The existence in appreciable quantity of combined silicon as a normal impurity in acetylene seems still open to doubt. Calcium carbide frequently contains notable quantities of iron and other silicides; but although these bodies are decomposed by acids, yielding hydrogen silicide, or siliciuretted hydrogen, they are not attacked by plain water. Nevertheless Wolff and Gerard have found hydrogen silicide in crude acetylene, and Lewes looks upon it as a common impurity in small amounts. When it occurs, it is probably derived, as Vigouroux has suggested, from "alloys" of silicon with calcium, magnesium, and aluminium in the carbide. The metallic constituents of these substances would naturally be attacked by water, evolving hydrogen; and the hydrogen, in its nascent state, would probably unite with the liberated silicon to form hydrogen silicide. Many authorities, including Keppeler, have virtually denied that silicon compounds exist in crude acetylene, while the proportion 0.01 per cent. has been given by other writers as the maximum. Caro, however, has stated that the crude gas almost invariably contains silicon, sometimes in very small quantities, but often up to the limit of 0.8 per cent.; the failure of previous investigators to discover it being due to faulty analytical methods. Caro has seen one specimen of (bad) carbide which gave a spontaneously inflammable gas although it contained only traces of phosphine; its inflammability being caused by 2.1 per cent. of hydrogen silicide. Practically speaking, all the foregoing remarks made about phosphine apply equally to hydrogen silicide: it burns to solid silicon oxide (silica) at the burners, is insoluble in water, and is spontaneously inflammable when alone or only slightly diluted, but never occurs in good carbide in sufficient proportion to render the acetylene itself inflammable. According to Caro the silicon may be present both as hydrogen silicide and as silicon "compounds." A high temperature in the generator will favour the production of the latter; an apparatus in which the gas is washed well in lime-water will remove the bulk of the former. Fraenkel has found that magnesium silicide is not decomposed by water or an alkaline solution, but that dilute hydrochloric acid acts upon it and spontaneously inflammable hydrogen silicide results. If it may be assumed that the other silicides in commercial calcium carbide also behave in this manner it is plain that hydrogen silicide cannot occur in crude acetylene unless the gas is supposed to be hurried out of the generator before the alkaline water therein has had time to decompose any traces of the hydrogen silicide which is produced in the favouring conditions of high temperature sometimes prevailing. Mauricheau-Beaupré has failed to find silica in the products of combustion of acetylene from carbide of varying degrees of purity. He found, however, that a mixture of strong nitric and hydrochloric acids (aqua regia), if contaminated with traces of phosphoric acid, dissolved silica from the glass of laboratory vessels. Consequently, since phosphoric acid results from the phosphine in crude acetylene when the gas is passed through aqua regia, silica may be found on subsequently evaporating the latter. But this, silica, he found, was derived from the glass and not through the oxidation of silicon compounds in the acetylene. It is possible that some of the earlier observers of the occurrence of silicon compounds in crude acetylene may have been misled by the solution of silica from the glass vessels used in their investigations. The improbability of recognisable quantities of silicon compounds occurring in acetylene in any ordinary conditions of generation is demonstrated by a recent study by Fraenkel of the composition of the deposit produced on reflectors exposed to the products of combustion of a sample of acetylene which afforded a haze when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by volume of phosphine.
PURIFYING MATERIALS.--When acetylene first began to be used as a domestic illuminant, most generator builders denied that there was any need for the removal of these carbide impurities from the gas, some going so far as to assert that their apparatus yielded so much purer an acetylene than other plant, where purification might be desirable, that an addition of a special purifier was wholly unnecessary. Later on the more responsible members of the trade took another view, but they attacked the problem of purification in a perfectly empirical way, either employing some purely mechanical scrubber filled with some moist or dry porous medium, or perhaps with coke or the like wetted with dilute acid, or they simply borrowed the processes adopted in the purification of coal-gas. At first sight it might appear that the more simple methods of treating coal-gas should be suitable for acetylene; since the former contains two of the impurities--sulphuretted hydrogen and ammonia--characteristic of crude acetylene. After removing the ammonia by washing with water, therefore, it was proposed to extract the sulphur by passing the acetylene through that variety of ferric hydroxide (hydrated oxide of iron) which is so serviceable in the case of coal-gas. The idea, however, was quite unsound: first, because it altogether ignores the phosphorus, which is the most objectionable impurity in acetylene, but is not present in coal- gas; secondly, because ferric hydroxide is used on gasworks to extract in a marketable form the sulphur which occurs as sulphuretted hydrogen, and true sulphuretted hydrogen need not exist in well-generated and well- washed acetylene to any appreciable extent; thirdly, because ferric hydroxide is not employed by gasmakers to remove sulphur compounds (this is done with lime), being quite incapable of extracting them, or the analogous sulphur compounds of crude acetylene.
About the same time three other processes based on somewhat better chemical knowledge were put forward. Pictet proposed leading the gas through a strong solution of calcium chloride and then through strong sulphuric acid, both maintained at a temperature of -20° to -40° C., finally washing the gas in a solution of some lead salt. Proof that such treatment would remove phosphorus to a sufficient degree is not altogether satisfactory; but apart from this the necessity of maintaining such low temperatures, far below that of the coldest winter's night, renders the idea wholly inadmissible for all domestic installations. Willgerodt suggested removing sulphuretted hydrogen by means of potassium hydroxide (caustic potash), then absorbing the phosphine in bromine water. For many reasons this process is only practicable in the laboratory. Bergé and Reychler proposed extracting both sulphuretted hydrogen and phosphine in an acid solution of mercuric chloride (corrosive sublimate). The poisonousness of this latter salt, apart from all other objections, rules such a method out.
BLEACHING POWDER.--The next idea, first patented by Smith of Aberdeen, but fully elaborated by Lunge and Cedercreutz, was to employ bleaching- powder [Footnote: Bleaching-powder is very usually called chloride of lime; but owing to the confusion which is constantly arising in the minds of persons imperfectly acquainted with chemistry between chloride of lime and chloride of calcium--two perfectly distinct bodies--the less ambiguous expression "bleaching-powder" will be adopted here.] either in the solid state or as a liquid extract. The essential constituent of bleaching-powder from the present aspect is calcium hypochlorite, which readily oxidises sulphuretted hydrogen, and more particularly phosphine, converting them into sulphuric and phosphoric acids, while the acetylene is practically unattacked. In simple purifying action the material proved satisfactory; but since high-grade commercial bleaching-powder contains some free chlorine, or some is set free from it in the purifier under the influence of the passing gas, the issuing acetylene was found to contain chlorine, free or combined; and this, burning eventually to hydrochloric acid, is hardly less harmful than the original sulphur compounds. Moreover, a mixture of acetylene, chlorine, and air is liable to catch fire of itself when exposed to bright sunlight; and therefore the use of a bleaching-powder purifier, or rather the recharging thereof, was not unattended by danger in the early days. To overcome these defects, the very natural process was adopted of diluting the bleaching-powder, such diluent also serving to increase the porosity of the material. A very unsuitable substance, however, was selected for the purpose, viz., sawdust, which is hygroscopic organic, and combustible. Owing to the exothermic chemical action between the impurities of the acetylene and the bleaching-powder, the purifying mass became heated; and thus not only were the phenomena found in a bad generator repeated in the purifying vessel, but in presence of air and light (as in emptying the purifier), the reaction proceeded so rapidly that the heat caused inflammation of the sawdust and the gas, at least on one occasion an actual fire taking place which created much alarm and did some little damage. For a time, naturally, bleaching-powder was regarded as too dangerous a material to be used for the purification of crude acetylene; but it was soon discovered that danger could be avoided by employing the substance in a proper way.
HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.--Setting aside as unworthy of attention certain compositions offered as acetylene purifying materials whose constitution has not been divulged or whose action has not been certified by respectable authority, there are now three principal chemical reagents in regular use. Those are chromic acid, cuprous chloride (sub- or proto-chloride of copper), and bleaching- powder. Chromic acid is employed in the form of a solution acidified with acetic or hydrochloric acid, which, in order to obtain the advantages (see below) attendant upon the use of a solid purifying material, is absorbed in that highly porous and inert description of silica known as infusorial earth or "kieselguhr." This substance was first recommended by Ullmann, and is termed commercially "heratol" As sold it contains somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is used as a solution in strong hydrochloric acid mixed with ferric chloride, and similarly absorbed in kieselguhr. From the name of its proposer, this composition is called "frankoline." It will be shown in Chapter VI. that the use of metallic copper in the construction of acetylene apparatus is not permissible or judicious, because the gas is liable to form therewith an explosive compound known as copper acetylide; it might seem, therefore, that the employment of a copper salt for purification courts accident. The objection is not sound, because the acetylide is not likely to be produced except in the presence of ammonia; and since frankoline is a highly acid product, the ammonia is converted into its chloride before any copper acetylide can be produced. As a special acetylene purifier, bleaching-powder exists in at least two chief modifications. In one, known as "acagine," it is mixed with 15 per cent. of lead chromate, and sometimes with about the same quantity of barium sulphate; the function of the latter being simply that of a diluent, while to the lead chromate is ascribed by its inventor (Wolff) the power of retaining any chlorine that may be set free from the bleaching-powder by the reduction of the chromic acid. The utility of the lead chromate in this direction has always appeared doubtful; and recently Keppeler has argued that it can have no effect upon the chlorine, inasmuch as in the spent purifying material the lead chromate may be found in its original condition unchanged. The second modification of bleaching-powder is designated "puratylene," and contains calcium chloride and quick or slaked lime. It is prepared by evaporating to dryness under diminished pressure solutions of its three ingredients, whereby the finished material is given a particularly porous nature.
It will be observed that both heratol and frankoline are powerfully acid, whence it follows they are capable of extracting any ammonia that may enter the purifier; but for the same reason they are liable to act corrosively upon any metallic vessel in which they are placed, and they therefore require to be held in earthenware or enamelled receivers. But since they are not liquid, the casing of the purifier can be safely constructed of steel or cast iron. Puratylene also removes ammonia by virtue of the calcium chloride in it. Acagine would probably pass the ammonia; but this is no real objection, as the latter can be extracted by a preliminary washing in water. Heratol changes, somewhat obscurely, in colour as it becomes spent, its original orange tint, due to the chromic acid, altering to a dirty green, characteristic of the reduced salts of chromium oxide. Frankoline has been asserted to be capable of regeneration or revivification, i.e., that when spent it may be rendered fit for further service by being exposed to the air for a time, as is done with gas oxide; this, however, may be true to some extent with the essential constituents of frankoline, but the process is not available with the commercial solid product. Of all these materials, heratol is the most complete purifier of acetylene, removing phosphorus and sulphur most rapidly and thoroughly, and not appreciably diminishing in speed or efficiency until its chromic acid is practically quite used up. On the other hand, heratol does act upon pure acetylene to some extent; so that purifiers containing it should be small in size and frequently recharged. In one of his experiments Keppeler found that 13 per cent. of the chromic acid in heratol was wasted by reacting with acetylene. As this waste of chromic acid involves also a corresponding loss of gas, small purifiers are preferable, because at any moment they only contain a small quantity of material capable of attacking the acetylene itself. Frankoline is very efficacious as regards the phosphorus, but it does not wholly extract the sulphur, leaving, according to Keppeler, from 0.13 to 0.20 gramme of the latter in every cubic metre of the gas. It does not attack acetylene itself; and if, owing to its free hydrochloric acid, it adds any acid vapours to the purified gas, these vapours may be easily removed by a subsequent passage through a vessel containing lime or a carbide drier. Both being essentially bleaching-powder, acagine and puratylene are alike in removing phosphorus to a satisfactory degree; but they leave some sulphur behind. Acagine evidently attacks acetylene to a slight extent, as Keppeler has found 0.2 gramme of chlorine per cubic metre in the issuing gas.
Although some of these materials attack acetylene slightly, and some leave sulphur in the purified gas, they may be all considered reasonably efficient from the practical point of view; for the loss of true acetylene is too small to be noticeable, and the quantity of sulphur not extracted too trifling to be harmful or inconvenient. They may be valued, accordingly, mainly by their price, proper allowance being made for the quantity of gas purified per unit weight of substance taken. This quantity of gas must naturally vary with the proportion of phosphorus and sulphur in the crude acetylene; but on an average the composition of unpurified gas is what has already been given above, and so the figures obtained by Keppeler in his investigation of the subject may be accepted. In the annexed table these are given in two forms: (1) the number of litres of gas purified by 1 kilogramme of the substance, (2) the number of cubic feet purified per lb. It should be noted that the volumes of gas refer to a laboratory degree of purification; in practice they may all be increased by 10 or possibly 20 per cent.
_________________________________________________
| | | |
| | Litres | Cubic Feet |
| | per Kilogramme. | per Lb. |
|______________|___________________|______________|
| | | |
| Heratol | 5,000 | 80 |
| Frankoline | 9,000 | 144 |
| Puratylene | 10,000 | 160 |
| Acagine | 13,000 | 208 |
|______________|___________________|______________|
Another method of using dry bleaching-powder has been proposed by Pfeiffer. He suggests incorporating it with a solution of some lead salt, so that the latter may increase the capacity of the calcium hypochlorite to remove sulphur. Analytical details as to the efficiency of this process have not been given. During 1901 and 1902 Bullier and Maquenne patented a substance made by mixing bleaching-powder with sodium sulphate, whereby a double decomposition occurs, sodium hypochlorite, which is equally efficient with calcium hypochlorite as a purifying material, being produced together with calcium sulphate, which, being identical with plaster of Paris, sets into a solid mass with the excess of water present, and is claimed to render the whole more porous. This process seemed open to objection, because Blagden had shown that a solution of sodium hypochlorite was not a suitable purifying reagent in practice, since it was much more liable to add chlorine to the gas than calcium hypochlorite. The question how a solidified modification of sodium hypochlorite would behave in this respect has been investigated by Keppeler, who found that the Bullier and Maquenne material imparted more chlorine to the gas which had traversed it than other hypochlorite purifying agents, and that the partly foul material was liable to cause violent explosions. About the same time Rossel and Landriset pointed out that purification might be easily effected in all generators of the carbide-to-water pattern by adding to the water of the generator itself a quantity of bleaching-powder equivalent to 5 to 20 grammes for every 1 kilogramme of carbide decomposed, claiming that owing to the large amount of liquid present, which is usually some 4 litres per kilogramme of carbide (0.4 gallon per lb.), no nitrogen chloride could be produced, and that owing to the dissolved lime in the generator, chlorine could not be added to the gas. The process is characterised by extreme simplicity, no separate purifier being needed, but it has been found that an introduction of bleaching-powder in the solid condition is liable to cause an explosive combination of acetylene and chlorine, while the use of a solution is attended by certain disadvantages. Granjon has proposed impregnating a suitable variety of wood charcoal with chlorine, with or without an addition of bleaching-powder; then grinding the product to powder, and converting it into a solid porous mass by the aid of cement. The material is claimed to last longer than ordinary hypochlorite mixtures, and not to add chlorine to the acetylene.
SUBSIDIARY PURIFYING MATERIALS.--Among minor reagents suggested as purifying substances for acetylene may be mentioned potassium permanganate, barium peroxide, potassium bichromate, sodium plumbate and arsenious oxide. According to Benz the first two do not remove the sulphuretted hydrogen completely, and oxidise the acetylene to some extent; while potassium bichromate leaves some sulphur and phosphorus behind in the gas. Sodium plumbate has been suggested by Morel, but it is a question whether its action on the impurities would not be too violent and whether it would be free from action on the acetylene itself. The use of arsenious oxide dissolved in a strong acid, and the solution absorbed in pumice or kieselguhr has been protected by G. F. Jaubert. The phosphine is said to combine with the arsenic to form an insoluble brownish compound. In 1902 Javal patented a mixture of 1 part of potassium permanganate, 5 of "sulphuric acid," and 1 of water absorbed in 4 parts of infusorial earth. The acid constantly neutralised by the ammonia of the crude gas is as constantly replaced by fresh acid formed by the oxidation of the sulphuretted hydrogen; and this free acid, acting upon the permanganate, liberates manganese peroxide, which is claimed to destroy the phosphorus and sulphur compounds present in the crude acetylene.