SODA.—Caustic.
839
Other works use two wrought-iron, or, again, twocast-iron pans ; and no fixed opinion seems to existwhich of the two kinds of metal is preferable for“ weak ” pans—that is, such as are heated by wastefurnace gases. The “ strong” pans, however, whichhave to finish the boiling previous to the “ pots ”themselves, are nearly always of cast iron.
The pans for strong liquor have fires of theirown—viz., two fire-places, about 4 feet 6 inches by2 feet 3 inches, arched over, and built in front ofthe two flues which have been described previouslyas running underneath the pans, so that each fluehas a separate fire. At the back of the pan the twounite in a common drop; but often the fire first passesunderneath a second or even a third pan—at leastin the case of east-iron pans, which are always muchshorter than wrought-iron ones. . The caustic liquorin the back pan does not get up to ebullition com-pletely, and only reaches about 28° Tw. In thefront pan it gets up to 45° or 50° Tw., but oven thenno salts are deposited, and it is then run into the“ strong ” pans.
Some manufacturers, however, attain a greaterstrength in the “weak” pans, and consequentlyhave to begin “fishing” the salts in the same.This must always be done from the strong pans,which are supplied by a constant feed from theweak ones; and when a specific gravity of 60°TV. has been arrived at, in some works theaddition of nitre is commenced with a few pounds.The evaporation is continued till the liquor reaches70° to 72° Tw., if for 60 per cent, caustic, or 80° to85°, if for 70°; then the fires are allowed to burndown, and the pans to cool for an hour or two ;the clear liquor is then drawn off through a swivelpipe into shallow settlers, and the salts remainingon the pan-bottom are taken out and placed on asmall drainer by means of a perforated shovel, fromwhich, after thorough draining, they are removedfor use in the black-asli furnaces. The strong pansare run off about once in twenty-four to thirtyhours, and about five of them are necessary to makeup one 10-ton pot of finished caustic soda.
In the settlers which feed the pots a small addi-tional quantity of “salts” is deposited, but oulysufficient to necessitate a weekly or fortnightlycleaning; if they are dispensed with altogether(which should not be done when absolute immunityfrom carbonates is desirable), the settling time in thestrong pans must be increased to six or eight hours(Morison).
Davies recommends strongly, instead of usingthe hydrometer, ivliose indications are rather un-reliable for such high degrees and temperatures,to use the thermometer tor fixing the time of run-ning off the liquors, since the boiling point risescontinually with the concentration, and in this case280° Falir. give the limit for running off the strongpans when 60 per cent, caustic is to be made. Healso recommends to run weak liquor on to the salts,which causes them to be more easily detached forfishing.
The following are analyses of fished salts:—•
Davies,
Morison.
A.
B.
Insoluble matter (chiefly FeS),
0-742
1-004
_
Sodium sulphide,.
0-116
0-097
_
“ hyposulphite,.
0-274
0-718
_
41 sulphite,.
3-723
2-695
_
“ chloride,.
7-024
5-834
0-50
“ sulphate, .
22-773
26-432
29-00
“ silicate,.
traces.
traces.
_
44 aluminate, .
traces.
traces.
_
“ carbonate . ..
21-559
24-218
39-00
“ hydrate, .
8-417
10-116
9-00
Water (by difF.),.
35-372
28-886
17-00
Total, .
100-000
100-000
-
The salts deposited in the settlers after runningfrom the strong pans, and before running into the
caustic pots (which operation is not always per-
formed), are more the result ofby cooling. The following areof these salts :—
a real crystallizationanalyses by Davies
A.
B.
Insoluble (mostly FeS),.
1-143
0-867
Sodium sulphide,...
traces.
0 077
44 sulphite,.
2-734
1-013
44 hyposulphite,.
0-316
0-146
44 sulphate,.
9-768
5-772
“ chloride,.
33-674
60-443
44 silicate,.
traces.
traces.
44 aluminate,.
traces.
traces.
44 carbonate,.
15-052
5-683
44 hydrate,.
13-440
10-773
Water,.
19-873
15-226
Total,.
100 000
100-000
These salts are more of a “sloppy” nature thanthose got out of the pan. They form in muchsmaller quantity.
It has been mentioned above that in some workssodium nitrate is added to the liquor in the boatpans, which is supposed to “throw down ” the salts;but care must be taken to add less than necessary,as if, on the contrary, more than necessary be used,the excess will be lost by separation with the salts.If the liquors have been thoroughly oxidised duringthe operation of causticizing, no nitre is necessary atthis stage. According to Pauli, as long as theboiling point of the liquid is between 138° and143° C. (280° and 290° Fahr.) the sulphide is quietlyoxidised to sulphate, with the formation of sodiumnitrite; at 155° C. (311° Fahr.) ammonia is evolvedwith violent effervescence and in large quantities.Davies doubts the correctness of this statement, asaccording to him, helow 280° Fahr. much ammonia isevolved and little sulphite is formed. Ilis researcheshave led him to believe that the oxidation of sodiumsulphide progresses through the gradual stages ofhyposulphite, sulphite, and sulphate, the reactionsbeing:—
(1) 2N%8 + NaNOg + 3II s O = Na 3 S a 0 3 + 3NaIIO
■f NH 3 .
(2) 2Na 2 S 2 0 3 + 3NaHO + KaNO, = 4Na 2 S0 3 + Nil,.
(3) 5Na a SO,' + 2NaN0 3 + II»0 = 5Na 2 S0 4 + 2NaHO
+ IS