Sulphur removal in
Iron making
ORIGIN AND DISTRIBUTION
• Sulphur contains in
1. In Iron ores – Ion Suplhide
2. Flux- Sulphide & Sulphates
3. Coke ash- Iron sulphide, Calcium sulphide and
Organic sulphur.
ORIGIN AND DISTRIBUTION
• Distribution is as such
• in ore : 0.05-0.10 %
Final sulphur load
• Coke : 0.6-1.5 %
Slag
Flue dust & top gas
Iron
Blast Furnace Sulphur Balance
Sulphur Input
Material
Sulphur Output
Kg
%
Kg
%
Coke (1%S) 600
Kg/THM
6.0
80
Charge materials
1.5
20
Iron (0.03%S)
0.3
4
Flue dust and gas
0.37
5
Slag
6.82
91
A. K. Biswas, Priniciples of blast furnace Ironmaking, Theory and Practice, SBA Publication, 1984.
Sulphur Reactions
1. FeS + 10Fe2O3 = 7Fe3O4+ SO2
SO2+2C = ½ S2 (g) + 2CO
[Fe] + ½ S2 (g) = [FeS]
2. CaSO4 + [Fe] + 3C = [FeS] + CaO +3CO
3. CaSO4 + 4C = CaS + 4CO
CaS + FeO = CaO + [FeO]
4. CaS + SiO (g) = SiS (g) + CaO
SiS (g) + 2 [Fe] = [Fe-Si] + [FeS]
Sulphur Reactions
Coke Sulphur Gasification
0.5 S2 + O2 = SO2
SO2+C = ½ S2 + 2CO
SO2+2C = CS + 2CO
In reducing Zone
SO2+1.5C = 0.5CO2 + COS
In presence of Natural Gas
SO2 + 3H2 = 2H2O + H2S
SO2 + CH4 = CO + H2O + H2S
Sulphur Reactions
Metal sulphur & Slag sulphur variation along with the furnace height
A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA publication, 1984
Sulphur control in Iron Making process
Schrama et al., Ironmaking & Steelmaking, 44 (2017) 333-343
Sulphur Reactions
A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA publication, 1984
Sulphur Reactions
The exchange of sulphur between iron droplets and hearth slag depends on
1. Sulphur reaction rate of the slag
2. Reaction surface between slag and metal
3. Contact time
4. Slag composition
5. Slag bulk, slag basicity & sulphur load
6. Metal composition
7. slag and metal viscosity
Sulphur Reactions
The reaction rate of sulphur transfer from metal to slag depends on
1. Basicity and temperature of slag
2. Slag bulk
3. Contact surface and time.
Sulphur Reactions
Slag transfer rate increases with increase in basicity and temperature of slag
A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA publication, 1984
Thermodynamic consideration of S removal
[S]Fe + (CaO) = (CaS) +(FeO)
𝐚𝐂𝐚𝐒 .𝐚𝐅𝐞𝐎
K=
𝐚𝐒 .𝐚𝐂𝐚𝐎
K=
(%𝐒).𝐚𝐅𝐞𝐎
/
%𝐒 .𝛄𝐒 .𝐚𝐂𝐚𝐎
/
𝛄𝐒 . 𝐁𝐚𝐬𝐢𝐜𝐢𝐭𝐲
(%𝐒)
Sulphur distribution ratio : 𝛈𝐬 =
∝ 𝐊.
[%𝐒]
𝐎𝐱𝐲𝐠𝐞𝐧 𝐩𝐨𝐭𝐞𝐧𝐭𝐢𝐚𝐥 𝐨𝐟 𝐬𝐥𝐚𝐠
Sulphur Reactions
The slag metal reaction can be accelerated by
1.
2.
3.
4.
Holding the slag longer in the hearth
Decreasing the amount of flushing slag, i.e tapping iron and slag together.
More frequent tappings.
A good and uniform furnace run with high degree of indirect reduction.
Sulphur Reactions
The contact surface between iron and slag is of considerable importance and
rate determining factor.
Sulphur transfer increases with increase phase contact surface.
Greater the thickness of the slag layer through the Iron droplets pass i.e
greater the contact time greater the desulphurization. Some researcher has
reported the vice versa effect.
Substantial sulphur removal occurs by more frequent tapping of iron.
Sulphur Reactions
Al2O3
10
10
15
15
SiO2
40
34
33.5
31.8
CaO
45
41
42.5
40.2
MgO
0
10
4
8
(CaO/SiO2) (CaO+MgO)/(SiO2+Al2O3)
1.13
0.90
1.21
1.16
1.27
0.96
1.26
1.03
Optimum slag composition for obtaining optimum desulphurization capacity.
A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA publication, 1984
Sulphur Load, Slag Volume & Slag Basicity
The theoretical sulphur content of the iron depends on sulphur distribution ratio, sulphur
load & slag bulk.
𝟎. 𝟏 𝑺
%𝑺 =
𝟏 + 𝟎. 𝟎𝟎𝟏 𝜼𝒔 𝒒
S : Sulphur load in Kg/THM
q= slag bulk, Kg/THM
𝜼𝒔 = (%S)/[%S]
Sulphur Control
1. Pre-reduction of ore results in higher bosh temperature.
2. High basicity and low-FeO content
3. High fluid bosh slag : slag thinning by MgO and / MnO.
4.
Low coke ash.
5.
Replacement of coke by hydrocarbon reduces coke ash.
6. Tapping of iron and slag together.
External desulphurization
1. Soda ash (Na2CO3) or Caustic soda (NaOH)
2. Lime (CaO), CaC2, Calcium cyanamide (CaNCN)
Sodium has higher affinity for sulphur
[FeS] + Na2CO3 (or NaOH) = Na2S + FeO + CO2 (H2O)
By Lime
[FeS] + CaO (s) + C (gr) = CaS (s) + CO (g) + [Fe]
2[FeS] + 2CaO (s) + [Si] = 2CaS (s) + SiO2 (s) + 2[Fe]
External desulphurization
Lime
Its low cost and easy availability make it an attractive reagent. But it has got some critical
disadvantages.
(1) During the process of desulphurization, lime particles are continuously being covered by two
precipitates namely calcium sulphide (CaS) and calcium silicate (CaSiO4). These compounds
impede the desulphurizing reaction by surrounding the lime and forming thick barriers at the
lime – hot metal interface.
(2) In order to reduce this growth, the grain size of the lime is to be restricted to 45 micrometer
maximum. The desulphurizing reaction with lime takes place as per equation:
2CaO + 2S =2 CaS + O2.
External desulphurization
Dr. Dmitri Kopeliovich, Desulfurization of steel, Substech
External desulphurization
Calcium carbide
Calcium carbide was widely used desulphurizing reagent but at present it is
less used.
Difficult material handling processes and stringent environment requirements
with the slag disposal have negatively impacted its use.
Calcium carbide also forms layer such as in lime, and inhibits the desulphurizing
reaction. The desulphurizing reaction is :
CaC2 + S = CaS + 2C.
External desulphurization
Magnesium
• Magnesium possesses high affinity for both oxygen and sulphur. During injection of Mg
into the hot metal, therefore it can quickly react with sulphur to form MgS.
• Only addition of magnesium is less used because of the violent reaction and the relatively
complex equipment requirement.
• Magnesium is soluble in hot metal and reacts with sulphur in solution.
External desulphurization
Magnesium
The desulphurizing reaction with magnesium takes place as per equation: Mg + S = MgS.
• Mg has low boiling point (1090 °C), magnesium vapourizes as it enters the hot metal.
This vapour is under high pressure.
• Once in ladle, the magnesium vapour forms bubbles which rise through the hot metal,
dissolve and react with sulphur in solution, forming magnesium sulphide (MgS).
External desulphurization
• The lime injection with the magnesium assists in dissolution by reducing the diameter of
the bubbles as well as providing precipitation sites for the MgS.
Lime addition to avoid resulphurisation by Mg addition
MgS (s) +1/2 O2 (g) = MgO (s) + [S]Fe
By adding CaO the formed CaS possesses higher stability than MgS.
Schrama et al., Ironmaking and Steelmaking, 2017.
J. Yang, M. Kuwabara and T. Teshigawara, et al. ISIJ Int., 2005
Issues in External Desulphurization
• During the desulphurizing process, the slag formation is proportional to the
amount of reagent added to the hot metal. Some hot metal gets trapped in the
slag.
• The loss of heat during the desulphurizing process is an important factor since it
reduces the sensible heat of the hot metal sent to the converters. The three primary
sources of heat loss are :
(i) radiation from the surface of the hot metal,
(ii) addition of cold reagents and
(iii) introduction of cold injection lances into the hot metal.
The largest temperature loss occurs during injection rather than skimming. A
temperature loss of 30 °C is expected during the desulphurization process.
Issues in External Desulphurization
• No major effect on the refractory lining life of the hot metal ladle as the
treatment time is small.
• Addition of reagent and slag removing operation generate fumes which
are to be collected and dedusted before their release to the
environment.
• The captured fumes are typically cleaned in a pulse jet type bag house
designed for metallurgical operations.
External desulphurization
Injections
Final effect
Lime ( 2% weight of HM) +N2
S reduction from 0.1 -0.01 (90% in 3 min)
Lime + 2% aluminium powder
S reduction from 0.3 -0.004 (99% in 4.5min)
Mg Powder
Final sulphur 0.001%
80% Lime+20% Soda + compressed Air
0.17-0.05 & 0.043-0.02%
Liquid metal+ lime powder (<1.5 mm) +
Soda in a refractory lined rotating cylinder
S reduction from 0.1 -0.01 (95% in 20-30 min)
Calcium carbide + N2
2-2.5 Kg for every 0.01%S drop (50 %
Desulphurization)
Lime powder + Methane + Mechanical
Stirring
<0.01 %S (9 Kg lime+ 30 liters of methane)
A. K. Biswas, Principles of Blast Furnace Ironmaking, SBA publication, 1984