Extraction of Mercury from Cinnabar

Symbol: Hg
Latin Name: Hydragyrum
Atomic Number: 80
Atomic Mass: 200.59
Valency: 1 and 2
Occurrence: Mercury is found in free state in trace quantity. Mercury generally occurs in combined state. The main ore of mercury is cinnabar (HgS).

Extraction process:

The following steps are involved for the extraction of mercury.

1. Crushing and Concentration: The sulphide ore is first crushed by jaw crusher whereas powdered in ball mills. Then the water is mixed with the water and pine oil and stirred by the current of air. Hence, all the sulphide ore floats as froth leaving behind impurities at the bottom. This process is called forth flotation process.

Froth flotation process

2. Roasting and distillation: The concentrated ore roasted in air when it gets oxidized to HgO and a portion of HgO decomposes to Hg and O2.
$$\ce{2HgS + 3O2->2HgO + 2SO2}$$
$$\ce{2HgO ->[{300^{\circ}C}]2Hg + O2 }$$

The modern furnace used in the roasting is shaft furnace. In this roasting process, the ore is mixed with coke and is heated in a shaft furnace at 1000oC.
$$\ce{2HgS + 3O2 ->[\Delta]2HgO + SO2}$$
$$\ce{2HgO->2Hg + O2}$$
$$\ce{HgO + C->Hg + CO}$$
The vapours of Hg and CO are led into a tank containing water, where mercury vapours condense to from liquid mercury.

Purification: Thus obtained mercury may contain impurities like copper, bismuth, lead, zinc etc. It is purified by the following steps.

  •  The impure mercury is first filtered through leather bag where solid impurities remain in the bag.
  • Mercury so obtained is dropped in a long tube filled with 5% HNO3. Impurities like zinc, iron, copper get dissolved in dilute HNO3 to give soluble nitrate while mercury does not react. During this, some of the mercury may be converted into mercuric nitrate which react with base metal and liberate mercury.
    $$\ce{4 Zn + 10HNO3->4Zn(NO3)2 + NH4NO3 + 3H2O}$$
    $$\ce{Hg2(NO3)2 + Zn->Zn(NO3)2 + 2Hg}$$

If purest sample of mercury is required, then it is subjected to vaccum distillation. After distillation, less volatile metallic impurities like like aluminium, silver, platinum etc will remain in the vessel whereas more volatile mercury distills off.

 

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