Mn alloys production with the use of natural gas. - Understanding the kinetics of Mn alloy production
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This research intends to develop the necessary understanding regarding the reaction kinetics involved in the production of manganese ferroalloys. The main reactions of interest are as follows:MnO_((l) )+C_((s,l))=Mn_((l))+CO_((g)) (1)SiO_(2(l))+2C_((s,l))=Si_((l))+2CO_((g)) (2)SiO_(2(l))+2Mn_((l))=Si_((l))+2MnO_((l)) (3)Reactions (1) and (2) represent the three phase i.e. slag-metal-gas whereas reaction (3) represent the two phase i.e. slag-metal equilibrium. A total of 11 experiments are performed; four via the induction furnace setup and seven using the sessile drop apparatus. The induction furnace experiments are performed using 15kW constant power input. A graphite crucible is used to contain the raw material mix. The crucible acts as the heating and reduction element. In some experiments, coke 2 in addition to the graphite crucible is utilized as reductant. A C-type thermocouple measures the temperature as a function of time. In the sessile drop, a fast heating rate is applied to reach 1200℃. The temperature at this point is maintained for 10 minutes to achieve melting. Beyond 1200℃, a slow heating rate of 10 ℃/min is utilized until 1600℃ is reached. The experiments are performed in a CO(g) atmosphere. The temperature is controlled using a pyrometer, Keller PZ40. The images are recorded using a fire-wire digital video camera (Sony XCD-SX910CR) with a tele-centric lens (Navitar 1-50993D).In the induction furnace, the first two experiments are performed using graphite crucible as reductant. The first experiment is performed using synthetic charge. FeS is added to enhance reactivity. The second experiment is performed using industrial charge. The 3rd and 4th experiments are performed using synthetic charge with and without FeS addition. In these experiments, coke 2 is added as an extra reductant along with graphite. The sessile drop experiments are also performed for synthetic and industrial charge. The first four experiments use synthetic charge on graphite, anthracite, coke 2, and carbon black substrates. These are performed by Bilal Nadir. The last three experiments utilize industrial charge and are performed by Sethulakshmy. Carbon black substrate is not tested for the industrial charge.A five component slag system i.e. CaO-MgO-Al2O3-SiO2-MnO was used in this investigation. The composition of synthetic charge was kept constant for both induction furnace and sessile drop experiments at 42%MnO-28.5%SiO2-8%FeO-11%CaO-3.5%MgO-7%Al2O3. The composition of industrial charge used in induction furnace experiment was quite lose to the synthetic charge at 42%MnO-32%SiO2-3%FeO-13%CaO-3%MgO-8%Al2O3. In the sessile drop, the composition of industrial charge was quite different to synthetic charge at 47.4%MnO-31.6%SiO2-5.2%FeO-7.6%CaO-1.3%MgO-7%Al2O3.The reduction of MnO has been reported to follow a first order reaction rate model. This model is mathematically expressed by the following equation: R=(-dm_MnO)/dt=k*A*(a_MnO-a_(MnO_eq ) )=k_o e^(-E⁄RT)*A*(a_MnO-a_(〖MnO〗_eq )) (4)For simplification, the rate equation can also be expressed as a function of the concentration of MnO rather than as activities. This modification results in the following rate equation: R=(-dm_MnO)/dt=k*A*MnO_moles=k_o A*e^(-E⁄RT)*MnO_moles (5)Similarly, there is an interest to observe the application of first order rate model towards the reduction of the sum of reducible oxides i.e. MnO+SiO2+FeO. The rate equation is then modified as follows: R=(-dm_((MnO+SiO_2+FeO)))/dt=k_o A*e^(-E⁄RT)*(MnO+SiO_2+FeO)_moles (6)The induction furnace and sessile drop experiments were modeled using the first order reduction rate model using equations 5 and 6. This model is utilized to observe the curve fitting of modeled curve with the experimental curve for reduction of MnO and MnO+FeO+SiO2 as a function of time. Based on the closest curve fitting, E (activation energy) and AKo values have been determined. Finally the k_o values are determined which are used to compare the reduction rates achieved in different experiments.The MnO and MnO+SiO2+FeO reduction rate followed very closely according to the first order reaction rate model as evident from the modelled curves for induction 3 and induction 4. The k_o values for induction furnace experiments are in the order of 10^3 whereas in case of sessile drop experiments the values are in the order of 10^7. Anthracite shows the highest reactivity as evident form the sessile drop k_o values. Coke 2 and graphite have almost the same reactivity whereas carbon black has the lowest reactivity. The high reactivity observed for induction 3 and 4 is not a consequence of the high reactivity of coke 2 over graphite. Rather the higher interface area available for reaction due to presence of coke 2 in addition to graphite in induction 3 and 4 results in a high reduction rate.