The Handbook of Soap Manufacture Part 19

_Treatment of Lyes--Evaporation to Crude Glycerine--Distillation--Distilled and Dynamite Glycerine--Chemically Pure Glycerine--Animal Charcoal for Decolorisation--Glycerine obtained by other Methods of Saponification--Yield of Glycerine from Fats and Oils._

As pointed out in Chapter II. the fatty acids, which, combined with soda or potash, form soap, occur in nature almost invariably in the form of glycerides, _i.e._, compounds of fatty acids with glycerol, and as the result of saponification of a fat or oil glycerine is set free.

In Chapter V. processes of soap-making are described in which (1) the glycerine is retained in the finished soap, and (2) the glycerine is contained in the lyes, in very dilute solution, contaminated with salt and other impurities. These lyes, though now const.i.tuting the chief source of profit in the manufacture of cheap soaps, were till early in last century simply run down the drains as waste liquor.

Much attention has been devoted to the purification and concentration of glycerine lyes; and elaborate plant of various forms has been devised for the purpose.

_Treatment of Lyes._--The spent lyes withdrawn from the soap-pans are cooled, and the soap, which has separated during the cooling, is carefully removed and returned to the soap-house for utilisation in the manufacture of brown soap. Spent lyes may vary in their content of glycerol from 3 to 8 per cent., and this depends not only upon the system adopted in the working of the soap-pans, but also upon the materials used. Although, in these days of pure caustic soda, spent lyes are more free from impurities than formerly, the presence of sulphides and sulphites should be carefully avoided, if it is desired to produce good glycerine.

The lyes are transferred to a lead-lined tank of convenient size, and treated with commercial hydrochloric acid and aluminium sulphate, sufficient being added of the former to neutralise the free alkali, and render the liquor faintly acid, and of the latter to completely precipitate the fatty acids. The acid should be run in slowly, and the point when enough has been added, is indicated by blue litmus paper being slightly reddened by the lyes.

The whole is then agitated with air, when a sample taken from the tank and filtered should give a clear filtrate.

Having obtained this clear solution, agitation is stopped, and the contents of the tank pa.s.sed through a filter press. The sc.u.m, which acc.u.mulates on the treatment tank, may be transferred to a perforated box suspended over the tank, and the liquor allowed to drain from it.

The filtered liquor is now rendered slightly alkaline by the addition of caustic soda or carbonate, and, after filtering, is ready for evaporation.

The acid and alum salt used in the above treatment must be carefully examined for the presence of a.r.s.enic, and any deliveries of either article, which contain that impurity, rejected.

Lime, bog ore, and various metallic salts, such as ferric chloride, barium chloride, and copper sulphate have been suggested, and in some instances are used instead of aluminium sulphate, but the latter is generally employed.

_Evaporation to Crude Glycerine._--The clear treated lyes, being now free from fatty, resinous, and alb.u.minous matter, and consisting practically of an aqueous solution of common salt (sodium chloride) and glycerine, is converted into crude glycerine by concentration, which eliminates the water and causes most of the salt to be deposited.

This concentration was originally performed in open pans heated by fire or waste combustible gases. In the bottom of each pan was placed a dish in which the salt deposited, and this dish was lifted out periodically by the aid of an overhead crane and the contents emptied and washed.

Concentration was continued until the temperature of the liquor was 300 F. (149 C.), when it was allowed to rest before storing.

This liquor on a.n.a.lysis gave 80 per cent. glycerol and from 9 to 10-1/2 per cent. salts (ash); hence the present standard for crude glycerine.

Concentration in open pans has now been superseded by evaporation _in vacuo_. The subject of the gradual development of the modern efficient evaporating plant from the vacuum pan, originated and successfully applied by Howard in 1813 in the sugar industry, is too lengthy to detail here, suffice it to say that the multiple effects now in vogue possess distinct advantages--the greatest of these being increased efficiency combined with economy.

The present type of evaporator consists of one or more vessels, each fitted with a steam chamber through which are fixed vertical hollow tubes. The steam chamber of the first vessel is heated with direct steam, or with exhaust steam (supplied from the exhaust steam receiver into which pa.s.ses the waste steam of the factory); the treated lyes circulating through the heated tubes is made to boil at a lower temperature, with the reduced pressure, than is possible by heating in open pans.

The vapour given off by the boiling liquor is conveyed through large pipes into the steam chamber of the second vessel, where its latent heat is utilised in producing evaporation, the pressure being further reduced, as this second vessel is under a greater vacuum than No. 1.

Thus we get a "double effect," as the plant consisting of two pans is termed. The vapours discharged from the second vessel during boiling are pa.s.sed through pipes to the steam chamber of the third vessel (in a "triple effect"), and there being condensed, create a partial vacuum in the second vessel. The third vessel may also be heated by means of live steam. The vapours arising from the last vessel of the evaporating plant, or in the case of a "single effect" from the vessel, are conveyed into a condenser and condensed by injection water, which is drawn off by means of the pump employed for maintaining a vacuum of 28 inches in the vessel.

In the most recent designs of large evaporative installations, the vapours generated from the last vessel are drawn through a device consisting of a number of tubes enclosed in a casing, and the latent heat raises the temperature of the treated lyes proceeding through the tubes to supply the evaporator.

It will thus be observed that the object of multiple effects is to utilise all the available heat in performing the greatest possible amount of work. Special devices are attached to the plant for automatically removing the condensed water from the steam chambers without the loss of useful heat, and as a precaution against splashing over and subsequent loss of glycerine through conveyance to the steam chamber, dash plates and "catch-alls" or "save-alls" of various designs are fitted on each vessel.

In working the plant, the liquor in each vessel is kept at a fairly constant level by judicious feeding from one to the other; the first vessel is, of course, charged with treated lyes. As the liquor acquires a density of 42 Tw. (25 B.) salt begins to deposit, and may be withdrawn into one of the many patented appliances, in which it is freed from glycerine, washed and dried ready for use at the soap pans.

Difficulty is sometimes experienced with the tubes becoming choked with salt, thereby diminishing and r.e.t.a.r.ding evaporation. It may be necessary to dissolve the encrusted salt with lyes or water, but with careful working the difficulty can be obviated by washing out with weak lyes after each batch of crude glycerine has been run away, or by increasing the circulation.

It is claimed that by the use of the revolving heater designed by Lewkowitsch, the salting up of tubes is prevented.

The salt having been precipitated and removed, evaporation is continued until a sample taken from the last vessel has a density of 60 Tw. (33.3 B.) at 60 F. (15.5 C.). When this point is reached, the crude glycerine is ready to be withdrawn into a tank, and, after allowing the excess of salt to deposit, may be transferred to the storage tank.

The colour of crude glycerine varies from light brown to dark brown, almost black, and depends largely on the materials used for soap-making.

The organic matter present in good crude glycerine is small in amount, often less than 1 per cent.; a.r.s.enic, sulphides and sulphites should be absent. Crude glycerine is refined in some cases by the producers themselves; others sell it to firms engaged more particularly in the refined glycerine trade.

_Distillation._--Crude glycerine is distilled under vacuum with the aid of superheated steam. The still is heated directly with a coal or c.o.ke fire, and in this fire s.p.a.ce is the superheater, which consists of a coil of pipes through which high pressure steam from the boiler is superheated.

The distillation is conducted at a temperature of 356F. (180 C.). To prevent the deposition and burning of salt on the still-bottom during the distillation, a false bottom is supported about 1 foot from the base of the still. With the same object in view, it has been suggested to rotate the contents with an agitator fixed in the still.

Every care is taken that the still does not become overheated; this precaution not only prevents loss of glycerine through carbonisation, but also obviates the production of tarry and other bodies which might affect the colour, taste, and odour of the distilled glycerine. The vacuum to be used will, of course, depend upon the heat of the fire and still, but as a general rule good results are obtained with an 18 inch vacuum.

There are quite a large number of designs for still heads, and "catch-alls," having for their object the prevention of loss of glycerine.

The distillate pa.s.ses into a row of condensers, to each of which is attached a receptacle or receiver. It is needless to state that the condensing capacity should be in excess of theoretical requirements. The fractions are of varying strengths and quality; that portion, with a density less than 14 Tw. (19.4 B.), is returned to the treated-lyes tank. The other portion of the distillate is concentrated by means of a dry steam coil in a suitable vessel under a 28 inch vacuum.

When sufficiently concentrated the glycerine may be decolorised, if necessary, by treating with 1 per cent. animal charcoal and pa.s.sing through a filter press, from which it issues as "dynamite glycerine".

The residue in the still, consisting of 50-60 per cent. glycerine and varying proportions of various sodium salts--_e.g._ acetate, chloride, sulphate, and combinations with non-volatile organic acids--is generally boiled with water and treated with acid.

The tar, which is separated, floats on the surface as the liquor is cooling, and may be removed by ladles, or the whole mixed with waste charcoal, and filtered.

The filtrate is then evaporated, when the volatile organic acids are driven off; the concentrated liquor is finally mixed with crude glycerine which is ready for distillation, or it may be distilled separately.

_Distilled Glycerine._--This cla.s.s of commercial glycerine, although of limited use in various other branches of industry, finds its chief outlet in the manufacture of explosives.

Specifications are usually given in contracts drawn up between buyers and sellers, to which the product must conform.

The chief stipulation for dynamite glycerine is its behaviour in the nitration test. When glycerine is gradually added to a cold mixture of strong nitric and sulphuric acids, it is converted into nitro-glycerine, which separates as an oily layer on the surface of the acid. The more definite and rapid the separation, the more suitable is the glycerine for dynamite-making.

Dynamite glycerine should be free from a.r.s.enic, lime, chlorides, and fatty acids, the inorganic matter should not amount to more than 0.1 per cent., and a portion diluted and treated with nitrate of silver solution should give no turbidity or discoloration in ten minutes. The specific gravity should be 1.262 at 15 C. (59 F.) and the colour somewhat yellow.

_Chemically pure glycerine_ or double distilled glycerine is produced by redistilling "once distilled" glycerine. Every care is taken to avoid all fractions which do not withstand the nitrate of silver test. The distillation is very carefully performed under strict supervision.

The distillate is concentrated and after treatment with animal charcoal and filtration should conform to the requirements of the British Pharmacopoeia. These are specified as follows: Specific gravity at 15.5 C., 1.260. It should yield no characteristic reaction with the tests for lead, copper, a.r.s.enium, iron, calcium, pota.s.sium, sodium, ammonium, chlorides, or sulphates. It should contain no sugars and leave no residue on burning.

_Animal Charcoal for Decolorisation._--The application of animal charcoal for decolorising purposes dates back a century, and various are the views that have been propounded to explain its action. Some observers base it upon the physical condition of the so-called carbon present, and no doubt this is an important factor, coupled with the porosity. Others consider that the nitrogen, which is present in all animal charcoal and extremely difficult to remove, is essential to the action. Animal charcoal should be freed from gypsum (sulphate of lime), lest in the burning, sulphur compounds be formed which would pa.s.s into the glycerine and contaminate it.

The "char" should be well boiled with water, then carbonate of soda or caustic soda added in sufficient quant.i.ty to give an alkaline reaction, and again well boiled. The liquor is withdrawn and the charcoal washed until the washings are no longer alkaline. The charcoal is then separated from the liquor and treated with hydrochloric acid; opinions differ as to the amount of acid to be used. Some contend that phosphate of lime plays such an important part in decolorising that it should not be removed, but it has, however, been demonstrated that this substance after exposure to heat has very little decolorising power.

Animal charcoal boiled with four times its weight of a mixture consisting of equal parts of commercial hydrochloric acid (free from a.r.s.enic) and water for twelve hours, then washed free from acid, dried, and burned in closed vessels gives a product possessed of great decolorising power for use with glycerines.

A good animal charcoal will have a dull appearance, and be of a deep colour; it should be used in fine grains and not in the form of a powder.

The charcoal from the filter presses is washed free from glycerine (which is returned to the treated lyes), cleansed from foreign substances by the above treatment and revivified by carefully heating in closed vessels for twelve hours.

_Glycerine obtained by other Methods of Saponification._--French saponification or "candle crude" glycerine is the result of concentration of "sweet water" produced in the manufacture of stearine and by the autoclave process. It contains 85-90 per cent. glycerol, possesses a specific gravity of 1.240-1.242, and may be readily distinguished from the soap-crude glycerine by the absence of salt (sodium chloride). This glycerine is easily refined by treatment with charcoal.

The glycerine water resulting from acid saponification methods requires to be rendered alkaline by the addition of lime--the sludge is separated, and the liquor evaporated to crude. The concentration may be performed in two stages--first to a density of 32 Tw. (20 B.), when the calcium sulphate is allowed to deposit, and the separated liquor concentrated to 48 Tw. (28 B.) glycerine, testing 85 per cent.

glycerol and upwards.

_Yield of Glycerine from Fats and Oils._--The following represent practicable results which should be obtained from the various materials:--

Tallow 9 per cent. of 80 per cent. Glycerol.

Cotton-seed oil 10 "