GATE Metallurgical Engineering Mineral Processing and Extractive Metallurgy Flashcards

24 sample cards from the Mineral Processing and Extractive Metallurgy deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Why are desulphurization and dephosphorization difficult to perform simultaneously?

   Desulphurization needs reducing (low oxygen potential) conditions, while dephosphorization needs oxidizing (high $\ce{FeO}$) conditions. Both require high basicity, but their opposite oxygen-potential requirements make simultaneous removal in one step thermodynamically conflicting.

2. What is metallurgical coke and what are its main functions in the blast furnace?

   Metallurgical coke is a hard, porous carbon made by destructive distillation (carbonization) of coking coal in the absence of air. In the blast furnace it serves as (1) fuel/heat source, (2) reducing agent (CO generation), (3) carburizer of iron, and (4) the permeable support that maintains bed permeability for gas/liquid flow.

3. Describe the by-product (recovery) coke-making process and the temperature range.

   Coking coal is heated to $\sim 1000$–$1100^{\circ}C$ in sealed slot-type ovens (coke-oven battery) in the absence of air for $\sim 16$–$24$ h. Volatiles are driven off and recovered (coke-oven gas, tar, ammonia, benzole), leaving solid coke that is then pushed out and quenched.

4. What is the single most important property required of coking coal and how is it characterized?

   The coal must have caking/coking ability — softening into a plastic mass on heating and resolidifying into coherent coke. It is characterized by free swelling index (FSI), Gieseler fluidity, and the plastic-range (dilatation). Only a limited rank range (medium-volatile bituminous) coke well.

5. List the main quality parameters used to judge blast-furnace coke.

   Ash content (low desired), sulphur and phosphorus (low), moisture, volatile matter (low), fixed carbon (high), mechanical strength (Micum/ASTM tumbler indices), and high-temperature strength after reaction with $\ce{CO2}$ (CSR — coke strength after reaction; CRI — coke reactivity index).

6. Why are alternative (non-blast-furnace) iron-making routes developed, and how are they broadly classified?

   They avoid dependence on metallurgical coke and large capital BF plants, using non-coking coal or natural gas. They are classified into: (1) Direct Reduced Iron (DRI) — solid-state reduction below melting (e.g. MIDREX, HYL, SL/RN, rotary kiln); and (2) Smelting Reduction — producing liquid hot metal (e.g. COREX, FINEX).

7. What is Direct Reduced Iron (DRI / sponge iron) and what distinguishes it from blast-furnace hot metal?

   DRI is iron produced by reducing iron ore in the solid state (below the melting point, $\sim 800$–$1100^{\circ}C$) using reducing gas ($\ce{CO}/\ce{H2}$) or coal. Unlike BF hot metal, it remains solid (porous 'sponge'), retains gangue, has very low carbon, and is used as scrap substitute in electric furnaces.

8. Describe the COREX smelting-reduction process and its two-reactor configuration.

   COREX produces liquid hot metal without coke. It has two stages: a reduction shaft where lump ore/pellets are reduced to DRI by reducing gas, feeding into a melter-gasifier below, where non-coking coal is gasified with oxygen, providing heat and reducing gas while the DRI is melted to hot metal and slag.

9. What is a key advantage of COREX over the conventional blast furnace?

   COREX uses non-coking coal directly (no coke ovens) and charges oxygen instead of hot air, eliminating costly coke-making and sinter plants, lowering emissions, and producing an export gas rich in $\ce{CO}+\ce{H2}$ usable for power or DRI production.

10. Describe the MIDREX direct-reduction process: reactor type, reductant, and product.

    MIDREX is the leading gas-based DRI process. A continuous vertical shaft furnace reduces lump ore/pellets in counter-current flow with reducing gas ($\ce{CO}+\ce{H2}$) produced by catalytic reforming of natural gas. The solid product is DRI/HBI (hot briquetted iron) with $\sim 90$–$94\%$ metallization.

11. What is metallization in DRI and how is it defined?

    Metallization is the fraction of iron present in the metallic state. $$\%\text{Metallization}=\frac{\text{metallic Fe}}{\text{total Fe}}\times 100$$ Commercial DRI typically reaches $90$–$95\%$ metallization; the remainder is unreduced iron oxide.

12. What is primary steelmaking, and how does it differ from secondary steelmaking?

    Primary steelmaking converts liquid hot metal (and/or scrap/DRI) into crude steel by oxidizing impurities (C, Si, Mn, P, S) — chiefly via the Basic Oxygen Furnace (BOF) or Electric Arc Furnace (EAF). Secondary steelmaking (ladle metallurgy) then refines composition, removes inclusions/gases, and adjusts temperature.

13. Describe the Basic Oxygen Furnace (BOF/LD) process and the role of the oxygen lance.

    In the LD/BOF, a water-cooled lance blows high-purity oxygen at supersonic speed onto molten hot metal plus scrap in a basic-lined vessel. Oxygen oxidizes C, Si, Mn, P to remove them; the exothermic reactions raise temperature without external fuel. A heat takes $\sim 15$–$20$ min.

14. Write the principal refining (decarburization) reactions in the BOF.

    $$\ce{[C] + \tfrac12 O2 -> CO}$$ $$\ce{[Si] + O2 -> (SiO2)}$$ $$\ce{[Mn] + \tfrac12 O2 -> (MnO)}$$ $$\ce{2[P] + \tfrac52 O2 -> (P2O5)}$$ The oxides report to a lime-fluxed basic slag; CO evolves as off-gas.

15. Why is lime ($\ce{CaO}$) added in the BOF, and what does the slag accomplish?

    Lime forms a basic slag (high $\ce{CaO/SiO2}$) that fixes acidic oxides $\ce{SiO2}$ and $\ce{P2O5}$, enabling dephosphorization, and removes some sulphur. A basic refractory lining is required because an acidic lining cannot withstand a basic slag.

16. Why can the BOF use only a limited amount of scrap (typically $\sim 20$–$30\%$) in the charge?

    The BOF is autogenous — it has no external heat source; heat comes only from oxidation of impurities in hot metal. Scrap acts as a coolant, so its proportion is limited to the amount the exothermic refining heat can melt while reaching tapping temperature.

17. In BOF process dynamics, describe the typical sequence of impurity removal during a blow.

    Silicon and manganese oxidize first (very early), forming slag. Carbon removal accelerates to a peak mid-blow (decarburization controlled by oxygen supply, then by carbon mass transfer at low C). Phosphorus removal occurs as slag basicity and FeO build up; refining ends near target carbon and temperature.

18. In BOF decarburization dynamics, what controls the carbon removal rate in the high-carbon versus low-carbon regimes?

    At high carbon, the rate is controlled by the oxygen supply (lance flow) and is roughly constant. Below a critical carbon (~$0.2$–$0.3\%$), the rate becomes limited by carbon mass transfer in the metal, so $-\dfrac{d[\%C]}{dt}\propto [\%C]$ and decreases as carbon is consumed.

19. What is the Boudouard reaction and why is it central to ironmaking process dynamics?

    The Boudouard (solution-loss) reaction: $$\ce{C + CO2 <=> 2CO}\quad \Delta H>0$$ It is endothermic and shifts toward CO at high temperature. It regenerates the reductant CO from $\ce{CO2}$ and governs coke consumption (direct reduction) in the high-temperature zone of the blast furnace.

20. State the rate concept of gas–solid reduction of iron oxide pellets and the controlling resistances.

    Reduction of an oxide particle follows a shrinking-core / topochemical model with series resistances: (1) gas film mass transfer, (2) diffusion through the product (porous Fe) layer, and (3) chemical reaction at the unreacted core interface. The slowest step controls the overall rate; for porous pellets, diffusion and chemical control often dominate.

21. What is fluxing index/self-fluxing in agglomerates and why is fluxed sinter preferred to acid sinter?

    Self-fluxing (basic) sinter contains enough $\ce{CaO}$ to flux its own gangue, so basicity $\ge 1$. It is preferred because it reduces the limestone charged to the furnace (less endothermic calcination in the BF), improves reducibility, gives better strength, and lowers coke rate.

22. Compare sintering and pelletizing in terms of feed size, equipment, and product strength.

    Sintering: coarser fines ($-10\,mm$), travelling-grate machine, irregular porous lumps, moderate strength, made near the plant. Pelletizing: ultrafine concentrate ($-45\,\mu m$), balling disc/drum + induration furnace, uniform spheres, high and consistent strength, transportable over long distances.

23. For froth flotation in mineral processing, what is the role of collectors, frothers, and depressants?

    Collectors adsorb on target mineral surfaces making them hydrophobic so they attach to air bubbles. Frothers stabilize the froth/bubble film for selective recovery. Depressants render unwanted minerals hydrophilic (non-floatable), improving selectivity (e.g. separating $\ce{CuFeS2}$ from pyrite).

24. State the Ellingham-diagram principle relevant to choosing reductants in extractive metallurgy.

    An Ellingham diagram plots $\Delta G^{\circ}$ of oxide formation versus temperature. A metal whose oxide line lies below another's can reduce that oxide. Because the carbon line ($\ce{2C + O2 -> 2CO}$) has negative slope, carbon becomes a stronger reductant at high temperature, crossing most metal-oxide lines — the basis of carbothermic reduction.

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