822 SODA. —Black-ash. Shanks’ Vats.
tank which has just been “cast,” that is, emptied ofits contents of exhausted balls. The tank is filledup to 12 inches below its top, and the pieces ofballs roughly levelled with a hook. Before tippingany balls into the tanks, the perforated plates aretaken up and the space below them is cleaned out,as any waste left there would greatly injure theliquors. The false bottom is replaced and a layerof about 4 inches of furnace cinders, from whichthe fine ashes have been separated by a-half-inchscreen, is spread upon it to serve as a filter andprevent the holes from being too quickly filled upwith tank waste. The balls are then tipped in.When a set of vats has to be started anew, ofcourse at least three of them have to be filled tobegin with. Supposing this to be the case, warmwater (90° to 100° Fahr. in summer, 100° to 115°Fahr. in winter) is run on to the top of the first tank,say that on the left-hand side of the drawing. Thewarm water in percolating through the balls dis-solves a portion of their soluble contents, andarrives below the false bottom charged with them.The “jug” communicating with the second tankhas been left open, and the ball solution will, there-fore, rise from underneath the false bottom in thejug; and as soon as the level of the water in thetanks is high enough, the liquor will overflow intothe second tank, running there over the balls fromthe top or “covering” them. In doing so andpercolating through the porous mass it becomesnaturally stronger, and indeed, when working withfresh tanks, will probably become strong enoughto be run out from the jugs of No. II. tank forevaporation. In the ordinary course, however, itwill again rise from underneath the false bottomin the jug and overflow into No. III. tank, andthence into No. IV. tank. In any case it will bestrong enough now, and the overflow jug of thistank will, therefore, be plugged, whilst the running-off jug is made to act by lifting its plug, and theliquor runs out of No. IV. tank through the openspout (shown in the drawing) into the strong liquorwell.
Meanwhile the first tank has received four dif-ferent waters, or rather sufficient water to fill itfour times over, but in a continuous flow. It willnow be entirely exhausted, as shown by the factthat the liquor rising in its tank jug only showsabout 1° Twaddle. The jug is now plugged; No.II. tank is made first tank, and fresh water is runupon it. No. I. tank is first emptied of water byopening the bottom cock; the water, which stillcontains a little soda, is run into the weak liquorwell and pumped up to serve as first water on No.II. tank. The contents of No. I. tank, which arenow greatly reduced in bulk, form what is called“tank refuse ” or “waste; ” they are at once shovelledout of it, mostly direct into waggons which carryit away to its destination. The cinders are castout along with the waste, the plates are lifted, thetank is washed out, the plates are replaced andcovered with cinders, and the tank is recharged withfresh balls.
The following is an analysis, by G-. E. Davies, ofweak liquor of 1° Twaddle :—■
Sodium hydrate,. 2*640
“ carbonate. 1*060
“ sulphide,. 2*690
“ hyposulphite,. 0*554
“ sulphate,. 0*284
“ chloride,. 6*780
“ silicate,. 0*100
u aluminate,... 0*108
Total solids per litre,. 14*216
In the meantime only three tanks have been atwork ; but these will have been quite sufficient. Itis usual to run the liquor away for evaporation tillit comes down to 1*23 or even 1*21 specifio gravity.The average specific gravity (after mixing up allrunnings) of a good tank liquor should not varymuch above 1*29, nor below 1*26. But when ithas come down to 1*23 (or even 1*21, according tothe judgment of the manufacturer), the running-offjug is stopped, and the overflow jug of No. IV. tankopened, which communicates through the long returnpipe with No. I. tank. The intermediate liquor ofNo. IV. tank will now “ cover” the balls in No. I.,and when rising from below its false bottom, it will,of course, become fully strong enough at first up to1*30; it is therefore at once run off from this tankfor evaporation. All the while No. IT. has beenfunctionating as “ weak ” tank, and it continues todo so until it is exhausted; then the water is divertedinto No. II.; the weak liquor of No. IT. is run off,and the whole operation repeated as before in No. I.It will thus be seen that each tank in its turn be-comes the first, an intermediate, and the last of theseries, the return pipe causing the set to act as ifit were mounted in a circle. It should be noticed,that the liquors always become stronger as they flowdownwards ; that therefore their increase in specificgravity does not cause any mixing up of the liquids,the weaker liquids always floating on the top of thestronger ones, but forcing the latter by their hydro-static pressure to rise in the jug and overflow toeither in order to be run off, or into the next tank.The side branches of the running-off jugs are a fewinches lower than those of the overflow-jugs, whichfacilitates the locomotion of the liquor in the wholesystem. Both the strong and the weak tank areworked according to their own requirements, andindependently of each other; the intermediate onestake their chance.
One very important matter is the proper regula-tion of the temperature. It has been mentionedthat the water (say the surface) of the weak tankought not to be warmer than about 100° Fahr.,with a downward margin of 10° in summer and anupward one of 15° in winter. The liquor becomeshotter as it gets stronger, partly owing to the heatof the balls, partly to the chemical action of thewater on the anhydrous oxides and salts of theblack-ash. The temperature of the strong liquorought in no case to exceed 150°, and if it rises abovethis the weak tank must at once be made colder.After what has been said before about Kolb’s ex-periments, it is unnecessary to enter again into the