONAL FEATURES OF SOME BFs IN THE WORLD Parameter
Posco (S. Corus Korea) BF 6 (Netherlands) BF 7
Kimitsu 3 (Japan)
Nippon Steel (Japan)
G Blast Furnace (Tata Steel)
Production, t/day
8600
6750
10,233
10,051
5150
Working volume, m3
3225
2328
3790
NA
2308
Productivity, t/m3/day*
2.66
2.9
2.7
2.47
2.2
Top pressure, kg/cm2
2.5
1.67
2.25
2.2
1.3
Oxygen enrichment, %
1.6/2.0
4.9
4.0
2.4
4.6
Burden, % Sinter (S), Ore (O), Pellets (P)
85(S) 15(O)
50(S) 50(P)
50(S) 50(P)
93(S) 7(P)
70(S) 30(O)
1.85
1.54
1.54
1.84
2.4
Coke ash, %
11
9.5
9.5
10.2
15.4
Coke rate, kg/thm
390
339
365
392
410
PCI rate, kg/thm
100
161
125
71(Oil)
120
Sl
320
236
236
286
300
l2O3 in sinter, %
k /h
AUXILIARY FUEL INJECTION INTO BLAST FURNACES Injection of hydrocarbons through the tuyeres generates H 2 and CO in the combustion zone. H 2 gives several additional benefits, such as: • Faster gaseous reduction of iron oxides. • Higher thermal conductivity of the gas and consequently, faster heat transfer to the solid burden. • Better bed permeability in the furnace, since hydrogen has a much lower density than CO and N2.
TOTAL REDUCING AGENTS IN 1995 Country
Coke, kg/thm
Coal, kg/thm
Oil, kg/thm
Others, kg/thm
Total, kg/thm
Japan
414
98.8
-
1.2
514
USA
413
36.5
10.0
40
499.5
France
351
125.6
3.5
-
480.1
Germany
359
51
63
-
473
Italy
353
129.5
15
-
497.5
Netherlands
357
141
-
-
498
UK
394
43
55
-
492
India
480
120
-
-
600
Today, in many countries, coke consumption even as low as 270290 kg/thm has been achieved at coal injection rates of 190-220 kg/thm, with a coke to coal replacement ratio in the range of
COAL INJECTION INTO BLAST FURNACE • Pulverised coal injection (PCI) is a of considerable current interest. • In most cases, 1 kg coal at best replaces 1 kg coke, referred to as Replacement ratio. Sometimes, RR can be more than 1. • Typically, coal is ground to about 80% below 75 micron (0.075mm). • Coal injection is normally accompanied by suitable oxygen enrichment of the air blast. • Coal injection rates above 100 kg coal/thm are quite common now-a-days and some modern furnaces have reached a level as high as 250 kg/thm. • Choice of appropriate coal in terms of its ability to combust easily in the raceway, depends on the nature of the coal (particularly its volatile matter content), particle size distribution and mode of injection. All these factors influence the Replacement ratio.
INCREASE IN GLOBAL AVERAGE PCI RATE
Higher PCI calls for better coke. Avg. PCI rate in 2008-09 : Japan, Korea, Taiwan – 120, China – 190, EU – 215,
PRESSURE DROP AT DIFFERENT COKE RATES
LIMITATION OF COAL COMBUSTION IN THE RACEWAY
FUEL RATE IN ‘G’ BLAST FURNACE,TATA STEEL 700
m h t / 600 g k , e 500 t a r n 400 o i t c e 300 j n i l a 200 o C / e k 100 o C
Coke rate
77
533
Coal rate
73 101
565
498
129
113
448
458
2006-07
2007-08
0 2003-04
2004-05
2005-06
Gradual decrease in coke and increase in coal is seen.
CHARGING SYSTEM: BELL TYPE
BELL-LESS TOP WITH ROTATING CHUTE
LATEST IS GIMBLE TOP CHARGING SYSTEM
ADVANTAGES ACCRUED FROM IMPROVED BURDEN DISTRIBUTION • Increased productivity, decreased coke rate, improved furnace life. • Improved wind acceptance and reduced hanging as well as slips. • Improved efficiency of gas utilisation and indirect reduction. • Lower silicon content in hot metal and consistency in the hot metal quality. • Reduces tuyere losses and minimisation of scaffold formation. • Reduced dust emission owing to uniform distribution of fines.
CENTRAL WORKING AND WALL WORKING BLAST FURNACES
MONITORING BURDEN DISTRIBUTION The monitoring system for assessing distribution includes: • Heat flux monitoring equipment to measure the heat flow in different zones (both above and under the burden). • Profile meters for the measurement of surface profiles. • Thermocouples in the throat, stack and bosh regions to measure temperature. • Stack pressure monitoring and pressure drop measurement along the furnace height. • Special instruments such as infrared probes to monitor the burden surface temperature, devices in the stack region to measure individual layer thicknesses and local descent rate, and tuyere probes to sample materials at the tuyeres level. • Mathematical models for charge distribution control, overall heat and mass balance and interpretation of probe data.
BLAST FURNACE PROBING AND CONTROL
PART - II IRONMAKING IN BLAST FURNACES Mechanism of Reduction, Blast Furnace Reactions, Zones in a BF
NOMENCLATURE OF REACTIONS IN A BF The reduction of iron oxides by CO and H 2 is traditionally known as Indirect Reduction in blast furnace ironmaking. This is meant to distinguish it from the reduction by solid carbon, which is called Direct Reduction. Gas-solid reactions are much faster than reactions between two solids. Therefore, maximum of indirect reduction is the goal. Utilisation of hydrogen as a reductant has definite advantage. Disadvantage is -----.
BLAST FURNACE REACTIONS As the solid charges descend downwards, major reactions may be classified into the following categories viz.: • Removal of moisture from the raw materials. • Reduction of iron oxides by CO. • Gasification of carbon by CO 2. • Dissociation of CaCO3 (where raw limestone added). • Reduction of FeO by carbon. • Reduction of some other oxides of ore by carbon. • Combustion of coke and coal in front of tuyeres. The outputs from the furnace are: • Molten iron (i.e. hot metal) • Molten slag • Gas at a temperature of around 200 OC, containing CO,
IMPORTANT BLAST FURNACE REACTIONS Gasification reaction: 2C + O2 = 2CO
Exothermic reaction
Boudouard reaction: 2CO = 2CO2 + C
Endothermic/ Exothermic (beyond 1000°C) reaction
Solution loss reaction: C + CO2 = 2CO
Endothermic reaction
Water gas shift reaction: CO + H2O = H2 + CO2
Mild Exothermic reaction
MECHANISM OF IRON OXIDE REDUCTION • Transfer of reactant gas to the solid surface (CO or H 2) across the gas boundary layer around the piece of solid. 3 Fe2O3(s) + CO (g) = 2 Fe3O4 (s) + CO2 (g) Fe3O4(s) + CO (g) = 3 FeO (s) + CO2 (g) 3 Fe2O3(s) + H2 (g) = 2 Fe3O4 (s) + H2O (g) Fe3O4(s) + H2 (g) = 3 FeO (s) + H2O (g) • Inward diffusion of reactant gas through the pores of the solid chemical reaction FeO (s) + CO = Fe (s) + CO 2 (g) FeO (s) + H2 = Fe (s) + H2O (g) • Outward diffusion of the product gas (CO 2 or H2O) through the pores. • Transfer of the product gas from the solid surface into the bulk
IRON OXIDE REDUCTION Reaction kinetics of iron ore reduction determines the rate at which iron oxides are converted to metallic iron. The rate of any chemical reaction increases as the temperature increases. As a result, reaction kinetics is not generally a matter of great concern in blast furnaces. This is contrary to the reaction rates in DR processes. Owing to lower temperatures, the reactions are slower. Hence, the production rate of DR processes is generally lower than that in blast furnaces.
IRON OXIDE REDUCTION Iron oxide reduction is complex because the oxide charged undergoes a series of changes, step-by-step before the conversion to the final product. The slowest step in the entire process chain determines the overall reaction rate and is referred to as the rate controlling step. Chemical reactions are either homogeneous (if a single phase is involved) or heterogeneous (if two or more phases are involved). The solid-state reduction of iron oxides is heterogeneous, involving solid and gas phases separated by an interface. For the chemical reactions to occur, the reactants must reach the interface and the products must move away. The movements of reactants and products are affected by
RATE CONTROLLING STEPS IN IRON OXIDE REDUCTION Rate of iron oxide reduction depends on the rates of heat and mass transfer across the gas-flow boundary layer at the outer surface of the solid phase. When the reaction rate is controlled by this factor, it is known as "Boundary Layer Control". Rate of diffusion of reducing gas inwards and product gas outwards through the reduced iron layer can control the rate of reduction. This phenomenon is generally associated with large ore particles, and known as "Gaseous Diffusion Control" or "Iron Pore Control." Chemical reaction at the wustite-iron interface can be rate controlling. In such a case, the rate of reduction per unit area of the remaining iron-oxide surface is found to be constant with time. Mechanism is known as "Interfacial Reaction Control" or "Phase Boundary Reaction Control". When both gaseous diffusion control and interfacial reaction control combine to influence the rate of reduction, the mechanism is referred to as
SCHEMATIC OF IRON ORE REDUCTION
REDUCTION OF IRON OXIDES
Topo-chemical type reduction
Partially reduced porous iron oxide
Importance of porosity assessed by microscopic examination of reduced iron oxide. Topo-chemical reduction has three concentric layers – magnetite, wustite, metallic iron – each layer same shape as outer surface in case of dense oxides. Porous oxides has similar structure in individual particles.
REDUCTION OF IRON OXIDE BY H2 VS. CO
Reduction by hydrogen
Reduction by CO
• Initially, reduction by hydrogen is faster than carbon monoxide. • Magnetite reduced by hydrogen contains grains of wustite completely enveloped in dense layers of metallic iron. In CO reduction, metallic iron layers consist of almost pure pearlite. • Carbon can diffuse very rapidly in austenite so that at the interface between austenite and wustite, carbon is available to complete the reduction. In case of hydrogen, reduction is incomplete. • Solid-state diffusion of ferrous iron through wustite much greater than gaseous diffusion of hydrogen or CO through ore particles. Therefore, solid-state diffusion in the stack region is not the rate controlling step in BF
STRUCTURAL CHANGES IN IRON OXIDE REDUCTION Hematite → Magnetite → Wustite → Metallic iron • • • • • • • • •
• • •
In hematite, oxygen atoms arranged in close-packed hexagon structure. In magnetite and wustite, the structure is FCC. In first stage of reduction, oxygen atoms undergo major readjustment. Results in 25% increase in volume, opens-up structure, facilitates redn. During magnetite to wustite transformation, oxygen lattice is unchanged. Iron atoms diffuse to fill vacant sites in lattice; volume change is small. Wustite has variable composition – composition changes from equilibrium with magnetite to equilibrium with metallic iron. Nucleation and growth of iron crystals results in shrinkage and large increase in porosity of the metallic phase. Transformation of hematite to magnetite – 25% increase in volume. Further, 7-13% increase during transformation to wustite. Followed by shrinkage to metal phase. Total increase in volume during complete reduction of hematite:25-27%. For magnetite ores, no volume increase; 4-5% shrinkage in final product. Explains why reducibility of magnetite is very poor.
REDUCTION OF METAL OXIDES BY CO
REDUCTION OF IRON OXIDE BY CO
REDUCTION OF IRON OXIDE BY HYDROGEN
BF NOMENCLATURE
F B f o t h gi e H
ZONES IN A BLAST FURNACE
DETAILS OF THE ZONES Stack: Wall slopes outwards in downwards direction Extends from the stock line to the mantle level. In this zone the burden is completely solid. The charge gets heated from 200°C at the stock line to 1100-1200°C at the bottom of the stack. Most of reduction occurs by gas-solid contact.
Belly: The cylindrical portion below the stack Metallic burden begins to soften and fuse as it travels.
Bosh: Below the belly and sloping inwards in downwards direction Burden begins to melt except coke. Gangue and flux combine to form the slag. The furnace walls are either parallel and then taper down, or are entirely tapering down resulting in reduction the sectional area by about 20-25% . This is because of decrease in the apparent volume of the charge.
Tuyere or Combustion Zone: End of slope; parallel walls Except central column of coke, entire charge is molten. Oxygen of the blast burns coke to CO. Number of combustion zones, in front of each tuyere exists. Thus, there is a ‘runway’ or ‘race-way’ in front of each tuyeres which is first horizontal and then becomes vertical while expanding.
Hearth: Below the tuyere region and cylindrical Some coke descends into hearth to form the ‘deadman’. Entire charge is molten and
TUYERE AREA IN A BLAST FURNACE
MECHANISM OF SILICON REDUCTION
CONCEPT OF RAFT From sensible heat of the flame, its temperature is calculated. This is known as Raceway Adiabatic Flame Temperature (RAFT). Heat content of flame = mass of gas in the flame average specific heat of gas (RAFT 298) Change in Operating Variable
Change in RAFT , 0C
Blast temperature raised by 100 0C
+82
Blast oxygen raised by 1%
+53
Blast moisture raised by 5g/Nm 3
-28
1% methane added to blast
-56
PRODUCTIVITY OF BLAST FURNACES BF productivity is defined as tonnes of hot metal produced per day per cubic metre of inner/working volume. Productivity can be increased by: • Screening of solid charges before charging into the furnace to eliminate fines below a certain size • Agglomeration of fines by sintering, pelletising • Proper top charging device to make the distribution of burden size as uniform as possible in any horizontal section. • Use of better quality coke. • Use of higher hot blast temperature. • Use of oxygen enriched blast. • Use of higher top pressure. • Use of superior quality iron oxide burden. • Improved facilities for metal and slag evacuation.
PRODUCTIVITY AS FUNCTION OF SLAG RATE
ACCEPTABLE COKE FOR BLAST FURNACES Suitability assessed in terms of: Room temperature strength High temperature strength Chemistry Size Reactivity For blast furnaces in India, ‘acceptable’ values are: • Room temperature strength : M 10 7.0 (max.) • High temperature strength: CSR 64 (min.), CRI 25 (max.). • Chemistry: Ash 15-17% min., Alkali 0.35% max. • Size : Suit iron oxide feed. Size at tuyere level? Lower productivity of Indian furnaces essentially on
TYPICAL COST BREAK UP FOR HOT METAL (1.2%) Fluxes
(25.4%) Ore & Scrap
(47.4%) Coke
(5.1%) Others (6.2%) Labour & Admn. (2.5%) Maintenance (2.5%) Relining (1.3%) (4.7%) Blowing Refractories cost
(3.7%) Coal & Tar injection
PART - III SMELTING REDUCTION ALTERNATIVE METHOD OF IRONMAKING
PROGRESSIVE REDUCTION IN BF COKE CONSUMPTION OVER THE YEARS Year Coke rate, Injectant, kg/thm kg/thm
Total reductant, kg/thm
Comments
1950
1000
0
1000
Lean local ores
1965
600
0
600
Rich seaborne ores
1970
525
50
575
Oil injection/high blast temperature/ oxygen enrichment
1980
500
50
550
High top pressure/burden distribution and permeability control
1990
400
125
525
Coal injection/improved sinter coke quality
2000
325
175
500
Increased coal/gas/oil injection
2010
250
250
500
Continued use of metallics in the burden
DEPENDENCE OF BLAST FURNACES ON COKE Parameter
Case A
Case B
Case C
Case D
Coal, kg/thm
0
106
149
173
Coke, kg/thm
482
376
334
305
Total fuel, kg/thm
482
482
483
478
59.4
59.2
61.1
58.8
1129
1141
1159
1177
Humidity, g/Nm3
30
16
10
6
Oxygen in blast, %
21
21
22.4
22.4
Gas utilisation, %
49.2
50.6
51.0
48.2
Hot metal temperature, oC
1487
1475
1478
1482
0 24
0 27
0 28
0 33
Production, area/ d
t/m2
hearth
Blast temperature, oC
Si i h t
t l %
FORECAST OF TECHNOLOGIES TO BE ADOPTED FOR HOT METAL PRODUCTION 100 Non-coking coal
New SR technologies Fine ore
75
Pellets Lump ore 50 Coking coal Conventional BF route 25
2000
Sinter Pellets Lump ore
2025-2030 YEAR
EMERGING TRENDS IN INPUTS TO IRON/STEELMAKING Future Recent Lump ore, sinter, pellets
Fine ores
Conventional
Lump ore
DRI using NG, non- coking coal
Coke from metallurgical coal and PCI
Coke from metallurgical coal Scrap
Scrap
Scrap
SR hot metal using noncoking coal
Non-coking coal, NG, synthetic gases
FUNDAMENTALS OF SMELTING REDUCTION Off-Gas (Critical in economics of all SR processes) Pre-reduction Unit
Iron Ore/ Pellets Hot Reducing Gas
Pre-Reduction
Pre-reduced Ore
Post Combustion Smelting Reduction Vessel
Final Reduction Coal Melting Coal Gasification
Oxygen/Air Hot Metal
Pre-reduction Degree, Extent of Post Combustion and Heat
RAW MATERIALS USED IN BLAST FURNACES AND IN SMELTING REDUCTION Process
Oxide Feed
Reductant
Product
Blast furnace including mini blast furnace
Lump ore, Coke, coal, Hot metal sinter, pellets oil, tar, essentially for BOF natural gas steelmaking
Smelting reduction
Ore fines, lump ore, waste iron oxides
Coal, oxygen, electricity
Hot metal (synthetic hot metal) for EAF / EOF steelmaking
FLOWSHEET OF SINGLE STAGE, TWO STAGE AND THREE STAGE SR PROCESSES Ore
Gas
Ore
Gas Reduction
Ore
Reduction
Gas
Coke Coal
Reduction and melting
O2
DRI
Gas
Metal
Gasification
DRI
Coal Melting
Slag
Gas
Coke
Gas
O2 Melting
Single stage Slag
Metal
O2
Two stage Slag
Metal
Three stage
Coal
SALIENT FEATURES OF SR (1) • SR involves both reduction and smelting, i.e. melting accompanied by chemical reaction(s). • In an ideal SR reactor, in the strictest sense, all the reduction reactions should take place in the liquid state in a single step. • Any SR process involves the extraction of metal values from the ore following liquid phase reduction by non-coking coal. • In actual practice, most SR processes utilise two steps – the removal of oxygen from the ore in the solid state to varying extents in step one, followed by removal of the remaining oxygen in liquid phase reduction reactions in step two. • Compared with DR processes, the principal advantage of high temperature operation in SR processes is: faster rates of reaction and prevention of sticking problems associated with solid-state reactions (intrinsic in DR processes).
SALIENT FEATURES OF SR (2) • Smelting reduction processes are thus either two-step processes with separate pre-reduction and smelting reduction steps (such as Corex and HIsmelt), • Simpler one-step processes involving simultaneous reduction and smelting still not fully proven. • All SR processes consume fairly large volumes of coal that generates large amount of export gas, the effective utilisation of the generated by-product gas is extremely important. • Generally, the use of export gas makes or breaks the cost structure of SR processes .
SALIENT FEATURES OF SR (3) • Unless the net export gas from any Corex plant can be utilised (extent of generation 1650 Nm 3 /thm; calorific value 7500KJ/Nm3) the process itself becomes totally uneconomical. • If no credit is given to the off- gas, the cost of the hot metal made can be as much as 50% higher than blast furnace hot metal. • Adequate credit can be obtained by selling cogenerated electrical power from the Corex off gas. • Another use of the export gas is in shaft furnace, DRI production adjacent to the Corex furnace.
REDUCTION STEPS IN SR • Reduction by solid carbon • Reduction by carbon dissolved in Iron • Reduction of molten FeO by CO
REDUCTION OF SLAGS BY SOLID CARBON A liquid boundary layer is expected to exist on the slag side of the slag / carbon interface, FeO must be transported to the nucleation site in the following manner: FeO (l) + C (s) = Fe (l) + CO (g) FeO (bulk) transported to FeO (slag /gas interface) A halo is formed, after which the following steps come into play: FeO + CO = Fe + CO 2 (at gas / slag interface) CO2 (gas /slag interface) transported to CO 2 (carbon /gas interface). CO2 + C = 2CO (at carbon / gas interface)CO (carbon / gas interface) transported to CO (gas / slag interface).
REDUCTION OF SLAGS BY CARBON DISSOLVED IN IRON Taking desulphurisation as an example, a three step mechanism has been proposed: FeS(iron) = FeS (slag) FeS (slag) + CaO (slag) = FeO (slag) + CaS(slag) FeO (slag) + C (iron) = Fe(l) + CO(g)
REDUCTION OF MOLTEN FeO BY CO • Reduction rate of slags is independent of the FeO concentration in the range 67.7 to 48.0%. • FeO less than 48%, the reduction is dependent on the fraction reacted and the partial pressure of CO. • Reduction rate of iron oxide by CO follows a first order rate law for the reversible reduction of stoichiometric FeO. The rate equation is: (R’ / Ao) = exp (-32300 / RT – 1.37). (1.0 - 0.7aSiO2). (aFeO. pCO - aFe. pCOk) where, R’ = Rate constant, mol/cm2 .s Ao = Reaction surface area, cm2 T = Reaction temperature, K The activation energy was determined to be 135 kJ/ mol.
NET ENERGY CONSUMPTION AND GAS CREDIT OF SR PROCESSES vis-a-vis BF 35 30 m h t / J G , y g r e n E
Ga s Credit Net Energy
25 20 15 10 5 0 Fastme lt
Hismelt
Corex
Redsmelt
Blast Furnace
MAJOR SMELTING PROCESSES Vertical Shafts • MBF – Capacity : 30,000-1,125,000 tpa (covers very wide range) • Corex – 5 operating plants, Capacity : 300,000-900,000 tpa First and leading SR process. Very high volumes of off-gas; some coke often used. Coal properties can be varied over a small range (Indian coal?) • Finex – Capacity : 1.2-1.5 mtpa. Process development complete. Very promising. • Tecnored – Capacity : 150,000 tpa. Process still under development
Bath Smelting Processes • HIsmelt – Capacity : 600,000-1,200,000 tpa. Process almost ready. • Ausmelt – Capacity : up to 2.5 mtpa. Process not proven. • Romelt – Capacity : 200,000-1,000,000 tpa . Russian process with tremendous promise, but no plant despite efforts, including in India and
Rotary Hearth Furnace (RHF)
Japan • ITmk3 – Capacity : 0.5 mtpa. Slag separation by partial melting is unique.
RHF Combined with Melting / Smelting
• Inmetco – Capacity - 60,000 tpa. Suitable for zinc-bearing iron ores. • FastMelt – 2 operating plants mainly for smelting solid wastes from ISPs .
COMMERCIALISED SR PROCESSES • Corex – Many operating units; most popular SR process. Typical iron oxide Lump ore
Pellets
Sinter
Fetotal
62 - 65
62 - 65
50 - 55
Grain size, mm
8 - 20
8 - 16
10 - 30
• HIsmelt – Nucor, Rio Tinto, Mitsubishi and Shougang (of PR of China) agreed to construct a 0.8 Mtpa plant at Kwinana in Western Australia after pilot plant tests were completed at the same location. The plant had produced more than 37,000 tonnes of hot metal in total till March 2006. Has been stopped thereafter.
FLOW SHEET OF THE COREX PROCESS
SOME KEY FEATURES OF COREX • Is a shaft furnace-based process. It can accept high alkali containing ores without any build-up inside the reactor. • Specific melting capacity is higher than in blast furnacesproductivity of the order of 3-3.5 t/m 3 /d can be achieved. • The process is capable of operating at 50-115% of its nominal capacity. • It takes only half an hour to stop the plant and four hours to restart it, whenever required. • Hot metal quality is comparable with blast furnaces (C=4.04.5%, Si=0.30-0.80%, S=0.02-0.09%, P depends on inputs). • It has outstanding environmental superiority in comparison with the blast furnace process in terms of generation of dust, SOx, NOx, phenol, cyanides, etc.
FLOW SHEET OF THE FINEX PROCESS
SOME KEY FEATURES OF FINEX • Posco developed Finex process to utilise iron ore/non-coking coal fines not suitable for charging to their Corex plant. • Corex in Posco uneconomical owing to restrictions on coal and iron oxide size (top size and under size). Also, often required 10% coke to ensure hearth permeability. • Finex actually able to utilise iron ore in the form of fines. • After fluidised bed reduction, fine DRI is hot briquetted before melting in a melter gasifier (similar to Corex). • Non-coking coal is also briquetted before use in melter gasifier. • Hot metal composition same as BF. • Range of complete scale of production yet to be established.
Finex is certainly of interest to ISPs. Whether it can be used to supply limited tonnages of hot metal to EAFs?
FLOW SHEET OF HISMELT PROCESS
HISMELT FLOW OVERVIEW
SOME KEY FEATURES OF HISMELT • Incorporates many BF features – hot blast stoves, air blast, etc. • Can be single stage process; better, with separate pre-reduction. • Pre-reduction of iron oxide and oxygen enrichment of hot air (1200°C) blast provide substantial productivity enhancement. • Hot metal contains 4.3 ± 0.15% carbon, phosphorus and silicon levels are extremely low viz. < 0.05% P and <0.05 Si%. • Process can use high phosphorus iron ore – can be of significance in India since many Indian ores have high phos. • Coals with 50-70% FC, 10-38% VM and 5-12% ash have been used in Pilot/Demo plant. Typical coal rate reported – 640-690 kg/thm. • Utilisation of export gas not as critical as in the case of processes like Corex on account of in-bath smelting. Hot metal production economics under Indian conditions still not proven. Demo plant not proven on sustained basis at f
PROCESS FLOW SHEET OF ITmk3
SOME KEY FEATURES OF ITmk3 • First step produces iron nuggets in a very short reduction time (10-12 min) along with some slag removal in a RHF. • High-purity iron nuggets contain 96-97% iron and 2.5-3.0% C, 0.05 S, i.e. nearly same composition as blast furnace pig iron. Coal consumption reported to be 500 kg/t nuggets. • The process is flexible as far as the type of iron ore that can be used. Magnetite, haematite as well as pellets made of taconite (low-grade iron ore found in USA) have been processed. • Emits at least 20% less carbon dioxide than blast furnaces. • Overall less NOx, SOx and particulate matter emissions. • Easy start-up, shut down and change in production rate.
Over last 10-12 years has gone through laboratory, small pilot plant and demonstration plant testing. Nuggets melted in EAFs. A commercial plant (0.5 mtpa) Mesabi Nuggets at North-Eastern Minnesota, USA has been started. Could be of
SUMMARY OF SMELTING REDUCTION • SR designates processes in which iron oxide is reduced at high temperature to liquid iron essentially using non-coking coal and oxygen.
• SR
processes configurations.
utilise
several
distinct
thermo-chemical
• In all SR processes, a substantial portion of the iron oxide charged enters into the slag phase and is reduced by solid carbon particles, gaseous CO, and carbon dissolved in iron.
• SR reactions are by nature quite complex – major reasons are elevated temperature, turbulence.
multiple
phases
and
intense
bath
• It is essential to optimise post-combustion ratio (PCR), degree of pre-reduction, and heat transfer efficiency (HTE). Ideal SR process should be of single-stage configuration to
