Since the molecular weight of caustic potash (56) is greater than that of caustic soda (40) more potash is required to saponify a pound of fat.

The resulting potash soap is correspondingly heavier than a soda soap.

When salt is added to a pota.s.sium soap double decomposition occurs, the pota.s.sium soap being transformed to a sodium soap and the pota.s.sium uniting with the chlorine to form pota.s.sium chloride. This was one of the earliest methods of making a hard soap, especially in Germany, where potash was derived from leeching ashes of burned wood and plants.

SODIUM CARBONATE (SODA ASH).

While carbonate of soda is widely distributed in nature the source of supply is entirely dependent upon the manufactured product. Its uses are many, but it is especially important to the soap industry in the so called carbonate saponification of free fatty acids, as a const.i.tuent of soap powders, in the neutralization of glycerine lyes and as a filler for laundry soaps.

The old French Le Blanc soda process, which consists in treating common salt with sulphuric acid and reducing the sodium sulphate (salt cake) thus formed with carbon in the form of charcoal or c.o.ke to sodium sulphide, which when treated with calcium carbonate yields a mixture of calcium sulphide and sodium carbonate (black ash) from which the carbonate is dissolved by water, has been replaced by the more recent Solvay ammonia soda process. Even though there is a considerable loss of salt and the by-product calcium chloride produced by this process is only partially used up as a drying agent, and for refrigerating purposes, the Le Blanc process cannot compete with the Solvay process, so that the time is not far distant when the former will be considered a chemical curiosity. In the Solvay method of manufacture sodium chloride (common salt) and ammonium bicarbonate are mixed in solution. Double decomposition occurs with the formation of ammonium chloride and sodium bicarbonate. The latter salt is comparatively difficultly soluble in water and crystallizes out, the ammonium chloride remaining in solution.

When the sodium bicarbonate is heated it yields sodium carbonate, carbon dioxide and water; the carbon dioxide is pa.s.sed into ammonia which is set free from the ammonium chloride obtained as above by treatment with lime (calcium oxide) calcium chloride being the by-product.

Sal soda or washing soda is obtained by recrystallizing a solution of soda ash in water. Large crystals of sal soda containing but 37% sodium carbonate are formed.

POTa.s.sIUM CARBONATE.

Pota.s.sium carbonate is not extensively used in the manufacture of soap.

It may be used in the forming of soft soaps by uniting it with free fatty acids. The methods of manufacture are the same as for sodium carbonate, although a much larger quant.i.ty of pota.s.sium carbonate than carbonate of soda is obtained from burned plant ashes. Purified pota.s.sium carbonate is known as _pearl ash_.

ADDITIONAL MATERIAL USED IN SOAP MAKING.

Water is indispensable to the soap manufacturer. In the soap factory _hard_ water is often the cause of much trouble. Water, which is the best solvent known, in pa.s.sing through the crevices of rocks dissolves some of the const.i.tuents of these, and the water is known as hard. This hardness is of two kinds, _temporary_ and _permanent_. Temporarily hard water is formed by water, which contains carbonic acid, dissolving a portion of calcium carbonate or carbonate of lime. Upon boiling, the carbonic acid is driven from the water and the carbonate, being insoluble in carbon dioxide free water, is deposited. This is the cause of boiler scale, and to check this a small amount of sal ammoniac may be added to the water, which converts the carbonate into soluble calcium chloride and volatile ammonium carbonate. Permanent hardness is caused by calcium sulphate which is soluble in 400 parts of water and cannot be removed by boiling.

The presence of these salts in water form insoluble lime soaps which act as inert bodies as far as their value for the common use of soap is concerned. Where the percentage of lime in water is large this should be removed. A method generally used is to add about 5% of 20 B. sodium silicate to the hard water. This precipitates the lime and the water is then sufficiently pure to use.

_Salt_, known as sodium chloride, is used to a large extent in soap making for "salting out" the soap during saponification, as well as graining soaps. Soap ordinarily soluble in water is insoluble in a salt solution, use of which is made by adding salt to the soap which goes into solution and throws any soap dissolved in the lyes out of solution.

Salt may contain magnesium and calcium chlorides, which of course are undesirable in large amounts. The products on the market, however, are satisfactory, thus no detail is necessary.

_Filling materials_ used are sodium silicate, or water gla.s.s, talc, silex, pumice, starch, borax, tripoli, etc.

Besides these other materials are used in the refining of the oils and fats, and glycerine recovery, such as Fuller's earth, bichromates of soda or potash, sulphate of alumina, sulphuric and hydrochloric acids and alcohol.

A lengthy description of these substances is not given, as their modes of use are detailed elsewhere.

FOOTNOTES:

[1] Seifensieder Zeit, 1913, 40, p. 687, 724, 740.

[2] Official Methods, see Bull. 107, A. O. A. C., U. S. Dept. Agricult.

[3] Journ. Coll. of Engin. Tokyo Imper. Univ. (1906), p. 1. Abs. Chem.

Revue f. d. Fett-u. Harz, Ind. 16, p. 84; 20, p. 8.

[4] Meyerheim--Fort. der Chem., Physik. und Physik. Chem. (1913), 8. 6, p. 293-307.

[5] Seifs. Ztg. (1913), 40, p. 142.

[6] Loc. cit.

[7] Les Matieres Graisses (1914), 7, 69, p. 3367.

[8] Zeit. f. Angew. Chem. (1914), 27, 1, p. 2-4.

CHAPTER II

Construction and Equipment of a Soap Plant.

No fixed plan for the construction and equipment of a soap plant can be given. The specifications for a soap factory to be erected or remodeled must suit the particular cases. Very often a building which was constructed for a purpose other than soap manufacture must be adapted for the production of soap. In either case it is a question of engineering and architecture, together with the knowledge obtained in practice and the final decision as to the arrangement is best solved by a conference with those skilled in each of these branches.

An ideal soap plant is one in which the process of soap making, from the melting out of the stock to the packing and shipping of the finished product, moves downward from floor to floor, since by this method it is possible to utilize gravitation rather than pumping liquid fats and fluid soaps. Convenience and economy are obtained by such an arrangement.

The various machinery and other equipment for soap manufacture are well known to those connected with this industry. It varies, of course, depending upon the kind of soap to be manufactured, and full descriptions of the necessary machinery are best given in the catalogs issued by the manufacturers of such equipment, who in this country are most reliable.

To know just what equipment is necessary can very easily be described by a brief outline of the process various soaps undergo to produce the finished article. After the saponification has taken place in the _soap kettle_ the molten soap is run directly into the soap _frames_, which consist of an oblong compartment, holding anywhere from 400 to 1,200 pounds, with removable steel sides and mounted upon trucks, in which it solidifies. In most cases it is advisable to first run the soap into a _crutcher_ or mixer which produces a more h.o.m.ogeneous ma.s.s than if this operation is omitted. Color and perfume may also be added at this point, although when a better grade of perfume is added it must be remembered that there is considerable loss due to volatilization of same. When a _drying machine_ is employed the molten soap is run directly upon the rollers of this machine, later adding about 1.0% zinc oxide to the soap from which it pa.s.ses continuously through the drying chamber and is emitted in chip form ready for milling. After the soap has been framed, it is allowed to cool and solidify, which takes several days, and then the sides of the frame are stripped off. The large solid cake is cut with wires by hand or by a _slabber_ into slabs of any desired size.

These slabs are further divided into smaller divisions by the _cutting table_. In non-milled soaps (laundry soaps, floating soaps, etc.), these are pressed at this stage, usually by automatic presses, after a thin hard film has been formed over the cake by allowing it to dry slightly.

In making these soaps they are not touched by hand at any time during the operation, the pressing, wrapping and packing all being done by machinery. For a milled soap the large slabs are cut into narrow oblong shapes by means of the cutting table to readily pa.s.s into the feeder of the _chipper_, the chips being spread upon _trays_ and dried in a _dry house_ until the moisture content is approximately 15%.

The process of milling is accomplished by pa.s.sing the dried soap chips through a _soap mill_, which is a machine consisting of usually three or four contiguous, smooth, granite rollers operated by a system of gears and set far enough apart to allow the soap to pa.s.s from a hopper to the first roller, from which it is constantly conveyed to each succeeding roller as a thin film, and finally sc.r.a.ped from the last roller to fall into the _milling box_ in thin ribbon form. These mills are often operated in tandem, which necessitates less handling of soap by the operator. The object of milling is to give the soap a glossy, smooth finish and to blend it into a h.o.m.ogeneous ma.s.s. The perfume, color, medication or any other material desired are added to the dried soap chips prior to milling. Some manufacturers use an _amalgamator_ to distribute these uniformly through the soap, which eliminates at least one milling. When a white soap is being put through the mill, it is advisable to add from 0.5% to 1% of a good, fine quality of zinc oxide to the soap, if this substance has not been previously added. This serves to remove the yellowish cast and any translucency occasioned by plodding. Too great a quant.i.ty of this compound added, later exhibits itself by imparting to the soap a dead white appearance. Inasmuch as the milling process is one upon which the appearance of a finished cake of toilet soap largely depends, it should be carefully done. The number of times a soap should be milled depends upon the character of a soap being worked. It should of course be the object to mill with as high a percentage of moisture as possible. Should the soap become too dry it is advisable to add water directly, rather than wet soap, since water can more easily be distributed through the ma.s.s. As a general statement it may be said it is better policy to overmill a soap, rather than not mill it often enough.

After the soap has been thoroughly milled it is ready for plodding. A _plodder_ is so constructed as to take the soap ribbons fed into the hopper by means of a worm screw and continuously force it under great pressure through a jacketed cylinder through which cold water circulates in the rear to compensate the heat produced by friction and hot water at the front, to soften and polish the soap which pa.s.ses out in solid form in bars of any shape and size depending upon the form of the _shaping plate_ through which it is emitted. The bars run upon a _roller board_, are cut into the required length by a special _cake cutting table_, allowed to dry slightly and pressed either automatically or by a foot power _press_ in any suitable soap _die_. The finished cake is then ready for wrapping and after due time in stock reaches the consumer.

Besides the various apparatus mentioned above there are many other parts for the full equipment of a modern soap plant, such as remelters, pumps, mixers, special tanks, power equipment, etc. As has been stated, however, practical experience will aid in judging the practicability as to installation of these. The various methods of powdering soap are, however, not generally known. Where a coa.r.s.e powder is to be produced, such as is used for common washing powders, no great difficulty is experienced with the well known Blanchard mill. In grinding soap to an impalpable powder the difficulties increase. The methods adapted in pulverizing soaps are by means of disintegrators, pebble mills and chaser mills. The disintegrator grinds by the principle of attrition, that is, the material is reduced by the particles being caused to beat against each other at great velocity; a pebble mill crushes the substance by rubbing it between hard pebbles in a slowly revolving cylinder; the chaser mill first grinds the material and then floats it as a very fine powder above a curb of fixed height. The last method is particularly adapted for the finest of powder (140 mesh and over).

CHAPTER III

Cla.s.sification of Soap-Making Methods.

In the saponification of fats and oils to form soap through the agency of caustic alkalis, as has been stated, the sodium or pota.s.sium salts of the mixed fatty acids are formed. Sodium soaps are usually termed hard soaps, and pota.s.sium soaps soft. There are, however, a great many varieties of soaps the appearance and properties of which depend upon their method of manufacture and the oils or fats used therein.

The various methods adopted in soap making may be thus cla.s.sified:

1. Boiling the fats and oils in open kettles by open steam with indefinite quant.i.ties of caustic alkali solutions until the finished soap is obtained; ordinarily named _full boiled soaps_. These may be sub-divided into (a) hard soaps with sodium hydrate as a base, in which the glycerine is recovered from the spent lyes; (b) hard soaps with soda as a base, in which the glycerine remains in the soap, e. g., marine cocoanut oil soaps; (c) soft potash soaps, in which the glycerine is retained by the soap.

2. Combining the required amount of lye for complete saponification of a fat therewith, heating slightly with dry heat and then allowing the saponification to complete itself. This is known as the _cold process_.

3. Utilizing the fatty acid, instead of the neutral fat, and combining it directly with caustic alkali or carbonate, which is incorrectly termed _carbonate saponification_, since it is merely neutralizing the free fatty acid and thus is not a saponification in the true sense of the word. No glycerine is directly obtained by this method, as it is usually previously removed in the clearage of the fat by either the Twitch.e.l.l or autoclave saponification method.

In the methods thus outlined the one most generally employed is the full boiled process to form a sodium soap. This method of making soap requires close attention and a knowledge which can only be obtained by constant practice. The stock, strength of lyes, heat, amount of salt or brine added, time of settling, etc., are all influencing factors.