21-Mat-A3 Structure and Characterization of Materials · May 2015
Question 4 of 7: Iron and Steelmaking (20 marks)
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Exams, May 2015 — 10-Met-A3, Metal Extraction Processes. Three hours, closed book, one approved calculator (Casio or Sharp). Seven problems of 20 marks each; the rubric asks for any five, and only the first five in the answer book are marked. All seven are solved here, because this set is a study resource rather than an exam script.
Note on the exam title. The printed exam header reads 10-Met-A3, Metal Extraction Processes. The content is extractive metallurgy — mineral processing, pyrometallurgy, iron and steelmaking, and magnesium and zinc production — and is answered as such.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
F. Habashi, Textbook of Pyrometallurgy — roasting, smelting, mass and heat balances.
F. Habashi, Textbook of Hydrometallurgy, 2nd ed. — leaching, purification, electrowinning.
T. Rosenqvist, Principles of Extractive Metallurgy, 2nd ed. — refining, molten-salt electrolysis.
B. A. Wills and J. Finch, Wills' Mineral Processing Technology, 8th ed. — comminution, classification, flotation, thickening, pulp density.
D. R. Gaskell, Introduction to the Thermodynamics of Materials — heat capacities and reaction/transformation enthalpies.
ASM Handbook, Vol. 2 (Nonferrous Alloys) — magnesium and zinc production practice.
(a) Advantages of oxygen over air. Air is roughly 79 % inert nitrogen; blowing air through molten pig iron carries that nitrogen through the melt for no metallurgical benefit while diluting the reacting oxygen partial pressure and absorbing sensible heat that has to be supplied by burning more fuel or oxidizing more iron. Pure oxygen blown at high velocity (as in the BOF's top-blown lance) reacts far faster because the oxidizing species is undiluted, refines a heat in 15–20 minutes instead of the many hours a Bessemer air-blow needs, generates enough exothermic heat from carbon and silicon oxidation to melt scrap additions and eliminates the external fuel needed by earlier open-hearth practice, and — critically — avoids dissolving nitrogen in the steel, which embrittles and ages the product. These combined effects are why the BOF displaced air-blown Bessemer and fuel-fired open-hearth steelmaking industry-wide by the 1970s.
(b) Deoxidizers. Dissolved oxygen left in the melt at the end of the blow (as FeO in equilibrium with the bath) must be removed before casting, or it reacts with dissolved carbon during solidification to evolve CO gas and produce blowholes ("rimming" defects). Aluminum, silicon (as ferrosilicon) and manganese (as ferromanganese) are the standard deoxidizers, added in that rough order of increasing addition rate and decreasing individual strength: Al has the strongest affinity for oxygen (forms stable $\text{Al}_2\text{O}_3$, used for a final "kill"), Si (as ferrosilicon, forming $\text{SiO}_2$) is a strong, cheaper intermediate deoxidizer, and Mn is the weakest but is nearly always present anyway to also counter sulphur (forming MnS instead of the more damaging FeS). Fully deoxidized ("killed") steel uses Al and/or Si to suppress essentially all the CO evolution; "semi-killed" or "rimming" steel deliberately leaves some reaction for specific casting behaviour.
Figure 4.1 — BOF mass balance envelope for parts (c) and (d): pig-iron impurities and the extra 2.5 t of oxidized iron leave as slag oxides; the oxygen demanded to do that oxidation is the boxed result of part (c).
Given (c, d).
Given data
Quantity
Value
Charge
100 t pig iron: 4 wt.% C, 1.5 wt.% Si, 0.75 wt.% Mn, rest Fe
Additional Fe oxidized
2.5 t
Carbon split
50 % to CO, 50 % to CO2
Atomic weights
Fe 55.8, C 12, O 16, Mn 54.9, Si 28.1
Find. (c) Mass of O2 gas required; (d) wt. % FeO, MnO, SiO2 in the slag.
Approach. Convert each oxidized element to moles, apply its own oxidation reaction and stoichiometric O2 demand, sum the O2, then form the slag oxide masses from the same mole counts and take mass fractions.
Extra iron → FeO. $n_{Fe}=2500/55.8=44.80$ kmol; $\text{Fe}+\tfrac12\text{O}_2\rightarrow\text{FeO}$ gives $n_{\text{O}_2,Fe}=22.40$ kmol.
Total oxygen (c). Sum the four demands and convert to mass ($M_{\text{O}_2}=32$):
$$n_{\text{O}_2}=250.0+53.38+6.83+22.40=332.6\ \text{kmol}$$
$$\boxed{m_{\text{O}_2} = 332.6\times32 \approx 10{,}644\ \text{kg}}$$
Slag oxide masses and composition (d). Using the same mole counts with each oxide's molar mass ($\text{FeO}=71.8$, $\text{MnO}=70.9$, $\text{SiO}_2=60.1$):
$$m_{\text{FeO}}=44.80(71.8)=3217\ \text{kg},\quad m_{\text{MnO}}=13.66(70.9)=969\ \text{kg},\quad m_{\text{SiO}_2}=53.38(60.1)=3208\ \text{kg}$$
$$m_{\text{slag}}=3217+969+3208=7394\ \text{kg}$$
$$\boxed{\text{FeO}=43.5\%,\ \ \text{MnO}=13.1\%,\ \ \text{SiO}_2=43.4\%}$$
