File:Silesian muffles furnace for zinc distillation.png

English: Silesian process. — In the zinc works of Silesia the furnaces employed differ considerably from those used in the Belgian process.

Fig. 691 represents an elevation, and fig. 692 a vertical section of the Silesian furnace. The distillation is effected in a sort of muffle of baked clay, m, fig. 692, and figs. 693, 694, about 3 feet 3 inches in length, and 20 inches in height. The front of this muffle is pierced with two apertures. The lower opening, d, serves to remove the residues remaining in the retorts after each operation, and is closed during the process of distillation by a small door of baked clay, firmly luted in its place. In the upper opening is introduced a hollow clay arm, beat at right angles, a, b, c, and which remains open at c. An opening at b permits of charging the retort by means of a proper scoop, and this, during the operation, is closed by a luted clay plug. From six to ten of these muffles or retorts are arranged in rows, on either side of a furnace provided with suitable apeitures for their introduction. They are securely luted in their places, and the openings closed by sheet-iron doors, by which the too rapid cooling of the pipe a, b, c is prevented. The fuel employed is coal, which is burnt on the grate G, situated in the centre of the furnace. The retorts are charged with a mixture of calamine and small coal, or more frequently coke dust, since, when coal is employed, the products of distillation are found to be liable to choke the pipe a, c, c. The zinc escapes by the opening c of the adapter, and is received into the cavities o of the furnace.

The furnace shown in figs. 695, 696, 097, is for remelting the metallic zinc. Fig. 696 is a front view; fig. 695 is a transverse section, fig. 697 a view from above; a is the fire- door ; b the grate ; c the fire bridge ; d the flue ; c the chimney ; f, f, f cast-iron melt- ing pots, which contain each about 10 cwt. of metal. The heat is moderated by the successive addition of pieces of cold zinc. The inside of the pots is sometimes coated with loam, to prevent the iron being attacked by the zinc.