Enormous quantities of glycerine are run to waste in the spent lyes of the soapmaker. One of the earliest attempts to extract it was a patent by H. Reynolds, June 10, 1S5S, for concentrating the spent lyes, and distilling off the glycerine by superheated steam between 360° and 400° F(193 1/2° to 2041/2°C) the largo quantity of sodium salts, especially sodium chloride, was found, howerer, to be an almost insuperable difficulty. On March 31, 1879, a patent was taken out by C. Thomas, W. J. Fuller, and S. A. King, of Broad Plain Soap Works, Bristol, by which process the first sucfrom spent soaprlyes was introduced into commerce, and several tons per week are now manufactured. The specification states that they " evaporate the spent or partially spent lyes until the boiling-point of the liquid rapidly rises, when nearly all the salts that can be thrown down by simple evaporation are deposited in the pan. The resulting liquor is chiefly composed of raw or impure glycerine. This we draw off into a second pan, and boil it with eicess of fatty acid, which, readily combining with some of the salts in solution, separates them from the liquor, and at the same time removes from it the fine crystals of salt formed during this operation.
After this treatment, we skim off the saponified fatty matter, allow the liquid to cool, and filter it to remove the gelatinous, albuminous, and, other impurities. The clear liquid may then be refined, distilled, or concentrated, as desired."
In his address to the first general meeting of the Society of Chemical Indnstry, the president called attention to the waste of glycerine in the soap trade, and pointed out that, in view of the growing demand for this substance for the manufacture of explosives, Sic, its then price was 120'. per ton. Already at that time soapmalcers had begun to tum their attention to the more complete recovery of the glycerine contained in the spent lyes, and since then the subject has been still further pursued. These lyes contain water, glycerine, common salt, sulphate of sodium, a small quantity of caustic amounts of albuminous, resinous, and soapy matters. To illustrate their general composition when concentrated to a certain degree (which is easily attained in practice), Kingzett states the composition of a sample recently examined by him. It showed a specific gravity of 1.236, the gallon being made up as follows: -
Glycerine and organic matter
Alkali (calculated entirely as carboante of sodium...
the eicess over 100 being caused by calculating all the alkali present as Na2CO3 the fact being that some of it was in the caustic condition.
Constant Victor Clolus (specification No. 681 of 1881) neutralizes soap lyes with hydrochloric acid, and evaporates the settled liquor till it registers 32° B. Heated air is then blown through to remove the rest of the water, the salts deposited during both stages being fished out and treated in a turbine. The final product may be finished by drying in vacuo. The crude glycerine thus obtained is said to contain but little salt, but the exact amount is not stated. It may be distilled in order to purify it. So far he fails to detect anything of an original character in this proceeding. Instead, however, of neutralizing the soap lyes with hydrochloric acid, the patentee may use carbonic anhydride, so that when evaporation has been carried to 25° B., after further exposure to carbonic anhydride, bicarbonate of sodium (being only slightly soluble in a glycerine solution of salt) is precipitated upon cooling. Another mode of freeing his crude glycerine from salt consists in treating it with excess of hydrochloric acid of 22° B., or in the form of gas. The solubility of the salt is said to be thus diminished, and in consequence more of it can be removed.
The excess of hydrochloric acid is subsequently got rid of by a current of air, or by an excess of plumbic oxide.
Benno, Jaflfe, and Darmstaedter (specification No. 1562 of 1881) employ sulphate instead of chloride of sodium for salting out the soap. They then neutralize the spent lyes with sulphuric acid, filter, evaporate, and thus get only sulphate of sodium separated. They say the sulphate is removed more easily than the chloride, and hence the value of this method seems to turn on whether the increased ease of removing the salts more than atones for the increased cost of material for salting out, because the only comparable difficulty in removing the chloride is one of cost.
George Payne (specification No.2816 of 1881) neutralizes the lyes with an acid, allows any precipitate to settle, and then adds a 10 per cent, (by weight) solution of tannin or tannic acid, until albuminous matter is no longer precipitated. The filtered liquor is concentrated by heated air or superheated steam, or direct by fire. He thus gets, he says, a crude solution of glycerine containing about 10 per cent, salts, which may be more easily refined than any other crude glycerine. In connection with this patent, Kingzett regards it as pertinent to ask whether the excess of tannin introduced is not as objectionable as the matter thereby removed.
0. Thomas and A. Domeier (specification No. 2462 of 1881) concentrate the lyes and add an excess of acid until there is present about 1 to l 1/2 per cent, free acid. This is to decompose any soap, eliminate resinous matters, and to so act on other matters that, when neutralized by alkali later on, they are precipitated. After this neutralization and settlement, the lyes are further concentrated, and the crude glycerine is extracted with about 33 per cent, of coal-tar oil or petroleum, or bisulphide of carbon, or amylic alcohol, or ether, or other menstruum in which the glycerine is insoluble, in order to remove any soluble matters which may be disagreeable to the smell or taste. After extraction, the glycerine is subjected to hot air or steam in order to get rid of traces of the solvent employed, and may then be used for commercial purposes, or further purified by distillation. They also describe an apparatus designed for the concentration of thin lyes, in which the liquor slowly descends a tower fitted with alternately-inclined shelves, and in which it encounters a current of hot air, much in the same manner as Leather proposed to make bleaching-powder, viz., by causing lime to encounter an ascending current of chlorine gas in its descent down a tower fitted with a continuous spiral shelf.