Acetylene tetrachloride, or tetrachlorethane, C_2H_2Cl_4, is now produced for sale as a solvent for chlorine, sulphur, phosphorus, and organic substances such as fats. It may be obtained by the direct combination of acetylene and chlorine as explained in Chapter VI., but the liability of the reaction to take place with explosive violence would preclude the direct application of it on a commercial scale. Processes free from such risk have now, however, been devised for the production of tetrachlorethane. One patented by the Salzbergwerk Neu-Stassfurt consists in passing acetylene into a mixture of finely divided iron and chloride of sulphur. The iron acts as a catalytic. The liquid is kept cool, and as soon as the acetylene passes through unabsorbed, its introduction is stopped and chlorine is passed in. Acetylene and chlorine are then passed in alternately until the liquid finally is saturated with acetylene. The tetrachlorethane, boiling at 147° C., is then distilled off, and the residual sulphur is reconverted to the chloride for use again in the process. A similar process in which the chlorine is used in excess is applicable also to the production of hexachlorethane.
Dependent upon price, again, are several uses for calcium carbide as a metallurgical or reducing reagent; but as those are uses for carbide only as distinguished from acetylene, they do not fall within the purview of the present book.
When discussing, in Chapter III., methods for disposing of the lime sludge coming from an acetylene generator, it was stated that on occasion a use could be found for this material. If the carbide has been entirely decomposed in an apparatus free from overheating, the waste lime is recovered as a solid mass or as a cream of lime practically pure white in colour. Sometimes, however, as explained in Chapter II., the lime sludge is of a bluish grey tint, even in cases where the carbide decomposed was of good quality and there was no overheating in the generator. Such discoloration is of little moment for most of the uses to which the sludge may be put. The residue withdrawn from a carbide-to-water generator is usually quite fluid; but when allowed to rest in a suitable pit or tank, it settles down to a semi-solid or pasty mass which contains on a rough average 47 per cent. of water and 53 per cent. of solid matter, the amount of lime present, calculated as calcium oxide, being about 40 per cent. Since 64 parts by weight of pure calcium carbide yield 74 parts of dry calcium hydroxide, it may be said that 1 part of ordinary commercial carbide should yield approximately 1.1 parts of dry residue, or 2.1 parts of a sludge containing 47 per cent. of moisture; and sludge of this character has been stated by Vogel to weigh about 22.5 cwt. per cubic yard.
Experience has shown that those pasty carbide residues can be employed very satisfactorily, and to the best advantage from the maker's point of view, by builders and decorators for the preparation of ordinary mortar or lime-wash. The mortar made from acetylene lime has been found equal in strength and other properties to mortar compounded from fresh slaked lime; while the distemper prepared by diluting the sludge has been used most successfully in all places where a lime-wash is required, e.g., on fruit-trees, on cattle-pens, farm-buildings, factories, and the "offices" of a residence. Many of the village installations abroad sell their sludge to builders for the above-mentioned purposes at such a price that their revenue accounts are materially benefited by the additional income. The sludge is also found serviceable for softening the feed-water of steam boilers by the common liming process; although it has been stated that the material contains certain impurities--notably "fatty matter"--which becomes hydrolysed by the steam, yielding fatty acids that act corrosively upon the boiler-plates. This assertion would appear to require substantiation, but a patent has been taken out for a process of drying the sludge at a temperature of 150° to 200° C. in order to remove the harmful matter by the action of the steam evolved. So purified, it is claimed, the lime becomes fit for treating any hard potable or boiler- feed water. It is very doubtful, however, whether the intrinsic value of acetylene lime is such in comparison with the price of fresh lime that, with whatever object in view, it would bear the cost of any method of artificial drying if obtained from the generators in a pasty state.
When, on the other hand, the residue is naturally dry, or nearly so, it is exactly equal to an equivalent quantity of quick or slaked lime as a dressing for soil. In this last connexion, however, it must be remembered that only certain soils are improved by an addition of lime in any shape, and therefore carbide residues must not be used blindly; but if analysis indicates that a particular plot of ground would derive benefit from an application of lime, acetylene lime is precisely as good as any other description. Naturally a residue containing unspent carbide, or contaminated with tarry matter, is essentially valueless (except as mentioned below); while it must not be forgotten that a solid residue if it is exposed to air, or a pasty residue if not kept under water, will lose many of its useful properties, because it will be partially converted into calcium carbonate or chalk.
Nevertheless, in some respects, the residue from a good acetylene generator is a more valuable material, agriculturally speaking, than pure lime. It contains a certain amount of sulphur, &c., and it therefore somewhat resembles the spent or gas lime of the coal-gas industry. This sulphur, together, no doubt, with the traces of acetylene clinging to it, renders the residue a valuable material for killing the worms and vermin which tend to infest heavily manured and under-cultivated soil. Acetylene lime has been found efficacious in exterminating the "finger-and-toe" of carrots, the "peach-curl" of peach-trees, and in preventing cabbages from being "clubbed." It may be applied to the ground alone, or after admixture with some soil or stable manure. The residue may also be employed, either alone or mixed with some agglomerate, in the construction of garden paths and the like.
If the residues are suitably diluted with water and boiled with (say) twice their original weight of flowers of sulphur, the product consists of a mixture of various compounds of calcium and sulphur, or calcium sulphides--which remain partly in solution and partly in the solid state. This material, used either as a liquid spray or as a moist dressing, has been said to prove a useful garden insecticide and weed-killer.
There are also numerous applications of the acetylene light, each of much value, but involving no new principle which need be noticed. The light is so actinic, or rich in rays acting upon silver salts, that it is peculiarly useful to the photographer, either for portraiture or for his various positive printing operations. Acetylene is very convenient for optical lantern work on the small scale, or where the oxy-hydrogen or oxy-coal-gas light cannot be used. Its intensity and small size make its self-luminous flame preferable on optical grounds to the oil-lamp or the coal-gas mantle; but the illuminating surface is nevertheless too large to give the best results behind such condensers as have been carefully worked to suit a source of light scarcely exceeding the dimensions of a point. For lantern displays on very large screens, or for the projection of a powerful beam of light to great distances in one direction (as in night signalling, &c.), the acetylene blowpipe fed with pure oxygen, or with air containing more than its normal proportion of oxygen, which is discussed in Chapter IX., is specially valuable, more particularly if the ordinary cylinder of lime is replaced by one of magnesia, zirconia, or other highly refractory oxide.