SODA. —Black-ash Liquor. Salting-down. 825

The tank liquor, as it runs off from the strongtank, never goes direct to the evaporating pans, butalways first to settlers. These are tanks in which it isallowed to stand for twelve hours or more, duringwhich time not only any mechanically carried awayportions of tank waste subside to the bottom, but aconsiderable quantity of a sodio-aluminium silicatecrystallizes out; a further quantity of this separatesas hard crusts upon the concentrating pans. Thefollowing are analyses by G. E. Davies of (A) thedeposit from settlers; (B) the crust from the concen-trating pans:—

A.

B.

Silica,.

23-60

39-572

Alumina,.

18-44

33-584

Sodium oxide,.

13-12

20-776

Water,.

Iron sulphide,...

—

5-844

5-84

—

Calcium sulphide, ..,.

38-00

—

Total,..

99-00

99-776

Sometimes large store tanks for holding the liquorare built underground; but as these cannot veryeasily be controlled as to their leakages, it is muchbetter to make the strong liquor well only just largeenough for a pump to work in (in this case it isgenerally made of cast iron), and pump the liquor, asit is made, into settling tanks placed at a higher level,where they are visible and accessible from below.In this case it is possible to give the liquor a pre-liminary heating in the settling tank by means ofthe waste heat of some furnace (for instance, thecarbonating furnaces); if applied over the top, thesettling itself will not be interfered with in the least.In any case the settlers must be placed high enoughto feed the boiling down pans; but where the liquoris first oxidised or carbonated in towers, they mustbe placed correspondingly higher.

This oxidising or carbonating process is resorted tofor several purposes, and sometimes one, sometimesthe other, sometimes both are used. The process,as first proposed by Gossage in 1853, and carried outin several Lancashire works, consists in exposing thetank liquor, whilst it trickles down in an iron towerpacked with cake, to the action of a current of air.The liquor, when arriving at the bottom, is testedwith a solution of lead, and if it should still show areaction upon sulphides, it is pumped up again andsubjected to the same process. The sodium sulphideis thus oxidised, and thciron sulphide combined with itis precipitated as oxide which can be settled out again.In some works, instead of a current of air, the gasfrom a lime kiln (impure carbonic acid) is employedfor this purpose, and evidently with much greatereffect, at the same time converting the caustic sodainto carbonate, and decomposing the sodium alum-inates with precipitation of Al 2 O s (which sometimescauses trouble by stopping up the interstices betweenthe pieces of coke). The best plan of all seems tobo that of employing the gas from some furnacecontaining both carbonic acid and free oxygen, andin this case the evaporation of the lye will be begunvery effectively in the oxidising tower itself.

VOL. II.

Instead of Gossage’s tower, Hargreaves has intro-duced (I860 ) a lower cylinder, fitted with a perfor-ated false bottom, below which he injects a currentof air by means of a steam blast; the double bottomdivides the current into many bubbles. This pro-cess is very much in use in factories where causticsoda is made, and where it is not intended to car-bonate but only to oxidise the liquor. It consumesa great deal of steam, more than the pumping inGossage’s plan, but it does not lead to the same lossof soda remaining with the coke, &c., as the latter.The most complete means for this purpose wouldseem to be Koehting’s blower, which works moreeconomically than a pump, and which can be usedwith either air or carbonic acid from any source.It would slightly dilute the liquid, whilst running itdown in a coke tower concentrates it; but thatwould be of less consequence than the apparentlyinevitable loss (as found in practice) of 1 or even 2per cent, of soda in the latter case. The majority ofworks, although irrationally, use none of these pro-cesses ; but either carbonate the ash later on bymeans of sawdust, or else separate the product into“fished” ash, which requires no carbonating, and“ caustic ash,” or caustic soda, as will be seenhereafter.

One of the constituents of tank liquor whichoccurs in such small quantities that it is generallyomitted in its analysis, nevertheless plays an im-portant part further on, when the colour of thesoda ash comes to be considered. This is sodiumferroeyanide. It is extremely difficult, if at allpossible, to destroy this compound. Although it canbe done, according to J. C. Stevenson, by heatingthe liquor under pressure to 150° C., there arepractical difficulties in the way of carrying this outon a large scale, apart from the expense, and it isnowhere done. In consequence of this all the ironcan never be removed from the tank liquors, evenby completely carbonating it and destroying theferro-sodium sulphide, and the destruction of thesodium ferroeyanide during the ultimate ignition ofthe ash (in the finishing or carbonating furnace)causes the ash to be coloured yellow by ferric oxide.This can only be avoided by using a mixing coalquite free from nitrogen. A portion of the cyanogencompounds formed in the ball-furnace is often de-composed during the cooling of the balls by theaction of the air, as evinced by the ammoniacalsmell they sometimes show.

Salting-down .—The evaporation itself is carried onby two systems, viz.—either by surface evaporationor by bottom heat. The first system is that univer-sally followed on the Tyne, and in use in manyplaces elsewhere; the second system is more usualin Lancashire and on the Continent. The pansgenerally used for surface evaporation are shown,both in connection with the ordinary and with therevolving ball-furnaces, upon the respective Plates,as they are always heated by the flame escaping fromthe ball-furnaces. They have been described indetail in connection with the ball-furnace, fromwhich they are inseparable, and it only remains to104