# Popular Science Monthly/Volume 61/May 1902/Sulfuric Acid and its Manufacture by the Contact-Process

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* SULFURIC ACID AND ITS MANUFACTURE BY THE CONTACT-PROCESS.
By Dr. R. KNEITSCH.
THAT the subject on which you ask me to speak is of the greatest interest from an industrial standpoint needs no argument. The sulfuric acid industry is rightly looked upon as the foundation of inorganic technology, but it has also in the last few years, aside from its importance in so many departments of the various textile branches, become of equal necessity in the manufacture of the organic dye-stuffs. This is especially the case in the field of the alizarin dyes, and more recently, as was shown just a year ago from this platform by Heinrich Brunck, in the manufacture of synthetic indigo. If then in industries of such importance a complete revolution is taking place, a description of the discoveries and experiments which have made such a revolution possible can not fail to be of interest. In the limited time at my disposal it would be of course impossible to treat exhaustively the great mass of material bearing on the subject, and hence I must confine myself chiefly to the results of the work which has been carried on at the Badische Anilin und Soda-Fabrik.
The chemistry of the sulfuric acid manufacture is exceedingly simple, indeed chemistry seems at first sight to play but a subordinate role. Nevertheless this simple process presents an exceedingly interesting and important example of a gas reaction which occurs only at high temperatures. The reaction between sulfur dioxid and oxygen, although it is exothermic, takes place, as is well known, with extraordinary slowness, and therefore every effort has been made from an industrial standpoint to discover methods of hastening it by the use of catalytic substances. Indeed the lead chamber process itself depends upon the use of nitric acid and the lower oxides of nitrogen as such catalytic agents.
There are, however, many solid substances which have this same catalytic action, though only at high temperatures, and which in virtue of their state of aggregation suffer no loss in the process. These also possess a further great advantage over their gaseous rivals, in that their action goes on in the absence of water, thus rendering possible the ​production, not of dilute acid only, as in the chamber-process, but of the strongest acids and of sulfur trioxid itself.
Historical.
In the historical development of the contact-process we recognize four periods, the first of which was ushered in by the discovery in 1831 by Phillips of the catalytic action of platinum in the manufacture of sulfuric acid. The second period dates from the discovery by Woehler and Mahla, in 1852, of a similar catalytic action on the part of a number of other substances, and the explanation of the mechanism of the reaction in some of these cases. The third period, which begins with Winckler, is characterized by the use of certain gaseous mixtures which, according to the conception of that time, were especially favorable for the reaction from a quantitative standpoint. In the fourth period there is a return to the use of the gases from the pyrites-burners. As their ultimate goal, the early efforts, like those of the present period, seek by the aid of the catalytic process, entirely to replace the lead chambers in the manufacture of ordinary sulfuric acid, while the workers of the third period, profiting by the large number of earlier failures, confined themselves to the attempt to make the expensive fuming acid.
The discoverer of the catalytic action of platinum in general was Sir Humphry Davy, who found that when a heated platinum wire was brought into a mixture of oxygen or air with hydrogen, carbon monoxid, ethylene or cyanogen, it became red hot, and the gas mixture was burned, generally slowly, but sometimes with great rapidity. Three years later Edmund Davy discovered that finely divided platinum, prepared by evaporating the nitrate and treating the residue with alcohol, was brought to a glow by moistening with alcohol, the alcohol itself being ignited. Doebereiner found in 1822 that the residue left on heating ammonium platinum chlorid, acted in the same manner, and in 1823 he discovered that when a stream of hydrogen impinged on finely divided platinum, it took fire. The next year he brought out the celebrated Doebereiner lamp, which depended on this phenomenon.
The honor of having applied this catalytic action of platinum to the manufacture of sulfur trioxid belongs, as has already been said, to Peregrine Phillips, Jr., a vinegar-maker of Bristol. In 1831 he received an English patent for his discovery. This discovery of Phillips' was confirmed in 1832 by two distinguished German scientists, Doebereiner and Magnus. Seventeen years passed with no further developments, and then the Belgian chemist, Schneider, announced that he had solved the problem of manufacturing sulfuric acid without the aid of lead chambers. He believed that he had found in a specially prepared pumice-stone a catalytic substance of extraordinary activity, ​but although great claims were made, no practical results followed.
The hopes of this period are well characterized in a letter which was written by Clement-Desormes to Schneider in 1835, only four years after Phillips' discovery. A portion reads as follows:
Thus early was the goal definitely pointed out. . . . In the year 1852 came the discovery of Woehler and Mahla, that the oxids of copper, iron and chromium exert a similar catalytic action upon a mixture of sulfur dioxid and oxygen, to that of platinum sponge and platinum foil. The mixture of copper and chromium oxids was found to be particularly active in this respect. This was followed by the discovery of many other catalytic substances, such as 'spent' pyrites, highly heated quartz, platinized pumice, platinized asbestos and platinized clay.
This second period in historical development is, like the first, characterized by the attempt to solve the problem of the manufacture of ordinary sulfuric acid without recourse to the chamber process. These efforts, however, were crowned by no practical success, indeed it was not found possible even to make fuming sulfuric acid cheaply enough to compete with that obtained by the distillation of the iron vitriol shales.
We now come to the turning point of our subject, the work of Clemens Winckler. By his experimental investigations he reached the conclusion that for complete conversion into sulfuric acid, it was necessary that the mixture of sulfur dioxid and oxygen should be in stoichiometric proportions, that is, two volumes of the oxid to one of oxygen and that all other gases, even oxygen in excess, exercised an injurious effect upon the reaction. This mixture Winckler prepared very simply by the decomposition of ordinary aqueous sulfuric acid with heat, subsequently removing the water. In this manner, by combining the gases, he obtained sulfur trioxid or fuming sulfuric acid at will.
This conception of the suitable conditions for the success of the contact process seemed at that time exceedingly obvious, and well calculated to explain all the failures which had attended every attempt to utilize the gases of the pyrites-burners by Phillips' process. Winckler's work attracted great attention and for a long time dominated all further experimentation in connection with the contact process. At about this time a similar process for the manufacture of fuming acid was discovered in Messel's works and was protected by patents. Winckler also brought out a more advantageous method of preparing ​the contact substance, that of reducing the platinum by the use of formic acid salts. This keen discovery of Winckler's brought at once into existence a new industry, that of the synthetic manufacture of fuming sulfuric acid. A number of works immediately adopted Winckler's process, among them first the Badische Anilin-und Soda-Fabrik, while the monopoly of Stark in Bohemia was broken. The whole industry therefore owes Winckler a debt of gratitude for this advance in technology.
Further work in this field was wholly under Winckler's influence. This is true of the patent secured ten years later by Haenisch and Schroeder, who replaced pure oxygen by atmospheric air, retaining, however, the use of pure sulfur dioxid, and compensating for the diluting influence of the nitrogen by carrying on the reaction under pressure, in order, as the patent reads, that the molecules of the gases may be brought closer together. This process also was put into practical application in the Badische Anilin-und Soda-Fabrik.
Messel and Lunge proposed a method of obtaining a stoichiometric mixture and at the same time excluding the atmospheric nitrogen by burning the pyrites with pure oxygen. All these processes were unsuited by their very nature to compete with the lead chamber process and were necessarily confined to the manufacture of the fuming acid. Nevertheless attempts at the solution of the larger problem were by no means wanting; little, however, regarding them made its way to the notice of the public. Winckler especially published nothing regarding his new work along this line, so that it only became known last year, through the striking lecture of Lunge and Winckler in Hannover on the development of sulfuric acid manufacture, that at his instance it had been found possible at the Mulden works to convert from two thirds to three fourths of the sulfur dioxid in the pyrites-burner-gases into sulfuric acid.
Purification of the Gases.
The solution of the problem of the complete conversion of the burner-gases into sulfuric acid remained unsolved, and indeed at that time, as far as was known, theoretically or practically, was unsolvable. Nevertheless, as I attacked the problem at the Badische works, it was chiefly theoretical considerations which made the possibility of attaining this great goal seem not absolutely out of the question.
It is well known that in the lead chamber process there is always an excess of six volumes per cent. of oxygen in the gases as they leave the chambers. In working with pyrites-burner gases under similar conditions, there would naturally always be this excess of oxygen, whatever contact-method was employed for making the acid, and it could not be understood why in spite of such an excess of oxygen the reaction should not proceed quantitatively. The question was tested ​experimentally with gases which were intentionally much diluted, pure sulfur dioxid with a large excess of air being used, and it could be demonstrated that the reaction did not fully cease, under certain circumstances, until the sulfur dioxid was almost completely converted into sulfuric acid. Strangely enough, even when double the theoretical amount of air was present, it seemed to have no untoward influence upon the reaction, indeed it rather appeared as if the large excess of oxygen exercised a favorable effect upon the quantity of sulfur trioxid obtained from a given quantity of the dioxid. From this it followed that the earlier conceptions, according to which the dilution of the sulfur gas was unfavorable for the contact-process, must be submitted to a critical examination.
From now on the experiments were carried out upon the gases which came directly from the pyrites-burners. For this purpose the gases were brought directly from the pyrites-burners to the laboratory through a long lead pipe. This pipe acted as a long dust catcher, and the gases in their passage through it were thoroughly freed from every mechanical impurity, such as ashes, burner-dust, etc. The gases were further passed through several bottles filled with sulfuric acid, before they reached the contact mass. The experiments were very satisfactory, for quite as favorable results were attained as had been the case with the mixture of pure gases. No diminution in the activity of the contact mass could be observed, although the experiments extended over several days, and the hope seemed well justified that in this simple manner it would be possible to manufacture sulfuric acid directly from the burner-gases without loss of sulfur.
The experiments were now carried out on a larger scale. Here, however, it soon appeared that the activity of the contact-mass rapidly diminished in strength and finally ceased. The results were the same, even after the gases had been not only purified as in the laboratory experiments, by cooling in long tubes and washing repeatedly with sulfuric acid, but in addition had been passed through a dry coke and asbestos filter; they were then as pure as was technically possible. We were, therefore, obliged to consider the experiments on a large scale as a failure.
Although by these unlooked-for results a hard blow was given to the hope of success, nevertheless further experiments were undertaken in the laboratory for the purpose of investigating the cause of the apparently inevitable deterioration of the contact-mass.
The surprising observation was soon made that there are substances which, when present in exceedingly small quantities, are capable of inhibiting the catalytic action of platinum to an extraordinarily great degree. Among these substances are first of all arsenic, mercury and phosphorus, while antimony, bismuth, lead, iron, zinc and the other ​substances, which are liable to be present in the burner-gases, are only injurious to the extent to which, when present in large quantities, they cover up and choke the contact mass. The injurious action of arsenic, for example, is so great that when present to the amount of only one or two per cent, of the platinum in the contact-mass, the latter becomes completely inert. Hence by these investigations, it was incontrovertibly proved that there are substances which are capable of exerting a specific action, 'poisonous,' one might almost call it, upon the contact-mass. The question now was as to whether there was present in the burner-gases, in spite of the efforts at purification, such a substance.
There was, in fact, in these gases a trace of white fume of sulfuric acid which could not be removed, and this was found still to contain arsenic derived from the pyrites. But even if the failure of the process on a large scale was owing to a now known cause, the remedy was not apparent. At that time the complete precipitation of this white fume, the so-called 'huttenrauch,' was considered by the most distinguished experts as technically impossible.
Although after such long and careful work, the prospects of future success had become very slight, nevertheless, since the cause of the previous failures was known, fresh energy was applied to the solution of the new problem. This was nothing less than the attempt to free the burner-gases completely from all impurities, so that finally there should remain absolutely nothing but the pure gases, that is, sulfur dioxid, oxygen and nitrogen.
With an enormous expenditure of time, care, money and patience, experiment after experiment was undertaken in the effort to reach this goal, and it may well be said without exaggeration that it has been one of the most difficult problems of modern industry which has had to be solved, in order to render possible the present revolution in the manufacture of sulfuric acid. It would carry us too far to go into the particulars of the various experiments. Even since the process has been put into operation on a large scale, it has demanded several years of the most assiduous work, before it has become possible to look upon the purification of the burner-gases as absolutely assured. The great difficulty of the task lay in the fact that it was a continual struggle with an invisible enemy, as one might say, and that every mistake paid the penalty of permanent damage to the plant as regards the amount of production. The final result of these labors was that it was in fact found possible to free the burner-gases from every trace of every impurity, if after appropriate treatment and cooling, they are made to undergo a thorough, systematically continued scrubbing with water or sulfuric acid. This must be continued until both optical and chemical examination of the gas assures us of complete purification from every injurious substance. How this thorough scrubbing is carried out with ​the purifying fluids is a matter of indifference in the final result. Thorough scrubbing and wet filtration, each by itself, or both together, bring us at last to the same point. Only a few of the difficulties which appeared in the application of gas purification on a large scale may be mentioned here.
For the success of the process it had proved necessary that the gases should be cooled slowly. It is a curious fact, and one as yet without due explanation, that the sulfuric acid fume from the pyrites-burners is far more difficult to remove when the gases are cooled rapidly than when they are cooled slowly.
For this purpose long iron conductors were used, which were kept cool by the circulation of the air. As far as our knowledge went at that time, these iron conductors could occasion no untoward influence upon the contact-process; for when dry pyrites are used in the burners, the sulfuric acid in the gases has a concentration of at least ninety per cent. Should this act on the iron at all, it would result in the formation of sulfur dioxid, and this could of course do no damage in the process.
But now in spite of the fact that the gases were freed from every mechanical impurity, so that the optical test, which was at that time considered sufficient, betrayed not the slightest sign of any solid or liquid particles (for the sake of absolute certainty the gases were finally passed through a kind of wet cloth filter, somewhat like a filter press), still the contact-mass gradually diminished in activity. This diminution was very slow it is true, being apparent only after weeks or perhaps months, but it was nevertheless certain to occur. It was only after long and difficult labor that the presence of arsenic was again proved to be present in the contact mass, and this, after it was supposed that every trace of the element had been eliminated in the process of purification. The contact-mass, however, showed unmistakably that arsenic was in evidence, and the suspicion arose that the cause of all the trouble might be due to the action of the condensed sulfuric acid upon the iron cooling-conductors. Further investigation showed that this was probably the case, and that by this action some gas which contained arsenic must have been formed. This gas was probably arsin, the hydrid of arsenic.
A change in the arrangements was now made so that the condensed sulfuric acid could no longer come into contact with the iron conductors, and from this time on the contact-mass remained undiminished in its activity. It appears from this that, contrary to the generally received ideas, hydrogen can be evolved by the action of concentrated sulfuric acid upon iron, and that when arsenic is present, arsin may be also formed.
​With this indeed ended the hardest struggle which met us in the attempt to introduce the new sulfuric acid process into technical industry. It was, however, by no means the last contest. When the process came to be put in operation on a larger scale, new difficulties in purifying the gases appeared, whose cause was almost as problematical and unforeseen as those which have been already described.
When the pyrites-burners were used to their full capacity, fumes were formed which seemed to mock every effort at absorption. Their cause was finally found to be the presence of unconsumed sulfur in small quantities. Like the quickly cooled sulfuric acid fume, this sulfur proved exceedingly difficult to remove. But how could this unburned sulfur be detrimental, when in the contact-apparatus it would burn to sulfur dioxid and sulfur trioxid? The explanation rests in the fact that this sulfur fume again contains traces of arsenic. This also it was unconditionally necessary to remove.
A radical means for this was finally found in thoroughly mixing the gases while still hot, so that the combustion of the last trace of sulfur was ensured. This mixing was accomplished by the injection of steam, which was found to have other and not less important advantages. It especially served to dilute the concentrated sulfuric acid in the gases, so that they were no longer condensed in the preliminary cooling conductors, and hence the evolution of arsin was avoided; when finally condensed in the chief cooling apparatus which was made of lead, they were so much diluted that they ceased to corrode the metal. Furthermore the formation of hard dust-scale in the various conductors was prevented, and danger of these becoming stopped up was avoided.
In closing this lecture I may be permitted to place before you what a development the sulfuric acid manufacture has enjoyed in our works alone, since the introduction of this contact-process. The annual production of sulfuric acid anhydrid has been:
| In 1888 | 18,500 tons. |
| In 1894 | 39,000 tons. |
| In 1899 | 89,600 tons. |
| In 1900 | 116,000 tons. |