Owing to the exceedingly variable character of the chief raw material, wood, it is difficult to give definite yields and costs. They may, however, be approximately ealculated from the follow ing data. One ton of wood, costing 14s. delivered to the ovens, will yield 2} to 3 ewt. of bark, and, if fairly dry, 5 ewt. of best charecal, 1i to 2 cwt. of best brown acetate, or 11. of grey, 1i to 2 gals. of " miscible " naphtha at 60° over proof, or 2} gals. of "solvent" naphtha at 45°. The labour in a works using, say, 45 tons of wood per week, will involve two men to charge and draw the ovens, two firemen, one neutralizer, one man to attend to the naphtha stills and condensers, one to work off the acetate of lime, carry it into the drying house, and spread and turn it while drying, and one general labourer. In these data no mention is made of the constantly varying incidental expenses entailed by wear and tear ; and it is assumed that the works are well planned and substantially built. The consumption of fuel in such a works should not exceed 12 tons per week, even where the waste gases are not utilized.
The following tables, drawn up by Stolze, are interesting, as showing the amount and strength of products obtained from various woods :— The amount of charcoal obtained depends to a remarkable extent upon the length of time allowed for carbonization. Karsten gives the following interesting results of a series of experi ments upon air-dried timber :— Many improvements and modifications of tho process of wood distillation have been pmposed, the most notablo baying D.1 renc, to the treatment of wood in a finely divided state—sawdust, and the exhausted residue of variuus manufactures, such as tanning and dyeing. These bye-products, in their spent condition only a nuisance to tbe manufacturer, still contain all the elements required for tbe production of acetic acid. Thcir finely divided state has, however, pre,sented a serious obstacle to their destructive distillation, as they form a dead mass in the retort, and allow of only partial surface carbonization. The processes of Messrs. Halliday and Bowers, both of which have met with considerable attention at the hands of manufilettwors, overcome this difficulty by keeping the woody material in the retort in a constant state of forward motion. Mr. Halliday's process is shown in front and back elevation and longitudinal section in Figs. 15, 16, and 17. The materials are put into a hopper A, Fig. 17, from which an endless screw B carries them forward into the cylindrical .retorts C, Figs. 16 and 17, Here they are taken up by a second screw D, and moved forward to the other end of the retort. The process of distillation is precisely the same as in the case of the ordinary timber, the charcoal issuing through a pipe E, into IA vessel F, filled with water, and the gases passing off np G to the condenser. The fireplace is shown at H, Figs. 15 and 17.
Mr. Bower's apparatus, adopted by four or five manufacturers in Lancashire and Yorkshire, is shown in Figs. 18, 19, and 20. Fig.18 gives a longitudinal section of the drying flat, heated by the waste heat of the furnace ; Fig. 19 a long,itudinal section of the furnace itself, and Fig. 20 a plan of drying flat and furnace arranged conveniently side by side.
Referring to Fig. 18, the sawdust, if wet, or waste dye-wood, is tipped into a hopper A placed at One end of the drying bed, and slightly above it. The material escapes from tho hopper, falls upon the drying bed, and is taken hold of by a series of scrapers fixed upon endless chains B B B which travel upon revolving drums C C. By these it is moved slowly over the heated plates to the far end of the flat, and falls over the edge into any convenient receptacle. It is then taken up by elevators and fed into the furnace or retort through a hopper D, Fig. 19. The grooved rollers shown at the bottom of the hopper, by which the supply of material to the furnace is regulated, are geared to revolve in contrary directions, and are set so that the points of the teeth approach each other very closely. They also revolve as nearly as possible in contact with the sides of the hopper, or with plates that can be " set up " towards the rollers so as to prevent escape of the products of distillation through the hopper. The woody material (admitted as naay be required, and in the manner shown, into the furnace) is taken hold of and moved slowly along the bed towards the fire end by an apparatus exactly similar to that already described as part of the drying arrangement, and is discharged, perfectly carbonized, into a receptacle E filled with water, so constructed as to form a luting and prevent the escape of gases from the furnace. The charcoal in a finely divided state soon fills tbis vessel, and is cleared off by the revolving seraper shown at F. The products of theiillation pass off to the condensers through the pipes U U, and are collected in the usual manner.
The many excellent points of this apparatus, which is very much to be preferred to the Halliday retort, will be readily appreciated, and when the drying bed and furnace are arranged side by side, as shown in Fig. 20, the machinery and steam-power required are reduced to a minimum. It will be noted that the material is never handled from the time of entering the drying bed to its exit from the furnace as charcoal, and as all the motions are slow the wear and tear of machinery are not great. One defect might be easily remedied. The scrapers now in use, and shown in cross section Figs. 18 and 19, move forward the woody material in small but unbroken heaps, the tops of which never get thoroughly dried. It would be better to serrate these scrapers alternately, as shown in Fig. 21. They would then act rather as claws, breaking up the lines of material into detached and small portions.