170 ALUM.— Bye-products.
pared alum flour. When the rocliing casks aredismantled, and the alum of the upper part removed,there is invariably found at the bottom a consider-able quantity of white slime mixed with smalloctahedral crystals of alum. A quantity of thisslime, well washed, gave the following centesimalcomposition:—
Ter Cent.
Sulphuric acid,... 39-40
Alumina,. 31-80
Potassa,. 1003
Moisture,. 1G-70
Insoluble matter, . 2-07
100-00 i
On examining the insoluble portion, it was found jto contain— I
Per Cent.
Silica. 73-79
Alumina, with a trace of iron,. 21-83
Lime, .,. 3*49
Loss,. -89
100-00
The mother liquor which remains after the separa- jtion of the alum, is composed of solutions of ferrousand ferric sulphates and chlorides, and magnesiumand other alkaline sulphates; and when soap-boiler’swaste has been used as the precipitating agent, itcontains, in addition to the foregoing, sodium alumin moderate quantities; if bisulphate of potash,saturated with wood ashes, has been employed, theliquor will contain some sulphuric acid and otheringredients in small proportions. From a chemicalanalysis of the alum shale, or ore, in the first instance,a good idea can be formed of the principal bodiesremaining in the mother liquor, arid their utility forthe formation of secondary products; for instance,if free acid should be detected in the lie, it may heemployed to neutralize crude ammonium carbonatefrom the gas-works, to procure ammonium sulphate;or if much iron be present in the acid liquor, afurther addition of borings of iron, or old iron, maybe added, and in this way ferrous sulphate for manu-facturing purposes formed. Should the motherliquor contain large quantities of the mixed chloridesof iron, it may be evaporated to dryness, and the drymass heated to redness in a reverberatory furnace todrive off the hydrochloric acid, and thus ferricoxide obtained of a quality fit for use as a pigment.
Magnesium sulphate, Epsom salts, is also manu-factured from the mother liquors of the ores.
The mass of alum from the crystallizing tub is cutinto square blocks for market, when it will be foundinteriorly to consist of beautiful large octahedralcrystals, similar to Fig. 11, which project from thesides and cover of the vessel.
For some purposes the alum is preferred in theform of a fine powder or flour; a portion is thereforeprepared for the market in this state. Fig. 12 is asketcli of the machinery usually employed at thealum-works for this purpose. The alum, reduced tocoarse fragments, is crushed beneath a pair of pon-derous wheels or rollers of cast iron, one of which isrough or fluted on the rim, and the other perfectly
smooth. These traverse in a circle, and the alum isconstantly pushed into the path of their revolutionby two vertical revolving blades, one of whichsweeps it inward, and the other outward. Thegrooved roller crushes the larger fragments beforethey are subjected to the action of the smooth surfaceof the other. When one charge is thus reduced topowder, it is shovelled out by an attendant to make
room for a new supply; and the powder is passed byhand into a hollow cylindrical sieve, shown at theright hand of the figure. The lower portion, orbelly of the cylinder, which lies in a slightly inclinedposition, consists of fine wire gauze ; the upper con-vexity is made of white iron. This is surroundedby wooden hoops, to which are attached brushes that
Fig. 12
pass into the interior of the cylinder. An oscillatingmotion is given to the latter, and that portion of thealum powder which has been reduced to a state ofsufficient fineness is driven by the action of thebrushes through the sieve, from which it falls in abeautiful white flour into a box below; the coarserparticles passing onward to the other end of thecylinder, to be again subjected to the action of thecrushing rollers.