all the base metals present; an excess of it would rapidly corrode the plumbago crucibles in the subsequent operations. The niter not only assists in furnishing the oxygen for oxidation, but it assists in fluxing the zinc oxide, forming zincate of potash, which is not so readily reduced as zinc oxide. 33. Melting the Oxidized Precipitate. The dry residue is broken into small lumps, weighed, transferred to a plumbago crucible, and mixed with suitable quantities of flux. The fluxes commonly used are bicarbonate of soda, borax, and clean quartz sand. There is considerable variation in the proportion of fluxes and precipitates. If there should be much sand present (which would give a glassy but thick flowing slag), the best corrective is more soda with a little flour. When the slag is too basic (that is, a dull, lusterless one), additional borax will neutralize the base and improve the slag. 34. Fluxing Precipitates.-Some of the fluxes used are given in Table I. Any one of them may have to be increased or decreased, according to the amount of impurities present. The whole of a charge as given in this table will go into two No. 35 plumbago crucibles.* * See Table V. The crucibles are placed in the fire and the contents fused until perfectly fluid. The crucibles are then withdrawn from the fire and the contents poured into molds. The metal settles to the bottom of the mold, and after cooling is turned out and the slag removed with a hammer. The bullion thus obtained is remelted with borax and run into an ingot. The second melting should be conducted at as low a temperature as possible, since gold forms a very imperfect alloy with zinc. The slags generally contain a considerable amount of gold, and should therefore be crushed and melted with a little borax glass and poured when fluid into an ingot mold. After cooling, the slag is removed from the bullion. MODIFICATION OF THE CYANIDE ACTION OF THE ELECTRIC CURRENT ON GOLD SOLUTIONS 35. Electrolysis. When the electric current decomposes a solution of a metallic salt, the metal is carried to the negative pole, or cathode, of the electrolytic cell and deposited, while the metalloid is liberated at the positive pole, or anode. In a given time a fixed quantity of current will release and deposit a definite quantity of metal. The quantity of metal released at the anode and deposited at the cathode varies with different metals, being in direct proportion to their electrochemical equivalents. This law does not hold good for solutions containing very small quantities of the metal in solution, as in cyanide solutions, for the current does not find sufficient metal present at the electrolytes, consequently the water is decomposed. To make the precipitation as efficient as possible, the solution is kept in constant diffusion by a steady flow through the precipitating box. 36. Conditions that the Siemens-Halske Cathode Must Fulfil.-The cathode should be a material to which the gold will adhere; it should be capable of being rolled into thin sheets, which are of such a character that the gold can be recovered without loss or great expense. The cathode should be more electropositive than the anode to prevent return currents being generated when the depositing current is stopped. Thin sheet lead has been adopted as the most suitable metal for the cathode of the SiemensHalske method of precipitation. The lead sheets are fastened to light wooden frames. Each frame contains three sheets of lead 2 ft. x 3 ft.; this gives each frame 18 square feet of surface. There are 87 frames in each precipitating box, giving an exposed surface of 1,566 square feet. The three sheets of lead in each frame weigh 3 pounds; this makes 261 pounds of lead in each box. 37. Carbon has been used for anodes, but it crumbles under the influence of the current that decomposes the cyanide Zinc used as an anode forms a white precipitate of ferrocyanide of zinc when the ores leached contain iron. Iron anodes form Prussian blue by the reaction of oxide of iron and ferrocyanide in the solution. The iron plates are covered with canvas to prevent short-circuiting and to collect the Prussian or Turnbull blue that is formed by the ferrous and ferric salts with cyanide solutions. plates are used in South Africa as anodes and lead cathode plates are suspended between them. Cyanide can be recovered from the Prussian blue by dissolving it in caustic soda, evaporating the solution, and melting the residue with potassium carbonate. 38. Electric Current Required for Precipitation. A weak current, one with a density of about .06 ampere per square foot, is required for precipitation. It can be produced by volts when the cathodes are about 14 inches apart. The advantages claimed for such currents are that there is a firm deposit of gold and that the iron anodes are decomposed very slowly, their waste being proportional to the current. Theoretically, 3 horsepower would be sufficient to run a plant having a monthly capacity of 3,000 tons; but the amount actually required, however, was 5 horsepower. 39. Advantages of Electrical Precipitation.-Gold may be precipitated from solutions by electrolysis independent of the strength of cyanide solutions. In the treatment of tailings, therefore, very dilute solutions can be used, the only limit being sufficient cyanide to dissolve the gold. Electrolysis does away with certain complications met with in zinc precipitation, such as the formation of alumina, lime, hydrate of iron, etc.; besides, it is cleaner and simpler, and gives a higher grade of bullion. Its application seems to be limited to those plants where very weak cyanide solutions can be used successfully. 40. Working Results.-At the Worcester works, South Africa, the strong solution contains from .05 to .08 per cent. of KCN and the weak solution about .01 per cent. There are four precipitating boxes, 20 feet long, 8 feet wide, and 4 feet deep. Heavy copper wires are fixed on the sides of the boxes to convey the current from the dynamo to the electrodes. The anodes are iron plates 7 feet long, 3 feet wide, and inch thick. They stand on wooden strips that are laid on the floor of the tank and they are kept in a vertical position by wooden strips on the sides of the box. To facilitate the circulation through the box, each alternate plate is raised 1 inch above the bottom, thus forming a series of compartments through which the solutions must rise and fall alternately through the successive compart ments. In a clean-up, the frames carrying the lead are removed one at a time, the lead is removed and replaced by a fresh sheet, and the frame returned to the box. The lead that contains from 2 to 12 per cent. of gold is melted into bars and cupeled. In treating 3,000 tons of tailings, 750 pounds of lead and 1,080 pounds of iron were consumed. PRECIPITATION WITH CHARCOAL 41. Johnson's Process.-One of the many methods proposed to take the place of zinc shavings in the precipitation of gold from auro-potassic cyanide solutions was the use of charcoal. It had been previously employed for the precipitation of gold from chlorine solutions, and there was no chemical reason why it should not be applied to a cyanide solution. The process patented by a Mr. Johnson consists in filtering a gold-cyanide solution through pulverized charcoal, from which the gold is recovered by burning the charcoal and smelting the residues with suitable fluxes. The process is considered too slow for large plants. The probable reaction that occurs may be expressed as follows: 2AuK(CN)+2CO, + 2H,0 = 2Au + 2KCO, + 4HCN 42. The charcoal filter used at the South German mine, Moldon, Victoria, is shown in Fig. 3. It consists of a tub a 2 feet 4 inches high, 2 feet 1 inch in diameter at the top, and 1 foot 9 inches in diameter at the bottom. In the center of each tub and resting upon wooden cleats c is a glazed drain pipe b 4 inches in diameter. The pipe and tub are nearly filled with charcoal, after which the drain pipe is placed under the mouth of a pipe e connecting with the gold-solution tank. The solution passes down the pipe b and out through the bottom, then rises through the charcoal in the tub and flows |