830 SODA.— Strength of Soda Asii.
Black
Salt
Used.
Seven
o'clock.
Eight
o'clock.
Nine
o'clock.
Carbon',.
I'll
0-32
0-38
0-10
nil.
Insoluble matter,.
0'95
0-93
1-00
0*97
0-97
Sodium carbonate,.
78-30
79-04
80-05
81-07
81-86
“ hydrate,.
7-59
6*30
5-42
4-40
3-97
“ sulphate,.
7-71
9-02
10-27
10-85
11-06
“ sulphite,.
0*82
1-77
0-83
0-38
0-21
“ hyposulphite, ..
1*05
trace.
nil.
nil.
nil.
“ sulphide,.
Per cent, undissolved1
0-33
trace.
trace.
trace.
trace.
sulphur on total sul- y-
phur,.)
Per cent. Na 2 0 in state )of NallO on total >•available Na 2 0, ....j
31
17
8-3
3-6
2-0
11’4
9-5
8-2
6'9
6-0
It -will be seen that the proportion both of unoxi-dised S and of Na 2 0 existing as NallO is reduced ina regular progression in the hourly stages, but theformer much more rapidly than the latter. Thesulphide and hyposulphite are oxidised -with com-parative ease, but the sulphite remains, although indiminished quantity, up to the last; and this isindeed the case not only with Moorhodse’s verybad salt, but with the very best carbonated saltsmade in the usual way, without separating redliquors, &c.
A mechanical carbonating furnace, built in theform of a staircase, has been devised for Stevensonby James Napier; but although it dried and car-bonated the salt, it could not be brought to the heatnecessary for finishing it, and it had to be given upagain. Probably Jones’ machine (described in thearticle Solpiiate of Soda) would do for carbonatingalso ; but as it has only just been brought out,no experience exists on this point. Another me-chanical furnace, however, patented lately by Mac-tear, of St. Itollox, seems to have really overcomeall practical difficulties in the way of carbonatingsoda ash without manual labour. Its principle isthat of a revolving circular hearth, with scrapers orploughs, similar to some extent to the mechanicalfurnace for the calcining of cupreous pyrites cinders,devised by Gibb and Gelstharpe.
When the soda ash has been obtained, eitherafter carbonating the tank liquor by gas in Gossage’stowers, or by “fishing” the salt and leaving themothers to be dealt with in some other way, it isgenerally both very strong and also very white.That, however, which has been obtained by surfaceevaporation, without separating the red liquors, isnever quite white, but always yellowish, and some-times reddish or brown ; it is therefore often knownas “ yellow carbonate.” A yellowish ash is, indeed,often found better carbonated than one of muchwhiter colour, and naturally so; for in the latterthe iron salts have not been properly decomposed,and their very slight colour does not influence thatof the bulk, whilst in the better fired sample all ironsalts are converted into ferric oxide, whose redcolour spoils the whiteness of the ash. A well-carbonated soda ash ought to be of yellowish, oreven better, of slightly bluish colour (either from atrace of ultramarine, or from one of sodium manga-nate), but not grey, which speaks of bad carbonat-
ing, as well as any remains of charred sawdust,which ought to be altogether absent. After grind-ing, it ought not to show too many black or other-wise coloured specks (owing to charcoal, brick-dust,&c.). The residue insoluble in water ought not toexceed 1-5 per cent.; sulphide ought to be entirelyabsent; sulphite, not exceeding '15 per cent;caustic, 2 per cent, (in ash made for dissolving forthe manufacture of soda crystals, it is often broughtdown to 1 per cent.) ; chloride, 2 per cent, to 3 percent.; the sulphate will vary from 5 to 10 per cent.,according to the strength of the ash. All this refersto ordinary soda ash (yellow carbonate)—say up to52 or 58 per cent, of available soda. The fishedash, made up to 56 per cent, or more, contains ofcourse much smaller quantities of any of the aboveimpurities. It would seem useless to repeat any ofthe published analyses of soda ash, as the propor-tions of the impurities vary very much, and theabove maximum values for good merchantable ashare more valuable than any chance analysis publishedby some author.
The soda ash is rarely brought into the market as itcomes from the furnaces, but is nearly always groundto powder; this is really much more economical, asthe grinding does not cost very much more than thepacking by hands, and there is a very considerablesaving of casks. An ordinary alkali “tierce,” madefrom 40-inch staves, holds only 8 to 9 cwts. of un-ground ash, but 12 to 14 cwts. ground ash, accordingto the way in which it is packed. The packing isoften assisted by mechanical contrivances for shak-ing the casks and pressing the ash in during thegrinding; in nearly every case the mills are made todeliver directly into the casks. This applies to grind-ing in horizontal mills, similar to flour mills,which are now generally preferred to the formerlyused edge-runners, because they work much morecheaply owing to the large amount of work they do,and allow sifting apparatus to be dispensed with.The stones of these mills are not the French “burrs”used in corn grinding, but blue lava stones, mostlyfrom the Lower Rhine, which resist the action ofthe hot ash without injury. A pair of edge-runnersis, however, very useful for crushing the hard flux-ings from the furnaces, which cause great wear andtear of the millstones. In Lancashire the ash isoften packed in hard-wood sugar hogsheads, whichhold up to 22 cwts. As customers frequentlyrequire ash of lower strength than that produced inthe works, the strength is reduced accordingly bymeans of common salt (previously dried in a fur-nace), immediately before grinding. Where kelp saltis cheap enough it is used instead of common salt,as its percentage of soda (8 to 12 per cent.), is madeavailable, and the sodium chloride in it costs littleor nothing.
Strength of Ash .—Soda ash is always sold accordingto its strength, as shown by test acid, and it is mostlyonly tested in this way; only exceptionally, moisture,caustic, insoluble, &c., are estimated as well. Thetest acid (usually standard sulphuric acid) saturatesnot only sodium carbonate, but also hydrate, silicate,