From the settlers, or " strong " pans, the liquor is run into the " pot." These pots are made of cast iron, about 9 ft. in diameter and 5 ft. deep. In shape, they resemble the soda cone shown in Fig. 262, but are deeper and much stouter. They are set after the fashion of a sulphate pan, the fire not being allowed to play directly upon the pot, but escaping from an under arch and passing round the sides by a circular flue. With all precautions, the breakage of caustic pots is a con stantly recurring evil. Sometimes the bottom rests upon a plate to facilitate the turning of the pot from time to time, by which operation a fresher surface is presented to the more direct action of the fire. An overhead crane is an advisable adjunct to assist in the turning or replacement of these pots, which weigh up to 61 tons each. Instead of working a charge off in one pot, a common arrangement is to have three pots, and bale the liquor, as it concentrates, from one to another. The pots should of course be set successively one below the other. When a temperature of 121° (250° F.) is attained, the liquor begins to froth up, and a strong smell of ammonia is given off, from the decomposition of the cyanogen compounds. Soon after, at 143° (290°F.), a dirty black scum rises to the surface, and is skimmed off, very carefully if 70 per cent. white caustic be required. At 155° (311° F.) the liquor boils rapidly, and turns a very dark colour. The deposition of salts goes on, and if a 70 per cent. product be required, the pot is cooled down when a temperature of 160° (320° F.) is reached, and the salts are fished out. Concentration is continued to 224° and finally to 260°. About 182° (360° F.), the pot is very liable to boil over, and the workman in charge has to give every possible care to prevent this by heating down the froth or adding a little grease or oil. Between 200° and 260°, the liquor boils very gently, and contains at the former temperature 60 per cent., and at the latter 65 per cent. of alkali. The ammoniacal vapours, mixed with a little caustic soda which is mechanically carried off, are at this stage exceedingly irritating. The cyanides are destroyed with separation of graphite and also nitrogen. Soon all motion ceases. The pot is then covered up and the fire urged until the contents are at a red heat. This process is termed "clearing." The sulphur compounds are now finally oxidized, either by the addition of nitre, or by blowing in air. The addition of nitre must be made very carefully so as not to overdo it, or turn the caustic green. From time to time a small sample is taken out, allowed to solidify, and tested with a few drops of acetate of lead. As a rule 1 ton of caustic will require 40 lb. or so of nitre. If the pot has been over-oxidized—a trace of sulphide should be always left—a few balefuls of fresh liquor are added. Oxidation by a current of air is now very generally practised, an ordinary blowing engine being employed for the purpose, with a short quick stroke. All water is perfectly discharged from the cylinder when starting to blow, and the delivery pipe is then connected with a strong le in. iron pipe, terminating in a perfomted ring which is pinnged into the liquor, and rtsto upon the bottom of the pot. The clearance of water is rendered necessary to avoid any possible explosion if it came in c,ontact with the red-hot liquor. The same blowing engine which supplies the causticizer may be nsed for the finishing process. Blowing is continued for about three houra in the caae of liquora which have been partially oxidized in the pans, and tor about eight hours if the sulphides have been all allowed to come through. The testing with acetate of teal for complete oxidation ia practised aa the operation proceeds. When it is finished a sample is taken from the pot, and tested for alkali. If only 60 per cent. be required the necessary reduction ia made by addiug common aalt in very email quantitiea at a time to prevent any violent deflagration. lf 70 per cent. be required, the sample abould test fully 72 in tile pot. Anything under 70 per cent. ia made into 60. After "aalting," the pot is heated np ag,ain, and then allowed to stand for eight or twelve hours'. The oxide of iron and aluminate of aoda settle down, leaving elear caustic supernatant. If a fine quality is required thia settling must be very thorough. The cauatic is now ready for packing into the well-known sheet iron "drums," which hold about 6 cwt. each. The drums are ranged round the pot and filled slowly, preferably at ahort intervala, by means of an iron apont. This slow and intermittent filling is necessary to givo absolutely full drums. The compoaition of GO and 70 per cont. cauatic is s.s follows :— Only the clear liquor ia baled out iuto the first sot of druina. The residue, irnpure caustic, LS
packed aeparately, and forms what is known as " bottoms "—an article containing from 54 to 62 per cent. of alkali—or is broken up when cool, diasolved to 48° Tw. hot, and the clear liquor, after settling out the iron, silica, and alumina, cauaticized again. Sometimes the "bottoma " are added to the red liquors, and go to make cream caustic.
Deacon haa patented a process for obtaining eryatala of caustic soda by evaporation uutil a temperature of 177° (350° F.) is attained, cooling down to 70° (158 F.), and crystallizing in cast iron cones. The hydrate that separatea has about the following composition :— Sodium oxide .. 50'5 I Sodium chloride .. 1.8 „ hyposulphite 0.5 Water .. 47.2 ' Parnell and Simpson have lately introduced -a high-pressure causticizer, which seems to be a decided improvement upon the older, opeu plan. The waste of heat cauaed by the ateam passing through the liquor and away from the top ia avoided, and a conaiderablo saving in fuel and labour effected.
Cream caustio is made either from roughly worked tank liquor, or red liquora—chiefly the latter. The colour of these liquors is probably due to the presence of a soluble double aulphide of iron and sodium. They are firat allowed to aettle well, then concentrated in any convenient aelf fired pan to 703 Tw.—a t,emperature of 121° (250° F.) being attained. They are then allowed to cool, aud the salta which have in the meantime separated, are fished out. The heat is again applied till the density reaches 95° Tw., and during this second stage, a quantity of nitre is added to oxidize the liquor& After eettling a little, the liquors are run off into pans, and allowed to atand till quite clear. They are then tmnsferred to a pot and fired until the batch tests 60 per cent., or thereabouts, for alkali. No oxidation in the pot is resorted to, and the contents are not fuaed aa in the case of the white caustic. The whole of the pot is then baled out into drums, leaving no " bottoms." The colour, due to the presence of iron oxide, is very variable. If too red, the product is sold aa inferior caustic, or worked up into white, by oxidation and fuaiou. Cream caustic uauttlly contains about five per cent of carbonate, 7 per cent. of chloride of sodium, 2 per cent. of aulphate, and 15 per cent. of water. The salts from the Battlers are washed and worked up in the carbonate process.
The cost of 70 per cent. caustic soda ia about 11/. per ton ; of white 60 per c,ent, 10/. per ton; of crearn GO per cent., 9/. 10s. per ton. The amount of bottoms average about 7 cwt. per 10-ton pot. 100 tone of salt cake should yield 55 tons of 60 per c,ent. caustic.
A few worda more may bo devoted to Bachet's process for manufaoturing caustic soda, which produced some considerable atir ten years ago, but ia now practically abandoned. It was carried out upon rather a large scale at the Walker-upon-Tyne works, and strangely enough, is based upon the old process of Dundonald and Losh, carried on in the same works eighty years ago, viz. the decomposition of salt by litharge. A mixture of 100 parts of litharge, 70 parts of salt, and 50 of lime, is ground into a paste in a mill. Decomposition ensues, chloride of lead and caustic soda being formed. The solution is pressed out, and freed from lead by filtration through hydrate of lime. It is then worked up to a 70 per cent. caustic in the usual way, the common salt which it contains being fished out during concentration. The cakes left in the press are dried at 300° to 350° F., the hydrate of lead being thereby converted into yellow oxide. The whole mass is then treated with boiling lime water to decompose the chloride, and the regenerated oxide used over again to decompose a fresh batch of salt. The reactions in this process have proved to be difficult and only partial. A considerable loss of lead also is sustained.
Snch are, briefly, the details of the several branches of the soda manufacture as carried on in this country upon an immense scale. Starting from the commencement of the present century, the growth of the trade has been almost beyond belief. As nearly as possible 700,000 tons of salt per annum are worked up into sulphate, and the total statistics of the trade are probably about as follows :-2,500,000 tons of raw material, pyrites, salt, chalk, timber, coal, manganese, &c., costing 2,000,0001., turn out finished products of the value of 3,500,0001., even at the depressed prices of the day. The value of the plant is about 4,500,0001., the number of factories about 120, and the number of hands employed about 40,000. In these estimates the allied branches of hydrochloric acid and bleaching powder, &c., are included.