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THE SEPARATION OF SILVER AND GOLD FROM BLACK
COPPER AT OKER.
BY T. EGLESTON, Ph. D.
The ores which are treated at Oker come for the most part from Rammelsberg, near Goslar, in the Hartz Mountains. They are lead ores mixed with pyrites, blende, chalcopyrite, barytes, and from 25 to 40 per cent. of sulphur. They contain, besides small amounts of man. ganese, cobalt, nickel, bismuth, arsenic, and antimony.
They are divided into five classes, two of which are treated as lead ores and three as copper ores. The poor lead ores contain about 9 per cent. of lead and 5 per cent. of copper; the rich ones 12 per cent. of lead and 4 per cent. of copper. The rich copper ores carry 15 per cent. of copper and 4 per cent. of lead; the poor ones 8 per cent. of copper and 2 per cent. of lead. They all contain silver and gold, the silver being from .01 to .02 per cent., and the gold between .00005 and .0001 per cent. All the ores are roasted for the manufacture of sulphuric acid except the lead ores. Of these, only those which contain 35 per cent. of blende and 25 per cent. of pyrites can be used for this purpose. The acid made is usually sold, the weak mother liquors being used for the separation of the silver from the copper. The working of these ores comprises seven distinct processes. They are mainly interesting for the separation of the silver and gold, which is done in several ways, only one of which, the separation from black copper, is to be described in this article.
The processes by which the ores are treated are:
1. The lead ores are first roasted and then leached with water to extract the zinc sulphate. This purifies the lead ores and gives a zinc sulphate containing 3 per cent. of impurities, the greater part of which is manganese. It is melted in its own water in a copper vessel, sifted to separate the large pieces, and sold.
The lixiviated ores contain lead, copper, and silver. They are treated in Piltz furnaces, which give lead and a lead and copper matte. The copper matte produced is roasted and is charged in a furnace 6 meters high and 1.50 meters in diameter at the throat. The throat is closed with double doors 1.25 meters square. The top of the furnace is made of iron, and constitutes the chimney. As the ore contains 20 per cent. of copper, the charge is made up of 75 centners of roasted ore, 25 of unroasted, 20 of rock with only a small amount of copper, 50 of slag, or 170 centners altogether; 213 centners are put through every 24 hours. For the charge 60 pounds of coke are used. The product is about 80 centners of mattes at 35 per cent. of copper. Krum furnaces were formerly used here. There are still four of them standing, but they are to be torn down and replaced by Piltz furnaces. The Piltz furnaces have a slag-hole and two tap holes, one for matte and one for slag. The slag runs into cones .45 meter in diameter and .75 meter deep. The matte is tapped into a basin. There are eight lead furnaces.
2. The mixed ores containing lead and copper are roasted and smelted, producing a lead which contains silver and a copper matte which is treated with the other copper mattes. The lead is cast in little round cakes .20 meter in diameter and about .16 meter thick. They contain a great deal of copper. To purify the metal it is melted in large iron kettles holding about 10 tons. The scums taken off are liquated in the next kettle and are treated for copper.
The silver lead is cupelled in cupel furnaces, of which there are three, two with movable hearths, and one with a fixed one. They all work with coal. As there is an easy sale for the litharge, the lead is purified in the kettles and all the pure litharge saved and sold. The screening is done in an ordinary trommel. The litharge is packed in barrels. There is no special arrangement for taking off the lead vapors, and cases of lead-sickness are quite common. It would be very easy to protect the men if the old ways of doing things were not kept up. Nothing is more remarkable about these works than the way old processes have been preserved.
3. The rich copper ores are smelted for a matte of 35 to 40 per cent. copper, concentrated to 66 to 70 per cent. smelted to black copper and treated for silver.
4. The ordinary copper ores are roasted with salt, leached with weak acid, and precipitated with iron or iodides.*
5. The concentrated mattes and the cement copper are treated in a reverberatory furnace. The silver is concentrated in bottoms, part of which is cast into plates for the electrolytic process, and part is granulated for the extraction of silver. The rest is copper poor in silver, which is sold.
6. The granulated copper is treated with weak acid for copper sulphate and the gold and silver residues smelted.
7. The silver is parted by solution in sulphuric acid and precipitated with copper.
The process of the separation of the silver from the black copper consists of several operations:
(1) Granulating the black copper. (2) Solution of the black copper granules in the acid mother liquor.
(3) Crystallization of the copper sulphates so as to retain the insoluble residues.
(4) Solution and classification of the sulphate crystals.
1. GRANULATION OF THE BLACK COPPER.
The silver is concentrated in bottoms in such a way as to make them almost entirely free from iron, cobalt, and nickel, whose presence is objectionable because these metals gradually increase in the mother liquors and make the quality of the product impure, unless they are from time to time withdrawn from the circulation and treated for these metals. The iron sulphate cannot be separated from the copper sulphate without a special oxidation to make insoluble sesquioxides. The nickel and cobalt could be crystallized out, but this complicates the process unnecessarils, so that they are separated as far as possible in the dry way. The silver having been concentrated in the copper in a reverberatory furnace, is ready for treatment and is granulated as it is cast from the furnace.
* Trans. Am. Inst. Min. Engs., May, 1885.
This concentration makes it possible to produce about half the copper made at these works to contain only .07 per cent. of silver, which is so. poor that it is sold. The other half contains .16 per cent., and this is granulated. The object of the granulation is so to multiply the surfaces of contact that the solution may be made in the shortest possible time. It is done by causing the copper to run in a thin stream from the furnace into a water tank into which a stream of water runs continuously to keep down its temperature. The copper is made to run across this stream of water. If it is at the proper pitch and temperature the granules are formed perfectly, but there are many precautions to be observed. The copper must be sufficiently refined, and at as low a temperature as it will run, to avoid the danger of not cooling the copper sufficiently. The depth of the water and the supply must be sufficient to have the water cool enough at the bottom to insure that the copper will remain solid. To make this certain, the cold-water supply in some works is divided ; one runs in at the top, crossing the copper stream to scatter the copper; the other is carried down by a pipe to the bottom of the tank, to insure the cooling of the mass there. If the copper is very hot, or the stream of metal too rapid, or the supply of water not sufficient, there might be danger of the copper being at such a temperature as to undergo only such a superficial cooling that, when it is altogether in mass at the bottom of the tank, it will readily become liquid again. The grains are sometimes cast hollow,* but tbis is done only when the copper is to be oxidized in a furnace, and not, as in this process, by means of air and sulphuric acid.
2. SOLUTION OF THE
BLACK COPPER GRANULES IN THE ACID
This is done in a large building with several stories. In the upper one there are four lead-lined vats, 3.60 meters long by 1.70 meters deep, by 2 meters wide, in the clear. (Fig. 1.) They are held together in frames which are 15 centimeters square. The whole vat before putting in the lead is painted inside and out with tar to prevent the absorption or loss of the liquors in case of a break in the lead lining. All the wood of the building is treated in the same way. These vats are heated by steam to 700 R. At the end of every row there is a tank one meter square in which the steam is condensed. The lead to line the vats weighs eight to twelve pounds to the square foot. The pipe for heating the liquor lies on the bottom of the tank and turns on itself seven times. It is of lead .05 meter in interior diameter and .005 meter thick; both ends of the pipe are turned up. In each one of these vats there are four lead siphons, each one of which has a stop.cock at the bottom to regulate the flow of the acid, and a funnel on the side with which to start it. They are supported from the roof. Each one of these siphons communicates with the copper-solution vats below. (Fig. 1.) The spent liquors from the crystallization-tanks are pumped up into the acid-tank with a Kortings injector made of lead, the points of which are made of very hard lead. The liquors are concentrated with fresh acid at 500 B. to 300 B., and the acid mother liquors made to flow over the granulated black copper contained in the leaching.vats below. These are conical, .88 meter in diameter above and .72 meter in diameter below and 1.62 meters high. On the bottom of the tank, on both sides of the bottom opening, pieces of wood about .15 meter high are placed. (Figs. 1 and
Trans. Am. Inst. Min. Engs., Vol. IV, p. 297.