Attempts have been made to do away with chambers altogether, the oxidation of sulphur dioxide taking place largely in the liquid phase by means of the nitrous compounds dissolved in vitriol, as in the Opl plant where the reaction between burner gas and nitrous vitriol takes place in a series of towers, the later ones of which act as absorbers for the nitrogen oxides which are liberated in the earlier ones, the nitrous acid being returned to the first tower. Water is fed down the middle towers to maintain them at the requisite strength for the reaction. Other types of apparatus have been introduced in which, instead of the gas merely coming into contact with films of liquor flowing over tower packings, the acid is bubbled or passed through nitrous vitriol which is simultane ously agitated by revolving drums which fill the vessels with a continuously renewed spray or film of acid. Where non-arsenical acid is required from an acid produced from arsenical pyrites, the impurities are removed by treatment with sulphuretted hydrogen; this precipitates arsenious sulphide and most other impurities, which are then removed by filtration or by oil flotation methods.
The concentration of chamber acid is of small commercial importance, because strong acid can be pro duced more cheaply by the contact process, but concentration is still of value in connection with such manufactures as dyes and explosives where, after nitration with mixed nitric and sulphuric acids, a dilute sulphuric acid remains which must be recovered. An installation suitable for small outputs consists of a number of basins of silicon-iron or fused silica arranged in two or three rows in cascade fashion and heated from below, but where larger quantities of acid are being concentrated, plant of the Kessler or Gaillard type is employed. The Kessler, of which there are many modifications, consists of a producer-gas fired, over-heat pan, gen erally built of volvic lava, inside a heavy lead dish and provided with baffles in the roof so as to cause the fire gases which enter at one end to strike down on the layer of acid which flows in the opposite direction. At the other end of the pan is a small packed tower down which the incoming vitriol is fed on its way to the pan. The exit gases are passed through a fine coke-packed scrubber to remove acid mist. These fumes are usually removed by a Cottrell electrical precipitator, lead electrodes being used. The Gaillard tower consists of an empty tower of 6' to 9' internal diameter and 40' to 50' high, built of volvic lava or acid-resisting brick, into the top of which weak sulphuric acid is sprayed meet ing the hot gases from a coke-fired producer. The exit gases from
the Gaillard tower are usually passed up a scrubbing tower to remove acid vapour and are then put through a Cottrell precipi tator. The Gaillard tower is suitable for dealing with dirty waste acids, but will not produce economically more than 93% vitriol, whereas the Kessler plant furnishes 95-96% vitriol.
The ordinary commercial grade of acid made from pyrites by the chamber process at 77-80% strength is known as common or brown oil of vitriol (B.O.V.) ; similar acid free from arsenic, either as the result of a de-arsenication process, or by preparation from a non-arsenical material such as brimstone, is called best brown oil of vitriol (B.B.O.V.). The stronger grade of 93-96% acid is known as rectified vitriol (R.O.V.). Acid of i00% strength, i.e., exactly
is called "monohydrate"; excess of sulphur trioxide readily dissolves in this, giving rise to still stronger acid known as "fuming" sulphuric acid or "oleum." The grade of oleum is expressed by stating the percentage of free sulphur trioxide,
which it contains apart from that com bined with water to form
thus 20% oleum means acid con taining 20 parts of
and 8o parts of
The usual commer cial grades of oleum are 20% and 6o-65%, both of which are liquids at ordinary temperatures, while intermediate and higher strengths are solids. More dilute grades of acid are sold for use in electrical storage batteries; these are sold by specific gravity as measured by the hydrometer, the usual strength for batteries for wireless sets and motor cars being 1.215 to 1.25o or 28-33% of sulphuric acid.
Sulphuric acid is generally sold in glass carboys holding i o gallons, which are protected by straw packing inside an iron frame work or steel drums, but chiefly in steel tank wagons. Acid should not be transported in steel, however, at less than 75% strength. For bulk transportation of weaker acid, lead-lined tanks are used.
Documents of the League of Nations International Economic Conference Geneva (1927) give the output of sulphuric acid in 1925 as follows:— Metric tons @ 62.5% H2SO4 Germany . . . 1,800,000 Denmark . . . 175,000 Gt. Britain . . 1,300,000 Sweden . . . 140,00o United States . . 6,300,00o Serbia . . . . ioo,000 France . . . . 1,84o,000 Czechoslovakia . . 210,000
Belgium . . . 740,00o Hungary . . . 80,000 Holland . . . i5o,000 Austria . . . . 70,000 Poland . . . 320,000 Switzerland . . . 50,00o Spain . . . . 230,00o Italy . . . . 1,075,00o Total 14,580,000 The output in Germany in 1913 was 2,700,000 tons. The plant capacity is doubtless much greater than the above figures : in Great Britain, for example, it is above 2,000,000 tons per annum.