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.
133. Collecting The Precipitate. After several precipitations, or when desired, the black mud, consisting of silver sulphide, mixed with free sulphur, and more or less base metal sulphide, is run out through the clean-up pipe, faucet, or plug-hole, to a filter, drained, washed by passing hot water through it, again drained, pressed into cakes or not, as desired, and dried.
134. Roasting The Precipitate. The dried precipitate is next roasted in the small reverberatory furnace, to burn off the greater part of the sulphur, beginning with only the heat required to set fire to it, and gradually increasing the temperature to dark redness, or to such a degree as the material will bear without melting. It must be stirred while roasting, and changed from end to end of the furnace, in the manner directed for the roasting of ore. If but little base metal is present, the roasting is continued until as little as possible of the sulphur remains; otherwise a prolonged roasting is avoided, on account of the formation of too much base metal sulphate and oxide, which are injurious to the black lead pot in the melting.
135. Melting The Silver. This is done in black lead crucibles, in a wind furnace, with coke or charcoal for fuel. If the precipitate, before roasting, contained but little base metal sulphide, the silver is seen in the form of threads traversing the roasted mass, which, however, still retains a considerable quantity of sulphur. The crucible, containing some scrap iron, is filled, and placed in the fire, standing on a piece of firebrick; for as the melting occupies a considerable time, even a thick layer of the best coke does not last long enough to prevent the crucible from settling down to the grate.
A little borax is added, and the whole is heated till there is room in the pot for more material, when it is refilled by means of a scoop and funnel. As in the case of gold, the refilling is done before the mass in the crucible has become fluid, in order to avoid loss by projection. As fast as the scrap iron disappears, more is put in; but, if such addition is made after full fusion, the iron is first heated.
If the roasted precipitate contains much copper or iron, more borax is required, and a little clean sand is useful, especially if the roasting has been excessive. Some charcoal is also added.
When the pot is full of thoroughly melted matter and pieces of iron, a test is made by placing the red hot end of a piece of nailrod, or thick iron wire in it. If, after a few minutes, on withdrawing the rod, it is found that a part of it has been melted, more time must be allowed. When iron is no longer consumed, the melting of that quantity of precipitate is finished, and slag and matte are dipped out, by means of a red hot assay crucible held with the crooked tongs, and poured into a mould, or iron pan. The pot is now refilled with roasted precipitate, taking the precaution to add it slowly until the melted mass is somewhat chilled.
When all the precipitate has been thus worked up, or the pot contains a sufficient quantity of metal, a part of the slag and matte is removed as before, and the remainder, with the silver, is poured into a warmed and greased mould. The overflowing of the slag and matte is of no consequence, if the mould is large enough to contain the silver, which will go to the bottorn in consequence of its greater specific gravity. After removal from the mould, it is usual to place the bar in a tub of water, for the purpose of cooling it, but when there is matte upon it this must not be done until the matte also has solidified; otherwise an explosion will occur.
If the melting has been properly conducted, the matte is brittle, and separates readily from the cooled bar. If it is tough, that which adheres to the metal must be beaten off, and the whole remelted in presence of iron, as it then contains a great deal of silver. After cooling, it is broken and examined for any large buttons of silver which may have been dipped out with it. All the slag and matte is preserved, the former to be sold, the latter to be crushed and reworked by roasting and leaching. If a handsome bar is desired, it must be remelted with borax, cleaned by skimming, and re-cast, covering the surface with powdered charcoal before solidification.