Page:The American Cyclopædia (1879) Volume XV.djvu/158

 150 SODA lidifies. The normal carbonate, existing in cer- tain lakes in Egypt and Hungary, and in the volcanic springs of Iceland and North Amer- ica, often containing sesquicarbonate, was long known in commerce as natron. Large quan- tities of it and of other soda salts occur in the form of an efflorescence on the " alkali plains " of the western territories. It was formerly pre- ji.-in-il artiticiiilly from kelp, or the ashes of sea- and fuci, and also from barilla, the semi- fused ash of the saltola goda, a plant which has been cultivated with great care by the Span- iards, especially in the vicinity of Alicante, the seed being sown in light low soils which are irrigated by sea water. Barilla yields much more soda than kelp, the latter being now prin- cipally used for obtaining iodine. But the quantity of soda obtained from barilla is small in comparison with that manufactured by the process of Leblanc, which consists in first con- verting chloride of sodium or common salt into sulphate of sodium or Glauber's salt, and then converting the sulphate into carbonate by heat- ing it with carbonate of lime and coal. The conversion of common salt into sulphate or "salt cake" is called the "salt-cake process," and is effected in a salt-cake furnace. One of the best forms of furnace contains two irdn vessels or retorts placed in separate heating apartments or furnaces, but connected with each other by a neck. Into the first vessel, called the decomposer, which is oval, are in- troduced 5 or 6 cwt. of common salt and a rath- er less weight of sulphuric acid of sp. gr. 1*78, and a gentle heat is applied. Hydrochloric acid is evolved and passes off by a flue to con- densing towers containing fragments of coke or stone, through which water is allowed to trickle. There are two towers, the first one receiving the vapors at the bottom, passing what are not absorbed to the top of the other, from the bottom of which the residue, mostly air and some impurities, issues and passes into a large chimney. In the first vessel about half the salt is decomposed, when the pasty mass, consisting of acid sulphate of sodium and undecomposed salt, is thrust into the second vessel or roaster, which is heated to a high- er degree, and the decomposition completed. The reaction in the first vessel is as follows 2NaCl + H,S0 4 = NaCl + NalI,S0 4 + HC1. In the second vessel the acid sodic sulphate re-
 * K-N upon the unchanged salt, the hydrogen

taking the chlorine to form hydrochloric acid, K-avinp two molecules of sodium to unite with tli- snlphion, SO 4 ; thus, NaCl+NaIIS0 4 = I Ifl + Na,SO 4. The hydrochloric acid gas from both vessels passes through the same flue and condensing towers. The neutral sulphate or " salt cake " is then removed from the sec- ond chamber, reduced to powder, and mixed with powdered chalk and coal, in the propor- tion of two parts each of sulphate and chalk and one part of coal. This mixture is then thrown in quantities of from 2 to 3 cwt. into a reverberatory furnace, and melted while be- ing stirred. The mass is then raked out into a mould from which it is turned when cold, forming ball soda, or black ash, which contains from 20 to 27 per cent, of pure soda or neu- tral carbonate, minus its water of crystalliza- tion, and mixed with calcium sulphate, quick- lime, and unburned coal. The reaction is represented as follows: Na a SO 4 -hCaCO s +40 =Na3C0 8 + CaS-|-4CO, the chemical changes consisting firstly in the deoxidation of the salt cake, and its conversion into disodic sul- phide with evolution of carbonic oxide,. and secondly in the formation of sodic carbonate and calcic sulphide by interchange of the con- stituents of the disodic sulphide and calcic car- bonate. The sodium salts are extracted in a series of vats, by warm water which passes from one to the other. Calcium sulphide, which is formed in large quantities, was for- merly a waste product, but is now partly util- ized in the preparation of hyposulphite of soda, which has been employed to a considerable extent as an "antichlor" for removing the last traces of chlorine from bleached paper pulp. The black solution obtained by the lix- iviation of the black ash is allowed to settle, when it is pumped into iron pans and evapo- rated by the waste heat from ' the furnaces. Much of the salt crystallizes during ebullition and is removed by perforated ladles. The mother liquor retains a portion of caustic soda, which may be converted into carbonate by mixing it with sawdust and roasting in a rever- beratory furnace. At present, however, this conversion into carbonate is not much prac- tised, but the caustic soda is extracted accord- ing to the plan of Mr. Gossage, already de- scribed. The crude carbonate is crystallized by redissolving it in hot Avater, allowing this to become clear by standing, and then running it into deep pans, having a capacity to yield about one ton of crystallized carbonate. The solution cools in five or six days, and large crystals are formed. The mother liquor yields an inferior ash. Sodic carbonate, or commer- cial neutral carbonate of soda, has a nauseous alkaline taste, and crystallizes in large trans- parent rhomboidal prisms, containing 10 mole- cules of water, which melt in their water of crystallization, are soluble in any proportion of hot water, and are also very soluble in cold water. The salt easily parts with its water, and melts at a red heat. If it is crystallized at a temperature of 4 F., 15 molecules of water of crystallization are taken up. Mit- scherlich obtained sodic carbonate with six molecules of water of crystallization. Above 93*2 the salt crystallizes in forms derived from the square-based octahedron, containing five molecules of water; but between 158 and 176 it crystallizes in four-sided prisms con- taining only one molecule of water. The max- imum solubility of soda in water is at 100*4. The principal uses of commercial carbonate of soda are in the preparation of the bicarbonate and of caustic soda ; in the manufacture of hard