This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
In a single hearth reverberatory furnace, each ton of ordinary silver ore requires from one-third to one-half a cord of dry pine wood. In a long furnace the proportion is materially reduced.
Concentrated auriferous sulphides, when roasted for chlorination, in a reverberatory furnace with three hearths, require one cord of wood, or one-half ton of Seattle coal, per ton of ore. In this roasting, one man on a shift is sufficient for one furnace, roasting 1 1/2 tons of ore in each 24 hours. The writer experimented with different numbers of men employed at one time, but found no advantage in more than one for all three hearths, thus proving that stirring the ore beyond a certain extent is useless, unless the supply of air be increased. In the mechanical furnaces the proportionate consumption of fuel is much dimished, especially in those which receive the ore continuously, in which one-tenth of a cord of wood to the ton of silver ore is a common proportion. The mechanical furnaces described are all in more or less extensive use, giving satisfactory results, with great economy of fuel and labor as compared with the old reverberatory furnace.
Those which receive the ore continuously are the most economical in operation, but the most costly in construction. They are especially adapted to the chlo-ridizing roasting of large quantities of silver ore of nearly uniform character.
For the roasting of gold-bearing sulphides, or exceptionally rich silver ores, for custom works in which the character of different small lots of ore is liable to extreme variation, or in small establishments, the furnaces which are charged periodically are usually preferred, because the roasting in them is more readily controlled, or because of their more moderate cost.
The furnaces which are charged continuously, as the Stetefeldt, and the White with its modifications, have not yet been tried for the dead roasting of gold-bearing sulphurets for the Plattner process, but there is no reason to doubt that, with proper care, they might be made to do the work. Possibly a modification, such as that used by Mr. Crosby (50) in connection with his reverberatory furnace, might be necessary on account of the large quantity of sulphur which the material in question contains.
One disadvantage of the continuous cylinder furnaces in which the ore progresses in an opposite direction to the draft is, that the heavier particles, which require the longest exposure, pass through more quickly than the lighter, because the force of the draft has less influence in retarding their horizontal progression. The action of the furnace must be adjusted to the requirements of these heavy particles. Between these and the dust which is completely controlled by the draft, are particles of all grades, some of which are barely massive enough to make headway against the current of air.
As the time required for the roasting of the larger particles is greater than that required for the smaller, it is clear that a large proportion of the ore which passes through the cylinder is detained within it longer than is necessary, causing useless consumption of power and, in all probability, loss of silver. This defect is obviated when the ore moves through the furnace in the same direction as the draft. The progression of the ore is then aided by the force of the draft, but in a less degree as the particles are heavier. The objection to this method is, that the ore in its progress encounters a gradually diminishing heat, and a decreased proportion of oxygen, while the reverse should be the case.
Mr. White, the original inventor of the application of this kind of furnace to the treatment of silver ores, has been much blamed for his persistence in this manner of working it, but it must be admitted that, if the objection pointed out could be overcome, the method would possess certain advantages.
In the Stetefeldt furnace, in like manner, the descent of the heavier particles is less impeded by the upward current of air and gas, and is therefore more rapid than that of the lighter, but in this case no waste of power results, because the ore is lifted, once for all, to the top of the shaft by an elevator.
In the O'Hara, all portions of the ore, except a small quantity of very light dust, are moved toward the hotter end of the furnace with practical uniformity. This furnace combines the advantages of the reverberatory, worked by hand, with those of automatic action. It probably costs more for repairs than the others.
A roasting furnace, of whatever description, should be kept in operation as constantly as possible, not only in order to economize fuel, but also to avoid the injurious effect of alternate heating and cooling. In the selection, therefore, regard should be paid to the quantity of ore to be treated daily, as well as to its character, and the financial resources at command.