To sum up the whole :—Probably the ammonia soda will gradually supersede the refined alkali of the Le Blanc process. But this covers very little ground. Soda ash, crystals, and caustic soda, will probably hold their own against the new comer. And it must be remembered that improve ments are daily being made in the old method. Weldon's manganese recovery process, Hargreaves' direct method of sulphate manufacture, revolving ball furnaces, and various other processes and improvements havo done very much in the way of reducing costs and utilizing waste products.
The arrangements of the ammonia process differ in the various works. The best method, perhaps, is to introduce the saturated solution of common salt, testing 23° or 24° Tw., into a. vessel fitted with a false bottom and saturate it with ammonia. A considerable amount of beat is generated, and the solution has to be cooled by a worm or any other convenient apparatus. It is then transferred to the absorbers—huge cylinders some 30 feet or so high—and a stream of carbon dioxide, obtained from limestone in the usual way, and washed, is introduced near the bottom. The liquid when saturated is drawn off, the bicarbonate of soda separated by filtration, waahed, and passed down a tower where it meets hot air or steam and is converted into the neutral carbonate. The disengaged carbon dioxide is conveyed to the absorbers.
Utilization of Tank Waste.—Of all the evils and difficulties of the Le Blanc process, perhaps the greatest has been the enormous waste residue from the lixiviating vats, and bow to deal with it. That it contains substances of great value has always been a patent fact, and many attempts at its utilization have been made. The most notable are the manufacture of hyposulphite of soda, and the various processes for the recovery of sulphur. The former will be treated of under the bead of Hyposulphite of Sodium. It may, however, be mentioned here, that the demand for byposulphite is so limited that only a. small portion of the tank waste produced can ever be em ployed in its production. Only a portion, too, of the sulphur compounds in the waste, which form the objectionable drainings from the heaps termed " yellow liquor," are destroyed in the process. The regeneration of the sulphur has been dealt with in the patents of Gossage, Delamure, Kopp, Losh, Noble, Fevre, Spencer, Mond, Leighton, Schaffner, Huffman, Fowler, Duclos, Bell, Jullien, 1VIactear, and others too numerous to mention. Only four, however, those of Mond,
Schaffner, Hoffman, and Mactear, have been worked with anything like success, and these only need be mentioned.
The process of Mond, patented in 1863, is the best known, and is as follows : —In addition to the tanks in ordinary use for lixiviating, one-and-a-half times as many more are provided. That is to say, six extra tanks for every set of four. In these extra tanks the waste is exposed to a strong current of air, forced through a false bottom with which the tanks are provided. This current is produced by any :convenient form of fan blast, and is passed through the waste for a period of from twelve to eighteen hours, depending upon the quality and texture of the material. During the oxidation process the temperature rises to about 140° (220° F.,) and clouds of steam are given off. The surface of the waste becomes covered with bright yellow spots, their appearance being a guide to the workman as to tbe progress of the operation. Weak liquor from a previous lixiviation is then run upon the waste for eight hours or so, until, starting at about 18° Tw., the density falls to 10°. Water is then run on, and the product collected as weak liquor to be used for the next tank. This process of blowing and washing is repeated three times, the quantity of strong liquor obtained being less each time. If the operation be conducted in the ordinary tanks, a sufficient number of extra ones not being provided, only two blowings and washings can be got through, but the waste is then anything but exhausted. Any of the sulphur liquor left in the tanks, too, spoils the ball lixiviation. The liquors from the bleaching operation consist of sulphide, sulphite, and hyposulphite of lime, together with small proportions of similar sodium compounds. They are allowed to settle thoroughly in a series of tanks, and then run into a wooden vessel lined with lead, some 10 ft. in diameter, and 5 ft. deep, treated with weak hydrochloric acid from the condensers, and heated to 65° (150° F.) with steam. Viewed in the simplest possible form, the following reaction takes place :— 2CaS CaS20, 611C1 = 3CaCl, + 311,0 + 4S.