1G2 ALUM.— Lixiviatiost.
level—see Fig. 2. The steeps are furnished withfalse bottoms formed of planks, which rest on trans-verse beams, and the roasted shale is laid on theplanks till it is about 18 inches above the bottom.Water, or a weak lie from a previous operation, ispoured upon the bed of ore till it is completelycovered over, and allowed to remain in contact withit for about eight hours, generally through the night.A plug at the bottom of the vat is then opened, andthe solution, which should have a specific gravity ofabout 20° Twaddle, or 1T00 specific gravity, is drawnoff into the settler, or supply cistern, where it isallowed to remain at rest, in order that any mattersheld mechanically may subside. A second wash ofa weaker liquor than that employed in the first oper-ation is then added, and left in contact with thepartly-exhausted shale as long as in the first instance,after which it is drawn off to the cistern as before ;the same operation is repeated a third tune if neces-sary, till the liquid becomes below 12° Twaddle instrength, after which it is not deemed economical toevaporate for alum, the further washings beingused for exhausting fresh quantities of ore. Thelast washings of the steep are made with water, andthe strongest of the several resulting weak liquorsfrom each steep is used the first in exhausting a freshone. After the ore is deprived of its soluble in-gredients, it is either removed to the waste heap, orreturned to the calcining ridges, where it is inter-mixed with the fresh ore for the purpose of check-ing a too rapid combustion, when such is apt totake place.
At Vahmmster, in the department of the Moselle,the tanks are constructed of burned stones and clay,and each has a capacity of 1728 cubic feet—1880cubic feet llhenish. At the bottom, at certain dis-tances from each other, beams are arranged, uponwhich a layer of straw and brushwood is laid, andupon this the false bottom of boards is placed.This arrangement forms the filtering apparatus, fromwhich the lie flows through apertures at the sides.Each cistern requires 1280 cubic feet of water forthe lixiviation. Lie is first poured upon the ore of20° 15. = sp. gr. IT57 ; when this has run off it cor-responds to 24° or 25° 15., and is fit for boiling; aweaker lie of 15° 15. = sp. gr. 1T13, and subse-quently others of 10° 15., = sp. gr. 1-072, and theweakest of 5° B., = sp. gr. 1-084, then follow; andlastly, the whole is washed out with pure water.All the lies that fall short of 24° or 25° B. arepoured upon more or less exhausted ores, accordingto their strength. By arranging these cisterns uponterraces, the one above the other, the lie can easilybe drawn from the upper into the lower cisterns,until it has acquired the proper strength. The solu-tion is then called crude liquor, and is preserved inlarge walled tanks, ready for further operations.
The density of the liquors is determined by aspecific gravity bottle peculiar to the alum manufac-turers ; it is capable of containing 80 pennyweightsof water at the ordinary temperature, and when thisbottle is filled with the alum liquor and weighed, theexcess of weight is denominated the strength in
pennyweights; thus, if a liquor should weigh 90pennyweights, it would be put as one of 10 penny-weights, or simply liquor of 90. A similar methodis the use of the hydrometer, for determining theamount of ingredients dissolved, and this even inthe pennyweights of the alum-maker; thus, dividingthe indications of Twaddle’s hydrometer by 2’6, givesthe alum-maker’s strength in pennyweights, withoutthe trouble of weighing.
According to Scanlan, eight different liquors aremet with in the alum-works on the Yorkshire coast.
1. Haw liquor. —The calcined alum shale is steepedin water till the liquor has acquired a specific gravityof 9 or 10 pennyweights, according to the languageof the alum-maker.
2. Clarified liquor. —The raw liquor is brought tothe boiling point in leaden pans, and suffered to standin a cistern till it has become clear; it is then calledclarified liquor. Its gravity is raised to 10 or 11pennyweights.
3. Concentrated liquor. —Clarified liquor is boileddown to about 20 pennyweights. This is kept merelyas a test of the comparative value of the potassiumsalts used by the alum-maker.
4. Alum-mother liquor. —The alum pans are fed withclarified liquor, which is boiled down to about 25 or30pennyweights, when a proper quantity of potassiumsalt in solution is mixed with it, and the whole runinto coolers to crystallize. The liquor pumped fromthese rough crystals is called alum-mothers.
5. Salt-mothers. — The alum-mothers are boileddown to a crystallizing point, and afford a crop ofrough Epsom, which is magnesium sulphate withferrous oxide.
6 and 7. Alum-washings. —The rough crystals ofalum—No. 4—are washed twice with water, the firstwashing being about 4 pennyweights, the secondabout 2£, the difference in’ gravity being due tomother-liquor clinging to the crystals.
8. Tun-liquor. —The washed crystals are now dis-solved in boiling water, and run into the roching-tuns—wooden vessels lined with lead—to crystallize. Themother-liquor of the roch-alum is called tun-liquor;it is of course not quite so pure as a solution of roch-alum in water.
With reference to the exhausted residues, two casesmay occur: they have either been rendered perfectlyporous by the roasting and decomposition, in whichcase they will have been completely exhausted, andmay be thrown away unless they can be used forcovering fresh heaps; or they still contain portionsof undecomposed ore, and may then be subjected toa second exposure—as at Buchsweiler, in Alsace —either by themselves or mixed with fresh ore.
The process in many other alum-works, where theores do not require roasting, is essentially the same asthat described above. The lixiviation, however, isthen carried on with the heaps themselves, andduring the process of decomposition; they are erectedfor this purpose either upon flat wooden boxes, termedBuhnen, or upon a foundation composed of brick-work or clay, which is completely impervious to theliquor, and where the water wMcb is pumped from