818 SODA.— Black-asii. Lixiviation.
for the insoluble soda, which he assumes to be allin the state of a double silicate of sodium andaluminium. The black ash was made with Buxtonlimestone. A and B are good samples, C was a“ red ” or “ burnt ” ball.
A.
B.
c.
Sodium carbonate,.
28-144
31-807
28-236'
“ oxide.
5-860
5-614
3-844
“ chloride,.
2-808
2-574
3-101
“ sulphate.
0*192
0-190
3-037
“ sulphite,.
0*151
0-072
none.
“ hyposulphite,.
. 0-189
0-853
0*126
“ sulphide,.
0-358
0-163
6-645
o
“ aluminate,.
0-344
0-752
0-923
“ silicate,.
1-026
0-914
0*758
“ cyanide,.
0*186
0-043
0-422
“ sulphocyauide, . .
0-074
0-021
0-077
Calcium sulphide, .
29-504
28*744
33-2451
“ carbonate, .
12-657
9-272
6-087
“ oxide, .
10-048
9-488
3-465
Iron sulphide, .
0*554
0-774
1-355
(U
Alumina, .
0-172
1-042
0-624
p
Silica, .
1-095
0-923
0-973 ( ?
Magnesia,.
0-266
0-322
0-146
a
Soda,.
0-344
0-546
0-577
Carbon,.
4-263
4-483
4-958
Sand,.
1-237
0-875
0-842
99-472
99:472
99-641
Soluble iron sulphide,.
—
—
0105
The analyses given in the above tables representthe silica as combined with one or two of the basesonly; this is, however, merely an assumption on thepart of the chemists who made the analyses. It iswell known that at least a portion of the silica existsas a compound with aluminium, calcium, magnesium,and sodium, which is insoluble in water, but solublein acids. The totally unavoidable formation of thiscompound is just the cause of the loss of soda in the“ tank-waste.” Another portion of the silica existsas soluble sodium silicate, which forms a constituentof every soda ash.
Black-ash is only very slowly changed by lying indry air at ordinary temperature. Kolb allowed asample made with special care to cool, protectedfrom the air, and lixiviated one part of it at once,the other after having exposed it for the space oftwo months to air free from moisture and cartfonicacid. Of the latter one portion was left thus at theordinary temperature, the other at 100° C,, and bothwere ultimately treated just like the first sample.The result was that the two last samples showed alittle more caustic; but on the whole about as muchavailable alkali as the first. Dry air at 100° C. onlycauses a slight increase of weight, but no injuriousdecomposition. If, however, the heat is increasedto 200° or 300° C., and still more, up to a red heat,the colour of the black-ash changes to brown red,and its percentage of available alkali decreasesquickly; indeed, it may be reduced to one-half ina very short time. The conclusion is that thecooling of the black-ash after coming out of thefurnace ought to be hastened ,as much as possible;and that, if possible, it ought to take place in closedboxes, as the cause of the loss is practically theoxidation of the calcium sulphide to sulphate, which
on lixiviation decomposes an equivalent quantity ofsodium carbonate and reconverts it into sulphate.In practice nothing more is done than to allow oneball to cool in the “pot” till it is about time todraw the next one, say three-quarters of an hour;it is then tipped, but not broken up till it is cool.
The above experiments only referred to air freefrom moisture and carbonic acid. In the ordinaryair containing both these the matter is different. Inthe first instance the caustic lime attracts moisture,and in being slaked to hydrate increases its volume,which makes the balls burst and fall to powder.The calcium hydrate gradually changes into car-bonate, the sodium sulphide into hyposulphite.Neither of these changes would be injurious, butthe moist air acts at the same time upon the calciumsulphide, and converts it to a certain extent intosoluble polysulphides, hyposulphite, sulphite, andultimately sulphate. On lixiviation all these reactupon the sodium carbonate and destroy it, so thatthe alkalimetrical test of black-ash, even allowingfor the increase of weight, goes back continually byexposure to the weather. Erdmann found in asample of black-ash, originally containing 29 percent, of sodium carbonate, only traces of that saltafter twenty-seven years’ lying in the air. Theconclusion is that the proper way of treating black-ash is this—to submit it to lixiviation as soon as ithas sufficiently cooled, for which forty-eight hours isquite sufficient. Thus the heat retained by the ballsis also utilized in the lixiviating process, and it ispractically found of great advantage to work up theballs after no longer exposure than two or three days.
Lixiviation of the Blavk-ash .—The object of thealkali manufacturer is to obtain as much and as puresodium carbonate as possible from his raw materials,and consequently from the black-ash. The onlyavailable means for doing this is to submit the blackash to lixiviation, that is, to separate the solublefrom the insoluble portion; the former being furtherworked up for soda ash, the latter being removedfrom the factory as “ tank waste ” (in some casesafter first submitting it to a process for recoveringa portion of the sulphur from it). The product ofthe lixiviation varies very much, indeed, quite apartfrom any variation in the quality of the balls, ac-cording to the way in which the process is carriedon. Three objects have always to be kept steadilyin view, viz.:—1st, To obtain all sodium compoundsfrom the black-ash as completely as possible ; 2nd,To obtain them as much as possible in the form ofsodium carbonate; 3rd, To obtain a solution asmuch concentrated as practicable.
The success of these operations depends verymuch, indeed, upon the manner in which the lixivi-ation is carried on ; but it will be better understoodif it is first explained in what manner water actsupon black-ash. It has been mentioned before thatblack-ash in itself does not contain any sodiumhydrate or sulphide, except in very minute quanti-ties, since alcohol, which would dissolve them, pracrtically does not act upon black-ash at all. On theother hand, it is quite impossible to make a solution