A very much better article, however, is obtained by the following process. A pure brown acetate of lime is first prepared by thoroughly settling, or even filtering, the saturated liquors, after the naphtha has been expelled, skimming off all the impurities which rise to the surface, and transferring only the clear liquor to an evaporating pan. Here it is evaporated to one-half its bulk and hydrochloric acid is added until litmus is just reddened. The resinous bodies still contained in the lime liquors are thereby further separated out and the creasote and other volatile compounds decomposed and driven off by further evaporation. The quantity of hydrochloric acid which it is necessary to acid varies of course with the strength and quality of the crude acetic acid, but may be averaged at 5 lb. to every 33 gallons of the liquor left after the naphtha has been expelled. The solution of lime salt is now boiled down to dryness, being stirred frequently during desiccation to allow of the free emission of all vapours. By this means all volatile empyreumatic substances are driven off, and an almost inodlorous acetate of lime, of a brownish colour but remarkably pure quality, is obtained.
Iron plates heated by a fire underneath are sometimes'etnployed for the final drying and char ring ;—a more certain clearing of the salt is thereby ensured, especially when the quantity operated upon is large. A brown acetate is preferred to a grey, because by saturation of the acid liquors before distillation a certain amount of the impurities are carried down by the lime.
Thu lime salt, carefully prepared iu this way, is next distilled with hydrochlotic acid of about 1.16 sp. gr., and a very good acetic acid of 1.06 sp. gr. with 40 per cent. of anhydrous acid obtained. If the &Tarot ii in of the resinous compounds which rise to the surface, by skimming or filtration, is carefully attended to, the acid conies over perfectly colourless, with a slight ethereal odour which may be get rid of altog,ether by digestion with aoimal charcoal, or distillation with 3 per cent. of bichrome. Inasmuch aa an acid of 1.06 sp. gr. is rarely required by consumers, the mixture of lime salt and hydrochloric acid may be advantageously diluted with water—preferably before dis tillation, as the acetic aeid 'comes over more freely from a weak than a concentrated solution. The following proportions may be recommended:— 100 parts of lime acetate ; 95 parts of hydrochlmic acid ; 23 parts of water; winch should yield 93 to 100 parts of acetic acid of 1.05 sp. gr.
A slight excess of hydrochloric acid in this process is nut of much consequence. It can be
readily got rid of, and indeed when the acetic acid is subjected to further purification, is no evil. The excess, however, must be nu more than to render the distillate slightly turbid when tested Ve ith nitrate of silver. This point should be carefully attended te.
With the reservation already set forth, the process of distillation with hydrochloric acid is very much to be preferred to that with sulphuric. In deciding, however, which method to adopt, tho manufacturer must take into account his position and the relative mists of the raw materials, as there is not sufficient advantage on either side to counterbalance these incidental eircumstanees. The wear and tear of plant with hydrochloric acid is slightly less titan with sulphuric, and the resinous eompounda aro kept in ti harmless state. The sulphate of lime too, form( d in the distilla tion of the litne salt with sulphuric acid, is apt to form a crust on the bottian of the retort, and cause the metal to crack, besides wasting a certain amount of heat. When the process just described is carried out in its entirety, and carefully, au acid is obtained little inferior to that produced in the ordinary way from am tate of soda and sulphuric- acid.
A similar process has been adopted te some extent on the Continent fer the produetion of acetic acid from brandy inegar, but in tbia ease of course there is not the same amount of tenacious impurity tu be got rid of. Strong brandy vinegar, containing up to 1'2 per cent. of anhydrous acid, is saturated with lime, and the turbid solution filtered aud evtftorated to dryness iu tin iron pun. 'rile dry salt obtained is perfectly white, as the colouring matters contained in the solution are oxidized by the action of the air. The decomposition of the acetate of lime is effected by hydro chloric acid in the manner described, but as there is less admixture of foreign substances than in the acetate obtained from pyroligueous acid, a larger propurtion of hydrochloric acid is required for the decomposition, viz. about 130 parts of acid to 100 parts of lime salt.
Tho final purification of tho acetic acid obtained may be effected by any convenient method. No extraordinary plant is required fur these proee,ses. The usual naphtha stills and evaporat ing pans of the brown acetate process may be used, and for the distillation with hydrochloric acid a copper still with leaden or copper condensing pipes laid in water. Earthenware has been tried, but copper and lead arc preferable US lasting longer, and io no way do they contaminate the acetic acid if the operation be conducted with care.