Page:The New International Encyclopædia 1st ed. v. 18.djvu/789

* SULPHURIC ACID. 687 SULPHUBIC ACID. substances used, wliile producing the required cliL'iuical reaction, romaiu themselves in the end unchanged. These two processes are termed, re- spectively, the chamber process and the con- tact process. TuE C'HAJiBEB Process. In the chamber proc- ess the burner gas is mixed in large lead cham- bers (and with more or less auxiliary appar- atus) with nitric acid, or with the products of the action of sulphuric acid on sodium nitrate, the catalytic agent being probably nitrogen tri- oxide, aiid the following reactions probably tak- ing place : 2S0, + NA + H,0 + 0, = 2S0,(0H) (X0„) ; 2S0,(0H) (NO,) + H,0 = 2H=S0, -f NA. The nitrogen trioxide is thus entirely repro- duced. In addition to the above principal reac- tions there are further reactions set up in the so-called Guy-Lussac and Glover towers, in which apparatus, respectively, the catalytic agent is recovered and made available (in the form of nitric acid) for further use. The prin- ciple of the chamber process, therefore, is the production of a dilute sulphuric acid by oxidiz- ing a burner gas containing say 5 per cent, to 8 per cent, of sulphur dioxide in presence of water vapor by the means of nitric acid in suit- able apparatus in such a way that the nitric acid is practically recovered for further use. This recovery, however, is rarely complete — a certain mechanical loss being unavoidable. The to 95.5 per cent, of H,SO, ; but the anhydrous can only be isolatcil by freezing. The Contact Puocess. In the contact process the method of production is directly synthetical, and acids of any desired strength may be made in one operation. In this process the burner gas must be first rendered absolutely free from eveijthing other than sulphur dioxide, oxygen, and inert nitrogen, the usual impurities of burner ga's, such as dust, moisture, arsenic, sele- nium, etc., derived from the raw material used, seriously interfering with the process. After purification, the gas is passed through a sub- stance possessing the faculty of causing sulphur dioxide to oxidize into sulphur trioxide, without apparently suffering any change whatever in its own condition. This contact must be brought about under conditions as to temperature ac- cording to the substance used. The contact substances used are finely divided platinum in various forms, such as platinized asbestos and pumice, etc., crusts formed of platinum and some soluble sulphate, ferric oxide, cupric sulphate, diatomaceous earth, etc. The transformation of sulphur dio.xide into the trio.xide takes place best between 200° and 450° C. (392°-842° F.). Above 450° C. the transformation grows incom- plete, for at that temperature commences the dissociation of sulphur trioxide into dioxide and free oxygen — a dissociation which is complete at 900° C. The formation of sulphur trioxide hav- Ki^JJ- „ DIAGRAM SHOWING CONTACT PUGCESB FOB MAKING SULPHURIC ACID. A is a bench of pyrites burners. The burner gas pas.ses through the flue .^.to the first cleaning: tower, B. "Weak sulphuric acid is constantly flowing down this tower, becoming'eoncentrated by the hot burner gas and absorption of the sulphur trioxide contained in the burner gas, and finally flows out at the bottom into the pooler C at a strength of from 62° to 64° Baum^. Froni the cooler the strong acid passes to the tank D andjis delivered by the pump D^ to the storage tank T, or to the tank F over the second cleaning tower. E. . constant stream of strong sulphuric acid from the tank F is kept flowing down this tower. In this tower the burner gas coming from the top of B is further cleaned and dried by the action of the strong acid and then passes to the filter tower. I ; the circu- lation of the gases through the train of apparatus is maintained by the (an J. Before entering the contact ovena, the mixed gases are reheated to the proper temperature for the combination of the sulphur dioxide and oxygen in the reheater. K. The contact oven, L, consists of cast-iron rings with perforated shelves, or diaphragms, upon which is placed the contact mass. The sulphur trioxide formed in the contact oven now passes through the absorption cylinders. M,. Mg, M3. M*. These are cylindrical iron tanks connected in such a way that the gas passes from end to end, meeting the weak acid fl<»wing in the opposite direction. Both the gas and the acid in M, are richest in sulphur trioxide, while in M* the gas and acid are weak. Sulphur trioxide is most readily absorbed by acid containing about 98<;^ H0SO4. The strong acid, which is reaily for the market as it comes from Mj. is collected in the tank Q and is delivered by the pump Qj to the storage tank. II. The gases coming from the last absorption tank. AT^, contain still a small amount of unabsorbed sulphur trioxide. In order to recover this, the gases are passed through the tower N, which is supplied with strong acid which absorbs the last traces of sulphur trioxide. The nitrogen and oxygen remaining pass into the air through the pipe O. The tank car S receives acid for shipment from the storage tank, R. strongest acid which can be produced by the chamber process will correspond to from 50 per cent, to 60 per cent, of sulphur trioxide. Strong- er acids must be produced by driving off the contained water by heat or by isolating the acid by freezing. It is thus possible to produce the concentrated acid (oil of vitriol) of commerce, containing from 93.5 to 97 per rent, of pure sul- phuric acid; also distilled acid, containing up ing been attained, it simply remains either to condense it at the proper temperature in its crystalline condition or to absorb it in sulphuric acid or water until a solution of the required strength is obtained. Uses of SrxpHtRic Acid. The uses of sul- phuric acid in the arts are extremely varied and of the highest importance. It forms the basis of manufacture of nearly all the other acids and