Precipitation of Copper contained Silver leached out with Copper

Precipitation of Copper contained Silver leached out with Copper

Having refractory ore under treatment, it is generally the case that copper is also found in it. While roasting, the presence of copper is favorable for the chlorination of the silver, but copper ores require some more salt, especially if it is intended to save the copper also. The more chloride of copper formed, the more will be found in the solution while leaching it with hot water. In order to convert all the copper into a chloride, it would take at least one and a half pounds of salt to each pound of copper; and considering other base metals, lime, etc., all of which absorb chlorine, while a considerable part escapes useless, the above quantity has to be doubled. For this reason no special attention can be paid to the copper ; only that part of it can be extracted which is converted into a chloride during roasting under the usual circumstances. The chloride of copper transfers a part of its chlorine to the silver and other metals (23), and is reduced thereby to a sub-chloride ; if there is sufficient salt in the furnace it is raised again to a chloride. This sub-chloride (Cu2 Cl) is not soluble in water; it remains in the ore during leaching.

The different chlorides, being removed in the first leaching (61), are principally those of copper, iron, lead, antimony and zinc, besides some undecomposed salt. The first quantity of hot water introduced into the leaching box is, of course, most saturated with the named salts, and they have the property of dissolving, also, some chloride of silver. The dissolved silver precipitates again as soon as it becomes diluted with more water. There is, therefore, no difficulty in regaining the silver which is thus leached out. The amount of silver carried out by the leaching water varies from 0.5 to three per cent. Not only the chloride of silver, but also those of lead and antimony, are precipitated by dilution with water. There are two ways of regaining this silver.

Mr. O. Hofmann adopted an ingenious plan for this purpose, by conveying the hot water, under a slight pressure from below, through the pipe, e, (Fig. 10), by attaching to it a rubber hose. The water rises through the ore from e up to d, and as soon as it reaches within two inches of the brim of the box, the hose is removed from e, and the water admitted through d. The concentrated solution, containing dissolved silver, is now above the ore, and being diluted with water from d, lets the chloride of silver fall as a precipitate on and throughout the ore.

The other plan is the precipitation of the silver, together with the chlorides of lead and antimony, outside of the leaching box. This mode is preferable to the former when a great deal of lead and antimony is in the ore ; for if precipitated in the box, a great part of it will be dissolved by the hyposulphite of lime and then precipitated as sulphides with the silver, making this impure and consuming much of the precipitating agent. As soon as the chlorides flow into the trough, f, below e, into which several leaching boxes discharge their fluids in different degrees of dilution, the gradual precipitation commences. The precipitate is white and adheres to the trough through the whole length of it. These chlorides are the richest, and contained, at Flint, Idaho, 9 per cent, of silver; the balance was principally lead and antimony. The precipitate deposits on all bodies offering a surface. For this purpose a box must be constructed, as represented in Fig. 13, which shows the top view or ground plan. The sides, a, of a wooden box, six feet by six, or ten by six, are six inches high, the front, b’, and partitions, b, four inches high, leaving a space of six inches between them. These spaces are filled with shavings representing an immense amount of surface for the chlorides to de-

gold silver roasting chlorides

posit on. The fluid entering the trough, c, contains now a purified copper solution. Chloride of iron is also with the copper in solution, but does not prevent the copper from precipitating.

The white precipitate, when accumulated, is taken out, placed in filtering bags, with or without the shavings, and washed with clear cold water, in order to get rid of the copper solution. The silver can be extracted in two ways : The simplest mode is the application of hyposulphite of lime.

The sediment is taken out from the filtering bags and charged, while wet, into a filtering box of a proper size, arranged like Fig. 11, 61. The hyposulphite of lime, in a cold condition, is poured over it and managed as with the ore with the second leaching, 61. The silver-holding fluid may be conveyed into the precipitating box, Fig. 12, 62, and treated with the solution from the ore. The liquid from the bags is examined from time to time with sulphide of calcium. In the beginning the precipitate appears dark, being mostly silver ; but when it is perceived that the precipitate assumes a light yellow color, too much of lead, zinc and antimony is being carried out, and the leaching must be stopped. The residue in the filter box contains still some silver.

The other mode of extraction is more perfect, but also more expensive and more troublesome. After the copper has been washed off, the contents of the bags are taken out and dried. It is then introduced into large crucibles and smelted with an addition of soda-ash. The reduced metal, if some lead occurs in the ore, must be separated by means of cupellation, resulting in clean silver and litharge.

The chloride of copper running from the box, Fig. 13, is led into a reservoir in which old iron is suspended. The copper precipitates in a metallic state on the iron, and about eighty-eight parts of iron go into the solution in place of one hundred parts of copper; consequently, as each one hundred pounds of pure copper require eighty-eight pounds of iron, the calculation as to the necessary amount of iron could be made easily if it were not for some other chlorides which may still be in solution, and which also require iron for precipitation. Wrought iron is preferable to cast iron, and gray cast iron is better than white; but all these sorts precipitate the copper, and it depends to a great extent on the price as to what kind of iron is chosen.

The most effective precipitant is the iron-sponge or finely divided iron, obtained by heating pulverized iron ore or roasted iron pyrites with charcoal powder in a proper reverberatory furnace, or in iron pipes or cylinders without admitting air. By these means the iron oxide is reduced to metallic iron, which precipitates the copper in a few minutes. Using old iron, the precipitation will be effected much more quickly than in tanks if an arrangement is made like Fig 13, putting the old iron in the place of the shavings. This would be a continuation of Fig. 13, but the box must be three or four times as long before it reaches the tank or reservoir.

To find out whether there is yet copper in the liquid, the best test is to take some drops on a piece of platinum, and to place a small, clean piece of zinc on it. The copper immediately appears of a bright red color. But for practical use, a clean piece of iron dipped into the liquid will also show a red coating if there is enough copper in it to make it remunerative to continue the precipitation. The water contains now principally chloride of iron, and is discharged. If by some cheap means the water could be evaporated, the remaining chloride of iron could be used in roasting ores without salt.

Where old iron commands a high price, the copper can be precipitated with a brine of ashes or of lime ; but in this case the iron also falls with the copper. The brine for this reason cannot be advantageously adopted where a great deal of iron is in the ore, or the roasting must be directed so as to decompose the chloride of iron.