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PEC EPE Metallurgical practice questions

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Question 1Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In copper smelting, what is the primary function of the silica flux added to the matte-smelting furnace?

  1. To reduce sulfur dioxide back to elemental sulfur
  2. To combine with iron oxide and form a fusible slag Correct answer.
  3. To lower the melting point of copper matte
  4. To oxidize copper sulfide into metallic copper

Explanation

Silica acts as an acidic flux that combines with iron oxide (FeO) produced by oxidation of iron sulfide, forming an iron silicate slag that is fluid at furnace temperature and separates from the matte. This slagging step removes iron from the matte, upgrading its copper grade.

Question 2Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

Which reaction best represents the roasting of a sulfide concentrate in an oxidizing atmosphere at moderate temperature?

  1. MS + 2HCl → MCl2 + H2S
  2. 2MS + 3O2 → 2MO + 2SO2 Correct answer.
  3. MS + H2 → M + H2S
  4. MS + O2 → M + SO2

Explanation

Roasting in air converts metal sulfides to oxides (or sulfates) while releasing SO2 gas. The generalized equation 2MS + 3O2 → 2MO + 2SO2 captures the partial oxidation that leaves a calcine suitable for reduction or leaching.

Question 3Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the Bayer process, what is the purpose of the hot caustic digestion step applied to bauxite?

  1. To convert gibbsite directly into aluminum metal
  2. To volatilize silica as SiF4
  3. To reduce iron(III) oxide to metallic iron
  4. To dissolve alumina as sodium aluminate while leaving most impurities as red mud Correct answer.

Explanation

Hot sodium hydroxide dissolves aluminum hydroxide minerals to form soluble sodium aluminate, while iron oxides, titania, and much silica remain as insoluble residue (red mud). The pregnant liquor is later clarified and precipitated to give pure Al(OH)3.

Question 4Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the Hall-Héroult process, why is cryolite (Na3AlF6) used as the electrolyte solvent rather than pure alumina?

  1. It reacts with alumina to form aluminum hydride
  2. It dissolves alumina and melts at a far lower temperature than alumina itself Correct answer.
  3. It provides the carbon consumed at the anode
  4. It prevents aluminum from oxidizing at the cathode

Explanation

Alumina melts near 2072 °C, which is impractical, but it dissolves in molten cryolite at roughly 950-970 °C. The fluoride melt therefore allows electrolysis to run at a workable temperature while supplying alumina continuously.

Question 5Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the cyanidation of gold ores, what is the role of dissolved oxygen?

  1. It oxidizes gold to Au+ so that cyanide can complex it Correct answer.
  2. It acts as the reductant that deposits gold on zinc
  3. It decomposes cyanide to ammonia
  4. It precipitates silver as AgCN

Explanation

Gold dissolution requires an oxidant; oxygen from air accepts electrons as gold is oxidized to Au+ and then complexed as Au(CN)2-. This is why cyanide leaching is aerated and why oxygen availability can limit leach rate.

Question 6Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

What is the principal purpose of the slag phase in a pyrometallurgical smelting furnace?

  1. To provide a reductant for the metal oxide
  2. To dissolve and carry away gangue oxides while protecting the metal or matte Correct answer.
  3. To act as a catalyst for sulfur oxidation
  4. To store heat for the next furnace campaign

Explanation

Slag is a molten oxide phase that absorbs gangue components and impurities such as iron oxide and silica, allowing them to be tapped separately from the metal or matte. Its composition and fluidity are engineered to control losses and protect the furnace lining.

Question 7Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In solvent extraction of copper from dilute leach solutions, what does the organic extractant such as LIX reagent accomplish?

  1. It electrolytically reduces copper to metal powder
  2. It precipitates copper sulfate crystals directly
  3. It oxidizes ferrous iron to ferric iron
  4. It selectively transfers copper ions into the organic phase by chelation Correct answer.

Explanation

Hydroxyoxime extractants form lipophilic chelate complexes with copper(II) at the organic-aqueous interface, transferring copper into the organic phase while rejecting iron and other impurities. The loaded organic is then stripped with acid to produce a concentrated electrolyte for electrowinning.

Question 8Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In a rotary kiln, what distinguishes calcination from roasting?

  1. Calcination removes chemically bound water or CO2 by heating, while roasting involves oxidation or other reaction with a gas Correct answer.
  2. Calcination dissolves the solid in a molten salt, while roasting dissolves it in water
  3. Calcination applies only to sulfides, while roasting applies only to carbonates
  4. Calcination always requires a reducing gas, while roasting requires an inert atmosphere

Explanation

Calcination is thermal decomposition of a compound, such as converting limestone to lime and CO2 or removing water of crystallization. Roasting is a gas-solid reaction, most commonly oxidation of sulfides, and changes the oxidation state as well as removing volatiles.

Question 9Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In pressure oxidation of refractory gold concentrates, why is the autoclave operated at elevated oxygen partial pressure and temperature?

  1. To oxidize sulfide minerals and liberate encapsulated gold for subsequent cyanidation Correct answer.
  2. To melt the sulfide matrix so gold floats
  3. To reduce arsenopyrite to metallic arsenic
  4. To volatilize gold as AuCl3

Explanation

Pressure oxidation uses high-temperature, high-pressure oxygen to oxidize pyrite and arsenopyrite, breaking down the mineral matrix that physically locks submicroscopic gold. The oxidized residue is then amenable to conventional cyanidation.

Question 10Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

A copper matte is smelted and converting is to follow. Which change in matte composition is expected as converting proceeds?

  1. Copper grade decreases as silica dissolves in the matte
  2. Sulfur content rises because air is blown in
  3. Copper grade increases as iron and sulfur are removed Correct answer.
  4. Iron content rises as slag is absorbed

Explanation

Converting blows air through molten matte, oxidizing FeS to FeO (slagged with silica) and then Cu2S to metallic copper, so the copper grade rises toward blister copper. Iron and sulfur are progressively eliminated.

Question 11Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In zinc electrowinning from acidic sulfate electrolyte, why is the current efficiency typically below 100%?

  1. Because sulfate is reduced to sulfide at the anode
  2. Because zinc is deposited as an oxide film
  3. Because the anode dissolves into the electrolyte
  4. Because hydrogen evolution competes with zinc deposition at the cathode Correct answer.

Explanation

Zinc deposition from acid sulfate competes with hydrogen ion reduction at the cathode, so part of the charge produces H2 rather than zinc. Current efficiency is therefore below unity, and it improves with higher zinc concentration, lower acid, and additives that suppress hydrogen.

Question 12Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

What is the function of a converter flux such as silica in copper converting?

  1. It absorbs SO2 to form a solid sulfate
  2. It lowers the freezing point of blister copper
  3. It combines with oxidized iron to form a fluid slag that is tapped off Correct answer.
  4. It reduces copper oxide back to copper

Explanation

In converting, iron sulfide is oxidized to FeO; silica fluxes this oxide into an iron silicate slag that can be poured off, separating iron from the copper-rich matte and metal. Without fluxing, the oxide would remain viscous and cause high copper losses.

Question 13Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the refining of blister copper by fire refining, why is air first blown through the molten metal and then a reducing agent such as natural gas or a pole is used?

  1. Air adds oxygen to make copper oxides that are then sold
  2. Air removes sulfur, and the reductant removes dissolved oxygen Correct answer.
  3. Air cools the metal, and the reductant reheats it
  4. The reductant removes sulfur, and air removes oxygen

Explanation

Oxidation during air blowing removes residual sulfur and oxidizes impurities; the subsequent reduction (poling) removes dissolved oxygen so that tough-pitch copper with acceptable conductivity is produced. The two stages are oxidation followed by deoxidation.

Question 14Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In heap leaching of oxide copper ore with dilute sulfuric acid, what is the main reason for agglomerating the ore with acid before stacking?

  1. To reduce the acid consumption of the gangue
  2. To precipitate copper as cement on the ore surface
  3. To bind fines into stable, permeable granules that allow uniform solution flow Correct answer.
  4. To convert chrysocolla into a sulfide mineral

Explanation

Agglomeration with acid and water binds fine particles into coarse granules, preventing fines from migrating and plugging the heap. This maintains permeability so leach solution can percolate evenly and contact the copper minerals.

Question 15Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In gold cyanidation, what is the effect of adding lead nitrate to a leach treating a sulfide-rich ore?

  1. It increases the solubility of silver chloride
  2. It oxidizes cyanide to cyanate
  3. It precipitates gold as AuCN
  4. It reduces sulfide mineral surfaces that passivate gold and can improve dissolution kinetics Correct answer.

Explanation

Lead nitrate can react with sulfide minerals to form a protective or less reactive layer, reducing their tendency to consume oxygen and cyanide and to passivate gold surfaces. It is used as a leach additive to improve gold recovery from certain sulfide ores.

Question 16Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the refining of nickel by the Mond process, what is the key chemical step that separates nickel from cobalt and copper?

  1. Nickel is leached in sulfuric acid under pressure
  2. Nickel is electrodeposited from a chloride electrolyte
  3. Nickel reacts with carbon monoxide to form volatile nickel tetracarbonyl Correct answer.
  4. Nickel is oxidized to nickel oxide in a converter

Explanation

Nickel reacts with CO at about 50-60 °C to form volatile Ni(CO)4, which is then decomposed at higher temperature to yield pure nickel. Cobalt and copper do not form volatile carbonyls under these conditions, so the separation is achieved by the selective volatility.

Question 17Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In a lead blast furnace, why is metallic iron added to the charge?

  1. To oxidize lead to litharge
  2. To reduce lead sulfide and precipitate lead, while iron sulfide forms matte Correct answer.
  3. To flux silica into a fluid slag
  4. To absorb sulfur dioxide from the gas

Explanation

Iron has a higher affinity for sulfur than lead does, so it displaces lead from PbS, yielding metallic lead and an iron sulfide matte. This is the classic iron reduction step in lead smelting of sulfide concentrates.

Question 18Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the purification of zinc sulfate electrolyte before electrowinning, why is zinc dust added to the neutral leach solution?

  1. To oxidize ferrous iron to ferric iron
  2. To cement out impurities such as copper, cadmium, and cobalt by displacement Correct answer.
  3. To raise the pH by generating zinc hydroxide
  4. To precipitate zinc as zinc carbonate

Explanation

Zinc dust is a more active metal that reduces dissolved impurity ions, depositing them as cement (e.g., Cu, Cd, Co, Ni) while zinc dissolves. Removing these impurities is essential because they would degrade current efficiency and cathode quality in electrowinning.

Question 19Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the refining of crude nickel matte by chlorine leaching, what advantage does chloride hydrometallurgy offer over sulfate processing?

  1. Chloride systems allow higher metal concentrations and more selective separation by solvent extraction Correct answer.
  2. Chloride systems avoid the formation of any waste residue
  3. Chloride leaching cannot dissolve nickel at all
  4. Chloride systems eliminate the need for any solvent extraction

Explanation

Chloride electrolytes can carry higher metal loadings and often give sharper separation factors in solvent extraction, which is why chloride routes are used for nickel-cobalt matte refining. They still require purification and residue management.

Question 20Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the electrowinning of aluminum, why must the alumina concentration in the cryolite bath be monitored and maintained within a narrow range?

  1. Too little alumina causes anode effect, and too much undissolved alumina forms sludge Correct answer.
  2. Alumina controls the purity of the product by alloying with iron
  3. Alumina reacts with the carbon lining to form aluminum carbide at all concentrations
  4. Excess alumina raises the bath conductivity beyond safe limits

Explanation

When dissolved alumina falls too low, the bath cannot sustain normal electrolysis and an anode effect occurs, with a sharp voltage rise. Excess undissolved alumina settles as sludge and disrupts operation, so feed is controlled within a narrow window.

Question 21Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the treatment of refractory gold ore by biooxidation, what role do chemolithotrophic bacteria such as Acidithiobacillus ferrooxidans play?

  1. They precipitate gold as a bacterial biofilm
  2. They directly oxidize gold to a soluble complex
  3. They catalyze the oxidation of ferrous iron and sulfide minerals, regenerating ferric oxidant Correct answer.
  4. They reduce arsenate to arsine gas

Explanation

These bacteria obtain energy by oxidizing ferrous iron and reduced sulfur species, regenerating ferric iron and acid that chemically attack pyrite and arsenopyrite. The mineral matrix is broken down biologically, exposing gold for later cyanidation.

Question 22Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

A smelter operator observes rising copper losses in slag. Which change in slag chemistry is most likely to reduce those losses?

  1. Increasing the matte grade by adding copper oxide
  2. Increasing slag viscosity by lowering temperature
  3. Adding more sulfur to the slag phase
  4. Adjusting silica and iron oxide ratios to lower slag viscosity and promote droplet settling Correct answer.

Explanation

Copper losses in slag occur partly as entrained matte or metal droplets and partly as dissolved oxide; both are reduced when the slag is fluid enough for droplets to settle. Controlling the SiO2/FeO ratio and temperature lowers viscosity and improves settling.

Question 23Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the pressure leaching of nickel laterite ores with sulfuric acid, why is the operating temperature typically kept below about 270 °C?

  1. Above this temperature the acid is completely neutralized
  2. Above this temperature the autoclave lining becomes a superconductor
  3. Above this temperature magnesium and aluminum dissolution increase sharply, raising acid consumption and scaling Correct answer.
  4. Above this temperature all nickel remains in the residue

Explanation

Higher temperatures accelerate the dissolution of gangue minerals such as magnesium silicates and aluminum phases, which consumes acid and promotes scaling and impurity loading. The process window is chosen to extract nickel and cobalt while limiting gangue attack.

Question 24Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the electrorefining of copper, why is the anode slime recovered and treated separately?

  1. It contains precious metals and selenium that are economically recovered after decopperizing Correct answer.
  2. It is pure copper that is returned directly to the anode furnace
  3. It is a waste with no metal value and is discarded
  4. It is used as flux in the converter

Explanation

Anode slimes contain gold, silver, selenium, tellurium, and platinum-group metals that do not dissolve during electrorefining and accumulate at the cell bottom. They are processed in a separate precious-metals circuit, making slime treatment a major revenue source.

Question 25Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In a top-blown rotary converter treating nickel matte, why is the endpoint of the blow controlled carefully by monitoring the off-gas SO2 and bath temperature?

  1. To ensure the matte freezes before tapping
  2. To avoid over-oxidizing nickel to nickel oxide and to maximize sulfur removal before casting Correct answer.
  3. To keep the matte fully molten by adding sulfur
  4. To prevent cobalt from volatilizing as CoCl2

Explanation

The converting endpoint must be stopped before excessive oxidation of metallic nickel to NiO, which would report to slag and lower recovery. Off-gas SO2 and temperature trends indicate when the sulfur has been sufficiently removed, allowing the metal to be tapped without overblowing.

Question 26Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the Bayer process for refining bauxite, what is the primary function of the hot sodium hydroxide solution in the digestion stage?

  1. It volatilizes silica as silicon tetrafluoride gas before the alumina is precipitated
  2. It reduces ferric iron to metallic iron so that it can be removed magnetically
  3. It selectively dissolves alumina minerals as soluble sodium aluminate while leaving iron and titanium oxides as insoluble residue Correct answer.
  4. It oxidizes sulfide sulfur to sulfate so that the liquor can be safely discharged

Explanation

Hot caustic soda digests bauxite at elevated temperature and pressure, converting hydrated alumina minerals such as gibbsite and boehmite into soluble sodium aluminate. Iron and titanium bearing phases are essentially insoluble under these conditions and report to the red mud residue, which is separated by settling and filtration before precipitation of pure aluminum hydroxide.

Question 27Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the Hall-Heroult cell, the alumina feed is dissolved in molten cryolite rather than melted on its own. Which property of cryolite best explains this design?

  1. It lowers the operating temperature far below the melting point of pure alumina while remaining an effective solvent for the oxide Correct answer.
  2. It supplies the carbon that is consumed at the anode during electrolysis
  3. It reacts chemically with alumina to form volatile aluminum chloride that decomposes at the cathode
  4. It prevents any aluminum from being deposited so that only sodium metal is produced

Explanation

Pure alumina melts near 2050 degrees Celsius, which is impractical for an industrial cell. Cryolite dissolves alumina and, with additives such as aluminum fluoride and calcium fluoride, allows electrolysis at roughly 950 to 970 degrees Celsius while still conducting ions to the electrodes.

Question 28Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the carbon-in-leach circuit of a gold plant, what is the main purpose of adding activated carbon to the leach tanks rather than only to a separate adsorption stage?

  1. It oxidizes sulfide minerals so that gold is liberated without any further comminution
  2. It adsorbs dissolved gold aurocyanide as it forms, reducing the risk that gold is re-adsorbed by carbonaceous or preg-robbing ore constituents Correct answer.
  3. It precipitates gold as metallic flakes that are recovered by gravity on the tank floor
  4. It decomposes cyanide to protect the downstream tailings facility from toxicity

Explanation

Carbon-in-leach combines leaching and adsorption in the same vessels, so dissolved gold is captured on carbon shortly after it enters solution. This shortens the residence time gold spends in solution and limits losses when the ore contains natural carbonaceous matter that would otherwise adsorb and retain the gold values.

Question 29Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In an electric arc furnace used for steelmaking, what is the dominant mechanism by which electrical energy is transferred to the scrap charge?

  1. Radiation from the electric arcs and resistive heating of the charge by the current passing through it Correct answer.
  2. Chemical energy released by the combustion of injected oxygen alone
  3. Mechanical stirring by an electromagnetic brake on the furnace shell
  4. Induction currents generated in the scrap by a surrounding water-cooled coil

Explanation

In an electric arc furnace the graphite electrodes strike arcs on the scrap, and the arc radiation together with resistance heating in the charge and slag transfers power to the metal. Chemical energy from burners and oxygen injection supplements but does not replace the electrical input.

Question 30Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the roast-leach-electrowin flowsheet for zinc, why is the calcine from the fluidized-bed roaster leached with dilute sulfuric acid rather than with concentrated acid?

  1. Concentrated acid would reduce zinc oxide to metallic zinc inside the leach reactor
  2. Dilute acid dissolves zinc oxide readily while leaving zinc ferrite largely unattacked, avoiding excessive iron in solution Correct answer.
  3. Dilute acid converts zinc sulfide directly to zinc sulfate without any roasting step
  4. Concentrated acid cannot wet the fine calcine particles and would leave them floating

Explanation

Neutral or slightly acidic leaching of roasted calcine dissolves the zinc oxide and leaves zinc ferrite in the residue, which keeps dissolved iron low and simplifies purification. The ferrite residue is then treated under more aggressive hot acid conditions in a separate stage to recover its zinc.

Question 31Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In a Peirce-Smith converter, the tuyeres are submerged in the matte and blow air through it. What is the main reason the tuyeres are kept submerged rather than blowing onto the bath surface?

  1. Submerged tuyeres allow the converter to be operated without any flux addition
  2. Submerged injection gives good gas-liquid contact and mixing so that oxidation of iron and sulfur proceeds rapidly through the bath Correct answer.
  3. Surface blowing cannot oxidize sulfur, only iron
  4. Surface blowing would freeze the matte because the converter is not refractory lined

Explanation

Submerged tuyeres disperse air through the matte, creating high interfacial area and vigorous mixing that accelerates oxidation of iron and sulfur. The exothermic reactions maintain temperature, and silica flux is added to slag the iron oxide.

Question 32Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the cyanidation of a silver-bearing ore, why is the leach often conducted at higher pH and with higher cyanide concentration than for a simple gold ore?

  1. Silver only dissolves when the pulp is acidic, so lime addition is used to buffer the acid
  2. Silver cyanide is volatile and must be kept alkaline to prevent it boiling out of the pulp
  3. Higher pH converts silver sulfide directly to silver metal without any oxidant
  4. Silver dissolves as a cyanide complex that is less stable than gold's, so more cyanide and higher pH are needed to keep silver in solution and suppress losses Correct answer.

Explanation

Silver forms stable but weaker cyanide complexes than gold and is sensitive to precipitation and adsorption effects, so plants often use higher cyanide strength and higher pH to keep silver in solution. Lime is added to protect against cyanide loss and to prevent formation of interfering species.

Question 33Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the Waelz kiln treatment of electric arc furnace dust, why is coke added to the charge along with the zinc-bearing oxide feed?

  1. It fluxes the silica so that zinc silicate slag can be tapped from the kiln
  2. It acts as a reductant that converts zinc oxide to zinc vapor, which is reoxidized and collected as crude zinc oxide in the baghouse Correct answer.
  3. It cools the charge so that zinc remains in the slag for later hydrometallurgical recovery
  4. It supplies the sulfur needed to form zinc sulfide fume that is captured in the off-gas

Explanation

The Waelz process reduces zinc oxide in the dust to metallic zinc vapor at high temperature in a rotary kiln. The vapor leaves the charge, reoxidizes in the oxidizing freeboard and off-gas system, and is collected as crude zinc oxide suitable for further refining.

Question 34Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the solvent extraction of uranium from acidic sulfate leach liquor using an amine extractant, what happens during the stripping stage with a sodium chloride or sodium carbonate solution?

  1. The uranium is reduced to insoluble uranium dioxide and filtered out of the organic phase
  2. The amine extractant is destroyed and must be replaced after each stripping cycle
  3. The uranium complex is transferred back to an aqueous phase by changing the chemistry of the aqueous solution so that the extractant releases the uranium Correct answer.
  4. The organic phase is distilled to separate uranium from the diluent

Explanation

Stripping reverses the extraction equilibrium by contacting the loaded organic phase with an aqueous solution whose composition favors release of the uranium complex. The regenerated organic phase is then recycled to extraction, and the concentrated strip liquor goes to precipitation or further purification.

Question 35Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the chloride leaching of a nickel-copper matte, why is the leach often carried out at a controlled redox potential using chlorine gas addition?

  1. The potential controls the pH of the solution, which must stay above 10 for chloride leaching to work
  2. Chlorine addition is used only to precipitate nickel as nickel chloride for sale
  3. The redox potential has no effect on dissolution and is monitored only for safety
  4. The potential controls which metals dissolve, so nickel and copper can be separated by keeping one in solution while the other reports to residue Correct answer.

Explanation

In chloride systems the dissolution of different metals depends strongly on the oxidation potential of the solution. By controlling potential with chlorine or another oxidant, operators can selectively dissolve one metal while leaving another in a residue, achieving a separation that would be difficult in simple sulfate leaching.

Question 36Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the matte smelting of a copper concentrate, why is the smelting temperature usually kept only modestly above the melting point of the matte and slag?

  1. Higher temperature freezes the matte and prevents it from being tapped
  2. The temperature has no effect on slag losses, so it is set only by the fuel available
  3. Lower temperature increases the sulfur content of the matte beyond the converter's capability
  4. Excessive superheat increases copper solubility in the slag and accelerates refractory wear, so the temperature is kept as low as practical while keeping the slag fluid Correct answer.

Explanation

Copper loss to slag rises with temperature because more copper oxide dissolves in the slag and more matte is entrained. Keeping the furnace as cool as practicable while still tapping a fluid slag and matte reduces losses and refractory attack.

Question 37Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the pressure acid leaching of a nickel laterite ore, why is the leach liquor often partially neutralized with limestone before the mixed sulfide precipitation step?

  1. Neutralization raises the pH and reduces free acid so that the subsequent precipitation of nickel and cobalt sulfides is effective and reagent consumption is controlled Correct answer.
  2. Neutralization oxidizes the nickel to a higher valence so that it can be precipitated as an oxide
  3. Neutralization is needed to convert sulfate to sulfide so that hydrogen sulfide can be generated on site
  4. Limestone addition removes all magnesium from the solution before any nickel is recovered

Explanation

Pressure acid leach liquor contains substantial free acid and dissolved impurities. Partial neutralization with limestone raises the pH and removes some iron and aluminum, creating conditions under which hydrogen sulfide addition can selectively precipitate nickel and cobalt sulfides.

Question 38Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

A copper electrorefinery observes that the cathode copper is becoming contaminated with antimony and bismuth. Which change in the tankhouse operation is most likely to address this problem?

  1. Reduce the anode spacing so that the cell voltage drops and impurities deposit on the anodes
  2. Increase the current density well above the normal range so that impurities are forced into the slime faster
  3. Raise the electrolyte temperature sharply so that antimony and bismuth plate preferentially with copper
  4. Tighten the control of anode impurity levels and adjust electrolyte circulation and addition agents so that antimony and bismuth report to the anode slime or bleed rather than to the cathode Correct answer.

Explanation

Antimony and bismuth are the classic troublesome impurities in copper electrorefining because they can dissolve and partially plate with the copper. Control relies on limiting their input in the anode, maintaining proper electrolyte circulation and temperature, using addition agents to promote smooth deposits, and bleeding impurities from the circuit.

Question 39Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the QSL or similar bath smelting process for lead concentrates, why is the oxidation and reduction stages separated within the reactor?

  1. The separation is needed only to control the sulfur dioxide content of the off-gas to meet emission limits
  2. Reduction must precede oxidation so that sulfur is removed before lead is volatilized
  3. Oxidizing conditions are needed to oxidize sulfide and form a low-lead slag, while reducing conditions are needed to recover lead from that slag, and the two cannot be maintained simultaneously in one bath Correct answer.
  4. Oxidation must occur at low temperature and reduction at low pressure, which requires separate vessels

Explanation

Lead bath smelting requires strongly oxidizing conditions to oxidize sulfide concentrate and produce a slag, followed by reducing conditions to recover lead from that slag. Because a single bath cannot be simultaneously oxidizing and reducing, the process uses separate zones or stages within the reactor to carry out each duty.

Question 40Extractive Metallurgy (Pyrometallurgy, Hydrometallurgy, Electrometallurgy)

In the chloride-volatilization route for treating tin-bearing materials, why is the charge often chlorinated with chlorine gas or hydrogen chloride at moderate temperature rather than with sodium chloride at very high temperature?

  1. Sodium chloride at high temperature produces tin metal directly, so chlorination is unnecessary
  2. Chlorine gas cannot react with tin oxide and is used only to fluidize the bed
  3. Moderate temperature is required because stannic chloride decomposes above 200 degrees Celsius
  4. Moderate-temperature chlorination with chlorine or hydrogen chloride selectively volatilizes tin as stannic chloride while limiting volatilization of iron and other gangue elements Correct answer.

Explanation

Stannic chloride is volatile and can be separated from less volatile chlorides by controlling chlorination temperature and the chlorinating agent. Chlorine or hydrogen chloride at moderate temperature favors tin volatilization while suppressing iron chloride volatilization, giving a cleaner tin product.

Question 41Physical Metallurgy and Phase Transformations

In the Fe-Fe3C equilibrium diagram, what is the eutectoid composition and temperature for the pearlite reaction?

  1. 0.02 wt% C at 727 °C
  2. 0.76 wt% C at 727 °C Correct answer.
  3. 2.14 wt% C at 1147 °C
  4. 4.30 wt% C at 1147 °C

Explanation

The eutectoid point of the Fe-Fe3C diagram lies at about 0.76 wt% C and 727 °C, where austenite transforms into pearlite (ferrite plus cementite). The 0.02 wt% value is the maximum carbon solubility in ferrite at that temperature, and the 4.30 wt% value is the eutectic composition at 1147 °C.

Question 42Physical Metallurgy and Phase Transformations

Which phase is stable in plain carbon steel at room temperature under equilibrium conditions?

  1. Liquid plus austenite
  2. Delta ferrite
  3. Ferrite plus cementite Correct answer.
  4. Austenite

Explanation

Below the eutectoid temperature, equilibrium in plain carbon steel consists of ferrite (alpha) and cementite (Fe3C). Austenite is only stable at elevated temperature, delta ferrite exists near the melting point, and liquid requires temperatures above the solidus.

Question 43Physical Metallurgy and Phase Transformations

A steel is slowly cooled from the austenite region such that proeutectoid ferrite forms first. Which composition range does this steel occupy?

  1. Between 0.76 and 2.14 wt% C
  2. Exactly 4.30 wt% C
  3. Less than 0.76 wt% C Correct answer.
  4. Between 2.14 and 4.30 wt% C

Explanation

Hypoeutectoid steels contain less than the eutectoid 0.76 wt% C, so proeutectoid ferrite forms along austenite grain boundaries before the remaining austenite transforms to pearlite. Above 0.76 wt% C, proeutectoid cementite forms instead.

Question 44Physical Metallurgy and Phase Transformations

What is the primary mechanism by which pearlite forms during the austenite-to-pearlite transformation?

  1. Martensitic shear transformation with no diffusion
  2. Cooperative growth of alternating ferrite and cementite lamellae by boundary diffusion Correct answer.
  3. Spinodal decomposition into two continuous solid solutions
  4. Homogeneous nucleation of equiaxed ferrite grains in the austenite matrix

Explanation

Pearlite grows by a coupled, diffusion-controlled mechanism in which ferrite and cementite lamellae advance cooperatively into austenite, with carbon partitioning by diffusion along the moving interface. This produces the characteristic lamellar morphology rather than isolated equiaxed grains or a diffusionless product.

Question 45Physical Metallurgy and Phase Transformations

In the Avrami-Johnson-Mehl equation for isothermal transformation, what does the exponent n primarily reflect?

  1. The nucleation and growth geometry of the transforming phase Correct answer.
  2. The equilibrium carbon content of the parent austenite
  3. The cooling rate imposed during quenching
  4. The activation energy for carbon diffusion in austenite

Explanation

The exponent n in the Avrami equation depends on the mode of nucleation and the dimensionality of growth, such as site saturation versus continuous nucleation and one-, two-, or three-dimensional growth. Activation energy and composition affect the rate constant, not the exponent itself.

Question 46Physical Metallurgy and Phase Transformations

On a TTT diagram for a eutectoid steel, the transformation curves have a C-shape. What is the principal reason for the upper and lower branches?

  1. Upper branch limited by solute drag, lower branch limited by coarsening
  2. Upper branch limited by grain growth, lower branch limited by recrystallization
  3. Upper branch limited by diffusion kinetics, lower branch limited by driving force Correct answer.
  4. Upper branch limited by driving force, lower branch limited by nucleation rate

Explanation

Near the eutectoid temperature the driving force is small so incubation is long, giving the upper branch. At low temperature the driving force is large but carbon diffusion becomes sluggish, so incubation lengthens again, producing the lower branch and the C-shape.

Question 47Physical Metallurgy and Phase Transformations

Which transformation product forms when austenite is quenched rapidly enough to suppress diffusion but not so rapidly as to avoid the martensitic transformation?

  1. Bainite
  2. Martensite Correct answer.
  3. Spheroidite
  4. Coarse pearlite

Explanation

Martensite forms by a diffusionless, displacive transformation when the cooling rate exceeds the critical cooling rate and the alloy is undercooled below the martensite start temperature. Carbon remains trapped in supersaturated solid solution, producing a body-centered tetragonal structure.

Question 48Physical Metallurgy and Phase Transformations

What is the crystal structure of freshly formed martensite in a plain carbon steel?

  1. Face-centered cubic
  2. Hexagonal close-packed
  3. Body-centered tetragonal Correct answer.
  4. Body-centered cubic

Explanation

Carbon trapped in interstitial sites distorts the body-centered cubic lattice into a body-centered tetragonal structure. The tetragonality increases with carbon content and is the reason martensite is hard and strained.

Question 49Physical Metallurgy and Phase Transformations

Which alloying element is a strong austenite stabilizer that expands the austenite phase field and lowers the martensite start temperature?

  1. Chromium
  2. Silicon
  3. Nickel Correct answer.
  4. Molybdenum

Explanation

Nickel is a classic austenite stabilizer: it expands the gamma phase field, lowers the A3 temperature, and depresses the martensite start temperature. This is exploited in austenitic stainless steels and in steels requiring retained austenite control.

Question 50Physical Metallurgy and Phase Transformations

What is the main effect of increasing the austenitizing temperature on the hardenability of a hypoeutectoid steel?

  1. It decreases hardenability by refining the prior austenite grain size
  2. It increases hardenability only if the steel is subsequently annealed
  3. It increases hardenability by coarsening austenite grains and dissolving carbides Correct answer.
  4. It has no effect because hardenability depends only on carbon content

Explanation

Higher austenitizing temperatures dissolve more carbides and coarsen the austenite grains, reducing nucleation sites for ferrite and pearlite and thus shifting the TTT curve to longer times. This raises the critical cooling rate requirement and improves hardenability.