Metallurgy (Occurrence And Concentration)

Occurrence Of Metals (from Class 10)

Metals are found in the Earth's crust and in the sea. They exist in nature in two main forms:

1. As Elements (Noble Metals):

Some metals are too unreactive to react easily with the common substances around them. Therefore, they are found in the free state or in native form.

• Examples: Gold (Au), Silver (Ag), Platinum (Pt), Copper (Cu) (sometimes found in native form).
• Occurrence: Often found as nuggets or grains in rocks or alluvial deposits.

2. As Compounds (in Ores):

Most metals are found in the Earth's crust in the form of their compounds, such as oxides, carbonates, sulphides, chlorides, etc. These compounds are called ores.

• Oxides: Metals like Aluminium (Al), Iron (Fe), Copper (Cu), Zinc (Zn) are found as oxides.
  • Examples: Haematite ($Fe_2O_3$), Bauxite ($Al_2O_3 \cdot 2H_2O$), Zincite (ZnO), Cuprite ($Cu_2O$).
• Sulphides: Metals like Copper (Cu), Zinc (Zn), Lead (Pb), Mercury (Hg), Silver (Ag) are found as sulphides.
  • Examples: Copper glance ($Cu_2S$), Zinc blende (ZnS), Galena (PbS), Cinnabar (HgS), Argentite ($Ag_2S$).
• Carbonates: Metals like Copper (Cu), Zinc (Zn), Lead (Pb), Calcium (Ca), Magnesium (Mg) are found as carbonates.
  • Examples: Malachite ($CuCO_3 \cdot Cu(OH)_2$), Calamine ($ZnCO_3$), Siderite ($FeCO_3$), Limestone ($CaCO_3$), Magnesite ($MgCO_3$).
• Chlorides: Sodium (Na), Potassium (K), Magnesium (Mg), and Calcium (Ca) are found as chlorides, especially in seawater and rock salt.
  • Examples: Sodium chloride (NaCl) in common salt and seawater.

The reactivity of the metal determines the form in which it is found. Highly reactive metals are usually found as oxides or chlorides, while less reactive metals can be found as sulphides or even in their native state.

Occurrence Of Metals (from Class 12)

Expanding on the occurrence of metals, we can classify them based on their reactivity series, which helps predict their natural occurrence:

1. Metals of Low Reactivity:

These metals (e.g., Copper, Silver, Gold, Platinum, Mercury) are found in their native or free state because they are unreactive and do not easily form compounds. They are also found in the form of their less stable compounds like sulphides and oxides.

• Copper (Cu): Found in native form, as copper pyrites ($CuFeS_2$), cuprite ($Cu_2O$), copper glance ($Cu_2S$), malachite ($CuCO_3 \cdot Cu(OH)_2$).
• Silver (Ag): Found in native form, as argentite ($Ag_2S$), silver glance ($Ag_2S$), as amalgam with mercury, and often in combination with other metals like lead and copper.
• Gold (Au): Found in native form (nuggets, grains) and as alloy with silver. Also found in some tellurides (e.g., calaverite, $AuTe_2$).
• Platinum (Pt): Found in native form, often alloyed with other platinum group metals.
• Mercury (Hg): Primarily found as cinnabar (HgS).

2. Metals of Medium Reactivity:

These metals (e.g., Zinc, Iron, Lead, Tin, Nickel) are moderately reactive. They are found in the Earth's crust mainly as their oxides, carbonates, and sulphides.

• Zinc (Zn): Found as zinc blende (ZnS), calamine ($ZnCO_3$), zincite (ZnO).
• Iron (Fe): Found mainly as haematite ($Fe_2O_3$), magnetite ($Fe_3O_4$), siderite ($FeCO_3$), iron pyrites ($FeS_2$).
• Lead (Pb): Found as galena (PbS), cerussite ($PbCO_3$), anglesite ($PbSO_4$).
• Tin (Sn): Found primarily as cassiterite ($SnO_2$).
• Nickel (Ni): Found as nickel glance ($NiAsS$), pentlandite ($(Ni,Fe)_9S_8$), and laterite ores.

3. Metals of High Reactivity:

These metals (e.g., Potassium, Sodium, Calcium, Magnesium, Aluminium) are highly reactive. They are never found in their free state in nature. They are always found in the form of their compounds, mainly as oxides, carbonates, and chlorides, due to their strong tendency to lose electrons.

• Sodium (Na): Found as sodium chloride (NaCl) in common salt and seawater.
• Potassium (K): Found as potassium chloride (KCl), potassium nitrate ($KNO_3$) in minerals like carnallite ($(KCl \cdot MgCl_2 \cdot 6H_2O)$).
• Calcium (Ca): Found as limestone ($CaCO_3$), gypsum ($CaSO_4 \cdot 2H_2O$), fluorspar ($CaF_2$).
• Magnesium (Mg): Found as magnesite ($MgCO_3$), dolomite ($CaCO_3 \cdot MgCO_3$), Epsom salt ($MgSO_4 \cdot 7H_2O$), carnallite.
• Aluminium (Al): Found abundantly in the Earth's crust as bauxite ($Al_2O_3 \cdot 2H_2O$), cryolite ($Na_3AlF_6$), and china clay (kaolinite, $Al_2(SiO_3)_2(OH)_4$).

Note: Some metals like Aluminium and Sodium are so reactive that their compounds are very stable and difficult to break down to obtain the free metal.

Concentration Of Ores

Concentration of an ore is the process of removing gangue (undesired rocky impurities) from the ore. The choice of method depends on the nature of the ore and the impurities.

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Hydraulic Washing (Gravity Separation):

This method is based on the difference in the gravitational force exerted on the ore particles and the gangue particles. It is mainly used for the concentration of ores of heavy metals, like haematite ($Fe_2O_3$) and cassiterite ($SnO_2$).

Process:

1. The powdered ore is treated with a stream of water.
2. The lighter gangue particles are washed away by the running water.
3. The heavier ore particles settle down at the bottom.

Principle: Difference in density between the ore and gangue particles.

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Magnetic Separation:

This method is used when either the ore or the gangue is magnetic.

Process:

1. The powdered ore is placed on a conveyor belt moving over a magnetic roller and a non-magnetic roller.
2. As the belt moves, the ore particles fall off. Magnetic particles are attracted to the magnetic roller and fall closer to the roller, while non-magnetic gangue particles are thrown further away.

Examples:

• Concentration of haematite ($Fe_2O_3$), which is magnetic, from non-magnetic gangue.
• Concentration of magnetite ($Fe_3O_4$) ore.
• Concentration of pitch blende (uranium ore, $UO_2$), which is slightly magnetic.

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Froth Floatation Method:

This is the most common method for concentrating sulphide ores. It is based on the difference in the wettability of the ore and gangue particles by water and oil (frother).

Process:

1. The powdered ore is mixed with water and a small amount of frother (e.g., pine oil, eucalyptus oil) and collector (e.g., potassium ethyl xanthate).
2. Air is blown through the mixture. The collector adsorbs onto the surface of the ore particles, making them hydrophobic (water-repelling).
3. The frother stabilizes the froth.
4. The hydrophobic ore particles attach to the air bubbles and rise to the surface, forming a froth, which is skimmed off.
5. The hydrophilic gangue particles remain at the bottom and are removed.

Principle: Difference in wettability of ore and gangue particles.

Stabilizer/Frother: Pine oil, eucalyptus oil.

Collector: Xanthates, fatty acids.

Depressant: Used to prevent certain minerals from floating (e.g., cyanide ions to precipitate ZnS in the case of PbS concentration).

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Leaching:

Leaching is a process in which the desired metal compound is selectively dissolved from the ore by a suitable reagent. This is used for ores that cannot be concentrated by other methods, particularly for low-reactivity metals and some oxides.

Process:

1. The powdered ore is treated with a leaching agent that dissolves the metal compound but not the gangue.
2. The solution containing the metal compound is separated from the insoluble gangue.
3. The metal is then recovered from the solution by a chemical reaction.

Examples:

• Concentration of Aluminium (from Bauxite): Bauxite ore is treated with a concentrated solution of sodium hydroxide (NaOH) at high temperature and pressure (Bayer's process). Aluminium oxide dissolves as sodium aluminate, while impurities like silica and iron oxide remain undissolved.
  • $Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2Na[Al(OH)_4](aq)$
  • The solution is then filtered, cooled, and acidified with $CO_2$ or by passing it through a stream of $CO_2$ to precipitate pure aluminium hydroxide ($Al(OH)_3$).
  • $2Na[Al(OH)_4](aq) + CO_2(g) \rightarrow 2Al(OH)_3(s) + Na_2CO_3(aq)$
  • The $Al(OH)_3$ is then heated to form pure alumina ($Al_2O_3$), which is subsequently electrolyzed to obtain aluminium.
• Concentration of Gold (Au) and Silver (Ag): These metals are leached from their ores using a dilute solution of sodium cyanide (NaCN) in the presence of air (oxygen).
  • $4Au(s) + 8NaCN(aq) + O_2(g) + 2H_2O(l) \rightarrow 4Na[Au(CN)_2](aq) + 4NaOH(aq)$
  • The dissolved metal is then recovered by displacement with a more electropositive metal like zinc.
  • $2Na[Au(CN)_2](aq) + Zn(s) \rightarrow Na_2[Zn(CN)_4](aq) + 2Au(s)$