Chamber Process.— In manufacturing sul phuric acid by the so-called °chamber process* the hot gases from the burners (consisting of air, sulphur dioxide and moisture) pass first over °niter pots,* which contain nitrate of soda and sulphuric acid, and which give off the nitric oxide gas that is to act as an oxygen carrier. Then, after passing through the dust removing apparatus, the gases are passed up ward through a tower (technically known as a °Glover tower))) that is loosely filled with fragments of coke, pumice, acid-proof stone ware or other inert material to distribute the flow, and here they are met by a downward stream of aqueous sulphuric acid obtained from a later stage of the process and containing oxides of nitrogen in solution. The precise reactions that occur cannot be discussed here, partly because they are complicated, and partly because they are not fully understood. The general effect, however, is to oxidize the SO2 to SO2, and the downward-flowing stream of weak acid dissolves the SO2 and thereby becomes stronger. In certain plants of recent design the oxidation of the sulphur dioxide and the absorption of the resulting trioxide are carried out in a series of Glover towers, without the use of chambers of any sort; but it is usual, after the gases have passed through one or two Glover towers, to cause them to enter large lead-lined chambers (from which the process takes its name), where the oxidation of the dioxide and the consequent strengthening of the acid are continued. Liquid sulphuric acid settles in the bottom of these chambers, and is drawn off from time to time. Steam or water is sprayed into •certain of the chambers, as •eeded, to provide the H20 that is required for the formation of the The gases coming from the last chamber are passed up through a °Gay-Lussac tower," which resembles the Glover tower in general construction. The liquid that is sent down through this tower, however, is concentrated sulphuric acid, and its purpose is to absorb the nitric oxide gas that is present, thereby preventing its loss and dimin ishing the quantity of nitre that must he used in the early part of the process. Upon leaving this tower the gases (which then consist mainly of nitrogen and oxygen) enter a stack and pass off into the atmosphere. The acid that is drawn off from the bottom of the final Gay Lussac tower contains oxides of nitrogen in solution, and is introduced (diluted, as may be necessary, with weaker acid) into the tops of the Glover towers. As the acid passes down through a Glover tower, however, the heat due to the reactions that occur, added to that which the entering gases already possess, drives off the nitrous oxides, and these keep returning upward through the tower with the sulphur oxides and air, while acid nearly free from nitrous oxides comes away from the bottom of the tower.
Sulphuric acid, as made by the chamber process (and especially when made from pyri tes) is likely to be contaminated with lead, arsenic, nitrous oxides and many other sub stances. Certain of these may be removed in considerable measure by treatment with sul phuretted hydrogen. If an acid of high purity is required, however, it is better to make it by the contact process, presently to be When treated with sulphuretted hydrogen for the removal of impurities, the acid should not have a greater specific gravity than 1.4, cor responding to about 50 per cent of actual and miiSt be diluted to this strength if it is already stronger. After the removal of the arsenic the punfied acid is concentrated by evaporation if a strong product is required. The evaporation may be carried ont in leaden pans, but a better product is obtained by effect ing the concentration in platinum stills. Owing to the high cost of platinum, however, it is com mon to perform the evaporation in a series of evaporating dishes constructed of fused silica.
These are arranged like a flight of steps, the lip of each one projecting out over the next dish. below. A slow stream of acid is kept running down through the cascades of dishes, while heat is applied to each dish from below. When an apparatus of this Idnd is properly arranged and operated it gives excellent re sults: Hoods should be arranged over the dishes, however, to take up the vapors that are given off and dispose of them in some proper way.
The Contact Process.-- In the contact proc ess for the manufacture of sulphuric acid, the sulphur dioxide is caused to combine with the oxygen of the air by bringing the mixed gases into contact with finely divided platinum, or with platinized asbestos. The catalytic ac tion of platinum (that is, its power of inducing combination in this way, without being itself consumed or otherwise permanently affected) was discovered by Sir Humphrey Davy, in 1818; and in 1E24 Doebereiner showed that finely divided platinum can effect the ignition of a jet of hydrogen, when this gas impinges upon it in contact with air. Peregrine Phillips, of Bris tol, England, first produced sulphyr trioxide by utilizing the catalytic effect of finely divided platinum upon a mixture of oxygen and sul phur dioxide, taking out a patent for this proc ess in 1831; and Schneider, in 1848, made a working model of an apparatus for manufac turing sulphuric acid by this method. Since that time many attempts have been made to rnake the contact process practicable for the manufacture of sulphuric acid, and many other catalytic agents have been tried besides platinum. It was not until about 1898, however, that the various practical difficulties involved in the process were satisfactorily overcome, largely through the la bors of Herr Knietsch of the Badische Anilin und Soda-Fabrik, a German company for the manufacture of chemical substances of nearly every lcind. It was found that the prime con dition of success in the application of the con tact method is that the gases that are treated shall be absolutely free from dust, arsenic, mercury and certain other substances. The gases from the pyrites-roaster are cooled very slowly and are then purified by filtration and washing. When passed to the tubes con taining the platinized asbestos that is used as the catalytic agent, 100 volumes of the roaster gas contain 7 volumes of sulphur dioxide, 10 volumes of oxygen and 83 volumes of nitrogen (from the air). The catalytic platinum is main tained at a temperature of about 750° F., since it is found that at this temperature the produc tion of sulphur trioxide is about 98 per cent of the theoretical production. The nitrogen that is present has no influence upon the reac tion, when the apparatus is working properly. The sulphur trioxide that is produced by this method needs only to be dissolved in previously prepared sulphuric acid containing more or less water, in order to yield an acid that is quite pure. It might naturally be supposed that water would be the best absorbent for the tri oxide; but it is found that an acid that contains from 97 to 99 per cent of lisS02 is a better absorbent ; and in the practical conduct of the process the trioxide is absorbed by an acid of this strength; the stronger acid that its solution yields being continuously drawn off and con tinuously replaced by fresh supplies of the 97 to 99 per cent acid, except when fuming acid of a very high degree of concentration is wanted. The minute details of the contact process are trade secrets and are carefully guarded.
The standard work on sulphuric acid manu facture is Lunge's 'Sulphuric Acid and Al kali.) Very good general accounts will be found, however, in Rogers' (Manual of Indus trial Chemistry) and Thorp's (Outlines of In dustrial Chemistry.)