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2.) Over these a few large pieces of copper are laid so as to prevent the grains from passing through them, thus forming a false bottom. The tank is then filled with the granulated copper.
Between the real and the false bottom there is an opening .10 meter in diameter, with spout 0.38 meter long, which projects over a shallow launder 0.88 meter wide. (Fig. 1.) These tanks are hooped on the outside and lined on the inside with lead, 16 to 18 pounds to the square inch. The acid mother liquor is turned on to the granulated copper by the siphon-cock until it is wet. The acid is then turned off and the air left to act on the grains of copper, which it does rapidly, as the surface of copper exposed to the action of the air which draws up through the mass is very large. The outside of each grain is thus transformed into a very thin, black film of oxide of copper. The acid is then turned on again and dissolves out the oxide thus formed. This is repeated every three quarters of an hour. The clear color of the liquor running out shows when the oxide of copper has been dissolved, and the liquor is then turned off until the black film forms again. As the copper dissolves out the tank is filled up.
3. CRYSTALLIZATION OF THE COPPER SULPHATES SO AS TO RETAIN
THE INSOLUBLE RESIDUES.
The solution vats are placed in a line (Fig. 2); in front of them are slightly inclined launders lined with lead, which communicate with a series of others at right angles to these. The liquor flows over the bottom in a thin stream, and as the temperature is very much lowered by the exposure to the air of such a very large surface, the crystals of copper sulphate deposit on the bottom of the launders.
These launders are arranged in three double rows for every six solution tanks, the first one being 20 meters long and the two others only 9 meters. Between each set of two troughs there is a doubly.inclined table (Fig. 3), covered with lead, on which the crystals as they form on the bottom are thrown to drain. The distance between the two troughs is 2 meters. The width of the aisle between each set of troughs is 1.50 meters. The total length of all the troughs is 106 meters, and they hare 93 square meters of surface. At every angle a little dam is made to make the liquors fall over it and thus further to cool it. When the crystals have accumulated on the bottom sufficiently they are detached with shovels and thrown up on the inclined tables to drain. After they have drained they are washed with water supplied by four tanks, three by 2 meters and 1 meter deep, placed on one side of the tables, in order to remove the acid mother liquors, and after they have again drained are collected in barrows to go to the solution vats. As no attempt is made to separate it, all the insoluble material which comes from the copper is taken up with the crystals as they form. These are cleaned up two or three times a day, the strength of the solution being kept as nearly uniform as possible. The spent liquors are run into a tank 2.40 meters square, from which they are again pumped up by an injector to be used again. The crystals which form nearest to the solution tanks are the richest in precious metals. Those which form at the end contain less silver and considerable gypsum. The fine material, which remains a long time in suspension, is composed mostly of compounds of arsenic, antimony, and lead. All of the crystals must be sorted according to their composition. This is done by keeping separate all those taken from the different parts of the launders. In this way the residues are classified at once into rich or poor, pure or impure.