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.
213. If any metallurgist had been told, prior to the introduction of the Stetefeldt furnace, that a chlorid-izing roasting of silver ore could be effected in the short space of a few seconds of time, he would probably have been incredulous. In the old reverberatory, the operation requires several hours; in the cylinder furnaces of continuous action it occupies from 10 to 30 minutes, but in the furnace now under consideration, the ore, if not of extremely refractory character, is almost completely roasted while falling from a height of little more than 20 feet.
The furnace consists of a stack or shaft of masonry, provided with fireplaces opening into it near the base, with a flue near its upper end, and with an apparatus for sifting the ore and salt into it at the top. The ore thus showered from the top of the stack, encounters in its descent, under the most favorable conditions for chemical action, the ascending flames and heated air from the fireplaces, as well as the fumes of its own combustion, and from the voltalization and decomposition of the salt. When it reaches the bottom, where a hopper is provided for its reception, not only is the oxidation completed, but the chloridation of the silver is so far advanced as to require for its completion only the further exposure to heat which the ore receives while accumulating in the hopper. The latter is emptied from time to time, by means of a slide, into iron cars and the ore removed to the cooling floor.
As in all other processes in which the pulverized ore is showered through the flames, the lighter portion, amounting sometimes to 30 per cent of the whole, is carried back by the current of air and gas. In order to prevent an accumulation of dust in the flue, the latter joins the stack at an acute angle, and is conducted almost vertically downward to the base, where the flames from an auxiliary fireplace enter it, and effect the roasting of the dust, which then, passing into a horizontal flue, settles mainly in a series of hoppers, while the hot air, smoke and gases pass through dust chambers to the chimney. The construction of the furnace involves many ingenious contrivances for regulating the feed and heat, for the removal of the roasted ore, the inspection of the interior, and the admission of air at suitable points, and in proper quantity.
In a large furnace, capable of roasting from 30 to 70 tons of ore in 24 hours, the roasting tower is 25 feet high and five feet square at the base. One man on a shift can attend to the firing.
The following extract from the Circular of the Stete-feldt Furnace Co. will assist, with the drawings, to an understanding of the arrangement:
"Of the accompanying drawings Figure I represents a vertical section of the Stetefeldt Furnace, showing its latest and most improvd mode of construction, and Figure 2 is a sketch of the Stetefeldt Feeder.