Metallurgy flash cards

1. 1.Define a mineral and an ore. How do they differ?

   A mineral is any naturally occurring inorganic solid in which a metal (or its compound) is present. An ore is a mineral from which the metal can be extracted profitably and conveniently. Thus every ore is a mineral, but every mineral is not an ore.

   Hint: Profit + convenience is the key distinction.

2. 2.What is metallurgy? Name its three broad stages.

   Metallurgy is the overall process of extracting a metal from its ore and refining it. Three stages: (1) concentration/dressing of the ore, (2) reduction of the concentrated ore to crude metal, (3) refining/purification of the metal.

   Hint: Dress → reduce → refine.

3. 3.What is gangue (matrix)?

   The unwanted, commercially worthless earthy/rocky impurities (sand, clay, rock) associated with an ore. It must be removed during concentration.

   Hint: The waste around the valuable mineral.

4. 4.Name the four main classes of ores by anion type.

   (1) Native (free metal), (2) Oxide ores, (3) Sulphide ores, (4) Halide ores. (Carbonate and sulphate ores are also common oxidised classes.)

   Hint: Native, oxide, sulphide, halide.

5. 5.Give the formula and metal for haematite and magnetite.

   Haematite $= Fe_2O_3$ (iron). Magnetite $= Fe_3O_4$ (iron); magnetite is a magnetic oxide.

   Hint: Both iron oxides; one is magnetic.

6. 6.Give formulas for the main iron ores siderite and iron pyrites.

   Siderite (carbonate) $= FeCO_3$. Iron pyrites (sulphide) $= FeS_2$. Iron pyrites is generally NOT used to extract iron (too much S).

   Hint: Carbonate vs 'fool's gold'.

7. 7.Name the principal ores of copper (at least four).

   Copper pyrites/chalcopyrite $CuFeS_2$, copper glance/chalcocite $Cu_2S$, cuprite/ruby copper $Cu_2O$, malachite $CuCO_3\cdot Cu(OH)_2$, azurite $2CuCO_3\cdot Cu(OH)_2$.

   Hint: Chalcopyrite is the chief ore.

8. 8.Name the principal ores of aluminium.

   Bauxite $Al_2O_3\cdot xH_2O$ (chief ore), cryolite $Na_3AlF_6$, corundum $Al_2O_3$, kaolinite (a clay) $Al_2(OH)_4Si_2O_5$.

   Hint: Bauxite for extraction; cryolite as flux/solvent.

9. 9.Name the principal ores of zinc.

   Zinc blende/sphalerite $ZnS$ (chief ore), calamine/smithsonite $ZnCO_3$, zincite $ZnO$.

   Hint: Blende is a sulphide.

10. 10.What is the chief ore of tin and of mercury?

    Tin: cassiterite/tin stone $SnO_2$. Mercury: cinnabar $HgS$.

    Hint: Both single-element sulphide/oxide names.

11. 11.Name two common ores of silver and the chief ore of lead.

    Silver: argentite/silver glance $Ag_2S$, horn silver $AgCl$. Lead: galena $PbS$.

    Hint: Glance = sulphide; galena is a sulphide too.

12. 12.What is concentration (dressing/benefaction) of an ore?

    The removal of gangue from a powdered ore to increase the percentage of metal-bearing mineral. Method chosen depends on the physical/chemical differences between ore and gangue.

    Hint: Enrich the ore before reduction.

13. 13.On what property does hydraulic (gravity) separation depend?

    On the difference in specific gravity (density) between the ore particles and the gangue. A stream of water washes away the lighter gangue, leaving the heavier ore behind. Used for oxide ores like haematite and tin stone.

    Hint: Heavy ore stays, light gangue floats off.

14. 14.Describe magnetic separation and give an example ore.

    Used when either ore or gangue is magnetic. The powdered ore on a belt passes over a magnetic roller; magnetic particles fall closer, non-magnetic ones farther. Examples: magnetite $Fe_3O_4$, chromite $FeCr_2O_4$; also separates magnetic tungsten (wolframite) impurity from cassiterite.

    Hint: Two heaps by the roller: magnetic vs non-magnetic.

15. 15.What is froth flotation and which ores does it concentrate?

    A process that concentrates sulphide ores. Powdered ore in water with oil and reagents is agitated with air; sulphide particles get wetted by oil, stick to air bubbles and rise as froth, while gangue wetted by water sinks.

    Hint: Sulphides love oil; gangue loves water.

16. 16.In froth flotation, what are the roles of collectors and froth stabilisers?

    Collectors (e.g. pine oil, fatty acids, xanthates) enhance the non-wettability of the mineral by oil so it attaches to bubbles. Froth stabilisers (e.g. cresols, aniline) stabilise the froth.

    Hint: Collector grabs mineral; stabiliser holds the froth.

17. 17.How can froth flotation selectively separate two sulphide ores, e.g. ZnS from PbS?

    By using depressants. Adding $NaCN$ selectively forms a soluble complex with ZnS, preventing it from forming froth, so PbS floats and is separated; ZnS stays behind. The depressant lets one sulphide be floated at a time.

    Hint: NaCN depresses zinc blende.

18. 18.What is leaching? State its principle.

    A chemical concentration method in which the ore is treated with a suitable reagent that selectively dissolves the metal (or its compound) as a soluble species, leaving gangue undissolved. The metal is later recovered from solution.

    Hint: Dissolve the wanted part, filter off gangue.

19. 19.Explain the Bayer/Hall leaching of bauxite for purification.

    Bauxite is heated with conc. $NaOH$ at $473-523\,K$ and $35-36$ bar; amphoteric $Al_2O_3$ dissolves as sodium aluminate, leaving $Fe_2O_3$, $SiO_2$ etc.: $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$. On cooling/diluting and seeding, pure $Al(OH)_3$ precipitates, then is calcined to $Al_2O_3$.

    Hint: Amphoteric alumina dissolves in hot alkali.

20. 20.Write the leaching of silver/gold by the cyanide (MacArthur–Forrest) process.

    $4M + 8CN^- + 2H_2O + O_2 \rightarrow 4[M(CN)_2]^- + 4OH^-$ (M = Ag or Au). The metal is then displaced by a more reactive metal: $2[Ag(CN)_2]^- + Zn \rightarrow [Zn(CN)_4]^{2-} + 2Ag$.

    Hint: Cyanide complex, then zinc displacement.

21. 21.Why is $O_2$ (air) essential in the cyanide leaching of silver and gold?

    Because the reaction is an oxidation of the metal from the 0 state to $+1$; dissolved $O_2$ is the oxidising agent that lets Ag/Au dissolve as the cyanide complex. Without air the metal does not dissolve.

    Hint: It is a redox dissolution, not simple complexation.

22. 22.After concentration, why must the ore be converted to oxide before reduction?

    Because metal oxides are the easiest to reduce to the free metal (by carbon, CO or other agents). So carbonate and sulphide ores are first converted to oxide by calcination or roasting.

    Hint: Oxides reduce most readily.

23. 23.Define calcination.

    Heating an ore strongly in limited air/absence of air below its melting point to drive off volatile matter (water of hydration, $CO_2$) and convert it to oxide. Used for hydrated oxides, carbonates and hydroxides.

    Hint: Drives off $H_2O$ / $CO_2$; no strong air.

24. 24.Give two reactions illustrating calcination.

    $Fe_2O_3\cdot xH_2O \xrightarrow{\Delta} Fe_2O_3 + xH_2O$ ; $ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$ ; $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$.

    Hint: Hydrate and carbonate decompositions.

25. 25.Define roasting.

    Heating an ore (usually a sulphide) strongly in excess of air below its melting point to convert it to oxide, expelling volatile impurities like S, As, Sb as their oxides.

    Hint: Sulphide + plenty of air → oxide + $SO_2$.

26. 26.Give the roasting reactions for zinc blende and for copper pyrites (partial).

    $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$. For copper pyrites: $2CuFeS_2 + O_2 \rightarrow Cu_2S + 2FeS + SO_2$ (controlled/partial roasting).

    Hint: $SO_2$ is always released.

27. 27.State three key differences between calcination and roasting.

    (1) Calcination: limited/absence of air; Roasting: excess air. (2) Calcination for carbonates/hydrated oxides; Roasting for sulphides. (3) Roasting is oxidation (removes S as $SO_2$); calcination mainly removes $H_2O/CO_2$ without added oxygen.

    Hint: Air supply, ore type, oxidation vs volatilisation.

28. 28.What is a flux? Distinguish acidic and basic fluxes.

    A flux is a substance added during smelting to combine with the infusible gangue to form an easily fusible slag. Acidic flux (e.g. $SiO_2$) removes basic gangue; basic flux (e.g. $CaO$, $CaCO_3$) removes acidic gangue.

    Hint: Acid flux for basic gangue and vice-versa.

29. 29.What is slag and why is it useful?

    Slag is the fusible, low-melting product formed when flux combines with gangue: $flux + gangue \rightarrow slag$. Being light and molten it floats on the denser metal and can be skimmed off, protecting the metal from oxidation.

    Hint: Fusible waste; floats on the metal.

30. 30.Write the slag-forming reactions in iron extraction (with limestone).

    $CaCO_3 \rightarrow CaO + CO_2$; then $CaO + SiO_2 \rightarrow CaSiO_3$ (calcium silicate slag). Here $SiO_2$ is acidic gangue and $CaO$ is the basic flux.

    Hint: Lime + silica → calcium silicate.

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