Roasting Furnaces

33. Roasting Furnaces. The essential condition of roasting pulverized ore is, that every particle shall be exposed to the action of heated air until certain chemical changes are effected, after which heat alone will complete the operation. This condition is fulfilled in the reverberatory furnace, in which a layer of ore of a certain thickness is acted on by a current of heated air from a fire. The surface of the layer is renewed from time to time by stirring the mass by means of implements operated by hand. As stirring by hand power is laborious and expensive, various means have been devised to dispense with it, and to substitute the automatic action of machinery, among the best of which are the furnaces known as the Stete-feldt, Bruckner, Brunton, White, Howell White, Pacific, and O' Hara, each of which has certain advantages over the other, so that a decided preference, under all circumstances, can be given to neither, and as the reverberatory is the original of all, as any kind of ore that is fit to be roasted can be roasted in it, and as the operator who knows how to use it, can easily adapt himself to the others, a complete description of its construction and operation will be given, but, as a work of this character would be imperfect without some account of the mechanical furnaces also, they will be briefly described in separate articles under "Addenda."

34. A Reverberatoiy Furnace is simply an oven. There is a fire-place at one end and a flue at the other, and the ore to be roasted is laid on a horizontal hearth between them. In the side walls are openings, which admit air, and allow the workman to turn the ore over, from time to time, with a long hoe, or rake. These openings can be closed, when necessary, with iron doors. The fuel must be such as will produce a flame; hence, wood, or flaming coal is suitable.

35. Reverberatory furnaces are of several kinds; the single, which has but one roasting chamber, the double, in which a second roasting chamber is constructed directly over the first, and is worked with waste heat; and the long furnace, which has two or more chambers, either in one horizontal plane, or, which is better where the ground is suitable, raised more or less one above another, in step form, but not superposed.

The single hearth is wasteful of fuel; the double hearth is inconvenient, so I will describe a long furnace with two hearths, represented by the horizontal section, Plate I, and vertical section, Plate 2. If more than two hearths are required, which with concentrated sul-phurets may be the case, a third or fourth can be added, but it will, in general, be necessary to add also an auxiliary fire-place to aid in heating them. The capacity of this furnace is three tons of average silver ore per twenty-four hours, or from one to one and a half tons of concentrated sulphurets in the same length of time.

36. The masonry may be built entirely of common bricks; with adobes in case of necessity; but it is better to make the inside of the fire-place, and the arch over it, of firebrick. If convenient the outside walls can be of any kind of stone, but, unless a good firestone can be procured for the inside work, that must be of brick. Lime mortar may be used with advantage for the outside work, but all parts which are exposed to much heat must be laid in clay. The brickwork consists of headers and stretchers alternately, appearances being sacrificed for the sake of strength. The masonry below the hearth is not solid, but the space inclosed by the walls is filled with sand, or with earth well tamped, and on this the hearth, or sole, is laid, of the hardest bricks on their narrow sides, without mortar, but afterwards grouted.

37. The walls are supported against the thrust of the arch as follows. At the points shown in the diagram are vertical backstays or "buckstays," of wrought iron, two inches wide by one and a quarter inch thick, and long enough to reach from below the hearth to just above the arched roof. Through holes punched in the backstays, near the ends, project the nutted ends of tie rods of 3/4 inch round iron. The lower tie rods, passing through the body of the furnace, below the hearth, and uniting the lower ends of opposite backstays, are put in before the hearth is laid. The upper ones extend across the furnace above the walls, uniting the upper ends of the backstays, and are not put in place till after the walls and arch are laid. Usually the furnace is stayed in the direction of its length, in the same manner, except that the lower longitudinal tie rods do not pass entirely through, but are comparatively short, being securely anchored in the body of the furnace, under the hearth. If the ends of the furnace are made masssive, these ties may be dispensed with, especially if the ends are supported by wooden posts well braced. The cross ties, which are indispensable in some form, may be of wood, uniting the upper ends of strong posts, the lower ends of which are sunk in the ground, close to the side of the furnace, and which replace the iron backstays, the objection being that the posts cannot be set very near the doors, both on account of being in the way, and because of the heat; but near the doors is precisely where they are most needed. Cast iron ribbed backstays are also used, but are liable to break.

38. When all is ready for raising the walls above the hearth, the cast iron door frames are set up, in the middle of the thickness of the wall, which should not be less than 16 inches thick, and a stirring-hoe is passed through each of them in succession, and its range of operation ascertained by trial, and marked on the hearth. The walls are then built up along the marks, cutting off the corners of the otherwise rectangular hearth, and giving it the form seen in the diagram. This, though entailing some loss of space within the furnace, is absolutely necessary, in order that every part of the hearth may be accessible with the hoe.