The returns from carbonizers of this description are somewhat unreliable. It is claimed that they not only get through very much more work, but also give more satisfactory results in the way of yields and costa than the ordinary process of timber distillation. In estimates of this kind it must be remembered that against a very large amount of work there is the increased cost of carrying the work on, and, probably from the more or less complete disintegration of the wood, the acid pro duced is contaminated with resinous and oily substances more intensely difficult to get rid of than is usually the case. Moreover, the charcoal produced is comparatively useless, and most of the woods used in dyeing, e. g. logwood, are not well adapted for distillation. Still, where an ample and low priced supply of sawdust or spent dye-woods is obtainable—the latter are often to be had for the carting away—the cost of the products of the distillation of such materials must be low, and a further saving is effected in carriage, inasmuch as the process can he advantageously adopted in large towns where the pyroligneous acid, naphthas, and tar compounds can be readily and imme diately utilized. It is unfortunate that the charcoal has.to be obtained in a wet state, as it does not pay to dry it, and even when dried and ground it makes but an inferior "blacking" Probably something might be clone by delivering it into tar and working it up into a patent fuel. Several attempts have been made to cool it in closed chambers without the aid of water, but its fine state of division renders it peculiarly liable to combustion when it is exposed to the air.
An interesting development of the treatment of sawduat and similar woody material would he the absorption by them of waste !ignore and the recovery of the absorbed substancea from the charcoal after distillation.
21.
Superheated steam has been occasionally tried as a carbonizing agent in aubatitution for the ordinary coal fire, and has been the subject of several patents, but the difficulty encountered by obtaining the distillates in'only a diluted form has never been overcome. Moreover, direct methods, such as those of Halliday and Bowera, have been devised and satisfactorily worked out, and the employment of waste wood producta as a source of pyroligneous acid has of late been very much restricted by their extended utilization in other directions—for bedding and building purposes, &c. The chief value of superheated steam, as will be readily apparent, lies in its adaptability to wood in a finely divided state, in defiling with which the ordinary oven or cylinder breaks down.
Vurioua processes have been set on foot for the extraction of acetic acid from the woody fibres used in paper making—in the preparation of "wood pulp "—the agent employed being steam at a high presaure, to avoid carbonization of the wood. The patent of Mr. George Fry, 1869, may be cited as an example. Insuperable difficulties have, however, been met with in the separation of the acetic acid from the methylic alcohol, formic acid, resins, &c., with which it is intimately
mixed, and the processes have never been worked on any large manufacturing scale. The same must be said of the proposals to separate the acid from the accompanying volatile products by preaenting to it, during carbonization of the wood, a aubstance with which it, and it alone, can combine. Desirable as some such process may be, and at first sight aeemingly easy of accomplish ment, only an imperfect product, of uncertain constitution, has been obtained, As a step, however, in the right direction, Mr. Steedman's pocess (patented 1873) for the purification of the crude product of diatillation ahould be noticed. He proposes to pasa the impure acid in a state of vapour through' a hydrocarbon, oil, or fat, kept sufficiently heated to throughout in a liquid state, and preferably at a alightly higher temperature than the acetic acid vapour, to prevent less by condensation. The process ia conducted is " a copper vessel, of a rectangular form, about 5 ft. long, 1 ft, wide, and 2 ft. 9 in. deep. Thia vessel hna fixed inside of it three partitions of copper or wood, horizontal in cross section, but slightly inclined lon gitudinally. The partitions are open at alternate ends, and the vessel being filled with paraffin or other purifying substance, the acetic acid, which is introduced from the usual distillatory apparatus by a pipe leading in beneath the closed end of the lowest partition, travels along through the paraffin from end to end beneath the partitions, and is filially led from the top of the vessel to an ordinary condensing apparatus. The paraffin or other purifying substance in the vessel ia kept heated by a coiled ateam-pipe or steam jacket, and is withdrawn from the veasel whenever it is fully charged with impurities from the acetic acid." Difficulties in regulating this process, its imperfect operation, except after repeated absorptiona and waste of the purifying agents, have militated against ita success. The worda of the patent are quoted in the hope of drawing the attention of manufacturera to the desirability of improving upon the pri.aent roundabout way of obtaining a pure acetic acid.
Pyroligneoua acid is chiefly prepared for the production of some of the acetates—lime, lead, iron, and copper ; also, but to a comparatively small extent, for use as an antiputrescent. About fifty makers are in the trade in England, the chief localities where the manufacture is carried on being Lancaabire, Yorkshire, and adjacent counties, and South Wales. There are also a few worka in Gloucestershire, Somersetshire, and in the neighbourhood of London. The cost of a plant to work, say, 45 tons of wood per week, with acetate of lime process complete, is about 5000/. Distillation is also largely carried on in certain parts of France, Belgium, Germany, Russia, and Sweden, the continental processes being somewhat different from the English, and having a more definite reference to the article which it is chiefly desired to produce.