Iron is generally produced from an iron ore resource by reducing it with carbon. The main production — Physical Chemistry — Thermodynamics Chemistry Question
Iron-making and crystal structure
Iron is generally produced from an iron ore resource by reducing it with carbon. The main production process is through a blast furnace (BF) and a convertor. Pig iron (molten Fe-C alloy) is produced by feeding iron ore resource, subsidiary materials (CaO) and cokes from the top of the BF and by blowing heated air from the bottom. Then, molten steel is obtained by oxidizing the pig iron in a vessel such as a convertor in order to remove impurities as well as carbon. Answer the questions 19.1 – 19.5 with the provision that carbon content of the pig iron is 4.50 mass % and that a coke contains 90.0 mass % of C, 7 mass % of SiO2, 3 mass % of Al2O3. The gas constant is 8.314 J K-1 mol-1, and the relative atomic masses of C, O, Ca and Fe are 12.0, 16.0, 40.1 and 55.8, respectively. The density of iron is 7.90 g cm-3 at room temperature.
The main component of an iron ore is Fe2O3 and it will be reduced to form Fe and CO2 by (i) CO gas produced by the reaction between the cokes and the heated air or (ii) carbon in the cokes. Describe each chemical reaction.
Model Answer
(i) Fe2O3 + 3 CO → 2 Fe + 3 CO2
(ii) 2 Fe2O3 + 3 C → 4 Fe + 3 CO2
Iron ore also contains gangue (impurity materials), such as 7 mass % of SiO2 and 3 mass % of Al2O3 other than 90 mass % Fe2O3. It will react with the gangue in the coke and the subsidiary material CaO, forming the oxide melts which will be exhausted as slag. How many kg of the slag is produced as a byproduct to obtain 1 kg of pig iron? Assume that the subsidiary material is added so that the mass of CaO is equal to that of SiO2.
Model Answer
Mass of Fe2O3 required to obtain 1.00 kg of pig iron is 955 × (159.6 / 111.6) = 1365.75 g. And the amount of the slag generated from iron ore is 17 / 90, namely 257.98 g. Coke (392.29 g) also produces slag from the gangue 0.17 times as much as the coke, namely 66.69 g. Accordingly, the total amount of the slag generated becomes 324.67 g.
m(Fe2O3) = 0.325 kg
In a convertor process, carbon is eliminated by blowing O2 gas onto pig iron. When the molar amounts of CO and CO2 are identical, what is the volume (dm3) of O2 gas required for the complete removal of carbon from 1.00 kg of pig iron at 27 oC and 2.026×105 Pa?
Model Answer
A half of the carbon (45 g) in the 1 kg of pig iron is oxidized into CO2 and the rest into CO. Hence, 3/4 times of 45/12 mol of O2 gas is required. Using p V = nRT, V can be obtained as 34.6 dm3.
V(O2) = 34.6 dm3
When 1.00 kg of iron is produced from iron ore through BF and a convertor only via process (i) in the question 19.1, how many kg of CO2 is generated? Assume that the CO gas (the same molar amount of CO2) generated from the convertor is oxidized and exhausted as CO2. In the calculation, include the CO2 generated in the calcinations of CaCO3 to produce the subsidiary material of CaO.
Model Answer
Considering 1 kg of pig iron, the amount of moles of C required for the reduction is 1.5 times as much as that of Fe. Then, (955 / 55.8) × 3/2 × 12.0 = 308.06 g, which becomes 353.06 g together with the carbon dissolved in pig iron, 45 g.
As derived in the question 19.2, 324.67 g of the slag containing 7/17 of CaO is generated. When this amount of CaO is produced from CaCO3, CO2 is generated as much as 44/56.1 of CaO in weight bases. Hence, the total amount of CO2 generation becomes (353.06 × 44 / 12) + (324.67 × 7 / 17) × 44 / 56.1 = 1399.41 g.
Then, divided by 0.955, a value per 1 kg of iron can be obtained.
m(CO2) = 1.47 kg
The crystal structure of iron is body center cubic (bcc). Derive the atomic radius of iron at room temperature.
Model Answer
The atomic radius of the bcc is 3 / 4 times of the unit length, a. The volume of the unit structure, a3 = 55.8/7.90 × 2/NA = 23.4543×10–24 cm3. Hence, a = 2.8625×10–8 cm.
r(Fe) = 1.24×10–10 m