SODA. —Black Ash. Mixing.
803
point, the exact attainment of which is the goal ofthe alkali-maker. The raw materials will be firstnoticed, then the construction of the furnace, and,lastly, the way in which the process is conducted.
The foremost of the three raw materials in thisprocess, sulphate of soda, has been treated of atlength before. It need only be said in this placethat the sulphate which is most adapted for theballing process contains no more than 1 per cent,undecomposed sodium chloride, and generally from^ to 1 per cent., or even more, of free acid (theacid is not really free, but exists as sodium bisul-phate). Although it would seem at first blush as ifit were more rational to use thoroughly calcinedsulphate, which is entirely free from acids, allpractical men agree that such sulphate, which caneasily be recognized by its red or “foxy” colour,from the complete oxidation of its- iron salts, doesnot generally make strong soda ash. The reason ofit is probably that, being too much calcined, it formshard lumps which are not so easily acted upon in thefluxing process. A small proportion of ferric oxideis not of much consequence, provided the subsequentoperations are properly carried out; but any largerproportion of calcium sulphate (which may bebrought into the salt cake by the rock-salt, if suchbe used in its manufacture) is extremely prejudicial,because a portion of it enters undecomposed intothe black ash, and during the lixiviation reducesits equivalent of sodium carbonate again to the stateof sulphate. It is most absurd to say (as is donein some books) that it is useful to leave some percents, of sodium chloride undecomposed. An alkalimanufacturer will always make his sulphate as“strong” (that is, free from sodium chloride) aspossible, since he can easily reduce his soda after-wards, if he likes, instead of carrying the sodiumchloride through all his processes.
The calcium carbonate is introduced into theprocess in the shape of limestone (as in Lancashire )or chalk (the universal material on Tyneside). Theformer is always used in the crushed state, the latterin most factories is also preferred in that state ; andit is certain that stronger ash can be made, and lesschalk will be used where it is employed, not roughlybroken, but crushed under edge-runners or betweenfluted rolls. The purer the limestone or chalk thebetter they are. Buxton or Welsh limestone is forthis reason preferred in Lancashire, and “London ”chalk on the Tyne. By the latter expression is under-stood the chalk from Greenhithe, Northfleet , andother places in the lower reaches of the Thames;the chalk from near the Medway, from Dover, &c.,contains too much silica and alumina, and makes badash. The chalk as received from the ships (whichalways bring it as ballast at very low rates) is fre-quently very wet, and is not easy to grind in thatstate, as the edge-runners convert it soon into a thick,tough, pasty mass, without reducing it to a finestate of division. It is therefore usual to dry aportion of the chalk, either by the waste heat ofsome furnace or in separate furnaces heated byrefuse coal, and to mix as much of this dried chalk
among the undried as enables the mill to work pro-perly. A proportion of one-quarter to one-seventhof dry chalk is generally used with the damp chalkin this manner. The organic substances frequentlyoccurring in limestone, and colouring it black orblue, are of no consequence, as they act similarly tothe coal; magnesia in anything like large quantitiesmakes any limestone unfit for balling, but a slightproportion of it does no great harm. Oxide of iron,silica, and alumina are very deleterious. They actlike the same substances in the coal ashes, by forminginsoluble double silicates of soda and earthy bases,and troublesome soluble sodium silicate and alum-inate. Richardson gives the following analysis ofLondon chalk as used in the Tyne (evidently dried).
Lime,. 55*20
Magnesia,.trace.
Carbon dioxide. 42*30
Sulphuric anhydride,. 0*29
Sodium chloride,.trace.
Alumina and iron,. 0’68
Sand,. 2'08
Water,. 0 - 30
100-85
CLAPHAM (ANALYSIS OP UNDRIED CHALK).
Carbonate of lime,. 78-000
Sulphate of lime,. 0*244
Silicate of lime,. 0*325
Phosphate of lime,. 0*119
Chloride of sodium,. 0 - 163
Magnesia,. trace
Alumina. 0*220
Oxide of iron. 0*200
Oxide of manganese. 0*150
Silica,. 0*600
Water,. 20*600
100*621
Organic matter,.
Alumina.
Ferrous carbonate, .. .Calcium carbonate, ...Magnesium carbonate,Manganese carbonate,Calcium phosphate,..
Buxton.
Minera.
Ruthin.
traces.
traces.
traces.
0-106
0-442
0-398
0-123
0-145
0-135
0-187
0-348
0-252
99-372
98*298
98-370
0*116
0*756
0*756
0*013
0*022
0*026
trace.
trace.
trace.
99*917
100.011
99*937
This is, however, only the composition of purelumps of the above ; the crushed limestone, as usedin the works, contains more clay, sand, and otherforeign matters.
The quality of the coal (slack) used for the sodamixture is of very great importance. Manufac-turers like to use always the same kind of coalwhich they have proved by experience to make goodash, and to which they and their men are accus-tomed. Whenever they have to try a new kind ofmixing coal it happens only too often that some-thing will go wrong, and that bad balls, bad liquors,and low strength ash will be the result. The veryfirst requirement in a mixing coal is that it shouldbe as free from uninflammable substances (ash) as