Page:The American Cyclopædia (1879) Volume I.djvu/389

ALUM 365 miles in width, and is covered by strata of allu- vial soil, sandstone, ironstone, shell, and clay. The alum schist is generally found disposed in horizontal lamina?. The upper part of the rock is the most abundant in eulphur, so that a cubic yard taken from the top of the stratum is five times more valuable than the same bulk 100 feet below. If a quantity of the schist be laid in a heap and moistened with sea water, it will take fire spontaneously, and continue to burn till the whole inflammable matter is con- sumed. Its color is bluish gray; sp. gr. 2*48. It imparts a bituminous principle to alcohol. The rock, broken into small pieces, is laid on a horizontal bed of fuel, composed of brush- wood, &c. When about four feet in height of the rock is piled on, fire is set to the bottom, and fresh rock continually poured upon the pile, until the calcined heap is raised to the height of90 or 100 feet. Its horizontal area is at the same time progressively extended, till it forms a great bed nearly 200 feet square, hav- ing about 100,000 yards of solid measurement. The rapidity of the combustion is allayed by plastering up the crevices with small schist moistened; but notwithstanding this pre- caution, a great deal of sulphuric or sul- phurous acid is dissipated. One ton of alum is produced, from 130 tons of calcined schist; this result has been deduced from an average of 150,000 tons. The calcined mineral is di- gested with water in pits usually containing about 60 cubic yards. The liquid is drawn off into cisterns, and afterward pumped up again upon fresh calcined "mine." This is repeated until the specific gravity becomes 1 - 15. The half exhausted schist is then covered with wa- ter to take up the whole soluble matter. The strong liquor is drawn off into settling cisterns, where the sulphate of lime, iron, and earth are deposited. At some works the liquid is boiled, which aids Us purification. It is then run into leaden pans 10 feet long, 4 feet 9 inches wide, 2 feet 2 inches deep at one end, and 2 feet 8 inches at the other. This slope facilitates the emptying of the pans. Here the liquor is con- centrated at the boiling heat. Every morning the pans are emptied into a settling cistern, and a solution of chloride of potassium (either pretty pure from the manufacturer, or the crude compound from the soap boiler) is added. The quantity of chloride necessary is deter- mined by a previous experiment in a basin, and is regulated for the workmen by the hydrome- ter. By this addition, the pan liquor, which nad acquired a specific gravity of 1'4 or 1*5, is reduced to 1-35. After being allowed to settle for two hours, it is run off into the coolers to be crystallized. At a greater specific gravity than 1-35, the liquor, instead of crystallizing, would on cooling solidify in a magma resem- bling grease. After standing four days, the mother waters are drained off, to be pumped into the pans on the succeeding day. The crystals of alum are washed in a tub and drained. They are then put into a lead pan, with as much water as will make a saturated solution at the boiling point. Whenever this is effected, the solution is run off into casks. At the end of 10 or 16 days the casks are un- hooped and taken asunder, when the alum is found exteriorly in a solid cake, but in the in- terior cavity in large pyramidal crystals, con- sisting of octahedrons, inserted successively into one another. This last process is called "rocking." Mr. Winter says that 22 tons of chloride of potassium, or an equivalent of 31 tons of the black ashes of the soap boiler or 73 of kelp, will produce 100 tons of alum. Where much iron exists in the alum ore, the alkaline chloride, by its decomposition, gives rise to an uncrystallizable chloride of iron. For this rea- son it is preferable to the sulphate of potassium. Alum may also be obtained from cryolite by heating the mineral with three times its weight of strong sulphuric acid, whereby anhydrous neutral sulphate of aluminum and acid sulphate of sodium are obtained ; treating the resulting mass with a small quantity of cold water to re- move the acid sodium salt ; then digesting the anhydrous sulphate of aluminum with warm water, to convert it into the hydrated salt, and adding the proper quantity of sulphate of po- tassium. As the alum from cryolite is remark- ably free of iron, it is highly prized by many manufacturers. The discovery of an aluminous earth in the neighborhood of Baux, France, and hence called bauxite, has added to the class of materials for the manufacture of alum. To this the addition both of sulphuric acid and of a salt of potash is necessary. Bauxite is exten- sively employed in the manufacture of the sul- phate of alumina (alum cake) and of the metal aluminum. At the chemical works of Harri- son Brothers, Philadelphia, ammonia alum is manufactured from a pure clay mostly ob- tained from ISTew Jersey. The clay is dried and then ground and calcined in a reverberatory furnace. When thoroughly calcined and puri- fied, it is while hot digested for some hours in sulphuric acid contained in large vats. The product is washed with water and concentrated, sulphate of ammonia having been previously introduced, and it is further purified by re- dissolving, then boiled by steam, and finally transferred to the crystallizing tubs. These are about eight feet high, and made of strong staves. At the end of eight or ten days, the staves of the tub being removed, a cylindri- cal mass of apparently solid alum is revealed. This being pierced near the bottom, the mother water at the centre flows off along the sloping floor into leaden subterranean cisterns, whence it is subsequently pumped and variously util- ized. Each crystallizing vat yields about 21 barrels of alum ready for market. The com- position of pure potash alum is : Potash per cent. 9-89 Alumina " 10-94 Sulphuric acid " 88-68 Water " 45-49 100-00