]
SODA.— Black-ash. Lixiviation.
819
of black ash free from either sodium hydrate or sul-phide. It is thus clear that both these compoundsare formed by the treatment with water, and it iswell known that they are formed in very varyingproportions, according to the variation in the con-ditions of the water treatment. It may be taken asa general thing that the alkali manufacturer wants
to avoid the operation of both sodium hydrate andsulphide during the lixiviation as much as possible,and it thus becomes of great importance to fix theconditions under which their formation is eitherfavoured or impeded. The most exhaustive experi-ments upon this matter have been made by Kolb,and the following table summarizes his results :—•
100 Grams. Black-ash treatedwith: c.c.
Water.
6 Hours at
24 Hours at
1 Week at
Yielded—
15° C.
40° C.
60° C.
15° C.
40° C.
60° C.
15° C.
40° O.
60° C.
Grams.
Na 2 C0 3 ,.
350
34-7
31-4
31-5
31-5
25-8
?G-4
Na 2 0,.
_
_
—
5-7
7-8
7-6
7-6
9-5
7*2
Na 2 S,.
—
—
—
trace.
•2
•3
trace.
2-2
3-7
Na 2 C0 3 .
500
38-5
37-4
31-5
38-6
35-4
26-8
37 0
34-2
25-5
Na 2 0,.
3-3
3-9
6*8
3*3
5-1
6*7
4*2
5-7
6*6
Na 2 S,.
•2
•3
•7
•3
•4
4-7
•3
•4
5-9
Na 2 CO s ,.
1000
40-2
35-9
34-2
38-6
34-2
31*1
35*2
31-4
23*2
Na 2 0,.
2-3
4-4
5-4
3-3
6-0
6*6
5*0
6*3
6*0
Na 2 S, .
•3
•5
■8
•3
•8
2-0
•3
■8
7*8
2000
38-2
36-1
37-9
37*6
34-7
331
34-5
31-5
19-5
Na 2 0,.
3-3
4-7
3 3
3-8
5*9
6*2
5-3
0-3
6*3
b T a 2 S,.
•2
■5
•6
•4
■5
1-0
•5
1-5
10-2
This table leads to the following conclusions:—-1. The quantity of sodium hydrate formed is notinfluenced very much by the quantity of water, butit increases with the time taken up and the tempera-ture employed. The difference between 40° and60° C. (104° and 140° Fahr.) is, however, not veryessential. 2. Sodium sulphide is hardly more influ-enced by the quantity of water; it also increaseswith the time and the temperature, but very little soup to 40° C., whilst there is a very large increasebetween 40° and 60° Fahr. 3. No fixed proportioncan be found between the quantities of sodiumhydrate and sulphide formed. 4. The decrease ofthe sodium carbonate is evidently dependent asmuch on the formation of the sulphide as on thatof the hydrate; that is, the sulphide is formed atthe direct expense of the carbonate, not that of thecaustic. 5. No more available soda can be extractedby 1000 or 2000 than by 500 c.c. (indeed, except inthe six hours’ trials, rather less); and the lower thetemperature the better. The worst results are ob-tained when an excess of temperature, of time, andof water, are all acting together.
Kolb’s special experiments upon the solubility ofcalcium sulphide also merit attention. He foundthe following results :—
1. CaS is very little indeed decomposed by purewater; the presence of NaCl slightly decreases, ofNa 0 S0 4 slightly increases its solubility. The de-composition, as far as it goes, takes place accordingto this equation :—
2CaS + 211,0 = CaH 2 S 2 + CaH 2 0 2
And 1 litre water dissolves by forty-eight hours’digestion:—■
At 10°, 18°, 40°, 60°, 90°,
•75, -23, -30, -48, -33 gram. CaS.
2. A saturated solution of sodium carbonate
scarcely acts at all on calcium sulphide; but theaction increases rapidly if the concentration of thesolution is diminished, and the temperature andtime of digestion are increased. The presence ofa slight proportion of sodium hydrate retards thedecomposition considerably. The equation is—
CaS + Na 2 CC* 3 = CaC0 3 + Na,S.
3. A dilute solution of sodium hydrate (= 3T5gram. Na a O per litre) decomposes only traces ofCaS, either in the cold or at a boiling heat. Moreconcentrated solutions, containing from 16 to 79grams. Na 2 0 per litre, if digested for forty-eighthours and heated to 40°, 60°, or 100° C., cause agradual decomposition, which increases proportion-ately to the quantity of sodium compounds in theliquid.
The practical conclusions from Kolb’s experi-ments are, that it is expedient to lixiviate the blackash—1st, as quickly as possible; 2nd, with as littlewater as possible; 3rd, at the lowest possible tem-perature. Unfortunately these conditions cannotbe applied to operations on a large scale, and it willbe shown afterwards what the practice on that scalehas shown to be'the best method.
The presence of sodium sulphide in the liquor isparticularly injurious, because it acts upon the ironoxide always present, and causes the production ofa ferro- sodium sulphide. This compound, evenwhen present in extremely minute quantities, coloursthe liquors green, yellow, or brown, and may simi-larly discolour the soda ash subsequently obtained.More will be said on this further on.
We shall say very little about the lixiviatingapparatus formerly in use, as the only one which isnow employed in works conducted on modern prin-ciples is that to be described at length hereafter.
The oldest apparatus consisted of several (usually