SODA.— Black Ash. Working.
805
—that is to say, as a coarse powder, not generallycontaining any pieces above the size of a walnut.A few manufacturers work with “ duif ”—that is,the fine dust screened off even of the “ smalls,” butthis is generally considered too impure for use asmixing coal.
With these figures (given from the writer’s practi-cal experience) should be compared the statementsof different authors. It will be found that the pro-portions given by Leblanc himself—viz., 1000 partseach of dry sulphate of soda and of carbonate oflime, along with 500 parts of charcoal—agree verynearly with what has been stated above as the mix-ture generally used in this country, and the followingare some of the mixtures recommended by severalmore modern authors:— Payen (see below) givesa proportion of 100 : 105 : 51; Scheurer-Kestneb(Hep. de Chim. Appl, 1862, p. 231) gives the mixtureof ten works, where the carbonate of lime (upon100 sulphate) varies from 90-2 to 121, and the coalfrom 35 to 73. In three German works mentionedby B. IIerl the proportions were 100 : 115 : 30to 35, 100 : 121 : 46-6, and 100 : 115 : 68. Allthese differences testify more to a difference in theraw materials than to anything else. Two figuresmentioned by Knapp as the proportions of someGerman works—viz., 100 : 108 : 183, and 100 :104-7 : 170-8—seem to contain some mistake in thefigure for coals.
If the furnacemen had their own way, they woulduse as much limestone or chalk as possible, as theycould then fire the balls much more without makingthem “ red,” and thus be saved the labour of workingthem thoroughly; but it is the manager’s businessto cut the limestone or chalk down to the lowestpossible limit, as he will undoubtedly get a largeryield by doing so, and have less caustic liquors aswell. On the other hand, the men dislike to usemuch coal, because they cannot “ clear ” theirballs so easily; whilst the manager wants them touse as much coal as can be fairly incorporated inthe ball, so as to insure a complete decompositionof the sulphate of soda.
The experiments of Kolb (1866) are among themost important ones in this respect. According tothe equation,
Na 2 S0 4 + CaC0 3 + 2C = Na 2 C0 3 + CaS + 2C0 2 ,the proportions of pure sodium sulphate : calciumcarbonate : carbon ought to be as 100 : 70-4 : 16-9(yielding in theory, 74-6 Na 2 C0 3 ), whilst in practicea large excess of the two latter ingredients is em-ployed—roughly speaking, Leblanc’s proportions(100 : 100 : 50) are still in use. Kolb made alarge number of comparative experiments on alarge scale, at Kdiilmann’s works at Amiens , withdifferent proportions, and treated the black ashobtained exactly in the same way. The followingtable shows the results:—
Mixture Employed.
Obtained.
NaaC03. NaaO.
Na 2 8.
Na. 2 S0 4 .
1.
Na 2 S0 4 + CaC0 3 + 2C
Sulphate, 106 parts =
100 Na 2 S0 4 )
38-20 3-10
•55
48-51
Chalk, .. 84 “
70-4 CaCO, ^
' -- '
Coal,.... 20 “
17 C j
= 43-5 Na 2 C0 3
2.
Na 2 S0 4 + CaCO s + 4C
Sulphate, 106 parts =
100 Na.,S0 4 !
59-51 3-39
2-16
6-71
Chalk, .. 84 “
70-4 CaC0 3 )-
V -— v - /
Coal,.... 38-4“
33-8 C )
= 65-3 Na 2 CO,
3.
Na 2 80 4 + 1J CaCO, +
40—
Sulphate, 106 parts =
100 Na 2 S0 4 )
G3-50 3-80
1-20
6-40
Chalk, .. 101 “
94 CaC0 3 >
' -y-- •
Coal,.... 38*4 “
33-8 0 )
= 70-0 Na 2 C0 3
4.
Na 2 S0 4 ■+■ CaC0 3 “l -
4JC-
Sulphate, 106 parts =
100 Na 2 S0 4 )
64-20 4-72
1*12
2-06
Chalk, .. 101 “
94 CaC0 3 V
V -y-- >
Coal,.... 53 “
44 C j
= 72-3 Na 2 C0 3
5.
Na.,S0 4 + l*CaCO, +
60—
Sulphate, 106 parts =
100 Na,S0 4 )
52-60 8-45
•15
•93
Chalk, ..116 “
103 CaC0 3
V-.. .«~y —- /
Coal,.... 73 “
50 C j
= 67-0 Na 2 C0 3
It will be seen that of mixture No. 1 nearly halfthe sulphate remains undecomposed, whilst theyield of available soda is correspondingly small.The latter is increased in Nos. 2, 3, and 4, but fallsagain a little in No. 5, where in consequence of theexcess of chalk the caustic soda increases. No. 4seems to be the most advantageous mixture ; butthe difference, both in materials and in the styleof work, will cause deviations from it in differentfactories. The excess both of chalk and coal pro-duces an increased yield of soda, because thus thepoints of contact with the sodium sulphate areincreased, and a more complete decomposition ofthe latter is insured.
Working .—The sulphate of soda, carbonate of lime,and coal are always brought together in reverberator//furnaces, called ball furnaces or black-ash furnaces; butthere is a great difference on this point, whether theoperation takes place by manual labour, as in the oldfurnaces, or by mechanical assistance, as in the “re-volvers.” We shall first describe the old furnaces,which even now are used in all small works and arenot quite extinguished even in large works, althoughtheir disuse is only a question of time, as will beseen later on when the advantages and drawbacksof both kinds of furnaces are weighed together. Butit is almost equally sure that in the smaller alkaliworks (and nearly all continental works belong to