The tun in which the saponification of the tallow takee place is made of oak or cedar, and is tightly bound with iron hoops. Steam is introduced by means of a spiral copper tube, laid on tbe bottom, and perforated with numerous small holes. An upright wooden shaft, carrying wooden arms fitted with teeth, is fixcd in the centre of the tun, and revolves during the precese. The tuns arc arranged iu rows in a Urge room, two being required for the completion of each batch.
It will be observed that in this process a very ls.rge amount of lime-16 per cent. on the weight of tallow employed—is used. The disadvantage of this is that much sulphuric acid is necessary to decompose the lime soap, thereby injuring the colour of the resulting fatty-acids. It was soon found that if the saponification were conducted in closed vessels, under Et steam pressure of 3 or 4 atmospheres, the amount of lime might be reduced to about 3 or 4 per cent. upon the tallow, thus reducing the cost, and improving the colour, of the product. This modification is still very largely worked, especially in America. Subsequently it was discovere,d that if sufficiently high tempera ture and pressure were employed, the litne might be dispensed with altogether, and that the resolu tion of the fat into fs.tty-acids and glycerine might be effected by steam alone. This process, known as the " Autoclave," hs.s been largely worked, both in Europe and America ; but iu consequence of numerous accidents, arising from the explosion of improperly constructed vessels, it is usual to decompose, tallow at a lower pressure, with the aid of 2 or 3 per cent. of lime, the sub.,equent °pers. tions of crystallization of the fatty-s.citla, hot and cold pressing, &c., remaining the same.
The next advaoce ws.s the discovery that when neutral fats are exposed to a very high tempera ture, 300° (572° F.), or above, in presence of superheated steam, they are decomposed, and the fatty azids Etre volatilized ; and that when these vapours are condensed, the fatty-acids are almost white : that, in fact, fatty-acids may be distilled, almost unchanged, in au atmosphere of superheated steam. It was impossible, however, to conduct this process on a large scale, in consequence of the simul taneous production of aeroleine, a vapour resulting from the decomposition of glycerine, and possess ing intensely irritating properties ; but, in 1841, it was discovered ths.t if neutral fats were treated first vvith concentrated sulphuric acid, and then boiled with water, they might be distilled without any such inconvenience, and the problem was thus solved by Dubruufaut. In 1842 and 1843, 1VIessrs. Jones and Wilson, under the name of Price and Co., took out two patents for the combined treatment of fatty bodies by sulphuric acid and water successively, and their subsequent distills.tion by the aid of superheated steam. Front that time to the present, this process has been worked, in its various modifications, on a most extended scale, especially in England. It gives a much larger quantity of material, of good colour for candle-making, from a given weight of fat, than any other known process. The es.ndles are not so hard, nor quite so white, as the continental bougies of stearie acid ; but while tsllow, treated by the saponification process, yields only about one-half its weight of candle material, tallow and palm oil, when distilled, give at least 75 per cent. of such material, of a slightly inferior quality.
The most perfected form of apparatus uow used in the distillation process, as made by Merryweather and Sons, Long Acro, London, is shown in Fig. 463. The process is conducted as follows :—The fEtt is melted from the casks in which it is stored, by means of a steam jet inserted in the bunghole, and runs into the underground wooden tank A, where it is left for some hours to settle the condensed water out of it. Hence it is pumped, by meEtns of the gun-metal lift snd force pump C, into a series of lead-lined collecting tanks B, fitted with steam coils, by which the material is boiled before being passed through the tap c to the vessel D. This latter, which is known as the " Acidifier," is made of stout copper, supported either on wrought-iron girders or on brickwork. It is fitted with a valved pipe a, for the admission of superheated steam; a copper pipe fitted with water shower pipe d, for condensing the vapours generated by the acidifying process ; a thermometer b, for guidance as to temperature ; and a gun-metal cover e, at the lower side, for cleaning out, and to which is affixed a tap f, for drawing off the acidified materials. On admission to D, the fats are heated for a certain time, by the introduction of superheated steam at a temperature of about 176° (350° F.), from the superheater F, constructed from the special design of Ed. Field. Sulphuric acid, in the proportion of from 3 to 6 lb. per cwt. of fat, is next supplied to the acidifier from the tank E by opening the plug g. When the acidification is complete, the material is left to stand for about six hours, and is then discharged into a series of lead-lined, open washing-vats G, provided NI ith copper steam coils, and containing water and a little sulphuric acid. Here it is
boiled with free steam for another two hours, and is left for about twenty-four hours to settle; it is then drawn off into the tank A, and pumped through the tap a' into a large, open, lead-lined tank H, placed at a sufficient elevation. This tank is fitted inside with a coil, which is charged with steam, to keep the contents in a liquid state. By means of the valve lz, about 5 tons of the material is ruo ioto the still I, consisting of an iron body, and copper dome ; it is fitted with a thermometer i, and the necassary taps of copper or gun-metal. The contents of the still are heated, by fire, to a temperature of about 116° (240° F.) ; superheated steam, at about 291° (560° F.), is then admitted by the pipe m from the superheater N, and the process of distillation commences. The temperature must be regulated according to the quality of the material operated upon. The vapours pass over by the pipe n to the refrigerator K, which consists of a series of vertical copper pipes, connected at top and bottom by gun-rnetal bends. These pipes are mounted on iron frames, over a set of six circular iron tanks k, into which they can be emptied. The tanks are furnished with pipes for the admission of steam, and with spiral copper cooling-coils, through which cold water may be passed. The " essence-tanli" / is fitted with an improved shower-pipe L, which prevents any vapour passing away uncondensed. The pipe' M conveys vapours to be burnt in the ilue. The fatty-acids are collected in pails from the mouths or outlets of the copper coils, the greater part in a fit state for candle-making, without the necessity for putting them through hydraulic presses. That part which is not fit for candle-making, as it comes direct from the still, is pressed and redistilled. As the result of distilling t dlow, it may be mentioned that out of every 100 lb. subjected to this process, 78 to 80 lb. of crude stearic acid is produced. Three-fourths of this, or about 60 lb., is ready for making stearine (i. e. stearic acid) candles without further treat ment ; the remaining fourth, about 20 lb., after being pressed and redistilled, yields about three fourths of stearic acid and one-fourth of oleic acid. Thus the total proportion of the latter product is only 5 lb. Besides the stearin and oleic acids, there is a large quantity of a third product, called " pitch." If allowed to get cold, this is a hard, black substance ; but provision is made for passing it at once to an iron vessel, where it ie submitted to great heat, and yields a product similar to that obtained by the distillation process, and which is often used in the preparation of " composite " candles, though much inferior to the pressed and purified material. The pitch, after this operation, becomes a commercial article of many uses, and will in all probability soon be recognized as an efficient substitute for " black-japan," for coating iron, the latter article being worth from 20s. to 30s. a gallon. The approximate cost of the plant required for distilling tallow or palm oil according to the above process, exclusive of steam boiler, may be stated at from 1700/. to 3150/., according to whether 1 ton or 3 tons are to be distilled at a time. ' The following table shows the quantity and value of tallow and stearine imported to this country in the year 1878 :— (N.B. Stearine, chemically speakiog, i.e. stearate of glycerine, is seldom or never made now, and the term " stearine" is somewhat loosely applied, commercially, to stearic or palmitic acids in various states of purity.) Pahn 0 s. —Palm oil is now used in enormous quantities for lhe production of palmitic and stearic acids at Price's Candle Company's woirks, as well as by almost every candle manufacturer in Great Britain, about 25,000 tons being annually consumed. In many continental countries, u prohibitive duty prevents its employment. The process employed consists in acting upon the fat with sulphuric acid, and then submitting it to distillation. The plant and the modus operandi ficarcely differ from those last described. The distilled mixture of stearic and palmitic acids is cut into shreds, by means of a revolving knife, and the shreds are wrapped in canvas or woollen cloths, spread in even layers between mats of cocoa-nut fibre, and submitted first t,o the cold press, and afterwards to the hot press, at a temperature of 29° t,o 32° (85° to 90° F.). The pressed cakes of fat s,re pared, and then melted again by steam, in large, wooden, iron-hound vessels, containing water and sulphuric acid. The whole is boiled for a time and is then allowed to stand, after which the acidulated water is drawn off. The melted fat is repeatedly washed with hot water, and then run into moulds; when cold, it is quite pure, and ready for manufacture into candles.