82 C
SODA. —Black-ash Liquor. Salting-down.
say something about the work performed in them.Each pan is supposed to do all the work necessaryfor the produce of its own furnace, that is, to eva-porate the liquor resulting from the lixiviation of asmany balls as the furnace produces. The liquor isrun in from the settler, and kept up by feeding thepan from time to time so that its level is pretty wellkept up. The supply is so regulated that after eachtwenty-four hours the pan is ready to be drawn,that is to say, the salt has accumulated in it to suchan extent that it reaches to the top of the liquid,and the whole forms, in fact, a pasty mass. It isnecessary during the evaporation to work from timeto time with a light rake in the pan, beating downany crusts formed on the top (which would, ofcourse, hinder the evaporation), and working thesalt from the neighbourhood of the furnace, where,of course, it forms in greatest abundance, towardsthe back-end of the pan. When it is ready fordrawing, one of the doors (which have been lutedwith clay) is slackened, so that the bulk of theliquid runs out into the drainer mounted in frontof the pan, and into which its discharging doorsproject; then the doors are taken off altogether,and the whole of the contents of the pan are rakedout as quickly as possible. The drainer is suppliedwith a perforated false bottom similar to that of thelixiviating tanks, and the real bottom of the draineris either sloped off at one end, or it contains at leasta depression (or well) for the drainer pump to standin. This is done so that the mother liquor (“redliquor”) may drain off as completely as possible andmay be pumped away. In many works it is alwayspumped back straight into the pans and boiled downalong with the next charge; in some cases it is, how-ever, pumped away into separate cisterns and boileddown by itself. This is especially often the case withrevolver pans, which (as well as the drainers) areprovided with a partition for this purpose. Thefront part of the pan never receives any red liquor,and therefore yields a soda ash testing up to 56per cent, available soda; the back part of the panreceives all the red liquor produced in the drainer,and only yields 50 to 52 per cent. ash. Where,as is usual on the Tyne, the red liquors are simplypumped back into the same pans from which theycame, the strength of the soda ash obtained in well-conducted works is not under 52 per cent.; butfrequently up to 53 per cent., and with revolvers 54per cent., or even more. This system is very cheap,simple, and effective, and causes very little wearand tear of the pans; but it is not at all adapted forworks where the red liquors are required for causticsoda, and it does not on the whole yield as pure anash as the “ fishing ” process, or rather it does notallow of such an easy and complete separation ofthe products. It is certainly a mistake to say (as itis sometimes done) that in this kind of evaporationno fuel at all is required, for every practical manknows that he must burn more coal to do his boil-ing down than when he has only balls to make, and,moreover, it must be a good bituminous coal, givinga long and hot flame. There is much less labour
required than with bottom evaporation, and thesalt does not easily burn fast to the sides andbottom of the pan. Where the liquor has not beenoxidised beforehand, it is usual to put sawdust intothe pan during the evaporation. This not only actsfurther on in the carbonating or finishing furnaceby means of its carbon, but it is also of very greatimportance for the draining process. The salt fromthese pans, being of much finer grain than the“fished” salts from boat pans, would part very badlywith its mother liquor, were it not that the sawdustmakes it porous and more permeable. In somecases, when it is used without mother liquor, it iseven washed with water to remove the latter morecompletely, and drained by means of a centrifugalmachine (first introduced by Gamble). This, how-ever, is only rarely done. In most works on theTyne the salt is wheeled from the pan-drainers tothe “ stock-drainers,” where it rests one or two days,and is more completely freed from red liquor. Itdistinctly improves by that operation.
The salt thus collected on the drainers (“blacksalt”) is essentially constituted according to theformula lSTa 2 C0 3 + H 2 0. When of very goodquality it is nearly white and of coarse grain; butwhen impure (from ferro-sodium sulphide) it ismore or less coloured yellow or brown, even red,and generally some of the red crystals of one of theseveral sulphides can be observed separately.
The action in the black-salt pan has been studiedby Mookhouse, who gives the following tables ofhis results:—
I.—PROPORTION OF CAUSTIC SODA TO TOTAL AVAILABLE SODAPRESENT IN THE LIQUOR, IN THE BLACK SALT AND IN TIIEASH MADE FROM IT.
Tank liquor,.
19*6
22*2
19-0
12-9
20-4
20*2
Black salt,..
13-3
13-7
14-3
11*6
17*6
11-8
Soda ash,.
—
6*0
3-2
3-5
3*0
4-4
Salt from front of pan,.
12.1
11-7
15-7
13-6
6-8
_
8-6
12-2
17-1
8*0
6*1
—
II.—TOTAL SULPHUR (IN ALL COMBINATIONS) CONTAINED
IN—
Tank liquor,.
1-59
2-24
214
2-08
2-13
2-05
Black salt,.
1'77
—
1-79
2-20
2-14
2*09
Soda ash,.
2T6
2*33
2-25
2-38
2-34
—
Salt from front of pan,.
—
—
1-80
2*50
1*90
2-42
Salt from back of pan,.
—
—
1*71
1*95
2*19
2*19
From No. I. it will be seen that, through theaction of the furnace gas, a large proportion of thesodium hydrate is converted into carbonate in thepan; the average diminution of Na 2 0 existing asNaHO, being 4-7 per cent, on the total available(that is, that which is shown by test acid) soda pre-sent. In carbonating a further average decrease of9'8 is produced; but it is unfortunate that eventhe best of the six samples tested by Moorhousewas quite inferior with regard, to this to the ordi-nary soda ash found in trade, and that, therefore,his experiments do not possess quite the same valueas if his furnace processes had been in good order atthat time.
The salt from the back end of the pan, which is