Whatever the nature of the raw material the resulting burner gas should preferably contain 7-8% (by volume) of sulphur dioxide with 9-91% of oxygen, the remainder being inert nitrogen. Actually it is easy to get as much as io or r r% of sulphur dioxide in the gas obtained by burning brimstone and still leave an ade quate excess of oxygen for the next stage. With blende it is difficult to maintain more than 6% of dioxide; in the case of this material and of pyrites some atmospheric oxygen combines with the zinc or iron originally associated with the sulphur, while in the case of spent oxide an appreciable amount is consumed by the carbonaceous matter. Burner gas from brimstone is the pur est ; that from pyrites always contains arsenic, lead, selenium and other impurities originally present in the ore ; that from blende has only a small amount of arsenic but often contains a little fluorine ; whilst that from spent oxide, though containing little more arsenic than the gas from brimstone, always includes in addition to harmless carbon dioxide a certain amount of tarry matter and ammonia which lead to a high consumption of nitrogen compounds in the next stage.
For the large-scale production of sulphuric acid from burner gas there are two processes : ( r) the contact process, wherein oxidation is brought about directly by passing the burner gas over a suitable catalyst, and (2) the chamber process, in which oxidation is brought about by an intermediate reaction with oxides of nitrogen acting as oxygen carriers between air and sul phur dioxide. The disadvantages of the latter process are that it is impossible to produce high-strength acid directly, and also that unless pure brimstone is used the impurities (e.g., arsenic) originally contained in the raw material are found in the finished acid. Further, except for certain modern modifications, the plant has the disadvantage of occupying a large ground space in proportion to output., but it has the great advantage of high efficiency, the yield regularly exceeding 98% of the sulphur in the burner gas; it is also controlled with very little labour. The contact process, on the other hand, will give directly acid of any required strength up to about 25% oleum. Moreover, in the preliminary purification of burner gas all harmful impurities are removed, so that a pure acid can be obtained directly from such material as arsenical pyrites or blende. The plant is, however, more costly, the labour and power charges heavier and the ef ficiency lower, so that whereas the Contact type of plant now holds the field almost exclusively for the manufacture of con centrated and fuming acid, the Chamber process can still corn pete in acid of no more than 8o% strength, as required by most consuming trades except oil refining and the manufacture of dye stuffs and explosives.
LYSIS.) It must be emphasized that a catalyst does not change the final equilibrium between the reacting substances at any given temperature: its function is purely that of an accelerator. For this reaction the various catalysts include metallic platinum and oxides of iron and vanadium, of which the first is much the most widely used. Platinum and vanadium operate effectively at 425 C, which is the optimum temperature. Iron oxide becomes effective only at 600° C, so that the maximum yield of S03 attainable is only about 7o%.
Catalysts are peculiarly susceptible to "poisoning" by impurities commonly present in burner gas. Arsenic and halogens even in minute quantities render a platinum catalyst quite inactive, al though the oxides of iron and vanadium are less sensitive. Con sequently a preliminary purification of burner gases is essential.
There are many types of contact plant of which the best known are the Badische (using this term to describe processes which were developed in England by Squire and Messel as early as in Germany, while actually the Badische Gesellschaft itself now employs a modification of the process developed in the United States), Grillo, Tentelew and Mannheim. Excepting the Mann heim plant, which employs ferric oxide as preliminary, all these employ platinum as catalyst, and they differ only in the nature of the plant used for scrubbing and purifying the gases and in the carrier employed for the platinum; in the Grillo process the platinum is spread over the surface of calcined magnesium sulphate, whereas in the other processes it is carried on asbestos fibre. All types of contact plant (excepting the Mannheim, where the hot burner gas passes directly up rectangular shafts packed with oxide of iron) consist first of coolers followed by or com bined with scrubbing towers in which the gases are washed with cold sulphuric acid, then another wash-tower, round which is circulated very weak sulphuric acid or an alkaline solution to remove traces of halogens (chlorine and fluorine). The washed gases pass through one or two filters which may be large rectangu lar boxes (as much as 3o' sq. x i 2' in depth) constructed of sheet lead packed with finely graded coke. The cold filtered gases are then dried by passing up another tower, again built of lead and lined with acid-resisting brick, through which pure 95% sulphuric acid is circulated. After filtering through graded coke to stop mechanically carried spray, the gases are passed through a Root's blower which creates a draught throughout the whole apparatus. Where brimstone is burned much of the foregoing plant may be omitted, but only after this complicated purification are the gases permitted to enter the contact chambers, which are cast-iron or steel vessels containing shelves or trays packed with platinised material. The heat generated by the reaction is utilized by tubular heat exchangers to heat the cold incoming gas, so that in a well designed plant no extraneous heat whatever is required to main tain the correct temperature.