The production of zieL tic acid from acetate of lime has assumed large proportions of late years, as much as 5000 to 6000 tons of brown acetate being anoually consumed by the trade. The chief beats of the industry are Lancashire, South Wales, and London. The product is known by the name of .,econd acid, or simply " seconds." The manufacture of acetic acid from the ac,etate, or, more correctly, the binacetate, of copper is of very old standing. From the time of the alchemists until a comparatively rec,ent date the bulk of the acetic acid used was obtained by this method, now pursued almost entirely for the subsequent manufacture of the well-known aromatic vinegar. The copper salt is prepared by dissolving verdigris in hot acetic acid and allowing the solution to cool. The acetate crystallizes out in dark-green crystals, which yield, upon the application of beat, strong acetic acid, slightly admixed with acetone. Tho process of distillation is shown in Fig. 25.
The crystals of acetate, which should be anhydrous and kept a, dry as p,ssible, so as to make a strong distillate, are intro luced into a suitable glass or earthenware retort A, and a gentle heat applied from a fire at B. The acid distils over, and collects in a scries of glass coolers, placed in cold water. The last of these is furnished with a 'Welter's tnbe, one arm of which dips into water or distilled vinegar in a vessel C, where all previously uncondensed vapours are collected. As soon as the acid comes over freely the temperature is raised, and gradually ioereased until distillation is comph•te, shown by the reeeivers becoming cool and the bubbles in the final condenser C ceasing altogether. The fire is then extinguished, the apparatus disconnected, and the receivers are emptied. The first acid that comes over is weak, diluted with whatever moisture may be in the copper salt.
H desirable, this may be collected separately. Various precautions should be observed during distillation, as the vapours come over exceedingly hot. The temperature may be easily regulated by observing the rate at which the air bubbles through the liquid in the final receiver D. If there is any vigorous displacement, the fire should be immediately checked. The water in the basins round the receivers should be renewed from time to time, but carefully, to prevent breakage, as the receivers get very hot. A trough or spout of running water may be advantageously substituted , 0 for the ordinary separate basins. The retort should be well filled with the copper salt, so as to have as small an admixture of air as possible, and both retort and receivers covered, the former with some non-conducting material, and the latter with cloths kept constantly damp. Finally, all joiutings should be perfectly dry before heat is applied.
During the distillation, fine portions of the copper salt are usually carried over, and give the distillate a pale-green colour. It is rectified by careful re-distillation, for which purpose the same, or a similar, apparatus may be used. If it be desired to obtain the whole of the acid from the copper salt, the last portions that come over should be collected in a separate receiver, as they are slightly empyreumatic. Twenty lb. of acetate should yield 10 lb. of crude, and si lb. of the re-distilled acid, of a specific gravity of about 1.08 when the contents of all three receivers are mixed together.
In the retort is left, after the first operation, a mixture of metallic copper with a little charcoal, amounting in all to about one-third of the weight of acetate used.
This process is still carried on in France to a considerable extent for the production of aromatic vinegars, for which purpose the acetic acid thus obtained is manipulated with various essential oils, camphor, and aromatic herbs, such as rosemary, thyme, &c. These preparations are known by several other names—acetum prophylacticum, Marseilles vinegar, vinazgre des quatre voleurs—the latter from the four thieves who, during a plague at Marseilles, plundered the sick and dead, escaping unhurt themselves through a plentiful use of the medicated vinegar. For further details of the manufacture see " Aromatic Vinegar." The last process for obtaining acetic acid by the distillation of its salts which it will be necessary to notice is that patented by Mr. H. B. Condy in 1868, and now in successful operation in Newcastle-upon-Tyne.
Proceeding from the already established fact that a solution of acetate of lime and chloride of calcium in equivalent proportions yields, when slowly evaporated, crystals of calcium aceto chloride (CaC1, + Mr. Condy demonstrated that the salt may be obtained readily and pure in quality even when a black or brown acetate of lime is employed. The patent may then be divided into two parts, (1) the preparing of the peculiar lime salt, (2) the production of acetic acid therefrom.
In the manufacture of the aceto-chloride of calcium, 12 cwt. of brown acetate of lime are dissolved iu 500 gals. of boiling water, and the solution is allowed to settle thoroughly, an operation usually requiring twenty hours, or thereabouts. The clear liquor is then transferred to another vessel and about 7 cwt. of dry chloride of calcium dissolved in it, the mixture being well stirred and heated during and after the addition of the chloride. This quantity required, 7 cwt., varies with the amount of impurity present ; or, if the chloride be used in the state of solution, the amount of water must be taken into account. In addition, however, to the quantity required to form the aceto-chloride, there should be an excess of about 20 per cent. The mixed solution of acetate of lime and chloride of calcium should stand at a specific gravity of 31° Beaumd; if weaker or stronger it should be concentrated or diluted until it registers the desired strength ; it is then drawn off to crystallize. The mother liquor from the first crystallization is evaporated till a strength of 31° Beaumd is attained, transferred to the crystallizing pans, and a second crop of crystals of aceto-chloride taken off. This process of crystallizing down the mothers is repeated until five crops are obtained, the liquor from the second crop being brought to 33° Beaumd, from the third and from tho fourth The last mother liquor retains nearly all the cmpyreu matie compounds of the original salt, and the five crops of crystals, which are of remarkably uniform character, contain all the lime salt worth extracting. The final "mothers" may bo distilled with sulphuric or hydrochloric acid in the manner already described for the production of a second quality of acetic acid.