838
The first three items bracketed together weresoluble when 5 grams, of the mud were digested in100 e.c. of water at 100° C. No other use than thatfor the ball mixture has been found practicable forthe lime mud, except where green glass works exist inthe neighbourhood; it costs far too much to recon-vert it into caustic lime.
As will be seen from the above analyses, thelime mud always contains a not inconsiderablequantity of soda, both in the soluble or insolublestate; but even the former cannot practically begot out on the large scale. Assuming it to amountto If per cent, of available soda (Na 2 0), the losswill be as follows :—For each ton of 60 per cent,caustic soda, about 11 cwts. of good lime are re-quired ; these yield about 36 cwts. mud, containing,at the rate of 1-g per cent., 60 lbs. of real soda, orequal to 100 lbs. of 60 per cent, caustic per 2240lbs. of the same actually obtained. If weak lime beused, the loss is proportionally greater, as more mudis then produced. Whether any of the soda, evenof its soluble portion, is actually recovered again byusing the lime mud for the ball mixture, appears atpresent still a questionable matter.
The following are analyses, by Davies, of dilutedtank liquor before causticizing (A), and of the sameliquor after causticizing (B), the agitation of theoperation having been produced by means of injec-tion of air:—
A.
B.
Iron sulphide,.
traces.
none.
Iron oxide,.
none.
traces.
Sodium sulphide,.
1*068
none.
“ sulphite,.
0*151
0*264
“ hyposulphite,.
1-137
1*864
“ sulphate,.
5-423
5-006
“ chloride,.
8-576
7-995
u silicate.
0-852
0*176
“ aluminate,..,.
2-490
1-438
“ carbonate,.
53-167
2 954
“ hydrate, .
“ ferrocyanide.
44*280
74*000
0*093
0*086
“ sulphocyanide,.
0*054
0*050
“ phosphate,.
traces.
traces.
Insoluble.
0-849
none.
Total solids per litre,.
118*140
93*833
A had a specific gravity of 20° Tw. at 56° Fahr., andB 18° Tw. at the same temperature. It will be seenthat the sulphides have, been quite removed, and thesilicate and aluminate greatly reduced in quantity.
The following is the composition of a liquorcausticized at 15° Tw., the agitation being performedmechanically; the finished liquor showed 13° Tw.:—
Iron sulphide,. traces
Sodium sulphide,. 1*092
“ hyposulphite,. 1*264
“ sulphite,. trace.
“ sulphate,. 3*264
“ chloride,. 4-212
“ silicate,. 0-151
“ aluminate,. 0*584
“ carbonate,. 7*844
“ hydrate,. 50*400
Other constituents, .. 0*200
Total solids per litre,. 69*011
It will be seen that in this case the sodium sul-phide still exists in the caustic. By blowing longenough, in the former kind of operation, all thelower compounds of sulphur may be converted intosulphate, but this is hardly ever attained in practice.The use of metallic oxides, of which many havebeen proposed for removing the sulphide from theliquor, is now made quite superfluous, as the oxidis-ing is both much cheaper and easy of execution.The oxides of iron, zinc, and manganese have beentried, but had to be abandoned from practicalreasons (sliminess of the metallic sulphides). Onlylead oxide (litharge) has been found practicable;but its use seems quite unnecessary, since the sameobject can be attained much more cheaply by usinga blower instead of a mechanical agitator in thecausticizing or “operation” pans. Certainly whenusing litharge stronger caustic can be obtained thanby any other method, as the sulphide is convertedinto caustic instead of into sulphate. At Dieuze (inLorraine) lead sulphate is used for the same purpose.
The caustic liquor is allowed to settle in specialtanks, and when perfectly “bright” is run off tothe concentrating apparatus. This was thought atone time to be done most economically by Dale’sprocess, in steam boilers, which thus produce thesteam for causticizing free of cost; but this modeof concentration is now very little employed, as theboilers suffer very much, and any negligence on thepart of the workmen might produce disastrousresults. It would appear as if the sodium sul-phide acted corrosively upon the iron, and there-fore certainly only liquor freed from it by blowingor by litharge ought to be used in this case. Anyapproach to the salting point must be strictlyavoided, and the strength of the liquor in the boilersought never to be allowed to exceed 30° Tw., or1*150 specific gravity, although J. IIofmann statesthat the liquids can be concentrated to 1*24 or1*25 “ without the slightest inconvenience.” Theevaporation of caustic liquors is, as well known,somewhat troublesome, in consequence of theirliability to froth and boil over all at once; and thisinconvenience, which in a steam boiler would bea very serious one, has been obviated by II. Deaconby placing within the boiler an inverted cast-ironfunnel, resting against the sides of the boiler by afew points only. The vapour, as it forms, causes theliquid to froth up, and forces it inside the funnel tothe top, where it pours over continually, and thusprevents the too violent ebullition of the liquidoutside the funnel.
In any case, whether direct from the caustic settlersor from the steam boilers, the liquor is now con-centrated in boat pans, of which there generally aretwo, placed end to end, so as to form a prolongationof the black-ash furnace, as it has been described inconnection with the same. In some works a cast-iron pan is placed next to the furnace, and awrought-iron one at the end of this, and the weakliquor for concentration is sometimes run into thofront and sometimes (much more frequently) intothe back pan first.