12
Modern Steelmaking Processes
1. Basic Oxygen Process:
Design of Converter and Materials Balance
Topics to discuss…
1. Basic oxygen process
2. Design of converter
3. Feed materials
4. Material balance
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Basic Oxygen Process
The dominant method of producing High purity oxygen is injected onto
steel from blast furnace hot metal. the surface of the bath by a water
cooled vertical pipe or lance
The process involves the treatment of inserted through the mouth of the
blast furnace molten iron in basic lined vessel.
BOF vessel
• Blast furnace hot metal contains In most of the steelmaking
C = 3-4%, Si = 0.8-1.0%, practices, hot metal is pretreated to
Mn = 0.6 - 0.8%, P = 0.15-0.20% remove Si, P and S from hot metal
• steel scrap are also used in some degree to the extent it is possible.
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In the basic oxygen steelmaking process, refining of hot liquid iron
is performed by top-, bottom-, or combined blowing of oxygen in a
converter
The top-blowing process has different names:
• in European steel plants, LD (Linz-Donawitz) process; in the UK, BOS (basic
oxygen steelmaking); in the Far East and America, BOF (basic oxygen furnace);
in the U.S. Steel, BOP (basic oxygen process)
The bottom-blowing processes:
• OBM in Europe and Q-BOP elsewhere
The combined blowing processes are used mainly to increase the
rate of production
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General BOF vessel classifications
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General BOF vessel classifications
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The Shop Layout
A BOF installation consists of the basic Shop Layout
oxygen furnace with
• requires rational arrangement of
• furnace support foundation, equipment to ensure smooth handling
• furnace tilt drive and controls, of solid raw materials, movement of
• furnace water cooling system, oxygen lance and hot metal.
• exhaust and cleaning system,
• it should ensure smooth flow of ladles
• oxygen injection system,
containing hot metal and steel.
• auxiliary furnace bottom stirring system,
• process additives system, • refining process is very fast and hence
• scrap and hot metal charging system, an efficient system of material transport
• molten steel delivery and slag disposal system, and weighing is required.
• furnace deskulling system, and
• other auxiliary steelmaking requirements such as
sampling, refractory inspection and relining systems,
process computers, etc.
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Layout of a typical two-converter BOF shop
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Design of Converter
From the metallurgical point of view, The inner volume is maximized to
an ideal converter keeps the liquid achieve an optimum metallurgical
steel in space and allows all process without sloping of slag.
necessary metallurgical reactions to • a ratio of 3 m3 internal volume/ ton of
take place within the temperature liquid is typical in converter design.
range of 1400−1600 °C.
The vessel is supported by a
The mechanical part, which keeps suspension system which transmits
the liquid steel in space, is a steel the load to the trunnion ring.
shell lined with refractory material.
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An operating BOF consists of
• the vessel and its refractory lining,
• vessel protective slag shields,
• the trunnion ring,
• a vessel suspension system supporting the vessel within the
trunnion ring,
• trunnion pins and support bearings, and
• the oxygen lance.
The size of BOF vessel varies between 30 – 400 ton
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The BOF vessel itself consists of
• the vessel shell, made of a bottom,
a cylindrical centre shell (barrel), and
a top cone;
• reinforcing components to the cone,
such as a lip ring and top ring;
• auxiliary removable bottoms for
bottom reline access, or for
individual bottom reline of bottom-
blown vessels;
• and a tap hole.
Typical components of a BOF vessel
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Converter design requires knowing height of molten steel bath, (hb)
diameter of bath (db), and working height of the converter, (hw)
Some correlations used in a typical
design process are given below:
ℎ𝑏
= 0.328 𝑇 −0.0148
𝑑𝑏
𝑑𝑏 = 0.704 𝑇 −0.386 𝑚
Nomenclature of the bath dimensions of a converter
ℎ𝑤 ≈ 3.5 x ℎ𝑏
For a 150-ton converter capacity,
hb = 1.48 m; db = 4.87 m; hw = 5.2 m; 𝑉 = 460 m3/min at STP 𝑉 = 3.07 𝑚3 /𝑚𝑖𝑛. 𝑡𝑜𝑛
Total height of converter = 6.7 m (excluding bottom refractory thickness) T = capacity of Converter in ton
Assuming bottom refractory thickness to be around 1 to 1.5 m,
total converter height from top to bottom becomes approx. 8 m.
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Typical dimensions of LD vessels of different capacities
(values are approximate and given for having some relative ideas
Vessel capacity, ton 30 75 200 300
Height of shell, m 7 7.5 9 9
Dia of shell, m 4.0 5.5 6.5 9.5
Dia of bath, m 2.5 4.0 5.0 6.5
Depth of bath, m 1.1 1.3 1.5 1.8
Dia of nose 1.2 1.65 2.3 3.5
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• compared to conventional processes of steelmaking the
refractories used in lining BOF vessels are expected to stand
more severe chemical and mechanical abuse.
• the attack of molten metal and slag is severe if the liquid iron
contains more silicon and manganese content and/or if steels
with low carbon steels in BOF vessel are to be produced
Safety lining
• burned pitch impregnated magnesite refractories
• typical thickness is 20 cm (45 cm on the bottom)
Working lining
• thickness varied on type of operation and wear rate
• normally lasts 300-1000 heats
• higher wear areas require greater thickness or higher quality materials
Normally 3-5 kg refractory consumed per ton steel made
Lining details of BOF vessel
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Typical example of lining used in LD vessel
Vessel Part Type of Lining Thickness of Lining, mm Materials required for lining
fire brick 1 x 75 a 50/60 ton converter :
magnesite brick 2 x 65
Bottom
magnesite brick 1 x 250 Dolomite brick
tarred dolomite brick 1 x 350 = (3821 nos.) x (31.2 kg/pc)
total thickness 805 = 121.4 ton
magnesite brick 1 x 125 Ramming mass = 9.0 ton
Side wall
tarred dolomite brick 1 x 350
total thickness 835 Magnesite lining = 57.9 ton
magnesite brick 1 x 125 Waste during laying = 6.0 ton
Nose tarred dolomite mass 1 x 190 max
tarred dolomite brick 1 x 350 Total weight refractory = 194.3 ton
total thickness 665 max. (475 min.)
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The Oxygen Lance
• 8-10 m long and 20−25 cm diameter
• designed to produce non-coalescing free oxygen jet
at an operating oxygen pressure of 10-12 kg/cm3
• water requirements are around 50−70 m3/hr at a
pressure of 5−7 kg/cm3
• Oxygen of high purity (at least 99.9 % purity)
is supplied at supersonic speed (about 1.5-2.5
Mach) on to the surface of the bath through a
water-cooled vertical lance, inserted through
(a) the mouth of the vessel.
(b)
(a) Adapter assembly of the BOF oxygen lance
(b) Various types of BOF lance tips
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• Nozzles are designed for a certain oxygen flow rate, resulting in a certain
exit velocity (Mach number), with the required jet profile and force to
penetrate the slag layer and react with the steel bath in the dimple area.
• Supersonic jets are produced with convergent/divergent nozzles.
The oxygen accelerates in the converging section up to sonic velocity,
Mach = 1, in the cylindrical throat zone. The oxygen then expands in
the diverging section. The expansion decreases the temperature,
density, and pressure of the oxygen and the velocity increases to
supersonic levels, Mach > 1.
Effect of nozzle design on impact angle
and jet thrust
Mechanics of supersonic jet formation 17/27
Multi-hole lances
• large volume of oxygen (typically 60 m3/ton at 109 m3/hr) can be blown with the
restricted total blowing time of 15-20 minutes.
• causes the total jet energy gets dispersed along the diameter of the vessel rather
than in the vertical direction
• this results more liquid metal to be exposed to oxygen, faster slag-metal reaction
and higher productivity
Lance life
• determined by the life of the nozzles.
• failures of the lance due to faulty cooling, manufacturing defects, and
differential expansion between copper tip and steel tube.
• the usual life of a lance does not exceed a few hundred heats.
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Feed Material
The major inputs for BOF steelmaking:
• Hot metal
• Cold pig iron
• Steel scrap
• Fluxes
• Gaseous oxygen
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Hot Metal Element Range Used
• Sulphur in the hot metal should be close to final Carbon 4.00-4.50
specification level Phosphorus 0.05-0.45
Sulphur 0.02-0.02
• Silicon content of hot metal determines amount of
Silicon 0.65-1.40
lime and slag.
Manganese 0.40-2.50
• A certain minimum level of manganese content is
necessary in the liquid charge (for heat generation). Other charges
MnO tends to retard the dephosphorization of the
bath. Mn content in the range 0.5 - 1.1% is tolerable. • Flux – Lime, Limestone, Dolomite
• Temperature of hot metal at charging is around • Scrap and Ore – used as coolant
1250°C to 1300°C.
• Oxygen – varies between 2.5-3 m3/min
• Proportion of hot metal in the charge is 75-90% depending on proportion of scrap and ore,
(i.e. the remaining 10-25% is steel scrap) and number of nozzles
• Deoxidisers (Al, FeSi, FeMn) and alloying
elements (Cr, Ni, V, etc.)
Material Balance
1. Hot metal of composition 0.8% Si, 0.2% P, 0.25% Mn, 4% C and
in-house steel scrap is refined in a converter to produce steel of
composition 0.1% C and rest iron. During refining scrap is charged
whose amount is 15% of hot metal.
Pure oxygen is blown.
The composition of slag is CaO 54%, FeO 18%, and MnO 2.5%,
with CaO/SiO2 = 3.5.
Exit gases analyses 15% CO2 and 85% CO.
Calculate amount of steel, slag, oxygen and waste gases per ton
hot metal.
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Basis: 1000 kg hot metal hot metal steel/ slag exit gas
steel scrap
0.8% Si CaO 54% 85% CO
Let, 0.2% P 0.1% C FeO 18% 15% CO2
a = mass of steel 0.25% Mn MnO 2.5%
4% C CaO/SiO2 = 3.5
b = mass of slag
Mn balance: Fe balance:
Mn in hot metal = Mn in slag Fe in hot metal + Fe in scrap = Fe in steel + Fe in slag
1000 x (0.0025) = b x 0.025 1000 x 0.9475 + (1000 x 0.15) x 0.999
b = 100 kg = a x 0.999 + (100 x 0.18) x (56/72)
a = 1084.45 kg
C balance:
C in hot metal + C in scrap = C in steel + C in gas
1000 x 0.04 + (1000x0.15) x 0.001 = 1084.45 x 0.001 + C in gas
C in gas = 39.06 kg
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C + O2 = CO2
C + O = CO
C in CO2 = 39.06 x 0.15 = 5.86 kg CO2 produced = (22.4 m3/12 kg) x 5.86 kg = 10.94 m3
C in CO = 39.06 – 5.86 = 33.20 kg CO produced = (22.4 m3/12 kg) x 33.2 kg = 61.97 m3
Exit gas volume = 72.91 m3 at STM (1 atm, 273 K)
Oxygen requirement
Mn + 1/2 O2 = MnO
For CO2: 10.94 m3 Fe + 1/2 O2 = FeO
For CO: (61.97 m3) x 0.5 = 30.98 m3 2P + 5/2 O2 = P2O5
For MnO: (1000 x 0.0025) x (22.4/2)/55 = 0.5 m3 Si + O2 = SiO2
For FeO: (100 x 0.18) x (22.4/2)/56 = 0.04 m3
For P2O5: (1000 x 0.002) x (5x22.4/2)/(2x31) = 1.81 m3
For SiO2: (1000 x 0.008) x (22.4/28) = 6.4 m3
Total oxygen requirement = 50.67 m3 at STP
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2. A Bessemer converter, lined with basic material, is charged with 20 tons
of pig iron of the following composition: Fe=91.2%, C=3.6%, Si=1.7%,
Mn=1.1%, P=2.4%. The blow oxidises all the C, Si, Mn, and P and also Fe
amounting to 5.6% of the pig iron. Assume that the Fe oxidises at a uniform
rate throughout the blow. Enough CaO is added to make 35% CaO in the slag.
Two-thirds of the carbon goes to CO, one-third to CO2. The blowing engines
furnish 580 m3 of air per minute.
Required:
1. The volume of air necessary to blow the charge.
2. The length of each period of the blow.
3. The weight of CaO to be added, and the percentage composition
of the slag.
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Basis: 20,000 kg pig iron
Si = 340 kg Si + O2 = SiO2 O required = 340 x (32/28) = 338.57 kg
Mn = 220 kg Mn + O = MnO O required = 220 x (16/55) = 64 kg
CCO2 = 240 kg C + O2 = CO2 O required = 240 x (32/12) = 640 kg
CCO = 480 kg C + O = CO O required = 480 x (16/12) = 640 kg
P = 480 kg 2P + 5O = P2O5 O required = 480 x (80/62) = 619.35 kg
Fe = 1120 kg Fe + O = FeO O required = 1120 x (16/56) = 320 kg
Total O required = 2621.92 kg
Volume of air required = (2621.92 kg) x (22.4 m3 / 32 kg) x (100/21)
= 8739.73 m3 at STP (1 atm, 273 K)
O supplied per minute = (580 m3) x (1.293 kg/m3) x (0.232 kg O / kg air) = 173.99 kg
density of air air contains 23.2%
oxygen by weight
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In basic Bessemer process, three periods of blow can be identified. During the first
period, Si and Mn are oxidised. During the second period, all carbon is oxidised. In the
third period, P is oxidised.
In all of these periods, Fe is oxidiesd to form FeO. For the ease of calculation, its is
assumed that Fe is oxidised at a uniform rate throughout these periods.
Time for 1st period (without Fe) = (338.57+64) kg / 173.99 kg/min = 2.31 min
Time for 2nd period (without Fe) = (640+640) kg / 173.99 kg/min = 7.36 min
Time for 3rd period (without Fe) = 619.35 kg / 173.99 kg/min = 3.56 min
Time for Fe oxidation = 320 kg / 173.99 kg/min = 1.84 min
Total time for 1st period = 2.31 + 1.84 x 2.31 / (2.31+7.36+3.56) = 2.63 min
Total time for 2nd period = 7.36 + 1.84 x 7.36 / (2.31+7.36+3.56) = 8.38 min
Total time for 3rd period = 3.56 + 1.84 x 3.56 / (2.31+7.36+3.56) = 4.06 min
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SiO2 in slag = 340 + 338.57 = 678.57 kg Slag analysis:
MnO in slag = 220+64 = 284 kg
P2O5 in slag = 480+619.35 = 1099.35 kg SiO2 = 678.67 kg = 14.22%
FeO in slag = 720+320 = 1040 kg MnO = 284 kg = 5.95%
P2O5 = 1099.35 kg = 23.04%
Total slag without CaO = 3101.92 kg, FeO = 1040 kg = 21.79%
which is (100-35) or 65% of the total slag CaO = 1670.26 kg = 35.00%
Total = 4772.18 kg = 100%
Total slag formed = 3101.92 kg / 0.65 = 4772.18 kg
Weight of CaO to be added = 4772.18 – 3101.92 kg
= 1670.26 kg
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