Metals rarely occur free in nature. Only the least reactive metals — gold, platinum and sometimes silver and copper — are found as native (free) elements; the rest occur combined in the Earth's crust.
A mineral is any naturally occurring substance in which a metal is present. An ore is a mineral from which a metal can be extracted profitably and conveniently. Thus every ore is a mineral, but not every mineral is an ore — both bauxite ($Al_2O_3\cdot 2H_2O$) and clay contain aluminium, yet only bauxite is an ore because clay does not yield the metal economically.
Classification of ores by anion:
- Oxides — haematite $Fe_2O_3$, magnetite $Fe_3O_4$, bauxite, cuprite $Cu_2O$, zincite $ZnO$.
- Sulphides — copper pyrites $CuFeS_2$, zinc blende $ZnS$, galena $PbS$, cinnabar $HgS$.
- Carbonates — calamine $ZnCO_3$, siderite $FeCO_3$, malachite, magnesite $MgCO_3$.
- Halides — rock salt $NaCl$, fluorspar $CaF_2$, cryolite $Na_3AlF_6$, horn silver $AgCl$.
The unwanted earthy impurity mixed with an ore is called gangue (matrix). Removing it to raise the percentage of metal is the first step of metallurgy, called concentration or dressing. The method chosen depends on the differences between the ore particles and the gangue.
1. Hydraulic (gravity) washing — used when the ore is denser than the gangue (native gold, haematite). Running water carries away the lighter gangue while the heavier ore settles; it relies on the difference in specific gravity.
2. Magnetic separation — used when the ore or the gangue is magnetic (magnetite, chromite, or tinstone $SnO_2$ contaminated with magnetic wolframite). The powdered ore drops onto a belt over an electromagnetic roller; the magnetic fraction falls nearer the roller, the non-magnetic farther away.
3. Froth flotation — the chief method for sulphide ores. The finely ground ore is mixed with water, a little oil (pine oil — the collector) and a frother, and air is blown through. The oil wets the sulphide particles, which become hydrophobic and rise with the froth, while the water-wetted gangue sinks. A depressant such as $NaCN$ can keep $ZnS$ in solution while $PbS$ floats, separating two sulphides from one lump.
4. Leaching — a chemical method used when the ore is soluble in a reagent but the impurities are not. Bauxite (Bayer process): $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$; iron oxide and silica are left behind, then alumina is reprecipitated and calcined. Gold/silver: the metal dissolves in dilute $NaCN$ in air, $4Au + 8NaCN + 2H_2O + O_2 \rightarrow 4Na[Au(CN)_2] + 4NaOH$, and is displaced by zinc.
Distinguish between a mineral and an ore using aluminium as an example.
Solution- A mineral is any naturally occurring compound in which a metal is present, irrespective of whether extraction is economical.
- An ore is a mineral from which the metal can be extracted conveniently and profitably.
- Aluminium is present in both clay (an aluminosilicate) and bauxite ($Al_2O_3\cdot 2H_2O$).
- Only bauxite yields aluminium economically, so bauxite is an ore while clay is merely a mineral.
Answer: Every ore is a mineral, but only minerals that allow profitable extraction (e.g. bauxite, not clay) are ores.
Galena ($PbS$) is contaminated with sandy gangue. Which concentration method is best and why?
Solution- Galena is a sulphide ore; sulphide particles are preferentially wetted by oil rather than water.
- Sandy (silica) gangue is wetted by water and sinks.
- Froth flotation exploits exactly this difference in wettability.
- On blowing air through the oil-water-ore mixture, the sulphide rises with the froth and is skimmed off.
Answer: Froth flotation, because sulphide ores are wetted by oil while the siliceous gangue is wetted by water.
Write the chemical equations involved in concentrating bauxite by the Bayer process.
Solution- Digest powdered bauxite with hot concentrated NaOH so that alumina dissolves while $Fe_2O_3$ and silica are filtered off: $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$.
- Dilute and seed the solution with fresh $Al(OH)_3$ to precipitate hydrated alumina: $NaAlO_2 + 2H_2O \rightarrow Al(OH)_3\downarrow + NaOH$.
- Heat (calcine) the hydroxide to get pure alumina: $2Al(OH)_3 \xrightarrow{\Delta} Al_2O_3 + 3H_2O$.
Answer: Bauxite is leached with NaOH to soluble $NaAlO_2$, reprecipitated as $Al(OH)_3$, then calcined to pure $Al_2O_3$.
How can magnetic separation be used to purify tinstone ($SnO_2$) contaminated with wolframite?
Solution- Tinstone ($SnO_2$) is non-magnetic, whereas the wolframite impurity (iron-manganese tungstate) is magnetic.
- The powdered ore is fed onto a conveyor belt running over a magnetic roller.
- The magnetic wolframite is attracted and falls in a heap close to the roller.
- The non-magnetic tinstone is thrown off farther away, giving two separate heaps.
Answer: Because $SnO_2$ is non-magnetic and wolframite is magnetic, an electromagnetic roller separates them into two heaps.
Why is $NaCN$ added during the froth flotation of an ore containing both $ZnS$ and $PbS$?
Solution- $NaCN$ acts as a depressant: it forms a soluble complex with zinc, $Na_2[Zn(CN)_4]$, keeping $ZnS$ in solution.
- This prevents $ZnS$ from floating with the froth.
- $PbS$ is unaffected, so only galena floats and is collected first.
- The two sulphide ores are thereby separated from the same mixture.
Answer: $NaCN$ depresses $ZnS$ (as $[Zn(CN)_4]^{2-}$) so that only $PbS$ floats, separating the two sulphides.
Gold is leached with $NaCN$ and then recovered. Write the two equations and name the redox roles.
Solution- Dissolution (oxidation of Au by air in cyanide): $4Au + 8NaCN + 2H_2O + O_2 \rightarrow 4Na[Au(CN)_2] + 4NaOH$.
- Here gold is oxidised from 0 to +1 and oxygen is reduced.
- Recovery (displacement): $2Na[Au(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Au$.
- Zinc, being more reactive, reduces $Au^+$ back to metallic gold.
Answer: Gold dissolves as $[Au(CN)_2]^-$ (oxidation by $O_2$) and is displaced by zinc (reduction of $Au^+$ to $Au$).