The Industrial manufacture of sulfuric acid is done mainly by two processes
1. The Lead Chamber process
2. The Contact process
1. The lead chamber process
The Lead Chamber process for the manufacture of sulfuric acid dates backabout 200 years. Although less efficient than the contact process, it is still of considerable commercial importance.
Raw Materials
Basis: 1000kg Sulfuric acid (98% yield)
Sulfur = 400kg
Air = 399kg
Reaction:
Sulfuric Acid Process Flow Diagram:
Figure: Manufacturing of Sulfuric acid by Chamber process
Sulfur dioxide is obtained by burning sulfur or by roasting pyrites. There are two function of burner
1. To oxidize sulfur to maximum extent
2. To produce and constant supply of gas containing maximum concentration of SO2
The burner of the furnace should expose large surface of melted sulfur and should be provided secondary air in order to burn sublimed burner. This is necessary due to low heat of combustion and high vapour pressure of sulfur. At about 4000C, pyrite (FeS2) decompose in to FeS and sulfur vapour, the later oxidized to SO2 in presence of excess air. The residual FeS also oxidizes to Fe2O3 and SO2. Iron oxide (Fe2O3) slightly catalyzed oxidation of SO2 to SO3. Burner gas should contain sufficient oxygen for carry out further oxidation of SO2 to SO3.
Now, burner gases are passed through niter oven made of cast iron in which equimolecular proportion of NaNO3 and H2SO4 is heated. Resulting nitric acid reacts with SO2 to give mixture of nitric oxide (NO) and nitrogen dioxide (NO2) which are carried with burner gases.
In modern plant oxides of nitrogen are produced by passing mixture of ammonia and air through heated platinum gauze acting as catalyst (same as manufacture of HNO3 by ammonia oxidation process)
After passing burner gases to dust chamber and niter oven, they pass through 5 meter square and 10 -15meter high Glower tower which is packed with flint stone, quartz, tile or acid resisting bricks. The packing in the tower is loosely stacked at the bottom to facilitate mixing of hot gases. The hot burner gases passes up this tower is at 450 - 6500C and dilute H2SO4 from the lead chamber and nitrosyl sulfuric acid from Gay-Lussac tower are made to trickle down the Glower tower by means of sprayers. Here, burner gases are cooled down to 70-800C, dilute chamber acid is concentrated up to 78% and nitrosyl sulfuricacid (nitrous vitriol) is denitrated by action of water.
The tower acid is drawn off from the bottom of the tower and collected in the container called acid egg. The acid from base of Glower tower is cooled to 400C by air coolers.
The mixture of SO2, Oxides of nitrogen and air is then passed to series of rectangular vessels made of lead (lead chamber) having 15-45 meter length, 6-7 meter width and 7 meter length. The number of chambers depends upon the size of plant, but usually they are 3 to 6 in number. The chambers are arranged in two parallel rows. Steam from low pressure boiler or pure filtered water is sprayed from top of the chamber. Mixture of gases is converted into H2SO4 having 65-70%v strength is collected at the bottom of the chamber. Dilute sulfuric acid obtained in any of the chamber is called chamber acid. A part of chamber acid is pumped to Glower tower, and the rest is sent for concentration.
The unabsorbed remaining gases contain oxides of nitrogen and SO2 from lead chamber are then passed through Gay-Lussac tower at the top of which Glower acid is sprayed to recover oxides of nitrogen.
The oxides of nitrogen recovered in the form of nitroso sulfuric acid are pumped to Glower tower to again regenerate oxides of nitrogen. When pyrite is used as raw material, the chamber acid may contain arsenious oxide (from pyrite), lead sulfate from lead chamber are removed by treatment of H2S and dilution of acid respectively. Dilute acid may be further concentrated into Glower tower.