Page:The American Cyclopædia (1879) Volume XVI.djvu/852

 822 ZINC times employed for the same purpose. Blende is crushed fine before roasting ; other zinc ores, which have been calcined in lumps, are crushed likewise, preliminary to distillation, finer for the Belgian than for the Silesian process. The crushing apparatus for hard ores may be rock breakers, rollers, &c.; for softer material, grind- ing or Chilian mills, or centrifugal pulverizers. The preparation of charges for distillation con- sists in mixing the calcined ore with a proper proportion (sometimes 100 per cent, but usu- ally 40 to 46 per cent, for calamine, and 55 to 60 per cent, for blende) of lean coal, as free as may be from pyrites and slate. It is well to substitute coke for half the coal, since coke forms no water, and thus diminishes that cause of the production of zinc powder. But coke alone is said to reduce less powerfully and to require a higher temperature. The distillation of zinc from the oxidized ores is always con- ducted in muffles (Silosian method), retorts (Belgian method), or crucibles (old English method), and hence involves much consump- tion of fuel and destruction of refractory ma- terial. Great losses of metal are also unavoid- able, ranging from 9 to over 30 per cent, of the zinc in the charge. All attempts to re- 'duce the cost of distillation by employing shaft furnaces, reverberatories, or very large muffles have hitherto failed, by reason of their pro- duction of zinc powder or zincic oxide, or both, which must after all be treated in retorts, to obtain liquid zinc. The old English method is a distillation per descensum, the ore being charged into crucibles placed in a circle about a central fire and under an arch. These cruci- bles are covered, and the zinc vapors, formed by heating, are obliged to pass down through the charge and by openings in the bottom of the crucibles, to be condensed below. The ca- pacity of such furnaces is small, the emptying of the crucibles inconvenient, the destruction of fire clay comparatively light, and the con- sumption of fuel very great (22 to 27 parts of fuel for one part of zinc produced). For these reasons the process is now generally abandoned, except for the redistillation of zinc residues, in localities where fire clay is dear and coal is very cheap. The Silesian and Belgian methods are now chiefly followed. The former em- ploys muffles, and, according as the flame out- side of the muffles is allowed to escape freely upward into the air through openings in the arch, or is conducted downward through open- ings in the hearth to a chimney, the process is known as the old Silesian or the Belgian-Sile- sian. The Belgian system employs cylindrical retorts, placed in rows, gently inclined, in the furnace. The old Silesian furnace consists of a square, arch-covered hearth, with a depressed ' fire box in the middle, on both sides of which muffles are arranged in rows. Such a furnace may contain 20 to 26 muffles, averaging 3 ft. 7 in. in length, about half as much in height, and a quarter as much in width. Lean coal, giving a short flame, is burned on the grate, and the flame plays around the muffles, subse- quently escaping, either through openings in the arch, or through side openings into cham- bers for "tempering" muffles, calcining ore, or remelting zinc. The zinc vapors from tho muffles escape through elbow-formed nozzles and fall, condensed into drops, into receptacles, where they congeal, to be subsequently rernelt- ed and cast into bars. The residues, withdrawn from the muffles, fall on the floor of the works, and, together with the gases and smoke esca- ping from the top, cause much inconvenience to the workmen. Moreover, the fuel consump- tion is large (17 parts to one part of zinc pro- duced), and the total loss of zinc may reach more than 30 per cent. Charging fresh ore into the adjoining calcination chambers may cool down the distilling space injuriously, as may also tho necessary poking of the coals, or opening of the furnace door to promote combustion, there being no chimney to give a draft. The addition of a chimney, on the other hand, is said to cause with certain coal, like the lean coal of Silesia, too quick a com- bustion ; and moreover the draft causes a leak- age of zinc vapors through defective places in such muffles as are nearly worn out. The muf- fles are made with care of fire clay and "cha- motte" (coarsely pulverized old muffle mate- rial), kneaded and moulded by hand or ma- chinery. The imperfections of this process have not prevented its continued use in Silesia, where habit and skill, cooperating with local conditions, seem to permit its employment with fair results. The charge per 24 hours for a 20- muffle furnace (apparently the best type) is 750 to 800 kilogrammes of calamine ; duration of muffles, six to eight weeks. The following aro the stages of the process of distillation : care- ful drying and heating of the furnace ; intro- duction of the glowing muffles from the tem- pering chamber, and placing of them upon a bed of sand in the hearth ; filling of spaces where the flame is not to pass with clay and fragments of brick ; attachment of the nozzles ; plastering of all cracks with loam ; gradual charging with long semi-cylindrical spoons, through openings in the elbows; closing of these and other openings; accumulation for the first two or three hours of zinc powder in the cold necks or nozzles, and its removal by means of an iron wire ; white heat in the fur- nace and an increasing production of drops of zinc for six or eight hours ; maintenance for an equal period of the maximum flow ; gradual diminution after 24 hours from charging ; open- ing of the furnace, and removal of the liquid zinc to a cooler space ; raking out of the resi- dues, scraping, repairing, or replacing of muf- fles ; fresh charge ; remelting of the zinc in iron kettles. In the Belgian-Silesian method, a long flame strikes from the fire box against the arch, reverberates around the muffles back to the hearth, and is drawn through openings in the latter into a flue below, which leads either direct to the chimney or to calcining chambers.