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 When this point is reached, the crude glycerine is ready to be withdrawn into a tank, and, after allowing the excess of salt to deposit, may be transferred to the storage tank.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.; arsenic, 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 coke fire, and in this fire space 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 356°F. (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 passes 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 passing 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.