Six ewt. of soda acetate is put into a jacketed copper still, heated either by direct fire under neath or by steam, and 312 lb. of sulphuric acid, of a specific gravity of added, and intimately mixed with it. The still is then closed in, luted, and connected with a condensing worm of earthen ware (preferably of porcelain or even silver), set in a convenient vessel. All the joiuts of the apparatus should be made of silver. A gentle heat being applied, distillation is allowed to proceed until a faint ompyroumatio odour comes from the distillate which rune from the condenser into a suitable receiver. The final products of distillation should thereafter be collected in a separate receiver and re-distilled. If the operation be carefully conducted with fairly pure materials, and at not too great heat, the acid comee over colourless, at about sp. gr., containing about 35 per cent. of anhydrous acetic aeid. Glacial acetic aeid is obtained by distilling the product of the first operation with fused calcium chloride, and cooling the distillation. Below 15°, crystals are deposited, which are re-dissolved in their own mother liquor and re-distilled with calcium chloride until tho whole of the acid crystallizes. Above 16°, these erystale deliquesce, yielding a very pure acetio acid of 1'063 sp. gr. Usually only the first part of the process described is followed, an acil at being all that is desired by the bulk of consumers. For culinary purposes, pickling, &c., the strong acid is reduced by the addition of five times its weight of water.
The sulphate of soda left in the retort is sold for the purpose of beiog worked up in the ordinary sulphate procese—the manufacture of the salt from chloride of sodium and sulphuric acid—and is worth about 258. per ton. Tho charge, as above, of 6 ewt. of acetate of soda and 312 lb. of acid, should yield 6i cwt. of etroog acetic acid. The cost of 1 ton of sp gr. acid is about as follows:— L 8. d.
17 cwt. acetate of soda.................18 0 0 7 cwt. eulphurie acid . 1 0 0 Fuel 1 5 0 10 0 Packages 12 0 Wear and tear, &c., and proportion of management expenses .. 2 0 21 9 0 Less sulphate of sods ., 1 5 f) 4 0 The selling price of the acid is 2d. per lb. The cost is usually enhanced by the small amount made.
If the acetic acid from this process does not come over perfectly colourless or free from empy remna, it may be purified by infusion with animal charcoal, or by allowing it to stand for ten days or so in tubs containing beeehwood cuttings.
The plant for the manufacture of, say, 2 tons of acetic acid per week, costs about 1000/. About eight makers are engaged in the trade in the neighbourhood of London, When it is not required to obtain an acid of great purity it may be prepared by distilling brown acetate of lime with sulphuric or hydrochloric acid. When the former is employed, the salt is first broken up and intimately mixed with the acid in any suitable arrangement, usually a cast-iron cylinder, about 5 ft. long by 2 ft. in diameter, set horizontally in brickwork, and having a revolving axis fitted with arms, whereby the acetate is brought into a finely divided state, and at the same time the mixture is thoroughly agitated. A convenient charge consists of 5 cwt. of the
salt to 3 cwt. of acid at sp. gr. The contents of the cylinder in a half liquid state are drawn off through an opening in the lower part of the front end of the cylinder and placed for distillation in cast-iron trays. These trays vary in size from 4 ft. long, 3 ft. wide, and 2i in. deep, to 18 in. square and 2i in. deep. They are transferred to the bed of any suitable furnace, and kept separate by means of rods of iron laid lengthways and transversely. Sometimes a special furnace is built, with a bed 8 ft. 6 in. long by 6 ft. 6 in. wide, and 3 ft. from sole to arch, but usually some unused iron retort is pressed into service, and answers all purposes. In either case heat is applied from a fireplace underneath, and a pipe from the further end of the furnace or retort carries off the products of distillation to any suitable condenser. The acetic acid is largely contaminated with sulphuric acid, sulphur, tarry, oily and various organic matters, from which it is purified by re-distillation with bicarbonate of soda or biehromate of potash. The charge of acetate and sulphuric acid, as above, should yield 7i cwt. of crude acetic acid of a specific gravity of For the re-distillation, cast-iron, copper, iron hued with lead, or earthenware retorts may be used, copper being upon the whole the best.
Distillation with sulphuric acid has been of late years to a great extent abandoned in favour of the newer process with hydrochloric acid, and is now carried on to a very small extent except by manufacturers of sugar of lead, &c. For some of these subsequent processes the impurities referred to are of slight moment ; indeed the crystals of sugar of lead obtained by treating litharge with acetic acid produced by the sulphuric acid process are better, finer, and of a purer colour than when hydrochloric scid has been used. A perfectly satisfactory reason for this has not been ascertained ; probably the sulphuric acid carbonizes the organic bodies contained to so large an extent in even the finest acetate of lime, and so renders them more easily got rid of. It must be remembered that, even when by repeated re-distillation the acetic acid is rendered to all appearance colourless these organic matters may still be present.
Distillation with hydrochloric acid upon a large scale is often carried out in the simplest possible manner, very little labour or plant being required. Good brown acetate, containing 70 per cent. of real acetate of lime, is dissolved in, and distilled with, an acid of sp. gr., the proportions varying with the quality of the lime salt and its constituents. Usually every 100 parts of good acetate will require 95 parts of acid. An ordinary jack( ted still and copper condenser-pipes may be used. The acetic acid comes over at 1.05 to 1.06 sp. gr., has a slight brown coloration, and a strong empyreumatic taste and smell. It is, however, sufficiently good to make a very fair sugar of lead, and is largely sold for the purpose,—also for further purification. Occasionally this rough acid is re distilled at a gentle heat before being sent into the market. It is worth about a penny per pound.