Q. Explain Lead chamber process?

Lead chamber process, is the older of the two processes. In this process SO₂, is produced by burning S or roasting pyrites.

 S + O₂------------------> SO₂

4FeS₂+ 11O₂----------> 2Fe₂O₃+ 8SO2

The oxidation of SO₂, to SO₃, is carried out by  using oxides of nitrogen. These are produced by oxidation of NH, using platinum as catalyst.

4NH₃+ 5O₂--------------------> 4NO+ 6H₂O

2NO+ O₂--------------------> 2NO₂

A mixture of air, SO₂, and the nitrogen oxides is passed into a set of chambers lined with lead sheets from the top of which water is sprayed. The exact nature of the reactions which take place in the lead chambers is not fully understood. The following scheme of reactions seems to be operative:

NO₂+ NO +2SO₂+ H₂O + O₂----------------> 2(NO.HSO₄)

2(NO. HSO₄) +H₂O-------------------------> 2H₂SO₄+NO₂+ NO

Nitrosyl hydrogen sulphate formed as an intermediate reacts with H₂, O to give H₂S0₄, and the nitrogen oxides. The oxides play the role of a catalyst.

Though chamber process involves relatively simple steps, yet it is not the method of choice where high purity of the acid is required. The chamber acid contains As, O, (from pyrites), PbSO4, (from chamber) and some oxides of nitrogen as impurities. This acid finds use in the preparation of fertilisers where removal of impurities is not necessary. The second drawback with chamber method is that it gives dilute acid (60-78%).