Page:The New International Encyclopædia 1st ed. v. 18.djvu/372

* SODA. 318 SODA. furnace.' Usually 100 pounds of salt cake, 100 pounds of calcium carbonate, and 50 pounds of coal dust form a charge. The hand-worked fur- nace is a long reverberatory with a hopper in the roof through which the charge is dropped into the first hearth near the flue where the heat is not Char ging Ho ppgr Flue toChimney n-ePlace ealimg Hearth / Ei aporating Pan i^ X ^^ FlO. 1. LOXGITUDIXAL SECTION OF A DLACK-ASH FrKSACK. very high ; after thorough drying and heating, the" materials are raked down onto the second or 'balling hearth,' where the temjierature is usu- ally about 1000° C, and thoroughly rabbled until it becomes a thick, pasty mass from which car- bonic acid gas escapes freely. As soon as the salt cake is decomposed, the charge begins to stiffen and carbonic acid gas (CO) is evolved, as is shown by jets of blue flame (the carbonic oxide is produced by the action of coal on the excess of calcium carbonate present). The charge is then laked into a ball and removed from the furnace to an iron truck, the escaping bubbles of gas causing the pasty mass to become porous. The shallow iron i)an' between the furnace hearth and the flue to the chimney is used for the evaporation of the tank liquor obtained by the lixiviation of the black ash in the third stage of the process. The furnace operation is quite difficult, and al- though the heavy tools are suspended by chains, the temperature "is so intense that the quantity a man can handle at one time is limited to 300 pounds. In order to eliminate expensive hand- labor and to work larger charges, revolving cy- lindrical black-ash furnaces are used: the com- mon size, 16 feet long and 10 feet diameter, can treat as much as two tons of salt cake in a single charge. The lixiviation of the black ash is ac- complished in a series of terraced tanks each with a false bottom perforated with small holes. The uppermost tank is charged with black ash, and water added to cover the charge; the solu- tion of sodium carbonate formed, being heavier than water, sinks to the bottom of the tank and is passed through the perforations, and is with- drawn by means of a pipe which delivers it to the second tank in the series, through which it passes to the third tank, etc. The operation is continuous, fresh water being added to the nearly exhausted ash in the uppermost tank to yield an unbroken flow of strong liquor. Good tank liquor contains approximately 23. .5 per cent, of sodium carbonate and sodium hydrate. The French Academy of Sciences in 1775 offered a prize for a method of making sodium carbonate from salt. Among the processes submitted was that of Nicolas Leblanc. Avhich was of promising merit, and, being granted a pat- ent in 1791. he began the manufacture on a com- mercial scale. The Leblanc process is regarded as the most important discovery in the entire range of chemical manufactures, and has fur- nished about one-half of the world's supply of soda. The fact that it produces both hydro- chloric acid and bleaching powder as by-products has enabled it to survive competition, but the re- cent introduction of electrolytic processes, which also yield bleaching powder as a by-product, is a serious menace to its future. The tiolriiy jiroctss, or "ammonia-soda process,' is based on the fact that hydrogen-ammonium carbonate, (NH, jHCOj, is decomposed l)y a strong solution of common salt, yielding sodium bi-car- bonate and ammonium chloride, as shown by the equation : (XHJHC03+XaCl=NaHC03+XH^Cl. The brine is first saturated with ammonia gas, and the cooled ammoniacal liquor is subsequently charged in carbonating towers with carbonic acid gas imder moderate pressure; the sodium bi- carbonate, being much less soluble, separates out and leaves the more soluble annnonium chloride in solution, from which the ammonia is subse- quently recovered bj' treatment with lime. The sodium bi-carbonate is converted into sodium car- bonate by calcination, and the carbonic acid gas evolved is again utilized to carbonate a second quantity of ammoniacal brine. In this cycle of operations no sulphuric acid is required and no hydrochloric acid is evolved. The reactions involved in the ammonia-soda process were discovered by H. C. Dyar and T. Hemming aliout 1838. but the process was not ])er- fected until 1873. In 1803 Ernest Silvay. a Bel- gian, constructed the first successful plant, which has led to an enonnous development of the indus- try. Sodium Hydroxide. Sodium Hydr.^te, or Caus- tic Soda, NaOH. This is of importance next to so- dium carbonate only, on account of its use in enormous quantities in refining fats and vegetable oils, and in the manufacture of soap. In appear- ance it is a white solid, strongly caustic and high- ly deliquescent. It is readily soluble in water, Avith evidution of heat, and by cooling a conci'n- trated solution to 8° C. a deposit of crystalline sodium hydrate (2NaOH + 7H,0) is obtaineil. Sodium hydroxide is one of the strongest alka- lies known. On a large scale it is manufactured by the action of milk of lime (calcium hydrate) upon a boiling solution of sodium carbonate, whereby calcium carbonate is preci|)itated, and sodium hydrate remains in the solution. The re- action is Na,C03+Ca (OH) „=CaC03+2XaOH. After the removal of the solid calcium carbonate the solution is evaporated, and finally yields the solid sodiiun hydrate. One of the chief sources of supply is the tank liquor, produced in the manufacture of sodium carbonate by the Leblanc process (see above). The tank liquor, contain- ing essentially sodium carbonate and sodium hy- drate, is heated to boiling and an excess of lime is stirred into the mixture. The sodium sulphide present in the tank liquor is oxidized to sodium sulphate by the combined action of air injected into the mixture and of sodium nitrate, which is added for the purpose. The solid calcium carbo- nate is separated by filtration. The action of so- dium nitrate is shown bv the following equation: NaNO,+2H,0='XaOH+NH,+40. The oxygen set free reacts upon the sodium sul- phides present, and converts them into the sul- phate. In recent years sodium hydrate has been manu- factured to a considerable extent by the electrol- ysis of brine, also by the direct electrolysis of ifused common salt. Tlie two most recent electro- lytic processes are the Aussig 'bell process' and