Another form of cast-iron retort, used in Germany, is given in Fig. 150, and is especially adapted to the manufacture of a strong acid. The charge consists of 700 kilos of sulphuric acid of a sp. gr. of 1681, and 600 kilos of nitrate of soda. A somewhat similar retort, closely resembling the acetate of lime pot shown in Fig. 35, is in use in England. The cover is usually formed of segments of stoneware, or flreclay " gnarls," bound together with iron. The difficulty of cleaning out the residual cake, and the continual breaking of the covers, are objectionable features.
A French furnace is shown in Fig. 151, and consists of a deep cast-iron boiler or kettle, about 11 yard in diameter, and 3 ft. deep. The advantage of this retort and method of setting is that the flames and heat envelop the whole of the vessel, and so, by keeping up a uniform temperature, preserve the metal. A double lid is employed—one of metal, fitting the kettle, and an upper one of earthenware, stone, or bricks, bound together, and luted into the brickwork setting of the furnace. It is well to line the iron tube which conveys the gases to the condensers with a glass or earthen ware tube, allowing this lining to project two or three inches into the retort.
The use of nitrate of soda has now almost entirely superseded that of nitrate of potash on acount of its cheapness. The former also contains, weight for weight, a greater amount of nitric acid—about 60 per cent. as against 53 per cent. The process is the seine whichever salt is used. Much diversity of practice exists with regard to the proportions of nitre and sulphuric acid. These may vary from the exact equivalents up to a large excess of acid. If the theoretical quantities are used, the operation takes longer and requires a greater heat. First the acid acts upon a portion of the nitre, disen gaging nitric acid and forming an acid sulphate of soda. Then this acid salt acts upon the remainder of the nitre, again liberating nitric acid and leaving, finally, a neutral sulphate. This residue forms au exceedingly compact mass which it is difficult to remove from the cylinder. More over the high temperature required decomposes a portion of the nitric acid, giving rise to peroxide of nitrogen, and oxygen, the former of which dissolves in the strong acid and imparts a red colour to it. This last disadvantage is more apparent when nitrate of potash is used. Nitrate of soda, even when only equivalent proportions are used, yields up its nitrio acid at a lower tempera ture, and the small portion that is decomposed only slightly colours the product, which may- be afterwards purified by dilution with water and the application of a gentle heat. Usually the rnannfaeturer uses a large excess of acid, proceeding entirely by rule of thumb—an excess of acid even heyniel equal weights. By this means ho saves fuel, gets a better product, and
by forming an acid, and always fused, sulphate in the retort, greatly facilitates the eleaning out operation.
Many descriptions of gl iss retorts are still used. The apparatus shown in Figs. 152 to 154 is to ho recommended, the construction of which will be readily understood. With this setting tho retorts can be removed at will. To prevent any portion being carried over into the receivers, the charge of nitro should be very carefully put into the retorts—shaken well down. Glass retorts are used when nitrate of potash is employed, and when a carefully made acid is required. At the com mencement of the operation red fumes are formed by the decomposition of a portion of the nitric acid, and the acid that first eomes over, impregnated with these fumes of peroxide, should be col lected separately. Gradually the coloration subsides, though rarely entirely absent. When the red fumes reappear it is a sign that the operation is completed, and distillation should be promptly stopped.
Passing now to the condensation of the product, the apparatus usually employed consists of an arrangement of Woulfe's bottles. These may be set as shown in Fig. 88 when treating of the con densation of hydrochloric acid. The acid that collects in the first bottle is usually very impure, in the last too weak for sale, esra cially when running water is employed to assist condensation. A good plan is to keep only a very small quantity of water in the Woulle's bottles, and connect the range with a low tower packed with coke, down which a stream of water is conducti.d. Tho weak acid may be added to the stronger, pure product of the middle bottles. The strength required depends, of course, upon the destination of the acid, and varies from to 1.5—or 100° Tw. All the acid as it first condenses is coloured by the peroxide of nitrogen, as described. For decolorization it is placed in bottles, similar to those used for condensing, sot over a furnace, or a sand-bath. A gentle heat— not moro than 85° —is applied from below until all evolution of red vapours ceases. Or, to obtain a thoroughly pure acid, the rough product may be distilled carefully in glass retorts. Peroxide of nitrogen and chlorine first come over and are separated, then a puro nitric acid is collected in a receiver, distillation being , checked when a small residue is left in the retort. This residuum contains all the iodic and sulphuric acids and whatever sulphate of soda may have collected. The iodine which helps to colour commercial nitric acid, comes from the original nitre.