To avoid repetition, it may be stated here that the treatments of all the products are pretty much the samo. Dilute acids are used to separate the bases, and alkaline solutions are enaployed to remove tbe acids, the process being called " washing." In particular cases, steaming is used to facilitate it, when special appliances are necessary to avoid evaporation.
The following synoptical arrangement will give a clear idea of the primary products obtained by ordinary fractionizing on the English system :— When the tar is distilled so as to yield a hard pitch, by removing as much as possible of the anthraceoe oils, it is converted into softer kinds by addition of the creosote oils.
This list gives only the principal constituents of each product ; by repeated fractionizings, their proximate constituents may be isolated, according to the purity required for the commercial article. The tar distiller rarely has to carry his operations so far as to obtain more than the approximate separation of the series contained in the second column. As the preliminary treat ment of most of these is almost identical, it will suffice to give one general description.
Washing.—Formerly, the crude naphtha and light oils, freed from their watery portions, were pumped into large, lead-lined, wooden vats, and agitated by perforated rotating fans, for some hours, with sulphuric, acid and caustic soda solutions, each washing being followed by a good agitation with water. The acid and other liquids must enter in a fine spray. Now, it is the general practice to re-run the crude naphtha before washing, as many impurities are thus removed, and a saving of material is effected. Sulphuric is preferable to other acids, because it more easily carbonizes the impurities, and causes them to separate out as a soft resinous mass, in addition to which, it forms, with the bases, salts which can be removed by washing. About 3 per cent. by measure of acid (e. o. v.) is gradually diffused through the liquids, and the whole is well agitated ; on standing, the impurities collect on the bottom ; the naphtha is drawn off into another tank, to be agitated' for five or six hours with 6 per cent. of acid. After settling, it is again drawn off, and agitated with repeated fresh supplies of water, until all the acid is removed. The water is drawn off, and the naphtha is then agitated with 10 per cent. of a solution of caustic soda, sp. gr. 1.03. Some times milk of lime is added ; but its use requires extra care. After standing for some time, the naphtha is treated with water as before, or is pumped into a still, and drawn over by superheated steam. Three washings with acid are given, the last being sometimes with a mixture of. sulphuric aud weak nitric. The acid sludge has been largely used for making superphosphate.
After the separation of the water by standing, the naphtha should be colourless, i. e. " water
white," and remain nearly ao on exposure to light ; this is the best teSt of thorough washing. For " second light oils," caustic solutions are first used, as the chief matters to be removed are carbolic and other acids. As these vary in different naphthas, a preliminary test is made, to ascertain how much caustic solution is required (see Carbolic Acid, p. 44). After removing the soda-salt formed, by washing with water, the whole is left to settle ; the liquid is drawn off and treated with acids, as above, or is first submitted to distillation.
Various ways of carrying out these rectifications are adopted by different manufacturers ; some have only ideal advantages, whilst others are more useful. As a rule, it is found that by pushing the distillation too far, in the preliminary treatment of the tar, little or nothing is gained. The object is to minimize the washing, which means saving acid and alkali, as well as labour. A saving of acid and alkali is effected by not allowing the heavy oils to go over with the second light oils, and, for the production of solvent naphtha, this is an important consideration. It is necessary to guard against loss from evaporizatiou, caused by the heat generated on mixing the naphtha with acid or alkali. The products, either crude or rectified, are redistilled for new products.
The stills used for fractionizing the crude naphtha are known as Coupier's and Clark's. The vapours are condensed in a leaden worm, and the liquids are received in a closed box with glass sides, containing some coloured " specific gravity beads " ; at the bottom, are several cocks communicating with separate tanks for the reception of the fractionized products. The still is covered with felt and cement. It converts the crude products into second-class benzols, toluols, and solvent naphtha, the residues being drawn off into the creosote tanks. The condensation must be thorough, to prevent escape of the vapour, which, from these portions of the distillate, is very inflammable. In the first fractionizing by Coupler's still, the proximate principles are only comparatively isolated, though more so than is usual with the English products ; but by submitting tbe products to a second or third rectification, it is possible to separate the principles very cornpletely. Thus Coupier's distillates have justly earned an enviable reputation. His still is used principally on the Continent. Clark's still, like the one previously mentioned, is, in many respects, similar to the Coffey still (see Ammonia). It may be used in a variety of ways ; the same 'results may be attained as by Coupier's still, but with the advantage that the liquid is broken up into several products at one operation.