Soap-Making Manual Part 14

It has already been pointed out that neutral fats or the glycerides are a combination of fatty acid with glycerine. These are split apart in the process of saponification. While by the term _saponification_ as used in soap making it is inferred that this is the combination of caustic alkalis with the fatty acids to form soap, this term is by no means limited to this method of saponification, as there are various other methods of saponifying a fat. The chemical definition of saponification is the conversion of an ester, of which glycerides are merely a certain type, into an alcohol and an acid or a salt of this acid. Thus, if we use caustic alkali as our saponifying agent for a fat or oil, we obtain the sodium or pota.s.sium salt of the higher fatty acids or soap and the alcohol, glycerine. On the other hand, if we use a mineral acid as the saponifying agent, we obtain the fatty acids themselves in addition to glycerine. While the former is by far the most generally employed for making soap, other processes consist in saponifying the fats by some method other than caustic alkalis and then converting the fatty acids into soap by either neutralizing them with sodium or pota.s.sium carbonate or hydrate.

It is important to again point out here that fats and oils develop free fatty acid of themselves and that the development of this acid represents a loss in glycerine. The selection of an oil or fat for soap making should therefore to a large extent be judged as to its adaptability by the free fatty acid content, as the higher this content is, the greater is the loss in the glycerine eventually obtained.

Glycerine often represents the only profit to a soap manufacturer. It is indeed necessary to determine the percentage of free fatty acid before purchasing a lot of stock to be made into soap.

In taking up the question of glycerine recovery we will consider the various methods thus:

1. Where the glycerine is obtained from spent lye by saponifying the fats or oils with caustic alkali.

2. Where the glycerine is obtained by saponifying the fats or oils by some other method than the above, of which there are the following:

(a) Twitch.e.l.l process.

(b) Saponification by lime in autoclave.

(c) Saponification by acid.

(d) Saponification by water in autoclave.

(e) Fermentative (Enzymes).

(f) Krebitz process.

RECOVERY OF GLYCERINE FROM SPENT LYE.

The spent lye obtained from the glycerine changes in making soap varies greatly, the quality depending upon the stock saponified and the soap maker's care in handling the operation. No two lyes run exactly alike as to proportion of the various ingredients, although they are all similar in containing the same substances either in solution or suspension.

Spent lye is a water solution of mainly glycerine, free alkali either as caustic alkali or carbonate and salt, including sodium sulfate, but furthermore contains some soap and alb.u.minous matter either in solution or suspension. Upon standing in the storage tank the greater part of the soap usually separates when the lye cools. In order to a.s.sure the greatest economical yield of glycerine by saponifying a fat with caustic soda it is necessary to obtain a proportion of three parts of water to every part of fat made into soap. Test runs have shown that this is the proper proportion and that it is not economical to greatly exceed this amount, and if a much less proportion is used the full yield of glycerine is not obtained.

The spent lyes contain varying amounts of glycerine, the first change being richest in glycerine content, and this being reduced in the subsequent changes. If the lyes always run high in glycerine it is an indication that it is not all being obtained. The usual percentage is from 0.5% to 5% or even more, although the average is somewhere around 2% to 3%. The lye as it comes from the kettle should not contain any more than 0.5% to 0.6% of free alkali calculated as sodium carbonate, Na_{2}CO_{3}. If the proportion is higher than this, it shows that the saponification has been conducted with too high a proportion of alkali, a condition which should be corrected in the kettle room. An excess of free alkali does not interfere to any great extent with the successful recovery of the glycerine, but is a waste of both alkali and the acid used in neutralizing this. It is, therefore, more economical to run a strong lye over fresh stock and neutralize the alkali thus, rather than treating the lye for glycerine recovery.

Before the spent lye can be run into the evaporator it is necessary to remove the alb.u.minous impurities and soap and to neutralize the excess alkali to between exactly neutral and 0.02% alkalinity. The lye should never be fed into the evaporator in the acid condition.

In order to treat the spent lyes for evaporation, they are first allowed to cool in the storage tank, after which any soap which may have separated is skimmed off and returned to the soap kettle. This lye is then pumped to the treatment tank, an ordinary tank equipped with some method of agitating the liquor, either by a mechanical stirrer, steam blower or compressed air, until it is about two feet from the top.

After the lye has been skimmed off it is thoroughly agitated and a sample taken. The amount of lye in the tank is then calculated. Spent lye is about 1.09 times heavier than water, or weighs about 9 pounds to the gallon. While the sample is being tested for alkalinity it is advisable to add sulfate of alumina, which may be dissolving while the sample is being t.i.trated. This substance should be added in the proportion of anywhere from 6 to 14 pounds per thousand pounds of lye, depending upon the amount of impurities contained therein. For a clean lye six pounds per thousand is sufficient, but for an impure lye a greater quant.i.ty is necessary. The sulfate of alumina used should be free from a.r.s.enic and sulfides and should contain a minimum amount of grit (silica), as grit reduces the life of the pump valves. This may be estimated with sufficient accuracy by rubbing the filtered-off portions, insoluble in water between the fingers and a plate of gla.s.s. The object of adding the sulfate of alumina is to transform the soap contained in the lye into the insoluble aluminum soaps, and at the same time to coagulate the alb.u.minous impurities. It must be remembered that the sulfate of alumina is added only for the fresh lye put into the tank.

Thus if there were 10,000 pounds of lye in the treating tank when the fresh lye was run in, and 50,000 pounds when the tank is filled, adding nine pounds of sulfate of alumina per thousand of lye, only 360 pounds would be added or enough for 40,000 pounds. Sulfate of alumina neutralizes one-third of its weight of caustic.

To determine the alkali in the sample, 10 cubic centimeters are pipetted into a beaker, a little distilled water added, then 3 or 4 drops of phenolphthalein indicator. From a burette, quarter normal (N/4) sulfuric acid is added until the pink color is just discharged. When this point is reached 4 to 5 c. c. more of acid are added and the solution is boiled to expel the carbon dioxide. Should the solution turn pink, it is necessary to add more acid. After having boiled for 3 to 4 minutes, N/4 caustic soda is added until the pink color just returns and the amount of caustic soda used is read on the burette. The difference between the number of cubic centimeters of N/4 sulfuric acid and N/4 caustic soda gives the amount of alkali in the sample. By using a 10 c. c. sample and N/4 sulfuric acid and N/4 caustic soda each c. c. obtained by the difference of these two solutions is equal to one-tenth of one per cent.

(0.1%) of the total alkali in the lye. As an example, say we first used 7.7 c. c. of N/4 sulfuric acid to just discharge the pink, then added 4 c. c. more, or 11.7 c. c. in total. After boiling it required 5.3 c. c.

to bring back a slight pink, the total alkalinity would be 11.7 c. c. - 5.3 c. c. = 6.4 c. c., or 0.64% total alkali in the lye in terms of caustic soda. If there were 40,000 pounds of lye to be treated then we should have to neutralize:

40,000 .0064 = 256 lbs. alkali. Since sulfate of alumina neutralizes one-third of its weight in caustic, and there are say 9 lbs. of this added per thousand pounds of lye we would add

40,000 9 = 360 lbs. of sulfate of alumina. This would neutralize 360 1/3 = 120 lbs of alkali. There are then 256 - 120 = 136 lbs. of alkali still to be neutralized. If 60 B. sulfuric acid is used it requires about 1.54 lbs. of acid to one pound of caustic. Therefore to neutralize the caustic soda remaining it requires:

136 1.54 = 209.44 lbs. 60 B. sulfuric acid to neutralize the total alkali in the 40,000 pounds of spent lye.

The acid is added and the lye well stirred, after which another sample is taken and again t.i.trated as before. From this t.i.tration the amount of acid to be added is again calculated and more acid is added if necessary. Should too much acid have been added, caustic soda solution is added until the lye is between exactly neutral and 0.02% alkaline.

The filtered lyes at this stage have a slight yellowish cast.

To be sure that the lyes are treated correctly the precipitation test is advisable. To carry this out filter about 50 c. c. of the treated lye and divide into two portions in a test tube. To one portion add ammonia drop by drop. If a cloudiness develops upon shaking, more alkali is added to the lye in the tank. To the other portion add a few drops of 1 to 5 sulfuric acid and shake the test tube. If a precipitate develops or the solution clouds, more acid is needed. When the lyes are treated right no cloudiness should develop either upon adding ammonia or the dilute acid.

The properly treated lye is then run through the filter press while slightly warm and the filtered lye is fed to the evaporator from the filtered lye tank. The lye coming from the filter press should be clear and have a slight yellowish cast. As the pressure increases it is necessary to clean the press or some of the press cake will pa.s.s through the cloths. Where sodium silicate is used as a filler, the silicate sc.r.a.p should never be returned to the soap kettle until the glycerine lyes have been withdrawn. This practice of some soapmakers is to be strongly censured, as it causes decided difficulty in filtering the lye, since during the treatment of the lye, free silicic acid in colloidal form is produced by the decomposition of the sodium silicate by acid.

This often prevents filtering the treated lye even at excess pressure and at its best r.e.t.a.r.ds the filtering.

As to the filter press cake, this may be best thrown away in a small factory. Where, however, the output of glycerine is very large it pays to recover both the fatty acids and alumina in the press cakes.

In some cases, especially when the lyes are very dirty and the total residue in the crude glycerine runs high, for which there is a penalty usually attached, a double filtration of the lye is advisable. This is carried out by first making the lye slightly acid in reaction by the addition of alum and acid, then filtering. This filtered lye is then neutralized to the proper point with caustic, as already described, and pa.s.sed through the filter press again.

While in the method of treating the lyes as given sulfuric acid is used for neutralizing, some operators prefer to use hydrochloric acid, as this forms sodium chloride or common salt, whereas sulfuric acid forms sodium sulfate, having 3/5 the graining power of salt, which eventually renders the salt useless for graining the soap, as the percentage of sodium sulfate increases in the salt. When the salt contains 25 per cent. sodium sulfate it is advisable to throw it away. Sulfuric acid, however, is considerably cheaper than hydrochloric and this more than compensates the necessity of having to eventually reject the recovered salt. It may here also be mentioned that recovered salt contains 5-7 per cent. glycerine which should be washed out in the evaporator before it is thrown away. The following tables give the approximate theoretical amounts of acids of various strengths required to neutralize one pound of caustic soda:

For 1 pound of caustic soda--

3.25 lbs. 18 B. hydrochloric (muriatic) acid are required.

2.92 " 20 B. " " " " "

2.58 " 22 B. " " " " "

For 1 pound of caustic soda--

1.93 lbs. 50 B. sulphuric acid are required.

1.54 " 60 B. " " " "

1.28 " 66 B. " " " "

It is, of course, feasible to neutralize the spent lye without first determining the causticity by t.i.trating a sample and this is often the case. The operator under such conditions first adds the sulfate of alumina, then the acid, using litmus paper as his indicator.

Comparatively, this method of treatment is much slower and not as positive, as the amount of acid or alkali to be added is at all times uncertain, for in the foaming of the lyes their action on litmus is misleading.

After the lye has been filtered to the filtered lye tank it is fed to the evaporator, the method of operation of which varies somewhat with different styles or makes. When it first enters the evaporator the lye is about 11-12 B. After boiling the density will gradually rise to 27 B. and remain at this gravity for some time and during which time most of the salt is dropped out in the salt filter. As the lye concentrates the gravity gradually rises to 28-30 B., which is half crude glycerine and contains about 60 per cent. glycerine. Some operators carry the evaporation to this point and acc.u.mulate a quant.i.ty of half crude before going on to crude. After half crude is obtained the temperature on the evaporator increases, the vacuum increases and the pressure on the condensation drain goes up (using the same amount of live steam). As the liquor grows heavier the amount of evaporation is less, and less steam is required necessitating the regulation of the steam pressure on the drum. When a temperature of 210 F. on the evaporator, with 26 or more inches vacuum on the pump is arrived at, the crude stage has been reached and the liquor now contains about 80 per cent. glycerine in which shape it is usually sold by soap manufacturers. A greater concentration requires more intricate apparatus. After settling a day in the crude tank it is drummed.

Crude glycerine (about 80 per cent. glycerol) free from salt is 33 B., or has a specific gravity of 1.3. A sample boiled in an open dish boils at a temperature of 155 C. or over.

TWITCh.e.l.l PROCESS.

The Twitch.e.l.l process of saponification consists of causing an almost complete cleavage of fats and oils by the use of the Twitch.e.l.l reagent or saponifier, a sulfo-aromatic compound. This is made by the action of concentrated sulfuric acid upon a solution of oleic acid or stearic acid in an aromatic hydrocarbon. From 0.5 per cent. to 3 per cent. of the reagent is added and saponification takes place from 12-48 hours by heating in a current of live steam. The reaction is usually accelerated by the presence of a few per cent. of free fatty acids as a starter.

Recently the Twitch.e.l.l double reagent has been introduced through which it is claimed that better colored fatty acids are obtained and the glycerine is free from ash.

The advantages claimed for the Twitch.e.l.l process as outlined by Joslin[12] are as follows:

1. All the glycerine is separated from the stock before entering the kettle, preventing loss of glycerine in the soap and removing glycerine from spent lye.

2. The liquors contain 15-20 per cent. glycerine whereas spent lyes contain but 3-5 per cent. necessitating less evaporation and consequently being more economical in steam, labor and time.

3. No salt is obtained in the liquors which makes the evaporation cheaper and removes the cause of corrosion of the evaporator; also saves the glycerine retained by the salt.

4. The glycerine liquors are purer and thus the treatment of the lyes is cheaper and simpler and the evaporation less difficult.

5. The glycerine can readily be evaporated to 90 per cent. crude rather than 80 per cent. crude, thus saving drums, labor in handling and freight. The glycerine furthermore receives a higher rating and price, being known as saponification crude which develops no glycols in refining it.

6. The fatty acids obtained by the Twitch.e.l.l saponifier may be converted into soap by carbonates, thus saving cost in alkali.

7. There is a decrease in the odor of many strong smelling stocks.

8. The glycerine may be obtained from half boiled and cold made soaps as well as soft (potash) soaps.