Sulphuric Acid

In the latest type of plant two catalyst chambers are used in series together with two heat-exchangers. A remarkable change takes place in the appearance of the gases after passage through the contact chambers ; whereas the incoming gas is colourless and invisible with an unpleasant sulphurous smell, the product issues as a dense, white, intensely acid fume of trioxide, practically without odour. After cooling, this fume is always absorbed in 98% sulphuric acid, since it is only very slowly absorbed by water. A portion of this solution is continuously diluted with water and returned to the absorbers. The surplus may be sold as "fuming" acid or oleum, or it may be diluted with water to any required strength. The absorption is carried out in steel towers packed with quartz, over which 98% acid is circulated by centrifugal pumps., or alternatively the gases are actually bubbled through the acid in a series of steel vessels. Although lead is generally used in plant construction up to the stage of the contact chambers, nothing but steel or cast iron can be employed in the later stages for lead is very rapidly attacked either by sulphur trioxide or by fuming acid.

Chamber Process.

In this process the oxidation of sulphur dioxide to trioxide is brought about by means of oxides of nitro gen, and the name is still applied generally, although the reaction is frequently brought about in plants differing considerably from the rectangular lead chambers formerly in exclusive use. There has been considerable controversy over the nature of the reactions occurring in the chamber process. The simplest theory, that of Berzelius, suggests that nitric oxide, NO, is the essential "carrier" which takes up oxygen from the air to form nitrogen peroxide, NO,. The latter then gives one of its atoms of oxygen to the sul phur dioxide, converting it into sulphur trioxide, which combines with water forming sulphuric acid. In yielding an atom of oxygen, nitrogen peroxide becomes again reduced to nitric oxide, which repeats the cycle of changes indefinitely except for losses which are inevitable under factory conditions. The foregoing interaction between nitrogen peroxide and sulphur dioxide only takes place in the presence of water. Nitrosyl sulphate, nitrosulphonic acid, or "chamber crystals," postulated by Lunge as an essential interme diate stage of the reaction, may appear under certain conditions in the later stages of the process when the supply of water to the chambers is insufficient.

A chamber plant of normal type includes burners to supply sulphur dioxide, with dust collecting chambers when necessary, followed as closely as possible (in order to avoid heat losses) by a Glover tower, which consists of a lead shell lined internally with vitrified brick and surmounted by two lead-lined cisterns. Inter nally the tower is packed with brick chequer work or flints sup ported on semi-circular arches. Down this tower are fed two qualities of acid—weak acid produced in the chambers at 6o-7o% strength which is concentrated by the heat of the burner gases (the steam driven off passing on to the chambers), and acid from the Gay Lussac tower at the other end of the process. The latter acid contains dissolved oxides of nitrogen which are expelled by the hot burner gas and returned to the leaden chambers, while the acid reaches the base of the Glover tower concentrated to about 8o% H,S0, and practically free from oxides of nitrogen.

This acid is then cooled in water-jacketed lead coolers, a portion being returned to the Gay Lussac towers for re-absorption of the nitrogen oxides escaping from the chambers, while the remainder is available for sale.

From the Glover tower the gases pass into the lead chambers which are large rectangular boxes of sheet lead, generally about 25ft. wide, 20 to 3oft. high, and up to i4oft. in length, and are usu ally arranged consecutively in sets of at least three. The lead sheets are supported in a frame-work of timber or steel, and the leaden joints are made by "burning" or heating the lead at the junction with a blowpipe flame so that it just melts and then reso lidifies. Lead of the very highest degree of purity, 99.99%, is to be preferred for this purpose. The chambers are generally erected on columns well above the ground so as to avoid unnecessary pump ing of acid. Excepting in the first chamber, to which steam is largely supplied by the evaporation from the Glover tower, it is necessary to supply water to convert the sulphur trioxide produced into sulphuric acid which falls like a shower of rain to the floor of the chambers. This water was formerly supplied in the form of steam, but is now generally sprayed in a finely atomised form. The lead chambers afford sufficient time and space for the alter nate cycles of oxidation and reduction to complete themselves until the whole of the sulphur dioxide has been converted into sulphuric acid, and they dissipate the heat evolved in the reaction.

By the time the residual gases leave the last chamber the whole of the sulphur dioxide should have been completely removed as sulphuric acid, whereas practically the whole of the nitrogen oxides should still be present unchanged. These are recovered by passing them up one or more Gay Lussac towers, which are similar to the Glover tower but of at least double the capacity, and of lighter lead since at this stage the acid is quite cold. The towers are lined with acid-resisting brick and packed either with coke, earth enware rings or narrow strips of glass with the alternate layers at right angles to one another. It is preferable to have at least two Gay Lussac towers with a fan between them controlling the draught of the whole process. These towers are fed from the Glover tower with cold strong sulphuric acid in which oxides of nitrogen are readily soluble. The nitrous vitriol leaving the first tower is returned to the Glover where its nitrogen oxides are re turned to the cycle. To compensate for inevitable loss through in complete absorption, quite apart from secondary reactions which may lead to destruction of a portion of the nitrogen oxides, it is necessary to replenish these oxides continuously. Formerly this was done by decomposing nitrate of soda with sulphuric acid in cast iron pots heated at the end of the pyrites burners. Alterna tively nitric acid is trickled down the Glover tower or a mixture of ammonia gas and air is passed over a hot platinum gauze which catalyses the oxidation of ammonia to oxides of nitrogen. The amount of fresh nitre required should not be more than 21-3% of the sulphur burned. Both in America and on the continent of Europe Glover and Gay Lussac towers have been constructed without any lead casing but simply of acid-resisting brick or volvic lava set in acid-resisting cement.