Manufacture of Sulphuric Acid

Manufacture from Sulphur

If sulphur is being used as the main feedstock, this must be converted to sulphur dioxide. Liquid sulphur is sprayed into a furnace and burnt in a blast of dry air. Excess air is used, and the resulting gases contain about 10% sulphur dioxide, and still about 10% oxygen.


Two further stages are needed to convert sulphur dioxide to sulphuric acid.

Oxidation of Sulphur Dioxide to Sulphur Trioxide

Cleaned and dried sulphur dioxide is passed with air over catalyst pellet beds in four cylindrical vessels called converters. The pressure is normally no more than 2 atmospheres.

The catalyst used is predominantly vanadium (V) oxide, V2O5, with alkali metal sulphate promoters (mainly potassium or caesium sulphate) on a silica ceramic base.

The catalyst components are mixed into a paste and extruded as small cylindrical pellets, rings or star-ring shapes, and baked at high temperatures. The catalyst bed may be some 8m in diameter and some 60 cm deep.


This reaction is exothermic, and the heat must be removed to prevent the equilibrium moving to the left. The heat can be used elsewhere in the plant.































Catalyst Replacement

The catalyst pellets will in time become clogged, and must be periodically replaced, though they can last for ten years. Vanadium is a useful metal for the steel industry, and so the spent catalyst can be processed to recover it. Alternatively it can be "locked" into concrete or glass and sent to controlled landfill.

Absorption of Sulphur Trioxide

Sulphur trioxide, SO3, will react with water, but the exothermic nature of the reaction means it generates a mist of sulphuric acid, which is more difficult to work with than a liquid. Consequently the SO3 is absorbed in 98% sulphuric acid, producing oleum, and water is added to this to generate sulphuric acid.

In practice this is carried out as a continuous process.

diagram: continuous production of sulphuric acid from sulphur trioxide

Energy Efficiency

The manufacture of sulphuric acid involves many exothermic reactions; the process as a whole actually generates more thermal energy than it uses. Whenever possible, excess heat is always used to reduce energy consumption in other parts of the plant.

Controlling Emissions

Even though the process is very efficient, there will be small amounts of unreacted sulphur dioxide and trioxide left, some sulphuric acid mist, and there will also be products from unwanted side reactions, including nitrogen monoxide and nitrogen dioxide (usually referred to as "NOx"). These "tail gases" cannot be vented to the atmosphere because of their environmental impact.

Double absorption will reduce the amount of sulphur compounds in the tail gas, and careful control of the conditions (pressure and temperature) will minimise the amount of NOx generated.

Any remaining sulphur dioxide can be removed by "scrubbing", reacting it with a base and generating additional useful products:

  • ammonia or ammonium hydroxide produces ammonium sulphate, a fertiliser
  • calcium hydroxide produces calcium sulphate ("gypsum") used in the manufacture of plaster and cement











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