Role of Catalysts in Steel Making
Increasing demand for Steel has also led to an increase in capacity. While new applications have been developed for use of Steel and consequently the consumption of Steel has increased, parallely there has been a capacity additions in existing Plants and green field projects have also been set up.
Improving the efficiency of the Steel making process has always been of paramount importance in Steel making. Consequently, major improvements have been brought about in design, engineering, production practices as well as in the metallurgy of the process. Some examples are
-Use of properly sized and good quality coke
-Larger Furnace volume
-Injection of liquid or pulverized solid hydrocarbons through the tuyres like pulverized coal injection
-Higher Blast temperature
-Computer aided process control.
Catalysts:
A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst may participate in multiple reactions at a time. Hence, by using a catalyst it is possible to increase the rate of a chemicalreaction by reducing theactivation energy. Activation energy is the amount of energy required to initiatea chemical reaction and is denoted as Ea. It can be determined by the equation
Ln (k 2 /k 1 ) = E a / R x (1/T 1 - 1/T 2 )
where,
Ea is the activation energy of the reaction in J/mol
R is the ideal gas constant = 8.3145 J/K·mol
T 1 and T 2 are absolute temperatures
k 1 and k 2 are the reaction rate constants at T 1 and T 2
A catalyst permits a different energy pathway for a chemical reaction which has a lower activation energy.
One molecule of a catalyst may transform several million reactant molecules a minute. Catalysts may be gaseous, liquid, or solid; they may be inorganic compounds, organic compounds, or complex combinations. They tend to be highly specific, reacting with only one substance or a small set of substances. Catalysts are essential to virtually all industrial chemical reactions, especially in Petroleum Refining and synthetic organic chemical manufacturing. The use of catalysts in industrial applications is well known and well documented.
There is extensive literature on the well-known Arrhenius equation (given below) and also the impact of activation energy on rate of reactions.
where EA is the Activation energy.
In recent times, the use of catalysts have been considered for application in Coke manufacturing and in Iron making through the Blast Furnace route.
A experiment was carried out at the Pilot Oven facility of RDCIS ( Research & Development Centre for Iron & Steel), Ranchi, to study the impact of addition of a specific catalyst on the Carbonisation process.
The blend composition used for pilot oven carbonization tests was; 48% Australian hard, 16% US hard, 6% New Zealand hard, 6% Australian soft, 6% each of Dugda, Moonidih, Rajrappa and Kathara coking coals. This base blend was subjected to carbonization and then the catalyst added to it in the dosage of 0.02%. After addition of the catalyst, carbonization tests were carried out and the following conclusions were drawn:
Addition of carbonization catalyst is capable increasing the rate of rising coke mass temperature effectively towards the heating wall of the oven. With 0.02% layer addition of catalyst in the coal charge, coke mass temperature of 1000 C is achieved about 1 hour before compared to base blend.
Coke mass temperature with catalyst in coal charge at one-fourth width of the oven at the end of carbonization period of 18 hours observed to be 10 to 15 C higher than that of base blend.
Addition of catalyst to coal charge also had a positive impact on Gieseler plastic properties – both maximum fluidity and plastic range got increased.
The figure given below clearly shows that the coke mass temperature has started rising earlier indicating that the catalyst is increasing the rate of reactions.
Temperature profile of Coke mass temperature with catalyst
In the above figure, the coke mass temperature starts rising earlier indicating that rate of reactions is faster and the final temperature is about 10 to 15 deg.C. higer.
The formulation of the catalyst can affect the physio-chemical changes in the coking process as well as the quality of coke produced.
Application in Reduction process:
A similar experiment was carried out in a Lab to study the effect of a catalyst in the reduction of Iron ore and coke to Iron. This experiment was carried out at the wee-known Central Institute of Mining & Fuel Research, Dhanbad.
Accordingly, the scope of work as proposed and subsequently carried out performance evaluation studies was ;
To study the effect of the catalyst on Coke and Iron Ore using Thermo-Gravimetric Analyzer (TGA) and Differential Scanning Calorimeter (DSC).
Study the reduction behavior of Iron Ore + Coke with & without the catalyst.
The sample of hard Coke & Iron ore was prepared as per procedure prescribed in IS 436 (PartII). Other tests like measurement of Gross calorific value on constant volume basis, proximate analysis etc were carried out to characterize the samples.
Compositional Analysis
The compositional analysis of Iron ore was done using chemical analysis method. The results are shown below in Table 1.
Compositional analysis of iron ore – Table 1
Sl # / Constituents / Percentage (%)1 / Fe (Total) 64.23 / 64.23
2 / Fe2O3 91.34 / 91.34
3 / Al2O3 2.64 / 2.64
4 / SiO2 1.32 / 1.32
5 / TiO2 0.08 / 0.08
6 / MnO 0.02 / 0.02
7 / Loss on Ignition 4.60 / 4.60
Thermo Gravimetric Analysis (TGA)
Thermo gravimetric analysis (TGA) is a thermal analysis technique which measures the amount and rate of change in the weight of a material as a function of temperature or time in a controlled atmosphere. TGA measurements are used primarily to determine the composition of materials and to predict their thermal stability up to elevated temperatures.
Differential Scanning Calorimeter (DSC) measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature in a controlled atmosphere. These measurements provide quantitative and qualitative information about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity.
∆H = Cp ∆T or in differential form
dH/dt = Cp dT/dt + thermal events
where:
Cp = specific heat (J/g°C)
T = temperature (°C)
H = heat (J)
dH/dt = heat flow (J/min.)
mW = mJ/sec
dT/dt = heating rate (°C/min.)
Reduction behavior
Generally, iron manufacturing involves reducing its ore to either sponge iron or pig iron. Pig Iron is produced using a blast furnace technique that uses iron ore and coke (reducing agent) as raw materials.
Degree of reduction of iron ore with and without catalyst was studied. The iron ore and hard coke was ground to 200 MESH and pellets using silicate binders and moisture of following compositions were manufactured:
i. Iron ore + hard coke (73:27 ratio)
ii. Iron ore + Hard coke + Catalyst
The iron ore pellets were allowed to heat at 250 oC and when achieved full strength, the pellets were kept at 950 OC for two hours for study the reduction behavior. The loss of weight was recorded after 10, 30, 60 and 120 minutes interval. The reduction behavior was measured as degree of reduction using following formulae:
DR = Wi x 100
TO
Where:
DR = Degree of reduction
W1 = Loss in weight of pellet
TO = Total oxygen content in the pellet
Reduction behavior of Iron ore in presence of coke at 950 oC
Sl # / Time / Initial Weight (g) / Final weight (g) / Loss in weight (g) / % Reduction60 / 3.0 / 2.12 / 29.33 / 87.73
90 / 3.0 / 2.12 / 29.33 / 87.73
120 / 3.0 / 2.11 / 29.66 / 88.14
Reduction behavior of Iron ore in presence of coke and catalyst at 950 oC
Sl # / Time / Initial Weight (g) / Final weight (g) / Loss in weight (g) / % Reduction60 / 3.0 / 2.08 / 30.66 / 89.40
90 / 3.0 / 2.07 / 31.0 / 89.85
120 / 3.0 / 2.07 / 31.0 / 89.85
It is inferred that ;
Based on the kinetic parameters studied, it is observed that the combustion
of coke starts at a lower temperature on addition of catalyst ascompared to base sample.
When the degree of reduction was measured, it was observed that the efficiency
of reduction of Iron Ore and Coke increased on addition of catalyst.
The activation temperature of reaction of Iron Ore with Coke has reduced on
addition of catalyst. Hence, it can be inferred that reaction(s) areinitiating at lower temperature.
The kinetic parameters of coke as well as iron ore changes positively in
relation to activation energy and heat of enthalpy upon addition of catalyst.
Conclusion:
Anticipated benefits in Coke making:
a)Reduced coking time
b)Improved Gieseler Plastic properties
c)Reduced energy input to Oven
d)Improved quality of Coke produced
Anticipated benefits in Blast Furnace:
a)Reduction in Coke rate / ton of Hot metal
b)Possibility of increasing
c)Reduced cycle time & hence increase in output
d)Improved thermal regime in the Blast Furnace
Catalysts could have significant impact on the process of Iron making.
It is known that these type of catalysts have been successfully tested on a commercial scale in Plants and have provided significant benefits. However, the economic feasibility and consistency of results achieved should be ascertained by carrying out trials since Plant characteristics and operational efficiencies vary.