804

SODA.— Black Ash. Mixing.

possible. The more ash, the more silica, alumina,iron, &o., will be introduced into the black ash, allof which in the lixiviation cause a loss of soda ash.Experience has shown that a good mixing coal oughtnot to contain beyond 7 to 10 per cent, of incom-bustible substances, certainly not exceeding 12 percent. Some collieries produce coal better fitted forsoda making than others, with no more ash; andit appears on the whole that a coal is the betterfitted for mixing the more bituminous substancesit contains. “ Gas coals ” are therefore preferredfor this purpose. Bituminous mixing coal becomessemi-fused, and greatly assists the furnace operations;but some alkali-makers believe that more per cent,is used of this kind of coal than of coals containinga larger amount of “ fixed ” carbon.

Leblanc prescribed wood - charcoal, but thisis nowhere used now; and Kolb has shown bytrials on a large scale that any substance rich incarbon may be substituted for it, such as coke, saw-dust, tar, and peat, all of which simply act in theproportion of their carbon, or rather of their re-ducing properties, as shown by Bebthiee’s methods,carbon and hydrogen, both acting in the black-ashprocess exactly in the same proportion as in reducinglitharge to metallic lead. It is therefore quiteright to use the cheapest source of carbon, that iscoal, but as free as possible from iron pyrites andincombustible matter.

Richardson gives the following analysis of mix-ing coal used on the Tyne :—

Carbon,. 74-34

Hydrogen,. 4-96

Nitrogen,. 1-42

Oxygen,. 10-16

Sulphur,. 1-39

Ash,. 7-73

100-00

G. E. Davies gives the following full analysis ofsmall coal (slack) used for ball mixtures in Lanca-shire :—

Sulphur,.,

Ash,.

Carbon, .Nitrogen,,

Per Cent.

1-432

10-000

70-122

0-286

Combustible matter,. 88-590

Volatile sulphur,. 1-387

Silica,. 4-936

Alumina. 2-575

Iron peroxide,. 1-511

.Lime,. 0-700

Magnesia,. 0-052

Sulphuric anhydride,. 0-243

Phosphoric do.,. 0-006

100-000

Slack is, however, generally only examined forash, sulphur, and moisture :—

Ash.

Sulphur.

Moisture.

1 .

4-46

0*724

2-41

2.

5-77

0-986

1-00

6.

7-41

0*937

10-14

4.

10-84

1-116

4*72

5.

10-96

1-736

5-11

6.

13-42

0-543

2-98

7.

16-76

0-431

1*22

8.

28-96

1-774

3-44

The nitrogen, which hardly ever fails to occur inthe mixing coal, is always more or less injurious inits effects. So far as it escapes in the form of am-monia during the balling process, and during thecooling of the balls, it is harmless; but another por-tion forms sulphocyanides and cyanides, the latterbeing transformed during lixiviation into ferro-cyanides.

It is not usual to go to any great trouble in mixingthe three ingredients of black ash. In most worksthey are tipped one after the other in front of theball furnace, and only mixed to a slight extent inshovelling them in ; or else into the hopper on thetop of the furnace, whence they fall down into theback-bed. In some works a little more is done, bypassing the mixture between fluted rolls before itis charged into the furnaces. In no case should themixture be finely ground, but the fragments oughtto remain somewhat coarse, so as to keep the ballsmore “ open ” or porous.

The proportion in which the three ingredients ofthe ball furnace are used cannot be laid down in acompletely fixed formula, since very much dependsupon the quality of the materials. On the Tyne itis usual (except in one or two works, where largercharges are made) to use for each charge of a “hand"furnace (“ revolvers ” will be treated of later on)3 cwts. of sulphate of soda; and this is also the casein the majority of Lancashire works, only a fewworks making smaller balls (from 2j or 2f cwts. ofsulphate). The charge of chalk required for 3cwts. of sulphate varies from 3| to 3f cwts., accord-ing to the more or less damp state of the chalk;that of limestone (which is practically dry to beginwith) varies only from 3 to 3^ cwts. * For reasonsafterwards given, it is requisite to confine the pro-portion of carbonate of lime to the lowest limitwhich allows the workman to work his ball properlywithout burning it, and a practised eye can easilytell by the colour of the ball whether to use any lesschalk in the next charge or not. The same observationapplies to the coal, but in this case it is not theobject of the alkali-maker to reduce the quantity;but, on the contrary, to use as much as can bethoroughly incorporated with the fluxing mixture;for only thus an approximately complete decom-position of the sulphate can be brought about, andthe ball be kept porous. When too much coal hasbeen used in the mixture, the workman soon findsit out by the fact that his ball works too “ stiff,” or“dry,” or “ fiery,” and does not “ clear whilst theforeman or manager perceives it by the unconsumedlumps dispersed through the substance of the ballon breaking up the latter. Different kinds of coal re-quire different proportions; and whilst in some casesthe proportion is as low as 1£ cwt. to 3 of sulphate,in others it is as high as 2 cwts. to 3 of sulphate. Onthe Tyne, with moderately good mixing coal, itdoes not vary much—about ll cwt. The coal isnever crushed, but always used in the state called“ small coal ” in the East, or “ slack ” in Lancashire

* The larger proportion is mostly used when the black ashis intended for the manufacture of caustic soda.