The naphtha—usually called "miscible "—coming over from the lime liquors, is at first very dilute. It is run from the first receiver into a cast-iron still heated by a fire underneath, condensed in a copper worm, and by three subsequent distillations in copper stills, jacketed, or with a steam coil inside, is " worked up," as it is technically termed, to 60° over proof. The direct application of heat from a fire under the stills is dangerous, though this plan is often adopted. Distillation by steam is to bo preferred. If a jacketed still he used, the jacket should be well up to the shoulder.
A good still of this description, made by Messrs. Robert Daglish and Co. of St. Helen's, is th shown in Fig. 86. Occasionally " plate " or whisky stills are used. Further details concerning e naphtha processes will be given under ' Pyroxylic Spirit." To obtain grey or white acetate of lime, the acid liquors after leaving the tar-settlers are pumped into a series of copper stills, heated pre ferably by a steam-coil inside. Here at a gentlo heat the naphtha is first expelled. The acetic acid next distils over, is condensed, and run into a tank to settle. The tarry and oleaginous de posit in the stills is drawn off through a stop cock at the bottom. From the receiving tank the clear, or fairly cleared, acetic acid is run off or pumped up into the neutralizer and mixed with a slight excess of lime. The subsequent pro cesses are precisely similar to those employed in the production of brown acetate, except that great care is taken to ensure purity,, and a spe cially heated drying house is often provided, the floor of the house being heated by circular flues from an independent fireplace. By this means the temperature is more accurately regulated than when only the waste heat from the ovens is uti lized. The naphtha from the grey acetate process is concentrated and purified by re-distillation in copper stills in a mariner similar to that already described, but is only worked up to about 45°, or such a strength as will readily dissolve resins and gums. It is called "solvent," in contradistinction to the " miscible" wood naphtha, obtained as a bye-product in the brown acetate process. A very pure " white " acetate may be made by dissolving the salt from the drying floors in hot water, filtering the solution through animal charcoal, and evaporating the solution to dryness. Good grey acetate
should contain 85 per cent. of real acetate.
The manufacture and sale of grey acetate has of late years considerably diminished, owing to the price, which averages about 80 per cent. more than brown. Inasmuch as the article at its best only contains 15 per cent. more real acetate, it is difficult to understand the very high price put upon it. There seems to be no reason why a good grey acetate should not be manufactured and sold at 121. 10s. to 131. per ton, in bags, free on rails, to yield a very fair profit.
The acetates of lime are used as mordants by printers and dyers ; for the production of other compounds, such as acetates of soda, iron, alumina, and manganese ; as a source of acetic acid by distillation with an acid, and in the manufacture of vinegar. They may be tested by dissolving in water and precipitating the lime by a slight excess of sulphate of soda, adding alcohol to prevent solution of the sulphate of lime formed. After filtration, the precipitate must be washed with a little dilute alcohol, and the lime determined from the sulphate. To estimate the acetic acid, the filtrate is evaporated to dryness, and calcined at a red heat to convert the acetate into carbonate of soda, the amount of which is ascertained by the alkalimeter, and the quantity of acetic acid calcu lated from it by reference to the combining proportions.
The advisability of securing more outlets than one or two for the products of wood distillation has already been noticed, the extra plant required for the production of a certain amount of grey as well as brown acetate, and fer distillation with sulphuric or hydrochloric acid, being of com paratively slight cost. It is worthy of mention that a very good grey acetate and solvent naphtha may be obtained by distilling the tar liquors—the bottoms of the tar-settlers.
A considerable number of makers are engaged in the trade in South Wales, Gloucestershire, Lancashire, and Somersetshiro. It is also becoming an important industry in the United States, where there are peculiar advantages for wood-distil]ation, but as yet the home article in the New York market fetches a lower price than the beat English brands. The present value of good 70 per cent. brown acetate is about 111. per ton in bags at the works; of grey, 15/. per ton. A considerable quantity is exported from this country.