840
The first reaction takes place at or about 280°Fahr., and the hyposulphite remains mostly in theliquor; the second commences about 305° Fahr.,and the sodium sulphite then formed rises and floatson the top of the pots as a white scum. The limitof the third reaction cannot be so clearly indicated;but, at any rate, the evolution of ammonia stops at360° Fahr., and nitrogen is evolved in its place. Allthese reactions occur only with an excess of nitre,and some secondary reactions also take place.
The settled liquor from the strong boat pans is nowfurther concentrated in “ pots ” (Figs. 32, 33). Theseare circular pans, constructed to hold 10 tons of fin-
Fig. 32.
Fig.30.
ished soda ; they are 9 feet in diameter and 5 feet 6inches in depth, and weigh about 6£ tons each. Thereis no fixed rule for setting them, but the common aimis always to divide the heat, so that the flame may playupon the pot all round; otherwise they would soonburn through in particular spots. The side of thepot against which the fire is built is nearly alwaysprotected from the direct action of the flame by ashield of brickwork, against which the flame splits intwo portions, which play round the pot and uniteagain at its other side to pass into the flue. Generallythe pots rest on a metal plate, and are stayed in anupright position with brickwork. The chief supportis given by the circular wall surrounding it, andforming the side flues, into which three lugs, cast onto the pot in equal distances about a foot from thetop, project. The metal plate just mentioned facili-
tates the turning of the pots, which is advisable toprevent them wearing too much in particular places,and the same object is assisted by an overheadtravelling crane, which also greatly facilitates theirputting in when new and removal when worn out.Sometimes pots are cast in two pieces, viz.—a lowersemi-circular piece, and an annular piece bolted tothe upper side of it; the latter lasts much longerthan the former. Heating the pots by generatorgas has been tried, but has been found to beuneconomical.
The liquor in the pots is kept boiling till a tem-perature of 320° Fahr. is reached. Beginning from290° Fahr. a scum forms on the top, sometimes of asemi-transparent straw colour, sometimes red, andeven black; at the same time the evolution ofammonia becomes very strong. This froth is some-times not removed, when only 60 per cent, causticis to be made; but when 70 per cent, has to be made,its removal is imperative. It is then taken off bymeans of a perforated scoop. Its compositionseems to be the same whether nitre has been addedin the boat pans or not, as shown by the followingtwo analyses (by Davies), of which A refers toscum from a liquor boiled to 320° Fahr., oxidisedduring causticizing, but not nitred ; whilst that in Bhad been nitred in the boat pans :—
A.
J3.
Iron peroxide,.
traces.
traces.
Iron sulphide,.
0*139
0-346
Sodium sulphite,.
51-408
57-120
44 hyposulphite,.
none.
traces.
44 sulphate,.
6-831
5-377
“ chloride,.
5*265
6*172
44 silicate,.
0*225
traces.
44 aluminate,.
traces.
traces.
44 carbonate,.
15*408
13*013
44 hydrate,.
20*800
17*672
Total,..
100*076
99-700
For the prepartion of 70 per cent, caustic thereare still too many “salts” in the liquor, and toreach that strength with certainty it is best to allowthe pot, after reaching 320° Fahr., to cool and settleagain.
The following shows the composition of thesettlings:—
Iron peroxide,. 1*054
Sodium sulphite,. 3*003
44 hyposulphite,. 0*442
44 sulphate,. 1*146
“ chloride,. 41*535
“ silicate,. 0*174
“ aluminate,. 0*544
“ carbonate,. 11*488
“ hydrate,. 19-840
Water, (by difi.). 20474
The boiling of the liquor is then continued, andwhen the temperature has reached 355°, it will soli-dify well on cooling, and contain 53 per cent, ofalkali. At this stage the liquor is exceedingly aptto rise in the pot and boil over; this has to be keptdown by ttie workmen beating down the scum, or bypouring in some oil. It is especially risky at thisstage to run in any fresh liquor'. When the temper-