Lead Chamber Process - The Process

The Process

Sulfur dioxide is introduced with steam and oxides of nitrogen into large chambers lined with sheet lead where the gases are sprayed down with water and chamber acid. The sulfur dioxide and nitrogen dioxide dissolve and over a period of approximately 30 minutes the sulfur dioxide is oxidized to sulfuric acid. The presence of nitrogen dioxide is necessary for the reaction to proceed. The process is highly exothermic, and a major consideration of the design of the chambers was to provide a way to dissipate the heat formed in the reactions.

Early plants used very large lead-lined wooden rectangular chambers (Faulding box chambers) that were cooled by ambient air. The internal lead sheathing served to contain the corrosive sulfuric acid and to render the wooden chambers waterproof. Around the turn of the nineteenth century, such plants required about half a cubic meter of volume to process the sulfur dioxide equivalent of a kilogram of burned sulfur. In the mid 19th century, French chemist Gay-Lussac redesigned the chambers as stoneware packed masonry cylinders. In the 20th century, plants using Mills-Packard chambers supplanted the earlier designs. These chambers were tall tapered cylinders that were externally cooled by water flowing down the outside surface of the chamber.

Sulfur dioxide for the process was provided by burning elemental sulfur or by the roasting of sulfur containing metal ores in a stream of air in a furnace. During the early period of manufacture, nitrogen oxides were produced by the decomposition of niter at high temperature in the presence of acid, but this process was gradually supplanted by the air oxidation of ammonia to nitric oxide in the presence of a catalyst. The recovery and reuse of oxides of nitrogen was an important economic consideration in the operation of a chamber process plant.

In the reaction chambers, nitric oxide reacts with oxygen to produce nitrogen dioxide. Liquor from the bottom of the chambers is diluted and pumped to the top of the chamber and sprayed downwards in a fine mist. Sulfur dioxide and nitrogen dioxide are absorbed in the liquid and react to form sulfuric acid and nitric oxide. The liberated nitric oxide is sparingly soluble in water and returns to the gas in the chamber where it reacts with oxygen in the air to reform nitrogen dioxide. Some percentage of the nitrogen oxides are sequestered in the reaction liquor as nitrosylsulfuric acid and as nitric acid, so fresh nitric oxide must be added as the process proceeds. Later versions of chamber plants included a high temperature Glover tower to recover the nitrogen oxides from the chamber liquor, while concentrating the chamber acid to as much as 78% H₂SO₄. Exhaust gases from the chambers are scrubbed by passing into a tower through which some of the Glover acid flows over broken tile. Nitrogen oxides are absorbed to form nitrosylsulfuric acid which is then returned to the Glover tower to reclaim the oxides of nitrogen.

Sulfuric acid produced in the reaction chambers is limited to about 35% concentration. At higher concentrations, nitrosylsulfuric acid precipitates on the lead walls as chamber crystals and is no longer able to catalyze the oxidation reactions.