SODA 



553 



From this solution large crystals of almost pure 

 carbonate of soda separate. Ordinary washing- 

 soda consists of these crystals, which are of uniform 

 composition and easily dissolved. They contain 

 ten molecules of water that is to say, they are 

 composed of 37 per cent, of carbonate of soda and 

 63 per cent, of water. Crystal soda being quite 

 free from caustic-soda and other compounds acting 

 on the skin, the hands of washerwomen suffer less 

 from it than from other kinds of alkali. 



Bicarbonate of Soda, 2NaHCOs. As will be 

 presently explained, this salt is now prepared on a 

 large scale as a stage in the ammonia-soda process. 

 See SODIUM. 



Sulphur Recovery. The recovery of sulphur from 

 the exhausted black-ash, which forms the waste 

 heaps of the alkali-maker, is now, after many unsuc- 

 cessful attempts to do so economically, practised on a 

 large scale by Chance's process patented so recently 

 as 1888. This residue, as has been stated, is 

 essentially calcium sulphide, which when brought 

 into contact with carbonic acid in the presence of 

 water is converted into carbonate of lime, and 

 sulphuretted hydrogen is liberated. The practical 

 difficulty had long been the getting of hydrogen 

 sulphide in a sufficiently concentrated state. Mr 

 Chance's process is as follows : The vat mud 

 (black-ash waste) has the coarser extraneous pieces 

 removed by a sifting process, and is then made 

 into a creamy consistency with water. In this 

 state it is distributed into a series of cylindrical 

 iron vessels for the purpose of having carbonic acid 

 passed through it. These cylinders have three 

 main pipes passing over them, with branches to 

 each. By one pipe the carbonic acid is introduced 

 at the bottom of the liquid, and the other two lead 

 the gases away from the top of 'the cylinder. 



The carbonic acid is produced in a limekiln, and 

 passes, unavoidably mixed with nitrogen, into the 

 cylinders, which successively become richer in 

 sulphur compounds. The result is that for a time 

 little else than nitrogen comes away from the 

 last cylinder. But when the reactions in the 

 cylinders are completed the final vessel of the 

 series gives off gas rich in sulphuretted hydrogen. 

 By means of stopcocks one of the pipes at the top 

 of each cylinder conveys the nearly unmixed nitro- 

 gen to an open chimney, and the other takes the 

 rich mixture of nitrogen and sulphuretted hydrogen 

 to a gasholder. When the carbonic acid from the 

 limekiln passes into the first cylinder containing the 

 black-ash waste, carbonate of lime is produced with 

 evolution of sulphuretted hydrogen, HuS. The 

 latter passes on with the excess of nitrogen into the 

 second cylinder, where there is formed sulphydrate 

 of calcium, CaHjSj, which is a compound of HzS 

 and CaS. In this way we have the sulphur con- 

 centrating from first to last in the series of cylin- 

 drical vessels until it is finally expelled as 

 sulphuretted hydrogen gas. In the process the 

 carbonic acid combines with the calcium of the 

 sulphydrate, giving off the two atoms of sulphur 

 an HiS, go that for a given amount of carbonic acid 

 used we get a double quantity of sulphur. All the 

 time the vessels at the beginning of the series 

 remain nnsaturated, the nitrogen, amounting to 

 about 70 per cent, of the gases pumped in, passes 

 away in pipes, and is allowed to escape. It 

 contains little or no sulphur; but by-and-by the 

 gas in the vessels consists of from 30 to 3o per 

 cent, of sulphuretted hydrogen and from 1 to 2 

 per cent, of carbonic acid, the remainder being 

 nitrogen. This mixed gas is collected in a gas- 

 holder to enable it to be treated as required. 

 The carbonated mud left in the vessels is drained, 

 and used in place of limestone in the black-ash 

 furnaces, so that any soda this dried mud contains 

 is recovered. 



The sulphur is obtained from the gas in the gas- 

 holder in a very pure state in cakes and flowers of 

 sulphur by this operation. A definite mixture of 

 the sulphuretted hydrogen (present in this gas) and 

 air is passed through a layer of anhydrous oxide of 

 iron in a Claus kiln, the oxygen present being only 

 enough to unite with the hydrogen (of the BUS) to 

 form water, the sulphur being set free. Iron oxide 

 has the power of producing the combination without 

 itself suffering change, the bed of this material 

 becoming (without the use of fuel) sufficiently 

 hot, by the chemical change which goes on, to 

 volatilise the sulphur vapour along with the steam 

 produced. The change is represented by the 

 formula HjS + O = HjO + S. But the sulphuretted 

 hydrogen may also be itself burned to make vitriol, 

 which is obtained of the same purity as when made 

 from sulphur. 



The Ammonia-soda process has within the last 

 few years come into competition with and threatens 

 to supersede that of Leblanc. It is based on the 

 mutual reaction which takes place at ordinary 

 temperatures between common salt and bicar- 

 bonate of ammonia in strong aqueous solutions. 

 The sodium of the salt combines with the carbonic 

 acid and the chlorine with the ammonia, giving 

 bicarbonate of soda, which is insoluble, and chloride 

 of ammonium, which remains dissolved in the 

 liquid, thus : 



NaCl + (NH 4 ) HCOs =NEUC1 + NaHCOj. 



The ammonia is recovered from the chloride and 

 one-half of the carbonic acid from the bicarbonate 

 for future use. Where possible natural brine is 

 used, and this is brought to a specific gravity of 

 nearly 1200, either by the addition of salt if too 

 weak or by adding water if too strong. Ammonia 

 in the free gaseous state is now passed into the 

 brine until the required quantity is present, which 

 is known by the amount of increase in the volume 

 of the liquid. The ammonia enters a mixing tank 

 under a perforated diaphragm, and the liquid is 

 kept in agitation. A great rise of temperature 

 is caused by the condensation of the gaseous 

 ammonia, and this necessitates the running of a 

 stream of water through a coil of piping inside 

 the mixer to keep the heat as low as possible. The 

 brine in running off passes through a filter to retain 

 solid impurities, and then through another worm of 

 piping surrounded by cold water. To form the 

 bicarbonate of soda the ammoniacal brine requires 

 to be saturated with carbonic acid. Air-pumps 

 draw the carbonic acid from the limekiln and force 

 it (after being properly cooled) at a pressure of 

 nearly two atmospheres in at the bottom of a tower 

 50 feet high, which is kept nearly full of the liquid. 

 This tower has perforated plates at every three feet 

 of height to make sure that the gaseous bubbles 

 are spread equally through the liquid. Every 

 half-hour some of the pasty mixture in the tower 

 is run off at the bottom. This is full of the 

 small crystals of bicarbonate of soda, and these 

 are separated by running the mass over a wire- 

 gauze filter covered by a cloth, a vacuum being 

 maintained below. The bicarbonate of soda on the 

 filter is nearly pure, and the liquid which passes 

 through is ammonium chloride. The bicarbonate 

 thus obtained is washed with water and carefully 

 dried in apparatus of which there are various forms. 

 As there is a comparatively limited demand for 

 this kind of soda, it is afterwards heated in close 

 vessels in which half of its carbonic acid is given 

 off, thereby reducing it to the normal or common 

 carbonate of soda (soda-ash). The gas given off is 

 pumped back to the tower and used along with the 

 kiln gas for carbonating fresh material. To expel 

 any ammoniacal salts adhering to the carbonate of 

 soda and render it denser for packing, the heat is 



