December, 1900.] 



519 



Timbers. 



A special form of kiln is said to be 

 adopted on many of the large wood- 

 distillation plants in Sweden, capable 

 of treating wood in all conditions — 

 sodden forest timbers, saw-mill waste 

 and ordinary air-dried logs. The general 

 arrangement of the furnace is in the 

 form of a long tunnel through which 

 pass open steel cars, on which the 

 wood is vertically stacked. The carts 

 are taken through the furnace at an 

 average rate of 22 per diem for per- 

 fectly wet wood, and about 36 per diem 

 for air-dried timber. Uncondensed gases 

 from the distillation are returned to 

 the furnace and burnt, thus practically 

 dispensing with the addition of any 

 further fuel. 



On the application of heat to the 

 retort the substance of the wood is 

 charred with the formation of volatile 

 products, which are driven off. Those 

 which are condensable are liquified 

 again in the condensers, and collected 

 in suitable receivers. 



Products op Distillation. 

 Although the process of distillatiou 

 is practically identical whatever the 

 kind of wood employed, the products 

 obtained are different according to 

 whether "hard" or "soft" wood is 

 used. 



Products from Hard Wood.— Most of 

 the wood distilled is hard, such a3 

 beech, birch, maple, et2., and is practi- 

 cally free from resinous constituents. 

 Four chief products are obtained, viz., 

 (1) an inflammable gas which escapes 

 from the condenser, and should, if possi- 

 ble, be returned to the fire to aid in 

 heating the retorts ; (2) a watery liquid 

 known as " pyroligneous acid "; (3) wood 

 tar, which is condensed with the 

 pyroligneous acid ; and (4) charcoal, 

 which remains behind in the retorts. 



The tar itself may be used as the fuel 

 to create the heat necessary for distil- 

 lation, and in this case it is sprayed with 

 a jet of steam and used in a similar 

 manner to " oil fuel." In this manner 

 the use of coal as furl may be avoided 

 completely. 



The charcoal is allowed to cool for a 

 day or two either before removal or in 

 specially-devised " cooling chambers " 

 out of contact with the air, or else it is 

 drenched with water immediately after 

 extraction from the retorts to prevent 

 its spontaneous combustion in the air 

 and consequent loss. 



Purification of the Products.— -The tar 

 and the reddish-brown pyroligneous 

 acid are run off together into large 

 settling vats, where separation is effected 

 by the tar collecting at the bottom. 



Each is then distilled separately. As 

 the tar and the pyroligneous acid are to 

 a certain extent mutually soluble, the 

 residue from the latter contains a quan- 

 tity of tar, and the distillate from the 

 former is distinctly acid in character. 

 If the tar is to be utilized as fuel the 

 acid is removed by passing the acid tar 

 downwards over " bafflers," where it is 

 met by an upward current of steam or 

 vapours from the stills to which the 

 pyroligneous acid is given up. The chief 

 products obtained by distilling the wood 

 tar(which is itself an article of commerce) 

 are light and heavy wood oils, wood creo- 

 sote and the well-known product wood 

 pitch, which is left behind in the stills. 



In the most modern treatment of the 

 pyroligneous acid an arrangement of 

 plant known as the " three-still " system 

 is adopted. In the first and largest still 

 the crude acid is heated, whereby the 

 volatile acetic acid and "wood spirit" 

 are driven off, and most of the dissolved 

 tar is left behind. The outlet", pipe 

 carries the vapours into the second still 

 and passes them through a thin cream of 

 lime and water. This absorbs the acetic 

 acid with the production of calcium 

 acetate, but does not affect the wood 

 spirit, which passes on and is treated 

 afresh in the third still in order to 

 remove the last traces of acetic acid. In 

 this manner the distillate eventually ob- 

 tained is free from acid, and by suitable 

 rectification can be made to yield pure 

 methyl alcohol, commercial wood spirit 

 and wood naphtha. The thin paste of 

 calcium acetate remaining in the stills 

 is run out and concentrated in large iron 

 pans until it contains about 84 per cent, 

 of "acetate of lime." This is the pro- 

 duct which comes on the markets as 

 " grey acetate of lime." 



Application of the Products.— The uses 

 of charcoal have been enumerated above. 

 Most of the acetate of lime is subjected to 

 dry distillation, and is thus converted 

 into acetone aud calcium carbonate 

 (chalk). The increasing consumption of 

 acetone in chemical industries has ren- 

 dered this process one of considerable 

 importance. Besides being a useful 

 solvent for varnish resins, acetone is 

 largely used in the manufacture of 

 smokeless explosives and celluloid arti- 

 cles, and its use will very probably be 

 greatly extended in the near future. 

 The present price of acetone is about £60 

 per ton, and it takes about 40 tons of 

 wood to produce one ton of acetone. Tlie 

 by-products in the distillation, so-called 

 " acetone oils," are also useful as " paint 

 removers." Their formation is due to 

 the presence in the pyroligneous acid of 

 organic acids higher than acetic acid. 



