828
SODA.— Carbonating-furnace.
seven-twelfths of its volume, and the salt precipitat-ing in the meantime is fished out, a product testing57 per cent, real soda is obtained. If the remainingliquor be boiled down to three-sevenths of its volume(= one-quarter of the original one), the productfished during this time will yield 50 per cent. ash.The residual liquor, dried in the furnace, yields ahighly caustic ash. Kuiilmann for this purposemarks the sides of his pan correspondingly. Kolbhas made special experiments on this subject, whichdo not quite harmonize with Kuiilmann’s statements.According to Kolb, when a liquor containing sodiumsulphate, carbonate, chloride, and caustic, is boileddown, the sulphate is generally separated first—allthe more quickly the more caustic the liquor is. Atthe same time the precipitation of carbonate com-mences; this, however, is influenced by the presenceof sodium chloride. If none of the latter salt ispresent, the first precipitates are poorer than thesubsequent ones, because they contain more sul-phate. If, however, much sodium chloride bepresent, the behaviour is an opposite one. Thefirst precipitates are richest in soda, and later onmore and more chloride is mixed with them. If theliquor contains, at the same time, a great deal ofsulphate and chloride, the percentage of the pre-cipitates in carbonate first increases, then it becomesstationary, and at last it decreases again. Thecaustic soda, which favours the precipitation of allthree salts, remains in the mother liquor up to thelast.
Carbonating or Finishing. —This operation com-pletes the manufacture of ordinary soda ash. Itwould seem to those not acquainted with prac-tical alkali making, as if both the furnace and themode of operation in converting the drained “black-ash” must be of the simplest character. This is,
however, only the case with very pure “ fished ” salts.These have merely to be dried by moderate heatto expel the water of crystallization, and the prin-cipal matter to be observed is this, that the heatmust be kept sufficiently low not to melt the sodiumcarbonate; but this is easily avoided, as sulphideand caustic are practically absent. The matter isvery different when the “black salt” from openpans (that is, those with surface evaporation) has tobe treated, since it contains a considerable propor-tion of sodium hydrate and sulphide, and at thesame time the sawdust, which is intended to supplythe carbon for carbonating them; and a great dealof care has to be bestowed upon carrying out theseoperations successfully, so as to obtain ultimatelya good ash.
A carbonating furnace of very good constructionis shown in Figs. 25 to 28. Fig. 25 is a sectionthrough e — f, Fig. 26; Fig. 26 is a horizontal sectionthrough A — B, Fig. 27 ; Fig. 27 is a section through
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C—d, Fig. 26; Fig. 28, front elevation. It will be seenthat the fire-bridge in this furnace is unusually high,and indeed only leaves a crescent-shaped slit for thefire to go through. Sometimes only one bed is used,
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but more frequently two, as shown in the drawings.The black salt, where the levels allow it, is notcharged through the door, but through a hopper,not indicated in the drawings, over the centre of theback bed. It is not necessary to bind these furnacesquite so strongly, or to provide them with hollowbridges and bridge plates, as the heat in them is
nothing like that of ball-furnaces. When a singlebed is used, it should be about 12 feet long by 8 feetwide ; when two beds are used, they are about 8 feetsquare each. In a few cases the furnace bed is madeof thick cast-iron plates; in the majority of works,however, it is made of fire-bricks on edge, closelywedged together. The waste heat from this furnace